Many people know that the two-stage compressor is suitable for high pressure, and the first stage is suitable for large gas production. Sometimes, more than two compressions are required, so why do you need graded compression?
When the operating pressure of the gas is high, it is not only uneconomical, but sometimes even impossible, to use single-stage compression, and multi-stage compression must be used. Multi-stage compression is to start from the gas inhalation and go through several pressurization boosts to reach the required working pressure.
Save power consumption
Using multi-stage compression, the method of setting an intercooler between stages can make the compressed gas be isobaric cooled first after the first-stage compression, so as to reduce the temperature, and then enter the next cylinder. The temperature decreases and the density increases, which makes it easy to compress further, which can greatly save power consumption compared to primary compression. Therefore, under the same pressure, the area of work done by multi-stage compression is less than that of single-stage compression. The more stages there are, the more power savings there are, 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 compression continues to cool after reaching the saturation state, condensate will precipitate. If this condensate enters the oil-air separator (oil tank) together with the compressed air, it will emulsify the cooling oil and affect the lubrication effect. As the condensate increases, the oil level will continue to rise, and finally the cooling oil will enter the system with the compressed air, contaminating the compressed air and 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 greater than the condensation temperature. For example, if the 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 be precipitated. Therefore, the exhaust temperature of the oil-injected screw air compressor should not be too low, that is, the application of isothermal compression in the oil-injected screw machine is limited due to the problem of condensate.
Improve volume utilization
Due to the manufacturing, installation and operation of three reasons, 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 also expand to the suction pressure, the cylinder can begin to inhale fresh gas, so that it 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 dramatically, and the effective volume of the cylinder will be smaller. In the extreme case, even after the gas in the clearance volume is completely expanded in the cylinder, the pressure is still not lower than the suction pressure, and then it is impossible to continue to 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 can be slightly expanded to reach the suction pressure, so that the effective volume of the cylinder can naturally be increased, so as to improve the utilization rate of the cylinder volume.
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 discharge temperature, but too high the exhaust temperature is often not allowed. This is due to the fact that in oil-lubricated compressors, the high temperature of the lubricating oil will reduce the viscosity and aggravate the wear, and when the temperature rises too high, it is easy to form carbon deposits in the cylinder and valves, aggravate the wear, and sometimes even explode. For a variety of reasons, the exhaust temperature is greatly limited, so it is necessary to reduce the exhaust temperature by means of multi-stage compression.
Note: Staged compression can reduce the exhaust temperature of the screw air compressor, and at the same time, the thermal process of the air compressor can be as close as possible to the constant temperature compression to achieve energy saving effect, but it is not absolute. Especially for the oil-injected screw air compressor with an exhaust pressure of less than 13 bar, because it sprays low-temperature cooling oil in the compression process, the compression process is close to the constant temperature process, and there is no need for secondary compression. If the graded compression is carried out on the basis of this oil injection cooling, the structure is complex, the manufacturing cost is increased, and the flow resistance of the gas and the additional power consumption are increased, which is a bit more than worth the loss. In addition, if the temperature is too low, condensate will form during the compression process, which will lead to a deterioration of the system state, which can have serious consequences.
Reduces the gas forces acting on the piston rod
On the piston compressor, when the compression ratio is high and the single-stage compression is adopted, the diameter of the cylinder is larger, and the gas final pressure on the piston area is higher, and the gas force on the piston is larger. Multi-stage compression can significantly reduce the gas forces acting on the piston, making it possible to make the mechanism lighter and more mechanically efficient.
Of course, multi-stage compression is not always better. Because the more stages, the compressor structure tends to be more complex, the size, weight and cost will increase, the gas passage increases, the pressure loss of the valve and the management increases, etc., so sometimes the more stages will reduce the economy, the more stages will increase, the moving parts will increase, and the chance of failure will also increase. Due to the increased friction, the mechanical efficiency will also be reduced.
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