Refrigerant migrates to the coldest point of the system and condenses there. A compressor standing outdoors on a winter night is mostly the coldest part of the system; the refrigerant comes to the crankcase and dissolves in the oil. When the compressor starts, the crankcase pressure falls suddenly, the refrigerant in the oil boils, the oil foams and goes with the foam into the cylinder or the scroll set. The result is bearings left without oil and liquid slugging. The damage mostly accumulates not from a single start but from repeated cold starts.
The crankcase heater prevents refrigerant from accumulating in the oil by keeping the oil 10–20 K above the system’s saturation temperature at that moment. Its power is small: 30–90 W in hermetic and scroll compressors, 100–200 W in large semi-hermetic compressors. On scroll and hermetic shells the belt type wrapped around the lower part is used, on semi-hermetics the immersion type inserted into a pocket in the crankcase. Some models regulate their own power according to temperature. Its consumption is low and insignificant compared with the price of the compressor it protects; that is why it is never disconnected.
The heater is supplied through the normally closed auxiliary contact of the compressor contactor; that is, it works while the compressor is stopped and is cut off while the compressor is running. At first commissioning, and in plants that have been without power for a long time, the heater needs to be energised for 8–12 hours before the compressor is started; most compressor manufacturers stipulate this period. This is why the main switch is not turned off for short shutdowns and between seasons. The same rule applies after a long power cut; no immediate start is made with a crankcase that is cold and full of refrigerant.
Checking it is simple: while the compressor is stopped the lower part of the crankcase should feel warm to the hand; if it is cold, the heater or its connection is faulty. A belt-type heater must sit fully against the shell, with no gap in between. A crankcase heater alone does not solve every situation; in systems with a large refrigerant charge and long lines it is used together with pump-down and, if necessary, with a suction accumulator added to the suction line. Heavy foaming of the oil in the sight glass at start-up shows that refrigerant has migrated to the crankcase and that the heater is inadequate.