Compressor lubricants
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Frequently asked questions
Are piston and screw compressor oils the same?
No. Under ISO 6743-3, DAA and DAB are for reciprocating compressors, while DAH and DAJ are for rotary types such as screw compressors, where the oil also cools and seals the compression chamber. Screw compressor oil needs good air release and water separation and a long service life.
Mineral or synthetic oil for a screw compressor?
A good mineral oil may run roughly 2,000 to 4,000 hours; PAO or ester synthetics usually last much longer and tolerate heat better, suiting hot compressor rooms. The choice also depends on the compressor maker's warranty requirements.
What should I watch when switching from OEM oil to an equivalent?
Match the base stock and viscosity grade, usually VG 32 or 46. Drain fully and change the separator and oil filter at the same time, since some different base oils are incompatible and leave varnish in the separator.
What signs show compressor oil needs changing early?
Rising discharge temperature, darkening oil, increasing separator pressure drop or more oil carried into the air. Heavy condensate in the oil, often from running underloaded in a humid climate, also emulsifies the oil and destroys its lubricity.
Can air compressor oil be used in a refrigeration compressor?
No. Refrigeration compressors use oil selected for the refrigerant, for example POE for R134a and R410A, and mineral or alkylbenzene oil for some older refrigerants. The oil must be miscible with the refrigerant so it returns to the compressor, and must keep its dielectric properties in hermetic units. Air compressor oil in a refrigeration system can block the expansion valve and damage the compressor.
How should oily condensate from compressors be handled?
Condensate drained from receivers, dryers and filters contains oil and must not go straight to the drain. Use an oil water separator to remove oil before discharging the water, and treat the collected oil as hazardous waste. PAG based synthetics can form stable emulsions, so choose a separator suited to the oil type.
How does running a compressor underloaded affect the oil?
When a screw compressor spends much time unloaded or at low load, the oil does not get hot enough to drive off condensed water. Water builds up in the oil, causing emulsions, rust and shorter oil life. Keep the discharge temperature above the dew point, typically around 75 to 85 °C, by adjusting the thermostatic valve or sizing the compressor to demand.
Should the oil filter be changed with every compressor oil change?
Yes. The old element holds deposits and varnish from the used oil, and keeping it contaminates the new charge from the start. Besides the oil filter, check the air filter and separator element by pressure drop or running hours. Record machine hours at each change to set the next interval.
