Compressor oil (POE, mineral, PAG)
Also known as: refrigeration oil, polyolester oil, compressor lubricant
Short definitionThe oil that protects the rubbing parts of the compressor, seals the clearances and carries heat away. Its type depends on the gas: mineral with R22, POE with HFC and HFO gases, PAG in car air conditioning.
In a retrofit quotation the oil change is a separate item, because the oil travels round the whole circuit with the gas and has to be able to mix with it. Oil that does not mix stays in the evaporator and does not return to the compressor. In today’s commercial refrigeration the rule is POE; mineral and alkylbenzene oils are used with R22 and ammonia, and PAG in car air conditioning and some CO₂ plants. Viscosity is given as a class such as ISO VG 32 or 68; the right value is on the compressor nameplate.
POE takes up moisture from the air within minutes; when moist oil heats up it forms acid, which attacks the winding varnish and the copper. So the can is opened at the last moment, what is left over is thrown away, the circuit is not kept open for more than a quarter of an hour and the drier is renewed at every intervention. In the sight glass the level should stand between one quarter and three quarters. Oil that has darkened or smells burnt is given an acid test. Foaming means liquid is returning to the crankcase. Oils of different types are not mixed.