Crankcase heater
Also known as: oil heater, sump heater
Short definitionA low-power heater that keeps the oil warm while the compressor is stopped. It stops refrigerant from mixing into the oil and collecting there, so the oil does not foam at start-up and no liquid enters the compressor.
Refrigerant likes to gather at the coldest point of the circuit. A compressor standing outdoors on a cold night is exactly such a point: the refrigerant settles at the bottom of the shell and mixes with the oil. When the motor starts, the pressure drops suddenly, the oil foams and escapes upwards, and the bearings run dry. The heater keeps the oil 10–20 K above saturation temperature and prevents this build-up. Scroll and hermetic compressors use a belt type, semi-hermetics an immersion type.
It is wired through the normally closed auxiliary contact of the contactor, so the heater comes on when the compressor stops. On commissioning day, or after a long power cut, the heater is energised for 8–12 hours first and the compressor is started afterwards. For this reason the main switch of the plant is not turned off in winter. Touch the bottom of the shell while it is stopped; if it is not warm, the heater has a fault. Systems with long lines and a large charge also get pump-down.