Why does air compressor need to be compressed in stages?

2020-06-05 10:00:53 keepwin

Many people know that two stages of compressor are suitable for high pressure production and one stage is suitable for large gas production. Sometimes, more than two times of compression are needed. Why do we need hierarchical compression?

When the working pressure of gas is required to be high, it is not only uneconomical, sometimes even impossible to use single-stage compression, but also necessary to use multi-stage compression. Multistage compression is to start from the inhalation of gas, after several times of pressure rise to reach the required working pressure.

Why does air compressor need to be compressed in stages

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1. Save power consumption

With multi-stage compression, the compressed gas can be cooled under equal pressure after being compressed in the first stage by setting an intercooler between stages, so as to reduce the temperature, and then enter the next stage of cylinder. With the decrease of temperature and the increase of density, it is easy to further compress. Compared with one time compression, it can save a lot of energy. Therefore, under the same pressure, the work area of multistage compression is less than that of single-stage compression. The more the number of stages, the more the power consumption is saved, and the closer it is to isothermal compression.

Note: the air compressor of oil injection screw air compressor is very close to the constant temperature process. If compression continues to cool down after reaching saturation state, there will be condensate precipitation. If these condensed water and compressed air enter the oil-gas separator (oil tank), it will make the cooling oil emulsified and affect the lubrication effect. With the continuous increase of condensate, the oil level will continue to 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 an air compressor with an exhaust pressure of 11bar (a), the condensation temperature is 68 ℃. When the temperature in the compression chamber is lower than 68 ℃, there will be condensate precipitation. Therefore, the exhaust temperature of oil injection screw air compressor can not be too low, that is, the application of isothermal compression in oil injection screw air compressor is limited due to the problem of condensate.

2. Improve the volume utilization rate

Due to the reasons of manufacture, 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 gas must expand to the suction pressure, so that the cylinder can start to suck in fresh gas, which is equivalent to further reducing the effective volume of the cylinder.

It is not difficult to understand that the larger the pressure ratio is, the larger the expansion of residual gas in the clearance volume is, and the smaller the effective volume of the cylinder is. In the limit case, even after the gas in the clearance volume expands completely in the cylinder, the pressure is still not lower than the suction pressure, at this time, it is impossible to continue suction and exhaust, 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 gas in the clearance volume expands slightly to reach the suction pressure, so that the effective volume of the cylinder can be increased naturally, so as to improve the utilization ratio of the cylinder volume.

3. Reduce exhaust temperature

The temperature of the exhaust gas of the compressor increases with the increase of the 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 the oil lubricated compressor, the temperature of the lubricating oil will reduce the viscosity and increase 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 even cause explosion. For various reasons, exhaust temperature is greatly limited, so multistage compression must be used to reduce exhaust temperature.

Note: staged compression can reduce the exhaust temperature of screw air compressor, and also make the thermal process of air compressor close to the constant temperature compression as much as possible, so as to achieve energy saving effect, but it is not absolute. Especially for the oil injection screw air compressor with the discharge pressure below 13bar, because it is injected with low-temperature cooling oil in the compression process, the compression process is close to the constant temperature process, so it is unnecessary to carry out secondary compression. For example, on the basis of the injection cooling, the staged compression makes the structure complex, increases the manufacturing cost, increases the flow resistance of the gas and the additional power consumption, which is not worth the loss. In addition, if the temperature is too low, the formation of condensate in the compression process will lead to the deterioration of the system state, resulting in serious consequences.

4. Reduce the gas 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 larger, so there is a higher final pressure on the larger piston area, and the gas on the piston is larger. If multi-stage compression is adopted, the gas force acting on the piston can be greatly reduced, which may make the mechanism light and improve the mechanical efficiency.

Of course, multi-level compression is not as good as more series. Because the more stages, the more complex the compressor structure, the more the size, weight and cost; the more the gas channel, the more the pressure loss of the gas valve and management, etc., sometimes the more stages, the lower the economy, the more the stages, the more the moving parts, the more the failure will also increase. Due to the increase of friction, the mechanical efficiency will also be reduced.

The internal leakage of compressor head is the main factor affecting the efficiency of compressor. Multi-stage compression can reduce the pressure ratio of each stage, and the natural internal leakage will be improved if the pressure ratio is small, so multi-stage compression has 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 components of the single-stage compressor head are significantly less than that of the multi-stage compressor, and the reliability of the machine is much higher. From the perspective of reliability, the single compression of high-precision manufacturing is more in line with the interests of users.

Suggestion: in the case of low machining accuracy of the machine head, multi-stage compression is considered to improve the energy efficiency of the machine. If there are high-precision processing conditions, we should adhere to the single-stage compression, and use the number of single-stage compression parts to strengthen the reliability selling point. After all, users pay more attention to reliability.