Carl W. Hall - Drying and storage of agricultural crops-The AVI Publishing Company (1980)

AND STORAGE OF — AGRICULTURAL CROPS Pipes Cet.) Dryeny ond 4Q) ADT iO; Ai. soe 4 “A 1774 Don Gresswell Ltd., London, N.21 Cat. No, 1207 Ii TROCTAM ;CESSION ea RILIMQEH ead Ee NUMBER nae ELAINE RAMer 5 Se . DRYING AND STORAGE OF AGRICULTURAL CROPS Carl W. Hall, P.E. Dean College of Engineering Washington State University Pullman, Washington & AVI PUBLISHING COMPANY, INC. Westport, Connecticut © Copyright 1980 by THE AVI PUBLISHING COMPANY, INC. Westport, Connecticut Copyright is not claimed in any portion of this work written by a United States Government employee as a part of his official duties. All rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means— graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems—without written permission of the publisher. Frontispiece Courtesy of Butler Manufacturing Co. Library of Congress Cataloging in Publication Data Hall, Carl W Drying and storage of agricultural crops. Includes index. 1. Field crops—Drying. 2. Grain—Drying. 3. Hay—Drying. 4. Field crops—Storage. 5. Farm produce—Drying. I. Title. SB186.2.H34 631.5'6 80-21074 ISBN 0-87055-364-X Printed in the United States of America by Eastern Graphics, Inc., Old Saybrook, Connecticut Preface The predecessor of this book published in 1957 was Drying Farm Crops, which was one of the early books in the field. The later book Drying Cereal Grains was more technical and provided more recent coverage of the subject. At the previous writing, drying of farm crops was gaining recognition as an important part of the normal farm operation. Drying is now an established practice in many parts of the country, particularly in the midwest corn belt and the eastern and southern United States. In the last 20 years, the attention has turned to energy source and costs, environmental impacts, and handling systems for drying. These issues are covered throughout this book. Major emphasis is placed on the common cereal grains and hay, but the principles discussed apply to other crops. The drying of some agricultural products other than farm crops is also presented. Problems are included at the end of each chapter to make the book useful as a text. A complete, updated reference list is furnished to aid those who are interested in studying drying and related topics in more detail. The chapters are nearly complete in themselves; thus, a reader without an understanding of mathematics and physics can learn the procedures of grain and hay drying without becoming involved in the theory. The reader may also, if he or she wishes, omit the theoretical material within a chapter and obtain only the practical recommendations. Topics relating to a successful drying program, such as moisture movement, measurement of moisture content, storage, and handling, are covered. Heat transfer, fluid flow, and mass transfer are covered from a unit operations approach, making the book readily adaptable as a text for general agricultural processing, as well as drying, courses. CARL March 15, 1980 W. HALL ARE Eee AG <i Nomi ee te REN) = a: cvs ran site Ce Le ee Les , id & eee igs viel f oo meEy Gs Waa ; , 4 f° ie & "els ie rT rae Mel nd Poe) a VTi ta? 94 el ube aia ai | by as a : Ve 2 hem hae Ee Sb yn — ba ba va ae: m1 1 Ee a rata “. Jae sdelges ey A AK if uneBi rind dsiensds " ye - &,<# ea Ma ‘a SS rs ' hee: Sar Tohng ‘mere a ves ; lhe Lal : Nee ra kL (We hese tee? ¢ — we Seeman nly! me Agra Sot ig rare. al ah y ee bt eF bas MS «5 " 0031 [* SRPj id Bl a i : rn eety. Cae Yon ce cardia ney wie Pw | ? oe a nr te bik on pe al cif i es a Oe t Bien mg: : abs ; oe saeewUse % Wey ipa eS ee cn ae Aa ate Busy oh aes tye al pt ' Pim fha etum 4 “ae te N rr els ieSIE Say x. ere cl Bc 48 (Wiis taismar'y DoW |i 0 “iLVed inte Aces. wae, ei i : | Oe ont a ja ais ie OF ae : rT vale ta ad WH ire we malty vee ; eRe ke ae ik \ ois es) Ly rT fi: , oS vs, Ais Nh ae Seal bog cee iC oa%s re . wets Hears z ye a emt eee ~~ “iheBa hele Savi ri¢ ape i Pee a0" eet ; SCRA ING h ah oie Aarredt 7 ea nae hehe: = 7, Tuas An ae 8 $date 7? Ui Sol eho Wy saconoaec ad eked Niven. ; | Peet ae Jon yar) Adsit vt! payin Pere an 1 7 2 ahs pilesiy oe 7 phy ee eye el - cy - Tete Hee ciety a4 gales ners ' hal XS, Mea cae preks {94 tli " ' 7 engi vite : a onya he i fh ols 2 Acknowledgments The original book Drying Farm Crops was written with the encouragement and assistance of colleagues and students at the time (1955-1957). The high demand for and interest in that book, the first of its kind, helped promote an association and working relationship with others which further developed the field and generated more books devoted to drying. Included among these were Drying of Milk and Milk Products, co-authored with T.I. Hedrick, and Drying Cereal Grains, co-authored with Donald Brooker and Fred Bakker-Arkema. The assistance of several colleagues in preparing chapters for this book is gratefully acknowledged: D.L. Calderwood, U.S. Dept. of Agriculture and Texas A & M Univ.; D.C. Davis, Washington State Univ.; O.R. Kunze, Texas A & M; R.L. Maddex, Michigan State Univ.; and G.C. Shove, Univ. of Illinois. Thanks also go to the many additional people who provided information incorporated in the book. Numerous individuals reviewed chapters or sections of the book, providing valuable suggestions. vil ee, Boren ii, ken wi‘ = md a ones, _ of hee vibe we 'e ’ trope AM a¥ Sam) ie | payee pod J in? ah ” eats ies ry Nie A re an mola wi £ a 5 he 447 e “4 5 cell ’ ins viv Ly hae veaews . 4 ye WAP u yi4 i © oa) i Lan ‘A tobe AAT ea m2 MDA wii : 609 1Hiw inveees i e 5 ¥ yen 7 f a ' LY diy tae isfy? big ' bee ,“ nee omienng 2 {cere mate ‘lends Pevine (een ; ‘ eC OF ea e Git sae, if is 7 tases ab te winnie Re. a atebeal ¥ Naoios i Ls seceT bik: @& & dened) Sie veri ia well nil’ 9 oe leila ef wryoe wilt ’ Contents PREFACE ACKNOWLEDGMENTS History and Importance of Drying 1 Equilibrium Moisture Relationships 16 Ondo Moisture and Temperature Changes and Effects 39 Moisture Content Determination Airflow and Air Distribution Theory and Principles of Drying 68 92 120 Heated Air Dryers 151 oO-, OAD Natural and Forced Air Drying of Grain and Ear Corn 180 Systems for Drying of Rice, Otto R. Kunze and D. L. Calderwood 209 10 Systems for Handling of Grain, Carl W. Hall and R. L. Maddex 234 258 1 Systems for Drying and Handling of Hay 12 Systems for Solar Energy Drying on the Farm, Gene C. Shove PASI 13 Moisture Control and Storage Systems for Vegetable Crops, Denny C. Davis 310 APPENDIX 361 INDEX aie JORGARD: cy é ; ; ae a 4 : ay, Te yak <br asi i a i <a YM ol 7 LS La 4 ai)Lenya apie ) 14913) a>% is »{ ‘ ¥ A lb Sh) ‘Te iP fe 7 pe 18 7 ie hel} baal ert Mf longed re ood i : yap? a r mad ¢ jt : aon i] | a ey: a tga +0 : cA ve! al are gar ry: yP4 at beth Pt) Os ie errs LES > Saul Way = gern eb tat battens ht , oa ent “i © Yuiel te cog? ca TPR ive : St |. | 8 vow, tbe an ‘ ie “6 : pe ee ide), WA an a wii OS 2h et: a j Tk", ¥ oer mle eMiowal 1 we ana mal ‘ DB a renal anh inate eee! Liwreh Te. gree Meee 4) Pais shales fra, Layvitae - ijl Lae rz hie % iC y , tise ry tae¢t¢ Mgrs vie Lar (da) IESa ible oa sons” cet i ; | aise 73 WOT ty a : ; 2 ry ig o] ut * fr “ * Gs . / ’ History and Importance of Drying HISTORY Drying as an Early Art Drying of agricultural crops dates back to the beginning of civilization. The use of solar energy and air movement provided the major method of moisture removal in the field. Crops for human consumption were occasionally dried in ovens or by hanging in heated rooms. Cereal grains were dried in the shock before husking or threshing. Sprouting of corn, wheat, barley, and rye was not uncommon. Molding of kernels often occurred. Natural air drying of ear corn in the field, for example, first occurred on the stalk, then in the shock, and if slightly wet, continued drying in the husked pile formed from each shock. Further natural drying occurred in a narrow crib, through which air moved. Experimental Dryers Between World War I and World War II, several experimental mechanical drying units were built and a few commercial units were in operation. Commercial dryers were primarily for dehydration of fruits, vegetables, and hay; drying of seed corn with heated air; and drying hay in the barn; usually with unheated forced air. Commercial Dryers Commercial and large scale farm drying became a common practice after World War II. The increase in mechanization Large quantities of moist requiring moisture removal increase in drying was coupled to the rapid and increase in land and labor productivity. or wet products were produced at harvest to avoid loss during subsequent handling and storage. The picker-sheller replaced the picker, in which shelled instead of ear corn was produced at harvest. Shelled corn, in the place of ear corn Askham Bryan of Asricuitre eins * 5) College and Horticulture a = Tay: g i Jeeirs j 2 DRYING AND STORAGE OF AGRICULTURAL CROPS that dried out between harvest and use, was too wet for normal storage and had to be dried. Speed of operations from harvest to storage forced the consideration, study, and use of heated air for drying. Rapid drying in shallow layers, usually with temperatures below 93.3°C (200°F) predominated. Grains for seeds were dried at 46°C (115°F) or less. Slow drying, using low temperatures, in deep beds up to 3.65 m (12 ft), usually the storage beds, provided another approach. As handling the products to be dried became more important, the use of wagon dryers with the heated forced air developed. Bins and containers for drying were designed to minimize handling and facilitate the movement of grains, forages, and fruits and vegetables. Quality of Grains The trend to higher temperatures of air for more rapid drying was limited by the use to be made of the product. Low temperatures, below 46°C (115°F), were used for grains for seed, medium temperatures, below 54.4°C (130°F), for milling and processing, and temperatures below 82.2°C (180°F) for animal feed. Studies showed that cereal grains developed checks and often cracked if dried rapidly, decreasing quality and ability to store without spoilage. Various methods evolved for decreasing damage to grains being dried. Wet grains were dried in stages. Grain was circulated or stirred during drying. Low Cost Energy The end of World War II and the 1950s witnessed the availability of cheap energy. During that period, even though many researchers evaluated the thermal efficiency of systems, there was little incentive for greatly decreasing energy costs. However, fuel costs increased and as fuel availablity became less assured, considerable emphasis on efficiency of energy use developed. Recycling of product and air, airflow direction, air velocity, cooling product, and use of solar energy received attention. Greater risks were chanced by leaving the product in the field longer in order to reduce the moisture content before harvest. Drying is now considered as a part of the system of grain production to a greater extent than previously. Handling and storing operations are important aspects of drying, as well as the total production system. Not to be overlooked also are the effects of each operation not only on drying, but also on wetting, cracking, germinating, and grade. HISTORY IMPORTANCE OF AND IMPORTANCE OF DRYING 3 DRYING Loss The annual loss from harvest to use of grain is estimated at 10% and of hay at 28% of the production. The loss of fruit and vegetables is estimated at 35 to 40% of the production. Moisture control, primarily by drying, provides an opportunity to prevent losses which occur during harvesting, handling, and storing. The world production of grains and potatoes is 1% billion metric tons (MT), of which the United States produces about 15%. The carryover of corn has varied between 15 and 30 million MT in the 1970s; of wheat, up to 25 million MT; and of rice, 0.52 million MT. Approximately 40% of the carryover is stored on farms. To prevent excessive losses, the products must be stored and maintained at the proper moisture content, and proper moisture content for storage is usually below the moisture content at harvest. Prices of products usually increase during the storage season thus encouraging farm storage. 400 350 350 Coworld — United aoe States 250 250 200 200 150 y| (MILLION) TONS (MILLION) TONNES 100 #0 A\Y as=ee A\Y AV y , Z Homies ooOF Se RS ae Se FT ean a Tl See ae ese ee ee 2 Se ae aS Vii), We Wheot AVBES ANGLALALALAL: Al) Dao fh niMIAIA|Alblp|MIYI 8 aa 3 ° a TY aes Lenk SR ee aan | Conn > PE oo eek cugitae te tee igps ee Te Eis Re eo os In TO MesD te ee eo a 9 Se Oe BES ” From Hall et al. (1977) FIG. 1.14. FOOD WORLD, 1975 AND FEED PRODUCTION FOR THE UNITED STATES AND THE 4. AND STORAGE DRYING OF AGRICULTURAL CROPS Preservation Various methods are available of preserving crops or increasing their usable economic life. These methods include the following: Canning.—This is a food preservation method in which food contained in a permanently sealed container is subjected to an elevated temperature for a definite period of time and then cooled. When heated, spoilage microorganisms are destroyed or their growth inhibited. Refrigeration or Chilling.—The rate of respiration of the food product and of the associated microorganisms is reduced in a low temperature atmosphere. By reducing respiration rate, storage life of the product is increased. Mold growth in grains is reduced by reducing the temperature, generally, from a practical standpoint, to 17°C (63°F) or below (Burges and Burrell 1964). Controlled Atmosphere.—CA (controlled atmosphere) storage and chemical gas treatment are becoming increasingly important and presently are in limited use for storage of wet shelled corn and apples as a method of preserving the product or increasing the storage life. Controlled atmospheric storage for apples is used in conjunction with a refrigerated storage. The normal atmosphere surrounding a product in storage is 21% oxygen, 0.03% carbon dioxide, and 79% nitrogen. In the controlled atmosphere storage the oxygen level is reduced and the carbon dioxide level increased. There are several engineering considerations involved in the design of a controlled atmosphere storage. McIntosh apples, when stored under controlled atmosphere conditions of 3% oxygen and 5% carbon dioxide at 3.3°C (38°F), may have twice the storage life of apples stored in air at 0°C (32°F) (Pflug and Dewey 1955). Dehydration or Drying.—These terms refer to the removal of moisture, usually by heat and air movement, to provide an environment in which organisms—yeasts, molds, and bacteria—that cause spoilage cannot grow, or grow very slowly. The use of a brine in which moisture is moved from the product to the brine is a form of drying. Chemical Treatment.—The environment surrounding agricultural crops and products can be made unfavorable for the growth of microorganisms by excess saltiness or acidity. The development of acidity, for example, limits the growth of microorganisms in the preparation of sauerkraut. The process of making silage is an example of chemical treatment in which carbon dioxide is produced by decomposition of carbohydrates, set into motion naturally or by addition of preservatives. The aerobic microorganisms are inactivated by the lactic acid produced (Bohnstedt 1944). Wet shelled corn sealed storages combine the effects HISTORY AND IMPORTANCE OF DRYING 5 of surrounding the product with carbon dioxide produced by the corn and the chemical changes of the corn. Organic acids, such as propionic, acetic, butyric, and formic acids, and anhydrous ammonia are chemicals used for preserving stored or baled moist farm crops. The amount of propionic acid suggested for wet grain is given in Table 1.1. Propionic acid is effective in delaying the growth of mold and reducing heating, dry matter losses, and moisture losses in baled hay. Hay in bales of 27% moisture content or less was completely protected from molding by using 2 to 3% propionic acid. Three percent acid in baled hay with 35 to 45% moisture content did not entirely prevent molding (Nash and Easson 1977). TABLE 1.1. APPLICATION RATES OF PROPIONIC ACID FOR PRESERVING Level of Propionic Acid by Wet Weight, % Level of Moisture For 1 Month For 6 Months 18 20 25 0.35 0.40 0.53 0.45 0.50 0.75 3D Veils) 1.40 40 1.40 legiey % Storage GRAIN Storage Source: Jones et al. (1974). Mixtures of organic acids are often more effective than a single acid. The preservative must be uniformly mixed throughout the grain. Care must be taken in handling organic acids to avoid damage to the skin, eyes, and lungs. Acids can cause rapid deterioration to metal storages, which can be protected with chlorinated rubber, plastic paint, vinyl or plastic cover or lining, or epoxy paint. Use of Subatomic Particles.—This is a process, more commonly called cold pasteurization or sterilization, in which beta or gamma rays are used for treating the product so that the microorganisms are ionized. Artificially accelerated electrons are known as cathode rays (beta) and naturally occurring radiations such as from radioactive substances are gamma rays. The ionization may sterilize the microorganisms so that reproduction does not take place, or if the dosage is large enough, kill the microorganisms. The approximate effect of different levels of energy treatment on various products is shown in Table 1.2. Moisture Removal Dehydration refers to the removal of moisture from a product until it is nearly bone dry. Drying in agricultural work refers to the removal of 6 DRYING AND STORAGE OF AGRICULTURAL CROPS TABLE 1.2. APPROXIMATE EFFECT OF DIFFERENT ENERGY LEVELS OF IONIZING RADIATION Product Energy, rad Effect Enzymes Virus 107 2a Oe to destroy to destroy Spore forming bacteria and molds Non-spore forming 108 ms to sterilize bacteria and molds 10° to pasteurize Insects 10° to kill Insects 104 Plants Man ; 103 5 X 102 to sterilize to induce mutations to kill Rad (radiation absorbed dosage) is the unit of absorbed radiation of 100 ergs per gram of irradiated material. moisture until the moisture content of the product is in equilibrium with the surrounding air, usually 12 to 14% moisture, wet bulb (w.b.). Reducing the moisture content of a crop to a value between 0 and 12% is called either drying or dehydration, depending on the author. Conditioning is very closely related to drying. In some instances, the grain is conditioned by removing or adding a small amount of moisture, about 1 or 2%. Generally, however, conditioning refers to maintaining uniform moisture and temperature throughout the stored product by moving a small amount of air through the grain, or moving the grain through air. There are many different methods of drying solids, liquids, and gases. Most of these methods are used to some extent in the drying of agricultural products. Freeze-drying.—This is a method for removing water from fluids or solids in which the product is first frozen. For freeze-drying liquids, such as removing water from fruit juices, a frozen slush is centrifuged to remove the ice crystals, or the product is slowly heated so the frozen water crystals melt and water is removed by draining. For freeze-drying solids, the ice crystals sublime under vacuum, thus removing the water. Decomposition of Water by Chemical Means.—Water is removed from a solid by chemical reaction, such as the reaction of calcium carbide or calcium hydride. Calcium carbide reacts with water to produce acetylene and calcium hydroxide. Adsorption.—Water is often removed from a gas by adsorption. The molecular forces on the surface of a solid are unbalanced and an ionexchange takes place, as in water treatment by the zeolite method. When the solid adsorbs the gas, heat is evolved and the rate of adsorption decreases rapidly as the unaffected surface decreases (Maron and Lando 1974). The adsorbed material remains on the surface of the adsorbent only. Alumina and silica gel are used for drying air by adsorption. HISTORY AND IMPORTANCE OF DRYING 7 Absorption.—Water or water vapor may be removed from solids or gases by the capillary action of porous materials. The water, in liquid or vapor form, passes through the surface of the absorbent and is distributed throughout it. As an example, water is absorbed by calcium chloride in the same manner as water is absorbed by a sponge. Com- bination adsorption-absorption phenomena often occur. Mechanical Separation——Moisture can be removed from materials using centrifugal force or gravity. Centrifugal force is used for removing free water from wool, textiles, and beet sugar. Vaporization.—The most important method of drying pertaining to farm crops is by vaporization. Heat must be supplied to the wet product to change the water to a vapor. The amount of heat required depends on the pressure and temperature at which vaporization occurs. Heat is taken from unheated air at atmospheric temperature for natural air drying. Vaporization may occur under standard atmospheric conditions or in a vacuum chamber. Where the flavor of the product is affected by using high temperatures, vacuum drying at low evaporation temperatures is practiced. Chemical Desiccants——Chemical plant desiccants may be used to cause plants to mature earlier, thus reducing the subsequent problem of drying following harvesting. Changes in Stored Products There are many changes which occur to a product during transportation, handling, storage, and preserving. Chemical Changes.—The effect of canning upon the minerals, proteins, and vitamins of various food products is a large scientific field in itself. In stored hay and grains, changes occur in the fat acidity, enzymes, color, and vitamins. These changes are influenced greatly by the moisture content and temperature which are often used as a means of indicating the quality of stored products. Respiration and Heating.—In hay, grain, fruit, and vegetable products respiration or breathing continues after storage. Heat is produced by the respiration process. The quantity of heat produced is greatly influenced by the moisture content and temperature of the product. Loss of viability or germination often occurs after heating. Movement of air through the products helps to prevent excessive temperatures by removing heat and excess moisture. Microorganisms.—Changes occur in the amount of mold and yeast growth in the stored product. These changes are largely dependent on 8 DRYING AND STORAGE OF AGRICULTURAL CROPS the temperature and moisture availability and are used as an indication of the potential storage life of many products. Insects and Rodents.—The population of insects is affected by the moisture and temperature of the environment of the stored product. Ventilation presents a method of controlling growth of insects, but after they get out of control, chemical means are commonly used. Importance of Drying The drying of farm crops offers the following advantages by permitting: Early harvest which reduces the field loss of products from storm and natural shattering and permits working the soil early for fall seeding. The field conditions (dry and fewer weeds) are often better for harvesting earlier in the season. Planning the harvest season to make better use of labor. Farm crops can be harvested when natural drying conditions are unfavorable. Long-time storage with little deterioration. Extended storage periods are becoming increasingly important with the large amount of grain being stored and carried over through another storage year by the farmer, government, and industry. The farmer’s taking advantage of higher price a few months after harvest. The increase in value of corn and wheat from harvest to the market peak price is usually about 10% of the market price at time of harvest, although in some years there is no price advantage. Maintenance of the viability of seeds. By.removing moisture the possibility of the grain heating with subsequent reduction or destruction of germination is decreased. The farmer’s selling a better quality product which is worth more to him and to those who must use those products. Use of waste products. Beet pulp can be utilized over a long period of time after it has been dried; otherwise, the storage life would be short. Losses of Farm Crops The losses of farm crops can be used to indicate the importance of proper harvesting, storing, and drying. About 10% of the grain grown on the farm never reaches the market because of losses during harvesting and farm storage. Many of the losses which occur in the field can be eliminated by harvesting at the proper stage of maturity followed by drying (Table 1.3). About 28% of the field hay crop never reaches market or its intended use because of loss. The total estimated annual loss during HISTORY AND IMPORTANCE OF DRYING 9 harvesting and storing of cereal crops and hay in the United States is over $1% billion. The losses during harvesting and storage of several agricultural products are shown in Table 1.4. TABLE 1.3. HAYMAKING LOSSES AS RELATED TO WEATHER Loss! of Starch Details of Treatment and Weather Equivalent, % No mechanical loss,? no rain Mechanical loss, no rain Rain 1—2 showers for 1—20 min 5—6 showers for 12—63 min Average of all trials 23 39 50 44 54 45 Source: Bohnstedt (1944). ‘Loss during haymaking based on nutrients in fresh hay. *Mechanical loss refers to loss of stems, leaves, etc. TABLE 1.4. ESTIMATED LOSS OF PRODUCTION DURING HARVEST AND STORAGE! Product Cereals (wheat, oats, rye, barley, rice) Corn Cotton, lint Harvesting, % Storage, % 5.0 4, Des 4.5 6.0 0.25 7.0 8.0 Seeds of grasses Seeds of lezumes Wt) 30.0 = = Sorghum, grain Soybeans for beans 15.0 50) 6.0 — Hay Potato 21.0 7.0 'Percentage of expected production without loss. The loss of seed from hay standing in the field is often from 20 to 30% of the production. Most of this loss can be prevented by early harvest followed by drying. sun and moving the In the past years to dry in the shock. Drying can be done either by natural drying in the seed periodically or by mechanical drying. many crops were left in the field after harvesting After drying in the field, ear corn was husked and placed into the crib and there was little problem of excess moisture because the moisture was removed in the field. However, now it is more common to move high moisture grain into storage immediately after harvest (Table 1.5). The nutritional changes which take place in forage products during growth in the field, drying, and storage give valuable guides for proper handling and drying: (1) More than 5 times as much carotene in alfalfa is preserved with artificial drying as compared to sun drying, with a negligible carotene loss during artificial drying. 10 DRYING AND STORAGE OF AGRICULTURAL CROPS TABLE 1.5. APPROXIMATE MOISTURE CONTENT DURING HARVEST AND STORAGE, % (W.B.) Maximum Usual When ; Reported Optimum During Harvested Approximate Maximum Harvest Minimum Loss Harvest for in Central States For1 Year Oats 35 28 = 32 24—30 16-20 16-20 14-30 ale 10-18 13 13-14 13 Grain sorghum — — 10-20 13 TOS tat Rice Soybeans Hay 30 —= ioe — 70-80 16-24 9—20 — 13 13 20-25 = 10 15—20 Crop Corn Wheat Barley Pea beans During 28 — 22 10-18 17-20 for Safe Storage 13 1 For 5 Years 11 2 iB 11 — (2) By early and frequent clipping of forages, from 40 to 60% more protein can be obtained. Protein purchases by the farmer can be reduced. Early harvest and frequent clipping combined with drying provide a method of preserving the protein. When hay can be field dried without being damaged by rain, there is little difference in the chemical constituents obtained, based on feeding trials, as compared to barn dried. California research workers reported a leaching loss of 67% of the minerals, 35% of the carbohydrates, and 18% of the protein from rain. (4) There are 3 sources of loss in field drying: (1) normal respiration loss, (2) weathering losses, including leaching, and (3) mechanical losses, including leaf loss (MacDonald 1946). An evaluation of the losses and feed value as handled by different methods is given in Table 1.6. = In order to conserve a maximum amount of dry matter in the hay, it is necessary to begin preservation as soon as possible after the plant is cut (Miller 1947). Prevention of Crop Losses Most harvesting losses of cereals are due to (1) shattering of grain to the ground, (2) breakage of straw when harvested too dry, (3) wind and insect damage, (4) improper operation of harvesting machinery, and (5) poor growing conditions. Harvesting losses of corn are due to (1) ears dropping to the ground, (2) corn shelled from ears, (3) stalks broken, (4) harvesting when too dry or too wet, and (5) wind and insect damage in the field. Losses of hay include shattering of leaves and lowering of grade from rain. A considerable reduction of the harvesting losses can be accomplished by timely harvest followed by drying. Most storage losses of cereal grains and hay are caused by insect dam- HISTORY TABLE 1.6. FEED VALUE FERENT METHODS OF ALFALFA AND IMPORTANCE HAY HARVESTED Dry MatMethod of Harvest and Storage Field cured, damage Field cured, Barn dried, Barn dried, OF DRYING AND seipeibor 11 = pipe / Total Di- Decline gestible in Milk Leaves Retained ter Retained Protein Retained Carotene Retained Nutrients Retained Production! 38 62 ie me 60 80 80 84 55 2: 75 78 2 3 6 10 735) 59.1 byl) 58.0 13.6 6.7 8.1 8.8 % % % % % % rain no rain no heat heat Dehydrated hay 94 90 81 23 59.8 6.4 Source: Shepherd et al. (1955). ‘During 30 day feed trials on controlled rations. age, mold, and heating due to excess moisture. The storage losses can be practically eliminated by drying and aeration (Hall 1956). Often loss occurs because of an accumulation of moisture in grain, even though dry when placed in the storage. Two terms are used for the preservation of grain through moisture control, drying and aeration. Drying is the procedure used to remove excess moisture from the grain to reduce the moisture to a level acceptable for safe storage or for commercial sale. Drying may be accomplished by using either heated or unheated air. Aeration refers to moving a small amount of air through the grain to cool and ventilate the grain at frequent intervals. The reverse operation, turning, refers to moving the grain through the air by transferring the grain from one bin to another. Representation of Moisture Content The amount of moisture in a product is designated on the basis of the weight of water and is usually expressed in percentages. Two methods are used to represent the moisture content: (1) wet basis, MCw., and (2) by dry basis, MC,». The moisture content on a wet basis is obtained the dividing the weight of water in the material by the total weight of material [Equation (1-1)]. % MCw where MC,, Wy Wa = Ww ee (100%) isk W.+W, moisture content, wet basis weight of water weight of dry matter 1-1 (1-1) 12. DRYING AND STORAGE OF AGRICULTURAL CROPS The moisture on a dry basis is determined by dividing the weight of water by the weight of dry matter [Equation (1-2)]. % MCay = Wy W; ne (100%) ig (1-2) \ The moisture content on a dry basis and wet basis are related by Equa- tion (1-3) % MCay = OL MG, (100%) oo MCyr A es (1-3) GR - The wet basis moisture content is used for commercial designation and by the Federal Grain Standards. Prices for grain and hay are based upon moisture content on a wet basis. The dry basis moisture content is used mainly by research workers and in equations dealing with moisture variations. Note that the denominator of Equation (1-2) will remain the same regardless of the amount of moisture in the product. The dry basis moisture content is more likely to be used for drying equations. The moisture content expressed on the dry basis is always larger than the wet basis. A conversion chart provides a convenient means for converting from the moisture content on one basis to the other (Fig. 1.2). The following example illustrates the use of moisture content on either wet or dry basis: Given: 1 MT of grain initially at 25% moisture, w.b. To find: Amount of moisture removed in drying to 14% moisture, w.b. Solution: Method A (using moisture content, wet basis, to solve) 0.25 X 2205 lb = 551.25 lb of water in product The final weight of the grain with 14% moisture is the weight of dry matter multiplied by the pounds of moist material per pound of dry matter which is “ 1653.75 100 xX ce 7 eee < alga Giana ee eae pe Se Thus, 2205 — 1923 = 282 lb water removed. Method B (determining amount of water removed using dry basis) Change 25 and 14%, wet basis, to 33.33 and 16.28%, dry basis, respectively. The amount of moisture removed is equal to dry matter multiplied by the change in dry basis moisture content, OruGs3erar x 83.58 —16.28) = 282 1b water removed. HISTORY d.b, 9 5 wb, CPA meer esi Se db !2 db eM db*5 10 56 7 13 15 8-9) TOs. Il, © Nee 20 25 13 30 35 40 45 50 55 60 apygte ggPe Par ieee ete sreert db.&° SHEA 38 db 75 wb OF DRYING ieee aaa OTe aie testtett masi aes w.b. AND IMPORTANCE CAEP 43 db 9° HGS a EE 65 39 El 80 ELL 44 a 95 HE 40 70 A ae 41 S5ee, a 45 el 46 a 75 bet 42 pe lt 47 90 100 105 ee FIG. 1.2. CONVERSION SCALE FOR MOISTURE CONTENT REPRESENTATION QUESTIONS L Make a graph illustrating the relationship of average monthly price of wheat from the time of harvest until the new crop is harvested for the last 5 years. Corn. Alfalfa hay. Apples. Discuss. . Determine the quantity of hay at 40% (w.b.) required to give 1 MT at 20% (w.b.). Work on wet basis and check answer by using dry basis calculations. . The loss during harvest and storage of a grain is 10% of the gross production. The value of the loss is $200 million. Determine the value of gross production. Determine the value of net production. > . Distinguish between losses by leaching and bleaching of forages. . Make a list of design and management practices which should be followed to eliminate rodent and other animal damage to grain. List all possibilities of loss of nutrient value for alfalfa hay from cutting and field curing to mowing. Discuss measures for greatly reducing or eliminating these losses. . What must be the original weight of material at 30% moisture, w.b., to provide 100 kg of material at 12% moisture, w.b.? Derive the relationship represented by Equation (1-3). REFERENCES ANON. 1928. Status of grain drying investigation. Agric. Eng. 9 (1) 14-16. 14. OF AGRICULTURAL DRYING AND STORAGE ANON. CROPS Stretch your season with grain preservatives. 1974. Farm Chem. 137 (7), 26-28: BOHNSTEDT, G. 1944. Nutritional values of hay and silage as affected by harvesting, processing, and storing. Agric. Eng. 25 (9) 337-340. BURGES, H.D. and BURRELL, N.J. 1964. Cooling bulk grain in the British climate to control storage insects to improve keeping quality. J. Sci. Food Agric. 15, 32-50. CAMPBELL, J.K. 1972. Preserving grain with organic acids—from an engi- Meet. N. Atlantic Region, Am. Soc. Agric. Eng., Univ. neering standpoint. Md., College Park. CHRISTENSEN, Aug. 15, 1972. C.M. and KAUFMANN, Role of Fungi in Quality Loss. HALL, C.W. 1956. H.H. 1969. Univ. of Minnesota Grain Storage: The Press, Minneapolis. Preventing crop losses by drying. Agric. Eng. 37 (6) 414— 415, 421. HALL, C.W. et al. 1977. World Food and Nutrition Study, Vol. 3. National Academy of Sciences, Washington, D.C. HODGSON, R.E., SHEPHERD, H.B., SCHOENLEBER, L.B., TYSDAL, H.M. and HOSTERMAN, W.H. 1946. Progress report on comparing the efficiency of three methods of harvesting and preserving forage crops. Agric. Eng. 27 (a2 19—222. HODGSON, R.E. et al. 1947. Comparative efficiency of ensiling, barn curing, and field curing forage crops. Agric. Eng. 28 (4) 154-156. HUKILL, W.V. 1957. Evolution of grain drying. Agric. Eng. 38 (7) 526-527. HURLBUT, L.W., PETERSEN, G.M., YUNG, F.D. and OLSON, E.A. 1952. Harvesting and conditioning grain for storage. Agric. Eng. 33 (7) 421—425. JONES, G.M., MOWAT, D.N., ELLIOT, J.I. and MORAN, E.T., JR. 1974. Organic acid preservation of high moisture corn and other grains and the nutritional value: a review. Can. J. Anim. Sci. 54 (4) 499-517. KAUFMANN, H.H. 1964. Quality control practices reduce losses and costs in stored grain. Agric. Eng. 45 (8) 432. MACDONALD, H.A. 1946. Factors affecting the nutritional value of forage plants. Agric. Eng. 27 (3) 117-120. MARON, S.H. and LANDO, J.B. 1974. Fundamental Principles of Physical Chemistry. Macmillan Co., New York. MASSIE, D.R., OLVER, E.F. and SHOVE, G.C. 1964. Extending the time of drying corn with controlled atmosphere. Trans. ASAE 7 (1) 32-33. MILLER, H. 1947. Dry matter loss in haymaking due to bacterial action. Agric. Eng. 28 (6) 242-244. NASH, M.J. and EASSON, D.L. 1977. Preservation of moist hay with propionic acid. J. Stored Prod. Res. 13 (2) 65—75. NATL. ACAD. SCI. 1977. World Food and Nutrition Study. National Acad- emy of Sciences, Washington, D.C. PATERSON, H. 1967. 19 (216) 19-20. Chilled grain storage. Farm Mechanization Buildings HISTORY PFLUG, I.J. and DEWEY, Eng. 36 (3) 171-172. D.H. 1955. AND IMPORTANCE OF DRYING Controlled atmosphere storage. PRUTTON, C.F.and MARON,S.H. 1944. Chemistry. Macmillan Co., New York. 15 Agric. Fundamental Principles of Physical SAUER, D.B., HODGES, T.O., BURROUGHS, R. and CONVERSE, H.H. 1975. Comparison of propionic acid and methylene bis propionate as grain preservatives. Trans. ASAE /8 (6) 1162-1164. SHEDD, C.K. 1949. Storage of small grains and shelled corn on the farm. U.S. Dep. Agric. Farmers’ Bull. 2009. SHEPHERD, J.B. et al. 1955. alfalfa forage for dairy cows. Relative merits of harvesting and preserving U.S. Dep. Agric. Circ. 963. STEWART, J.A. and CLARK, B.S. 1949. The Canned Food Reference Manual, 3rd Edition. American Can Company, New York. U.N. 1977. Statistical Yearbook (28th Issue)—1976. United Nations, New York. U.S. DEP. AGRIC. 1954. Losses in agriculture. U.S. Dep. Agric. ARS 20-1. U.S. DEP. AGRIC. 1975. Agricultural Statistics. U.S. Dep. Agric., Washington, D.C. Equilibrium Moisture Relationships The equilibrium moisture content is directly related to the drying and storing of farm crops. The equilibrium moisture content is used to determine whether a product will gain or lose moisture under a given set of temperature and relative humidity conditions. A product is in equilibrium with its environment when the rate of moisture loss from the product to the surrounding atmosphere is equal to the rate of moisture gain of the product from the surrounding atmosphere. The atmospheric conditions are defined by temperature and relative humidity. The moisture content of the product when it is in equilibrium with the surrounding atmosphere is called the equilibrium moisture content or hygroscopic equilibrium. The relative humidity of the surrounding atmosphere during equilibrium is known as the equilibrium relative humidity and is dependent on and specified at a particular temperature. Thermodynamically, equilibrium is reached when the free energy change for a material is zero. The adsorption process is accompanied by a decrease in entropy. The equilibrium moisture content of a particular product may be expressed on either a wet or a dry basis. For use in mathematical calculations it is customary to express the moisture content on a dry basis. The relationship between the moisture content of a particular material and its equilibrium relative humidity at the particular temperature can be expressed with an equilibrium moisture curve. These curves are often called isotherms because the values plotted for each curve usually cor- respond to a specific temperature (Fig. 2.1). Unless stated otherwise, equilibrium moisture content curves are commonly plotted for a temperature of 25°C (77°F). According to the Brunauer classification, grain isotherms are of Type II, called S-shaped or sigmoid isotherms, and attributed to multimolecular adsorption (Brunauer 1945). For each 0.1 kg (0.22 lb) of dry matter of shelled corn, approximately 2.6 ha (6.5 acres) are available for adsorption (Rodriguez-Arias 1956). An empirical equation is used to represent the equilibrium moisture content, equation (2-1): 16 EQUILIBRIUM 1 - RH MOISTURE RELATIONSHIPS = e=<™e" 17 (2-1) in which RH, the relative humidity, is represented as a decimal; T, the absolute temperature, °R; M., the equilibrium moisture content, %, d.b.; and c and n are constants varying with the materials (Henderson 1952). From Fig. 2.1 and equation (2-1) it can be noted that the (1) equilibrium moisture content is 0 at 0 relative humidity (2) equilibrium relative humidity approaches 100% as the moisture content approaches infinity (3) slope of the curve approaches infinity as the moisture content approaches infinity and increases rapidly as the moisture content approaches zero. MOISTURE CONTENT PERCENT (d.b) 10) 10 20 30 40 RELATIVE 50 60 HUMIDITY, FIG. 2.1. EQUILIBRIUM MOISTURE CONTENT 70 80 90 100 PERCENT CURVE for proThe equation is valuable for extrapolating limited data and ly general is viding a complete curve with a minimum of two points. There a reduction in moisture content for a fixed relative humidity as the as the temtemperature is increased. The moisture content decreases 18 DRYING AND STORAGE OF AGRICULTURAL CROPS perature is increased for a fixed vapor pressure. For wheat and corn both the equation and actual tests show a loss of about 3% moisture for each 10°C (50°F) rise in temperature. For peanuts, lettuce seeds, flax seeds, pine seeds, tomato seeds, pea beans, and onion seeds, the equilibrium moisture content is higher at 10°C (50°F) than at 5°C (41°F) and 20°C (60°F) (a lower and higher temperature) at relative humidities of 55 and 76% (Table 2.3). Equilibrium moisture content values for various agricultural products are summarized in Tables 2.1, 2.2, 2.3, and 2.4. The composition of the product determines the characteristic of adsorption of moisture. With feedstuffs, the relative amounts of soluble carbohydrate and protein largely determine the equilibrium moisture curve. At 63% relative humidity the water adsorption varies directly with the carbohydrate content and inversely with the protein content, and at 90% relative humidity the relationship is reversed (Snow et al. 1944). Determination of Vapor Pressure The equilibrium moisture content information can be used for determining the vapor pressure of the material. If the vapor pressure of the material is higher than the vapor pressure of the surrounding atmosphere, moisture will move from the material to the atmosphere. Conversely, if the vapor pressure of the material is lower than the surrounding atmosphere, moisture will move from the atmosphere to the material. The vapor pressure of the material in question can be readily determined by superimposing the equilibrium moisture content data ona psychrometric chart. If the vapor pressure of the product is below that of the atmosphere, the product will gain moisture and sometimes may gain enough moisture so that mold growth in storage increases to the extent that the product is damaged. The vapor pressure, usually in pounds per square inch or millimeters of mercury, is determined from Fig. 2.2 and 2.3 by locating the point on the curve corresponding to the moisture content of the product at the appropriate temperature. Thus, pea beans at 15% moisture and 25°C (77°F) have a vapor pressure of 2300 Pa (0.33 psi). Another method of determining the vapor pressure*is to locate the equilibrium relative humidity at 15% moisture and 25°C (77°F). The equilibrium relative humidity is defined as the ratio of the vapor pressure of water in the product to the pressure of saturated water vapor at the specified temperature. Therefore, the vapor pressure of a product can be determined by multiplying the equilibrium relative humidity times the saturated water vapor pressure, at that temperature. The equilibrium relative humidity is 72%, and from Table 2.5, the saturated vapor EQUILIBRIUM MOISTURE RELATIONSHIPS 19 pressure is 3.165 kPa (0.46 psi). The vapor pressure of the 15% moisture pea beans is 3165 X 0.72 = 2279 Pa (0.33 psi) at 25°C (77°F). Equilibrium moisture content values at temperatures other than room temperature are desirable for heated air drying calculations. Shelled corn values are given in Fig. 2.4. Determination of Equilibrium Moisture Content Two general methods are used for determining the equilibrium moisture content: The static method, in which atmosphere surrounding the product comes to equilibrium with the product without mechanical agitation of the air or product; and the dynamic method, in which the atmosphere surrounding the product or the product itself is mechanically moved. The dynamic method is quicker but presents problems in design and instrumentation. Therefore, the static method has been used more extensively. Several weeks may be required using the static method, whereas with the dynamic method the data may be obtained in a couple of days or less. The speed at which equilibrium is approached, of course, depends upon the amount of change which must take place in order to reach equilibrium for a particular product. The usual procedure consists of using wet grain and drying to the equilibrium condition, in which case the term desorption isotherm applies. When using the static method for determining equilibrium moisture content, a saturated salt solution or an acid solution may be used for maintaining the desired relative humidity at the temperature of storage. Another method is to permit the product to come to equilibrium with an enclosed surrounding, the relative humidity of which is then measured. In both cases the final moisture content of the product is determined by an accepted method. With the static method, because long periods are required for the product to come to equilibrium, mold is apt to develop on most agricultural products at relative humidities above 80%. The moisture content obtained for relative humidities above 80% for the product is usually not a true one, because the moisture from the mold gives an apparent high equilibrium moisture content. With the dynamic method, air is bubbled through absorption towers containing an acid or saturated salt solution which controls the humidity and then around the product. Also, an air conditioning system can be used to obtain the desired conditions. In another method the air is moved around the product in a closed container and the final relative humidity and temperature of the air are determined to define equilibrium conditions. The method used for determining the moisture content of the product NIVHD ‘suvaq Pat UBOIXe|A] UIOY}ION jeaI13 st]dy par Aaupry yep pel Agupry “4eTF [[BUS a}IYM oyurd paasxe]y $180 “aory aJOyM UleLs afoym ules parr aot ysnor aky paT[ays U109 GA ; GZ GZ GZ GZ GZ GZ GZ GZ GZ 8g GZ LG eY GZ = 0 OL 1Z GZ IL v om “Oe GS GS GZ i OL GZ gE PG J, De LL Thi bs iy LL LL Hy Ny, 0OT jb, 08 OIL LL 0Z as 0S OL LL 09T OF 09 cin VL GC?) GL Wik VL 67 99 08 OL 9 OL OL Z9 9°8 SL ZS 1G 6s e'9 cche VL 9g 09 6S 19 VS 09 19 ee I? 6S* 6. Vg 0Z OL 9 OT VP 0's ainjeraduia ce iy LL LL OF 0S phy 00T OST BOY oAT} 18°6 1, 78 9b meSG B06 OF AGRICULTURAL 6G SAN MO 226 €'8 28i6 RS (OUUL) ‘On 06 OOT GiGTe 8°96 6k. 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Rodriguez-A andmold (1955). (1956): (1925). aicten Fellows Fenton(1941); (1956); (1972); Hall ASAE Sources: Sallans and al. Becker Dexter et (1944); “2%? AIGVL ‘Avy BJPITV ‘IBAOC[D CULT 91N} MBIS oleAvy poy “Taaopo ‘ON T ONZ BAY MeIys OUIpeT 62 62 LZ 62 GZ GS Noyes v8 ¥8 08 v8 oe ey OOT ST LG Lg 09 LG 19 OL 89 VL 89 OL ED, ay 18 Tia; 89 eg Lg 89 SL Lg 9°9 9 SL v8 28 ER es 9°9 VL Ger OOL OOPS GIL GL el TIL OIl 26 wile LOL Cae COL ere 006 6GIeae9ST €STVGl 9&6 6 18 8 Galay5 v8 OIE 96 66 GL OO 68 OO Veis FAK GSt ‘ApIpTUIny % 09 SGI JLO GZ VL Sy LG aArzelay 0S SAMA 8g sseidaulolg Lil ne ip LL Di 99 6€ 6g OF LOL ‘“I9A0T/) OUIPR] GZ GZ GZ GZ GZ 78 SL SL 09 0g 18°66 18°81 66 16 1G 19° 1V 8% 113 G616 19'S 06 Areutpio pepeys peyore] ways dozJo W10}}40q JO W1a4S suory}OR SNOLIeA IJ 6% 92 9% 9T = 8g OOT OL 08 6 Gon er Ci 99 OST sg exe 9 ie 02 OF AGRICULTURAL 72 ‘se0Inog STAB AUIZ SU0T 72 104x9q “(LP6T) ‘(GE61) ‘(TG6T) ‘(OG6T) ‘ejduies uo YAMOIS ploul JO asnedeq 9s[qeIjatuy) | ainjerieduie ile OI DRYING AND STORAGE g WNIYEITINOA 3DVWHOS J, Oy "aM °% ‘LNALNOO SYNLSIOW ey] [ele Bs[eITV Avy TON ON ON€ TV ese Aey 22 CROPS OMMr-wmoowotTNOLMN ANNAN GLIGoel: SOLGETVELOCT EQUILIBRIUM TABLE 2.3. SEED EQUILIBRIUM MOISTURE {eae Temperatu re C p Flax : Lettuce Onion Peanut Pine Tomato 5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30 5 10 20 30 41 50 68 86 4] 50 68 86 41 50 68 86 41 50 68 86 41 50 68 86 41 50 68 86 MOISTURE RELATIONSHIPS 23 CONTENT, %, W.B. usi Seay idi a 7.8 7.6 BED 5.0 6.5 6.3 Bes) 5.0 10.3 10.0 8.7 8.0 5.8 5.6 4.8 4.2 8.2 8.3 All 6.2 9.1 9.2 8.0 ell 10.5 Ib? 9.0 Wes 8.7 9.9 7.8 6.1 13.6 15.8 1272 9.8 7.9 8.7 6.2 nee 10.9 13.9 9.5 7.9 1223 13:5 10.8 9.0 11.4 13.9 Laer Tee 9.7 13.5 TES 9.9 15.0 Ue 15.6 14.7 8.5 10.7 8.9 9.0 11.6 T3e2 12.9 11.9 13.9 19.6 13:7 13.0 Source: Barton (1941-1942). is aS important as the method used for determining the relative humidity. Standard methods are described by the U.S. Dep. of Agriculture and American Association of Cereal Chemists (see Chapter 4). Maintaining Relative Humidity with a Saturated Salt Solution Research in agricultural engineering often requires controlling temperature and humidity conditions. Refrigerators and coolers are usually available for temperature control. Mechanical humidity control equipment is expensive and seldom available in sufficient quantities to run a large number of tests simultaneously. A chemical means of controlling the humidity is usually satisfactory. Either an acid or saturated salt solution may be used for chemically controlling the relative humidity in a closed container. Sulfuric acid will corrode metal easily. A salt solution is more stable, less corrosive, and often less expensive. A given saturated salt solution will often maintain practically the same relative humidity at different temperatures. A solution will exert a certain vapor pressure depending upon the chemical, its concentration, and temperature. It is easier to maintain a saturated salt solution than to attempt to maintain a solution of less than 100% of saturation. 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O'9T € Or OL ‘aM “% “LNALNOO SYNLSIOW uelg Ve] [Be HTN NTR mt Nt St rt o N 6D ELD 1 Se el eee eee mor dtinion Ea COC SS estat SS E- OO NR NA DNONMANTODOCO SCO .C0 IGE S co MAH NANNANNNAN TY GBT Lg HESSOY EXO ace OHORAHGDOSHOD AOA Norn 06 L6G Vol VIE CROPS OF AGRICULTURAL AND STORAGE DRYING 24 suvaq yo}00¢ J, Ty, J, 08—OL 08—-OL 08—0L LL LL LL 08 OOT OGT OFT 08 OOT OdI OFT 08 OOT OdT OFT LG—IG LG—-1G LG—-IG GS GG GG LG 8E 67 09 LG 8E 67 09 LG 8E 67 09 NouIe’T 72 70 6L v9 LS OME VT ant aE GG TZ 8T qT VE 9% ST OCT LOU OOT (arat OF Lgé Sits ns AZ Ov GE 8S 9G GG LY Ty 86 VG G8 0'OT 06 Est OAL Gaal GG 67 97 ov 69 VG GPT Lg G9 09 os rg 06 Tepe 9 ‘(PHET) Ae[YOO"] 22 ‘72 Vor 8 66 6G LG VG UG Ge ee LZ GG 0S eV qé LG SL LY 8'OT oGI 0:31 LYt Tor L8t O'OT 66 v8 LL OTT TOL 68 6L G6 €8 Sik 89 Girt St6 L8 OCT TV OFT OT SOL GaLG SET 6<GI OCI 8)E JE) VGT LST E<GI Ost Cac Vall L’6 g8 O'sT LST GOL (LG6T) SUIOW] (LPET) 0'6 OE GOT EI6 sel GOT TL 219 0'9 OS €8 VL v9 VG VL 9°9 69 09 GOL GGL VL LTL Lt L IG T&G LtL VeEl GgGé G96 O'9€ cle ZABMYS{(EPET)MOUS 42 ‘JD GOT sel IT6 SLT 8ST 6G LI 991 qGT TP gGT OLT gst Lvl VGG GIT MOISTURE :seomn0g HAGE (ZPHET) AoTeg ‘(ZET) {(FP6T) MA BANQUse(LZ6T) ‘000eqO Aaying diiys o998qQO], Ystyin 08—-O0L LG—TC LE—-IZ yied [ ‘utezoid ‘yoreys [| ‘qaed ‘uteyoid Z sqied Yoleys MEIYS pouue ea] Toy} ‘oD0BqO a1BSI9 997 ON Je2] QIEGOT, ys drys O8—OL ‘yo1eyS Z sqied GE sel LL LL OLT GG GG O¢e urysdaays duray 6&& [esis EQUILIBRIUM RELATIONSHIPS 25 DRYING AND STORAGE 26 OF AGRICULTURAL CROPS & % ov | Ko MOISTURE EQUILIBRIUM ae 70} CONTENT, ov? :$, w.b. oe 2 feow a: 060+ 1 6 | & a 050 158 oa| ac : > 0304 — a ec 1 o<¢ Ww = Ww ° Ww a olo4 lhe po coe |e | SUPERIMPOSED ON Jt 0.00 DRY BULB FIG. 2.2. EQUILIBRIUM MOISTURE PSYCHROMETRIC CHART See Appendix \ Se TEMPERATURE CONTENT — DEGREES F. OF PEA BEANS for metric conversions. evaporates from the system the solution will remain saturated, although some of the salt may precipitate. Thus, a saturated solution will give the same relative humidity from beginning to end of an experiment. The effect of temperature variation on the relative humidity of a given concentration of solution depends on the chemical used. The percentage relative humidity of a given concentration decreases with an increase in temperature. A saturated cobaltous chloride (CoCl,) solution has a relative humidity of 65.2% at 25°C (77°F) and 57.2% at 40°C (104°F), while potassium sulfate (K2SO,) has a relative humidity of 97.9% at 10°C (50°F) and 96.2% at 40°C (104°F). The cobalt chloride has a greater variation than the potassium sulfate with the same temperature range. Table 2.6 gives the relative humidity of several chemicals for different : Neate: zB temperatures. The relative humidity is calculated from —, where P, = vapor pressure of saturated air, and P = partial vapor pressure of water. The exact desired relative humidity cannot always be obtained with a saturated salt solution but a wide range of humidities is available. In a chamber controlled at a given temperature, tests at several different relative humidities can be run simultaneously. Each test sample is sup- EQUILIBRIUM iS) MOISTURE RELATIONSHIPS 27 3 nNoO nN oO SORGHUM GMO (d.b) PERCENT o aO° ~ MOISTURE , (d.b) PERCENT , MOISTURE fo) 10 20 30 40 50 60 70 80 RELATIVE HUMIDITY, PERCENT fe) 90 10 20 30 40 50 60 70 80 RELATIVE HUMIDITY, PERCENT 90 w fe) ol [e) nN on ROUGH n RICE a DS(e) p a De)—fe) a fo) 90°F a 3 III°F oO oO MOISTURE, (d.b) PERCENT MOISTURE, PERCENT (d.b) nNoO uo 30 40 RELATIVE 50 60 70 HUMIDITY, FIG. 2.3. DESORPTION WHEAT 80 90 i100 °0 PERCENT ISOTHERMS 20,30) RELATIVE FOR SHELLED 440 50 60 HUMIDITY, CORN, SORGHUM, 70 80 90 PERCENT RICE, AND ported in a mesh basket in a closed container with the saturated salt solution which will give the desired relative humidity. In order to carry out tests at several different temperatures, it is convenient to use a saturated salt solution which has the same relative humidity for a wide range of temperature as given by sodium chloride (NaCl) in Table 2.6. Note that there is a much greater variation for potassium chloride (KCl) for the same temperature variation. A saturated salt solution is easily prepared by dissolving all of the salt a solution will hold at a temperature above the temperature at which the tests are to be run. It is necessary that some of the solid salt be always present; otherwise, a supersaturated solution occurs. If there is an excess of salt, it will crystallize out of solution when the solution cools. The amount of solute required to saturate a solution depends on the temperature for any given salt. 28 DRYING AND STORAGE TABLE 2.5. SATURATION Temperature iC 38 0 4.4 10.0 15.6 Zale! 26.7 32.2 37.8 43.3 48.9 54.4 60.0 65.6 (lel 76.6 82.2 87.8 93.3 98.8 100.0 104.4 110.0 115.6 PAI 126.7 132.2 137.8 143.3 148.9 0.01 10 20 30 40 50 60 70 80 90 100 110 120 130 140 OF AGRICULTURAL VAPOR PRESSURE AND LATENT HEAT OF WATER Pressure 10° Pa psia By 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 22 220 230 240 250 260 270 280 290 300 50 68 86 104 122 140 158 176 194 PAs 230 248 266 284 CROPS Latent Heat kJ/kg Btu/lb 0.08854 0.12170 0.17811 0.2563 0.3631 0.5069 0.6982 0.9492 1.2748 1.6924 2.2229 2.8886 3.718 4.741 5.992 7.510 9.339 11.526 14.123 14.696 17.186 20.780 24.969 29.825 35.429 41.858 49.208 57.556 67.013 0.006 0.01227 0.02337 0.04242 0.07375 0.1233 0.1992 0.3116 0.4736 0.7011 1.0132 1.432 1.985 2.701 3.614 €? 1075.8 1071.3 . 1065.6 » LOGTa 1054.3 1048.6 1041.9 1037.0 1631.6 1025.8 1020.0 1014.1 1008.2 1002.3 996.3 990.2 984.1 O77-9% OTe 970.3 965.3 958.8 952.2 945.5 938.6 931.8 924.7 917.5 910.0 2500.8 2477.2 2453.7 2430.0 2406.2 2382.1 2357.9 2333.3 2308.3 2282.8 2256.9 2230.2 2203.1 2174.0 2145.0 A small sample of from 10 to 15 kernels should be used for equilib rium moisture content determinations for best results. The surface area of the kernels should not exceed the exposed surface area of the chemical, particularly if the time of reaction is involved in the data. In order to obtain quick recovery of the relative humidity in a closed container after the lid has been removed and then replaced, it is desirab le that the volume above the chemical solution be as small as practic al in relation to the surface area of the solution. The sample should be far enough above EQUILIBRIUM MOISTURE RELATIONSHIPS 29 LATENT HEAT OF GRAIN HEAT LATENT OF FREE WATER . MOISTURE CONTENT, PERCENT (d.b.) FIG. 2.4. VARIATION OF LATENT HEAT WITH MOISTURE CONTENT the solution so that the chemical will not splash on the kernels when the container is moved. Maintaining Relative Humidities with Acid Solutions A common practice is to use an acid to obtain desired relative humidities in a closed container, varying the percentage of acid to obtain different relative humidities. Sulfuric acid is usually used. There is justification for using one acid instead of two or more acids or several different salts to obtain the various relative humidities desired since the moisture equilibrium value might be affected by the salt or acid, thus giving erroneous results. By using one acid the data would be on a comparable basis. The partial vapor pressure of the acid is small as compared to the partial pressure of the water vapor and is probably insignificant for most practical applications. The data presented in Table 2.7 show the relationship between the partial pressures of various acid and water solutions. The partial pressure of sulfuric acid as compared to water is much lower than the other acids commonly considered. The partial pressure of sulfuric acid is less for lower temperatures and lower concentrations. The partial pressure of sulfuric acid is so small that CROPS OF AGRICULTURAL DRYING AND STORAGE 30 (ai 0S Oot 98 89 0S GE OOT 98 LL 89 gSLe 0€ 0% OT 0 BLE O€ GS 06 Ov L'06 «=«2'S6 G'G6 9°L6 6 SV OG LY o8V 067 vOT 6'8E 618 0'S8 (OG6I Sivag pue YUIM) (PG6T emesaseyy pur 1o[xem) 0€ (eq eueADOIY} WUNIssez0) 0S OV 98 06 0 BLE O€ GS 06 BLE 0€ GGL VOT SIT 0G GZ VAAGIE SET jh 89 GE OOT 98 LL 89 OOT 98 (OG6T SIB9g pue YUL ) (9}8j} 9008 WINISsey0) 20°H20M (Apoyo wntyyWT) O°HIO'NI Vol Lvl V'0G 06S LCS GES Ly, 8 eV (PS6I eredaseyy pue Ja[xo\) (878171 WINIsse}0 ) ON” (OG6T SIBag pue YUIM) (0}11}1U UINISSByOq) 7ONM 0z (VS6I Ppeys pue uosdwoy,[ ) O*H¥-*(*ON)®0 (OG6T Sivag pus YUIM) (87 e141U WINTOTBD)) IG G€é 0€ GG 89 OL G6 98 Loe «(9S OL O0°HZ:*OO*H Os 0 Hs ja (97 B11 UINIOTB)) %FON)BO MA) (LO6I UINnqyse (av es[NS UNTOTeD)) 07HS-"OS®0 Mm) (L261 UINnqyse SNOM MA) (LZ61 UANqyse (e}yeUOgIed UINISSe40 ) gg 02 VG (@PHoTY urntoye/)) 0GP 9°97 v0S 89 OLY VVG 6S (4S 06 0G (VS6I ppeys pue uosdu0y,y,) 9L v9 v9 89 OL ¢ IG OL 89 GGL VOL 98 89 0S 06 OS OF 0€ 06 OT (PS6T BMesasey pue Ja[xXeM) (e7eJ[Ns WINIssejOg) Igy (epruoig WnIsse}0g) (LZ6I UnqysemM) 86 1G 89 0 OOT ESE a5 6'8T OS IV OL 0S GIS 0 8'6E OSE 0'0V WeS “9'¢ AIaVL O°HS-710 8a (aplroyyo wuntreg) 1080 \) (LE6T Urnqyse (epuoryo untoyeD) O°H9-41080 (Gs SCOT 99 Gy. eV vv 669 166 08 8°S6 696 9°96 6 L6 6 L6 ‘OID*A. V'66 4O ALIGINNH SAILV1SY Do ue ain} e1eduia J, G8 lp Ge (OG6T Steeg pue YUL ) (e}eWOIYS WINISsezOq) (OG6T Steeg pue YUL ) 88 06 SLE 0€ GG 0Z (ey eU0gIed WINISse}Og) °00*H % AY LNSYSSSIG LV SNOILNTIOS LIWS GALVYNLVS HES OVP OOT 98 LL 89 9°98 €'°98 G98 9°98 SLE 0€ GS OOT 98 LL VEV EV SEV 89 06 6 EV Jo A. ainyeieduia J, SSYUNLVYSdW3L % AY 0°H9-*(°ON)3IN 0 83S 0€ SLE UINIpoS) (9783908 YUM) (9¥6T ZE 98 OOT CE 89 98 VOT GSI as 89 98 vor GGL 89 ii 98 OOT 89 LL 98 OOT €L 98 OOT GE 89 98 POT GGL €L 98 OOT 9°09 VIL LL9 SPL GG), = 9°C), HG,= GPL 6°¢L 67S ~°ZC Z6F OV 26S g°1.¢ 9g€ L’&G SIT IT= oIt oItl 6ZE HE=—=s_-w = §©BTE 0'SE 9'EE SCE TGE FTE GEG "TG 0'6P ainqyereduroHAY a. % wntuowUTYW (e}eJ[Ns ) pueIe[xeM)BMesasey ‘VG6I yu pue SIveg (OG6T "OS°(HN) 0 LO= 0¢ 06 GS 0€ BLE 06 GG 0€ SLE 86S 0€ SLE 0 06 0€ OV J, Oo 06 og OV 0S 0 OT T1Z 04 (eyeydsoy CZdouow 0€ SLE wntuomMUIY (ePMo[yo ) pueuosduo Ppeys (PS6T 0yy) *Od°H’HN wntuowuy) IO°HN “ON®N WINIPOS) (9}1}U AUIM) ‘OPEL YUM pue Sevag (OG6T ‘OND*®N WNIpos) (e}eWOTYSIp YUM) (9P6T O°H3:'OMD*®N WINIpos) (9}eUOTYoIp layxem) pur BPMeseseH (S61 O*HE“O°H2RN WUNIPOS) (ayeye0R YUIM)pur Steeg (OG6T WeS 89 98 vor GGL o& 0Z Ze 0g OL 89 Ni 98 OOT 231 89 Jin 98 OOT 89 LL 98 OOT €L 98 OOT GE 89 98 VOT oF 9°08 0°08 9.6L T6L L'€8 68 eg 18 GL GE6 9°26 0°66 T'T6 9L Leh el 9L9 €99 79€ €°€9 819 TVS 03S 00S 9°09 ogg GOS 867 ainqyereduio‘HY ce % MOISTURE 70°H“OR8N 0G og OV 0S 0 0G og OF 0S 0G CZ 0€ SLE 06 GG O€ SLE 86S 0€ SLE 0 06 0€ OF OS 83S O€ SLE J, Oo uNntpos) (9PMoTYo Je[xe) pur BMeSasey ‘PG6I HUI pue Sevag (OG6T lO8N CeENII wntpog) (eptwosg yUIM)puke SI¥ag (OG6T UINIsSaUseT\ 81}1U(9}]) Ja[xo\) pur BMesesey (PS6T (ON) UINISaUseT\ (9}8171U ]) YUIM) (QP6T wintsausey/(ePMoTYo y) AIM) (9P6T 0°H9-1031N winiseusey(@epuo[Yyo y) Ja[xaM) pur BMesaseH (PS6T 103 SIN WeS (panuijuod) umtyy17) (@puopyo yu)pue SI¥ag (OG6T IO'T AIaVL ‘9°% EQUILIBRIUM RELATIONSHIPS 31 32 DRYING AND STORAGE OF AGRICULTURAL CROPS TABLE 2.7. PARTIAL PRESSURE OF ACID AND WATER IN AQUEOUS ACID SOLUTIONS Temperature Concentration Partial Pressure, Pa Water Acid % °F iG Acid 10,477 66.7 89.25 356 180 H,SO, (sulfuric) 1,053 36 50 68 20 HNO; (nitric) (PAL 1413, 30 68 20 HCl (hydrochloric) Source: Perry and Chilton (19783). it is not recorded for the normal range in which work would be done to determine moisture equilibrium values of agricultural products. Conditioning with Salt or Acid Solutions Products can be conditioned to the desired moisture content for drying tests by placing them in an atmosphere of the appropriate temperature and humidity, either acid or salt solutions (Table 2.8). The previous history of the product will determine to some extent the moisture content reached. Wheat and corn predried to 8% decreased the equilibrium content about 0.5% in comparison to grain premoistened to 16% (Thompson and Shedd 1954). The difference in moisture content is known as the TABLE 2.8. RELATIVE HUMIDITY OF DIFFERENT CONCENTRATIONS ACID SOLUTIONS AT VARIOUS TEMPERATURES, % Temperature Acid H,SO, (sulfuric) Acid by Weight, % XG °F 20 40 60 80 -17.8 10 20 30 40 44 0 50 68 86 104 122 87.3 87.4 87.7 87.5 87.6 88.8 55.7 56.6 56.7 56.6 BRO) 58.2 15.0 15.8 16.3 17.0 17.8 18.8 3.14 3.88 4.76 5S 6.88 8.2 20 HNOs (nitric) OF AQUEOUS -17.8 Acid by Weight, % 30 40 50 0 89.2 78.4 65.3 45.7 10 20 50 68 86.7 86.6 77.0 Toe 63.0 WAGIS 45.6 40 104 85.9 74.1 60.5 30 44 60 86 I? 140 86.6 86.5 86.9 74.9 74.6 75.6 61.3 Acid by Weight, % HCI (hydrochloric) -17.8 10 20 30 Calculated by = from basic data. 0 50 68 86 10 20 30 40 83.5 83.5 83.2 84.2 56.0 27.4 8.9 EQUILIBRIUM MOISTURE RELATIONSHIPS 33 hysteresis effect. The time required to reach equilibrium varies considerably. From a practical standpoint, the product will approach equilibrium in 4 to 8 weeks for 5% moisture change. Equilibrium is reached more slowly at the lower temperatures. Popcorn can be conditioned to the proper moisture content by using a saturated salt solution. Popcorn should contain about 13.5% moisture for best popping, which is a relative humidity of approximately 75% (Dexter 1946). Sodium chloride (common table salt) will maintain the relative humidity of 75% over a wide range of temperatures. The saturated salt solution may be absorbed by porous materials such as blotters and corn cobs. Latent Heat In most drying applications the latent heat is determined by considering the evaporation of free water as given in the steam tables. The use of these data for latent heat, especially with crops at a low moisture content, presents considerable error. The equilibrium moisture data may be used as a basis for determining the latent heat. A simple and accurate method was first proposed by Othmer and is based on the Clapeyron equation, equation (2-2). dP L aT (WowT where P= via vapor pressure, lb/ft? T = the absolute temperature, °R V = the specific volume of water vapor, ft?/Ib L = the latent heat of vaporization, ft-lb/lb v = specific volume of liquid water, ft®/lb Equation (2-3) relates the vapor pressures and latent heats of two substances temperature, at the same namely a farm crop and water vapor (Gallaher 1951). it ae L’ A ‘log P, — log P, log P2 — log Pi (2-3) where L and P represent the latent heat and vapor pressure for the farm crop, and L’ and P’ represent the latent heat and vapor pressure of free water, respectively. The ratio of the numerator to denominator of equation (2-3) is the ratio of the latent heat of the product to free water. Nearly straight lines are obtained when the vapor pressures of crops are plotted on the ordinate against the vapor pressure of water on the abscissa for each moisture content, d.b., on logarithm paper. The slopes of these lines at different moisture contents give the ratio of the latent 34 AND STORAGE DRYING OF AGRICULTURAL CROPS heat of wheat to that of water, a value greater than 1.0. The latent heat ratio of a product and water can be expressed in terms of the moisture content if equilibrium moisture content data are available for several temperatures (Hall 1957; Brooker et al. 1974). The relationship of the latent heat of wheat to the latent heat of free water is shown in Fig. 2.4 and is represented by equation (2-4) (Gallaher 1951). This equation is based on different equilibrium data from those in Table 2.9, and gives only slightly different values. day a L’ = Latent heat of wheat 1429 (2-4) -9.40M Latent heat of free water where M is the moisture content in %, d.b. For example, if the moisture content of wheat is 14%, d.b., the latent heat of water in the wheat is 1.1 times the latent heat of water at the temperature at which drying occurs. If drying occurs at 65.6%°C (150°F), the latent heat is 1.1 times 2245 X 103 J/kg (1008 Btu/Ib) or 2574 X 10° J/kg (1108 Btu/Ib). For shelled corn the latent heat is 1.16 times 2245 X 103 J/kg at 65.6°C (150°F) or 2713 X 103 J/kg (1169 Btu/lb) water. Other Methods of Equilibrium Moisture Representation A curve representing a plot of constant moisture content is called an isostere; of constant pressure, an isopiestic line or isobar, although the former is preferred; and of constant temperature, an isotherm. TABLE 2.9. LATENT OF WATER Moisture, %,w.b. HEAT OF VAPORIZATION 0 32 FOR CORN Temperature of Evaporation 10 Pasi 37.8 50 AND WHEAT, 70 100 65.6 150 Corn ; 10 11 17 13 14 1265 1220 LS 1145 1120 1250 1210 1165 1135 1110 1240 1200 eS 1125 1100 1220 1180 1135 1105 1080 1180 1140 1105 1070 1050 1S Wheat Lal 1095 1085 1075 1055 1025 1250 1240 122% 1200 1170 12 1b) 1200 1160 1190 1150 1s 1140 TESS 1120 1120 1090 14 15 Water 1130 1110 1075 1120 1100 1065 1110 1090 1055 1090 1070 1037 1060 1040 1008 Source: Thompson and Shedd (1954). Conversion: 1000 Btu/lb = 2326 kJ/kg. ~C sk BTU/LB EQUILIBRIUM MOISTURE RELATIONSHIPS 35 Several methods have been developed to relate the relative humidity (relative vapor pressure), moisture content, and temperature. Because of the many mechanisms of adsorption and desorption it appears that no one equation applies over the entire range of values. The reader is referred to the references to get detailed information on the use of these relationships (Hall and Rodriguez-Arias 1958; Gustafson and Hall 1974), Probably the most representative isotherms are given by the Brunauer, Emmet, and Teller (B-E-T) theory of multimolecular adsorption. The B-E-T isotherm is generally valid in the range of 5 to 50% humidity (Brunauer 1945). relative The Harkins-Jura method of plotting isotherms gives reasonably accurate results from 10 to 90% relative humidity, but certain constants needed for the calculations are not readily available (Harkins and Jura 1944). The Smith equation is found applicable for isotherms in the higher relative humidities, from 50 to 95% (Smith 1947). The Smith and B-E-T equations complement each other. QUESTIONS 1. List the steps which are followed for determining the latent heat of moisture in grain using the equilibrium moisture content data. 2. Design an experiment to determine by the static method the equilibrium moisture content of a product at 10°, 21°, and 32°C (50°, 70°, and 90°F) and approximately 10, 40, 70, and 90% relative humidity. List equipment and supplies needed and outline the procedure to be followed. 3. What is the vapor pressure in Pascals of wheat at 21°C (70°F) and 40% relative humidity? At 21°C and 60% relative humidity? At 10°C (50°F) and 40% relative humidity? At 10°C and 60% relative humidity? 4. Contrast the effect of mold on samples used for equilibrium moisture determination when moisture determination is based on (1) original sample and (2) final sample when (a) dry matter is not greatly changed and (b) dry matter is greatly reduced by mold growth. 5. A method of obtaining the temperature in degrees Fahrenheit from the temperature in degrees Celsius is as follows: Add 40° to the °C temperature, take 9/5 of the summation, and subtract 40° from the product to obtain °F. Explain the method mathematically. How would the above procedure be followed to determine °C from °F? 6. Calculate the values of equilibrium constants c and n of No. 2 alfalfa hay, using the data of 78°F and 40 and 70% relative humidity. DRYING AND STORAGE 36 OF AGRICULTURAL CROPS 7. Contrast the effect of starch, protein, and fiber on the equilibrium moisture content, Table 2.4, at low and high relative humidities. Illustrate by examples of products with various proportions of protein and fiber. 8. Plot the equilibrium moisture content data of yellow dent shelled corn on a psychrometric chart. Discuss the value of\this method of representation. 9. Make a list of the moisture contents, w.b., of the common crops at an equilibrium relative humidity of 75% and at 25°C (77°F). Discuss importance of relationships. 10. From the equilibrium moisture content curve, equation (2-1), de- d ; termine (a) the slope of the curve, a where y is the percentage moisture, d.b., and x is the relative humidity, decimal, and evaluate d2 at (0,0) and (b) the point of inflection, i.e., where = = 0. 11. Using the equation, H = F + TAS, discuss the changes which take place in each of the terms in the system as grain absorbs moisture from the atmosphere to reach equilibrium. REFERENCES ASAE. 1972. Agricultural Engineers Yearbook. tural Engineers, St. Joseph, Mich. BABBIT, J.D. 1942. American Society of Agricul- On the adsorption of water vapor by cellulose. Can. J. Res. 20A, 143-172. BAILEY, C.H. 1920. The hygroscopic moisture of flour exposed to atmospheres of different relative humidity. Ind. Eng. Chem. 12, 1102-1104. BAKKER-ARKEMA, F.W. and HALL, C.W. 1965. Static vs. dynamic equilibria in the drying of biological products. J. Agric. Eng. Res. 10 (4) 308-311. BARTON, L.V. 1941-1942. ty to viability of seeds. Relation of certain air temperatures and humidi- Contrib. Boyce Thompson Inst. 12, 85-102. BECKER, H.A. and SALLANS, H.R. 1956. A study of the desorption isotherms of wheat at 25°C and 50°C. Cereal Chem. 33 (3) 79-91. BROOKER, D.B., BAKKER-ARKEMA, F.W. and HALL, C.W. 1974. Drying Cereal Grains. AVI Publishing Co., Westport, Conn. BRUNAUER, S. 1945. The Adsorption of Gases and Vapors, Vol. 1. Physical Adsorption. Princeton Univ. Press, Princeton, N.J. COLEMAN, D.A. and FELLOWS, H.C. 1925. Hygroscopic moisture of cereal grains and flaxseed exposed to different relative humidities. Cereal Chem. 2, Phi has ih DAVIS, R.B., JR., BARLOW, G.E. and BROWN, D.B. 1950. heat in mow drying of hay (III). Agric. Eng. 31 (5) 223-226. Supplemental EQUILIBRIUM MOISTURE RELATIONSHIPS 37 DEXTER, S.T. 1946. Conditioning popcorn to the proper moisture content for best popping. Mich. Agric. Exp. Stn. Q. Bull. 29. DEXTER, S.T., ANDERSON, A.L., PFAHLER, P.L. and BENNE, E.J. 1955. Responses of white pea beans to various humidities and temperatures of storage. Agron. J. 47, 246—250. DEXTER, S.T., SHELTON, W.H. and HUFFMAN, C.F. hay. Agric. Eng. 28 (7) 291—293. FENTON, F.C. 1941. Storage of grain sorghums. 1947. Better quality Agric. Eng. 22 (5) 185-188. GALLAHER, G.L. 1951. A method of determining the latent heat of agricultural crops. Agric. Eng. 32 (1) 34, 38. GUSTAFSON, R.J. and HALL, shelled corn from 50 to 155°F. HALL, C.W. suds. 1956. G.E. 1974. Equilibrium moisture content of Trans. ASAE 17 (1) 120-124. Drying temperatures and storage problems of sugar beet J. Am. Soc. Sugar Beet Technol. 9 (2) 161-166. HALL, C.W. 1957. Drying Farm Crops. AVI Publishing Co., Westport, Conn. HALL, C.W. and RODRIGUEZ-ARIAS, J.H. 1958. Equilibrium moisture content of shelled corn. Agric. Eng. 39 (8) 466-470. HARKINS, W.D. and JURA, G. 1944. A vapor adsorption method for the determination of the area of a solid. J. Am. Chem. Soc. 66, 1366. HENDERSON, S8.M. Eng. 33 (1) 29-31. 1952. A basic concept of equilibrium HENDERSON, 1970. Equilibrium moisture content of small grain hysteresis. S.M. moisture. Agric. Trans. ASAE 13 (6) 762—764. HENDERSON, S.M. and PERRY, R.L. 1976. Agricultural neering, 3rd Edition. AVI Publishing Co., Westport, Conn. HOGAN, J.T. and KARON, M.L. 1955. Process Engi- Hygroscopic equilibria of rough rice at elevated temperatures. Agric. Food Chem. 3 (10) 855-859. HOUSTON, D.F. and KESTER, E.B. 1954. Hygroscopic equilibria of whole grain edible forms of rice. Food Technol. 8 (6) 302-304. KARON, M.L. 1947. Soc. 24 (2) 56-58. LANG, N.A. 1956. Hygroscopic equilibrium of cottonseed. The Handbook of Chemistry. Handbook J. Am. Oil Chem. Publisher, San- dusky, Ohio. LARMOUR, R.K., SALLANS, H.R. and CRAIG, B.M. 1944. Hygroscopic equilibria of sunflower seed, flaxseed, and soybeans. Can. J. Res. 22F, 1-8. LOCKLAIR, E.E., VEASEY, L.G. and STANFIELD, M. 1957. Equilibrium desorption of water vapor in tobacco. Agric. Food Chem. 5 (4) 294-298. LONG, D.R. 1951. Threshability of ladino clover as affected by moisture. Agric. Eng. 32 (12) 674-676. MORRIS, York. OTHMER, T.N. D.F. 1947. 1940. The Dehydration of Food. D. Van Nostrand Co., New Correlating vapor pressure and latent heat data. Eng. Chem. 32, 841-846. Ind. 38 DRYING AND STORAGE PERRY, OF AGRICULTURAL R.H. and CHILTON, McGraw-Hill C.H. 1973. CROPS Chemical Engineers’ Handbook. Book Co., New York. RAMSTAD, P.E. and GEDDES, W.F. 1942. Respiration and storage behavior of soybeans. Minn. Agric. Exp. Stn. Tech. Bull. 156. RODRIGUEZ-ARIAS, J.H. 1956. Desorption isotherms shelled corn in the temperature range of 40° to 140°F. and drying rates of PhD. Dissertation. Mich. State Univ. SCHWARTZ, packages. T.A. 1943. Improvement needed in technique for testing food Food Ind. 15 (9) 68-69, 124-125. SEIDELL, A. 1940. Solubility of Inorganic and Metallic Organic Compounds. D. Van Nostrand Co., New York. SMITH, J.E. 1947. The sorption of water vapor by high polymers. J. Am. Chem. Soc. 69 (3) 646-651. SNOW, D., CRICHTON, M.H. and WRIGHT, N.C. of feeding stuffs in relation to humidity of storage. 1944. Mold deterioration Ann. Appl. Biol. 31, 102— 116. THOMPSON, H.J. and SHEDD, C.K. 1954. vaporization of shelled corn and wheat. WASHBURN, E.W. 1927. International Graw-Hill Book Co., New York. WESTON, W.J. and MORRIS, H.J. Equilibrium moisture and heat of Agric. Eng. 35 (11) 786-788. Critical Tables, Vol. 1 and 2. Mc- 1954. Hygroscopic equilibrium of dry beans. Food Technol. 8 (8) 353-355. WEXLER, A. and HASEGAWA, S. 1954. Relative humidity-temperature relationship of some saturated salt solutions. 19-26. J. Res. Natl. Bur. Stand. 53 (1) WINK, W.A. 1946. Determining the moisture equilibrium curves of hygroscopic materials. Ind. Eng. Chem. Anal. Ed. 18, 251. WINK, W.A. and SEARS, G.R. 1950. Instrumentation studies. LVII. Equilibrium relative humidities above saturated salt solutions at various temperatures. Tappi 33, 96A—99A. ZINK, F.J. 1935. Equilibrium moisture content of some hays. Agric. Eng. 16 HigUme war Gee Moisture and Temperature Changes and Effects Moisture Migration Moisture migration in stored farm crops results from changes. The quantity of moisture present in grain is one important factors influencing its storage life. High moisture temperature promote mold and insect growth and increase temperature of the most and a warm the respiration rate of the product. These factors tend to reduce the quality of the grain either through direct damage to the product or by a decrease in the viability of the seed. Moisture migration may take place in a bin even though the grain is at a moisture level generally considered safe when stored. It is not uncommon for grain to be placed in the bin at 12.5% moisture. This is considered low enough moisture for safe storage in most areas, but the moisture accumulation in the top layer may build up to as much as 30% moisture content. Moisture accumulation is noted in bins holding over 70.5 m? (2000 bu). A loss of 17.5 m® (500 bu) from an 880 m? (25,000 bu) storage has been noted after 1 year in government storages in Indiana (Holman and Carter 1952). Greater losses have occurred in bins with stored grain when (1) there was a high moisture content of the product at the beginning of the storage, (2) a tall structure was used, (3) a large difference in atmospheric and grain temperatures existed, and (4) product was placed in storage during warm weather as compared with placing in storage in cold weather. Convection Air Movement The moisture movement in storage is primarily a result of convection air movement. Some moisture movement occurs as a result of diffusion. Most grains are placed in storage in the fall when the grain is warm. Wheat, oats, and early harvested shelled corn are usually warm when 39 40 DRYING AND STORAGE OF AGRICULTURAL CROPS placed in the bin. In this condition the air in the grain near the sur- the face of the storage cools and moves downward along the edge of where bin the of center the bin, across the bottom and then up near or at the the air and grain are warm. The air moving through the center of top the across moves or bin bin gathers moisture until the air leaves the to the sides. The surface of the grain is cold on top and\the moisture condenses on the grain, raising its moisture content (Fig. 3.1). The spoiled grain is often 0.3 to 0.6 m (1 to 2 ft) below the surface and extends to about 1 m (3 ft) from the edge of the bin. MOISTURE ACCUMULATION FIG. 3.1. CONVECTION AIR CURRENTS THROUGH WARM GRAIN IN BIN SURee WITH COLDER | For grain which is cold when placed in the bin, such as late harvested shelled corn and pea beans in the late fall and winter, the air rises along the surface edge of the bin during the late winter and early spring. The air moves downward through the center of the bin to the bottom where moisture condenses on the cold bottom. The air then moves to the walls and upward along the walls upon being heated. Under this condition grain spoilage occurs at the bottom of the bin (Fig. 3.2). The most critical time from the standpoint of controlling moisture accumulation is in the winter and early spring. The majority of moisture which accumulates in the top layer of the bin does not come from the outside atmosphere. The moisture is picked up by the air currents moving through the grain due to MOISTURE AND TEMPERATURE eas CHANGES PSS AY | GOLD a EFFECTS \ | GRAIN WARM GRAIN 41 gt || WARM GRAIN i FIG. 3.2. CONVECTION AIR CURRENTS THROUGH COLD GRAIN IN BIN SURROUNDED WITH WARMER AIR aJ eg a / WARM AIR AND | | WARM AIR by COOL GRAIN MOISTURE ACCUMULATION changes in temperature between the outside air and the center of the grain mass (Carter and Farrar 1943). The factors which contribute to greater losses in storage are caused by greater convection air currents. When emptying a bin which includes some spoiled grain, it is quite possible that the grade of the entire bin will be lowered because of mixing the spoiled grain with the grain in other portions of the bin. Moisture Content and Storage The moisture contents given in Table 1.5 are the maximum values recommended for safe storage. These recommendations must be applied TABLE 3.1. RECOMMENDATIONS Moisture %,w.b. FOR STORING SOYBEANS Condition 15 Not safe for storage 14 13} Safe storage limited to winter months Safe storage for one winter season 12 U.S. No. 1 and 2 grades held up for 10 Good condition for 4 years storage nearly 3 years but germination declined Source: Holman and Carter (1952); Holman (1955). 42 DRYING AND with judgment STORAGE OF AGRICULTURAL because of varying atmospheric CROPS and grain conditions. The moisture content of the wettest grain in a bin of grain should be used as an index as to whether it is safe for storage. The length of time which crops can be stored varies with the moisture content, crop, and atmospheric conditions. To store a crop for 5 years the moisture content of the product should be approximately,2% below the moisture level which is considered safe for 1 year storage. The relationship of moisture content to storage life of soybeans is presented in Table 3.1. The same relationship, not necessarily the same values, is approximately applicable for other grains. One factor used for setting the moisture content for safe storage is the effect of the moisture content on the loss of viability of the product. Other factors affect the viability of grain, such as age and condition of the grain when stored, but the effect of high relative humidity is very important even for short storage periods. Serious damage occurs to viability of wheat, oats, and barley stored at 21°C (70°F) and 90% relative humidity or higher for less than 1 month, while if stored for 3 years at 21°C (70°F) and 57% relative humidity there is only a small decrease in viability (Robertson et al. 1939). The time required for the product to reach equilibrium from a high relative humidity to one safe for storage will also affect the viability. The equilibrium moisture content is of value for estimating the possibility of grain going out of condition. Samples of grain taken from bins in which moisture has accumulated in the top layer have shown a definite decrease in germination (Carter and Farrar 1943). The reduction in germination was greatest at 0.3 m (11 in.) depth with 45% decrease. The accumulation of moisture in the grain because of rain and snow should not be overlooked. The excess moisture which usually occurs on the top layer could lead to the deterioration of the grain. TABLE 3.2. RELATIONSHIP OF INITIAL MOISTURE CONTENT REAL GRAIN TO FINAL BULK STORAGE TEMPERATURE ; Grain Moisture, % 17 18 To Ensure Unimpaired Germination xe ay 10 50 19 Th 8 46 20 222, 5 4 41 39 45 OF UNINFESTED CE- To Be Satisfactory for Feeding Animals “€ “1p 13 55 8° 46 8. 8 46 46 8 46 Source: Paterson (1967). Grain Changes When moisture is removed from grain with heated air, the surface con- tracts more rapidly than the interior and checks or cracks develop in the surface of the grain. The relationship between the rate of moisture MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 43 removal and cracking has not been worked out in detail, but checks develop in corn with rapid removal of about 6% moistur e, and in pea beans with 3 to 4% moisture removal. As moisture is added to dry grain, expansion takes place and the lateral pressure exerted on the storage unit is increased. The increas e in pressure amounts to at least 6 times that of dry grain when the moisture content is increased about 4% by passing high humidity air through the grain (Dale and Robinson 1954). The lateral pressure of shelled corn at a depth of 1.5 m (5 ft) increased from 2.415 to 13.1 kPa (0.35 to 1.9 psi) as the moisture content was increased from 13 to 15%. When increased to 22% moisture by soaking the corn, the lateral pressure was 22 kPa (3.2 psi). An overall reduction in volume of the grain occurs during drying. A nearly straight-line relationship exists between the percentage volume shrinkage and percentage moisture removed down to 25% (w.b.) for corn. Reducing the moisture content of corn 25% gave a bulk volume shrinkage of 33% (Wileman 1941). A similar relationship exists for drying wheat. Shrinkages of 25 and 10% were obtained when drying from 30 to 15 to 10% moisture content, w.b. (Johnson 1958). Temperature Changes Moisture movement occurs through the grain as a result of diffusion as distinguished from air movement by convection. The importance of diffusion in the movement of moisture deserves additional investigation. The thermal diffusivity is a measure of the rate of temperature change and indicates the speed at which temperature equilibrium will be reached. The germination of seed is reduced if wet grain is frozen before the moisture is removed. With corn at a moisture content above 25%, germination will decrease upon exposure to temperatures below 0°C (32°F) for 24 hr (Table 3.3), and decrease upon exposure to —-16°C (4°F) for only 4 hr (Kiesselbach 1923). TABLE 3.3. GERMINATION OF CORN AT VARIOUS MOISTURE CONTENTS EXPOSED TO DIFFERENT TEMPERATURES, % Temperature for 4hr Moisture Content of Grain, %, w.b. “C °F =1.1 —5.5 -10.0 -14.4 -16.4 30 22 14 6 2:5 Source: Kiesselbach (1923). A5—40=40—35 69 Wy 0 0 0 go 13 0 0 0 05-00" (s) 67 12 0 0 30-25) 85 We 34 ik 0 25-20 20-15 15-10 100 96 88 47 0 100 100 100 98 68 100 100 100 100 Oni 44 DRYING AND STORAGE OF AGRICULTURAL CROPS The linear coefficient of thermal expansion, a, for corn is 0.34 X 104 per °C (0.187 X 10-4 per °F) and for steel is 0.11 X 10™4 per °C (0.0608 X 104 per °F) (Stahl 1950). Thus, 1 in. of corn would expand to 1.0000187 in. when the temperature is increased %C (1°F). The increase in length is L.aAt where L, is the original length and At is the increase in temperature. The surface or area coefficient, 8, is 2a, and the volume coefficient, y, is 3a. The change of temperature throughout a storage varies considerably with different products, kinds of storages, and geographical locations. The surface responds quickly to temperature variations but the center of the grain mass lags considerably. There are internal and external sources of heat which cause temperature changes to occur within bulk stored grain. The internal sources of heat are respiration, mold growth, insect infestation, and the heat of the product when placed in the bin. The external sources of heat changes are due to the external atmosphere and are caused by daily temperature temperature. differences and seasonal changes in The following variations in the temperature of the product were found for large unventilated storages (Babbitt 1945): (1) For a temperature difference of 11.1°C (20°F) from day to night, the temperature is changed %°C (1°F) at a depth of 0.12 m (5 in.). Daily fluctuations do not penetrate greater than 0.17 m (6 in.) to any appreciable extent. (2) The annual temperature variation of the product at a depth of 3.9 m (138 ft) is not greater than %°C (1°F). (3) At the end of 1 year the temperature 6.1 m (20 ft) from the surface is practically unchanged and remains at the temperature at time of storage. Low temperatures retard the deterioration of wet products. When wet products become warm, molding and heating will proceed rapidly and spoilage may occur. Products should be at a temperature as low as possible when placed in the storage. Cereal grains have a large thermal capacity. Shaded bins are the most effective type of storage when the outside temperature is high as compared to the grain temperature (Kelly 1941). Insulation of an unventilated storage structure is a disadvantage because the removal of excess heat is decreased. The use of underground bins for storage helps to maintain a low temperature. Prevention of Moisture Accumulation Moisture accumulation can be prevented by moving air around the grain kernels to maintain uniform temperature or by reducing the tem- MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 45 perature variations on the surface of the product to reduce convection and diffusion. The procedures which follow describe methods for preventing moisture accumulation. Natural Ventilation.—By using the velocity of the wind to move air through the grain, either with or without ducts, the undesirable effects of natural convection currents can be prevented. Mechanical Ventilation.—By using a fan and a suitable duct arrangement, air can be moved through the product to maintain a cool temperature. Only enough air should be moved through the product to maintain cool temperatures. When only enough air is moved through the product to prevent moisture accumulation, it is called aeration. Aeration airflow usually amounts to less than 0.104 m?/min-t (0.1 cfm/bu) of grain in the bin. Heated Forced Air.—Using heated forced air, the moisture content of the grain is reduced to a level well below that considered safe for storage. By using heated air, preferably for drying before the grain is placed in the bin, moisture migration is held to a minimum. Unfortunately, it is uneconomical to dry the grain to a low moisture content, perhaps 6%, for the purpose of eliminating migration. Move the Grain.—A practice of many commercial concerns is to move the grain periodically on schedule, perhaps every 3 weeks. The cost of conveying, elevating, providing extra storage space, and damage to the grain for this method of preventing moisture accumulation is usually greater than the cost of aeration. Stir Surface of Grain.—To prevent excessive accumulation of moisture and the resultant spoilage, the surface of the grain is stirred during the late fall and winter to help dissipate the moisture. The moisture content of the grain next to the outside atmosphere is usually less than the moisture content a few centimeters below the surface of the grain. Surface Covering of Bin.—The surface covering for the bins can be selected to avoid large fluctuations of temperature and to keep the grain from cooling and heating excessively. Large fluctuations of the temperatures of the grain and the building material are avoided by using surfaces having a low emissivity at night and low absorption of heat during the day. A low emissivity at night prevents the surface from cooling greatly below the air temperature when there is a clear sky. A low absorption during the day prevents the surface from heating up greatly above the air temperature when there is a clear sky. Moisture accumulation in the upper layer of grain in northern areas of the United States presents the greatest problem during the winter months (Holman and 46 DRYING AND STORAGE OF AGRICULTURAL CROPS Carter 1952; Holman 1955). For temperatures below 0°C (32°F), the lowest to highest emissivity was obtained in the following order: aluminum paint, galvanized surfaces, and oxide paints (Henderson 1947). The lowest to highest absorption at a temperature of 10°C (50°F) was obtained in the following order: oxide paint, aluminum paint, and galvanized surfaces. Frost accumulation on any of the surfacés increases the emissivity. Painted exterior had little effect on the temperature of soybeans (Holman and Carter 1952; Holman 1955). The temperature varied a maximum of 2.8°C (5°F) near the surface and 1.1° to 1.6°C (2° to 3°F), 0.15 m (6 in.) from the wall for unpainted steel, black surface plywood, and aluminum-painted plywood. Heat Transfer To determine or to predict temperature changes resulting in a stored product, it is desirable to know the thermal conductivity, k, and the specific heat, c. Equations are available for predicting the temperature at any time if certain basic data are available. Data on specific heat and thermal conductivity are given in Tables 3.4 and 3.5. The rate of heat transfer, g, Btu/hr, is as follows: dt q = kA — dx 2. iy (3-1) Btu-ft where k = thermal conductivity, GnChGe) A = area of the cross section perpendicular to path of heat flow, fb? U ae temperature gradient, F/ft. Corresponding metric units can be used. See Appendix Table A.6. The thermal conductivity values depend upon the material and vary as follows: (1) The thermal conductivity of alloys rises with a rise in temperature. (2) There is an increase in thermal conductivity with an increase in density. (3) The thermal conductivity of air increases slightly with an increase in temperature and pressure. (4) There is an increase in thermal conductivity of grain with an increase in moisture content of grain. AND TEMPERATURE CHANGES AND EFFECTS (7) (9) (9) (9) (9) (Z) (T) GV 0 6690 00¢°0 8670 68 6S IWHYNLINOISV SLONGOYd 660 GLE0 00'T VLZO vv0 6V'0 vv 0 FG 6G 6°} 9} 06 89 ZI 0FGE 0GIS 00T°} 89 0GIG 00T°} 93 ZI OOT 8E 0S °F FZ % O€T GL VG 07 EL 9%} O6T 88 &§&@ °F 9} 66 FG °%} ZI 6G OFS V8 %} GI 66 FPG V8 °} OOT 09 9ST BLE 93 9ST 09 OZT 68h 94 OT CTE hs QSOy OL OF V8 GE 0EPE0} OP OM 462 OG 085 ©} TG 86 ‘0, O€ FOLVV€'0G LIZ 0G GE FTG 68 9F OT GE OF1G 68 9} GE TS 68 OT 9} OT qc Da 0%09 OT AIGVL “¥'€ sIN4STOPAy “JU9}UOD suveq [eleqyeyy uo) aqyarou0) UY,ABI AeH aryhog INO] 044072, a0] yeoUM Taye resng Yoieys [OOM POOM "M34/£4 OLETP = A. A1/M T :worsreauop “Q's °FTS ST GALO3TIAS LV'0 L‘0 v0 v6e'0 6980 TES°0 asuey oinjeiedura OlsIOAdS LVAHAO g0g'0 889°0 OT oe)8966 vscheSTV T 69'T€8T6669V'% Tv8LETSOG LGT6L'T T6Git ITZ 60%61%GLSour 066¢9'0 V8 Oe d. ueezey (9) “(PS61) Seusiq (S) “(8F6T) THPIS (7) (LET) UOITYD pue AIJEg (€) “(6LET) ISP (Z) {(6E6T) UOSPNH (T) :201n0g 001% O Z1ZZE (T) 870 vor'0 EN) 6G 0 860 Ms84/e¥ yeay oytoads 4.4/4 (9) (1)(2)(8)(9)(9) (9)(T)(V7) (¥) (¥) (Z) (Z) aouerlejoy MOISTURE (G96T) T@H pue 47 AND STORAGE OF AGRICULTURAL CROPS (3) (8) (S) (T) (€) (8) (S) (T) (S) (S) (2) (€) (S) (L) (S) (9) (S) (¥) (T)(3)(€) st 0L'0 v8'0 v60 BVT96°0 vor 680 96°0 €0'T 620 G9L'0 T9€°00v0°0 TOTO 8cT0 OST—06 € Or cvs L@tl 11e Arp Vr£06 GALOAIAS IWHNLINOISV93919U0) SIVIYSALVW 3IEVL “Se IVWHSHL ALIAILLONGNOOD 4O ul09 :sjany pmbiy aT ueury ‘pues Arp $1eQ “Vea }JOS ‘[10g AIp ‘MeIIS qNd AA ‘poo,auld ‘mous IaVeM AIS ‘JOOM SSUIABYS “4JsNPMeG Jeyyea'T [eee qoeduroo “W09409 €45/AI aIny4Sloyy ‘yUI}YUOD % ‘UoIsteAUoDTU/MQ_T 235 “U/A, = vV—1G vy—I1zd vyV—I1zd vv—IzG v—-0 IZ TE OF L— 0€ EE 0 0O0T—0 [, aInjeledula UBATAy 8V—IZ 8V—IZ 8V—1Z OL ITT—OL TIT—OL ITT—-OL TTT—OL ‘(9P61) ‘(8P6T) (L) A2TOOM (9) THP4IS ‘(6L6T) 88-06 00€—-89 oO: i. 98 O€ GIS—GE 6TT—OL 6ITT—OL 61TT—OL OV—ZE (6s SBP “MU/M LOGS G€c0'0 LVT0O 06ST 6LT0 06ST 09LT 880°0 0O€T TS0°0vIZ0 06ET €89°0 0IT 96T'0 08ET 06vT 6v00 qsB9MA (G) (ET6T) [QOZ pue [[ossesuy (F) ‘(G96T) [2H pue uBlezey (g) ‘(ELET) UOIYO pue A1sJeg (Z) (6861) UOSpNH (T) :Se0IN0g goualajoy DRYING ny 48 MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 49 The steady-state heat transfer equation is rather simple as is shown by equation (3-2). k q pte’ (At) (3-2) where x = thickness of material parallel to path of heat flow, ft Transient Heat Transfer Steady-state conditions do not normally occur in storages. The temperature conditions will change with time. For unsteady or transient heat flow in one direction, an instantaneous heat balance gives the relationship as shown in equation (3-3) with the heat removed from the surroundings on the right side of the equation and the heat gained by the grain on the left side of the equation. 6 6t/6 cp dx dy dz a = kdydz Ae (3-3) It is assumed in most heat transfer calculations that the thermal properties, specific heat, and conductivity remain constant. Equation (3-4) represents the heat transfer relationship. ot = Ge) 66 cp (3-4) bx? The general transient heat transfer equation is represented by equation (3-5). eck b t.—ts ké hr. a CPT mn? k Ks where t t, = temperature at any time, F = temperature of surroundings, °F 6 = t, = initial temperature, °F time,hr I'm = cylinder radius, ft cigs =Bage aetecarl iach ean (st—————_ comme thermal conductivity, (hr)CF)(ft?) 1 For metric conversions, see Appendix Table A.6. (3-5) DRYING 50 h AND STORAGE OF AGRICULTURAL CROPS = surface conductance, Btu/(hr)(ft2)(F), usually 1.65 for still air, and 6.0 for 15 mph wind In SI units, k is in W/m:K for which 1 Btu-ft/hr ft? °F = 1.730735 W/mK. A graphical solution of the transient heat transfer equation is presented in Fig. 3.3. The chart can be used for determining the temperature at the center of a bin of grain with the other conditions given; or if the various temperatures are known, including temperature changes in the bin at a given time, the value of k can be determined. The results of transient heat transfer in a storage are presented in Fig. 3.4. Periodic Temperature Variations The thermal conductivity varies greatly from the laboratory to the field. As mentioned previously, convection is not important in small bins but probably plays a very important part in large bins. The temperature at a given location in a bin of grain is difficult to determine under field conditions. Considerable temperature variation occurs under field conditions and does not permit simple analysis with the standard transient heat transfer equations. The temperature can be obtained at a given depth from equation (3-6) based on a periodic temperature variation (see Fig. 3.5). i= i sin(raul =X Pf a aP + ta (3-6) = temperature at any depth, x, °F cto = amplitude (half range) of external fluctuations, °F = depth, ft = period, hr = time, hr Dry © = 2.718 a Peak hr “cp =diffusivity,— =— tm = mean of external fluctuations, °F The equation is also applicable using a in m?/s with units in °C, m, and sec. The damping of temperature fluctuations is represented by equation (3-7). ta a as ee ei w where t, is the amplitude at depth, x = (3-7) MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 51 A reasonable result is obtained using this method but is not entirely accurate because (1) the outside temperature does not necessarily fluctuate according to a sine function; (2) the temperature of the air rather than the grain is usually measured; and (3) the previous treatment of the product, its temperature, moisture, and heat produced, will affect the results. 0.04 0.02 cms 0.2 0.4 FOURIER No., O06 0.8 10 Ke za Cp hm ln t= temperoture ot center, °F tg= surrounding temperature, °F to= original temperoture, °F k=thermal conductivity, 'm= radius of cylinder, btu.-ft. Ahr)(sq. ft.) feet C = specific heat, btu /(Ib)(°F) p=specific weight, Ib./cu.ft O= time, hrs. enMeee) A) CE) FIG. 3.3. TRANSIENT HEAT TRANSFER IN CENTER OF A CYLINDER pO RITE 12 1.4 DRYING 52 STORAGE AND OF AGRICULTURAL CROPS 80 70 20 60 10 iL ° «4 ° ae LJ x E < 50 ) cr A- 25cm FROM CIRCUMFERENCE (tin,) WW x B-12 cm FROM CIRCUMFERENCE m= aq 30 C- 23cm FROM CIRCUMFERENCE (9in.) (Sin) auJ uJ rw a = = uJ - ud - D- CENTER ©Cm =0 20 STEEL DRUM DIAMETER = 58 cm(23in.) =29.4°C(85°F) SURROUNDING TEMP. MOISTURE CONTENT = 16.0 % (w.b.) -10 10 0 -20 -10 (6) 20 40 TIME, 60 80 100 HR FIG. 3.4. TIME-TEMPERATURE RELATIONSHIP FOR UNSTEADY STATE HEATING OF GRAIN IN A CYLINDER Calculation of Temperatures in the Bin The transient heat transfer equation (3-5) can be used for estimating the temperature in a bin at a particular location at a given time. The equation may be used to determine the apparent thermal conductivity of a material by measuring the temperatures at a specific time. This approach would be beneficial for large masses of grain or hay. The thermal conductivity is usually determined with small lots. The thermal conductivity determined by equation (3-5) would include the transfer which MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 53 PERIOD,HR AMPLITUDE TEMPERATURE FIG. 3.5. TEMPERATURE VARIATION ON WHICH EQUATION (3-6) IS BASED takes place by convection, a value of more significance for practical field problems. By comparing the thermal conductivity for small and large lots of the same material, the influence of convection can be determined. The solution of equation (3-5) can best be made using Fig. 3.5 as follows: (1) From experimental t -t, a value for the temperature ‘ ratio (; Oo data, determine )at the center of the bin. Ss (2) From experimental and physical properties, determine 3 m terms of k. : kes, in terms of k. (3) From experimental data determine i m (4) Assign values to k, and determine value of (ae = ie ——— from the graph in’ Fig: 3:5: (5) Determine the value of k by graphical methods to obtain temperature ratio. Example: (1) After 40 hr the temperature at the center of a 23 in. diameter drum decreased to 40°F from an initial uniform temperature of 85°F when placed in an environment at a temperature of 3.5°F. 40-85 —_ 3.5 — 85 (2) Use the following values: 6=40hr c= 0.45 Btu/lb °F = —45 =S1.0 = 0.55 54 DRYING AND STORAGE OF AGRICULTURAL CROPS p =48lb/ft? tm = 11.5/12 ft = 0.96 ft h = 1.65 Btu/(hr) (ft?) CF) ke" cprn? acehr, k40 (0.45)(48)(0.96)? ss (1.65)0.96) \ eee! (4) Assign k values (5) Solve for k, for a temperature ratio ef 0.55, illustrated in Fig. 3.6. Btu-ft k = 0.098 (ante) 18 "& Btu-in. = 017W/mk (hnt)CF) The temperature at a particular location can be found values of the product are known. If the temperature is te at a location other than the center of the bin, use the lines r Fig. 3.7 represented by it the values of which are 0 at if the physical be determined on the graph in the center and 1 at the circumference. Temperature Measurement The temperature of stored grain gives an indication of the condition of the grain. If overheating is noticed, the grain should be moved or mechanically cooled to prevent excessive grain loss. Temperature measurements are also desirable to check the performance of a dryer. MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 55 to-t, bet FIG. 3.6. USE OF TEM- PERATURE RATIO TO SOLVE FOR k k, conductivity | Btu-ft/hr ft@°F = 1.730 W/mk Farm installations are usually too small to justify the cost of expensive electronic equipment. The temperature of grain may be sensed by placing one’s hand as far into the bin as possible. A bulb glass thermometer provides a reasonably accurate method with inexpensive equipment. The thermometer bulb is often protected with a metal shield, which decreases the sensitivity. These thermometers should remain in the grain and be removed for reading. Thermometers which are heavy enough for inserting in grain may not come to a constant reading for 30 to 40 min. Commercial elevator installations can usually justify electronic systems using thermocouples to sense the temperature. A thermocouple consists of two small wires of different metals joined together by soldering or welding. The thermocouple is placed where the temperature is to be determined. Copper and constantan or iron and constantan are used for thermocouples. The voltage output of a thermocouple varies with the temperature and is measured by a potentiometer. Flexible cables enclosing several thermocouples and covered with nylon are inserted in bins to get the temperature at several locations. Various thermocouple spacings are used, with a 2 to 3 m (5 to 10 ft) spacing normally used for grain storage. 56 DRYING \o AND STORAGE OF AGRICULTURAL CROPS eses WH EaE a ra 0 asSa ARR ee BB\NANNNNGBESS WL ARR RE Bae, 522 L) OX i iE eE : eeaoF EN L, NINO Soo weSEN LL MA Os, Ay nar ae VAAN SABES\\SARANNIBAN SL et WEP Tat AEE NRE PPC IE NS A, FIG. 3.7. UNSTEADY STATE HEAT FLOW INA CYLINDER . Thermocouples and thermometers should be shielded to determine temperatures for heated air installations to prevent erroneous readings due to heat from radiation. Grain Deterioration The deterioration of grain is dependent on the moisture content, temperature, oxygen supply, and microorganisms involved. The kind and MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 57 quantity of microorganisms present depend on the previous treatment of the grain. The relative deterioration rate increases with an increase in temperature, up to about 27°C (80°F) (Steele 1963). At higher temperatures, many of the microorganisms could be killed, thus reducing the deterioration rate. The relative deterioration rate also increases with an increase in moisture (Fig. 3.8). 2a je) = Pee o ti ee 10 on<q eg 0.5 = <x — = 0 4¢ 8 2 26 £30 GRAIN MOISTURE, PERCENT (w.b.) az fe) = <q 5... &rp 15 Lil aa 1.0 LJ > FE 05 mal a 0 30 A O 40 ee 50 60 70 80 90 TEMPERATURE , °F er ea | a ee fe) 20 30 TEMPERATURE , °C FIG. 3.8. RELATIVE GRAIN DETERIORATION FECTED BY MOISTURE AND TEMPERATURE RATES AS AF- 58 DRYING AND STORAGE OF AGRICULTURAL CROPS Maintaining Grain Quality Damp grain from the combine or from a storage may be cooled or maintained at a uniform condition by (1) aeration or mechanical ventilation, using a fan moving unheated air, (2) natural ventilation, moving air through the grain with wind pressure, (3) refrigerated air forced through the product, (4) turning, moving the grain from one bin to another, and (5) cleaning, removal of weed seeds, chaff, straw, and other foreign material. Aeration or Mechanical Cooling Aeration or mechanical cooling should be distinguished from drying because of the difference in objectives and procedures involved. Aeration is the moving of air through stored products for the primary purpose of maintaining a uniform moisture content and/or temperature throughout the storage. Airflows of 0.104 m3/min-t (0.1 cfm/bu) or less are moved through the product. Using aeration, safe storage can be provided for grain 1 to 2% moisture above that normally considered safe for storage. Aeration is used to prevent the accumulation of moisture migration from, warm to cold layers. Although about 2% moisture is removed in cooling grain from 21° to 4.4°C (70° to 40°F), the purpose is not to dry the product. Aeration provides a positive method of conditioning grain at a low cost. The cost of maintaining grain per bushel with aeration equipment in large installations is about the same as by turning the grain once, without the attendant cracking which might accompany turning. Less bin volume is required when using aeration as compared to turning the grain. Aera- tion provides a positive means of cooling grain, reducing the possibility of insect growth, mold growth, and chemical changes, and with aeration, undried grain can be held in cool weather with low airflow rates. Air Requirements The quantity of air used for aeration is from 0.104 to 0.021 m3/mintonne (%9 to ¥%50 cfm/bu) of grain. Successful cooling of shelled corn has been accomplished with 0.02 m*/min-t (%o cfm/bu) in Indiana (Mayes 1955). Recommended airflow rates are given in Table 3.6. The static pressure encountered with these low air volumes in moving air through the grain is relatively low and it is possible to economically move air through depths up to 46 m (150 ft). The static pressures of various products, based on different airflows for various depths, are presented in Table 3.7. Most systems are designed for less than 3.73 kPa (15 in.) static pressure. MOISTURE TABLE 3.6. AND TEMPERATURE CHANGES AND EFFECTS 59 AIRFLOW RATES USED FOR AERATION Southern States Northern States m?/m’sX 1078 cfm/bu m/m’s X 107? cfm/bu Farm storage! O61 =3:35 0.05-0.25 0.67—-1.34 0.05-0.1 Horizontal storage Vertical storage 0.67—3.35 0.67—-1.34 0.05—-0.25 0.05—0.10 0.67-1.34 0.34—0.67 0.05-0.1 0.025—0.05 Source: Holman et al. (1957). Conversion: 1 cfm/bu = 3.4 m3/m3s X 107. Above data based on intermittent operation with suitable air conditioners. 1 For continuous operation (0.0167—0.033 cfm/bu). etn STATIC PRESSURES Airflow m3/m3s FOR AERATING Depth of GRAIN (NOT INCLUDING DUCT Static Pressure, in. Water cfm/bu Storage, ft Wheat Shelled Corn Rough Rice 034 *% 107% 1/10 OPMTEX 10°3 1/20 OI 10=3 1/30 0.09 X 10-3 1/40 50 100 150 50 100 150 50 100 150 50 4.0 18.0 42.0 2.0 8.0 19.5 1.4 5.3 12.0 1.0 1.5 5.0 12.0 0.5 Pap 5.25 0.32 1.35 3.30 0.23 2.65 12.0 Tbe 1.25 5.30 12.5 0.90 3.40 7.95 0.65 220 0.20 0.738 1.50 5.92 0.50 2.00 4.65 0.07 X 10-3 100 150 50 100 150 1/50 Some designers add 25—50% and packing. 4.0 9.0 0.80 3.20 6.60 0.95 2.50 to the above static pressures to account for fine material Source: Holman et al. (1957). Conversion: 1 in. water = 0.2488 kPa. The horsepower requirements will depend on the quantity, depth, and kind of grain for a given airflow. Typical horsepower requirements are 1.2 kW (1% hp) for 1230 to 1400 m3 (35,000 to 40,000 bu) flat storage for shelled corn, 1 hp for 704 m3 (20,000 bu) flat storage, /2 hp for 352 m° (10,000 bu) circular storage, and % to % hp for 112 m° (3200 bu) circular storage. A 2100 m? (60,000 bu) and 45 m (150 ft) high silo with 2000 cfm requires a 9 kW (12 hp) motor, and a 1050 m® (30,000 bu) silo 18 m (60 ft) high with 84 m*/min-t (3000 cfm) requires a 5.6 to 7.5 kW (7% to 10 hp) motor (Holman 1955). For deep bins, the motor horsepower can be estimated by multiplying (0.0004) X (cfm) X (static pressure, in. water) if the friction from ducts is not excessive. A 88 m? (2500 bu) storage was lowered from 12° to 6°C (54° to 20°F) with a ventilation rate of 0.26 m?/min-t (4 cfm/bu) in 71 hr of ventilation with an average atmospheric temperature of —8°C (17°F) (Robinson et al. 1951). Approximately 5 to 10 days are required for the grain 60 DRYING AND STORAGE OF AGRICULTURAL CROPS to approach atmospheric temperature by using 0.104 m3/min-t (“io cfm/bu). The grain should be cooled within 1 month to prevent deterioration. The time required for cooling is changed by increasing or decreasing the airflow rate. The air is usually moved downward through the grain in aeration systems using a suction or exhaust system. Thus, the exhaust air which has been heated while passing through the grain is exhausted through warm grain, thereby avoiding the possibility of condensation on the cold surface grain. The convection currents caused by temperature changes are overcome by pulling the air downward. Equipment for Aeration The fan, duct, and bin for aeration are very similar to those used for forced air drying except the air requirements are much less and deep bins can be ventilated practically. The fan and the duct are much smaller for cooling a given quantity of grain. It is desirable to have a duct crosssection of 1 m2 for each 10,000 m*/min-t (1 ft? for each 1000 cfm)of airflow, although for short ducts of 7.6 m (25 ft) or less, velocities to 10 m/ sec (2000 ft/min) are used. The minimum open surface area in ft? for a flat storage is cfm/20 and for an upright storage cfm/50 (Holman et al. 1957). A very simple system for aeration of grain consists of installing a cylindrical duct vertically in the bin with the fan on the top of duct exhausting air from horizontal storages or vertical storages under 6 m (20 ft) high (Fig. 3.9). The length of the vertical duct can be changed according to the depth of the bin. A 0.2 or 0.38 m {8 to 12 in.) diameter stovepipe can be used with a screen-covered or perforated metal or slotted metal opening on the bottom half of the duct for air passage. Satisfactory operation can be obtained if the center duct is 3 or 4 ft from the bottom of the floor. More rapid and uniform cooling is obtained if the duct extends to the floor. Manufacturers have exhaust fans available which can be either the centrifugal or the propeller type. The cost is about $50 to $75 for each installation for cooling from 2 to 3.5 m3 (70 to 115 ft? or 2000 to 3200 bu) of grain with 0.047 m3/s (100 cfm) of air. The duct for distributing the air can be installed after the bin is filled with grain, although it is much easier to install before. The ducts can be installed when the grain is placed in the bin and the fan moved from duct to duct. The installation can be made so that the air above the grain is partially recirculated through the grain or it can be designed so that the air is exhausted to the outside. The installation which exhausts the air above the grain, shown in Fig. 3.10A, is recommended for cooling because the moisture content of the surface of the grain will be from 1 to 2% MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 61 FAN (PROPELLER OR CENTRIFUGAL TYPE) GRAIN STOVE PIPE (8" TO 12"DIA.) LOWER HALF OF DUCT PERFORATED SCREEN COVERING, PERFORATED METAL,OR SLOTS IN METAL (MINIMUM OF BOTTOM FIG. 3.9. VERTICAL 7% OPENING) OF BIN DUCT USED FOR AERATION lower than in the installation exhausting air to the outside. If, however, the installation is built for slight moisture removal from grain in warm weather, the air should be exhausted to the outside, as shown in Fig. 3.10B. An aeration system with 0.104 m?/min-t (1/10 cfm/bu) is not rec“ommended for removing over 1% excess moisture. The vertical tube duct is ideal for small round or rectangular grain bins. It can also be installed in a flat storage by placing the aeration units 4.5 to 6.1 m (15 to 20 ft) apart in the bin. Horizontal ducts (Fig. 3.11) can be placed in the bottom of the grain bin and may be used with or without the lateral or branch ducts as shown. Branch ducts are necessary for shallow storages to obtain uniform air distribution. The ducts are usually made of 0.2 to 0.3 m (8 to 12 in.) diameter perforated or slotted material. Any of the duct systems used for drying can be used for cooling. A deep storage [over 6 m (20 ft)] is usually cooled through one perforated pipe along the bottom of the bin. The fan can be moved from bin to bin or connected with air ducts and valves 62 DRYING AND STORAGE OF AGRICULTURAL CROPS A FAN x Shien Jan e pier Pa BIN FIG. 3.10. AIR EXHAUSTED FROM AERATION SYSTEM A—INSIDE BIN AND B—OUTSIDE BIN 8"- 12" PERFORATED LATERALS STEEL DUCTS GRAIN PLACED OVER DUCT — — LATERALS OPTIONAL Sf SYSTEM FIG 3.11. HORIZONTAL DUCTS FOR AERATION to direct the air to the appropriate bin. The temperature reduction is more rapid and more uniform by using the horizontal ducts as compared to the vertical ducts. The horizontal ducts can be adapted to any shape or size of bin (Fig. 3.12). A mechanical cooling system for a deep bin, holding over 352 m? (10,000 bu), could be installed at a cost of 1 to 2 cents/bu. } Grain Chilling Cooling grain with refrigerated air in an aeration system provides a method of preserving high moisture grain without using chemicals or holding ahead of the dryer. To avoid wetting of grain from the cooled air, the relative humidity of the air entering the ducts should be less than 75%. As an example, 245 t (270 tons) of wheat at 21°C (70°F) were MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 63 (a) (e) (b) (f) (c) (g) (d) (h) <a> —— FIG. 3.12. TYPICAL AERATION SYSTEMS A—Flat bottom, no basement. B—Flat bottom, perforated floor. C—Vertical tube. D—Flat bottom, high grade level. E—Hoppered bottom, low grade level. F—Hoppered bottom, high and low grade level. G—Flat bottom, no basement. H—Flat bottom, circulation of fumigant. lowered to 9.9° to 12.7°C (50° to 55°F) after 94 hr cooling. During cooling, a total of 1661 kWh were consumed at 17.7 kW/h, an airflow rate 64 DRYING AND STORAGE OF AGRICULTURAL CROPS of 5200 m3/h at 7460 Pa (30 in. water) static pressure was used, and a 20 hp compressor with 5 hp fan on the compressor was used. Refrigerated Air Circulation An aeration system using refrigerated air provides a method of holding high moisture grain. Corn at 22% moisture content in an insulated bin was aerated with 0.52 m3/min-t (0.5 cfm/bu) air at 1.7° to 4.4°C (35° to 40°F). Cooling, during which a reduction of 0.3% moisture content per week occurs, should begin immediately upon storage to avoid mold growth (Converse et al. 1973). Operation of Aeration System The fan can be operated continuously with small flows of 0.052 cm?/ min-t (% cfm/bu) or less when the air is at least 5.6°C (10°F) cooler than the grain. Approximately 300 hr of fan operation are required to cool a deep bin of grain. It would be desirable to turn off the fan during periods of high humidity, rain, and heavy fog. In many areas of the United States, high humidity prevails between 2 and 9 A.M., and a time control can be used to turn off the system during these hours. The labor required for manually controlling the operation of the fan usually does not justify the saving obtained in electricity. Humidity controls can also be used. Units which operate with 0.1 to 0.052 m3/min-t (%o to Yo cfm/bu) are often used for cooling the grain close to freezing and the fan is moved to other bins for repeating the operation. The grain is usually cooled to between 1.7 and 4.4°C (35° and 40°F). Codling to a lower temperature increases the possibility of moisture condensing on the grain when it is handled. By using a humidistat set so that the fan does not operate when the humidity is above 85%, about one-half of the power cost can be saved in Indiana or areas of similar climatic conditions. The cost of the electricity for aeration of the grain, usually within a 1 month period, ranges from ¥%9 to 42%/bu for continuous fan operation. Wheat, in a deep bin in Georgia was aerated at % cent/bu for power cost and maintained from December to September. The electrical cost for the motor on a fan delivering 50 cfm of air would be about 20 to 25 cents/week for continuous operation with electricity at 2% cents/kWh. An aeration system is valuable for fumigation of stored grain. The fumigant can be placed in the bin over the grain and circulated for the appropriate time depending on the fumigant, usually 30 min. The fumigant can then be flushed out, for example, in 24 hr. MOISTURE AND TEMPERATURE CHANGES AND EFFECTS 65 QUESTIONS 1 How could k be determined by using equation (3-6) as a basis? Answer for the bin in question 2. 2 A steel bin 1.2 m (4 ft) in diameter containing shelled corn at 13% moisture is at 25°C (77°F) throughout. The bin is placed in a refrigerated room maintained at -15°C (5°F). How long would be required to get the center of the bin to 5°C (41°F)? -5°C (23°F)? How long would be required to get the grain 1 ft from the edge to o CA4I F)?:—5 €.(23°F)? Determine the average daily temperature fluctuation for January and July using local weather bureau data. Using a daily temperature high of 21°C (70°F) and a low of 40°C (39°F) with wheat at 13% w.b., at 1.7°C (35°F), and 0.15 m (0.5 ft) from the surface, calculate the time required to get the same location to 5°C (41°F)? 10°C (50°F)? What would be the maximum temperature attained? How long would be required? (Ignore convection air currents and surface radiation.) . Adm (16.4 ft) diameter galvanized steel 0.13 cm (0.050 in.) circular bin is filled to a depth of 1.5 m (5 ft) with shelled corn at 13% moisture, w.b. The moisture content is increased to 20%. Will the sheets be pushed off or ruptured by the grain at the higher moisture content? . Plot the hourly temperature for several different days. For the average of 2 weeks hourly temperature? Do the temperatures follow a sine curve? Discuss. An aeration system is to be installed in a 3520 m3 (100,000 bu) silo storage 30.5 m (100 ft) high for shelled corn with an airflow of 0.42 m?/min-t (1/25 cfm/bu). Estimate (a) fan specifications, (b) electric motor horsepower, (c) installation cost, and (d) annual operating cost. Discuss. How long is required to obtain one air change in the installation described in question 7? REFERENCES BABBIT, J.D. 1945. Thermal properties of wheat in bulk. Can. J. Res. Sect. F: 23 (6) 388-401. BOYCE, D.S. 1966. Heat and moisture transfer in ventilated grain. J. Agric. Eng. Res. 11 (4) 255-265. CARTER, D.G. and FARRAR, M.D. 1943. Redistribution of moisture in soybean bins. Agric. Eng. 24 (9) 296. CHRISTENSEN, C.M. and KAUFMANN, Role of Fungi in Quality Loss. H.H. 1969. Grain Storage—The Univ. Minnesota Press, Minneapolis. / 66 AND DRYING AGRICULTURAL H.H., SAUER, D.G. and HODGES, CONVERSE, moisture corn. DALE, OF STORAGE T.O. 1973. Aeration of high Trans. ASAE 16 (4) 696-699. ROBINSON, and A.C. CROPS Pressures 1954. R.N. in deep grain storage structures. Agric. Eng. 35 (10) 570-573. DISNEY, R.W. 1954. The specific heat of some cereal grains. Cereal Chem. 31 \ ()9229= 239. grain. Trans. of aeration during FOSTER, G.H. 1967. Moisture changes ASAE 10 (3) 344-347, 351. HENDERSON, S.M. 1947. Negative radiation—its relation to farm building design. Agric. Eng. 28 (4) 137-140. HOLMAN, L.E. Aeration of stored grain. 1955. Agric. Eng. 36 (10) 667-668. HOLMAN, L.E. and CARTER, D.G. 1952. Soybean storage in farm-type bins. Ill. Agric. Exp. Stn., Urbana, Bull. 552. HOLMAN, L.E. et al. 1957. Aeration of grain in commercial storage. U.S. Dep. Agric. Mark. Res. Rep. 178. HUDSON, R.G. 1939. Sons, New York. The Engineers’ Manual, 2nd Edition. John Wiley & INGERSOLL, L.R. and ZOBEL, O.J. 1913. Introduction to the Mathematical Theory of Heat Conduction. Ginn and Co., Boston. JOHNSON, W.H. 1958. Machine and method efficiency in combining wheat. Agric. Eng. 39 (4) 244. (abstract) . KAZARIAN, E.A. and HALL, C.W. 1965. Thermal properties of grain. Trans. ASAE 8 (1) 33-37, 48. KELLY, C.F. 1941. Temperature of wheat in experimental farm type storages. U.S. Dep. Agric. Circ. 587. KIESSELBACH, T.A. Lincoln, Bull. 188. MADDEX, 1923. Productive R.L. and HALL, C.W. 1956. seed;corn. Neb. Agric. Exp. Mechanical cooling of grain. Stn., Mich. State Univ., E. Lansing, Ext. Bull. 316. MAYES, H. F. 1955. Aeration for the control of moisture migration in stored corn. Annu. Meet. Am. Soc. Agric. Eng., Univ. of Illinois, Urbana, June 1955. Mimeo Rep. U.S. Dep. Agric. (unnumbered). McCUNE, W.E., PERSON, N.K., JR. and SORENSON, J.W., JR. ditioned air storage of grain. Trans. ASAE 6 (3) 186—189.. 1963. Con- NAVARRO, S., DOINAHAYE, E. and CALDERON, M. 1973. Studies on aeration with refrigerated air. I. Chilling of wheat in a concrete elevator. J. Stored Prod. Res. 9 (3) 253-259. PATERSON, H. 1967. Chilled grain storage. Farm Mechanization Buildings 19 (216) 19-20. PERRY, R.H. and CHILTON, C.H. McGraw-Hill Book Co., New York. 1973. Chemical Engineers’ Handbook. ROBERTSON, D.W., LUTE, A.M. and GARDNER, R. 1939. Effect of relative humidity on viability, moisture content, and respiration of wheat, oats, and barley seed in storage. J. Agric. Res. 59, 281-291. MOISTURE AND TEMPERATURE ROBINSON, R.N., HUKILL, W.V. and CHANGES FOSTER, G.H. AND 1951. EFFECTS 67 Mechanical ventilation of grain. Agric. Eng. 32 (11) 606-608. SAUL, R.A. and LIND, E.F. 1958. Maximum time for safe drying of grain with unheated air. Trans. ASAE / (1) 29-33. SOWELL, R.S. and HODGES, T.O. 1963. pressures in grain storage structures. The effects of moisture changes on Am. Soc. Agric. Eng., St. Joseph, Mich., Pap. 63-425. STAHL, B.M. 1948. Engineering data on grain storage. Agric. Eng. Data 1. Am. Soc. Agric. Eng., STAHL, B.M. 1950. Grain bin requirements. U.S. Dep. Agric. Circ. 835. STEELE, J.L. 1963. Deterioration of shelled corn during drying as measured by carbon dioxide production. M.S. Thesis. lowa State Univ., Ames. SUTER, R.C. 1964. The Courage to Change. Interstate Printers and Publishers, Danville, III. WEAST, R.C. 1979. CRC Handbook of Chemistry and Physics, 60th Edition. CRC Press, West Palm Beach, Fla. WILEMAN, W.H. 1941. Shrinkage of artificially dried seed corn. Agric. Eng. 22 (4) 256. WOOLEY, J.C. 1946. Farm Buildings. McGraw-Hill Book Co., New York. WRATTEN, F.T., POOLE, W.D., CHESNESS, J.L., BAL, S. and RAMARAO, V. 1969. Physical and thermal properties of rough rice. Trans. ASAE 12 (6) 801-803. Moisture Content Determination As mechanization of harvesting operations has increased, it has allowed less time in the field for grain and hay to lose moisture. Products with a high moisture content will not keep for extended periods in storage, so it is important, therefore, that equipment be available for determining the moisture content of grain and forage to determine whether they will keep in storage. The same equipment might also be used for determining the time of harvest or cutting of forage. Equipment for precise moisture measurement is usually not necessary since moisture content determination within 1% is usually sufficient to establish whether a product will keep in storage. The method of pricing agricultural products, in which the moisture content is one important criterion, increases the importance of having methods for determining moisture content. Considerable money is lost each year because of over-drying. When the product is over-dried, the farmer or rancher sells dry matter for the price of water and thus loses economically. Commercial concerns often take grain of a low moisture content from one farmer and grain of a high moisture content from another and mix or blend the two together to get the approximate moisture content on which the price is based. By following this procedure, the commercial operation can gain economically, because the farmer with high moisture grain is penalized directly by a lower price for excess moisture products and the farmer with a low moisture product is penalized indirectly for supplying dry matter instead of water to the purchaser. Most farmers do not own a meter of sufficient accuracy for determining grain and/or forage moisture content to meet official standards. In most cases, the farmer relies upon experience to estimate the moisture content. For grain, this might entail biting, rattling, feeling, and observation. Rather indefinite methods are used to determine the moisture content of forage products. Farmers normally use observation of physical properties 68 MOISTURE CONTENT DETERMINATION 69 such as feel, twist, squeeze, rattle, break, and curl of leaves to determin e moisture content of forage. The moisture content is an index of the probable keeping quality of the product and can be expressed on either the wet or dry basis (Chapter 1). Moisture meters for grain and hay are designed to give the percentage moisture content on a wet basis. The methods of determining the moisture content of products may be divided into two broad classifications: (1) direct and (2) indirect. With the direct method, the amount of moisture weighed or measured is related to the amount of dry material present or to the original amount of material. Thus, the moisture content can be expressed in either the wet basis or the dry basis. The direct methods may not be any more accurate than the indirect methods but are usually accepted as standards for calibration for and comparison with the indirect methods. Regardless of the method used for determining the moisture content, there are possibilities of errors in making the determination. The major problem is that of securing a sample which is representative of the entire lot of material. To reduce the possibilities of error, several samples should be obtained from different locations in the bin, field, or mow. Usually a large sample is obtained which is taken to the laboratory for determining the moisture content. If the sample which is taken to the laboratory is not properly divided, errors may again occur. The sample can be divided equally into 2 parts with the Boerner sampler. These can be further subdivided for more samples. The standard deviation of the moisture content of individual kernels in a sample taken from a bin of grain of uniform moisture content is about + 0.2% (Oxley 1948). Under the best conditions in the laboratory, a standard error of sampling of + 0.11% for replicate samples of wheat has been obtained (Cook et al. 1934). It is important that the moisture content of the product be maintained from the time the sample is obtained until the determination is made. Standard metal containers and film bags have been prescribed by the government and are available through commercial concerns for holding the sample. The familiar cardboard type container will permit considerable moisture loss. For some methods of moisture determination, it is necessary to grind the sample. Grinding is normally done when the moisture content is near equilibrium with the atmosphere so that the moisture change will be kept at a minimum. DIRECT METHODS Oven Methods Several different oven procedures are available for moisture determination of different materials. The usual procedure is to remove the mois- 70 DRYING AND STORAGE OF AGRICULTURAL TABLE 4.1. PARTIAL LIST OF MOISTURE Beckman METERS CROPS FOR AGRICULTURAL CROPS Instruments, Inc., Illustrated, Cedar Grove, N.J. 07009 is da Model HMP-1 System (moisture above head space sensed, equilibrium moisture) Moisture Sensor (measures relative humidity) Burrows Equipment Co., Evanston, IL 60204 ; Burrows Model 700 Digital Moisture Computer (capacitance) \ Burrows Moisture Recorder (dielectric) Universal Moisture Tester (electric resistance) Brown-Duvel Moisture Tester (distillation) Delmhorst Moisture Detector (moisture probe, contact resistance) Wood Moisture Detector (moisture probe, contact resistance) Brabender Moisture Tester (drying chamber, loss of weight) Air oven (electric heat, loss of weight) Safecrop Moisture Tester Quicktest Moisture Tester (stem or probe) Ohaus Moisture Determination Balance DICKEY-John Corp., Auburn, IL 62615 Grain Moisture Tester Insto I and II Forage Moisture Tester Continuous-Flow Moisture Meter Dupont Instruments, Wilmington, DE 19898 303 Moisture Monitor (measure moisture level in gas, electrolyzes moisture in gas stream) Eaton Controls, Carol Stream, IL 60187 Dole 400 Moisture Tester (capacitance) EPIC, Inc., New York, N.Y. 10038 Grain Stem Hydrometer (Wilh. Lambrecht grain, limited to 70°F and below) KG Gottingen) (Humidity in Aqua Boy Electronic Moisture Meters (electrical conductivity) (numerous electrodes available for many different products) Koster Crop Tester, Inc., North Randall, OH 44128 Koster General Purpose Moisture Tester (evaporation of water, weight) (used for grains and forages) Lab-Line Instruments, Inc., Melrose Park, IL 60160 Lab-Line Electro-Hydrometer (measures relative humidity in grain interstices) Motomceo, Inc., Patterson, N.J. 07513 Motomco Moisture Meter (dielectric) (formerly known Seedburo Equipment Co., Chicago, IL as Halross meter) 60607 Steinlite Moisture Tester (manufactured by Fred Stein Lab.) (capacitance) Steinlite Digital Moisture Tester (capacitance) Dole Moisture Tester (capacitance) Ohaus Moisture Determination Balance (IR heating, measure moisture loss) Cenco Infrared Moisture Balance (IR heating, weight determination) Brown-Duvel Moisture Tester (distillation principle) Brabender Moisture Tester (sample dried and weighed) Westberg Manuf. Co., Sonoma, CA 95476 Westach Moisture Meter (electric prod; forage, grain) Weston Instruments, Newark, N.J. Weston Moisture Meter 07114 MOISTURE CONTENT DETERMINATION 71 ture from the product in an air-oven. When warm or hot water is circulated around the walls to heat the oven by the circulating water, it is called a water-oven. This method than the air-oven. Two general grain and dry it in the oven from the whole grain in the oven at a 96 hr. maintains a more uniform temperature procedures are available: (1) grind the 1 to 2 hr at 130°C (266°F), and (2) place temperature of 100°C (212°F) for 72 to The usual oven methods for moisture determination of grain are: A. Air-oven Method, 130°C + 1° (266°F).— (1) One-stage, for grain under 13% moisture. Grind duplicate samples of 2 to 3 g each. Heat 1 hr at 130°C (266°F). Place in desiccator; then weigh. Samples should check within 0.1% moisture. (2) Two-stage, for grain above 13% moisture. Remove moisture until 25 to 30 g sample is below 13% (usually about 14 to 16 hr are required). Continue as discussed under (1). B. Water-oven or Air-oven Method, 100°C (212°F).— Place duplicate 25 to 30 g samples in oven, heated to 99° to 100°C (211° to 212°F) for 72 to 96 hr. Place in desiccator; then weigh. Samples should check within 0.1% moisture. The AOAC has set up slightly different requirements which give a slightly higher value for moisture determination of grain and stock feeds. The material is ground and heated at 135°C (275°F) for 2 hr (AOAC 1950). The latest recommended procedure by AOAC and U.S. Dep. Agric. for determining the moisture content of different products should be consulted. To remove the moisture from a high moisture sample, the two-stage drying method has been advocated (Anderson 1936). With this method, a sample of whole grain is first dried in an oven and then the sample is ground and further dried in an oven. The moisture content is determined on the basis of water loss. The sample should be left in the oven until weight loss stops. It is practically impossible to remove all the moisture from a sample without deterioration of the product. If the sample remains in the oven too long, the organic material will be reduced and a loss of weight occur which will appear as moisture loss and give an inaccurate value. For most grains, it is considered that the deterioration of dry matter occurs quite extensively after 96 hr in an oven at 100°C (212°F). For some air-oven determinations, at 100°C (212°F), it is specified that the sample be kept in the oven a minimum of 72 hr. The oven may be heated with electric resistance heaters, infrared lamps, steam, or a high frequency electric field. The vacuum oven method is used for determining moisture contents. For grain, the product is ground and placed in the oven at approximately 72 DRYING AND STORAGE OF AGRICULTURAL CROPS 100°C (212°F) and the oven is maintained at 25 mm pressure for about 5 hr. By using a lower temperature or a shorter time, there is less possibility of a is particularly For accurate protected from loss of weight due to deterioration of the dry matter. This important for fruits and vegetables. moisture content determinations the sample should be absorbing moisture after the moisture has been removed from the sample. Moisture absorption can be prevented by placing a glass cover plate over the sample and taking the weight after the sample and container have cooled. Distillation Methods With the distillation methods, moisture is removed by heating the grain in oil and determining the volume of the weight of water removed from the grain in condensed vapor or from the loss of weight of the sample. The Brown-Duvel distillation method (Fig. 4.1) was one of the early accepted methods for determining moisture content of grain (Brown and Duvel 1907). The whole grain is heated in oil, the weighed sample heated, and the vaporized moisture condensed and measured in a graduated cylinder. The procedure for different grains varies, and it is necessary to follow the recommended procedures for different grains (Coleman and Boerner 1936). A sample of 100 g is heated in a flask with 150 ml of oil. About 1 hr is required to determine the moisture content. The equipment is rather simple, and accurate results can be obtained without expensive equipment such as balances. Proper calibration assures more accurate results. re. The official toluene (methy] benzene, CsH;CHs) distillation method for grain requires heating finely ground grain in an apparatus that collects the condensed water. The toluene boils at 111°C (233°F), and thus all of the water and other substances which have a boiling point below this temperature are evaporated. Boiling is continued as long as water accumulates in the graduated tube. Usually about 25 g of material are boiled in the flask with 75 ml of toluene. With the modified Brown-Duvel method, the product is placed in a vegetable oil bath and heated to a predetermined temperature. This process takes only 15 to 20 min. The use of oil permits rapid evaporation with large quantities of heat without scorching or burning the product. Starting from room temperature, the oil is heated to 145°C (293°F) for grasses and silage and 190°C (374°F) for grains. As soon as these temperatures have been reached, the moisture has been driven off, and the dry weight of the product remains. The oil and 100 g of product in the oil are balanced before heating, and the loss of weight of the product after heating is then determined. The change of weight is used to calculate the moisture content of the original product. MOISTURE CONTENT DETERMINATION Courtesy of Seedburo FIG. 4.1. BROWN-DUVEL MOISTURE TESTER Equipment Co. 73 74 DRYING AND STORAGE OF AGRICULTURAL CROPS Drying with Desiccants The moisture content of a product is determined by placing the sample near an efficient drying agent in a closed container. The material should be finely ground in order to provide rapid response. The desiccant maintains a very low vapor pressure within the container. The vapor pressure of the material is higher than that of the desiccant, and the moisture moves from the material to the drying agent. One standard procedure is to place the sample in a vacuum oven with anhydrous sulfuric acid until constant weight is obtained. This method is particularly useful for materials where dry matter decomposition would be great if the product were heated. With some products, however, the product may decompose because of the mold and bacteria growth before the equilibrium vapor pressure is reached. INDIRECT METHODS Indirect methods involve the measurement of a property of the material including mechanical, electrical, or thermal property, which is related to the moisture content. One of the direct means is required to calibrate the indirect method. The moisture content is usually expressed on a wet basis for the indirect methods. Electrical Resistance Methods The electrical resistance or conductivity of a material depends upon its moisture content. This principle is used as a’basis for a number of moisture meters. In wheat it has been found that there is a linear relationship between the moisture content and the logarithm of its electrical resistance from approximately 11 to 16% moisture. These moisture meters must be calibrated for each grain against a standard method. Inasmuch as temperature affects the electrical resistance of a material, corrections for variations in temperature must be made for tests conducted at temperatures other than those at which the calibration is reported. Electrical resistance meters reported as being suitable for grain are the Universal (Fig. 4.2), DICKEY-John’s combination resistance-dielectric meter, and the Marconi (Anderson and Alcock 1954). The electrical resistance units are rather simple in design and require 1 min or less for making a moisture determination. The resistance of grain may be measured between two steel rolls which serve as electrodes in the Tag-Heppenstall meter. A different spacing of rolls can be made or a different roll used for various products such as grain, peanuts, walnuts, wood, etc. The standard error of estimati on in MOISTURE CONTENT DETERMINATION Courtesy of Burrows FIG. 4.2. UNIVERSAL MOISTURE Equipment 75 Co. TESTER testing hard spring wheat is about + 0.23%. One of the problems with the meter is the difficulty of maintaining calibration because wear of the bearings and rolls changes the spacing between the rolls and gives a lower moisture content. By proper maintenance, however, improper moisture content value due to spacing can be minimized. 76 DRYING AND STORAGE OF AGRICULTURAL CROPS The pressure exerted on the sample with the electrical resistance method affects the resistance of the product. The relationship of moisture content, pressure on sample, and resistance of various materials is available (Isaacs 1954). The Universal moisture tester presses a sample of grain to a specified thickness in order to get the same pressure each time. The unit incorporates its own power source, thus eliminating the necessity for electrical outlets or batteries. Units are made to cover various ranges of moisture content. A representative of the manufacturer of the Universal tester points out that the instrument uses a high pressure on a sample which permits a standard calibrated dial for all materials. The only difference between calibration from one material to another is in the pressure exerted on the sample in terms of thickness of the sample. A straight line must result when Universal moisture content determinations are plotted against oven determinations, so calibration consists only of determining the compression data which will give a straight line on a 45° slope. This principle is covered by patents of the company. The Marconi moisture meter consists of a test cell in which great pressure is applied to the specimen. The circuit is powered by dry cells or alternating current. The Shafer moisture tester is a portable inexpensive unit of unique design. The sample is placed in a grain compartment where the electrical resistance, R, is measured. The circuit is so designed that a neon lamp flashes, the speed of which is determined by the resistance of the sample between the plates. The number of flashes seen through a viewing window are counted for a period of 1 min. By referring to a calibration card for the particular grain being tested, the moisture content of the sample can be determined. A small battery powers the unit. The Delmhorst moisture detector is used for determining the moisture content of baled hay and straw. It is a device which measures the electrical resistance obtained by forcing an electric probe into the bale and reading the moisture content on the meter. The moisture content measured is that of the wettest fibers in contact with the’two electrodes near the end of the probe. The electrical resistance of hay decreases with an increase in pressure. Alfalfa hay at 17.3%, w.b. has a DC resistance of 750 kilohms with slight pressure, 180 kilohms with 34.5 kPa (5 psi) pressure, and 90 kilohms with 68.9 kPa (10 psi) pressure (Isaacs 1954). The 60 cps DC resistance is slightly higher. The electrical resistance is greater with current flowing perpendicular to the stems than with current flowing parallel to the stems. The electrical resistance of grain decreases when the pressure is increased. For 15.2% shelled corn, with a pressure of 689.4 kPa (100 psi), the DC resistance is 1.0 megohm. MOISTURE CONTENT DETERMINATION 77 Above 17% moisture content, there is a parabolic relationship between the moisture content and the logarithm of the electrical resistance. Most meters do not give readings below 7% moisture, because there is very little change in the electrical conductivity. Grain that has been recently dried with heated air gives lower readings than the actual moisture content of the product. This occurs because the tendency of these meters is to measure the resistance on the surface. If moisture has been added to the grain, the readings are higher than the average moisture content of the product. Dielectric Methods (Capacitance) The dielectric properties of products depend on the moisture content. The capacity of a condenser is affected by the dielectric properties of the material placed between the condenser plates. Wet materials have a high dielectric constant,! and dry materials have a low dielectric constant. Water has a dielectric constant of 80 at 20°C (68°F). Most grains and hay have a value less than 5, and air in a vacuum has a value of 1. The capacitance, which is an indirect measurement of the moisture content, may be obtained (Fig. 4.3, 4.4, and 4.5). The Steinlite meter is used by many commercial concerns for grain. A sample of 0.15 kg (0.3 lb) is placed into a grain chamber. The grain is then moved to a chamber formed by two plates of a condenser. A standard error of estimate for the Steinlite meter when compared to the vacuum oven is + 0.4% (Hlynka et al. 1949). Meters of this type are less subject than resistance meters to any errors which might result from uneven moisture distribution due to drying or wetting of the kernels of the grain being tested (Zeleny 1954). The capacitance method for determining the moisture content of hay has been used (Sherwood 1951). It was found that: (a) Capacitance did not change with frequency in the range of 170 to 700 kilocycles. (b) Capacitance varied directly with moisture content but was dependent on the initial weight of the sample. (c) There was no consistent relationship between the capacitor power factor and hay moisture content. (d) A standard error of estimate of 3% was obtained from the regression line between 21 and 56% moisture content with 0.45 kg (1 lb) samples. (e) The dielectric constant of the hay was about 1.8 at 11.4% moisture and 500 kg/m? (31.2 Ib/ft*). 78 AND DRYING OF STORAGE AGRICULTURAL CROPS spend NE aE Fe wi IS CELL ON! weeRIAL NUMBER Courtesy of Motomco, FIG. 4.3. MOTOMCO DIELECTRIC MOISTURE Inc. METER The Motomco (formerly Halross) meter (Fig. 4.3) consists of electronic components of two miniature vacuum tubes and associated circuits. The measuring cell is designed to (1) provide leveling in the cell, (2) reduce the packing factor, and (3) reduce the effect of liter (bushel) weight variations in the materials. These claims are covered by patents. The error of estimate reported by the company is about + 0.22%. Various configurations of moisture testers are shown in Fig. 4.3 to 4.7. 'The dielectric constant, K = . . 2 , where Q; and Q» are charges, r is distance between r2 charges, and F the force in dynes of the electric field. . . MOISTURE CONTENT DETERMINATION 79 Courtesy of Eaton Controls FIG. 4.4. DOLE MOISTURE TESTER, CAPACITANCE iYPE Chemical Methods or combines Water is removed by adding a chemical which decomposes which can ed with the water. From the chemical reaction a gas is produc 80 DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Seedburo Co. FIG. 4.5. STEINLITE MOISTURE TESTER be measured volumetrically or which decreases the original weight of the sample. After suitable calibration curves are established the moisture content can be determined. Calcium carbide acetylene gas. reacts with water to form calcium hydroxide and MOISTURE CONTENT DETERMINATION CaC, + 2H,O > Ca(OH): + CyH, t 81 (4-1) This method is used for determining the moisture content of forages and grains by shaking an excess of calcium carbide with 30 g of material. About 10 to 25 min are required for the reaction. The loss of weight in the sample is due to the evolution of acetylene and is used for an index of plant moisture from 10 to 85%. A hay sample is cut into approximately 1.3 cm (0.5 in.) lengths and grain samples are crushed. A balance may be used for getting a reading of percentage of moisture in the sample. This method is accurate to within 3% of the actual moisture content. Calcium hydride has been proposed for the above reaction in place of calcium carbide, giving the reaction described in equation (4-2). CaH. se 2H,O a Ca(OH), ap 2H. t (4-2) The chemical method can be used to determine the moisture content by the gas pressure exerted. The higher the gas pressure exerted in a closed container, the higher will be the moisture content. Put another way, the volume of gas at a constant pressure would indicate indirectly the amount of moisture in the sample. This method utilizes a receptacle with a gauge and safety valve and is recommended for use by farmers on hay and grain. A reading can be made in about 3 min when grain samples are ground. Note that when calcium hydride is used instead of calcium carbide, twice the volume of hydrogen as compared to the volume of acetylene is produced. The Fischer method offers considerable possibility for determining the moisture content by chemical means (Fischer 1935). The method utilizes chemical solvents which penetrate the organic tissues and aid in quickly removing water from the material. The material must be finely ground, and the moisture is extracted with anhydrous methy] alcohol. Hygrometric Methods Grain or hay in a closed container comes to equilibrium with the air at a certain relative humidity, dependent upon the moisture and temperature. The relative humidity of the air in equilibrium with the material is used as a measure of the moisture content. A simple device has been developed for use on grains utilizing the principle of humidity measurement to obtain the moisture content. The apparatus consists of two thermometers mounted in a rubber stopper, a saturated salt solution, and a container such as a milk bottle wrapped for insulation. Agitation is provided by shaking the container (Dexter 1949). The wet bulb thermometer is wetted with a saturated solution of a salt 82 DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Burrows FIG. 4.6. DIGITAL MOISTURE Equipment Co. COMPUTER which maintains approximately the same equilibrium relative humidity as the material being tested. By using a salt solution on the wet bulb thermometer, the wet bulb depression or wet bulb rise is reduced, and the time required to take each stabilized temperature is reduced. Agitation is required to bring the atmosphere to equilibrium with the products. If a salt solution which had an equilibrium relative humidity of 75% MOISTURE CONTENT DETERMINATION 83 B Courtesy of Epic, Inc. C Courtesy of Lab-line Instruments FIG. 4.7. OTHER MOISTURE TESTERS. A—Grain stem hygrometer. B—Aqua boy electric moisture meter. C—Lab-line electro-hygrometer. D—Koster crop tester. Courtesy of Koster Crop Tester, Inc. 84 DRYING AND STORAGE OF AGRICULTURAL CROPS were used, and a wet sample were being tested, there would be a wet bulb rise because moisture would leave the sample, go through the atmosphere, and saturate the wet bulb. If the grain sample were quite dry, moisture would go from the wet bulb to the sample, thus cooling the wet bulb due to evaporation of moisture from the salt solution. Certain salts have the property of changing color when exposed to various humidities which can be used to indicate the moisture content of the product. A method has been developed in which a mixture of ferric ammonium sulfate and potassium ferrocyanide is used in a carrier of sodium chloride as a colorimetric indicator of humidity (Dexter 1948A). In the dry state the mixture is blue, but in the presence of water it turns red. When used in a range of 8 to 12% for wheat, it is accurate to within 1%. Even though the hygrometric method may not give quite as accurate values of moisture content of products as some of the other indirect methods, it may actually give a better indication of the storage quality of the product than the accurate moisture content. The growth of microorganisms on the surface of the grain or hay is influenced by the relative humidity and temperature of the surrounding atmosphere. The hygrometric method gives an accurate value of the environment in which these microorganisms would grow and hence a more accurate indication of the storage quality of the product. It is generally considered that mold growth occurs on grain when the relative humidity surrounding a product is above 75%. A very simple method of determining whether hay or grain crops will keep in storage has been developed using a salt (Dexter 1947A). Common salt (non-iodized), sodium chloride, has an equilibrium relative humidity of about 75% when saturated with water. Common salt is mixed with the sample. If the sample is too wet for safe storage, that is, above 75% equilibrium relative humidity, salt crystals will lump together because they become saturated. If the sample is dry enough for storage, the salt will not lump, because the equilibrium relative humidity is below 75%. All that is needed to determine whether a product will keep in storage is a small test tube, a cover of cloth to prevent heat from the hand or exterior getting to the sample while the test is being run, and common salt. The test is run by merely shaking the salt through the product sample. One of the problems in grain storage is that of determining the moisture content of the product at various locations without disturbing the material in storage. One approach, using the hygrometric based on relative humidity measurement, is to place a mining the relative humidity at various places in the thermocouple which will give the temperature reading method, which is device for deterbin along with a at that location. Thus, by measuring the relative humidity and temperature of the air MOISTURE CONTENT DETERMINATION 85 surrounding the product at various locations, the moisture content of the product may be determined during storage or drying. For a ventilated bin, it is necessary to turn off the fan from 1 to 2 hr before the readings are taken so that the relative humidity of the air is at equilibrium with the grain (Hukill 1957). It appears that an accuracy of within 0.5% can be obtained. By drawing air from the center of bales of cotton and passing it over wet and dry bulb thermometers for humidity measurement, the standard deviation was 1.45% moisture and required from 2 to 3 min (Gaus et al. 1941). The accuracy of the hygrometric methods depends upon the previous drying history of the product. It has been found that the wetting and drying curves from zero to equilibrium moisture content do not coincide. If the moisture content is based upon a reading taken between the zero and equilibrium moisture content points, the value will depend upon whether the material is dried or wetted (Anderson and Alcock 1954). A unique method of following a drying front during tests has been developed (Hukill 1957) using pipe cleaners. The method provides a moisture content with an accuracy of + 2% which is useful for determining the location of the drying front. A pipe cleaner consists of two wires with cotton fiber separating them. The electrical resistance of cotton fiber varies as moisture is gained or lost. By measuring the electrical resistance across the wires of the pipe cleaner, an indication of the humidity of the drying air is obtained. Other Methods With the advent of the atomic age, new methods have been proposed and used to indicate the moisture content, particularly for soils. In one of these methods, the presence of hydrogen nuclei of water in the product is used as an index of moisture content. Two methods are used for measuring the presence of hydrogen nuclei: (1) the neutron scattering method and (2) the nuclear resonance-absorption method. In the neutron scattering method, neutrons emitted from a radioactive source are directed into the product being tested. In passing through the material, the neutrons bombard the atomic nuclei of the material. When a neutron collides with a heavy nucleus of a solid material, less energy is transferred from the neutron than if it hits a light nucleus, as that of the hydrogen in water. When more hydrogen nuclei are present in the material, a greater number of the slow neutrons will be scattered back to the vicinity of the source which can be measured by a sensitive radiation counter. An accuracy with 1% has been obtained in soils (Bel- cher et al. 1950). The nuclear resonance-absorption method utilizes a double electrical field: one a magnetic field, the second a radio-frequency field. The hy- 86 DRYING AND STORAGE OF AGRICULTURAL CROPS drogen nuclei absorb energy from the radio-frequency field. The maximum absorption of energy occurs when resonance occurs, which is shown by an increased flow of current through the coil producing the field. The greater the amount of water present, the greater is the energy absorption from the field (Shaw and Elsken 1950). A unique method of drying moisture samples has been developed using the engine exhaust from a tractor (Dexter 1947B). The unit consists of an open-ended cylinder which contains a sample and is dried as exhaust gases are passed directly through it. A scale is required to determine the weight of the sample before and after drying. The temperature of the sample is controlled at 140°C (282°F) by adjusting the engine speed. The sample is turned about once each minute to prevent burning. About 6 min are required to dry a sample of grass of 15.8% moisture and 10 min to dry a sample of grass of 38% moisture. Ten minutes are required to dry a sample of oats at 27.3% moisture. Improvements were made on the exhaust oven to include an injector which would temper the engine exhaust temperature by utilizing outside atmospheric air. By adding outside air, the drying temperature is reduced and the airflow increased in the exhaust oven to prevent burning of the 150 g sample. This attachment would be desirable only on those tractors which operate at high exhaust temperatures under no load conditions. Mechanical shear on alfalfa hay stems, as a method of moisture determination, is not related directly to the moisture content. Many other factors, such as stem size, location on stem, and maturity of hay, affect the shearing force. SAMPLING The method of sampling, size, and number of samples are important to assure that the moisture content is representative of the product. A device for sampling baled hay has been developed in which a cutting tool of 0.05 m (2 in.) diameter stainless steel tubing with teeth is placed on an electric drill for cutting out a sample “plug.” A, hand-operated device, of similar design but using 0.076 m (3 in.) diameter tubing, was previously developed. Probes are also available for deep sampling of bins and also for sampling from bags of grain (Fig. 4.8). Some probes have open centers while others have dividers in the tubes to hold grain at the level the sample is taken. STANDARD ERROR OF ESTIMATE A regression, or least squares, line can be obtained relating the moisture content to the measured variable (Fig. 4.9). For the resistan ce moisture meter, the measured variable is the resistance, R, which gives a straight MOISTURE feSe . CONTENT DETERMINATION 87 WOODEN HANDLE WHICH TURNS INNER TUBE DOUBLE TUBE LENGTH: 8 IN. to 10 FT MATERIALS: ALUMINUM BRASS BRASS W// NI OR CR PLATE AVAILABLE WITH OR WITHOUT PARTITIONS BETWEEN OPENINGS MAY HAVE BRONZE POINT A, B, D Courtesy of Seedburo Equipment Co. C Courtesy of Gamet Manufacturing Co. FIG. 4.8. SAMPLING DEVICES. , A—Grain probe. B—Boerner sampler. C—Series 6800 Diverter Type Sampler diverts a 100% cross-cut section of all material flowing through a spout. D—Rice trier. 88 DRYING AND MOISTURE 12 STORAGE La aa of percent points 10 CROPS CONTENT 68 y AGRICULTURAL OF foll between lines i) two---— Tested Being Unit S' Standard Error of Estimate, SE Moisture Content, Percent, Oven Standard x FIG. 4.9. ANALYSIS OF DATA AND REGRESSION LINE line when log R is plotted against the moisture content, MC, obtained by standard oven methods. A straight line is represented by y=mx+t+b (4-3) where y is the ordinate, log R. _ m is the slope of the line x is the abscissa, MC, moisture content (w.b. or d.b.) b is the intercept on y-axis The regression line is defined by log R= m MC +b ‘ (4-4) where ae me 2YxZy eee ee (4-5) 2 Dx? b= y n - (2x) n » -m=n n = number of values (4-6) MOISTURE CONTENT DETERMINATION 89 Thence, the standard error of estimate, S.E., in the y-direc tion is S.E., a= + ="m tea ob - UxDy pana (4-7) n= 2 and S.E., 1 = + — m |S.E.,| QUESTIONS 1. Determine the regression line and standard error of estimate for the following data of a moisture meter: Moisture Content|Moisture Content|Moisture Content\Moisture Content % % % Vacuum Oven Vacuum Oven 2. Compare S.E. with averages for each method, question 1. 3. Compare results obtained in question 1 with scatter diagram and graphical interpretation. 4. The following data are obtained for a two-stage procedure of drying grain: Weight of portion of original sample 26002 2 Weight of portion after air drying 24.402 g Weight of subportion of air-dried sample 2.961 g Weight of subportion after oven drying 2.670 g Determine percent moisture, w.b. and d.b. 90 AND DRYING STORAGE OF CROPS AGRICULTURAL 5. How is the dielectric constant, K, of an agricultural product determined? What are the units of K? 6. The regression line equation for a moisture meter for hay by the method of least squares is log R = 0.08016 MC + 4.995 and the standard error of estimate of the log R = 0. 237. What is the standard error of estimate in terms of the moisture content? 7. Design an experiment for calibrating a moisture meter with respect to changes in temperatures. 8. Can the moisture content of a product exceed 100%? Explain. REFERENCES ANDERSON, J.A. and ALCOCK, A.W. 1954. Storage of Cereal Grains and Their Products. Am. Assoc. of Cereal Chemists, St. Paul, Minn. ANDERSON, J.E. 1936. Some facts concerning vacuum-oven moisture determination. AOAC. Cereal Chem. 1950. 13, 487-452. Official Methods of Analyses. Assoc. of Official Agricultural Chemists, Washington, D.C. BELCHER, D.J., CUYHENDALL, T.R. and SACK, H.W. 1950. urement of soil moisture and density by neutron and gamma USS. Civ. Aeronautics Admin. Tech. Dev. Rep. 127. The meas- ray scattering. BROWN, E. and DUVEL, J.W.T. 1907. A quick method for the determination of moisture in grain. U.S. Dep. Agric. Bur. Plant Ind. Bull. 99. CHRISTENSEN, C.M. 1974. Storage of Cereal Grains and Their Products. Am. Assoc. of Cereal Chemists, St. Paul, Minn. COLEMAN, D.A. and BOERNER, E.G. 1936. The Brown-Duvel moisture tester and how to operate it. U.S. Dep. Agric. Bull. 1375 (rev.). COOK, W.H., HOPKINS, J.W. and GEDDES, W.F. 1934. Rapid determination ot moisture in grain. I. Comparison of 130°C air oven and Brown-Duvel methods with vacuum oven method. Can. J. Res. 11, 264-289. DEXTER, S.T. 1947A. A method for estimating whether hay or grain will keep in storage. Mich. Agric. Exp. Stn., E. Lansing, Q. Bull. 30, 150-152. DEXTER,S.T. 1947B. A method for rapidly determining themoisture content of hay or grain. Mich. Agric. Exp. Stn., E. Lansing, Q. Bull. 30, 158-166. DEXTER, S.T. 1948A. A colorimetric test for estimating percent moisture in the storage quality of farm products or other dry materials. Stn., E. Lansing, Q. Bull. 30, 422—426. DEXTER, S.T. 1948B. content of farm crops. An oil distillation method Mich. Agric. Exp. for determining moisture Mich. Agric. Exp. Stn., E. Lansing, Q. Bull. 31, 248— 253. DEXTER, S.T. 1949. The wet and dry bulb method of moisture content determination. Mich. Agric. Exp. Stn., E. Lansing, Q. Bull. 31, 275. MOISTURE _ FISCHER, K. 1935. CONTENT DETERMINATION 91 A new method for analytical determination of the water content of liquids and solids. Angew. Chem. 48, 394—396. GAUS, G.E., SHAW, C.S. and KLIEVER, W.H. 1941. A practical seed-cotton moisture tester for use at gins. U.S. Dep. Agric. Circ. 621. HLYNKA, I., MARTENS, V. and ANDERSON, J.A. 1949. A comparative study of the electrical meter for determining moisture content of wheat. Can. J. Res. 27F, 382-397. HUKILL, W.V. 1957. Grain moisture indicator. Agric. Eng. 38 (11) 808. ISAACS, G.W. 1954. The design of simplified equipment for the rapid determination of moisture content of grain. Ph.D. Dissertation. Mich. State Univ. MATTHEWS, J. 1963. The design of an electrical capacitance-type moisture meter for agricultural use. J. Agric. Eng. Res. 8 (1) 17-30. NORRIS, K.H. 1964. Design and development of a new moisture meter. Agric. Eng. 45 (7) 370. OXLEY, T.A. 1948. Scientific Principles of Grain Storage. The Northern Publishing Co., Liverpool, Eng. RASMUSSEN, H.E. and ANDERSON, J.C. 1949. A simple electrical meter for estimating the moisture content of grain. Can. J. Res. 27F, 249-252. SHAW, T.M. and ELSKEN, R.H. tion in hydroscopic materials. SHERWOOD, tent. E.M. 1951. 1950. Nuclear magnetic resonance absorp- J. Chem. Phys. 18, 113-114. The Rapid Determination of Hay Moisture Con- M.S. Thesis. Mich. State Univ. STEVENS, G.N. and HUGHES, M. 1966. Moisture meter performance in the field and laboratory. J. Agric. Eng. Res. 11 (3) 210-217. U.S. DEP. AGRIC. 1941. Air-oven and water-oven methods specified in the official grain standards of the U.S. for determining the moisture content of grain. US. Dep. Agric. Serv. Regulatory Announc. 147 (rev.). ZELENY, L. Do2=256: 1954. Methods of grain moisture measurement. Agric. Eng. 34, Airflow and Air Distribution The principles involved in the flow of air through grain or forage are the same for natural ventilation, forced ventilation, and heated air drying. Basically, a drying system consists of a means of moving the air through the product, the wind for natural ventilation or a fan for mechanical ventilation, and ducts to distribute the air from the fan to the grain. The main duct or manifold is the part of the system which distributes air or collects air from two or more smaller (lateral) ducts or a perforated or slatted floor. In some cases, the floor acts as the main duct for distributing air to the product. The relationships among the volume of air moved through the product, the static pressure developed, and the depth of product over the air distribution system are important. All of the basic units of a drying system must be integrated from the standpoint of design, selection, and operation. i The following terms are commonly used in drying work. Static Pressure (AP).—The resistance to flow of air through a product, usually represented in Pa or kPa (inches of water). The static pressure is negative for a drying system in which the fan exhausts air. cfm (Q).—The quantity of airflow in cubic meters per. minute, cubic meters per second, or cubic feet per minute. The volume of airflow is usually related to a volume of product in a bin or cross-sectional area through which the air passes as m?/tonne‘min, m3/m*:s, m3/m? min, cfm per bu, cfm per ft’, cfm per ton, or cfm per ft?. A bushel is 0.03524 m3 (1.25 ft?) of small grain or 0.074 m3 (2.5 ft) of ear corn. A ton of hay occupies various volumes depending on its form. Forced Air Drying.—Natural air forced through grain or hay by a fan for drying products. Heated Air or Heated Forced Air Drying.—Air to which heat is added prior to being forced through the grain or hay by a fan. a2 AIRFLOW AND AIR DISTRIBUTION 93 FANS Fans may be placed into one of two general classes: those that operate by centrifugal force, or radial flow, where the flow is perpendicular to the axis of rotation; and those that operate by axial flow where the flow is parallel with the axis of rotation of the fan blade. The radial flow fans can be constructed with blades which are straight, forward-curved or backward-curved. The axial flow fans may consist of propeller fans or other axial flow fans with an impeller, or propeller fans with an enlarged hub, known as a disc fan. The discussion will be directed to those fans which are normally used in various phases of drying or aeration. The terms fan and blower are often used interchangeably. A fan usually refers to a mechanical device which will deliver a large volume of air or other gases at low pressures up to 6.9 kPa (1 psi). A blower is designed to deliver small volumes with pressures up to 241.3 kPa (35 psi). A compressor will deliver small volumes at even higher heads of pressure. In some cases, reciprocating compressors may operate at pressures as high as 34.5 X 103 kPa (5000 psi). There is considerable overlapping of the pressure for fans, blowers, and compressors. The manufacturers of fans generally used for crop drying are in Table 5.1. A fan may be used to move a small volume of air against large pressure or a large volume of air against low pressure. Generally, all types of fans can be made to operate under various conditions, but noise, efficiency, and other factors may limit the applications. CENTRIFUGAL (RADIAL FLOW) FANS A centrifugal fan consists of a wheel which is rotated in a spiral housing. The air pressure at the center of the rotating wheel is less than the surrounding air pressure. The air pressure is increased in the spiral housing as the wheel rotates. Air flows axially into the wheel and is then moved from the center to the periphery of the fan by the centrifugal force of the rotating wheel. It is possible to get higher pressure on a centrifugal blower by increasing the length of the blades because greater centrifugal force is exerted. The forward-curved fans are constructed of curved blades mounted on the outer periphery of a revolving wheel (Fig. 5.1). There are usually from 20 to 64 curved blades in this type of fan with the curved blades so arranged that the leading edge of the blade is on the outer edge of the periphery. The best mechanical efficiency that can be obtained by this fan is about 80%. The backward-curved fan may have from 8 to 50 curved blades located around the outer periphery of the wheel and usually operates at a speed 94 DRYING AND STORAGE TABLE 5.1. MANUFACTURERS OF AGRICULTURAL CROPS OF FANS USED FOR CROP DRYING’ Acme Engineering & Manufacturing, Muskogee, OK 74401 Aerovent Fan and Equipment Co., Lansing, MI 48906 AgStar, Goshen, IN 46526 Air Systems Div., Zurn Industries, Kalamazoo, Air-Care, Inc., Dorval, Quebec, Canada MI 49001 Airfoil Impeller Corp., College Station, TX 77840 \ American Fan Co., Cincinnati, OH 45216 American Farm Equipment Co., Lake Zurich, IL 60047 American Standard, Dearborn, MI 48121 Baughman-Oster, Inc., Taylorville, IL 62568 Bayley Propellair, Lebanon, IN 46052 Behlen Manufacturing Co., Columbus, NE 68601 Buffalo Forge Co., Buffalo, NY 14240 Bush Hog-Eaton, Omaha, NE 68110 Butler Manuf. Co., Agri-Products Div., Kansas City, MO 64126 Caldwell Manuf. Co., Div. of Chief Industries, Kearney, NE 68847 Campbell Industries, Des Moines, IA 50317 Carrier Air Conditioning Co., Syracuse, NY 13201 Champion Blower and Forge, Inc., Lancaster, PA 17604 Chelsea Fans and Blowers, New Britain, CT 06050 Chicago Blower Corp., Chicago, IL 60137 Chicago Eastern Corp., Marengo, IL 60152 Chore-Time Equipment, Inc., Milford, IN 46542 Cincinnati Fan and Ventilator Co., Cincinnati, OH 45242 Circle Steel Corp., Taylorville, IL 62568 Clarage—see Zurn Industries Combustion Equipment Co., Kansas City, MO 64132 Doerr Electric Corp., Northbrook, IL 60062 Farm Fans, Inc., Indianapolis, IN 46203 Farm Systems Corp., Marengo, IL 60152 Garden City Fan & Blower Co., Niles, MI 49120 G.M.T. Co., Rising City, NE 68658 Harrington Manuf. Co., Lewiston, NC 27849 Hart-Carter Co., Mendota, IL 61342 Hartzell Propeller Fan Co., Pigua, OH 45346 Harvestall Industries, New Hampton, IA 50659 Hutchinson Div., Royal Industries, Clay Center,’ KS 67432 Hy-Mark Industries, Henderson, NE 68371 ILG Industries, Div. Carter Corp., Chicago, IL 60641 Industrial Air, Inc., Amelia, OH 45102 Joy Manuf. Co., Pittsburgh, PA 15222 S.F. Kennedy, New Products, Inc., Taylorville, IL 62568 Krenz & Co., Germantown, WI 53022 Long Mfg. Co., Davenport, IA 52808 Long Mfg., NC, Inc., Tarboro, NC 27886 Martin Steel Corp., Mansfield, OH 44901 Middle State Mfg. Co., Columbus, NE 68601 Modern Farm Systems, Webster City, IA 50595 The Moore Co., Marceline, MO 64658 New York Blower Co., Chicago, IL 60616 H.K. Porter Co., Pittsburgh, PA 15219 Quietaire, Houston, TX 77003 Reed-Joseph Co., Greenville, MS 38701 Robinson Industries, Zelienople, PA 16063 George A. Rolfes Co., Boone, IA 50036 Sioux Steel Co., Sioux Falls, SD 57101 Stormor, Inc., Fremont, NE 68025 Strobie Air Corp., Trenton, NJ 08611 Sukup Mfg. Co., Sheffield, IA 50475 Superior Equipment Mfg. Co., Mattoon, IL 61938 Trane Co., LaCrosse, WI 54601 Twin City Fan & Blower Co., Minneapolis, MN 55414 AIRFLOW TABLE 5.1. AND AIR DISTRIBUTION 95 (Continued) Unico, Alliance, OH 44601 Wahoo-Built Div., Economy Housing Co., Wahoo, NE 68066 Westeel-Rosco, Ltd., Toronto, Ontario, Canada Western Engineering & Manuf. Co., Marina del Rey, Los Angeles, CA 90291 Westinghouse Electric Corp., Sturtevant Div., Boston, MA 02136 Wheat Belt Supply Co., Dodge City, KS 67801 A.R. Wood Mfg. Co., Northco Farm Automation System, Luverne, MN 56156 Woods Fan Div., The English Corp., Elmsford, NY 10523 York Div., Borg-Warner Corp., York, PA 17405 Zurn Industries, Inc., Kalamazoo, MI 49001 ‘Note: There are several more manufacturers of fans, some of which are used for crop drying. This partial list represents those manufacturers most active in crop drying applications, many of whom are included in the ASAE (1978); HPAC (1978); and AMCA (1978). — 100 80 60 40 20 PRESSURE PERCENT STATIC HORSEPOWER BRAKE PERCENT 20 40 PERCENT 60 80 100 VOLUME FIG. 5.1. FORWARD-CURVED CENTRIFUGAL FAN about twice that of the forward-curved fan (Fig. 5.2). The fan wheel is made with the leading edge of the curved blade on the inner edge of the s outer periphery. A backward-curved fan has a wide range of usefulnes may occur. but is usually used where a large variation in airflow volumes The high speed requires heavy construction and the volume occupied DRYING 96 AND STORAGE w OF AGRICULTURAL CROPS 20.8 HP = 31007—S* a* 80 100 & 80 Sr ro) WW wn 2 WW B- 60 60 x -= 40 a Y 20 = a [S) eb a fe) 20 40 PERCENT 60 80 100 VOLUME FIG. 5.2. BACKWARD-CURVED CENTRIFUGAL FAN by the fan is often twice as large as the forward-curved fan. The best mechanical efficiency which may be obtained with this fan is about 80%. A straight-bladed fan is sometimes called a radial-blade or paddle- wheel fan because the blades of the fan lie in the radius of the wheel. The radial fan is used for low volume displacement against high pressure. The radial fan consists of 5 to 12 blades and is usually operated in a range of 500 to 3000 rpm. It has a particular advantage where materials are carried in the airstream. The backward-curved fans give the smallest resultant air velocity, V,, when the fan wheels are operated at the same blade tip speed (Fig. 5.3). The velocity of the air along the blade tip is represented by V;, and the blade tip velocity by V,. The vector resultant is shown by V,. Operational Characteristics of the Centrifugal Fan The principal factors governing the fan operation are the pressure, volume of flow, and power. In order to obtain stability of operation, a reduction of air pressure should be accompanied by an increase of air volume in the working range. The backward-curved fan best meets the AIRFLOW FIG. 5.3. AND AIR DISTRIBUTION 97 ANALYSIS OF CENTRIFUGAL FANS (A) FORWARD (B) BACKWARD (C) STRAIGHT requirements of stability and does not overload the motor. The backward-curved fan may be operated at high speeds without excessive noises. The pulsating point of blowers normally occurs at an airflow of 40 to 50% of the designed airflow. Pulsation seldom happens if the pressure is below 3.7 kPa (15 in. water). The forward-curved fan is usually used in commercial ventilation work. It has a low peripheral speed and is quiet in operation. It has a connected load to prevent overloading. These fans are often available through surplus outlets or wrecking companies at comparatively low prices. They are usually designed to operate at static pressure of 0.25 kPa (1 in. water) and are satisfactory for hay or ear corn drying, but usually not for forcing air through deep layers of grain unless operated above rated speed. Note that the horsepower increases as 100% full volume is approached at low static pressure. In drying installations there is low static pressure with a shallow depth of material over the duct. To prevent overloading without using an oversize motor, a restriction in the form of a valve or constricted canvas duct on the discharge side (or intake) of the fan may be used when the duct system is covered with a low depth. An ammeter can be placed on the motor to check overloading of the motor. AXIAL FLOW FANS The axial flow fan moves the air through the fan nearly parallel to the impeller shaft. For high pressure applications multistaging is practiced. A propeller fan consists of two or more blades mounted on a hub. The radial mounted blades are set at an angle with the plane of rotation, usually at a 15° pitch angle. The stability of a propeller fan is maintained for pitch angles of about 17° although in some cases may go as high as 20°. Propeller fans now available are not designed to work against high resistances. About 1.24 kPa (5 in. water) static pressure is the limit of most fans presently available for crop drying. The propeller fans are suited for ventilation of rooms and air ducts of low resistance. The propeller fan is characterized by excessive noises at high speeds. Structural 98 DRYING AND STORAGE difficulties are encountered OF AGRICULTURAL CROPS in designing and constructing high-speed propeller fans. Fans with steep pitches may be used for large volumes and can be built for pressures up to 3.5 kPa (0.5 psi). The vaneaxial fan is distinguished by large hubs and short blades, in addition to stationary straight vanes on the discharge sides of the fan to prevent rotation or swirling of the air. It is normally used for delivery of air for pressures to 2.24 kPa (9 in. water) and for ventilation systems where space is valuable or head room is limited. The tubeaxial fan is similar to a vaneaxial fan except that it does not contain guide vanes (Fig. 5.4). The tubeaxial fan is simpler and somewhat less efficient than the vaneaxial fan. The horsepower and static pressure curves for axial fans are very similar (Fig. 5.5 and 5.6). As the capacity increases, the horsepower decreases, opposite to the relationship for a centrifugal fan. The working range of an axial fan is less than fora corresponding centrifugal fan, but its efficiency is 4 to 5% higher. FIG. 5.4. GENERAL TUBEAXIAL FAN FAN PRINCIPLES’ The mechanical efficiency of a fan is defined as the ratio of the fluid or air horsepower output to the brake horsepower input of the fan and is in the range of 40 to 70%. The fluid horsepower is the work output of the ‘See Appendix for metric conversions. AIRFLOW AND AIR DISTRIBUTION Ke)o i uu F x = ee vo) a loo a H.P ° 80 oO 9 so = wW = x < 60 60 & ” S.P roa) = 40 40 WwW o = Ww oO & 20 20.0; a. a O FIG. 5.5. HIGH SPEED PROPELLER FAN CHAR- 20 ACTERISTICS 40 PERCENT 60 80 100 VOLUME oh z 2 2 100 100 z 80 80 9 = <= 60 “ a. WWJ SP 60 & o 5 40 40 - W [S) 2 Ww & 20 fo) 20 & o 20 40 PERCENT FIG. 5.6. VANEAXIAL 60 80 100 VOLUME FLOW FAN fan, ft-lb per min divided by 33,000. If the volume of flow is in pounds per minute, the head must be in feet of flowing fluid, in this case air. The static pressure is usually given in inches of water, which must be con- ity of water The fluid horseverted to feet of air by multiplying by dens density of air 100 DRYING AND STORAGE OF AGRICULTURAL CROPS power, with the cfm based on air at 20°C (68°F), is given by equation (5-1). Fluid hp (cfm)(AP;) Sani (5-1) where AP, is the total operating pressure in inches of water, although the static pressure is usually used in calculations. The motor horsepower is approximately twice the fluid horsepower. The usual variations in fan characteristics are approximately as follows: The capacity of the fan varies directly as the speed ratio, the pressure (static, velocity, total) varies as the square of the speed ratio, and the horsepower varies as the cube of the speed ratio. Fans can be connected in tandem to increase the static pressure and in parallel to increase the airflow output. The same relationships exist for the radial and axial flow fans. The percentage volumetric efficiency of a centrifugal fan is equal to the volume of air delivered divided by the cubicle displacement of the fan wheel multiplied by 100%. The fan performance is usually figured at standard conditions of 20°C and 100 kPa (68°F and 29.92 in. Hg) barometric pressure and 50% relative humidity. The horsepower and pressure vary inversely with the absolute temperature. The horsepower and pressure vary directly with the density. The fan selection should be based on space requirements, installation costs, initial cost, controls necessary, power requirements, method of driving, position of mounting, flexibility of use, hazards involved in use, maintenance requirements, and noise produced. The rate of air movement through the product, the depth of the product, and the volume of the storage are the three factors which determine the size of the fan and power unit needed for removing the moisture from the grain. Fans are rated by the manufacturer according to their ability to deliver air against different static pressures. Specifications should be based on the Air Moving and Conditioning Association, Inc. (AMCA) ratings or equivalent recognized body. The performance rating is available from fan manufacturers and should be used as a basis of selecting fans for drying. The performance characteristics of an adjustable blade propeller fan are given in Table 5.2. The propeller blade is adjustable in increments up to 30°. A reliable manufacturer will supply the performance characteristics for the fans merchandized. An electric motor or internal combustion engine can be used to drive the fan. Most farm installations use a 3.73, 5.6, 7.46, or 11.2 kW (5, 7%, 10, or 15 hp) electric motor. AIRFLOW AND AIR DISTRIBUTION 101 TABLE 5.2. AIRFLOW AND HORSEPOWER REQUIREM ENTS FOR A PROPELLER FAN WITH AN ADJUSTABLE BLADE. = Airflow in cfm, horsepower Blade Setting Numbers 0.5 in () for blade setting. 1.0 : Static Pressure, in. Water 1.5 2.0 US (7.0) 25,300 (5.5) 21,200 (4.5) 17,400 (3.5) 14,000 (3.0) 10,400 (1.8) 7,500 (1.2) 4,700 (15.5) 30,400 (12.5) 26,400 (9.2) 22,300 (7.6) 18,400 (6.5) 14,600 (5.2) 11,200 (4.2) 7,800 (2.8) 4,800 (1.5) 2,000 0 2 4 6 8 10 12 14 16 (325) 18,400 (2.5) 14,800 (2.0) 11,200 (1.4) 8,100 (0.7) 5,200 (11.5) 28,000 (8.0) 23,900 (7.0) 19,800 (5.3) 16,200 (4.5) 12,800 SES) 9,400 (25) 6,500 (2.0) 3,800 Source: Courtesy of Joy Manufacturing Blade setting No. 16 corresponds to an Conversion: 1 in. water static pressure 1 cfm airflow = 0.0004716 1 hp = 0.746 kW (16.5) 28,500 (13.5) 24,400 (10.5) 20,400 (8.8) 16,600 (7.0) 12,400 (5.0) 8,500 3.0 Sh) (18.5) 26,300 (14.5) 22,200 (11.5) 18,000 (9.5) 13,600 (20.0) 23,300 (16.0) 18,200 Co., Pittsburgh, PA. angle of 30°. = 0.248 kPa m3/s Gasoline Engine as a Source of Power The gasoline engine offers some advantages in spite of its higher cost and the attention required. The speed of the fan can be varied, making it possible to change the quantity of air. Overload of engine from the fan is less probable than with an electric motor. Some units with gasoline engines for power are designed to supply heat through the exhaust and cooling system. Duct, Floor, and Bin Design It is important that a uniform length of path of air travel be provided to get even air distribution throughout the product. The distance the air travels through the product should be the same through the bin from the duct. As the air travels from the fan it may enter a main duct and then through the product or be distributed through a manifold to lateral ducts and then through the material. The main and lateral ducts may be rectangular, circular, telescoping, or tapered. Combinations of these are used. A rectangular main duct with triangular lateral ducts is often used (Fig. 5.7). The main duct may be inside or outside the storage and placed along one side or through the center of the storage. When placed outside, the main duct is often called a manifold. j k ‘& t am ft Askh : Veena ' i> , 24 | da picts ae , Calle een re a DRYING 102 AND STORAGE OF AGRICULTURAL CROPS LATERALS x8" BOARDS ON 2x4" BLOCKS MAIN FIG. 5.7. DUCT SYSTEM FOR RECTANGULAR DUCT BIN The procedure used by ventilation engineers for duct design may be used for designing tapered ducts. Telescoping ducts which would start as a large size and then reduce to smaller sizes in steps have been found to cause eddy currents and therefore loss of power and poor air distribution. By tapering a duct to maintain a constant friction loss per meter of length, much better airflow is obtained as compared with a telescoping duct. With such a design the velocity is theoretically zero at the end of the duct. The duct has a constantly decreasing static head due to flow and a constant air gap for escape of the air providing uniform air distribution throughout the length. For a tapered duct, the width is maintained the same throughout the length, and the height of the duct is decreased from the end next to the main duct to the outer end. Such a system has been worked out using the pressure drop relationship due to the friction of air in straight ducts. In applying these relationships it is necessary to assume a fixed friction loss per unit length. For example, assume 1.27 cm (0.5 in.) of water, then plot correspondingly the vertical dimensions of the duct for full volume of air at the end next to the main duct and zero volume at the outer end. When these are plotted, the shape of the duct is obtained. It is necessary to relate diameter of circular ducts to rectangular dimensions for rectangular ducts because the standard charts present the fraction loss information for circular ducts. Reynolds number, Re, is used to describe the characteristics of the fluid flow, equation (5-2). ReemM = diameter of round pipe or equivalent (5-2) diameter for rectangular ducts, m (ft) =| average velocity of flow, m/s (ft/sec) = density, kg/m: (lb/ft?) ee Ss a = viscosity, Pa:s (lb/ft-sec), which for air at 21.2°C (70°F) is 1.2 X 10 > lb/ft-sec and air at 100°C (212°F) is 1.5 X 10-5 Ib/ft-sec (1 Ib/ft-sec = 1.488 Pa-s) AIRFLOW AND AIR DISTRIBUTION 103 For laminar flow, Re < 2100, use Table 5.3 to convert from rectangular to circular ducts. For turbulent flow, Re > 4000, the equivalent diameter of a rectangular duct is 4 times the hydraulic radius. The flow between Reynolds numbers of 2100 and 4000 is unpredictable. TABLE 5.3. EQUIVALENT DIAMETER OF ROUND DUCTS TO CARRY THE SAME CAPACITY WITH EQUAL FRICTION AS RECTANGULAR DUCTS Rectangular Duct . Danicasiona Diameter of Round Pipe (in.) (in.) 12 18 24 30 36 42 48 54 60 ae 84 96 ly 18 24 {aya LEOW 19.7, S32 O cose, 202 2552203 WACOM S214 2 Oe 234 294 844 24.0 5 oly sOOO ZO 29 Moon | 21.6 345 404 43.8 Source: ASHRAE 30 36 eee8 OO-8 mara OLA: 38:6 42:4 41 seedse 40.60) 848.0 45:8 504 49.7 549 Doc) OS ON DG. (62:4) 42 48 45.9 48:9 5216 1eS = 55.6 4546 585° 596 63.9 64e 68.8 68:2) 73525 56 638 69.2 (74.5 7194) 60 " 65:7 71.7 977.2 82:6 iP 78.8 84is 905 (1977). A central duct is often used. However, the central duct system is usually such that there are large openings for passage of air from the duct to the grain. Prefabricated ducts may be obtained. A circular shape of expanded steel with screen covering is often used for grain. For hay, triangular and rectangular shapes are more common. The central duct system functions satisfactorily as long as the height of product over the duct does not become too great as compared to the width of the bin. A wooden slatted or perforated metal floor may be used for hay or grain. The perforated floor is usually more expensive than other types (Fig. 5.8). In order to prevent escape of air along the sides of the bin or mow, the slatted or perforated floor should not extend to the edge of the storage. The distance of the duct opening from the wall will vary with the depth and type of product over the duct, but is usually from 0.6 to 2 m (2 to 6 ft). For low airflows, such as used for cooling of grain in flat storages, uniform air distribution has often been difficult to attain where an open duct system cylindrically shaped with at least 30% openings has been used. With a suction system the greatest quantity of air is moved through the grain next to the fan (Fig. 5.9). The openings in the duct should be selected to throttle the air to provide uniform airflow throughout the length of the duct. Airflows are throttled by the duct if there are less than 7% openings. One approach consists of providing a duct with 104. DRYING AND STORAGE AGRICULTURAL OF CROPS PERFORATED FLOOR FIG. 5.8. PineUrae METAL BIN WITH FALSE FLOOR OF PERFORATED I] Roe Oe : JOISTS 2"x4 8" APART METAL From U.S. Dep. Agric. (1952) SUCTION SYSTEM ZONE OF HIGH AIRFLOW PRESSURE SYSTEM ZONE OF LOW AIRFLOW FIG. 5.9. VARIATION OF AIRFLOW FOR SUCTION AND PRESSURE SYSTEM WITH VELOCITY ABOVE 7.6 M/S (1500 FT /MIN) ZONE OF LOW AIRFLOW ZONE OF HIGH AIRFLOW AIRFLOW AND AIR DISTRIBUTION 105 perhaps 1% openings at points of otherwise high airflows varying to perhaps 7% openings at the opposite end. The air velocity in the duct is usually from 305 to 914 m/min (1000 to 3000 ft/min). Designs should consider the relationship of airflow direction of flow and volume, openings in duct, and pressure drop through product to design airflow systems with uniform air distribution (Shove 1959). The pressure drop in a pipe as related to airflows is given in Table 5.4. Pressure Loss Through Floors The effect of the perforated floor or ducts on the static pressure of the system is important. If the opening is too small, excessive static pressure will be developed requiring additional horsepower for operating the fan, and in many cases, may cause a reduction in airflow below the minimum requirements. The approximate pressure drop through a floor covered with grain is given in equation (5-3) (Henderson 1943): Q. = 30(% opening) AP°:*2 (5-3) where Q. = airflow, cfm/ft? floor area AP = pressure drop, in. water % opening = opening of floor, % No additional resistance is encountered from the floor when the openings are 20% or more with shelled corn. Seven to 10% in openings of the floor area is sufficient from a practical standpoint for drying. Effect of Material on Airflow The volume of airflow is usually expressed in terms of m*?/m?-min, m?/m2min, m?/tonne-min, cfm/bu, cfm/ft’, cfm/ton, or cfm/ft? of crosssectional area (gross area including grain and void space) through which the air is forced. Recommendations regarding the amount of airflow for various products and conditions are given in Chapters 6, 7, 8, 9, and 11. The static pressure against which the fan and motor must operate varies approximately as the square of the quantity of air moved through the grain at any one particular time. The static pressure varies directly as the depth of the product, with a linear relationship between the static pressure and depth. If there is a static pressure of 0.014 kPa for 0.3 m (0.06 in. for 1 ft) of material, the static pressure will be 0.028 kPa for 0.6 m (0.12 in. for 2 ft). It would be expected that static pressure per 9 TI 6 GT ST 16 VG LG 0€ ‘1ayaW “UlIBIC] HSLVYSd M 001 44 €& 9€ 820 7800 S600 LOS I 890 ZEe0 LITO 60:0 6900 09 GG TT 8coO 180 10 210 1400 T6 ¥, (CiGe Gl CLIOR geo 8z0 018 LOST oT OL a ‘Sadi NId JO GV FIO LHO 6E $00 810 890 920°0 600 9S0°0 €€0°:0 €20'0 80 020 ZITO SS0'0 eT CEO 970 O€0 STORAGE OF :90In0g AAHSV (6761) UOISIaAUOD :s10jOeJ | Wyo= 9TL¥000'0 ~8/eM “UT= 79200 UW [ “ur 914848 ainssoid = 88hZ'0®q¥ 000'T €8 000°¢ 000‘0T 000‘02 000‘04 000‘08 ujo NOILO 4OIdSHIVNI AND JIEVL “b'S DRYING v6) 106 AGRICULTURAL CROPS AIRFLOW meter would increase considerably AND because AIR DISTRIBUTION 107 of packing as material is placed in the bin. The packing is not as variable or as important as the method of filling and amount of foreign material on the static pressure . Therefore, it is possible to plot the pressure drop per meter of depth in kPa (foot depth of grain in inches of water) (Fig. 5.10) and to find the pressure for any other depths. At a given airflow, the depth in m (ft) is multiplied by the pressure drop per m (ft). It can be noted in Fig. 5.9 that when the static pressure vs. airflow data are plotted on logarithmic paper nearly a straight line results. There is a section of the curve in which a straight line expresses the relationship between pressure and flow. ; Q. = aAP”! Equation (5-4) expresses the relationship between (5-4) the static pressure drop AP’ per unit depth of grain (inches of water), and airflow, Q,, expressed in volume per unit time—cross-sectional area (ft?/ft2 min). If the exponent b; is a positive value, the curve is a parabola, and if a negative number, the curve is a hyperbola. A positive value of b; is obtained for the static pressure relationship in crop drying. The value of Q, when AP’ = 1 is the value of the constant a,. The airflow pressure curve is slightly convex upward and the slope of the curve is greater for fine materials than for coarse materials. If the foreign material in the grain is larger than the grain kernels, the static pressure decreases, while for small particles the static pressure increases. Measurement of Static Pressure A manometer is a device in which the difference in elevation of the liquid in the tube can be measured to determine the pressure developed. A convenient material to use for the liquid in the manometer is water, although liquids of lower density might be used for greater accuracy. When material of lower density is used, the amount of change of liquid level is greater for a given pressure, thus providing greater accuracy (Fig. 5.11). For small readings the U-tube may be inclined and the reading of the change in elevation of the fluid taken along the U-tube at R. By making a substitution in equation (5-5) the static pressure may be calculated. h = AP =r sin 0 (5-5) It is advisable to calibrate the gage because of possible variations in the diameter of the tube and inclination of the two legs. ® J ! t MBLVM s o vd z z ‘NIVUD 4O ie MO1SHIV Y3d OL SGSSS GNV SNIVYD 4O JONVISISSY dO 20° 20° 3YUNSS3IUd IYNSS3IUd dOYO £0" £o° 3d vo 10° 400° 10° 800° 600° 900° 00° £00" €00° 200° 200° ‘01'S ‘Did 100° Loe : € a n eo eee ra ‘ - « ‘urer3 Ap AjaayVRPeI “UvayD Jo (paxoed you) [IlJ aSOO] B JO} SAMBA SAATS yzeYS SIYT, :SE}ON 20° 1004 so" YILIW 40° 60 H1d30 ‘NIVYD JOH1d30 S3HONI € ia “MOTJIIe JO a3e1 UaAIs e 10} dorp ainsseid pazeorpul ut) Jue}UOO aunqstour YY SurAvy UleIS UwBEIO JO [IJ eSOo] B JOY “UMOYS SAN[BA ay} UBY} MOTJITE 0} doULAsISAI JaYsIY JUeoIed QG esnvo ABUT UIq B UI UTeIS BY JO suryoeg “uleI3 ay} ULY} JasIeOO SI [IIa}VUL UFIB10J ay} JI Pasveroap St MO]JITE 07 BOUL4SISAI PUB ‘UTeIS 9Y) URY} JOULJ SI [Blaze usIa10J aU} Jl pasvaroUl SI MOTJATe 07 aoUeySISeI JEY} paiou eq P[Noys 4 ‘JaAeMO} ‘papuawMULOdaI aq Ud UOTJdeII09 oLsoads OU UTeIS YIM PaXTU SI [eIT9}VUL usIa10j UdyA z ay} Jo Juaorad yg ATuO asn ‘(quecsed gg Zurpasoxe saryIprumy eAyeiar YIM, WNQIMbe ) 2 } Ul iB SS i € . ? = 5 oO as ae ¢ ~ s = ae = ior eas~ & § 4 oO »2 oat ce nie Clean oz 8 3 32 Od¥ BBHZ'0 = 10j0M “Uy | $2UU/¢W BOGOO'O = 214/W49 | wepor'o = 4 | ' POS StesEE HSneh eS PhRLS CROPS AGRICULTURAL OF STORAGE AND DRYING 108 AIRFLOW AND AIR DISTRIBUTION 109 A ee FIG. 5.11. MANOMETER U-TUBE The draft gage is commonly used for low pressure heads of gases and consists of a U-tube arrangement in which one leg of the tube is a reservoir of large diameter as compared to the diameter of the tube that forms the inclined leg (Fig. 5.12). Small variations in the level of the inclined tube produce very little change in the level in the reservoir of large diameter. Commercial draft gages are usually designed to be provided with a liquid other than water. The instructions of the manufacturer should be followed for zeroing the instrument before use and leveling throughout use. al EF FIG. 5.12. DRAFT GAGE For very precise measurements a micromanometer is used, which operates on the same principle as a regular manometer, but is provided with an accurate method of reading. A swinging vane meter may be used in which the impact of the moving air hits an air vane which is connected to a calibrated static pressure scale. These meters are quite useful for field work and should be calibrated against manometer readings. 110 DRYING AND STORAGE OF AGRICULTURAL CROPS The quantity of airflow may be determined by multiplying the average velocity times the cross-sectional area. A pitot tube connected to a manometer is often used for determining the velocity of the air. How- ever, it is necessary to obtain the velocity at various locations in the cross-section of the airstream from which an average is obtained. The accuracy of the pitot tube method depends upon the elimination of turbulence in the airstream. To avoid turbulence it is recommended that the location of the pitot tube be preceded by from 10 to 20 diameters of straight pipe and followed by about 5 diameters of straight pipe. A vane type anemometer provides a good means of determining the velocity of air for field observations. The vane anemometer consists of a propeller connected to a revolution counter mounted in a frame. The number of revolutions of the propeller depends upon the velocity of air. The vane anemometer must be calibrated against other methods. An outlet may be built on the air outlet of a bin so that all the air is directed through the anemometer. A restriction placed over the bin outlet will change the static pressure, so it should be left in place for drying tests where airflow measurement is important. The hot-wire anemometer operates on the principle that the cooling effect of the air passing a wire heated by electrical means varies according to the amount of air passing over the wire. The resistance of the wire varies with the temperature and the amount of current flowing through the wire and will change according to the velocity of air past the wire. After calibration this instrument can be used for determining the velocity of air according to the electric current required to maintain the hot wire at a constant temperature. This device can be used for velocities as low as 0.03 m/s (6 ft/min). 4 A low velocity range hot-wire anemometer was developed for measuring air velocities over the surface of drying bins. The unit is a self-contained direct reading portable instrument which is capable of measuring velocities from 0 to 0.10 m/s (20 ft/min). The device is very useful for checking the airflow characteristics of dryers for hay and grain. A unique device has been developed measuring airflow with perforated metal sheets used for orifice plates (Polson and Lowther 1932). A perforated sheet of metal is calibrated by cutting two plates from opposite corners of the sheet. The two plates are calibrated for airflow per hold at various pressure drops. Data are placed on a chart relating airflow per hold and pressure drop. From these charts the number of holes exposed to the airflow to give a desired air volume can be determined. A standard error of estimate of + 1.8% was reported for this method. Orifice plates provide simple and accurate devices for measuring rates of flow of air. These can be constructed in the shop or purchased. The flow of air through a circular thin plate orifice discharging to atmospheric air is based on equation (5-6). AIRFLOW AND AIR DISTRIBUTION w = C 37.6907 diam? (=) : 111 (5-6) where w is lb/min of air, diam is the diameter of orifice in inches, AP is the differential head in inches of water, and T is the absolute temperature, “R. The coefficient of discharge, C, is about 0.61. It is necessary to refer to tables included in severalengineering handbooks to determine the coefficient of discharge for various orifice plates at various differential pressure and cross-sectional area relationships. A straight section of pipe of 10 pipe diameters should be provided on the upstream (high pressure) side and a straight section of 5 pipe diameters should be provided on the downstream (low pressure) side of the meter device—orifice plate, venturi, and anemometer. For an orifice plate, the high pressure tap should be located 1 pipe diameter and the low pressure tap a variable distance from the orifice. The distance, Ls, that the low pressure tap is located from the orifice is determined by equation (5-7). d L2 = 0.90 D where E = (=) 2 (5-7) Lz, = distance of pressure tap from orifice on low pressure side, in. d = diameter of orifice, in. D = diameter of pipe, in. The airflow in 7.6 cm (3 in.) pipe with different static pressures and orifice plates is presented in appendix Fig. A.2. Other general rules for installation and design are that the orifice diameter should not be greater than 70% of the pipe diameter and the low pressure should be no less than 40% of the high pressure. The pressure difference obtained with the above design can be substituted into equation (5-8), using appropriate C-values, to find the quantity of airflow. C-values can be determined from Table 5.5, interpolating between values. Another procedure consists of placing pipe taps 1 in. from and on each side of the orifice plate and referring to standard tables for the constants related to the conditions. Q=1096.5 CA / AP — (5-8) where v = 1096.5 (a2 p (5-9) 112 DRYING AND STORAGE OF AGRICULTURAL CROPS where C is the coefficient of discharge, A is the area of the orifice, ft’; p is weight of the air, lb/ft3; Q is the airflow, cfm; v is velocity, ft/min. Equations (5-6) and (5-7) give the same results for practical purposes. A pitot tube can be used for determining the airflow, Q, if readings are taken throughout the cross-section of the pipe at points representing concentric circles of equal areas. Five equal areas are obtained in the pipe at points located 32, 55, 71, 84, and 95% of the distance from the center to the edge of the pipe. TABLE 5.5. C-VALUES Pipe Diameter FOR ORIFICE PLATES d/D m in. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.076 0.305 3 1, 0.619 0.610 0.631 0.620 0.653 0.637 0.684 0.663 0.728 0.700 0.788 0.756 0.880 0.846 Source: ASHVE (1949). Airflow Analysis The analysis of data collected for non-parallel lines of airflow during grain drying, by existing methods, requires considerable time. There is need for a quick, accurate method of making such determinations. The method presented was developed to meet these requirements. The commonly accepted measure of the rate of airflow is cfm/bu. The rate for non-parallel lines of airflow may be expressed as an average for the entire bin, but would not serve to determine the airflow rate at different locations in the bin. The airflow per accumulated bushel of grain (cfm/acc bu) is appropriate. At the location of the entrance of air to the grain, the airflow rate approaches infinity and decreases as the air moves through the grain. It is necessary to obtain a relationship between static pressure lines and airflow in cfm/bu. The static pressure is measured at the intersections of a grid system perpendicular to the air tunnel of the bin. These pressures are plotted at appropriate positions on a cross-section sheet of the building. Lines connecting points of equal static pressure (isopressure or isopiestic lines) are drawn, normally for 0.2 cm or 0.1 in. intervals. The airflow is then obtained in cfm/ft? of cross-sectional area perpendicular to the airflow. An airflow chart for wheat in which the airflow in cfm/ft? is plotted against the depth in inches and is represented by a straight line on logarithmic paper for various static pressures is shown in Fig. 5.13. The data are represented in this form so that the relationships for depths less than 0.3 m (12 in.) are obtained easily and rather accurately. AIRFLOW 40 ; aS AND AIR DISTRIBUTION im 113 STATIC PRESSUR CIN, WATER) pee ARIST EPsCe 9 aac 3 Sa iiee AE GRATE | in. = 0.0254m iN aiebAd olasia 1 cfm/ft2 = 0.00508 m2/m2@s (CFM FLOW AIR PER FT.) SQ | in, water \ 2 FIG. 5.13. = O.2488 3 Bll ipooeCOU ALpbine kPa 4 AIRFLOW THROUGH | ‘XI 5 678910 DEPTH, INCHES 2 oO 30 40 5060 80 100 CLEAN WHEAT Procedure for Determining Airflow from Static Pressure Data (U.S. Customary Units).— (1) Draw isopressure lines at 0.1 in. intervals. (2) Use the coordinates, X, Y, Z, as designated, in in. (Fig. 5.14). (3) Measure X, the distance in in., represented between isopressure lines (Fig. 5.13). (4) Read airflow from 0.1 in. static pressure line for X value from airflow chart for grain in bin (Fig. 5.12). (5) Substitute in equation for airflow/unit volume of grain which can be used for all similar problems. (a) Derive equation (1) Average cross-sectional area, ft?, perpendicular to flow of air, represented (Avg Y) (Z)/144. in the YZ-plane with values in in., is 114. AND DRYING OF STORAGE AGRICULTURAL AIR FLOW STATIC Z CROPS LINES ISOPRESSURE IS THE LENGTH LINES PARALLEL TO TUNNEL \ FIG. 5.14. DIMENSIONS OF A SECTION IN GRAIN BIN (2) Volume, ft?, of section of grain is (X) (Avg Y) (Z)/1728. (3) Airflow/unit of volume of grain, cfm/ft? = (cfm/ft?) X (average cross-sectional area)/(volume of section). Substituting, (cfm/ft2) (Avg Y)(Z)/144] cfm/ft* = —~CxyAvg Y\(Z)/1728) = 12(cfm/ft?)(1/X) (5-10) (Multiply equation (5-10) by 1.25 to get cfm/bu.) cfm/bu = 15 (cfm/ft?) (1/X) (5-11) (b) Substitute values in equation (5-11) Note that only the distance X between isopressure lines, drawn for each 0.1 in. of pressure, is required to obtain airflow from Fig. 5.12. Thus, when X = 10 in., in wheat, the airflow is 5.7 cfm/ft?. Also, cfm/bu= 15(5.7) (1/10) = 8.6 (5-12) 1 cfm/bu = 0.134 m3/m? sec Note that the distance between airflow lines in Fig. 5.13 was not used, which indicates that the airflow lines are unnecessary, except for determining shape of airflow pattern. (6 Determine the airflow per accumulated bushel of grain as follows: —" Let X’ (Fig. 5.15) equal the distance in inches from the point where the air enters the grain and the point where airflow per accumulated bushel is to be found. The value for cfm/ft? is obtained from Fig. 5.12 at the X’ value for depth and static pressure drop from the entrance of the air to X’. cfm/acc bu = 15 (cfm/ft?) (1/X’) (5-13) The airflow represented by Fig. 5.14 is shown in Table 5.6 as determined by graphical means. The accuracy of the proposed method is AIRFLOW AND AIR DISTRIBUTION 115 0.6 6" fe) STATIC ISOPRESSURE LINES FIG. 5.15. A SECTION FOR DETERMINING TABLE 5.6. RESULTS OF GRAPHICAL (For section represented METHOD AIRFLOW OF DETERMINING AIRFLOW. in Fig. 5.14.) Static pressure—line number Static pressure drop, in. water Tk 0.1 X, in. grain between pressure lines Airflow, cfm/bu X’, in. grain accumulated cfm/acc bu, equation (5-12) 2 0.2 if 8 LEO = glee 7 15 17.0 7.4 3 0.3 11 all 26 3.6 4 0.4 12 6.0 38 2.3 5 0.5 13 yl bi 1.6 6 0.6 14 4.5 65 iL? Source: Hall (1955). Conversion factors: 1 in. water = 0.2488 kPa 1 cfm/bu = 0.0134 m3/m3s greatest when the airflow lines are straight. To get accurate results for curved airflow lines, several short intervals should be taken in the straight portion of the curve. Similar data as given in Table 5.6 are obtained using equation (5-14). bases cfm L wh i (+) (=) ? we) where L is distance of air travel, in.; and v is the air velocity, ft/min, determined from Fig. 5.9. Traverse Time.—Another method of designating the flow of air is to use the traverse time, i.e., the length of time required for the air to pass through the grain, usually in seconds (Hukill and Shedd 1955). The traverse time, 6;,, for parallel airflow lines for grain with 40% voids, is given in equation (5-15). oe bu ie G.. (5-15) Thus the traverse time for an airflow of 3 cfm/bu would be 10 sec. The by average linear velocity v, ft/min, of air through a product is given equation (5-16). 116 DRYING AND STORAGE OF Vv = AGRICULTURAL CROPS Qe SSS porosity Valving a Bin with Non-rectangular (5-16) Cross-section Moving air through a bin with center duct only and non-parallel airflow lines when partially filled presents a problem of providing adequate uniform airflow through the entire bin. This problem is caused by the irregular cross-section through which the air passes. In some years the bin may not be filled to capacity and therefore uniform airflow and uniform drying are not provided. The air will be short-circuited through the grain duct flow duct mass directly over the duct while grain located at the sides of the will not receive adequate ventilation. This non-uniformity of airis greatest with small amounts of grain in the bin with a center system. One method of ventilating such a bin is to cover the mass of grain immediately over the duct through which the air is short-circuited with paper or canvas and thereby divert part of the air to the remaining grain. This method is known as “valving.” The static pressure will be increased and the total airflow through the bin will be reduced with valving. Valving is easily accomplished when an exhaust system is used. QUESTIONS 1. Plot the data for fan performance, Table 5.2, of static pressure (% of maximum), vs. volume (% of maximum), for various blade positions. oa 2. Derive the relationship for kW (horsepower) output of a fan, equation (5-1). 3. What is the fluid horsepower for moving 7.08 m3/s (15,000 cfm) of air (15 ft? = 1 lb) against a total head of 5.4 cm (2.12 in.) of water? Give total head in kPa. 4. Using Table 5.5, compare the pipe diameter and cross-sectional area to the friction head, for a given length of pipe for an airflow of 4.7 m°/s (10,000 cfm). Plot and discuss relationship. 5. Using the method of Kramer (1947), determine the height of a tapered duct 3 m (10 ft) long, having a constant width of 0.15 m (0.5 ft), for a capacity of 0.23 m3/s (480 cfm) of air, assuming a constant friction loss of 12.44 Pa (0.05 in. water)/ft of length (which is equivalent to a total friction head of 124.4 Pa (0.5 in. water). Are the assumptions justified? 6. Based on Shedd (1945), prepare an airflow graph for clean ear corn with 16% moisture, relating airflow in m?/m3s (cfm/ft3) and AIRFLOW AND AIR DISTRIBUTION 117 depth for static pressures of 2.49 Pa (0.01 in. water) and 12.44 Pa (0.05 in. water). 7. Based on Guillon (1946), relate in graphical form the depth, airflow, and static pressure for alfalfa hay with 30% moisture. Place the depth up to 20 ft on the ordinate and static pressure on the abscissa for airflows of 10, 15, and 20 cfm/ft?. 8. A 0.9 m (36 in.) propeller fan operating at 2000 rpm with a 3.73 kW (5 hp) motor has the following performance data: Static pressure, in. water Airflow, cfm Y% V4 1 1% 1% 1% 2 20,800 19,400 17,700 16,000 14,000 11,500 8400 (1 in. water = 0.2488 kPa; 1 cfm = 0.000472 m3/s) (a) Draw the static pressure-airflow curve for fan speeds of 2000, 2500, and 3000 rpm, (b) calculate fluid horsepower requirement for each, and (c) calculate efficiency for the various airflows at 2000 rpm. 9. Determine the percentage error in calculating the airflow using equation (5-8) when the specific weight of dry air is used instead of an actual specific weight of air at 75% relative humidity and 37.8°C (100°F); 10% relative humidity and 71.1°C (160°F). Using equation (5-6) as a base, what is the percentage error obtained by using equation (5-8)? 10. In equation (5-6), what is the value of line AP as Q, approaches infinity? As Q, approaches zero? 11. Sketch and explain the operation of an airflow and distribution system which could be used for recirculating air, reversing direction of airflow, and for admitting outside air. REFERENCES AMCA. 1978. Directory of Licensed Products. Assoc. Publication 261, Arlington Heights, III. ASAE. 1978. Agricultural Engineers Yearbook. Air Movement and Control Am. Soc. Agric. Eng., St. Jo- seph, Mich. ASHRAE. 1977. Handbook. Fundamentals. Am. Soc. Heat., Refrig. Air-Con- ditioning Eng., New York. ASME. 1946. Test code for fans. Am. Soc. Mech. Eng., New York, PTC 11. ASME. 1949. Flow measurement by means of standardized nozzles and orifice plates. Am. Soc. Mech. Eng., New York, PTC 19.5. BAUMEISTER, T. et al. 1976. Standard Handbook of Mechanical Engineers. McGraw-Hill Book Co., New York. 118 DRYING AND STORAGE OF AGRICULTURAL CROPS BROOKER, D.B., BAKKER-ARKEMA, F.W. and HALL, C.W. 1974. Drying Cereal Grains. AVI Publishing Co., Westport, Conn. CHURCH, A.H. 1944. (1972.) Centrifugal Pumps and Blowers. John Wiley & Sons, New York. (Reprinted by Robert E. Krieger Publishing Co., Huntington, N.Y.) COLLINS, T. 1953. Flow patterns of air through grain ome drying. Agric. Eng. 34 (11) 759-760. CRANAGE, T. 1946. Analysis of types of fans for curing hay in the mow. Agric. Eng. 27 (11) 509-512. DAVIS, R.B. and BAKER, V.H. 1951. The resistance of long and chopped hay to air flow. Agric. Eng. 32 (2) 92—94. GUILLON, R. 1946. Forced air flow in drying hay. Agric. Eng. 27 (11) 514— 520. HALL, C.W. 1955. Analysis of air flow in grain drying. Agric. Eng. 36 (4) DENT, PISO) HENDERSON, S.M. 1943. Resistance of shelled corn and bin walls to air flow. Agric. Eng. 24 (11) 367-369, 374. HENDERSON, S.M. and PERRY, R.L. ing, 8rd Edition. HPAC. 1978. 1976. Agricultural Process Engineer- AVI Publishing Co., Westport, Conn. Info-dex. Heating/Piping/Air Conditioning. Penton/IPC Pub- lishers, Stamford, Conn. HUKILL, W.V. and IVES, N.C. 1955. Radial air flow resistance grain. Agric. Eng. 36 (5) 332-335. HUKILL, W.V. and SHEDD, C.K. 1955. Non-linear air flow in grain drying. Agric. Eng. 36 (7) 462—466. KAZARIAN, E.A. and HALL, C.W. 1955. oe air flow in drying bins. Agric. Eng. 36 (12) 801-802. KRAMER, A. 1947. Improved duct design for mow hay finishers. 28 (2) 58, 61. MacDONALD, J.B. and HEDLIN, C.P. 1954. Agric. Eng. 35 (9) 658, 660. MADDEX, R.L. and HALL, C.W. 1954. State Univ., E. Lansing, Ext. Bull. 316. MADISON, R. MOSS, S.A. POLSON, orifices SHEDD, 19-20; SHEDD, 1949. 1916. Agric. Eng. Air-flow meter for crop driers. Drying grain with forced air. Mich. Fan Engineering. Buffalo Forge Co., Buffalo, N.Y. Trans. ASME 38, 761! The impact tube. J.A. and LOWTHER, J.G. 1932. The flow of air through circular in thin plate. Univ. Ill., Urbana, Eng. Exp. Stn. Bull. 240. C.K. 1945. Resistance of ear corn to air flow. Agric. Eng. 26 (1) 25. C.K. 1953. Resistance of grain and seeds to air flow. Agric. Eng. 34 (9) 616-619. SHEDD, C.K. 1954. Eng. 35 (6) 420. Measuring air flow with perforated metal sheet. Agric. AIRFLOW SHOVE, G.C. 1959. Airflow sertation. lowa State Univ. AND AIR DISTRIBUTION analysis of grain ventilation ducts. Ph.D. 119 Dis- U.S. DEP. AGRIC. 1952. Drying shelled corn and small grains with unheated air. U.S. Dep. Agric. Leafl. 332. Theory and Principles of Drying Much of the research in drying of agricultural products reported from 1940 to 1955 did not delve into the theory of drying but was concerned mainly with field results. From 1955 to the present, considerable research has dealt with the theory, as presented in many technical articles in Agricultural Engineering, Journal of Agricultural Engineering Research, and Transactions of ASAE. Several books cover the theory and principles of drying in detail. Among the recent books are Brooker et al. 1974; Hall and Hedrick 1971; Lykov 1968; Nonhebel and Moss 1971; Van Arsdel et al. 1973; and Williams-Gardner 1971. The most recent comprehensive summary of the theory and principles of drying is Bakker-Arkema et al. 1977. Several research workers have presented the vapor pressure theory of drying. As the temperature of the product increases while the moisture content is maintained, the vapor pressure inside the product increases. The flow of moisture is from locations of high to low vapor pressure and is approximately proportional to the vapor pressure difference between the product and the surrounding atmosphere. It is only approximately proportional because the resistance to the movement of moisture on the surface of the grain is less than the resistance in the interior of the product The drying of farm crops, biological in nature with a seed coat or cover, differs from many other products which are dried, such as dust, paper, sand, stone, chemicals, etc. Many ideas presented on drying have been obtained after analysis of the research literature pertaining to drying in other fields, which must be modified to apply to farm crops. Rate Periods of Drying The two major periods of drying are (1) the constant rate period and (2) the falling rate period. In the constant rate period drying takes place from the surface of the grain or forage and is similar to evaporation of 120 THEORY AND PRINCIPLES OF DRYING 121 moisture from a free water surface. The rate at which moisture is evaporated is determined largely by the surroundings and is affected only a small amount by the material from which moisture is being evapora ted. The point marking the end of the constant rate period occurs when the rate of moisture diffusion within the product decreases below that necessary to replenish the moisture at the surface. Most of the drying of sand, washed seed, and washed grain takes place in the constan t rate period. The constant rate drying period is short in duration for farm crops. The magnitude of the rate of drying during this period is dependent upon: (1) area exposed, (2) difference in humidity between airstream and wet surface, (3) the coefficient of mass transfer, and (4) velocity of the drying air. These variables are related according to equation (6-1) (Henderson and Perry 1955): dW dé = 7 f,A (p, _— Pp.) — k A(t, —t,) hr, (6 = 1) where os = drying rate, kg/s (lb/hr) of water f, = water vapor transfer coefficient, kg/s m2 Pa (1 lb/hr ft? psi) A water surface area, m? (ft?) ps = water vapor pressure at t,, kg/m? (lb/in.”) Pp. = water vapor pressure in the air, kg/m? (lb/in.2) kp = thermal conductance of air film, W/m?K (Btu/hr ft2°F) t, = temperature of air, °C (°F) t, = water temperature, wet bulb, °C (°F) hy, = latent heat of vaporization, J/kg (Btu/Ib) The velocity of the drying air would affect the value of k; in the equation. The falling rate period is entered after the constant rate period. The critical moisture content occurs between the constant rate and falling rate periods. The critical moisture content is the minimum moisture content of the grain that will sustain a rate of flow of free water to the surface of the grain equal to the maximum rate of removal of water vapor from the grain under the drying conditions. In grain and forage the initial moisture content is usually less than the critical moisture content so that all of the drying occurs in the falling rate period. The falling rate interval is therefore the most important period from the standpoint of drying of hay and harvested grain. Even when the constant rate period is in effect at the start of drying it is often neglected by researchers because of its short duration and the small amount of moisture to be removed before 122 DRYING AND STORAGE OF AGRICULTURAL CROPS entering the falling rate period. The critical moisture content for wheat is between 69 and 85%, d.b. (Simmonds et al. 1953A). The falling rate period of drying is controlled largely by the product and involves the (1) movement of moisture within the material to the surface by liquid diffusion and (2) removal of moisture from the surface. The falling rate period often can be divided into two stages: (1) unsaturated surface drying and (2) drying where the rate of water diffusion within the product is slow and is the controlling factor. These intervals are sometimes called the first falling rate period and the second falling rate period, respectively. Carefully controlled research indicates that there are generally more than two falling rate periods (Rodriguez-Arias 1956). There are four falling rate periods for shelled corn above 25%, d.b. The variation in falling rate periods can be explained as follows: the surface area of the micropores of wet grain is covered with water. The thickness of water in the micropores increases with an increase in moisture content. The thickness of a water molecule is about 2.9 A units. With four molecular layers of water, the top layer will be removed first, then the next layer, etc., and as a layer of water is removed the drying constant changes. Drying of farm crops involves two fundamental processes: (1) a transfer of heat to evaporate liquid and (2) a transfer of mass as internal moisture and evaporated liquid. The mass is transferred as liquid and/or vapor within the solid and as vapor from wet surfaces. The external conditions and the internal mechanisms which control drying are both important. However, the internal mechanism of moisture movement is more fundamental although less generally used because of the empirical nature of the knowledge of the internal mechanisms controlling drying of agricultural products. Some of the possible mechanisms which may control interna. movement of moisture are as follows: (1) Diffusion as (a) liquid and/or (b) vapor (2) Capillary action (3) Shrinkage and vapor pressure gradients (4) Gravity (5) Vaporization of moisture Determining Periods of Drying First obtain a drying curve of a thin layer of grain fully exposed to the airstream by plotting moisture content in percent, d.b., vs. the time in hours (Fig. 6.1). It is difficult to ascertain the range of the moisture content periods by observing the drying curve. These periods can be shown more easily by differentiating the drying curve with respect to time. Because of the difficulty of writing an equation for the drying THEORY AND PRINCIPLES OF DRYING 123 MOISTURE, (D.B.) M, PERCENT 20 40 60 80 100 120 140 6, TIME, MINUTES FIG. 6.1. DRYING RATE CURVE curve, it is easier to differentiate the curve graphically to obtain the drying rate. The drying rate can then be plotted vs. time and moisture content on separate graphs as a means of determining various periods of _ drying (Fig. 6.2 and 6.3). In these figures A-B is the heating or cooling period B-C is the constant rate period of drying C is the critical moisture content C-—D is the first falling rate period D-—KE is the second falling rate period. Vapor Pressure Changes When drying by forcing heated air through a product, the vapor pressure varies at different stages of heating and drying and is shown in Fig. 6.4. The vapor pressure in the product is increased as the temperature is increased with a constant moisture content. The product does not dry appreciably until it reaches a temperature nearly equal to the heated air. The vapor pressure of the surface of the grain increases rapidly during 124 AND DRYING STORAGE OF AGRICULTURAL 30 =40 CROPS HOUR PER MOISTURE PERCENT de gM, °o p 0.2 10. 20 50 60) 70°80) | 90) 100 6, TIME, MINUTES FIG. 6.2. DRYING RATE VS TIME ro) HOUR PER MOISTURE PERCENT * dM me) de {e) io 2) 4p Gabe 8s OM eloan Rlseeunl Gwen (8hee2OTa 22 M, MOISTURE CONTENT, FIG. 6.3. DRYING RATE VS MOISTURE PERCENT (0.8) CONTENT the initial stages of heating. Moisture movement is then from the outside edge of the grain to the drying air and also some toward the center of the grain. Moisture does not move from the center of the grain to the drying air until the product is heated throughout or the outside layer is dried. The warming-up period is shown in Fig. 6.4B and under practical conditions may take from 15 to 20 min. The cooling period accomplishes THEORY AND PRINCIPLES OF DRYING 125 WwW a 2) Bw VAPOR a OF S a a > DRYING PRESSURE HEATED AIR RADIUS OF KERNEL (A) INITIAL CONDITION (B) BEGINNING OF DRYING (C)PART WAY THROUGH FIG. 6.4. VAPOR PRESSURE (D) END OF ORYING DURING STAGES OF DRYING some drying and it might prove economical to stop heating the air before the product reaches the moisture content desired and thus remove moisture by cooling. About 3 to 4% moisture can be removed by the heat in the product at 22% moisture heated to 54.4°C (130°F). Moisture removal in some products has been done more economically by running the heater intermittently during heated air drying, and by intermittent operation of the fan for unheated air drying. Best results in the amount of water removed per unit of drying time were obtained when drying in a series of short drying runs of 3 to 6 min alternately with relatively long periods of rest of 1 to 3 hr (Fig. 6.5). CONTINUOUS cS) oO DRYING % WATER CONTENT HOURS DRYING TIME (EXCLUDING “RESTING” PERIODS) From Oxley (1948) FIG. 6.5. EFFECT OF INTERMITTENT DRYING 126 DRYING AND STORAGE OF AGRICULTURAL CROPS Thin Layer Drying Thin layer drying refers to the drying of grain which is entirely exposed to the air moving through the product. The equation representing movement of moisture during the falling rate period of drying is based on Newton’s equation. Newton’s equation refers to the heating or cooling of solids and is stated as follows: The rate of change in temperature of a body surrounded by a medium at constant temperature is proportional to the difference in temperature between the body and the surrounding medium when the temperature difference is small, equation (6-2) dt a = -k(t-t,) (6-2) where k is the heating or cooling constant and t is the temperature at any time 6 and t, is the external or outside temperature, °F. Equation (6-2) is a differential equation because it expresses a relation between a function and its derivative. By separating the variables and putting those containing temperature on the left-hand side and those containing the time on the right-hand side, equation (6-3) is obtained. dt Sehga l Equation (6-4) is obtained gh cae (6-3) by integrating, obtaining an exponential equation of the form shown in equation (6-5). =e (6-4) y = A,3* (6-5) An exponential equation will plot as a straight line on semilogarithmic paper, either a parabola or hyperbola resulting according to equation (6-6). y = Ax® : (6-6) A parabola results if the exponent B is positive and a hyperbola if the exponent B is negative. In Newton’s equation the k-value is represented by the slope of the line plotted on semilogarithmic paper. The slope of the line, k, on semilogarithmic paper is determined from 2 = (0.4343) (k) (f) Xi where f is the scale factor as shown in Fig. 6.6. The scale factor is the number which is represented from the origin on the x-axis equal to the width of one cycle on the y-axis, or f = 6.35 cm (2.5 in.) in Fig. 6.7. The THEORY AND PRINCIPLES OF DRYING 127 Y= Ae kx SO = (0.4343)(kUF) f ' 20 YN oP @Owo°d a FIG. 6.6. SLOPE, K, FROM SEMILOGARITHM PLOT values of x and y can be obtained on any length scale, i.e., millimeters, centimeters, meters, inches, feet, etc., because the distances are expressed as a ratio. By substituting moisture contents, dry basis, for the temperature in equation (6-4), equation (6-7) is obtained. oo =e (6-7) where M is the moisture contents, dry basis, at any time in hours, #, M. is the equilibrium moisture content; M, is the original moisture content, and k is the drying constant. RECN. is known as the moisture content M aie M. a * if ratio. Different k-values would be obtained for the various rate periods they were discernible. The k-value is proportional to Q,, c, and p;. The relationship expressed in equation (6-7) approximately represents the data is drying relationships. Another method of representing the drying 128 DRYING AND STORAGE OF CROPS AGRICULTURAL {Se SS SSS Set 1 i if ifiseeeae Set 4b 2 i 3 G, Time, FIG. 6.7. DETERMINATION hr. OF DRYING CONSTANT given in equation (6-8), where mw is an experimental constant value less than 1. ve = eK" (6-8) The p-value for shelled corn varies with the relative humidity of the drying air and values of 0.60, 0.65, and 0.83 were obtained experimentally at relative humidities of 35, 50, and 70%, respectively (Page 1949). THEORY AND PRINCIPLES OF DRYING 129 The original moisture content of the grain and the temperature of the drying air had no effect on the form of the equation or on the values of the constants, k and yw, but did affect the time unit in that a decrease in either the original moisture content or in the temperature of the drying air increased the time unit. The time of response of drying may be compared to the time of radioactive decay. The rate of radioactive disintegration is proportional to the number of nuclei present. At a time when just one-half of the original material still remains, the time elapsed in hours from the beginning is known as the time of one-half life or one-half response. Likewise, the time for one-fourth life or one-fourth response occurs when one-fourth of the original material still remains. Note that the time for one-fourth response is longer than the time for one-half response. A more descriptive term is “time of retention.”! The same movement. terminology: may be applied to heat transfer or moisture In these cases the difference between the initial and final equilibrium values represents the original material available for change. Thus, the time of one-half response in a drying process would be the number of hours necessary to obtain a moisture content ratio of one-half. This is obtained from equation (6-7) as shown in equation (6-9). TABLE 6.1. DRYING RATE CONSTANTS, Temperature °F £6 Relative Humidity, % 4.4 40 14 4.4 40 60 15.6 60 76 K, 1/HR k, 1/hr 50 ; ce ae eet Yucam gene Hae2 LS) Source: Rodriguez-Arias (1956). Suggested by F.H. Buelow, Univ. of Wisconsin. Range of Applicability, % Moisture, d.b. 0.075 0.0315 0.016 0.0072 0.0622 0.0325 0.0231 29.96 24.79 20.50 16.25 27.01 25.2 20.2 to 24.79 to 20.50 to 16.25 to — to 25.2 to 20.2 to18.0 0.0149 0.0583 0.0396 0.0291 0.1047 18.0 DD) 25.2 20D OO to ReM Akos PAay to 20.5 eto 8 & LOMO: 0.0424 0.0143 18.00 to 13.00 13.00 inFe 4 to 19% 0.338 Ad.) 0.0503 0.0184 0.271 Le Sie COMM (ou tO ve 43.6 tol13. 0.074 0.052 10.4 to 9.70 9.70 to — 0.149 0.139 19.0 to11.5 13.00 to 10.4 130 DRYING AND STORAGE OF AGRICULTURAL CROPS log.” ( Gis k — iO aan wee F Vp (6-9a) = Yy (6-9b) A very complete analysis of the mechanism of drying exposed kernels of wheat has been published (Simmonds et al. 1953A,B). These results can be considered as some of the most accurate of work reported on drying theory based on diffusion in wheat inasmuch as each of the drying curves was constructed from no fewer than 30 experimental readings and drying was continued to a point close to equilibrium moisture content of the samples. The rate of drying was independent of the air velocity for a range of 0.15 to 0.8 m/s (30 to 160 ft/min) for wheat. For wheat, there was no constant rate drying period and all of the drying of the grain which was previously tempered to 66%, dry basis (39.5%, wet basis) took place in the falling rate period. Straight lines were obtained when the free moisture (M — M.) was plotted semilogarithmically against the time of drying from 50 to 0% moisture with only one falling rate curve. Upon investigating the influence of air humidity and temperature upon the rate of drying it was found that a three- to four-fold increase in humidity was roughly equivalent to a drop in temperature of 28°C (50°F). Figure 6.8 shows the effect of temperature on the rate of drying for thin layers. There is a lag in grain temperature of 5.4° to 18°C (3° to 10°F) as the 50 40 AIR TEMPERATURE 90°F i ° Air BASIS DRY MOISTURE, PERCENT velocity Inlet O.16m/s humidity 0.5 DRYING (32 ft/min) 0.007 kg/kg at 32°C(90°F) to 65°C(150°F) Lo 1.5 TIME, HOURS 2.0 25 From Simmonds Fiber EFFECT OF TEMPERATURE 3.0 et al. (1953A) ON THE RATE OF DRYING EXPOSED THEORY AND PRINCIPLES OF DRYING 131 moisture is reduced from 50 to 5% in 1 hr with an air velocity of 0.53 m/s (105 ft/min) (Becker and Sallans 1955; Simmonds et al. 1953A). The rate of drying is equal to the vapor pressure driving force divided by the resistance to drying, as shown in equation (6-10). dM = = (p, = Da) Se: eer ewes he ky am (pg — Pa) (6-10) where M is the mean moisture content, kg water/kg dry grain (lb water/lb dry grain); k, is the mass transfer coefficient, kg water/s m? (lb water/hr ft?); a, is the effective area, m? (ft?); p, is the vapor pressure of the grain, Pa (in. Hg); and pa the vapor pressure of the air, Pa (in. Hg). The driving force of moisture movement is (p, — p,) and the resistance to 1 drying is a The values of the mass transfer coefficients k, a, are gfm shown in Fig. 6.9 and 6.10 in English units. KEY RANGE OF RESULTS—RUNS COVER HIGH AND LOW HUMIDITIES AND AIR TEMPERATURES OF 70° TO 170°F. EXTREME POINTS ARE DISTINGUISHED AS LX A O @ g%m K Ol 02 FREE 03 0405 MOISTURE 06 07 Low High Low Low humidity humidityhumidity humidity - 08 CONTENT LB./LB. From Simmonds FIG. 6.9. CORRELATION 21°C (70°F) 65°C (149°F) 77°C (170°F) 32°C(90°F) OF DRYING et al. (1953B) RUNS The diffusion of moisture through a kernel of grain is important in the theory of drying. In Chapter 3 the diffusion of air and moisture between the kernels of grain is discussed. A diffusion equation is presented as well as the procedure for determining the diffusion coefficient. In applying this method to moisture contents below 10.3%, it was found that the calculated diffusion coefficient at 44.4°C (112°F) was 0.023 X 10° cm2/sec, and at 80°C (176°F) it was 0.254 X 10 “6 cm?/sec for a sample of 9.65% initial moisture content. The corresponding coefficients in the 132 DRYING OF STORAGE AND T 100 AGRICULTURAL ME =a(ea) (Toa fe —T T CROPS 3 os 02 MOISTURE, db FREE AVERAGE (M-M,) = 5 2 10% Kg Om, FIG. 6.10. 2 10% MASS MASS TRANSFER TRANSFER 10 5 2 a 5, COEFFICIENT COEFFICIENT range above 10.3% moisture were 0.565 X 10 ° cm2/sec and 3.1 X 10 6 cm2/sec, respectively. These data were obtained for wheat, assuming spherical shapes changing from 9.7 to 5.2% moisture. Thus, in passing from lower to higher moisture levels, diffusion coefficients increased by 10 to 25 times in the temperature range from 44.4°C (112°F to 176°F) (Becker and Sallans 1955). Deep Layer Drying When using heated air, the crop is dried to a low moisture level and the air enters at the time the grain on the discharge side is at a moisture content safe for storage. The greater the depth the greater is the overdrying for a particular airflow. Therefore, for heated air drying at temperatures above 43.3°C (110°F), it is usually recommended that the thickness of the layer be 0.45 m (1.5 ft) or less. The majority of the drying takes place in a volume called a drying zone which moves through the grain in the direction of air movement (Fig. 6.11). The drying of grain in a deep layer can be thought of as several thin layers in which the humidity and temperature of the air entering and leaving each layer vary with time depending upon the stage of drying. This procedure is sometimes used for estimating the time of drying and the amount of moisture removed. Additional moisture will be removed from the dry layer until the equilibrium moisture content is reached. Little moisture is removed and some may be added to the wet zone until the drying zone reaches it. The THEORY AND PRINCIPLES OF DRYING DRYING DRYING 133 FRONT ZONE DRY pate et FIG. 6.11. DEEP LAYER DRYING AIR volume of the drying zone has not been defined by equation, but varies with the temperature and humidity of entering air, the moisture content of the grain, and the velocity of air movement. The higher the airflow the less the difference in moisture content between entering and leaving air. Drying will cease as soon as the product comes to equilibrium with the air. In thin layer drying, which was discussed previously, the air velocity is not critical as for deep layer drying. For shelled corn at 25% moisture, d.b., and an air velocity of 0.2 m/s (40 ft/min) the drying front moved at a rate of 2.5 cm (1.0 in.) per hr and 70% of the free moisture at the initial moisture content was removed. The movement of the drying layer is proportional to the velocity ratio. Two drying rate periods may be considered for deep layer drying. The maximum rate drying period occurs until the drying front reaches the top of the bed and is represented by equation (6-11). The rate of drying during this period depends solely on the moisture carrying capacity of the drying air. Mi - M. _ dM, _ AG(H, - Hi) 6; whence, dé W (M, a M,) ee Bae 6; a ietenn AG (HL — H,) W (6-11a) (6-11b) DRYING 134. AND OF STORAGE AGRICULTURAL CROPS of drying As soon as the drying front reaches the top of the bin, the rate drying layer of rate ing starts to decrease and is designated as the decreas as given by equation (6-12). dM. rT) 6, whence, 2 = IlII S555 * = 2,303 m'(M, — M.) = (6-12a) logio (6-12b) 6 = 6,+86, (6-12c) = — average moisture content in bed, d.b. initial moisture content in bed, d.b. equilibrium moisture content of bed, d.b. final moisture content of bed, d.b. average moisture content of bed at end of maximum drying rate, d.b. cross sectional area of dryer, m? (ft?) mass rate of airflow, kg of dry air/m? s (lb/ft? hr) £8 ZorHonweight = ll of grain in bin, kg (Ib) dry matter humidity of saturated air in the grain bin, kg water/kg dry air (1b/Ib) “a. | humidity of air entering dryer, kg water/kg dry air (Ib/lb) rate of drying constant for deep bed, 1/hr (2.3 m’ =k) lI time of drying during maximum rate’period, hr time of drying during decreasing rate period, hr total time for drying ~ Sas Ss > | The drying time for the product in a deep layer is the sum of the time required for the maximum decreasing rate of drying. rate of drying and the time required for the The amount of drying is indicated by the temperature of the exhaust air which is cooled upon giving up its heat to evaporate moisture from the product. However, the wet bulb temperature remains fairly constant throughout the drying process. Another method for determining the rate of drying of deep layers is presented. The computed relationships among drying time, grain moisture, and grain depth units are shown in Fig. 6.12 and are generalized to apply to any drying problem. The time unit is the time of one-half response of grain fully exposed to drying air. THEORY 28.2- 100 26.354 90 2444 80 22.54 os AND PRINCIPLES OF DRYING 135 70 cies 2064 = '=60 Ww a o D x > 1871 ° 16885 a 5 i491 & 50 = gE = 0 Ww je) ae 13.04 z a q oO 40 3 = 20 nt 9.2 O TIME T T O 35 T = 70 105 UNITS T + + + 140 178 210 245 < 280 + 315 350 HOURS From Hukill (1947) FIG. 6.12. COMPUTED RELATION MOISTURE, AND GRAIN DEPTH AMONG DRYING TIME, GRAIN Application of Hukill’s Analysis to Drying The computed relationships among drying time, grain moisture, and grain depth units have been generalized to make them applicable to any drying problem (Fig. 6.12) (Hukill 1947). (a) Drying Time.—Each time unit is the time of one-half response of grain fully exposed to the air entering the drying system. (b) Grain Moisture.— Moisture is expressed as moisture content ratio, M - M. M, —- M. (c) Grain Depth.—A depth factor is described as “containing enough grain that if all the theoretical available heat could be used it would all dry to equilibrium” in a period of time equal to the time of one-half response. The kilograms (pounds) of grain (G) in each depth are computed by use of the following heat balance: DRYING 136 AND AGRICULTURAL OF STORAGE dQ 40sa c At AM = CROPS dw rT, he, ——- (6-13) in which, ae rae volume of air, kg/s (lb/hr) = specific heat of air at constant pressure, J/kg °K (Btu/Ib °F) maximum temperature drop of the air in passing through the At = AM grain, ‘C (°F) = M, - M.,%, d.b. “ = dé moisture removed from volume under consideration, kg/s (b/hr) hr, = latent heat of drying, J/kg (Btu/lb) of water dw then, G = “de x 6 Y% (6-14) With appropriate data the following information can be computed for a drying problem at any given time after drying has started: (1) Moisture content at any depth (2) Pounds of moisture removed (3) Average rate of drying (4) Temperature and relative humidity of the exhaust air (5) Thermal efficiency of drying. Sample Problem (U.S. Customary Units),—A circular metal bin 18 ft in diameter is filled to a depth of 6 ft with shelled corn at a moisture content of 22%, w.b. It is to be dried with air at 90°F, relative humidity 30%. Assume time of one-half response of 3.5 hr. Air is to be supplied at a rate of 15 cfm/bu. Determine the 5 items listed above after 31.5 hr of drying. Solution: There are 1512 ft? or 1210 bu of corn. Considering 47 lb dry matter/bu, the crib contains 56,870 lb of dry matter. Air at 90°F, RH = 30%, occupies 14.1 ft?/lb, therefore, dQ 15ssX a 60 X 1210 fice, b Ah 77,235 lb/hr of air M, = 22%, w.b. = 28.2%, d.b. M. 8.39%, w.b. = 9.2%, d.b. 2See Appendix for metric conversions. THEORY AND PRINCIPLES OF DRYING 137 Shelled corn at 22% moisture, w.b., is in equilibrium with air at 96% relative humidity. Therefore, the drying air will cool to 68°F (from the psychrometric chart at the point where the initial wet bulb temperature of the air intersects the relative humidity line of 96%) (1) To Compute G.— dQ a “a9 “At dw “do _ = dw AM—q hte 77.235 X 0.24 X (90-68) (282 — 0.092) X 1170. G = 1836 ~ 1886 lb/hr X 3.5 = 6416 lb Knowing G, the number of depth factors can be computed: 56,870 6416 6 + 8.85 8.85 depth factors = 0.68 ft in each factor The ordinate on the graph may be changed to agree with the moisture content of the corn. A moisture ratio of 100 is equivalent to 28.2%, a ratio of 0 is equivalent to 9.2%, a ratio of 50 is 18.7%, etc. The abscissa may also be changed to agree with the actual hours by letting each unit represent 3.5 hr. The ninth unit represents 31.5 hr. The items sought may now be determined. The moisture content at any depth can be read directly from the graph. For example, if the moisture content at 4 ft from the bottom is desired, 4 rs. = 5.9 depth factors and from the graph the moisture ratio is 8%, which is equivalent to 10.8%. (2) Pounds of Moisture Removed.—The moisture content of each depth factor may be read as indicated above. These readings multiplied by the pounds of dry matter in each factor (6416 lb except in the top layer) will give the pounds of water remaining in the grain. Subtracting this from the original water content will give the moisture removed. 138 DRYING AND Depth Factor OF STORAGE CROPS AGRICULTURAL Moisture Content xX Dry Matter Water, lb 1 0.092 X 6416 = 590.3 2 3 0.092 X 6416 0.096 X 6416 = = 590.3 615.9 4 5 0.100 X 6416 0.101 X 6416 = = 641.6 641.6 6 0.109 X 6416 = a 8 8% 0.128 x 6416 0.155 X 6416 0.178 X 4812 = = = 699.3 821.3 944.5 856.5 6451.3 lb water in corn 56,870 X 28.2% = 16,037 lb water originally in grain 16,037 — 6451 = 9586 lb water removed Average moisturecontent = = 11.3% db. (3) Average Rate of Drying.—The slope of the drying curves at a particular time represents the rate of drying (%/hr) for each depth factor. The average of these slopes will be the rate of drying for the entire crib at that time. Depth Factor ~ %/hr * 0.00 0.00 0.05 0.08 0.13 0.50 0.69 0.84 TI BMH Fe ConA FrwWN 4 0.87 Total = 3.16 dM — 7 Atom X 0.0035 cox At dM pe oes "ia X 56,870 xX 1170 11,235 'X 0.24 bared = 12.55°F THEORY AND PRINCIPLES OF DRYING 139 (4) Temperature and Relative Humidity of Exhaust Air.— 90 - 12.55 = 7.45°F (exhaust temperature) The relative humidity may be read at the point where the initial wet bulb temperature intersects a dry bulb reading of 77.45°F. The relative humidity of exhaust air is 58%. (5) Thermal Efficiency of the Drying System.—The pounds of water evaporated from the corn is known. The total pounds of air used may be computed, as well as the enthalpy of the air above the dew point. Heat utilized Heat available Hay Drying Theory The overall drying rates were determined for baled hay in test chambers and correlated with the drying equation (6-15). dM et dé lb dry air/hr ee 3 lb dry matter i (nn. (P— p.) 6-15 \ The constant, k’, was found to be 1.45 for alfalfa, with p and p, in in. of mercury, and saein % moisture/hr, for variations of airflow from 6.8 to 16.2 lb/(hr) (Ib dry matter), temperatures of 100° and 120°F, and original moisture contents from 29.7 to 48.1%. The value of p will change as drying proceeds. Based on the above results with the drying rate directly proportional to the weight of dry air passing through the hay per unit of time and inversely proportional to the dry matter present, equation (6-16) can be written. aes Sea bGs) (MIM) (6-16) and by substituting in equation (6-16), equation (6-17) is obtained. M,-M. M, i =e (a- bG (6-17) M. where b is a constant, which depends upon the vapor pressure and temperature of the drying air, and Gp is the mass velocity, lb of dry air/Ib 140 DRYING AND STORAGE AGRICULTURAL OF CROPS of dry matter. This relationship exists because there is a linear relationship between k’ and Gp, from expression k’ = a + bG2 (Hopkins 1955). Data are presented for these relationships (Fig. 6.13). Drying curves for crushed and uncrushed hay are presented in Fig. 6.14 and 6.15. 0.07 = 0.06 — 0.05 0.04 0.03 002 [oXo}} ie) 4 8 12 16 20 Go, kg of air /hr-kg dry matter “From Hopkins (1955) FIG. 6.13. RELATION OF DRYING CONSTANT TO MASS AIR VELOCITY FOR HAY Psychrometric Chart for Analyzing Drying The psychrometric chart (humidity chart) can be used as a basis fo analyzing the drying of a product by studying the external factors. Th psychrometric chart provides the thermodynamic properties of the ai and vapor. By knowing any two of the three temperatures—dry bulk wet bulb, dew point—the physical properties can be described. Definitions.— Dry Bulb Temperature.—The temperature of the air or product indicated by a thermometer which is not affected by water vapor content of the air. THEORY AND PRINCIPLES OF DRYING 141 SM PERCENT PER MINUTE 5 TEMP RH. ie) 10 20 MOISTURE 30 CONTENT (DRY = 123°F = 28% 40 50 60 BASIS) — PERCENT From Hopkins (1955) FIG. 6.14. RELATION MOISTURE CONTENT OF DRYING RATE OF ALFALFA TO Wet Bulb Temperature.—The temperature given by a thermometer with its sensing bulb covered with a thin layer of water and moving through the air until a steady temperature during evaporation is obtained. The difference between the dry bulb and wet bulb temperature is called the wet bulb depression. Dew Point Temperature—The temperature at which condensation of water vapor begins if a mixture of air and water vapor is cooled. Humidity (Absolute Humidity, Specific Humidity)—Weight of water vapor, usually in pounds or grains per pound of dry air (7000 grains = 1 Ib), or kg per kg of dry air. Relative Humidity.Ratio of actual partial pressure of the water vapor to the pressure at saturation at the dry bulb temperature, usually expressed as a percentage. 142 DRYING AND STORAGE OF 1.0 fea ay18 lL O7 Eas AGRICULTURAL CROPS SLTe Ee a ea 2A Ed TEMP = 123°F R.H. = 28 % le) 20 40 ELAPSED 60 80 100 120 TIME — MINUTES From Hopkins (1955) FIG. 6.15. RELATION OF MOISTURE CONTENT RATIO TO TIME IN THIN LAYER DRYING OF ALFALFA HAY Humid Heat.—The specific heat of the air with water vapor it contains. It is helpful for understanding drying to note the following relationships from the psychrometric chart: (1) The dry bulb, wet bulb, and dew point temperatures are equal when the relative humidity is 100%. (2) The dew point < wet bulb < dry bulb temperature when the relative humidity is less than 100%. (3) The rate at which heat is transferred from the air to the water is proportional to the wet bulb depression. (4) All values are based on a barometric pressure of 1 atmosphere. (5) The water vapor pressure nearly doubles for each 11°C (20°F) increase in temperature. (6) The density of air saturated with vapor is less than the density of dry air at a given temperature. (7) The difference between the dew point and dry bulb temper ature is nearly constant for a given relative humidity. (8) The latent heat of vaporization increases as the temper ature of evaporation decreases. (9) The dew point of a given air condition is the same regardl ess of amount of heating of the air. THEORY AND PRINCIPLES OF DRYING 143 Heat is supplied for vaporization of water during drying with unheated or heated forced air. As the air moves by the product, heat is transferred to the product and vaporization of water occurs, increasing the relative humidity of the air. This process is known as simultaneous heat and mass transfer. The dry bulb temperature of the air decreases during drying, approaching the dew point, with the wet bulb remaining constant. If the air reaches the dew point, condensation will occur. Condensation may occur on cold grain or hay on the air discharge side of a drying bin. For drying systems where heat is supplied by the air an adiabatic relationship exists—where the total heat change is considered as zero for a system. Note the changes as air at 21.1°C (70°F) and 50% relative humidity is heated to 66°C (150°F) (Fig. 6.16). The changes during heating occur on a horizontal line on the psychrometric chart, at constant vapor pressure, and constant absolute humidity. During drying, the changes occur on the wet bulb lines, the heat moves from the air to the water (only a slight amount is used for heating the dry matter of grain), and vaporization takes place. The air originally contains 0.008 lb water vapor/lb of dry air, and finally (after heating to 150°F) holds 0.023 lb water vapor/Ib of dry air. If the air were not heated, it would hold 0.011 Ib water vapor/lb of dry air. In the temperature range for crop drying, the moisture holding capacity of 1 lb of air is increased 0.0002 lb/"F increase in temperature. The minimum volume of air required to hold the moisture can be calculated which assumes that air is discharged from the dryer at 100% relative humidity. FIG. 6.16. PSYCHROMETRIC CHART RELATIONSHIPS FOR COOLING, HEATING, AND CHEMICAL DEHYDRATION 144 DRYING AND STORAGE OF CROPS AGRICULTURAL Ib H,O Hh alt = ap eal) 3 a] Ss 6-18) lb H,O/min cm wr HG yeaa Ele SEOs ey ibaeag (6-19) pH In practice the amount of air is usually 2 to 5 times the calculated : dW minimum values. The rate of evaporation, —, dé —— where : lb/hr, is: = (a+ by) (Ap) (6-20) aisaconstant representing evaporation in still air b is a constant representing evaporation in moving air v is velocity of drying air, usually ft/min Ap is the difference in vapor pressure in product and in air, psi, or mm Hg Note the similarity between equation (6-20) and equation (6-16). The maximum rate at which moisture can be removed from a hygroscopic material is controlled by the diffusion of moisture within the product to the surface and the capacity of air on the exterior to hold the moisture. An excess of air is supplied for drying so that the controlling factor is the internal diffusion of moisture. The drying effect of air is related to the ratio of the moisture content of the air to the saturation content at the same temperature. Thus, as the temperature is increased, the drying effect is increased and the relative humidity is decreased, but the absolute humidity remains constant. The difference between the dew point and dry bulb temperature for air can be called degrees of superheat. The greater the degrees of superheat, the greater the drying effect. The heat utilization factor is defined as the ratio of temperature decrease due to cooling of the air during drying (evaporative cooling) to the temperature increase due to heating the air. THEORY Heat Utilization Factor AND PRINCIPLES OF DRYING 145 = air temperature (dry bulb) decrease during drying air temperature (dry bulb) increase during heating Bo = ee Gis Ge ee (6-21) where t; is the original dry bulb air temperature, t» is the temperature of air after heating, and tz is the dry bulb temperature, °F, of air exhausted from dryer (Fig. 6.16). The heat utilization factor is usually less than 100% but can exceed 100%. QUESTIONS 1. Wheat at 25% moisture, w.b., is heated to 65.6°C (150°F) and cooled to 21.1°C (70°F). Assume that all the heat is used for removal of moisture. What is the final moisture content, w.b., of the product? 2. Calculate the rate of drying of wheat, given the following information: Moisture content of grain, 25%, w.b. Drying air temperature of 49°C (120°F), 10% relative humidity Airflow of 0.4 m/s (80 ft/min) 3. Using the data given in the sample problem for Hukill’s analysis, determine the time of drying, 6, equation (6-12c). 4. Determine the drying rate values, k, for drying thin layer crushed hay with air of 50.6°C (123°F), 28% relative humidity. Repeat for uncrushed hay. 5. A product with an original moisture content of 20%, w.b., has an equilibrium humidity of 87%. Drying air is heated to 65.6°C (150°F), so that equilibrium moisture content is 5%, d.b. At the end of 1 hr the moisture content is 15.3% w.b. a. Determine value of the drying constant for the preceding data using equation (6-8), with w = 1 and pw = 0.60. b. What is the rate of drying at the end of 30 min for both cases? 6. An iron rod of infinite length is at 538°C (1000°F) at time zero. The rod is exposed to air at a constant temperature of 93.3°C (200°F). The temperature of the rod at any time is defined by Newton’s equation, with a cooling constant of 0.3 with the time in hours. a. Plot the temperature vs. time curve for the first 7 hr of cooling. b. Plot the rate of change of temperature vs. temperature differences. c. Write the equation for the curve in part b. 146 STORAGE AND DRYING OF ‘isd ~©oe NANA 9 NNNNN CROPS AGRICULTURAL ‘eunsseud sodpDA CHR mROHMTEMN-DAQRROMEMA- A NW SS He Sf SHS Ses Habe lft iV | ekTTT PA a AYALA CCN TS CH HRA SEITE Pes Pal TA PN le Sa ERENT He ME B meee ae 1gAL salad A esWa BA ce Ue<< a = iL) Pias, L/ LV — as ww 4 be OD LYS . ra Oana | ~a 4. FDry degtemperature, bulb 0.31 0.30 0.29 0.28 0.27 0.26 0.25 0.24 / (Btu) (°F) (Ib. air) Specific dry Heat De) GOH ee Ae COP” Oe AD. ae SE = 410 LGN a TO Foe ned ba lanes eas Aap Q\/ 4835 09 CHART. HUMIDITY OR PSYCHRO 6.17. FIG. units. Custom A—U.S. = s o8'0 OL _ \\ St Aug ging 0z Sz 9, eumexadwia) 580 \ OE SA GE TV | * Ov \ Sb ois 060 ho TAS o Pe a) ln] | il! LY i f sl 1 Tle ta N = io q { t t . in ical\ t a teeta Nia Td 0s | | 4 00°0 ss 0603 W004 s604© o10 ates ot 201984 Yea} aqusu = “sun |S—@ se0 : $y d 22] 104 aay saury voNBAdG Adjeyug pue saedosy 9.9 Mojag 2 e ’ ; 0 Baas + IPSS - ; \ Se Pk BOS c ; <S Cae ; oe es o1- ! te t | © a- & % ay Aig By/_us auinjon OE ast iz cain tt mS aa 5 AR - 1/9 ‘2 = es 5 > = as ¥$ | + oaWOIegeunsseig SZELOL ed t — to S SS = % O1dLaW SLINO ( al = ky 13431 vas IS ~Z fae +6 18 AKI el | i n Sioa | — i r SAYNLVYEAdWALq a scala: t t } tH} : | VV + am Ly + Aan a TVINYON eis NYE Tey aE > = VAUAP % GL61 Uones0dso3 4011129 rybUUAdog ; mal : t RE SKA wi : aS t+ HP Se ii 0z1 % "dI0D salsseg yo AsayinopD 100'0 z00'0 : 7 = LSA s|= AZ | = di SE Se 90°04 400'0 8000 600'0 : a fs | Sz} 2> a+ = > z10'0 £100 ; TTT ov & eS oe % ++ g10'0 en ; T s100 Say sy % | ny Aon 4 ES re ‘aa + Pf Set a Se e a a Fas PRT mie) sth % v1.00 $905 ovo" + NS 4 zoos 0s 9 $0, £000 14s Ov % yf oe 3 4 sit wer 507 i200 = ai e2004 6z0'04 ze0'0 1£0°0 0f0'0 ovo) = oso] 8200 sv Sot %, 500'0 p00" Pe0'OG Sy 80 a 9 Ob Jy Arg 84/84 uaquog aimsiow 580 9€°0 147 DRYING OF PRINCIPLES AND THEORY Sd “LYVHOD ‘Sls “219 OIWLAINOYWHOA 148 DRYING AND STORAGE OF AGRICULTURAL CROPS fe A very thin layer of shelled yellow dent corn with a moisture content of 25%, w.b., is to be dried with air at 25°C (77°F) and 45% relative humidity during drying to follow the law based on Newton’s equation, with a drying constant of 0.07. a. Plot moisture content of corn, %, d.b., vs. time for all values of 6 from 0 to 60 hr. \ b. Plot rate of drying against vapor pressure differences. Write the equation of curve obtained in part b. io) d. How long would be required to dry a sample of corn at 25%, w.b., to 15%, w.b., if dried under these conditions? A thin layer of seeds with a moisture content of 22%, w.b., is to be dried with air at 29.4°C (85°F) and 50% relative humidity. The k value is 0.10, when the time is in hours. What is the time in hours for the seeds to be dried halfway to equilibrium moisture? The following data were obtained for drying a product with air at 93.3°C (200°F). The equilibrium moisture content is 1.8%, w.b., and the original moisture content was 19.8%, w.b. Time, min Weight, g 0 13 29 45 62 75 158.0 150.4 145.5 142.1 139.5 138.2 Determine k by graphical method. What is the rate of drying at 15, 30, 45, 60 min? Predict the time in minutes when the sample will be at equilibrium moisture content. 10. Calculate the factors for converting vapor pressure from inches of 11% mercury to inches of water, from lb/in.2 to mm of mercury. Explain why the lines of adiabatic humidification (or dehumidifica- tion) on the psychrometric chart are not exactly lines of constant enthalpy even though shown as such. 12 Illustrate an example of drying where the heat utilization factor is over 100%. £3. Compare the effectiveness of using chemicals for dehydration of air as compared with heating air for drying. THEORY AND PRINCIPLES OF DRYING 149 REFERENCES BAKKER-ARKEMA, grain drying. F.W., BROOK, R.C. and LEREW, L.E. 1977. Cereal Jn Advances in Cereal Science and Technology, Vol. 2. Y. Pom- eranz (Editor). Am. Assoc. of Cereal Science, Minneapolis. BARRE, H.J. 1938. Vapor pressures in studying moisture transfer problems. Agric..Eng. 19 (6) 247-249. BECKER, H.A. and SALLANS, H.R. 1955. A study of internal moisture movement in the drying of the wheat kernel. Cereal Chem. 32 (5) 212-226. BROOKER, D.B., BAKKER-ARKEMA, F.W. and HALL, C.W. 1974. Drying Cereal Grains. AVI Publishing Co., Westport, Conn. FENTON, F.C. 1941. Storage of grain sorghum. Agric. Eng. 22 (5) 185-188. FOSTER, G.H. 1950. Methods of conditioning shelled corn. Agric. Eng. 31 (10) 497-502. GALLAHER, G.L. 1949. Fundamentals of drying agricultural crops. Unpublished M.S. Thesis. Dep. Agric. Eng., Mich. State Univ. HALL, C.W. and HEDRICK, T.I. 1971. Drying of Milk and Milk Products, 2nd Edition. AVI Publishing Co., Westport, Conn. HENDERSON, S.M. and PERRY, R.L. ing, John Wiley & Sons, New York. 1955. Agricultural Process Engineer- HOPKINS, R.B. 1955. Some effects of chemical and mechanical treatments in haymaking. Unpublished Ph.D. Dissertation. Dep. Agric. Eng., Mich. State Univ. HOUGEN, O.A., McCAULEY, H.J. and MARSHALL, W.R., JR. 1940. Limitations of diffusion equations in drying. Trans. Am. Inst. Chem. Eng. 36, 183-209. HUKILL, W.V. 1947. Basic principles in drying corn and grain sorghum. Agric. Eng. 28 (8) 335-338, 340. (Also Jn Anderson, J.A. and Alcock, A.W. 1954. Storage of Cereal Grains. Am. Assoc. of Cereal Chemists, St. Paul, Minn.) LYKOV, A.V. 1968. Theory of Drying. State Power Press, Moscow. (Russian) NONHEBEL, G. and MOSS, A.A.H. 1971. Drying of Solids in the Chemical Industry. OXLEY, T.A. Butterworth 1948. and Co., London. The Scientific Principles of Grain Storage. Northern Pub- lishing Co., Liverpool. PAGE, G.E. 1949. Factors influencing the maximum rates of air drying corn in thin layers. Unpublished M.S. Thesis. Dep. Mech. Eng., Purdue Univ. RODRIGUEZ-ARIAS, J.H. 1956. Desorption isotherms and drying rates of shelled corn in the temperature range of 40° to 140°F. Ph.D. Dissertation. Mich. State Univ. SIMMONDS, W.H.C., WARD, G.T. and McEWEN, E. 1953A. The drying of wheat grain. Part I. The mechanism of drying. Trans. Inst. Chem. Eng. 32; 265-278. 150 DRYING AND STORAGE OF AGRICULTURAL CROPS SIMMONDS, W.H.C., WARD, G.T. and MCEWEN, E. 1953B. The drying of wheat grain. Part II. Through-drying of deep beds. Trans. Inst. Chem. Eng. aL, 279-288. VAN ARSDEL, W.B. 1947. Approximate diffusion calculations for the falling rate phase of drying. Trans. Am. Inst. Chem. Eng. 43 (1) 138-24. VAN ARSDEL, W.B., COPLEY, M.J. and MORGAN, A.I. 1973. Food Dehy- dration, 2nd Edition, Vol. 1 and 2. AVI Publishing Co., Westport, Conn. WILLIAMS-GARDNER, A. 1971. Industrial Drying. Leonard Hill, London. Heated Air Dryers A complete drying system, including fan, heater, ducts, and bin, is called a heated air dryer. It is not uncommon to designate the burner or heater part of the system as the heated air dryer. However, the dryer isa system of which the heater is a part. The applications of heated air dryers for various crops are discussed in other chapters. Based on a 1973 estimate, 3804 X 103 m? (1005 X 108 gal.) of liquid propane gas (LPG) equivalent (92,000 Btu/gal.) were used for farm crop drying in the United States, of which over half was used for drying corn (Cast 1975). Operation of Heated Air Dryers The factors to be considered for heated air drying include: (1) The cost of fuel and power for drying grain and hay is about the same for heated air as for forced air drying depending on weather conditions. During the fall in northern areas the cost of fuel and power for heated air systems is less than the cost of power for forced air drying. The initial cost of the heated air drying system for farm use is two to three times as great as for a forced air system. A greater volume of product should be dried with a heated air sys- (2) tem to decrease the fixed cost of operation. The rate of evaporation of moisture increases as the vapor pressure of the product is increased by heated air. In addition, the heated air is greater than that air drying system with a grain heating. The product is heated with moisture carrying capacity of the of unheated air. The typical heated layer of 18 in. of corn is designed to remove 15% moisture in about 3 hr at 60°C (140°F). An additional Y% to ¥% hr is required for cooling the grain. (3) The rate of airflow per volume of product is greater for heated air than for unheated air. An airflow of at least 1.7 m?/m?s (100 cfm/ ft?) is used for layer Crys, es meer alr. Relehory (ike £AS of As aaa ae a LiBR, BevanC . in ture AG i F * 9 m @ i - < ae % oF aw 152 DRYING AND STORAGE OF AGRICULTURAL CROPS (4) Safety of the product must be considered. The temperature used for drying is largely dependent on the intended use of the product. When grain is used for animal feed, air temperatures as high as 149°C (300°F) have been used. If the product is to be milled, it is usually recommended that the air temperatures be kept below 60°C (140°F). If the product is to be used for seed, air temperatures are kept below 46°C (115°F) so that germination is not reduced. Although recommendations are based on the air temperature, the temperature of the product is of prime importance. Much higher air temperatures than those stated can be used if the product temperature can be kept within limits. (5) Safety of the equipment and buildings is a factor to be considered. The hazards involved in using a heated air drying system are greater than with an unheated system because of the possibility of fire. Adequate controls and safeguards are built into commercial units to minimize the risks of fire if the units are properly used. CLASSIFICATION Classification of Heater According to Method of Heat Transfer Two types of heaters based on method of heat transfer are used: (1) direct and (2) indirect. In a direct heater the products of combustion are forced through the product with the drying air (Fig. 7.1). With the direct-fired unit there is a greater possibility that the product might be damaged because of smoke if improperly operated. However, the directfired unit is less expensive and makes more efficient use of the energy in the fuel. The direction of flow of heated air is often changed as it leaves the burner so that carbon particles will not normally be carried into the drying bin. A screen might be used to intercept carbon particles that might be in the heated air. With the indirect heater the heat transfer surface is heated and the air used for drying the product is circulated around the outside of the heat transfer surface. The products of combustion from the burner are removed from a stack in the unit and do not mix with the drying air (Fig. 7.2). This unit is claimed to be safer because sparks or hot pieces of carbon emitted from the discharge side of the burner cannot pass into the drying air and then into the crops. Classification According to Type of Oil Burne r Two types of burners are used: (1) the vaporizing burner, commonly called the pot type, where the oil is heated, vaporized, burned, and the heat transferred through a large heater surfac e; and (2) the atomizing HEATED AIR DRYERS 153 A me suse ae eoek dre: ~~ - Coes cpamensn!” _ SSeeneesetty’ e LS ' : " i Ss ‘ % Courtesy of Aerovent Fan and Equipment Co. B HEATED AIR ——— oo DAMPER —— COLD AIR BURNER ~=— FIG. 7.1. A—DIRECT B—DIRECT PUEL HEATER, PORTABLE HEATER FOR CROP DRYING (NO HEAT EXCHANGE) burner, where the oil is first atomized either by forcing a large quantity of air to the burner, commonly called the air atomizing type, or by forcing the oil through a small orifice, commonly called the jet, gun, or mechanical pressure atomizing burner. The oil pressure in the air atomizing type is usually 7 to 35 kPa (1 to 5 psi) and in the gun type is usually above 154. DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Sukup Manufacturing Co. B STACK ——> HEATED —_— AIR He SS FUEL Sr. FIG. 7.2. A—FUEL OIL HEAT EXCHANGER B—INDIRECT FOR BIN DRYING HEATER (HEAT EXCHANGER) 517 kPa. After the fuel is atomized there is considerable surface area exposed to the air and heat which provide quick and’ efficient vaporization. The airflow to the air atomizing burner can be varied, thus giving a means of control. The capacity of the high pressure atomizing burner is changed by the size of openings and number of nozzles. Changing the pressure will give a small change in capacity. The atomizing type is used for on-off control. Because of low initial and operating cost and operational safety, the high pressure atomizing burner is the most common type used for oil-fired crop drying systems. HEATED AIR DRYERS 155 Classification According to Fuel Burned Fuel oil, gas (natural or manufactured), or solid fuels, such as coal, wood, and crop residues, may be used in heated air crop dryers. For a fuel oil burner it is recommended that: The burner be of an approved atomizing type. The fuel nozzles be kept free of carbon deposits. The drying air be free of smoke or odors which might affect the quality of the product being dried. Automatic spark. ignition of the burner take place continuously by electric The heating unit be adjustable in capacity to provide the required heat output. The fuel oil be delivered from the tank to the burner by pump action. There be an automatic control interlock with fan control which would shut off fuel supply if the fan stops. A filter or strainer be provided in the fuel line between the tank and the burner. A manual shut-off valve be supplied in the fuel line at the tank end when a metal pipe is used. For a gas-fired burner it is recommended that: The gas burner be approved by the American Gas Association. The gas heater output be adjustable to match required heat. For a coal-fired unit a bin should be available for coal supply to stoker feed the furnace at a rate consistent with the heat required. Available units are of the indirect heater type and have a heat output of approximately 527,500 kJ/hr (500,000 Btu/hr). Both anthracite and bituminous coal burning units are available. Classification of Dryer According to Flow of Air as Related to Flow of Product The air movement may be concurrent flow, counterflow, or crossflow in relation to the product being dried. The air movement pattern can be applied to the drying and cooling section, with same or different patterns for each. There may be several stages for drying. In addition, air may be recirculated to increase the water removed per heat unit. A method of comparing the heat requirements is on the basis of kJ/kg (Btu/Ib). Considerable research is being devoted to computer simulation of various combinations of airflow patterns for cooling and drying, multistaging, and recirculation of air to improve the efficiency and economy of operation of dryers. Warm air leaving the cooling section may be used for inlet to the heater for the drying section. In a dryer with several stages (multistage), warm 156 DRYING AND STORAGE OF AGRICULTURAL CROPS Another air leaving one section may be used to enter another section. the through way part airflow of n directio the reverse technique is to air the of ation recircul of amount the in increase An drying process. increases the efficiency, up to a point. Another technique is to move or direct the grain or product from the air-entering side of the column to the air-exit side through a “turn-flow” device. Crossflow dryers are the most common. Continuous crossflow dryers usually have a crossflow cooling section. The techniques discussed previously are used to increase the energy efficiency. A major advantage of the crossflow dryer is the straight-through flow of product. Recently, concurrent flow grain dryers have been increasing in prominence. The hot air enters with the entering wet grain and flows in the same direction. Higher airflows, 150 as compared to 100 cfm/ft?, and higher temperatures, up to 260°C (500°F) as compared to 1022s: FD, are possible with the concurrent versus the crossflow dryer, providing for higher energy efficiencies (Hawk et al. 1978). In general, energy use varies from 3000 to 8140 kJ/kg (1300 to 3500 Btu/lb), with the usual requirements between 4185 and 6510 kJ/kg (1800 and 2800 Btu/Ib). Because of the many design combinations possible, and economic considerations, it is difficult to state categorically that one of three types of dryers based on airflow pattern is necessarily better than the others. SAFETY Safety Features of Burner Farmers, insurance companies, equipment’ manufacturers, and equipment dealers are interested in the safe operation of burners used for heated air drying systems. Important considerations of an installation designed for safe operation include the following: (1) a flame control to shut off the fuel supply in the case of ignition failure, (2) a high temperature limit switch which will stop the burner but allow the fan to continue to operate, (3) a temperature control on the ‘bonnet of the burner to prevent overheating of heater, (4) proper electrical wiring connecting the fan and burner to the electric circuit (Fig. 7.3). All units should be designed to “fail safe.” There is a tendency to overemphasize the danger of starting a fire from particles of trash getting into the open flame of a heated air dryer operated outside. The velocity of the air passing the flame is so great that straw, chaff, and even cotton lint carried into the airstream do not remain in contact with the flame long enough to ignite, although such a circumstance is not recommended (Montfort 1947). HEATED : Y) Z = AIR & i ‘ai ryaaa DRYERS 157 Bi 4 YZ. © y . V7 . LEGEND: relay; 4— i— alve ry Oil burner; Ignition Temperature exhaust air. ans base 2— Photocell; transformer; controls; 7 and 9— 5— 3— Electronic Ignition spark; Solenoid valves; Protecto6 10— and Bulbs 8— in Courtesy of Honeywell, Inc. FIG. 7.3. SCHEMATIC DIAGRAM OF AUTOMATIC CONTROL SYSTEM FOR A PORTABLE DRYER HEATER Safety Features of Installation A properly designed heater can be used in an unsafe manner. Safety features of an installation should include the following: (1) fuel pump and piping located a safe distance from the flame of the burner, (2) the fuel feed line from the tank to the fuel pump protected from mechanical injury, (3) the fuel tank located at least 5 m (16 ft) from the bin and other buildings, (4) oil drums refilled a safe distance from the drying unit or the drying unit shut down when the drums are refilled, (5) separate drying and storage installations provided for safe and efficient grain drying by heated air, (6) if the crop is dried in batches, on wagons, or in a batch bin, drying equipment separated from the main building by 3 m (10 ft), (7) the drying unit connected to the bin by a duct of flameproofed canvas or other noncombustible material. Insurance companies may require that a special permit be obtained to install and use a heated air crop dryer. The insurance company representatives will determine if the installation is reasonably safe, and if approved, a permit may be purchased. Each 158 DRYING AND STORAGE OF AGRICULTURAL CROPS insurance company establishes housekeeping regulations and standards. These are often based on guidelines established by the Association of Mill and Elevator Mutual Insurance Co., Chicago. Most states have air pollution control regulations which must be met. DESIGN ; Bin Arrangements Drying bins are designed so that a thin layer of 0.5 m (1.5 ft) or less of grain is exposed to the heated air. A thin layer drying bed or column is used to avoid over-drying of the grain which might otherwise occur in deep bins. A recirculating grain bin proivdes a method of moving the grain to reduce effects of over-drying. Over-drying reduces the financial returns to the farmer or the elevator for a cash crop. The grain is normally fed into the top of the bin and fills a space of from 0.1 to 0.5 m (4 to 18 in.) between perforated sheets or expanded metal. The cross-section of the drying chamber is in the form of a vertical column, square, or oval space (Fig. 7.4). The air moves into the center of the bin and is forced out through the perforated sheet through the grain and then leaves the grain through another perforated sheet. Dryers for farm use are usually designed for batch operation. After drying it is necessary to cool the grain before it is placed in the bin for storage. If grain or hay is to be dried in a storage bin where the depth might be 2 m (6 ft), only a limited amount of heat should be added to the air to prevent over-drying—heated only 3° to 10°C (5° to 10°F). Commercial drying installations usually provide a bin for continuous feeding of the grain into, through, and out of the bin. One type consists of vertical columns (Fig. 7.4 and 7.5), in which the rate of flow of grain into and out of the bin controls the rate of drying. Another type is a horizontal moving steel chain or conveyor on which the wet product is fed at one end and removed from the other end after drying and cooling. The rate of drying is controlled by the speed of movement of the chain or belt (Fig. 7.5). The principle of countercurrent movement of air is utilized with the moving belt where the incoming heated air contacts the dried product first and is exhausted from the dryer where the incoming wet grain is placed. The continuous dryers are provided with a section for cooling the grain after it is dried. Comparison of Dryers Several manufacturers make portable and stationary dryers (Table 7.1). In general, the oil-fired portable units burn from 31.5 X 10 ~5 to 63.1 xX 10 “5 m3/s (5 to 10 gal./hr) of fuel and the stationary units from 63.1 X HEATED PERFORATED OR EXPANDED METAL OR SCREENED COVERING HEATED SS AIR — FORCED AIR DRYERS 159 BIN GRAIN 4" TO 18" SPACE IN HEATED FORCED PERFORATED OR EXPANDED METAL OR SCREENED COVERING AIR IN a GRAIN 4'To 18" SPACE WOVEN WIRE -|| SCREEN eee HEATED AIR FORCED IN HEATED AIR INLET ae Bh HELICOID SCREW CONVEYOR “—AUGER FOR REMOVING GRAIN FIG. 7.4. BATCH DRYING BINS FOR HEATED AIR SYSTEM 10-5 to 118.6 X 10~> m3/s (10 to 18 gal./hr). The portable units are mounted on two wheels or skids and may be used as stationary units. Certain minimum design standards have been established by a group of several manufacturers of crop drying equipment called the Crop Dryer Manufacturers Association (CDMA), which is a part of FIEI (Farm and Industrial Equipment Institute). Dryers meeting these minimum standards may display the CDMA emblem. The approval of the heater unit 160 DRYING AND STORAGE CROPS OF AGRICULTURAL HEATED COOLING AIR SECTION A HEATED ———_ AIR BELT UNHEATED e)) AIR MOVEMENT 2 GRAIN B REMOVED RECEIVING GARNER HEATED gece COOLER SECTION COOLING SECTION VARIABLE SPEED DISCHARGER ft AUGERS 5 FOR REMOVING FIG. 7.5. CONTINUOUS GRAIN /\ AIR INLETS “-s AIR OUTLETS FLOW DRYERS by the Underwriters Laboratories is desirable and indicates that an effort has been made to make the unit safe although some manufacturers have not submitted their equipment for test. The American Gas Associa- tion (AGA) establishes standards for gas burners. Air/Product Flow Drying units are classified according to flow of air as related to product flow, crossflow, counterflow, and parallel flow (Fig. 7.6). Various dryers are illustrated in Fig. 7.7 to 7.11. HEATED TABLE 7.1. HEATED AIR DRYERS 161 AIR CROP DRYERS ‘Aeroglide Corp., Raleigh, NC 27611 ‘Aerovent Fan & Equipment, Inc., Lansing, MI 48906 American Drying Systems, Inc., Miami, FL 33169 American Farm Equipment Co., Zurich, IL 60047 Baughman-Oster, Inc., Taylorville, IL 62568 ‘Beard Industries, Frankfort, IN 46041 'Behlen Manuf. Co., Columbus, NE 68601 Benthall Div., Harrington Manuf. Co., Suffolk, VA 23434 Bush Hog-Eaton, Omaha, NE 68110 ‘Butler Manuf. Co., Kansas City, MO 64126 Caldwell Manuf. Co., Division of Chief Industries, Kearney, NE 68847 Campbell Industries, Inc., Des Moines, IA 50317 Chicago Eastern Corp., Marengo, IL 60152 Chromalloy-Shunk Blade Diy., Bucyrus, OH 44820 Circle Steel Corp., Taylorville, IL 62568 Clayton and Lambert Manuf. Co., Buckner, KY 40010 Combustion Equipment Assoc., New York, N.Y. 10022 Diamond International (Dehydrator), Farmington, MI 48024 Driall, Inc., Attica, IN 47918 Farm Fans, Inc., Indianapolis, IN 46203 Farm Systems Corp., Marengo, IL 60152 Ferrell-Ross, Saginaw, MI 48602 General Dryer Corp., Clarkfield, MN 56223 Gilmore and Tatge Manuf. Co., Clay Center, KS 67432 Harrington Manuf. Co., Lewiston, NC 27849 Harvestall Industries, Inc., Hampton, IA 50659 The Heil Co., Dryer Div., (Dehydrator), Milwaukee, WI 53201 Hy-Mark Industries, Inc., Henderson, NE 68371 Industrial Engineering and Equipment Co., St. Louis, MO 63144 Jetstream Systems Co., Hayward, CA 94540 S.F. Kennedy, New Products, Inc., Taylorville, IL 62568 Krenz & Co., Germantown, WI 53022 Long Manuf. Co., Davenport, IA 52808 Long Manuf. Co., Inc., Tarboro, NC 27886 Martin Steel Corp., Mansfield, OH 44901 Mathews Co., Crystal Lake, IL 60014 M&W Gear Co., Gibson City, IL 60936 Middle State Manuf., Inc., Columbus, NE 68601 Modern Farm Systems, Inc., Webster City, IA 50595 'Moridge Manuf. Co., Moundridge, KS 67107 Powell Manuf. Co., Bennettsville, SC 29512 Ransome Gas Industries, Inc., Leandro, CA 94577 Reed-Joseph Co., Greenville, MS 38701 Sioux Steel Co., Sioux Falls, SD 57101 Specialized Products, Inc., Taylorville, IL 62568 1Stormor, Inc., Fremont, NE 68025 'Sukup Manuf. Co., Sheffield, IA 50475 Superior Equipment Manuf. Co., Mattoon, IL 61938 T-L Irrigation Supply Co., Hastings, NE 68901 Westeel-Rosco, Ltd., Toronto, Ontario, Canada Westlake Agricultural Engineering, St. Marys, Ontario, Canada Wiegard Div., Emerson Electric Co., Pittsburgh, PA 15208 ! See illustrations in this chapter. Energy Effectiveness! The ability to remove moisture from grain can be represented by bushels which can be dried by a gallon of fuel (sometimes called ef1 See Appendix for metric conversions. SLNSWSDNVYYV Y¥31009 MO14 AN3Y¥YND YSLNNOD Y¥3Aud MO14S (1371VuVd) LNAYYNONOD SNMO009 yIV Lonaoud AUG MO1d LONGOYd GNV MO1SHIV “9°2 ‘Sls (YIV Y431V3H) ONIANG 9NITI009 GNV _ (ONIAYG) ONILV3H MO14 SSOUD L9ndoud LONGO’d 14M SNINO09 SNI1009 MO14 SSOYD HLIM YaAud MO14 LIN3Y¥YyND YALNNOD SNI1009 L9NGOd Aug SNIAYG OL G319354uI0 YyIV Y3SLVSH G3sn ulV YO4 Lam 19NdOud NI “SNVINI SNI1009 YyIV 38 AVW “G3.1LV 1NDYID3Y Je AVW YlV “S13A371 AN3uY3ssI0 LV S39VLS NI G3LV3H 38 AVW LoNdO’d SNIAUG LAM LONdOwd CROPS OF AGRICULTURAL DRYING AND STORAGE 162 HEATED AIR DRYERS 163 Courtesy of Stormor, Inc. FIG. 7.7. EEZE-DRY UNIT WHICH CAN BE USED AS A BATCH OR A PART OF CONTINUOUS FLOW SYSTEM ficiency). With a bin dryer, about 9.2 bu of corn can be dried (through 10 points) per gal. of LPG; batch or continuous flow with dryeration, 8.1 bu/gal.; batch or continuous flow with cooling, 6.5 bu/gal.; and by combining batch or continuous flow (5%), dryeration (2%), and aeration (3%), 12.6 bu/gal. (Cast 1975). Thermal Efficiency The overall thermal efficiency (heat utilized for drying divided by heat input) of heated air dryers will be greater for: (1) direct-type heaters, Courtesy of Mathews Co. FIG. 7.8. PORTABLE TYPE GRAIN DRYERS A—Batch type, PTO driven. B—M.C. portable dryer. HEATED AIR DRYERS 165 Co. Courtesy of Moridge Manufacturing FIG. 7.8. C—Portable, batch grain dryer. contents, (4) high atmospheric (2) low airflows, (3) high initial moisture of grain. There is a disagreement in temperatures, and (5) thick columns of the heated air temperature on the literature regarding the effect 166 DRYING AND STORAGE CROPS OF AGRICULTURAL ie See (hie £8 Se Na, ae ee a y aa a, ee Courtesy of Butler Manufacturing Co. FIG. 7.8. D—Portable dryer. efficiency. For drying ear corn, the average overall thermal efficiency for an oil-fired direct heater was 34.6% (7.7 cfm/bu), for an oil-fired indirect heater was 28.5% (6.2 cfm/bu) and for a coal-fired indirect heater was HEATED AIR DRYERS 167 17.2% (4.6 cfm/bu) (Ramser 1951). These data compare closely with comparative studies (Cooper 1948). Although the ear corn depths were not given, the installations were probably in cribs where the depths were from 1.5 to 3.0 m (5 to 10 ft). Equation (7-1) can be used to calculate the overall thermal efficiency. Overall thermal efficiency, % = (water evaporated, lb) (latent heat, Btu/Ib) (100%) (fuel used, lb) (net heating value of fuel, Btu/Ib) a The latent heat values can be approximated from Table 7.2. The net heating value of the fuel can be approximated from Table 7.3. TABLE 7.2. LATENT HEAT OF WATER Temperature Latent Heat © °F 15.6 26.7 ilies: 48.9 60.0 TACs 60 80 100 120 140 160 2465.8 1060.1 2439.0 1048.6 2399.5 ~~10316 2386.0 1025.8 2358.9 1014.1 W331 389 eLO023 93:3 200 2274.6 977.9 100.0 104.4 115.6 126.7 137.8 148.9 212 220 240 260 280 300 2256.9 2245.3 2214.8 2183.2 2150.9 2116.7 970.3 965.3 952.2 938.6 924.7 910.0 Note: The latent kJ/kg heat for evaporating Btu/lb water from grain and hay is greater than evaporation from a free water surface. For practical applications add 290 kJ /kg (125 Btu/Ib). The heater thermal efficiency is the heat output of the heater divided by the heat input. For indirect heaters the thermal efficiency is from 70 to 85% (Table 7.4). The efficiency of a heated air dryer using a conveyor moving grain opposite the direction of airflow is about 65% and of a commercial rotary high temperature dryer is 65 to 80% (Williams-Gard- ner 1971). Precise determination of thermal efficiency of a drying system must consider several additional factors such as initial product temperature, , final product temperature, air inlet and air outlet conditions. Generally the high temperature dryers use about 2100 Btu/lb and bin drying systems 1500 Btu/Ib of water evaporated. Improving Thermal Efficiency in deep The thermal efficiency of drying may be improved by (1) drying incoming air or layers, (2) removing heat from the outgoing air to heat 168 DRYING AND STORAGE OF AGRICULTURAL CROPS A Courtesy of Mathews FIG. 7.9. PORTABLE A—M.C. CONTINUOUS Co. DRYING UNITS continuous dryer. incoming product, and (3) drying in stages in which the product moisture content equalizes between stages. HEATED AIR DRYERS _ 169 Courtesy of Behlen Manufacturing Co. FIG. 7.9. B—Pre-heat module for use with continuous dryer. Supplemental Heaters Units designed for a limited temperature rise, usually less than 11°C (20°F) are designated as supplemental heaters. A heat output of about 170 DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Aeroglide Corp . FIG. 7.10. AEROGLIDE™ CONTINUOUS FLOW DRYER HEATED WET GRAIN WET GRAIN UPPER INTAKE HEAT AIR DRYERS 171 = COLUMN GRAIN COLUMNS (All four sides) NEAR SATURATION AIR EXHAUST LOUVERS ADJUSTABLE WARM wo (OISERRE EXHAUS (Lower |--}~ "AVERAGING SENSOR NP "MOISTURE EQUALIZER™ © (Grain turn) WEATHER dtees. ( )) HOT MEDIUM MOISTURE AIR I] 1 SHIELD AIR GAP (Lower % RECYCLED VERTICAL COLUMNS IN BASE PLENUM LEVEL MOISTURE SENSORS CORNER RETURN << _| AIR OUCT AIR peu IRN BORER @ "DRYING OPENINGS (CLO ineea COIL (LP Models ) R.H. than 2) THROUGH TUBULAR ro BLOWER 4 COLUMN AIR OPENINGS (OPEN) (For recycling air to main blower) UPPER % reH. than |) MOISTURE————qt--+0) <<a OPTIMIZER" (Plenum divider) osee) eee OC Tk VERY HOT LOW MOISTURE AIR (Lower % R.H. than 3) RECYCLED THROUGH VERTICAL TUBULAR COLUMNS TO BLOWER IN BASE C{{}e-— COOLING _AIR DRAWN THROUGH GRAIN COLUMN WEATHER SHIELD WEATHER SHIELD BUTTERFLY VALVES BUTTERFLY (OPEN) TRANSITION GAS VALVE SAFETY LADDER BURNER COOLING AIR DRAWN THROUGH GRAIN COLUMN BLEND OF HOT EXHAUST, WARM COOLING, & COOL BLOWER FREES AIR FREE AIR INLET VALVE mS FREE AIR INLET VALVE AUXILIARY GRAIN oe CONVEYOR fp SE : Ny f + Courtesy of Beard Industries FIG. 7.11. SUPERB™ ENERGY MISER 105,000 to 263,750 kJ/hr (100,000 to 250,000 Btu/hr) is provided. Supplemental heaters are particularly valuable for drying crops in deep bins during periods of high humidity. The operation of the heater can be 172 DRYING AND STORAGE TABLE 7.3. APPROXIMATE OF AGRICULTURAL CROPS NET HEATING VALUES OF VARIOUS FUELS Density Fuel kg/m? Heating Value lb/ft? kJ/kg Agricultural and crop residues cae, dry Corn stalks, dry Cotton batting Cottonseed hulls Newspaper Btu/Ib 18,600 16,600 16,550 20,000 8,000 7,150 7,114 8,600 20,680 13,950 17,445 8,890 6,000 7,500 27,750 21,115 11,930 9,080 26,240 19,580 11,280 8,420 DAUD) 31,401 33,960 30,355 12,800 13,500 14,600 13,050 18,330 Pecan shells Straw Wheat 45 Alcohol—gas Ethyl (C2.H;OH) Methyl] (CH;0H) 1.956 1.363 0.122 0.085 7,880 Alcohol—liquid (pure) Ethyl (C.H-OH). Methyl (CH;0H) Coal Anthracite Bituminous Semi-bituminous Manufactured briquets 815.3 796.1 50.9 49.7 Specific Gravity = 1.12—1.35 Fuel Oil Ib/gal. No.1 No. 2 No. 4 No.5 No. 6 813.6 863.9 932.2 938.2 964.2 6.79 PAL 7.78 7.83 8.05 43,473 42,590 41,263 41,030 39,960 18,690 18,310 17,740 17,640 17,180 Gasoline Kerosene 736.9 817.2 6.15 6.82 43,960 43,403 18,910 18,600 Gas (CSE: ce 2) Natural, methane (CH,) 0.680 lb/ft? 0.0424 50,055 21,520 0.0803 0,048 47,488 24,610 20,416 10,580 Ethane (C,H) Manufactured 1.287 0.769 Butane (C,H) 1.924 0.120 252 46,390 0.158 AMO 705) dry 20,934 Propane (C3Hs) Wood 19,944 19,680 9,000 Sources: Anon. (1972); Baumeister et al. (1976): Fryling (1966); Kajewski et al. (1977); Perry and Chilton (1973); Weast (1977). The net heating value (nhv) of fuel oils is based on gross heating value minus 1120 Btu/Ib. TABLE 7.4. APPROXIMATE THERMAL EFFICIENCY OF HEATED AIR SYSTEMS, % Direct Heater Overall Summer Fall Winter Indirect Heater: Overall (Burner) (Dryer) (Burner) (Dryer) 85 70 50 60 50 36 60 50 35 45 36 28 Source: Hall (1979). controlled by a time clock, thermostat, or humidistat. The relative humidity of the air is approximately halved by heating the air 11°C (20°F). HEATED AIR DRYERS 173 FUEL Fuels for Drying Coal, wood, crop residues, fuel oil, and natural and manufactured gas are used for fuels. In some commercial installations the air is heated by steam formed by heating with one of these fuels. The potential energy output of the fuel is expressed in heating values, Btu/unit weight or volume. The high (gross) heating value, as reported in many references, includes the heat available in the moisture in the products of combustion and would be utilized only if the combustion products were condensed. The net heating value is more useful for drying calculations because the temperature of the air exhausted by the heater is considerably above the original air temperature, particularly for the indirect unit. No. 1, 2, or 4 fuel oil grades are usually used for portable heaters. The heating value of the heavy fuel oils is higher than the lighter fuels. Commercial operators can take advantage of this relationship by using a No. 5 or No. 6 fuel oil in which the oil is preheated in the tank. Storage facilities must be adequate. By installing a large tank, oil may often be secured for a lower price. A carload is 49.2 m? (13,000 gal.) and a truckload is 11.4 m3 (3000 gal.) and larger. The factors which are important in fuel selection, particularly applicable to commercial operations, are (Skrotski and Vopat 1945): (1) (2) (3) (4) (5) (6) (7) (8) Energy content Price of fuel Overall efficiency Cost of handling and storage Disposal of refuse Operating labor Maintenance Safety Combustion Carbon, hydrogen, and sulfur are the components of fuel which can be burned to provide heat and upon complete oxidation in the presence of adequate oxygen give carbon dioxide (CO,), water (H,O), and sulfur dioxide (SO,). The presence of carbon monoxide (CO) in the exhaust indicates incomplete combustion. The gross (high heat value) amount of heat produced by burning of fuel is equal to the heat produced from burning the component parts minus the portion of the hydrogen which combines with the oxygen to form water. A pound (0.45 kg) of sulfur produces 4220 kJ (4000 Btu) when burned, but produces an undesirable 174. DRYING AND STORAGE OF AGRICULTURAL CROPS exhaust product; 0.45 kg (1 Ib) of carbon, 15,403 kJ (14,600 Btu); 0.45 kg (1 Ib) of hydrogen, 65,400 kJ (62,000 Btu). The net (low heat value) heating value is more applicable because the products of combustion normally are not cooled to the temperature at which the moisture will condense. The net heating value per 0.45 kg (1 Ib) of fuel, nhvy, is given in equation (7-2) in which the heating value of hydrogen is 54,860 kJ (52,000 Btu) per 0.45 kg (1 Ib) instead of 65,100 kJ (62,000 Btu). nhv = 14,400 C + 52,000 (1 = .) +4500S+1100w (7-2) where nhv = net heating value, Btu/lb C H O S w = weight =weight = weight = weight = weight of carbon/|lb of fuel of hydrogen/|b of fuel of oxygen/|b of fuel of sulfur/lb of fuel of moisture/|b of fuel (1 Btu/Ib = 1.055 kJ/lb = 2.454 kJ/kg) The amount of air in excess of the theoretical requirements needed to give complete combustion varies considerably. With stoker-fed coal, the excess air is 30 to 50%; fuel oil, 20 to 30%; and gas, 15 to 20%. Equation (7-2) applies to complete combustion of the fuel. Electricity For electric bin drying, about 0.35 kwh of electricity is used per bu for each point of water removed. MANAGEMENT Proper management of a heated air drying system consists of: (1) Drying in a separate building or bin designed only for use of heated air. (2) Preventing over-drying of part of the product by -drying in thin layers or continuous moving of the grain. (3) Controlling rate of drying to prevent case hardening and cracking of some grains. (4) Determining when the product is dry, which can be done with a moisture meter, by measuring the amount of fuel, or measuring the time required to dry the grain. (5) Cooling the grain after drying. HEATED AIR DRYERS 175 (6) Preventing condensation from accumulating on the top layer of grain by adequate air movement. (7) Preventing damage to product when handling, drying, and storing. (8) Requiring good housekeeping around drying installation. Insurance Regulations Assoc. 1946) for Commercial Dryers (Natl. Fire Protect. Location—Dryers should be in a room separated from elevators or storage tanks by as much space as is practical. Louvers—Permanent openings where air enters or leaves the dryer should be protected by corrosion-resistant wire screens not exceeding 0.85 cm (% in.) mesh to exclude birds, paper, etc. Garner and hoppers—Shall be dust-tight and provided with adequate positive air aspiration or effective vents to the outside. If the grain is brought to the dryer by belt, the belt shall not enter garner but shall discharge into spout in closed top of garner. Removal of refuse from grain—Where operating conditions permit, all the grain shall pass over a coarse screen immediately ahead of the dryer to remove cobs, papers, sticks, etc. Top of dryer—Should be open or inclined at a steep angle so dust will not lodge on the surface. Steam coils—Coils shall be designed so that dust will not lodge on them and the coils should be installed in a room separated by a dusttight partition. Fire heated dryers—Furnace shall be located in a fire-resistant room or division separated from dryer and elevator by masonry walls and it must be automatically controlled by reliable means. It would be desirable to prevent combustion products from going into the dryer. Oil tank—There are several provisions regarding the location of the oil tank with respect to size, underground or aboveground installation, and location within the building. These should be checked with the local fire inspection authorities also. Handling A good drying system will include efficient handling equipment which requires a minimum of energy and avoids damage to the product. Auger, belt, flight, and bucket conveyors are used. Grain settles as it dries in a batch drying bin. It is desirable to place grain into the bin during drying or have a greater thickness of the top layer to maintain even airflow and prevent excessive air loss through the top of the batch bin. Continuous 176 DRYING AND STORAGE OF AGRICULTURAL CROPS circulation of the grain with a slow speed conveyor is used on some units to provide uniform and faster drying (Fig. 7.11). (also see Chapter 9). Automatic loading, drying, and unloading, controlled by a timer, provide excellent handling with a minimum of labor. Other Types of Heated Air Dryers Infrared heat has been used to dry a depth of 0.6 cm (0.25 in.) of red fescue seed on a continuous rubber belt conveyor 6.1 m (20 ft) long, 2 ft wide, and equipped with 60 lamps of 250 watts each for heating (Nich- olas and Musser 1941). With a belt speed of 0.02 m/s (4.2 ft/min) (exposure of 4 min, 50 sec), the moisture removal from seeds was 2.9% with 40 lamps (10 kw) and 3.6% with 60 lamps (15 kw) for % to 2 kg (150 to 250 Ib) of seed. The lamps were about 7.5 cm (3 in.) above the belt. Plow fingers were placed 73 m (2 ft) apart to turn the seed. A continuous vertical dryer was developed in which the grain moves downward over coils heated with hot water as air is blown upward through the grain (Hukill 1948). The heat was supplied within the bulk of the grain and the air temperature did not drop in its passage through the grain as in a batch dryer. Grain may be heated by conduction in a drum and dried by forcing atmospheric air through the heated grain in a drying compartment. In a dryer of this type up to 3% moisture can be removed during one passage. Many indirect dryers, particularly commercial units operated in main buildings, are heated with steam or hot water. The combustion unit, the boiler, is placed in a separate building. Overall heat transfer coefficients, U, as found in practice for steam pipes in moving air vary from 33 to 90 W/m?K (6 to 16 Btu/hr ft? °F) and for hot water pipes from 12 to 57 W/m?2k (2 to 10 Btu/hr ft? °F) (Perry and Chilton 1973). An estimate can be made of the amount of heat transfer based on the temperature difference and area of pipe. (1 Btu/hr ft? °F = 5.678 W/m?K.) QUESTIONS 1. Compare the cost of fuel for drying 352 m3 (10,000 bu) (at 15%) of ear corn from 25 to 15% moisture (w.b.) using fuel oil, coal, natural and liquid propane (LP) gas where overall efficiencies are 35, 20, 40, and 50%, respectively. The fuel costs are: No. 3 fuel oil at $1.00/gal., bituminous coal at $30/ton, natural gas at $3.20/1000 ft, and LP gas (propane) at $23.00/100 lb tank and bulk LP gas at 45¢/gal. (4.35 lb/gal.). Assume evaporation takes place at 120°F. 2. Assume that the combustion or oxidation of fuels takes place as (a) primary, 2C + O, = 2CO and (b) secondary, 2CO + O. = 2COs. If air HEATED AIR DRYERS 177 containing 21% oxygen by volume (23.1% by weight) is supplied for combustion, how many kilograms (pounds) of air are required for the theoretical complete combustion of 45 kg (100 Ib) of fuel for primary, secondary, and total combustion of anthracite coal containing 80% carbon, fuel oil containing 87% carbon, and natural gas containing 95% methane (CH,). How much air is supplied in practice? . Using the fuel costs given in question 1, determine the comparative costs for 100 therm of energy. . Air is supplied to a heater at 2.36 m°/s (5000 cfm). Assume 100% heater thermal efficiency. The air is heated to 82°C (180°F) and all of the moisture in the air is evaporated. How much heat, in kJ/hr, is lost in the water in the air upon heating, with air originally at 27°C (80°F) and (a) 20, (b) 40, (c) 60, and (d) 80% relative humidity? Give the equivalent loss in liters/hr of No. 2 fuel oil. . The major constituents of the products of combustion of fuels are carbon dioxide and water. The water produced from burning 2.83 m3 (100 ft?) of natural gas is 18.9 liters (5 gal.). If 0.47 m3/s (1000 cfm) of air were heated from 26.7° to 82.2°C (80° to 180°F), considering 100% combustion efficiency, what would be the percentage increase in relative humidity from the products of combustion at (a) 82°C (180°F) and (b) 27°C (80°F) after the air is cooled. . Compare the costs for 1 million kJ of the various fuels available in your area. Discuss the use of various energy sources for drying, considering also the efficiency in conversion to usable heat. . A 25 hp gasoline engine is used for driving a fan delivering 8.46 m?/s (10,000 cfm). The fuel consumption is 7.5 liters (2 gal.)/hr. Twenty percent of the fuel energy is used for propelling the fan with the remainder being discharged in cooling, exhaust, and friction. Assuming that 80% of the heat is recovered for heating the 18°C (65°F), 70% relative humidity air, calculate the final temperature and relative humidity. . Prepare a table estimating the liters (gallons) of No. 4 fuel oil required to dry 35.2 m? (1000 bu) of ear corn to 15% moisture (w.b.) with a direct-fired heater from 30, 27, 21, and 18% moisture. Use an overall efficiency of 36% for calculations. . Based on the theoretical equation for complete combustion of 1 m? of CHy,, C2H», and C,He, determine the volume of the products of combustion. Note whether volume is increased, decreased, or the same after combustion. 10. Calculate the quantity of heat produced by burning 1 kg of coal containing 75% carbon, 5% hydrogen, 6% oxygen, 1% sulfur, and 4% water. 178 CROPS OF AGRICULTURAL DRYING AND STORAGE REFERENCES ANON. York. Babcock and Wilcox Co., New Steam—Its Generation and Use. 1972. The oil burning crop drying unit. Agric. Eng. 32 (12) , 1951. BAKER, V.H. 657-660. BAKKER-ARKEMA, F.W. et al. 1973. Energy performance in grain dryers. Annu. Meet. Am. Soc. Agric. Eng. Univ. Kentucky, Lexington, June 17—20, 1973. Pap. 73-324. BAUMEISTER, T. et al. 1976. Standard Handbook for Mechanical Engineers. McGraw-Hill Book Co., New York. F.W. 1977. Design of commercial conR.C. and BAKKER-ARKEMA, BROOK, Soc. Agric. Eng., N.C. State Univ., Am. Meet. Annu. current grain dryers. Raleigh, June 26—29, 1977. Pap. 77-3017. BROWN INSTRUM. DATA. semi-portable grain dryers. delphia. BUELOW, F.H. 1956. energy air heaters. 1949. Sheet No. 3.6-1. Automatic control of Minneapolis-Honeywell Regulator Co., Phila- The effect of various parameters on the design of solar Ph.D. Dissertation. Mich. State Univ. CAST. 1975. Potential for energy conservation in agriculture. Sci. Technol., Iowa State Univ., Ames, Rep. 40. CLYDE, A.W. L27=~ 129: 1933. New developments in hay driers. Counc. Agric. Agric. Eng. 12 (5) COOPER, A.W. 1948. Mechanical drying of ear corn on Indiana farms. Purdue Univ. Dep. Agric. Eng. Mimeo 14. FRYLING, G.R. 1966. Combustion Engineering. New York. HALL, C.W. Combustion Engineering, 4a 1979. Dictionary of Drying. Marcel Dekker, New York. HAWK, A.L. et al. 1978. The present status of commercial grain drying. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, June 24—28, 1978. Pap. 78-3008. HUKILL, W.V. 1948. Types and performance of farm grain dryers. Agric. Eng. 29 (2) 53-54, 59. KAJEWSKI, A.H., MARLEY, S.J. and BUCHELE, with a crop residue fired furnace. W.F. 1977. Drying corn Winter Meet. Am. Soc. Agric. Eng., Chicago, Dec. 13-16, 1977. Pap. 77=3a25. MADDEX, R.L. and HALL, C.W. 1954. Farm crop dryers. Mich. State Univ. Folder F-184. MONTFORT, P.T. (3) 96-97, 108. MOYER, J.A. 1934. N. AM. MANUF. Cleveland. 1947. Supplemental heat in barn curing. Agric. Eng. 28 Power Plant Testing. McGraw-Hill Book Co., New York. 1952. Combustion Handbook. N. Am. Manuf. Co., CO. HEATED AIR DRYERS _ 179 NATL. FIRE PROTECT. ASSOC. 1946. National Fire Codes, Vol. 2. The Prevention of Dust Explosions. Natl. Fire Protection Assoc., Boston. NICHOLAS, lamps. NOYES, J.F. and MUSSER, H.B. 1941. Agric. Eng. 22 (12) 421-423. R.T. 1977. Superb—‘“Optimum” Seed drier uses infrared electric continuous crossflow commercial grain dryer. Annu. Meet. Am. Soc. Agric. Eng., N.C. State Univ., Raleigh, June, 1977. Pap. 77-3015. PERRY, R.H. and CHILTON, C.H. McGraw-Hill Book Co., New York. RAMSER, J.A. 1951. 1973. Chemical Engineers’ Handbook. Some results of artificial drying of corn and small grain in Illinois. Univ. Ill. Dep. Agric. Eng., Mimeo Rep. AE 662. SKROTSKI, B.G.A. and VOPAT, W.A. 1945. Applied Energy Conversion. McGraw-Hill WEAST, R.C. Book Co., New York. 1977. CRC Handbook of Chemistry and Physics. CRC Press, Cleveland. WILLIAMS-GARDNER, A. 1971. Industrial Drying. Leonard Hill, London. Natural and Forced Air Drying of Grain and Ear Corn NATURAL AIR MOVEMENT Grain and ear corn can be ventilated by natural or mechanical means. The use of a natural ventilation system is practical for ear corn. The pressures are too great to move by natural means an adequate quantity of air for drying through the depth of grain and hay as normally stored. The loss of income due to high moisture products can be economically prevented by natural or mechanical ventilation. Natural Ventilation Natural ventilation involves the use of wind or change in temperature of the air to obtain air movement to remove moisture from grain. Natural ventilation can be accomplished by: (1) air movement through the storage, as with ear corncribs, (2) a downdraft or pressure ventilator, (3) an updraft or suction ventilator, (4) an A-type frame or vertical duct which provides an air movement through the center of the bin or crib, and (5) tubes made of steel spring or screened channels placed horizontally in the bin. The quantity of air which moves through the grain in the storage by natural ventilation depends upon the wind velocity, direction of the wind, resistance to airflow offered by the material, and difference in temperature of air in storage. The resistance of airflow depends upon the material in the storage, the product, fines and contaminants, and the length of the path through which the air must pass in going through the product. The relationship in U.S. Customary units between the wind speed and the pressure exerted is given in equation (8-1). 180 NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN p = 0.00256 w? 181 (8-1) where p = pressure normal to surface, lb/ft? w = wind speed, mph Natural Ventilation of Ear Corn The quantity of air in cubic feet per minute which will move through ear corn in a crib is given in equation (8-2) (Barre and Sammet 1950). 303A (A P)9-442 Q= L°.540 (8-2) where Q is the quantity of airflow, ft?/min; A is the area perpendicular to the wind through which the air passes, ft2; L is the length of the path through the grain, ft; and AP is the mean pressure difference, in. water. To use equation (8-2) it is necessary to calculate the pressure difference, AP, from p as determined from equation (8-1). The pressure determined from equation (8-1) can be converted to static pressure in inches of water by equation (8-3). AP _ (pressure coefficient) (pressure, lb/ft?) (144 in2/ft?) (0.036 Ib/in.*) (8-3a) AP = (pressure coefficient) 0.19 p (8-3b) The pressure coefficient is 1.0 for rectangular buildings and 0.8 for cylindrical structures. The quantity of airflow through a particular building can be determined in cubic feet per minute from equation (8-2). Whether a high moisture product will keep in a naturally ventilated storage can be determined by comparing the airflow per bushel to the amount recommended for mechanical ventilation, which is normally 0.06 to 0.13 m?/m3s (5 to 10 cfm/bu) of ear corn (1 bu of ear corn will produce 1 bu of shelled corn; 2 baskets of ear corn equal 1 bu). Ventilator Cowls Natural ventilation can be provided with a cowl on a central, vertical, or horizontal ventilation duct. The rotating type cowls are divided into two types: (1) the pressure cowls, in which the open end faces the wind and develops a positive pressure in the ventilation duct; and (2) the suction cowls, which utilize the wind for producing negative pressure and often utilize a rotating turbine on the ventilator (Fig. 8.1). 182 DRYING AND STORAGE OF AGRICULTURAL CROPS B A FIG. 8.1. A—SUCTION COWL. B—PRESSURE COWL. The pressure type cowl develops nearly the full theoretical velocity pressure and the suction and rotating turbine cowls develop only about one-third of the theoretical negative pressure (Kelly et al. 1942). A comparison of performance of cowls is provided by equation (8-4), in U.S. Customary units. AP = w? (8-4) K K = = 0.000483 (theoretical) 0.00043 (pressure, 89% theoretical) K = 0.000170 (section, 35% theoretical) where AP is the pressure, in. water, and w is the wind speed, mph. The main disadvantage of the pressure cowl is that more snow and rain may get into the storage if means are not provided to prevent it. A gooseneck with a drip hole may be provided for preventing moisture from getting into the bin (Fig. 8.2). Cribbing Ear Corn Ear corn can safely go into a crib at about 20 to 25% moisture content. If ventilators are provided for the crib and the ear corn is above 25% moisture only a sample grade can be obtained (Shedd 1946). Other important factors that should be remembered which will certainly influence the amount of air and the distribution of the air are the width of the crib, the foreign material in the grain, and the method of filling. Thirty-two percent of the ear corn was damaged from mold when stored NATURAL AND FORCED AIR DRYING OF GRAIN AND | DRIP HOLE | 1 CORN 183 ie Wino ex =e — -—— —— WIND ———> ———— EAR ) | || 1 GRAIN 4 | oe FIG. 8.2. PRESSURE COWL INSTALLATION TO KEEP OUT SNOW AND RAIN with the husks left on from mechanical picking as compared to 5.6% damage when the ear corn was rehusked by hand. The A-frame ventilator helps to remove the moisture content of ear corn around the ventilator to about 1 ft above the frame. The A-frame should be placed to provide sufficient air so that the recommended minimum airflow is obtained or exceeded. In many areas such as northern Ohio and southern Wisconsin, spoilage will occur in the center of the ear corn in a bin if it is over 1.8 m (6 ft) wide and in other areas such as Missouri, Illinois, and southern Indiana, 2.4 m (3 ft) wide (Shedd 1955). Horizontal tubes embedded in the corn with the ends of the tubes exposed to the wind pressure will cause a definite reduction in damaged corn, particularly in the immediate area about the tube. Commercial devices of this type are available which can be placed in the corn during cribbing. The open ends of the horizontal tubes are placed next to the sides of the crib and can be made of supported screen wire or coils of steel springs. It is generally agreed that an open crib with good ventilation will remove from 3 to 5% excess moisture in ear corn. The problems encountered in southern states are different from those in the cold areas. The control of insect infestation is the major problem and crib design should permit fumigation. Corn with an initial moisture content of 20% was stored in slatted and tight sidewall structures up to 3.6 m (12 ft) wide in North Carolina (Usry 1956). Slatted sidewall cribs are satisfactory for moisture control but do not permit adequate fumigation easily. The tight sidewall structure utilizes natural ventilation through a horizontal floor duct, slatted vertical ducts along sides of the crib between the studs, and slatted vertical ducts of 43m X % m (1 ft X 1 ft), 2.4 m (8 ft) apart in the center of the bin. 184 DRYING AND STORAGE OF AGRICULTURAL CROPS Soft Corn Corn which is frozen before it matures, with an ear moisture content above 40%, is called soft corn and is difficult to dry. After corn has reached maturity, about 2 days standing in the field is required to lose 1% moisture. Corn which is frosted before it matures dries more slowly on the stalk than corn which has not been frosted. Corn which is frosted after it matures dries more rapidly than unfrosted corn. Ear corn with more than 25% moisture should be stored in well-ventilated cribs 1.2 m (4 ft) wide or less, dried by artificial means, or left unharvested until the moisture is down to a safe level for storage. Corn above 30% moisture content cannot be shelled without excessive kernel damage with shellers now available. It is difficult to grind ear corn that is above 30% moisture content. To avoid molding and heating of a grain mixture containing wet corn, only small amounts should be ground at one time. The feed value of soft corn is equivalent to dry corn on a dry matter basis. Soft corn can be “hogged off” or made into snapped ear corn silage or corn and stalk silage. FORCED AIRFLOW Forced Air Drying of Grain Farmers have suffered considerable cash losses during storage of high moisture grain crops. High moisture grain brings reduced prices at the elevator because it is likely to deteriorate in quality when put into storage if not dried. The moisture content is often too high to be reduced to safe limits by natural ventilation before molding occurs. The time of harvesting has been shortened from a matter of weeks to a matter of minutes with the advent of the combine and corn picker. The time for desirable harvest conditions is relatively short. The corn picker, in some cases, has added to the problem of drying because of incomplete removal of husks and trash which are placed in the bin with the stored corn. Early harvest can be practiced to avoid losses which occur because of shattering and weathering, but a high moisture crop is obtained during early harvest. Forced air drying permits a method of removing excess moisture from the grain without the use of added heat. Components of a Forced Air Drying System Forced air drying is usually done in the storage, although batch operations are occasionally used for removing a limited amount of moisture with good drying weather. A forced air drying system consists of a bin for storage of the grain, a fan and motor to force the air through the prod- NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 185 uct, and an appropriate duct system for uniformly distributing the air through the product. Moisture is evaporated by using the heat from the atmospheric air. The success of a forced air drying system depends on proper selection and design of component parts, proper management of the system, and desirable atmospheric air conditions. Most storage bins can be adapted to forced air drying. However, there are certain _ limitations regarding the use of storage bins. The maximum economical depth of storage for drying of small grains is 1.8 m (6 ft); for shelled corn and pea beans, 2.4 m (8 ft); and for ear corn, 6 m (20 ft). It is important that these maximum depths not be exceeded unless special consideration be given to the design. The horsepower required to operate the fan becomes quite large for greater depths and is generally considered uneconomical for drying. The capacity of the fan required depends on the depth of grain and the rate of airflow. The rate of airflow depends upon the crop and its moisture content. Airflow Through Grain The rate of drying grain depends on (1) the temperature and humidity of air while drying, (2) the rate of air movement through the grain, and (3) the moisture content. If the air is forced into ducts on the bottom of the bin floor, the grain dries from the bottom to the top. When the moisture on the top layer has been reduced to the amount required for safe storage, all the grain in the bin is safe for storage. The amount of water that must be removed from high moisture grain to make it safe for storage for 1 year is shown in Fig. 8.3. Excess moisture can be removed by additional drying in the spring. The influence of temperature and humidity on drying can be noted as they affect the amount of moisture removed per 0.48 m?/s (1000 cfm) of air. Although little or no moisture is removed when the temperature is below 10°C (50°F) and the humidity is above 85%, the fan can be operated to cool the grain and prevent deterioration. The time required to dry a bin of grain can be approximated by using Table 8.1 and Fig. 8.3. The fan and horsepower requirements for drying wheat, oats, and shelled corn can be obtained from Table 8.2. Note that the recommended airflow rate varies with the moisture content of the crop. An airflow of 0.03 to 0.06 m*/mis (3 to 5 cfm/bu) is recommended for small grains, although lower airflows will suffice for low moisture grain. The static pressure increases as the depth of grain is increased and as the rate of airflow is increased. The maximum quantity of grain that can be dried per horsepower on the fan is listed in the right-hand column of Table 8.2. These data are useful in estimating power requirements for the conditions represented. 186 DRYING AND STORAGE OF AGRICULTURAL CROPS TABLE 8.1. MOISTURE REMOVAL Relative Temperature, AG; °F Water Removed in 1 hr with Humidity, % 0.4716 m3/s (1000 cfm) of Air kg lb 26.7 26.7 211, 15.6 80 80 70 60 0} 50 60 60 1.0 5.4 PASS) 1.8 10.0 50 or _- No appreciable drying! 85-100 No appreciable drying! \ 22 12.0 5.0 4.0 less Source: Maddex and Hall (1954). ‘From a practical standpoint, it is not economical to operate a dryer under these conditions for the purpose of moisture removal. The fan may be operated to keep grain cool. 20 | Bu. Oats, 32 Ib. ot 145% | Bu. S. Corn, 56 Ib. at H,0 (wb) 15.5 | Bu. Wheat, 60 Ib. at 14.0 | Bu. Eor Corn, 70 Ib. ot 15.5 © 15 y, | Bu. Rice, 45 Ib. at 13.5 of o ‘< 5 X = 4 4 Ww ray a lJ fea} (2) & of ® on 10 \) * sy s CS [eg Z oy es = <q > 5 | Ib = 0.454 kg | bu = 0.035m3 | ft3= 0.028 m3 15 20 MOISTURE FIG. 8.3. WATER REMOVED PRODUCTS AT 13% 25 CONTENT, FROM PERCENT 30 (w. b.) 1 BU OF cae a Rc NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 187 Fan Selection The rate of air movement through the grain, as determined by the grain and its moisture content, the depth of grain, and the size of the storage, are the factors which determine the size of the fan and power unit required for drying grain. Either a centrifugal or propeller type fan may be used to force air through the grain. Fans are rated by the manufacturer according to their ability to deliver various quantities of air against different static pressures. This information should be available from the fan manufacturers and is to be used in selecting fans for crop drying. The fan should be operated 24 hr per day when the high moisture grain is placed in storage. Operation is required on a 24 hr basis at the beginning of drying to prevent heating of the grain. After the moisture content is reduced to about 18%, the fan can be operated during the day only when the relative humidity is low, generally from 10 o’clock in the morning until 7 o’clock in the evening. Method of Air Distribution The main principle in the design of drying systems is to provide the same air path distance from the duct to the outer surface of the grain throughout the bin. One method of adapting a circular metal storage bin is to use a perforated metal floor placed at least 0.45 m (16 in.) above the floor or ground. The floor can be supported on wood framing and concrete blocks or on metal framing with steel posts. In the design of this system, it is important that the floor supports not obstruct the airflow. Concrete blocks should be laid so that the air can circulate around them with one support for each 1.76 m? (50 bu) of grain. Perforated metal is usually used for wagon box dryers. Hardware cloth can be used as a material for false floors in drying bins. It is important that the bottom of the bin equipped with a false floor be sealed to prevent leakage of air between the false floor and the ground or bottom of the bin. The openings should comprise at least 10% of the total area for a false floor for drying. wal Two systems of air distribution might be used in buildings with curved or arched ribs (Fig. 8.4 and 8.5). A center duct will provide uniform air distribution for buildings 4.8 m (16 ft) or less in width. For widths over 4.8 m (16 ft) there are usually lateral ducts extending from the center main duct or side manifold toward the outside of the bin for air distribution (Fig. 8.6). The cross-sectional area of the main duct or manifold should be about 0.093 m? (1 ft?) for each 0.4 m3/s (1000 cfm) of air. If a bin holds 70 ft? (2000 bu) of grain 1.5 m (5 ft) deep, the fan should deliver 3 to 4.7 m?/s (6000 to 10,000 cfm) of air at a static pressure of 622 Pa DRYING AND STORAGE OF AGRICULTURAL ag ue) 00ST 0g8OPPO88T 009002 lemodasioyy nq yeu,T, 08ST OSZI 06S09ST 0&8Ogrl 000T Ogee CROPS 098 (ers OZII OL9 00SZ OLOT 089 0009 0008 O88T GALVSHNN HIV WOuS 188 80 an eu ST rI 0%60 61 €'0LG‘02090€0G0 CT ST Gi V%VS og a ST ST VG VS ORSVz 9g ‘dy paT[eys UIOD ‘ I € Zz “AT I I z z nq MOTJALY pepusmuloday /wjo g i 9 9 8 9 8 ZI 8 ral 91 Pv 9 9 8 8 Or 8 all € w $1e8Q v 8 8 OT cl cS ST ST VS ST VZ 9°¢ VZ 9° ST ulely yeauM Pp 9 VW syydeq [eotqoeig Zr0 20 LZ‘0cr0Ge'0S‘0200LY'0 G eq¥y LV0 v0 60 Lg'0 STO 0 Zo €'0 ZT'0 GZ'0 v0 J 913849 €10'0 GZ oT oT 9Z0'0 ST ST 02 700 % 9IN4SIOfy ulei4)‘yUE7IUOD r0'0 920'020'0 €10'0 02 ¥0'0 9Z0'0€10'0 490°0 /,W GZ VUNUITUTPAT 0Z ST OT o1ye4¢ ainsseid Sapnpout GZ’Q“UI soURMOTTE 1OJ SSO] WLOIJ JONP ‘UOJ MOTJITY JaMmodesi0Yy/(WYd) paseq UO QYOE WO JO IIe 4B | “Ul 013848 ‘eansseid 9PL'0MY= T 80cd eT 0% S&PL'02OLY16210sOSEGCTT €'°GG ; z I9}e paInssel Gi 4 MA “UT SUL JIGVL ‘2'8 NV4 LNSYSSSI0 0°62VSI8'S9 O'1%S08GZS 10g CST Z'6E G'eZ G'L8 Gg 8'kZ 0012 0'SOT 8°99 eu L‘0 oT 60 qT 9°0 60 TL G0 OT oT ue y /paid wunuixepy ‘(AZS61) ousy ‘daq ‘s'Q :e0In0g SLNSWSYINOSY YO4 DNIAYG ‘LVSHM ‘SLVO GNV G3T11SHS NYOO HLIM SSDVLN30u"Sd 4O SYNLSIOW LNALNOO GNV LV SNOIYVA IWOILOVYd SH1d3d Ayueny NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 189 CORN LEVEL SLATTED WALL DUCT REMOVABLE SLATTED AIR DUCT ON FLOOR THROUGH CENTER OF BUILDING AIR OUTLETS A FIG. 8.4. GENERAL PURPOSE BUILDING EQUIPPED WITH AIR DUCT AND FAN UNIT FOR DRYING EAR CORN WITH UNHEATED AIR GRAIN LEVEL MAIN DUCT =—_- = LATERAL OUTSIDE INTAKES AIR FIG. 8.5. GENERAL PURPOSE BUILDING EQUIPPED FOR DRYING AND STORING GRAIN (2 % in. water). The main duct delivering the air should have a crosssectional area of 0.9 m2 (10 ft2). The inlet end of the lateral duct should have a cross-section of 0.645 X 10 ~3 m2/0.0047 m3/s (1 in.?/10 cfm) to 190 DRYING AND STORAGE OF AGRICULTURAL CROPS FIG. 8.6. DRYING SYSTEM FOR ROUND CRIB provide self-balancing characteristics for coarse products, such as ear corn. Grain in sacks may be dried with forced air. A tunnel may be constructed by placing 4 to 6 layers of sacks of grain in a half circle about the air passage. The fan is placed at one end and the opposite end is closed with sacks of grain. It is important that the sacks be stacked as closely as possible to eliminate paths of air escaping through air channels. For seeds, a duct with doors or openings the size of a filled bag is provided to direct air through the bagged seeds. Storage of Small Grains and Shelled Corn The product should be properly stored after it is dried. The basic requirements of bins for grain are (Shedd and Cotton 1949): (1) there should be no openings or cracks permitting loss of product, (2) rain, snow, and soil moisture must be excluded, (3) reasonable protection must be provided against thieves, rodents, birds, poultry, insects, and objectionable odors, (4) facilities must be permitted for effective fumigation to control insects, and (5) reasonable safety must be provided from fire and wind damage. Grain can be kept for considerable time, 3 to 5 years, in proper storage under favorable conditions. Twenty-eight m° (800 bu) of wheat, at an estimated initial moisture content of 12%, were stored in a 35 m3 (1000 bu) steel bin in Kansas. The bin was tight in construction with a 0.15 m (6 in.) center vertical perforated duct connected to a horizontal duct along the floor and extending to the outside. The wheat was not turned, ventilated, or moved while in the storage. It showed no signs of heating, was never fumigated, and had no trace of any damage from insects NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 191 _ or other causes. The milling and baking characteristics compared favorably with those of the adapted varieties grown in thesame locality at the same time. Drying Ear Corn with Forced Air Ear corn has been successfully dried from a kernel moisture content of 40%, w.b., with 0.27 m3/m3s (20 cfm/bu) of air. The minimum ommendation rec- of airflow for ear corn in northern areas is 0.06 to 0.13 m'/m*s (5 to 10 cfm/bu) or 0.03 to 0.05 m3/m3s (2.5 to 5 cfm/basket). The static pressure for ear corn will usually be from 0.142 to 18.7kPa (% to %4 in. water) for depths up to 5 m (16 ft) for airflows of 0.06 to 0.13 m?/ms (5 to 10 cfm/bu). It may be impossible to get the moisture content of the kernels below 20% in some of the northern areas if low temperatures and high humidities occur before the corn is dried. During this season of the year there are often only a few days during which temperatures rise to the high teens (sixties). Enough moisture can be removed by operating the fan during these periods to allow safe storage through the winter. Moisture can be removed from the corn with fan operation in the spring. The fan should be operated when the temperature is above 10°C (50°F), and the relative humidity below 80% for moisture removal. If the temperature is below 10°C (50°F) at the time of cribbing wet corn, the fan should be operated several hours a day to prevent heating. A factor involved in drying ear corn is the moisture in the cob (Fig. 8.7). When the kernel moisture content is 20%, the cob moisture content at equilibrium with the kernel is 32%. The cob has a much lower density than shelled corn. In a bushel of ear corn, at 20% moisture, w.b., there would be 6.8 kg (15 lb) of cobs and 26.75 kg (59 lb) of shelled corn. At a kernel moisture content of 40% there would be 10.9 kg (24 lb) of cobs and 35.8 kg (79 lb) of shelled corn which would yield 0.035 m? (1 bu) (56 Ib) of 15.5% shelled corn. The drying of ear corn is usually done by adapting a corncrib previously designed for natural ventilation. New storage building costs and development of picker-shellers have decreased interest in new crib storage. The same principles involved for grain drying systems apply to ear corn. The major types of ear corn storage adapted for drying are shown in Fig. 8.8 to 8.12. The common shed-type corncrib can be adapted to drying ear corn by placing an air duct, constructed of wood, metal, or canvas, along one side of the crib. The air duct, the floor, and the wall of the crib adjacent to or above the duct and the end next to the fan must be tight. The distance the air must travel from the duct to the top or opposite side of the crib is the same. Another method of using a shed-type corncrib is DRYING AND STORAGE mv OF AGRICULTURAL CROPS 7 a eS} = KERNEL CT] SSS SSS — =Seeag oO (e) a ie) UIT | a oO {e) ph 20 — WET WEIGHT OF MATERIAL L8S./BUSHEL % MOISTURE COB BASIS) (WET CONTENT 3 5 10 20 30 40 50 20 KERNEL MOISTURE CONTENT (WET BASIS) % 30 KERNEL MOISTURE CONTENT (WET BASIS)% FIG. 8.7. POUNDS OF EAR CORN REQUIRED TO YIELD 56 LB OF SHELLED CORN AT 15.5% MOISTURE See Appendix MAKE WALL for metric conversions. AIRi TIGHT LEAVE SLATS OPEN TIGHT KyA 0 e\\ FIG. 8.8. DRYING IN SHED-TYPE CORNCRIB FLOOR NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN ‘bas BACK WALL NOT COVERED | | AIR DUCT Dimensions in Feet DUCT HEIGHT (KH) FRONT WALLS SHOULD BE AT LEAST 1/3 OF CRIB WIDTH (W) & END COVERED FIG. 8.9. CRIB EQUIPPED FOR FORCED VENTILATION DOORS FOR FILLING & VENTILATION — ea "x6" CRIB SIDING SPACED |" APART FILL WITH CORN THIS HEIGHT TO [| UA | ,\aArae Tt R © xe BrPeWy,Stem | 6=00 | NOTCHED FOR. 2°x 10° I-9 SET FAN @ MOTOR IN TUNNEL “pi ans * 6' AIRTIGHT SECTION Construction of gutters with removable covers for drying crib FIG. 8.10. DRYING CRIB FOR EAR CORN 193 194. AND STORAGE DRYING OF AGRICULTURAL CROPS DOORS FOR FILLING @ VENTILATION SET FAN & MOTOR IN TUNNEL 6' AIRTIGHT SECTION FIG. 8.11. ADAPTING CORNCRIB MAKE FLOORS AIR TIGHT FOR FORCED AIR DRYING SNOW FENCING MAIN AIR DUCT, SLATTED AIR ENTRANCE TIGHT SIDED FIG. 8.12. AN INEX- PENSIVE CRIB FOR MECHANICAL DRYING OF EAR CORN WOOD PLATFORM as DUCT ON TREATED WOOD SILLS. LAY BUILDING PAPER TO MAKE FLOOR AIRTIGHT to use a slatted floor with an air plenum below and to make the walls tight by using plywood, metal, or tarpaper. The walls should be slanted so that the ear corn will remain against the wall after drying, thus preventing unnecessary escape of air. A combination machinery shed-corn crib (Fig. 8.11) can be adapted to drying ear corn by placing a slatted duct through the center of the crib. NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 195 Corn can be placed in the entire building, considerably increasing the storage capacity of the building. Bracing in the shape of an X should be placed every 1 m (3 ft) along the length of the bin to provide adequate strength. Round bins with perforated sides or round bins constructed from snow fence can be adapted for mechanical drying of ear corn by using a vertical duct, as shown in Fig. 8.12. It is important that the top of the duct be about the same distance below the surface of the corn as the distance from the duct to the side of the crib. If the duct is extended to the top of the corn, air will escape without drying the grain. A solid circular bin can be adapted to ear corn drying by using a perforated or slatted false floor and blowing the air vertically through the product. The round crib with solid sides or rectangular bin may be adapted to forced air drying by using a duct system with a main duct used through the center or side of the bin with removable lateral ducts, Fig. 8.6. Kar corn can be dried on a floor without using the building walls. A triangular slatted duct, as used for hay drying or for wide corncribs, can be used. It is important that shelled corn, husks, and silks be removed from the ears as the crib is filled. This material will fill the space between the ears and interfere with the movement of air. A screen should be used in the elevator to remove the foreign material. A fan blowing through the ear corn discharge from the elevator will remove most of the light material. Special attention should be given to the area where the elevator drops the ear corn in the crib, because shelled corn will accumulate at this point. The elevator should also have a spout on it which can be moved to direct the corn to various locations in the bin. The minimum airflow rates recommended for ear corn are shown in Table 8.3. A rate of airflow which is too low may permit spoilage of corn by heating and growth of mold. Somewhat faster drying can be accomplished by increasing the rate of airflow. Doubling the airflow does not necessarily double the rate of drying. It is best to use near the minimum effective rate of airflow to keep the power cost low. TABLE 8.3. RECOMMENDED MINIMUM RATE OF AIRFLOW AND MAXIMUM NUMBER OF BUSHELS OF EAR CORN THAT SHOULD BE DRIED PER FAN HORSEPOWER BY MECHANICAL VENTILATION WITH UNHEATED AIR Kernel Moisture, % Minimum Airflow Rate m3/m3s_ _—cfm/bu Maximum Quantity of Ear Corn Dried per Fan 0.75 kW (hp) m3 bu 30 0.083 5 28 25 0.050 3 45.5 800 1300 Source: U.S. Dep. Agric. (1952D). ; é Calculated on the basis of a static pressure of 0.19 kPa (3/4 in. water) and fan air delivery rate of 1.89 m/s (4000 cfm)/0.75 kW (hp). 196 DRYING AND STORAGE OF AGRICULTURAL CROPS The principles of design for adapting cribs for ear corn drying can be summarized as follows: (1) A uniform flow of air through the corn is essential. (2) There must be approximately the same distance for the air to travel from the duct to the atmosphere through all parts of the bin. (3) There should be a cross-sectional area of 1 m? of duct area for each 5.24 m3/s of air (1 ft? of duct area for each 1000 cfm of air). (4) There must be adequate area for outlet of air (for a pressure system) from the crib through doors, louvers, and ventilators, or inlet of air if a suction system. Ear corn should be free of foreign material when placed in bins. (5) (6) Narrow cribs will require that one or more sides be made air tight with reinforced paper or tight sheathing. (7) A shelling trench, which can be used for a conveyor for removing the corn, is seldom large enough to be used as a main air drying duct. Operation with Suction or Pressure The drying system may be designed or operated so that the fan exhausts the air from the bin as a suction system or so that the fan moves air from the outside through the fan and into the bin as a pressure system. There appears to be little difference between the two methods for drying applications. However, for the operation of a grain drying system on the farm the following advantages for the pressure system have been advanced. (1) It is advantageous to have the wettest grain where it can be readily (2) inspected. Drying starts at the duct and the top surface of grain is the last to dry with a pressure system. The first grain to dry. in the pressure system is next to the duct and the shrinkage loosens the grain after drying is started. This results in a net decrease in air resistance. In a suction bin the grain next to the duct does not dry and shrink first. As a result, with a given fan, more air is supplied to the pressure bin than to the suction bin (3) during most of the drying period. Heat energy from the fan and motor aids in drying grain in the pressure system. The temperature rise will be about 1°C (2°F) for airflows commonly used. Harvesting and Storing High Moisture Wheat The use of unheated forced air, heated forced air, and natural ventilation for high moisture harvest is generally limited to wheat at 20% NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 197 moisture or below. Grain above 20% moisture may deteriorate before adequate drying is obtained with unheated air. Heated air drying must be used with caution to avoid damage to the wheat. Wheat is generally below 16% moisture when combined. There is little problem of combining the high moisture wheat, although as the moisture content increases, the grain is more difficult to thresh. It is desirable to use slightly higher than normal straw moisture content to minimize breakage of straw and to keep the chaff on the straw in order to minimize rack and shoe losses. Early harvest is desirable to minimize cutter bar and shatter losses. Use of Heated Forced Air for Grain Drying Drying can be done quickly with heat with minor dependence on the temperature and humidity of the atmospheric air (Table 8.4). More expense is involved with a heated air system and the operator must be aware of the possibilities of a fire hazard and of damaging the grain if a high temperature is used. Commercial elevators have used heated air systems for several years. It has been only in the last 25 years that the drying of crops with heated air on farms has become widely practiced. Several reliable manufacturers merchandise heated air drying equipment which is easy and safe to operate (see Chapter 7). The grain can be dried in a batch or continuous flow bin. One of the characteristics of drying with heated air with which the operator should be aware is that the grain dries to a much lower moisture content at the place the heated air enters the bin, as compared to the moisture content where the air leaves the bin. A thin layer of 0.5 m(1 % ft) or less can be dried or the heating effect can be kept small to avoid unevenness in drying. The thin layers can be provided in a batch or continuous dryer or on a wagon by limiting the drying temperature rise of 11°C (20°F) (called supplemental heating) instead of a temperature rise of 21°C (70°F), which will extend the time required for drying about 5 times. More heat will be required for removing the moisture with a limited temperature rise—about 10 to 12% additional heat is required—but the variation in moisture content will be much less. Heated air is usually used with wagon box drying. Most units are capable of removing 10% moisture from 3.5 m? (100 bu) of shelled corn per hr. Effect of Drying Temperature on Product The effect of the drying temperature on the product must also be considered. If the grain is to be used for seed, a temperature limit of the air of about 43°C (110°F) is recommended. 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TELcStv8I GalSSTPst 9STL8T QT sulAIq FIEVL 8 CROPS Ue) uoljduinsuo*d jo 198 NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN _ 199 is killedat 52°C (125°F) but by maintaining a limit of about 43°C (110°F) air temperature, the heat will not affect the germination when used for an extended time. If the grain is to be milled, it is important that high temperatures [from 60°C (140°F) and above, depending on the grain] be avoided because of the effect of the heat on the chemical structure of the grain. It is common practice for millers to test a sample of the grain for milling properties before purchasing if it is to be used for baking or other food processing. Higher air temperatures could be used for drying since the grain remains cool as long as rapid evaporation of moisture occurs. Temperatures above 52°C (125°F) could be used if the heated air were withdrawn when the grain was dry. To date, a practical method has not been developed to use these higher air temperatures for drying seed. Another possibility for decreasing the variation of moisture content when using heated air is to reverse the direction of airflow after the bin is about one-half dried. First the bottom would be dried and then the top. Although not a standard practice, it offers some possibility of providing more uniform moisture content throughout the bin. When heat drying grain in thin layers of about % to 4% m(1 to 1 % ft) thick, the airflow per bushel is usually high, from 0.3 to 1.6 m3/m3s (20 to 120 cfm/bu). It is impractical, however, to use these high rates of airflows when drying in a deep bin of 2 m (6 ft) and over because of the additional power requirements for forcing the air through the grain. The airflow rates. as recommended for unheated air are used or doubled if possible, for heated air for deep bins, 0.06 to 0.13 m°/m3s (5 to 10 cfm/bu). With a low volume of airflow for heated air, the air picks up a large amount of moisture as it moves through the product. The moisture will often condense near the top layer of the cold grain as it leaves the bin. Thus, the upper layer of the grain may actually gain moisture and molding may occur before drying is completed. Higher airflow rates or decreased depths are followed to prevent or greatly reduce condensation. Drying temperatures up to 71.1°C (160°F) can be used without affecting pearling index, particle size index, break flour yield, flour yield, mixogram area, cookie diameter, and loaf volume for soft and winter wheats grown in Illinois (Ramser 1954). Safe hot air temperatures recommended by the Malt Product Manufacturers in England are as follows (Muntona 1952A,B): Wheat for milling, 66°C (150°F); for malting and seed grain, over 24%, 43.3°C (110°F); and for malting and seed grain, below 24%, 49°C (120°F). They further recommend that air speeds from 0.2 to 0.3 m/s (40 to 60 ft/min) be used for heated air drying. An average of 12 min is needed to remove each 1% moisture under best drying conditions. Lower temperatures are recommended for high moisture products. There is a relationship between the temperature and duration of heating to the moisture content as it affects germination. The relation- 200 DRYING AND STORAGE OF AGRICULTURAL CROPS ship is rather involved but the temperatures mentioned previously apply for the normal period of heated air drying, which is from 30 min to 2 hr. When the initial moisture content of corn is over 50%, the germination is decreased at all drying temperatures over 40.6°C (105°F) (McRostie 1949). When using heated air, it is important that the grain be cooled after drying. Usually the temperature can be reduced to a few degrees from atmospheric within % to 1 hr after the heat is turned off. The temperature of the grain coming from a heated air dryer can be over 37.8°C (100°F), but should be cooled to within 11°C (20°F) of atmospheric temperature for storage to prevent overheating in the bin. Other recommendations for maximum temperatures of heated air for drying of wheat and malting barley which check closely with the English recommendations are presented in Table 8.5. TABLE 8.5. HEATED AIR TEMPERATURES Type of Drying FOR DRYING WHEAT Maximum Air Temperature “© °F AND BARLEY Grain Temperature 46 10 Batch type dryer Seed grain and malting barley Milling grades of wheat Continuous flow Seed grain and malting barley Milling grades of wheat Bin bulk drying 43.3 48.9 110 120 43.3 110 60.0 510) 140 PelO= 20 ile led 43.3 48.9 70 (final) 70 (final) 110 120 above outside air Source: Harris and Sibbitt (1952). The most economical rate of drying grain Sorghum with the lowest costs for power and fuel was obtained with an air velocity of 0.4 to 0.5 m/s (80 to 90 ft/min) (Sorenson et al. 1949). Air heated to 66°C (150°F) is recommended for drying grain sorghum to 12% moisture, w.b., from an original moisture content of 14 to 16%. Grain sorghum was dried with air temperatures as high as 79°C (175°F) without detrimental effect on germination and up to 93.3°C (200°F) without impairing the wet milling characteristics (Sorenson et al. 1949). Ear Corn Drying with Heated Air The same items of equipment for heated air drying are needed for ear corn as for bin drying of small grains. The bin may be constructed as discussed under the section on drying ear corn with unheated air. The temperature of the drying air should not be greater than 60°C (140°F) and in cold weather should be at least 33°C (60°F) warmer than the outdoor temperature (U.S. Dep. Agric. 1952C). Drying can be stopped NATURAL AND FORCED AIR DRYING OF GRAIN AND EAR CORN 201 after the wettest corn is below 20% moisture if the ear corn is in a crib where natural ventilation is effective. When using heated air, the driest corn may be 5 to 10% moisture, w.b., as compared to the wettest corn of 20% in the same bin. If the corn is to be sold, it should be dried to a kernel moisture content of about 15%. If it is to be shelled and stored after drying, the moisture content should be not more than 13.5%. It is recommended that the heat be discontinued but the fan kept in operation in a large crib from 4 to 7 hr before the end of the drying period, because drying will continue until the grain has cooled to the outside air temperature. The relationship between heater and fan capacities is shown in Table 8.6. The temperature increase, °F, with heated air equals 108,000 X the amount of fuel in gal./hr divided by the cfm of the fan. TABLE 8.6. HEATER AND FAN CAPACITIES (To dry 1000 bu of ear corn having 30% moisture) Heater Fuel Capacity gal./hr Fan Capacity cfm Approximate Drying Time Days 4.0 2.0 1.0 0.5 5400 3600 2700 2700 4—6 Saale 16-24 32-48 Source: Cleaver (1946). Conversion factor: 1 gal./hr = 1.05 X 106 m3/s 1 cfm = 0.472 m3/s The airflows recommended for satisfactory heated air drying (Table 8.7) would provide drying in about 3 days from 30 to 12% moisture. TABLE 8.7. AIRFLOW Depth of Ear Corn m ft 132) 4 1.8 6 2.4 8 FOR DRYING SEED CORN (EAR) Flow of Heated Air cfm/ft? 40 50 65 Bil! 10 80 ull i 90 Source: Barre (undated). Conversion factor: 1 cfm/ft? = 0.00508 m3/m?2s Hybrid Seed Corn Drying The systems discussed for ear corn drying serve for the basic elements of hybrid seed corn drying. Hybrid seed is normally dried on the ear. The critical factor is the maximum temperature which can be used without 43.3°C reducing germination. The normal safe operating temperature is be (110°F). At temperatures of 49°C (120°F) and above, the corn must ee EIN NER Riei-beynn, TADS IT F 202 DRYING AND STORAGE OF AGRICULTURAL CROPS below certain critical moisture contents: roughly 48.9°C (120°F), 40%; 51.7°C (125°F), 35%; 54.4°C (130°F), 25%; and 65.6°C (150°F), 20% (Reed and Dungan 1940). Recirculating the drying air to improve uniformity of drying is feasible in deep cribs of high moisture ear corn where mold growth on top layers may occur. The relative humidity of the air is increased, its ability to evaporate moisture is decreased, but the moisture removed will be from a greater depth. The fuel cost is reduced by recirculating some of the air. Good results have been obtained by recirculating enough air to maintain a wet bulb of 21° to 24°C (70° to 75°F) (at a maximum of 50% relative humidity) (Reed 1939). Cost of Forced Air Drying Grain and Ear Corn The cost of drying grain with unheated forced air will vary greatly with atmospheric temperature, humidity, and grain moisture content. The cost of electricity for a fan motor is about 3 to ¥%C/bu for each 0.45 kg (1 lb) water removed (about 1% moisture) with electricity at 242¢/kwh. A rule of thumb for the total cost of drying, including all costs, is 1 to 2¢/bu for each 1% moisture removed. The cost of electricity at 2.5¢/kwh, for drying ear corn with unheated forced air, is about 3¢/bu for reducing the moisture content from 30 to 20% with an airflow of 0.06 m°/m’s (5 cfm/bu). The cost of equipment will usually be from 2 to 3¢/bu making a total of 4 to 5¢/bu for drying ear corn from 30 to 20% moisture with unheated air with reasonably good atmospheric air conditions. Cost of Drying with Heated Air The average total cost for drying shelled corn from 20.1 to 15.5% moisture, w.b., for 29 elevators in Indiana in 1952 was 4C/bu (Davis and Shute 1954). The variable costs are as follows: for fuel, 0.9¢; labor ($1.16/hr), 0.7¢; power, 0.2¢; repairs, 0.1¢/bu; making a total variable cost of 1.9¢/bu. A commercial grain dryer can probably be operated profitably wth a minimum of 1760 m3 (50,000 bu) annually in Nebraska (Whitney 1952). Recent comprehensive data have not been published, but most of the costs have tripled (1978) since 1952. The shrinkage provides another important cost which must be considered that depends upon the amount of moisture removed and the price of the corn. With corn at $1.75/bu at 20% moisture content, the cost of shrinkage when drying to 15.5% is 9¢/bu (Fig. 8.13). Table 8.8 presents data for farm operations. One method of calculating shrinkage can be calculated according to equation (8-5) (Van Arsdall 1956), or, including dry matter loss, equation (8-6). NATURAL AND FORCED AIR 1.00 DRYING a) OF GRAIN AND “| ae a) Sali PRICE EAR CORN = 203 1 SPERSBU 4.00 0.80 3.50 3.00 0.60 2.50 040 DOLLARS 1.50 0.20 18 20 22 MOISTURE 24 26 28 1 1 30 32 34 CONTENT , PERCENT FIG. 8.13. COST OF SHRINKAGE FOR CORN DRIED TO 15.5% FROM VARIOUS MOISTURE CONTENTS TABLE 8.8. DRYING GRAIN AND EAR CORN ON FARMS Temperature Product Ear corn (Illinois) Moisture Range, % Airflow, cfm/bu Dios 20:8 12:0 PNET BE 5 ZAG No.6 20.8-16.3 A iene 205 eal 19.6—-11.4 25.9-18.0 20.4-—14.6 22.6-17.5 28.0—-14.0 Above Atmosphere, °F “& 2.4 Dit 3.4 ee, 12.6 5.0 9.8 4.0 10.2 o.0 6.7 Gril 10.0 46 46 45 38 38 31 29 Ol, 28 2 28 °F Drying Time, 102! hr 27.4 82.9! 82.9! 80.3! 69.0! 69.4! Doyle 5y3}-(0) 48.0? 50.02 49.4? 50.0! 71.0 31.8 22,\5. 38.3 28.0 147.3 43.6 101.0 40.0 27.8 150.0 cfm/ft? Ear corn 23.0—20.0 30.0 (16) (60)4 65.0 (Illinois) 23.0-14.0 26.0—23.0 20-150 35.0 5)Vit) 13.0 (13) 4 28 (55)4 8 50 60.0 57.0 212.0 Soybeans (Illinois) Oe TA 13.8-10.5 18.0 18.0 (21) (7) (70)4 (44)4 308.0 494.0 42 18.0 (22) Shelled corn (Illinois) Sorghum (Texas) le, 56 (72)4 1003 180.0 39 bu/hr AY lites) Os sles 26.0 51 92.0! 50.0 18.5— 9.0 DO GAL? 16.0—12.0 Teco — 1240 DNPH) 32.0 26.0 80.0 80.0 80.0 33 6 39 39 39 60.0! 10.0 70.0! 70.0! 70.0! 40.0 91.0 1.0 1.5 2.9 Sources: Holman and Carter (1947); Ramser (1951); Sorenson et al. (1949). ! Direct heater. 2 Indirect heater. 3 Hot water 99°C (210°F) heated by coal. Water circulated in pipes. Grain heated by conduction. 4 Unheated forced air (air temperature). 204. DRYING AND STORAGE OF AGRICULTURAL CROPS Cost of shrinkage, cents = ~ (100 — initial moisture content) (100 — final moisture content) [Price/bul (8-5) Another approach to shrinkage is to consider, in addition to the moisture loss, a dry matter (D.M.) loss of 0.5%. Equation (8-6) and Table 8.9 are based on that premise (Frederick 1974). Moisture and dry matter losses = D.M. in wet grain, % Se TABLE 8.9. D.M.in dry grain, % SHRINKAGE WHEN GRAIN x 100] + 0.5% D.M.loss (8-6) IS DRIED, % (Table applies to all grains and includes %2% dry matter shrink) Initial Moisture, % 13.0 14.0 Final Moisture, % 15.0 16.0 18.0 16.0 17.0 18.0 19.0 20.0 21.0 22.0 23.0 24.0 25.0 26.0 27.0 28.0 29.0 30.0 3.95 5.10 6.25 7.40 8.55 9.70 10.84 11.99 13.14 14.29 15.44 16.60 ae 18.89 20.04 2.83 3.99 5.15 6.31 7.48 8.64 9.80 10.97 113) 13:29 14.45 15.62 16.78 17.94 19.10 1.68 2.85 4.03 DAL 6.38 1:56 8.74 9.91 11.09 12.26 13.44 14.62 15.79 16.97 18.15 1.70 2.88 4.08 BT 6.46 7.65 8.84 10.03 1122 12.41 13.60 14.79 15.98 VAG le 2.94 4.16 5.38 6.60 E82 9.04 10.26 11.48 12.70 13.91 iB alss Source: Frederick (1974). Wet Corn Stored in Sealed Bins Hermetically sealed units can be used for storing wet shelled corn up to 30% moisture content (w.b.) (Foster 1964). Oxygen is excluded so that the spoilage microorganisms, which are principally aerobic, cannot live. The air surrounding the shelled corn at time of storage is quickly depleted of its oxygen which is replaced by carbon dioxide through respiration. The corn can be crushed or ground before storing to decrease NATURAL AND the oxygen ditions. FORCED AIR DRYING OF GRAIN AND EAR CORN 205 surrounding the stored product and improve storage con- Corn stored at 18% moisture developed a sour odor and darkene d in 1 year of storage. Corn at 27% moisture developed a sour odor and dark- ened in less than 1 month. The viability of the corn decreased to zero in from 1 year to 3 months for 18 and 27% corn, respectively. The fat acidity (mg KOH/100g corn) increased from 50 to 200 in 1 year. There was no great loss of dry matter during storage. The use of shelled corn stored in sealed bins is limited to feeding when removed from the storage. A white powdery mold will develop on corn stored above 14.5% moisture within 24 hr after removal from storage. QUESTIONS 1. What would be the maximum width of a crib which would provide 0.65 m*/ms (5 cfm/bu) for ear corn by natural ventilation with an average wind velocity of 0.45 m/s (8 mph)? 2. Make a graph which illustrates the amount of water in kilograms (pounds) which must be removed from 1 bu of oats, shelled corn, and wheat at 30% moisture (w.b.) to obtain products of 13% moisture. Consider 1 bu of oats as 14.5 kg (32 Ib) at 14.5%, shelled corn as 25.39 kg (56 Ib) at 15.5%, and wheat as 27.2 kg (60 lb) at 14.0%. Construct the graph with the moisture content (w.b.) on the abscissa and kilograms and pounds of water on the ordinate. Show sample calculations. 3. Based on the data in Fig. 8.9, calculate and plot in graphical form the static pressure in inches of water for clean ear corn for depths from 2 to 6 m in 0.5 m increments for airflow rates of 0.065, 0.134, and 0.20 m?/m?s (5, 10, and 15 cfm/bu). Plot depth in meters and feet on the ordinate. 4. Show that equations (8-1) and (8-2) are approximately the same by mathematical analysis. 5. How much moisture is removed from 1 bu of ear corn with a kernel moisture content of 40% (w.b.) when dried to 20%? Assume that equilibrium conditions exist between kernels and cobs at both conditions. 6. Write the fan specifications for drying 35.2 ft? (1000 bu) of 20% wheat, 1.8 m (6 ft) deep in the bin. What size electric motor is required? Compare with depth of 1.2 m (4 ft). 7. A 4.88 m (16 ft) diameter circular bin is to be used for drying 23% moisture shelled corn with a depth of 1.8 m (6 ft). Design an air distribution system consisting of a rectangular center main duct with triangular laterals. Give complete dimensions. 206 DRYING AND STORAGE OF AGRICULTURAL CROPS 8. It is planned to install a mechanical cooling system in a 3500 ft? (100,000 bu) silo storage 30.48 m (100 ft) high for wheat with an air flow of 0.000536 m3/m3s (1/25 cfm/bu). Estimate (a) fan specifications, (b) electric motor horsepower, (c) installation cost, and (d) annual operating cost. 9. A 4.8 X 9.14 m (16 X 30 ft) wooden stave silo is to be used for drying and storing ear corn. Give recommendations for adapting the silo. 10. Sketch a drying bin arrangement where the air plenum, instead of being located at the bottom of the bin, is located halfway between the top and bottom to decrease the distance the air is forced through the grain, and thereby reduce the static pressure. Discuss. 11. Five hundred bushels of 24% moisture shelled corn are to be dried with 0.94 m/s (2000 cfm). Estimate the number of hours of fan operation required to reduce the moisture content to 15% with air at 15.6°C (60°F) and 60% relative humidity. 12. How long is required to obtain one air change in the grain described in question 8? REFERENCES ASAE. 1977. Agricultural Engineers Yearbook. Am. Soc. Agric. Eng., St. Joseph, Mich. BARRE, H.J. (Undated.) Seed ear corn drying with forced heated air. In Farm Electric Service Handbook. BARRE, H.J. and Sons, New York. SAMMET, CLEAVER, T. 1946. 32 (9) 426-427. DAVIS, Northern States Power Co., Minneapolis. L.L. 1950. Farm Structures. John Wiley & Forced drying of a farm crop with heated air. Agric. Eng. W.K. and SHUTE, J.A. 1954. Artificial corn drying by selected elevators in Indiana. U.S. Dep. Agric. FCS Gen. Rep. 5. FOSTER, G.H. 1964. Dryeration—a corn drying process. U-S. Dep. Agric. Agric. Mark. Serv. Bull. 532. FOSTER, G.H. et al. 1955. Effects on corn of storage in air-tight bins. Agric. Food Chem. 3, 682-686. FREDERICK, A.L. hattan, C-503. HARRIS, R.H. and 1974. Selling Kansas grains. SIBBITT, L.D. 1952. Agric. Coll. Bimonthly Bull. 14, 171-175. HOLMAN, PS ia. L.E. 1948. Kan. Coop. Ext. Serv., Man- Watch Adapting cribs for corn drying. that grain drying. N.D. Agric. Eng. 29 (4) 149— HOLMAN, L.E. and CARTER, D.G. 1947. The conditioning of corn and grain with and without heat. Agric. Eng. 28 (9) 397-401. HUKILL, W.V. 1954. Grain drying with unheated air. Agric. Eng. 35 (6) 393— 395, 405. NATURAL AND JOHNSON, FORCED AIR DRYING R.T. and MARVIN, V. OF 1943. GRAIN AND EAR CORN 207 Mechanical ventilation of ear corn. Agric. Eng. 29 (2) 76. KELLY, C.F., CROPSEY, M.G. and SWANSON, W.R. 1942. Performance of cowls for ventilated grain bins. Agric. Eng. 23 (5) 149-151. MADDEX, R.L. and HALL, C.W. 1954. Drying grain with forced air. Mich. State Univ. Ext. Bull. 316. McROSTIE, G.P. mature grain corn. MUNTONA, LTD. 1949. Some factors influencing the artificial Agron. J: 41 (9) 425. 1952A. Malt and Malt Products. Muntona, drying of London. (Monograph) MUNTONA, LTD. 1952B. The Drying and Storage of Combine Harvested Muntona, London. (Monograph) Grain. RAMSER, J.H. 1951. Some results of artificial drying of corn and small grain in Illinois. Univ. Ill. Dep: Agric. Eng. Mimeo Rep. A E662. RAMSER, J.H. 1954. Effect of drying temperatures Univ. Ill. Agric. Exp. Stn. Circ. 730. REED, R.H. 1939. Recirculation Agric. Eng. Mimeo Rep. 200. REED, R.H. and DUNGAN, G.H. on quality of wheat. of air in a seed corn drier. 1940. Univ. IIl. Dep. Drying hybrid seed corn. Univ. III. Dep. Agric. Eng. Mimeo Rep. 279. ROBINSON, R.N., HUKILL, W.V. and FOSTER, G.H. 1951. Mechanical ventilation of grain. Agric. Eng. 32 (11) 606-608. SHEDD, C.K. 1946. Drying ear corn in farm cribs by natural ventilation. Agric. Eng. 27 (9) 426-427. SHEDD, States. SHEDD, corn on C.K. 1955. Storage of ear corn on the farm in the North Central U.S. Dep. Agric. Farmers’ Bull. 2076. C.K. and COTTON, R.T. 1949. Storage of small grains and shelled the farm. U.S. Dep. Agric. Farmers’ Bull. 2009. SORENSON, J.W., JR. et al. 1949. Drying and its effects on the milling characteristics of sorghum grain. Texas A&M Univ. Agric. Exp. Stn. Bull. 710. SWANSON, A.F. 1939. Long time storage of winter wheat. Agron. J. 31, 896— 897. SWANSON, C.O. 1941. The effect of low temperature in preventing damage to wheat stored with high moisture content. Cereal Chem. 18 (5) 299-315. U.S. DEP. AGRIC. 1952A. Drying shelled corn and small grain with heated air. U.S. Dep. Agric. Leafl. 331. U.S. DEP. AGRIC. 1952B. Drying shelled corn and small grain with unheated air. U.S. Dep. Agric. Leafl. 332. U.S. DEP. AGRIC. 1952C. Drying ear corn with heated air. U.S. Dep. Agric. Leafl. 333. U.S. DEP. AGRIC. 1952D. Drying ear corn with unheated air. U.S. Dep. Agric. Leafl. 334. USRY, S.H. 1956. Ear corn storage in North Carolina. N.C. Agric. Exp. Stn. Tech. Bull. 114. 208 DRYING AND STORAGE VAN ARSDALL, companying R.N. 1956. field shelling OF AGRICULTURAL CROPS Farm management and economic problems acand artificial drying of corn. Proc. Conf. Field Shelling and Drying of Corn. U.S. Dep. Agric. Agric. Res. Serv., Chicago, May 15-16, 1956. WHITNEY, R.C. Res. Bull. 55. 1952. Artificial grain drying in Nebraska. Univ. Neb. Bus. \ Systems for Drying of Rice Otto R. Kunze! and David L. Calderwood? Equipment designed for drying grain generally is suitable for drying rice, but methods of operating grain dryers must be changed to accommodate the unique requirements of rice. Rapid moisture adsorption or desorption causes stress cracks that weaken the kernels in rice. Thereafter, the grains may break more easily during harvesting, handling, and processing. Generally, rice must be dried slower than feed grains in any type of dryer if milling quality is to be preserved. In 1977, nearly 4.54 billion kg (10 billion lb) of rice were harvested in the United States from about 910,000 ha (2,250,000 acres) of land. This rice was produced in three distinct geographic regions that consist of (1) an area extending inland about 100 mi from the gulf coast of Louisiana and Texas, (2) the Mississippi River delta region, mostly in Arkansas but including considerable hectarage in Mississippi and smaller areas in Louisiana and Missouri, and (3) the Sacramento and San Joaquin Valley areas in California. Because of variation in climatic conditions and of physical properties of rice, the procedures recommended for drying rice differ for different areas. Three types of rice are produced in the United States. Short-grain rice is plump in appearance and its diameter is about one-half its length. This type is grown mostly in California. The medium-grain type is less plump and is grown to some extent in all of the major production areas. The long-grain type is slender in appearance and is grown in Louisiana, Texas, and Arkansas. Grain type affects drying characteristics. Of the three United States-produced types of grain, the long-grain varieties dry the fastest; the medium-grain, slowest. However, next fastest; and the short-grain, the the milling yields of long-grain varieties are more 1Texas A & M Univ. 2Texas A & M Univ. and USS. Dep. of Agriculture, Agricultural Research Service. 209 210 DRYING AND STORAGE OF AGRICULTURAL CROPS likely to be reduced by a given drying procedure than the milling yields of either medium- or short-grain types. Although manual harvesting methods are common in some parts of the world, rice in the United States is harvested by combines that cut heads from stalks and thresh the grain. Ordinarily, rice is combined at a moisture content above safe storage levels, so additional drying is needed after harvest. Storage moisture of rice in the United States generally is considered to be 12.5% or below. The rice grain (rough or paddy), as harvested, consists of the hull, pericarp, germ, and endosperm. The threshing operation separates the rough rice grain from the panicle. The hulling or shelling operation removes the hull to yield brown rice. Milling converts brown rice into polished rice by removing the bran and germ from the grain. The endosperm that remains is the primary product of rice production. After broken grains are separated (into second heads, screenings, and brewers’ rice), the remaining kernels, which are threequarters of their original size or larger, are classified as whole kernels of milled rice (head rice). The head rice yield is the major factor determining the value of a lot because a broken grain loses about one-half of its economic value. In this respect the marketing of rice is different from the marketing of other cereal grains, which generally are processed and used in a form other than the whole kernel. More than two decades ago, the cost to the rice industry of grains broken during milling was estimated to be $15 million (about 5% of total value) annually (Autrey et al. 1955). Although milling was thought to have caused the kernel breakage, the damage may have previously existed in the grain and milling simply made it apparent. Damage may begin in the field before the rice is harvested and then continue develop during harvesting, drying, storing, milling, and packaging. to Terminology for physical damage to rice kernels, such as surface cracks, cracks, sun-cracks, sun-checks, checks, faults, internal faults, splits, fractures, partial fractures, vacuoles, crack rings, and fissures can be found in the literature. For citations, we generally use the original authors’ terminology. Otherwise, the term “fissure” is used to designate the large crack that usually forms perpendicular to the long axis of the grain. Moisture content values mentioned in this chapter are those given by the original authors. The identification of these values as to wet basis (w.b.) or dry basis (d.b.) was not always given. Generally, we use wet basis for our discussions. Whenever dry basis is used, it is so specified. The metric ton (2204.6 Ib) is used to designate a mass of rice having a nominal volume of 1.74 m*. Airflow is expressed in cubic meters per second per tonne (m?/s-t). This expression can be converted to U.S. Customary system of units, cubic feet per minute per hundredweight (cfm/ cwt), by applying a multiplication factor of 96.1. The nominal volume of SYSTEMS FOR DRYING OF RICE 211 a hundredweight of rough rice is 2.78 ft?. Other conversion units are given in the Appendix. Modern technology is providing an explanation for many of our current drying practices and should provide the information for development of new and more efficient procedures for harvesting, handling, drying, storing, and milling of rice. Optimum Harvest Moisture Both the quality and quantity of rice yield are affected by the date of harvest. The best time for harvesting depends upon climatic conditions, variety characteristics, field conditions, and the stage of rice maturity. The proper stage of maturity for harvesting (U.S. Dep. Agric. 1973) has been reached when the grains are fully mature in the upper portions of the panicle and in the hard dough stage at the base. McNeal (1950A) reported that under Arkansas conditions a rice moisture content between 17 and 23% provided the highest head rice yield for Zenith (a medium-grain variety), and that a moisture content between 16 and 22% provided the highest head rice yield for Rexark (a long-grain variety). In California, it was determined by Kester et al. (1963) that head rice yields of medium- and short-grain varieties were maximum at moisture contents between 25 and 32%. Morse et al. (1967) determined that total grain yield of Caloro (a short-grain variety) increased as the moisture content decreased to about 20% and that the percentage of head rice peaked at harvest moistures between 26 and 30%. Tests were run in Texas by Calderwood and Bollich (1978) to determine the economic feasibility of harvesting rice at a low moisture content as a method of reducing energy for drying. Harvesting of four varieties started at about 25% moisture and was continued at 11 intervals during a 30- to 36-day span. The amount and rate of drying varied with weather conditions. Moisture content dropped steadily during dry weather, but rain and dew caused it to fluctuate. Rice eventually dried to 16% or lower in 10 tests and to 14% or lower in 5 tests. Loss of milling quality was prohibitive for field drying to 14% moisture in all tests, but both the long-grain variety (Labelle) and the medium-grain variety (Nato) retained good milling yields when harvested at 16% moisture. Types of Kernel Failure Stermer (1968) distinguished between moisture desorption, or drying damage, and moisture adsorption, or wetting damage. Desorption cracks were irregular, whereas adsorption cracks were straight. Damage from moisture adsorption was thought to be more serious than damage from 212 DRYING AND STORAGE OF AGRICULTURAL CROPS moisture desorption since visible damage from moisture desorption sometimes disappeared. An example of the two types of damage is shown in Fig. 9.1. Fissures are believed to be caused by (1) the grain surface readsorbing moisture from the environment, (2) the grain surface adsorbing moisture from the center of the kernel, or (3) the grain surface adsorbing moisture from both the center of the kernel and the environment. Under certain conditions, temperature gradients may also be in- fluential in causing the development of fissures. Fissured kernels usually break during hulling and milling operations. FIG. 9.1. LEFT—CRACKING PATTERNS RESULTING:FROM RAPIDLY DRIED MILLED RICE. RIGHT—FISSURES RESULTING FROM RAPID MOISTURE ADSORPTION IN BROWN RICE. Stresses and strains in the kernel should be related to the damage that develops, and attempts have been made to use strain gages to obtain such measurements. Gages can be attached to a milled rice kernel, but, when the gages are energized, the kernel usually fissures. Kunze and Choudhury (1972) developed hypothetical stress diagrams for kernels that were either adsorbing or desorbing moisture. Stresses that are expected in a kernel subjected to a drying environment are illustrated in Fig. 9.2. The illustration is a simplified one in which only one dimension of a three-dimensional kernel was considered. For the illustration, the drying rate around the periphery of the kernel was assumed to be constant. When the total compressive forces within the kernel exceed the tensile strength of the kernel at the surface, desorption damage develops. According to Henderson (1958B), the damage would be expected to progress from the surface toward the center of the kernel. Similar reasoning can be used for a kernel that is adsorbing moisture. When the SYSTEMS FOR DRYING OF RICE 213 RESIDUAL DESORPTION STRESS GRADIENT Cereal Chemistry 49(6):689, 1972. COMPRESSION piesa From Kunze and Choudhury (1972) FIG. 9.2. A HYPOTHETICAL WITHIN A RICE KERNEL STRESS DISTRIBUTION OF RESIDUAL DESORPTION compressive forces around the periphery of the kernel exceed the tensile strength of the kernel in its central region, a fissure develops. Reference is made in the literature to the sun-checking or sun-cracking of rice. The term implies that the sun is responsible for checking the grains, but generally the definition of sun-checking within the industry is sufficiently broad to include any reduction in head rice yield before the grain is harvested. Stahel (1935) recognized the term to be a misnomer and stated that rough, high-moisture rice could be dried in a thin layer in the sun to below 10% moisture without the formation of cracks. He said that the cracks that subsequently developed were caused by a rise in moisture content after the grain had dried. When rice was dried to 14% or below and then allowed to readsorb moisture, grains developed fissures. Grain Fissuring by Moisture Readsorption Readsorption of moisture by rice has been identified as a cause of fissured grains. The ambient air is a convenient source from which low-moisture rice can readsorb moisture. Kondo and Okamura (1930) found that rice at storage moisture can be made to fissure when exposed to dry days and humid nights. One of their experiments indicates that 72% of rough rice kernels fissured after being exposed to the ambient air for only 24 hr. Kunze (1964) and Kunze and Hall (1965, 1967) moved brown rice through 10% increments of increasing relative humidity in 24-hr periods without causing fissures to develop. However, fissures 214. DRYING AND STORAGE OF AGRICULTURAL CROPS developed when the difference between the lower and higher humidities approached 20%. Observations of similar moisture adsorption conditions and subsequent fissuring in wheat have been reported by numerous researchers (Kunze 1977A). Before Harvest.—The moisture content to which rice grains must dry before they will fissure when moisture is rapidly readsorbed can be designated as a critical moisture level. It is commonly believed that grains are sufficiently plastic to prevent the development of fissures when moisture is above 14 to 16%. Different rice varieties probably have different critical moisture levels. The suddenness and magnitude of an environmental change from a relatively dry to a moisture-adsorbing environment might cause different threshold moistures. McDonald (1967) observed fissures in grains with moisture contents up to 21%. Kunze and Prasad (1978) reported that 14 of 60 of the most mature grains harvested from a plot were fissured when the average moisture of a field sample was 29%. Obviously, the field sample consisted of grains with much higher and much lower moisture contents than 29%. McDonald found that the moisture of grains within a panicle differed more than 10%. Thus, the moisture of grains in different panicles can differ much more than 10%. Current technology indicates that fissured grains in the field have at some time dried to the critical moisture level and later readsorbed moisture. During a typical 24-hr cycle, grains lose moisture in the daytime and readsorb moisture at night. After Harvest.—Diversity of moisture among rice grains in the field needs further definition. The moisture content of each maturing grain is continuously changing. After the crop starts to mature, initially the moisture content of most grains is above the critical level. Conditions are transient from day to day and even within a given day. According to Stahel (1935), changes in moisture content from 20.9% at 8 A.M. to 15.2% at 4 P.M. have been observed. Experiments with low-moisture rice by Kondo and Okamura (1930) indicated a 3.1% change in an 8-hr period, but only a 0.2% net moisture loss in 24 hr. Twice a week during the harvest season, Chau and Kunze (1978) collected the 10 driest and the 10 wettest panicles (by observation) from a plot. Samples of 10 grains were taken from the top and bottom of each panicle. On the day before the plot was harvested, the driest sample had a moisture content of 14.9% and the wettest sample 52%. At harvest, average moisture of the grain was 22%. According to Wratten and Ken- drick (1970), rough rice at a moisture of 22% anda temperature of 26.7°C (80°F) produces an interstice relative humidity of 97%. Rice at a moisture of 14% and a temperature of 26.7°C produces an interstice relative humidity of 75.6%. Thus, low-moisture grains in a harvested mass could be subjected to a relative humidity more than 20% SYSTEMS FOR DRYING OF RICE 215 higher than their equilibrium relative humidity at the given temperature. This environment would be a moisture-adsorbing one for low-moisture grains and might cause them to fissure. Such conditions could exist in a combine hopper, transport cart, truck, or holding bin before rice is dried. The high- and low-moisture kernels would equilibrate toward the average moisture of the grain mass. Fissuring damage, if it developed, would commence within a few hours. During Drying.—The foregoing discussion has indicated the potential for low-moisture grains to fissure while they are in the field or while they are held in a mixture of freshly harvested high-moisture rice. Holding of the grain mixture could be eliminated by immediate drying, but this is impractical with current harvesting procedures. Even if the interval between drying and harvesting could be eliminated, the reactions of the low-moisture grains during drying would need to be observed. Kunze and Prasad (1978) placed rice at storage moisture on top of a 40.6 cm (16 in.) column of high-moisture grain in a laboratory dryer. While the column of grain was dried with heated air, the low-moisture grains on top of the column adsorbed moisture from the humid warm exhaust air and fissured. Results of their experiments are shown in Table 9.1. TABLE 9.1. HIGH MOISTURE ROUGH ON TOP OF DRYING COLUMN Type and Number High Moisture Rice, % w.b. 14.5 15.0 28.0 RICE DRIED WITH LOWER of Low Moisture Kernels MOISTURE RICE Fissured. Variety Temp, 5 Drying, hr Column Depth, cm Column Top % Fissured Grains Brown Rough Lebonnet Lebonnet Lebonnet 59.4 59.4 57.8 2.26 2.50 oO) 3015 40.6? 40.6 16 92 100 8 18 100 16.8 22.0 25.3 28.0 Nato Nato Nato Nato 59.4 59.4 58.9 58.9 3.20 5.29 5.75 6.25 40.6 40.6 40.6 40.6 98 100 100 100 24 78 90 100 20.5 32.0 Brazos Brazos 56.7 58.9 4.75 5.75 40.6 30.5 100 100 92 100 Source: Kunze and Prasad (1978). 130.5 cm = 12 in. 240.6 cm = 16 in. Three forms of rice were used to determine the magnitude of the fissuring potential in a given drying procedure. Milled rice at storage moisture was found to be more sensitive to a moisture adsorbing environment and to fissuring than brown rice. Rough rice was more resistant to fissuring than either milled or brown rice. If milled rice could not be made to fissure, then the potential for development of damage in brown or 216 DRYING AND STORAGE OF AGRICULTURAL CROPS rough rice was small. The exhaust air was usually near 100% relative humidity when drying commenced and then became less humid with time. Drying was stopped when the exhaust air reached 37.8°C (100°F). More than 80% of the low-moisture rough rice on top of the drying column fissured when rice at harvest moisture (20.5 to 25.3% w.b.) was dried for several hours. This result indicated that low-moisture grains in a mixture or mass of rice in a dryer may fissure if they are above or ahead of the drying front. The low-moisture grains readsorb moisture until the drying front reaches them. Thereafter, they start to dry. Kunze and Prasad (1978) reported that a 4 to 5% moisture difference between high- and low-moisture rice in a mass to be dried was sufficient to start development of fissures in low-moisture rough rice. According to Wratten and Kendrick (1970), the interstice air for rough rice at a moisture content of 11% and 26.7°C (80°F) will have a relative humidity of 56%, but the air for rough rice at a moisture content of 15% and the same temperature will have a relative humidity of 80.4%. The 4% difference in moisture causes a 24.4% change in relative humidity. The previous discussions concerned large portions of high-moisture grain being mixed or dried with small portions of low-moisture grain. If a large volume of low-moisture grain were mixed with only a small volume of high-moisture grain, no fissures would be expected to develop in the low-moisture grains. Grain Fissuring After Rapid Drying Rice cracking was related by Ban (1971) to high rates of moisture removal and to the moisture span through which the rice was dried. The grains did not necessarily crack during drying or immediately thereafter. Rapidly dried rice was stored under airtight conditions in which the grain neither dried nor adsorbed moisture. Several hours after drying, the grains began to fissure and then continued to do so for about 48 hr. Henderson (1958B) used X-ray methods to inspect rice for fractured grains. He found that many apparently sound kernels contained internal fractures that originated at the center and developed along the minor axis toward the outside. He also found that (1) exposure of rice to dew caused a reduction in half checks and an increase in whole checks and (2) Poe rice yield was highest when the lowest drying air temperature was used. Kunze (1977B) also studied the fissuring of rice grains after heated air drying. His objective was to determine the extent of fissuring in rough rice immediately after it was dried and then to document fissures that developed in dried whole grains within the next 24 to 48 hr. When 1000 grains of brown rice from 4 varieties were dried for periods ranging from SYSTEMS FOR DRYING OF RICE 217 2 to 10 hr at a temperature of 58°C (137°F), 7.5% of the grains were fissured after the drying treatments. Only 3.2% of the rough rice grains fissured from the same treatments. Other rough rice at storage moisture was dried for 10 hr at 60°C (140°F). After the rice was dried, the grains were shelled and some were found to be fissured. Some of the remaining good grains were formed into a rosette and kept in an air- conditioned room (Fig. 9.3). Pictures were taken periodically during the next 24 hr to provide a record of fissure development. As indicated in the figure, the grains were not fissured immediately after drying but had fissured after 24 hr. Ban (1971) stated that crack rings (fissures) develop even if the grains are sealed in a container and have no environmental exposure. From Kunze (1977B) FIG. 9.3. ROUGH RICE DRIED AT 60°C FOR 10 HR. LEFT—Good grains 1 hr after drying. RIGHT—Same grains 24 hr later. In another approach used by Kunze (1977B), field-harvested, highmoisture rice was dried continuously for 12 hr at 59°C (138°F). A batch dryer with a column depth of 63.5 cm (25 in.) was used. Rice originally at 29.8% moisture was reduced to 9.1% in a single pass. The rice was milled rice shortly after it was dried and gave a total yield of 76% with a head yield of 74%. The milled rice was left exposed to the ambient air and soon damaged. commenced fissuring, which continued until all the grains were It is commonly believed that fissuring due to a severe drying treatment fissuring may occurs while rice kernels are in the heated air dryer, but test results Milling drying. air occur for as long as 48 hr after heated 218 DRYING AND STORAGE OF AGRICULTURAL CROPS from samples milled immediately after drying are often not consistent with those from samples milled several days later because of the additional fissuring that may occur. Samples milled from rice after several days of storage are quite stable for an extended period of time. The common element in the research efforts of Ban (1971) and Kunze (1977B) is a rather steep moisture gradient of which the higher moisture has been at the grain interior and the lower moisture at the surface. A low-moisture rice grain apparently can withstand a small gradient, but large gradients cause the development of fissures when the gradient decreases. This information can be related to current drying practices. Air for deep-bed drying should be heated only enough to provide a slow moisture removal rate which will not cause a steep moisture gradient. Specific recommendations are discussed in the section “Bin Drying— Supplemental Heat.” In multipass operations, rice is subjected to heated air for short periods and then allowed to temper for relatively long periods, a practice that prevents the formation of steep moisture gradients. Non-mixing-type dryers have the potential to establish high moisture gradients in the grain next to the warm air plenum. Mixing-type dryers subject the grain alternately to hot, dry air and to warm, humid air, a procedure which prevents the formation of a steep moisture gradient. Mechanical Damage Mechanical equipment used to harvest, handle, convey, and process rice has the potential to break the grain. McNeal (1950B) reported that combines hulled an average of 4.91% of the harvested grains and broke an average of 3.59% of them. Matthews and Spadaro (1975) reported that the combine operation broke about 6% of the rough rice, whereas only 0.4% of the rough rice in hand-harvested lots was broken. The rice industry should accurately assign damage to the production or process operation in which it develops. All the sources of grain damage have probably still not been identified. Further research to delineate the different forms of damage and their sources is necessary. The foregoing information should provide readers with a background for the study of rice drying systems. Drying Systems Two distinctly different systems for handling and drying rough rice are commonly used in the United States. One system is commercial drying, in which large, continuous flow heated air dryers are used in SYSTEMS FOR DRYING OF RICE 219 multipass operations. The other is bin drying, in which rice is both dried and stored in the same bins. Bin drying is the most widely used on-farm drying system. Commercial Rice Drying Facilities for commercial rice drying include both privately owned dryers and farmer-owned cooperative dryers as well as some of the rice mills that purchase freshly harvested rice. A farmer-owned cooperative dryer is shown in Fig. 9.4. Generally, each facility is equipped with one or more continuous flow heated air dryers, several storage bins, and a conveying system for moving rice as required for receiving, drying, and shipping. At some drying facilities, the identity of each lot is preserved, but at other facilities, lots of similar varieties, quality, and moisture content are commingled. Preserving identity is a common practice at commercial drying facilities in Louisiana and Texas and requires many small bins, while commingling is the common practice at marketing cooperatives in Arkansas and California. When several lots of rice are commingled, the dryers are emptied and refilled fewer times than would be necessary if identity of individual lots had to be preserved. With commingling, productive drying time is increased and the utilization of space in large bins is improved. RICEGROWERS | ‘AMERICANCO-OP, ASSN. FIG. 9.4. COMMERCIAL RICE DRYING FACILITY, BEAUMONT, TEXAS 220 DRYING AND STORAGE OF AGRICULTURAL CROPS Handling Undried Rice.—Since commercial dryers are likely to be a considerable distance from fields where rice is grown, hauling green (freshly harvested) rice for distances up to 32 km (20 mi) in farm trucks is common. Generally, the loading of a truck directly from a combine is impractical, so self-propelled, self-unloading field carts are used to transport rice from the combine to the nearest point at which a truck can approach the field. Upon arrival at a dryer, trucks are weighed both full and empty to determine the product weight. Rice is sampled only for a moisture test if it is part of an identity-preserved lot, but it must be sampled for quality if rice is to be commingled. After the rice is weighed and sampled, it is dumped into a receiving pit, then conveyed to a bin for temporary storage until the entire lot is received and drying begins. Storing Undried Rice.—The rate at which a commercial dryer can receive green rice is much faster than the rate at which it can dry the product. Commercial dryers commonly receive a large amount of rice whenever it is available. They store some of it “undried” until it can be scheduled into the drying routine. High moisture content rice is subject to quality deterioration as evidenced by discolored kernels after it is milled. Increasing moisture contents, storage temperatures, and storage times each increase the rate of quality deterioration. McNeal (1960) found storage damage in 24% moisture content rice stored at a temperature of 10°C (50°F) for a period of only 24 hr. Activity of microorganisms, principally molds, causes high moisture content rice to heat spontaneously when stored in bulk. Spontaneous heating causes rapid quality deterioration. Aeration at a suitable rate dissipates heat and keeps rice temperature at or near the ambient air temperature. When a heat buildup is prevented, the rate of quality deterioration is slowed. Tests on storage of high-moisture rice (Calderwood 1966) provide data from which safe storage time can be predicted. Results of these tests are presented in Fig. 9.5. Rice moisture content, average ambient temperature, and aeration system airflow rates are factors in predicting a safe storage time. Assays for aflatoxins in rice samples from these tests indicated that samples which were graded No. 1 seldom contained more than a trace of aflatoxins (Calderwood and Schroeder 1968). Continuous Flow Dryers.—The continuous flow heated air dryers are upright columns through which rice flows by gravity and is exposed to heated air while descending. When rice follows a straight path, the dryer is classified as a nonmixing-type. If the direction of rice flow is diverted by baffles, the dryer is classified as a mixing-type. The rate of rice flow through either type of dryer is regulated by feed rolls or oscillating rockers at the bottom of the column. In a nonmixing-type dryer, rice flows between two parallel screens spaced 15 to 23 cm (6 to 9 in.) apart. Heated air flows horizontally SYSTEMS FOR DRYING OF RICE 221 STORAGE TIME, DAYS Is I9 20 al ce MOISTURE CONTENT, % (wb) ae) From Calderwood FIG. 9.5. SAFE STORAGE TIME MAINTAINING U.S. NO. 1 GRADE FOR MOIST RICE IN AERATED 24 (1966) BINS BASED ON through the screens and rice. High air velocities are normally used because the kernels cannot be blown away. Since the kernels tend to flow straight downward, the kernels nearest to the screen through which air enters are always exposed to hotter and drier air than are the kernels near the exhaust screen. Kernels from both locations are mixed as rice is discharged and conveyed to bins for tempering or storage. The mixing-type dryer commonly used for rice drying consists of a vertical compartment across which rows of baffles (sometimes called “racks” or “air channels’) are installed. One end of each baffle is open and the other end is closed. Alternate rows are open to heated air and intervening rows are open to the exhaust-air plenum. Alternate rows are offset to split the grain stream as it flows downward, thereby causing a mixing action. Individual kernels encounter successively hotter and cooler air and no kernel is continuously exposed to the hottest air. Chaff and light material may be carried away by the exhaust-air stream. Rice kernels also may be blown into the exhaust plenum and provisions should be made for recovering sound kernels that are so displaced. 222 DRYING AND STORAGE OF AGRICULTURAL CROPS Heated air in a nonmixing-type dryer characteristically has a higher velocity and lower temperature than does the heated air in a mixing-type dryer. Typical airflow rates are 1.87 to 4.39 m3/s:t (180 to 422 cfm/cwt) in the nonmixing-type dryer and 0.93 to 2.19 m°/s:t (89 to 211 cfm/cwt) in the mixing-type dryer. Air temperatures seldom exceed 54°C (130°F) in the nonmixing-type dryers, whereas temperatures of about 66°C (150°F) are used in mixing-type dryers. Multipass Drying.—In a typical multipass operation with either type of dryer, rice is exposed to heated air for 15 to 30 min, and moisture content is reduced by 2 to 3% of the dry weight during each pass. Between passes, rice is removed from the dryer and stored in a bin for 4 to 24 hr for tempering. The number of passes required for drying any given batch of rice depends upon the initial moisture content and the amount of moisture removed per pass. The total elapsed time for drying a batch of rice by the multipass method generally exceeds 24 hr and may be as long asa week, but individual rice kernels are in the dryer for only 1 to 2 hr. Tempering.—Storage of rice in a bin after a dryer pass provides time for moisture within a kernel to move from the interior to the external surface where it is readily available for evaporation. This process commonly is called tempering. Observations in Louisiana (Wratten 1959) indicated that drying efficiency increased with tempering time up to 24 hr. Drying efficiency increased rapidly during the first 6 hr of a 24-hr tempering period, but only slowly during the final 12 hr. California tempering studies (Wasserman et al. 1964) indicated that rice temperature affected tempering time. Rice that was rapidly cooled to 24°C (76°F) immediately after drying required 6 hr for adequate tempering, but rice stored at 40°C (104°F) required only 4 hr for the same amount of tempering. When period should not molding, souring, more than 24 hr, practice. rice is stored at a high temperature, the tempering exceed 24 hr because kernel damage may occur due to and yellowing. If the next dryer pass must be delayed cooling rice in a bin by aeration is the recommended Supplementary Drying with Aeration.—Whenever warm and moist rice is slowly cooled by aeration, it gives up moisture even when the relative humidity of ambient air is high. This process was investigated by Calderwood and Hutchison (1961). Their tests indicated the average amount of drying to be about 1% (d.b.) per cooling period and 30 to 50% of the total drying occurred during aeration periods of lots of rice that were aerated after each pass through a dryer. Each cooling period should be long enough for rice temperatures in all locations in a bin to approach SYSTEMS FOR DRYING OF RICE 223 the average ambient temperature. Although the time require d to completely cool a bin of rice depends on both the amount of moisture removed and the airflow rate, Calderwood and Hutchison (1961) determined that cooling time can be estimated with a suffici ent degree of accuracy for most practical purposes by considering the airflow rate to be the only variable. An estimate of cooling time vs. airflow rate is provid ed in Fig. 9.6. 100 80 fe)ie) S oO COOLING TIME, HOURS nN oO O 2 AIRFLOW 4 6 8 10 l2 4 RATE, m°/s:‘t X 107° (cfm/cwt X 0.096) From Calderwood and Hutchison (1961) FIG. 9.6. ESTIMATED COOLING TIME VS AIRFLOW RATE FOR ROUGH RICE AFTER A PASS THROUGH A HEATED-AIR DRYER Dryer Adjustments.—The operator of a continuous flow, heated air dryer can adjust the heated air temperature and the feedroll speed that controls the flow rate of rice. Sometimes the dryer operator will monitor the temperature of rice as it flows from a dryer and make an adjustment either of feedroll speed or heated air temperature while holding the other control at the same setting in order to keep the rice temperature at some maximum value that is considered safe. Calderwood and Webb (1971) conducted tests on the operation of a mixing-type dryer in which they 224 DRYING AND STORAGE OF AGRICULTURAL CROPS compared (1) results when a high air temperature and fast feedroll speed were used with (2) results when a lower air temperature and slower feedroll speed were used. With both methods rice was removed from the dryer at the same temperature. With the high air temperature and fast feedroll speed, drying time was less and moisture removal and milling yields greater than when the lower air temperature and slower feedroll speed were used. In another test reported by Calderwood and Webb (1971), the feedrolls were set such that rice would be retained in the dryer for 15 min during each pass and the air temperature control setting would remain unchanged for a particular lot of rice during each of the required number of passes through the dryer. Six lots of rice were dried with the air temperature control set at a different temperature between 43° and 78°C (110° and 172°F) for each lot. The temperature of rice as it flowed from the dryer increased with each successive pass. Drying time was about 40% less and head rice was 1% less when the air temperature control was set at 78°C (172°F) than when it was set at 43°C (110°F). In another series of tests, the air temperature control was adjusted such that each lot of rice would be removed from the dryer at a particular temperature between 35° and 49°C (95° and 120°F) after each dryer pass (Calderwood 1972). The feedrolls were set such that rice remained in the dryer 16 min per pass. Then the rice was stored in a bin to be cooled by aeration before another dryer pass. Drying rice from 20 to 12.5% moisture content required 7 passes when the rice temperature was 35°C (95°F), but only 3 passes when its temperature was 49°C (120°F). Rice that had been removed from the dryer at 49°C (120°F) had a subsequent head rice yield that was 2 to 3% less than that of rice that had been removed at 35°C (95°F), and its germination rate was 4% lower than that of 35°C (95°F) rice. Fuel consumption was not appreciably affected by the drying temperature, but more electric energy was required for drying rice at a low temperature than at a high temperature because more dryer passes were needed. The airflow rate of a continuous flow dryer generally is fixed by the manufacturer’s design and is not adjustable. However, tests were run with an experimental mixing-type dryer (Calderwood and Webb 1971) in which the airflow rate was varied. In those tests the air temperature control was set at 66°C (150°F), and the feedroll speed was set such that rice would remain in the dryer for 15 min per pass. The airflow was varied from 1.87 to 3.03 m*/s:t (180 to 291 cfm/cwt) by changing the dryer fan speed from 800 to 1300 rpm in increments of 100 rpm. The temperature of rice as it flowed from the dryer increased as airflow rates increased. Increasing the airflow rate from 1.87 m3/s-t (180 cfm/cwt) to 3.03 m*/s't (291 cfm/cwt) increased the amount of drying per pass from 2.2 to 3.0% (d.b.). SYSTEMS FOR DRYING OF RICE 225 Combination Drying.—Heated air, multipass drying can be combined with deep-bed drying. This practice has become common in Califor nia (Wasserman et al. 1969). The rice is dried to below 18% moisture by the multipass method and then to 12.5% moisture with ambient air while in storage. The combination system provides an inexpensive way to expand plant capacity because continuous flow dryers can handle a much larger volume of rice when their use is discontinued at a rice moisture of 16-18% than when their use is continued until moisture is reduced to 12.5%. The deep-bed dryers provide both low cost storage facilities and additional drying capacity. California finish-dryers hold as much as 4536 tonnes (100,000 cwt), and beds are up to 10 m (33 ft) deep. On-farm Drying The common method of drying rice on farms is deep-bed drying in a storage bin equipped with an air distribution system and a fan. Portable grain dryers are not widely used for rice drying and only a few large farms use continuous flow heated air dryers. Bin Drying.—A round, steel bin having a perforated metal floor and a plenum chamber below the floor is the structure most commonly used for bin drying of rice. The perforated metal floor provides uniform air distribution throughout the cross-sectional area of the bin and the round shape is well adapted to mechanical unloading methods, although rectangular buildings equipped with an air distribution system and fan are sometimes used for deep-bed rice drying. Both axial flow (propeller) and radial flow (centrifugal) fans are suitable for bin drying, but axial flow fans are most commonly used because of their simple construction, easy installation, and low cost. The direction of airflow has little effect on dryer performance; however, air is custom- arily moved in an upward direction so the wettest layer of rice will be at the top where it may be sampled easily. Airflow Rate—The airflow rate required for successful bin drying must be adequate to reduce the moisture content of rice in all parts of the bin to about 15% within a relatively short time after harvesting to prevent quality loss from mold activity, which would be evidenced by discolored (“damaged”) milled kernels. Drying tests in Texas (Sorenson and Crane 1960) indicated that rice sustained damage resulting in grade reduction when the moisture content in the wettest layer of rice at temperatures above 26.7°C (80°F) remained above 15% moisture content for 8 to 10 days; however, rice at 18°C (65°F) remained above 15% moisture for as long as 15 days without evidence of damage. After rice moisture had 226 DRYING AND STORAGE OF AGRICULTURAL CROPS been reduced to 15%, the length of drying time within which moisture was reduced to 12.5% was not critical. Sorenson and Crane (1960) recommended an airflow rate of 58.2 X 1073 m3/s-t (5.6 cfm/cwt) for rice having an initial moisture content of 20% (w.b.). With this airflow rate, the rice bed usually should be no deeper than 2.44 to 3.01 m (8 to 10 ft) for economical use af power. If the initial moisture content of rice exceeds 20%, the bed depth should be reduced. The design depth of 2.44 to 3.01 m (8 to 10 ft) may be exceeded if the initial moisture content of rice is well below 20%. Greater depths also may be used after moisture content of rice in the top layer is reduced to 15%. For example, 15% moisture rice from another bin could be added to the top of 15% moisture rice in a given bin, resulting in a depth greater than 3.01 m (10 ft). With this greater depth the airflow rate would be reduced and a longer period of fan operation would be required before the moisture content would be reduced to 12.5% in all parts of the bin. In the California rice-growing area where the climate is drier and cooler than it is in the Texas rice-growing area, lower airflow rates are suitable for bin drying. Henderson (1958A) recommended airflow rates of 26.0 X 10°? and 41.6 X 1073 m?/s:t (2.5 and 4.0 cfm/cwt), respectively, for rice having initial moisture contents of 20 and 25%. Fan Operating Schedule.—Sorenson and Crane (1960) recommended the following operating schedule for unheated air drying in Texas: Start the fan as soon as the air distribution system is completely covered; operate the fan continuously as the bin is filled and until the moisture content of rice 0.3 m (1 ft) below the top surface is reduced to 15%; then operate the fan only when the relative humidity of ambient air is 75% or less; continue drying until the moisture content of rice 0.3 m (1 ft) below the top surface is reduced to 12.5%. Drying Time.—The time required to dry a bin of rice with unheated air depends upon the initial moisture content of the rice, the airflow rate, and weather conditions. In Texas, Sorenson and Crane (1960) dried rice from 18 to 12.5% moisture content using an airflow rate of 58.2 X 1073 m3/s‘t (5.6 cfm/cwt) in 40 days during a period of unfavorable drying weather and in 19 days during a period of good drying weather. Supplemental Heat.—Supplemental heat may be used during periods of high relative humidity or cold weather, or both, to increase the rate of drying. Excessive heat will overdry the rice that is first exposed to the heated air (rice near the bin floor), resulting in unnecessary weight loss. Excessive heat may also reduce head rice yields and the percentage of kernels that will germinate. Since rough rice at 12.5% moisture is in equilibrium with 25°C (77°F) air at about 65% relative humidity, the extent to which air is heated should be the minimum needed to reduce its SYSTEMS FOR DRYING OF RICE 227 relative humidity to 65%. A study of weather records for the Texas rice growing area by Sorenson and Crane (1960) indicated that when relative humidity is 75% or above, ambient air temperature usually is 27°C (80°F) or lower. Therefore, the temperature rise of ambient air should not exceed 7C (12°F) because this amount of temperature rise will reduce relative humidity of 27°C (80°F) air from 95 to 65%. When supplemental heat is used, fan operation may be continuous throughout the entire drying operation. Supplemental heat is commonly controlled by a humidistat in the plenum chamber of a rice dryer. This controller actuates a heater (generally an LP-gas burner) when the relative humidity is above a preset value. Heat is applied only when the condition of the ambient air is unsuitable for unheated air drying. Sorenson and Crane (1960) compared the times for drying rice from 19 to 12.5% moisture content with unheated air and with supplemental heated air having a 6°C (10°F) temperature rise during periods of high relative humidity. This operation required 31 days with unheated air and only 17 days with supplemental heat. Solar Heat.—Most of the time when solar heat can be collected, the relative humidity of ambient air is less than 65% and low enough to dry rice to 12.5% moisture content without additional heat. Under these circumstances, immediately applied solar heat is not adaptable for use as supplemental heat with the method recommended by Sorenson and Crane (1960). Tests were run in Texas (Calderwood 1977) to determine if immediately applied solar heat affected drying time and milling yield of rice as compared with unheated air drying. A solar collector having an absorber surface area of 2.15 m?/t (1.05 ft?/cwt) of stored rice provided an increase in the plenum air temperature above ambient air temperature averaging 6°C (10°F) for an airflow rate of 58.2 X 10-3 m*/s't (5.6 cfm/cwt) during a 10 hr period on clear days. Drying comparable lots of rice from 19.6 to 12.0% moisture content required 15 days with solar heat and 20 days with unheated air. In another test, drying comparable lots of rice from 16 to 12% moisture content required 10 days with solar heated air and 19 days with unheated air. There was little difference in the percentage of head rice for rice dried with unheated air or with solar heated air. Handling Equipment—Round drying bins commonly are positioned on the circumference of a large circle with a cone-shaped dump pit in the center (Fig. 9.7). The bins are filled by a portable auger loader that moves rice from the pit and discharges it into any of the bins or into a truck. Rice is unloaded through a center opening in the floor of a round bin. Rice that will not flow to the center opening by gravity is moved there mechanically by a sweep auger. A screw conveyor, installed below 228 DRYING AND STORAGE OF AGRICULTURAL CROPS FIG. 9.7. RICE DRYING BINS INSTALLED ON THE CIRCUMFERENCE OF A LARGE CIRCLE WITH A PORTABLE LOADING AUGER TO MOVE RICE FROM CENTRALLY LOCATED DUMP PIT the perforated metal floor, moves rice from the center opening to the dump pit. The portable auger loader can be used to transfer rice from one bin to another or to load it into a truck. The use of spreaders to level rice in drying bins eliminates the need for manual labor to perform that task. Some models of spreaders throw grain at a high velocity, and such handling may cause grain damage and packing. Heavy material tends to be thrown toward the bin walls; whereas, light material generally settles in the center of a bin. As a result, airflow and drying are nonuniform throughout the cross-section of such a bin. Vertical stirring augers elevate rice from near the floor of the bin to the top surface as they move about the bin in a set pattern that affects all of the rice in the bin periodically. This action has the beneficial effect of reducing the packing that results from loading the bin. Stirring prevents the development of a typical drying front and the moisture content of the rice in a bin is nearly uniform throughout the entire depth (Calderwood 1977). Use of a stirring auger reduces the risk that rice in any part of the bin will remain at a high moisture content long enough to sustain damage by molding. Devices are available that convert drying bins into continuous-flow dryers. One of these is a tapered sweep auger that uniformly unloads SYSTEMS FOR DRYING OF RICE 229 layers of dry rice from the bottom of the dryer while green rice is loaded into the top of the dryer. Another device for converting a bin dryer intoa continuous flow dryer is a revolving radial arm that distributes wet grain on one side of the arm while dry grain is unloaded on the other side. The retention time of grain being dried is the time required for the radial arm to move around the circumference of the bin. Portable Grain Dryers.—Portable grain dryers, either continuous flow or batch, are not widely used for rice drying. Drying of rice is excessively slow because low air temperatures must be used to avoid kernel checking. Therefore, a very large size dryer is needed to keep up with the amount of rice harvested by a single combine. In a test of a batch dryer (Calder- wood 1970), drying time varied from 6 hr with 49°C (120°F) air to 3 hr with 66°C (150°F) air. Rice dried with 49°C (120°F) air had a milling yield equal to that of rice dried with unheated air, but the head rice yield was reduced by 8% when 66°C (150°F) air was used. A portable, continuous flow (concurrent flow) dryer (Calderwood 1970) required 3 passes using an air temperature of 93°C (200°F) and a rice flow rate of 0.13 t/min (288 lb/min) to dry rice from 22 to 14.6% moisture content. The milling yield of rice receiving this treatment was reduced by 2.7% head rice compared to a control sample dried with unheated air. In an attempt to dry rice in a single pass, rice was dried from 22 to 16% moisture content using an air temperature of 57°C (135°F) and a rice flow rate of 0.02 t/min (45 lb/min). This treatment resulted in a 12% head rice reduction compared with a control sample dried with unheated air. Summary Rice is hygroscopic and it reacts with the moisture in any environment to which it is exposed. Moisture reenters the grain whenever the vapor pressure within the kernel is lower than the vapor pressure in the surrounding air. Freshly matured low-moisture rice may encounter moisture adsorption environments (1) in the field, (2) in the combine hopper, grain cart, transport truck, or holding bin, and (3) ahead of the drying front in certain types of dryers. Rapid moisture adsorption causes low-moisture rice grains to fissure, and fissured kernels usually break during hulling and milling operations. Rapid drying is another cause for the fissuring of rice grains, even though much of this damage may not develop until after drying. The grains may fissure for 48 hr after drying has stopped. The decreasing moisture gradient after rapid drying apparently causes the fissures to develop. Two systems for handling and drying rough rice commonly are used in the United States. Most of the rice crop is dried at commercial facilities 230 DRYING AND STORAGE OF AGRICULTURAL CROPS in which large, continuous flow heated air dryers are used in multipass operations. The heated air temperature and the feedroll speed of these dryers are adjusted such that 2 or 3 percentage points of moisture (d.b.) are removed during each 15- to 30-min pass in the dryer. After a pass through the dryer, rice is stored in a bin for tempering, usually for 6 to 24 hr. Four to 6 dryer passes generally are required to dry rice to the 12.5% moisture content required for marketing. The other rice drying system commonly used is on-farm drying in a storage bin equipped with a fan and air distribution system. Unheated air is recommended for this drying method, except during prolonged periods of high humidity. During those times, supplemental heat that increases ambient air temperature by no more than 7°C (12°F) may be used. An airflow rate of 58.2 X 1073 m3/s:t (5.6 cfm/cwt) is recommended under Texas conditions for rice having an initial moisture content of 20% (w.b.) and a bed depth of 2.44 to 3.01 m (8 to 10 ft). Portable, heated air dryers, either continuous flow or batch, are not widely used for rice drying. Modern technology is giving us more insight into our current drying practices and should provide ideas for development of new and more efficient procedures in the postharvest processing of rice. QUESTIONS 1. Select two dozen sound and vitreous kernels of unprocessed raw milled rice from a package of purchased rice. Place one dozen kernels in a beaker with tap water and the other dozen in a beaker with water initially heated to 60°C. Use a‘penlight or some other small light to inspect the grains. Record the number of fissured grains in each of the samples every 2 min. Continue the inspections until all grains in each of the samples are fissured. 2. Since a rice grain is a free body, discuss how compression caused by moisture adsorption on the surface of the grain will affect the interior of the kernel. In relation to these compressive stresses, when will the rice grain fissure? 3. Sketch the hypothetical moisture gradient across a rice grain if the kernel has been subjected to a drying period and the stresses are as shown in Fig. 9.2. 4. Assume that a low-moisture rice grain is in a stress-free condition. The kernel is then exposed to a moisture-adsorbing environment and uniformly adsorbs moisture over its surface. Using a one-dimensional illustration similar to Fig. 9.2, illustrate: (1) the hypothetical moisture gradient in the grain and (2) the hypothetical stress conditions that result as a consequence of the moisture gradient. SYSTEMS FOR DRYING OF RICE 231 5. How many kilograms of water are lost from a tonne of rice (12.5% w.b.) if the grain moisture content changes from 20.9 to 15.2%, w.b.? If the above moisture change occurs (at a constant rate) in the field between 8:00 A.M. and 4:00 P.M., what is the rate of moisture loss in grams of moisture per kilogram of dry matter per hour? 6. What is the estimated time that rough rice at 20% moisture content may be held in aerated storage before drying is started when the aeration rate is 6.5 X 1073 m°/s:t, and the average ambient temperature is 30°C? 25°C? 7. An aeration system provides an airflow rate of 6 X 10-3 m3/stt. What time will be required for cooling a bin of rough rice after a pass through a heated air dryer? 8. What static pressure must a fan work against in a deep-bed rice dryer installation when the airflow rate is 58.2 X 107? m3/stt, rice depth is 2.5 m, and a system pressure loss of 62 Pa is assumed? REFERENCES AUTREY, H.S., GRIGORIEFF, W.W., ALTSCHUL, A.M. and HOGAN, J.T. 1955. Effects of milling conditions on breakage of rice grain. J. Agric. Food Chem. 3 (7) 593-599. BAN, T. 1971. Rice cracking in high rate drying. Jpn. Agric. Res. Q. 6 (2) 113-116. CALDERWOOD, D.L. 1966. Use of aeration to aid rice drying. Trans. ASAE 9 (6) 893-895 and Rice J. 69 (6) 22-27, 36. CALDERWOOD, D.L. 1970. Operating characteristics of two kinds of portable grain dryers. Proc. 13th Rice Tech. Work. Group, U.S. Dep. Agric., Beaumont, Tex., Feb. 24—26, 1970. ~ CALDERWOOD, D.L. 1972. Effect of rice temperature on continuous-flow dryer operation. PR-3101 In Rice Research in Texas, 1971. Tex. Agric. Exp. Stn. Consolidated PR-3092-3105. CALDERWOOD, D.L. 1977. Bin drying with stirring: rice. Proc. Solar Grain Drying Conf., Univ. Ill. Dep. Agric. Eng., Champaign, Jan. 11-12, 1977. CALDERWOOD, D.L. and BOLLICH, C.N. 1978. Field drying. Proc. 17th Rice Tech. Work. Group, U.S. Dep. Agric., College Station, Tex., Feb. 14—16, 1978. CALDERWOOD, air dryers with D.L. and HUTCHISON, aeration R.S. as a supplementary 1961. Drying rice in heated treatment. U.S. Dep. Agric. Mark. Res. Rep. 508. CALDERWOOD, D.L. and SCHROEDER, H.W. 1968. Aflatoxin development and grade of undried rough rice following prolonged storage in aerated bins. U.S. Dep. Agric. Agric. Res. Serv. 52-26. CALDERWOOD, D.L. and WEBB, B.D. 1971. Effect of the method of dryer operation on performance and on the milling and cooking characteristics of rice. Trans. ASAE 14 (1) 142-146. 232 AND STORAGE DRYING OF AGRICULTURAL CROPS CHAU, N.N. and KUNZE, O.R. 1978. Moisture content of driest and wettest rice grains in a field during the harvest season. Proc. 17th Rice Tech. Work. Group, College Station, Tex., Feb. 14-16. HENDERSON, S.M. 1958A. Deep-bed drying on the ranch with unheated air. Calif. Agric. Exp. Stn. Leafl. 103. HENDERSON, S.M. 1958B. The causes and characteristics of irice checking. Rice J. 57 (5) 16, 18. KESTER, E.B. et al. 1963. Influence of maturity on properties of Western rices. Cereal Chem. 40 (4) 323-336. KONDO, M. and OKAMURA, T. 1930. Fissuring of the rice grain due to moisture adsorption. Ohara Inst. Agric. Res. Bericht 24, 163-171. (German) KUNZE, O.R. 1964. Environmental conditions and physical properties which produce fissures in rice. Ph.D. Dissertation. Mich. State Univ. KUNZE, O.R. 1977A. Moisture adsorption influences on rice. J. Food Process Eng: f (2) 167-181. KUNZE, O.R. 1977B. Fissuring of the rice grain after heated air drying. Soc. Agric. Eng. Winter Meet., Chicago, Dec. 13-16, Pap. 77-3511. KUNZE, O.R. and CHOUDHURY, M.S.U. 1972. Am. Moisture adsorption related to the tensile strength of rice. Cereal Chem. 49 (6) 684-696. KUNZE, O.R. and HALL, brown rice to crack. KUNZE, C.W. 1965. Relative humidity changes that cause Trans. ASAE 8 (3) 396-399, 405. O.R. and HALL, C.W. 1967. Moisture adsorption characteristics of brown rice. Trans. ASAE 10 (4) 448-450, 453. KUNZE, O.R. and PRASAD, S. 1978. Grain fissuring potentials in harvesting and drying of rice. Trans. ASAE 2] (2) 361—366. MATTHEWS, J. and SPADARO, J.J. 1975. Rige breakage during combine harvesting. Rice J. 78 (7) 59, 62—63. McDONALD, D.J. 1967. “Sun-cracking” in rice, some factors influencing its development and the effects of cracking on milling quality of the grain. M.S. Thesis. Univ. of Sydney, Australia. McNEAL, nation. X. 1950A. When to harvest rice for best milling quality and germi- Ark. Agric. Exp. Stn. Bull. 504. McNEAL, X. 1950B. The effect of combine adjustment on harvest losses of rice. Ark. Agric. Exp. Stn. Bull. 500. McNEAL, X. 1960. Rice Storage: Effect of moisture, temperature and time on grade, germination and head rice yield. Ark. Agric. Exp. Stn. Bull. 621. MORSE, M.D. et al. 1967. The effect of grain moisture at time of harvest on yield and milling quality of rice. SORENSON, Agric. Exp. STAHEL, G. the paddy. STERMER, Rice J. 70 (11) 15. J.W. and CRANE, L.E. 1960. Drying rough rice in storage. Tex. Stn. Bull. B-952. 1935. Breaking of rice in milling in relation to the condition of Trop. Agric. Trinidad 12 (10) 255-260. R.A. 1968. Environmental conditions and stress cracks in milled rice. Cereal Chem. 45 (4) 365-373. SYSTEMS FOR DRYING OF RICE 233 STOUT, B.A. 1966. Equipment for rice production. Food Agric. Organ. U.N., Rome, Development Pap. 84. U.S. DEP. AGRIC. 1973. Rice in the United States: varieties and production. U.S. Dep. Agric. Agric. Res. Serv. Agric. Handb. 289. WASSERMAN, T. et al. 20-22: WASSERMAN, 1964. Tempering Western rice. Rice J. 67 (2) 16-17, T., HOUSTON, D.F., FERREL, R.E. and HAMILTON, W.E. 1969. Combination system for rice drying in California. Proc. 12th Rice Tech. Work. Group, U.S. Dep. Agric., New Orleans, March 5-7, 1968. WRATTEN, F.T. 1959. Cited by W.C. Dachtler (Editor). ditioning and storage of rough and milled rice. Research on con- U.S. Dep. Agric. Agric. Res. Serv. 20-7. WRATTEN, F.T. and KENDRICK, J.H. 1970. Cited by Wasserman, T. and Calderwood, D.L. Rough rice drying. In Rice Chemistry and Technology. D.F. Houston (Editor). Am. Assoc. Cereal Chemists, St. Paul, Minn. 10 Systems for Handling of Grain ° Carl W. Hall and R.L. Maddex! The total system—handling, drying, storing— should be considered as related to the particular grain, economics, fuel, method of marketing, and use of product, such as for feeding or manufacturing. The details of drying systems are covered in Chapters 5, 6, 7, 8, and 9, immediately preceding this chapter, but some duplication of dryers already covered is given to provide a more chapter. coherent discussion of the systems in this Considerable analysis is needed to determine the optimum handling, drying, and storing system for a particular situation. Equipment and storage available, type and moisture content of the product, labor available, and feeding or marketing possibilities, all considered on the basis of costs and returns, are involved. Although drying is only one operation in the total system, the capacity of the drying unit often limits the overall volume of grain handled. Thus the drying unit establishes the overall rate of handling. The system should be designed on the basis of speed of harvesting but the drying equipment must be large enough so as not to unduly limit the system capacity. Handling of Grain and Ear Corn A drying and storing system should incorporate a handling system to minimize work requirements. A well-planned grain storage should permit the circulation of grain from the truck into a bin for permanent storage and back to the loading area without manual handling. The handling of grain to cleaning equipment, dryer, and storage should be considered ‘Professor R.L. Maddex, Michigan State University, originally agreed to write this chapter. His notes and mimeographed material were used as the basis of the content. The chapter is dedicated to his memory tributions to drying. 234 in recognition of his many years of outstanding con- SYSTEMS FOR HANDLING OF GRAIN 235 (Fig. 10.1). A large grain storage of several thousand bushels should include the proper planning of the three operations. With some grain storages, only the first and second operation will be used, while with the small storages of 35.2 m$ (1000 bu) or less, only the third operation may be needed, with the drying equipment installed in the storage bin. The same vertical elevator and portable elevator conveyor can be used for all operations. The use of wagon box dryers provides a means of decreasing some of the handling if properly organized. Gutters can be added to ear corncribs for removal. The planks on top of the gutter are removed as the grain is removed from the crib, so the conveyor in the gutter will not be loaded with ear corn when the unloading operation is started. Another procedure is to build the floor on an angle, either sloping to one side or to the middle, where draglines or portable elevators can be used for emptying. The cost of handling systems may be more than the cost of a drying system, but the additional cost can usually be justified. An understanding of the technology of drying, plus an understanding of the equipment and methods, is important in selecting the method and equipment include: (1) (2) (3) (4) (5) (6) (7) for a grain handling Identify Identify Identify Identify Identify Identify Identify the the the the the the the system. Steps in planning a system annual and daily volume. flow rate of grain to the system. harvest and storage moisture content. drying method. storage size and type. handling equipment. system components. (8) Check system for grain quality control. A guide for harvesting shelled corn in the Midwest (Maddex 1966), where there is generally a period of 30 days of reasonable harvest weather in the fall, is as follows: (1) Select harvesting equipment that has the capacity to harvest the crop in one-half of the 30 days available as the reasonable harvest period, or in 15 days. (2) Select drying equipment that has the capacity to dry in 16 hr the grain that can be harvested in 10 hr or 1 day. (3) Select handling equipment to move the grain from harvest to drying to storage based on avoiding delays in operation of either the harvesting equipment or drying equipment. (4) Select storage equipment to receive the dry grain without delay from the dryer. 236 DRYING AND STORAGE OF AGRICULTURAL CROPS ELEVATOR VERTICAL ye \< ELEVATOR PIT HORIZONTAL CONVEYOR From Maddex FIG. 10.1. SCHEMATIC OF A GRAIN HANDLING and Hall (1954) SYSTEM A flow diagram (Fig. 10.2) assists in developing a layout and selecting appropriate equipment to make a system of operation. A number of combinations of components can be assembled to provide a satisfactory handling, drying, and handling and storage system. The components of a system provide the following functions separately or in combination: Wet grain receiving Wet grain elevation Wet grain holding Grain drying with heated air Grain cooling with unheated air Dryer unloading Moving to storage Storage Aeration during storage Dry grain elevation Grain quality can be maintained in systems using different components as long as the limitations of particular components are recognized and correct procedures followed for operating the drying and storage system. Grain flow rates and dryer throughput are important considerations in planning a drying, handling, storage system. At some point in each system there is a maximum or critical flow rate of product. Failure to recognize this maximum or critical flow rate can tie up grain transport SYSTEMS FOR 4 Row, self-propelled HARVEST HANDLING OF GRAIN 237 combines (240 bu/hr),6 hr/day 8.5 m3/hr | person TRANSPORT ELEVATE -| Truck, 2 trips/hr | person Dump pit with cross auger 35 m?/hr (1000 bu/hr) bucket elevator Gravity to holding bin HOLDING BIN Wet grain holding, above dryer | 172 times dryer capacity Gravity to holding bin Load, dry, cool, unload 3-5 m?/hr (1000 bu/hr) for drying from 30 to 13% moisture, or 5m7/hr (140 bu/hr) 25 to 13% moisture 15 cm (6 in. auger) to dump pit, 35 m2/hr (1OOO bu/hr) ELEVATE -2 Use elevator -| Gravity to storage Two 282m* (8000 bu) and one 176 m?(5000 bu) bins Use grain spreader Aeration unit in each bin Underfloor screw for bins unloading ELEVATOR WORKING -3 BIN BLENDER - GRINDER MILL Use elevator -| To gravity truck Bin level Gravity or working bin indicator flow to mill Meter, grind, mix 4 in. auger to feed bin GROUND FEED BIN FIG. 10.2. FLOW DIAGRAM, 50 M’ (1400 BU) PER DAY 705 M’ (20,000 BU) HANDLING SYSTEM PER SEASON, vehicles, increase down time on a drying unit, reduce field harvesting capacity, increase mechanical breakdown, and add additional labor requirements for the operation. The peak flow rate for in-bin dryers and continuous-flow dry units is usually at the time of unloading wet corn or wet product. Low flow rates at this point tie up vehicles and manpower. Tying up a transport vehicle for an additional 5 to 10 min can result in a down time on the combine 238 DRYING AND STORAGE OF AGRICULTURAL CROPS the combine waiting to unload its grain tank. The time required to fill the combine from and to vehicle the of time travel the grain tank minus :dentifies the time allowable for unloading the transport vehicle. The wet le grain conveyors must be sized to unload the vehicles in the allowab there because rates flow critical become often rates flow time. Wet grain is a reduced capacity of a conveyor when handling wet grain. A batch dryer has a high flow rate as it discharges dry grain. Conveyors handling grain from the batch dryer to storage usually must have a capacity of over 35.2 m3/hr (1000 bu/hr). The high flow rate is also desirable in transferring dry grain from a batch and bin dryer to the storage. Continuous-flow dryers have a low flow rate at the dryer discharge, unless gravity unloading is used. The load out time of dry grain from a large operation can be reduced with higher capacity conveyors or by combining an elevated load out bin with a gravity drop. Screw conveyors are widely used for handling grain. Generally, capacity and horsepower increase with diameter, with capacity decreasing and horsepower increasing as the angle of elevation is increased and as the speed of the screw is decreased (Table 10.1). The capacity decreases and horsepower increases as the grain moisture content increases. Capacity will drop 40 to 60% and the horsepower requirements double or triple for grain 25% or more in moisture content as compared to 15% moisture. Minimum grain damage results when the auger or screw is running full of grain and the damage can increase significantly when the tube is less than half full. The amount of exposed screw at the inlet or intake affects both capacity and horsepower requirements. Bucket elevators are more commonly used for vertical lifting of the grain principally for medium and large size operations. The bucket elevator capacity is determined largely by the size and number of buckets. The belt size is a function of bucket size. Bucket spacing on the belt will range from 0.15 to 0.38 m (6 to 15 in.). Capacities can often be more than doubled by reducing bucket spacing to the minimum TABLE Screw 10.1. REPRESENTATIVE SCREW for a particular AUGER CAPACITIES auger capacity at 600 rpm for dry corn and wheat. Auger Power, Angle, Diameter, Capacity, deg. 0 45 90 cm 10 10 10 ton/hr 9=10 Gaul 3.5 Wheat 93 118 90 W/m Corn 71 86 64 0 45 90 15 15 15 29 sili 18.8—20.9 Lieut 144 218 247 120 201 189 Source: Abstracted from Sinha and Muir (1973). Conversion factors: 1 ton = 907 kg; 1 kW = 1.34 hp. W/m is Watts/meter length of auger. These data are not applicable to all auger or screw conveyors; they are presented for comparison rather than for design. SYSTEMS FOR HANDLING OF GRAIN 239 product. Horsepower varies with bucket size, facing, height of elevator, and type of grain. Tables are available from manufacturers providing these relationships of capacity and configurations. The size of the dryer should be based on the daily volume of product to be dried. Most literature for dryers gives the drying capacity based on removing 5 to 10 percentage points of moisture per m? (bushel). As the amount of moisture removed increases, the capacity is decreased. Generally in the Midwest, drying equipment should be selected on the basis of an average moisture removal of 13 to 15% for corn and 5 to 10% for wheat. Storage bins can be purchased in a wide range of sizes, usually with the cost per m* (bushel) decreasing as the bin size increases. Horizontal or flat storages and vertical or upright storages may be selected. Two smaller bins, even though more costly, might be more functional than one large bin, particularly where more than one product is handled. Upright or tall structures will involve more equipment and costs for elevating the grain, but, on the other hand, might be more adaptable to gravity unloading. DRYING METHODS Dryers have two general configurations—bin and column. Bin manufacturers generally also market a bin dryer. Other manufacturers usually market a column-type drying unit. There are four rather distinct methods for drying grain, upon which can be imposed a number of variations. The variations are in heater unit size, airflow through the grain, configuration of the drying unit, and equipment added to permit automation of the drying operation. Characteristics of different drying systems are listed in Table 10.2 with the system components in Table Ts: Deep Bin Drying For deep bin or in-bin drying, grain is placed to a depth of 1.22 m (4 ft) or more, usually in a circular bin, for drying. For deep layers of 3.6 to 5.5 m (12 to 18 ft), low temperature of the air at 1.6° to 2.8°C (3° to 5°F) above atmospheric temperature is used. Grain is placed in the bin either in layers or filled to final storage depths (Table 10.4). The grain depth dictates low airflows, which in turn dictate a low drying temperature and a low rate of drying. Increasing drying temperatures results in overdrying lower levels of the grain in the bin. A minimum airflow of 0.0134 m/m*s (1 cfm/bu) is required with grain depths not exceeding 5.5 m (18 ft). Stirring devices increase the rate of drying by 15 to 25% and give more even drying throughout the grain depth (Fig. 10.3). DRYING AND STORAGE 240 OF AGRICULTURAL CROPS TABLE 10.2. CHARACTERISTICS OF DIFFERENT DRYING SYSTEMS I. Deep Bin (In-bin, Layer, Storage) . Low airflow—0.04—0.07 m?/m's (3—5 cfm/bu) . Low drying temperatures—15°—27°C (60°—80°F) _ Low capacity—8.75 X 10 ~* to 14.5 X 10 ~$m3/s (15 to 25 bu/hr) to prevent overdrying Humidity control and heat unit—reduces drying but usually reduces volume dried Temperature control on heat unit—increases volume dried, but can cause overdrying (below 10% m.c.) in colder weather Lowest investment—bin, fan and heater unit, portable cone, electrical service, fuel supply Season volume—175—350 m? (5000—10,000 bu) . Not generally recommended for corn production over 350 m’ (10,000 bu)/year Il. at ch (In-bin) Low to medium airflow—0.134—0.268 m3/m’s (10—20 cfm/bu) Medium drying temperatures—38 —49°C (100°—-120°F) Medium capacity —0.023—0.035 m3/s (40—60 bu/hr), 1 batch/day . Drying volume increased by increasing floor area (bin diameter) Minimum supervision required—can be one-man operation bin, storage bin, aeration unit, moisture tester, OoTO MMOOWEw OD swMedium investment—drying portable conveyors or vertical elevator and return conveyors, electrical service, fuel supply Q . Season volume—280—525 m3 (8000—15,000 bu) WL. Batch-column (Manual, Automated) Medium to high airflows—0.536—1.21 m3/m3s (40—90 cfm/bu) High drying temperatures—60°—88°C (140°—190°F) Medium to high capacity —0.038—0.073 m3/s (65-125 bu/hr) . Drying volume increased by using larger batch bin Maximum supervision required for manual control of loading, unloading, and fan operation Minimum supervision required for automated cycling of loading wet grain, drying, cooling, and unloading dry grain. More handling components required. ByOW> Q3 . Medium to high investment—dryer, wet holding bin, storage bins, aeration units, moisture tester, bucket elevator and return conveyors, electrical service, an) . fuel supply, center building Season volume—350—700 m3 (10,000—20,000 bu) = manual batch, 525—2100 m? (15,000—60,000 bu) = automated batch IV. Continuous Flow High airflow—0.94—1.21 m3/m3s (70—90 cfm/bu) High drying temperatures—60°—88°C (140°—190°F) Medium to high capacities—0.043—0.088 m/s (75—150 bu/hr) . Drying volume increased by using larger drying bin Minimum supervision required—has more handling equipment than batchcolumn dryer moO A High investment—dryer, wet holding bin, storage bins, aeration units, moisture tester, two bucket elevators and return conveyors, electrical service, fuel supply, center building G. Seasonal volume—525—3500 m? (15,000—100,000 bu) . Dryeration . Drying unit can be either column or bin dryer . Cooling unit—70—105 m? (2000-3000 bu) bin with low airflow—0.008—0.0167 m*/m*s (0.5—1 efm/bu) (2 bins are desirable for flexibility in unloading) . High capacities—1.4—1.6 times the capacity of the comparable batch or continuous flow unit 0.073—0.18 m?/s(125—300 bu/hr) S16 So, . Moisture removal— Drying unit—down to 17 or 18%—removed hot Cooling unit—down to 14-15% . High supervision—most handling equipment required SYSTEMS TABLE 10.2. FOR HANDLING GRAIN 241 (Continued) F. High investment—same equipment for batch or continuous ing bin and fan unit and additional conveyors G. Seasonal volume— 1050-3500 m? (30,000—100,000 bu) VI. OF flow ai d i 2 a Modified Dryeration . Drying unit is usually a bin dryer with bottom sweep unloader eels . Cooling unit and storage unit—210—350 m3 (6000—10,000 bu) with airflow of approximately 0.007 m3/m’s (0.5 cfm/bu) . Capacity increased somewhat over comparable batch or continuous flow oa - More careful bin management required to avoid moisture damage to grain in storage bin Thick Layer Drying A thick layer of 0.6 to 1.2 m (2 to 4 ft) depth uses limited temperature of 21° to 27°C (70° to 80°F) or slightly higher temperature of 27° to 37.8°C (80° to 100°F) if a stirring device is used. A higher airflow is required for higher temperatures. The stirring device permits a higher temperature by mixing the incoming wet grain on top with the drier grain on the bottom next to the incoming heated air. Power requirements for air movement limit the amount of air per m? (bushel) to low or medium airflows, 0.0134 to 0.04 m?/m3s (1 to 3 cfm/bu), which in turn limit the air temperatures that can be used without resulting in unacceptable moisture difference between the bottom and top layers in the batch. A bin dryer may be equipped with a floor auger which takes the hot, partially dried grain from the floor, lifts the grain, and spreads it over the top of the bin. The unit functions as a continuous flow batch bin (Fig. 10.4). Thin Layer of Column Dryer Most thin layer drying units handle the grain in vertical columns which are 0.45 m (18 in.) thick or less. For thin layer drying with horizontal layers in a bin, a given depth of 0.3 to 0.6 m (1 to 2 ft) of grain is added at a time. The layer is partially dried, then a a new layer of wet grain is added. The depth of layer and speed of filling are highly dependent on the moisture content of the grain and the drying capacity based on airflow and temperature, which are established by the manufacturers’ guides. An airflow rate of 0.04 to 0.067 m3/m°s (3 to 5 cfm/bu) with temperatures of 21° to 27°C (70° to 80°F) is usually used. A humidity control can be used such that the heat is added only if the relative humidity exceeds some value, usually 55%. Column dryers are commonly designed with a thickness of 0.3 m (12 in.) grain. Some commercial dryers have thinner columns and some greater. 242 DRYING AND STORAGE TABLE 10.3. ERS) CHARACTERISTICS OF AGRICULTURAL CROPS OF SYSTEM COMPONENTS (OTHER THAN DRY- I. Storage Bins . Range in size from 175 to 1750 m3 (5000 to 50,000 bu) : lech . Some variation in bin size is desirable to provide flexibility of storage for grains © other than corn C. All bins should be equipped with an aeration system ely D. Allround bins should be provided with a bottom unloading unit II. Wet Holding Bins : A. Required for automated batch and continuous flow drying units B. Hoppered bottom bins desirable C. Can be located over dryers for gravity flow rate dryer or placed beside dryer D. III. with a connecting auger automatically controlled Should have a capacity for 4-10 hr at the harvesting rate Conveyors A. Augers 1. Best for dry grain 2. Vertical lift 6—7.5 m (20-25 ft) 3. Volume reduced approximately one-half when handling wet grain 4. 15cm (6in.) unit best for grain center B. Bucket Elevators C. 1. Lowest horsepower requirements 2. Capacity about same for wet or dry grain 3. Total cost should include erection and guying Belt Conveyor 1. High speed, horizontal transfer conveyors IV. Cleaners A. Low flow units—low capacity. Screen cleaner at top of leg, elevator medium to high capacity B. Ifused on dry side, fines can be stored C. Biggest advantage is to keep fines out of storage bin D. Gravity flow and screened auger units available V. Moisture Testers A. Only accurate method of knowing grain moisture B. Should be used before and after drying C. Price range—$100—$400 VI. Dump Pits . Deep pits not recoommended—high in cost, hard to clean, expensive to build, hard to keep dry - Shallows pits—0.5 cm to 1 m (2 to 3 ft) deep—with drive-over grating desirable . Capacity of cross conveyor to bucket elevator critical Daw . Depressed or swing-away belt conveyor available with high capacity VII. Center Building A. Polestructure suitable B. Width—6—7 m (20-22 ft) C. Height—4—5 m (14-16 ft)—room for a dump truck bed VIII. Angle of Gravity Drop Tubes A. Wet grain—approximately 60° B. Dry grain—approximately 45° SYSTEMS TABLE IX. 10.3. FOR HANDLING OF GRAIN 243 (Continued) Feed Processing A. Electrical mill—will grind 90-136 kg/hr (200-300 lb/hr)/0 .746 kW (hp) B. Provide overhead buffer bins—1.75—5.25 m3 (50—150 bu) X. Electric Service A. Single phase motors up to 30 kW (40 hp) available B. Three-phase service desirable if available at reasonable cost C. 100-400 amp service may be needed if dryer is powered with an electric motor D. XI. Most drying and storage units have a number of motors—a central control panel protected from weather is desirable Temperature Sensing Units A. Recommended for bins of 700 m? (20,000 bu) or larger Limiting the grain layer column to 0.3 m (12 in.) permits higher flows and high drying air temperatures. These units can be controlled automatically providing better quality control of the product. (See batch and continuous drying units.) Dryeration Dryeration is a process where grain is removed while hot from the heated air drying unit and placed into a holding bin. The holding bin has a fan to move air to provide for additional drying and cooling, after which the product is moved into a storage bin (Fig. 10.5). Grain is removed from the column dryer at 17 to 18% moisture when the temperature is between 54° and 66°C (130° and 150°F). Unheated air at 0.013 to 0.019 m?/m?s (1 to 1.5 cfm/bu) is moved through the grain removing 1 to 3% moisture while cooling the grain. The grain can be allowed to equalize in temperature before moving the air through or air can be moved through the product immediately upon placing in the dryeration bin. After 10 to 20 hr the dried grain is placed in storage. The dryeration bin is not used as a storage bin because during the dryeration process moisture may condense and collect on the bin walls which would lead to spoilage if the grain were stored in the same bin. Drying equipment and installation can be classified into two broad categories: (1) Batch (2) Continuous systems In both cases, materials and the equipment before and after drying are involved. In the continuous system, however, additional opportunity is provided for automation. CROPS OF AGRICULTURAL AND STORAGE DRYING 244 isGe Oe: G6 Wy Gee SOUL LYO Ww O0SCame OUSIE O8L nq QUE ONY GLE rae peasy og Avy 78 yjdeq pue Aeq sed a1eyy[I Gg Vie) v6 skeq ‘guy, suldiq e101, 97 Go LE Wy OFT 910 6810" ul Avqied peuq yideq O891 026 009 nq 26S veg Lic ju Aeq sed peuqgAiqueny 0088 ZLV0 0088 ZLYO ‘cl000S8a wy CGY SIF STF ST s/w MOTJITY 06 GZ 0€ % AIaVL 8's GL 8S GL acu = es MY dy Ol ‘ain stoyy jeniuy SO W115 Allva Iamog [e101 ‘ouy ‘“queudinby 2 ue JUeACIEYW JO AsajInog :301nN0G 6 61 wey aye 019819 “Ul ‘einsseld ainjeioduiaJ,SuUIAIG (4,01) 0.87 ONIAUG HOLWA HOS NHOO G3713HS HLIM Nid YALAWVIG (14 yz) Wed SYSTEMS FOR HANDLING OF GRAIN 245 Courtesy of Sukup Systems FIG. 10.3. STIRRING DEVICE FOR PERMITTING DRYING DEEP LAYERS OF GRAIN Batch Drying or Batch in Bin Batch drying may be accomplished in a bin or in a portable or stationary vertical column and with temperatures of 37.8° to 87.8°C (100° to 190°F). Bins up to 12 m (40 ft) diameter with grain depth of 0.2 to 0.3 m (20 to 36 in.) in airflows of 0.134 to 0.54 m3/m's (10 to 40 cfm/bu) at 37.8° to 60°C (100° to 140°F) temperature are used. Stirring devices may be used to provide more uniform and more efficient drying. These units are designed to dry a batch in 16 hr and to handle a batch each day with the batch matched to the harvest volume. Column units, usually vertical columns with grain thickness of 0.15 to 0.6 m (6 to 24 in.), use high temperatures of 37.8° to 87.8°C (100° to 190°F) and an airflow of 0.27 to 1.34 m3/m3s (20 to 100 cfm/bu). Batch columns hold from 2.5 to 35.2 ft? (70 to 1000 bu) with farm units usually in the range of 8.75 to 14.1 ft? (250 to 400 bu). As an example, a 10.6 ft? (300 bu) unit with an airflow of 0.27 to 1.21 m3/m's (20 to 90 cfm/bu) 246 DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Sukup Systems FIG. 10.4. BINDRYER EQUIPPED TO REMOVE HOT GRAIN FROM BIN CAN BE USED AS A CONTINUOUS FLOW DRYER and an air temperature at 71.1°C (160°F) will average 3.52 m?/hr (100 bu/hr) of dried grain when removing 10 to 12 percentage points of moisture in a 0.45 m (18 in.) column. The drying cycle is 2 hr for batch drying, % hr for cooling, and ’% hr for loading and unloading batch bin. Batch drying with the layer in a horizontal position can be done on a wagon. Some wagons have a heater mounted on the unit and others use a portable heater which is attached to the wagon to move air into the plenum. The wagon plenum may be under the grain or above the grain. The depth of grain or seed in the wagon may be from 0.05 to 0.66 m (2 to 26 in.). A perforated or slatted floor is usually used. SYSTEMS FOR HANDLING OF GRAIN 247 WET CORN IN (20% TO 30%) M Ww [ORYER BATCH OR CONTINUOUS 0.0134 m3/m3s5 93°C (200°F) TO OR STORAGE MARKET (1/2-| cfm / bu) From Hirning et al. (1974) FIG. 10.5. DRYERATION PROCESS Continuous Flow Drying Continuous flow drying may be carried out in bins or columns. In bins the dried product is continuously removed from the bottom of a 0.6 to 1 m (2 to 3 ft) depth, with the wet product entering the top layer. The more common continuous flow dryer is of a column type, 0.15 to 0.5 m (6 to 18 in.) thick, like the batch column, but is mechanized and automated to provide continuous flow of the grain entering at the top and falling by gravity through the dryer. Air is directed through the grain in a path of 0.15 to 0.5 m (6 to 18 in.) long. The air may be forced in the same direction (concurrent), opposite to the flow of the grain (countercurrent), or perpendicular to the direction of flow of grain (crossflow). Air temperatures up to 99°C (210°F) with airflows of 0.67 to 1.3 m3/m’s (50 to 100 cfm/bu) are used. The continuous flow column dryer has two sections, one for the heated air (drying) and one for unheated air (cooling) driven by fans. The outgoing heated air may be used for heating the incoming air. The cycle time on a batch dryer includes 25 to 35% down time of the heating unit while cooling the grain, loading, and unloading the dryer. The continuous flow dryer has a division in the plenum and inside chamber which permits the flow of heated air and unheated air at the same time to different sections of the grain column, eliminating the down time on the heating unit. These units may be operated up to 24 hr a day and can be sized accordingly. Horizontal continuous dryers with product from 0.15 to 0.5 m (6 to 12 in.) deep, although used, are not common. These units have a heating and cooling section. 248 DRYING AND STORAGE OF AGRICULTURAL CROPS HANDLING The handling units connect harvesting-drying-storage together as a system. Generally it is desirable to minimize the amount of handling to reduce the risk of damage. However, one unit poorly designed and man- aged can cause more damage to the product than several components properly matched and properly utilized. The speed or rate of volume of handling must be kept reasonably low to avoid excessive damage. Damage during handling is related to kind of product, moisture content, temperature, and history of treatment of product. Handling by itself does not have the potential of increasing the value of the product, as do drying and storage, but it is needed to provide efficient drying and storage. Although commercial handling systems generally are larger than many farm materials handling systems, there are many farm installations of the same or larger size and capacity as commercial operations. Harvest Materials handling at the combine consists of a screw conveyor or gravity to unload the combine bin. Usually the grain is handled in bulk with a conveyor, but some feed grains and seeds are collected in bags. Transport and Emptying The bulk grain or grain in bags is transported from the field to the dryer or storage in dump wagons, trucks, special hopper bottom containers, or auger unloaded or vacuum unloaded containers. Portable grain wagons, self-unloading type, may be as large as 10.5 m? (300 bu) per load. Unloading (Based on Stewart and Britton 1973) The bulk transport can be unloaded from a self-dump truck of 3.5 m3 (100 bu) or a lift which raises the front of the truck or wagon in 6 to 10 min. The bulk is dumped into a pit or hopper which should handle 1000 bu/hr. For horizontal conveying a belt conveyor has a higher capacity than a chain conveyor. Screw conveyors or flight conveyors for the hopper and a bucket conveyor for vertical lift move the grain to the batch dryer or to the holding tank preceding the continuous flow dryer. Flight conveyors may be used for an angle of 20° or less. Stationar y or portable holding bins might be used. a capacity of 35.2 m3? (1000 bu). A common portable holding bin has Bags, if used, may be placed ona bag or sack conveyor or placed directly in storage. Special bag conveyors are available. Bags may be untied and SYSTEMS FOR HANDLING OF GRAIN 249 the grain dumped into a conveyor, bucket, or belt, or into a pneumatic handling system. Holding Bin Although the dump pit could serve as a holding bin the dump pit is usually below the dryer and the rate of moving the grain to the dryer is limited by the capacity of the materials handling system. It is usually desirable to gravity fill the dryer from a holding tank above. For continuous dryers, conveyors may move the grain from a holding tank to the dryer. With gravity filling the dryer can be loaded very quickly in the case of a batch dryer, thus keeping the capacity of the dryer in use through a longer period rather than being idle while filling. Spreader Grain placed in a bin for drying or aeration should be of a uniform depth over the air distribution system. A device which keeps the product at nearly uniform depth and uniform distribution is preferred over a unit which levels the surface after filling. The device which continuously levels the product will have a more uniform product density and a more uniform distribution of fines and foreign matter, providing for better drying than if leveled after filling. Dryer/Bin Unloading Dryers can be emptied by gravity or with auger conveyors usually across the bottom of a drying unit. The dried product is usually elevated from the dryer to a bin, although the dryer might be mounted above the storage or holding bin so that gravity could be used from the dryer to the bin. The bin dryer can be unloaded or partially unloaded using sloped floors or hopper bins which permit gravity flow. Floor mounted, sweep screw conveyors can be used to remove the grain from the floor. The grain is moved by the screw conveyors to the center of the bin and discharged. Transportation Transportation for receiving and shipping of grain can be carried out by several methods (Halter 1973). Wagons and trucks predominate for moving the grain from the harvester to the dryer or storage. Trucks, in capacities up to 24 to 28 m3 (700 to 800 bu), are used for large volume transport. These may be emptied from the bottom, using a sling hoist, 250 DRYING AND STORAGE OF AGRICULTURAL CROPS elevator, or hydraulic lift, or a platform to lift the front of the unit so the material can move by gravity. Boxcars with a capacity of 107 m3 (3040 bu) or hopper bottom cars which in the old style held 135 m3 (3835 bu) and in new units 160 m? (4545 bu) can be used. With a dumper (entire boxcar is dumped), a boxcar can be emptied in 8 to 10 min. Using augers the time might be as much as 1 hr. The hopper bottom cars drop the grain by gravity through a grating between the rails. Unit trains are often used which are made up of 50 to 100 rail cars to handle one type and grade of grain. These are usually used for a one-way load. With large installations these cars can be loaded continuously. For water transport, barges on rivers, lakes, and oceans and ships on lakes and oceans are used for handling large quantities of grain, usually in bulk. Bucket or chain elevators or pneumatic handling devices predominate. Unit loads of bags or pallets or unit bulk loads in bulk containers may also be used, particularly where the entire ship is not devoted to a single product or grade. STORAGE Types of Storage Storage units can generally be classified as horizontal or vertical and can be further classified as round, rectangular, cylindrical, box shaped, etc. Horizontal structures are usually defined as those in which the height is less than the length, whereas the vertical storage is defined as one where the height is greater than the diameter or width of storage. Larger commercial units tend to be more of the vertical type. The selection of the type depends primarily on the following: Storage structure costs Land costs and land availability Life expectancy Periodicity of filling and emptying (once/year, or once/ week) Whether or not and how the storage is to be used in connection with drying and conditioning Materials handling Storages may further be classified according to the construction material. Large centralized units tend to be concrete. Although a few concrete storage structures are used on farms, wood and steel predominate. Steel structures are usually thin for farm applications and are galvanized or have some other coating, particularly at the joints. SYSTEMS FOR HANDLING OF GRAIN 251 Of major importance is the appropriate design of the storage, not only to restrain and properly hold the material while in storage, but also to prevent or minimize the damage to the grain which might occur due to moisture condensation or excessive temperatures. The storage must also be appropriately designed to withstand the change in pressures of the grain during filling and emptying of the storage unit. Farm Storage’ The value of grain normally increases after harvest if the quality is maintained. Quality can be maintained by drying, protecting against rodents, preventing insect infestation, aerating, and protecting the grain against the weather elements. The cost of maintaining the quality is usually more than justified by the increase in value of the product after harvest. It is not unusual for wheat to increase 10 to 20% in value from harvest to the January—March period. The annual cost of providing a farm storage is approximately 10 to 20% of the initial cost. This annual cost includes depreciation (2.5 to 5% of the initial cost), interest (6.0 to 15%), repairs and maintenance (1.0%), and taxes and insurance (1.0%). These costs vary considerably with the structure and with the economy. In English units, the lateral pressure, L, lb/ft?, for a shallow bin under 20 ft deep can be calculated from Rankine’s equation (10-1). 4 = 1-sing ——— 1+sing = (10-1) where w = weight of material, lb/ft? (40 to 50) y = distance from top to point in question, ft g@ = the angle of repose, deg. (25 to 35) The lateral pressure for a bin which is 15 ft deep, with material at 50 lb/ft?, and a 30° angle of repose, is 50 (15) iak ists = 2501b/ft? or 1.7 psi (1 psi = 6.89 kPa) Commercial Storage Satisfactory operation of a commercial storage depends greatly on the handling facilities. The grain should be checked regularly for heating, infestation, and damage from rodents and weather. 2See Appendix for metric conversions. DRYING 252 AND STORAGE OF AGRICULTURAL CROPS A charge of 1 to 3¢/bu-mo is usually made for storage. It is often more economical to store grain than to sell it at harvest to take advantage of the increase in price after harvest. The lateral pressure, in English units, in deep storages can be estimated from Janssen’s equation (10-2) (Fig. 10.6). 100-7} a 144L +2" [jp ken | a 90 oHol wu 5 “ - ; aS r = radius, ft. 80 2 2 = oi 2 70 = ay {e) Oo nN 2 2 wW ao) tne) a 5,60 2 ‘5 50 ; = 3Ss =e ° 2 < p= 50 Ibs. per cu. ft. = 30 wo b= 0.4 ((grain k = 0.6 Ae on wall) | ft =_0.3048m “4 | Ib/ft>= 16.02 kg/m> | psi = 6.895 kN/m2 . 2 3 Lateral FIG. 10.6. LATERAL PRESSURE L = 5 SB 4 pressure, 5 6 7 8 L,psi IN CYLINDRICAL, BIN - ¢ ~Kuh : (10-2) where L = lateral pressure of grain, lb/ft? w = weight of grain, lb/ft?, (40—50) “ =coefficient of friction of grain on walls, (0.30—0.48) R = hydraulic radius of bin, ft, area of cross-section, ft? ; circumference, ft h = depth at any point, ft Y = vertical pressure of the grain, lb/ft? K = ratio of lateral to vertical pressure, L approximately 0.60 for grain e = 2.718 Y SYSTEMS FOR HANDLING OF GRAIN 253 Kinds of Storage Many farm storages are as large or larger than commercial elevators. Halter (1973) classifies elevators as follows. (1) Country Elevators—These hold 1760 m? (50,000 bu) and up and are used for receiving grain from wagons shipped by truck or rail. and trucks, from which grain is (2) Subterminal Elevators—These have a capacity of 17,600 m3 (500,000 bu) and serve a much larger geographical area than the country elevator. The subterminal receives grain from large trucks and farm vehicles. The subterminal may include a dryer and automatic dumping equipment as well as cleaning equipment. The subterminal elevator is a truck-rail interface. (3) Terminal Elevator—The terminal elevator includes the various functions of receiving, handling, storing, and shipping and is primarily a rail or a truck-ship interface. A wide variety of products can be handled. (4) Trans-shipment Elevator—The trans-shipment elevator is used primarily to transfer products from inland shipping units, such as ships, to ocean-going vessels or to rail, and has as its main function unloading, storing, and loading out. (5) Importing Elevator—This unit receives grain from ships and is involved in handling and shipping out to trucks or by rail. (6) Manufacturing Elevator—This is a specialized elevator which will process a specific crop for a particular industrial purpose, such as cereal grains, flour, feed, and grain sugars. Layout of Storage The storage must be laid out so as to accommodate efficiently the handling and drying system. A schematic for a wet grain and drying storage center is shown in Fig. 10.7. A representative commercial installation is presented in Fig. 10.8. Farm units do not only store the grain but are often for mixing supplements, drying the product, and delivering to self-feeders. Shown is a 52.8 m? (1500 bu)/hr unit. This unit uses a 0.15 m (6 in.) auger to move the grain from a dump pit to the elevator. Either a 1.5 m? (43 bu) capacity pit with cross auger or a 7.4 m3 (210 bu) gravity feed dump can be used for quick unloading of trucks or wagons. Over the driveway are four 16 m? (465 bu) bins for gravity flow. A central elevator leg provides communication for handling of grain or mixing for loading and unloading. A commercial warehouse with receiving and shipping is shown in Fig. 10.9. 254 DRYING AND STORAGE OF AGRICULTURAL CROPS 880 m° 530 m°> (25,000 (15,000 BU.) STORAGE BIN BIN FIG. 10.7. FLOW OF WET GRAIN—DRYING BU.) AND STORAGE CENTER Dust and Explosions Explosions in grain elevators must be considered in the design and operation of grain handling and storage systems. Explosions result from ignition of fine dust particles in the air (Hall et al. 1971). The basic factors involved in an explosion are a suitable mixture of dust and oxygen (in the air), and a spark. Grain dust is considerably more explosive (50 times) than coal dust. Generally, a concentration of organic dust less than 0.177 kg/m? (5 g/ft) is considered safe from explosion. Air and grain circulating systems must be designed and operated so as to avoid increasing the concentration of dust. The possibility of explosion from dust has increased in recent years when dust cannot be rejected to the atmosphere. With more handling additional fines are produced and dust is produced. Dust should be removed from the circulating air. The dust can be pelleted and used for feed. Granaries and elevators are constructed with caps, doors, windows, or panels which “blow out” if an explosion occurs, to minimize damage to the structure and to help keep the effect from spreading to other areas. A spark may result from static electricity in the machinery, switch, broken light bulb, and any number of sources. Explosions are prevented by providing a well-ventilated build- SYSTEMS FOR HANDLING OF GRAIN 255 Courtesy of Butler Manufacturing Co. FIG. 10.8. A REPRESENTATIVE COMMERCIAL UNIT FOR 1500 BU/HR ing, dust removal equipment, dustproof electrical equipment and lights, and properly grounded equipment. Proper operation and maintenance of equipment are equally important. Construction and building regulations, air pollution control requirements, insurance requirements, and national fire and safety codes all are established to prevent explosions. Aldis and Lai (1979) have prepared a comprehensive publication on grain dust explosions. QUESTIONS 1. Contrast a grain handling system for a one crop operation with a grain handling system for several grains for a grain-livestock enterprise. 256 DRYING CROPS OF AGRICULTURAL AND STORAGE < IMJ SCALE GARNERS aii HRA eh SCALE Fill FLOOR DISTRIBUTING FLOOR CONVEYORTO STORAGE BIN FLOOR | t ii ORK HOUSE BINS t ++_ SZ) 1 NI TO SHIPPING GALLERY TRUCK SHIPPING the AA ies rH SHIPPING LEG SHIPPING RECEIVING RECEIVING BELT L LEG she From Sinha and Muir (1973) FIG. 10.9. COMMERCIAL GRAIN STORAGE AND HANDLING 2. Obtain a copy of the latest EPA memoranda for preventing elevator explosions and list the essential requirements. 3. Obtain the latest OSHA requirements for elevator operations and list the essential requirements. 4. Make a time study at a grain receiving operation. Develop a set of recommendations for improving the operation. 5. What changes should be considered in operating a receiving-drying- handling-storing system when changing from wheat to corn? 6. Compare the lateral pressure versus depth for a 6 m (20 ft) diameter circular bin using the Rankine equation and the Janssen equation. 7. Select two grain drying-handling-storage systems in your area. Make a flow diagram of both. Contrast the two systems, giving advantages and disadvantages. How could the design be improved, considering the system? 8. Compare three methods of unloading a railroad boxcar considering rate of unloading, cost, operation following unloading, use for various grains, etc. SYSTEMS FOR HANDLING OF GRAIN 257 REFERENCES ALDIS, D.F. and LAI, F.S. 1979. aspects of grain dust explosions. FLEMING, J.M. 1968. Review of literature related to engineering U.S. Dep. Agric. Misc. Publ. 1375. Canadian grain terminals: design and construction features. Eng. J. 51 (1) 4-15. FOSTER, G.H. 1964. Dryeration—a corn drying process. U.S. Dep. Agric. Cire. AMS-582. HALL, C.W., FARRALL, A.W. and RIPPEN, A.L. 1971. Encyclopedia of Food Engineering. AVI Publishing Co., Westport, Conn. HALTER, G.S. 1973. Design of country and terminal elevators. In Grain Storage: Part of a System. R.N. Sinha and W.E. Muir. (Editors). AVI Publishing Co., Westport, Conn. HIRNING, H.H., OLVER, E.F. and SHOVE, G.C. 1974. Drying grain in IIlinois. Univ. Ill., Urbana, Ext. Cire. 1100. KETCHUM, M.S. 1919. The Design of Walls, Bins and Grain Elevators. McGraw-Hill Book Co., New York. MADDEX, R.L. 1966. Drying shelled corn. Mich. State Univ. Agric. Eng. Dep. Info. Ser. 174. MADDEX, R.L. and HALL, C.W. State Univ. Ext. Bull. 3/6. SINHA, R.N. and MUIR, W.E. 1954. 1973. Drying grain with forced air. Mich. Grain Storage: Part of a System. AVI Publishing Co., Westport, Conn. SMITH, R.W. and BALDWIN, E.D. 1975. Economics of farm drying and storage systems in Ohio. Ohio State Univ. Coop. Ext. ESS 519 and MM 358. STAHL, B. 1950. Grain bin design. U.S. Dep. Agric. Circ. 836. STEWART, B.R. and BRITTON, M.G. 1973. Design of farm grain storages. In Grain Storage: Part of a System. R.N. Sinha and W.E. Muir (Editors). AVI Publishing Co., Westport, Conn. Ld Systems for Drying and Handling of Hay The value of the hay produced annually in the United States (1975) is approximately $5 billion, making it the most important farm crop. The leading states in quantity of hay produced are Wisconsin, Minnesota, California, Nebraska, and Iowa. In hectarage the leading states are Iowa, Illinois, Texas, Kansas, Minnesota, and North Dakota. There is a greater loss of production of hay than of any other farm product except fruits and vegetables. Approximately 21% of the production is lost during harvesting of the crop and another 7% is lost during storage, making a total loss of 28%. The loss amounts to approximately $1% billion per year in the United States. Although it would not be economical to prevent all the loss, the potential for saving a part of the loss commercially is substantial. The forage must be cut at the proper stage of maturity to obtain the highest nutrient value of the crop. Alfalfa should be cut when one-tenth to one-fourth bloom; alsike clover, full bloom; sweet clover, beginning of bloom; red and mammoth clover, one-half to full bloom; cowpeas, first pods matured; annual lespedeza, full bloom; lespedeza sericiz, before bloom and less than 0.5 m (20 in.) tall; soybean hay, beans fully developed and bottom leaves starting to yellow; brome grass, just after full bloom; Johnson grass, when heads are emerging but before one-fourth have emerged; prairie grass, before plants turn brown; sudan grass, when heading; timothy, fully headed to early bloom. Good drying weather does not often prevail in the Midwest when hay is at the proper stage of maturity for harvesting. Waiting for the hay to dry during rain and heavy dew causes loss of nutrients through bleaching from sun, leaching from rain, and loss of leaves. To permit timely harvest without the hazards of inclement weather, drying of hay by forced ventilation has been successful and economical. With a properly arranged schedule of harvesting and storing, coupled with drying, only enough hay should be mowed so that the cut hay is in the field for one drying day. 258 SYSTEMS FOR DRYING AND HANDLING OF HAY 259 The hay is field dried to approximately 40% moisture and additional moisture removed with the barn hay dryer until the moisture content is down to 20%. This procedure of removing moisture from hay and other forages is known as barn drying, barn curing, mow curing, or finishing hay. Drying hay as it is cut in the field would require a heating unit of 0.378 m*/hr (100 gal./hr) to dry 1360 kg (1% tons)/hr and is used for dehydration units, not generally for farm use. The danger of spontaneous combustion is eliminated by proper use of a barn hay dryer. The product will remain as cool as or cooler than the air blown into the hay. Barn hay drying systems may use unheated forced air or heated air. Long hay can be stored at 28% moisture without danger of spontaneous combustion while chopped hay must be at 25% or less and even then discoloration will occur (Duffee 1942). Drying hay to 25% in the field before chopping presents an added weather hazard and possible additional loss from handling. The quantity of nutrients in alfalfa hay harvested by different methods gives an indication of the effect of different methods of curing. The feed nutrients normally compared are dry matter, protein, and carotene. Results of one study are presented in Table 1.6, which check closely with data presented by previous researchers (Davis 1951). Another method of determining the value of the forages dried by different means is by using livestock feeding trials (Chapter 1). Results are generally reported per kg (Ib) of hay fed, rather thar. per kg (lb) of hay grown, and may not consider the loss in quantity and quality between cutting and feeding. The amount of carotene in alfalfa and other materials is a measure of the vitamin A content. Pure carotene is oxidized when exposed to air, with the process being accelerated in sunlight and at higher temperatures. Market hay frequently contains only one-tenth to one-fifth of the carotene ordinarily found in the growing plant, and the carotene is lost completely if the hay is kept in storage for a few months. Results of the effects of temperature and fineness on the loss of carotene in alfalfa hay are summarized as follows (Kane et al. 1937): (1) Hay stored when the outside temperature was 7.2°C (45°F) lost an average of 3% of the carotene content per month. (2) Hay stored when the average outdoor temperatures ranged from 7.2° to 20°C (45° to 68°F) lost an average of 6.5% of the carotene content per month. (3) Hay stored when the outside temperature was 20°C (68°F) lost an average of 11 to 21% of the carotene content per month. (4) Alfalfa meals ground to pass 0.31, 0.63, and 1.9 cm (%, %4, and 34 in.) mesh screens lost carotene at the same rate during storage regardless of the degree of fineness, which was practically the same as for baled alfalfa hay. 260 DRYING AND STORAGE OF AGRICULTURAL CROPS The leaf loss from alfalfa hay during handling becomes great as the moisture content is reduced below 30%. The leaf loss in a series of tests ranged from 10 to 65% for a moisture range of 28 to 32% (Zink 1936). The leaves contain 60 to 90% of the proteins and vitamins of the hay. Drying in the Field \ Windrowing of hay within a couple of hours after cutting in northern climates decreases the drying rate of cut hay. If hay yielding 1815 kg (2 tons) per 0.4 ha (1 acre) is double-windrowed after mowing, spoilage will occur before drying occurs unless excellent drying conditions prevail. Hay is normally partially dried to about 40 to 50% moisture in the swath, and then raked into windrows for drying in the field or mow to 25 to 30%. In southern climates the most rapid drying of cut forage was obtained by windrowing after cutting. The stomata of the hay reopen following windrowing thus allowing exit of plant moisture (Jones 1939). In hay which was double-windrowed 2 or 3 hr after cut, the temperature surrounding the leaves inside the windrow gradually fell while the relative humidity increased. The temperature inside the windrow was 5°C (9°F) lower in double windrows 3 hr after cut than in the swath, while the relative humidity was 10% higher. The moisture loss from the leaves was kept to a minimum in the higher humidity atmosphere. The leaf functions in its normal capacity under these conditions so the water in the stem has a free outlet for evaporation, giving a greater overall moisture loss (Table 11.1). The values would be expected to be different under other air temperature and humidity conditions. gE TABLE 11.1 NATURAL DRYING OF ALFALFA HAY IN MISSISSIPPI CUT AT 8—9 A.M. Moisture, % w.b. Method of Handling As Cut 2hr After Cut 4hr After Cut 8hr After Cut 20 hr After Cut 25 hr After Cut 70 60 46 26 46 25 70 62 38 PAI 70 64 34 26 = 38 a 27 70 60 40 70 60 32 fs 19 Re 30 17 70 58 43 70 58 44 at 20 z 30 x 21 Swath Single windrow as cut Double windrow as cut Bas windrow 2 hr after cut Double windrow 2 hr after cut TG windrow 4hr after cut Double windrow 4hr after cut Source: Jones (1939). SYSTEMS FOR DRYING AND HANDLING OF HAY 261 Drying time in the field can be reduced by use of chemical desiccants before or after cutting and by use of mechanical conditi oning after cutting. The speed of drying in the field, based on a given air environment, is dependent on type of plant, stage of maturity, leaf to stem ratio, density of product in the swath, and the moisture content of soil on which the product is placed. The environmental conditions greatly affecti ng the drying include solar radiation, air movement, and humidity and temperature of the air within and surrounding the product. The following relationships exist for field drying of forage (Klinner and Shepperson 1975): (1) Respiration of the plant stops at a moisture content of 38 to 40%. (2) A wind speed of 2.2 m/s (7.22 ft/sec) is considered as most effective for drying in the windrow. (3) Drying rate decreases with decreasing crop (4) Solar energy absorption decreases with the and with crop moisture. (5) Respiration losses make up 2 to 8% of the losses under good drying conditions, and 16 conditions. moisture. intensity of radiation total crop dry matter to 20% in poor drying Crushing Hay The practice of crushing hay has gained considerable interest because it provides a method of decreasing the drying time in the field and therefore reduces the weather hazard. The drying time has decreased 50% or more in good drying weather by crushing. Typical drying curves for ‘crushed and uncrushed alfalfa after alfalfa hay was crushed which was cut and then raked ture content. The crushed hay are shown in Fig. 11.1 and were obtained one morning and compared with the hay into windrows when wilted to 50% moiswas baled the afternoon of the second day of drying while the uncrushed hay was baled the third day. On a good drying day in Wisconsin, at the end of 5 hr drying time, crushed alfalfa hay was at 25% moisture and uncrushed hay near 45% (Bruhn 1955). On an average drying day, crushed alfalfa hay dried to 35% in 5 hr as compared to 48% for uncrushed hay. Crushing has proved satisfactory in the curing of alfalfa, Johnson grass, soybeans, and pea hay. Data show that 72 hr of drying time were required for non-crushed Johnson grass while the crushed hay was baled in 24 hr with an improvement of one grade. Crushed soybean hay was dry enough to bale at the end of 48 hr whereas uncrushed soybean hay required 120 hr of drying in the field. Nearly the same results were 262 DRYING AND STORAGE 12:00 4:00 8:00 OF AGRICULTURAL 12:00 Aug.9 4:00 CROPS 8:00 12:00 4:00 7:00 Aug. |O From Montford (1947) FIG. 11.1. FA MOISTURE CONTENT FOR CRUSHED AND UNCRUSHED ALFAL- obtained with pea vine hay (Jones 1939). The greatest advantage or gain from crushing is obtained in good drying weather. As the drying weather becomes poorer, the advantage of crushing diminishes. The pressure on the rolls of the crusher should be properly adjusted. A faster drying rate is obtained by passing the material through the crusher more than once. Two sets of rollers with the second set running at 80% of the speed of the first is recommended. Opening, twisting, rearranging, and tearing of the stems occurs as they pass between the rollers (Bruhn 1955). The higher the pressure per lineal inch of roller, up to 205 kPa (30 psi), the faster the drying rate of crushed hay. Tearing and stripping of the leaves occurs with the higher pressures. If too much pressure is applied, the juice may be squeezed from the hay, especially if it is in the succulent stage, causing a loss of the nutrient material in the plant juices. Hay that has been crushed will also dry faster during barn curing because of the greater surface area of the crushed stems. Present practices do not justify crushing for reducing mow drying time. If hay is rained on after it is crushed it will absorb moisture as fast or faster than uncrushed hay. Respiration and Mold Growth There is a close relationship between the moisture content or environmental relative humidity of the product and the rate of mold growth. SYSTEMS FOR DRYING AND HANDLING OF HAY 263 Safe storage for 1 year occurs when the hay is 14% moisture, which is - equivalent to approximately 68% relative humidity or less. Hay with equilibrium relative humidity of 70% or less (15% moisture) can be stored without development of mold for 200 days (Dawson and Musgrave 1950; Wright 1941). When drying hay with natural air above 21.1°C (70°F) or with supplementary heat, the temperature of the air being discharged is often in the range of most rapid mold growth. When hay is dried with air of 60°C (140°F), a large amount of mold results. Even though temperatures greater than 49°C (120°F) kill most molds, the temperature of the product during evaporation is much cooler than the heated air temperature (Terry 1947). Few data are available on the amount of heat given off during respiration of hay. One estimate is that 11,630 kJ (5000 Btu/Ib) of heat is produced for 1 kg of dry matter consumed. The heat generated in 40% w.b. hay is roughly 232 kJ/kg (100 Btu/Ib) of dry matter for each kg (Ib) of water evaporated; for 50%, 696 kJ/kg (300 Btu/Ib); 60%, 1068 kJ/kg (460 Btu/Ib); and for 70%, 1300 kJ/kg (560 Btu/Ib) (Davis and Barlow 1948). The amount of respiration from hay stored in a mow will depend on the moisture content and the amount of mold or microorganisms on the hay. The utilization of the heat of respiration for drying when forcing air through the product will depend upon the temperature, relative humidity, and airflow. By using a low volume of drying air with a high relative humidity at a moderate temperature, so the microorganisms are not killed, heating of the stored hay due to the microorganisms is responsible for a major portion of the evaporation. The contribution of the microbial respiration for evaporation of moisture during forced air drying is variously reported as 15, 40, 63, to as high as 75% of the total heat (Frudden 1946; Hendrix 1947; Strait 1944; Terry 1948). These variations are undoubtedly due to differences in temperature and amount and kinds of microorganisms at the time of making the tests. In carefully controlled tests these variables were identified and it was found that during barn drying of hay, 25% of the heat absorbed by water evaporated was obtained when entering air had a low relative humidity and there was little if any microorganism development. Respiration provided more than 60% of the heat absorbed for water evaporation when the air was 75% or higher relative humidity and microorganism development had occurred in the hay (Dawson and Musgrave 1950). With chopped hay it was found that during forced air drying approximately 63% of the heat necessary for drying was supplied by heat generated by the hay and microorganisms (Hendrix 1947). Even though mold may grow, spontaneous combustion does not normally occur if hay is dried to 25% before storage. At 25% moisture, the diffusion of moisture and heat from the high 264 DRYING AND STORAGE OF AGRICULTURAL CROPS moisture, high temperature location is sufficiently rapid to prevent spontaneous combustion with the oxygen normally present. The relationship of time and temperature and mold formation on high moisture hay can be seen in Fig. 11.2. It is necessary to dry the hay to about 20% moisture in 42 hr or less to prevent mold formation since it is not economical to use the hay as a source of heat for drying.as respiration decreases dry matter content. There is a loss of 0.012 kg/kg (25 Ib/ton) of hay per day when 50% of the water evaporated during drying is from the heat of respiration of the hay (Dawson and Musgrave 1950). 140 55 50 120 45 40 100 oie SSloue uJ oe eae eS 394 pS w =" 720 uJ uJ & *<a 80 a. > uJ KE KE 15 60 10 S 40 40 80 120 160 TIME , HR From Terry (1947) Ba ne RELATIONSHIP OF TEMPERATURE AND TIME FOR MOLD FORMATION ON The rate of drying of hay or grain can be represented by the wet bulb and the dry bulb temperatures. The temperature of the product will SYSTEMS FOR DRYING AND HANDLING OF HAY 9265 approach the wet bulb air temperature during drying and will rise to the dry bulb temperature when about half of the water is removed . Thus, a thermometer in the product will indicate the stage of drying because the product temperature will decrease due to the cooling effect during drying (evaporation of moisture). When a majority of the moisture is removed the temperature will increase, approaching the temperature of the drying air. Barn Drying Through appropriate management of barn drying systems, harvest losses can be decreased, product can be harvested at its optimum stage of growth, storage losses can be minimized, and the organic matter can be kept higher (Klinner and Shepperson 1975). Although these advantages are generally accepted, barn drying systems have not increased in recent years because of the difficulty of providing a handling system compatible with the harvesting method. Long Hay An air distribution system composed of a fan and duct system is required for mow drying hay. The fan must provide an adequate quantity of air against the static pressure caused by the hay. The general recommendations for quantity of air are either 0.07 to 0.10 m3/m3s (15 to 20 cfm/ft?) of mow floor area or 0.156 to 0.26 m*/s:t (300 to 500 cfm/ ton). A tonne (ton) of long hay will occupy about 11.3 m? (400 ft?) of mow space. With airflows of approximately 0.051 to 0.076 m’/m3s (10 to 15 cfm/ft?) the static pressure for design of hay drying systems is 0.18 to 0.25 kPa (0.75 to 1.0 in. water) for 4.6 m (15 ft) depth and 0.13 to 0.15 kPa (0.5 to 0.6 in. water) for 2 to 3 m (6 to 8 ft) depth of hay. The duct system selected depends upon the shape of the mow, depth of the hay, and form of hay—long, chopped, or baled. The distribution system consists of a main duct which may be placed along one side of the mow with laterals or slatted floor to distribute the air or which may be placed in the center of the mow with or without laterals or slatted floor, depending upon the depth of hay and shape of mow (Table 11.2). For shallow mows, 3.66 m (12 ft) or less, the best air distribution, regardless of the width of the mow, is obtained with the main duct along one side of the mow and laterals (Fig. 11.3). For storages less than 11 m (36 ft) wide and under 7.6 m (25 ft), a main center duct is advisable (Fig. 11.4). A main center duct can be an A-frame or rectangular framework covered with slats, wire fencing, or plain steel cables for long hay (Fig. 11.5). There should be 3.8 m? of open duct surface for each m°/s of air fy ‘QUINOA eSUOT, 09 09 09 OZTOZTOZ 0ZTOZOZ J 1Z IZ LE 83 IZ 61 LT tI €I MOW éT eI 6 GT €I IT 9T I ral yideq fy ‘papueulul0je ” ; yy peddoyo pereq peTeq pereq pereq Su0] suo, SUuo] suol peddoyd peddoyd peddoyo WLIOABH jo @pIg ‘ell ATEVL ureulsieyue0 uleul IOOTJ pe97eIs ‘WS6)) TP Eies2aneS J, = == [ese7e]IOOTJuTeU yesaqe] Jo poyye]s Io Je}ua0 sjonp Jajuad Y}IM YIMsjonp BNsueyz9y joodAy, w9a38h yong Jepnsuel UreU = NI AVH SNIAWG HOS SWSALSAS LONG 4O NOSIYVdNOS Vy 9¢ 9gi“ 9€ 9&8Z OF 4 OF vp vy vy WNUIIXxe ‘UYPIM 9T €I 31 aAogy ‘yond eet LT v1 al aAaogy ‘I00| vv-ZE 9€—02 96-08 0F—02 Bs UIPIM OZTOZOZ 4 ‘1dJBM ‘Ul T Ue Ssay ATTeNsn ov paleyUNOoUA sainsseid 914849 « ‘Jad1e] opeul oie sjonp pue parddns st ie [euorjIppe jl Jeyeaid oq ued YYsUET, ‘ramodasioy Jed palip oq uBo (su0} Q[—/) seuuo G—¢g'9 AoyeUIxoIddy , 2 ‘wiayshs jonp Ul UsISep SutIeeuIsUe Iedoid sutye10d10ouI Aq pepesdxe aq you Aew syydop asoy,y, z ‘yonp IepNZuezda1 IO Wd OOO‘SP PUB JONpP Ie[NSueII} 10} WI YYO‘FZ JO MoOTjIIe ‘api 43 g pue YsIY 4J g SJoNp UO paseg ; "M1 9PL'0 = dy T ‘eq¥ 88770 = 10}BM “Ul T ‘S/W ¢ OT X QILF'O = WP T ‘WT = 33 BHOEO ‘S1OJIBJ UOISIAAUOD iJ OF OV 0g OL GL 06 09 09 09 Gg Gg Gg WNUIIXe ‘yysue'T CROPS OF AGRICULTURAL DRYING AND STORAGE 266 SYSTEMS FOR DRYING AND HANDLING OF HAY FIG. 11.3. MAIN DUCT AT SIDE OF MOW FIG. 11.4. MAIN DUCT AT CENTER OF MOW FOR DEEP HAY STORAGE FOR SHALLOW 267 HAY STORAGE SELECT CROSS-SECTIONAL AREA, A, SO THAT THERE IS | SQ.FT. FOR EACH 1000 CFM OF AIR 2"x 4" STUDS SPACED 2 FEET (" MESH FOR WIDTH, FT. HEIGHT, FT. HEIGHT, FT. 2"x4" hae: CROSS- SECTIONAL FIG. 11.5. FLOOR WIDTH, FT. AREA = =—_—“NO HEIGHT x WIDTH CROSS — SECTIONAL CENTER MAIN DUCT WITHOUT BRANCHES See Appendix for metric conversions. SOLE AREA ®= Se HEIGHT Se x WIDTH OR LATERALS 268 DRYING AND STORAGE OF AGRICULTURAL CROPS entering the duct (1 ft? for each 50 cfm) at 0.13 kPa (0.5 in.) static pressure. The main duct should have a cross-sectional area of 0.2 m?/m’s of air (1 ft? for each 1000 cfm of air) that is moved through the duct. The duct is normally at least 1.8 m (6 ft) high and is large enough for a person to work inside. The major objective in any drying system is to have a uniform length of path through which the air passes in getting from the duct to the outside of the forage being dried. With the rectangular or triangular duct it is desirable, if considerable hay is to be dried with a shallow depth, to have doors in the top of the duct which can be closed to prevent air escape (Fig. 11.6). This system is usually used for mows wider than 11 m (36 ft). The duct doors are opened for depths over 4.6 m (15 ft). For very deep forage over the ducts, the doors to the floor ducts can be closed when the hay on the bottom is dry. The branch or lateral ducts may be constructed as either a slatted floor or a duct system. It is important that the lateral ducts do not extend closer than 2.5 m (8 ft) to the sides or 1.5 m (5 ft) to the ends of the haymow so there is little air loss. Lateral ducts are commonly 1.2 to 2 m (4 to 6 ft) apart. Details of laterals and slatted floor plans are in Fig. 11.7 and 11.8. FIG. 11.6. ARRANGEMENT OF DOORS IN MAIN DUCT A round structure with a forced air drying system that incorporates a vertical duct is used for commercially built hay storages (Fig. 11.9). Doors are provided which can be closed for shutting off the air to the hay previously dried. For deep narrow mows it is advisable to install two-level ducts through the center of the mow, one above the other approximately 4.5 m (15 ft) apart. A two-level system of ducts would provide air movement for a SYSTEMS FOR DRYING AND HANDLING OF 6 ts SOL "x3" A ‘oped FIG. 11.7. OF MOW——— —— SLATS 3" OPENINGS id ; “g" x 9" —+ 20" 2" OPENING— LATERAL DUCT DETAILS See Appendix for metric conversions. SLATTED FLOOR MAY BE IN SECTIONS =——— WIOTH OF MOW ———™ "x 3" SLATS INGS lys OPENINGS Lot" |415-25 FIG. 11.8. SLATTED FLOOR LATERAL DUCT DETAILS See Appendix for metric conversions. HAY 269 DRYING 270 AND STORAGE OF AGRICULTURAL CROPS AIR oureTs DOORS CLOSED DOORS OPEN FOR AIR PASSAGE THROUGH HAY HAY BEING DRIED—| = [ees — PREVIOUSLY DRIED HAV DOORS AIR CLOSED DUCT FAN FIG. 11.9. ROUND STRUCTURE AIR DRYING SYSTEM mow WITH FORCED 9.1 m (30 ft) deep. To move 0.077 m3/m2s (15 cfm/ft?) of air through 4.9 m (16 ft) of chopped hay requires a static pressure of 0.5 kPa (2 in. water). Vertical flues rising from the center duct can also be used. A flue-form may be moved vertically through the hay as the mow is filled to provide a vertical hay-formed center duct. Chopped Hay The requirements of a drying system for long hay apply to chopped hay. The various systems can be adapted to chopped hay. The major difference is that a shallower depth of chopped hay is required for the same quantity. About 4 m (13 ft) of chopped hay is equivalent to 4.9 m (16 ft) of long hay. The static pressures involved for the two depths would be about the same. Static pressure relationships are presented in Fig. 11.10. For chopped hay a center main duct should be covered with 2.5 cm (1 in.) mesh fencing material. A wagon dryer or separate drying building should be used for heated air drying from which the dried product is moved to storage. A drying SYSTEMS bie ° FOR DRYING AND WALKED ON DURING LOADING HANDLING OF HAY 271 © Seog H20wo oO IN ke ° ny 6] is)= a° AP, STATIC PRESSURE, ° | in, water = 0.2488 | ft =0.3048 m fe).) uo kPa | cfm/ft2 = 0.00508 m3/m2s 2 3 4 5 DEPTH 6 uf BEFORE 8 2) DRYING, 10 =I 12 FT. From Davis and Baker (1951) FIG. 11.10. RELATIONSHIP OF STATIC PRESSURE, DEPTH, AIRFLOW, AND PERCENTAGE MOISTURE FOR LONG AND CHOPPED HAY building incorporating use of heated air and a mechanical handling system can be built which can be used for both hay and grain. A perforated floor for air distribution and an endgate-type unloader to move the product into a flight conveyor beside the perforated floor are the principal features of this system. A building 13.4 m (44 ft) long, 6 m (20 ft) wide, and 4.3 m (14 ft) high will dry 14,512 to 18,140 kg (16 to 20 tons) of chopped hay or 21 to 28 m? (600 to 800 bu) of grain in 24 hr. Baled Hay Both the density and moisture content of the bale at time of storage affect the final hay quality. Hay must be dried to about 20% moisture content, w.b., before baling and the bale density should be about 96 kg/m? (6 lb/ft?) to obtain completely mold-free hay (Hopkins et al. 1954) (Fig. 11.11). A bale 0.34 m X 0.46 m X 1 m (14 in. X 18 in. X 40 in.) weighing 16 kg (35 lb) would have a density of 96 kg/m? (16 lb/ft*). These bales would be very difficult to handle and impractical. Although the complete absence of mold is not necessary when hay is fed on the farm, the amount of must gives a measure of the importance of density and moisture content. 272 DRYING AND STORAGE CROPS OF AGRICULTURAL kg/m? lb/ft? 16.02 |= FT. DENSITY, CU. PER BALE LB. 8 20 MOISTURE 22 24 26 CONTENT (WET 28 BASIS),PER 30 32 CENT From Hopkins et al. (1954) FIG. 11.11. BALE DENSITY EFFECT ON QUALITY AND MOISTURE It is desirable when hay is to be baled and dried by forced air to field dry to 35% moisture content before baling. It is desirable to bale the hay as loosely as is consistent with good handling practices. To permit good handling requires that the same bale weigh between 25 and 30 kg (55 and 65 lb) or approximately 176 kg/m? (11 lb/ft’). If loosely baled, it is possible to bale at a moisture content of 35%. Hay with a moisture content of 30%, weighing about 176 kg/m? (11 lb/ft’), heated up to as high as 41°C (105°F) immediately after placement in the mow in the absence of forced ventilation. The temperature dropped to below the entering air temperature within 30 min after the fan was started. An airflow of 0.1 m’/m?s (20 cfm/ft?) was used with most of the air going through 0.05 m (2 in.) spaces provided between the bales. The static pressure for forcing air around the bales is only about one-third that required for forcing air through the bales when drying. There are three basic methods of placing bales in a mow for mow curing: (1) tight-packed, (2) loose-stacked, and (3) helter-skelter. A comparison of the three methods of placement in the mow for a 30—70% moisture of alfalfa-brome is listed in order of desirability from best to poorest as follows: (1) Tight-packed. Hay with a density of 225 kg/m? SYSTEMS (14 lb/ft?) was tions 3 m FOR DRYING AND HANDLING OF HAY 273 tightly packed with rows packed in alternate direc- (10 ft) deep at 29.5% moisture content with an airflow of 0.10 m*/m?s (20 cfm/ft?) and a mean static pressure of 0.2 kPa (0.77 in. water). (2) Loose stacked. Bales were placed in the mow in alternate directions with about 0.5 m (2 in.) between bales, 7 layers or 3 m (10 ft) deep, 190 kg/m? (11.9 lb/ft) density of hay at 31.5% moisture and with an airflow of 0.10 m*/m?s (20 cfm/ft?) with a mean static pressure 0.7 kPa (0.28 in. water). (3) Helter-skelter. The bales were dropped from a track at random to the dryer to a depth of 3.2 m (10.5 ft) with an original moisture content of 33% and an overall density of 136 kg/m? (8.5 lb/ft?) with an airflow of 0.12 m’/m?s (23.5 cfm/ft2) with a mean static pressure of 0.05 kPa (0.20 in. water). Tight packing with the bales staggered and placed in alternate directions is the best system of stacking for curing with forced ventilation while use of helter-skelter or random piling is desirable for natural ventilation. Bales should be placed on edge so that the air is forced into the narrowest dimension of the bale to obtain good airflow. However, the problem of handling causes some to overlook the advantage of airflow from stacking on edge and to recommend placing the bales on their flat side. Heated air drying is done in a building, on a platform, or wagon designed specifically for drying. One heated air unit will dry 4 wagon- loads 9070 kg (10 tons) at a time and can be used for grain. Some units have a false floor in the wagon box. The bales are moved from the drying wagon or building to storage. A platform batch dryer has been developed for baled hay. The hay should be reduced to 40 to 50% moisture in the field and baled into loose bales weighing about 176 kg/m? (11 lb/ft’). The following specifications describe a platform unit: Floor area Floor Depth of loading Air capacity Heat capacity Capacity Time to dry Cover 52 m?(4.3 m X 12.2 m), 560 ft? (14 ft X 40 ft) Slatted, perforated metal, woven wire 3 bales 0.15 m3/m?s at 0.12 to 0.18 kPa (30 cfm/ft? at 0.5 to 0.75 in. water) 293 kJ/s with 33°C increase (1 million Btu/hr with 60°F increase) 7250 kg (8 tons), 3 bales deep 16 to 22 hr A tarpaulin should be placed over hay when drying for weather protection, uniform drying, and to prevent excessive loss of hot air 274. DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Sperry-New Holland FIG. 11.12. TWO SIZES OF LARGE ROUND BALES Large Round Bales A recent development is the large, or big, round bales which can be stored in the field or feedlot and moved when convenient to storage or for use (Fig. 11.12). The little large round bales 1.8 m X 1.3 m (4% ft X 4 ft) weigh up to 386 kg (850 lb) and the regular large bales 1.8 m X 1.8 m (5% ft X 5% ft) weigh 680 kg (1500 lb). Variations in these dimensions exist for different manufacturers. To avoid heavy leaf loss at the lower moisture contents and internal spoilage at the higher moisture contents, bales should be made when the moisture content is 15 to 22%. Some evidence exists that organic acid preservatives can be used to reduce spoilage that would otherwise occur in large round bales made with forage in the range of 20 to 30% moisture content. Mechanical or artificial drying has not generally been practiced with large round bales. To minimize spoilage or loss of hay when large round bales are stored outside: (1) have a uniform density throughout, (2) produce a bale which SYSTEMS FOR DRYING AND HANDLING OF HAY 275 will maintain a cylindrical shape, neither barrel-shaped nor flattened wider bale; (3) use higher density bales to have less weathering because there is less penetration of moisture, the bales retain their shape, and they have less surface contact with the ground and less spoilage; (4) use appropriate tension (for feeding in the field little or no twine can be used); and (5) when storing in the field, orient the center axis of the bale north-to-south, or face a round side of the bale toward prevailing winds. Bales can be handled individually with forklifts (Fig. 11.13), which will suffice for moving 158 X 103 kg (175 tons)/year. A multibale unit can pick up 3 to 5 bales in the field, transport the bales, and unload in a couple of minutes (Fig. 11.14). Drying Large Round Bales Solar energy was used for drying wet (40% moisture content) large round hay bales placed on a drying platform (Baker and Shove 1978). The solar collector heated the air to about 19°C (35°F) and the fan moved the air through the collector and forced the air through the bale. The aS Courtesy of Sperry-New Holland FIG. 11.13. TRANSPORTING A SINGLE BALE ON FORK LIFT 276 DRYING AND STORAGE OF AGRICULTURAL CROPS Courtesy of Sperry-New Holland FIG. 11.14. TRANSPORTING SEVERAL LARGE ROUND BALES bales are placed on end over a perforated floor with a cylinder ring around the lower part of the bale, extending up about 0.3 to 0.6 m (1 to 2 ft). A 3.7 kW (5 hp) fan provides air for 4 bales with an airflow of 2 m3/s (4560 cfm) at a static moisture content was days of drying. Corrie bales with a density m3/m2s (45 cfm/ft?). pressure of 0.5 kPa (2 in. water). Hay at 40 to 45% dried to 20% with’air heated 19°C (35°F) over 2 and Bull (1969) reported favorable drying of large of 90 kg/m? (5.6 lb/ft?) using an airflow of 0.23 The large round bale densities with conventional machines in the United States are in the range of 160 to 240 kg/m? (10 to | 15 lb/ft’). Wafers, Pellets, and Cubes Forage is often formed into wafers, pellets, or cubes to provide a compact material, in a small unit, which can be easily handled. These are defined by ASAE Standard 8269.2 (ASAE 1979): Wafer: An agglomeration of unground ingredients in which some of the Pellet: fibers are equal to or greater than the length of the crosssectional dimension of the agglomeration. An agglomeration of individual ground ingredients, or mixture of such ingredients, commonly used for animal feeds. SYSTEMS FOR DRYING AND HANDLING OF HAY 277 _ There is not a standard definition of a cube. A cube is formed by compressing fibers, usually by an extrusion process, into 31 mm X 31 mm (1% in. X 1% in.) sizes. Cubes are denser than wafers, and wafers have a larger cross-section than pellets and cubes. A film of water is sprayed on forages while cubing to bind the fibers, with the moisture up to 17%, followed by drying to about 13% for storage. TABLE 11.3. Size BASIC DATA ON PELLETS, CUBES, AND WAFERS WITH HAY Pellets Cubes Compressed Compressed Wafers (Extruded) (Extruded) Rolled 38 X 38mm (1%in. 31 X 31mm (1% in. 60-100 mm diameter X 1% in.) and less X 1% in.) X 20-100 mm long Density! 480-560 kg/m? (30—35 lb/ft’) 480-560 kg/m? (25-30 lb/ft?) Pressure used 238 X 105 N/m? 340 X 105 N/m? Energy to produce? 160—256kWh/100kg (5—8 hpchr/ton) 480-800kWh/100kg (15-25 hp-hr/ton) including propulsion Reference Bellinger and McColly (1961) Curley et al.(1973); Dobie (1975) (3500 psi) (5000 psi) 240-480 kg/m} (15-30 lb/ft?) — 224-320kWh/100kg (7-10 hp:hr/ton) | Molitorisz and McColly (1970) Source: Hall and Davis (1979). ' Bale density of field machines, 144—160 kg/m’ (9-10 lb/ft). 2 Baler i 48-96 kWh/100 kg (1.5—3 hpshr/ton) for alfalfa (20 hp:hr/ton = 1.64 kWh/ 100 kg). Insect pests are unable to infest compressed forage products in the form of wafers, cubes, and pellets if the atmosphere is less than 75% relative humidity and the forage less than 14% (Woodroffe 1973). Organic acids, such as propionic acid, provide a means of protecting compressed forage products from spoiling when made at the higher moisture contents than are safe for storage. Approximately 4% of propionic acid by weight mixed with a wet forage to be dried or with a nearly dry forage to be placed in a high humidity environment greatly extends the storage life of compressed forage (Fig. 11.15). (Chaplin and Tetlow 1971). Alfalfa is normally field cured or artificially dried to 12% or less with surface moisture applied at the time of cubing to provide a cube moisture of about 15%, which provides a better cube than cubing at 15% internal moisture. Generally a better quality cube is obtained if made at a moisture content of 16 to 23% moisture. These will not keep unless dried or protected with a preservative. Cubes made from 20% moisture content alfalfa, treated with 1% by weight of propionic acid added before cubing, were effectively protected from serious molding (Dobie and Carnegie 1973). Heated or natural air can be forced through stored wafers, pellets, and 78 ie) DRYING OF AGRICULTURAL AND STORAGE 90 CROPS TREATED WITH 4% PROPIONIC ACID. 80 70 60 PERCENT HUMIDITY, RELATIVE AT 21°C (70°F) 10 | LOG OF MINIMUM 1000 DAYS 2] 3 MOLD FREE STORAGE DAYS From Chaplin and Tetlow (1971) FIG. 11.15. EFFECT OF PROPIONIC TIMOTHY, AND RYEGRASS ACID ON STORAGE OF WAFERED ALFALFA, cubes to reduce the moisture content to approximately 14% to provide a product which will store safely. The pressure drop of air flowing through compressed forages varies greatly depending upon the size of the wafer, pellet, or cube and upon the fines included. With an airflow of 6 to 12 m3/m?-min (20 to 40 cfm/ft?), pellets with 10% fines will have a pressure drop of 0.4 to 3.0 kPa/m depth (0.5 to 3.5 in. water/ft depth) (Kjelgaard 1965). Dried products may have poor density and less desirable handling characteristics. One approach is to use binders so that the dried compressed forage will have better handling characteristics, particularly durability, after drying. Cubes made from 20% moisture alfalfa molded in storage, while those made with 1% propionic acid by weight added before cubing did not mold seriously (Dobie and Carnegie 1973). Selection and Use of Air Distribution Systems The triangular center main duct is used for mows 6 to 11 m (20 to 36 ft) wide; rectangular center main for mows, 9 to 11 m (30 to 36 ft) wide; side SYSTEMS FOR DRYING AND HANDLING OF HAY 279 main with laterals or slatted floor for mows, 6 to 12 m (20 to 40 ft) wide; and center main with lateral or slatted floor for mows, 9.7 to 13.4 m (32 to 44 ft) wide. Limitations of width are in Table 11.2. Hay may settle away from rectangular ducts while drying and provide escape of air along the duct and out the end of the mow. The depth of hay stored over the center main duct depends on the distance from the duct to the edge of the mow. In general, the recommended distance from the duct to air discharge is 4.8 m (14 ft) for long, 3.9 m (13 ft) for chopped, and 3.7 m (12 ft) for baled hay. The first cutting may supply all of the hay if placed on the dryer over a 2 week period. For a mow 10.7 m (35 ft) wide, with a 2.1 m (7 ft) X 1.5 m (5 ft) wide rectangular center main duct, there would be 4.6 m (15 ft) of hay on each side of the duct. Nearly the same depth of hay can be placed over the duct giving a maximum depth of storage in the mow of 6.7 m (22 ft). If the dimensions of the main duct were reversed, the maximum depth of storage would be 5.8 m (19 ft). Air movement across the mow takes place easier than vertically through the hay, although detailed data are not available on this subject. The hay should be placed around the duct with the center main system maintaining a uniform distance in all directions from the duct; with the lateral or slatted floor system, the hay should be kept level over the ducts. Prefabricated ducts are available. Two fans can be used with one at each end of the main duct. Pressure systems are normally used. For a small volume of hay one fan can be operated with a cover or door over the other fan. The main duct needs to be only about one-half as large with two fans at opposite ends as compared to one large fan at one end. However, a smaller duct may be economical from the standpoint of cost but might not be practical for a wide mow. The starting current of two small motors started independently is less than for a larger motor. Designing a Barn Hay Dryer The following steps should be followed in designing a forced unheated air drying system for a mow. Method A is used for determining the airflow on the basis of mow area. It is easier to use than Method B, which is used for determining the airflow on the basis of volume of hay. Both methods give approximately the same results for minimum and maximum air volume, although Method B is more accurate, particularly if the depths vary from the recommended values. Select a fan to give an airflow approximately midway between the minimum and maximum values for static pressures of 0.19 kPa (% in.) for long hay and 0.37 kPa (1.5 in.) for baled hay. DRYING 280 AND STORAGE OF AGRICULTURAL CROPS Method A.—[Based on floor area in meters (square feet) in mow] (1) Determine floor area of mow. Multiply the length in m (ft) X width in m (ft) to determine the mow area in m? (ft?). (2 Calculate the quantity of air required, m*/s, by multiplying the mow floor area by 0.075 m?/m°s (5 cfm/ft?) (minimum) and 0.10 — m?/m?2s (20 cfm/ft2) (maximum). The fan must be selected and duct system designed to deliver this quantity of air at a static pressure up to 0.25 kPa (1 in. water). [Baled hay may have a static pressure of 0.38 kPa (1% in. water).] (3 Select type of air distribution system based on dimensions of mow and depth of filling (Table 11.2). a (4 — Calculate cross-sectional dimensions of main duct, and laterals if used, so for each m?/m?/s 50 cfm) (5 — that there is 1000 cfm) of (1 ft? of duct of air through at least 0.23 m?/m3/s (1 ft? cross-section air through the duct. There should be 4.5 surface opening for air discharge for each duct. Estimate fan horsepower requirements on basis of mow area of 21.8 to 31.2 m2/kW (175 to 250 ft?/hp). Method B.—(Based on quantity of hay, tons)! (1) (a) Calculate the volume of first cutting or second cutting of hay, whichever is the larger, which will be placed in the mow. The maximum depths are shown in Table 11.2 and are 4.9, 3.9, 3.7 m (16, 13, 12 ft) for long, chopped, and baled hay, respectively, for a wide mow. Volume for a rectangle mow Ss equals length X width X depth in meters for volume in mt = 2.83 m? ="907 kg. Calculate quantity of hay in tons, using 10.5 to 12.6 m3 (375 to 450 ft?) per ton (2000 lb) for long hay, 8.4 to 9.8 m? (300 to 350 ft*) per ton for chopped hay, and 4.9 to 7 m3 (175 to 250 ft?) per ton of baled hay. Volume per ton (2000 lb) figures commonly used for long, chopped; and baled hay are 11.2, 8.4, and 5.6 m® (400, 300, and 200 ft), respectively. Calculate the quantity of air required, m3/s (cfm), by multiplying the number of m? (tons) by 0.141 m3/s/2.83 m3 (300 cfm/ ton) (minimum) and 0.24 m3/s/2.83 m3 (500 cfm/ton) (maximum). (3-4) Same as Method A. 'See Appendix for metric conversions. SYSTEMS (5) FOR DRYING AND HANDLING OF HAY 281 Estimate fan horsepower requirements on basis of 8.4 to 12 t/ kW (7 to 10 tons/hp), for hay of 40% moisture. Placing Hay on Dryer Long hay may be placed on the dryer in the same way asit is placed ina mow, with forks and slings or a blower. However, this procedure places the hay in large bunches and makes drying difficult. Chopped hay can be placed on the dryer with a blower or conveyor. However, leaves and stems tend to separate as they are blown onto the dryer, making uniform drying impossible. The conveyor can be mounted so that its discharge point can be moved to drop the hay at various locations over the dryer to provide uniform loading. The conveyor requires less power but the initial cost is higher than a blower. A 1.5 to 2.2 kW (2 to 3 hp) electric motor _would suffice for a conveyor where a 12 kW (15 hp) motor would be required on a blower to do the same job. Baled hay is commonly placed on dryers with an elevator, although an overhead fork can be used. Walking on wet hay on a dryer should be avoided, although it may be necessary to tramp hay around posts to prevent air leakage. The hay should be placed uniformly over the drying duct so uniform drying may result, unless some means is provided for blocking the air outlet from the duct so that air does not escape through areas where there is little or no hay. The track and carrier in many barns used for moving hay by slings or forks may be used to support a deflection board for aid in obtaining uniform hay distribution. The hay from the blower is directed toward the deflection board which is approximately 2 m X 1.2 m (6 ft X 4 ft). The deflection board can be moved up or down and along the track on the hay carrier to obtain more uniform hay distribution. Building Requirements A hay mow should be investigated for strength before it is filled with wet chopped hay or baled hay. The weight per cubic meter (cubic foot) of wet hay is greater than field dried hay normally put into the mow and if the hay is chopped or baled the density is usually considered to be greater. A mow of dry hay originally at 35% moisture will contain about 10% more hay if filled with field dried hay. Any supporting member should be strengthened if it shows signs of being overloaded. Another important requirement is that the floor must be tight. If not, the forced air will go through the floor instead of through the hay. A tongueand-groove floor in good condition or a dirt floor is satisfactory. Floors 282 DRYING AND STORAGE OF AGRICULTURAL with cracks should be covered material. CROPS with roofing paper, plastic, or similar Supplemental Heat in Hay Drying Supplemental heat is often used to decrease the time required for drying hay. By raising the air temperature approximately 11°C (20°F), the time required for drying is reduced to one-fourth the time for unheated air; by raising the air temperature 16.7°C (30°F), to one-sixth; and raising the air temperature 33.3°C (60°F), to one-ninth of the time required when no additional heat is supplied. The quality of the hay is increased by using supplemental heat because the time of drying is decreased, thereby reducing the growth of mold, loss of carotene, and loss of dry matter which occurs during respiration. It is not recommended that heated air drying equipment be used in major storage buildings. It is desirable to have a separate structure used only for drying and a mechanical means of handling the product from the dryer to the storage building. The use of heated air is more satisfactory for baled hay under these conditions because of the handling involved. If a heated air dryer is to be used for drying hay in the main storage building, the heater should not be placed in the building but should be located outside and connected by a flame-resistant canvas duct. Alternatively, a boiler may be placed in a separate building and steam piped to a heat exchanger for heating the air. A better quality product can be secured by using heated air as compared to forced unheated air because the product can be dried before molding takes place. A check of the top 2 m (6 ft) of alfalfa hay dried with forced unheated air was graded moldy and by the use of supplemental heat the depth of moldy hay was reduced to 0.6 m (2 ft) top, which increased the value of the hay (Davis 1947). If 21.1°C (70°F) air at 50% relative humidity is heated to 37.8°C (100°F), its moisture holding capacity is increased 170%. If heated to 93.3°C (200°F), the moisture holding capacity is increased 810%. The increase in moisture carrying capacity of the air cannot be realized in practice because some of the heat is transferred to the hay as sensible heat to raise the temperature of the mass or to replace heat loss through the walls. Approximately 3133 kJ/°C-t (1500 Btu/°F-ton) are required to heat hay containing 65% moisture. When the drying air is heated, the volume of airflow should be increased to carry the moisture from the hay and prevent condensation in the upper layer of the hay mass. The general practice has been to use 0.07 to 0.15 m*/m?’s (15 to 30 cfm/ft2) for heated air. The heating equipment should be of sufficient size to provide approximately 1.35 kJ/m? for each 1°C temperature rise (20 Btu/1000 ft? of air for each 1°F SYSTEMS FOR DRYING AND HANDLING OF HAY 283 rise). Thus, for 18,000 cfm for a 25°F temperature increase , a heater should provide (20) Xx (18) x (25) X (60) or 540,000 Btu/hr. (See Chapter 7 for a discussion of heaters.) The drying zone is approximately 1 m (3 ft) for hay with initial moisture content of 41% with an airflow of 0.076 m3/m?s (15 cfm/ft2) at 54.5°C (130°F) and 16% relative humidity. _ The quality of the hay obtained is a function of the time required to complete the drying process. The quality of hay improves from the bottom to the top with excellent quality hay occurring in the lower portion of the mow which is dried first. By using air heated 14°C (25°F), an increase of one-half grade can be expected as compared to drying with unheated forced air (Davis et al. 1950). Additional heat may be supplied to the drying air with a heat exchanger on the exhaust when a gasoline engine is used. An increase in temperature of 3.3°C (6°F) occurs when moving 7.24 m3/s (15,400 cfm) of air with a heat exchanger on a 21 kW (28 hp) engine exhaust. This amount of air would be used for a mow area of about 92 m? (1000 ft2). Fan for Forced Air Drying of Hay Much more water has to be evaporated from a mow of hay than from a bin of grain for most general farm drying installations. The fans used for hay drying deliver larger volumes of air at lower static pressures than those for grain drying. Fans for hay drying are selected on the basis of delivering a large quantity of air against 0.25 kPa (1 in. water) static pressure. When the same fans are used to dry grain, a different set of conditions normally exists. A smaller amount of air is needed for grain drying, but the static pressure against which the fan should deliver the air is up to 0.75 kPa (3 in. water). As the static pressure increases, the volume of air delivered by a fan decreases. Limiting the depth of grain in the storage bin to 2 m (6 ft) permits the use of hay drying equipment for grain drying if the fan operates over the static pressures encountered. Management of a Hay Dryer Satisfactory operation of a forced or heated air system for hay depends upon the management of the system, which begins when the hay is in the field. If the hay is cut in the morning, its moisture content will be approximately 40% in the afternoon of a good drying day and can be placed on the dryer. The specifications for farm hay drying installations are usually based on drying 40% hay. Almost twice as much water must be removed if hay is placed on the dryer at 50% instead of 40% (Table 11.4). If hay with a moisture content above 40% is placed on a dryer, DRYING 284 AND STORAGE OF AGRICULTURAL TABLE 11.4. MOISTURE TO BE REMOVED 25, 20, 15, AND 10%., W.B. CROPS 0.9 T (1 TON) OF HAY AT TO PRODUCE Water to Be Removed to Give Final Moisture Content Of 10% 15% 20% lb kg lb kg lb kg lb Initial Moisture, % w.b. kg Uw 70 65 1814 1358 1039 4000 2995 2290 1995 1509 1168 4400 3328 2576 2177 1660 1298 4800 3661 2862 60 55 794 605 1750 13338 907 680 2000 1555 1020 806 45 40 727 330 221500 A12 SO2e 909 266i 494 378 35 139 2307 209 461 27D eis: 349 769 65) 143 0 93 60 206 133 195 L21 429 267 259 181 aoe 400 25% 30 DAs) — 20 15 10 — — — _ _ — _ — 5200 3994 3148 2250 1778 1134 907 2500 2000 1090 834 Hin 454 woe 1000 Seles 0 — == 2358 1811 1444 \ 0 _ _ — 13250 BAe = 0 the quantity of hay must be reduced proportionately so that the total amount of water to be removed is approximately the same. If the hay is baled, it should be placed in loose bales and if chopped, the length of cut should be 0.07 m (3 in.) or longer. Moisture meters are available for hay. The condition of the hay gives a clue to the moisture content (Table 1¥5): TABLE 11.5. PHYSICAL CONDITION OF HAY AT VARIOUS MOISTURE CONTENTS Moisture Content, % w.b. 75 50 40 35 32.5 30 DU 25 Condition Freshly cut Wilted, very heavy to handle Heavy to handle Handles like hay . Leaves still hang on Tough, leaves rattle Would heat in ordinary storage Slightly tough, leaves shatter The hay should be placed uniformly over the drying duct. The hay will dry in about 7 days with forced air drying under good conditions. A good rule-of-thumb to regulate placing hay on a dryer is no more than 1 m (3 ft) of wet hay on any one day and no more than the maximum depth permitted 5 m (16 ft) for long hay, etc., in a 2 week period. At least 1 m (3 ft) depth of hay should be on the dryer when starting so that uniform air distribution may be secured. Overloading the dryer will seriously impair drying efficiency and result in moldy and musty hay with a reduced grade. The fan should be operated continuously for the first 4 or 5 days SYSTEMS FOR DRYING AND HANDLING OF HAY 285 or until the hay is dry on top. The fan should even be operated at night when the humidity conditions are high because the air movement and heat generated from the hay will continue to evaporate moisture from the wet hay and prevent damage to the hay. If the fan is not operated, the hay will heat, mold growth will occur, and the quality of hay will be reduced. After the hay has started to dry on the top surface the fan need not be operated during the day except in periods of rain or heavy fog. Doors, windows, ventilators, and louvers should be left open during drying to permit exhaust of moisture-laden air. Slightly different management is followed when heat is used for drying hay. The fan can be operated continuously when heat is supplied, but excessive fuel consumption results if the temperature of the air leaving the hay is more than 3°C (5°F) above the outside temperature (Davis 1951). There is no advantage in operating the fan after the hay is dry. To avoid excessive condensation on the surface, the discharge of a heated air system should operate under a slight pressure to keep the moisture- laden air separated from the outside air. A simple means of determining when the hay is dry enough to keep safely is to check the air blown through the hay after the fan has been turned off for a period of 12 hr. If the air coming out of the mow is warm in any spot the fan should be operated for another 12 hr and the test repeated until no warm air comes from the mow. An important part of the successful operation of drying equipment depends upon the facilities for mechanical handling of the product. Available machinery, blowers, and conveyors may be used economically to handle the hay, with their selection depending upon the form in which the hay is handled. Self-feeding of the hay from the mow usually located on ground level is a simple method of reducing the labor required for removing the product from the mow. Cost of Drying Hay A heated air drying system will cost two to three times more than a forced air system. The major expense with the forced air system is for the fan and motor and with the heated air system for the heater, fan and motor, air distribution system, and handling system. The electric energy for drying 30% moisture hay with forced unheated air with good drying weather is about 297 X 103 kJ/t (75 kWh/ton) and the power and fuel costs for a heated air system will be twice the cost of the forced unheated system. The cost of drying with forced air will increase, however, if poor drying weather occurs during the drying period. 286 DRYING AND STORAGE OF AGRICULTURAL CROPS Handling Hay General statements on the comparative labor requirements and cost of handling hay are difficult to make. Of prime importance is the relationship of the handling system to the field operations and the storage or feeding operations. A comparison of labor requirements of the two principal handling procedures, baled and chopped forage, is presented in Table 11.6. These data are indicative of comparable labor requirements under similar conditions. Comparable data for all the possible methods of handling are not available. TABLE 11.6. HAY HANDLING, MAN-HR/TON, Operation 1970-1975 Baled Chopped Unloading 0.23 0.25 0.08 Removal 0.41 0.50 Total 1.57 Ishi Distributing Moving Feeding/ distributing 0.17 0.51 0.20 0.13 0.46 Reconditioning Overdried Hay It is desirable to add moisture to overdried hay to prevent loss of leaves and to improve palatability. Hay absorbs moisture rapidly during the first 4 days of moving high humidity air through it and the hay first contacted by the high humidity air absorbs most of the moisture with little for the remaining layers. In deep mows, the layer originally wetted will mold and spoil before the remainderis wetted. Hay originally at 11.5% moisture in a mow 2.3 m (7% ft) deep was wetted to 26.2% on the bottom, while the bottom 0.15 m (0.5 ft) averaged 23.5%, the center 16%, and the top 15.4%; air was used at 94% relative humidity and 16°C (61°F) for 35 hr with 0.0009 m°/s (19.3 cfm/ft?) (Ball and Barger 1948). Alfalfa Dehydrators By definition of the American Dehydrators Associations, dehydrated alfalfa is obtained by drying the product artificially at a temperature above 100°C (212°F). The chopped alfalfa is fed into the dehydrator at about 75% moisture and is dried to approximately 5 to 8% moisture. If above 10% moisture, the power requirements for the hammer mill are excessive. Most of the alfalfa dehydrators are of the rotary drum type consisting of (1) single or (2) multiple drums. These units are usually 2.50 to 3.75 m (8 to 12 ft) in diameter and 9 m (30 ft) long with an output of approximately 0.9 t (1 ton)/hr of dry meal. Temperatures up to 927°C SYSTEMS FOR DRYING AND HANDLING OF HAY 287 (1700°F) are used in the dehydrators used in the preparation of alfalfa meal. The air, including the products of combustion, dries the hay and leaves the opposite end of the drum at about 121°C (250°F). If the exhaust air is above 149°C (300°F), the sudden mixture with oxygen may cause combustion. In these units the speed of the drum should be slow enough that the centrifugal force does not hold the alfalfa against the _ inside of the drum. Baffles are used to make the alfalfa fall through the stream of hot air. One long drum makes up the drying chamber of the single drum unit. The multiple drum unit has the same apparent construction from the outside, but consists of 3 concentric drums which are used to obtain 3 different drying conditions. Efficiencies for these dehydrators are between 50 and 85%. For each 0.9 t (1 ton) of dehydrated alfalfa meal, 227 to 265 X 1073 m3 (60 to 70 gal.) of fuel oil or 226 to 283 m? (8000 to 10,000 ft®) of gas are required. For general comparisons, tonne (2240 Ib) can be substituted for ton (2000 Ib) (Schrenk et al. 1953). In the rotary drum dryer about one-half of the heat loss occurs from radiation and leakage and the remainder of the loss is in the exhaust and hay. The web or conveyor type dehydrator is used to some extent. Its efficiency is about 65%. QUESTIONS 1. If hay at 75% moisture, w.b., is dehydrated to 5% moisture, how much water is removed per ton of dehydrated product? If dried from 40% moisture to 20% for storage, how much water is removed per ton? Estimate the cost for both systems. 2. Compare the quantity of air for different haymows using the two recommendations of 15 to 20 cfm/ft? and 300 to 500 cfm/ton. Also compare in SI units. Discuss the advantages and disadvantages of these methods of representing air recommendations. 3. Sketch an air distribution system for forced air drying in a haymow 9.75 m X 15.25 m (32 ft X 50 ft) with (1) laterals, (2) slatted floor, and (3) center A-frame. Prepare a bill of materials of each with wood construction and estimate the cost. 4. Refer to a commercial fan catalog and used for forced air drying of both grain 0.0352 m3) of wheat and 40 t of hay. recommended. Give model number and select a fan which can be and hay for 25 m? (1 bu = Assume depths commonly complete specifications of fan and motor. 5. Write the equation for pounds of water to be removed to produce 1 t of 20% hay with various initial moisture contents. Use all moisture contents on wet basis (Table 11.4). 288 DRYING AND STORAGE OF AGRICULTURAL CROPS 6. Make a chart showing the frequency of obtaining 3 consecutive days of good hay drying weather during harvest from official weather records. Compare with the number of 2 consecutive and single days of good drying weather. Discuss. Compare for first and second cutting. 7. Sketch an air distribution system for a haymow 7.3\m wide, 9.1 m deep, and 12.2 m long (24 ft X 30 ft X 40 ft). Specify duct dimensions. 8. Check the specifications for a commercially available duct system for hay. Make a table showing the duct sizes and arrangements for different mow sizes. 9. Why are the center main ducts usually built with the largest dimension for the height? Show by example. 10. Set up a time schedule for a one-man drying operation using a mower, field chopper, and barn hay dryer. Give specifications of the dryer for the operation. 11. German engineers suggest 1 m? of air/g of moisture to be removed. Compare with the U.S. airflow recommendations. REFERENCES ANON. 1954. Losses in agriculture. ANON. 1976. land, Pa. ASAE. 1979. U.S. Dep. Agric. AMS-20. Farmer’s guide to round baling. Wafers, pellets, and crumbles. Sperry-New Holland, New HolJn Agricultural Engineers Year- book. Stand. $269.2. Am. Soc. Agric. Eng., St. Joseph, Mich. BAKER, J.L. and SHOVE, G.C. 1978. Solar drying of commercially produced large round bales. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1978. Tech. Pap. 78-3066. BALL, C.E. and BARGER, E.L. 1948. Reconditioning overdried hay. Agric. Eng. 29 (7) 229-300. BELLINGER, P.L. and McCOLLY, H.F. 1961. Energy requirements for forming pellets. Agric. Eng. 42 (5) 244-247, 250. BRUHN, H.D. 1955. Status of hay crusher development. Agric. Eng. 36 (3) 165-170. CHAPLIN, R.V. and TETLOW, R.M. 1971. Storage of dried grass wafers. J. Stored Prod. Res. 7 (2) 171-180. CORRIE, W.J. and BULL, of large bales. CURLEY, D.A. 1969. Investigation into the characteristics J. Agric. Eng. Res. 14 (4) 323-331. R.G., DOBIE, J.B. and PARSON, P.S. 1973. Comparison of stationary and field cubing of forage. Trans. ASAE 18 (5) 864-866. DALE, J.G., HOLT, D.A. and PEART, R.M. 1978. A model of alfalfa harvest and loss. Am. Soc. Agric. Eng., Utah State Univ. Tech. Pap. 78-5030. SYSTEMS FOR DAVIS, R.B. 1951. Drying Farmers’ Bull. 2028. DRYING forage AND by forced HANDLING ventilation. OF USS. HAY 289 Dep. Agric. DAVIS, R.B. and BAKER, V.H. 1951. The resistance of long and chopped hay to airflow. Agric. Eng. 32 (2) 92-94. DAVIS, R.B. and BARLOW, G.E. 1948. Supplemental heat in mow drying of hay (Part II). Agric. Eng. 29 (6) 251-254. DAVIS, R.B., BARLOW, G.E. and BROWN, D.P. 1950. Supplemental heat in mow drying of hay (Part III). Agric. Eng. 31 (4) 223-226. DAVIS, R.B. and BARLOW, G.E. 1947. Supplemental heat in mow drying of hay. Agric. Eng. 28 (7) 289-290, 293. DAWSON, J.E. and MUSGRAVE, R.B. 1946. Respiration in hay as a source of heat for barn drying partially cured hay. Agric. Eng. 27 (12) 565—567. DAWSON, J.E. and MUSGRAVE, occurrence of mold in hays. DOBIK, J.B. 1975. Cubing R.B. 1950. tests with grass sources. Trans. ASAE 18 (5) 864—866. DOBIE, Effect of moisture potential on Agron. J. 42 (6) 276-281. J.B. and CARNEGIE, E.J. 1973. forages and similar roughage Curing and storage of moisture alfalfa. Trans. ASAE 16 (4) 766—768, 722. DUFFEE, J.W. 1942. The chopping and storing of hay. Agric. Eng. 23 (6) 195-196. FRUDDEN, C.E. 1942. Factors controlling the rate of moisture removed in barn curing systems. Agric. Eng. 27 (3) 109-111. HALL, C.W. HALL, 1957. Drying Farm Crops. C.W. and DAVIS, D.C. 1979. AVI Publishing Co., Westport, Conn. Processing Equipment for Agricultural Products, 2nd Edition. AVI Publishing Co., Westport, Conn. HENDRIX, A.T. 1947. Heat generated in chopped hay and its relation to drying effect. Agric. Eng. 28 (7) 286-288. HOPKINS, R.B., WIANT, D.E. and PETTIGROVE, H.R. 1954. The relation of moisture content and bale density on hay quality. Mich. Agric. Exp. Stn. Q. Bull. Article 32-37. JONES, T.N. 1939. Natural drying of forage crops. Agric. Eng. 20 (3) 115116. KANE, E.A., WISERMAN, H.G. and CAREY, C.A. 1937. The loss of carotene in hays and alfalfa meal during storage. J. Agric. Res. 55, 837-947. KJELGAARD, W.L. 1965. Airflow resistance of hay pellets and wafers. Trans. ASAE 8 (4) 560—561. KLINNER, W.E. and SHEPPERSON, G. 1975. nology—a review. The state of haymaking tech- J. Br. Grass. Soc. 30 (3) 259-266. MAISH, L.J. CUYKENDALL, C.H. and HASBARGEN, P.R. 1969. Economic comparison of hay harvesting, storing and feeding systems for beef cow herds. Univ. of Minn. Agric. Ext. Serv. Ext. Folder 246. MENEAR, J.R. and HOLDREN, hay in humid areas. R.D. 1965. Handling, storing, drying wafered Trans. ASAE 8 (2) 256-258, 263. 290 DRYING AND STORAGE OF AGRICULTURAL CROPS MOLITORISZ, J. and McCOLLY, H.F. 1970. Analysis of chemical systems for rolling-compressing hay wafers. Am. Soc. Agric. Eng. Pap. 70-621. MONTFORD, P.T. 1947. Supplemental heat in barn hay curing. Agric. Eng. 28 (3) 95-97, RENOLL, E., SMITH, L.A. and STALLINGS, J.L. 1977. Round bale hay feeding systems evaluation. Ala. Agric. Exp. Stn., Auburn, Circ. 238. RICHEY, L.F. Cires 2: 1956. Cooperative alfalfa dehydrators. U.S. Dep. Agric. FCS RIDER, A.R. and MCMURPHY, W.E. 1977. Roll bale storage. Winter Meet. Am. Soc. Agric. Eng., Chicago, Tech. 1977. Pap. 77-1515. SCHRENK, W.G., MITCHELL, H.L., SILKER, HONSTEAD, W.H. and TAECKER, State Coll. Bull. 356. SHEDD, C.K. and BARGER, E.L. R.G. 1947. 1953. R.E., GRANDFIELD, C.O., Dehydrated alfalfa. Kan. Curing baled hay. Agric. Eng. 28 (6) 257-258, 260. SHELDON, W.H., WIANT, D.E., KLEIS, Barn hay driers in Michigan. STRAIT, J. 1944. R.W. and DEXTER, S.T. 1951. Mich. Agric. Exp. Stn. Circ. Bull. 219. Barn curing of hay with heated air. Agric. Eng. 35 (11) 421-428. TERRY, C.W. 1947. Relation of time and operating schedule to hay quality, mold development and economy of operation. TERRY, C.W. 1948. Some Agric. Eng. 28 (4) 141-144. 1947 results of barn hay drying research. Agric. Eng. 29 (5) 208-209. WEAVER, J.W., JR., GRINNELLS, C.D. and LOVLORN, R.L. baled hay with forced air. Agric. Eng. 28 (7) 301-304, 307. WELLS, G.D. and BEAUDRY, J. 1977. 1947. Drying Comparing field losses from conventional square and big round bales. Am. Soc. Agric. Eng. Tech. Pap. 77-1514. WOODROFFE, G.E. 1973. Observations of the susceptibility of compressed dried grass and legumes to infestation by some storage insects. Res. 9 (4) 235-239. WRIGHT, N.C. OAS Pls ZINK, F.J. 329. 1936. 1941. The storage of artificially dried grass. Moisture content at which leaves shatter. J. Stored Prod. J. Agric. Sci. 31, Agric. Eng. 17 (8) V2 systems for Solar Energy Drying on the Farm Gene C. Shove! Energy consumption to dry agricultural crops often approaches and can exceed the energy required to produce the crop. Since crop drying is a high energy use operation, the cost of drying is directly related to energy availability and cost. As energy costs rise, energy conservation and the use of alternative energy sources become more important. Petroleum based fuels have been the dominant heat energy source for crop drying, particularly the drying of cereal grains. These fuels are also the energy source for moving the high volume of air used in batch and continuous flow high temperature grain dryers that are tractor Pto driven. Electric powered fans are utilized on bin dryers and to some extent on high temperature dryers where three phase electrical service is available. Petroleum fuels are used to heat the air on most high and intermediate temperature bin dryers; however, electric heat is generally used on low temperature (air temperature raised only a few degrees) corn drying systems. POTENTIAL APPLICATION OF SOLAR ENERGY Generally, crop drying takes place at a given location, i.e., the crop drying facilities on most farms are permanently positioned in one or more locations. This nonmobility of drying equipment lends itself to the application of solar energy. In particular, low temperature drying adapts well to solar energy since a temperature rise of only a few degrees is required and drying usually takes place over an extended period of several weeks. An extended drying time increases the probability that '\Professor of Agricultural Engineering, Univ. Of Illinois, Urbana-Champaign. AS) 292 DRYING AND STORAGE OF AGRICULTURAL CROPS of the sufficient solar energy will be available for satisfactory completion drying. The possibility of using solar energy to dry agricultural crops was investigated by Buelow in 1958. In the early 1960s, Buelow (1962) and Sobel and Buelow (1963) presented designs of solar energy collectors for heating air that could be used for drying crops or modifying the temperature in livestock buildings. Hall (1968) reported on the preheating of ventilation air by passing it along the underside of the roofs of livestock buildings, and an apparatus in which the solar radiation was collected directly by the material to be dried has been described by Bailey and Williamson (1965). SOLAR CROP DRYING RESEARCH Even though many solar energy grain drying experiments had been described and considerable information was available on the design of solar collectors, very few actual operating crop drying systems employing solar energy were in use prior to 1973. An operating system which created renewed interest in applying solar energy to crop drying was developed in 1973 by William H. Peterson, Extension Agricultural Engineer, South Dakota State University, Brookings. Peterson and Hellick- son (1976) in their report on solar collectors mounted on grain bins concluded: (1) black-painted bare sheet corrugated collectors can be just as efficient for collecting solar energy (for small temperature rises required for low-temperature drying) as plastic covered collectors and can be installed typical, amount savings at a lower cost; (2) solar collectors, mounted on the outside of a round, low-temperature drying, bin, can supply an appreciable of the energy needed for drying shelled corn; and (3) energy of 1.18 kWh per m? (0.042 kWh per bu) per percentage point of moisture removed were noted when comparing solar supplemented with conventional low-temperature shelled corn drying systems. The solar bin used 26% less energy per percentage point of moisture removed than the conventional drying bin. Foster (1977) described solar grain drying research initiated by funds from the National Science Foundation in 1974 in response to the 1973 fuel shortage and as a part of the United States effort toward energy self-sufficiency. Solar drying research and demonstration program sponsorship was assumed by the U.S. Department of Energy (DOE) in 1977 and includes funding for projects on the drying of cereal grains, forages, tobacco, peanuts, lumber, etc. The crop drying research program is man- aged by the U.S. Department of Agriculture (USDA) and early results of the grain drying program were reported by Foster and Peart (1976), Shove (1977), and Hartsock (1978). Many of the projects funded during 1974—1975 were proof of concept tests involving various solar collector configurations and materials. SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM 293 These tests were followed with specific research experiments, includin g some simulated drying and economic studies. Heid (1978) made an economic analysis of 8 experiments and concluded that solar drying of corn might be economically feasible, i.e., given current cost comparisons (1976-1977), a solar grain drying system might be considered if an additional dryer needs replacing, or if fossil fuels are no longer available. SOLAR ENERGY AVAILABILITY Buelow (1977) outlined a procedure for estimating the average daily radiation on sloping surfaces and commented that even though methods of estimation have their shortcomings, they do give values that are valuable for the design and evaluation of solar grain drying systems. The standard value of the solar constant (1353 W/m?, 428 Btu/hr: ft?) may be considered as the basic parameter in developing information on availability of solar energy. However, it is of little practical value because of the variable attenuation of the earth’s atmosphere, the changes in angle of incidence on any fixed-in-place solar collector, and the diffusions and reflections that may occur at a specific location and collector orientation. For these reasons, most of the approaches to determine solar energy availability are based on actual data gathered by researchers for specific purposes or by the various governmental meteorological agencies throughout the United States and the world. With the aid of punched cards obtained from the National Weather Records Center at Asheville, North Carolina, Baker and Klink (1975) analyzed the solar radiation records of 20 stations in the U.S. northcentral region. Their publication includes numerous charts, maps, and tables of probabilities of solar radiation at each of the stations scattered throughout the corn belt and adjacent states. This information is very useful in estimating solar radiation if actual data are not readily available for a given location. Handbooks published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. also include insolation data. Table 12.1, from ASHRAE data, gives the total insolation striking 1 ft? of solar collector surface per day for each month at different collector orientations and the 40° north latitude. For design purposes these values must be modified to account for sunshine probability and collector efficiency. SOLAR COLLECTORS Concentrating vs Flat Plate Two basic solar energy collectors that can be used to heat air or a liquid, such as water, are the flat plate collector and the concentrating collector 294. DRYING TABLE 12.1. AND STORAGE CROPS DAILY INSOLATION FOR 40° NORTH LATITUDE Insolation Btu/ft? Daily Insolation on Surfaces! Values Given for : the 21st Day of Each Month OF AGRICULTURAL : South-facing Surface Angle with Horizontal 0 30 40 50 60 90 948 1660 1810 1906 1944 1726 February March April May June July August September 1414 1852 2274 2552 2648 2534 2244 1788 2060 2308 2412 2442 2434 2409 2354 2210 2162 2330 2320 2264 2224 2230 2258 2228 2202 2284 2168 2040 1974 2006 2104 2182 2176 2174 1956 1760 1670 1728 i894 2074 1730 1484 1022 724 610 702 978 1416 October November 1348 942 1962 1636 2060 1778 2098 1870 2074 1908 1654 1686 December 782 1480 1634 1740 1796 1646 January \ Source: ASHRAE (1978). 11 Btu/ft?-day = 0.131 W/m?. (Fig. 12.1). The concentrating collector focuses the direct rays of the sun which are incident on a curved reflector onto a smaller absorbing area. Because of the focusing effect, a concentrating collector can heat an absorber to a much higher temperature than can a flat plate collector; but, because of the use of only direct rays, the concentrating collector must track or follow the sun. Since tracking is complicated and grain drying does not require extremely high air temperatures, the flat plate collector is a better choice for solar grain drying. Flat Plate The flat plate collector absorbs the direct rays of the sun at different angles of incidence during the day since flat plate collectors are usually stationary. However, the flat plate also absorbs diffuse radiation, that is, the portion of the sun’s energy which is filtered through clouds or is reflected from other objects. Duffie and Beckman (1974) note that flat plate collectors can be designed for applications requiring energy delivery at moderate temperatures, up to perhaps 100°C (180°F) above ambient temperature. Before beginning their detailed discussion of the thermal performance of flat plate collectors, they also noted that flat plates have the advantages of using both beam and diffuse radiation, not requiring orientation toward the sun, requiring little maintenance, and are mechanically simpler than the concentrating reflectors, absorbing surfaces, and orientation devices of focusing collectors. Flat plate collectors are being applied to solar water heating systems, building heating and cooling, and other processes requiring heat. Flat plate collectors have the potential for many agricultural applications, including the drying of crops. The basic design of a flat plate solar collector includes: (1) a plate to absorb the sun’s radiant energy, causing an increase in plate temper- SYSTEMS FOR SOLAR ENERGY SOLAR DRYING ON THE FARM) 295 RADIATION FLAT PLATE CONCENTRATING FIG. 12.1. TWO BASIC SOLAR ENERGY COLLECTORS ature, and (2) a cavity for circulating a fluid, such as air, which picks up the heat absorbed by the plate and transfers it to a point of use or storage. An air collector can be as simple as a bare sheet of metal painted black to absorb the maximum amount of energy, with air forced through an underneath cavity (Fig. 12.2A). Convection and radiation losses from the surface of the bare plate collector cause it to have a low efficiency, perhaps collecting less than 30% of the solar energy striking the collector. Covered Plate The more efficient covered plate collector (Fig. 12.2B) utilizes a transparent cover with air circulated between the cover and the absorber plate. Convection heat loss due to wind blowing across the surface of the collector is reduced by the cover, which acts as a barrier between the wind and absorber plate. Radiation losses are also greatly reduced since the transparent cover provides a nearly opaque barrier to longwave radiation from the absorber while readily transmitting shortwave solar radiation. This phenomenon is sometimes referred to as the “greenhouse effect.” Suspended Plate A third collector, which provides twice as much surface area for heat transfer from the absorber plate to the air, called a covered, suspended plate (Fig. 12.2C), has air circulated on both sides of the absorber plate. 296 DRYING AND STORAGE OF AGRICULTURAL CROPS Rie Solar Airflow Radiation \ \ \ Sheet Metal ~ Absorber (a) BARE PLATE Airflow Solar Radiation 24 Transparent Cover’. SE x SS s e Sheet Metal ee Absorber Insulating Material (b) COVERED PLATE Airflow Solar Radiation Transparent Cover By es ms Sheet Metal we Absorber Insulating Material 4 (c) COVERED SUSPENDED FIG. 12.2. TYPICAL CONFIGURATIONS ENERGY COLLECTORS PLATE OF AIR FLAT PLATE SOLAR SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM 297 The transparent cover serves the same purpose as in the covered plate collector. Heat Loss Insulation on the back of the collector is necessary to reduce the conduction heat loss in most collectors. The importance of insulation becomes greater as the difference between the outside air temperature and the collector fluid temperature increases. Efficiency may also be increased by adding additional covers to collectors operating with large temperature differentials. Each additional cover reduces the convection and radiation heat losses; however, cost is increased and reflection losses from the cover are increased. For home heating applications, two covers may be appropriate, but for most lower temperature agricultural applications, one-cover collectors will generally be sufficient and will have acceptable cost-benefit ratios. Airflow The amount of useful heat collected of the total amount of solar radiation striking the collector surface is affected by the rate of flow of air through the collector. If the heat transfer fluid flows slowly, it becomes hotter and loses more heat to the surroundings since there is a large difference in temperature between the fluid and its surroundings. On the other hand, if the fluid flows too rapidly, it may not have enough contact time to absorb ample heat to be useful or efficient. Fortunately, small air temperature rises can be effectively utilized in drying many crops; therefore, moving the large volumes of air required for drying _ through solar collectors provides efficiency advantages. Selective Surfaces Constructing a collector with a selective absorbing surface is another possibility of increasing collector efficiency. The absorber is treated chemically so that it will absorb most of the radiation striking it but will not re-radiate very much of the absorbed energy. The treatment is fairly expensive and probably is not justified for solar crop drying. Reflective Surfaces Reflectors can be used to increase the amount of radiation striking a collector. Collector enhancement by the use of reflectors has considerable potential for vertical collectors installed on the walls of agricultural 298 DRYING AND STORAGE OF AGRICULTURAL CROPS buildings since horizontal space for reflectors is often available. Various ground covers, i.e., white crushed rock, may provide some reflective characteristics while serving another intended use. \ Cost vs Efficiency Improvements in efficiency of solar energy collectors must be weighed against the increase in cost and/or maintenance requirements of the system. Usually, the more efficient a collector is, the more costly it is. A less efficient, but also less costly, collector may prove to be the more costeffective in delivering a given amount of energy. Heat energy for drying crops is still (1979) relatively inexpensive; therefore, careful thought and planning are necessary in adapting and applying solar collectors to crop drying facilities. Construction Materials Materials used in the construction of solar collectors must withstand the adverse effects of the sun’s radiation and be capable of withstanding weather the same as the roof or wall of a building. There should be a resistance to corrosion or clogging in liquid collectors and to the deposi- tion of dust and moisture in air collectors. Breakage of the transparent cover (glazing) must also be avoided. Collectors should be designed to give a useful and economical output of heat with a minimum of repair, maintenance, and component replacement. Absorber Materials Desirable characteristics of an absorber plate are (1) to absorb as much of the sun’s radiant energy as possible, (2) to lose as little heat as possible to the surroundings, and (3) to transfer the heat retained to a fluid. Absorber plates are usually painted black for maximum absorption of solar radiation. A flat black paint is used to reduce possible reflection. Materials generally used for collector plates are copper, aluminum, and steel. Copper is the most expensive but also has the highest thermal conductivity. Steel is the least expensive but has a lower conductivity than either copper or aluminum. In an air collector where the entire area of the absorber is swept by the heat transfer fluid, the conductivity of the plate is much less important. For air collectors, particularly in agricultural applications, wood, be used as absorber plates. fectively as the metals but is less. Surface area of the fiberboard, plastics, and even insulation can They generally do not perform quite as efare acceptable since the cost of the collector plate also affects heat transfer; therefore, SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM 299 many designs include fins or corrugations to increase the area available for heat transfer. Cost is always a factor, particularly for collectors used in agricultural production; however, corrugated metal is a common agricultural building material and may be justified for an absorber while some finned arrangement may be too expensive. Cover Materials Glass is often used as glazing for solar collectors used for space heating of homes or for industrial or public building applications. It is a good transmitter of the incoming solar radiation but transmits very little radiant heat from the absorber surface to the surroundings. If there is danger of glass breakage from hail or vandalism, a wire mesh can be used to protect the glass. This hail screen does act as a partial shade, which decreases the effective collector area. Clear fiberglass, used for many years in greenhouse construction, is most often used as a cover for agricultural solar collectors. It performs nearly as well as glass except the transmissivity to solar radiation is somewhat less, but it is much more resistant to breakage or cracking. Clear fiberglass is available in the same configuration as steel sheets used in building construction; therefore, fiberglass can be easily adapted as roof and wall material. A clear fiberglass roof or wall allows a covered plate solar collector to be incorporated into an agricultural building with little, if any, modification in design or construction. In this way, large areas of collector surface can be obtained at a reasonable cost. SOLAR COLLECTORS INCORPORATED INTO BUILDINGS Farm buildings, including machine sheds, livestock shelters, grain bins, and hay barns, have been constructed as solar buildings, i.e., solar collectors have been incorporated into all or part of the walls and/or roofs (Fig. 12.3). Most often the collector is built to be utilized for one purpose, such as crop drying; however, solar collectors incorporated into farm buildings have the potential for other uses. For example, a collector incorporated into the roof of a swine building can be used to dry grain in the fall before heat is required in the building during the colder winter months. Or a collector incorporated into the wall and roof of a machine shed for crop drying purposes can also provide heat to a repair shop in the shed during the winter (Fig. 12.4). Multiple use of a collector should always be considered when a solar building is planned since multiple use will make the collector more cost effective. 300 DRYING AND STORAGE FIG. 12.3. EXISTING DRYING BINS GRAIN OF AGRICULTURAL DRYING CROPS BINS CONVERTED TO SOLAR COLLECTOR Roof and Wall Collectors Roofs and walls.of buildings can become bare plate collectors by providing some means of moving air along the back side of the roof and wall surfaces. Sheet metal roofing and siding commonly used in farm building construction become the bare plate energy absorber. An air chamber below the roof or behind the wall can be created by attaching a lining material to the structural members supporting the roof or wall covering (Fig. 12.5). The use of the building will determine whether or not there is a need for lining material which offers resistance to heat transfer. If the building is to serve as a cold machinery storage shed, any loss of heat through the lining material can be captured by moving some air through the entire building space. Because crop drying requires large volumes of air, there will be sufficient airflow available for the collector even though some air is moved through the entire building. If the building is to be used as a heated repair shop or a livestock shelter, insulation should be used for the lining to keep the building heat losses to a minimum. SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM 301 _ Covered plate collectors can be incorporated into the roof and walls of buildings by substituting a clear material for the metal roofing and siding. Clear, corrugated fiberglass panels can be used for the exterior of _ the building to create a covered plate collector (Fig. 12.6). The lining material used to create the air space should have a black surface exposed _to the sun’s rays. Attic Collectors Solar collectors can also be incorporated into buildings utilizing the complete attic space as the air chamber. When the complete attic space is utilized, an attic floor serves as the black energy absorbing surface (Fig. 12.7). Even though complete attic spaces create large chambers for the movement of air, reasonably high air velocities can still be maintained because of ‘the large volume of air required in crop drying. The orientation of buildings with attic collectors is not critical since the solar absorbing surface is in a horizontal position. If the length of the building is oriented north-south, the complete roof should be clear fiberglass. However, if the building is oriented east-west, it is only necessary to install clear fiberglass on the south roof slope. During the grain drying season in the major corn producing states, the angle of the sun will be such that most of the horizontal energy absorbing surface will be exposed to the sun’s rays through the clear south roof slope. Conventional metal roofing can be placed on the north roof slope. FIG. 12.4. WALLS OF FARM BUILDING DESIGNED TO COLLECT SOLAR ENERGY de ace ne re CEEOL ry 302 DRYING AND STORAGE OF AGRICULTURAL CROPS METAL SIDING AIRFLOW FIG. 12.5. BARE PLATE SOLAR WALL OF A BUILDING ENERGY COLLECTOR INCORPORATED : INTO THE Grain Bin Collectors Bare plate and covered collectors can be installed on the sidewalls of grain drying bins by installing a secondary wall on the sunny side of the bin. For example, a covered plate collector can be constructed on a grain bin by installing a clear fiberglass wall around a black painted bin. The drying fan then pulls air through the space between the clear cover and SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM — 303 CLEAR CORRUGATED FIBERGLASS AIRFLOW LINING SERVES AS ENERGY ABSORBER FIG. 12.6. COVERED PLATE THE ROOF OF A BUILDING SOLAR ENERGY COLLECTOR INCORPORATED INTO the black energy absorbing grain bin wall. Cost of materials is kept to a minimum because the existing grain bin wall serves as the energy absorbing plate. Since grain bins are circular in shape, only a portion of a bin sidewall collector is exposed to the sun at any given time. However, because of the circular shape, essentially a constant area is exposed to the sun throughout the day. Suspended Plate Collectors More Costly Suspended plate collectors can also be incorporated into building roofs and walls or placed on grain bin sidewalls. Suspended plate collectors require more materials and labor for construction since the absorber plate must be arranged so that air moves along both of its sides. The added costs of materials and construction labor may be difficult to justify for crop drying purposes. CROP DRYING AIR VOLUME BENEFICIAL Since keeping solar collector temperature differentials at a minimum is beneficial to the efficiency of operation of collectors, it is desirable to move as much air as possible through collectors applied to crop drying. However, frictional losses in moving air through collectors must be considered. The air volume of many crop drying fans begins to diminish as 304. DRYING AND STORAGE OF AGRICULTURAL CROPS AIRFLOW METAL ROOFING — ATTIC FLOOR SERVES AS ENERGY ABSORBER CORRUGATED FIBERGLASS ROOFING — FIG. 12.7. COVERED PLATE SPACE OF A BUILDING SOLAR ENERGY SOUTH = COLLECTOR UTILIZING THE ATTIC the static pressure against which the fan must operate approaches 100 mm (4 in.) of water column. If high air velocity through the collector consumes more than approximately 20% of the pressure capability of the fan, the solar drying system may not have sufficient airflow for satisfactory drying. However, because of the high airflow used in crop drying, solar collector efficiency can be maintained without moving all the air through the collector. Frictional losses in the collector and any connecting duct can be adjusted to less than 20 mm (0.8 in.) of water column in the field after the collector is in operation. A simple sliding door installed in the fan housing or in the connecting duct near the fan can be used to divert some air directly to the fan, bypassing the collector. After the crop to be dried is in place and the solar drying system is operating, a manometer is used to measure the static pressure, including the loss through the collector, against which the fan is operating. The amount of air bypassed is then adjusted to keep the fan within a satisfactory op- SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM 305 erating range and yet maintain as high an airflow as possible through the collector. SOLAR GRAIN DRYING SYSTEMS IN OPERATION Flat plate solar collectors incorporated into farm buildings and placed on the sidewalls of grain drying bins can be constructed in a variety of ways. Examples of some successful solar grain drying systems are listed in Table 12.2. TABLE 12.2. AIR TEMPERATURE RISES AND POWER FROM SOLAR COLLECT INCORPORATED INTO FARM BUILDINGS FOR DRYING GRAIN IN OCTOBER ORS AND NOVEMBER Collector Configuration Area Air Temperature Rise Max. Approx. 24hrAve. ° °F Car ft? Drying Fan,kW 140 1506 10 6 10.8 1s and complete attic? 420 4520 10 ie O56 3B) hes 15 Vertical wall and south roof, 20° slope? 130 1399 20 SPe6.2 2 3.6 20 Horizontal roof? 560 6027 10 17 30.6 Awaie2, 23 Wall at 60° angle? 2h a290 5 8 14.4 2 m2 Approx. Ave. Power, kW Bare plate South and north galv. metal roof, 3° slope (nearly horizontal)! 6 Covered plate Vertical wall 3.0 4 ! Located in Wisconsin at about 43° north latitude. ? Located in Illinois at about 41° north latitude. 3 Located in Illinois at about 40° north latitude. Maximum air temperature rise achieved in a crop drying solar collector is an interesting value; however, it is the total energy collected that must help evaporate moisture from the wet crop. Therefore, the energy (or air temperature rise) obtained during the daylight hours must be averaged over the entire 24 hr day since most crop drying equipment operates continuously. The daily solar energy collected can also be considered in terms of electrical resistance heater wattage necessary to provide the same energy over a 24 hr period. Since the drying of many crops continues for several days or perhaps a few weeks, it is even more appropriate to average the energy collected over the entire drying period. The longer the period over which solar energy is applied to a process, 306 DRYING AND STORAGE OF AGRICULTURAL CROPS the more likely sufficient solar energy will be available to complete the process. This is particularly true in an agricultural process, such as crop drying, which is tolerant of a variable heat energy input. Since crop drying can be functional with a variable heat energy input, it is not necessary to provide heat storage as a part of a solar drying system. Therefore, simple flat plate solar collectors incorporated into building surfaces without provision for heat storage can be cost effective supplying heat energy for the drying of agricultural crops. LOW TEMPERATURE GRAIN for DRYING Since solar energy collected at or near the earth’s surface is essentially a low temperature heat source, it is ideally suited to the low temperature method of drying grain. Drying grain with a low temperature system is a simple and economical method of producing high quality grain with minimum kernel stress-cracks and mechanical damage. It is a flexible system which can be matched to nearly any harvesting rate, uses energy efficiently, and can effectively utilize different sources of available heat energy. Electricity is generally used to power fans for forcing air through the wet grain. Airflow Requirement for Corn Drying Although low temperature drying has been applied to many different grains, it has been used most extensively for the drying of shelled corn in the major corn producing states of the, United States. Corn moisture content is reduced over an extended period of time with airflows that dry corn before deterioration takes places; therefore, the method works best at ambient air temperatures below 10°C (50°F). Fortunately, average daily temperatures approach or are below 10°C during much of the corn harvest season. Specific airflow requirements are determined by the moisture content of the harvested grain (Table 12.3). Heat Requirements for Corn Drying During the normal corn harvest season from late September through late November, the relative humidity of ambient air is often nearly low enough to dry corn to desired moisture levels. When relative humidity must be lowered, only small amounts of heat are generally required, i.e., if relative humidity averages 80% or above, a 3°C (5.4°F) temperature rise will be required to dry corn to 15.5% moisture content. Even when relative humidity approaches 100%, the air temperature need be increased by only about 6°C (10.8°F). SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM 307 TABLE 12.3. AIRFLOW FOR LOW TEMPERATURE SHELLED CORN DRYING Corn Moisture Content, Air Changes per min Approx. % w.b. Based on Grain Volume m'/m*min cfm/bu 20 0.8 1.04 il DE 1.0 a3) a5) 24 1.6 2.08 2 26 2.4 3.12 3 28 4.0 5.2 5 Solar Heat for Corn Drying Heat for low temperature corn drying systems is generally obtained from electrical resistance heaters. The heaters are often sized to provide approximately 1 kW of heat for each kilowatt of fan power to produce an air temperature rise of about 2°C (3.6°F). An additional temperature increase of about 1°C (1.8°F) will be generated as the air passes through the fan. Solar collectors can replace electric heaters on low temperature corn drying systems by providing 6 to 9 m? (approx. 65 to 100 ft2) of collector surface for each 35 m3 (approximately 1000 bu) of corn to be dried. If more collector surface is available, enough heat is likely to be collected to require the installation of a grain stirring machine to prevent overdrying of the corn. SUMMARY Solar energy collected by flat plate solar collectors can be a practical, economical source of heat energy for drying agricultural crops. Solar crop drying systems have rapidly increased in number since the late 1970s with some systems relying entirely on the sun’s radiation for heat energy while others combine solar energy with fossil fuel and/or electrical energy. Morrison (1977) combined an electrically powered heat pump with solar collectors in an attempt to provide a constant drying potential by using solar energy to heat the drying air during sunshine hours and the heat pump during the night and periods of low insolation. In a low temperature corn drying experiment, he concluded that the solar collector-heat pump combination effectively reduced the electrical energy requirements as compared to an electric heat low temperature drying system. Solar crop drying systems also function exceptionally well when used in combination with more conventional drying methods, i.e., solar drying can be used to complete the drying of grain following partial moisture reduction in a high temperature-low temperature combination system. As alternative energy sources become more attractive and/or necessary, the incentive to install solar crop drying facilities will become greater. 308 DRYING AND STORAGE OF AGRICULTURAL CROPS Certainly, solar energy is a renewable resource that can be expected to provide an increasing amount of the future energy requirements for agricultural crop drying. QUESTIONS , 1. A machine shed to be erected on a corn producing farm in central Illinois (40° north latitude) will have a south wall solar collector of 130 m2? and a 30° south roof collector of 260 m?. The energy collected is to be used to heat air for a corn drying bin during October. Assuming the collectors have an efficiency of 60%, determine the equivalent output of the total collector in kW for: (a) maximum sunshine, and (b) 70% probability of maximum sunshine. 2. Grain is to be dried by doubling the drying potential of ambient air at 10°C and 70% relative humidity by the addition of solar heat. The incident solar energy on a collector with a 60% collective efficiency is 300 W/m?. Calculate the area of the solar collector required when 7 m*/s of air is supplied to the dryer. Assume saturated air exhausts from the wet grain. 3. A 400 m’ low temperature corn drying bin located at 40° north latitude is filled with 22% moisture content corn in November when a 3°C air temperature rise will be required to dry corn to 15.5% moisture. Determine the theoretical size of a solar collector tilted 60° from the horizontal to heat the drying air. 4. Considering the theoretical size of the solar collector obtained in question 3, suggest the actual size of a-collector that would be appropriate to apply to the 400 m* low temperature drying bin. 5. Calculate the efficiencies of the horizontal roof and the 60° wall solar collectors listed in Table 12.2 for the month of October. Assume an airflow of 5 m°/s for each kW of fan power. REFERENCES ASHRAE. 1978. Handbook. Applications. Am. Soc. Heat. Refrig. Air Cond. Eng., New York. BAILEY, P.H. and WILLIAMSON, W. F. 1965. grain by solar radiation. Some experiments on drying J. Agric. Eng. Res. 10, 191-196. BAKER, D.G. and KLINK, J.C. 1975. Solar radiation reception, probabilities, and areal distribution in the north-central region. Univ. Minn. Agric. Exp. Stn. Tech. Bull. 300. BUELOW, F.H. 1958. Drying grain with solar energy. Mich. State Univ. Agric. Exp. Stn. Q. Bull. 41 (2) 421—429. BUELOW, F.H. 1962. Solar energy collector design. Trans ASAE 5, 1-2, 5. SYSTEMS FOR SOLAR ENERGY DRYING ON THE FARM _ 309 BUELOW, F.H. 1977. Solar energy availability. In Solar Grain Drying Conf. Proc., Jan. C. Shove (Editor). Univ. Illinois, Urbana-Champaign. DUFFIE, J.A. and BECKMAN, W.A. 1974. Solar energy thermal processes. John Wiley & Sons, New York. FOSTER, G.H. 1977. Overview of solar grain drying research-field tests. In Solar Grain Drying Conf. Proc., Jan. G.C. Shove (Editor). Univ. Illinois, Ur- bana-Champaign. FOSTER, tential. HALL, G.H. and PEART, R.M. 1976. Solar grain drying progress and po- U.S. Dep. Agric. Agric. Inf. Bull. 401. M.D. 1968. Solar-heated ventilation air for swine buildings. Agric. Eng. 49, 7, 81. HARTSOCK, J.G. 1978. West Lafayette, Ind. Solar Grain Drying Conf. Proc., May. U.S. Dep. Agric. ; HEID, W.G., JR. 1978. The performance and economic feasibility of solar grain drying systems. U.S. Dep. Agric. Agric. Econ. Rep. 396. MORRISON, D.W. 1977. Solar energy-heat pump low temperature grain drying. M.S. Thesis. Univ. of Illinois. PETERSON, W.H. and HELLICKSON, corn in South Dakota. SHOVE,G.C. 1977. Trans. ASAE M.A. 1976. Solar-electric drying of 19, 349-353. Solar Grain Drying Conf. Proc., Jan. Univ. Illinois, Urbana- Champaign. SOBEL, A.T. and BUELOW, F.H. 1963. Galvanized steel roof construction for solar heating. Agric. Eng. 44, 312—313, 316-317. 13 Moisture Control and Storage Systems for Vegetable Crops Denny C. Davis! Vegetable crops, in contrast to cereal crops, may require storage conditions that minimize moisture loss rather than enhance drying. Vegetable crops such as onions may require controlled drying or curing, whereas potatoes, sugar beets, and other root crops require storage environments that minimize moisture loss. This chapter addresses the environmental requirements of three specific vegetable crops and presents design criteria for use in establishing optimum environmental conditions. Each commodity is discussed in a separate section of the chapter. POTATOES Average annual per capita consumption of potatoes in the United States is nearly 52 kg (115 lb,,), the largest of any vegetable (U.S. Dep. Agric. 1977). In recent years the American diet has shifted from home prepared foods to larger percentages of preprocessed foods, including potato products. The potato processing industry has demanded a high quality potato supply throughout an 11-month period of the year to fully utilize its capital investments, attract a dependable labor force, and supply the processed potato product market. The industry has therefore increased its demands on acceptable performance criteria for potato storages. Storage Criteria Storages for potatoes may range from earth-covered trenches having no environmental control to highly refined aboveground air-conditioned ‘Department of Agricultural Engineering, Washington State Univ. 310 MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 311 structures. Earthen trenches will store potatoes satisfactorily for short (3- to 4-month) periods if the potatoes have been harvested and placed into storage under ideal conditions. However, potatoes that are injured, diseased, immature, or placed into storage at high (above 25°C) temperatures will not store well without proper environmental control. Even in storages with elaborate environmental quantities of potatoes in a single storage tatoes for frequent inspection a storage portance. Potato storages must be designed to They must have adequate control, the risk of losing large makes easy access to the podesign criterion of utmost immeet at least three criteria. capacity (volume), have adequate structural strength, and provide a suitable storage environment. Structural strength and environmental requirements will be discussed in subsequent subdivisions of this chapter. The required storage volume can be calculated from the total potato yield (mass) and a bulk volume of approximately 1.6 m?/t (50 ft3/ton) for potatoes stored in bulk. For example, 1000 t of potatoes would require a storage volume of 1600 m? (i.e., 1.6 X 1000). The floor plan for a potato storage should be developed with consideration given to both potato handling and storage management functions. The arrangement must be convenient for loading and unloading potatoes from all points in the storage and must provide the proper environment for potatoes located at any point in the storage. The air handling system must provide adequate ventilation to all potatoes in the storage. Throughout the duration of the storage period, both the potato pile and the controls for the air conditioning and handling system must be accessible for inspection and maintenance. These considerations will be discussed in other subdivisions of the chapter. Structural Considerations Potato storages may be constructed in numerous sizes and shapes within the limits of the total storage capacity desired and reasonable lengths for ducts used in the air handling system. Considerations of the size and operating reach of potato handling equipment are also important. The sizing of individual structural members must include consideration of the maximum loadings expected for each member. Dead weight of materials, snow and wind loads, and wall loadings from the potatoes are all essential components in determining maximum loads. Because some of these loadings are discussed in detail in many engineering texts, only those loads exerted by the stored potatoes will be discussed here. Potatoes in bulk storage apply distributed loadings over each wall that restrains the potatoes. Lateral pressures at individual points on a wall 312 DRYING AND STORAGE OF AGRICULTURAL CROPS vary with location, time, moisture status of the potatoes, and relative height-to-width ratio for the potato pile. Pressures increase from zero at the top of a pile to maximum near the bottom of the pile. Pressures increase as potatoes settle in storage. Pile height-to-width ratios (H/W) less than 1 produce greater wall pressures than do relatively narrower (larger height-to-width ratio) storage piles. In all cases lateral pressures on walls are less than fluid pressures that would occur if the storage contained a liquid whose density were equal to the bulk density of potatoes, 670 kg/m? (42 lb,,/ft?). Magnitudes of lateral wall forces exerted by potatoes in narrow bins were first reported by Edgar (1960). These forces were revised by Will- son (1968) to include bins whose height-to-width ratios approach a value of unity. Lateral forces for wide bins have been reported by Torabi et al. (1977) and Yaeger and Schaper (1978). Because the size of bins varied for these studies, their data have been reduced to dimensionless form for comparison and selection of design values. Lateral wall pressures may be expressed as a fraction of the fluid pressure that would occur if the same depth of fluid (having a specific weight equal to that of bulk stored potatoes) existed in the storage bin. At any depth from the top of the pile, h, the fluid pressure for a fluid of specific weight, w, is defined as p=wh (13-1) Thus, the lateral pressure, p,, due to the same depth of potatoes above a point, can be expressed as pL=kipwh (13-2) The lateral pressure coefficient, k,, does not remain constant for all depths due to influences of the floor, ventilation ducts, and other walls. It can, however, be defined for relative depths (fractions of the total pile height) when pile height-to-width ratios are constant. The lateral pressure coefficient, k,, (actual pressure/fluid pressure) is derived from the published data. Different coefficients are required to define the lateral pressures for wide and narrow (those with H/W <1.0), the lateral pressure is pr =0.30wh bins. For wide bins (13-3) For narrow bins (those with H/W > 1.0), the pressure coefficient depends upon the height-to-width ratio and other factors in an unknown MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 313 manner. Thus, the design equation for narrow bins should be used with caution. For bins with H/W = 2.2, the lateral pressure is pi =0.22wh (13-4) where w is the specific weight and h is the distance from the top of the pile to the point of the lateral pressure. Actual pressure measurements have indicated that the maximum lateral pressure for wide bins occurs at h/H = 0.8, rather than at h/H = 1.0 as indicated by equation (13-3). Thus, although these design equations do predict adequate wall loads, wall joints (the weak point in many walls) should not be located at this point of maximum lateral pressure. Until more accurate pressure relationships are developed, equations (13-3) and (13-4) will be used for designing vertical walls for storages. Total lateral force on a bin wall is obtained by integrating the pressure equation over the area being considered. Thus for wide bins, the lateral wall load, p,, per unit of wall width is pi = J #0.30whdh (13-5) pr = 0.15 w H2 (13-6) which yields with the load center occurring at a distance H/3 from the bottom of the pile. For example, a 6 m high pile of potatoes (w = 6.60 kN/m?) exerts a lateral force of 35.6 kN (0.15 X 6.6 X 62) per meter of wall width. The load center would be 2 m (6 m/3) from the bottom of the pile. (In English units, the lateral force of 2500 lb is applied 6.6 ft from the floor for each 1 ft width section of wall.) Storage Environment The environmental conditions around stored potatoes affect both the potential duration of the storage period and suitability of the tubers for a desired end use. Environments that are suitable for one period of storage and one end use are not necessarily suitable for another storage duration or another end use for the potatoes. Thus, recommendations for specific conditions are presented in Table 13.1 and discussed individually in this section. contain Field Heat Removal.— When potatoes are harvested, they may period. storage the of n initiatio the at field heat that must be removed 314 DRYING AND STORAGE TABLE 13.1. RECOMMENDED POTATO STORAGES Phase of Storage Suberization Short-term storage OF AGRICULTURAL TEMPERATURES End Use of Potatoes HUMIDITIES Temperature F EC FOR Relative Humidity, % 90 to 95 Fresh Processed 8.9tol10.0 8.9 Seed Wee 45 90 to 95 Fresh 4.4 to 5.5 40 to 42 90 to 95 72 45 90 to 95 5.5 42 90 to 95 358 38 90 to 95 Processed (fried) Processed (dehydrated) Seed Reconditioning RELATIVE 50to60 \ 48to50 48 Any Long-term storage AND CROPS Processed (fried) 10.0t015.6 12.8to18.3 55to65 90 to 95 90 to 95 40 to 70! 1 Relative humidity should be low enough so that the dew point temperature of the air is below the lowest tuber temperature. The tubers are living organisms that continue to respire, consuming sugars in the tuber and generating heat, water, and carbon dioxide. The rate at which tubers respire is a function of temperature, tuber maturity, and tuber condition. Injured, infected, or immature tubers respire more rapidly than do those stored in good condition. A tuber temperature reduction of 10°C results in a 20 to 60% reduction in the tuber respiration process. Tubers should be cooled from their field temperature to a temperature of 10°C (50°F) within 24 hr if possible. High relative humidity air should be used for the cooling process to minimize tuber weight loss. The relative humidity should reach or exceed 90% by the end of the 24-hr cooling period. Suberization Period.—If the potatoes are to be stored for a period that is greater than 1 month, then a curing or healing period is recommended. Holding the tubers at temperatures above those that produce minimum respiration levels allows the tubers to heal or suberize any injuries that would otherwise remain unhealed. The healing process reduces weight loss from those injuries and makes them less susceptible to infection during storage. The increased respiration during the suberization period is an investment in improved tuber storability for extended periods of storage. The storage environment during the suberization period should have a relative humidity above 90% (preferably 95%) to facilitate wound healing. The recommended temperatures are between 10.0° and 15.6°C (50° and 60°F) and the durations are 10 days to 2 months. If the tubers are mature when placed in storage, then the cooler temperatures and short to moderate suberization periods are recommended. Immature tubers MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 315 should be exposed to the higher temperatures for the moderate length periods. If tubers are in good condition, the long suberization periods at 10.0°C (50°F) may be used. Short-term Storage.—Potatoes placed in storage for short periods (up to 10 weeks) are stored at temperatures between 7.2° and 10.0°C (45° ~ and 50°F). When tuber temperatures fall below 10.0°C (50°F), reducing sugars accumulate in the tuber. Large quantities of these sugars in potatoes that are processed (fried) produce dark colored potato products which are undesirable. Potatoes that are to be processed (fried) prior to sale to the consumer should be stored at a temperature of 8.9°C (48°F). Those that will be sold fresh may be stored at temperatures between 8.9° and 10.0°C (48° and 50°F). Seed potatoes may be stored at 7.2°C (45°F). In each case the relative humidity should be maintained above 90%. Long-term Storage.—Potatoes that are to be stored for long periods (over 10 weeks) are stored at lower temperatures than those stored for short periods, although destined for the same end use. The lower temperatures not only reduce respiration losses but also cause increased accumulations of reducing sugars in the tubers. Thus, tubers stored under these conditions may require a reconditioning period prior to removal from storage (see next section). Potatoes that are to be sold fresh after long term storage are stored at temperatures between 4.4° and 5.5°C (40° and 42°F). Seed potatoes may be stored at 3.3°C (38°F) for long periods. Potatoes that are to be processed commercially should be stored at 7.2°C (45°F) if they are to be fried and 5.5°C (42°F) if they are to be dehydrated. The relative humidity of the storage environment should remain above 90% to minimize tuber weight loss. Reconditioning.—Potatoes that have been in storage may require reconditioning prior to their removal from storage. Reconditioning, warming the tubers, is performed to remove concentrations of reducing sugars in the tuber and to minimize damage to the tubers when they are handled. Warmer tuber temperatures increase the rate of respiration, thereby consuming the reducing sugars, and also reduce the brittleness of the tuber. To reduce tuber damage during removal from storage, the storage environment should be warmed slowly to 12.8° to 18.3°C (55° to 65°F) and maintained at that temperature until the tubers are uniformly warmed to the same temperature. This warming period may last 1 to 4 weeks. The longer reconditioning periods and higher reconditioning temperatures may be required to remove reducing sugars that have ac- 316 DRYING AND STORAGE OF AGRICULTURAL CROPS cumulated during the longer and colder storage conditions. The humidity of the warm air must be low enough so that the dew point temperature of the air is lower than the tuber temperature. If the air dew point rises to the tuber temperature, moisture will condense on the tubers. Caution must be used in reconditioning potatoes. If tuber quality has begun to deteriorate prior to reconditioning, the period of warmer temperatures may hasten tissue breakdown. Managers of very large storages seldom risk a long reconditioning period, but instead stop the fans during the storage unloading and allow the potatoes to warm from their own respiration heat for the duration of the unloading period. Except in potato chip processing where chip color is extremely critical, blending potatoes from different storages may make reconditioning of a high reducing sugar lot of potatoes unnecessary. Air Distribution System An air distribution system is an essential part of a potato storage. This system must distribute air of proper temperature and relative humidity to tubers located at any point in the potato storage. The system must function properly during each of the stages mentioned in the previous section and provide adequate airflow when the storage is partially or completely full. An air distribution system includes a fan (or fans) for moving the air plus a plenum chamber and/or ducts for distributing air to the tubers. Fan selection follows the duct and plenum sizing which in turn depends upon the physical size and shape of the storage. Selection of multiple fans may be desirable when widely different airflow conditions are sought during different phases of the storage season. Layout Options.—The physical layout of a potato storage depends to a large extent upon the desired storage capacity. As discussed previously, the storage volume is determined by the mass of potatoes to be stored. The storage volume, in conjunction with limiting values for pile height and airflow requirements, sets maximum limits for the length of ventilation ducts. Typically, the maximum air velocity in ducts is maintained below 300 m/min (1000 ft/min) to minimize friction losses. Recommended center-to-center distance between ventilation ducts at the base of potato piles is 2.5 m (8 ft) to provide adequate air distribution between ducts. Typical ranges for pile ventilation rates, potato pile heights, and duct cross-sectional areas are: 0.2 to 0.4 m3/min-m? (or cfm/ft’), 5 to 6 m (16 to 20 ft), and 0.15 to 0.30 m? (1.6 to 3.2 ft2), respectively. Maximum ventilation duct lengths associated with these constraints are given in MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 317 Table 13.2. Practical considerations of potential product loss that could result from the failure of a single duct have caused the industry to keep duct lengths less than 18 m (60 ft). TABLE 13.2. MAXIMUM DUCT LENGTHS FOR TYPICAL VENTILATION RATES, PILE HEIGHTS, AND DUCT AREAS FOR POTATO STORAGE! Ventilation Rate Pile Height Duct Area Duct Length (min7!)2 m ft m ft m 0.2 Os 0.2 0.2 0.4 0.4 0.4 0.4 5.0 5.0 6.0 6.0 5.0 5.0 6.0 6.0 16.4 16.4 19.7 OE 16.4 16.4 19.7 We 0.15 0.30 0.15 0.30 0.15 0.30 0.15 0.30 1.6 3eZ 1.6 3.2 1.6 SP 1.6 Bh 18 36 15 30 9 18 7.5 15 59 118 49 98 30 59 25 49 1 Duct spacing is 2.5 m (8 ft). 2 May be m3/min-m? or cfm/ft?. The maximum duct length, in conjunction with the selected duct arrangement, defines at least one dimension for the storage. Figure 13.1 illustrates three typical duct arrangements and the corresponding dimensional constraint set by the duct length, L. Thus, if the maximum duct length is 15 m (50 ft), then the length of a storage using 1 plenum and longitudinal ducts would be approximately 15 m (50 ft). Similarly, for the single plenum lateral duct design, the maximum storage width would be 15 m (50 ft). The twin plenum system with lateral ducts could be as wide as 30 m (100 ft). When the duct length, pile height, and storage capacity have been selected, the remaining dimension of the storage can be defined. Approximate storage capacities for various pile dimensions are given in Table 13.3. For example, if the pile width is determined by the duct length to be 15 m (49 ft), the desired storage capacity is 2500 t (2900 tons), and the maximum pile height is 6 m (20 ft), so the required storage length is 45 m (148 ft). If the same width and height constraints existed but the desired storage capacity were twice as large, then the required storage length would also be twice as large or 90 m (296 ft). Duct Selection.—Air distribution ducts may be of either portable above-ground or permanent below-ground types. Portable ducts of corrugated metal or slotted wood construction may be laid on the floor of the storage as the storage is filled. These ducts require storage space and may be damaged by machinery, but are less expensive than permanently installed ducts. Permanent ducts of concrete cast-in-the-floor construction are less subject to machinery damage and do not require installation 318 DRYING AND STORAGE OF AGRICULTURAL CROPS D p D L Si P A. SINGLE PLENUM, LONGITUDINAL B. SINGLE PLENUM, LATERAL DUCTS DUCTS p D =| C. TWIN PLENUM, LATERAL DUCTS FIG. 13.1. P—Plenum. TYPICAL DUCT ARRANGEMENTS D—Duct. L—Limiting duct length. FOR POTATO STORAGES. 3/e¥10G) 0F8 OOTT O0FT OOLT 6y 99 Z8 86 *(W01/e3J 082 O0FT 86 91 08% OLP 00L 91 g& 6h (06) (00ZT) (00ST) (008T) (008) (00ST) (0&2) (028) (OLL) OOLT 00% 008% O0FE 09¢ 008% OLP 0F6 O0FT WOE 86) (33 (006T) (009%) (00Z8) (008) (0€9) (DOTS) (008) (000T) (00ST) eq SAILIOVdVO NI SANNOL (SNOL) HOd UST 6h) (33 SADVYOLS IWOldAL 00G% O0%€ 0027 O0TS 0*8 002% 00L OOFT 00TS (006Z) (0068) (008%) (008S) (096) (009%) (09L) (009T) (008%) 00% 00SF 0098 0089 OOTT 009g 06% 006T 008% SHLONAT (006) (002) (0069) (OOLL) (00ST) (0029) (O00T) (001Z) (OOTE) 409 LET) (33 ‘SLHDISH “SHLGIM GNV 433ueT WGP SVT) (33 SYSTEMS paseg UodnQT g (OO€T) (09) (000) (009%) OrOOTT ee 09g 00% OOLT GT 0z GZ 0g OLVLOd & STORAGE 0Z) (33 OTLg UO =WYSIEY g Or ST (001Z) (O00T) (002) (OOTE) 0z0F6 99 006T 008 008% (009%) (OO€T) (0028) (006) Q%O00GE Z8 O0ZT 00€% OOLF 0g w UIPIM Vy CONTROL (JF 9T) aT Wg =14SI8Y Ald JIAVL‘El MOISTURE FOR VEGETABLE CROPS 319 320 DRYING AND STORAGE OF AGRICULTURAL CROPS at harvest time, but they do require occasional dirt removal and do require a high initial investment. Both types of ventilation ducts perform satisfactorily if properly designed, installed, and maintained. Ducts must be capable of withstanding both potato pressures and machinery loadings. Above-ground ducts are subjected to impact by potato loading equipment in addition to potato pressures. Underground ducts must also support concentrated loads from trucks in the storage. Vertical pressures exerted by potatoes on ducts with potatoes piled to a height, h, above the duct are approximately py =1.3wh (13-7) where w is the bulk specific weight of the potatoes and p, is the vertical pressure. For example, at a distance 6.0 m (19.7 ft) below the surface of a potato pile, the pressure exerted on a horizontal surface is 51.5 kPa (1080 Ib;/ft?).! A ventilation duct must provide both adequate airflow to the section of the pile that it serves and uniform distribution of the air along the length of the duct. Ventilation requirements for potatoes depend upon the function of the air and the difference between the air and potato temperatures. During the cool-down period, the potatoes require maximum airflow to provide maximum cooling. If ambient or nighttime air is used for cooling and the air temperature is controlled by refrigeration and/or evaporative cooling, then ventilation rates may be fixed for given initial potato temperatures. After the cool-down period, fixed or slowlychanging potato temperatures are to be achieved, so lower airflow quantities of constant temperature and relative humidity are required. Table 13.4 summarizes airflow rates that are recommended for various conditions. The uniformity of air distribution along a duct depends upon only the duct geometry when moderate airflow occurs in the duct (Davis et al. 1978). When the number, N, of openings or orifices in the duct is large (N > 160), the uniformity of airflow from the orifices depends upon the duct length-to-diameter ratio, L/D, and the total-orifice-area-to-duct-cross- sectional-area ratio, A,/Ap. Uniformities are best for area ratios ranging from 0.25 to 1.25 when L/D is between 60 and 80 and remain good for L/D as low as 40. The smaller area ratios require greater fan pressures and, therefore, greater power to provide a given airflow. The recommended duct parameters are as follows: 40 < L/D < 80 and A,/Apy = 1.0. Recommended center-to-center distance between ducts was given previously as 2.5 m (8 ft). ‘Potato specific weight, w, is 6.6 kN/m (42 Ib;/ft’). WI, UI UO) ,W.UIUI/, IO /UWjo"ej UOT} uorouny SnonuT}u0+ MOTJIO 3997119} MOT} SNonUT} AOTJ ‘ginjeiaduie} palisep 1V 0.01 (4,0$) o1n} > quaurdojaAep 401 sul0S UOT}IPUOD poos SNonuT}UOD ‘MOTJ -BIadule} (4.08) 0,01 > a1nzeredure4 ‘MOTI 99} qTUIIAUT A -U1a} JUBYSUOD ainieied UMOP-[00D ‘ainzei1eduie} palisep VW (7.09) 0.91 = einqyereduie, perqy1uy GAGNANWOOSY MOTSHIV HOA OLVLOd ADVYOLS OV'0 (4.98) 0.ET < aanqeredurs} perytuy UOI}IPUOD 03810g (A.9S) 0.81 = ernyeredurey [ery TU] IOJ SII} JIEVL¥El OV'0 0G Or 02°0 /WI UI U0} /W1Jo }.UTUI MO[JILY pepusurIUL0seyYy IY 2781S T€'0 T€0 T€'0 690 690 OT 0G OL 02°0 0c°0 *1- 321 CROPS FOR VEGETABLE SYSTEMS & STORAGE CONTROL MOISTURE 322 DRYING AND STORAGE OF AGRICULTURAL CROPS The duct diameter can be selected using the required duct length and the recommended L/D ratio. For example, if the required duct length, Ls, is 18 m (59 ft) and an L/D ratio of 40 is selected, then the duct diameter is L/40 or 0.45 m (1.48 ft). The exact duct diameter used would depend upon sizes that are either commercially available or easily constructed. The number and size of openings in each duct are determined by the duct cross-sectional area and the recommended area ratio of A./Ap = 1.0. An additional consideration is that a minimum opening size of 25 mm (1.0 in.) diameter be used to prevent plugging if sprout inhibitor is introduced into the pile through the ventilation system. It is also apparent that larger numbers of openings provide the better uniformities. Consider the duct selected above with a diameter of 0.45 m (1.48 ft). The recommended opening area would be equal to 1.0 times the duct area: A,=Ap = 0.785 D?, or 0.159 m? (1.72 ft?). If N = 200 openings are selected, then the area of each would be a, = A./N or 795 mm? (1.24 in.?). The 31.9 but that diameter of a circular opening of this area would be D, = 1.13]a, or mm (1.26 in.). The actual opening shape and size would be flexible, openings should be equally spaced along the duct and oriented so air is directed toward the region between ducts. Common practice for corrugated metal ducts is to locate circular holes in the minimum diameter corrugations so that sharp edges do not injure the potatoes. The holes are also located in two rows located 90° apart and oriented to direct the air downward 45° from the horizontal to each side of the duct. Plenum Design.—A plenum chamber is required to reduce air turbulence and distribute air to individual ducts when more than two or three ducts are used in an air distribution system. As shown in Fig. 13.1, plenum chambers are used in three common arrangements for potato ventilation systems. The proper design and maintenance of a plenum chamber are essential to providing equal airflow to each duct. A plenum may also be required to provide equal airflow or prescribed unequal airflow to some of the ducts during periods when the storage is being loaded or unloaded. Design criteria similar to those used in duct selection may be used in plenum chamber design. The air velocity should remain below 300 m/ min (1000 ft/min) in all regions of the plenum. The plenum cross-sectional area (perpendicular to the flow in the plenum) should not be less than the combined cross-sectional areas of the ducts that it supplies. The surfaces of the plenum should be smooth and free of obstructions to avoid producing turbulence. Turbulence near duct entrances disrupts the airflow and causes reduced airflow to those ducts. Smooth rounded transitions between the plenum and the duct reduce friction losses and turbulence in that region of the duct and avoid low-discharge points in MOISTURE CONTROL the duct. Any new & STORAGE SYSTEMS FOR VEGETABLE CROPS 323 air distribution system should be checked and adjusted to obtain uniform flow patterns before the storage is filled. Fan Selection.—Fan selection for a potato ventilation system requires knowledge of appropriate fan types and the necessary air pressures and flows for the storage. Two basic fan types are used in potato storages— propeller (axial flow) fans and centrifugal fans. Propeller fans are typically high airflow, low pressure fans with discharges that vary markedly as the pressure against which they operate changes. Some modern large- hubbed propeller fans do produce higher pressures and flows that remain relatively uniform for significant changes in operating pressures and are suitable for potato storages. Propeller fan noise levels may be excessive when located near residential areas. Centrifugal fans (sometimes called squirrel-cage fans) of backwardcurved design are suitable for potato ventilation systems. These fans are of heavy construction and are more expensive than propeller fans. They deliver air against higher pressures and maintain nearly constant flow even when pressures vary markedly. This constant-discharge characteristic is especially important if the ventilation airflow will be varied or concentrated into fewer ducts during different periods of time. The total airflow required by a given potato storage is given by the product of the storage volume and the required airflow per unit volume (see Table 13.4). This airflow requirement must be met by the fan or fans that operate at one time. For instance, if a 10,000 m? (350,000 ft?) storage requires 0.4 m/min-m3 (or cfm/ft?), then the combined output of the fans must equal or exceed 4000 m?/min (140,000 cfm) at the existing operating pressure. When all of the fans move air from a low pressure region to the same higher pressure region and the pressure difference between these two regions is the same for each fan, then the fans are working in parallel. Thus, each fan in a parallel arrangement moves air against the same pressure, and the combined airflow from all of them is the sum of the individual fan discharges against that pressure. For example, two fans in parallel, each of which would deliver 1000 m3/min (35,000 cfm) against a pressure of 250 Pa (1.0 in. water), would deliver 2000 m3/min (70,000 cfm) against the same pressure. Determination of the pressure against which the fans will operate requires the development of a system head-vs-discharge characteristic curve. The system characteristic curve is obtained by calculating the total pressure drop (head loss) that occurs through the system as a function of the airflow. The total head loss includes that which occurs in the plenum, duct, and any other obstructions between the intake side of the fan and the air discharge to either the outside (if the fan is drawing outside air) or the intake of the fan (if air is recirculated). Under the 324. DRYING AND STORAGE OF AGRICULTURAL CROPS pressure conditions that occur in potato ventilation systems, the air may be considered as an incompressible fluid. The head loss in a potato pile due to ventilation rates in the range of 0.20 to 0.40 m3/min-m? (or cfm/ft) is less than 12.5 Pa (0.05 in. water) when 6 m (20 ft) high piles of potatoes are relatively clean and free of trash. Increased airflow and accumulations of dirt and\trash could increase the head loss significantly. In potato storage ventilation, the head loss due to the pile of potatoes is rather insignificant in comparison to the head loss that occurs in the ducts and other components of the air handling system. Head losses that occur in common duct sections, such as elbows, entrances, and straight unperforated lengths, may be determined by standard techniques as described in many fluid mechanics texts. Head losses that occur in straight sections of perforated duct having unobstructed, constant cross-sections are derived from data presented by Davis et al. (1978) for perforated corrugated ducts. The head loss, Hi, for a total duct flow is H, = ? (138-8) where H, is the head loss, Pa (Ib;/ft?) pis the air density, kg/m? (Ib;-sec2/ft4) Ap is the cross-sectional area of the duct, m? (ft?) A, is the total area of the openings, m2 (ft?) Q, is the total airflow in the duct, m/s (cfs) The total airflow required per duct to obtain a specified ventilation rate can be determined from the following relationship: Q=Q, HSL (13-9) where Q. is the airflow required per unit volume of potatoes, m?/min-m3 (cfm/ft?) H_ is the height of the pile, m (ft) S fih center-to-center distance (spacing) between adjacent ducts, m ft Lis the length of the duct, m (ft) Q is the total airflow for the duct, m?/min (cfm) MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 325 The airflow for equation (13-8), Q., is Yo times Q. For example, consider a potato storage with ducts of 18 m (59 ft) length, spaced 2.5 m (8.2 ft) apart, and with potatoes piled 6 m (19.7 ft) high. If the design airflow is 0.40 m3/min-m3 (or cfm/ft?), then the airflow per duct, Q, is 108 m3/min (3810 cfm). The head loss in the perforated duct can be determined from equation (13-8) when the air density and duct and orifice areas are known. The density of air at 90% relative humidity and 10.0°C (50°F) is 1.23 kg/m’ (2.39 X 10-3 lb;sec2/ft4) under standard barometric pressure. (Air properties at other conditions can be obtained from the psychrometric chart in Fig. 6.17.) If the duct diameter is 0.450 m (1.48 ft) and the area ratio (A,/Ap) is 1.0, then A, = Ap = 0.159 m? (1.72 ft?). The calculated head loss for this duct then becomes: H, = 97.3 Q2in metric units (13-10) H, = 1.62 X 10-2 Q? in U.S. customary units (13-11) or And for the specific airflow in the duct, Q., of 1.80 m/s (63.5 cfs), the head loss is 315 Pa (6.53 lb;/ft?). Using a conversion factor of 5.196 lb;/ft?/in. water yields a head loss of 1.26 in. water. The total head loss for a potato ventilation system in which good air handling design practices are observed will be only slightly greater than the perforated duct losses. If refrigeration coils or air washers are not used in the system, total head losses are typically between 250 and 375 Pa (1.0 and 1.5 in. water) for adequate airflow. The head losses of refrigeration coils and air washers can be obtained from manufacturers and should be added directly to the other head losses to obtain the total system operating pressure. Fan selection is made by matching the performance characteristics of a fan to the system head loss relationship. Ideally, the fan performance curve and the system head loss curve should be plotted on the same set of axes for analysis before the fan is selected for a particular application. For example, if 1 fan is to provide air to 6 ducts, then the fan performance curve and the system head loss curve for 6 ducts in parallel should be plotted for analysis. Consider the case in which the head loss for 1 duct (including entrance losses, etc.) is calculated to be: H, = 100 Q2in metric units (13-12) H, = 1.66 X 1073 Q2 in U.S. customary units (13-13) or 326 DRYING AND STORAGE OF AGRICULTURAL CROPS Because the 1 fan serves 6 ducts, the head loss of the 6-ducts-in-parallel system is given by: H,, = 100 (Q;/6)? in metric units (13-14) or H, = 1.66 X 1073 (Q;/6)? in U.S. customary units where Q; is the fan discharge, m?/s (13-15) (cfs). These head loss relationships are plotted in Fig. 13.2 together with a typical fan performance curve. The airflow units shown are volumes per minute, the typical units used for fan performance curves. (Note also that the units for air pressures in USS. customary units are inches of water.) The performance of the ventilation system with the proposed fan can be evaluated from the fan performance and system head loss curves as presented in Fig. 13.2. The intersection of the two curves defines the operating point, which in this case is 550 m?/min (19,400 cfm) of air moved by the fan against a static pressure of 232 Pa (0.93 in. water). Thus, each duct would receive one-sixth of the total flow or 92 m?/min (3200 cfm). If this airflow is not adequate, then the performance curves of other fans should be considered similarly to select the appropriate fan. These plots also provide a mechanism for evaluating the system performance when changes occur. For example, if only 5 ducts were operational or material buildup in the ducts increased the head loss, then new system head loss curves (above the original one) could be drawn to identify the new operating points. These curves are valuable for numerous management and design decisions. Temperature Control Control of potato temperature is very important throughout each stage of the potato storage season. Inadequate cooling or fluctuating temperatures can allow rot development and sprout development in addition to increased quantities of heat from tuber respiration. Both adequate cooling capacities and appropriate control and management of cooling equipment are required in potato storages. Cooling Requirements.—The cooling requirement for a potato storage is equal to the net amount of heat gained by the storage during a given period of time. Contributors to the heat gain are as follows: (1) Field heat of the potatoes (2) Heat of respiration MOISTURE CONTROL INCHES OF WATER & STORAGE SYSTEMS FOR VEGETABLE CROPS 327 Pa 400 Coan PERFORMANCE 1.50 SYSTEM HEAD LOSS 1.25 - Le? 300 lJ 5 : 1.00 or a 0.75 S) : = 0.50 D (op) OPERATING <> POINT 20 . 100 Ww 0.25 O 0 O 200 ee (e) 400 10,000 FAN FIG. 13.2. FAN PERFORMANCE 600 ee 20,000 AIR AND SYSTEM 800 m/min 30,000 cfm FLOW HEAD LOSS CURVES (3) Heat conducted through the walls and roof (4) Heat from air infiltration (5) Miscellaneous heat production in the storage. Each of these is discussed briefly. Field Heat.—Field heat or heat of product is that heat contained in the potatoes when placed into storage that must be removed to lower the tuber temperature to the desired level. The field heat is proportional to the difference in the tuber temperatures before and after the cool-down stage. The rate at which this cooling occurs is the field heat contribution to the storage heat gain. The field heat gain is calculated from the relationship. qr = Beet (13-16) where qr is the rate of field heat contribution to the cooling load, W (Btu/hr) m c is the mass of potatoes being cooled per unit time, kg (Ib,.) is the specific heat of the tubers, J/kg°C (Btu/Ib,,’F) 328 DRYING AND STORAGE OF AGRICULTURAL CROPS T, is the desired storage temeprature, °C (°F) T; is the tuber temperature from the field, °C (°F) t is the period of time for cooling this mass of tubers to T., sec (hr) Consider a storage in which 100 t (100,000 kg or 220,000 lb) of potatoes at 15°C (59°F) are placed into storage each day. They are ‘to be cooled uniformly to 10°C (50°F) in 1 day (86,400 s or 24 hr). The specific heat of raw potato is 3430 J/kg°C (0.82 Btu/lb,,’F). Thus, the rate of field heat removal is 19,800 W (67,600 Btu/hr) for that day and that mass of potatoes. Heat of Respiration.—The heat of respiration generated per unit mass of potatoes depends greatly upon the tuber temperature and time in storage as shown in Fig. 13.3. For example, a 100,000 kg (220,000 lb,,) lot of potatoes that has been in storage for 1 day and presently has a temperature of 10°C (50°F) will generate respiration heat at the rate of 100,000 X 0.055 (220,000 X 0.085) or 5500 W (18,700 Btu/hr). Thus, the respiration heat must be determined separately for each lot of potatoes placed in storage due to their different periods of storage and temperatures. At any given time the total cooling load due to respiration heat is the sum of the respiration heats generated by all of the lots. The heat of respiration for potatoes is also a function of variety, injury, and other factors, so the values given in Fig. 13.3 should be used with this variability in mind. BTU/Ib,,-hr O15 S <= W/ka : 09 08 0.10 & cr WL S 0.05 x 07 0 06 05 04 KR .03 iw ue 18.3°C (65°F) 216.6" ¢ (60°F) 7 —12.8°C (55°F) ° ——10.0°¢ (50°F) Ol 0 0 ° “~~ 7.2°C (45°F) 0 lo DAYS IN 20 30 STORAGE From Wilson (1971) FIG. 13.3. HEAT OF RESPIRATION FOR POTATOES AS S AAFUNCTION OF TEMPERA ; TURE AND STORAGE TIME MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 329 Conduction Heat Gains.—Heat conducted through the roof and walls of potato storages varies as the temperatures inside and outside and the solar radiation outside vary. Maximum heat conduction into the storage would occur when maximum outside temperature, minimum inside temperature, and maximum solar radiation occur. The heat conducted through any wall or roof (or section thereof having uniform temperature difference and insulation properties) can be calculated from the following relationship. (ETD) A, qi. = a ary k (13-17) where qx is the rate of conduction heat gain through the wall or roof sec- ETD tion, W (Btu/hr) is an “equivalent temperature difference” across the thickness of the wall or roof section, °C (°F) A, is the surface area of the wall or roof section, m? (ft?) R, is the thermal resistance or “R-value” for the wall or roof section, °C-m2/W (hr°F-ft?/Btu) For example, if a wall section of area 200 m? (2150 ft?) has an R-value of 3.5°C-m?/W (19.9 hr°F-ft?/Btu) and an equivalent temperature difference of 15°C (27°F), then the rate of heat conduction into the storage through this wall section is 855 W (2920 Btu/hr). R-values are available for various construction and insulation materials from either the manufacturer or standard engineering handbooks. Recommended R-values for potato storages are dependent upon the intended storage environment and the expected weather conditions at the location of the storage. In cold climates where potato freezing may be a problem, the R-value must be adequate to maintain the conduction heat loss from the storage below the sum of the potato respiration heat and heat produced by auxiliary sources. In milder climates where freezing of the potatoes is not a serious threat, the R-value should be chosen to prevent condensation of the high-humidity storage air on the walls or roof. R-values recommended for potato storages in the latter case are 3.5°C-m?/W (20 hr°F-ft?/Btu) for walls and 5.0°C-m?/W (28 hr°F-ft?/Btu) for the roof. Equivalent temperature differences are effective temperatures that can be used for given wall or roof sections to incorporate the effects of the air temperatures and surface temperatures (as affected by radiation) into the conduction heat transfer calculation. Obviously, the color, orientation, heat storage properties, and insulation properties of wall or roof 330 DRYING AND STORAGE OF AGRICULTURAL CROPS sections and the direction and intensity of solar radiation affect instantaneous conduction heat gains. Equivalent temperature differences that reflect these factors are tabulated for different locations, dates, and hours of the day (ASHRAE 1977). Maximum conduction heat gains occur at different times of the day for different wall or roof sections. Thus, the orientation of the potato storage would determine those sections that receive the greatest quantity of radiation during the warmest time of the day, and would therefore define the approximate time at which a conduction heat gain analysis should be made. The exact time is not critical, however, because conduction heat gains are quite small compared to respiration and field heat gains for well-insulated storages. Infiltration Heat Gains.—Heat gains from infiltration of outside air occur whenever doors are open or outside air is drawn into the storage and heat content of the air outside is greater than that inside. In- filtration heat gain can be calculated as follows: a = a (13-18) where qi is the infiltration heat gain, W (Btu/hr) m is the mass exchange per unit time, kg (Ib,,) h, is the heat content of outside air, J/kg (Btu/Ib,,) h; is the heat content of inside air, J/kg (Btu/Ib,,) t is the time period used for the heat exchange, sec (hr) The amount (mass) of air exchange between the inside and outside through open doors is difficult to determine but may be estimated to be one-half of an air change per hour. Thus, if a storage of 5000 m3 (177,000 ft®) volume experienced one-half air change per hour and the air density were 1.23 kg/m? (2.39 X 10° *Ibysec?/ft4 or 0.077 Ib,,/ft3), then the mass of air exchange would be 0.854 kg/s (6810 lb,,/hr). The heat content of a unit mass of air, a function of its temperature and moisture content, can be obtained from the psychrometric tables or charts. The heat content of inside air at 10°C (50°F) and 90% relative humidity air is 45,400 J/kg (19.5 Btu/Ib,,). If the outside air is at 20.0°C (68°F) and 30% relative humidity, its heat content is 49,300 J/kg (21.2 Btu/Ib,,). The infiltration heat gain due to one-half air change per hour then becomes 3330 W (11,600 Btu/hr). Miscellaneous Heat Gains.—Equipment, people, lights, and other sources of heat inside the storage also contribute to the cooling load. MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 331 These heat sources can be evaluated by estimates of their power consumption or by published data (ASHRAE 1977). In general these heat sources are negligible when loading or unloading of the storage is not occurring. Maximum Heat Gain.—Maximum heat gain will occur during the period in which the potato storage is being filled. At this time the conduction, infiltration, respiration, field heat, and miscellaneous contributions will be large. Each of these values should be calculated and summed for the last full day of the filling operation (when respiration of _ the total capacity is occurring). If large quantities of field heat are also assumed, then a maximum cooling requirement can be obtained for a properly designed cooling system. Cooling Methods.—Potatoes may be cooled by natural night air, by air that has been cooled by evaporation of water (evaporative cooling), by air that has been cooled by refrigeration, or by combinations of these methods. Cooling of the potatoes can be accomplished only when the air temperature is below that of the potatoes. Each of the cooling methods is discussed briefly. Night Cooling —The use of night air for cooling potatoes is limited to the periods of time when the outside temperature is cooler than the potatoes but above freezing. Because the greatest cooling requirements occur early in the storage season when the least night air cooling is available, this method is suitable for only a limited number of geographic regions. The amount of night cooling that is available can be determined by the following equation. Me m (h; — h,) (13-19) t where q_ is the rate of cooling, W (Btu/hr) m is the mass of air used per unit time, kg (Ib,,) h; is the heat content of the inside air that is exhausted, J/kg (Btu/Ib,n) h, is the heat content of outside air drawn into the storage, J/kg (Btu/ Ibm) t is the time period, sec (hr) The mass flow of the air is the product of air density and air volumetric flow. The air leaving the storage has the temperature of the potatoes and a relative humidity near 100%. Consider the case in which potatoes at 15.0°C (59°F) are being cooled by 10.0 m3/s (1.27 X 106 ft3/hr) of outside air at 5.0°C (41°F) and 60% rela- 332 DRYING AND STORAGE OF AGRICULTURAL CROPS tive humidity. The density and heat content of the incoming air are 1.26 kg/m? (0.0787 lb,,/ft?) and 31,000 J/kg (13.3 Btu/Ib,,), respectively, and the heat content of 15°C (59°F) air at saturation is 60,000 J/kg (25.8 Btu/lb,,). Thus, the rate of cooling during the period in which these conditions exist is 365,000 W (1,250,000 Btu/hr). To determine the total cooling available per day, air temperatures and relative humidities would be required each hour and calculated cooling available at each hourly period would be summed. Cooling by night air requires minimum investment for equipment but does not provide optional cooling at all times. It should also be noted that outside air removes moisture from the potatoes during the cooling process, so in most cases some form of humidification should be provided. Evaporative Cooling.—Air can be cooled by evaporation of water whenever the air is not previously saturated. When unsaturated air contacts free water for a sufficiently long period of time, it absorbs water and is cooled as the evaporation process removes heat from the air. The temperature at which the air becomes saturated is called the wet bulb temperature. Thus, unsaturated air can be cooled to its wet bulb temperature through an evaporative cooling process. Wet bulb temperatures for air at various temperatures and relative humidities are given in psychrometric tables and charts. Evaporative cooling can be used in potato storages during the time for which the wet bulb temperature of the air is less than the potato temperature. Thus, the cooling period is extended over that during which only night air is available for cooling. The available cooling for an evaporative cooling process is calculated using equation (13-19) as was done for night cooling. If chilled water is not used for evaporative cooling of the air, then the heat content of the saturated air (at its wet bulb temperature) is the same as it was before being evaporatively cooled. Some of its sensible heat (due to its temperature) was converted to latent heat (heat in the water vapor) during the evaporative cooling process. Although the cooling capacity of the air is not changed, it will not remove as much moisture from the potatoes and is therefore more desirable than natural night air for potato cooling. Consider the wet bulb and dry bulb temperature record of Fig. 13.4 for a comparison of night cooling and evaporative cooling ‘methods. Night cooling can be used during the 6.5 hr period (indicated by NC) when the dry bulb temperature of the outside air is less than the potato temperature. Evaporative cooling can be used during the 14 hr period (indicated by EC) during which the air wet bulb temperature is less than the potato temperature. Because the heat content of air is nearly constant for a constant wet bulb temperature and because the air leaving the po- MOISTURE oF CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 333 2G 30 80 70 c = DRY BULB 20 EC 60 WET BULB QO a i 50 lOR S47 ” X OOS KY Ces OR 40 RANA Of rrr XXX OXY 0 30 M NC =NIGHT COOLING EC =EVAPORATIVE COOLING 6a.m. TIME N OF 6p.m. M DAY FIG. 13.4. WET AND DRY BULB TEMPERATURE VARIATIONS FOR A 1-DAY PERIOD tatoes is approximately saturated (wet bulb and dry bulb temperatures are equal), the amount of potato cooling that occurs during both night air and evaporative cooling is nearly proportional to the respective areas shown in the figure. Thus, in this example, evaporative cooling, although used for a period over twice that of night air cooling, provides only 50% more cooling than does night air alone. It must be noted, however, that moisture loss from the potatoes can be excessive when unconditioned night air is used. Refrigerated Cooling.—Refrigerated air cooling may be considered when night air and evaporative cooling capacities are not adequate to offset the total heat gain. This may occur during early harvest periods or in storages that are operated during late spring and early summer months. In refrigerated storages optimal control of the air temperatures should be possible so that temperature fluctuations are avoided and long storage periods can be achieved. Thus, the added cost of refrigeration equipment may be offset by better market prices or a longer processing season for the potatoes. Air cooling by refrigeration cools the air without altering its moisture content until the air reaches saturation (at its dew point temperature). If the dew point temperature of the air is below the temperature of the potatoes (which is the normal case), then the air does not need to be cooled to the point of saturation. Most efficient cooling would include refrigerated cooling until the wet bulb temperature of the air is below the 334. DRYING AND STORAGE OF AGRICULTURAL CROPS potato temperature, then evaporative cooling to saturate the air. The available potato cooling is again defined by equation (13-19) in which the heat content of the cooling air is evaluated at its state following the evaporative cooler. The cooling, however, is provided at the cost of the refrigeration system cooling. The cooling capacity required for refrigeration cooling is\the difference between the total heat gain during a given period of time and the evaporative cooling that is available during that same period. The type of refrigeration system used may be either direct expansion or chilled water. A direct expansion system would have the cooling coil (evaporator) located directly in the airstream before the humidification equipment. Chilled water systems would be used to cool the water used in humidification (evaporative cooling) so that the air would be cooled below its wet bulb temperature during the humidification process. An attractive feature of the chilled water system is that portable water chillers could be leased and connected to the humidification the periods when refrigeration is required. Controls.—Automatic system for only control of the fan and cooling system provides more accurate and consistent environmental control than does manual control. Air circulation must be provided to cool the potatoes and to keep uniform temperature conditions throughout the storage. A lack of air circulation allows both heat and carbon dioxide to accumulate at localized regions, thus producing conditions that shorten storage life of potatoes. Temperature fluctuations at a point also cause sprouting to develop although sprout inhibitors do reduce the severity of this problem. Temperature sensors must be used in conjunction with differential thermostats to prevent the introduction of outside air into the storage when outside temperatures are higher than potato temperatures. Multiple sensors should be used inside the potato pile to sense the pile temperature at several locations so that “hot spots” do not develop. Both dry bulb and wet bulb temperature sensors should be used to sense outside air temperatures. A temperature sensor in the réturn air duct is also required if recirculated air from the potato pile is to be used for part or all of the ventilation air. Wet and dry bulb temperatures in the supply ducts to the potatoes should also be sensed. Numerous control options should be provided to meet the various conditions that occur in potato storages. These are listed below. (1) Fans should be started whenever the pile temperatures are too high or when temperature variations in the pile exceed 3°C (5°F). (2) Return air should be used wholly or in part when outside air is below 0°C (32°F). MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 335 (3) No outside air should be used when its temperature is warmer than the inside air. (4) Refrigeration should be used when the wet bulb temperatures of both return and outside air sources are above the potato temperature. (5) The mix of return and outside air sources should be controlled to provide the desired temperature for air supplied to the potatoes. Humidity Control The provision of adequate humidity is essential for potatoes in long- term storage. This is clearly shown in data for a 300 day storage period for potatoes held at 4.4°C (40°F). The cumulative weight loss for potatoes stored in air of 80% relative humidity was twice that for potatoes stored in air of 95% relative humidity (Cargill et al. 1971). In addition to the loss in salable weight, dehydration of tubers is suspected as being a major factor in causing pressure bruising of the tubers. Humidification of air is accomplished by maintaining contact between free water and a stream of unsaturated air. Humidification of air through water sprays or atomized air is the same process as evaporative cooling of air. Thus, equipment that provides optimum humidification to 90% relative humidity or greater also provides the maximum feasible amount of evaporative cooling. In general, air washers provide better humidification of air than do spray nozzles or atomizers due to the greater contact time of the former. Humidification equipment should be located downstream from fans so that heat from the fans is added prior to the addition of moisture. This arrangement results in maximum humidification. Water Requirement.—The quantity of water that must be provided to humidify air from one state to another can be determined using information from psychrometric tables or charts. When an airstream contacts water at approximately the same temperature, water is evaporated by the air and the air dry bulb temperature decreases. The wet bulb temperature of the air, however, remains constant as the air is humidified. Thus, for potato storages, the dry bulb temperature of the air should decrease until the relative humidity of the air reaches 90%. The amount of water required for this humidification of a unit mass of air is the difference between the moisture content or absolute humidity (mass of water per mass of dry air) of the air at the initial and final states. Water requirements for humidification of sample air states to 90% relative humidity are provided in Table 13.5. In actual practice humidification systems do not evaporate 100% of the potential water requirements of the air. Thus, a fraction as large as 40% 336 DRYING TABLE 13.5. HUMIDITY AND STORAGE WATER FOR REQUIREMENTS Bulb Wet oF ol @ cia 30 DGS Bi 4.7 40.5 8.3 47 5.0 41 6.4 43.5 8.9 48 11.4 52.5 8.3 47 10.0 50 13.3 56 TCMaew 11.4 52.5 11.4 56.5 11.4 63.5 11.4 69.5 ; RH 10 20 40 60 10 20 40 60 10 20 40 60 10 20 40 60 CROPS HUMIDIFYING Humidified Air Initial Air State od Pur 50 10.0 50 10.0 50 10.0 50 10.0 15.6 60 LER GO 60 15.6 60 15.6 ZALLS sw, AMG 7K) PaaS) AKO, Pa a 79 Oia OO 26." «80 2000 80 ZOU oe OF AGRICULTURAL Dry Bulb 2D) C Ke PP 3.3 38 5.3 41.5 7.2 45 5.8 42.5 7.2 45 9.7 49.5 12.2 54 9.2 48.5 eoleo, OS 14.4 58 17-2: 4163 12.5 54.5 14.7 58.5 18.9 66 DPA G(R RH % 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 AIR TO 90% RELATIVE Water Required a Liters Water! gph Water See OS cfm Air 1000 m*Air TS ee aa1000ee 8 4.0 1.5 3.3 1.0 2.2 0.6 1.3 4.9 2:2 ie) 4.2 29 1.3 0.8 1.8 6.0 2c 4.9 Dee, 3.6 1.6 0.8 1.8 6.9 3.1 5.8 2.6 3.8 1%, 2.2 1.0 Source: Pettibone and Iritani (1973). ! Liters/sec of water per 1000 m°/s of air. of the required water may remain in liquid form and collect in the humidification equipment or plenum. A mechanism for collecting and disposing of, or, preferably, recycling the remaining water must be provided. The region immediately downstream of the humidification equipment must have sufficient freeboard to retain the water even against the forces of the moving airstream. For example, consider a storage that has an outside airflow of 10.0 m3/s (21,200 cfm) at 15.6°C (60°F) and 20% relative humidity. From Table 13.5 it is evident that this air would require 4.2 X 1073 liters of water per m? of air (1.9 X 1073 gph/cfm) to humidify the air to 7.2°C (45°F) and 90% relative humidity. Thus, for the given airflow, the water flow would be 0.042 liters/s (40.3 gal./hr) for the desired humidification. In practice, the water consumption may be only 60% of these values. Vapor Barrier.—A vapor barrier is an important structural component of potato storages. Water vapor moves from regions of higher vapor pressure to regions of lower concentration, passing though permeable materials that separate these regions. During the colder periods of a potato storage season, the vapor pressure inside storages is greater than that outside. Thus, water vapor travels through the insulation material and other construction materials (especially through unsealed joints) en route to the outside. At some point along this path, the vapor reaches a material that has a temperature below the dew point temperature for that vapor pressure and some of the vapor condenses on that material. Mois- MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 337 _ ture accumulations then lead to deterioration of the materials or dripping onto the potatoes which can then lead to rotting. A vapor barrier serves to slow the movement of moisture to the regions where condensation can occur. Suitable vapor barriers can be highly impermeable plastic sheets placed on the inner side of insulation or can be insulation materials that are highly impermeable to moisture movement. Many closed-cell foam insulations provide sufficient resistance to moisture movement to serve as both a vapor barrier and insulation. Specific recommendations are dependent upon the storage environment, construction, and weather conditions, so suppliers of building materials or appropriate engineers should be consulted for vapor barrier recommendations for a given potato storage. SUGAR BEETS Forty percent of the world’s sugar (sucrose) production is derived from sugar beets; the remainder is derived from sugar cane. In 1977 the total world production of sugar beets was 260 million tonnes (290 million tons) (U.S. Dep. Agric. 1977). The sugar beet industry in the United States, however, has faced serious competition from non-sucrose sweeteners and has been threatened by the resulting decline in sugar prices. The economic pressure has forced the industry to identify steps that could make its production more efficient. In northern sugar beet producing areas of the United States, sugar beet storage has received attention due to its potential to extend the operating season for processing facilities and to reduce sucrose losses that occur during the storage of unprocessed sugar beets. Currently sugar beet processing facilities may operate up to 150 days on unprocessed beets and may experience sucrose losses less than 0.125 kg/t of beets per day of storage (0.25 lb/ton-day), about half that experienced by stored beets 10 years ago. Approximately two-thirds of the sugar beets processed in the United States are stored prior to being processed. Criteria for design and management of storage facilities for sugar beets are presented in the following sections of this chapter. Where suitable, contrasts and similarities between sugar beet and potato storages will be presented. Storage Criteria adi Storage facilities for sugar beets vary widely depending upon climatic region and planned duration of storage. In regions where extended periods of freezing occur, entire piles of beets are allowed to, or encouraged 338 DRYING AND STORAGE OF AGRICULTURAL CROPS to, freeze so that sucrose losses during storage will be minimized. Freezing is encouraged by forced ventilation at the beginning of the expected sub-freezing weather period. In more temperate in below-ground sugar beet growing regions, sugar beets are stored trenches, in above-ground outdoor piles, and inside permanent storage structures. In each of these storage methods, ventilation and protection from surface freezing are provided. Whereas only in the permanent storage structures is structural design pertinent, in all of the storage methods ventilation is a critical part of the storage problem. In determining the space requirements for sugar beet storage, an approximate sugar beet bulk density of 650 kg/m? (41 lb;/ft) may be used. Thus, the volume required per unit mass is 1.54 m3/t (49 ft?/ton), and the total volume required to store 1000 t (1100 tons) of beets would be 1540 m? (49,000 ft’). Considerations for appropriate floor plans for permanent storages are similar to those cited for potato storages. Convenience for operation and maintenance are factors of prime importance, as is the ventilation system design. Structural Considerations Sugar beet storage pile sizes are determined, in part, by the size of the piling equipment used. Common pile heights are 6 to 8 m (100 to 150 ft) at the top. Unrestrained piles spread at the bottom to form sides with a slope from the horizontal of approximately 57°. Structures which confine sugar beet piles would, therefore, be required to have sufficient vertical clearance for the piler boom above the pile and widths that are less than or equal to the arc of the boom. Lateral wall loadings are less than those measured for potatoes due to the larger angle of repose for sugar beets. Based upon wall pressure theory for semisolid materials, the sugar beet wall loadings should be 40% of potato wall loadings. Typically, sugar beet piles are wider than they are high, so the lateral pressures exerted by the beets on vertical walls are given by: pr =0.12wh (13-20) where p, is the lateral pressure, Pa (lb;/ft2) w is the bulk specific weight of the beets, N/m? (Ib;/ft3) h is the height of beets above the point of interest, m (ft) The bulk specific weight of sugar beets is 6.44 kN/m? (41 Ib,/ft’). MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 339 The total lateral load on a vertical wall that supports sugar beets is given by the integral of equation (13-20) over the total height of the pile which yields Pi = 0.06 w H? (13-21) where py is the total lateral force, N (lb;) H is the height of the sugar beet pile, m (ft) The centroid of the lateral loading occurs at a distance H/3 from the floor. Consider a pile of sugar beets 7 m (23 ft) high restrained by a vertical wall. The total lateral force exerted by the beets on the wall would be 18.9 KN per m of wall width with the load center occurring 2.33 m above the ground. This is equivalent to 1300 lb; per ft of wall width acting at a distance 7.67 ft above the ground. Storage Environment The environmental conditions surrounding stored sugar beets determine the respiration rate of the beets and their weight loss. About two-thirds of the sucrose loss that occurs during storage is traced to respiration losses. Weight loss of the beets does not cause a loss in salable weight because the beets are purchased by the processor prior to storage. Excessive desiccation of the beets, however, does make processing more difficult and may cause increased impurities in the beets. A suberization or wound healing period does not occur as distinctly for sugar beets as it does for potatoes. The storage periods that are appropriate for sugar beets are only the cool-down period for field heat removal and the long-term storage period for maintaining suitable storage conditions. Reconditioning of beets prior to removal from storage is unnecessary because losses from beet injury at this time are insignificant when the beets are processed immediately. Field Heat Removal.—Field heat should be removed from sugar beets as quickly as possible after they are piled. If beets are warm when they are harvested, their respiration rate is high and a significant quantity of sucrose is consumed before the beets are cooled to a suitable storage temperature. The field heat should be removed from stored sugar beets to lower their temperature below 10°C (50°F) during the first 2 or 3 days of storage. The relative humidity of air used for cooling the beets should be high (above 90%) to minimize desiccation of the beets during this period. 340 DRYING AND STORAGE OF AGRICULTURAL CROPS Long-term Storage. —After the sugar beets have been cooled initially, they should be stored at a constant storage condition until they are processed. The beets should not be allowed to freeze and thaw because this results in tissue breakdown, rot development, and large sucrose losses. Minimum respiration levels and minimum sucrose losses occur when beets are maintained within 1°C (2°F) of 4.4°C (40°F). Temperatures outside of this range or variations in temperature with time cause increased respiration and losses. Air Distribution System Air distribution systems for sugar beet storages are designed using the same principles as discussed previously for potato storages. The arrangement of ducts could be parallel to or depending upon the pile size. Typically, ducts to be perpendicular to the longest or all of the distance through the pile perpendicular to the pile length the large sugar beet piles require pile dimension and to extend half (as illustrated in Fig. 13.1). Duct Selection.—Appropriate duct spacings and duct cross-sectional area limits can be determined from Table 13.2. Recommended ventilation rates are 0.4 m3/min-m? (or cfm/ft’) and 0.2 m3/min-m? (or cfm/ft*) for cool-down and long-term periods, respectively. If a ventilation rate of 0.2 m3/min-m? (cfm/ft?3) is desired for ducts spaced 2.5 m (8 ft) apart and the beet pile height is 6 m (19.7 ft), then the maximum length for a duct having 0.30 m? (3.2 ft?) cross-sectional area is 30 m (98 ft). The lengthto-diameter ratio, L/D, that produces good air distribution along perforated corrugated ducts is 40 or greater. Thus, for a duct length of 30 m (98 ft) and L/D equal to 40, the duct diameter would be 0.75 m (2.5 ft) and the duct area would be 0.44 m? (4.9 ft?). This area, being greater than that used in Table 13.2, would maintain the air velocity in the duct below 300 m/min (1000 ft/min) as desired. If duct spacings greater than 2.5 m (8 ft) are desired, appropriate multipliers may be used together with Table 13.2 to set duct length limits. For example, if the duct spacing is doubled to 5 m (16 ft), then the duct length would be halved when all other conditions are unchanged. Thus, the air velocity (and friction loss) would be excessive if the duct length were not decreased when the duct spacing was increased and other conditions were unchanged. The size and number of orifices (or openings) in a duct should be chosen to keep the number large (greater than 160) and the total area of the orifices equal to the duct cross-sectional area. Thus, if the duct and orifices are circular, then the diameter of the orifices should be D,=D//N (13-22) MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 341 where D, is the orifice diameter, m (ft) D is the duct diameter, m (ft) N is the number of orifices For example, if 160 orifices are used, then the orifice diameter should be Y,) times the duct diameter. Plenum Design.—An air plenum may be used to supply air to the individual ducts in permanent storages. A plenum and central air moving (fans) and conditioning (cooling and humidification) equipment would provide the best and most efficient control for the sugar beet environment. However, the justification for permanent storage facilities of this type is questionable due to three factors: (1) The storage season is short relative to that for potato storages (2) Large piles require large airflows per duct (3) High humidity is not essential to preventing sucrose loss. Common practice for sugar beet storage is to use a separate fan for each duct, thus eliminating the need for a plenum. Cool night air is used as the source of ventilation air. This system does offer the advantages of lower cost and individual control of airflow to each duct. If an air plenum is used for a sugar beet ventilation system, then design criteria similar to those for potato ventilation systems should be used. Fan Selection.—Propeller, vane-axial, or backward-curved centrifugal fans may be used for sugar beet ventilation systems. Propeller fans, due to their simple construction and lower cost, are best suited to systems with a plenum requiring large capacity air moving equipment. Large hubbed vane-axial and centrifugal fans are well suited to systems that use a separate fan for each duct. The vane-axial fans may produce noise levels that are excessive for some locations. Selection of a specific fan is accomplished by matching fan performance curves to system head loss characteristic curves as discussed for potato ventilation systems. The fan curve used should define the characteristics of a fan that would provide the total quantity of air required by the number of ducts which it serves when the resulting system pressure exists. The head loss for a single perforated duct is given by equation (13-8). The head loss through the sugar beet pile is rather insignificant in comparison to duct losses. Temperature Control Control of sugar beet temperature is the primary storage requirement for minimizing sucrose losses when the beets are in good condition at the 342 DRYING AND STORAGE OF AGRICULTURAL CROPS start of the storage period. Sucrose losses are greatest at the initiation of the storage period due to the high rate of respiration for beets that have had their growth patterns disrupted, have been injured during handling, and have high temperatures. Cooling Requirements.—Maximum cooling is required by beets at the time that they are placed in the storage pile. At this time both field heat and heat of respiration are maximum and outside air temperatures may be high enough to prevent daytime cooling with natural air. If outdoor piles are used, only field heat and heat of respiration need to be considered for the cooling requirements. If the beets are stored in a permanent structure, then other heat contributors such as conduction heat gains, infiltration heat gains, and miscellaneous heat gains should be determined using methods discussed for potato storages and added to the field heat and heat of respiration. Field Heat.—Field heat contained in beets that are placed in storage can be calculated using equation (13-16). The specific heat of sugar beet tissue is 8550 J/kg°C (0.85 Btu/Ib,,°F). The mass, m, of sugar beets being cooled per unit time is specific to the duct or ducts served by the fan or fans for which the cooling requirement is being determined. For example, if 400 t (441 (59°F) to 10.0°C (50°F) unit time, m/t, is 2.31 field heat is 41,000 W tons) of beets are cooled by 1 fan from 15.0°C in 2 days (48 hr or 172,800 sec), then the mass per kg/s (18,400 lb/hr). From equation (13-16), the (140,000 Btu/hr). Heat of Respiration.—The heat of respiration generated per unit mass of sugar beets depends upon the beet temperature, the period of storage at a constant temperature, the injury that the beet has experienced, and the elapsed time since the injury has occurred. If the beets are not subjected to injury after being placed in storage, then the duration of storage and the storage temperature are the primary factors influencing respiration rate. The heat of respiration produced by sugar beets in storage is presented as functions of storage time and temperature in Fig. 13.5. Respiration rate does, however, depend upon sugar beet variety, so heat of respiration values may be above or below those illustrated. For example, if the 400 t (441 tons) of beets cooled by 1 duct are being considered, their respiration heat can be determined from Fig. 13.5. If they are placed into a storage that has 10°C (50°F) ventilation air, then their respiration heat during the first day is approximately 0.070 W/kg (0.11 Btu/hr'lb,,). Thus, the total respiration heat for the portion of the pile served by the duct is 28,000 W (97,000 Btu/hr). If the storage environment around the beets is 20°C (68°F) during the first day, then the heat of respiration is about 1.6 times that for the 10°C (50°F) environment. Because sucrose losses are proportional to the respiration MOISTURE CONTROL Btu & STORAGE SYSTEMS FOR VEGETABLE CROPS 343 W hr. Ib kg 0.12 0.175 0.10 0.150 0.125 0.08 0.100 0.06 0.075 0.04 20°C 0.02 lO°C (50°F) HEAT OF RESPIRATION (68°F) 0,050 0.025 O°C (32°F) oO O oO 5 DAYS 10 OF I5 20 STORAGE From Dilley et al. (1970) FIG. 13.5. SUGAR BEET HEAT OF RESPIRATION AS FUNCTIONS OF STORAGE TIME AND TEMPERATURE rate, it is important to provide an optimum cool-down for stored sugar beets. environment and rapid Cooling Methods.—Sugar beets can be cooled by the use of cool night air, air that has been evaporatively cooled, or by refrigerated air. Evap- orative cooling offers the advantage of providing air with high relative humidity and thereby reduces beet desiccation during storage. Also, in contrast to night air cooling, it provides a significantly longer period of time during which ventilation fans can be used to remove field heat and reduce the peak sucrose losses. Refrigeration cooling would be advantageous during the cool-down period to remove field heat from the beets as quickly as possible. Portable refrigeration units or portable water chillers or air washers could prove economically advantageous at this time. Units that could be moved 344. DRYING AND STORAGE OF AGRICULTURAL CROPS from duct to duct as the beet pile is formed would quickly cool the recently-piled beets at the most critical time and prevent a significant portion of the sucrose loss. After the cool-down is completed, night air cooling alone should be adequate to dissipate the lower levels of heat gain in the pile. Night air cooling, evaporative cooling, and refrigeration cooling are discussed more fully for potato storages. If cooling systems for permanent sugar beet storages are being considered, then the methods discussed for potato storage cooling should be followed. Control of the cooling system can range from the complexity of permanent storage systems (as discussed for potatoes) to simple thermostatic control of the fans. In outdoor storage pile ventilation systems using night air cooling, a thermostatically controlled switch that starts the fans when the outside air is above freezing but below a set temperature may prove adequate. However, a method of monitoring the beet pile temperature at numerous points is essential to detect “hot spots”; action to cool or immediately process those beets is then taken. In recent years aerial photos of outdoor beet piles using infrared film have proven useful in detecting “hot spots.” Humidity Control The relative humidity of the ventilation air significantly affects the weight loss of the stored sugar beets, but it does not significantly affect the sucrose loss (Andales et al. 1978). The severe desiccation of sugar beets exposed to low relative humidity ventilation air does, however, affect the processability of the beets arid may cause losses in sucrose extraction. Highest rates of moisture removal occur near the ventilation ducts and during the cool-down period when ambient air temperatures are warm and relative humidities are low. Beets stored in permanent storage facilities are those which will be stored for longer periods of time and, therefore, should be ventilated with high relative humidity (90% or greater) air to minimize desiccation. Methods and equipment for achieving humidity control are discussed fully under potato storages. Systems that use evaporative cooling would provide air with relatively high relative humidities and reduce beet desiccation. Portable water chillers used with air washers during the cooldown period or throughout the entire storage period would significantly reduce the beet desiccation and improve sugar beet processability. ONIONS Onion production in the United States in 1976 was 1.56 million tonnes (1.72 million tons). Of this, 49% was placed in storage for some time prior MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 345 to processing or sale (U.S. Dep. Agric. 1977). Not all varieties of onions are suitable for storage, but for varieties that can be stored, principl es of storage are somewhat similar. These are discussed in the followin g sections of this chapter. Storage Criteria Onion storages must provide specific environments for the onions if.a high quality product is to result. Storage losses are a function of the storage environment as well as the condition of the onions when placed in storage and the weather condition and cultural practices used during the growing season. Proper control of the storage environment can, however, significantly extend the storage season from that which would result from storage environments that were not matched to the condition of the onions when placed in storage. Onions may be stored in bulk or may be placed in palletized bins that are stacked in a storage structure. The use of bins may make handling of onions convenient, but it does make ventilation of the onions more difficult. Improved bin designs may alleviate the ventilation problems in the future. The bulk density of onions is approximately 640 kg/m? (40 lb,,/ft?). This means that the volume required to store a unit mass of onions is 1.56 m?/t (50 ft?/ton). Thus, the total volume of an onion storage can be determined for a known mass of onions. For example, 100 t (110 tons) of onions would require 156 m? (5500 ft?) of bulk storage or would fill about 345 0.45-m? (16-ft?) palletized bins. Bulk piles of onions should not exceed 3.0 m (9.8 ft) in height if resistance to airflow through the pile is to be maintained at a reasonable level. Onions in bulk storage exert lateral pressures on walls that restrain them. Lateral pressures for onions should be approximately equal to those for potatoes because the bulk densities and angles of repose are nearly equal for the two products (Williams and Franklin 1971). Thus, equations (13-2) through (13-6) may be used to estimate lateral pressures and total lateral forces exerted on vertical walls of bulk onion storages. For example, a 2.5 m (8.2 ft) high pile of onions would exert a total lateral force given by equation (13-6). If the bulk density of the onions is 640 kg/m? (40 lb,,/ft3), then the bulk specific weight, w, is 6.27 kN/m° (40 Ib;/ft3). The total lateral force is 5.88 kN per meter of wall width (403 lb; per foot of wall width). The centroid of the lateral load would occur 0.83 m (2.73 ft) above the floor. Storage Environment The environmental conditions around stored onion bulbs are critical to the storage life of the onions. Depending upon their initial condition, 346 DRYING AND STORAGE OF AGRICULTURAL CROPS onions may be subjected to different environmental conditions and different durations for the phases of the overall storage sequence. The periods of storage discussed here are drying, curing, cooling, holding, and conditioning. Drying Period.—A drying period is required only when onions are removed from the field with excess surface moisture. If the weather is suitable at harvest, the onions should remain in the field until all surface moisture is gone so that the drying step can be omitted. Onions that have surface moisture when placed in storage cause the relative humidity in the pile to be near 100%, thereby encouraging the growth of rot-causing organisms. The drying period is used to prepare the onions for the normal storage sequence. Ventilation air during the drying period should have a temperature below 35°C (95°F) and a relative humidity less than 75%. A ventilation rate of 2 m3/min-m? (2 cfm/ft®) is recommended for the duration of the drying period. The end of the drying period can be detected by a sudden decrease in the relative humidity of the exhaust air when air supply conditions remain constant. The relative humidity of the monitored continuously with a recording hygrothermograph tently by the use of a psychrometer. The drying period may few hours when heated air is used to 3 days when unheated air can be or intermitlast from a air is used. Curing Period.—A period of curing may be required to allow the development of natural dormancy when this is not fully accomplished through field curing. Onions that have been fully cured have necks that are dry and shrunk and outer onion skins that are dry and healed of any damaged areas. The curing process seals the onion bulbs against disease organisms and reduces the respiration rate of the bulbs. Normally field curing can be accomplished in 1 to 2 weeks. Air used in the curing step should be warm and dry to remove the moisture in the neck and skins quickly. Air temperatures less than 35°C (95°F) and relative humidities below 50% are recommended. The recommended airflow depends upon the stage of bulb maturity and the degree of field curing that has been accomplished. Fully mature bulbs that are partially field cured may be cured in bulk with an airflow of 1 m'/min-m? (1 cfm/ft?). Onions that are immature or have not been field cured need airflow rates of 2 m°/min-m? ( 2 cfm/ft*) to complete the curing process quickly. The air used for curing should at all times have a dew point temperature lower than the bulb temperature, otherwise condensation of moisture on the bulbs will occur. Thus, it is essential that bulb temperatures be monitored throughout the storage on a regular basis. MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 347 Cooling Period.—The cooling period is used to reduce the onion bulb temperature from its warm drying state to its cold holdin g state. The bulb temperature should be reduced in a stepwise manner to preven t uneven drying and produce a gradual temperature decrea se throughout the cooling period. A typical rate of cooling is a reduction in bulb temperature of 6°C (11°F) per month until a temperature of 0°C (32°F) is reached. If seasonal temperature averages for an onion growing area differ markedly from this cooling schedule, appropriate adjustments in the cooling rate may be made. If refrigeration is used, then a fixed cooling rate can be maintained without regard to seasonal temperatures. Airflow during the cooling period should be maintained near 1 m?/ min-m? (1 cfm/ft*) when refrigerated air is used to prevent temperature variations between different regions of the storage. Intermittent operation of fans may be required (when refrigeration is not used) to avoid ventilating the onions with air that is warmer than desired. For intermittent operation, airflow should be 2 m?/min-m? (2 cfm/ft?). When outside air is colder than desired, it should be mixed with inside air to achieve the desired ventilation air temperature. The temperature of ventilation air should be maintained within 3°C (5°F) cooler than the bulb temperature throughout the cooling period. Thus, it is essential that bulb temperatures be monitored during this period. Holding Period.—Once the onions have been cooled to the holding temperature, they should be maintained with a minimum of temperature fluctuation. Variations in bulb temperature tend to break the dormancy of the bulbs and may cause sweating and sprouting of the onions. Ideal conditions for holding onions are an air temperature of 0°C (32°F) and a relative humidity between 60 and 70%. Without refrigeration systems, this temperature may be too low to be maintained in some regions, so other temperatures less than 4.4°C (40°F) may be selected for the holding period. Ventilation of the onions during the holding period is required only to maintain the desired bulb temperature and to exhaust gas concentrations from the pile. Thus, ventilation fans may be used intermittently when the outside air temperature is lower than the bulb temperature. Mixing of outside and inside air is required when the outside air temperature is —1.0°C (30.2°F) or lower. The recommended airflow rate during the holding period is 1 m?/min-m? (1 cfm/ft*) on an intermittent basis or 0.25 m?/min-m? (0.25 cfm/ft*) for continuous operation. Unconditioned outside air should never be introduced into the onion pile when the dew point temperature of the air is above the bulb temperature. Conditioning Period.—Onions should be conditioned prior to packing or processing that requires the removal of the loose outer bulb scales. 348 DRYING AND STORAGE OF AGRICULTURAL CROPS Onions taken directly from cold storage to warmer environments will condense moisture on their scales, thereby making “shucking” of these scales difficult. However, a gradual conditioning of the onion bulbs to raise their temperature without moisture condensation will avoid this problem. Onions that have experienced wetting of the scales need to be dried prior to packing or further processing. \ Onion reconditioning can be accomplished by exposing the bulbs to air having temperatures only slightly warmer than the bulbs. The dry bulb temperature of the air should be warmer than the onion bulb temperature, but the dew point temperature of the air must be lower than the bulb temperature to prevent condensation. For example, if the bulb temperature is 5.0°C (41°F) and the ventilation air (dry bulb) temperature is 10°C (50°F), then the relative humidity of the air must be less than 70% to keep the dew point temperature below 5.0°C (41°F) and prevent condensation. It may be advantageous to recondition only a portion of the stored onions at one time to provide an ample supply for packing or processing. Thus, removal of the onions from storage may be required and a separate reconditioning area may be desirable. Air Distribution System Air distribution systems for onion storages are similar to those used for potato storages. Although onion storages may use palletized bins for the onions, the basic ventilation principles remain the same for these and bulk storages. A well-designed air distribution system will provide the proper quantity of air to onions at every ‘location in the storage. This may require capabilities for varying total airflow and diverting flow to selected regions of the storage. Layout Options.—Any of the duct arrangements shown in Fig. 13.1 may be used for onion storage ventilation. The fan or fans should be located where they can use recirculated and outside air if the system is not refrigerated or where appropriate refrigeration and/or heating of outside air can be accomplished prior to its delivery to the supply plenum. If palletized bin storage is used, then the ducts should be below the floor with slots directed upward. Bins should be stacked tightly in rows over the ducts with clearance between rows of 15 cm (6 in.) and they should be 30 cm (12 in.) from walls. Thus, the spacing between ducts for bin storage is determined by the bin size. Best air movement through the onions occurs when bins have a large area of openings, especially in the bin bottoms. Bin storage should not be used for onions that are immature or have not been fully field cured. A typical arrangement for a palletized bin onion storage is shown in Fig. 13.6. MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 349 OUTSIDE AIR a SUPPLY AIR PLENUM ] SLATTED FLOOR DUCTS EXHAUST FLOOR VENTS PLANS 5 alae i CENTRAL FAN | HOUSE aa eee S fil eh a ete The TheTL ha ThHN es AURV RU AUREUS Pa S WAAAAAAAAALA AA ‘ AIR ava¥ <¥.V avery, "aa" ELEVATION VIEW PERPENDICULAR TO DUCTS From Matson FIG. 13.6. PALLETIZED BIN ONION STORAGE LAYOUT et al. (1978) 350 DRYING AND STORAGE OF AGRICULTURAL CROPS Bulk storages may use ducts above or below the floor for distributing air to the onions. Due to greater resistance to airflow through onions (as compared to potatoes and sugar beets), the recommended maximum pile height for onions is 3.0 m (9.8 ft) and the maximum duct spacing is 1.8 m (6.0 ft). The maximum duct length depends upon the duct area, volume of onions served by the duct, the recommended ventilation rate, and a maximum recommended air velocity in a duct of 300 m/min (1000 ft/min). If the duct spacing and pile height are fixed, then maximum duct lengths can be determined for various ventilation rates. Table 13.6 contains maximum duct lengths for 3.0 m (9.8 ft) pile heights and a 1.8m (6.0 ft) width of onions served by each duct when duct areas are varied. TABLE 13.6. MAXIMUM DUCT LENGTHS FOR ONIONS WITH VARYING VENTILATION RATES AND DUCT AREAS! Ventilation Rate Duct Area Duct Length (min7!)* m2 ft? m ft 1.0 1.0 1.0 2.0 2.0 2.0 0.15 0.30 0.45 0.15 0.30 0.45 1.6 3.2 4.8 1.6 See 4.8 8.3 16.7 25.0 4.2 8.3 12.5 ilo? 54.4 81.7 13.6 Die 40.8 ‘ Duct spacing is 1.8 m (6.0 ft) and pile height is 3.0 m (9.8 ft). * May be m3/min-m? or cfm/ft?. Consider a bulk storage that has 1.8 m (6.0 ft) duct spacing, 3.0 m (9.8 ft) pile height, and a maximum airflow ‘requirement of 2.0 m?/min-m? (2.0 cfm/ton). For this system a 0.30 m2 (3.2 ft?) duct area would be adequate for lengths up to 8.3 m (27.2 ft). Longer ducts lengths could be used only if the duct area were increased. Duct Selection.—Air ducts may be portable above-ground or permanent below-ground types depending upon the storage type. Bulk storages may use either type, but palletized bin storages should use below-ground ducts. Below-ground ducts must be constructed to withstand loadings caused by both filled bins and machinery. Ducts must be designed to carry the required maximum airflow and to distribute the air uniformly along the duct length. Recommended airflow rates for the various storage periods are summarized in Table 13.7. The duct cross-sectional area required to keep the air velocity in the duct less than a maximum, V,,, is defined by the relationship Ap > QSHL v, (13-23) MOISTURE CONTROL & STORAGE TABLE 13.7. RECOMMENDED AGE PERIODS : Period or Phase Drying Curing - SYSTEMS AIRFLOW FOR VEGETABLE FOR ONIONS DURI NG DIFFERENT Holding Conditioning 351 STOR a Onion Quality or Fan Operation Continuous operation Onions immature or without Recommended Airflow m'/min‘t cfm/ton min7!* Sail 100 2.0 field curing Cooling CROPS Mature onions partially field cured Intermittent operation Continuous operation Intermittent operation Continuous operation Continuous operation oul 100 2.0 1.6 Sel 1.6 1.6 0.4 1.6 50 100 50 50 13 50 1.0 2.0 1.0 1.0 0.25 1.0 *May be m3/min-m? or cfm/ft3. where Ap is the duct cross-sectional area, m2 (ft2) Q, is the airflow required per unit volume of the onions, m?/min-m? (cfm/ft*) S is the center-to-center spacing of ducts, m (ft) H is the height of the pile, m (ft) L is the duct length, m (ft) Vin is the maximum air velocity in the duct, m/min (ft/min) The diameter of a uniform circular duct meeting this criterion would be eee — T (13-24) Vm where D, is the diameter of the circular duct. The side of a uniform cross-sectioned square duct would have a length given by pase ee aZ (13-25) where D, is the length of one side of the duct with a square cross-section. One criterion used to assure uniform air distribution along the length of a perforated duct is that the length-to-diameter ratio of the duct be greater than or equal to 40 (L/D 2 40). If the ratio of total opening area to the duct cross-sectional area is less than or equal to 1.0, then good uniformities may be achieved with L/D 2 20. Thus, for circular ducts, this restriction on the duct diameter is D. = L/20 (13-26) DRYING AND STORAGE 352 CROPS OF AGRICULTURAL D, that For ducts with a circular cross-section, the range of values for ned satisfy both equation (13-24) and equation (13-26) may be determi nt represe areas shaded the Only 13.7. Fig. in ted graphically as illustra conditions criteria. ft that satisfy both air velocity and length-to-diameter m Qv = 2.0 min! 1.25 4 S = 1.8 m(60ft) H = 3.0 m(9.8 ft) Vm m/min 1.00 favs 5 Z a: b: ft/min 300 400 974 300 ry Fe ratio a 0.75 = a Or r2 a 0.50 = a | 0.25 O 0.00 0 ee 5 lo ce 15 20 25 m re er DUCT LENGTH, L FIG. 13.7. DETERMINATION OF OPTIMUM DUCT DIAMETER FOR DUCT LENGTHS IN ONION STORAGES . Consider an onion storage with duct spacing at 1.8 m (6.0 ft) and pile height of 3.0 m (9.8 ft). If the maximum ventilation rate is to be 2.0 m?/min-m? (2.0 cfm/ft®), then the duct diameter consideration is illustrated in Fig. 13.7. The duct diameters satisfying length-to-diameter ratio criteria are those below the respective L/D = constant lines. The duct diameters that maintain maximum duct air velocities below 300 m/min (974 ft/min) and 400 m/min (1300 ft/min) are those above line a and line b, respectively. The acceptable duct diameters lie in the shaded MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 353 regions of the graph. The more heavily shaded region satisfies the lower _ system friction losses. If the duct length were 20 m (66 ft), then the minimum duct diameter satisfying the higher velocity limit and the L/D = 20 limit would be 0.83 m (2.7 ft). The high ventilation requirements for bulk stored onions have required the selection of ducts with length-to-diameter ratios less than 40. Under these conditions, good uniformity along the ducts requires that the total opening or orifice area be less than or equal to the duct cross-sectional area. When the number of openings is large (N > 160), then good flow uniformity can be achieved with the total orifice area equal to the duct cross-sectional area. For circular openings, the orifice diameter can be calculated from equation (13-22). The orifices should be oriented or shielded so that they are not obstructed by the onions. Plenum Design.—The air plenum which distributes air to the ducts should be designed to provide uniform flow to each duct. Two basic rules of thumb should be followed. The first is to keep air velocities in the plenum less than 300 m/min (1000 ft/min). The second is to keep the plenum cross-sectional area greater than the sum of the duct crosssectional areas. Additional discussion of plenum design and management considerations is provided in the potato storage section. Fan Selection.—The proper selection of a fan or fans for onion storages requires consideration of the total air volumes required and the pressure against which the fan must operate. Because different airflows are required for different phases of the storage, multiple fans or variable speed fans (fans with interchangeable pulleys) may be advisable. The maximum airflow requirement of the storage is defined by the product of the maximum ventilation rate and the bulk volume of the on- _ions stored. For example, a ventilation rate of 2.0 m?/min-m? (2 cfm/ft?) for a volume of 1000 m? (35,300 ft?) would require a total airflow of 2000 m?/min (70,600 cfm). This total airflow could be provided by fans acting in parallel if the sum of the individual fan deliveries at the common operating pressure were 2000 m?/min (70,600 cfm). The total operating pressure against which each fan will operate is the sum of the head losses along the path which the air follows. The primary head losses will occur in the pile of onions and in the perforated distribution ducts. The head loss in perforated ducts can be calculated as a function of the flow in the individual duct by using equation (13-8). Discussion of this calculation is presented in the potato storage section. The head loss for air passing through bulk stored onions is not insignificant as it was for potatoes and sugar beets. The head loss relationship for bulk stored onions is H, =KQ,H (13-27) 354. DRYING AND STORAGE OF AGRICULTURAL CROPS where H, is the head loss, Pa (in. water) Q, is the ventilation rate, m?/min-m? (cfm/ft*) H is the height of the pile, m (ft) K is a constant, 27.2 Pa‘min/m (0.0333 in. water-min/ft) Thus, the head loss in the pile increases linearly with both the ventilation rate and the air travel distance through the pile. The head loss for palletized bin stored onions is not well defined but would be less than that for an equal depth of bulk stored onions. Head losses that occur in unperforated ducting can be determined by standard techniques as described in fluid mechanics texts. Head losses occurring in perforated lengths of duct with constant cross-section may be calculated using equation (13-8). If refrigeration units are used, the head loss through the cooling coils can be obtained from the manufacturer and be added to the other losses. Additional discussions of these calculations and the determination of total system head loss for fan selection are provided in the potato storage section. A typical total static pressure requirement for an onion storage without refrigeration is 310 Pa (1.25 in. water). Temperature Control Temperature control for onion storages is critical to the success of long-term storages. Both cooling and heating considerations are important to the diverse phases of the total storage period. Heating Requirements.—The main heating requirement is to provide warm, dry air for the drying and curing periods. Thus, it may be necessary to heat outside air to reduce its relative humidity sufficiently for these phases of storage. The required heating rate, the power rating of the required heating unit, is defined in terms of the change in air temperature, q=cpQ(T,-T,) (13-28) where q is the heating rate, kW (Btu/hr) c is the specific heat of air, 1.005 kJ/kg°C (0.24 Btu/Ib,,’F) p is the air density, kg/m? (lb,,/ft3) Q is volume rate of air being heated, m3/s (ft?/hr) T, is the final air temperature, °C (°F) T; is the initial air temperature, °C (°F) The density of air is determined from psychrometric charts or tables. Consider heating 20 m?/s (2.54 X 108 ft?/hr) of air from 10°C (50°F) to MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 355 20°C (68°F). The density of air in this temperature range is 1.23 kg/m? (0.077 lb,,/ft?). The power requirement for heating this air 10°C (18°F) is 247 kW (845,000 Btu/hr). Cooling Requirements.—Cooling requirements for onions include many factors common to cooling potatoes or sugar beets in permanent storages. The maximum total heat gain of the storage must be determined to define the maximum cooling requirement. Field heat, heat of respiration, conduction heat gain, infiltration heat gain, and miscellaneous heat gains should be considered. _ Field Heat.—Field heat or heat of product is an important contributor to the heat gain. Onions that have been either field cured or cured in bulk storage will have temperatures well above the desired holding temperature. All of the heat associated with this product cooling must be removed. However, the recommended gradual cooling reduces the cooling capacity significantly from that which would be required for rapid cooling. Equation (13-16) defines the heat gain due to product cooling. The specific heat of onions is approximately 3810 J/kg°C (0.91 Btu/Ib,,°F). Consider an onion storage that contains 1000 t or 1 X 10% kg (2.2 x 106 Ib,,) of onions that are to be cooled 5°C (9°F) in 30 days. Thirty days are equal to 720 hr or 2.59 X 10° sec. Thus, the heat gain due to product cooling would be 7360 W (25,000 Btu/hr). Heat of Respiration.—Throughout the storage period, onions produce carbon dioxide, water vapor, and heat as products of respiration. The rate of respiration depends upon the maturity, temperature, variety, and degree of dormancy of the onions. All-encompassing data for heat of respiration are not available, but approximate values as a function of bulb temperature may be used for estimating the respiration heat gain. - Heat of respiration values for onions at 0°C (32°F) and at 10°C (50°F) are presented in Table 13.8. Values at other temperatures near this range and above freezing may be obtained by interpolating from the tabulated values. The total respiration heat gain for a storage is calculated by the product of the heat of respiration per unit mass or volume and the total mass or volume of onions in the storage. Other Heat Gains.—Heat gains due to conduction, infiltration, and miscellaneous sources are discussed fully under potato storages. After each heat gain is considered, the maximum heat gain is identified for use in sizing cooling equipment. Cooling Methods.—Onion cooling must be accomplished without adding moisture to the onions. Thus, only night air cooling, refrigeration cooling, and combinations of these are suitable for onion storages. These cooling methods are discussed in detail for potato storages. 356 DRYING AND STORAGE OF AGRICULTURAL CROPS HEAT OF RESPIRATION FOR ONIONS Temperature of Onions 10°C (50°F) 0°C (32°F) Heat of Respiration TABLE 13.8. W/kg W/m? Btu/Ib,,"hr Btu/ft*-hr 0.0118 7.05 0.0183 0.732 0.0252 16.1 0.0390 1.56 \ The cool-down period for onions requires the maximum amount of cooling capacity and requires a steady availability of this cooling capacity to produce the desired gradual cooling of the onions. Thus, if normal night air temperatures are not low enough to provide adequate cooling, then refrigeration should be available during this period. The holding period also requires a steady availability of air at temperatures near 0°C (32°F) to maintain uniform onion temperatures during this period. If occasional warm weather occurs during this time in a region where onions are stored, then refrigeration equipment should be available at the storage. Controls.—Automatic controls for an onion storage can provide consistent and accurate control of the environment and are recommended over manual control. Any set of controls must be capable of operating in any of the storage phases that may be used in the onion storage. Thus, they must be capable of controlling fans, heaters, refrigeration units, and air mixing chambers. The temperature of the onion bulbs and the dry bulb and dew point temperatures of the air inside and outside of the storage are important management parameters. Onion temperature should be monitored at numerous locations throughout the storage. The dry bulb and dew point temperatures of the air outside the storage, that returning for recirculation, and that traveling to the ducts should also be monitored. During the drying and curing phases, the heaters should be activated by a differential thermostat whenever the outside air dry bulb temperature is below the onion temperature. The heaters should not operate when the temperature of the air traveling to the distribution ducts exceeds 35°C (95°F). The fans should operate continuously during these two phases. During the cooling and holding phases, the fans may operate continuously or intermittently. If refrigeration is used, then the fans may operate continuously, but for energy conservation purposes, may instead operate on a regular periodic basis. The following are considerations that should be incorporated phases: into the controls for the cooling and holding MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 357 (1) Fans should be operated when the onion temperatures are too high or when temperature variations within the pile exceed 3°C (5°F). (2) Outside air can be used alone when its dry bulb temperature is below the onion temperature, its dew point temperature is at least 0.5°C (1°F) below the lowest onion bulb temperature, and when the outside air dry bulb temperature is at least -1.0°C (30.2°F). (3) When the outside air temperature is below —1.0°C (30.2°F), outside air can be mixed with return air to achieve the desired air temperature. (4) Outside air should not be used for ventilation when its dew point temperature is not at least 0.5°C (1°F) below the lowest onion bulb temperature. (5) Refrigeration should be used when the fans are operating and the dry bulb temperatures of both the outside and return air are greater than the lowest onion bulb temperature. During the reconditioning phase the fans should operate continuously. Outside air should be used only when its dew point temperature is at least 0.5°C (1°F) below the lowest onion bulb temperature. QUESTIONS 1. List factors that limit the maximum length for ventilation ducts. 2. List the advantages and disadvantages of large potato storages. 3. If a wide storage has potatoes piled 5 m high, what is the total lateral force exerted on a 1 m width of wall? At what depth would the maximum lateral pressure occur? 4, Explain the importance of rapid removal of field heat from stored potatoes. 5. How does suberization affect moisture loss from stored tubers? What environmental conditions aid in the suberization process? 6. What is the function of the reconditioning period? What humidity considerations are important during reconditioning? 7. Explain the differences between short- and long-term storage environment recommendations. 8. Given a potato storage with 12 m long ventilation ducts spaced 2.5 m on center. Determine the recommended duct diameter and the diameter of 200 circular orifices evenly spaced in pairs along the duct length when the height of potatoes in the storage is 5 m. 9. Determine the expected head loss curve as a function of airflow for a 12 m length of perforated corrugated duct. The duct diameter is 0.60 m and the total orifice area is equal to the duct cross-sectional area. Assume an air state of 10°C and 90% relative humidity. 358 DRYING AND STORAGE OF AGRICULTURAL CROPS 10. Specify the airflow and static pressure for a fan that serves 10 ventilation ducts, each of which requires 2 m*/s against a static pressure of 250 Pa. 11. Explain why maximum cooling requirements for potato storages frequently occur during the last day during which the storage is being loaded. \ 12. Define conditions for which outside air can be used to ventilate potatoes during long-term storage that uses evaporative cooling. 13. What is the function of a vapor barrier in a potato storage? 14. Why is humidification of sugar beet storages less critical than humidification of potato storages? 15. Why is a suberization period not recommended for sugar beet storages? 16. Consider a single ventilation duct in a sugar beet pile that has one fan attached directly to each ventilation duct. In an attempt to increase the air delivery in one duct, a second fan (identical to the first) is attached to the opposite end of one duct. Discuss the changes in total airflow and air distribution that would occur when the two fans on one duct are operated simultaneously in contrast to the initial operation situation. 17. What is the purpose of onion curing? 18. Why is the dew point temperature of the ventilation air especially important during the conditioning period for onions? 19. Why is the maximum duct spacing recommended for onion storages less than that for potatoes and sugar beets? 20. Why are the duct length and recommended L/D ratio for good air distribution not adequate by themselves to select the appropriate duct diameter for an onion storage? REFERENCES ANDALES, S.C., PETTIBONE, C.A. and DAVIS, D.C. 1980. Influence of relative humidity and temperature on weight and sucrose losses of stored sugar beets. Trans. Am. Soc. Agric. Eng. 23 (2) 477-480. ASHRAE. 1977. ASHRAE Handbook and Product Directory —Fundamentals. Am. Soc. Heat., Refrig., Air Conditioning Eng., New York. CARGILL, B.F., HELDMAN, D.R. and BEDFORD, C.L. 1971. Influence of environmental storage conditions on potato chip quality. Am Soc. Agric. Eng. Pap. 71-379. DAVIS, D.C., ROMBERGER, J.S., PETTIBONE, C.A., ANDALES, S.C. and YEH, H.J. 1980. Mathematical model for airflow from perforated circular ducts with annular corrugations. Trans. Am. Soc. Agric. Eng. (in press) MOISTURE CONTROL & STORAGE SYSTEMS FOR VEGETABLE CROPS 359 DILLEY, D.R., WOOD, R.R. and BRIMHALL, P. 1970. Respiration of sugar beets following harvest in relation to temperature, mechanical injury, and selected chemical treatment. J. Am. Soc. Beet Sugar Technol. 15 (8) 671-683. EDGAR, A.D. 1960. Pressures on walls of potato storage bins. US. Dep. Agric. AMS-401. IRITANI, W.M., PETTIBONE, C.A. and WELLER, L. 1977. Relationships of relative maturity and storage temperatures to weight loss of potatoes in storage. Am. Potato J. 54 306-314. MATSON, W.E., MANSOUR, N.S. and RICHARDSON, D.G. 1978. Onion storage: guidelines for commercial growers. Oreg. State Univ., Corvallis, E.C. 948. PETTIBONE, C.A. and IRITANI, W.M. the early storage period. 1973. Humidity requirements during Proc. Wash. State Potato Conf., Wash. State Potato Comm., Moses Lake, 1973. PETTIBONE, C.A. and IRITANI, W.M. 1977. Potato storage design and management. Proc. Wash. State Potato Conf., Wash. State Potato Comm., Moses Lake, 1977. SPARKS, W.C., SMITH, potato storages. TORABI, V.T. and GARNER, J.G. 1968. Ventilating Idaho Univ. Idaho, Moscow, Bull. 500. M.R., POWELL, A.E. and PETTIBONE, C.A. erted on the walls of larger bins by stored potatoes. Pap. 77-4062. U.S. DEP. AGRIC. 1977. Agricultural Statistics. 1977. Pressure ex- Am. Soc. Agric. Eng. U.S. Govt. Printing Office, Washington, D.C. WILLIAMS, L.G. and FRANKLIN, D.F. 1971. Harvesting, handling, storing yellow sweet Spanish onions. Univ. Idaho, Moscow, Bull. 526. and WILLSON, US. G.B. 1968. Lateral pressure on walls of potato storage bins. Dep. Agric. ARS 52-32. WILSON, E.B. 1971. Cooling potatoes with night air, evaporation, and refrigeration. Wash. State Univ., Pullman, E.M. 3463. YAEGER, E.C. and SCHAPER, patterns of stored potatoes. L.A. 1978. Horizontal and vertical pressure Am. Soc. Agric. Eng. Pap. 78-4057. “7 x yh ily i 7 ; | ti ‘ Teg ae "dra At oteng tae iis ; : is mene hh ek Sie DAWwel ; a Atl inti am HOT Asie | ’ a wsa i Fi ie ‘ ae i | , eee, A | ROA eee id We p ) : b Oy ’ | ' . : Ghee ortt eatbe ‘ | oe : 7 ;ale FB| . 7w . wim Sab-TY nt vube a TH ics mnie ‘de ~ ‘yr PHM aLaG saad | mlPe he > ry iS tio -. i ‘ . si bya ats a <iVe evr . % a Dyna: ee: 362 DRYING AND STORAGE OF AGRICULTURAL air Q, flow, cfm CROPS ALS PA kit IY 0 0.1 0.2 0.3 FIG. A.1. PRESSURE DROP THROUGH FOR DETERMINING AIRFLOW 0.4 ORIFICES 0.5 0.6 IN 3-IN. PIPE APPENDIX INPUT HEAT HOUR PER BTU 0 20 40 60 80 100 = 120 TEMPERATURE RISE °F 140 Courtesy of Lennox Furnace FIG. A.2. TEMPERATURE HEAT INPUTS See Appendix 160 Co. RISE WITH DIFFERENT AIRFLOWS AND for metric conversions. 363 364 DRYING AND STORAGE OF AGRICULTURAL SHELLED CROPS CORN TEMPERATURE Percent CONTENT, MOISTURE INITIAL DRYING TIME, hours Courtesy of Habco Manufacturing Co. FIG. A.3. TIME FOR DRYING SHELLED CORN TO 13% MOISTURE WITH HEATED AIR APPENDIX TABLE A.1. 365 DRYERS SOLD AND VALUE (UNITED STATES) FOR SELECTED YEARS 1972 Dryer Batch type PTO | electric motor Continuous-flow Heated-air units! Fans, over 15,000 cfm at 1 in? 1967 No. Value, $1000 4,511 12,972 3226 8,202 1,520 10,966 9,373 5,606 2681 9370 25,094 3,540 2,678 NA eaOtee 1965 16,008 1964 Value, No. Value, $1000 No. $1000 No. 1996 4226 8627 1680 265 3911 614 NA 1267 3615 1963 Value, $1000 No. 921 2508 4664 NA 433 3861 467 NA 151d 3375 Value, $1000 Batch type PTO electric motor Continuous-flow Heated-air units Fans, over 15,000 cfm at lin. 404 6737 1103 4045 3068 1962 1961 PTO electric motor Continuous-flow Heated-air units Value, $1000 No. Value, $1000 452 1092 279 1294 1548 1200 670 1243 406 1683 1723 3106 Fans, over 15,000 cfm at lin. NA NA 1242 5534 NA NA 1585 7362 1959 1958 No. Value, $1000 No. Value, $1000 Batch types 5938 7546 7488 11,696 Heated-air units Fans, over 15,000 cfm at 1 in. NA NA NA NA 4079 594 5loD 1534 1960 No. Batch type 1275 2061 3114 NA 709 Source: U.S. Dep. Commer. (1976). 1 Heated air units are usually coupled with a drying structure. 2 Data from some years list fans 24 in. in diameter separately. No. Value, $1000 4538 5099 440 2674 NA NA 3811 1137 366 DRYING AND OF STORAGE AGRICULTURAL GRAIN DRYING RECOMMENDATIONS Recommendations Content ) with Natural Air Moisture Maximum 1. ) with Heated Air of Crop at Harvesting for 2. Maximum Moisture Content of Crop for Safe Storage in a Tight Structure CROPS \ TABLE A.2. EarCorn 30% 35% Shelled Corn 25% 35% 13% 13% 3. Pounds of Water per Bushel Which Must ) 30% ) 25% Be Removed for Safe Storage When ) 20% Grain Is Harvested at Moisture ) 18% Content of 22.0 Ag 8.1 Deo gia 8.7 4.7 30 3.0 19 4. Maximum Relative Humidity of Air Entering Crop Which Will Dry Crop Down to Safe Storage Level When Natural Air Is Used for Drying 60% 60% 5. Maximum Safe Temperature of) 1. Seed Heated Air Entering Crop for Drying When Crop Is to Be ) 2. Sold for Used for ) Commercial Use! ) 3. Animal Feed? 110°F 110°F 130°F 130°F 180°F 180°F b= 2046 16-24 in. (If the above products are to be stored for long periods, the moisture content should be 1 to 2% lower than shown in this tabulation.) 16% 6. Preferred Depth of Crop for Batch Drying with Heated Air 7. Maximum Depth of Crop at Different Moisture, % Moisture Levels for Drying in Tight Structure with Fans Capable of Delivering ) Depth, ft the Required C.F.M. as Listed in 8. Below. ) (not critical) 3052 20 15 20 20 45 6 8 The above depths can be increased 8. Minimum Airflow to Dry Crop ) .CFM per Bushel at Moisture Level and Depth ) Natural Air? as Listed Above in 7. ) Heated Air with Not ) over 15°F Rise in ) Temperature a0 Zo 20S by bd SOMES somewhat— ED Bune (Shella ve is pl Source: CDMC (1956). *Not recommended. ' Higher temperatures than those listed may be used when the corn is dried under carefully controlled conditions so that the maximum temperature of the kernels does not exceed 130°F at any time. APPENDIX TABLE A.2. 367 (Continued) Wheat Oats Barley GrainSorghum Soybeans Rice Peanuts 20% 25% 20% 25% 20% 25% 20% 25% 20% 25% 25% 25% 45-50% 45—50% 12% 11% 12% 138% (13% (13% (12% for (12% for (seed wheat 13% (seed oats 5.2 3.7 2.1 5.0 251 129 Lat 4.2 209 1.8 9:2 5.3 3.9 2.5 11.0 6.6 5.1 3.5 6.5 3.2 2.0 1.0 60% 60% 60% 60% 65% 60% 75% 110°F 110°F 105°F 110°F 110°F 110°F 90°F 140°F 140°F 105°F 140°F 120°F 110°F 90°F 180°F 180°F 180°F 180°F = = = 16—24in. 16—24in. 16—24in. 16—24in. 16-24 in. 9-18 in. 4—6 ft 20 18 16 25 20 16 20 18 16 25201816 25201816 25201816 40-50% 4 6 810 4 6 810 A46,48" 14 6 especially 8 4) © at the lower 8ii4s4e68 moisture levels—provided 6 fan capacity will meet the airflow requirements in 8. at relatively high static pressures. Oueanelere Oac gees aes Ae l.p e215 de oe. 3.2 1 aa os ee enaaoe eda 5 A 3.2 La ey ec LES earn at 3 3 2Tf there is any possibility that the crop may be sold, use the lower temperatures as listed above for commercial use. et 3 For fall crops or in humid areas, natural air drying depends on weather conditions and may take days or months to complete. Heated air drying under same weather conditions can be completed in hours or a few days depending upon volume to be dried. Note: See Appendix Table A.6 for metric conversions. 368 DRYING AND STORAGE OF AGRICULTURAL TABLE A.3. TIME AND OIL REQUIREMENTS Starting Moisture lb Water lb Water Content pertonat RemovedinDrying (%) 60 Do) 50 45 40 35! 30 25 Start 2400 1958 1600 1310 1070 860 685 535 1 ton to 20% 2000 1558 1200 910 670 460 285 135 CROPS TO DRY HAY WITH HEATED AIR Approx. Time Approx. Approx. to Dry 20 tons gal. Oil gal. Oil While Burning UsedtoDry UsedtoDry 1 ton to 20% 20 tons to 20% 28.6 562 PBN 460 18.6 sive 14.6 292 11.1 222, 1.9 158 Sell 102 205 50 Source: Courtesy of Lennox Furnace Co., Des Moines, IA. ‘Maximum moisture for economical drying. Note: See Appendix Table A.6 for metric conversions. — 10 gal. per hr (hr) 56 46 37 29 22 16 10 5 ; nq 09 0°87 QP 0°82 SV 0°86 EG 8'9E 0'8F 09796 rr 3S—8F 0€—-FI 0S—PP O'8P 0'8F TIT OZ 9°GZ Ih—-¥'8E 9 OOFRS ysnol seayINOsS Ula peinseausjaysnq Jo ‘uUIOO (PaTNY) nq ike nq ine| ix nq | nq nq seq [| wie ne nq BG nq nq BGs nq | nq nq nq Ov 0970S 09706 ‘paysny UO ay} ‘1ea OF VS BY GE Sig 09 09 0G 09 09 0S CY OOT GOL oe wa 9F 10L Si 0'SE—-9'6Z yng ‘Ayisuaqd 43/1 JHL O'CE O'ZE 6T8S 0°98 0°87 0°87 0°07 087 0'8P 0°07 098 O'9€ 0°96 SvV 89E esneoaq 3 9°96 ZO00FO8F O8rRO00r 898 0°82 SvV 0'F AGG 9'LT 9°G6 GIT ‘xoiddy “JUSTO «= SAYNSVAW GSN NI S26 OV 0g 8% CG qu) ON ainsvayy] aI | 6& in| vI SLHDISM GNV Qeeym'd)yeds nq suvaqhog 0810g ‘sory ysnod ‘sory ysnor aAy CESSES) ‘ory possedey dopey Addog Bed u1oodog OVE-Z 1820 0'0F—-Z'GE Peifeys ystuedg ‘sIauUNI ag ceZ uepng sseis 0g 0'0V CALLS ENO 87 3 0S PF8EO00 AYOWULL 929G0G hrBS OOP APPA suveq OvV—GE —Oi8Gies TPT 09 0'8P MA S}NUTE O0S—87 VANS ONO EMU 09—-8¢ 0'87—V'9P 9¢ 10L 9g G€ 6& 9V 09 09 OOT 0g 9¢ ce GaSva NO yng ‘A}ISUOC, /OIJ Ulery 10 pees O'8P SE) V8E preyoig SsBis ‘synuveg peTjeysun OPT eIUTsItA adAq Sd33aS Ajfensn paziusooer se Sureq ynoqe z sertmnber QL q] 07 pyaIA T ‘nq Io 9g ‘q] Jo paT[ays‘UI0D :a30Nvag xIpueddy a[qey,Q°V IOJ IIIJOUW “SUOISIAAUOD plepueysJYSIaeM JO 0), q] st :e0I1n0g AVSV oy], ‘(8L61) nq nq) inte BG BO nq Bei ng peesdwia ATOyoT}yy synu sun} ueIe[turjo] Le y yode TL SEM TNA preysnyy yy nq nq ng in| nq nq nq nq nq poesxepy SEsdNo)) peysny Peileus Uses JoeMs pe9su0}}0) ‘ree ulo) 10}sSe,) SuBveq IOAOID yeaymyong IL ule s10s SUNY | EG nq yors Bel wel nq nq nq ‘xoiddy “VY319 SAILISNSG 4O NIVHDGNV yu) 19NMA aInseayy aI pS 09 ee 8Y TNOWS Gay ‘OlWDV pe]jeysun ‘Iay}0 Arp ‘19q}0 Alp sseisan[q URAOBREOON deus ‘eury urery Jo pees BTESTV Agqrieg suvag “eurAp 3IGVL“vv ‘S'n ‘dadAO APPENDIX 369 DRYING 370 TABLE GRAIN A.5. AND SPECIFIC STORAGE GRAVITY OF AGRICULTURAL OF GRAIN Variety Grain Wheat, soft Oats Harvest Queen Victory Oats Red Texas Oats lIowar Oats Kanota Barley Barley White Hulless Svansota Coast (6 rows) Trebi (6 rows) Barley Barley Barley Manchu Soybeans Grain sorghum Grain sorghum Rye Rice Wilson Yellow Milo Blackhull Kafir Common Honduras Rice Wataribune Flaxseed Corn, No. 1 Mixed yellow and white Millet Siberian Japanese Buckwheat TABLE A.6. 9.8 Air Space or Voids in Bulk, % 42.6 Kernel Specific Gravity 1.30 40.1 1.29 9.8 9.8 39.6 47.6 1.32 1.05 10.3 Dow 0.99 9.7 51.4 0.95 9.4 50.9 1.06 10.4 9.8 39.5 45.4 1.33 1.21 TNS) 1.24 10.3 10.7 57.6 47.9 O71 44.5 36.1 1.18 7.0 9.5 9.9 9.7 19 33.8 37.0 36.8 41.2 50.4 els 122 1.26 123 Sigil 5.8 9.0 34.6 40.0 1.10 1.19 9.4 36.8 LE 6.9 12.4 10.1 46.5 41.0 1.26 ele 1.10 (1978). CONVERSION FACTORS FOR DRYING—U.S. CUSTOMARY Airflow 1 cfm = 0.0283 m?/min = 0.0004716 m3/s = 0.4716 mm?/s 1 cfm/bu = 1.0405 m3/min-t = 0.804 m3/m? min = 0.0134 m3/m's 1 cfm/cwt = 0.01058 m?/t's 1 cfm/ft? = 0.3048 m3/m? min = 0.00508 m3/m?s 1 cfm/ft? = 1 cfm/ton = = = IN BULK 9.8 Hannchen Soybeans Source: ASAE Moisture Content, %ow.b. Turkey, winter (yellow) Wheat, hard VOIDS PERCENTAGE AND Turkey, winter Wheat, hard CROPS 1 m3/m3 min = 0.01667 m3/m3s 0.0000312 m?/kg min = 0.52 ym3/kg s 0.00000052 m?/kg s = 0.00052 m3/t:s 1.12 cfm/t 1 Ib/hr ft? = 0.813 kg/m?s 1 ft3/hr = 0.007866 X 107!° m3/s Area and Length 1 ft = 0.3048 m 1 in. = 0.0254 m 1 acre = 4046.87 m? = 0.404687 ha 1 ft? = 0.0929 m? 1 in.2 = 0.6452 X 1073 m? = 6.452 cm? 1 mi = 1.6093 km TO SI UNITS APPENDIX TABLE A.6. (Continued) Density 1 lb/bu = 12.872 kg/m3 1 Ib/ft? = 4.883 kg/m? 1 lb/ft? = 16.0185 kg/m} 1 Ib/gal. (U.S.) = 119.8 kg/m? 1 lb/in.2 = 0.00927 kg/m? Mass 1 Ib = 0.4536 kg 1 ton = 907.18 kg 1 tonne (t) = 1000 kg 1 lb/min = 0.00756 kg/s 1 ton/hr = 0.252 kg/s 1 ton/acre = 2241.7 kg 1 long ton = 2240 lb = 1016.05 kg Pressure 1 atm = 1.013 X 10° N/m? = 101.3 kN/m2 = 760 torr = 101.3 kPa 1 bar = 100 kN/m? = 750 mm Hg = 10.20 in. water = 0.9869 atm 1 1 1 ib 1 in. Hg = 3.376 kN/m?2 = 3.376 kPa = 0.03342 atm in. water = 248.8 N/m? = 248.8 Pa = 0.2488 kPa = 0.00246 atm lb/ft? = 47.88 N/m2 = 47.88 Pa /im- = 1 psi = 6.895 kN/m?2 = 6.895 kPa mm Hg = 133.33 N/m? = 0.133 kPa Thermal 1 Btu = 1055.1 J = 1.055 kJ 1kW=1kJ/s 1 kWh = 3.6 X 108 J 1 hp = 0.746 kW = 746 W 1 Btu/ft? s = 1.1349 k 104 W/m? 1 Btu/Ib = 2.326 kJ/kg 1 1 1 1 Btu/hr = 0.2931 W Btu/ft? = 1.1356 X 104 J/m2 Btu/lb °F = 4.1868 kJ/kgK Btu-ft/hr-ft? °F = 1.731 W/mK 1 Btu/hr ft? °F = 5.678 W/m2K 1 Wh = 3.6 kJ 1 Btu/hr ft? °F/in. = 0.14423 W/mK 1 Btu/hr ft? = 3.1546 W/m2 Viscosit 1 cm?/s (poise) = 1.000 X 107! kg/ms (or Ns/m?2) 1 ft?/s = 92.903 X 1073 m2/s 1 lb/ft-sec = 1.488 Pa:s = 1.488 Ns/m? 1 lb-sec/ft? = 47.88 Pa-s Volume 1 ft? = 1 1 1 1 1 0.02832 gal. (U.S.) = in? = 1.6387 liter = 0.001 ton = 2.8317 bu = 0.03524 m? = 28.32 dm? 0.00379 m* = 3.79 dm? = 3785 cm? X 10°5 m3 = 16.387 cm’ = 0.0164 dm? m? m3 m3 = 35.24 liters 1 bu/acre = 0.0871 m3/ha 1 bu/hr = 0.009785 1 yd? = 0.76455 m3 X 1073 m/s 371 DRYING 372 TABLE A.6. AND STORAGE (Continued). OF AGRICULTURAL TEMPERATURE CROPS CONVERSION The numbers in boldface type in the center column refer to the temperature, either in degree Celsius or Fahrenheit, which is to be converted to the other scale. If converting Fahrenheit to degree Celsius, the equivalent temperature will be found in the left column. If converting degree Celsius to Fahrenheit, the equivalent temperature will be found in the column on the right. Temperature Celsius °CorF -39.4 -38.9 -38.3 -37.8 -37.2 -36.7 og -35.6 -35.0 -34.4 -33.9 -33.3 -32.8 32.2 317 =p -30.6 - 30.0 -29.4 -28.9 - 28.3 -27.8 -27.2 - 26.7 - 26.1 -25.6 -25.0 24.4 -23.9 -23.3 -22.8 -22.2 -21.7 -21.1 -20.6 20.0 -19.4 -18.9 -18.3 -17.8 -17.2 16.7 16.1 15.6 -39 -38 -37 -36 -35 =34 =33 -32 =31 -30 -29 - 28 -27 - 26 -25 -24 -23 22 -21 -20 -19 -18 -17 -16 -15 =14 -13 -12 -11 -10 -9 -8 -7 =6 =5 -4 =3 -2 =1 0 +1 +2 +3 +4 15.0 +5 -14.4 13.9 +133 -12.8 -12.2 -11.7 “11a -10.6 -10.0 -9.4 -8.9 -8.3 7.8 -7.2 -6.7 6.1 -5.5 -5.0 -4.4 -3.9 -3.3 28 2 <17 Sig 08) : +06 11 Temperature Fahr Celsius : -38.2 -36.4 34.6 -32.8 -31.0 -29 2 -27.4 -25.6 -23.8 - 22.0 - 20.2 -18.4 Fics -14.8 -13.0 -11.2 -9.4 =mG -5.8 -4.0 2.2 -0.4 +1.4 +3.2 +5.0 +6.8 +8.6 +104 +12.2 +14.0 +15.8 +17.6 +19.4 +21.2 +23.0 $24.8 +26.6 +28.4 +30.2 +32.0 +33.8 +35.6 +374 +39.2 +41.0 +6 +7 +8 +9 +10 +11 +12 +13 +14 +15 +16 +17 +18 +19 +20 +21 +22 +23 +24 +25 +26 +27 +28 +29 +30 +428 +446 +46.4 +48.2 +50.0 +518 +53.6 +55.4 +57.2 +59.0 +60.8 +62.6 +64.4 +66.2 +68.0 +69.8 +71.6 +73.4 +75.2 +77.0 +78.8 +80.6 +824 +84.2 +86.0 +33 +34 +91.4 +93.2 +31 | +978 : +1.7 +2.2 +2.8 +3.3 +3.9 +4.4 +5.0 +5.5 +6.1 +6.7 +7.2 +7.8 +8.3 +8.9 +9.4 +10.0 +10.6 +111 +11.7 +12.2 +12.8 +13.3 +13.9 +144 +15.0 +15.6 +161 +16.7 +17.2 +17.8 +18.3 +18.9 +19.4 +20.0 +20.6 +21.1 $21.7 +22.2 +22.8 +23.3 +23.9 +244 +25.0 +25.6 +261 +36 +37 +38 +39 +40 +41 +42 +43 +44 +45 +46 +47 +48 +49 +50 +51 +52 +53 +54 +55 +56 +57 +58 +59 +60 +61 +62 +63 +64 +65 +66 +67 +68 +69 +70 +71 +72 +73) +74 +75 +76 +717 +78 +79 +26.7 +27.2 +27.8 +28.3 +28.9 +29.4 +30.0 +30.6 +311 +31.7 +32.2 +32.8 +33.3 +33.9 +344 +35.0 +35.6 +36.1 +36.7 +37.2 +37.8 +38.3 +38.9 +394 +40.0 +40.6 Temperature °CorF +80 | | | | | | +81 +82 +83 +84 +85 +86 +87 +88 +89 +90 +91 +92 +93 +94 +95 +96 +97 +98 +99 +100 +101 +102 +103 +104 +105 Fahr Celsius +96.8 +98.6 | +100.4 | +102.2 | +104.0 | +1058 | +107.6 | +109.4 | +1112 | +113.0 «| +1148 | +1166 | +1184 | +120.2 | +122.0 | +1238 | +1256 | +1274 | +129.2 | +131.0 | +1328 | +134.6 | +1364 | +138.2 | +140.0 | +1418 | +143.6 | +1454 | +1472 | +149.0 | +1508 | +1526 | +1544 | +156.2 | +158.0 | +1598 | +161.6 ||+1634 | +165.2 | +167.0 | +1688 | +170.6 | +1724 | +1742 +43.9| +444 +45.0 +45.6 +46.1 +46.7 +47.2 +478 +48.3 +48.9 +494 +50.0 +50.6 +51.1 +51.7 +52.2 +52.8 +53.3 +53.9 +54.4| +55.0 +556 +561 +56.7 +57.2 +57.8 +58.3 +58.9 +594 +60.0 +60.6 +61.1 +61.7 +62.2| +62.8 +63.3 +63.9 +64.4 +65.0'| +65.6 +661 +66.7| +67.2 +67.8 +176.0 +68.3 | +1778 | +179.6 | +1814 | +183.2 | +185.0 | +1868 | +188.6 | +1904 | +1922 | +194.0 | +195.8 | +1976 | +1994 | +201.2 | +203.0 | +2048 | +2066 | +208.4 | +2102 | +212.0 | +2138 | +2156 | +2174 | +2192 | +221.0 +689 +69.4 +70.0 +70.6 +71.1 +71.7| +72.2| +72.8| +73.3| +73.9| +74.4| +75.0| +75.6 +76.1| +76.7| +77.2 +77.8| +78.3| +78.9| +79.4| +80.0| +80.6| +81.1| +81.7| +82.2| ral1| 106 |+2228 : 7 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |, | 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+359.6 +828 | +181 |+357.8 4 +42.2 | +108 | +2264 *42.8 | +109 | +2282 °CorF +83.3 | +83.9| +844] +183 | +361.4 +184 | +3632 Temperature Celsius +85.0 +85.6 +86.1 +86.7 +87.2 +87.8 +88.3 +88.9 +89.4 +90.0 +90.6 +91.1 +91.7 +922 +92.8 +93.3 +93.9 +94.4 +95.0 +95.6 +96.1 +96.7 +97.2 +97.8 +98.3 +98.9 +994 +100.0 +100.6 +101.1 +101.7 +102.2 +102.8 +103.3 +103.9 +104.4 +105.6 +106.7 +107.8 +108.9 + int +112.2 +113.3 +114.4 4S °CorF | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | +185 +186 +187 +188 +189 +190 +191 +192 +193 +194 +195 +4196 +197 +198 +199 +200 +201 +202 +203 +204 +205 +206 +207 +208 +209 +210 +211 +212 +213 +214 +215 +216 +217 +218 +219 +220 +222 +224 +226 +228 + nasa | +234 | +236 | +238 aed +117.8 | +118.9| +120.0| + eee +123.3| +1244 | |. +1255| + v1a7 6 +128.9| +130.0| +131.3 | + 13a +134.4| +135.6 | +136.7| + +4389 +140.0| +1411 | +1422] 4143.3] tiaaa| +145. 34464 +147.8| + sre +244 +246 +248 + aoe +254 +256 +258 | Fahr | +365.0 | +366.8 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(SL61) 12 72 Leneq{yNyy 00ST Ire Surdrp ysneyxXy (SL61) 72 42 Tayoorg 000Z—008T suor}Ipuod sur}e1edo aseIIAV pezeinzes (SLE) 12 72 toxoorg 0061—009T ; (LL6T) UeeTeISOM SUOT}IPUOD SuTZVIEdo aseIBAY A.0F 0} €€ 72 12 SUOT}IPUOD 0} JeTNo1puedied MoTJITY WIOLJ PalIp UIOD ‘SUOTIIES Surjooo Z% ‘sutArp 9 ‘ures 1,06 38 ‘VV PI 04786 108% 28 ‘eAoge sv aueg -BINdUId} ‘dINJSIOW Y%FT 03 0%] queiquie $7 .GeZ—,0G% Seiny 006Z—0022 00TZ—SZ8I (LL6T) UOAPTIISOM (8L61) 72 72 ¥MeH OSEZ qyum¢g 00GE (LL6T) 70 72 Jeneg (LLOL) 72 72 Laneg g0anog A1IaVL “Lv CROPS AGRICULTURAL OF STORAGE AND DRYING 374 APPENDIX 375 REFERENCES ANDERSON, R.J. 1972. Commercial concurrent flow heating-counterflow cooling grain dryer: the Anderson Model. Am. Soc. Agric. Eng. Pap. 72-846. ASAE. 1978. Agricultural Engineers Yearbook. Am. Soc. Agric. Eng., St. Joseph, Mich. BAKKER-ARKEMA, F.W., BROOK, R.C. and LEREW, L.E. 1978. Cereal grain drying. In Advances in Cereal Science and Technology, Vol. 2. Y. Pomeranz (Editor). Am. Assoc. Cereal Chemists, St. Paul, Minn. BAKSHI, A.S., SINGH, R.P., WANG, C.Y. and STEFFE, J.F. 1978. costs of a conventional and air recycling cross flow rice dryer. Energy Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1978. Pap. 78-3011. BAUER, W.W., FOSDICK, S., WALKER, L.P. and BAKKER-ARKEMA, F.W. 1977. Testing of a commercial sized conventional crossflow and modified crossflow grain dryers. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1977. Pap. 77-3014. BROOK, R.C. and BAKKER-ARKEMA, F.W. 1977. Design of commercial counterflow grain dryers. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1977. Pap. 77-3017. BROOKER, D.B., MCKENZIE, B.A. and JOHNSON, H.K. status of on-farm grain drying. CDMC. 1956. Grain drying 1978. The present Am. Soc. Agric. Eng. Pap. 78-3007. recommendations. Crop Dryer Manufacturers Council, Farm and Industrial Equipment Institute, Chicago. CLARK, R.G. and LAMOND, W.J. 1968. Drying wheat in two foot beds. J. Agric. Eng. Res. 13 (3) 245. CONVERSE, J.O. 1972. A commercial crossflow-counterflow grain dryer: the HC. Winter Meet. Am. Soc. Agric. Eng., Chicago, 1972. Pap. 72-828. HAWK, A.L., NOYES, R.T., WESTELAKEN, C.M., FOSTER, G.H. and BAKKER-ARKEMA, F.W. 1978. The present status of commercial grain drying. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1978. Pap. 78-3008. MOREY, R.V., CLOUD, H.A. and LUESCHEN, W.E. 1974. Practices for efficient utilization of energy for drying corn. Winter Meet. Am. Soc. Agric. Eng., Chicago, 1974. Pap. 74-3541. MUHLBAUER, W., KUPPINGER, H. and ISAACS, G.W. 1978. Design and operating conditions of single-stage concurrent flow and two-stage concurrentcounterflow grain dryers. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1978. Pap. 78-3008. U.S. DEP. COMMER. 1976. 1972 Census of Manufacturers, Vol. 2. Industry Statistics. U.S. Dep. of Commerce, Washington, D.C. WESTELAKEN, C.M. 1977. Concurrent flow commercial grain dryers. Annu. Meet. Am. Soc. Agric. Eng., Utah State Univ., Logan, 1977. Pap. 77-3016. _ sea Pars —= a. ‘ 2 ~ ¢ a nea ¥ ~~ . /in 20 —— \ i a TS nq ' ' Fo ti 1 ; f } { ‘ 4 a 4 ad ¥f ; ie Ve fs —m" rt of ~ as a . 7 a + — = _ ~ 4 ! ~ = XN Ubi? ‘ ra aw bse pe a? seed ary Lee he i (ont ACY ie TST waee b> ot a i EUS Seer 4 TD — a ake j Alas OD ait AY glee te Se i a eee te. ; , Index Absorption, 7 Acid solutions, 29-32 Behlen, 164 Belt dryer, 160 BET isotherm, 35 Bin arrangement, 158, 225, 228 Bin drying, 225, 239-240, 244-245, See also Batch Blending, 68 Adjustable blade, 101 Adsorption, 6, 18 Aeration, 58-62, 222 Aeration systems, 63—64 Aeroglide, 170 Aerovent, 153 AGA, 160 Air distribution, 187-196, 302 Boerner sampler, 87 316—320, 340, 348-351 Air movement, 39 Air plenum, 322, 341, 353 Air/product flow, 160, 162 Air requirements, 58, 188, 195 Airflow, 59-60, 64, 92-117, 181, 185, 201, 203, 224—226, 241, 265, 271, 340, 351, 362 Airflow measurement, 109-112, 362 AMCA, 100 AOAC, 71 Aqua-Boy, 83 Arrangements of storage, 253 Attic collectors, 301, 304 Auger conveyor, 238 Automatic control, 156 Axial flow fans, 97-99 Brine, 4 Brown-Duvel, 72-73 Building, 281, 299-301 Bulk density, 345 Burrows, 75 Butler, 166, 255 Calcium carbide, 80 Canning, 4 Capacitance, 77—80 Carotene, 259 Carrier Corp., 147 Carryover, 3 CDMA, 159 CDMC, 366 Centrifugal fans, 93 Cfm, 92 Characteristics, drying systems, 240 Characteristics, handling and storage Backward-curved fans, 93—97 Bale density, 271-272 Baled hay, 271-273 Bales, density, 271 large round, 274 Bare plate collector, 302 Barn drying, 265 Batch drying bins, 159, 163-166, 189-193, 225, 240, 243, 246. See also Bin systems, 242—243 Chemical decomposition of water, 6, 79-81 Chemical desiccants, 7 Chemical methods, 79 Chemical treatment, 4 Chilling, 4, 62 Chopped hay, 270 Classification of heated air dryers, 152-156 Classification of storages, 253 377 378 DRYING AND STORAGE OF Collectors, 292 airflow, 297 configuration, 296 covers, 299 heat loss, 297 materials, 298 part of building, 299-301 surfaces, 295—296 Column dryers, 160, 162-171, 220, 239, 241, 245 Combination drying, 225 Combination resistance-dielectric, 74 Combustion, 173-174 Commercial dryers, 1, 151-170 Commercial storage, 251 Concentrating collectors, 293-295 Conditioning, 32, 347-348 Conduction, 55, 329 Constant rate, 120-121, 123-124 Continuous flow dryers, 160, 162, 168-169, 220, 223-224, 240, 247 Controlled atmosphere, 4 Controls, 334, 356-357 Convection, 39-41 Conversion factors, 370 Conveyors, 249 Cooling, 54, 58, 103, 174, 326-334, 344, 730D-—OOlk Cost, 285 Covering, 45 Cowls, 181-183 Cribbing, 182-183, 191-194 Crushing hay, 261—262 Cubes, 276-278 Curing, 346 Cutting forages, 258-259 AGRICULTURAL CROPS Direct methods, 69 Distillation method, 72 Diverter, 87 Dole, 79 Draft gage, 109 Dry bulk, 149 Dryeration, 240-241, 243, 247 Dryers sold, 365 , Drying, advantages, 8 definition, 4—5 Drying constant, 126-131, 140 Drying front, 85, 133-134 Drying in field, 1, 9, 260 Drying rate, 121—131, 133-138, 140-144 Drying rate periods, deep layer, 131 Drying recommendations, 366-367 Drying system, 151, 184, 234, 279-281, 348-356 Drying time, 203, 364 Drying zone, 132-133 Ducts, 61—62, 101-103, 189, 265-270, 316-322, 340, 350—353 Dust, 254 343-— Eaton, 79 Effect of drying temperature, 197 Efficiency, 373—374 Electric resistance, 71, 74—77 Electricity, 59, 64—65, 174 Energy costs, 2 Energy effectiveness, 161, 163 Epic, 83 Equilibrium moisture content, 17—23 Equivalent diameter ducts, 103 Evaporative cooling, 332 Excess air, combustion, Damage, 2, 210, 216 Decreasing rate of drying, 134 Deep layer drying, 132-133 Dehydration, 1, 4, 286 definition, 5 Dehydrators, 286—287 Delmhorst, 76 Density of seed and grain, 369-370 Depth units, 134-135, 137 Desiccants, 74 Deterioration, 44, 56—57 Dew point, 141 DICKEY-John, 74 Dielectric, 77—80 Diffusion, 131 Diffusivity, 50 Digital moisture computer, 82 174 Experimental dryers, 1 Explosions, 254—255 Falling rate, 120-124, 130 Fan characteristics, 95-97, 99 Fan operation, 64, 226 Fan performance, 100, 327 Fan principle, 98 Fan requirements, 188 Fan selection, 187, 323-326, 341, 353 Fans, 93-101 FIEI, 159 Field drying, 1, 9 Field heat, 313, 327, 339, 342, 355 INDEX Filling rate, 241-253, 266-271, 317 Fischer, 81 Fissures, 212—217 Floors, 105 Flow diagrams, 237, 254 Fluid horsepower, 100 Forced air drying, 92, 276 Forced airflow, 184—205 Forward-curved fans, 93, 95—97 Freeze-drying, 6 Friction, 103, 106, 353 Fuel, 155, 172-174, 198 Fuel heating value, 172 Fumigation, Hot-wire anemometer, 110 Hukill’s analysis, 135 Humid heat, 142-143 Humidistat, 172 Humidity control, 335-336, Hygrometric, 81, 84—85 379 345 Importance, 1—8 Indirect methods, moisture measurement, 64 74 Infiltration, 330 Infrared, 176 Insects, 8 Installation, 60 Insurance regulations, 175 Intermittent drying, 125 Ionizing, 5-6 Isopressure, 114 Isotherms, 27 Gamet, 87 Gasoline engine, 101 Germination, 43, 197, 199 Grain changes, 42 Grain depth, 188 Grain probe, 87 Grain quality, 58 Gravity, 249 Janssen, 252 Joy, 101 Habco, 364 Halross, 78 Handling, 2, 175, 220, 234-239, Harkins-Jura, 35 Harvest, 9, 68, 211, 213-215 248, 286 Kernel, rice, failure, 211 Koster, 83 Hay storage, 259, 263 Head loss, 324, 353-354 Heat, 306 Heat gain, 328—330 Heat of respiration, 263, 342—343, Heat pump, 307 Heat transfer, 46, 49, 176 Heat utilization factor, 144-145 Heated air, 151-176, 197, 202 Heated forced air, 45, 92 Heating, 7, 263, 354 355 Heating value fuels, 172-174 Henderson, 17, 121 Higher temperature, 2 History, 1 Holding bin, 249 Honeywell, 159 Horizontal storage, 250-251 Horsepower requirements, 59-60, 100, 188, 195, 305 Lab-line, 83 Large round bales, 274—276 Latent heat, 28-29, 33-34, 167 Latent heat, corn and wheat, 34 Lateral (wall) pressure, 251-252, 311-313, 3838-339, 345 Layer, 126-131, 199 Layout of storages, 253 Least squares, 86, 90 Lennox, 363, 368 Loading hay dryer, 281 Long hay, 265 Long-term storage, 315 Loss, 3, 8-10, 39, 258 Low temperature, 306-307 380 DRYING AND STORAGE Management, 174, 283 Manometer, 107, 109 Manufacturers, 70, 161 Marconi, 74, 76 Mass transfer, 131-132 Mathews, 164 Mechanical damage, rice, 218 Mechanical removal of moisture, 7 Mechanical shear test, 86 Mechanical ventilation, 45, 58 Microorganisms, 7 Milling yields, 209-210 Mixing type dryer, 221 Moisture accumulation, 39-41, 44 prevention of, 45 Moisture content, 10 determination, 68—86 equilibrium, 16 KAO mG a Ai Meko 4 aloo representation, 11, 13 storage, 41—44, OF AGRICULTURAL CROPS Periodic temperature variations, 50 Physical conditions, 284 Picker-sheller, 1 Platform dryer, 273 Portable dryers, 158, 229 Potato storage, 310-337 Power, 59-60, 100, 188, 195, 305 Power, solar drying, 305 Preservation, 4—5 Pressure coefficient, 181 Pressure drop, 362 Pressure loss, 105, 324, 353-354 Pressure system, 104, 196 Pricing, 68 Product flow, 160 Production, 3 Propeller fan, 97, 100 Propionic acid, 5 Psychrometric, 140-141, 143, 146-147 Pulsating fan, 97 210, 221, 259 Moisture control, 3 Moisture meters, 68—86 Moisture migration, 39 Mold growth, hay, 262—264 Moridge, 165 Motomco, 78 Moving grain, 45, 54 Multipass drying, rice, 222 Quality, 2, 58 Radiation, 5 Rankine, 251 Rapid drying, rice, 216 Rate periods, 120-123 Natural drying, hay, 260 Natural ventilation, 45, 180-184 Reconditioning, Nuclear, moisture determination, Nutritional losses, 9, 11 Oil burner, 152-154, 368 Oil requirements, 368 Onion storage, 344-347 286, 315, 357 Recycling, 2 Newton equation, 126 Night air cooling, 331 Refrigerated air, 62, 64, 333 85 Regression, 86 Relative humidity, moisture tester, 84—85 Resistance, moisture tester, 74—77, 353 Respiration, 7, 261—263, 325, 355 Reynolds number, 102 Rodents, 7 Roof and wall collectors, 300—304 R-value, 329 Organic acids, 5 Orifice plate, 110—112, 363 Oven methods, 69-72 Overall heat transfer, 176 Overdrying, 68, 174 Safety, burner, 156-158, Salt moisture tester, 84 175 Sample, 71 Sampling, 86—87 Saturated salt solution, 23, 26-27, 32 Palletized bin, 349 Pellets, 276-278 Saturation vapor pressure, 28 Screw conveyor capacity, 238 Sealed bins, 204 INDEX Seed loss, 9 Seed storage, 315 Seedburo, 73, 87 Semilogarithmic plot, 126-127 Shafer, 76 Shrinkage, 203-204 Smith equation, 35 Soft corn, 184 Solar, 227, 291—307 Solar collector, 292—304 Solar constant, 293 Specific gravity, 369-370 Specific heat, 47 Sperry-New Holland, 274—276 Spontaneous combustion, 263—264 Spreaders, 228, 249 Sprouting, 1 Standard error, 69, 74, 86, 89 Static pressure, 58-59, 92, 108, 113, 323 Static pressure measurement, 107 381 Temperature of drying, 2, 158, 197-200, 305, 363-364 Temperature scales, conversion, 374 Tempering, 222 Theory, 120-145 Thermal conductivity, 46, 48, 55 Thermal efficiency, 139, 163, 167-168 Thermal expansion, 44 Thermostat, 172 Thin layer drying, 126-131, 199, 241 Time of response, 129 Toluene, 72 Transient heat transfer, 49, 51 Transportation, 249 Traverse time, 115 Trier, 87 Tubeaxial fan, 98 Two-level ducts, 268 Types of storage, 250 Stationary dryers, 158 Steinlite, 77, 80 Stirring grain, 45, 227-229, 245 Storage, 4, 9, 39, 196, 220—221, 234-237, 250-253, 310-348 Storage arrangement, 253 Storage capacity, 319 Storage center, 254 Storage criteria, 310, 337-338, 345 Storage dimensions, 319 Storage environment, 313-316, 339, 345— 348 Storage losses, 39 Storage temperature, 314, 340, 354 Stormor, 163 Straight-bladed fan, 96 Stress cracks, 209, 213 Suberization, 314 Suction systems, 104, 196 Sugar beet storage, 337-344 Underwriters Laboratory, 160 Universal, 75-76 Unsteady-state heating, 52—53, 56 Vacuum oven, 71 Valving, 116 Vane anemometer, 109-110 Vaneaxial fan, 99 Vapor barrier, 336 Vapor pressure, 18, 28, 123, 125, 131 Vaporization, 7 Vegetable crops, 310 Sukup, 154, 245-246 Supplemental heaters, 169, 171-172, 226— 227, 282 Ventilation cowls, 181-183 Ventilation ducts, 316—322 Ventilation systems, 316 Vertical storage, 250 Suspended plate collectors, 295 Systems, 2, 209, 218, 234, 240, 252, 316 Tag-Heppenstall, 74 Temperature, storage, 41—44, 50, 52, 314— 316, 323, 328, 348 Temperature changes, 43-44, 50, 54, 305, 363 Temperature control, 341, 354—357 Wafers, 276-278 Wagon dryer, 273 Wall pressures, 251-252, 311—313, 339, 345 Water, air humidification, 336 Wet bulb, 141 Wet grains, 2 Windrowing, 260 338— Other AVI Books AGRICULTURAL ENERGETICS Fluck and Baird DRYING CEREAL GRAINS Brooker, Bakker-Arkema, Hall ENCYCLOPEDIA OF FOOD ENGINEERING Hall, Farrall, Rippen ENCYCLOPEDIA OF FOOD TECHNOLOGY Vol. 2 Johnson and Peterson FOOD DEHYDRATION Vol. 1 and 2 2nd Edition Van Arsdel, Copley, Morgan FOOD ENGINEERING SYSTEMS Vol. 1 and 2. Farrall FOOD PROCESSING WASTE MANAGEMENT Green and Kramer FUNDAMENTALS OF ELECTRICITY FOR AGRICULTURE Gustafson FUNDAMENTALS OF FOOD ENGINEERING, : 3rd Edition Charm : GRAIN STORAGE: PART OF A Sinha and Muir HANDLING, TRANSPORTATION VEGETABLES SYSTEM AND STORAGE OF FRUITS AND Vol. 1 2nd Edition Ryall and Lipton Vol. 2 Ryall and Pentzer AN INTRODUCTION TO Roth, Crow, Mahoney NUTRITIONAL 2nd Edition EVALUATION OF Harris and Karmas POSTHARVEST BIOLOGY Haard and Salunkhe POTATOES: PRODUCTION, 2nd Edition Smith PRINCIPLES 3rd Edition PROCESSING 2nd Edition AGRICULTURAL AND FOOD ENGINEERING PROCESSING HANDLING STORING, OF FRUITS AND PROCESSING OF FARM MACHINERY Kepner, Bainer, Barger EQUIPMENT FOR Hall and Davis AGRICULTURAL SOYBEANS: CHEMISTRY AND TECHNOLOGY Vol. 1 Revised Edition Smith and Circle THE TECHNOLOGY OF FOOD PRESERVATION 4th Edition Desrosier and Desrosier PRODUCTS VEGETABLES

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