The cooling of fresh fruit and vegetables by forced convection has been practiced for more than a century. The ice bunker rail cars developed after the US Civil War used ice as the cooling agent and, when coupled with air movement, generated cooling as the car moved down the track. In the early 1900s, Clarence Birdseye, the developer of the frozen food industry, used blasts of very cold air to quickly freeze meat and vegetables. Birdseye had observed Eskimos in Labrador who quickly froze freshly caught fish in 40°F weather. The frozen fish retained its fresh taste and texture almost indefinitely. From this observation came blast freezers that quickly freeze all types of foods in a high-speed stream of very cold air. Very quick freezing does not allow time for large ice crystals to form in the cells. Very tiny ice crystals do not puncture the cell walls, while large ice crystals puncture cell walls and ruin the texture of the food. This process is known as individual quick freezing (IQF) because the items are frozen individually before packaging.

Forced-air cooling was first developed at the University of California – Davis in the 1950s by Guillou and Parks. Since that time, forced-air cooling (sometimes called tunnel cooling or pressure cooling) has been studied extensively. The research results have focused on produce type, packing, and fan arrangement.

In California, the techniques of forced-air cooling evolved and spread to Florida and other produce growing areas in the US. The first forced-air cooling installation was built in North Carolina in the early 1980s. There are many reasons for the popularity of forced-air cooling. It is relatively simple, efficient, fast, clean, and does not require special packaging. In recent years, with the greater emphasis on food safety and energy efficiency, forced-air cooling has become the preferred method of produce cooling. Today, forced-air cooling is the workhorse of the postharvest cooling industry around the world.

Forced-air cooling is simply the application of forced convection to produce cooling. Fans are employed in a refrigerated space in one of two basic arrangements to create an air pressure differential across containers of produce. This pressure differential induces a cool airflow through the entire container and past the produce. Although the actual cooling rate depends on a number of factors, this method of forced convection is significantly faster than room cooling that uses natural convection. Even bulk containers of tightly packed produce such as string beans, small peppers, or cucumbers have sufficient spaces that allow the cool air to penetrate the mass and remove the heat.

Room cooling is generally sufficient for maintaining temperature, but is often too slow and inadequate for the timely removal of field heat in highly perishable produce. This is particularly true for produce in large containers such as bulk pallet bins or bulk boxes. The same is true for pallet loads of packaged produce and for produce such as strawberries, blackberries, or blueberries that require immediate cooling after harvest. Forced-air cooling has advantages when compared to other types of cooling:

1. Faster cooling reduces the time that produce must remain at elevated temperatures. This reduces deterioration and increases shelf life. In some cases, produce may be cooled almost as quickly by forced-air as by hydrocooling.
2. Faster cooling results in shorter turnaround times and more efficient utilization of floor space inside the cooler. Postharvest cooling facilities should be viewed as a piece of equipment rather than a structure. A fast cooler is an efficient cooler. The three goals of postharvest handling are keep it cool, keep it moving, and keep it clean. Forced-air cooling helps to accomplish all three goals.
3. Produce may be cooled effectively in both large and small unopened containers. Produce may be cooled in inexpensive fiberboard containers, while hydrocooling requires wax or plastic containers that are expensive and difficult to re-cycle. Air is very efficient at penetrating closed containers and tight spaces to remove heat.
4. Produce may be cooled without wetting with hydrocooling. Wetting spreads postharvest diseases and requires chlorination or the application of fungicides to the cooling water. Wet produce must not be allowed to re-warm because this contributes to the four requirements for postharvest rot to grow and survive: liquid water, warmth, darkness, and a food supply. In addition, hydrocooling produces a major wastewater disposal problem at the end of cooling.
5. Forced-air cooling does not subject the produce to excessive or rough handling. Cooled produce may be safely left in place as it awaits shipment.
6. Forced-air cooling is more energy efficient than room cooling or hydrocooling. A refrigerated room with forced-air cooling and one without (room cooling), held at the same temperature, have the same conduction losses. Forced-air cooling is much faster, and the conduction losses per unit cooled are less.
7. Forced-air cooling can often be retrofitted into an existing room in a cooling facility with sufficient cooling capacity. The fans can be mounted permanently on the walls or can be portable.
8. Forced-air cooling is very versatile and can be used effectively for a wide range of packaged and bulk produce. Many very delicate and perishable items cannot be cooled effectively by any other method.

Forced-air cooling is simply cooling by the forced convection of cold air past warm produce. Forced-air cooling must be used in a room cooling facility of sufficient refrigeration capacity to displace the heat removed from the produce, while also keeping the ambient temperature sufficiently low and steady. To increase the cooling rate, fans that are separate from the fans of the refrigeration system are used to pull the cool air through the containers (forced convection) and past the surface of the produce. Although the cooling rate depends on factors such as the velocity of the air, forced-air cooling is significantly faster than plain room cooling, and is also more thorough and uniform. With a properly designed and operated forced-air cooling facility, the produce in the center of a large container will be cooled at the same or nearly the same rate as the produce on the outside of the container.

Forced-air cooling works by arranging the fans and containers of produce in a way that creates an air pressure differential between one side of the container and the other. This difference in air pressure forces the cool air past the produce where it attracts heat and carries it away. The forced air cooling fans may be wall or floor mounted depending on the specific needs of the facility. Portable floor fans may be easily moved during the off-season when the room is used for other purposes.

The graph of time and temperature in Figure 3a-1 illustrates the response of a typical commodity to airflow rates. The beginning temperature of the produce (pulp temperature) at time zero is 95°F. This temperature varies with ambient conditions and the amount of field heat in the produce (which normally ranges from 60°F to over 90°F). The desired air temperature inside the cooling room depends on the commodity being cooled. As shown below, the room temperature is 41°F and is assumed to remain constant during the period of cooling.

As shown, airflow has a very pronounced effect on the rate of cooling. The heat moves through the individual produce items primarily by conduction but leaves the produce surface by either natural or forced convection. The coefficient of surface convection, h, is directly proportional to the rate of airflow past that surface. Thus, in still air, convection is minimal but up to a certain limit, the more rapid the airflow, the greater the heat transfer (cooling).

The upper curve in Figure 3a-1 (still air) is representative of the relatively slow rate of cooling for packaged and palletized produce that would be expected in a typical room cooling situation with little or no air movement. After two hours in the cooler, the temperature on the outside of the produce would only have decreased about 7 F° or 8 F°. It is common to find the temperature increasing in the center of the pallet for produce with a high respiration rate even after it is placed in a cooler. When heat is only shed from the produce by conduction, the heat cannot be conducted to the surface as quickly as it is generated. Thus, the temperature rises. The higher the temperature, the more quickly the heat is generated. This creates a positive feedback loop. In addition, although the packaging materials inhibit both conduction and convection, the effect is much greater with conduction.

The other two curves demonstrate the increase in cooling rate that is possible with airflow rates of 2 and 5 cu ft per minute per pound of produce. The rate of cooling, represented by the slope of the curve, decreases as the temperature of the produce approaches the temperature of the room air. In this way, the heat transfer function is bounded by the room temperature. Reducing the temperature the last few degrees may take a very long time (or never) and is of little practical importance. When comparing cooling times for various methods, the time required to decrease the pulp temperature ⅞ (87.5%) of the difference between the beginning temperature and the room ambient temperature is the value most often cited.

In Figure 3a-1, the ⅞ cooling temperature (⅞ from starting to room temperature) is calculated as:

Starting temperature of the produce: T₀ = 95°F

Room temperature: T_R = 41°F

Temperature difference: \(∆T=T_O-T_R=95-41=54\ F°\)

⅞ cooling temperature: \(95-[(95-41)(0.875)]=47.75\ ℉\)

Also in Figure 3a-1, the ⅞ cooling time at the higher airflow rate (5 cfm/lb) is about 30 minutes, whereas at 2 cfm/lb, the ⅞ cooling time is three times as long at about 90 minutes. An estimate of the ⅞ cooling time for still air would be many hours.

Figure 3a-1. Representative cooling curves of warm produce with varying airflows in a 41°F room.

Source: M. Boyette.

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Figure 3a-1. Representative cooling curves of warm produce with varying airflows in a 41°F room.

Source: M. Boyette.

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Forced-air cooling works by arranging the fans and containers of produce in a way that creates an air pressure differential between one side of the container and the other. The air moves horizontally through the container and picks up heat as it moves. This requires that a tunnel or plenum be built between parallel rows of containers as shown in Figure 3a-2. The containers can be individual boxes or crates of produce, pallets of produce, or bulk bins. The photo shows an arrangement with a small fan and crates of string beans. There are six crates in two parallel rows with about a one ft space (plenum) between them. The black canvas cover attached to the plywood fan mount goes over top of the plenum and down to the floor on the back side. This encloses the plenum. The fan pulls air out of the plenum.

In all forced-air applications, the air is pulled from the plenum. This ensures that the negative pressure is uniform throughout the entire plenum and the airflow through all containers of produce is uniform, regardless of distance from the fan intake.

The cover is drawn down into the plenum by the negative air pressure. In practice, a wooden slat or a small PVC pipe must be attached to the cover at regular intervals to prevent the cover from being drawn completely into the plenum. Since the cover does not extend over the entire top of the container, some cool air is able to enter. This is not a negative issue and may actually speed the cooling rate since a larger volume of cool air can enter each container.

The design of forced-air cooling facilities can vary depending on the produce to be cooled, the throughput (units cooled per unit of time), and the nature of the operation. In general, all facilities can be grouped into one of two forms based on the size of the operation and the throughput. For small scale growers who may only need forced-air cooling for a short harvest season, one or more pallet-mounted portable fan arrangements like that shown in Figure 3a-2 or Figure 3a-3 below will be adequate. A box fan that has been permanently mounted on a pallet is a simple way to obtain adequate forced-air cooling with minimum expense and a maximum space utilization.

In forced-air cooling facilities that handle large volumes of produce over a long harvest season, it is more convenient to mount the fans on a plenum wall as shown in Figure 3a-4. The white stripes on the floor mark the position of the parallel rows of pallets. Each pair of rows is called a “cooling lane.” For each cooling lane, there is an opening through the plenum wall that is 18 in. wide by 72 in. high. The fan above the opening draws air through the opening from the plenum and pushes it back out into the room. The refrigeration coils are customarily hanging from the ceiling, 15 ft to 25 ft away from the wall mounted fans, but blowing in the same direction. In this way, the warmed air from the produce is blown directly into the cooling coils, which provides maximum thermal efficiency.

In forced-air cooling facilities that handle large volumes of produce over a long harvest season, it is more convenient to mount the fans on a plenum wall as shown in Figure 3a-4. The white stripes on the floor mark the position of the parallel rows of pallets. Each pair of rows is called a “cooling lane.” For each cooling lane, there is an opening through the plenum wall that is 18 in. wide by 72 in. high. The fan above the opening draws air through the opening from the plenum and pushes it back out into the room. The refrigeration coils are customarily hanging from the ceiling, 15 ft to 25 ft away from the wall mounted fans, but blowing in the same direction. In this way, the warmed air from the produce is blown directly into the cooling coils, which provides maximum thermal efficiency.

After the two rows of pallets have been put into place, a worker will walk down the 18 in. to 20 in. plenum gap between the rows, and roll out the cover over the top of the pallets and down to the floor on the near end. This will completely enclose the plenum. There is a partition behind the plenum wall between each opening and a forced-air cooling fan so that each cooling lane operates independently. There is also a 12 in. space between the outside wall and the first pallet and the same space between each parallel row of pallets to allow for the movement of air. The total width required for each cooling lane is approximately 10 ft. In large facilities, as many as 10 cooling lanes may be inside one refrigerated room that would be approximately 60 ft wide by 100 ft long. Some very large cooling facilities may have numerous cooling rooms.

Forced-air cooling is most frequently used to cool palletized boxes, crates, flats, or pallet boxes as shown in Figure 3a-5. It is customary to fill a cooling lane with 24 to 26 pallets (two rows of 12 or 13 pallets each) before starting to fill the next lane. The pallet footprint most often used in the produce industry is 42 in. wide by 48 in. long. The longer dimension is the direction of the fork truck slots.

The number of pallets in each parallel row can vary although the number of pallets in each row must be the same so that both rows are of equal length. The fewer the pallets, the more air will be pulled through each one and the faster the cooling. It is customary to build forced-air cooling rooms so that 10 to 12 pallets can be placed in each row for a total of 24 to 26 pallets per fan. This is the number of pallets that can be placed into a standard refrigerated truck, which are known in the shipping industry as “reefers.” The standard interior of a reefer is approximately 104 in. wide by 96 in. high by 48 ft to 53 ft long.

Reefer: In the trucking industry, a reefer is a refrigerated van equipped with its own diesel-powered refrigeration unit that is mounted on the front of the van behind the truck. The refrigeration unit removes the heat coming in through the walls by conduction. Reefers hold produce at the temperature it was at loading and do not provide active cooling.

The weight of a full pallet can range from 350 lb for less dense items such as bell peppers to over 1000 lb for tomatoes or strawberries. Since floor space in the cooler is limited, it is sometimes necessary to stack cartons on the pallet as high as 8 ft or higher if the pallets are double stacked. This makes the rolling the cover by hand very difficult. In this case, the covers are frequently raised and lowered on cords operated by a pulley system (Figure 3a-7).

It is difficult to calculate the accurate cooling times for any combination of fans, produce, and containers, other than testing the setup empirically. There are some published data that can be helpful in estimating times when designing facilities. These studies will be discussed in later chapters of this publication.

Figure 3a-4. Wall mounted forced-air cooling fans in a large melon packing facility.

Source: M. Boyette.

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Figure 3a-4. Wall mounted forced-air cooling fans in a large melon packing facility.

Source: M. Boyette.

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Figure 3a-5. Top view of fan and pallets arranged for forced-air cooling with a portable box fan.

Source: M. Boyette

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Figure 3a-5. Top view of fan and pallets arranged for forced-air cooling with a portable box fan.

Source: M. Boyette

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Figure 3a-6. Reefers are a critical part of the cold chain from farm to consumer.

Source: M. Boyette.

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Figure 3a-6. Reefers are a critical part of the cold chain from farm to consumer.

Source: M. Boyette.

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Figure 3a-7. Cord and pulley cover system for extra tall pallets.

Source: M. Boyette.

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Figure 3a-7. Cord and pulley cover system for extra tall pallets.

Source: M. Boyette.

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Figure 3a-2. Arrangement of fan and produce for small forced-air cooling.

Source: M. Boyette.

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Figure 3a-2. Arrangement of fan and produce for small forced-air cooling.

Source: M. Boyette.

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Figure 3a-3. Portable pallet with mounted box fan.

Source: NC State University.

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Figure 3a-3. Portable pallet with mounted box fan.

Source: NC State University.

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The heat transfer rate (Btu/hr) of forced-air cooling is much greater compared to room cooling. When forced-air cooling is added to a facility that was previously used for room cooling, it may be necessary to increase the capacity of the refrigeration system to accommodate the increased cooling load. Although the selection of proper refrigeration coils limits the amount of water removed from the air by condensation, it may still be necessary to install supplemental humidification equipment.

Although a higher airflow rate decreases the time required to cool, further increases in the airflow rate may not able to substantially reduce the time further. The rate of heat transfer by conduction inside the produce can become the limiting factor. Conduction is not directly affected by the airflow rate. Oversized forced-air cooling fans waste energy.

The time required to cool depends on several factors. With all other factors held constant, the beginning temperature of the produce and the room temperature do not change the ⅞ cooling time. A factor that does affect the cooling time is the amount of produce. For a given fan’s capacity, the fewer the number of pallets, the more air will be pulled through each one and the faster it will cool. Less dense items such as bell peppers will cool more quickly than more dense items such as tomatoes or squash. Another critical factor is the fan capacity. Fans are rated at a certain number of cu ft per minute (CFM) at a certain pressure differential. In fan curves, the pressure differential is normally indicated in inches of water (in. H₂O). One in. of water pressure is equivalent to 0.036 psi.

It may require some experimentation but a good rule of thumb is to provide approximately 2 to 3 cfm/lb of produce at a static air pressure of 0.10 in. of water. Larger fans will yield more airflow and faster cooling. However, the larger the fans, the greater the refrigeration load that is consumed in offsetting the heat generated by the fan. Oversized forced-air cooling fans waste energy because all the electrical power going into the room to power the fans is eventually turned into heat. This heat then adds to the refrigeration load, decreases the capacity available for cooling the produce, and requires removal from the room. This is a triple waste. The operator of the cooling facility is paying for electrical power to operate the fans and the refrigeration system to remove the fan heat and is probably losing produce weight by overdrying the produce.

Propeller fans are almost always used since their volume and pressure relationships are better suited than squirrel cage fans. In addition, with the same size electric motor, the larger the propeller diameter, the more efficient the fan in terms of electric power consumed. Since forced-air cooling facilities may be used to cool a range of products, it is best to size the fans for the heaviest pallets that are anticipated.

The size of the openings in the sides of the produce container can also have a profound influence on cooling times. Too few or too small openings will restrict the movement of air. If the cool air cannot enter the package of produce, the package will not cool. In contrast, the stacking strength of the carton will be compromised if there are too many openings or if they are too large. Container manufacturers have struggled with this issue for years. The opening shape that results in the least amount of weakening in the stacking strength is a vertical slot. Ordinarily, slots that are 5% of the face perpendicular with the flow of air have been shown to be sufficient.

Each forced-air cooling fan should be controlled individually by an “open on fall” thermostat that is positioned in the airflow just before it enters the fan. The thermostat, which is usually set a few degrees above the room temperature, senses the air coming from the loaded pallets. At the beginning of the cooling cycle, the temperature of this air can be 20 F° to 30 F° above the room temperature because the hot produce is warming the air. As cooling proceeds, the air temperature coming from the produce falls. When the produce has lost most of its heat, the temperature will approach that of the room air. To operate the forced-air cooling fans past this point is a waste of time and energy, and may contribute to needless loss of water from the produce.

In most cases, forced-air cooling is the best choice for produce cooling. Even with difficult to cool, high respiration items such as sweet corn in wire bound crates that have traditionally been hydrocooled or cooled with ice, research has shown that forced-air, with the correct humidity, can cool pallet loads thoroughly and in a reasonable amount of time. Since the packages are not wetted during forced-air cooling, some sweet corn growers have switched to much less expensive (and recyclable) fiberboard cartons (Figure 3a-8).

Figure 3a-8. Sweet corn field packed in wire-bound crates and cooled with forced-air.

Source: M. Boyette.

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Figure 3a-8. Sweet corn field packed in wire-bound crates and cooled with forced-air.

Source: M. Boyette.

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