One of the most common applications of refrigeration systems is to produce and maintain space temperatures by circulating air through a refrigerated coil. If the temperature of the refrigerant in the coil is below 0°C, water in the air freezes and accumulates on the coil. The ice blocks airflow and acts as an insulator, penalizing coil performance. For efficient performance, the coil must be defrosted periodically. The defrost cycle is a necessary and important part of the design of the refrigeration system.
Over the years, various defrost methods have been used. One of the first methods was to arrange the coil in such a manner that it could be isolated from the cold room. Warm air was circulated over it until the ice melted. Another method is to run water over the coil. Careful design of the water lines into and out of the cold room prevents freezing of the defrost water. Electric heater rods inserted into formed holes through aluminum fins work effectively, and this type is common for halocarbon systems. All of these have been used for ammonia coils, but the most common method is hot gas from the compressor discharge. Hot gas defrost is simple and effective, it removes ice rapidly, and is relatively inexpensive to install. However, the control valves selection and the sequence of operation must be correct for reliable and efficient defrosts.
Defrost systems vary with the size and type of evaporator, with some choices possible for the larger size coils. Electrical heating defrost via elements in the drip trays under the evaporator or as elements through the coil fins are the most common and economical for small evaporators. Hot gas systems that pump hot refrigerant gas through the coils or defrosting by running ambient water over the coils are more common on larger systems.
Auto cycle defrost is not as complicated as it sounds. In fact, cycle defrost systems are the least complex in operation and most effective defrost systems available. Cycle defrost systems are feasible only on all-refrigerator units because these units do not contain a freezer compartment. Cycle defrost units contain a special thermostat which senses the evaporator plate temperature. At the completion of each compressor run cycle, the thermostat disconnects the electrical power and turns off the compressor. The thermostat will not connect the electrical power again to initiate the next compressor run cycle until the evaporator plate reaches a preset temperature well above freezing.
During this evaporator warm-up period, the frost which has accumulated during the previous compressor run cycle melts and becomes water droplets. These water droplets run down the vertical surface of the evaporator and drop off into the drip tray located just underneath the evaporator, which then empties into a drain tube. The drain tube discharges the water droplets into the condensation pan located in the mechanical assembly under and outside the refrigerator compartment. There, the compressor heat and the air flow from the condenser fan evaporate the moisture.