The preservation of food depends not only on the temperature, but also on the relative humidity within a case. When rh is too high (above 95 percent), mold and slime conditions may occur, particularly on stored meats. When the rh in the refrigerated case is too low, excessive dehydration will occur. Proper case rh is maintained by achieving the correct evaporator temperature difference.
An evaporator’s temperature difference (TD) is the difference between the saturation temperature of the refrigerant inside the coil and its entering air temperature. The greater the temperature difference, the more moisture will be removed from the air and the lower maintained rh in the case. The smaller the temperature difference, the less moisture is removed from the air and the higher the maintained rh in the case.
Many walk-in coolers are designed based on an evaporator TD of approximately 10˚F (35˚ entering air over a 25˚ coil). This temperature difference will cause the air to not give up much of its moisture and the case’s rh would be maintained at approximately 85 percent. This is ideal for long-term storage of food products.
Since many food products have high water content, maintaining a high rh within a case is good for business. Many products (like meats) are sold by the pound and moisture is part of that weight, so maintaining the right rh is good for business. When the rh is too low, food products dry out, reducing the product’s shelf life and its weight, both of which result in lower profits.
This design temperature difference may not be used on all medium temperature cases. Reach-in coolers may not have the room for large coils required for a 10˚ temperature difference. To save space, these evaporators are more compact, which leads to a higher evaporator temperature difference. Reach-in coolers may incorporate an evaporator with a designed temperature difference of 20˚. This results in a lower maintained rh of approximately 65 percent within the case. To prevent product dehydration, items stored in these cases are either wrapped or only stored in the case for short durations, usually less than 24 hours.
Understanding these differences may help to explain why on some refrigeration systems you may measure lower suction pressures when compared to another perceived similar refrigerated case. For example, if you were to compare a walk-in cooler to a reach-in cooler and both systems had an entering air temperature of 35˚ and both were using HFC-134a as their refrigerant, assuming no suction line pressure drop, the suction pressure of the walk-in cooler would be 22.1 psig. This is based on a coil temperature of 25˚ and an evaporator temperature difference of 10˚. The reach-in cooler would have a suction pressure of 15 psig, based on a coil temperature of 15˚ and an evaporator temperature difference of 20˚. Both cases maintain the same case temperature, but different rh values and slightly different operating suction pressures.
Publication date: 03/05/2012