Synthetic oil cuts cost of compressor operation

With international standards such as ISO8573 placing increasing demands on the purity of compressed air, compressor operators need to improve the quality of the compressed air they generate.

With international standards such as ISO8573 placing increasing demands on the purity of compressed air, compressor operators need to improve the quality of the compressed air they generate.

Compressor oil plays a very important role as it determines not only the oil vapour content in compressed air but also the costs for subsequent air treatment.

With screw-type compressors, lubricating oil is injected directly into the compressed air.

During this process oil aerosols (droplets) and oil vapour (mist) are formed, and both are expelled from the compressor together in the compressed air flow.

The oil separator downstream mechanically separates the oil aerosols from the air flow with a filter element and returns them to the oil circuit.

The oil vapour remains in the compressed air and can only be removed by activated carbon filters or similar.

As activated carbon filters can only absorb (saturate) the oil and do not feed it back into the circuit, the oil vapour content determines the oil consumption of a screw-type compressor (along with possible leakage).

Oil vapour content largely depends on the base oil and the vapour pressure of the compressor oil used.

As a real-world example of the problem, one German company was using an oil injected screw compressor rated at 110kW.

The oil fill was 35 litres of a mineral-oil-based product that was discharged at a temperature of 90 to 100C, depending on the time of year.

This was producing an oil consumption of 4 litre every 300h of operation.

However, after changing to Kluber Summit SH-68, the compressor required just a 6-litre refill after 3300h operation.

The quantity of oil vapour in the compressor depends to a large degree on the molecular distribution of the oil.

In a gas-chromatographic analysis, mineral oil based products show a typical broad, bell-shaped distribution of the molecular weight with a high proportion of short molecular weights, whereas PAOs have a pointed curve, in which the individually mixed base oil portions can be seen.

The major difference is that PAO contains very few short-chain molecules compared to the mineral oil.

It is these short-chain molecules that evaporate most easily.

With this high oil consumption the customer, in this example, could run for 9000h without a single oil fill, but such high oil ingress (oil vapour condenses and becomes liquid) may have negative effects on the compressed air ductwork system and on the connected machinery.

For example, compressed air with a high oil content can escape from the tool or deposit itself in the system.

This is less important with normal compressed air, but must be avoided in painting and control applications, in instruments and in particular in the food-processing industry.

The oil can gradually cause gumming, clogging of valves and tubes (pressure drop) in the compressed-air ductwork system.

The service life of activated carbon filters is limited, ie saturation occurs.

This means that that only a limited quantity of oil can be absorbed.

And, if the activated carbon filter is not changed in time the filter can break thus allowing ingress of the oil in the compressed-air ductwork system.

In industry, and in particular in the food-processing sector, oil vapour after the oil separator is separated from the compressed air flow either by activated carbon filters or by activated carbon adsorbers.

These oil vapour separators are, as a rule, very expensive and have to be replaced between 400 and 8000h operation depending on the oil vapour content (saturation).

The costs for changing the oil vapour separators are between Eur 30 and 350 per unit.

The use of Kluber Summit SH or HySyn FG oils considerably reduces the oil content in compressed air when compared to mineral oil based products.

Oil vapour measurements and comparisons between various compressor oils are available from Kluber Lubrication on request.

In 2002, Kluber designed and built a compressor test rig to record oil vapour content in compressed air and compare compressor oils.

The test rig consists of an oil-injected 7.5kW screw-type compressor, a compressed air treatment line with integrated filters and oil separators (without activated carbon filters) and monitoring equipment to record the hydrocarbon content in compressed air.

Various compressor oils have been tested on the rig for 100h at a final discharge temperature of 100øC.

In brief, the oil vapour content of synthetic oils is four to five times lower than same-viscosity mineral oils.

And, looking at trends it can be said that the higher the viscosity of the compressor oil, the lower the oil vapour content.

Field tests performed with an ISO VG 46 mineral oil and Kluber Summit SH-68 in oil-injected screw compressors confirmed that oil vapour content may differ by factor 20.

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