OIL-INJECTED ROTARY-SCREW COMPRESSORS

How do they work? As its name implies, oil is injected in the compressor element (where the two rotors turn), during the compression of the air. What we end up with is a mixture of oil and air under pressure (commonly about 7 bar).

In a special oil separator, the oil is separated from the air. Most of the oil is removed by the centrifugal force, the remaining less percentage of oil is separated by the separator (filter) element (it looks just like a big air filter). The separator element should be renewed every 2,000 running hours or so (depending on the manufacturer/model).

The oil is cooled and fed back to the compressor element to do its job again. The compressed air, now without the oil, is directed to the pressure outlet of the compressor, usually through an after-cooler (the air gets very hot when it’s compressed).

There is no special oil pump to do all this; the oil flows by the pressure differences inside the compressor.

Pros:
• Quiet operation
• High volume of air, steady flow
• Low energy cost
Cons:
• Expensive compared to piston compressors
• More suitable for continuous operation only

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