A Guide to Compressor Oils

A compressor oil is used to lubricate both the working parts and the pressurized spaces in a compressor. 

The purpose of the lubricant in the pressurised section is not only to reduce friction and wear but also to improve sealing of the pressurized spaces and to provide cooling.  

The compression process generates a lot of heat and in general, the higher the final compression temperature of the air at the outlet from the compression space, the higher are the requirements placed on the oil with respect to its oxidation stability and resistance to deposit formation.

For Reciprocating Compressors

Lubrication is required for the bearings, pistons, rings, cylinders and valves. 

The pistons and valves reach temperatures of 2200C to 3000C and the oil film on the surfaces of these components must not form carbon. Hard carbon can jack open valves, which can cause a rapid increase in air delivery temperature. This can lead to glowing carbon particles in the delivery pipework and valve chamber and ultimately lead to an explosion.

Soft, oil carbon is less of a problem but can also eventually dry out if it is in the delivery pipework and cause the same problem. It can also build up in the pipework causing restrictions and reductions in efficiency.

As such, a lubricant with exceptionally low carbon forming properties is required.

Antiwear additives are required for the bearings, pistons, rings and cylinders.

Good demulsibility properties are also necessary to aide water/oil separation as the oil containing air is cooled and the water vapour condenses.

Viscosity selection is also important to maintain adequate oil film thickness and minimise friction losses as well as minimising oil consumption.

One of the main degradation of the oil is by oxidation, which can be detected by an increase in viscosity, acidity and acrid smell.

Allowing an oil to stay in use above its recommended operational time would result in thermal breakdown of the oil and possible breakdown of the compressor as a result of deposits forming on the valves.

For Screw Compressors

Rotary Screw are either geared or non-geared.

Lubrication of the gear design is easier since the role of the lubricant is to lubricate the bearings and gears and to act as a sealing medium between the rotors.

Non-geared units required good lubrication of the rotors as well as the bearings and usually require higher FZG performance than the geared designs.

Due to the intimate mixing of oil and air, an oil with excellent resistance to oxidation is required.

Low foam and good air release are also important otherwise excessive foam will form in the receiver and significantly increase oil carry over.

Good air release is essential to ensure adequate lubrication of gears and bearings etc. and to minimise foaming on shut down when the air entrained in the oil under pressure is suddenly released.

Antiwear is important to minimise bearing, gear and rotor wear

Good water separation is necessary to avoid emulsions being formed, which will not drain from the condensate drain tap and be carried over into the compressed air system of the factory causing restrictions and blockages downstream and increase air tool wear and tool exhaust problems.

The lubricant must also possess good anti-corrosion properties due to the ever-present condensate.

Another factor that is important in rotary compressors is the coalescing property.  The fine mists, which are produced by injecting the oil into the compressed air have to be removed again before the air leaves the compressor.  A coalescing element is installed in the receiver to achieve this but the oil has a significant influence on the efficiency of the element.

Poor recoalescing properties lead to higher oil carryover, increased oil consumption and oil in the condensate drain.

For Rotary Vane Compressors

There are two types; water-cooled and oil-cooled.

Water Cooled

For water-cooled systems, the compressor oil serves only to lubricate and seal.   Oil is fed mechanically to rotor and bearings in small, metered quantities that are eventually discharged from the system with the outgoing air.

Oil Cooled

For oil-cooled systems, the compressor oil cools the system as well as lubricates and seals.  Oil is flood-injected into the intake air, recovered, and recirculated.

For both types of rotary vane compressors, the compressor oil must be able to withstand severe oxidising environments and possess good demulsibility characteristics.  For oil-cooled systems, the oil carry over must also be low so that large amounts of oil are not discharged with air – a low volatile oil is therefore required.

This information has been sourced, reviewed and adapted from materials provided by Castrol.

For more information on this source, please visit Castrol.

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