In my last article, we discussed the limitations of a single-stage air compressor and the problems associated with it. But just cooling the compressed air delivered out of each stage to near atmospheric temperature leads to increased overall operating efficiency and overcomes plenty of mechanical problems associated with the compressor. In this article, let us discuss the multi-stage air compressor and its advantages.
How do multi-stage compressors overcome this problem?
A multi-stage compressor is one in which there are several cylinders of different diameters. The intake of air in the first stage gets compressed and then it is passed over a cooler to achieve a temperature very close to ambient air. This cooled air is passed to the intermediate stage where it is again getting compressed and heated. This air is again passed over a cooler to achieve a temperature as close to ambient as possible. Then this compressed air is passed to the final or the third stage of the air compressor where it is compressed to the required pressure and delivered to the air receiver after cooling sufficiently in an after-cooler.
Advantages of Multi-stage compression:
- The work done in compressing the air is reduced, thus power can be saved
- Prevents mechanical problems as the air temperature is controlled
- The suction and delivery valves remain in cleaner condition as the temperature and vaporization of lubricating oil is less
- The machine is smaller and better balanced
- Effects from moisture can be handled better, by draining at each stage
- Compression approaches near isothermal
- Compression ratio at each stage is lower when compared to a single-stage machine
- Light moving parts usually made of aluminum, thus less cost and better maintenance
Refer to the diagram of a multi-stage compressor, where it is evident that the work done by the compressor is less when compared to a single-stage machine for same delivery pressure.
Practical Understanding of Multi-stage Compressor
We know that PV^n = C, where n is the polytropic compression index.
If we want to compress air from atmospheric pressure to a pressure of 30 bar, and say the ambient temperature is 27 degree Celsius:
The compression index n = 1.35 and the compression ratio for single-stage compressor would be 30:1.
Also we know that T1/T2 = (P1/P2) ^ ((n-1)/n).
Thus when calculated using the above expression, T2= 450 degree Celsius. Thus it is evident that the delivery temperature of compressed air is 450 degree Celsius.
Do you think of any harm to compressor will happen at this temperature?
The main issue is the lubricating oil mist and associated explosion. The lubricating oil at this temperature will ignite and cause very severe problem. Thus if multi-staging is used, the delivery air temperature is controlled very close to the ambient air and there is no possibility of lubricating oil and associated problems.
Why Cylinder Diameter reduces as Pressure Increases
Now let us understand a very practical issue.
Have you ever noticed in a multi-stage compressor, the diameter of cylinder liners of each stage is different? To be more elaborate, the first stage cylinder diameter is biggest and the intermediate stage is the second biggest and the third or final stage is the smallest of all. Do you know the reason for this?
As the pressure of compression increases, the cylinder diameter decreases. The mass flow rate or the amount of air passing through each stage is same. Thus as the pressure increases with the same mass flow rate, the volume occupied by air must reduce. This is accomplished by reducing the diameter and thus the volume of the cylinder.