Why do air compressors use two-stage compression?

The compressor has two stages, with the first stage suitable for generating high pressure and the second stage suitable for handling large gas volumes. Sometimes, multiple compressions are required. Why is staged compression necessary? 

When the working pressure of the gas needs to be higher, using single-stage compression is not only uneconomical but sometimes even impossible to achieve. Therefore, multi-stage compression must be adopted. Multi-stage compression means that starting from the gas's intake, it undergoes several stages of pressure increase to reach the required working pressure. 

The principle of the two-stage compression screw air compressor is as follows: The air passes through the air filter and enters the first stage of compression. In the compression chamber, it mixes with a small amount of lubricating oil and is compressed to the inter-stage pressure. The compressed gas then enters the cooling channel and comes into contact with a large amount of oil mist, thereby significantly reducing the temperature. The cooled compressed gas then enters the second stage rotor for secondary compression, which is compressed to the final exhaust pressure. Finally, it is discharged from the compressor through the exhaust flange, completing the entire compression process. The principles of different designs are largely similar.

Reduce power consumption

By using multi-stage compression, through the method of setting intermediate coolers between stages, the compressed gas can first undergo isobaric cooling after the first stage compression to lower the temperature, and then enter the next cylinder. The temperature drops and the density increases, which makes it easier to further compress. Compared to single-stage compression, this can significantly reduce the power consumption. Therefore, under the same pressure, the working area for multi-stage compression is less than that of single-stage compression. The more stages there are, the more power is saved and the closer it approaches isothermal compression. 

Note: The air compressor of the oil-injected screw air compressor is already very close to the constant temperature process. If the compression continues and the cooling continues after reaching the saturation state, condensate water will be formed. If this condensate water mixes with the compressed air and enters the oil separator (oil tank), it will emulsify the lubricating oil, affecting the lubrication effect. As the amount of condensate water increases, the oil level will also rise continuously. Eventually, 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 formation of condensate water, the temperature inside the compression chamber must not be too low and must be higher than the condensation temperature. For example, for a compressor with an exhaust pressure of 11 bar (A) and a condensation temperature of 68℃, if the temperature inside the compression chamber is lower than 68℃, condensate water will precipitate. Therefore, the exhaust temperature of the oil-injected screw compressor cannot be too low, and the application of isothermal compression in oil-injected screw machines is restricted due to the problem of condensate water.

Improve the utilization rate of volume 

Due to the reasons of manufacturing, installation and operation, the clearance volume within the cylinder is always inevitable. This clearance volume not only directly reduces the effective volume of the cylinder, but also the high-pressure gas remaining in it must expand to the intake pressure before the cylinder can start to inhale fresh gas. This is equivalent to further reducing the effective volume of the cylinder. 

It is easy to understand that if the pressure is greater, the residual gas in the clearance volume expands more severely, and the effective volume of the cylinder becomes smaller. In the extreme case, even if the gas in the clearance volume completely expands within the cylinder, the pressure is still not lower than the intake pressure. At this point, it is impossible to continue inhaling and expelling, and the effective volume of the cylinder becomes zero. If multi-stage compression is used, the compression ratio of each stage is very small. The residual gas in the clearance volume can slightly expand to reach the intake pressure, which naturally enables the effective volume of the cylinder to increase, thereby improving the utilization rate of the cylinder volume. 

Reduce exhaust temperature 

The temperature of the gas discharged by the compressor increases as the compression ratio increases. The higher the compression ratio, the higher the exhaust temperature. However, excessively high exhaust temperatures are generally not allowed. This is because: In oil-lubricated compressors, the temperature of the lubricating oil will reduce its viscosity, accelerate wear, and when the temperature rises too high, it is prone to form carbon deposits in the cylinder and valves, further accelerating wear, and in some cases, even causing an explosion. Due to various reasons, the exhaust temperature is greatly limited, so multi-stage compression must be adopted to reduce the exhaust temperature. 

Note: Grade-based compression can reduce the exhaust temperature of the screw air compressor and also enable the air compressor's thermodynamic process to approach a constant-temperature compression as closely as possible, achieving energy-saving effects. However, this is not absolute. Especially for oil-injected screw air compressors with an exhaust pressure below 13 bar, since they inject low-temperature cooling oil during the compression process, the compression process is already close to a constant-temperature process. There is no need for further secondary compression. If secondary compression is carried out on this basis while injecting oil for cooling, the structure will become more complex, the manufacturing cost will increase, and it will also increase the gas flow resistance and additional power consumption, which is somewhat not worth it. In addition, if the temperature is too low and condensate water forms during the compression process, it will deteriorate the system state and cause serious consequences.