Why do compressors need staged compression? Principle of two-stage compression screw air compressor

The two-stage compressor is suitable for high pressure, and the one-stage compressor is suitable for large gas production. Sometimes, more than two compressions are required. Why is staged compression necessary?

When the working pressure of gas is required to be high, single-stage compression is not only uneconomical, but sometimes even impossible to achieve, so multi-stage compression must be used. Multi-stage compression is to increase the pressure of gas from suction to several times to reach the required working pressure.

The principle of two-stage compression screw air compressor is that the compression process is that natural air enters the first stage through the air filter, mixes with a small amount of lubricating oil in the compression chamber, and compresses the mixed gas to the interstage pressure. The compressed gas enters the cooling channel and contacts with a large amount of oil mist, which greatly reduces the temperature. The compressed gas after cooling enters the second stage rotor for secondary compression and is compressed to the final exhaust pressure. Finally, it is discharged from the compressor through the exhaust flange to complete the entire compression process. The principles of different designs are similar.

Save power consumption

By adopting multi-stage compression, an intercooler can be set between the stages so that the compressed gas can be cooled isobarically after the first stage of compression to reduce the temperature before entering the next stage of cylinder. The temperature is reduced and the density is increased, which makes it easier to compress further, and can greatly save power consumption compared to a single compression. Therefore, under the same pressure, the area of ​​multi-stage compression work is smaller than that of single-stage compression. The more stages there are, the more power consumption is saved and the closer it is to isothermal compression.

Note: The air compressor of the oil-injected screw air compressor is very close to the constant temperature process. If the compression and cooling continue after reaching the saturated state, condensed water will precipitate. If this condensed water enters the oil-gas separator (oil tank) together with the compressed air, it will emulsify the cooling oil and affect the lubrication effect. As the condensed water continues to increase, the oil level will continue to rise, and finally the cooling oil will enter the system along with the compressed air, polluting the compressed air and causing serious consequences to the system.

Therefore, in order to prevent the generation of condensed water, the temperature in the compression chamber cannot be too low and must be greater than the condensation temperature. For example, for an air compressor with an exhaust pressure of 11 bar (A), the condensation temperature is 68°C. When the temperature in the compression chamber is lower than 68°C, condensed water will precipitate. Therefore, the exhaust temperature of the oil-injected screw air compressor cannot be too low, that is, the application of isothermal compression in the oil-injected screw compressor is limited due to the problem of condensed water.

Improve volume utilization

Due to manufacturing, installation and operation reasons, the clearance volume in the cylinder is always unavoidable. The clearance volume not only directly reduces the effective volume of the cylinder, but the residual high-pressure gas therein must also expand to the suction pressure before the cylinder can begin to inhale fresh gas, which is equivalent to further reducing the effective volume of the cylinder.

It is not difficult to understand that if the pressure ratio is larger, the residual gas in the clearance volume will expand more violently, and the effective volume of the cylinder will be smaller. In extreme cases, even after the gas in the clearance volume is fully expanded in the cylinder, the pressure is still not lower than the suction pressure. At this time, it is impossible to continue to inhale and exhaust, and the effective volume of the cylinder becomes zero. If multi-stage compression is used, the compression ratio of each stage is very small, and the residual gas in the clearance volume can reach the suction pressure with a slight expansion, which naturally increases the effective volume of the cylinder, thereby improving the utilization rate of the cylinder volume.

Reduce exhaust temperature

The temperature of the exhaust gas from the compressor increases with the increase of compression ratio. The higher the compression ratio, the higher the exhaust temperature. However, too high exhaust temperature is often not allowed. This is because: in oil-lubricated compressors, the temperature of the lubricating oil will reduce the viscosity and aggravate wear. When the temperature rises too high, it is easy to form carbon deposits in the cylinder and on the valve, aggravating wear and even explosion. For various reasons, the exhaust temperature is greatly limited, so multi-stage compression must be used to reduce the exhaust temperature.

Note: Staged compression can reduce the exhaust temperature of the screw air compressor, and also make the thermal process of the air compressor as close to the constant temperature compression as possible to achieve energy saving effect, but it is not absolute. Especially for the oil-injected screw air compressor with an exhaust pressure below 13 bar, since low-temperature cooling oil is injected during the compression process, the compression process is close to the constant temperature process, and there is no need to perform secondary compression. If staged compression is performed on the basis of this oil-injection cooling, the structure will be complicated, the manufacturing cost will be increased, and the flow resistance of the gas and the additional power consumption will be increased, which is a bit not worth the loss. In addition, if the temperature is too low, the formation of condensed water during the compression process will cause the system state to deteriorate, resulting in serious consequences.

Reduce gas forces acting on the piston rod

In a piston compressor, when the compression ratio is high and single-stage compression is used, the cylinder diameter is large, and a higher gas final pressure acts on a larger piston area, and the gas pressure on the piston is larger. If multi-stage compression is used, the gas force acting on the piston can be greatly reduced, which may make the mechanism lighter and improve mechanical efficiency.

Of course, multi-stage compression does not mean the more stages, the better. Because the more stages, the more complex the compressor structure, the larger the size, weight and cost; the more gas channels, the greater the pressure loss of the gas valve and management, etc., so sometimes the more stages, the lower the economic efficiency, the more moving parts, the greater the chance of failure. Due to the increase in friction, the mechanical efficiency will also be reduced.