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Gas compressor

A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume. Compression of a gas naturally produces heat.

Compressors are loosely 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.

Compressor designs

Some important designs of compressors include:

• Reciprocating compressors - use pistons driven by a crankshaft
• Centrifugal compressors - a rotating disk in a pressure vessel forces the gas to the rim of the vessel increasing the pressure
• Axial-flow compressor - a fan or series of fans spinning on a shaft in a tube draw in gas from one end, compress it and output it at the other end.

Applications

Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed:

• in pressurised aircraft to provide a breathable atmosphere of higher than ambient pressure
• in jet engines to provide a high enough concentration of oxygen for combustion of the air and fuel mixture. The power to turn the compressor comes from the jet's own turbines.
• in medicine and manufacturing to store purified or manufactured gases in a small volume
• as a medium for transferring energy, such as to power pneumatic equipment
• in refrigeration equipment to move heat from one place to another
• in pipeline transport of domestic gas to move the gas from the production site to the consumer
• in SCUBA diving, hyperbaric oxygen therapy and other life support devices to store breathing gas in a small volume such as in diving cylinders
• in submarines to store gas for later use as buoyancy
• in turbochargers and superchargers to increase the performance of internal combustion engines by concentrating oxygen

Temperature

Charles Law says "when a gas is compressed heat is generated".

There are two possible relationships between temperature and pressure in a gas undergoing compression:

• isothermal - gas at final stage of compression is same temperature as at beginning of compression. This is impractical for a working machine.
• adiabatic - the rise in air and temperature ratio means compression does not follow a simple pressure to volume ratio. This is less efficient, but quick.

Staged compression

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 bar (7 x 7 x 7 = 343).

Prime movers

There are many options for the "prime mover" or motor which powers the compressor:

• jet turbines power the axial flow compressors that are part of jet engines
• steam turbines or water turbines are possible for large compressors
• electric motors are cheap and quiet for static compressors. Small motors suitable for domestic electrical supplies use single phase alternating current. Larger motors can only be used where an industrial electrical three phase alternating current supply is available.
• diesel engines or petrol engines are suitable for portable compressors