Compressor oils | Frontier

Why compressor oil is a case of its own

In a screw compressor the oil is mixed continuously with hot air under pressure for thousands of hours — oxidation conditions no other industrial lubricant meets. The result of that oxidation is not wear but varnish: a hard deposit on the separator element that raises pressure drop, pushes discharge temperature up and, if ignored, ends in a temperature trip. On a reciprocating compressor the problem is different — oil reaches the discharge valve, and deposits there can burn. This is why DIN 51506 classifies these oils by permitted discharge temperature rather than by viscosity. Refrigeration compressors are a different world again: the oil has to be miscible with the refrigerant and to return from the evaporator, which ties the choice to the refrigerant rather than to the compressor.

Types, specifications and service life

Oil-injected screw compressors typically run PAO synthetics in ISO VG 46, sometimes 32 or 68 depending on design, with service lives of 8,000 hours and beyond; diester and polyol-ester versions go further still but demand attention to seal compatibility. Reciprocating compressors align to DIN 51506, the VDL classes being those intended for high discharge temperatures. Vacuum pumps have oils of their own, chosen on the ultimate pressure required — an oil with volatile components raises the limit pressure and makes the pump look faulty. On refrigeration, the choice follows the refrigerant: mineral or alkylbenzene on ammonia, polyol ester on HFC refrigerants, where a mineral oil simply does not return from the circuit.

What shortens a compressor's life

The compressor room is where the same mistake gets made most often:

Topping up a PAO synthetic with mineral oil. The additives are incompatible, and the result is deposits in the separator and rising discharge temperature.
Taking the drain interval from the oil's datasheet rather than from actual conditions. Hot intake air, dust and high humidity halve the declared life.
Changing the separator element only when oil consumption rises; by then the compressor has run for months at raised pressure drop, wasting energy.
A screw compressor oil used in a reciprocating machine, where the requirement is discharge temperature, not service life.
Condensate left in the sump: on compressors that run intermittently, water accumulates and corrodes exactly what should be protected.