Sizing and Laying Out a Short-Term (Summer) Refrigerated Storage for Fruits and Vegetables
Table of Contents
Growers know that cooling fruits and vegetables quickly after harvest helps prolong the produce's shelf-life. Removing field heat reduces respiration activity, the growth of microorganisms, moisture loss, and helps keep produce firm during handling. Refrigerated storages are an integral part of the post-harvest cooling process, and are built as a management tool to:
The disadvantages of storages are their high capital and operating costs, and management requirements. As a result, the importance of good planning before building cannot be stressed enough. Not only is the stored produce often worth thousands of dollars, but because buildings are costly and used for relatively short periods each year, the cost per unit of produce storage can be high. Although the costs of a storage should be amortized over a period of about 10 - 15 years, a well built storage may last 25 years or more.
A complete storage and handling facility is comprised of more than just the storage. There may be: shipping and receiving areas; washing, sorting, grading and packing areas; storage space for equipment and supplies; a separate compressor room; and a lunch room, washroom, office, and sales area (Figure 1). In planning, one must consider future expansion, utilities, security, accessibility, and aesthetics.
Figure 1. Plan view of product flow on a typical peach operation. Arrows indicate the direction of product flow.
Ontario produces dozens of different types of fruits and vegetables over the growing season, starting with asparagus in May, to cole crops such as cabbage and cauliflower as late as November. Many summer (May to September) harvested crops such as asparagus, broccoli, corn, peaches and strawberries can be successfully held for brief periods (1 to 30 days) in short-term storages that are cooled using refrigeration and held at 0°C to 10°C, depending on the crop.
Autumn (September to November) harvested crops such as apples, cabbages, carrots, onions, potatoes and others, can be stored for much longer periods of up to 6 months or more in long-term storages that are cooled using refrigeration and/or cold outside air. This Factsheet discusses only the sizing and layout of short-term refrigerated storages, (or simply called storages in this Factsheet), although many of the same layout and planning principles apply for long-term storages as well.
Rapid cooling is important for virtually all summer fruits and vegetables. Although hydrocooling (cold water), icing (a mixture of water and ice, or just ice), and vacuum cooling (cooling to 0C by evaporating water from produce under a reduced pressure) are more appropriate for some crops, many Ontario growers find that forced-air cooling is the most cost effective method. (See OMAFRA Factsheet, Forced-Air Rapid Cooling of Fresh Ontario Fruits and Vegetables, Agdex 202/736).
Properly designed forced-air cooling systems use high capacity airflow fans to pull refrigerated storage air through pallets or racks of produce. Rapid cooling is accomplished by forcing the refrigerated air to come in contact with the hot produce.
Produce is either cooled immediately after being harvested in field containers, or after it is packed in a shipping container. Forced-air cooling right after harvest in the field containers is preferred, as it removes the field heat promptly and permits more orderly packing. Sometimes the field container is also the shipping container as is the case for berry crops. For produce such as asparagus or peaches, cooling often takes place in field containers such as baskets before further packing.
Unfortunately, with this procedure, the cooling capacity used for the 5% to 25% culls of the produce harvested will be wasted. The produce may also rewarm somewhat during the packing operation. Cooling after packing in corrugated cardboard shipping containers may be done instead. Regrettably, forced-air cooling is difficult in many of these containers because there are often too few and poorly located air vents.
Cooling of produce in storage can be accomplished in three ways (Figure 2):
Figure 2. Plan view of three storage layout options. Arrows indicate the direction of airflow blowing from the evaporator coils at the ceiling.
Option 1: Provide two storages; one for forced-air cooling hot produce, with the other storage to hold cold produce after cooling;
Option 2: Provide one storage where hot produce is forced-air cooled in one area, while cold produce is held in another area of the same storage;
Option 3: Provide one storage where hot produce is room cooled in one area, while cold produce is held in another area of the same storage.
Option 1 is preferred, but it usually has the highest capital cost since it requires a lot of floor area for air tunnels between pallets, and has extra wall area and doors. Hot produce is forced-air cooled either before or after packing in the forced-air cooling storage with plenty of refrigeration. After it is cooled, it is transferred to a holding storage which has less refrigeration capacity since cold produce is easy to keep cold.
Option 2 uses a forced-air cooling device (for hot produce) that is built in one end of the storage under the evaporator coils. Cold produce is then stored on the opposite side of the storage. This layout should be utilized if retrofitting an existing storage is the only alternative. This option may not be much less expensive than option 1 anyway, and the disadvantage is that adding a forced-air cooling system may require more refrigeration capacity. If the refrigeration is not adequate, the storage temperature may rise during the rapid cooling period. This may cause unwanted condensation, or sweating, on produce that is already cold, since warmer air could be contacting it. Furthermore, produce could fluctuate in temperature.
Option 3 is least preferred, but is also the least expensive one to build since it has the least floor area. Because no forced-air cooling is used, produce containers must be vented very well, and high airflows are needed from the evaporator fans. Unfortunately, cooling rates may be unacceptably slow using this method.
The flow of produce into and out of a refrigerated storage and related areas should be kept in one direction if possible. (Figure 1 and Figure 3). Produce should be arranged to allow a first-in, first-out policy.
Figure 3. Plan view of air spacing for optimum cooling.
Exterior hinged or sliding doors are expensive, but two or more may be necessary on walls parallel to the room airflow direction. They should line up with aisles inside the storage. Doors must be wide and high enough to accommodate forklift dimensions, loaded pallets and racks, or other off-season uses. There must be at least 2.1 m (7') of clear door height to accommodate most farm size fork-lifts, however 2.4 m (8') is preferred.
Aisles in the storage must be wide enough to permit right angle turns. For most farm size fork-lifts, these aisles must be at least 3.4 m (11') wide. Because unused aisle space is so costly, most growers use this area for temporary storage during heavy picking periods. Aisles should be sized and located to help accommodate the overflow. If one-way entry pallets are used, remember to allow room for entry with a fork-lift. If hand operated fork-lifts are used, much less turning room is required. Fork-lift suppliers should be consulted for turning radius information during the planning stage. Carbon monoxide (CO) gas is a deadly by-product of the combustion of propane, diesel, or gasoline fuelled engines. The 'Propane Storage, Handling, and Utilization Code, RSO 825/82' was designed to minimize worker exposure to it. The Ontario Ministry of Labour has a regulation that specifies the maximum permissible worker exposure levels to CO gas under different conditions. To reduce risks, fork-lift operators should: minimize the engine running time inside refrigerated storages; keep engines in good working order; install exhaust fans if necessary; or install catalytic converters on new or older equipment.
Good airflow in a storage is almost as important as having enough refrigeration capacity. As an analogy, consider your house's forced-air heating system. The size of the furnace makes little difference if there isn't a good air ducting distribution system for heat addition to each room and for cold air return. Likewise, refrigerated cold air must be evenly distributed to the produce to both cool it and keep it cool. Warmed air then returns to the evaporator coils for recooling.
Ceiling-mounted evaporator coils with high-capacity fans will direct cold air across the ceiling a distance of at least 15 m (50 ft), providing there are no obstructions. Leave at least 0.5 m (1.5 ft) of space between the top of the pallets or racks of produce and the ceiling to allow unrestricted air passage to the back of the room. In storages kept at or near 0°C, the air coming off the evaporator coils will likely be below 0°C. The space above the produce helps temper this air, and prevent produce from freezing.
Produce should be spaced at least 0.3 m (12") from walls downstream
and upstream of the evaporator coils. This layout will allow the cooled
air from the evaporators to pass down the wall, around the produce, and
finally back to the coils (Figure 4). Leave at
least a 0.2 m (8") air-gap on walls parallel to the room airflow.
Air along the walls is often warmer, and the walls may need protection
from forklift abuse. To ensure these spacings are provided, bumpers can
be made using 5 cm (2") thick lumber attached to the floor and/or
wall to push pallets or racks against. Be certain that these bumpers do
not interfere with room airflow. Pallets or racks should be spaced at
least 0.1 m (4") apart. Painting bright lines on the floor helps
indicate exactly where to place the pallets or racks.
Figure 4. Cross-sectional view of a storage showing airflow patterns.
In rooms where both hot and cold produce are stored together, the hot produce should be placed under the evaporator coils, with cold produce placed near the downstream part of the room on the opposite wall. This allows cold air from the evaporator coils to contact cold produce first, then return through the hot produce. If produce was arranged in an opposite fashion, the cold air would contact hot produce first, warm up, then contact cold produce. This could cause undesirable condensation, or sweating, on cold produce.
Avoid inside aisles that are parallel with the room airflow, since air could short-circuit directly down the aisle rather than around the containers. Air will always take the path of least resistance and will not go through or between pallets if there is a direct open area for it to flow instead.
Most new Ontario short-term (summer) refrigerated storages have floor areas between 90 to 185 m2 (1000 ft2 to 2000 ft2). Storages that are nearly square in floor layout are more efficient than rectangular layouts since there is less wall perimeter per floor area. This results in lower construction costs, less wall area for conductive heat gains, and less distance to move product around.
Small storages are more expensive per unit storage as there is relatively more vacant floor area around the perimeter to allow good airflow. Large storages are more cost effective per unit storage, but might not allow enough flexibility for different crop environment needs such as temperature and relative humidity. Because more produce is stored in large storages, there is often more time lost maneuvering pallets or racks. Wider aisles may be needed in this case. It might be more effective to have two smaller storages, one used for forced-air rapid cooling of freshly picked hot product, and the other used for cold packed product.
Most storages have walls between 2.4 m to 4.8 m (8' to 16') high. Low walls under 2.4 m (8') restrict airflow, future changes in stacking arrangements, and other uses during the off-season, and should not be used. In general, making the walls a little higher doesn't cost that much more, and it barely affects the refrigeration needs from the standpoint of more heat gain due to more wall surface area. For instance, increasing the wall height from 2.4 m to 3.6 m (8' to 12') in a storage with a floor area of 90 m2 (1000 ft2) represents a 50% increase in wall height. However, the increased construction and refrigeration costs would likely rise only about 15%.
For similar volumes of produce stored, it is usually less expensive to stack produce higher than to spread it out on the floor. That is, for containers that allow stacking on each other, the floor area and construction costs are reduced. Stacking higher is not always possible for some containers, and forklift capital and operating costs increase with lifting height requirements. In some cases, steel shelving racks can be installed to accommodate pallets that will not stack. Figure 5 shows three holding storage sizes that will each accommodate 192 bulk bins, 1.2 m x 1.0 m x 0.6 m (48" x 40" x 24"), for produce such as cantaloupes, peaches, or sweet corn.
Figure 5. Plan view of three storage sizes and layouts for the same volume of produce.
In storage A, four layers of bins are stored. A high percentage of this storage's cost is for concrete in the floor and foundation, and for the large roof system. In storage B, six layers of bins are stored. Concrete costs are reduced and the roof area is smaller, however the higher walls may need more structural considerations. In storage C, eight layers of bins are stored. Although concrete and roof costs are reduced, the high walls will require much more structural considerations.
Several pieces of information are needed, such as:
It is difficult to decide how large to build a storage when more than one type of produce is stored, and harvest or picking seasons overlap. (Growers must be careful to check the temperature, relative humidity, and compatibility of different types of produce if they are stored together. See OMAFRA Factsheet, Storage Incompatibility, Agdex 202/65).
A high-volume crop like peaches may be the one that dictates the storage size. Many Ontario peach growers pick into 12.5 litre (11-quart) baskets, then place them on movable steel shelving racks holding up to about 72 baskets to cool overnight, sometimes on a forced-air cooling system. The next day, the peaches are culled, graded, packed into masters, palletized, then returned to storage. The peaches are often shipped within a day or two of packing. In the following case study, the three storage systems shown in Figure 2 are demonstrated.
Suppose a grower has 20 ha (50 acres) of peaches that are produced over about a 65-day season. Production is about 14.6 t/ha (6.5 tons/acre), with average production over the season of 4.5 t/day (5 tons/day). However, on a heavy day, production could be twice that, or 9 t/day (10 tons/day). There are usually a couple of days each season when this still would not be enough, but the grower cannot justify building any larger. The plans are to use the aisle inside the storage for some of the production overflow on heavy days.
A decision has been made to store 3 days' worth of production which includes: l day of freshly picked product in 11-quart picking baskets, holding 8.2 kg (18 lbs) each, on 1.0 m x 1.2 m x 1.8 m (40" x 48" x 72") high steel racks holding 72 baskets on 6 layers; and 2 days of packed product in masters that each hold 7.7 kg (17 lbs) of peaches, and are stacked on pallets the same size as the racks, holding 70 masters. About 5% of the peaches are culled during grading and packing.
Peaches only need to be cooled down over about an 8 hour period. In the case of forced-air cooling, this could be accomplished using a series of tunnel coolers where refrigerated air is pulled through individual racks of hot peaches. The forced-air cooler would be turned off after about 8 hours, then the cold peaches would be packed either that day or the next, before more hot peaches are brought in.
The number of racks and pallets can be calculated as follows:
Three possible storage system layouts are shown in Figure 2. Since stacking of pallets of peach masters is not currently possible, there is little need to make the storage height any more than 2.4 m (8'). This height could be a limiting feature, however, for alternate uses in the off-season such as equipment or other crop storage. To reduce the storage dimensions and cost, fewer days of production could be stored, or packed produce could be stacked higher on steel shelving racks. Air gaps between pallets or racks and along walls should never be reduced to cut down on floor area.
A storage that doesn't cool produce effectively, or uses space inefficiently can reduce profits. Every m2 should have an assigned use so that it is economically justified. Clearly, the most efficient and cost-effective layout requires many considerations. One of the best methods is to use scale-sized pieces of paper representing pallets or racks, arranging them to arrive at many different layouts. You can quickly determine the optimum arrangement using this strategy. Remember, graph paper is inexpensive, while buildings are not. A few hours planning and laying out a refrigerated storage will save time, dollars, and frustration in the long term.
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