Learn how a rotary screw compressor works and what to look for when buying one.

On this page I will tell you about the pros and cons of the rotary screw compressor, what to look for when buying one and common breakdowns that might occur during operation

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 gets '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.

Compressor element (oil-free type). Photo: Atlas Copco

Because this is a continuous process, this kind of compressor doesn't make a lot of noise; it runs quiet and smoothly.

Compared to 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.

Oil-free or lubricated

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 machine.

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. I've mostly seen them on big factories like oil/gas or chemical refineries, big food factories or other places where the compressed air must be 100% oil-free (otherwise it could contaminate the food, product or chemical process).

Oil-injected rotary screw compressors

How do they work? As its name implies: there's oil injected in the compressor element (where to 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 centrifugal force, the remaining few % of oil is separated by the separator (filter) element (it looks just like a big air filter). The separator element should be renewed every 2000 running hours or so (depending on manufacturer / model).

The oil is cooled in the oil 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.

Example of small rotary screw
compressor. Photo: Atlas Copco

Pros:

• Quiet operation
• High volume of air, steady flow.
• Low energy cost

Cons:

• Expensive compared to piston compressors.
• More suitable for continuous operation only

Oil-free rotary screw compressors

The basic workings are the same as the oil-injected screw compressor, only this time, there's no oil.. only air! Because of this, the rotors used are of superior quality with very little space in between them. They do not touch each other though; otherwise they would wear-down too quickly.

Because there's no oil injected during compression, the compression is usually done in two stages. Why because if we would compress the air in one go from 1 to 7 bars, it would get really, really hot.

Stage one compresses the air to a few bars (say 3,5 bars). The air will be very hot at this time, so it flows through an inter-cooler first before entering the second stage. Stage two will compress the air further from 3,5 bar to the end-level, mostly 7 bar.

Normally the two stages will be built on 1 gearbox, with one electro motor driving them at the same time.

Pros:

• 100% oil-free air

Cons:

• More expensive than oil-injected type
• Servicing/repairing more difficult, and more expensive than oil-injected type
• More noise than oil-injected compressors

Example of big rotary screw compressor (old fashioned oil-free type).
Photo: Atlas Copco

Failures:

Rotary screw compressors can fail in a number of ways. The most common 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 vapor), and compresses it to 7 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 is 'it just won't give me any pressure' .. the problem 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 'unloaded' condition and wont supply any air.