Compressor oil does not stay only in the compressor; some of it is carried into the system with the discharged vapour. If oil accumulates in the evaporator it impairs heat transfer and cuts capacity, and if it does not return to the compressor, the compressor runs dry and seizes its bearings. The oil separator separates the oil from the vapour in the discharge line and returns it to the compressor crankcase through a float mechanism.
It is not needed in every system. In small medium-temperature cold rooms with short lines and a single evaporator the oil returns by itself; if pipe diameters and velocities are chosen correctly no separator is required. On long or rising lines, at low evaporating temperatures (−25 °C and below; oil thickens in the cold), in multi-compressor central systems and with screw compressors, however, an oil separator is mandatory.
In central systems the oil separator is accompanied by an oil reservoir and an oil level regulator on every compressor; oil management is the most critical issue in these systems. The separator is selected on compressor capacity, refrigerant and discharge line diameter; its heater prevents the refrigerant inside from mixing with the oil during standstill.