Why does an air compressor need graded compression?

Many people know that two-stage compressors are suitable for producing high pressure, while the first stage is suitable for producing large amounts of gas. Sometimes, it is necessary to perform two or more compressions. Why is hierarchical compression necessary? When the working pressure of the gas is required to be high, using single-stage compression is not only uneconomical, but sometimes even impossible to achieve, and multi-stage compression must be used. Multi stage compression refers to the process of increasing the pressure of a gas several times from inhalation to reach the required working pressure.
1. Save power consumption

By using multi-stage compression, intermediate coolers can be installed between stages to allow the compressed gas to undergo isobaric cooling after being compressed in one stage, in order to reduce temperature before entering the next stage cylinder. The temperature decreases and the density increases, making it easier to further compress, which can greatly save power consumption compared to a single compression. Therefore, under the same pressure, the area of work done by multi-stage compression 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.

Attention: 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 saturation, condensation water will precipitate. If these condensed water enters the oil and gas separator (oil tank) together with compressed air, it will emulsify the cooling oil and affect the lubrication effect. As the amount of condensed water continues to increase, the oil level will also rise, and eventually 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 condensation water, the temperature inside the compression chamber cannot be too low and must be greater than the condensation temperature. If the exhaust pressure of an air compressor is 11 bar (A) and the condensation temperature is 68 ℃, condensation water will precipitate when the temperature inside the compression chamber is below 68 ℃. Therefore, the exhaust temperature of the oil injection screw air compressor cannot be too low, that is, the application of isothermal compression in oil injection screw machines is limited due to the problem of condensation water.

2. Improve capacity utilization rate

Due to manufacturing, installation, and operation reasons, the clearance volume inside the cylinder is always inevitable. The clearance volume not only directly reduces the effective volume of the cylinder, but also requires the residual high-pressure gas to expand to the suction pressure before the cylinder can start inhaling fresh gas, which is equivalent to further reducing the effective volume of the cylinder.

It is not difficult to understand that the larger the pressure ratio, the more severe the expansion of residual gas in the clearance volume, and the smaller the effective volume of the cylinder. In extreme cases, it is even possible for the gas within the clearance volume to fully expand in the cylinder, but the pressure is still not lower than the suction pressure. At this point, it is impossible to continue suction 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 expand slightly to reach the suction pressure, which can naturally increase the effective volume of the cylinder and improve the utilization rate of cylinder volume.

3. Reduce exhaust temperature

The temperature of the gas discharged by the compressor increases with the increase of compression ratio, and the higher the compression ratio, the higher the exhaust temperature. However, excessively high exhaust temperature is often not allowed. This is because in oil lubricated compressors, the temperature of the lubricating oil will decrease viscosity and exacerbate wear. When the temperature rises too high, it is easy to form carbon deposits in the cylinder and valves, exacerbating wear and even causing explosions. Due to various reasons, the exhaust temperature is greatly limited, so multi-stage compression must be used to reduce the exhaust temperature.

Attention: Graded compression can reduce the exhaust temperature of screw air compressors and also make the thermal process of the air compressor as close as possible to constant temperature compression to achieve energy-saving effects, but it is not absolute. Especially for oil injected screw air compressors with exhaust pressure below 13 bar, due to the injection of low-temperature cooling oil during the compression process, the compression process is already close to a constant temperature process, and there is no need for secondary compression. If graded compression is carried out on the basis of fuel injection cooling, the structure will become more complex, the manufacturing cost will increase, and the flow resistance of gas and additional power consumption will also increase, which is somewhat counterproductive. In addition, if the temperature is too low and condensation is formed during the compression process, it will lead to the deterioration of the system state 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 cylinder diameter is larger, resulting in a higher final gas pressure acting on the 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 lightweight and improve mechanical efficiency.

Of course, multi-stage compression is not necessarily better with more stages. As the number of stages increases, the structure of the compressor becomes more complex, resulting in an increase in size, weight, and cost; The increase in gas channels leads to an increase in pressure loss of gas valves and management, so sometimes the more stages there are, the lower the economic efficiency. With more stages, the number of moving parts increases, and the chance of failure also increases. Due to increased friction, mechanical efficiency will also decrease.

The main factor affecting the efficiency of the compressor is the internal leakage of the compressor head. Multi stage compression can reduce the pressure ratio of each stage, and a smaller pressure ratio naturally improves internal leakage. Therefore, multi-stage compression has a certain energy-saving effect. However, if the machining accuracy of the machine head is high, it can effectively control the internal clearance, reduce internal leakage, and also achieve significant energy-saving effects. The components of a single-stage compressor head are significantly fewer than those of a multi-stage compressor, and the reliability of the machine is much higher. From a reliability perspective, high-precision manufacturing of single machine compression is more in line with customer interests. Suggestion: When the machining accuracy of the machine head is low, consider improving machine energy efficiency through multi-stage compression. If there are high-precision machining conditions, it is still advisable to adhere to single-stage compression and use the number of single-stage compressed parts to enhance the reliability selling point.