Why Is Multi-Stage Compression Necessary for Air Compressors?

Most people understand that a two-stage compressor is ideal for producing higher pressure, while a single-stage unit works better for delivering higher air volumes. But in certain situations, multiple stages of compression may be required. Why is this the case?

The main reason is that achieving higher gas pressures through single-stage compression becomes inefficient and, in some cases, unfeasible. Multi-stage compression allows the gas to be pressurized in gradual stages until it reaches the desired operating pressure.

Energy Efficiency

With multi-stage compression, an intercooler can be placed between stages to cool the air after it is compressed in the first stage. This cooling reduces the air’s temperature, allowing it to be compressed further in the next stage with less effort. As a result, multi-stage compression can significantly reduce energy consumption compared to single-stage compression. The more stages, the closer the process gets to isothermal compression, which is the most energy-efficient process.

Important note: In oil-injected screw compressors, the process is already close to isothermal compression. If the cooling process goes too far, condensate water will form. This condensate can mix with the cooling oil, causing it to emulsify and lose its lubrication properties. If the condensate continues to increase, it could raise the oil level, eventually contaminating the compressed air system and leading to operational failures.

To prevent condensation, the temperature inside the compression chamber must stay above the dew point. For example, in a compressor with a discharge pressure of 11 bar (A), the dew point is around 68°C. If the compression chamber temperature falls below this, water will condense, creating issues for the system.

Increasing Cylinder Efficiency

In compressors, there’s always a bit of clearance volume left in the cylinder, which reduces the overall effective volume. The high-pressure air trapped in this space expands before new air can be taken in, further decreasing cylinder efficiency.

The higher the pressure ratio, the more significant the expansion of leftover gas, leading to lower effective cylinder capacity. Multi-stage compression reduces the pressure ratio in each stage, allowing the gas to expand less and improving the efficiency of the cylinder.

Reducing Exhaust Temperatures

As the pressure ratio increases, so does the exhaust temperature. Excessive exhaust temperatures can cause serious problems, especially in oil-lubricated compressors, where high temperatures thin the oil and increase wear. They can also lead to carbon buildup, which, in extreme cases, could result in explosions. Multi-stage compression effectively controls and lowers exhaust temperatures, enhancing the compressor’s safety and reliability.

Note: While multi-stage compression can lower the exhaust temperature and improve efficiency, it’s not always necessary, especially for oil-injected screw compressors with pressures below 13 bar. In such compressors, the oil injection already provides significant cooling during compression. Adding more compression stages would make the design more complicated and costly without providing additional benefits. It may also increase the flow resistance and energy consumption.

Lowering Gas Forces on Piston Rods

In piston compressors, high-pressure ratios require larger cylinder diameters, which in turn apply greater force to the piston. Multi-stage compression distributes the load across smaller cylinders, reducing the mechanical stress on each component and improving overall machine efficiency.

That said, more compression stages do not always mean better performance. Increasing the number of stages makes the system more complex, leading to higher costs, more components, and increased risk of failure. The key is to balance the number of stages with operational efficiency and cost.

To learn more about air compressors, please visit the Sayi official website at www.sayiair.com.