The reason is that when higher working pressure is needed, using a single-stage compressor is not only inefficient, but sometimes even impossible. Multi-stage compression allows the gas to reach the required pressure gradually by increasing pressure in stages.
Saving Power Consumption
Using staged compression, we can install an intercooler between stages to cool the gas after the first stage of compression, reducing its temperature before entering the next stage. The lower temperature makes it easier to compress further, saving energy compared to single-stage compression. The more stages, the closer the process gets to isothermal compression, and the more energy is saved.
Note: In oil-injected screw air compressors, the compression process is already close to isothermal. If cooling continues past the saturation point, condensate will form. If this water enters the oil separator, it can emulsify the cooling oil, affecting lubrication. As condensate increases, oil levels will rise, leading to oil entering the compressed air system, which can cause serious issues.
To avoid condensate, the temperature in the compression chamber must remain above the dew point. For example, if the exhaust pressure is 11 bar (A), the dew point is 68°C. If the chamber temperature falls below 68°C, condensate will form, limiting the application of isothermal compression in oil-injected compressors.
Improving Volume Efficiency
Due to manufacturing, installation, and operational reasons, there is always some clearance volume in the cylinder, which reduces the effective cylinder volume. The high-pressure gas remaining in this clearance volume must expand to intake pressure before new air can enter, further reducing cylinder efficiency.
The higher the pressure ratio, the more the remaining gas expands, reducing the effective cylinder volume. In extreme cases, the cylinder may not intake any air. By using staged compression with lower pressure ratios per stage, the remaining gas only needs slight expansion to reach intake pressure, increasing cylinder efficiency.
Lowering Exhaust Temperature
As the pressure ratio increases, so does the exhaust temperature. High exhaust temperatures can cause issues such as lower lubricant viscosity, increased wear, and carbon buildup, leading to possible explosions. Staged compression reduces the exhaust temperature, ensuring safer operation.
Note: Staged compression lowers exhaust temperature and brings the process closer to isothermal compression, which improves efficiency. However, for oil-injected screw compressors with discharge pressures below 13 bar, the cooling oil already keeps the process close to isothermal. Adding extra stages would increase complexity, cost, and energy loss due to increased airflow resistance, making it less efficient.
Reducing Gas Force on the Piston Rod
In piston compressors, when using a high compression ratio with a single stage, the large cylinder diameter creates high gas forces on the piston. By using multi-stage compression, these forces are reduced, allowing for a lighter design and improved mechanical efficiency.
However, multi-stage compression is not always better. More stages increase the complexity, size, weight, and cost of the compressor, and lead to more friction and potential mechanical failures. Therefore, it's important to find the right balance for optimal performance.