Rotary screw compressor

A rotary screw compressor is a type of gas compressor which uses a rotary type positive displacement mechanism. The mechanism for gas compression utilises either a single screw element or two counter rotating intermeshed helical screw elements housed within a specially shaped chamber. As the mechanism rotates, the meshing and rotation of the two helical rotors produces a series of volume-reducing cavities. Gas is drawn in through an inlet port in the casing, captured in a cavity, compressed as the cavity reduces in volume, and then discharged through another port in the casing.^[1]

The effectiveness of this mechanism is dependent on close fitting clearances between the helical rotors and the chamber for sealing of the compression cavities.

Rotary screw compressors are used in a diverse range of applications. Typically, they are used to supply compressed air for general industrial applications. Trailer mounted diesel powered units are often seen at construction sites, and are used to power air operated construction machinery.

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History

The principle of the screw compressor was first patented by Heinrich Krigar of Hannover, Germany in March 1878.^[2] He improved his designs later that year and lodged a second patent in August 1878. Both of these patents are amongst the earliest on record. However, the manufacturing capabilities available at that time prevented the commercial development of his designs.

Oil-flooded screw compressors

In an oil-flooded rotary screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling sink for the gas charge. The oil is separated from the discharge stream, then cooled, filtered and recycled. It is usual for some entrained compressor oil to carry over into the compressed gas stream. In some applications, this is rectified by coalescer/filter vessels.^[3]

Standard oil-flooded compressors are capable of achieving output pressures over 200 psig, and output volumes of over 1500 cubic feet per minute (measured at 60 °C and atmospheric pressure).

Oil-free screw compressors

In an oil-free compressor, the air is compressed entirely through the action of the screws, without the assistance of an oil seal. They usually have lower maximum discharge pressure capability as a result. However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psig, and output volume of over 2000 cubic feet per minute (measured at 60 °C and atmospheric pressure).

Oil-free compressors are used in applications where entrained oil carry-over is not acceptable, such as medicial research and semiconductor manufacturing.

See also

• Gas compressor
• Variable speed air compressor
• Vapor-compression refrigeration

References

1. ^ Screw Compressor Describes how screw compressors work and include photographs.
2. ^ The History of Screw Compressors
3. ^ Technical Centre Discusses oil-flooded screw compressors including a complete system flow diagram