In water cooled compressors, freshwater is circulated through jackets that are built into the walls of the cylinders and cylinder heads.
when comparing with a single stage compressor work can be reduced by inter cooling in a multistage compressor.
Work can be reduced further by increasing number of stages and inter cooling, but as the number of stages increases design becomes complex, constructional cost increases, maintenance cost also increase, which may nullify the effect of work saved during operation. This is the limiting factor for more number of stages.
Isothermal Compression
During the process of compression, whatever heat produced is taken away by a cooling medium. In other words, it is the compression , keeping temperature of the gas constant. For a process to be isothermal, the process must be very slow, which is impractical. From the indicator card, it is clear that, work of compression is minimum in isothermal compression.
Adiabatic Compression
Whatever heat produced during compression is kept inside the gas only, or heat transfer is zero in an adiabatic compression. For a perfect adiabatic process, process must be very fast. All the thermodynamic process resembles adiabatic process. It can be seen from the indicator card that, work of compression is maximum in adiabatic compression.
Specific heat is defined as the heat energy required to raise the temperature of unit mass of substance by unit degree.
Polytropic Compression
Polytropic compression is neither isothermal nor adiabatic. It comes in between.
Work of compression can be minimized by isothermal compression. But compression is practically a fast process. So it better resembles an adiabatic process. Jacket cooling of compressor makes the compression polytropic.