Screw Compressors

Screw Compressor

A short time ago I picked up a book about the principles and operation theory of air compressors.  We have a wealth of knowledge floating around the department and company as a whole, so I thought this book may provide an opportunity for me to contribute.  As I read through, I find myself “geeking out” over the level of technicality with which compressors are designed.  Perhaps I’m biased based on experience and familiarity, but my compressor preference tends to lean toward a screw compressor.  This isn’t to say these are the best or most suitable compressor for every application (they most suitably aren’t), only that they sit with me the best.

A screw compressor, like a reciprocating compressor, is technically a positive displacement device (unlike an axial or centrifugal compressor which is classified as dynamic device).  A volume of gas becomes trapped in an enclosed space and then that volume is reduced resulting in an increase in pressure.  Within a screw compressor there are two screws with mating profiles which we can refer to as screw “A” and screw “B”.  Of these screws, screw “A” has concave inlets, and screw “B” has convex inlets.  These screws rotate in opposite directions and as screw “A” receives power from the outside source (motor), it transmits power through to screw “B” through a set of synchronization gears.  (Think of the way air is forced into an engine through a supercharger…)

As the screws rotate, gas is drawn into the inlet/suction port and compressed by rotary motion.  This gas is moved axially from the suction to the discharge port.  The location of a discharge port determines when compression is complete, and this location can be changed to regulate the discharge pressure.

The efficiency of such a setup is dependent on the clearance between the screws and the quality of the seal surrounding the air as it travels through this process.   To aid with efficiency and to increase seal quality, oil is sometimes injected into the inlet cavity, and is later separated for reuse.  (Aging components can result in oil contamination within the compressed air, which is why we recommend an oil removal filter when in doubt.)

One of my favorite things about screw compressors is the ability to control output through a slide valve.  As the valve is adjusted to shorten the working length of the rotor (screw), less horsepower is required to maintain operation.  This type of control is unique to screw compressor design and can yield significant reduction in operating cost, which we tend to like at EXAIR.

If you have compressed air or application related questions, give us a call.

Lee Evans
Application Engineer

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