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.
Planning Before You Build
Although small, commercial, contractor-built cooling rooms are available,
they are often much more expensive than owner-built structures. In addition
to avoiding a substantial amount of the construction cost, you can tailor
a cooling facility to your individual needs by designing and building it
yourself. Whether the cooling facility is built or bought, you can ensure
its effectiveness by giving careful thought to the topics discussed in
the following sections.
Types of Produce. Different types of produce have different
cooling requirements. For example, strawberries, apples, and broccoli all
require near-freezing temperatures, whereas summer squash or tomatoes can
be injured by low temperatures (Table 1). If small quantities of produce
with different cooling requirements must be cooled or stored together,
the temperature will have to be set high enough to prevent chill injury
of susceptible produce. This temperature, however, will not provide optimum
quality and storage life for other types of produce.
Some fruits and vegetables produce ethylene gas as a natural product
of ripening and respond to this gas by accelerating their ripening. Others
do not produce ethylene but are very sensitive to it (Table
2). For sensitive
produce, minute quantities of ethylene gas will greatly accelerate the
ripening process even at low storage temperatures. It is very important
not to store items sensitive to ethylene with those that produce this gas.
Table 2. Products That Produce Ethylene or Are Ethylene Sensitive.
In addition to ethylene sensitivity, some types of produce generate
odors that are readily absorbed by other items. The odor of apples and
onions in particular are easily transferred to other produce items. Consequently,
care should be exercised when storing other products with either of these
items.
Most conflicts in storing mixed produce are avoidable, but serious
problems may develop if the unique requirements of each commodity are not
kept in mind. Complete information on postharvest cooling and storage requirements
may be obtained from Introduction to Proper Postharvest Cooling and
Handling Methods.
Quantity and Movement of Produce. Although the primary function
of any cooling facility is to remove field heat, an important secondary
function is to provide cold storage space. Cooling capacity and storage
capacity are separate characteristics, but together they determine the
size of the facility. Cooling capacity and, to a lesser degree, storage
capacity depend upon the size of the facility and the capacity of its refrigeration
system. Therefore, it is important to determine the amount of produce you
are likely to cool and store.
A refrigeration system can be thought of as a pump that moves heat from
one place to another. Refrigeration capacity, a measure of the rate at
which a system will transfer heat energy, is normally expressed in tons.
A ton of refrigeration capacity is the ability to transfer the amount of
heat required to melt 1 ton of ice in a 24-hour period (288,000 Btu). Said
another way, a refrigeration system of 1-ton capacity is theoretically
capable of freezing 1 ton of water in 24 hours. That is, it can transfer
288,000 Btu in 24 hours or 12,000 Btu per hour.
The correct size for a refrigeration unit is determined by three factors,
the first of which is the weight of produce to be cooled. Since most produce
is sold by volume (by crates, boxes, or bushels) you may have to determine
its weight per unit of volume. Obviously, the more produce to be cooled,
the larger the refrigeration unit must be.
The second factor is the minimum time required from start to finish
of cooling. Ideally, cooling should take place fast enough to prevent serious
degradation of the produce but no faster. Cooling produce faster than necessary
is unduly expensive because the refrigeration system must be larger and
the demand cost for electrical energy is greater. To cool a load of produce
in 2 hours instead of 4 may require twice the refrigeration capacity, and
the cost of electricity may be three times as high.
The third factor is the nature of the refrigerated space: its size,
how well it is insulated, and how it is to be operated. Because as much
as one-half of the refrigeration capacity in a typical facility is used
to overcome heat gained through the floor, walls, ceiling, and doors, it
is important to minimize these gains. Selecting a refrigeration unit of
the proper size will be discussed in a later section.
Storage Capacity. The decision to cool and ship produce immediately
or to store it for a time often depends not only on the type of produce
and market conditions but also on the availability of space in the storage
facility. The type of produce you grow will determine, in part, how much
storage space you need. Obviously, highly perishable produce requires less
storage space than less perishable items simply because it cannot be held
for long periods without losing quality.
If the construction budget will allow, it is advisable to construct
enough storage space for at least one day s maximum harvest of the most
perishable commodities and even more for the less perishable items. It
is much easier to build adequate storage space initially than to add space
later. Cost per square foot decreases and energy efficiency increases with
the size of the facility. Adequate storage space should not be overlooked,
since one of the major benefits of a postharvest cooling facility is the
marketing flexibility it allows by providing short- term storage. On the
other hand, excess (unused) storage space is a waste of energy and money.
To determine the amount of refrigerated space to build, use the following
formula:
V = 2.5 X (C + S)
Where:
V = volume of the refrigerated space in cubic feet
C = maximum number of bushels to be cooled at any one time
S = maximum number of bushels to be stored at any one time
After you have determined V, divide it by the ceiling height in feet
to obtain cooling room floor area in square feet. Keep in mind that the
ceiling height should be no more than 18 inches greater than the maximum
stacking height of the produce you intend to cool. For produce packaged
in bulk, volume must be converted to bushels before applying the above
equation.
Produce Packages. The fresh produce industry uses a bewildering
array of packaging containers, such as fiberboard cartons, bulk boxes,
baskets, wirebound cartons, and trays (Figure 1). The types of package
you select should always conform to the standard type used by the wholesale
market. Produce marketed in nonstandard containers often does not sell
easily.
Figure 1. Common produce packages.
Be sure to allow enough space in your floor plan for aisles and walkways.
They must provide ready access to all the produce stored in the room. For
small to medium cooling rooms, devoting 25 percent of the total floor space
to aisles and walkways is not excessive. Since produce containers should
never be allowed to touch interior or exterior walls, reserve at least
an additional 6 inches of space for good air circulation.
There are also limits as to how high produce containers may be stacked.
The maximum height varies with the commodity and type of package but should
not exceed a safe level nor damage the produce. To allow for good air circulation,
produce should never be stacked closer than 18 inches to the ceiling. Whether
or not you anticipate the use of forced-air cooling, allow sufficient space
for active cooling. That is, allow space to install forced-air fans or
to spread the produce out for rapid cooling.
If the produce volume is sufficient to justify the use of a lift truck
and hence palletized loads, their dimensions should be considered in the
design of the facility. Doors and aisles should be no less than 1 1/2 times
the width of the lift trucks. Ramps leading to floors above grade should
be inclined at a slope of no more than 1 to 5. It is also convenient to
include a raised dock for loading trucks and trailers.
Location and Layout. The location chosen for the cooling facility
should reflect its primary function. If you plan 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 the cooling
facility is used in connection with a pick-your-own operation, it is best
to locate it near restrooms and retail sales areas.
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.
Before you begin construction, familiarize yourself with any applicable
laws, regulations, and codes pertaining to construction and electrical
systems, worker health and safety, and the handling and storage of food
products.
Design and Construction
Plan number 6145 for a moderate-sized refrigerated storage building
with adjacent space for packing and sorting is included at the end of this
publication. The plan includes an optional retail sales room that can be
built if needed. This facility provides approximately 7,400 cubic feet
of refrigerated space with a design storage capacity of approximately 3,000
bushels. The nominal design cooling capacity is 300 bushels per day with
a 3 1/2 ton refrigeration unit. As explained earlier, however, the required
refrigeration capacity cannot be accurately determined unless the types
and amounts of produce to be cooled are known.
These plans illustrate only one of many workable designs. What may
be ideal for one operation may not be suitable for another. In developing
the overall design, therefore, give serious consideration to how the produce
will move through your facility. Doors, loading docks, and adjacent
work areas should be conveniently located to accommodate the flow of material.
Remember the cardinal rule of industrial engineering, Always make it easy
to do the right thing.
The construction of a produce cooling and storage facility is an investment
in quality maintenance. Therefore, the materials and workmanship of the
facility itself should be of the best possible quality. Many different
construction materials are suitable for such a project. The difficulty
is deciding which ones are the most appropriate and cost effective for
your application. Many problems can be avoided by paying particular attention
during the construction phase to the items discussed in the following sections.
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 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 (Figure 2). 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 the walls.
Figure 2. Bumper guard
Insulation. 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 product s R-value, its
cost, and the effects of moisture on it.
R-Value. A measure of an insulation s 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 wall s overall thermal resistance.
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 3.
Table 3. Insulation R-Values for Common Building
Materials
R-Value
Full thickness
Material 1 inch thick of material
Batt and Blanket Insulation
Glass wool, mineral
wool, or fiberglass 3.50
Fill-Type Insulation
Cellulose 3.50
Glass or mineral
wool 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
Urea-
formaldehyde 4.20-5.50
Building Materials
Solid concrete 0.08
8-inch concrete block, open core 1.11
8-inch lightweight concrete block,
open core 2.00
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
Masonite particleboard 1.06
25/32-inch insulated sheathing 2.06
1/2-inch Sheetrock 0.45
1/2-inch wood lapsiding 0.81
Cost. The cost of insulation varies considerably with the type,
whether expressed in dollars per square foot per inch of thickness or dollars
per square foot per unit of thermal resistance (R). For example, even though
the R-value per inch of polyisocyranuate (Table 3) is more than twice that
of loose-fill cellulose, the cellulose may actually be less expensive in
terms of insulating value per dollar.
Of the insulation materials commonly used for refrigerated rooms, loose-fill
cellulose is usually the least costly, followed by batts and blankets,
then various foam sheet materials, and finally sprayed or foamed-in-place
materials, which are the most expensive. Sprayed-on and foamed-in-place
insulations have the added advantage of sealing an otherwise leaky structure
and thereby greatly reducing infiltration. Sprayed-on foam also significantly
reduces labor and material costs because an interior panel wall is not
required. Certain types of foam insulation may constitute a fire hazard
if carelessly handled, and care should therefore be exercised. Check local
fire and building codes. In selecting any insulation material, carefully
consider the cost associated with installation and any additional material
costs.
Effects of Moisture. In most types of insulation the flow of
heat energy is impeded by small cells of trapped air distributed throughout
the material. When the insulation absorbs moisture, the air is replaced
by water and the insulating value is greatly reduced. For this reason,
insulation should be kept dry at all times.
With the exception of most plastic foam insulations, which are essentially
waterproof, all insulation materials must be used with a suitable vapor
barrier. A 4-mil polyethylene sheet is normally installed on the warm side
(outside) of the insulation, the opposite of the normal practice for house
construction. This placement prevents the formation of condensation on
and within the insulating material. The vapor barrier sheet must be continuous
from floor to ceiling. Where two sheets join, they should overlap 12 inches
and be positively sealed (for example, with duct tape).
Doors and Other Hardware Items. The door is a critical part of a cooling
facility. Improperly built or maintained doors can waste large quantities
of energy. Doors should have as much insulation as the walls and should
be well weather stripped to reduce the infiltration of warm air. Door gaskets
should always provide a good seal.
Door seals can be checked by inserting a thin sheet of paper between
the door and the seal area and then closing the door. The seal is acceptable
only if resistance is felt when the paper is pulled out. Remember that
a large single sliding or swinging door is much easier to keep tight than
a set of double swinging doors.
All large doors have a tendency to sag over time unless they are diagonally
braced and well supported. Use only the best grade of hinges and latches.
Be sure that the door can be opened from the inside. Plastic strip curtains
are often added to reduce energy loss when the door must remain open for
long periods. These curtains allow free entry and exit by people, produce,
and fork trucks but block the mixing of inside and outside air, which can
waste a substantial amount of energy. Although there are many ac-ceptable
designs, three door section details that have been proven in actual use
are shown in Figure 3.
Figure 3. Three acceptable door designs.
Calculating the Heat Load
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, calculate the required refrigeration capacity
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.
Example. The following example illustrates the method for calculating
the amount of refrigeration needed to operate a cold storage facility.
Two thousand bushels of summer apples are loaded into a storage room
23 feet square and 14 feet high (inside dimensions) at the rate of 200
bushels per day. The walls have an insulation value of R-16, the ceiling
a value of R-20, and the floor a value of R-11. The cold storage room will
be operated at 32 F during August when the mean maximum temperature is
80 F. The average maximum temperature for all 12 months for locations throughout
North Carolina can be obtained from Table 4.
Table 4. Average Monthly Maximum Dry-Bulb Temperatures for Various
North Carolina Cities
Note: Select the city that best approximates the climate of your
location.
Heat Conduction. Heat is conducted into the cooling room through
the walls, ceiling, and floor. The amount of heat flowing through these
surfaces is a function of their thermal resistance (R-value), their area,
and the temperature difference between one side and the other.
The walls have an R-value of 16 and an area of 4 X (23 X 14) or 1,288
square feet. The temperature difference between the inside and outside
is 80 F - 32 F, or 48 F. The heat conduction, HC, through the walls is
found by multiplying the area of the walls by the temperature difference
and dividing the product by the R-value.
Wall area (sq ft) X Temp. Difference (F)
HC (Btu/hr) = ----------------------------------------
R-value (sq ft F/Btu)
1,288 sq ft X 48 F
= ------------------
16 sq ft F/Btu
= 3,864 Btu/hr
Heat conducted through the ceiling is calculated using the same equation.
However, since the ceiling usually has a more direct exposure to sunlight
and hence a higher temperature, more insulation is installed in it, and
the temperature difference in the calculation is increased by 10 F (that
is, 80 F + 10 F = 90 F). If applicable, the installation of an attic fan
will reduce the temperature considerably, but the cost of the fan and the
electricity to run it should be weighed against the saving in heat conduction.
529 sq ft X 58 F
1,534 Btu/hr = ----------------
20
Heat conducted through the floor is also calculated with this equation.
However, since the ground has an average year-round temperature of approximately
55 F in most parts of North Carolina, less insulation is put in the floor,
and the temperature differential used in the equation is decreased to 23
F.
529 sq ft X (55 F - 32 F)
1,106 Btu/hr = -------------------------
11
The total amount of heat conducted through the walls, ceiling, and floor
is the sum of these three calculated values:
Field Heat. The second source of heat is the warm produce brought
into the cooling facility. The heat energy it contains is called field
heat. The amount of field heat is usually calculated from the mean
maximum monthly temperature in this example, 80 F. The apples are to be
cooled from a field temperature of 80 F to the optimum cold storage temperature
of 32 F within 24 hours.
Field heat, FH, is the product of the specific heat, SH, of the crop
(the amount of heat energy it holds per degree), the difference, DT, between
the field temperature and the storage temperature, and the weight, W, of
the produce.
FH (Btu/hr) = SH (Btu/lb/F) X DT (F) X W (lb)
The specific heat of water is 1 Btu per pound per degree Fahrenheit.
Since fruits and vegetables are mostly water, their individual specific
heat is directly related to their water content and can for practical purposes
be estimated as 1. For actual specific heat values, see Table 5. For this
example, we will use the actual value of 0.87. This means that 0.87 Btu
of heat must be removed to cool 1 pound of apples 1 F.
Table 5. Specific Heat and Heat of Respiration for Horticultural
Crops Grown in North Carolina
Although a bushel of apples weighs about 45 pounds, we will assume a
weight of 50 pounds to take into account the weight of the container, which
must also be cooled. The amount of field heat that must be removed is:
0.87 Btu/lb X (80 F - 32 F) X 200 bu X 50 lb/bu = 417,600 Btu
Since the apples are to be cooled to 32 F in one day, the hourly field
heat load is 417,600 Btu divided by 24 hours, or 17,400 Btu per hour.
Heat of Respiration. The third source of heat is the respiration
of the crop itself. Horticultural crops are alive and give off heat as
they respire. The amount of heat produced depends on the temperature, the
crop, and the conditions and treatment the crop has received. The heat
of respiration at various temperatures for horticultural crops grown in
North Carolina is given in Table 5. The amount of heat given off approximately
doubles for each 18 F increase in temperature. Therefore, less refrigeration
is required to remove the heat of respiration when produce is cool than
when it is warm.
On the last day of harvest, the storage facility will contain 1,800
bushels of cool apples and 200 bushels of field- warm apples. At this time
the respiration heat load generated by the stored crop will be the largest.
Table 5 shows that field-warm summer apples pro-duce about 0.340 Btu per
pound per hour, whereas properly cooled summer apples produce only 0.018
Btu per pound per hour. Thus field-warm apples pro-duce nearly 19 times
as much heat by respiration as do cool apples. Since the freshly harvested
(warm) apples will be cooled to their proper storage temperature within
one day, the heat of respiration used in the calculation will be the average
of the values. The average is:
0.340 + 0.018
------------- = 0.179 Btu/lb/hr
2
The weight, W, of the 1,800 bushels of cool apples is approximately:
W = 1,800 bu X 50 lb/bu = 90,000 lb
The heat of respiration of the cool apples, HRc, is:
HRc = 90,000 lb X 0.018 Btu/lb/hr
= 1,620 Btu/hr
The weight, W, of the 200 bushels of warm apples is approximately:
W = 200 bu X 50 lb/bu = 10,000 lb
The average respiration rate, as given previously, is 0.179 Btu per
pound per hour. Therefore the heat of respiration for the warm apples,
HRw, is:
HRw = 10,000 lb X 0.179 Btu/hr
= 1,790 Btu/hr
To obtain the total heat of respiration, HRt, we add the value for cool
and warm apples:
HRt = 1,620 Btu/hr + 1,790 Btu/hr
= 3,410 Btu/hr
Service Load. The fourth source of heat comprises a number of
miscellaneous items and is called the service load. It includes
heat given off by equipment such as lights and fans and by people working
in the storage room, heat brought into the storage area by warm air when
the door is opened, and heat that enters by air infiltration past faulty
door gaskets and through other cracks. The amount of heat contributed by
these sources is usually very difficult to deter-mine accurately. Service
load is therefore dealt with collectively and estimated to equal 10 percent
of the heat from the other three sources: conductance, field heat, and
heat of respiration. The service load, SL, is therefore:
The total heat load is the sum from all four sources, as shown in Table
6.
Table 6. Total Heat Load for Example Cooling Facility
1. Heat conductance
Walls 3,864
Ceiling 1,534
Floor 1,106 6,504 Btu/hr
2. Field heat
200 bushels per day at 80 F 17,400 Btu/hr
3. Heat of respiration
200 bushels warm apples 1,790
1,800 bushels cold apples 1,620 3,410 Btu/hr
Subtotal 27,314 Btu/hr
4. Service load
10% of subtotal 2,731 Btu/hr
Total heat load 30,045 Btu/hr
The approximate proportion of the total heat load contributed by each
of the four sources is:
As discussed previously, refrigeration systems are rated by how much
heat they will move or displace in a given length of time. The standard
unit of rating is the ton. Since 1 ton of refrigeration equals 288,000
Btu per 24 hours, or 12,000 Btu per hour, the refrigeration capacity, RC,
for a unit that would just handle the heat load in the previous example
under the conditions described would be:
A 2.51-ton refrigeration unit would need to run continuously to keep
the temperature in this storage facility at 32 F under the conditions listed
above. If the conditions used in this calculation were changed, a 2.51-ton
unit could be either too large or too small to maintain the desired storage
temperature. The capacity needed to maintain a temperature of 32 F would
increase if:
The refrigeration system were to run only part of the day,
More than 200 bushels of fruit were stored every day,
The building were larger,
The incoming fruit were warmer than 80 F,
The outside temperature were higher than 80 F,
The amount of fruit in storage were greater.
For example, if harvesting were increased to 400 bushels of apples per
day, the total heat load would increase to nearly 51,000 Btu per hour.
A 4 1/4-ton unit running continuously would be required to maintain a storage
temperature of 32 F under these conditions.
In practice, it is advisable in selecting a refrigeration system to
add reserve capacity to the calculated rating as a protection against overloads.
Of particular interest is the fact that refrigeration systems, for reasons
to be discussed later, are operated for a total of only 16 to 20 hours
per day. The total capacity of the system must be increased, therefore,
to compensate for the "off time." For example, if the system
described in the example were to operate for only 16 hours per day, it
would need to be 24/16, or 1.5 times as large--that is, 3.77 tons instead
of 2.51 tons.
Furthermore, it is good practice to increase the capacity of the system
by a chilling rate factor. The rate at which heat is removed from the produce
is not constant during the cycle but is greatest at the beginning. If the
capacity of the system is not sufficient to overcome the thermal inertia
of the produce, cooling time may increase above the specified limits for
the product. To compensate, a chilling rate factor of 1.5 is applied to
most fresh fruits and vegetables. This factor is applied in addition to
the on-time factor. Thus the actual refrigeration system capacity, RC,
required for the example facility would be:
RC = 2.51 X 1.5 X 1.5 = 5.65 tons
Reducing the Refrigeration Load
In the example, field heat accounts for 58.9 percent of the total heat
load. Once field heat has been removed from the crop, much less refrigeration
capacity is required to maintain the storage temperature. While 5.65 tons
of refrigeration are needed to cool the apples in the example, about 2
1/2 tons are needed to maintain a constant temperature after they have
reached 32 F. Therefore, anything that can be done to lower the temperature
of the crop brought into the storage facility will significantly reduce
the initial heat load, thus reducing the cost of the refrigeration equipment
needed and the electrical energy to operate it.
Harvesting early or late in the day or even at night helps reduce refrigeration
cost. Some large growers of highly perishable crops have started harvesting
"under the lights" to reduce cooling costs and preserve quality.
Although not usable for all crops, hydrocooling is an effective way to
remove the first 20 to 30 degrees of heat quickly, lessening the load on
the system. Even cooling with well water is a very energy efficient method
of precooling produce before it is placed in the cooling room. Note, however,
that some types of produce are sensitive to wetting, which encourages the
growth of microorganisms.
If the 200 bushels of apples in the example facility were cooled just
20 F before entering the cooling room, the total heat load would be reduced
to about 22,080 Btu per hour, or 26 percent. In this case, only 4.14 tons
of refrigeration would be required instead of 5.65. Table 7 shows the calculations
involved.
Table 7. Total Heat Load at a Field Temperature of 60 F
1. Heat conductance
6,504 Btu/hr
2. Field heat 10,158 Btu/hr
3. Heat of respiration 3,410 Btu/hr
-------------
Subtotal 20,072 Btu/hr
4. Service load 2,007 Btu/hr
-------------
Total heat load 22,079 Btu/hr
Although doubling the insulation value in the walls, ceiling, and floor
would reduce the heat conductance by one-half from 6,504 to 3,252 Btu per
hour, the percentage reduction of the total heat load would be small. The
only way to reduce the heat load substantially is to start the refrigeration
cooling process with fruit as cool as possible.
Other Factors to Consider in a Cooling Facility
Condensation and Humidity. The cooling coils of the refrigeration
system (Figure 4) must be colder than the air in the room if the air is
to be cooled. The larger the temperature difference, the greater the rate
of heat transfer and the smaller (and less expensive) the cooling coils.
However, the colder the coil surface, the more water vapor from the air
will condense on the coils, either as a liquid or as ice. Evaporator coil
condensation represents wasted refrigeration capacity and should be minimized.
Allowing hot, humid air to enter the cooling room is particularly costly
because the refrigeration system must not only cool the air but also condense
the additional water vapor.
Figure 4. Evaporator coils inside a cooling room.
Besides substantially reducing the energy efficiency of the system,
condensation of water reduces the relative humidity of the air. Since the
optimum storage humidity for most produce is 90 percent or greater, either
moisture must be added with a humidifier or the difference between the
air and coil temperatures must be minimized. The temperature difference
can be reduced by increasing the size of the coils enough that the air-to-coil
temperature differential is 5 F or less. Not all refrigeration contractors
are aware of the special needs of produce cooling, so when purchasing a
system be sure to specify a 5 F maximum air-to-coil differential. The system
will be slightly more expensive, but the benefits will soon pay for the
difference in cost. Table 8 lists the recommended maximum coil-to-air temperature
differential for various relative humidities.
Table 8. Maximum Evaporator Design Temperature Differential
Desired Relative Design Temperature
Humidity Differential
(percent) ( F)
If the cooling room temperature is below 36 F, the coil temperature
must be below freezing, and ice will therefore accumulate on the coil surface.
The ice acts as an insulator, greatly reducing the coil's capacity to absorb
heat from the air. This ice must be removed periodically by some type of
defrosting mechanism, such as electrical resistance heaters, a warm water
spray, or the momentary reversing of the refrigerant flow. If the refrigeration
system has sufficient capacity, it may be allowed to remain off for 6 to
8 hours to let the ice melt naturally without adding supplemental heat.
Mechanisms designed specifically to aid de-frosting introduce heat into
the room to melt evapor-ator ice, thus adding to the total cooling load.
The amount of heat added, although small, should be taken into account
when calculating refrigeration unit size.
Sanitation and Maintenance. It is essential that storage rooms
and containers be clean and sanitary. All accumulations of condensation
water should be piped outside. Clean all storage rooms thoroughly before
filling them. If molds are found growing inside the room, disinfect the
surfaces with a 0.25 percent solution of sodium hypochlorite (1 gallon
of household chlorine bleach in 20 gallons of water) applied with a high-pressure
washer. Allow the room surfaces to air dry for several days. Refrigeration
coils, fans, and ducts should be inspected and cleaned regularly. Refrigeration
coils in particular can become clogged with dust and dirt, substantially
decreasing their thermal efficiency.
Temperature Control. In controlling a cooling facility, the
most important temperature is that of the produce, not the air. Measuring
the air temperature will not correctly indicate produce temperature because
the heat of respiration always raises the temperature of the produce
above that of the surrounding air. Put thermometers and thermostat sensing
elements into the produce or at least into the produce container. Directly
measuring the temperature of the pulp (interior of the product) is the
only accurate way to determine the produce temperature. It is good practice
to use several pulp thermometers in various locations to obtain an accurate
temperature reading. Thermostats and wet-bulb thermometers should be recalibrated
from time to time with an accurate mercury thermometer. Humidistats can
be checked for accuracy with a sling psychrometer. If possible, avoid positioning
the sensing elements of controllers on the exterior walls or ceiling. Typical
temperature controllers are shown in Figure 5.
Figure 5. Temperature controllers.
The produce temperature and the humidity must be monitored frequently
during cooling and storage to prevent undercooling and chill injury. Also,
maintaining the proper temperature and humidity becomes more important
with increasing time in storage.
Container Design and Positioning. The movement of air inside
the cooling room helps conduct heat away from the produce. Produce containers
should be designed and stacked to allow sufficient air circulation to enhance
the cooling rate and keep the crop at the optimum storage temperature.
Evaporator coil fans may be positioned to aid air circulation inside the
cooling room. For the rapid cooling required by many horticultural crops,
it may be necessary to move cool air past the produce with additional fans.
Fans designed and operated specifically to force cold air through produce
containers can reduce cooling time by more than 80 percent if sufficient
refrigeration capacity is available.
Summary
Although cooling increases production cost, it is essential to maintaining
high product quality. No amount of cooling, however, will improve poor-quality
produce. If you wish to have high-quality produce after cooling and storage,
you must start with high-quality produce. Maintaining quality requires
harvesting the crop at the correct stage of maturity; handling it with
tender, loving care; and quickly cooling it to the proper storage
temperature.
Many factors must be considered when planning and building a cooling
facility. Among them are how it is to be used, the types and amounts of
produce to be cooled and stored, and the desired refrigeration capacity.
The correct size of a cooling facility cannot be determined strictly on
a "square foot per acre" basis. Similarly, the capacity of the
refrigeration system needed cannot be accurately determined solely on the
basis of the floor area or volume of the facility. By taking the time to
follow the design procedures described in this publication, you can assure
yourself that the facility you build will be adequate to meet your needs
under all foreseeable conditions.
![[ Produce Packages ]](design1.gif)
![[ Bumper Guard ]](design2.gif)
![[ Acceptable Door Designs ]](design3.gif)
![[ Evaporator Coils ]](design4.gif)
![[ Temperature Controllers ]](design5.gif)
![[ Perspective ]](design6a.gif)
![[ Floor Plan ]](design6b.gif)
![[ Longitudinal Section ]](design6c.gif)
![[ Section Through Cold Storage Room ]](design6d.gif)