An air compressor is a mechanical equipment that compresses the volume of gas, increases gas pressure, and transports gas. As a consumable product of power energy, its application range and industry are very wide.

According to the working principle, air compressors are divided into volumetric and velocity types. The volumetric type can be divided into three types: piston type, screw type, and slide type; The speed type can be divided into two types: centrifugal and axial flow. This article discusses the working principles, advantages and disadvantages, and application scope of six common air compressors.

1 piston compressor

The work of a piston compressor is achieved by continuously changing the working volume composed of a cylinder, an air valve, and a piston that reciprocates in the cylinder.

If the volume loss and energy loss in the actual operation of the piston compressor (i.e. ideal working process) are not considered, the work completed by the crankshaft of the piston compressor per rotation can be divided into suction, compression, and exhaust processes.

Working principle of piston compressor:

Compression process: The piston moves upward from the bottom dead center, and the suction and exhaust valves are closed. The gas is compressed in a closed cylinder. As the cylinder volume gradually decreases, the pressure and temperature gradually increase until the gas pressure in the cylinder is equal to the exhaust pressure.

Exhaust process: The piston continues to move upwards, causing the gas pressure in the cylinder to be greater than the exhaust pressure. The exhaust valve opens, and the gas in the cylinder is evenly discharged into the exhaust pipe under the push of the piston until the piston reaches the top dead center. At this point, due to the force of the exhaust valve spring and the gravity of the valve plate itself, the exhaust valve closes and the exhaust ends.

Advantages and disadvantages: Piston air compressors are inexpensive, easy to maintain, but prone to breakage and produce loud noise.

2 screw compressor

Screw compressors are divided into single screw compressors and twin-screw compressors.

Working principle of screw compressor:

The cylinder of the screw compressor is equipped with a pair of helical male and female rotors that mesh with each other. Both rotors have several concave teeth and rotate in opposite directions to each other.

The gap between rotors and between the casing and rotors is only 5-10 threads. The main rotor (also known as the male rotor or convex rotor) is driven by an engine or electric motor (mostly electric motor), while the other rotor (also known as the female rotor or concave rotor) is driven by an oil film formed by the main rotor through fuel injection, or by synchronous gears at the main and concave rotor ends.

Advantages and disadvantages: High gas production, low noise, high reliability, but relatively high cost, cannot be used in ultra-high pressure applications.

3 Centrifugal compressors

Centrifugal compressor, also known as turbine compressor, is mainly used to compress gases and consists of two parts: rotor and stator.

The rotor includes an impeller and a shaft, with blades, a balance disk, and a portion of the shaft seal on the impeller; The main body of the stator is the cylinder, as well as devices such as diffusers, bends, refluxers, inlet pipes, and exhaust pipes.

Working principle of centrifugal compressor:

When the impeller rotates at high speed, the gas is thrown into the diffuser behind it by centrifugal force, forming a vacuum zone at the impeller. At this time, fresh gas from the outside enters the impeller.

The impeller continuously rotates, and gas is constantly sucked in and thrown out, thus maintaining the continuous flow of gas.

Main advantages: large and continuous gas transmission, smooth operation; The unit has small external dimensions, light weight, and occupies less space; The equipment has fewer vulnerable parts, longer service life, and less maintenance workload; Due to its high rotational speed, it can be directly driven by a steam turbine, which is relatively safe and easy to achieve automatic control.

Main disadvantages: lower efficiency than axial and reciprocating compressors; The stable operating condition zone is relatively narrow, and there is a surge phenomenon occurring.

4 rotor compressor

The rotary compressor is driven by an engine or an electric motor (mostly an electric motor), while the other rotor (also known as a female rotor or concave rotor) is driven by an oil film formed by the main rotor through fuel injection, or by synchronous gears at the main and concave rotor ends.

Principle of rotary compressor:

When the rotor rotates and compresses the lubricant+gas (referred to as the oil gas mixture), the volume of the compression chamber decreases, and the oil gas mixture is compressed towards the exhaust port. When the compression chamber passes through the exhaust port, the oil and gas mixture is discharged from the compressor, completing a suction compression exhaust process.

5 axis compressor

Axial flow compressor is a type of large-scale air compressor, with a maximum power of 150000 kW and a displacement of 20000 cubic meters per minute. Its compressor energy efficiency ratio can reach about 90%, which is more energy-efficient than centrifuges.

Axial compressors and centrifugal compressors are both speed compressors, also known as turbine compressors.

Advantages: High efficiency, single machine efficiency can reach 86% to 92%, 5% to 10% higher than centrifugal compressors, large flow capacity per unit area, small radial size, suitable for flow rates greater than 1500m ³/min, low single-stage pressure, single cylinder multi-stage pressure ratio can reach 11, simple structure, and easy maintenance.

6 plate compressor

The sliding vane compressor is driven directly at a very low speed, and the rotor is the only continuously running component with several slots cut along the length direction, in which sliding vanes that can slide on the oil film are inserted. The rotor rotates in the stator of the cylinder.

Working principle of slide compressor:

Air is drawn in through a filter and a proportional valve, which is mainly used to regulate the pressure chamber formed by the rotor and slide of the air cylinder. The rotor rotates eccentrically relative to the cylinder, and the valve plate is installed in the groove of the rotor. The sliding plate is pushed to the cylinder wall by centrifugal force. The efficient oil injection system can ensure the cooling of the compressor and minimize the loss of lubricant. The thin oil film formed on the cylinder wall can prevent direct contact between metal parts and wear.

Advantages and disadvantages: Durable, requiring minimal maintenance. The main disadvantage is that there is significant mechanical friction between the slider and the rotor cylinder, resulting in significant energy loss and therefore lower efficiency.