When using an air compressor, when the working pressure of the gas is required to be high, single-stage compression is not only uneconomical, but sometimes even impossible to achieve, so many people use multi-stage compression. Multi-stage compression is to start from the suction of the gas and increase the pressure several times to reach the required working pressure.
1 Save power consumption
By using multi-stage compression, the compressed gas can be isobarically cooled after the first stage of compression by setting an intercooler between the stages to reduce the temperature before entering the next stage of the cylinder. The lower the temperature and the higher the density, the easier it is to further compress, which 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 already very close to the constant temperature process. If the compression and cooling continue after reaching the saturated state, condensed water will precipitate. If these condensed water enter the oil-gas separator (oil tank) together with the compressed air, the cooling oil will be emulsified, affecting 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.
2 Improve volume utilization
Due to the three 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 gas must expand to the suction pressure before 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 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.
3 Reduce the 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, but too high an 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 explosions. 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 energy saving effect, but it is not absolute. Especially for the oil-injected screw air compressor with an exhaust pressure below 13 bar, since the 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.
4 Reduce the gas force acting on the piston rod
For piston compressors, when the compression ratio is high and single-stage compression is used, the cylinder diameter is larger, and a higher gas final pressure acts on a larger piston area, and the gas 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 light and improve the mechanical efficiency.
Of course, multi-stage compression is not the more stages the better. Because the more stages there are, the more complex the compressor structure becomes, the larger the size, weight and cost will be, the more gas channels there are, the more pressure loss of the valve and management will be, etc., so sometimes the more stages there are, the lower the economy will be, the more moving parts there are, the more chances of failure will be, and the mechanical efficiency will also be reduced due to increased friction.