Why do dry goods 〡 air compressor need staged compression? Can't it be compressed once?

Why do dry goods 〡 air compressor need staged compression? Can't it be compressed once?

Many people know that two compressor stages are suitable for high pressure production, and one stage is suitable for large gas production. Sometimes, more than two compressions are needed. Why do we need hierarchical compression? I will organize it for everyone today.

　　 When the working pressure of the gas is required to be high, the use of single-stage compression is not only uneconomical, but sometimes even impossible to achieve, and multi-stage compression must be used. Multi-stage compression means that the gas is sucked in, and then the pressure is increased several times to reach the required working pressure.

　　1 Save power consumption

　　Using multi-stage compression, it is possible to install an intercooler between the stages, so that the compressed gas will undergo isostatic cooling after one stage of compression to lower the temperature before entering the next stage of cylinder. The temperature decreases and the density increases, so that it is easy to compress further, which can greatly save power consumption compared with one compression. Therefore, under the same pressure, the area of ​​multi-stage compression work is less than that of single-stage compression. The more the number of stages, 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 very close to the constant temperature process. If you continue to compress and continue to cool after reaching the saturation state, condensate will precipitate. If the condensed water enters the oil-air separator (oil tank) together with compressed air, it will emulsify the cooling oil and affect the lubrication effect. With the continuous increase of condensate, 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, an air compressor with an exhaust pressure of 11 bar (A) has a condensation temperature of 68°C. When the temperature in the compression chamber is lower than 68°C, condensate 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.

Why do dry cargo 〡 air compressors need staged compression? Can't it be compressed once?

　　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 the remaining high-pressure gas must be expanded to suction Pressure, 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 if the pressure ratio is larger, the residual gas in the clearance volume expands more vigorously, and the effective cylinder volume becomes 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, and then 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 can be expanded slightly to reach the suction pressure, which naturally increases the effective cylinder volume, thereby increasing the utilization rate of the cylinder volume.

　　3 Reduce exhaust temperature

　　The temperature of the discharged gas of the compressor increases with the increase of the compression ratio. The higher the compression ratio, the higher the discharge temperature, but too high discharge temperature is often not allowed. This is because in an oil-lubricated compressor, 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, aggravate 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: The 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 as possible to the constant temperature compression to achieve energy saving effect, but it is not absolute. Especially for an oil-injected screw air compressor with an exhaust pressure of 13bar or less, since it sprays low-temperature cooling oil during the compression process, the compression process is close to the constant temperature process, and there is no need to perform secondary compression. If the staged compression is performed on the basis of the oil injection cooling, the structure is complicated, the manufacturing cost is increased, and the flow resistance of the gas and the additional power consumption are increased, which is not worth the gain. In addition, if the temperature is too low, the formation of condensed water during the compression process will cause the system to deteriorate and cause serious consequences.

　　4 Reduce the gas force acting on the piston rod

　　 On a piston compressor, when the compression ratio is high and single-stage compression is used, the larger the cylinder diameter, the higher the gas final pressure acts on the larger piston area, and the larger the gas on the piston. If multi-stage compression is used, the gas force acting on the piston can be greatly reduced, which may make the mechanism lighter and improve the mechanical efficiency.

　　 Of course, multi-stage compression is not as many stages as possible. Because the more stages, the more complicated the compressor structure, the increase in size, weight, and cost; the increase in gas passages, and the increase in the pressure loss of the valve and management, etc., so sometimes the more stages, the economy will decrease. As more moving parts increase, the chance of failure will increase. As friction increases, mechanical efficiency will also decrease.

　　The main factor that affects the compressor efficiency is the internal leakage of the machine head. Multi-stage compression can reduce the pressure ratio of each stage. If the pressure ratio is small, the internal leakage will be improved. Therefore, multi-stage compression has a certain energy saving effect. However, if the machining accuracy of the machine head is high, the internal clearance can be effectively controlled and internal leakage can be reduced, and significant energy saving effects can also be obtained. The single-stage compressor head has significantly fewer parts than multi-stage compression, and the reliability of the machine is much higher. From the perspective of reliability, the single-machine compression manufactured with high precision is more in line with the interests of users. Recommendation: When the machining accuracy of the machine head is low, consider improving the energy efficiency of the machine through multi-stage compression. If there are high-precision processing conditions, we still insist on 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.