A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally increases its temperature.
Compressors are closely related to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas, whereas the main result of a pump raising the pressure of a liquid is to allow the liquid to be transported elsewhere.
* Reciprocating compressors—uses pistons driven by
a crankshaft. They are both stationary and portable, can be single or multi-staged,
and can be driven by electric motors or internal combustion engines. Small
reciprocating compressors from 5 to 30 HP are commonly seen in automotive applications
and are typically for intermittent duty. Larger reciprocating compressors up
to 1000 HP are still commonly found in large industrial applications, but their
numbers are declining as they are replaced by less costly rotary screw compressors.
Discharge pressures can range from low pressure to very high pressure (>5000
psi or 35 MPa).
* Rotary screw compressors—uses two meshed rotating positive-displacement helical screws to force the gas into a smaller space. These are usually for continuous, commercial and industrial applications and are both stationary and portable. Their application can be from 5 hp (3.7 kW) to over 500 hp (375 kW) and from low pressure to very high pressure (>1200 psi or 8.3 MPa). They are commonly seen with roadside repair crews powering airtools. This type is also used for many automobile engine superchargers because it is easily matched to the induction capacity of a piston engine.
* Oil Free 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 (56.634 cubic meters) 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 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.
* Scroll compressor—similar to a rotary screw device, this one includes two interleaved spiral-shaped scrolls to compress a gas. Its output is more pulsed than the latter and this factor has caused its declining industrial use. Can be found in automotive use as a supercharger.
Air compressors sold to and used by the general public are often attached on top of a tank for holding the pressurized air. Oil-lubricated and oil-free compressors are available.
Since compression generates heat, the compressed air is to be cooled between stages making the compression less adiabatic and more isothermal. The inter-stage coolers cause condensation meaning water separators with drain valves are present. The compressor flywheel may drive a cooling fan.
For instance in a typical diving compressor, the air is compressed in three stages. If each stage has a compression ratio of 7 to 1, the compressor can output 343 times atmospheric pressure (7 x 7 x 7 = 343).
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