Why do high-pressure air compressors need staged compression?

Many people know that the two-stage compression of the compressor is suitable for producing high pressure. Sometimes, it is necessary to perform more than two compressions. Why do you need many stage compression?

When the working pressure of the air is required to be high, the use of single-stage compression is not only uneconomical, but sometimes even impossible, and multi-stage and segmented compression must be used. Multi-stage and staged compression is to start the initial pressure gas from the inhalation, and to achieve the required working pressure after several times of boosting.

1. Save power consumption

By adopting multi-stage and sub-stage compression, the method of setting intercooler between stages can make the compressed gas undergo isobaric cooling after first-stage compression to reduce the temperature, and then enter the next stage of compression. The temperature is lowered and the density is increased, so that it is easy to further compress, which can greatly save power consumption compared with one-time compression. Therefore, the work area of multi-stage compression is less than that of single-stage compression under the same pressure. The more the number of stages, the more power consumption is saved, and the closer it is to isothermal compression.

Note: Oil-injected screw air compressors are already very close to an isothermal process. If you continue to compress and continue to cool after reaching the saturated state, condensed water will be precipitated. If the condensed water enters the oil-air separator (oil tank) together with the compressed air, it will emulsify the cooling oil and affect the lubrication effect. With the continuous increase of the condensed water, 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 higher than the condensation temperature. For example, an air compressor with an exhaust pressure of 11 bar (A) has a condensing temperature of 68 °C. When the temperature in the compression chamber is lower than 68 °C, condensed water will be precipitated. 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 air compressor is limited due to the problem of condensed water.

2. Improve volume utilization

Due to three reasons of manufacture, installation and operation, the clearance volume in the cylinder is always unavoidable, and the clearance volume not only directly reduces the effective volume of the cylinder, but also the residual high pressure gas must be expanded to the suction pressure, the cylinder can start 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 rapidly, 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, the suction and exhaust cannot be continued, 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 expands slightly to reach the suction pressure, which naturally increases the effective volume of the cylinder, thereby improving the utilization rate of the cylinder volume.

3. Reduce the exhaust temperature

The temperature of the exhaust air of the compressor increases with the increase of the compression ratio. The higher the compression ratio, the higher the exhaust temperature, but the excessively high exhaust temperature is often not allowed. This is because: in an 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, aggravate the wear, and even explode. 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 at the same time, it can also make the thermal process of the air compressor as close to the constant temperature compression as possible to achieve the effect of energy saving, but it is not absolute. Especially for oil-injected screw air compressors with an exhaust pressure of 13 bar or less, because of the low-temperature cooling oil injected during the compression process, the compression process is already close to the constant temperature process, and there is no need for secondary compression. If the staged compression is carried out on the basis of this oil injection cooling, the structure is complicated, the manufacturing cost is increased, and the flow resistance of the gas and the extra power consumption are also increased, which is a bit of a loss. In addition, if the temperature is too low, the formation of condensed water during the compression process will lead to 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 used, the cylinder diameter is larger, and there is a higher final gas pressure acting on the larger piston area, and the gas on the piston is larger. If the multi-stage compression is adopted, the gas force acting on the piston can be greatly reduced, so it is possible to make the mechanism light and the mechanical efficiency improved.

Of course, multi-stage compression is not the more the better. Because the more the number of stages, the more complex the compressor structure, the increase in size, weight and cost; the increase in the gas passage, the increase in the pressure loss of the gas valve and management, etc., so sometimes the more the number of stages, the lower the economy. With more moving parts, the chance of failure will also increase. Mechanical efficiency will also be reduced due to increased friction.

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