Many people know that the two-stage air compressor is suitable for high pressure production, and the first 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, and sometimes even impossible to achieve, and multi-stage compression must be used. Multi-stage compression means that the gas is sucked in, and the pressure is increased several times to reach the required working pressure.
1. Save power consumption
With multi-stage compression, an intercooler can be set between the stages, so that the compressed gas will undergo isostatic cooling after one stage of compression to lower the temperature, and then enter 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-time compression. Therefore, the area of work done by multi-stage compression under the same pressure 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, condensed water 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, 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 be expanded to the 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. 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 gas in the clearance volume expands slightly to reach the suction pressure, which naturally increases the effective volume of the cylinder, thereby increasing the utilization rate of the cylinder volume.
Third, reduce the exhaust temperature
The temperature of the exhaust gas of an air compressor increases with the increase of the compression ratio. The higher the compression ratio, the higher the exhaust temperature, but too high exhaust temperature is often not allowed. This is because in oil-lubricated air 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, aggravate wear, and even explode. . For various reasons, the exhaust temperature is greatly restricted, 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 make the thermal process of the air compressor as close to the constant temperature compression as possible to achieve energy-saving effects, 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 already 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.
Fourth, reduce the gas force acting on the piston rod
In a piston air 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 structure of the air compressor, the increase in size, weight and cost; the increase in gas passages, the increase in pressure loss of the valve and management, etc., so sometimes the more stages there are, the economy will decrease. As the number of moving parts increases, the chance of failure will increase. As friction increases, mechanical efficiency will also decrease.
The main factor that affects the efficiency of the air compressor is the internal leakage of the nose. Multi-stage compression can reduce the pressure ratio of each stage. If the pressure ratio is small, the internal leakage will be improved. Therefore, the multi-stage compression has a certain energy-saving effect. However, if the machining accuracy of the machine head is high, the internal gap can be effectively controlled, and internal leakage can be reduced, and significant energy-saving effects can also be obtained. The single-stage air 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 you have high-precision processing conditions, you 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.