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Compression in stages

Theoretically a gas can be compressed isentropically or isothermally. This can take place as a part of a reversible process. If the compressed gas could be used immediately, at its final temperature after compression, the isentropic process would have certain advantages. In reality the gas can rarely be used directly without being cooled before use. Therefore the isothermal process is preferred, as this requires less work. In practice attempts are made to realise this process by cooling the gas during compression. How much you can gain by this is shown, for example, with an effect- ive working pressure of 7 bar that theoretically requires 37% higher output for isentropic compression compared with isothermal compression. A practical method to reduce the heating of the gas is to divide the compression into several stages. The gas is cooled after each stage, to then be compressed further. This also increases the efficiency, as the pressure ratio in the first stage is reduced. The power requirement is at its lowest if each stage has the same pressure ratio. The more stages the compression is divided into the closer the entire process gets to be isothermal compression. However there is an economic limit for how many stages a real installation can be designed with.