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Design of cold storage for fruits and vegetables
Technical Report · October 2002
DOI: 10.13140/RG.2.2.14335.82082
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DESIGN OF COLD STORAGE FOR FRUITS AND VEGETABLES
1. Introduction
Cold storage is the one widely practiced method for bulk handling of the
perishables between production and marketing processing. It is one of the methods of
reserving perishable commodities in fresh and whole some state for a longer period by
controlling temperature and humidity with in the storage system. Maintaining adequately
low temperature is critical, as otherwise it will cause chilling injury to the produce. Also,
relative humidity of the storeroom should be kept as high as 80-90% for most of the
perishables, below (or) above which his detrimental effect on the keeping quality of the
produce.
Most fruits and vegetables have a very limited life after harvest if held at normal
harvesting temperatures. Postharvest cooling rapidly removes field heat, allowing longer
storage periods. Proper postharvest cooling can:

Reduce respiratory activity and degradation by enzymes;

Reduce internal water loss and wilting;

Slow or inhibit the growth of decay-producing microorganisms;

Reduce the production of the natural ripening agent, ethylene.
In addition to helping maintain quality, postharvest cooling also provides marketing
flexibility by allowing the grower to sell produce at the most appropriate time. Having
cooling and storage facilities makes it unnecessary to market the produce immediately
after harvest. This can be an advantage to growers who supply restaurants and grocery
stores or to small growers who want to assemble truckload lots for shipment. Postharvest
cooling is essential to delivering produce of the highest possible quality to the consumer
Cold storage can be combined with storage in an environment with added of
carbon dioxide, sulfur dioxide, etc. according to the nature of product to be preserved. The
cold storage of dried/dehydrated vegetables in order to maintain vitamin C, storage
temperature can be varied with storage time and can be at 0°-10°C for a storage time of
more than one year, with a relative humidity of 80-95 %.
The cold storage of perishables has advanced noticeably in recent years, leading to
better maintenance of organoleptic qualities, reduced spoilage, and longer shelf lives.
These advances have resulted from joint action by physiologists to determine the
requirements of fruit and vegetables, and by refrigerating specialists to design and run
refrigerating machines accordingly.
Care should be taken to store only, those kinds, which does not show in
compatibility of storage, when storing multi produce in the same room. For example,
apple can be stored with grapes, oranges, peaches, and plums and not with banana.
However with potato and cabbage slight danger of cross actions can occur. Contrary to
this, grape in compatible to all other vegetables except cabbage.
To resolve the
incompatibility during cold storage, foodstuffs are grouped into three temperature ranges
Based on their thermal incompatibility the produce are classified into
1. Most animal products (or) vegetable produce, not sensitive to cold (0-4°C)
E.g. Apple, grape, carrot and onion
2. Vegetable produce moderately sensitive to cold (4-8°C)
E.g. Mango, orange, potato and tomato (ripened)
3. Vegetable produce sensitive to cold (>8°C)
E.g. Pineapple, banana, pumpkin and bhendi
Based on the purpose the present day cold stores are classified into following groups:
1. Bulk cold stores: Generally, for storage of a single commodity which mostly
operates on a seasonal basis E.g.: stores for potatoes, chilies, apples etc.
2. Multi purpose cold stores: It is designed for storage of variety of commodities,
which operate practically, throughout the year.
3. Small cold stores: It is designed with pre cooling facilities. For fresh fruits and
vegetables, mainly for export oriented items like grapes etc.
4. Frozen food stores: It is designed for with (or) without processing and freezing
facilities for fish, meat, poultry, dairy products and processed fruits and vegetables.
5. Mini units /walk in cold stores: It is located at distribution center etc.
6. Controlled atmosphere (CA) stores: It is mainly designed for certain fruits and
vegetables
2. GENERAL ARRANGEMENTS AND CONSIDERATIONS
If produce is to be stored, it is important to begin with a high quality product. The
produce must not contain damaged or diseased units, and containers must be well
ventilated and strong enough to withstand stacking. In general proper storage practices
include temperature control, relative humidity control, air circulation and
maintenance of space between containers for adequate ventilation, and avoiding
incompatible product mixes. Commodities stored together should be capable of tolerating
the same temperature, relative humidity and level of ethylene in the storage environment.
High ethylene producers (such as ripe bananas and apples) can stimulate physiological
changes in ethylene sensitive commodities (such as lettuce, cucumbers, carrots, potatoes,
sweet potatoes) leading to often undesirable color, flavor and texture changes.
The general features of a cold store operational programme (products, chilling and
chilled storage and freezing) include total capacity, number and size of rooms,
refrigeration system, storage and handling equipment and access facilities. The
relative positioning of the different parts will condition the refrigeration system chosen.
The site of the cold chambers should be decided once the sizes are known, but as a general
rule they should be in the shade of direct sunlight. The land area must be large enough for
the store, its annexes and areas for traffic, parking and possible future enlargement. A land
area about six to ten times the area of the covered surface will suffice.
There is a general trend to construct single-storey cold stores, in spite of the
relatively high surface: volume ratio influencing heat losses. The single storey has many
advantages: lighter construction; span and pillar height can be increased; building on
lower resistance soils is possible; internal mechanical transport is easier. Mechanical
handling with forklift trucks allows the building of stores of great height, reducing the
costs of construction for a given total volume.
The greater the height of the chambers the better, limited only by the mechanical
means of stacking and by the mechanical resistance either of the packaging material or of
the unpackaged merchandise. The length and width of the chambers are determined by the
total amount of merchandise to be handled, how it is handled (rails, forklift trucks), the
number of chambers and the dimensions of basic handling elements.
There is no advantage in building many chambers of a small size. Thermal and
hygrometric requirements are not so strict as to justify a lot of rooms: the accuracy of the
measuring instruments and the regulation of conditions inside the chamber always produce
higher deviations than those of ideal storage conditions for different products. This is
particularly true for frozen products.
A design that opts for fewer, larger chambers represents in the first place an
economy in construction costs as many divisional walls and doors are eliminated.
Refrigeration and control equipment is simplified and reduced, affecting investment and
running costs. Large chambers allow easier control of temperature and relative humidity
and also better use of storage space. Only in very particular situations should the cold store
be designed with more than five or six cold chambers. Store capacity is the total amount
of produce to be stored. If the total volume of the chambers is filled, the quantity of
produce by unit of volume will express storage density.
Several parameters must be defined within a cold store. The total volume is the
space comprised within the floor, roof and walls of the building. The gross volume is the
total volume in which produce can be stored, that is excluding other spaces not for storage.
The net volume represents the space where produce is stacked, excluding those spaces
occupied by pillars, coolers, ducts, air circulation and traffic passages inside the chambers
that are included in the gross volume. Storage density referred to as net volume is
expressed in kg/useful m3, but is most commonly referred to as gross volume.
An index of how reasonably and economically the cold store has been designed is
the gross volume divided by the total volume. It must be in the range of 0.50 to
0.80.Similarly gross volume is about 50 percent greater than net volume, and gross area
(same concept as volume) is about 25 percent greater than net area. The extent of
occupation is the ratio between the actual quantity of produce in storage at a given
moment and that which can be stored. Equally the extent of utilization is the average of the
extent of occupation during a given period — usually a year, but it can also be per month
Temperature management during storage can be aided by constructing square
rather than rectangular buildings. Rectangular buildings have more wall area per square
meter of storage space, so more heat is conducted across the walls, making them more
expensive to cool. Temperature management can also be aided by shading buildings,
painting storehouses white or silver to help reflect the sun's rays, or by using sprinkler
systems on the roof of a building for evaporative cooling. The United Nations' Food and
Agriculture Organization (FAO) recommends the use of Ferro cement for the
construction of storage structures in tropical regions, with thick walls to provide
insulation. Facilities located at higher altitudes can be effective, since air temperature
decreases as altitude increases. Increased altitude therefore can make evaporative cooling,
night cooling and radiant cooling more feasible.
The air composition in the storage environment can be manipulated by increasing
or decreasing the rate of ventilation (introduction of fresh air) or by using gas absorbers
such as potassium permanganate or activated charcoal. Large-scale controlled or modified
atmosphere storage requires complex technology and management skills; however, some
simple methods are available for handling small volumes of produce.
3. HEAT LOAD CALCULATION FOR TAMARIND STORAGE
The optimal storage temperature must be continuously maintained to obtain the full
benefit of cold storage. To make sure the storage room can be kept at the desired
temperature, calculation of the required refrigeration capacity should be done using the
most severe conditions expected during operation. These conditions include the mean
maximum outside temperature, the maximum amount of produce cooled each day, and the
maximum temperature of the produce to be cooled. The total amount of heat that the
refrigeration system must remove from the cooling room is called the heat load. If the
refrigeration system can be thought of as a heat pump, the refrigerated room can be
thought of as a boat leaking in several places with an occasional wave splashing over the
side. The leaks and splashes of heat entering a cooling room come from several sources:

Heat Conduction - Heat entering through the insulated walls, ceiling, and floor;

Field Heat - Heat extracted from the produce as it cools to the storage
temperature;

Heat of Respiration - Heat generated by the produce as a natural by-product of its
respiration;

Service Load - Heat from lights, equipment, people, and warm, moist air entering
through cracks or through the door when opened
4.0. MODEL CALCULATION FOR STORAGE OF TAMARIND OF 100 TONNES
CAPACITY
Calculation of the heat load involves considerations of various parameters and some of
them are presented below:
Harvesting season
: April-June
Optimal storage temperature
: 7°C
Optimal relative humidity (%)
: 90-95%
Approximate cold storage
: 3-4weeks
Quantity to be stored
: 100 tonnes
Ambient conditions
: 30°C and 70 % RH
Latitude
: North 11.00°
Altitude
: 409 MSL
TAMARIND PROPERTIES:
Bulk density
: 850 kgm-3
Heat of respiration
: 700 Kcal/ton/24 h
Specific heat (20%M.C)
: 0.524 Kcal/Kg°C
4.1. DESIGN OF BOX FOR STORAGE OF THE PRODUCE:
Volume of the product = Total Weight of the Produce / Bulk Density of Produce
= 1, 00,000 kg /850 kgm-3
= 117.64 m3
Assumed size of each box = 0.554 x 0.304 x 0.228m
Therefore volume of each box = 0.0383 m3
*Bulk density of the hard wood used for the storing the tamarind = 850 kgm-3
Weight of produce in each box = (Volume of each box) (B.D of Hard wood)
= 0.0363 m3 x 850 kgm-3
= 30 kg/box
Total number of boxes = Total weight of the Produce
Weight of the produce in each box
= 100,000 kg / 30 kg
= 3226 boxes
Thickness of each box = 0.004 x 0.004 x 0.008m
Actual volume of wood used per box = 0.0020 m3/box
Total volume of boxes = (volume of each box) (total number of boxes)
= 0.002 x 3226
= 6.452 m3
Total volume of boxes and produce = (Total volume of tamarind +box volume)
= 117.64 + 6.452
= 124.092 m3
0.228
0.554
0.304
4
DIMENSIONS OF THE WOODEN BOX (m)
4.2. INTERNAL DIMENSIONS OF THE COLD STORAGE:
The efficiency of the cold storage as well as for easy handling and movement of
the produce during loading and unloading of the tamarind can be improved by stacking the
boxes or containers in proper way. The boxes can be stacked in row and columns on the
Standard pallets as given in the general considerations. Proper stacking helps in uniform
cooling of the produce, also spacing should be considered for the movement of air and
handling equipments. Assumed dimensions based on the total capacity of the tamarind to
be stored, are given below:
Length = 13.972 m
Breadth = 7.648 m
Height = 4.42 m
Total internal volume = (13.972 x 7.648 x 4.42) m
= 472.3 m3
Free volume available inside the Cold storage = (Product volume – Internal volume)
= 472.3 m3 -124.4 m3
= 348.3 m3
Inner dimensions = 13.972 x 7.648 x 4.42
5.0
VOLUME =472.3 m3
8.648
14.972
EXTERNAL DIMENSIONS OF THE COLD STORAGE (m)
4.3. EXTERNAL DIMENSIONS OF THE COLD STORAGE:
1. Length = 13.972 m + (0.5 x2)m (walls) = 14.972 m
2. Breadth = 7.648 m + (0.5 x2)m (walls) = 8.648 m
3. Height = 4.42 m + 0.6m (floor & ceiling) = 5.0 m
4. Total external volume = 647.38 m3
5. Outer dimensions = 14.972 x 8.648 x 5.0 m3
6.
Total building volume = (External volume – Internal volume )
= 647.38 – 472.3
= 175 m3
4.4. HEAT TRANSFER THROUGH THE BUILDING:
The R (for resistance) number, is always associated with a thickness; the higher the Rvalue, the higher the resistance and the better the insulating properties of the material.
There are three alternatives for insulating the facility. Alternative A uses 10-20-30 Rvalues for the floor, walls and ceiling respectively. Alternative B uses 0.4-20-30 R-values,
which are equivalent to no insulation in the floor and only a concrete slab 4 inches thick.
Alternatives A and B correspond to grower self-built units. Alternative C corresponds to a
new prefabricated walk-in cooler with an insulation of 30-30-30 R-values for the walls,
ceiling, and floor.
This calculation is based on the first option i.e. R-value 10-20-30 as this would be suitable
for most of the tropical countries, where the losses through the buildings is higher.
4.4.1. HEAT TRANSFER THROUGH THE WALLS:
If the steady state flow is considered than, the heat flow is
Q = UA (To – Ti) Kcal / hr
Where,
U --- Over all heat transfer coefficient (Kcal/m2 hr° C)
A --- Surface area through which heat is transferred (m2)
To --- Temperature of outside air (°C)
Ti --- Temperature of inside storage space( ° C)
The overall heat transfer coefficient is given by
u
1
1 x1 x 2
1
   ....
ho k 1 k 2
hi
Where,
ho ---- heat transfer coefficient on the out or surface
hi ---- heat transfer coefficient on the inner surface
X1, X2 --- Thickness of wall and insulating material respectively (cm).
K1, K2 --- Thermal conductivity of wall and insulating materials (Kcal/m.hr. °C)
With thick wall and low conductivity, the resistance X/K makes U so small that 1/hi and
1/ho have little effect and can be omitted from the calculation. The values of U for
different types of walls and ceilings various from 1.00 to 4 Kcal /m2.hr. °C.
*Surface area = A = [2 x 13.972] x 4.42 = 123.5 m2 (Length)
[2 x 7.648] x 14.972 = 229.0 m2 (Breath)
*Total surface area = (length x Breadth) = 352.5 m2
*Ambient temperature (To) = 30oc
*Cold storage temperature (Ti) = 7oc
* Insulations resistance to the movement of heat (R) = 20
* Thickness of the brick = 0.44m
* Thermal conductivity of the brick = 0.62 kcal/m/hoc
* Thickness of the cement plaster = 0.10m
* Thermal conductivity of the cement plaster = 1.488 kcal/m/hoc
Overall heat transfer coefficient
1/u = 0.44/0.62 + 0.01/1.48
u = 1.398 kcal/m2/hoc
Therefore heat transfer through building material
Q = 1.398 x 352.5 x (30-7) x 24
= 272139.81 Kcal/24h
Heat transfer through insulation material
Q = [A (To – Ti) x 24]/R
= [352.5 x (30-7) x 24]/20
= 9729 Kcal/24h
Total heat transfer through walls = 272139.81 + 9729
= 281868.81 Kcal/24h
4.4.2. HEAT TRANSFER THROUGH CEILING:
* Surface area = A = 13.972 x 7.648 = 106.85 m2
* Insulations resistance to the movement of heat (R) =30
* Thickness of the cement concrete = 0.20m
* Thermal conductivity of the cement concrete = 1.488 kcal/m/hoc
Therefore heat transfer through ceiling material, can be generally taken as 20% more than
wall overall coefficient
i.e. Q = (1.398 x1.2) x 106.85 x (30-7) x 24
= 98946.859 Kcal/24h
Heat transfer through insulation material
Q = [A (To – Ti) x 24] /R
= [106.85 x [30-7] x 24]/30
= 1966.04 Kcal / 24 h
Total heat transfer through walls = 98946.859 + 1966.04
= 100912.89 Kcal/24h
4.4.3. HEAT TRANSFER THROUGH FLOOR:
* Surface area = A = 13.972 x 7.648 = 106.85 m2
* Insulations resistance to the movement of heat (R) =10
* Thickness of the cement concrete = 0.15m
* Thermal conductivity of the cement concrete = 1.488 kcal/m/hoc
Heat transfer through insulation material
Q = A (To – Ti) x 24 /R
= 106.85 x [30-7] x 24/10
= 5898.12 Kcal / 24 h
Total heat transfer = Heat transfer through walls +ceiling + floor
= (281868.81 + 100912.89 + 5898.12) Kcal / 24 h
= 388679.00 Kcal /24 h
Based on the above R-value the most appropriate insulation material can be selected
considering various parameters like Availability of the material, Cost on insulating
material, conductivity, Quality and life of the material. The most commonly used building
and the insulating materials with their properties are presented in the appendix.
4.5. PRODUCT LOAD:
Product cooling = (Weight of the fruit) (Specific heat of fruit) (Temperature difference)
= (100000) x (0.524) x (30-7)
= 1205200 Kcal / 24 h
Box heat load (Hard wood) = (weight of the box) (sp heat of box) (temp. difference)
= (0.002 m3 x 3226 boxes x 720 kg/m3) (0.571) (30 – 7)
= 4645.4 x 0.571 x 23
= 61008.562 Kcal / 24 h
Total product load = 1205200 +61008.562
= 1266208.5 Kcal/ 24 h
4.6. RESPIRATION LOAD DURING COLD STORAGE:
Average temperature = (30 + 7) / 2 = 18.5 C
Respiration heat load = wt. of the fruit x heat of respiration
= 100 tonnes x 700 Kcal/ton/24 h
= 70000 Kcal / 24 h
Total heat load = Heat transfer through surface + Product cooling + Respiration load
= 388679.00 + 1266208.5 + 70000
= 1724887.5 Kcal/ 24 h
VII. Miscellaneous load calculation
*Service load can be taken as 10 per cent of the total load i.e., lights, fans, forklift and
working men. Therefore, total heat load during cooling = 1897376.2 Kcal/ 24 h
*Including 10 % of the total heat load as a safety factor, the overall heat load
2087113.8 Kcal/ 24h
TOTAL HEAT LOAD CALCULATION
*Assuming refrigeration operates for about 16 hours/ day, the refrigeration capacity
requirement = 2087113.8 / 16
= 130444.61 kcal/ h =546041.13kJ/h
*One ton of refrigeration = 12660 kJ / h
Therefore, refrigeration required = 546041.13/ 12660
= 43.0 tons of refrigeration
So based on this cooling load calculation we can select the refrigeration unit capacity for
particular product to be stored.
5. FUNDAMENTALS FOR IMPLEMENTING A COLD STORAGE PROJECT
The design of cold storage facilities is usually directed to provide for the storage of
perishable commodities at selected temperature with consideration being given to a proper
balance between initial, operating, maintenance, and depreciation costs. The basic
procedures for constructing (or) implementing the cold store units are should have the
following requirements:
a) Process Layout
The most important requirement for any food project using insulated envelopes is
to determine the process layout of the operation which is to be housed by the envelope. In
the case of a meat plant, this can be a carcass dressing line or a boning room, or for a cold
store, the pallet layout and mode of operation must be established. It is simply no good
building an envelope and then attempting to place the processing machinery inside it.
b) Planning Drawings and Application
It is only after concluding the process layout that a planning application can be
made when the dimensions of the envelope and supporting buildings can be frozen.
c) Design Drawings and Specifications
Once planning approval has been obtained then the preparation of design drawings
and specifications can proceed. For a competitive design and construct tender, it is
essential to prepare some 15 - 20 detailed drawings covering, at the minimum, the process
layout, elevations and sections, the refrigeration system layout, mechanical and electrical
systems reticulation and the lighting layout.
In addition to make up package at least six separate detailed specifications are required
covering the project's requirements on:
1. Contractual requirements
2. Building specification
3. Refrigeration specification
4. Insulation panel supply and erection
5. Electrical requirements
6. Mechanical services.
6. LOCATION CONSIDERATIONS IN DESIGN OF COLD STORAGE
GENERAL
CONSIDERATION SPECIFIC FACTOR
DETAILED INFORMATION
Location
Environment
Altitude
Latitude
(For calculating solar loads)
Elevation above sea level
1) North or south of equator
2) Degree Line
Place
1) Outside design conditions
2) Unusual surroundings
Water
1) Corrosion and scaling
properties of local water
Atmosphere
Outdoor contaminants which could
affect outdoor equipment, air
handling equipment filtration
Labor
1) Availability, skill and costs
2) Design should be based on use
of local labor
Materials Transportation
Availability and costs Shipping,
receiving and storage availability
of equipment
Local Factors
6.1. LOCATION AND LAYOUT
The location chosen for the cooling facility should reflect its primary function. If
the plan is to conduct retail sales of fresh produce from the facility, it should be located
with easy access to public roads. A retail sales operation located away from the road,
particularly behind dwellings or other buildings, discourages many customers. Adequate
parking for customers and employees, if any, must be provided.
If, however, the primary function of the cooling facility is to cool and assemble
wholesale lots, ease of public access is less important. In this case, the best location may
be adjacent to the packing or grading room. In addition to housing grading and packing
equipment, the space could be used to store empty containers and other equipment and
supplies when it is not needed for cooling. All cooling and packing facilities should have
convenient access to fields or orchards to reduce the time from harvest to the start of
cooling.
Regardless of how it is used, the facility will need access to electrical power and
water. For larger cooling rooms requiring more than about 10 tons of refrigeration in a
single unit, access to three-phase power will be necessary. The location of existing utility
lines should be carefully considered, as connection costs can be prohibitive in some rural
areas. Consult your local power company for details.
In addition, it is a good idea to anticipate any future growth when locating and
designing your facility. The cold storage unit should be built on a site, a where the ground
in clean, well drained and preferably leveled and near to supplies of energy and water. If
possible, it should be in the shade of prevailing wind and direct sunlight. A refrigerated
store, with one (or) more thermally insulated places, and refrigerating machines can be
planned with the aim of assuring certain services. The details about:1. Nature of the products
2. Frequency of loading and unloading
3. Calendar for harvest and dispatch
4. Field heat of the produce
5. Daily tonnage of produce to be handled
6. Daily tonnage of ice to be manufactured
7. Nature and dimension of packages
The above particulars are to be collected before initiating the cold storage unit
work. The conditions to be considered for planning, a cold storage are temperature and
duration of storage, handling and stacking method, type of; commodities to be stored
together, prevailing climatic factors like temperature, relative humidity, rainfall, wind and
water. Availability of skilled and unskilled labor from the local area is the major factor to
be considered for the successful operation.
7. CONSTRUCTONAL DETAILS REQUIRED FOR COLD STORAGE DEISGN
Category
Factor
Architectural Design
Scale Drawings
Structural Design
Type of Structure:
Columns, Beams Bracing
Seismic Effects Expansion
and Settlement
Joint
Specific information required
Plans, elevations, sections
Orientation
Size, depth, location
Record and Pattern
Location and expected movement of
joints
Type of Construction
Walls, Roof, Floors
Insulation
Materials, thickness
Type, thickness, "k" or "C" value,
"R" factor
Surrounding
Condition
Outside
Design conditions, summer and
winter
Adjacent Spaces
Conditioned or Unconditioned
temperature
Adjacent Buildings
Shading
Doors
1) Location, type, size and usage
2) Doors for access to and removal
of conditioning equipment
3) Access of lift trucks
Stairways and Elevators
1) Location and size
2) Temperature of connecting spaces
3) Equipment horsepower
4) Ventilation requirements
Access
CALCULATION OF THE COLD STORAGE DIMENSIONS
The useful volume of a chamber is calculated as a function of the maximum mass
of produce, in store at the same time, taking account of the useful densities of storage,
expressed respectively as net mass of goods, per useable m3 (or) in kg of carcasses
suspended per linear in of rail.
The gross volume of a cold room is equal to the useful volume, increased by the
volumes necessary to allow for circulation of air and for handling. For a preliminary,
assume that the gross internal volume is twice the useful volume, (or) alternatively for
rooms to be used for miscellaneous products that it is of the order of 160kg/m3 gross for
chilling (or) 300 kg/m3 for freezing. The internal height depends on the means of
handling and stacking in very large stores, (or) where stacking is done by lift trucks, the
internal height is of the order of 8.5 cm for 4 super imposed pallets. If stacking is manual,
the maximum height of stacks does not exceed 3 m, which gives an internal height of 3.50
to 4 m.
7.1. FOUNDATION AND FLOOR
Almost all postharvest cooling facilities built nowadays are constructed on an
insulated concrete slab with a reinforced, load-bearing perimeter foundation wall. The
slab should be built sufficiently above grade to ensure good drainage away from the
building, particularly around doors. The floor should also be equipped with a suitable
inside drain to dispose of wastewater from cleaning and condensation.
The ground loads from a cold store are in the order of 5500-8000 kg/m2. This
consists of static loads due to merchandise, structure and concentrated rolling loads
transmitted by e.g., forklift trucks and other handling equipment. It is of importance that
those loads are investigated in detail for each special project.
STANDARD FLOORS
Standard floors shall be a minimum 125mm concrete. A 150mm hardcore base
shall be provided, compacted with vibrating or heavy roller, and topped with fine sand. All
floors shall incorporate 1000 gauge polythene D.P.C. membrane with 600mm overlaps
laid on the sand under concrete, and taken up along walls to meet D.P.C., where this has
been installed. In stores for certain forms of produce, or with floors subject to heavy
mechanized traffic, reinforced floors shall be installed. The design shall meet the
requirements of the specific loading. In the absence of specific design data an A393 mesh
to BS4482/BS 4483 [10mm @ 200mm c/c : 6.16 kg/m2] shall be placed 40mm below the
finished floor surface. Depending on specific requirements the top surfaces of floors may
require proprietary hardeners and/or sealing agents.
MOBILE RACKING FLOOR & TYPICAL FLOOR AND DOOR DETAILS
UNDER FLOOR DUCTING
Stores for certain forms of produce may require underfloor ducting. Design of
ducting (size, spacing, and construction) is specific to the type of produce stored and the
mechanical plant installed. Lay-out and design details shall be provided by the mechanical
plant supplier or consultant.
LAYING OF CONCRETE FLOORS
Laying of concrete floors shall be done in alternate bays measuring not more than
4.5m wide by 6m long where there is no fiber additive, and not more than 4.5m wide by
8m long with fiber additive. In the case of mesh reinforced floors joint spacing can be
extended to 12m by 8m. Concrete shall be placed about 20mm proud of the shuttering and
tamped to the correct level using a tamper or vibrating screed. Concrete may also be laid
in one operation as above and bays to the dimensions specified shall be cut by concrete
saw 25mm deep x 12mm wide in the hardened concrete within 24 hours of pouring. All
joints shall be brushed out and filled with mastic as per manufacturers’ instructions.
CURING OF CONCRETE FLOORS
As soon as concrete surface is firm enough (within about 1 hour) the slab shall be
sprayed lightly with water and maintained in a damp condition for seven days. This is best
achieved by covering the wetted slab with a polythene sheet. Care should be taken to
ensure that polythene firmly fixed at the edges of the slab to avoid wind draught between
the polythene and the concrete surface.
In the case of a single-storey building, a reinforced raft is usual, including ground
beams at the edges or bases for the structural frame. This can rest directly on the existing
ground or a supported slab. The floor wearing surface requires particular care. In addition
to the wear other industrial floors have to stand, it is exposed to low temperature. All other
parts of the cold store can be repaired whilst most of the space is still used for storage, but
not the floor. Most commonly the floor wearing surface is a concrete slab cast on the floor
insulation with a thickness of 100-150mm. In cases where intensive traffic is foreseen a
special hard wearing top-finish is recommended. Before casting the wearing surface, the
floor insulation should be protected by bituminous paper or plastic sheeting, the function
of which is twofold. Firstly, to prevent the water from the fresh concrete penetrating into
the floor insulation and secondly, to provide a slip-sheet, which will reduce the friction
when the concrete when contracts. It is of great importance that the floor wearing surface
be level to enable high stacking and easy traffic. The top-finish should provide a
reasonable anti-slip surface.
Special attention must be given to floor joints. It is recommended that a device
which allows horizontal displacement, but not vertical movement, is used between the
joints. If the joints open too much after lowering of the temperature, they must be filled
with a suitable jointing compound. If the pallet layout is painted on the floor (the
conventional way for easy location) a special hard-wearing, alcohol-based paint should be
used.
The floor of a refrigerated room must support heavy loads and withstand hard use
in a wet environment but still provide an acceptable measure of insulation. The slab floor
should be at least 4 inches of wire-mesh-reinforced concrete over 2 inches of waterproof
plastic foam insulation board such as DOW Styrofoam or equivalent. Five or even 6
inches of concrete may be necessary for situations where loads are expected to be
unusually heavy. The need for floor insulation is often poorly understood and therefore
neglected to cut cost. This is false economy, however, since the insulation will pay for
itself in a few seasons of use. If the room is to be used for long-term subfreezing storage, it
is essential that the floor be well insulated with at least 4 inches of foam insulation board
(having a rating of R-20 or greater) to prevent ground heave.
Any framing lumber in contact with the concrete floor must be pressure treated to
prevent decay, especially the sill plates and lower door frames, which may be in long-term
contact with water. Although no produce would normally come into contact with it, the
lumber must be treated with an approved nontoxic material. Information on the toxicity of
treated lumber should be obtained from the building materials supplier.
During construction, the interface between the underside of the sill plate and the
floor must be sealed to prevent the movement of water. This is easily done by completely
coating the underside of the sill plate with a heavy layer of suitable sealant before securing
it to the foundation pad with anchor bolts. The sill plate must be adequately secured to the
floor to prevent the building from moving off the foundation in a high wind.
Although the plan shows a treated 4-by-4-inch bumper guard adjacent to the sill
plate, a 4-by-6-inch guard is recommended. This essential component serves two
important purposes. First, it protects the walls from being damaged by movement of
loaded produce pallets and lift trucks. It also ensures that a suitable air gap is maintained
between the produce and te walls.
7.2. STRUCTURAL SUPPORT SYSTEMS
Steel or concrete frames usually support-insulated envelopes, the envelope either
being suspended as a box within the frame (external steelwork) or, alternatively, being
attached to the outside of the frame (internal steelwork). The two diagrams below show
the different arrangements.
Each system has its advantages and disadvantages. The more commonly used
external steelwork arrangement ensures the panels are permanently weather protected,
although subject to planning requirements, usually the wall sheeting is left off to save
capital. The internal steelwork arrangement has been and continues to be used, as
generally there can be a saving in capital cost, and the steelwork design can be integrated
with the racking steelwork. The steelwork, if held at sub-zero temperatures, will suffer
little corrosion, unlike the external design which requires corrosion protection either by
galvanizing or high quality paint work.
The internal steelwork arrangement also eliminates the problem of roof space
condensation. This problem comes about from the roof space steelwork and the roof of the
external steel structure reaching a temperature below the roof space air's dew point. This
problem can however be significantly reduced by arranging air openings at the bottom of
the wall cladding sheeting and further roof apex cowlings, thus ensuring a natural airflow
through the roof space. In very large floor area stores, this arrangement may need
enhancing by the introduction of fans and heaters particularly during autumn and spring
weather conditions.
7.3. INSULATION
GENERAL CONSIDERATIONS
Thermal energy always flows from warm objects to cold ones. All materials, even
good conductors like metals, offer some resistance to the flow of heat. Insulation,
however, is any material that offers high resistance to the flow of energy. Hundreds of
different materials have been used at one time or another for thermal insulation. Since
selecting the proper insulation is one of the most important building decisions you will
make, it is important that the material be not only cost effective but also correct for the
job. The characteristics of insulation materials differ considerably. Suitability for a
particular application, not cost, should be the deciding factor in choosing a material. Some
of the important characteristics that should be considered are the products R-value, its
cost, and the effects of moisture on it.
The choice of insulation is very important as it accounts for a large proportion of
the total construction cost. The insulation material and thickness is also important from an
energy point of view. Besides a satisfactory thermal conductivity coefficient the insulation
material should also be odour-free, anti-rot, vermin and fire-resistant and impermeable to
water vapour. Any type of building or facility used for storage of horticultural crops
should be insulated for maximum effectiveness. A well insulated refrigerated building will
require less electricity to keep produce cool. If the structure is to be cooled by evaporative
or night air ventilation, a well insulated building will hold the cooled air longer.
Control of moisture entry into the insulation is of primary importance to limit
water and ice collection in the structure. Deterioration of the insulation system and the
structure will result if moisture flow is not controlled. There are many construction
methods used to build and insulate low temperature storage areas. Most insulation
methods can be classified as insulated structural panel, mechanically applied insulation,
adhesive, (or) spray applied foam system.
PIPING AND DUCT INSULATION
Insulation is required on piping and duct work to conserve energy and prevent
condensation. Wherever insulation is used to prevent condensation, a suitable vapor
barrier shall also be used on the warm side to prevent absorption of moisture by insulation
and corrosion of equipment. The surface of the insulation shall not be exposed in the
refrigerated spaces. Outdoor installation requires a waterproof protective covering.
(1) Piping to be insulated. The following types of piping are to be insulated:
(a) All brine supply and return piping.
(b) Refrigeration suction piping.
(c) Drain piping.
(d) Condenser water piping where insulation is required to protect outdoor piping from
freezing or where cooling towers are used year round.
(e) Refrigerant discharge piping where people can come in contact.
(f) Refrigerant liquid piping when temperature of surrounding space is higher than
condensing temperature.
(2) Equipment to be insulated. The following is a list of equipment that is normally
insulated:
(a) Brine pumps.
(b) Unit cooler fans and casings located external to cooled space.
(c) Brine chillers.
(3) Connections. Piping connections to equipment coils and drain pans, valves, and
unions shall be fully insulated and arranged so that all condensation flows to and through
the drain pan.
COLD STORAGE INSULATION
Economic factors, minimum thickness of insulation, and installation of insulation
are as follows:
a. Economic Considerations: Factors such as cost of insulation and amount of insulation
must be carefully considered.
(1) Cost of insulation. In cold storage plants, the cost of insulation of cold storage
rooms amounts to a substantial part of the total cost of the installation. As the thickness of
the insulation is increased, its cost goes up, but the cost of refrigeration decreases.
(2) Selection. The thickness and type of insulation shall result in minimum total
life cycle costs comparing the operational expenses over the 25 year life of the structure
with first cost.
b. Minimum Thickness of Insulation: Walls, floors, and doors will be insulated.
Problems of conductivity, solar radiation, and heat gain must be given adequate
consideration.
(1) Conductivity. The insulation material generally used for cold storage
applications should have conductivity between 0.16 and 0.36 British thermal unit hour per
square foot per degree F per inch of thickness.
(2) Heat gain. Using the recommended insulation material, the heat gain should be
calculated based on an optimum insulation thickness that has been determined by life
cycle costing including energy costs, maintenance costs, and construction costs of
installation.
(3) Solar radiation. Solar radiation shall be considered in heat gain calculations
and in determining insulation thickness and equipment sizing.
(4) Smoke and fire safety. Interior finish and insulation shall comply with the
requirements of DOD 4270.1-M (June 1978), fire protection criteria, and other criteria
required to assure satisfaction in service performance.
(5) Shading. Solar heat gain to the storage facilities should be reduced by
providing external shading wherever possible. This shading can be obtained from nearby
buildings, trees, or the external construction of overhangs or projections on the buildings.
c. Installation: Insulation shall be installed in a minimum of two layers with staggered
joints.
(1) One side vapor barrier. A vapor barrier is essential on the warm side to
prevent moisture condensation in the insulation.
(2) Two side vapor barrier. The enclosures subjected to alternately warmer and
cooler temperatures than the surrounding temperatures may require a vapor barrier on both
sides of the insulation, particularly if high humidity’s inside the enclosure are encountered.
(3) Leakage of moisture at junctions of floors, walls and ceilings of cold storage
warehouse shall be prevented with vapor barriers with adequate expansion provisions.
Currently, with existing energy costs, the thermal conductance should not
exceed 0.15 kcal/m2h°C for cold stores. However in the future with ever increasing
energy costs this figure may have to be improved.
R-Value. A measure of an insulations resistance to the movement of heat is its R-value.
The R (for resistance) number, is always associated with a thickness; the higher the R-
value, the higher the resistance and the better the insulating properties of the material. The
R-value can be given in terms of a 1-inch-thick layer or in terms of the total thickness of
the material.
The total resistance to the flow of heat through any insulated wall is simply the
sum of the resistances of the individual components. That is, in addition to the thermal
resistance of the insulation, the inside and outside sheathing, layers of paint, and even the
thin layer of air next to the surface contribute to the walls overall thermal resistance.
TABLE SHOWING R-VALUE FOR DIFFERENT MATERIALS
Material
1 inch thick
Batt and Blanket Insulation
Glass wool, mineral wool, or fiberglass
3.50
Fill-Type Insulation
Cellulose
Glass or mineral wool
3.50
2.50-3.00
Vermiculite
2.20
Wood shavings or sawdust
2.22
Rigid Insulation
Plain expanded extruded polystyrene
5.00
Expanded rubber
4.55
Expanded polystyrene molded beads
3.57
Aged expanded polyurethane
6.25
Glass fiber
4.00
Polyisocyranuate
8.00
Wood or cane fiber board
2.50
Foamed-in-Place Insulation
Sprayed expanded urethane
6.25
Building Materials
Solid concrete
Full thickness of material
0.08
8-inch concrete block, open core
1.11
8-inch concrete block with vermiculite in core
5.03
Lumber, fir or pine
1.25
Metal siding
<0.01
3/8-inch plywood
1.25 - 0.47
1/2-inch plywood
1.25 - 0.62
25/32-inch insulated sheathing
2.06
1/2-inch Sheetrock
0.45
1/2-inch wood lapsiding
0.81
Source: Boyette, M.D. et al. No date. Design of Room Cooling Facilities: Structural and Energy
Requirements. North Carolina Agricultural Extension Service.
Although they are highly weather resistant and require little upkeep, metal
sheathing materials are very poor insulators. When specifying building materials, be sure
to select those with the best combination of economic value and thermal resistance.
The R-values of common building materials are listed in table. For frictional insulants
(cork, glass, wool) and plastic foams with closed cells (polystyrene, polyurethane) its
thermal conductivity lies between 0.05 and 0.22 (W/ m° k).
The final quality of any insulation is not only a matter of the properties of the
material itself, but of the way it is erected or fitted to the external building. Heat bridges
should be avoided, e.g., those normally created by pipes, cable joints, etc. Piping which
carries low pressure refrigerant or other liquids at low temperature must be insulated. The
provision of an efficient vapour barrier on the outside of the finished insulation with joints
properly sealed is of utmost importance, as moisture vapour penetrating the insulation will
form ice and gradually destroy the insulation material. The thickness of insulation depends
upon the internal temperature, heat conductivity of the insulation material and the dew
point of the ambient air, in order to avoid condensation. The insulation material should be
protected against moisture and mechanical damage. Where uncovered insulation material
is used, the internal walls and ceiling can be protected by sheets of aluminium, galvanised
steel, reinforced plastic, etc., or with materials such as plaster and cement. The choice of
material should be related to the use of the store, e.g., need for washing down. Painting of
plastered walls is not recommended unless special paint is used as it will quickly peel off.
MINIMUM THICKNESS OF INSULATION AT DIFFERENT TEMPERATURES
Room temperature (°C)
Equivalent thickness of
cork board (cm)
Minimum heat
transfer coefficient
(kcal / m2 .hr. c)
-28 to –23
20
0.205
-22 to –20
17
0.229
-19 to –15
15
0.268
-14 to –7
12
0.327
-6 to +2
10
0.405
+3 to +7
7
0.542
+7 and over
5
0.815
The selection of an insulating material for a particular purpose depends upon the
required properties of insulating material.
Desirable properties of low temperature
insulators are low thermal conductivity, durability when moistened, lightweight, water
repellent case of application, sanitation, odorless, fire proof and low cost.
In this case, the vapour barrier must be provided outside the insulation to prevent
the moisture from entering and condensing on the insulation. The vapour will try to come
inside, as the vapour pressure of the ambient air is higher than the inside vapour pressure.
The Vapour barrier should be provided on the hotter side of the insulation when the
insulation separator two spaces of air which is at different temperatures. When the
buildings are constructed of concrete, masonry black (or) porous stone, outer surfaces
should be coated, whenever possible, with cement, plaster (or) other coatings to reduce air
infiltration.
THERMAL CONDUCTIVITY OF INSULATING MATERIALS
Materials
Density (kg/m3)
Mean temp (°C)
Conductivity
(Kcal/m.hr.°C)
Asbestos, packed
701.6
0
0.200
Cement mortar
-
-
1.488
Cork board, typical
133
2
0.034
Cotton
81
0
0.048
Glass wool
40
18
0.032
Sawdust
192
32
0.050
Brick, low density
-
-
0.620
Brick, high density
-
-
1.140
Cement, plaster
-
-
1.488
Concrete, typical
-
-
1.488
Sand and gravel
-
24
1.562
Lime stone
-
24
1.339
INSULATION PANEL TYPES
There is much work proceeding with respect to developing new insulation
materials to improve fire risk. Nevertheless, the panels available at present for practical
use together with their principal characterization can be summarized as follows:
The most commonly used panel for cold storage use is Polystyrene, which has
been used now since the middle1960s. It is more economical to erect and is lighter in
weight than the other materials. Styrofoam has much higher load bearing characteristics
and is therefore mainly used in cold store floors, although the material listed above has
recently been developed for panel use. Polyurethane, although more expensive, has a
better U value but it has been shown that some 15% deterioration in this value can take
place over 10 years of use.
DETAILS OF INSULATION PANEL TYPES:
Panel type
U value
(kcal / m2 .hr. c)
Weight
(kg/m2)
Possible water
absorption (%)
Polystyrene
0.34
11.2
1.0
Styrofoam
0.24
12.3
0.5
Polyurethane
0.2
13.3
2.0
Mineral wood
0.38
19.0
50
Consequently, insulation envelopes such as containers must have increased
refrigeration capacity built into the plant to ensure long-term effective performance.
Polyurethane is used more usually on the European Continent.
Mineral wool panels are used in high fire risk situations such as process cooking
areas. In our opinion, it is not cost effective to use this material for cold store construction,
except for special firewall requirements. In particular, its ability to absorb water means
that a vapour leak could cause excessive ice formation and unacceptable weight increase,
if the panel was used throughout the store, and could result in a total ceiling collapse with
the steel structures becoming overloaded.
VAPOUR SEALING
Vapour sealing of envelopes subject to temperature variations below ambient
temperatures is one of the most important requirements in the construction of an insulated
envelope. Vapour penetration into the envelope will occur as vapour pressures are lower at
lower temperatures and warm air drawn into the envelope will condense its moisture,
which in turn will form ice, which may damage the panels. Panel penetrations for panel
and refrigeration equipment support and the introduction of refrigeration and electrical
services must be carefully designed to ensure long-term vapour sealing is maintained for
the insulated envelope. What is required for long-term successful operation is to ensure
that every joint of the paneling and every penetration is so designed and constructed to
ensure that each and everyone of these penetrations are water and vapour tight. The figures
below show the relevant aspects of what is required.
The sub-zero detail above relies upon a vapour seal, which is continuous under the
floor insulation usually effected by a heavy duty Visqueen which joins a bituthene or
equivalent continuous seal which passes under the open end of the wall panel and is
lapped around the outside of the panel. In the case of the chill store, the vapour sealing
material must seal the open end of the panel and also seal its fixing to the concrete
beneath, to prevent vapour penetration into the store.
Penetrations are required for
evaporator supports and electrical wiring and refrigeration pipes. In all cases our policy is
to bore out a sufficiently sized hole in the panel which is then sleeved with a PVC sleeve.
The piping, electrical wiring or the support steel is then taken through this sleeve and the
penetration is then foamed and sealed with mastic together with a suitable end cap.
DIAGRAM SHOWING TYPICAL VAPOUR SEALING ARRANGEMENTS
INSULATION OF THE FLOOR: This is done by insulant panel is two layers with
staggered joints on a concrete sub-base and an impermeable barrier of pitch (1cm). This
must carefully be joined to that of the walls. The panels may be in expanded polystyrene
(or) polyurethane in cork (or) in any other material with good compression resistance.
INSULATED DOORS:
The insulation of the cold store doors should be the same
standard on the store wall. The most common insulation material for doors is polyurethane
and door heaters should be fitted to prevent ice forming at the seal thus jamming, and
ultimately causing damage to the door.
Insulated doors must be chosen with cane, since they are one of the most
vulnerable parts of a store. Doors for handling produce are vertically hinged leaf types,
for small doors if not sliding. It is necessary to choose the doors, as soon as the store is
being planned, so as to fix the size of the opening to be made in the main work. What ever
the type of door, the leaf is fitted with a peripheral joint and a scraper (or) the gill, which
are compresses (or) closure of the door.
8. INFORMATION REQUIRED ABOUT COLD STORAGE OPERATION:
Factors
Details
Lighting:
Wattage at Peak
People:
1) Average number in space
2) Activity
3) Duration of time in space
Equipment:
1) Heat output of motors and machines used in space
2) Load factor or hours of operation
3) Weight, temperature, specific heat of material handling
equipment brought into space
Product:
1) Type, weight, specific heat of product
2) Temperature of product prior to storage
3) Final temperature of product
4) Maximum allowable chilling or freezing time
5) Frequency of loading
6) Type, weight, specific heat of containers
Ventilation:
Air quantity requirements for product
Infiltration:
Size and usage of doors
Purpose of Building :
Chilling, freezing, storage, single temperature, multiple
usage
Location of Equipment:
Ductwork, unit coolers, refrigeration,
heat rejection equipment
Electrical energy dissipated in the refrigerated space (light, motors, etc) must be included
in the heat load. Heat given off by light: Each watt is equal to 0.86 Kcal/hr
HEAT CAPACITY OF ELECTRIC LIGHTS:
Capacity of electric lights
Heat
(watts)
B.t.u/hr/electric light
25
85.25
50
170.50
100
345.00
200
682.00
400
1,364.00
600
2,046.00
Heat given off by Electric motors: Heat equivalent of electric motors varies from 290 to
1071 Kcal/hp-hr.
Heat given off by occupants (Body heat): People give up heat at varying rates
depending upon the temperature, type of work, clothing size etc. For convenience in
calculation, the body heat is taken as normally adult 100 Btu/hr. When people go into the
cold stores for short durations, they will carry with them a considerable amount of heat.
AIR CIRCULATION AND CHANGES
To maintain the circulation of air in a partly filled room the stack alignment must
be perpendicular to the direction of air movement and the stacks placed close to the cooler.
Fans must be operating when the refrigeration system is running and it is advisable to stop
them only during the defrosting period. Two-speed fans should be used to adjust to air
circulation needs in the room. Stacking must follow exactly the layout prescribed,
respecting loading limits and allowing space between the stacks and walls, and below the
pallets. The palletization layout plan must take account of distances between store
elements. They are in the range of 5–10 cm between pallets, 15–20 cm along the walls and
a stacking limit of 40–60 cm below the ceiling. The gangways for forklift truck circulation
depend on the type of truck, but are in the range of 2.15 to 3.0 m.
Air circulation inside the store is expressed by the air speed (m/s) through an
empty cross-section of the store and also by the chamber coefficient of air circulation,
which is the number of times the air equivalent to the total internal volume of the empty
chamber passes through the cooler in one hour. Both are obviously related, but the latter is
more commonly used for chambers than for tunnels as it gives a clearer idea of air
movement.
STORE LOADING PLAN
Correct system for alley loading is 1, 2, 3…
PALLETS FOR COLD STORAGE
For pallets and similar stacking elements the layout of the chamber is based on the
pallet module, including the size of the pallet, tolerance of air circulation and ease of
manoeuvre. Different lot sizes may require different spacing of gangways. Pallets, which
can be made of different materials, are becoming standardized, the most usual dimensions
being 0.80×1.00×1.20 m. The shorter and longer dimensions can be increased by 5 and 15
cm respectively to set up the recommended pallet module.
Different types of pallet used for forklift truck handling and stacking
Stacking width is influenced by the width of the gangway and the length of the
pallets. The width of the gangway depends on the forklift truck used and the depth of the
pallets depends on stock rotation the slower the rotation the deeper the pallets. Pallet
stacking depth is three to four pallets for a high rotation and seven to eight pallets for a
low rotation Several layers of boxes can be used on a pallet, the number being determined
mainly by the mechanical resistance of the packages and their shape for ease of piling.
Five to six layers are usual and sometimes seven are possible. The number of pallets in a
pile is also dependent on the mechanical resistance of the packages and on the type and
reach of the forklift truck used for stacking. A stacking height of two to four pallets is the
most common, but for large stores with a low rotation up to five pallets would be suitable.
LOADING DOCKS
Loading docks ease the handling and transfer of pallets to and from the cold stores
and transport vehicles, so most stores are provided with loading/ unloading docks adapted
to road or railway transport. For road transport the problem is to determine the height of
the dock to correspond with average vehicle height: for trucks it will be about 1.40 m, but
for distribution vans it will be as low as 60 cm. Moreover when the vehicle is loaded or
unloaded its height changes, and this is particularly awkward when the forklift truck has to
enter it. Levelling facilities will adjust the dock to any vehicle height; the dock and truck
platform thus corresponding at any time of the loading/ unloading operation.Docks for
railway transport can be built to a standard height.
The length of a loading bank should allow the simultaneous handling of an
adequate number of vehicles; it will depend on the size of the cold store and its rotation of
stored produce, which also influence the depth of the bank. The minimum recommended
depth is 6 m, but one of 8–10 m is considered to be more suitable. Loading docks are
usually under cover, sometimes simply an extended canopy open all around and
sometimes enclosed with a surrounding wall and doors. The choice of open or enclosed
docks is mainly influenced by climate and the handling system employed.
Enclosed docks are usually cooled and they should be used where temperature and
humidity are high, and when the merchandise is handled excessively with a long exposure
in conditions that are very different from those of storage. Any delay in transfer from
trucks to cold store in an open dock is obviously more detrimental than in a cooled
enclosed dock. Cooled loading docks must be insulated and are equipped with a
refrigeration system; the floor should be heated to prevent condensation. The height of the
canopy is determined by the height of the store doors plus the mechanisms above the
lintels for door opening and/or air curtain. Where for economy of handling two pallets are
superimposed for transfer to the cold store, this unit load height will decide the free height
of the loading dock roof.
Cooled dock doors should be equipped with a perimeter cushion seal to adjust the rear of
the truck to the loading door, reducing the cold air leakage. This system is usually
provided with a displacement mechanism which, together with the levelling device, will
ease handling and the maintenance of the loading dock temperature
9. TYPES OF COLD STORES
9.1. STORES WITH UNIT COOLERS:
The most widely used method of cooling modern cold stores is by means of unit
coolers with fan designed with good air flow characteristics. This type of cooler is
generally the cheapest to install; it contains a relatively small charge of refrigerant, it can
be readily defrosted without interfering too much with the store conditions and it does not
require a heavy structure for support. The main disadvantage is that many designs using
this type of cooling unit do not allow for uniform distribution of the air within the store.
This gives rise to poor storage conditions where the air circulation is either too high or too
low (Figure). By suspending the unit cooler from the ceiling (Figure) or installing the unit
outside the store (Figure) and ensuring that pallets are stacked with suitable head space
and floor spacing, uniform air distribution can be achieved.
Uneven Air Distribution In A Store With A Unit Cooler With Fan Circulation
Multiple units are usually better than large single units for a number of reasons. A
multi-unit system gives some insurance in case of breakdown. The store can usually be
maintained at its design value without the need for all units to be in operation provided
there is not a high additional refrigeration load due to product and heavy traffic in and out
of the store. Multiple units also allow each unit to be defrosted in sequence and this
arrangement has the least effect on storage conditions. If a hot gas defrost system is used,
then a multiple unit system is essential so that the units in use provide the necessary
refrigeration load for the refrigeration compressor.
Cold Store with Suspended Unit Cooler and Head Space above Pallet Stacks
Cold store with cooler unit outside the main store
With small units, electrical defrosting is more common. The defrosting of unit
coolers in small cold stores is usually automatic and operated by a time clock. With this
mode of operation, the timing of defrosts should be arranged to coincide with times when
the refrigeration load is low, usually during the night.
9.2. PREFABRICATED COLD STORES:
Besides prefabricated panels and the structural components used in the
construction of cold stores, there are "building kits" available on the market today for
small modular cold stores. The most complete "kits" include wall and roof panels, loading
ramp, as well as refrigeration plant. A typical example is a cold store with a nominal
storage capacity of some 200t measuring 12 x 12 x 6m built with self-supporting
polyurethane insulated panels faced inside and out with galvanised and plastic coated steel
sheeting, as well as a prefabricated floor. The only local requirement is a concrete floor
slab on which the building is erected. Normally the assembly is carried out by specialists
and the erection time varies between 4 and 8 weeks depending on local conditions. The
material for the store is shipped in three ordinary containers one of which contains the
engine room which can be contained in a weatherproof building adjacent to the cold store.
A possible cross-section of such a prefabricated cold store with a simple overhead crane is
shown in Figure.
CROSS - SECTION OF PREFABRICATED COLD STORE.
10. REFIGERATION SYSTEMS
There are essentially two types of cold storage refrigeration plants. These are the
Direct and Indirect Systems.
In the direct system, direct expansion coils are used in the cold rooms with the
compressor and high side equipment concentrated in a central machine room. The indirect
system utilizes this same type of machine room, and in addition, adds brine chillers. Brine
is chilled in the machine room and is circulated by pumps through pipe lines to the cold
rooms where it is circulated through the cooling units in the various rooms.
The indirect brine system has advantages in simplicity of operation, ability of
system to absorb short load peaks, ease of control, avoidance of possible leakage of
refrigerant into storage areas, and flexibility of piping system design. Brine is particularly
desirable for convection coil installation and for large spread out systems. The higher
initial cost due to the need for pumps, motors, valves, and control equipment; the
maintenance costs required for this equipment, and monitoring the condition and strength
of the brine; and higher power cost due to added pumps and lower compressor suction
temperatures more than offset the advantages.
Plants can be either or both types. One type or the other will usually predominate.
The normal cold storage plant will have at least two and sometimes more refrigerant
temperature levels available with one refrigerant piping system at a proper temperature
level for cooler storage and another for freezer storage. The large machine room shall
contain a minimum of 2 and possibly up to 4 or 5 refrigerant lines to the storage space.
One of the chief advantages of the central machine room is flexibility. By cross
connections and the use of two stages, it is almost impossible for a breakdown to occur
that will seriously affect the overall operation of the plant. This is particularly true with a
multiplicity of machines. Freezer storage is most often handled by booster compressors,
with the boosters normally discharging into an intermediate pressure that is also used for
cooler storage. The optimum booster compressor discharge may be at variance from the
suction pressure required for the coolers, but usually is so close that it is impractical, from
an efficiency standpoint, to carry the optimum intermediate pressure plus that required for
cooler storage. All high stage machines in the machine room shall be valved so that any
machine can operate on any of the high or intermediate pressures. Boosters serving
freezers should be interconnected so that they can be valved into whatever suction duty is
required. Variations will be found in valving of machines depending on the individual
plant design.
10.1
BRINE
CIRCULATING
SYSTEMS
AND
LIQUID
REFRIGERANT
RECIRCULATING SYSTEMS
1. BRINES: In an indirect refrigeration system, water is generally used as a secondary
cooling medium for temperatures down to 40 deg. F (4 deg. C). Applications require
cooling medium temperatures below 40 deg. F (4 deg. C) employ chemical solutions of
water having freezing temperatures substantially below the operating temperature.
a. Calcium Chloride. Calcium chloride brine is the most common secondary refrigerant
down to -40 deg. F (-40 deg. C). Corrosion is the principal problem for which
chromate treatment is recommended.
b. Sodium Chloride. Sodium chloride brine is used for applications where, due to
hygienic reasons, contact with calcium chloride brine may not be permitted. It is also
preferred for spray-type unit coolers for cold storage rooms. Sodium chloride brine
should not be used below 10 deg. F (-12 deg. C).
c. Propylene Glycol. Propylene glycol solution can be used for temperatures down to
-35 deg. F (-37 deg. C). This brine may be more expensive than the calcium or
sodium chloride brines and shall be inhibited to neutralize corrosive properties.
Usage: Brines are used in larger systems where safety and easy piping is considered of
prime importance. Brines are desirable for transmitting refrigeration because:
(1) In the event of leakage, brine is less objectionable than refrigerant gases
(2) Individual temperature control of each space or fixture may be simpler than
with a direct expansion refrigerant
(3) Sharp but short load peaks may be absorbed in a brine system, particularly if it
is designed as a storage system
(4) If brine sprays are used, a defrost system is not required because any moisture
condensed in the cooling is dissolved in the brine.
Undesirable Features. Brine systems have certain undesirable features including:
(1) Corrosion of equipment is often possible due to chemical or electrical action
(2) Additional equipment and maintenance due to the need for pumps, motors,
valves, and control equipment, and the required attention to correcting the
condition and strength of brine;
(3) Possibility of equipment damage due to freezing if brine condition and
refrigerant plant operation are not properly supervised
(4) normally higher power cost due to added pumps and lower compressor suction
temperature.
Where circulating brine systems are required below -35 deg. F (-37 deg. C), the
usual brines are unusable. Below this temperature, the following brines have been used:
trichloroethylene, methylene chloride, Refrigerant 11, methanol, ethanol, and acetone.
These are specialized systems and require pressurization of the refrigerant to prevent
evaporation.
BRINE PIPING SYSTEMS:
a. Piping, Pumps, and Valves. The piping, pumps, and valves shall be of materials
and sizing to suit the brine used.
b. System Frictional Pressure Drop. For charts and tables on viscosity, specific
gravity and other required information
c. Make-Up. Make-up for brine system shall be mixed in a tank to proper
proportions and pumped into the system. Brine systems shall not be connected directly to
potable water supplies.
10.2. LIQUID REFRIGERANT RECIRCULATING SYSTEMS:
A very excellent method of feeding refrigerant where direct expansion is used is by
means of the liquid recirculation method. In this system refrigerant liquid is fed into a low
pressure receiver connected to the suction of the load being worked upon. The liquid
refrigerant flashes down to the temperature corresponding to the suction pressure and the
chilled liquid is then pumped to the evaporator units in the cold storage rooms. Instead of
boiling off all of the liquid refrigerant in the evaporator unit as is done with flooded or
expansion valve operation, an excess of liquid is fed in to the evaporator unit. In ammonia
plants, this flow will be as high as 4 or 5 times more refrigerant pumped through the
evaporator unit than is evaporated. In the case of the halocarbon refrigerants, slightly less
liquid is normally pumped. With the increased flow of liquid, the liquid refrigerant
becomes, in part, a brine flowing through the evaporator and giving very high performance
by keeping the entire inner surfaces of the refrigerant tubes wet with refrigerant which
increases the heat transfer ability of the tubes.
The excess liquid refrigerant flows back to the low side receiver along with gas
from evaporation. At the receiver, the gas separates and is pumped back to the compressor
and on to the high side. The return cold liquid drops into the liquid pool in the low side
receiver and is again circulated through the system. Liquid flow may be accomplished
either with mechanical pumps designed for liquid refrigerant flow or by patented pressure
pumping systems in which regulated high side pressure is used to force the liquid
refrigerant through the low side units. In some of these systems the chilled liquid is
isolated in relatively small drums or vessels, and the high side pressure applied to force the
liquid into the system. The high side pressure is normally reduced so that no more pressure
than that required to force the liquid is applied.
Alternating drums can be used so that a continuous flow of liquid can be assured
by allowing one drum to fill while the other is feeding. In either system, pump or pressure
feed, the end result is to overfeed the low side units. Liquid recirculation has a number of
advantages over either flooded operation or direct expansion. Control is simplified in that
all of the refrigerant flow controls are outside of the chilled areas and at one location at the
low pressure receiver. A simple solenoid valve in the liquid inlet to low side unit is
sufficient to shut off the flow of liquid for temperature control or for defrost application.
Other controls can be used on the low side unit if more sophisticated control is desired, but
the basic flow controls are concentrated at one vessel which is of some advantage in any
type plant. Close temperature differences between refrigerant and room temperature can
be maintained by this system. Evaporator surface is used more efficiently than with other
methods of direct refrigerant cooling and a minimum amount of surface is required for
good results.
Since more liquid refrigerant is circulated than in the conventional refrigeration
system, larger liquid and suction lines are necessary. In a large plant, the first cost of a
recirculated liquid system will be very little, if any more than with any other good
refrigerant cycle, direct expansion or flooded. In any recirculating system utilizing
refrigerant pumps, a spare pump should always be included as insurance with each system.
Any refrigerant system can successfully use a liquid recirculation system, but the most
commonly used refrigerant with these systems is ammonia.
[
11. PARTS OF THE REFRIGERATION SYSTEM
11.1 COMPRESSOR:
Three
types
of
compressors
are
generally
used;
these
are
the
OPEN
RECIPROCATING, SCREW, and CENTRIFUGAL TYPE. Open reciprocating
compressors are used in the 25 through 250 ton (88 to 880 kW) range. Screw machines
may be the best choice in a range of 200 through 800 tons (700 through 2800 kW).
Centrifugal units are usually used for the largest installations. Many cold storage
operations use the medium speed multicylinder compressor, either direct driven or belt
driven at operating speeds of 900 to 1200 RPM. The first cost is less than the slower speed
machines and maintenance is low and machine life would be long when these machines
are used in a properly designed plant. Adequate safeguards are a necessity in the plant,
with large suction accumulators and intercoolers being required for proper operation.
Booster compressors most commonly used are the reciprocating type and the
screw type. Both render excellent service. In a constant suction pressure plant where the
low, or booster, suction is kept at a relatively constant level, the screw and reciprocating
types of compressors will both operate efficiently. This type of service is used in freezer
storage rooms and in continuous freezers. Where suction pressure can vary widely, as in
some batch freezing plants, the screw compressor can cause some problems when
compression ratios become too high, but the reciprocating compressor is not particularly
bothered by this condition. The reciprocating machine with internal unloading, can more
efficiently match compressor capacity with system requirements than the screw
compressor. Both types of compressors are used with success in cold storage operations.
Automation of the cold storage facility is required because the cold storage plant must
operate continuously 24 hours. Heavy duty equipment, although having a greater first cost,
is justified by the longer life and longer maintenance free periods. In sizing electric motors
for compressor drives, the horsepower required should be checked for all possible
conditions of load that may be encountered in the operation of the plant and a motor
selected that will not be overloaded under any conditions that may be imposed upon it by
the compressor it is driving.
11.2. CONDENSERS:
The larger cold storage plants will use cooling towers or evaporative coolers for
condensing.
(i) Evaporative condensers. Condensing temperatures and their corresponding
pressures should be kept to the practical minimum for long equipment life and economical
operation. If evaporative condensers are used, desuperheater coils for the incoming
refrigerant gas can be used to good advantage, especially in an ammonia installation. A
good desuperheater coil (for ammonia) and proper bleed-off of water, will minimize
scaling problems. With parallel operation of evaporative condensers, proper trapping of
the outlets and sufficient height of the condenser outlet above the receiver should be
observed to prevent liquid backup in the condensers which reduce capacity.
(ii) Cooling tower with water-cooled shell and tube condensers. Condensers
shall be oversized sufficiently to assure adequate heat transfer.
11.3. RECEIVERS FOR REFRIGERANT:
Receivers for refrigerant should be sized generously. It may not be necessary to
install receiver capacity for the pump-down of the entire plant. Most large plants are
somewhat sectionalized so that various sections of the plant may be pumped down and
adequate receiver capacity should be installed to hold the charge from the largest section.
It is also good practice to use a standby receiver capable of handling a bulk truck shipment
of several thousand pounds of refrigerant, since cost can be lessened by buying refrigerant
in larger bulk quantities. Receivers shall be installed adjacent to one another if they are to
be operated in parallel for best operation. Parallel receivers shall have equalizing lines
between them.
11.4. ACCUMULATORS AND INTERCOOLERS:
Large accumulators and intercoolers should be used in any ammonia plant and are
useful in Halocarbon Refrigeration Systems. Suction line accumulators shall be provided
with liquid refrigerant return systems. Accumulators should be large enough to keep
refrigerant velocity below a point where any liquid carry over to the suction line will
occur. Adequate baffling should be built into the accumulator to prevent splashing or
turbulence of the liquid refrigerant from causing liquid refrigerant to enter the suction line.
Liquid return systems may be either powered by pressure or by liquid refrigerant pumps.
From the accumulators, some systems return the excess refrigerant directly to the plant
receiver, while some return it to the liquid line periodically after shutting off the main flow
from the liquid receivers.
11.5. REFRIGERANT PUMPS:
When using a liquid refrigerant recirculating system, liquid flow is accomplished
with mechanical pumps or by a gas pressure pumping system. The mechanical pumps
include open, semi-hermetic, magnetic clutch, and "canned rotor" arrangements with
either positive rotary, centrifugal or turbine vane construction. Cavitation and Net
Positive Suction Head (NPSH) are considerations when selecting the pump. Sealing of
shafts usually requires double mechanical seals with an oil feed from an oil reservoir.
Motors are selected with a service factor to take care of operation with cold, stiff oil.
Surrounding temperatures, heat gains, operating pressures, internal bypasses, operation of
automatic valves, and evaporation of refrigerant are all to be considered in selecting a
mechanical pump.
11.6. TWO-STAGE REFRIGERATION SYSTEMS:
The main operating economy in two-stage plants is obtained by prechilling the
liquid refrigerant at the intermediate pressure before using it in the low stage evaporators.
This requires the use of some type of intercooler. This intercooler also serves the function
of chilling the booster discharge gas to a saturated condition. For efficient and economical
operation, the liquid chilling feature should not be eliminated from an intercooler.
Intercoolers should be generous in size and with some reserve for future plant growth.
11.7. EVAPORATORS:
Evaporator equipment in the various storage rooms is mostly confined to some
type of forced air evaporator unit, either floor or ceiling mounted. Pipe coils should not be
used because their first cost is high and defrosting is difficult. The typical ceiling type
evaporator consists of a cooling coil with fins at various spacings depending on the
temperature of the room and manufacturer of the coil. Sizes of coils will usually vary from
2 to 20 tons (7 to 70 kW) refrigeration capacity. Air circulation is obtained by a propeller
or squirrel cage fan, either blowing through the coil or pulling air through the coil. Drain
pans under the coil are used to catch drip from condensation or defrost as the case may be.
Drain pans are sometimes insulated to prevent external drip from a cold pan. A
number of ceiling fan units placed in a line and blowing out from one wall of a cold
storage room can cover wide rooms without duct work and
even temperatures and
uniform air flow can be maintained. The more units in the line, the wider the room that can
be spanned. With high ceilings in a cold storage room, a distance of 100 to 150 feet (30 to
46 m) may be spanned by the blower coils along one wall of the room with the blower
units evenly spaced. The multiplicity of units all blowing in the same direction tends to get
the entire mass of room air circulating in a parallel pattern so that the entire room is well
covered with adequate air circulation. Care should be exercised when using ceiling type
blowers that maintenance considerations be included in the design. These coils are up and
out of the way and there is sometimes a tendency to forget about them until trouble
develops.
A maintenance inspection schedule shall be posted in a conspicuous place to
prevent breakdown of equipment. In cooler and freezer storage rooms, propeller fans are
provided when no duct work is involved, since the propeller fan is more efficient than the
centrifugal fan when very small pressures are needed. Fans may be direct connected to the
shaft of the driving motor or belt driven depending on the size and horsepower required to
drive the fan. The larger units will employ slow speed belt driven fans with standard
motors. The smaller units will utilize smaller higher speed direct motor mounted fans.
Many times the smaller fan units will not have replacement parts when they need
replacement after a few years and will require new unit purchases. Good practice in
freezer operation is to use units with the fan or fans mounted to pull air through the coil
and discharge it out into the room. The coil defrosts the air and the fan is less likely to get
frosted. In most instances the floor-type unit consists of a coil and fan or fans mounted
above a drain pan and all encased in a suitable housing.
Centrifugal fans are normally used since air must normally be conducted up to the
ceiling level of the cold room and turned to spread out in the room. This imposes some
resistance and more horsepower is usually required for the floor-type unit than for a
comparable ceiling unit. Air entering a floor-type unit also makes a 90 degree turn to flow
through the coil and in a standard ceiling unit passes straight through the unit without
turns. The main advantage of the floor unit is ready accessibility for maintenance and
repair. The disadvantages are that it takes up floor space that could otherwise be used for
merchandise storage and that it, if not heavily guarded, is subject to damage from
materials handling equipment. Since floor-type units are usually larger than the ceiling
type, fewer are used per room and piping costs will normally be less in a total installation.
This will about offset the normally higher cost of the floor-type unit so that the total
installation cost and equipment cost will not vary significantly regardless of the type units
used.
11.7. REFRIGERATION SYSTEM FOR SMALL COLD STORAGE BUILDINGS
(UP TO 25 TONS)
The small plant refrigeration system is usually the single package unit containing
evaporator, compressor, air-cooled condenser, receiver, halocarbon refrigerant, and control
devices if suitable outside walls are available for the through-the-wall condenser section.
Split system package units with remote air-cooled condenser should be used if suitable
outside wall is not available. One condenser should be used with each compressor. The
condenser can be located outside at ground level, on the building roof, or in a common
equipment room with adequate forced ventilation. Compressors in package units are
reciprocating hermetic or semi-hermetic type. Refrigerant system is the direct expansion
halocarbon type; refrigerants R-12, R-22 and R-502 are all used depending on application.
Evaporator fans are the direct mounted propeller type either blowing through a coil bank
or pulling air through the coil bank. Larger than 25 ton compressors may be used if so
dictated by an economic and energy analysis. The smallest rooms will usually consist of a
single air-cooled condensing unit with a single direct expansion coil in the cold room.
Control will be off and on from a thermostat either starting and stopping the compressor or
operating a liquid line solenoid valve which will allow the compressor to pump down or
shut off on a pressure control. Various means of automatic defrost may be used. As larger
rooms are encountered and also a multiplicity of rooms, a number of condensing units may
be employed with multiple evaporators to each unit. The unit package system for wall or
the split package system for roof or ground mount of the condensing unit of the factory
fabricated type may be used, singly or in multiple on a single room. Life cycle cost
analysis should be used to determine most economical choice.
11.8. REFRIGERATION SYSTEM FOR INTERMEDIATE AND LARGE COLD
STORAGE BUILDINGS (OVER 25 TONS).
The recommended system for intermediate and large cold storage buildings is the
recirculated ammonia type. This system should utilize reciprocating compressors,
evaporative condensers, fan coil evaporators and necessary accumulators, intercoolers, and
recirculators. Evaporators in all spaces that operate at temperatures above freezing with
chill spaces 33 to 50 deg. F (1 to 10 deg. C) shall be on one set of compressors in one
system, and evaporators in spaces below freezing with freezer spaces 32 to -20 deg. F (0 to
-29 deg. C) shall be on a second set of compressors in another system.
A standby compressor shall be provided in both the freezer system and in the chill
temperature system for pull-down after unloading and for emergency operation during
outage of a compressor unit. For parallel operation, piping shall be provided to equalize
crankcase oil levels. In addition to oil equalizers, parallel systems often use an oil
reservoir and oil level floats on each compressor.
11.9. INTERMEDIATE AND LARGE COLD STORAGE BUILDINGS.
In the warehouse system, reliability and low operation cost resulting from efficient
design and application of machinery should be the first consideration of the owner. Year
round operation and maintenance of temperatures and conditions in the warehouse rooms
within narrow limits must be maintained. For this reason, reliability is paramount, which
means heavy duty machines and non-over-loaded motors and equipment. Operating costs
are also important.
These costs consist of power cost and maintenance, replacement, and repair costs.
For best results, all of these add up to obtaining a system containing the best and most
efficient components available.
11.10. EQUIPMENT LOCATION:
The refrigerating equipment for large refrigerated rooms should be located in a
separate machine room which should include ample space for the equipment and its
maintenance. It should have adequate ventilation, be segregated from other areas, and be
located on a outside wall and have separate exits. Small and medium size prefabricated
rooms may have refrigeration equipment mounted on top or alongside.
(1) Air-cooled condensers, evaporative condensers or water cooling towers may be located
on the roof or at grade adjacent to the machine room.
(2) The evaporator equipment may be located in the conditioned space or in a penthouse
over the refrigerated rooms. The penthouse offers many advantages:
(a) Storage area is more fully utilized.
(b) Defrost water drains can be piped through penthouse walls to discharge on the main
storage roof.
(c) Equipment is not subjected to physical damage by stocking trucks.
(d) Service on cooling equipment and controls can be handled by a single individual from
floor or roof deck location.
(e) Maintenance and service costs are minimized.
12. REFRIGERANTS:
Refrigerant selection affects both first costs and operating costs. The most
common refrigerants used in cold storage refrigeration systems is one of the halocarbon
compounds R-12, R-22, R-502 or ammonia (R-717).
12.1. HALOCARBON REFRIGERANTS:
(1) Halocarbon refrigerants are rated as group 1 according to Safety Code for Mechanical
Refrigeration and are considered nonflammable. Refrigerants R-22 and R-502 are
classified as group 5a according to Underwriters' Laboratories classification of
comparative hazard to life of gases and vapor while R-12 is classified as group 6. These
ratings indicate the halocarbons are safer to use than ammonia and are generally preferred
for safety reasons.
(2) The halocarbon refrigerants can be used satisfactorily under normal conditions with
most of the common metals such as steel, cast iron, brass, copper, tin, lead, and aluminum.
(3) Because of their physical properties, the halocarbons are better suited to air-cooled
condensing. The required lower compression ratios allow the use of lighter equipment.
12.2. TYPES OF HALOCARBON REFRIGERANTS:
(i) Refrigerants 12, 22, and 502 should be used within their saturated suction temperature
range for single stage compressors, but they may be used down to -80 deg. F (-62 deg. C)
with compound or cascade systems.
(ii) Refrigerant 22 is preferred over R-12 in single stage systems because only
approximately 60 percent of the compressor displacement is required. Additional
advantages are: less refrigerant circulated per ton, smaller pipe sizes, and higher suction
pressures.
(iii) Refrigerant 12 is used by compressor manufacturers to increase the number of
available units without adding sizes of compressors. It does have a slightly lower brake
horsepower per ton than Refrigerant 22.
(iv) The main disadvantage with Refrigerant 22 is oil return, and therefore, an efficient oil
separator must be used.
(v) R-502 has lower brake horsepower per ton than R-22.v
12.3. AMMONIA REFRIGERANT:
(1) Ammonia is rated group 2 according to ASHRAE 15-1978 Safety Code for
Mechanical Refrigeration and is considered explosive when present in a range of 16 to 25
percent by volume in air. It is rated as group 2 according to Underwriters' Laboratories
classification of comparative hazard to life of gases and vapors. Although ammonia is a
toxic material, it is considered a self-alarming refrigerant. Its smell makes leaks quickly
detectable.
(2) Most of the common metals can be used with ammonia with the exception of copper,
brass, bronze, and zinc.
12.4. REFRIGERANT PERFORMANCE:
Table lists comparative refrigerant performance for four common refrigerants
based on 5 deg. F (-15 deg. C) evaporation and 86 deg. F (30 deg. C) condensation.
Because of the higher required compression ratio, ammonia compressors are heavier in
construction than halocarbon compressors; but because of the greater refrigeration effect,
the compressor size is smaller or the operating speed is less. The heavy duty machinery
generally has a long life and low operating costs.
COMPARATIVE REFRIGERANT PERFORMANCE:
Refrigerant BHP/Ton
(W/W)
Compression Refri. Circulated
Ratio
lb./min./ton (kg/s/W)
Typical Saturated
Suction Temperature
Deg. F, (Deg. C)
R-717
0.989 (0.210)
4.94
0.422 (11)
-10 to 40(-23 to 4)
R-12
1.002 (0.212)
4.08
4.00 (108)
30 to 50(-1 to 10)
R-22
1.011 (0.214)
4.03
2.86 (77)
30 to 50(-1 to 10)
R-502
1.079 (0.229)
3.75
4.38 (118)
-40 to 0(-40 to -18)
12.5. LEAK DETECTION OF HALOCARBON REFRIGERANTS:
There are several methods of leak detection, the most common being the
electronic detector and the halide torch. The operation of the electronic detector depends
on the variation in current flow due to ionization of decomposed refrigerant between two
oppositely charged platinum electrodes.
The halide torch is a fast and reliable method of detecting leaks of halocarbon
refrigerants. Air is drawn over a copper element heated by a methyl alcohol or
hydrocarbon flame. If halocarbon vapors are present, they will be decomposed and the
color of the flame will change to bluish green. The electronic detector is the most sensitive
although the halide torch is suitable for most purposes.
* Leak Detection of Ammonia: Ammonia leaks are quickly detected by smell. Location
can be found by burning a sulfur candle in the vicinity of the suspected leak or by bringing
a solution of hydrochloric acid near the object. If ammonia vapor is present, a white cloud
or smoke of ammonia sulfite or ammonium chloride will be formed. Ammonia can also be
detected with an indicating paper which changes color in the presence of a base.
REFRIGERANT REQUIREMENTS PER TON OF REFRIGERATION:
Refrigerant
Refrigera
ting effect
Latent heat
of vaporization
(kcal/kg)
(kcal/kg)
Volume of
liquid
Mass of refrigerant
circulated/std.ton
Circulated/std ton
(kg/min)=
(lit/min)
50/refrigeration effect
R-717
264.28
315.53
0.1895
0.138
R-40
77.93
94.15
0.642
0.712
R-22
38.46
31.995
1.3
0.105
R-11
37.51
46.65
1.335
0.913
R-113
29.00
38.95
1.725
1.11
R-12
28.31
38.75
1.765
1.369
The horse power requirement varies for refrigerant for effect of one ton of
refrigeration. The following table shows the horse power requirement for different
refrigerant.
HORSEPOWER REQUIREMENT PER TON OF REFRIGERATION:
REFRIGERANT
Hp Requirement/Ton Of Refrigeration
R-17
0.998
R-11
0.9325
R-12
1.01
R-22
1.02
R-744
1.855
R-40
0.97
12.6. REFRIGERATION EQUIPMENT SELECTION:
Refrigeration equipment is designed to operate continuously without ill effect and
it is the defrost problem it determines the compressor operating time. When the
refrigerant temperature is 0°C (or) higher, there is no frost and to general practice has been
to select equipment based on 20 (or) 22 hr operation .The equipment must be selected to
meet the following requirements.
1. Proper cooling of products loaded in chambers to the desired temperature and
maintenance of the temperature and the desired relative humidity. The daily product
loading is an important factor is the cooling load estimates.
2. Proper air distribution in the cold chambers for uniform cooling and maintenance of
desired condition.
3. System design to achieve the best possible energy efficiency. Since energy bills
constitute the biggest factor of running expenditure.
4. System shall have automatic/ semi automatic control and instruments for recording
storage conditions. Facility for setting the desired temperature level in the chambers,
depending on the product requirement should be provided.
5. For high humidity storage requirements, provision for external humidification (or) use
of sprayed coil air handlers can be made.
6. The system shall be easy to maintain with easy availability of spares, refrigerant gas
and services etc.
12.7. REFRIGERATION CONTROLS
There are three common methods of controlling the operation of refrigeration
compressors.
1. On-Off Based on Suction Pressure: Cold storage rooms use this method. The
compressor is cycled on and off based on the refrigerant pressure in the cooling coil. The
system responds slowly, and a narrow-range chart recorder will show the "saw-tooth"
pattern generally acceptable in storage areas. It is the least expensive form of control.
2. On-Off Based on Air Temperature: Environmental rooms with specified uniformity
greater than ±0.5ºC and gradients larger than 1.0ºC often turn compressors on and off in
response to air temperature in the room. The time constant of the system is shorter than
with suction pressure control, so heat load changes can be dealt with more quickly. Older
high-quality environmental rooms used this control with reasonably good results.
3. Time-Proportioning PID Control: Formerly used only in the most sophisticated
industrial processes, PID control is now nearly the same cost as on-off air temperature
controls. Microprocessors calculate the rate of temperature change, and cycle the
refrigeration equipment in minor increments (less than one second). This provides
exceptionally close temperature control--well within the classic 0.5ºC uniformity and
1.0ºC gradient as long as the equipment has enough cooling capacity and air has been
properly distributed throughout the room. Compression machines work with ammonia (or)
with halocarbon mostly R12, R22 and R502.The most commonly used compressor types
are multi cylinders reciprocating compressors always open for ammonia.
12.8. NOISE:
In storage rooms, noise is not generally an issue, so the complete refrigeration
system is often mounted on the room wall. This design is usually too noisy for
environmental rooms. A typical cold storage room system generates 90 decibels at a
distance of one foot from the fan. In contrast, an environmental room system seldom
generates more than 70 dB.
The decibel scale is logarithmic--an increase of 10 decibels represents a ten-fold
increase in sound power. In other words, the cold storage room system generates about
100 times more sound power than a well-designed environmental room. Normal human
conversation generates about 70dB--shouting generates about 90 dB.
The designer can ensure a quiet room through specifying four features:
1. Remote-mounted, vibration-isolated compressors, low speed fans, and manualoverride fan speed controls.
2. Placing the compressors in a remote mechanical room moves the noise away from the
room, and vibration isolators reduce their structure-borne noise.
3. Specifying fan speed to be no greater than 1140 rpm reduces the noise produced by the
fan tips, which travel through the air much faster than the motor shaft?
4. Manual-override fan speed control allows the room occupant to further reduce fan
speed. A lower speed--less air--can be perfectly acceptable when the sensible heat
loads in the room are below the maximums used for system design.
12.9. SAFETY COMPONENTS:
Often the designer must be concerned with the durability of the refrigeration
system since the room temperature will rise if the system is down for repairs. Several
inexpensive system components can be specified to increase system reliability.
1.
Refrigeration filter-driers: Inside the refrigerant piping, dirt and moisture
eventually corrodes the system and damages components. A small filter-drier removes this
contamination and can double the life of a compressor.
2.
Suction line accumulator: The compressor is a gas pump--not a liquid pump.
Since liquid cannot be compressed, it can burst the compressor seals or break the
connecting rods on the piston when it enters the compressor. A suction line accumulator is
a small tank located in the suction line--the piping between the cooling coil and the
compressor. It prevents liquid refrigerant that may have bypassed the cooling coil from
being pulled into the compressor. Liquid often bypasses the coil when loads fall rapidly,
preventing the entire refrigerant from evaporating to a gas inside the coil. If a system is
constantly losing compressors, this is often the reason. The problem is preventing with a
low-cost suction line accumulator.
3.
Multiple independent sensors: In cold storage rooms, a single temperature sensor
controls the refrigeration system, the temperature recorder and alarms when these are
provided. This has the advantage of low cost and avoiding confusion between sensors.
Environmental rooms, however, are not normally specified with such risk in sensors. If a
single sensor fails or moves out of calibration, the system does not control, the alarm
system is ignorant of the problem, and the chart recorder provides a false sense of security.
Generally, environmental rooms are specified to contain separate, independent sensors for
temperature control, recorders, and alarms. This minimizes the consequences of sensor or
instrument malfunction
12.10. OPERATING AND SERVICING THE REFRIGERATING EQUIPMENT
1. Temperature: The temperature of the cold rooms is taken daily by thermometers. The
simplest and crudest form is mercury (or) alcohol thermometers, graduated to 1/5°C.
Further, an automatic switch may control the temperature of a space, which, most often, is
a thermostat.
2. Relative humidity: The relative humidity of a cold room indicates the equilibrium
between the water evaporated form the produce and its elimination by the evaporator. For
example, the shriveling reaches 1% per month for relatively large fruits such as apples (or)
pears stored at 0°C. In practice, shriveling (or) wilting of produce is limited if the
following indications are observed.
i)
Store the produce at the lowest temperature compatible with its needs.
ii)
Limit the duration of storage and organize rotation of stock so that the first lots in
are the first out.
iii)
Keep the store 50 to 100% full, the less is the loading of the room the more rapid
the shriveling.
iv)
Keep the small difference between to temperature of the air and that of the
refrigerant in the evaporator.
v)
Avoid fluctuations of product temperature, especially in chilling.
vi)
Bring the running of the fans of the air cooler under control of the compressor unit.
vii)
Limit the duration and frequency of opening the doors.
12.11. AUTOMATIC CONTROL:
General Purpose Cold Storage Control : Operation of the entire refrigeration plant
shall be completely automatic.
a) Type of Control: Generally, it is only necessary to control the room temperature for
above freezing temperatures; defrost control shall be added for storage temperatures below
32 deg. F (0 deg. C). The room shall be provided with a room type or remote bulb type,
electric thermostat with adjustable differential 3 to 5 deg. F (2 to 3 deg.C).
b). Control Arrangement: Controls for a single compressor will be as follows:
i) One compressor for one room. A room or remote bulb thermostat shall control the
compressor motor. The refrigerant liquid line solenoid valve should cycle with the
motor.
ii) One compressor for more than one room. A thermostat shall control the liquid line
solenoid valve of the respective room, while the compressor shall be under control of a
low pressure switch. If rooms are at different temperatures, evaporator pressure
regulators shall be provided.
c).Thermostat Location: Thermostats or sensors shall sense average room temperature
and shall be located in a place having good air movement. For example, on the intake side
of circulating fans avoid locating thermostats or sensors in a direct stream of supply air.
d). High Limit Thermostat: An alarm to indicate excessive temperature in cold storage
space or a compressor fault shall be located locally and also located in the office or other
similar supervisory area. One alarm light shall be provided for each room and for each
compressor on panels with audible alarm with manual silencing switch.
e. Relative Humidity Control:
(1) When maintaining high relative humidities, the temperature rise from the coil
leaving temperature to the room temperature must be at a minimum. The temperature
difference between the coil leaving temperature and the refrigerant temperature must
also be small.
(2) For rooms requiring conditions such as 60 deg. F (16deg. C) and 40 percent RH or
50 deg. F (10 deg. C) and 60 percent RH, the relative humidity is controlled by
maintaining a proper coil leaving temperature or dew point and maintaining the space
temperature with reheat. Since this is not energy efficient, it is necessary to establish the
design operating conditions as realistically as possible.
(3) For rooms with relative humidity design of 75 percent to 90 percent, control is
obtained by controlling the coil refrigerant temperatures with an evaporator pressure
regulator and maintaining the room temperature by cycling a coil solenoid valve.
f. Control Diagram: A control diagram with sequence of operation shall be framed under
glass and mounted on wall of mechanical equipment room.
g. Programmable Controller: Considerations shall be given to providing a
programmable controller when dictated by an economic and energy analysis.
APPENDIX
RECOMMENDED OPTIMUM STORAGE TEMPERATURE FOR DIFFERENT
FRUITS AND VEGETABLES
Recommended Temperature and Relative Humidity, and Approximate Transit and Storage
Life for Fruits and Vegetable Crops
Product
Amaranth
Anise
Apples
Apricots
Asian pear
Asparagus
Avocados, Lula, Booth-1
Bananas, green
Barbados cherry
Bean sprouts
Beans, dry
Beans, green or snap
Beans, lima, in pods
Beets, bunched
Beets, topped
Belgian endive
Bitter melon
Black sapote
Blackberries
Blood orange
Blueberries
Breadfruit
Broccoli
Brussels sprouts
Cabbage, early
Cabbage, late
Carrots, bunched
Carrots, mature
Carrots, immature
Cashew apple
Cauliflower
Celeriac
Celery
Cherries, sour
Cherries, sweet
Chinese broccoli
Chinese cabbage
Coconuts
Corn, sweet
Cucumbers
Custard apples
Dates
Temperature
°C
°F
0-2
32-36
0-2
32-36
-1-4
30-40
-0.5-0
31-32
1
34
0-2
32-35
4
40
13-14
56-58
0
32
0
32
4-10
40-50
4-7
4045
5-6
4143
0
32
0
32
2-3
36-38
12-13
53-55
13-15
55-60
-0.5-0
31-32
4-7
4044
-0.5-0
31-32
13-15
55-60
0
32
0
32
0
32
0
32
0
32
0
32
0
32
0-2
32-36
0
32
0
32
0
32
0
32
-1 to -0.5 30-31
0
32
0
32
0-1.5
32-35
0
32
10-13
50-55
5-7
41-45
-18 or 0
0 or
32
Relative Humidity
(%)
95-100
90-95
90-95
90-95
90-95
95-100
90-95
90-95
85-90
95-100
40-50
95
95
98-100
98-100
95-98
85-90
85-90
90-95
90-95
90-95
85-90
95-100
95-100
98-100
98-100
95-100
98-100
98-100
85-90
95-98
97-99
98-100
90-95
90-95
95-100
95-100
80-85
95-98
95
85-90
75
Approximate storage
life
10-14 days
2-3 weeks
1-12 months
1-3 weeks
5-6 months
2-3 weeks
4-8 weeks
14 weeks
7-8 weeks
7-9 days
6-10 months
7-10 days
5 days
10-14 days
4-6 months
24 weeks
2-3 weeks
2-3 weeks
2-3 days
3-8 weeks
2 weeks
2-6 weeks
10-14 days
3-5 weeks
3-6 weeks
5-6 months
2 weeks
7-9 months
4-6 weeks
5 weeks
34 weeks
6-8 months
2-3 months
3-7 days
2-3 weeks
10-14 days
2-3 months
1-2 months
5-8 days
10-14 days
4-6 weeks
6-12 months
Durian
Eggplants
Figs fresh
Garlic
Ginger root
Gooseberries
Grapes, Vinifera
Grapes, American
Greens, leafy
Guavas
Jackfruit
Kiwifruit
Lemons
Lettuce
Lychees
Mangoes
Mangosteen
Melons:
Casaba
Crenshaw
Honeydew
Persian
Mushrooms
Okra
Olives, fresh
Onions, green
Onions, dry
Onion sets
Oranges, Calif. & Ariz.
Oranges, Fla. & Texas
Papayas
Passion fruit
Peaches
Pears
Peas, green
Peas, southern
Peppers, Chili (dry)
Peppers, sweet
Pineapples
Plantain
Plums and prunes
Pomegranates
Potatoes, early crop
Potatoes, late crop
Pumpkins
Radishes, spring
Radishes, winter
Raspberries
Rhubarb
Spinach
Squashes, summer
Squashes, winter
Strawberries
Sugar apples
Sweet potatoes
Tamarinds
Tangerines, mandarins, and related
4-6
12
-0.5-0
0
13
-0.5-0
-1 to -0.5
-0.5-0
0
5-10
13
0
10-13
0
1.5
13
13
39-42
54
31-32
32
55
31-32
30-31
31-32
32
41-50
55
32
50-55
32
35
55
55
85-90
90-95
85-90
65-70
65
90-95
90-95
85
95-100
90
85-90
90-95
85-90
98-100
90-95
85-90
85-90
6-8 weeks
1 week
7-10 days
6-7 months
6 months
34 weeks
1-6 months
2-8 weeks
10-14 days
2-3 weeks
2-6 weeks
3-5 months
1-6 months
2-3 weeks
3-5 weeks
2-3 weeks
2-4 weeks
10
7
7
7
0
7-10
5-10
0
0
0
3-9
0-1
7-13
7-10
-0.5-0
-1.5 to 0.5
0
+5
0-10
7-13
7-13
13-14
-0.5-0
5
10-16
4.5-13
10-13
0
0
-0.5-0
0
0
5-10
10
0
7
13-15
7
4
50
45
45
45
32
45-50
41-50
32
32
32
3848
32-34
45-55
45-50
31-32
29-31
90-95
90-95
90-95
90-95
95
90-95
85-90
95-100
65-70
65-70
85-90
85-90
85-90
85-90
90-95
90-95
3 weeks
2 weeks
3 weeks
2 weeks
34 days
7-10 days
+6 weeks
34 weeks
1-8 months
6-8 months
3-8 weeks
8-12 weeks
1-3 weeks
3-5 weeks
2-4 weeks
2-7 months
32
4041
32-50
45-55
45-55
55-58
31-32
41
50-60
40-55
50-55
32
32
31-32
32
32
41-50
50
32
45
55-60
45
40
95-98
95
60-70
90-95
85-90
90-95
90-95
90-95
90-95
90-95
50-70
95-100
95-100
90-95
95-100
95-100
95
50-70
90-95
85-90
85-90
90-95
90-95
1-2 weeks
6-8 days
6 months
2-3 weeks
24 weeks
1-5 weeks
2-5 weeks
2-3 months
10-14 days
5-10 months
2-3 months
34 weeks
24 months
2-3 days
24 weeks
10-14 days
1-2 weeks
2-3 months
5-7 days
4 weeks
4-7 months
3-4 weeks
24 weeks
citrus fruits
Tomatoes, mature-green
18-22
65-72
90-95
1-3 weeks
Tomatoes, firm-ripe
13-15
55-60
90-95
4-7 days
Turnips
0
32
95
4-5 months
Turnip greens
0
32
95-100
10-14 days
Watermelons
10-15
50-60
90
2-3 weeks
White sapote
19-21
67-70
85-90
2-3 weeks
White asparagus
0-2
32-36
95-100
2-3 weeks
Winged bean
10
50
90
4 weeks
Yams
16
61
70-80
6-7 months
Source: McGregor, B.M. 1989. Tropical Products Transport Handbook. USDA Office of Transportation,
Agricultural Handbook 668.
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