! !
" # $ % # &$ %' % !"## ! " # $ !% ! & $ ' ' ($ ' # % % ) % * %
' $ ' + " % & ' !# $,
( $ - . ! "- ( % . % % % % $ $ $ - - - -
// $$$ 0 1"1"#23." 4& )*5/ +)
678%99:::& %
2 ; ) -
* &
< "-+ % -= -+ %
/- >7?:94& )*5/ +)
"3 " & 7?:9
AKNOWLEDGEMENT
All thanks and glory be to Allah (SWT) for all his blessings that he showered on us
from the first day of our lives up to this moment and I pray that may his blessings be
with us throughout our lives in this world and here after.
I grateful to my supervisor Dr.D.B. Yahaya for sparing his time and effort to go through
my work, I wish to also thank the H.O.D and the entire members of staff of the
department of Mechanical Engineering, Bayero University Kano for their kindness and
assistance throughout my study. I also wish to thank Mall. Mustapha Usaini, General
Manager, Pioneer Technical services Ltd Kaduna for assisting me tremendously in the
construction part of this research. Finally I wish to everybody who in one way or the
other assisted me in carrying out this work.
/s
DEDICATION
This work is dedicated to my beloved father; Alh. Abubakar Garba, Mother; hajiya
Aishatu Abubakar, wife: Amina Jamila Usman and my daughters Maryam (Ummi) and
Aisha (Siddiqa)
s
TABLE OF CONTENTS
Contents
Page No.
Acknowledgement----------------------------------------------------------------------- IV
Dedication--------------------------------------------------------------------------------- V
Table of contents------------------------------------------------------------------------- VI
List of figures ----------------------------------------------------------------------------- IX
List of tables------------------------------------------------------------------------------ X
Abstract-----------------------------------------------------------------------------------
XI
CHAPTER ONE: INTRODUCTION
1.0
Background of the Study------------------------------------------------------------
1
1.1
Statement of the Problem------------------------------------------------------------- 1
1.2
Aim of the Study----------------------------------------------------------------------
1.3
Objectives of the Study--------------------------------------------------------------- 2
1.4
Significance of the Study-------------------------------------------------------------- 2
1.5
Scope of the Study--------------------------------------------------------------------- 3
2
CHAPTER TWO: LITERATURE REVIEW
2.1
Economic Importance of Onions---------------------------------------------------- 4
2.2
Temperature Effects and Metabolic Activity of Onions during Storage------ 5
2.3
Onion – Bulbs Size and Shape------------------------------------------------------- 6
2.4
Methods Employed In the Storage of Onions-------------------------------------
7
2.4.1 High Temperature Storage-----------------------------------------------------------
7
2.4.2 Refrigeration/ Low Temperature Storage------------------------------------------
7
2.5
9
Refrigeration of Fruits and Vegetable---------------------------------------------s/
2.5.1 Water Loss of Stored Product------------------------------------------------------ 13
2.5.2 Heat Transfer of Root Product----------------------------------------------------- 16
2.6
Termination of Processing Time and Heat Load during Refrigeration
of Foods------------------------------------------------------------------------------- 20
2.7
Thermal Properties of Foods------------------------------------------------------- 25
2.8
Refrigeration and Refrigeration Cycles------------------------------------------- 27
2.8.1 Refrigerator Efficiency-------------------------------------------------------------- 28
2.8.2 The Ideal Vapor Compression Refrigeration Cycle----------------------------- 29
2.8.3 Actual Vapour Compression Refrigeration Cycles------------------------------ 31
2.9
Refrigerants--------------------------------------------------------------------------- 34
2.9.1 Selection of the right refrigerant --------------------------------------------------- 35
CHAPTER THREE
3.1
Design Procedure--------------------------------------------------------------------- 37
3.2
Calculation of Heat Load------------------------------------------------------------ 37
3.3
Determination of the Refrigeration Capacity Required-------------------------- 49
3.4
Selection and Sizing of Refrigerant and Refrigeration Equipment------------- 49
3.4.1 Compressor Power Requirement---------------------------------------------------- 52
3.4.2 Sizing and Selection of Condenser------------------------------------------------- 53
3.4.2 Sizing and Selection of Evaporator------------------------------------------------- 55
3.4.4 Refrigerant Piping--------------------------------------------------------------------- 56
3.4.5 Selection of Expansion Valve-------------------------------------------------------- 56
3.5
Construction, Testing and Cost Analysis------------------------------------------- 56
3.5.1 Physical Description of the Storage System---------------------------------------- 56
3.5.2 Components of the System------------------------------------------------------------ 57
3.6
Testing of the Storage System--------------------------------------------------------- 58
3.7
Cost Analysis --------------------------------------------------------------------------- 66
s//
CHAPTER FOUR: DATA PRESENTATION AND ANALYSIS
4.0
Results from Testing Of Storage System-------------------------------------------- 68
4.1
Data Analysis and Inferences--------------------------------------------------------- 70
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION
5.0
Conclusion------------------------------------------------------------------------------ 75
5.1
Recommendation----------------------------------------------------------------------- 75
5.2
Suggestions for Further Studies------------------------------------------------------- 75
Plate 1: Frond view of the constructed storage system---------------------------- 76
Plate II: Side view photograph of the constructed storage system--------------- 77
References------------------------------------------------------------------------------- 78
APPENDICES
APPENDIX I: Wet and dry thermometer chart------------------------------------- 82
APPENDIX II: Heat Leakages------------------------------------------------------- 84
Appendix III: Properties of Common Refrigerants-------------------------------- 85
s///
LIST OF FIGURES
Figures
Page Number
Fig. 2.1:
Shapes of Onion Bulb---------------------------------------------------------- 6
Fig 2.2:
Schematic of a typical vapor Recompression or Mechanical
Refrigerator System------------------------------------------------------------ 10
Fig. 2.3:
General system for unsteady heat conduction------------------------------ 21
Fig 2.4:
Newtonian heating or cooling------------------------------------------------- 21
Fig 2.5:
Transient temperature response----------------------------------------------- 23
Fig 2.6:
Equivalent thermal circuit for lumped capacitance solid------------------ 23
Fig. 2.7:
A Refrigerator------------------------------------------------------------------- 28
Fig. 2.8:
Schematic Diagram; Ideal Vapor Compressor Refrigerator Cycle------- 28
Fig. 2.9:
T-S Diagram for the Actual Vapor Compression Refrigeration Cycle-- 32
Fig. 3.1a:
Pictorial View of the Storage System (closed) ----------------------------- 60
Fig. 3.1b:
Pictorial View of the Storage System (opened) ---------------------------- 61
Fig. 3.2(a&b): Sectional view of the storage system showing onion in storage and
chemical containers respectively. -------------------------------------------- 62
Fig. 3.3:
Sectional view of the storage system showing evaporator---------------- 63
Fig. 3.4a:
Back view of the storage system---------------------------------------------- 64
Fig. 3.4b:
Air cooled condenser and compressor arrangement------------------------ 64
Fig. 3.5:
Automatic expansion valve---------------------------------------------------- 65
Fig. 3.6:
Compressor---------------------------------------------------------------------- 65
Fig. 3.7:
Air – cooled condenser--------------------------------------------------------- 65
Fig. 3.8:
Evaporator----------------------------------------------------------------------- 65
Fig. 4.0:
Respiration rate of onions under storage and in normal air--------------- 72
Fig. 4.1:
Weight loss measurements for onions in storage and in control---------- 73
Fig. 4.2:
Average relative humidity recorded----------------------------------------- 74
/y
LIST OF TABLES
Tables
Table 2.1:
Respiration Rates (Co2 Production) of Onions in mg.kg-1h1--------------- 5
Table 2.2:
Recommendation Refrigerated Storage Condition for Onion Bulbs------ 8
Table 2.3:
Heat of Respiration of Some Fresh Fruits and Vegetables at various
Temperatures. ------------------------------------------------------------------ 12
Table 2.4:
Coefficients for CO2 production by Commodities [17] ---------------------- 13
Table 2.5:
Thermal Conductivity of Some Root Crop Products [18] ---------------- 19
Table 3.1:
Frequency Table for Diameters of Onion Bulbs---------------------------- 38
Table 3.2:
Summary of Cost Analysis---------------------------------------------------- 66
Table 4.0:
Summary of readings obtained during testing of the storage System---- 69
y
ABSTRACT
This work is a study of a refrigerated and controlled atmosphere onion storage system.
In the process of this study a system was constructed for refrigerated onion storage. The
system constructed uses a vapour – compression refrigeration unit, to reduce and
maintain the temperature inside the system at 1oC and 87% relative humidity. Soda lime
and silica gel were used to control moisture and carbon dioxide production rate of
onions in storage. Refrigerated unit components were sized and selected based on heat
load calculated. The system was tested for a period of four months with 10kg of onion
in storage and exact quantity as control. Results obtained shows that carbon dioxide
production rate is significantly lower than normal respiration rate of onions exposed to
open air, while weight loss of onions in storage was reduce with only 0.3% weight loss
after four months, as compared to 1.67% weight loss for onions in open air. It was
concluded that combination of refrigeration and controlled atmosphere is a very
efficient method of onion storage.
y/
CHAPTER ONE
1.0
Introduction
Onion (Alliums cepa L) are highly valued World Wide for their flavor
and for their Nutritional value in supplying minor constituents such as
minerals and trace elements. The bulbs are boiled and used in soups and stews,
fried, or eaten raw. They are also preserved in the form of pickles. Onion
leaves, especially from the spring onions are also used in salads and soup.
Onions are a major crop in the tropics which account for nearly 30% of total
global production (1). Nigeria being a tropical country also contributes to this
account. According to Natural Resources Institute (NRI) bulleting of 1997,
Nigeria which is located on latitude 4 – 140N produced the (Kano red), (Kano
White), (Gindin Tasa), (Wuyan Bajimi) and (Wuyan Mokorowa), varieties of
onions.
Out of these varieties the Kano Red and the Kano White are widely
produced in Kano State of Nigeria. Despite the economic viability of onion
of the local farmers and people who seal in the wholesale and retail of the
crop are faced with high losses of their crop due to poor storage (1). This is
Abecause onion is a perishable crop and thus its storage longer than the
maximum period of 23 to 30 days (refer to appendix IV) is difficult. Although
various storage techniques are employed by the local people traditionally, this
project work was carried out in order to develop and construct a system that
can be used to store onions for a period longer than the period after which it
will perish.
1.1
Statement of the Problem
Onion is a Biennial crop and it shows a distinctly marked dormancy
between the vegetative and the generative growth period. Therefore onion
bulbs are natural storage organs well for long term crop storage (2). The
problems in onion storage is mainly due to climatic conditions (especially
ϭ
temperature) and how to prevent water loss, exclude pathogens i.e. germs that
can cause diseases, prevent sprouting i.e. germination during storage, bulb
softening, shriveling and weight loss.
Therefore a system where the atmosphere or the climatic conditions
and the problems above can be controlled will provide an effective storage of
onion for a long period. Since according to NRI Bulletin (1978) the Kano Red
onion specie is being produced at a rate of 20 to 30 tones per hectare and can
only be stored for 23 day to 1 month, this will constitute a great loss if the
onions cannot be stored beyond the period.
1.2
Aim of the Study
The aim of the study was to develop an onion storage system in which
the temperature and relative humidity within the storage area is 10C and 87%
respectively, and to slow rate of respiration of onions during storage.
1.3
Objectives of the Study
The objectives of this study are:
i.
To develop a functional refrigeration system for onion storage.
ii.
To test and determine the efficiency of the storage system.
iii.
To determine and select size of refrigeration equipment for onion
storage system.
1.4
Significance of the Study
For the Local farmers and traders of onion, the construction of a storage
system that is cheap and accessible to them will be very useful. This will
enhance and improve their economic status. In the cause of the study new
knowledge encountered will help in reducing loss of onions during storage.
Thus the study is highly significant.
Ϯ
1.5
Scope of the Study
There are different varieties of onions grown all over the world and
each type of specie has different physical appearance. Some long, some oval
others round etc. also their storage periods (refer to fig. 2.1 and appendix IV).
That is the period over which they can be stored without any defects on them.
Due to this the Kano red species of onions was selected for this project work.
Therefore information and data collected during testing of the storage system
for a period beyond reported maximum storage life of 23 days to 1 month was
done using Kano red onion specie.
ϯ
CHAPTER TWO
LITERATURE REVIEW
2.1
Economic Importance of Onions
Edible onions are important vegetables World Wide. Pre – eminent
among them in terms of volume grown and traded is the common onion grown
for bulbs. In terms of global weight of vegetables produced, at nearly 28
million tons per annum, only tomatoes and cabbages exceed bulb onions in
importance (2). International trade is estimated at about 2 million tons
annually, worth about 400 million, U.S. dollars equivalent to about 5.6 billion
Naira (2).
Onions are a major crop in the tropics, which account for nearly 30%
of total global production (1), estimated loss of total crop in tropical countries
is high and can reach 20% to 95% (3). Losses between Wholesale and retail
of over 9% have been reported for spring onion (4). Although some tropical
countries are not importers, export potential of onions is developing in several
tropical regions partly because if dried and parked properly the bulbs can be
transported for considerable distances without deteriorating. Storage for
several months also is possible if suitable bulb temperature can be maintained.
Proper storage environment is critical to minimize bulb softening;
shriveling, weight loss and development of storage rot decay. Different
cultivars have variable storage life. In general poor – keeping cultivars are
less pungent (5) and have a low dry matter content (6), a low refractive index
(7,8), and high relative rate of water loss and total water loss, especially in
period immediately following harvest. Poor storing cultivars also are more
susceptible to storage rots. Sprout more readily (9) and benefit more from
“curing”.
ϰ
2.2
Temperature effects and metabolic activity of Onions during Storage
During dormancy at lower and higher temperatures sprouting is
depressed. The rate of elongation of sprouts within the bulb and the rate of
leaf initiation were much faster at 15oC than 0 or 30oC (2). Therefore, sprout
development in onion bulbs unlike most physiological processes, does not
increase in rate progressively as temperature increases (2). Once sprouting
has occurred in rooted bulbs like onion, sprout growth rate increases
progressively with temperature (2). Respiration rates of produce indicate the
degree of metabolic activity and provide useful insights in the design of
storage systems for environmental and atmospheric control. The rate of post
harvest deterioration (Spoilage) of produce generally is proportional to the
rate of respiration (10). Bulb onions have a low respiration rate (3 – 4mg Co2.
Kg-1. h-1 at 5 0C) (10) and this increases with corresponding increase in
temperature. Green onions have higher respiration rates, comparable to leafy
vegetables; at the same temperature (refer to table 2.1 below) a low oxygen
level in the storage chamber halves the rates respiration (10).
Table 2.1: Respiration Rates (Co 2 Production) of Onions (in mg.kg -1h1) (3)
Temperature
Dry Onions
Green Onions
0 oC
4 - 5 oC
10oC
15 - 16 oC 20 - 21oC
3
3–4
7 -8
10 – 11
14 - 198
10 - 32
17 – 39
36 – 62
66 – 115
79 – 178
In storage as time progresses, the rate of respiration increases. If bulbs
are wounded, their rate of respiration increases and reaches a maximum after
about 12hours (12). The higher respiration level is measurable over the whole
storage period (12), if the dry outer skins of onions are removed, the
respiration rate of bulbs increases nearly two fold and the rate of water loss
also increases. Bulbs with the skin removed also sprout more rapidly than
those with intact skins (2).
ϱ
2.3
Onion – Bulbs Size and Shape
Bulb size and shape are important attributes both for marketing and
retention of post harvest quality. Onion bulb shapes can be described by
various standard geometrics (refer to fig. 2.1 below). Bulb size affects both
sprouting and water loss during storage. Studies have shown that during
storage at 110C large bulbs sprouted at a faster rate than small ones, but small
onions lose weight more rapidly (12,13).
1. Flat
5. High Globe
2. Thick flat
6. Spindle
3. Flattened globe
7. Cylinder
4. Globe
8. Flat top
Fig. 2.1: Shapes of Onion Bulb
ϲ
9. High top
2.4
Methods Employed In the Storage of Onions
There are various methods used in storage of onions. These methods
are numerous, for the purpose of this project work only two methods, whose
information are relevant to this work are discussed here. These are:
2.4.1 High Temperature Storage
Onions may be stored at high temperatures of over 25°C at a range of
relative humidity's (75% to 80%) that are sufficient to minimize water loss
[1]. Storage at temperatures of 25 to 30°C has been shown to reduce sprouting
and root growth compared to cold storage at 10 to 20°C, however, weight
loss, desiccation of bulbs, and rots occurred at high levels of temperature
making the system uneconomic for long periods [1, 14]. High-temperature
storage of onions can be achieved under both ambient and heated storage
conditions. However ventilation must be applied carefully inside the store to
achieve the required temperature and humidity levels.
2.4.2 Refrigeration/ Low Temperature Storage
Ranges of temperatures and relative humidity have been recommended
for storage of onions (refer to table 2.2) below. Most storage facilities use
mechanical refrigeration to control storage temperature. This system utilizes
the fact that a liquid absorbs heat as it changes to a gas. The most common
mechanical refrigeration systems use a refrigerant such as ammonia or a
variety of halocarbons i.e. chlorofluorocarbons (CFC's) fluids (sometimes
referred to by the trade name 'Freon') whose vapor can be recaptured easily
by a compressor and heat exchanger. Fig 2.2 below shows the components of
a typical vapor- recompression (or mechanical) refrigeration system.
ϳ
Table 2.2: Recommendation Refrigerated Storage Condition for Onion
Bulbs [11]
Temperature
Relative Humidity (%)
Length of Storage (Month/ Days)
-3-0
70-75
6 mo
-3
85-90
5-7mo
-2
75-85
300 d
-3 - (0.6)
75-80
6 mo
-1-0
70-80
6 - 8 mo
-0.6
78-81
6-7mo
0
75-85
6 mo
0
65-75
-
0
70-75
20 - 24 wk
0
70-75
-
0
65-70
1 -2 mo
0
65-70
6-8 mo
0
-
230 d
0
70 - 75 or 90 - 95
Up to 120 d
0
80-85
30 -35 wk
1-2
80-85
30-35wk
1
87
-
1.1
70 – 75
16-20wk
4
-
170 d
8
-
120 d
12
-
About 90 d
20
-
25 d
(°C)
The refrigerant fluid passes through the expansion valves, where the
pressure drops and the liquid evaporates at temperatures low enough to be
ϴ
effective in removing heat from the storage area. Heat from the material to be
cooled is transferred to the room air, which is then forced past the evaporator
(cooling coil located in the room). This is usually a finned tube heat
exchanger, which transfer the heat from the air to the refrigerant causing it to
evaporate. After fully changing to a gas, it is pressurized by the compressor
and then passes through a condenser, where it is cooled to a liquid. The
condenser is located outside the storage area and releases hot liquid which is
stored in the receiver and is metered out as needed for cooling.
Onion bulbs freeze below - 3°C, green onions store best at about 0°C
and very high humidity greater than 95%. Maximum length of storage at these
conditions varies from just a few days to about 3 weeks [15]. Low temperature
storage of onions can be achieved under both ambient and refrigerated storage
conditions; however, ventilation must be applied carefully inside the store to
achieve the required temperature and humidity levels without inducing
condensation of water on the surface.
2.5
Refrigeration of Fruits and Vegetable
Fruits and vegetables are frequently cooled to preserve pre-harvest
freshness, flavor and to extend storage and shelf life. Fruits and vegetables
are mostly water, and thus their properties are close in value to those of water
[16]. Initially, all of the heat removed from the product comes from the
exterior of the products, causing a large temperature gradient within the
product during fast cooling. But the mass average temperature of the product
at a given time is used in calculations for simplicity.
The heat removed from the products accounts for the majority of the
refrigeration load and is determined from:
Qproduct= mcp(Tinitial – Tfinal) /'t
(w).........................................(2.1)
ϵ
Fig 2.2: Schematic of a Typical vapour Recompression or Mechanical
Refrigerator System (11)
ϭϬ
Where
Qproduct = Average rate of heat removal from the fruits and vegetables.
m
= The total mass of the fruits and vegetables
m
= The average specific heat produce.
Tinital = mass average temperature before cooling.
Tfinal = Mass average temperature after cooling.
't
= Cooling time.
Fresh fruits and vegetables are live products and they continue to
respire for days and even weeks after harvesting at varying rates [16]. During
respiration a sugar like glucose combines with oxygen to produce carbon IV
oxide and water. Heat of respiration is released during this exothermic
reaction, which adds to the refrigeration load during cooling of fruits and
vegetables. A simplified representation of the process is given as:
C6H12O6+6O2 o 6C020 + 6H20 +Heat of respiration
The rate of respiration varies strongly with temperature, an increase in
temperature results in increased reaction rates but not all reactions have the
same rate of change in temperature [16]. The change in respiration rate due to
a change in temperature represents the overall effect of temperature on the
different chemical reaction of the respiration process. Keeping the product
quality throughout a storage period as near as possible to the quality at harvest
time requires that the metabolic processes be slowed down as much as
possible. This is best achieved by storage at low temperature, provided that
no other adverse effects such as cell-membrane damage occur at low
temperature.
Refrigeration load due to respiration is determined from [16]:
Qrespiration = 6mqrespiration
(w).......................................(2.2)
This is the sum of the mass times the heat of respiration for all the food
products stored in the refrigerated space. The heat of respiration for some
fruits and vegetables was given as shown in the table below.
ϭϭ
Table 2.3: Heat of Respiration of Some Fresh Fruits and Vegetables at
various Temperatures. [16]
Product
Heat of Respiration mW/kg
5°C
20°C
Apple
13-36
44-167
Straw berries
48-98
303-581
Broccoli
102-475
825-1011
Cabbage
22-87
121-437
Carrots
20-58
64-117
Cherries
28-42
83-95
Lettuce
39-87
169-29
Watermelon
*
51-74
Mushrooms
211
782 - 939
10-20
50
Onions
In another related publication based on a data from U.S. department of
Agriculture, the carbon dioxide production of products can be expressed as a
function of temperature, by a least-square regression fit, of the form [17]:
݉௖௢మ ൌ ݂ ൬
ͻܶ௠
൅ ͵ʹ൰ ௫ ሺ݉݃݇݃ ିଵ݄ିଵሻ ǥ ǥ ǥ ǥ ǥ ǥ ǥ ǥ ሺʹǤ͵ሻ
ͷ
Where
݉௖௢మ =
Is the carbon dioxide production per unit mass of a product
ሺ݉݃݇݃ ିଵ݄ ିଵሻ
Tm = the mass average temperature (°C)
f and x are respiration coefficients given in table 2.4 below.
ϭϮ
Table 2.4: Coefficients for carbon dioxide production by Commodities [17]
Skin
Mass Transfer Coefficient
Respiration coefficients
2
Ks (g/m .s.mpa)
Low
Mean
High
VPL
Carrots
31.8
156.0
361.0
0.99
Onions
-
0.888
-
0.98
Potatoes
-
0.635
-
0.98
0.01709
1.769
9.09
33.6
87.3
0.96
8.591x10-3
1.888
Sugar beets
f
X
0.05002
1.793
3.668X10 -4 2.538
The rate of heat generation by respiration qresp of equation (2.2) is given
by [17]
qresp = 10.7 ݉௖௢మ (Jkg-lh-l) .................................................(2.4)
It should be noted that even at low temperatures the product is still
characterized by a low metabolic activity and subsequently heat is still
generated. This internal heat generation in the product causes the product
temperatures to be slightly higher than the storage -room air temperature. This
may not have a large effect on the respiration activity, but it can greatly affect
the vapour-pressure difference between the product and the surrounding air,
thus influencing the moisture balance of the product.
2.5.1 Water Loss of Stored Product
Apart from respiration, another process that the temperature of the
product influences is transpiration. Transpiration of root crops is a mass
transfer process in which water is lost from the product. It involves the
transport of moisture through the outer layers of the product, the evaporation
of the moisture from the product surface, and the convective mass transport
of the moisture to the surroundings. Moisture loss affects the product quality
causing changes in appearance (the surface starts to shrivel), texture and
flavor. In addition moisture loss also reduces the mass salable product. The
ϭϯ
driving force for transpiration is the vapor-pressure difference between the
surface of the product and the surrounding air. The basic mathematical model
to describe transpiration is given by [18]:
mw = kt (ps - pa) (g/kgday)................................................(2.5)
Where:
kt
= The transpiration coefficient.
mv
= Moisture loss, per unit product Surface.
ps
= Vapor pressure at the product Surface, and is given by [18] :
ళǤఱ೅
ps = ͳͲቀଶǤ଻଼ହ଻ାమయళǤయశ೅ቁ .....................................................(2.6)
Where
T
= Temperature at which product is stored
Pa
= Vapor pressure in the surrounding air, and is given by [18]
Pa = Pa
ோு
……….....................................................(2.7)
ଵ଴଴
Where RH = Relative humidity at which product is stored.
To take into account the skin resistance of the product and the effect of
airflow rate the value of kt is evaluated from [18] :
ଵ
௄೟
ൌ
ଵ
௄ೌ
൅
ଵ
௄ೞ
...............................................................(2.8)
ka
= Convective mass transfer coefficient
ks
= The skin mass transfer coefficient, which Accounts for the diffusion
resistance of the skin towards moisture migration.
In model calculations it is convenient to use values of ks that have been
obtained experimentally [18] (refer to Table2.4). Values for the convective
mass transfer coefficient at the surface are calculated from SherwoodReynolds -Schmidt correlation given as.
Sh = ʹǤͲ ൅ ͲǤͷͷʹ ܴ݁ ଴Ǥହ ܵܿ 033 ............................................. (2.9)
Where
Re = is the Reynolds number
Sc= Schmidt number
Sh = Sherwood number
ϭϰ
So that Ka can be determined from
Sh = k a +
ௗ
.....................................................(2.10)
ఋ
Where
G = diffusion coefficient of water in air
The diffusion coefficient is strictly a function of pressure and
temperature and that for water vapor in air Schirmer's equation can be used to
determine G [19] Using the equation
ଵ
்
௉
ଶ଻ଷ
G = ʹǤʹ͸ܺͳͲିଷ ቀ
ଵǤ଼ଵ
ቁ
(m/s2) ...................................(2.11)
Where
T = water temperature in Kelvin
p =ambient pressure in bar
d = average diameter of the product.
In storage conditions in which natural convection is significant, the
airflow rate is caused by internal heat generation and by evaporative cooling
resulting from transpiration. A higher fraction of surface permeable to water
results in a higher rate of total moisture loss and thus in lower temperature
differences and lower air velocities under steady state conditions [18]. This
results in a lower temperature, while the total rate of moisture loss appears to
be only slightly affected.
Diso [19], on convective mass transfer, stated that consider the case of
a plane water surface at a temperature Tw with corresponding properties of
water concentration Cw, and saturation pressure Psat, and if air flows over the
water with concentration C0, partial vapor pressure Pvo and initial
Temperature T0 there will be equilibrium at water level. Due to equilibrium
water concentration Cw corresponds to the saturation concentration at Tw.
Therefore, because of the temperature difference, there is going to be
"Sensible heat transfer" and due to the difference in partial pressures there
will also be "mass transfer" which will be companied by "Latent heat
transfer", and that the different transfers can be characterized as follows:
ϭϱ
(i)
Sensible heat transfer qs is expressed as
dq s=h(Tw-Ts)ds (W)...........................................(2.12)
Where
Ta - Ambient air temperature
Tw = Water temperature.
(°C)
(°C)
qs - Sensible heat transfer
(W)
h = Sensible heat transfer coefficient (W/m2 °C)
S = Area of the water surface (m2)
(ii)
Mass transfer m is expressed as:
dm = hcc(Cw -C0)ds (Kg/s).............................................(2.13)
hcc = Concentration mass transfer coefficient in m/s
Cw and C0 = Respective water and air concentrations expressed in
kg/m3
The rate of latent heat transfer qL is expressed as:
(iii)
dq L = dmLv(Tw)
(W)
...........................................(2.14)
Where
qL = Latent heat transfer. (W)
Lv = latent heat of vaporization at the water temperature. (J/kg)
m = The rate of evaporation (kg/s)
2.5.2 Heat Transfer of Root Product
It has been mentioned that the product temperature influences
processes of respiration and transpiration. As a consequence, the temperature
history of the product affects the quality of the product after storage. One of
the most important steps after harvest is the removal of heat stored in the
product. Indeed, at harvest time the product is at a temperature at which the
respiration rate is high and the quality degrades rapidly. Heat transfer inside
the product can be described by the Fourier equation, a partial differential
ϭϲ
equation involving the change in heat content as a function of temperature
and heat generation [18].
ܲ௣ ‫ܥ‬௣
ఋ்ು
ఋ௧
ൌ ‫׏‬൫݇௣ ‫ܶ׏‬௣ ൯ܹ௥௘௦௣ ……………………. (2.15)
With Tp (x, y, z, t) = Tp,ini (x, y, z) at t = t0
Where
pp = The product density (kg m-3)
c = The specific heat capacity of the product (Kg kg-loC-l)
kp = The thermal conductivity of the product (Wm-1oC-1)
Tp = The product temperature (°C)
Tp.ini = The initial temperature of the product (°C)
t=
time (s)
t0 =
Initial time (s)
wresp= The volumetric respiration rate (Wm-3)
x, y, and z = Cartesian co-ordinates (m)
Also
௉೛௤ೝ೐ೞ೛
Wresp =
ଷ଺଴଴
………………………………………. (2.16)
Where
qresp = specific respiration rate (Jh-1kg-1)
In order to obtain the temperature distribution inside the product as a
function of time, the boundary condition describing the energy transfer should
be specified. The different terms in the boundary condition can be:
(i)
Heat transfer by radiation absorbed or released to the surroundings.
This is expressed as
qrad = HV(Ts -Tw)4 ............................................. (2.17)
Where
qrad
= Heat transfer by radiation (J)
H
= The emissivity of the product.
V
= Stefan Boltzman constant (Wm-2 C-4)
Ts
= the surface temperature (°C)
ϭϳ
Tw
(ii)
= the surrounding temperature (°C)
Heat Lost or Gained by Convection
This is expressed as:
qconv = h(Ts-Ta).................................................(2.18)
Where
qconv = heat transfer by convection (J)
h = surface heat transfer coefficient (WM-2.0 C-1)
Ts= surface temperature of the product (°C)
Ta = the ambient (air) temperature (°C)
(iii)
Heat lost or gained as latent heat of evaporation or condensation
This is expressed as
qevap = mw(CmTs - CvTa - hfg) ................................. (2.19)
Where qevap = heat gained or lost as latent heat of evaporation or
condensation.
mw = Moisture loss per unit surface (kgs-1 m-2)
hfg = Latent heat of Vaporization of water (JKg-1)
Cm =Specific heat capacity of the moisture in the product (JKg-1 0C-1).
Ts = The surface temperature (°C)
Cv = Specific heat capacity of the vapour in the air (J Kg-1 oC-1).
Ta = The Ambient air temperature. (°C)
(iv)
Heat lost or gained by conduction to other units or to the side walls
of the container or of the storage room
This is expressed as
‫ݍ‬௖௢௡ௗ ൌ െ݇௖௢௡ௗ
ߜܶ௖௢௡ௗ
ߜ݊
Where
‫ݍ‬௖௢௡ௗ ൌHeat lost or gained by conduction (J)
݇௖௢௡ௗ = Wall thermal conductivity (Wm~ ° C" )
ܶ௖௢௡௧ ൌWall temperature
ϭϴ
n = outward normal
The thermal conductivity of some root products is given in the Table
below
Table 2.5: Thermal Conductivity of Some Root Crop Products [18]
S/N
Products
Water
Apparent Temperature
Fraction
Density (kg
(Wet Basis)
m-3)
(°C)
Thermal
Conductivity
(Wm-1 K-1)
1.
Potato
0.763
1127
40.0 °C
0.410
2.
Potato
0.763
1117
50.0 °C
0.470
3.
Potato
0.835
—
130.0°C
0.641
4.
Onion
0.873
970
28.0 °C
0.574
5.
Beet
0.895
1530
28.0 °C
0.601
6.
Carrot
0.923
—
25.0 °C
0.571
7.
Carrot
0.923
—
105.0oC
0.649
The data in the table are based on experiments conducted by
Rhaman,[20] It is possible to approximate the values of these properties for a
given product if the chemical composition of the product is known, [18] for
simple geometry and linear boundary conditions, the analytical solution of the
equation (2.15) above is available and can be found in text books on heat
transfer [21, 22],In general, root crops are complex shaped products with
temperature dependent material properties (pp, Kp, Cp) which are subjected to
non-linear or time varying process conditions. No analytical solutions are
available for such complicated heat transfer processes [18], and an
approximate solution technique becomes mandatory. In practice, a numerical
solution of heat equation (2.15) can be obtained by means of finite difference
or finite element methods. When considering heat transfer problems with
ϭϵ
complex shaped products and complex boundary conditions, the finite element method is better and more advised [23,24].
2.6
Termination of Processing Time and Heat Load During Refrigeration of
Foods
Rajput [25] mentioned that when a body whose initial temperature is
Tj throughout and which is placed suddenly in ambient air or any liquid at a
constant temperature ta is considered (refer to fig.2.3) the transient response
of the body can lime determined by relating its rate of change of internal
energy with convective heat exchange at the surface that is:
Q = - pvc
ௗ்
ௗ௧
= hAs (T-Ta) ................................................ (2.21)
Where Q = convective heat exchange
p = density of solid kg/m3
v = Volume of the body m3
c = Specific heat capacity of the body
h = heat transfer coefficient / unit surface conductance w/m2°C
T = Temperature of the body at any time, °C
A = Surface area of the body m2
T = ambient temperature, °C
t = time in s
After re- arranging the equation (2.21) and integrating the result is:
න
݀ܶ
݄‫ܣ‬௦
ൌ
න ݀‫ݐ‬
ሺܶ െ ܶ௔ ሻ ߩ‫ܿݒ‬
Ÿ‫݊ܫ‬ሺܶ െ ܶ௔ ሻ ൌ ௛஺ೞ
ఘ௩௖
The boundary conditions are
At t = o, T = ti (initial surface temperature)
? c1 = In (Ti – Ta)
Hence
In (T-Ta)=
௛஺ೞ
ఘ௩௖
‫ ݐ‬൅ In (Ti – Ta)
ϮϬ
‫ ݐ‬൅ ܿଵ
Ÿ In (T-Ta) - In (Ti – Ta)=െ
Ÿ ‫ ݊ܫ‬ቀ
Ÿ
்ି்ೌ
்೔ ି்ೌ
்ି்ೌ
்೔ ି்ೌ
ൌ
ቁ=െ
ఏ
ఏ೔
௛஺ೞ
ఘ௩௖
௛஺ೞ
ఘ௩௖
‫ݐ‬
‫ݐ‬
ൌ ݁‫ ݌ݔ‬ቀെ
௛஺ೞ
ఘ௩௖
ቁ …………………………… (2.22)
Where T = final temperature and Ti = initial temperature.
Ϯϭ
The following points are worth noting.
(i)
The equation (2.22) above gives the temperature distribution in the
body Newtonian heating or cooling and it indicates that temperature
rises exponentially with time (refer to fig 2.4).
(ii)
The quantity
ఘ௩௖
௛஺ೞ
has the dimensions of time and is called thermal time
constant denoted by tth its value is indicative of the rate of response of
a system to a sudden change in its environmental temperature i.e. how
fast a body will respond to a change in environmental temperature tth
can be expressed as
tth = ቀ
ଵ
௛஺ೞ
ቁ ሺߩ‫ܿݒ‬ሻ ൌ ܴ௧௛ ‫ܥ‬௧௛ …………………….. (2.23)
Where
Rth = ቀ
ଵ
௛஺ೞ
ቁ= resistance to convective heat transfer
Cth=pvc = lumped thermal capacitance of solid.
Any increase in the thermal resistance Rth or thermal capacitance Cth
will cause a solid to respond slowly to changes in its thermal
environment and will increase the required time to attain the thermal
equilibrium (T = 0) (refer to fig 2.5).
ϮϮ
Ϯϯ
Figure (2. 6) shows an analogous Electric network for a lumped heat capacity
system, in which Cth = pvc Represents the thermal capacity of the system.
The value of Cth can be obtained from the following thermal and electrical
equations by similarity [25]:
Q = (pvc) t = Cth t
S = CE
Where
S = capacitor change
C = capacitance of the condenser
E = voltage.
When the switch is closed, the solid is changed to the temperature T.
On opening the switch, the thermal energy stored as Cth is dissipated through
the thermal resistance and the temperature of the body decays with time. From
this analogy it can be concluded that Resistance-capacitance electrical circuits
may be used to determine the transient behavior of thermal systems.
௛஺ ೞ
The power on exponential i.e
௣௩௖
‫ ݐ‬can be
Arranged in dimensionless form as follows:
௛஺ೞ
௣௩௖
‫ݐ‬ൌቀ
௛௩
௞஺ ೞ
ቁቀ
஺మೞ ௞
௣௩ మ ௖
‫ݐ‬ቁ ൌ ቀ
௛௅೎
௞
ఈ௧
ቁ ቀ మ ቁ ………………….(2.24)
௅೎
Where
௞
D = ቀ ቁ = thermal diffusivity of the solid.
ఘ௖
L = characteristic length =
௩
஺ೞ
=
௏௢௟௨௠௘௢௙௧௛௘௦௢௟௜ௗ
ௌ௨௥௙௔௖௘௔௥௘௔௢௙௧௛௘௦௢௟௜ௗ
The values of characteristic length Lc for simple geometric shapes are
given below [25].
(i)
Flat plate: L =
௩
஺ೞ
=
௅஻ு
ଶ஻ு
௅
= = Semi thickness
ଶ
Where L, B, and H are thickness, width, and height of the plate
(ii)
Cylinder (long): Lc =
(iii)
Sphere: Lc = య
ర
గோయ
ସగோమ
గோమ ௅
ଶగோ௅
=
ோ
ଶ
ோ
= where R = Radius of the sphere.
ଷ
Ϯϰ
where R = Radius of cylinder
(iv)
Cube: Lc =
௅య
଺௅మ
Further from the dimension less equation:
(i)
଺௅೎
The non-dimensional factor
௞
is called Biot number (Bi )
It gives an indication of the ratio of the internal (conduction)
resistance to surface (convection) resistance, when the value of Bi is
small; it indicates that the system has a small internal (conduction)
Resistance i.e. relatively small temperature gradient or the existence of
practically uniform temperature within the system. The convective
resistance then predominates and the transient phenomenon is
controlled by the convective heat exchange.
If Bi < 0.1 the lumped heat capacity approach can be used to
advantage, with simple shapes such as plates, cylinders, spheres and
cubes; the error associated is around 5% [25].
(ii)
The non dimensional factor
ఈ௧
௅మ೎
is called the Fourier number (F0)
i.e Fourier number
F0 =
ఈ௧
௅మ೎
.....................................................................(2.26)
It signifies the degree of penetration of heating or cooling effect
through a solid. Hence using non dimensional terms Equation (2.1)
takes the form
ఏ
ఏ೔
2.7
ൌ
்ି்ೌ
்೔ ି்ೌ
ൌ ݁ ஻೔ி೚ …………………………………… (2.27)
Thermal Properties of Foods
Refrigeration of foods offers considerable challenge to engineers since
the structure and composition of food and their thermal and physical
properties vary considerably. Furthermore, the properties of foods also
change with time and temperature. Fruits and vegetables offer an additional
Ϯϱ
challenge since they generate heat during storage as they consume oxygen
and give off carbon dioxide, water vapor and other gases [16].
The thermal properties of foods are dominated by their water content.
In fact, the specific heat and the latent heat of foods are calculated with
reasonable accuracy on the basis of their content alone. The specific heats of
foods can be expressed by Siebel's formula as.
Cpfresh = 3.35 a + 0.84 (KJ Kg-l °C)........................................(2.28)
Cpfrozen = 1.26 a + 0.84 (Kg Kg-l °C).......................................(2.29)
Where
Cp fresh = specific heat of food before freezing.
Cpfrozen = specific heat of food after freezing.
a = fraction of water content of the food (a =0.65 if the water content
is 65%) While the constant 0.84 KJ Kg-1 °C represents specific heat of the
solid (none-water) portion of the food. Siebel' s formulae are based on the
specific heats of water and ice at 0 C and 2.10 kJ Kg-1 °C respectively and
thus they result in the specific heat values of mater and ice at 0°C for a =
100% (i.e. pure water) therefore Siebel's formulas give the specific heat
values at 0°C. However they can be used over a wide range of temperature
with reasonable accuracy[16].
The latent heat of a food product during freezing or thawing (the heat
of fusion) also depends on its water content and is determined from:
hlatent = 334a (KJ/kg).....................................................(2.30)
Where
a = is the fraction of water content
334 KJ/Kg-1 is the latent heat of water during freezing at 0 oC at
atmospheric pressure. Perishable foods are mostly water in content, which
turns to ice during freezing therefore; we may expect the food items to freeze
at 0°C, which is the freezing point of pure water at atmospheric pressure. But
the water in foods is far from being pure, and thus the freezing temperature of
food will be somewhat below 0°C, depending on the composition of a
Ϯϲ
particular food. In general, food products freeze over a range of temperatures
instead of a single temperature since the composition of the liquid in the food
changes (becomes more concentrated in sugar) and its freezing point drops
when some of the liquid water freezes. Therefore, it is often spoken about the
average freezing temperature or, for foods like lettuce that are damaged by
freezing, the temperature at which freezing begins. The freezing temperature
of most foods is between -0.3°C and - 2.8°C. In the absence of the exact data,
the freezing temperature can be assumed to be -2°c for meats and -1°C for
vegetables and fruits [16].
2.8
Refrigeration and Refrigeration Cycles
Refrigeration is the transfer of heat from a lower temperature region to
higher temperature one, and that devices that produce refrigeration are called
refrigerators (or heat pumps), and cycles on which they operate are called
refrigeration cycles. The working fluids used in the refrigeration cycles are
called Refrigerants [16]. Figure2.7 shows a schematic diagram of a
refrigerator. Here QL is the magnitude of heat removed from the refrigerated
space at temperature TL, QH is the magnitude of heat rejected to the warm
space at temperature TH and Wnet,in is the net work input to the refrigerator.
QL and QH represent. Magnitudes and thus are positive quantities.
Ϯϳ
Fig. 2.7: A Refrigerator
Fig. 2.8: Schematic Diagram; Ideal Vapour Compressor Refrigerator
Cycle
Ϯϴ
2.8.1 Refrigerator Efficiency
The efficiency of a refrigerator is expressed in terms of the co-efficient
of performance (COP) denoted by COPR, The objective of a refrigerator is to
remove heat (QL) from the refrigerated space. To accomplish this objective,
it requires a work input of W net in. Then the COP of a refrigerator can be
expressed as
COPr =
஽௘௦௜௥௘ௗை௨௧௣௨௧
ோ௘௤௨௜௥௘ௗை௨௧௣௨௧
ൌ
஼௢௢௟௜௡௚௘௙௙௘௖௧
ௐ௢௥௞௢௨௧௣௨௧
ൌ
ொಽ
ௐ೙೐೟ǡ೔೙
This relation can also be expressed in rate form.
Refrigerators like heat pumps are cyclic devices and the conservation
of energy principle for a cycle device requires that:
Wnet,n =QH - QL (KJ) ............................................................ (2.31)
This implies therefore that the C.O.P. relation can also be expressed as:
COPr =
ொಽ
ொ ಹషೂಽ
ൌ
஼௢௢௟௜௡௚௘௙௙௘௖௧
ௐ௢௥௞௢௨௧௣௨௧
ൌ
ଵ
ೂಹ
ೂషభಽ
It should be noted that the value of COP& can be greater than unity.
That is the amount of heat removed from the refrigerated space can be greater
than the amount of work input. This is in contrast to the thermal efficiency,
which can never be greater than 1. In fact, one reason for expressing the
efficiency of a refrigerator by another term coefficient of performance (COP)
- is the desire to avoid the oddity of having efficiencies greater than unity.
The cooling capacity of a refrigeration system i.e. the rate of heat
removal from the refrigerated space is often expressed in terms of tons of
refrigeration. The capacity of a refrigeration system that can freeze 1ton (2000
Ibm) of liquid water into ice at 0°c in 24 hrs is said to be 1ton. One ton of
refrigeration is equivalent 211KJ/min (2000 BTU/min).
The most frequently used refrigeration cycle is the Vapor Compression
Refrigeration in which the refrigerant is vaporized and condensed alternately
and is compressed in the vapor phase. There are two different forms of vapor
compression refrigeration cycles [17].
i.
The Ideal Vapor Compression Refrigeration Cycle,
Ϯϵ
ii.
The actual vapor Compression Refrigeration cycle.
2.8.2 The Ideal Vapor Compression Refrigeration Cycle
As earlier mentioned the vapor compression refrigeration cycle is the
most widely used cycle for refrigerators, air conditioning systems, and heat
pumps. This is as shown in the schematic and the Temperature - Entropy (Ts)
diagrams, (refer to figs2.8 and 2.9).
1- 2
Isentropic compression in a compressor
2- 3
Constant pressure heat rejection in a condenser.
3- 4
Throttling in an expansion device
4- 1
Constant pressure heat absorption in an evaporator.
In an ideal vapour compression refrigeration cycle, the refrigerant
enters the compressor at state I as saturated vapour and is compressed
isentropically to the condenser pressure. The temperature of refrigerant
increases during this isentropic compression to well above the temperature of
the surrounding medium. The refrigerant then enters the condenser as super
heated vapour at state 2 and leaves as saturated liquid at state 3 as a result of
heat rejection to the surroundings. The temperature of the refrigerant at this
state is still above the temperature of the surroundings.
The saturated liquid refrigerant at state 3 is throttled to the evaporator
pressure by passing it through an expansion valve or capillary tube. The
temperature of the refrigerant drops, below the temperature of the refrigerated
space, during this process. The refrigerant enters the evaporator at state 4 as a
low quality saturated mixture and it completely evaporates by absorbing heat
from the refrigerated space. The refrigerant leaves the evaporator as saturated
vapour and re-enters the compressor completing the cycle.
The authors went on further to explain that all four components
associated with the vapour compression refrigerator cycle are steady flow
devices, and thus all four processes that make up the cycle can be analyzed as
ϯϬ
steady flow processes. The kinetic and potential energy changes of the
refrigerant are usually small relative to the work and heat transfer terms, and
therefore they can be neglected then the steady flow energy equation on a unit
mass basis reduces to
(qi – qout) + (Win - Wout) = h out – hin ……………………(2.32)
The condenser and the evaporator do not involve any work and the
compressor can be approximated as adiabatic. Then the C.O.P’s of
refrigerators and heat pumps operating on the vapour – compression
refrigeration cycle can be express as
COPr =
௤ಽ
ௐ೙೐೟ǡ೔೙
ൌ
௛భ ି௛ర
௛మ ି௛భ
……………………………. (2.33)
Where
h1 = h gas
h4 = hfluid
2.8.3 Actual Vapour Compression Refrigeration Cycles
An actual vapour – compression refrigeration cycle differs from the
ideal one in several ways, owing mostly to irreversibility that occurs in
various components [17]. Two common sources of irreversibility are fluid
friction (causes pressure drops) and heat transfer to or from the surroundings
(refer to fig. 2.8 and 2.9).
In the ideal cycle the refrigerant leaves the evaporator and enters the
compressor as saturated vapour. In practice, however, it may not be possible
to control the state of the refrigerant so precisely. Instead, it is easier to design
the system so that the refrigerant is slightly superheated at the compressor
inlet. This slight over design ensures that the refrigerant is completely
vapourised when it enters the compressor.
Also the line connecting the evaporator to the compressor is usually
very long thus the pressure drop caused by fluid friction and heat transfer from
the surroundings to the refrigerant can be very significant. The result of super
ϯϭ
heating, heat gain in the connecting line and pressure drops in the evaporator
and the connecting line is an increase in a specific volume, thus an increase
in the power input requirements to the compressor since steady-flow work is
proportional to the specific volume.
d
Fig. 2.9: T-S Diagram for the Actual Vapour Compression
Refrigeration Cycle
The compression process in the ideal cycle is internally reversible and
adiabatic and thus isentropic. The actual compression process, however, will
involve frictional effects, which increase the entropy and heat transfer, which
may increase or decrease the entropy, depending on the direction. Therefore
the entropy of the refrigerant may increase (process 1-2) or decrease (process
1-2) during an actual compression process, depending on which effects
dominate. The compression process 1-2 may be even more desirable than the
isentropic compression process since the specific volume of the refrigerant
and thus the work input requirement are smaller in this case. Therefore the
refrigerant should be cooled during the compression process whenever it is
practical and economical to do so.
In the ideal case the refrigerant is assumed to leave the condenser as
saturated liquid at the compressor exit pressure. In actual situations, however,
it is unavoidable to have some pressure drop in the condenser as well as in the
lines connecting the condenser to the compressor and to the throttling valve.
ϯϮ
Also, it is not easy to execute the condensation process with such precision
that the refrigerant is a saturated liquid at the end and it is undesirable to route
the refrigerant to the throttling valve before the refrigerant is completely
condensed.
Therefore the refrigerant is sub cooled somewhat before it enters the
throttling valve. However, since the refrigerant in this case enters the
evaporator with a lower enthalpy and thus can absorb more heat from the
refrigerated space. The throttling valve and the evaporator are usually located
very close to each other, so the pressure drop in the connecting line is small.
ϯϯ
2.9
Refrigerants
Ethyl ether was the first commercially used refrigerant in vapour
compression systems in 1850, followed by ammonia, carbon dioxide, methyl
chloride sulphur dioxide, butane, ethane, propane, isobutene, gasoline and
chloroflurocarbons, among others [16].
The industrial and heavy commercial sectors were very satisfied with
ammonia, and still are, although ammonia is tonic, the advantages of
ammonia over other refrigerants are its low cost, higher coefficient of
performance (and thus lower energy cost), more favorable thermodynamic
and transport properties and thus higher heat transfer co-efficient (requires
smaller and lower cost heat exchangers) greater delectability in the event of a
leak and no effect on the ozone layer the major drawback of ammonia is its
toxicity which makes it unsuitable for domestic use. Ammonia is
predominantly used in food refrigeration facilities such as the cooling of fresh
fruits, vegetables, meat and fish, Refrigeration of beverages and dairy
products such as beer, wine, milk and cheese; freezing of ice cream and other
foods; ice production and low temperature refrigeration in the pharmaceutical
and other process industries.
It is remarkable that the early refrigerants used in the light commercial
and house hold factors such as sulphur dioxide, ethyl chloride, and methyl
chloride were highly toxic. The wide spread publicity of a few instances of
leaks that resulted in serious illness and death in the 1920's caused a public
cry to ban or limit the use of these refrigerants creating a need for the
development of a safe refrigerant for household use at the request of Frigidaire
corporation, general motors research laboratory developed R-21, the first
member of the CFC family of refrigerants, within three days in 1928. Out of
several CFCS developed, the research team settled on R-12 as the refrigerant
most suitable for commercial use and gave the CFC family the trade name
"Freon". Commercial production of R-11 and R-12 was started in 1931 by a
company jointly formed by General motors and E.I du pont de Numours and
ϯϰ
G.inc. The Versatility and low cost of CFCS made them the refrigerants of
choice. CFCS were also widely used in aerosols, foam insulations and the
electronic industry as solvents to clean computer chips.
R-11 is used primarily in large capacity chillers serving air
conditioning systems in building R-12 is used in domestic refrigerators and
freezers, as well as automotive air condition. R-22 is used in window air
conditioners of commercial buildings, and large industrial refrigeration
systems, and offers strong competition to ammonia.
R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in
commercial refrigeration systems such as those in super markets because it
allows low temperatures at evaporators while operating at single stage
compression. The Ozone layer crisis has caused a major stir in the
refrigeration and air conditioning industry and has triggered a critical look at
the refrigerants in use. It was realized in the mid-1970's that CFCs allow more
ultra violate radiation into the earths atmosphere by destroying the protective
Ozone layer while preventing the infrared radiation from escaping the earth
and thus contributing to the green house effect that causes global warming.
As a result the use of some CFCs was banned by international treaties fully
halogenated CFCs (such R-11, R-12 and R-115) do the most damage to the
Ozone layer. The none fully halogenated refrigerants such as R-22 have about
5 percent of the Ozone depleting capability of R-12. CFCs that are friendly to
the Ozone layer that protects the earth from harmful ultraviolet rays and at the
same time do not contribute to the green house effect have been developed.
Currently R-12 is being replaced by the recently developed chlorine free R134a.
2.9.1 Selection of the Right Refrigerant
When designing a refrigeration system, there are several refrigerants
from which to choose, such as chlorofluorocarbons (CFCs), Ammonia,
hydrocarbons (Propane, ethane, ethylene e.t.c) carbon dioxide, Air (in the air
conditioning of aircraft) and even water (in applications above the freezing
ϯϱ
point) the right choice of refrigerant depends on the situation at hand of these
CFCs such as R11, R12, R22, R134a and R502 account for over 90% of the
market in the united states [16].
The important parameters that need to be considered in the selection of
a refrigerant are the temperatures of the two media (The refrigerated space
and the environment) with which the refrigerant exchanges heat. To have heat
transfer at a reasonable rate, a temperature difference of 5 to 10°C should be
maintained between the refrigerant and the medium with which it is
exchanging heat. If a refrigerated space is to be maintained at -10°C, for
example the temperature of the refrigerant should remain at about -20°C while
it absorbs heat in the evaporator.
The lowest pressure in a refrigeration cycle occurs in the evaporator
and this pressure should be above atmospheric pressure to prevent any air
leakage into the refrigeration system. Therefore a refrigerant should have a
saturation pressure of 1atm or higher at -20°C in this particular case.
Ammonia and R134a are two such substances.
The temperature (and thus the pressure) of the refrigerant on the
condenser side depends on the medium to which heat is rejected. Lower
temperatures in the condenser (Thus higher C.O.Ps) can be maintained if the
refrigerant is cooled by liquid instead of air. Other factors based on which the
selection of refrigerant to use should be considered are
(i)
Cost
(ii)
Efficiency i.e. C.O.P
(iii)
Flammability
(iv)
Compatibility with piping materials
(v)
Toxicity [26].
ϯϲ
CHAPTER THREE
3.1
Design Procedure
Review of literature was conducted in the previous chapter. Since the
main objective of this project work is to design and construct a refrigeration
system for onion storage this chapter will discuss the method that was used in
the design and construction of the storage system, based on the information
obtained from the previous chapter.
The methods employed were as follows:
3.2
Calculation of Heat Load
This is the total heat required to be removed from the refrigerated
space and was used to determine the size of the refrigeration equipments that
was used. This quantity was determined from the sum
(1)
The total heat of the product
(2)
The total heat gain and service load.
(3) Defrost heat; That is, heat generated as a result of stoppage of operation.
In order to determine the quantities the following parameters were defined
and determined; these are
Ts = Temperature at which the storage area was maintained = 1°C.
Tp = Temperature of the product before cooling which was taken generally
as room temperature (27°C to 29°C)
RH = Relative humidity at which the storage area was maintained
= 87% (from table2.2) chapter two.
Ks =
Skin mass transfer coefficient = 0.888 (from table2.4)
f
Respiration coefficient = 3.668X10-4 (from table2.4)
=
x =
Another respiration co-efficient = 2.538. (from table2.4)
Kp=
Thermal conductivity of the product = 0.574 wm-1 K-1 (from table
2.5).
Lv = Latent heat of vaporization of water at the product temperature before
cooling.
ϯϳ
pp =
Density of the product = 970kg/m-3 (from table2.5)
a
Water fraction of the product = 0.873(from table2.5).
=
S
= Storage area
h1 = Specific enthalpy of the product at room temp (28°C) before cooling
this is determined from psychometric chart at RH (87%) = 83 KJ/Kg
h2 =
Specific enthalpy of the product within the storage area at storage
temperature (1°C) and RH (87) = 9KJ/kg.
h3 =
Sensible heat transfer co-efficient.
Cp =
The product's average specific heat capacity which is determined
using siebel's formula Cp = 3.35a+0.84KJKg-10C Where
a = fraction of water content of the product = 0.873.
i.e. Cp= 3.35x0.873 + 0.84 = 3.764KJKg-l0C
m = Total mass of the onion stored = 50kg.
d = Average diameter of product which was determined
Statistically using raw data collected. The raw data was collected by
measuring the diameter of 50 samples of onions using vernier's caliper in
(mm). The average diameter d was determined statistically using grouped data
approach thus:
Table 3.1: Frequency Table for Diameters of Onion Bulbs
Variable
Frequency
D
f
61.00 - 70.50
dm
fdm
15
65.75
986.25
71.00 - 80.50
16
75.75
1212.00
81.00 - 90.50
5
85.75
428.75
91.00 - 100.50
4
95.75
383.00
101.00 - 110.50
10
105.75
1057.50
6
50
Summation
Therefore Average diameter d =
4067.25
σ ௙ௗ೘
σ௙
ϯϴ
=
ସ଴଺଻Ǥଶହ
ହ଴
= 81.35mm.
Using the value of the parameters above the heat load calculation was
carried out thus:
1.
The Total Heat of the Product
This quantity was determined from the sum of;
i.
Heat of respiration
ii.
Latent heat which is accompanying mass transfer
iii.
Sensible heat due to mass transfer between the water inside the onions
and the atmosphere inside the storage system
iv.
Heat removed from the product.
(i)
Heat of Respiration
The refrigeration load due to heat of respiration is given by equation
(2.2) as
ܳ௥௘௦௣௜௥௔௧௜௢௡ ൌ ෍ ݉‫ݍ‬௥௘௦௣௜௥௔௧௜௢௡ ሺܹሻ
Where m = total mass of product = 50kg.
qresp = specific respiration rate and is given by equation (4) as
qresp = 10.7 ݉௖௢మ (Jkg-1h-1)
݉௖௢మ is also given by equation (3) as
݉௖௢మ ൌ ݂ ቀ
ଽ்೘
ହ
௫
൅ ͵ʹቁ ሺ݉݃݇݃ ିଵ ݄ିଵሻ Where
F = 3.668 X 10-4
x = 2.538
Tm = Mass average temperature of the product = 28oC
Thus
݉௖௢మ ൌ ͵Ǥ͸͸ͺ‫ିͲͳݔ‬ସ ቀ
ଽ௑ଶ଼
= 26.73݉݃݇݃ିଵ ݄ିଵ = 26.73 x
= 7.425 x 10-3 ݉݃݇݃ ିଵ‫ି ݏ‬ଵ
? qresp = 10.7 x 7.425 x 10-3
= 0.0794‫ି݃݇ܬ‬ଵ ‫ି ݏ‬ଵ
Hence Qresp = mqresp
ϯϵ
ହ
ଵ
ଷ଺଴଴
൅ ͵ʹቁ
ଶǤହଷ଼
= 50 x 0.079
= 3.972W.
(ii)
Latent Heat due to Mass Transfer
Before calculating the latent and sensible heat due to mass
transfer, the rate of mass transfer was determined first and this, as
discussed in chapter two, is caused due to transpiration which is given
by equation (2.5) as:
Mw = kt(Ps – Pa) (g/kgday)
Where Ps = Vapour pressure at the product’s surface evaluated from
equation (2.6) given as
ళǤఱ೅
ps = ͳͲቀଶǤ଻଼ହ଻ାమయళǤయశ೅ቁ
ళǤఱ೉భ
ps = ͳͲቀଶǤ଻଼ହ଻ାమయళǤయశభ ቁ
ps = ͳͲଶǤ଼ଵ଻
= 656.15Ua
Pa = Vapour pressure in the surrounding Air and is
Evaluated from equation (2.7) given as
Pa = Pa
ோு
ଵ଴଴
RH = Relative humidity at which product is stored = 87%
Thus
Pa = 656.15 x
଼଻
ଵ଴଴
=570.85Pa = 5.7085 x 10-4Mpa
While
Kt = Transpiration co-efficient and is evaluated from equation (2.8)
given as:
ଵ
ଵ
௄೟
௄ೌ
ൌ
൅
ଵ
௄ೞ
Where
Ks = Skin mass transfer co-efficient
ϰϬ
Given in table 2.4 as 0.888g/m2s Mpa
And
Ka = Convection mass transfer co-efficient at the products surface
calculated using the Sherwood – Reynolds Schmidt correlation given
by equation (2.9) as:
Sh = ʹǤͲ ൅ ͲǤͷͷʹܴ݁ ଴Ǥହ ܵܿ 033
Since in this system there will be no flow that is no fan within
the storage area then Re is considered to be Zero. Thus, the above
relationship becomes
Sh = 2.0
So that Ka can be evaluated from equation (2.10) given as:
Sh = k a +
ௗ
ఋ
d = average diameter of the product
= 81.35 x 10-3m as determined statistically above
G = diffusion coefficient of water vapour in air which can be determine
from Schirmer’s equation (2.11) given as:
ଵǤ଼ଵ
ଵ
்
௉
ଶ଻ଷ
G = ʹǤʹ͸ܺͳͲିହ ቀ
ቁ
(m/s2)
Where
T = Water temperature in this case to be taken as average temperature
of this product.
P = Ambient (total pressure) in bar, in this case taken as 1bar.
Thus:
ଵ
ଷ଴ଵ ଵǤ଼ଵ
ଵ
ଶ଻ଷ
G = ʹǤʹ͸ܺͳͲିହ ቀ
ቁ
= 2.69 x 10 -5 m2/s)
Thus Ka can now be evaluated from
2.0 = Ka +
଼ଵǤଷହ௫ଵ଴షయ
ଶǤ଺ଽ௫ଵ଴షఱ
Ÿ 2.0 = Ka + 3,024.16
Ÿ Ka = 3,024.16 - 2.0
ϰϭ
= 3,022.16
Therefore Kt can now be evaluated
ଵ
i.e
Ÿ
௄೟
ଵ
௄೟
ൌ
ൌ
ଵ
ଷǡ଴ଶଶǤଵ଺
൅
ଵ
଴Ǥ଼଼଼
଴Ǥ଼଼଼ାଷǡ଴ଶଶǤଵ଺
ଶǡ଺଼ଷǤ଺଻
= 1.126
Hence the rate of mass transfer or rate of evaporation
Mw = Kt (Ps – Pa)
= 1.126 (6.516 x 10-4 – 5.708 x 10 -4) x 3600 x 24
= 4.35g/kgday
Now the latent heat accompanying mass transfer can be
calculated using equation (2.14) given as
qL = mw Lv (Tp) Where
mw = Rate of evaporation = 4.35g/kg day
Lv = Latent heat of vaporization (at the water temperature)
Since Lv is the same with hfg, and this quantity was determined
from the difference between h1 and h2 given amongst the parameters
defined above. Thus Lv = hfg = h1 – h2 = 83 – 9 = 74KJ/kg
Therefore
qL =
ସǤଷହ௫ଵ଴షయ
ଷ଺଴଴௫ଶସ
‫ݔ‬͹Ͷ‫Ͳͳݔ‬ଷ‫ʹݔ‬ͺ
3600 x 24
= 0.104W
(iii)
Sensible Heat due to the Mass Transfer
The sensible heat transfer due to mass transfer is determined
from equation (2.13) gives as
qs = h 3 (Tp - Ta)S
Where
Tp = Initial temperature of the product = 28oC
Ts = Ambient air temperature of the storage area = 10C as obtained
from literature
ϰϮ
S = Storage surface area which was considered as the water surface
area given in standard handbook of engineering calculations as
S = (length – thickness of insulation) (Breadth – thickness of
insulation)
i.e.
(60 – 2.5) (50 -2.5) = 57 x 47.5 = 2,731.25cm
= 2731.25 x 10 -2m2
h3 = Sensible heat transfer coefficient. This quantity was determined
thus in a general sense Nusselt and Sherwood number characterizes
heat and mass transfer respectively so if
Nu = DRem Prn œ Sh = DRem Scn
Therefore this means that the Sherwood – Reynolds – Schmidt
correlations stated above can also be written as:
Nu = 2.0 + 0.552 Re0.53 Pr0.33 hence
Nu = 2.0 since Sh = 2.0
Since Nusselt number is expressed as
Nu =
௛௅
௄
Ÿ
௛ೞ ௅
௄
where
h3 = Sensible heat transfer coefficient
L = Length along the direction of air movement. In this case taken to
be just 1cm above the products in storage.
k = Thermal conductivity of the air. In this case, evaluated from steam
table at the ambient air temperature of the storage area 1oC, i.e. 273+1
= 274K hence from the table k by interpretation.
௞
T(k)
ଵ଴షమ ሺௐȀ௠௞ሻ
250
2.227
274
k
275
Ÿ
ଶ଻ହିଶହ଴
ଶ଻ସିଶହ଴
2.428
ൌ
ଶǤସଶ଼ିଶǤଶଶ଻
௞ିଶǤଶଶ଻
ϰϯ
Ÿ
ଶହ
ଶସ
ൌ
Ÿk=
଴Ǥଶ଴ଵ
௞ିଶǤଶଶ଻
ሺଶସ௫଴Ǥଶ଴ଵሻାହହǤ଺଻ହ
ଶହ
= 2.419 x 10-2W/mk
h3 =
ே௨௞
௅
=
Thus
ଶǤ଴௫ଶǤସଵଽ௫ଵ଴షమ
ଵ௫ଵ଴షమ
ൌ ͶǤͺ͵ͺܹȀ݉ଶ݇
Hence, the sensible heat load qs can now be determined thus, From
qs = h (Tp - Ta)S
= 4.838 (28 - 1)x 2731 x 10-2
= 3, 567.72W
(iv)
Heat Removal from the Product
The removed from the product is determined from equation (2.1)
Qproduct= mcp(Tini – Tfinal) /'t
Where
m = The total mass of the onions = 50kg
Tini=Mass average temperature of the product before cooling=28 oC
Tfinal =Final mass average temperature of the product after cooling
Cp = Specific heat capacity of the products determined from the
expression, equation (2.28)
Cp = 3.35a + 0.54(kgkg-1oC)
Where
a = Fraction of water content of the onions = 0.871 thus
Cp = 3.35 x 0.871 + 0.84
= 3.758 x 103JKg-1oC
't = cooling time
The cooling time is the time it will take for the product to be cooled
from its harvest temperature to the desired storage temperature and this
is determined from the Fourier equation (2.15) giving the heat transfer
inside the product expressed as:
ܲ௣ ‫ܥ‬௣
ߜܶ௉
ൌ ‫׏‬൫݇௣ ‫ܶ׏‬௣ ൯ܹ௥௘௦௣
ߜ‫ݐ‬
ϰϰ
With Tp(x,y,z,t) = Tp,ini (X,Y,Z) at t = to
Where
Wresp = The volumetric respiration rate determined from
௉೛௤ೝ೐ೞ೛
Wresp =
ଷ଺଴଴
where
qresp = Specific respirations rate computed above as 0.0794JKg-1h-1
Pp= Density of the onion given as 970kgm-3
Therefore
Wresp here becomes
ଽ଻଴௫଴Ǥ଻ଽସ
Wresp =
ଷ଺଴଴
= 0.02Wm-3
Cp = The specific heat capacity of the product computed above as
3.758x103JKg-10C
Kp = The thermal conductivity of the onion given as 0.574Wm-1K-1
Tp = The product temperature = Tp,in and
Tp,in = Initial temperature of the product = 28oC
t = Time
Now, simplifying equation (2.15) i.e. ܲ௣‫ܥ‬௣
ܲ௣ ‫ܥ‬௣
ఋ்ು
ఋ௧
Taking
ൌ ݇௣ ‫׏‬ଶܶ௣ ൅ ܹ௥௘௦௣ = ௄೛
ఘ೛ ஼೛
?
ൌ ‫ ܥ‬ଶ ο்೛
ο௧
ఋ்ು
ఋ௧
ൌ
௄೛
ఘ೛ ஼೛
ఋ்ು
ఋ௧
ൌ ‫׏‬൫݇௣‫ܶ׏‬௣ ൯ܹ௥௘௦௣
‫׏‬ଶܶ௣ ൅ ܹ௥௘௦௣
ൌ ‫ ܥ‬ଶ‫׏‬ଶ ܶ௣ ൅ ‫ݍ‬௥௘௦௣
Now the equation
ఋ்ು
ఋ௧
ൌ ‫ ܥ‬ଶ ‫׏‬ଶܶ௣ ൅ ‫ݍ‬௥௘௦௣ ………………………… (3.1)
Is the heat equation which gives the temperature of a body T(x,y,z,t) of
homogenous material where C2 is the thermal diffusivity and ‫׏‬ଶ ܶ௣ is
the laplacian of temperature Tp with respect to Cartesian coordinates
x,y,z .
οଶ ܶ௉ ൌ ఋ మ ்ು
ఋ௑ మ
+
ఋ మ ்ು
ఋ௒ మ
൅
ఋ మ ்ು
ఋ௓ మ
Kreyszig (2002).
Equation (3.1) above can further be expressed as:
ϰϱ
ఋ்ು
ο்೛
ൌ ‫ ܥ‬ଶ ߜ‫…………………… ݐ‬.. (3.2)
Where ‫ ܥ‬ଶ ൌ ௄೛
ఘ೛ ஼೛
= thermal diffusivity of the onion
This heat equation is analogous to the heat equation solve by Rajput
(2005) since the boundary conditions are that
Tp(x,y,z,t) = Tp,ini (X,Y,Z) at t = to as in equation (2.27) thus equation
(3.2) above becomes:
்೛
்೛ǡ೔೙೔
ൌ ݁ ஻భ ி೚
Where
Bi = Boit number given by equation (2.25) as =
௛௅೎
௞
2
And h = h3 = 4838w/m K
K = 0.574Wm-1K-1
The shape of the onion is considered to be a sphere therefore the
characteristic length of the onion is:
Lc =
ோ
ଷ
where
R = Radius of the spherical onion =
? Lc =
଴Ǥ଴ସ଴଺଻ହ
ଷ
଼ଵǤଷହ௫ଵ଴షయ
ଶ
= 0.040675m Thus,
= 0.035558m
Hence Biot number is evaluated as
Bi =
ସǤ଼ଷ଼௫଴Ǥ଴ଵଷହ
଴Ǥହ଻ସ
= 0.114
Fo = Fourier number given by equation (2.26) as F0 =
Where D = thermal diffusivity
=
௄೛
ఋ೛ ஼೛
଴Ǥହ଻ସ
ଽ଻଴௫ଷǤ଻ହ଼௫ଵ଴య
= 1.575 x 10-7
Thus, F0 =
ଵǤହ଻ହ௫ଵ଴షళ ௫௧
଴Ǥ଴ଵଷହହଷ଼మ
= 8.573 x 10-4 t
Therefore equation (2.27) was evaluated as:
ϰϲ
ఈ௧
௅మ೎
்೛
்೛ǡ೔೙೔
ൌ ݁ ି଴Ǥଵଵସ௫଼Ǥହ଻ଷ௫ଵ଴
݁ ି଴Ǥଵଵସ௫଼Ǥହ଻ଷ௫ଵ଴
షర ௧
ଵ
షర ௧
ଶ଼
?
ଵ
ଶ଼
షర ௧
݁ ି଴Ǥଵଵସ௫଼Ǥହ଻ଷ௫ଵ଴
ൌ
ൌ
ଵ
షర೟
௘ షబǤభభరೣఴǤఱళయೣభబ
ൌ ʹͺ
-4
0.114 x 8.573 x 10 t = In28 = 3.332
T=
ଷǤଷଷଶ
hence
଴Ǥଵଵସ௫଼Ǥହ଻଼௫ଵ଴ షర
The cooling time
't =34,093.16s
= 9.47h this gives the time required to cool the product from 28 oC to
1oC i.e. cooling rate =
ଶ଼೚ ஼
ଽǤସ଻
= 2.9oC/h
Therefore the heat removed from the product was then calculated as
follows:
Neglecting Wresp i.e.
ܳ௣௥௢ௗ௨௖௧ ൌ ହ଴௫ଷǤ଻ହ௫ଵ଴య ሺଶ଼ିଵሻ
ଷସǡ଴ଽଷǤଵ଺
= 148.81W
Thus
The total heat of the product is =
Heat of respiration + sensible heat due to mass transfer + latent heat
which is accompanying the mass transfer + heat removed from the
product i.e.
3.972 + 3.567.72 + 0.104 + 148.81 = 3,720.606W | 3721W
(2)
Heat Gain and Service Load
This is the heat gain into the storage system through the
insulated surfaces caused by the difference between the inside and
outside temperatures. Also there is a service load, that is, a heat gain
caused by the opening and shutting of the storage system’s door, since
the onion will be loaded only once a day, it is safe to assume that the
ϰϳ
service load is a normal one i.e. the door will be opened less than 5
times per hour.
For product storage, cooling, heat and service load is determined
from [27]: Heat gain and service load = (Total outside area of cooler)
(Maximum outside temperature - minimum inside temperature) (a
factor from a given table: refer to Appendix II.
i.e. Heat and service load Q = A0 (T0 – Ti)K..................................(3.1)
Where
A0 = Total outside area determined thus
2(60x10 -2xl50x10-2) + 2(50x10-2x150x10-2) + 2(50x10-2x60x10-2)
= 1.8 + 1.5 + 0.6 = 3.92
To = outside temperature = 28°C
Ti= inside temperature = 1°C
Heat leakage factor, since the storage system was measured to
be of 2.5cm insulation thickness then the value of the factor for Heat
leakage plus normal service load is given as 0.0012KJ°Cm2 from the
table (refer to appendix II).
Hence, the heat gain and service load 0 = 3.9(28 - l) x 0.012x103
= 1.263w
| = 1.264w.
(3) Defrost Heat
This is the heat generated as a result of stoppage of equipment
operation. An assumed figure of 450W [27] is added to the heat load to serve
to take care of defrost heat.
Total Heat Load
After computing all the above heat loads the total heat load or heat to
be removed from the cold storage space is therefore the sum of:
Total heat of the product + heat gain and service load + Defrost heat. That is
3721W + 1264W + 450W = 5435W
ϰϴ
3.3
Determination of the Refrigeration Capacity Required
From literature, the cooling capacity of a refrigeration system that is
the rate of heat removal from the refrigerated space is often expressed in terms
of tons of refrigeration. The capacity of a refrigeration system that can freeze
1 ton of liquid water into ice at 0°C in 24 hours is said to be 1 ton. And 1 ton
of refrigeration is equivalent to 211 Kg/min. This is equivalent to 211 x 60 =
12,660KJ/h. Since the heat load calculated above is 5,435 watts, this implies
that:
5435 x
ଷ଺଴଴
x24 = 469,584KJ/day
ଵ଴଴଴
Assuming that the refrigeration equipment works 18h/day this implies
that 469,584 y 18 = 26,088KJ/h required to be removed.
Now since,
1 ton of Refrigeration = 12,660 KJ/h
Then
26,088KJ /h is
ଶ଺ǡ଴଼଼
= 2.0 tons
ଵଶǡ଺଺଴
Hence 2.0 tons of refrigeration is required for the storage system.
3.4
Selection and Sizing of Refrigerant and Refrigeration Equipments
Before sizing and selection of the refrigeration equipment, the
refrigerant was first selected because it is based on the properties of the
working fluid that the equipments can be sized. The selection of the
refrigerant was based on the following main factors.
i.
Availability
ii.
Cost
iii.
Coefficient of performance (C.O.P)
The most commonly and cheaply available Refrigerants encountered
in the cause of this project work are R12 and R22 commonly known to the
local vendors as Freon 12 and Freon 22. Thus the selection of working fluid
is between either of the two available refrigerants. In order to select a
ϰϵ
refrigerant the temperatures of the two media with which the Refrigerant will
exchange heat are considered i.e. the temperature of refrigerated space and
that of environment (Room air).
From literature it has been mentioned that to achieve a reasonable
exchange of heat at the refrigerated space and the environment
a
temperature difference of between 10°C - 20°C between refrigerant and
medium must be maintained. Thus selection of refrigerant was carried out as
follows:
i. Since desired temperature of refrigerated space is 1°C then temperature
of the refrigerant at evaporator must be between 10°C - 20°C lower
than the desired temperature of 1°C.
ii. The temperature of refrigerant at condenser must be at a level such that
room air temperature is less than the temperature of saturation of the
refrigerant by a difference of between 10°C to 20°C.
The application area of a refrigerant is restricted by critical temperature (tfr)
and freezing temperature [28]. Above critical point a refrigerant vapour
cannot be condensed and beyond freezing point the liquid does not exist.
Hence this means that any refrigerant in application must operate within this
temperature range. So using figure from table (refer to appendix III) where
the properties of common refrigerants was .given by GutiowsJa, (1996), the
refrigerant was selected.
The notations used in the table above are defined as follows:
qv
=
Volumetric refrigeration effect at to = -15°C and
tk
=
30°C
to
=
temperature of evaporation
tk
=
temperature of condensation.
n
=
Thermodynamic efficiency
C.O.Pc =
Theoretical Coefficient of performance for the reversible Ideal
cannot cycle. It depends only on the temperatures. This is given
as
ϱϬ
்೚
C.O.PC =
ሺ்ೖି்బ ሻ
To =
Temperature of evaporation (K)
Tk =
Temperature of condensation (K) Then
Then
C.O.Pc =
ଶହ଼
ሺଷ଴ଷିଶହ଼ሻ
ൌ ͷǤ͹
C.O.P = Actual coefficient of performance of the cycle which Depends on the
refrigerants properties and working conditions: 30° condensation temperature
and - 15°C evaporation temperature.
COP =
௤೚
௘೎
qo
= Refrigerating effect = h1 - h 4
ec
= compression work
COP
=
= h2 - h1 Thus
௛భ ି௛ర
௛ మ ି௛భ
Kn = Exponent of isentrope of saturated vapour for normal temperature.
Snv
tn
=
=
normal density of saturated vapor (Kg/m3)
normal boiling point (°C),
tcr
=
critical temperature (°C),
tfr
=
freezing temperature (°C),
Both R12 and R22 are suitable to be applied for the storage system but
R12 was selected because:
i.
Its value of t n is higher than that of R22 and thus more suitable for a
system that requires only cooling to 1°C .
ii.
Its low toxicity percentage in air compared to R22.
The refrigerant, R12 selected is used at the following working conditions.
Temperature of evaporation (t0)
=
-15°C
Temperature of condensation (tk)
=
30°C
Hence:
The refrigerant will enter the compressor as a superheated vapour at -15°C.
Therefore in order to determine compressor power, a temperature of 50°C
ϱϭ
which is 20°C higher than the condensing temperature of 30°C is chosen. That
is the refrigerant will enter the compressor at -15°C it is then compressed and
leave at a temperature of 50°c. The refrigerant is then cooled to a temperature
of 30°C and then throttled to the evaporator pressure of 1.826 bar. Thus the
properties of the refrigerant from table of thermodynamic and transport
properties of fluids arranged by Rogers and May hew are obtained as follows:
T1
=
-15°C
P1
=
1.826 bar
h1
=
180.97 KJ/Kg
T2
=
50°C
P2
=
12.19 bar
h2
=
206.45 KJ/Kg
T3
=
30°C
P3
=
7.449bar
h3
=
64.59 KJ/Kg
h3 (throttling) =
h4
=
64.59 KJ/Kg
3.4.1 Compressor Power Requirement
The compressor power requirement was calculated from the expression
P = m (h2 - h 1) = mW --------------------------
(3.1)
Where
m
=
mass flow rate of refrigerant (kg/s).
W
=
h2 - h1= unit work of isentropic compression (KJ/Kg).
While m is expressed as:
m =
ொబ
௛భ ି௛ర
...............................................................(3-2)
Where
Qo
= Refrigeration Capacity i.e amount of heat required to be Removed
from the storage area calculated as 5435 W = 5.44KW.
Therefore m =
ହǤସସ
ଵ଼଴Ǥଽ଻ି଺ସǤହଽ
= 0.05kg/s
ϱϮ
Hence
P
= 0.05 (206.45 - 180.97)
= 0.05x25.48
= 1.3kW =1.743hp | 2.0 hp
This implies that the coefficient of performance of the refrigerator is
COP =
ொబ
ௐ
=
ହǤସସ
ଵǤଷ
= 4.18 W
Hence a compressor of 1.3kW power was selected.
3.4.2 Sizing and Selection of Condenser
The selection of each type of condenser should be based on the data
provided by the manufacturer's catalogue [28]. If the working conditions are
out of the catalogue range, it is necessary to carryout analytical calculations
based on proper formulae for given heat transfer pattern, and that the
necessary heat exchange surface area Ak of a condenser is given by a general
formula:
Ak = m =
ொಽ
ሺ௎ೖ ିο௧ೖ ሻ
…………………………………….. (3.4)
Where
Qk
= The amount of heat to be removed during condensation process and
is expressed in terms Q0 and COP as
Qk
= Qo (1 + COP-1)(kW)
= 5.44(l +4. 18 -1)
= 6.74kW.
Uk
= Overall heat transfer coefficient of the condenser (W/m2.K),
The following heat flux figures were given by Gutkowski [28] for rough
estimation of condenser capacity.
Shell and tube condensers,
4,500 - 5000W/m2
Evaporative condensers
1,500 - 2500 w/m2
Air cooled condensers
150 - 200 w/m2
ϱϯ
Since for the storage system Air cooled was used a heat flux figure of 165
w/m2 [28] was chosen Thus Uk was determined as:
Uk =
ு௘௔௧௙௟௨௫ௐȀ௠మ
஼௢௡ௗ௘௡௦௔௧௜௢௡்௘௠௣௘௥௔௧௨௥௘௄
=
ଵ଺ହ
= 0.54w/m2
ଷ଴ଷ
'tm = Logarithmic mean temperature difference (LMTD) expressed in (oC)
as: ο‫ݐ‬௠ ൌ ο௧భ ିο௧మ
ο೟
ூ௡ο೟భ
మ
Where ο–ଵ and ο– ଶ are maximum and minimum temperature difference
between cooling medium and condensing refrigerant. The cooling medium
temperature is Room air temperature taken as 28°C, condensation temperature
of refrigerant is 30°C its temperature when entering the condenser is 50°C.
This is represented in LMTD graph thus:
ϱϰ
Fig: Graphical Representation for Determining LMTD
ο‫ݐ‬௠ ൌ ͵ʹ െ ʹ
͵ʹ
‫݊ܫ‬
ʹ
= 10.8°C hence the heat exchange area of the condenser is
‫ܣ‬௞ ൌ ଺Ǥ଻ସ
= 1.15m2
଴Ǥହସ௫ଵ଴Ǥ଼
Thus a condenser with the calculated area was selected.
3.4.3 Sizing and Selection of Evaporator
As for condenser the necessary heat exchange surface area of an
evaporator is given by a general formula for heat exchangers.
‫ܣ‬௘ ൌ ொ೚
…………………………………………… (3.5)
௎ο௧೘
Where
U (W/m2. K) is the overall heat transfer coefficient of an evaporator in this
case also figures were given by Gutkowski [28] , for R22 at heat flux of 1000
W/m2 and mentions that for ammonia, the values are much bigger than for
R22 and for R22 are lower.
Dry evaporators
350 W/m2.K
Flooded evaporators
450 - 500 W/m2.K
Wet evaporators
750 W/m2.k
Considering the evaporator used is a dry evaporator a figure of 350
2
W/m was chosen Thus,
U = 350W/m2.
'tm =
is logarithmic mean temperature difference calculated by means
and Qo =
heat removed by the evaporator in (W)
of the same formula as for condensers. Thus = 10.8°C
Therefore
‫ܣ‬௘ ൌ ହସଷହ
ଷହ଴௫ଵ଴Ǥ଼
=
1.43m2
ϱϱ
=
5435W
Therefore an evaporator having heat exchange surface area of 1.43 m2
was then selected.
3.4.4 Refrigerant Piping
The selection of return pipe diameter must always be based on the
vapor flow resistance. This resistance causes static pressure drop along the
flow which must not exceed the corresponding difference in saturation
temperature of 2°C [28]. The total pressure drop is the sum of the pressure
drop in piping. The pressure drop in the above apparatus is given in the
manufacturer's catalogue. The pipes used were selected based on
compatibility with compressor and condenser size.
3.4.5 Selection of Expansion Valve
The selection of all expansion devices should be strictly based on
manufacturer's catalogue with consideration to refrigerant, refrigerant
capacity and operating conditions [28]. Based on this statement the automatic
expansion valve was selected because of its principle of operation which is
based on the difference between atmospheric pressure and pressure of
evaporation. The evaporation pressure to open the valve is always pre-set in
a factory for a given refrigerant and indicated in the catalogue.
3.5
Construction, Testing and Cost Analysis
After the design calculations were conducted and the figures obtained
were used to select components, the construction of the storage system was
carried out.
3.5.1 Physical Description of the Storage System
The storage system is a container of length 60cm (2ft), Breadth 50cm
(1ft 20") and height 150cm (4ft 59") the container is fitted with insulation
between the inside and outside surfaces of thickness 2.5cm. Onions are stored
ϱϲ
inside system at the recommended temperature and relative humidity which
are 1°C and 87% respectively. In order to achieve and maintain this
temperature a vapor-compression refrigeration unit, comprising of
compressor, condenser, expansion valve and an evaporator was incorporated
to the system size of which was determined from the total heat load calculated.
The source of power utilized is electricity. The system is also fitted with two
thermometers one giving dry bulb temperature of storage area while the other
is giving the wet bulb temperature. The two readings were used to determine
the relative humidity of storage area. The system was also fitted with small
containers inside the storage area containing soda lime and silica gel. Their
purpose is to control respiration rate of onions thereby preventing them from
shrinking and loosing their quality.
3.5.2 Components of the System
The system can be divided into two main parts namely:
i.
The storage container
ii.
The refrigeration unit.
i.
The Storage Container: An old scrap household refrigerator of
dimension 60cmx50cmx 150cm, thickness 2.5cm was used as the
storage container. The storage area was divided into five shelves using
wooden plates. On both sides of each shelve perforated containers were
fitted to the wall in which soda lime and silica gel were placed inside
to absorb the carbon dioxide and moisture produced. Wet and dry bulb
thermometer was fitted to the door of the container which protrudes
through the door and then covered with glass from the outside such that
readings can be taken from outside without opening the door. The
description of the container is as shown in figure 3.1(a and b).
ii.
The Refrigeration Unit: This unit comprises of the following
components
ϱϳ
i.
The Compressor: This is a 2.0 hp compressor, selected and
bought based on figure obtained from design calculations. It
was mounted at back and bottom of constructed container. The
compressor was powered by electricity. Its description is as
shown in fig (3.6).
ii.
The Condenser: This is also selected based on the figures
calculated compatible with the compressor. It was mounted
close to compressor with a fan to increase condensing
efficiency of the condenser. Its description is as shown in fig
(3.7) while installation arrangement of compressor and
condenser is as shown in figure (3.4b)
iii.
Expansion Valve: This is an automatic expansion valve it was
selected based on its compatibility with evaporator and above
mention components. Its description is as shown in Fig (3.5).
iv.
Evaporator: This is also selected based on figures calculated
compatible with other components. Its description is as shown
in fig (3.8).
3.6
Testing of the Storage System
After construction the storage system was tested using 10kg of onions
which were placed into the middle shelf, after allowing the storage system to
reach the storage temperature of 1°C. simultaneously another 10kg of onions
were kept outside in open air. This arrangement was monitored for a period
of 4 months during which the following information or data were collected
each day.
i.
Mass of onions in control storage and onions in open air and their
average value recorded.
ii.
The dry bulb temperature of storage area was measured three times
daily and their average value recorded.
ϱϴ
iii.
The wet bulb temperature of storage area was measured three times
daily and their average value is recorded.
iv.
The relative humidity of the storage area, using the dry and wet bulb
temperatures obtained above from wet and dry thermometer chart was
measured three times daily and their average value is recorded.
v.
The carbon dioxide production rate of onions in control and in open air
storage using the regression fit given by equation (2.3)
݉௖௢మ ൌ ݂ ൬
௫
ͻܶ௠
൅ ͵ʹ൰ ሺ݉݃݇݃ିଵ݄ିଵሻ
ͷ
Where Tm = Average dry bulb temperature recorded daily.
ϱϵ
ϲϬ
Fig. 3.1b: Pictorial View of the Storage System
ϲϭ
Fig. 3.2 (a and b): Sectional View of the Storage System
Showing Onion in Storage and Chemical Containers Respectively
ϲϮ
Section X – X
Fig. 3.3: Sectional View of the Storage System
Showing Evaporator
ϲϯ
ϲϰ
ϲϱ
3.7
Cost Analysis
The cost involved in production of the storage system can be divided
into the cost of materials and labor cost. The cost of materials is summarized
as shown in the table below:
TABLE 3.2: Summary of Cost Analysis
S/N
Material Description
Unit
Total Cost
Required
Cost(N)
(N)
1.
Pomeka wood sheet (60 x70 x 160)cm
1 sheets
4,500
4,500
2.
Insulation material (fiber glass)kg
1kg
200
200
3.
Plastic lining sheet (70x80cm)
5 sheets
700
3,500
4.
Wet and Dry thermometer (a pair) °C
1 number
1,100
1,100
5.
Soda lime
2kg
500
1000
6.
Silica gel
1kg
500
1000
7.
Compressor (2.0 hp)
1 number
4,500
4,500
8.
Condenser (compatible with item 7)
1 number
1,500
1,500
9.
Expansion valve (compatible with item 7)
1 number
150
150
10.
Evaporator (compatible with item 7)
1 number
4,500
4,500
11.
Refrigerant 12
1kg
1,100
1,100
12.
½ inch Nails
1kg
200
200
13.
5mm screws
1kg
100
100
14.
1 inch angle bar (2 x l)cm
1 number
1,500
1,500
15.
Electrode (gauge 12)
10 number
10
100
16.
5mm Rivet
1 packet
1,200
1,200
TOTAL
22,260
ϲϲ
Quantity
26,150
Labour Cost
Labour cost is 10% of total cost of material
i.e.
ଵ଴
ଵ଴଴
‫ʹݔ‬͸ǡͳͷͲ = N2,615:00
Over head Cost is 5% of total cost of materials
i.e.
ହ
ଵ଴଴
‫ʹݔ‬͸ǡͳͷͲ = N1,307:50
Therefore the Overall total cost is Cost of material + Labor Cost + Overhead
cost. i.e. N26, 150:00+ N2, 615:00+N1, 307.50:00
= N30,072.50
ϲϳ
CHAPTER FOUR
DATA PRESENTATION AND ANALYSIS
4.0
RESULTS FROM TESTING OF STORAGE SYSTEM:
During the testing of the storage system readings within and outside
the storage system were measured. The results obtained from these reading
were presented, in the table below. Each data presented is aN average value
of three days reading.
The symbols used in the table below are defined as follows:
A
=
Average mass of onions in control storage (kg)
B
=
Average mass of onions in open air (kg)
C
-
Average dry bulb temperature of storage area. (°C)
D
=
Average wet bulb temperature of storage area (°C)
E
=
Average relative humidity within storage area obtained from
wet and dry thermometer chart, (refer to appendix I)
Q
=
Average carbon dioxide production rate of onions in Control
using equation (2,3) given as:
݉௖௢మ ൌ ݂ ൬
௫
ͻܶ௠
൅ ͵ʹ൰ ሺ݉݃݇݃ିଵ݄ିଵሻ
ͷ
Where
F And x are respiration co-efficient for onions given in table (2.4) as f = 3.668
x 10-4 and g = 2.538.
Tm is the average temperature of the onions in storage and in control.
Z =
Average carbon dioxide production rate of onions in storage
determined using the same formula as above.
Tmc
= Average temperature of onions in open air.
Tms
= Average temperature of onions in control storage.
ϲϴ
Table 4.0: Summary of readings obtained during testing of the storage
System
Date
A
B
Kg
Kg
C
D
E
o
o
%
C
C
Tmc
o
C
Q
Tms
Mgkg -1h-1
o
C
Z
Loss
Loss
Mgkg -1h-1 in Wt in Wt
for A for B
kg
kg
1
2.78
0
0
29.79
1
2.78
0
0.001
29.79
1.50
2.97
0
0.001
30
29.79
1.50
2.97
0.001 0.002
30
29.79
1.50
2.97
0.001 0.002
80
30
29.79
1.50
2.97
0.001 0.002
80
30
29.79
1.50
2.97
0.001 0.002
1
80
30
29.79
1.50
2.97
0.001 0.002
1.50
1
80
30
29.79
1.50
2.97
0.001 0.002
1.50
1
80
30
29.79
1.50
2.97
0.001 0.002
9.078
1.50
1
80
30
29.79
1.50
2.97
0.001 0.003
9.075
1.50
1
80
30
29.79
1.50
2.97
0.001 0.003
24/3/06 9.090
9.072
1.50
1
80
30
29.79
1.50
2.97
0.001 0.003
27/3/06 9.089
9.068
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
30/3/06 9.088
9.064
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
2/4/06
9.087
9.060
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
5/4/06
9.086
9.056
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
8/4/06
16/2/06 10.000 10.000
1
0
80
30
29.79
19/2/06 10.000 9.099
1
0
80
30
22/2/06 10.000 9.098
1.50
1
80
30
25/2/06 9.099
9.096
1.50
1
80
28/2/06 9.098
9.096
1.50
1
80
3/3/06
9.097
9.093
1.50
1
6/3/06
9.096
9.090
1.50
1
9/3/06
9.095
9.087
1.50
12/3/06 9.094
9.084
15/3/06 9.093
9.081
18/3/06 9.092
21/3/06 9.091
9.085
9.052
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
11/4/06 9.084
9.048
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
14/4/06 9.083
9.044
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
17/4/06 9.082
9.040
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
20/4/06 9.081
9.036
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
23/4/06 9.080
9.032
1.50
1
80
30
29.79
1.50
2.97
0.001 0.004
26/4/06 9.079
9.028
2.00
1
81
30
29.79
2.00
3.17
0.001 0.004
29/4/06 9.078
9.024
2.00
1
81
30
29.79
2.00
3.17
0.001 0.004
1/5/06
9.019
2.00
1
81
30
29.79
2.00
3.17
0.001 0.005
9.077
ϲϵ
4/5/06
9.076
9.013
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
7/5/06
9.075
9.007
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
10/5/06 9.074
9.001
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
l#/5/06 9.073
9.095
2.00
1
81
#0
29.79
2.00
3.17
0.001 0.006
16/5/06 9.072
9.088
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
19/5/06 9.071
9.082
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
22/5/06 9.070
9.076
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
25/5/06 9.069
9.070
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
28/5/06 9.068
9.064
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
31/5/06 9.066
9.058
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
3/6/06
9.064
9.052
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
6/6/06
9.062
9.036
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
9/6/06
9.060
9.030
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
12/6/06 9.058
9.024
2.00
1
81
30
29.79
2.00
3.17
0.001 0.006
TOTAL
4.1
0.037 0.162
Data Analysis and Inferences
From the data presented in the table above two parameters of major
interest are carbon dioxide production rate (Respiration rate) of onions in
storage and their loss in weight. The data indicated that respiration rates of
onions in control storage varies between 2.78mgkg-1h-1 and 3.17mg kg-1 h-1
over the period through which the experiment was conducted at the storage
temperature (averagely 1 to 2°C), a comparison of this response to that of
normal respiration rate of onions, Fig (4.0) suggests that onions respire much
more slowly under control storage condition than when stored in open air.
As shown respiration rates of onions has been slowed by about 9 times under
control storage system's conditions.
Analysis of loss in weight of onions stored in the constructed system
and those in open air was carried out thus:
Total average loss of weight in onions exposed to open air = 0.162kg.
Therefore percentage weight loss was calculated as:
ϳϬ
ͲǤͳ͸ʹ
ܺͳͲͲ ൌ ͳǤ͸ʹΨ
ͳͲ
Total average loss of weight in onions in control storage =0.037kg.
Therefore percentage weight loss was calculated as:
ͲǤͲ͵͹
ܺͳͲͲ ൌ ͲǤ͵͹Ψ
ͳͲ
Weight loss measurement studied in this work was presented in fig.
(4.1). It was found that weight loss in onions stored in open air is higher,
with a total of 0.162kg loss over the period of storage; constituting about
1.62% monthly, while loss in weight of onions in control storage is lower
with a total of 0.037kg loss over the period of storage constituting an average
of about 0.37%) monthly. In a similar work, 9% weight was reported [29],
on onions stored using combination of refrigerated and controlled
atmosphere storage over a period of 7 months after which the onions were
considered marketable.
Brice [30] also reported that weight loss under cool controlled
atmosphere storage is limited to 15% over 7 months compared with 40%
typical of onions held under heated forced air ventilated storage in tropical
environment. Therefore from results obtained using the storage system
constructed it is inferred that for onions stored in the constructed system a
forecast of a loss of only 2.59% after seven months is expected, hence an
indication of good result from combination of cold and controlled
atmosphere storage of onions. Relative humidity in storage system remained
constant at 80% with only an increase by 1% percent after 23 days. After
which it remains the same throughout the experiment this is as shown in
figure (4.2).
ϳϭ
ϳϮ
ϳϯ
ϳϰ
CHAPTER FIVE CONCLUSION AND RECOMMENDATION
5.0
Conclusion
Based on the results obtained and analyzed as shown in the previous
chapter, the following conclusions were reached.
(1)
That with a forecast of 2.59%, approximately 3% weight loss of
onions if stored in the constructed system for seven month, about 97%
of onions will be available for marketing.
(2)
That the achievement of the forecast above may depend on the size of
the storage system.
(3)
That a combination of refrigeration and controlled atmosphere is a
very efficient method for onion storage.
5.1
Recommendation
Following the above conclusion it is therefore recommended that:
(1)
The storage system's capacity be increased or developed to room sizes
to accommodate more onion bulbs.
(2)
A different and more accurate method of measuring carbon dioxide
production rate of onions should be developed and used
5.2
Suggestions for Further Studies
The followings are hereby suggested for further studies.
(1)
Similar storage
system should be designed and constructed using
absorption refrigeration system or solar energy.
(2)
Similar storage system should be developed for other crops like
Tomatoes, e.t.c.
ϳϱ
Plate 1: Frond view of the constructed storage system
ϳϲ
Plate II: Side view photograph of the constructed storage system
ϳϳ
REFERENCES
1.
Thompson A.K., Booth R.H. and Proctor. F.J. (1972) "Onion Storage in the
tropics". Tropical Science 14(1): 19-34
2.
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ϴϭ
APPENDIX I
DRY AND WET THERMOMETER TABLE
Humidity Table in ConUlgrade
Humidity Table in Fahrenheit
Balance Wet A dry Point
Balance Wet A dry Point
0
1
2 3 4 5
6 7 8 9 10 Wet Point
0
1
2
3
4 5
*
6
7 8 9 10 Wet Point
0
80 63 49 37 28 20 13 8 4
46
100 91 84 76 70 64 59 53 49 44 40
1
81 65 51 4030 22 18 11 7 4
47
100 91 84 78 70 64 59 54 49 45 41
2
82 66 53 4233 25 19 14 10 6
48
100 91 85 77 71 65 €0 54 504642
49
100 91 85 77 71 65 61 55 51 46 42
83 67 55 44 35 27 21 16 12 9
50
100 92 85 78 72 66 61 56 52 47 43
4
83 6956 463/30 24 19 14 11
51
100 92 85 78 73 66 62 57 53 48 44
5
84 70 58 4839 32 26 21 17 13
52
100 92 85 78 73 67 62 57 53 49 45
6
84 71 59 4941 34 28 23 19 IS
3
53
100 92 86 79 73 68 63 SB 54 50 46
54
100 92 86 79 74 68 63 58 54 50 46
55
100 92 86 80 74 69 64 59 55 51 47
7
85 72 61 51 43 36 30 25 21 17
56
100 92 86 80 74 69 64 60 56 51 48
8
85 73 62 52 44 37 32 27 23 19
57
100 92 87 80 75 69 85 60 58 52 48
9
86 74 83 54 46 39 33 28 24 21
58
100 93 87 80 75 70 65 61 57 5349
59
100 93 87 81 76 70 66 61 57 53 49
10
86 74 64 5547 41 35 30 26 23
60
100 93 87 81 76 70 66 62 58 54 50
11
67 75 65 5649 42 36 32 28 24
61
100 93 87 81 76 71 67 62 58 54 51
12
87 76 66 57 50 43 38 33 29 28
62
100 93 88 81 77 71 67 63 59 55 51
63
100 93 88 82 77 72 68 63 59 55 52
13
87 76 67 58 51 45 39 34 30 27
64
100 93 88 82 77 72 68 64 60 55 52
14
68 77 68 59 52 46 40 36 32 28
65
100 93 88 82 77 72 68 64 60 56 53
15
88 78 68 6053 47 42 37 33 2$
66
100 93 88 82 78 73 69 64 60 58 53
67
100 93 88 82 78 73 69 65 61 57 54
68
100 94 88 83 78 73 69 65 62 57 54
69
100 94 89 83 78 74 70 65 62 58 55
16
88 78 69 61 54 48 43 38 34 36
17
89 79 70 6255 49 44 39 35 31
70
100 94 89 83 79 74 70 68 82 58 55
18
89 79 70 6356 50 45 40 36 32
71
100 94 89 83 79 74 70 66 63 58 55
19
89 80 71 6357 51 46 41 37 33
72
100 94 89 83 79 74 71 66 63 59 56
20
89 80 72 64 58 52 47 42 38 34
73
100 94 89 84 79 75 71 67 63 59 56
74
100 94 89 84 80 75 71 87 64 60 57
ϴϮ
75
100 94 89 84 80 75 78 67 64 60 57
21
90 8072 6558 53 47 43 39 35
76
100 94 89 84 80 75 72 68 6461 57
22
90 81 73 66 59 53 48 44 40 36
77
100 94 90 84 80 76 72 68 65 61 58
23
90 81 73 6660 54 49 45 40 37
78
100 94 90 84 80 76 73 88 65 61 5fl
24
90 82 74 67 60 55 50 45 41 38
79
100 04 90 85 80 76 73 68 65 62 58
80
100 94 90 85 81 76 73 89 66 62 58
25
90 82 74 6761 56 50 46 42 38
81
100 94 90 85 81 77 73 69 68 62 59
28
91 82 75 6862 56 51 47 43 39
82
100 94 90 85 81 77 73 69 66 62 59
83
100 94 90 85 81 77 73 69 86 63 59
27
91 83 75 6862 57 52 47 43 40
84
100 85 90 85 81 77 74 70 67 63 60
28
91 83 75 6963 57 52 48 44 40
85
100 95 90 95 82 77 74 70 87 63 6O
29
91 63 76 6963 58 53 49 44 41
86
100 95 90 86 62 78 74 70 87 83 80
30
91 83 76 7064 58 S3 49 45 41
87
100 95 90 88 82 78 74 70 87 64 61
88
100 95 90 86 82 78 74 71 68 64 $1
89
100 95 91 86 62 78 75 71 68 64 61
31
91 83 76 7064 59 54 50 45 41
90
100 95 91 86 82 78 75 71 68 64 62
32
91 84 77 7065 59 54 50 45 42
91
100 95 91 86 83 78 75 71 68 65 62
33
92 84 77 71 65 80 55 51 46 43
92
100 95 91 86 83 78 75 72 68 65 62
34
92 84 77 71 65 60 55 51 46 44
93
100 95 91 86 83 79 75 72 69 65 62
94
100 95 91 87 83 79 75 72 69 65 62
95
100 95 91 87 83 79 76 72 69 66 63
35
92 84 78 71 65 61 56 51 47 44
ϴϯ
leakage
plus
SOURCE: (27)
normal service load
Heat
Heal leakage only
0.216
0.178
1
0.163
0.127
2
0.110
0.079
4
0.090
0.059
6
In
0.077
0.046
8
0.069
0.038
10
0.0012
0.0010
2.5
0.0009
0.0007
3.1
Insulation thickness
15.2
0.0006 0.0005
0.0005 0.0003
10.2
cm
Btu and kJ degree temperature difference per ft (m ) of outside surface
Heat Leakage Factors
APPENDIX II
0.0004
0.0003
20.3
0.0004
0.0002
25.4
ϴϱ
-180.0
-160.0
-100.0
R 13
R22
R502
-45.56
-40.8
-81.5
-29.8
23.7
-33.35
82.2
96.0
28.78
112.04
197.78
132.4
1.125
1.195
1.170
1.122
1 .904
1.3
2086.76
2097.5
N.A
1281.7
203.63
2168.7
6.25
4.77
N.A
6.2
5.88
0.8613
0.76
0.81
N.A
0.82
0.89
0.83
NIL
NIL
NIL
NIL
NIL
Very limited
Aluminium plastics
except tephlon
Magnesium zinc, lead
40
Plastics except tephlon
(T)
(T)
40
30
Lead tin zinc
aluminium natural rubber
20
Zinc natural rubber
Copper with its alloys
Reactivity with materials
(T)
(T)
40
(L)
0.25
in air %hr
in air %
16-25
Toxic
concentration
Explosive
concentration
Appendix III: Properties of Common Refrigerants
Refrigeration and air conditioning ppg 15.
% hrs - time exposition in hours to refrigerant in (L) - Lethal in %, (T) -Toxic in %, Source: Gutkow
-15.1.0
-111.0
R11
R 12
-77.9
R 717
Refrigerants