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Compressor Oils

Background

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.