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Compressed Air Encyclopedia

Rotary Screw Compressors
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Rotary screw compressors are one of the most popular types of compressors. They are ideal for industrial applications between 20 -300 hp.

The rotary screw compressor is one of the most popular types of air compressor.

The design has been around for about 50 years, but due to the low efficiency of the design, rotary screws were used almost exclusively in engine-driven or low-power applications.

With the development of the two-stage rotary screw compressor and the refinement of the screw profile, the rotary screw compressor is now the compressor of choice for industrial applications between 20-300 hp.

A rotary screw compressor consists of two meshing helical screws. One screw has a male profile the other has a female profile. There are no valves as with piston compressors, which creates a more simpler, quieter-running compressor.

Air at atmospheric pressure enters one end of the compressor and gets trapped between the two meshing screws. As the screws rotate together, the gap between them decreases which compresses the air that is still trapped between them. At the opposite end of the compressor is the discharge where the air is released from between the screws at a significantly higher pressure than it originally entered.

Two-stage compressors have two sets of screws – a low-pressure set and a high-pressure set. Compressed air that is discharged from the ‘low-pressure’ screws passes through an intercooler before being compressed by the second set of ‘high pressure’ screws. A two-stage rotary screw compressor can operate at efficiency levels up to 15% higher than a single stage unit. The reduced pressure differential in a two stage unit significantly reduces blow-by and thrust bearing loads.

Rotary screws are usually internally oil-cooled. Cooling oil is injected into the air stream to absorb the heat generated by the compression of the air. The hot oil is taken away and cooled in a heat exchanger. Because the rotary screw compressor is cooled from within, it keeps the internal components of the compressor from reaching excessive temperatures. This allows rotary screw compressors to be rated for continuous duty. In fact, oil-cooled rotary screw compressors can run at full load and full pressure 24 hours a day, 365 days a year.

Some advantages of rotary screw compressors:

  • Discharge air is clean – free from burned oil
  • Compressed air is relatively cool – about 180°F
  • Rotary screw compressors offer a large flow capacity in a small package
  • Due to their simple design, rotary screw compressors run quieter than other designs. Electrically-driven models are available that operate at 75-85 dB at one meter (per the CAGI Pneurop specifications)

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