Types and characteristics of air compressors and their application choices

Types and characteristics of air compressors and their application choices

Air compressor (English: aircompressor) is the main body of the air source device. It is a device that converts the mechanical energy of the prime mover (usually an electric motor) into gas pressure energy, and is a pressure generating device for compressed air.

Multi-unit combined medium pressure air compressor

Types of air compressors

There are many types of air compressors, which can be divided into positive displacement compressors, reciprocating compressors, and centrifugal compressors according to their working principles. The working principle of positive displacement compressors is to compress the volume of gas and increase the density of gas molecules per unit volume. To increase the pressure of the compressed air; the working principle of the centrifugal compressor is to increase the movement speed of the gas molecules, so that the kinetic energy of the gas molecules is converted into the pressure energy of the gas, thereby increasing the pressure of the compressed air. The working principle of a reciprocating compressor (also called a piston compressor) is to directly compress gas and discharge it when it reaches a certain pressure.

The commonly used air compressors are piston air compressors, screw air compressors, (screw air compressors are divided into twin screw air compressors and single screw air compressors), centrifugal compressors and sliding vane air compressors Machine, scroll air compressor. The following is the definition of various compressors. Compressors such as cam type, diaphragm type and diffusion pump are not included because of their special purpose and relatively small size.

Positive displacement compressor-a compressor that directly relies on changing the gas volume to increase the gas pressure.

Reciprocating compressor-is a positive displacement compressor, its compression element is a piston, reciprocating in the cylinder.

Rotary compressor-is a positive displacement compressor, compression is achieved by the forced movement of rotating elements.

The sliding vane compressor is a rotary variable capacity compressor, the axial sliding vane slides radially on the rotor eccentric to the cylindrical cylinder. The air trapped between the sliding plates is compressed and discharged.

Liquid-piston compressor-is a rotary positive displacement compressor in which water or other liquid acts as a piston to compress gas and then discharge the gas.

Roots double-rotor compressor-a rotary positive displacement compressor, in which two Roots rotors mesh with each other to intercept the gas and send it from the inlet to the outlet. There is no internal compression.

Screw compressor-is a rotary positive displacement compressor in which two rotors with helical gears mesh with each other to compress and discharge gas.

Speed ​​type compressor-is a rotary continuous flow compressor, in which high-speed rotating blades accelerate the gas passing through it, thereby converting speed energy into pressure. This conversion occurs partly on the rotating blades and partly on the fixed diffuser or reflux baffle.

Centrifugal compressor-a speed compressor, in which there are one or more rotating impellers (blade usually on the side) to accelerate the gas. The main air flow is radial.

Axial compressor-a speed compressor in which the gas is accelerated by a bladed rotor. The main air flow is axial.

Mixed flow compressor-also a speed compressor, the shape of its rotor combines some of the characteristics of both centrifugal and axial flow.

Jet compressor-use high-speed gas or steam jet to take away the inhaled gas, and then convert the speed of the mixed gas into pressure on the diffuser.

Features of air compressor

The compressor is directly driven by the electric motor, which causes the crankshaft to rotate, and drives the connecting rod to cause the piston to reciprocate, causing the cylinder volume to change. Due to the change in the pressure in the cylinder, the air enters the cylinder through the air filter (muffler) through the intake valve. During the compression stroke, due to the reduction of the cylinder volume, the compressed air passes through the exhaust valve and passes through the exhaust pipe. The valve (check valve) enters the gas storage tank, and when the exhaust pressure reaches the rated pressure of 0.7MPa, it is controlled by the pressure switch and automatically stops. When the pressure of the gas storage tank drops to 0.5-0.6MPa, the pressure switch is automatically connected and activated.

Selection of air compressor

The choice of air compressor is mainly based on the working pressure and flow rate of the pneumatic system.

The working pressure of the gas source should be about 20% higher than the maximum working pressure in the pneumatic system, because the loss along the way and the local loss of the gas supply pipeline should be considered. If the working pressure requirements in some parts of the system are low, a pressure reducing valve can be used to supply air. The rated discharge pressure of the air compressor is divided into low pressure (0.7~1.0MPa), medium pressure (1.0~10MPa), high pressure (10~100MPa) and ultra high pressure (above 100MPa), which can be selected according to actual needs. The common use pressure is generally 0.7-1.25.

First, select the type of air compressor according to the characteristics of the air compressor. Then determine the output pressure pc and suction flow qc of the air compressor according to the working pressure and flow parameters required by the pneumatic system, and finally select the air compressor model.

(1) The output pressure of the air compressor pc

pc=p+∑△p

pc: output pressure of air compressor

p: the highest working pressure of pneumatic actuator

∑△p: The total pressure loss of the pneumatic system.

Under normal circumstances, another ∑△p=0.15~0.2MPa.

(2) The suction flow rate of the air compressor qc

No gas tank, qb=qmax

Set gas tank, qb=qsa

qb: Flow rate provided by pneumatic system

qmax: the maximum air consumption of the pneumatic system

qsa: Average air consumption of pneumatic system

The suction flow of the air compressor, qc=kqb

qc: suction flow of air compressor

k: correction factor. Mainly consider the leakage of pneumatic components, pipe joints, etc., the estimation error of pneumatic system air consumption, the utilization rate of multiple pneumatic equipment at different times, and the possibility of adding new pneumatic equipment. Generally k=1.5~2.0.

(3) Power P of the air compressor

p=(n+1)*k*p1*qc*(pc/p1)^{[(k-1)/[(n+1)*k]-1}/(k-1)*0.06

Use of air compressor

a. Traditional air power: pneumatic tools, rock drills, pneumatic picks, pneumatic wrenches, pneumatic sandblasting

b. Instrument control and automation devices, such as tool replacement of machining centers, etc. c. Vehicle braking, opening and closing of doors and windows

d. In the air-jet loom, compressed air is used to blow the weft yarn instead of the shuttle

e. Food and pharmaceutical industries, using compressed air to stir slurry

f. Start-up of large-scale marine diesel engine g, wind tunnel experiment, underground passage ventilation, metal smelting

h. Oil well fracturing

i. Coal mining by high-pressure air blasting

j. Weapon system, missile launch, torpedo launch

k. Submarine ups and downs, sunken ship salvage, seabed oil exploration, hovercraft

l, tire inflation

m, spray paint,

n. Bottle blowing machine