Medium Temperature Evaporator Air Defrost
Medium-temperature commercial refrigeration usually operates at box temperatures between 34°F to 40°F with evaporator temperatures about 15°F to 25°F. Because the coil temperature is well below freezing, it is normal for frost to build up on the evaporator fins during the “on cycle” when the compressor is running.
When the tstat is satisfied, the compressor shuts off and the evaporator fans continue to circulate air from the space through the coil fins. Because the space and product temperatures are above freezing, the evaporator warms up and the frost melts. This process, called random, or off-cycle, defrost, occurs each time the tstat turns off the compressor.
To help ensure enough defrost time, commercial refrigeration tstats are designed with a relatively wide temperature differential. Differential is the difference between a control’s cut-in and cut out. The cut-in of a tstat is the setting at which the contacts close when there is a rise in box temperature. The cut out is the setting at which the contacts open, stopping the refrigerating cycle. Tstats that sense box temperature usually have a differential of 4°F or 5°F, wide enough to air defrost the evaporator coil.
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