Refrigeration Manual - HVAC and Refrigeration Information Links

DEFROSTING OF BLOWER COILSIce and frost will accumulate continuously on coilsoperating below freezing temperatures, and air flowthrough the coil will be eventually blocked unless thefrost is removed. To allow continuous operation onrefrigeration applications where frost accumulation canoccur, periodic defrost cycles are necessary.If the air returning to the evaporator is well above 32°F., defrosting can be accomplished by allowing thefan to continue operation while the compressor is shutdown, either for a preset time period or until the coiltemperature rises a few degrees above 32° F., themelting temperature of the frost.For low temperature applications, some source of heatmust be supplied to melt the ice. Electric defrost systemsutilize electric heater coils or rods in the evaporator.Proprietary systems using water for defrosting areavailable. Hot gas defrosting is widely used, with thedischarge gas from the compressor bypassing thecondenser and discharging directly into the evaporatorinlet. In hot gas defrost systems, the heat of compressionor some source of stored heat provides defrost heat,and adequate protective devices such as re-evaporatorsor suction accumulators must be provided if necessaryto prevent liquid refrigerant from returning to thecompressor. Other systems may utilize reverse cycledefrosting, in which the flow of refrigerant is reversedto convert the evaporator temporarily into a condenseruntil the defrost period is complete.To prevent refreezing of the melted condensate in theevaporator drain pan, a drain pan heater is required onlow temperature systems.© 1967 Emerson Climate Technologies, Inc.All rights reserved.6-3