Why does the air compressor need hierarchical compression?

2023-11-17

As many people know, two grades of the compressor are suitable for high pressure, the first grade is suitable for large gas production. Sometimes, you need to do the compression more than two times. Why need hierarchical compression?

The reason is that when the working pressure of gas is required to be high, Single-stage compression is not only uneconomical, but sometimes even impossible, so hierarchical compression must be adopted. Hierarchical compression means that the gas starts from inhalation, after several boosting then reaches the required working pressure.

1. Save power consumption

After hierarchical compression, the compressed gas can be cooled by the cooler between stages after the primary compression, to reduce the temperature, and then enter the next stage cylinder. The temperature decreases and the density increases, so it is easy to further compress, witch can greatly save the power consumption compared with single compression. Therefore, the area of hierarchical compression under the same pressure is less than the single-stage compression. The more stages, the more power consumption can be saved and closer to isothermal compression.

2. Improve the volume utilization rate

Due to manufacturing, installation and operation these three aspects, the clearance volume in cylinder is always inevitable, and clearance volume not only directly reduce the effective volume of the cylinder, and the residual high pressure gas must expand to the suction pressure, then the cylinder can start inhaling fresh gas, this is to further reduce the effective volume of the cylinder.

It is not difficult to understand that if the pressure ratio is larger, the more sharply the residual gas in the clearance volume expands, and the smaller the effective cylinder volume is. In the extreme case, even after the gas in the clearance volume can fully expand in the cylinder, the pressure is still not lower than the suction pressure, at this time, it cannot continue to absorb and exhaust, and the effective volume of the cylinder becomes zero. If multistage 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 inspiratory pressure, so that the effective volume of the cylinder can naturally increase, so as to improve the utilization rate of the cylinder volume.

3. Reduce the exhaust temperature

The temperature of the excluded gas of the compressor increases with the increase of the compression ratio. The higher the compression ratio, the higher the exhaust temperature is. But too high exhaust temperature is always not allowed. This is due to: in the oil lubrication compressor, high temperature of lubricating oil will reduce the viscosity, aggravate wear. When the temperature rise is too high, it is easy to form carbon deposition in the cylinder and the valve, leading to wear aggravation, and sometimes even explosion. For various reasons, the exhaust temperature is greatly limited, so multistage compression must be used to reduce the exhaust temperature.

4. Reduce the gas force acting on the piston rod

On the piston compressor, when the compression ratio is high and the single-stage compression is adopted, the cylinder diameter is larger, and there is a higher gas pressure on the larger piston area, then the gas force on the piston is larger. If multistage compression is used, the gas force acting on the piston can be greatly reduced, therefore the mechanical efficiency can be improved.

Of course, multistage compression is not the more stages, the better. Because the more stages, the more complex the compressor structure tends to be. The size, weight and cost will all increase; the gas channel increases, and the pressure loss increases. So sometimes the more stages will reduce the economy. More stages increase the moving parts, then the chance of failure will also increase. There will also reduce mechanical efficiency due to increased friction.