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Why Do Air Compressors Need Staged Compression? Can't You Compress It Once?

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Author : Lily JIang
Update time : 2022-10-09 11:39:57
Many people know that the two stages of the compressor are suitable for producing high pressure, and the first stage is suitable for producing large amount of air. Sometimes, you need to perform more than two compression operations. Why do you need to perform hierarchical compression?
 
When the working pressure of the air is required to be high, single-stage compression is not only uneconomical, but sometimes even impossible, and multi-stage compression must be adopted. Multi stage compression refers to the process that the air is sucked into the air to reach the required working pressure after several times of pressure rise.
 
1.Save Power Consumption
Multi stage compression is adopted. By setting an intercooler between stages, the compressed air will be cooled at equal pressure after being compressed at the first stage to reduce the temperature before entering the next cylinder. The temperature decreases and the density increases, so it is easy to further compress. Compared with one compression, it can greatly save power consumption. Therefore, under the same pressure, the work area of multi-stage compression is less than that of single-stage compression. The more stages, the more power consumption will be saved, and the closer to isothermal compression.
 
Note: The air compressor of oil injection screw air compressor is very close to the constant temperature process. If the compressor continues to compress and cool after reaching the saturation state, condensate will be separated. If the condensate enters the oil-air separator (oil tank) together with the compressed air, the cooling oil will be emulsified and the lubrication effect will be affected. With the continuous increase of condensate water, the oil level will also rise. Finally, the cooling oil will enter the system with the compressed air, polluting the compressed air, causing serious consequences to the system.
 
Therefore, in order to prevent the generation of condensate, the temperature in the compression chamber must not be too low, and must be higher than the condensation temperature. For example, for the air compressor with the exhaust pressure of 11bar(A), the condensation temperature is 68. When the temperature in the compression chamber is lower than 68, there will be condensed water. Therefore, the exhaust temperature of oil injected screw air compressor cannot be too low, that is, the application of isothermal compression in oil injected screw air compressor is limited due to the problem of condensate.
 
2.Improve Volume Utilization
Due to the reasons of manufacturing, installation and operation, the clearance volume in the cylinder is always inevitable, and the clearance volume not only directly reduces the effective volume of the cylinder, but also the residual high-pressure air must expand to the suction pressure before the cylinder can start to inhale fresh air, which is equivalent to further reducing the effective volume of the cylinder.
 
It is not difficult to understand that if the pressure ratio is greater, the residual air in the clearance volume will expand more and the effective volume of the cylinder will be smaller. In the extreme case, even after the air in the clearance volume has fully expanded in the cylinder, the pressure is still not lower than the suction pressure. At this time, the suction and exhaust cannot continue, and the effective volume of the cylinder becomes zero. If multi-stage compression is adopted, the compression ratio of each stage is very small, and the residual air in the clearance volume will expand slightly to reach the suction pressure, which will naturally increase the effective volume of the cylinder, thus improving the utilization rate of the cylinder volume.
 
3 Reduce Exhaust Temperature
The temperature of the discharged air of the compressor increases with the increase of the compression ratio. The higher the compression ratio is, the higher the exhaust temperature will be. However, excessive exhaust temperature is often not allowed. This is because: in the oil lubricated compressor, the high temperature of the lubricating oil will reduce the viscosity and aggravate the wear. When the temperature rises too high, it is easy to form carbon deposits in the cylinder and on the valve, which will aggravate the wear, and sometimes even explode. For various reasons, the exhaust temperature is greatly limited, so multi-stage compression must be used to reduce the exhaust temperature.
 
Note: The staged compression can reduce the exhaust temperature of the screw air compressor, and also can 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 injection screw air compressor with the exhaust pressure below 13bar, the compression process is close to the constant temperature process because of the injection of low temperature cooling oil during the compression process, so there is no need for secondary compression. If the staged compression is carried out on the basis of this injection cooling, the structure is complex, the manufacturing cost is increased, and the flow resistance of air and additional power consumption are also increased, which is a bit more than worth the loss. In addition, if the temperature is too low, the formation of condensate during the compression process will lead to the deterioration of the system state, causing serious consequences.
 
4 Reduce The Air Force Acting On The Piston Rod
On the piston compressor, when the compression ratio is high and single-stage compression is adopted, the cylinder diameter is large, and there is higher air final pressure acting on the larger piston area, and the air force on the piston is larger. If multi-stage compression is adopted, the air force acting on the piston can be greatly reduced, which may make the mechanism lightweight and improve the mechanical efficiency.
 
Of course, multi-level compression is not as good as more stages. Because the number of stages is more, the compressor structure tends to be complex, and the size, weight and cost will increase; With the increase of air channels, the pressure loss of air valves and management increases, so sometimes the economy decreases with the increase of the number of stages. With the increase of the number of stages, the number of moving parts increases, and the chance of failure increases. As friction increases, the mechanical efficiency will also decrease.
 
Summary:
The main factor affecting the compressor efficiency is the internal leakage of the head. Multi stage compression can reduce the pressure ratio of each stage, and the internal leakage will be improved if the pressure ratio is small. Therefore, multi stage compression has a certain energy-saving effect. However, if the machining accuracy of the head is high, it can effectively control the internal clearance and reduce the internal leakage, which can also achieve significant energy-saving effect. The parts of the single-stage compressor head are significantly less than those of the multi-stage compressor head, and the reliability of the machine is much higher. From the perspective of reliability, the single machine compression of high-precision manufacturing is more in line with the interests of users.
 
Suggestion: In the case of low machining accuracy of the head, consider improving the machine energy efficiency through multi-stage compression. If there are high-precision processing conditions, single-stage compression is still insisted, and the number of single-stage compression parts is used to strengthen the reliability selling point. After all, users pay more attention to reliability.