This is a resistance heater. If externally mounted it is located on the underside of the compressor crankcase. It may be mounted internally as an integral part of the compressor; this is standard for larger commercial equipment.
The presence of liquid refrigerant in a compressor crankcase is wholly undesirable. It could cause dilution of the lubricating oil, resulting in poor lubrication. Liquid refrigerant vaporizing in a crankcase will cause foaming when the compressor operates after an off cycle. Excess oil will then be discharged by the compressor; this could drastically reduce the amount of oil in the crankcase. Non-condensible slugs of oil and liquid refrigerant could enter the compressor cylinders to cause considerable damage to valves, pistons and connecting rods, and even to fracture crankshafts.
Crankcase heaters are essential for compressors installed for low temperature applications, where evaporating temperatures and crankcase temperatures can be extremely low. They are also necessary for remote installations where the compressor is exposed to low ambient temperatures (winter conditions). Whenever the crankcase temperature falls below that of the evaporator, refrigerant vapour will migrate and condense in the crankcase unless a heater is employed to maintain a temperature in the crankcase above the temperature of the refrigerant vapour.
Because of the tendency of oil to absorb miscible refrigerant, a certain amount of refrigerant will always be present in the crankcase.
