HOW A ROTARY-SCREW COMPRESSOR WORKS

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 HOW A ROTARY-SCREW COMPRESSOR WORKS

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• The rotary-screw compressor uses two rotors (helical screws) to compress the air. There’s a “female” rotor and a “male” rotor. The rotors are of different shape, but fit each other exactly.

• When the rotors start turning, air will get sucked in on one side and get “trapped” between the rotors. Since the rotors are continuously turning, the air gets pushed to the other end of the rotors (the “pressure side”) and new fresh air gets sucked in. 

• Because this is a continuous process, this kind of compressor doesn’t make a lot of noise it runs quietly and smoothly. Compared to the piston-type reciprocating compressor, the rotary screw compressor is much more expensive, but it will use a lot less energy over the years, resulting in a lower overall cost.

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OIL-FREE OR LUBRICATED COMPRESSORS:

• The rotary-screw compressor is available as oil-injected and oil-free versions. The basic principle is the same (the rotors “push” the air to one side), but they are quite different machines.

• Oil-inject models are by far the most common oil-injected screw-type compressors. When you need a lot of air in your workshop or factory, this type of compressor is usually the best choice Oil-free models are used for specific special applications. 

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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.

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FAILURES:

• Rotary-screw compressors can fail in a number of ways. The most com￾mon problem is oil in the compressed air. Most of the times this happens because the oil separator is not doing its job properly. The chances are that the separator element is saturated with oil (read you didn’t service the compressor in time There’s a fixed time limit [running hours] to change the element).

• Another problem often encountered is water in the compressed air. Since the compressor takes in a huge amount of air (with water vapour) and compresses it to seven times a smaller volume, a lot of water will be produced.

•  Normally this water is drained using an electronic or mechanical automatic drain. If this drain is broken, the water will stay in your compressed air and fill up your air receiver and piping.

• If the problem persists, it will most probably be a defective pressure switch (which will start/stop, load/unload the compressor), or a defective inlet valve (which opens and closes the air inlet of the compressor). If it’s closed, the compressor is running in an “unloaded” condition and won’t supply any air.

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