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A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.

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; while some can be compressed, the main action of a pump is to pressurize and transport liquids.

Many compressors can be staged, that is, the fluid is compressed several times in steps or stages, to increase discharge pressure. Often, the second stage is physically smaller than the primary stage, to accommodate the already compressed gas. Each stage further compresses the gas and increases pressure. Those that are powered by an electric motor can also be controlled using a VFD or power inverter, however many (hermetic and semi-hermetic) compressors can only work at certain speeds, since they may include built-in oil pumps. The oil pumps are connected to the same shaft that drives the compressor and forces oil into the compressor and motor bearings. At low speeds, insufficient quantities or no oil is forced into the bearings, eventually leading to bearing failure, while at high speeds, excessive amounts of oil may be lost from the bearings and compressor and potentially into the discharge line due to splashing. Eventually the oil runs out and the bearings are left unlubricated, again leading to failure, and the oil may contaminate the refrigerant, air or other working gas.[1]

## Types

The main and important types of gas compressors are illustrated and discussed below:

### Positive displacement

A positive displacement compressor is the system which compresses the air by the displacement of a mechanical linkage reducing the volume (since the reduction in volume due to a piston in thermodynamics is considered as positive displacement of the piston).[vague]

Put another way, a positive displacement compres

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; while some can be compressed, the main action of a pump is to pressurize and transport liquids.

Many compressors can be staged, that is, the fluid is compressed several times in steps or stages, to increase discharge pressure. Often, the second stage is physically smaller than the primary stage, to accommodate the already compressed gas. Each stage further compresses the gas and increases pressure. Those that are powered by an electric motor can also be controlled using a VFD or power inverter, however many (hermetic and semi-hermetic) compressors can only work at certain speeds, since they may include built-in oil pumps. The oil pumps are connected to the same shaft that drives the compressor and forces oil into the compressor and motor bearings. At low speeds, insufficient quantities or no oil is forced into the bearings, eventually leading to bearing failure, while at high speeds, excessive amounts of oil may be lost from the bearings and compressor and potentially into the discharge line due to splashing. Eventually the oil runs out and the bearings are left unlubricated, again leading to failure, and the oil may contaminate the refrigerant, air or other working gas.[1]

The main and important types of gas compressors are illustrated and discussed below:

### Positive displacement

A positive displacement compressor is the system which compresses the air by the displacement of a mechanical linkage reducing the volume (since the reduction in volume due to a piston in thermodynamics is considered as positive displacement of the piston).[vague]

Put another way, a positive displacement compressor is one which operates by drawing in a discrete volume of gas from its inlet then forcing that gas to exit via the compressor's outlet. The increase in the pressure of the gas is due, at least in part, to the compressor pumping it at a mass flow rate which cannot pass through the outlet at the lower pressure and density of the inlet.

#### Reciprocating compressors

A motor-driven six-cylinder reciprocating compressor that can operate with two, four or six cylinders.

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.[2][3][4] Small reciprocating compressors from 5 to 30 horsepower (hp) are commonly seen in automotive applications and are typically for intermittent duty. Larger reciprocating compressors well over 1,000 hp (750 kW) are commonly found in large industrial and petroleum applications. Discharge pressures can range from low pressure to very high pressure (>18000 psi or 180 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, and more costly than comparable rotary units.[5] Another type of reciprocating compressor, usually employed in automotive cabin air conditioning systems,[citation needed] is the swash plate or wobble plate compressor, which uses pistons moved by a swash plate mounted on a shaft (see axial piston pump).

Household, home workshop, and smaller job site compressors are typically reciprocating compressors 1½ hp or less with an attached receiver tank.

A linear compressor is a reciprocating compressor with the piston being the rotor of a linear motor.

This type of compressor can compress a wide range of gases, including refrig

A positive displacement compressor is the system which compresses the air by the displacement of a mechanical linkage reducing the volume (since the reduction in volume due to a piston in thermodynamics is considered as positive displacement of the piston).[vague]

Put another way, a positive displacement compressor is one which operates by drawing in a discrete volume of gas from its inlet then forcing that gas to exit via the compressor's outlet. The increase in the pressure of the gas is due, at least in part, to the compressor pumping it at a mass flow rate which cannot pass through the outlet at the lower pressure and density of the inlet.