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 similar 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. Liquids are relatively incompressible, so the main action of a pump is to transport liquids.

Types of compressors

As shown above, there are many different types of gas compressors. The two primary categories are:
Positive displacement compressors with two sub-categories:
Reciprocating
Rotary
Dynamic compressors also with two sub-categories:
Centrifugal
Axial
The more important types in each of the four sub-categories are discussed below. .

Centrifugal compressors use a vaned rotating disk or impeller in a shaped housing to force the gas to the rim of the impeller, increasing the velocity of the gas. A diffuser (divergent duct) section converts the velocity energy to pressure energy. They are primarily used for continuous, stationary service in industries such as oil refineries, chemical and petrochemical plants and natural gas processing plants. Their application can be from 100 hp (75 kW) to thousands of horsepower. With multiple staging, they can achieve extremely high output pressures greater than 10,000 psi (69 MPa).

Many large snow-making operations (like ski resorts) use this type of compressor. They are also used in internal combustion engines as superchargers and turbochargers. Centrifugal compressors are used in small gas turbine engines or as the final compression stage of medium sized gas turbines.

Diagonal or mixed-flow compressors are similar to centrifugal compressors, but have a radial and axial velocity component at the exit from the rotor. The diffuser is often used to turn diagonal flow to the axial direction. The diagonal compressor has a lower diameter diffuser than the equivalent centrifugal compressor.

Axial-flow compressors use a series of fan-like rotating rotor blades to progressively compress the gasflow. Stationary stator vanes, located downstream of each rotor, redirect the flow onto the next set of rotor blades. The area of the gas passage diminishes through the compressor to maintain a roughly constant axial Mach number. Axial-flow compressors are normally used in high flow applications, such as medium to large gas turbine engines. They are almost always multi-staged. Beyond about 4:1 design pressure ratio, variable geometry is often used to improve operation.

Reciprocating compressors use pistons driven by a crankshaft. They can be either stationary or 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 horsepower (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 various other types of compressors. Discharge pressures can range from low pressure to very high pressure (>5000 psi or 35 MPa). In certain applications, such as air compression, multi-stage double-acting compressors are said to be the most efficient compressors available, and are typically larger, noisier, and more costly than comparable rotary units

Gas compressors are used in various applications where either higher pressures or lower volumes of gas are needed:
in pipeline transport of purified natural gas to move the gas from the production site to the consumer.
in petroleum refineries, natural gas processing plants, petrochemical and chemical plants, and similar large industrial plants for compressing intermediate and end product gases.
in refrigeration and air conditioner equipment to move heat from one place to another in refrigerant cycles: see Vapor-compression refrigeration.
in gas turbine systems to compress the intake combustion air in storing purified or manufactured gases in a small volume, high pressure cylinders for medical, welding and other uses.
in many various industrial, manufacturing and building processes to power all types of pneumatic tools.
as a medium for transferring energy, such as to power pneumatic equipment.
in pressurised aircraft to provide a breathable atmosphere of higher than ambient pressure.
in some types of jet engines (such as turbojets and turbofans) to provide the air required for combustion of the engine fuel. The power to drive the combustion air compressor comes from the jet's own turbines.
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 air for later use as buoyancy.
in turbochargers and superchargers to increase the performance of internal combustion engines by concentrating oxygen.
in rail and heavy road transport to provide compressed air for operation of rail vehicle brakes or road vehicle brakes and various other systems (doors, windscreen wipers, engine/gearbox control, etc).
in miscellaneous uses such as providing compressed air for filling pneumatic tires.