The theory of compression and different types of compression

Theory of compression:

When the upward movement of the piston compresses the air in an airtight cylinder, the volume of the air decreases and its pressure increases. This change in pressure and volume can be represented by a p-v diagram.


When volume decreases from V1 to V2, the pressure increases from P1 to P2.

Different types of compression:

During compression of air, if the heat generated dissipates, the compression will take place at a constant temperature. This type of compression and the pressure-volume relation is given by the product of pressure and volume equal to c, which is a constant.


In a P-V diagram, the work done in compressing the air is represented by the area under the isothermal curve. You cannot achieve isothermal compression in practice, as not all the heat generated during compression can be dissipated.

During compression of air, if the cylinder is insulated so that no heat dissipates, the process is referred to as adiabatic compression.


The pressure-volume relation for this compression is PV √𝜸=C, where C is constant and gamma is the specific heat capacity ratio.

For air, the value of  𝛄 is 1.405.

You can see from the PV diagram that the work done for adiabatic compression is more. The heat generated in compressing the air is stored as heat energy.

The actual compression process for an air compressor takes place between the isothermal and the adiabatic compression. This type of compression is called polytropic compression.


The pressure-volume for this compression is PV power n equal to C, where C is constant and the value of n varies between 1 and 1.4.



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