Principles of Forced-air Cooling

The simplest form of forced-air cooling uses a portable blower. Two rows of stacked containers or palletized produce are lined up on either side of the blower inlet. The open area between the containers is covered with a canvas or plastic tarp, sealing the top and back of the stacks. The blower, of a high air volume design, is then started and it directs the air away and upward from the containers, forming a slight pressure differential inside the container stack interior. This negative pressure pulls air though all openings equally. In this simple system the air is chilled by the storage room refrigeration system. If already cooled produce is in the room, sufficient refrigeration power must be available to ensure that the room air does not warm up to an undesirable degree, causing damage to the produce that has already been cooled.

More advanced pressure cooling systems use a separate room with additional refrigeration power to increase efficiency and reduce cycle time. Evaporator coil temperature differential should be held low to minimize dehydration and reduce moisture loss. Humidification devices can also be added.

Room Cooling

Room cooling is most widely used in the fruit industry when post-harvest respiration is slow enough that quality loss is very small. Growers of apples and melons often employ room cooling.

Portable Units

Forced-air cooling units can be designed to fit within a standard 40-foot (ca. 12 m) sea container. Although auxiliary refrigeration is required, these units can be towed to remote sites and operated with no set-up needed. Other portable units can be used as centralized sources of cold air refrigeration and movement.

Forced-air cooling features:

Advantages:
●　Inexpensive options are available for small farms
●　Cooling can take place in containers packed and ready for distribution
●　Especially good with products that have low respiration rate
●　Systems can handle large volumes and large containers

Disadvantages
●　Moisture and weight loss can be problems
●　For sensitive products, humidification systems are required
●　Not suitable for many products, especially where respiration rate is high

Containers and Stacking Considerations for Forced-air Cooling

Many types and sizes of fresh produce containers can be used successfully with forced-air cooling. Popular types include fiberboard cartons; and unlined plastic containers, including large bins. The containers should stack together tightly so that the air must flow through the openings in walls of the container and thence the produce. The openings must be designed with size and arrangement to ensure that the produce in inside does not block the air flow. The ratio of orifices to container surface should be at least 5%. Placement must consider air flow and the effect on container strength.