Rotary-screw compressor

The gas compression process of a rotary screw is a continuous sweeping motion, so there is very little pulsation or surging of flow, as occurs with piston compressors.

In a dry-running rotary-screw compressor, timing gears ensure that the male and female rotors maintain precise alignment without contact which would produce rapid wear.

[2] The capacities of rotary-screw compressors are typically rated in horsepower (HP), Standard Cubic Feet per Minute (SCFM)* and pounds per square inch gauge (PSIG.)

Rotary-screw compressors tend to be smooth running with limited vibration, thus not requiring a specialized foundation or mounting system.

[4][5] The modern helical lobe screw compressor was developed in Sweden by Alf Lysholm who was the chief engineer at Ljungstroms Angturbin.

[4][6] In 1952, the first Holroyd cutting machine was used, by the Scottish engineering company Howden, to produce helical lobe compressor rotors greatly reducing both cost and manufacturing time.

[4][5] Slot valves were developed by SRM in the 1950s, allowing for improvements in capacity control which had been a limiting factor for screw compressor application.

In addition to fixed units, rotary-screw compressors are commonly mounted on tow-behind trailers and powered with small diesel engines.

Construction compressors are used to provide compressed air to jack hammers, riveting tools, pneumatic pumps, sand blasting operations and industrial paint systems.

However, multi-stage oil-free compressors, where the air is compressed by several sets of screws, can achieve pressures of over 150 psi (10 atm) and output volume of over 2,000 cubic feet per minute (57 m3/min).

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

In an oil-injected rotary-screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling for the gas charge.

Oil flooded screw compressors are used in a wide variety of applications including air compression, gas refrigeration, hydrocarbon processing and power utilization from low-grade heat sources.

One manufacturer has rated its pneumatic high speed 4-way valves with a life of 50 million cycles, if not exposed to polyglycol oils.

In conjunction with the decreasing diameter of the cone shaped rotor this also allows much higher compression ratios in a single stage with lower output pulsation.

However, when the demand for compressed air is satisfied or reduced, instead of disconnecting power to the compressor, a device known as a slide valve is activated.

[citation needed] Instead of starting and stopping the compressor, a slide valve as described above continuously modulates capacity to the demand rather than being controlled in steps.

Due to the limited adjustment in compressor power consumption relative to compressed-air output capacity, modulation is a generally inefficient method of control when compared to variable-speed drives.

The continuously variable production rate also eliminates the need for significant storage if the load never exceeds the compressor capacity.

While this does reduce power consumption when compared to a modulation control scheme, a load/unload system can be more effective with large amounts of storage (10 gallons per CFM).

[20] One way that variable displacement may be accomplished is by using multiple lifting valves on the suction side of the compressor, each plumbed to a corresponding location on the discharge.

In harsh environments (hot, humid or dusty) the electronics of variable-speed drives may have to be protected to retain expected service life.

[23] The requirement of high-precision computer-controlled manufacturing techniques makes the screw type supercharger a more expensive alternative to other forms of available forced induction.

A clear example of the technology applied by the twin-screw in companies like Ford, Mazda, Mercedes and Mercury Marine can also demonstrate the effectiveness of the twin screw.

The term "blower" is commonly used to define a device placed on engines with a functional need for additional airflow, such as a 2-stroke Diesel engine, where positive intake pressure is needed to "scavenge", or clear spent exhaust gasses from the cylinder and force a fresh intake charge into the cylinder before the compression stroke.

The term "blower" is applied to rotary screw, roots-type, and centrifugal compressors when utilized as part of an automotive forced induction system.

The term 'cabin blower' is also used for the pressurisation of aircraft for high altitude flight, which used Roots type compressors particularly in the 1950s (see Marshall supercharger).