The USDA Portacooler is a portable insulated box on a trailer that uses a one ton (12,000 BTU/hr) window air conditioner for the cooling source. This window air conditioner is not a typical unit and one should understand what makes the AC unit work in this application. This AC unit was designed with an evaporator coil (cooling coil) that operates down below 40 deg F. Most AC units are not designed to cool the evaporator coil lower than about 50 deg F because for most applications that is cold enough air.

The minimum design cooling temperature of most evaporator coils is purposely limited to prevent moisture from being condensed out of the air onto the evaporator coils where it might freeze. If the coils frost up, the airflow is restricted through the coils and the efficiency of the system drops.

In the USDA Portacooler, an air conditioner unit with a low evaporator (cooling) coil temperature was selected. The thermostat was replaced with one that would operate the AC unit at a lower temperature. A room AC unit thermostat needs to operate in the human comfort range of maybe 60 to 80 deg F. For cooling fruits, vegetables, or cut flowers, a thermostat with a range of 35 to 80 deg F. was more appropriate. Since the AC unit selected could cool to the lower temperatures, changing the thermostat was an option.

One of the problems with operating a cooling system at temperatures near the freezing point is that frost will form on the evaporator coils if conditions are right. In the USDA Portacooler, the evaporator coil temperature was low enough for frost to occur. A defrost cycle was created by installing resistance heating strips close to the cooling coil and operating the heating strips for a few minutes out of each 10 minutes using a simple time clock. The heat melted the frost and allowed the moisture to drain away.

Good airflow is important around the produce or product in the Portacooler. In the USDA Portacooler a separate blower fan circulates the air from the plenum where the cold air is exhausted from the AC unit through the insulated box in which the produce is placed. The air moves through and around the produce and then returns in a separate plenum (ductwork) under the floor to the AC unit. The air can be pulled through the AC unit for further cooling or can simply be recirculated by the blower fan. The thermostat controls when the AC unit operates. Use low velocity airflow for cut flowers.

The USDA Portacooler was designed to remove field heat from highly perishable fruit, vegetable, or cut flower product. It is not the best type of cooling unit for moderate or long term storage. This cooling system is a dehumidifier and, as such, it will gradually dry out the product being cooled, if used for very long. For best cooling and long-term storage, select cooling equipment with an evaporator coil that operates just below the desired storage temperature so little moisture is condensed out of the air.

in MSWord