Screw air compressors do not have valves or other mechanical forces that can cause unbalance. This allows a rotary screw compressor to operate at high speeds while combining a large flow rate with small exterior dimensions. The ideal applications of rotary air compressors are continuous, workplace, and industrial applications. There are two primary versions, oil-free and oil-injected, with options of fixed speed or variable speed drive operation.
The main benefit of rotary screw compressors is their energy efficiency. But this type of compressor has many other benefits. These include:
- continuous operation at temperatures of up to 46 degrees C
- low oil carryover (as little as 3 ppm) in oil-lubricated machines
- zero loss of capacity over time low noise output
- small footprint for point-of-use installations
- no duty cycle
There are two main types of screw compressors: Oil-free and Oil Inject
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 before the point of use. Consequently, air treatment identical to that used for an oil-flooded screw compressor is frequently required to ensure quality compressed air.
In an oil-injected rotary-screw compressor, oil is injected into the compression cavities to aid sealing and provide cooling for the gas charge. The oil is separated from the discharge stream, 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 into the compressed gas stream downstream of the compressor.
Oil-Injected 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.
oil-free compressors have higher prices compared to oil-injected ones.