Rotary-screw compressor Working principle

Oil-free

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 maximal 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 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. However, this does not preclude the need for filtration, as hydrocarbons and other contaminants ingested from the ambient air must also be removed prior to the point of use. Subsequently, air treatment identical to that used for an oil-flooded screw compressor is frequently still required to ensure a given quality of compressed air.

Oil-injected

Diagram of a rotary-screw compressor

In an oil-injected 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. The oil captures non-polar particulates from the incoming air, effectively reducing the particle loading of compressed-air particulate filtration. It is usual for some entrained compressor oil to carry over into the compressed-gas stream downstream of the compressor. In many applications, this is rectified by coalescer/filter vessels.^[3] In other applications, this is rectified by the use of receiver tanks that reduce the local velocity of compressed air, allowing oil to condense and drop out of the air stream to be removed from the compressed-air system by condensate-management equipment.

Control schemes

Among rotary-screw compressors, there are multiple control schemes, each with differing advantages and disadvantages.

Start/stop

In a start/stop control scheme, compressor controls actuate relays to apply and remove power to the motor according to compressed air needs.

Load/unload

In a load/unload control scheme, the compressor remains continuously powered. 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. This device uncovers part of the rotor and proportionately reduces capacity of the machine down to typically 25% of the compressors capability, thereby unloading the compressor. This reduces the number of start/stop cycles for electric motors over a start/stop control scheme in electrically-driven compressors, improving equipment service life with a minimal change in operating cost. This scheme is utilised by nearly all industrial air-compressor manufacturers. When a load/unload control scheme is combined with a timer to stop the compressor after a predetermined period of continuously unloaded operation, it is known as a dual-control or auto-dual scheme.