Single Stage Air Compressor Basic Theory With PV Diagram Explanation

Basic theory of air compressors 
         When the piston moves from BDC to TDC air gets compressed, as a result, pressure increases and therefore volume decreases. The work done to compress the air is converted to heat energy in the air so that, the air temperature is increased. 

         During compression, if all the heat generated is taken by cylinder wall then it is called Isothermal compression. Here, further temperature rise is avoided and the compression is taking place at constant temperature. The relationship between the pressure and volume would follow Boyles law (PV =C)

        If there is no heat transfer from the compressed air, then all the work done during compression would appear as stored heat energy. This is known as the Adiabatic compression and the relationship between pressure and volume would be PVγ = C (γ= 1.4 for air)

        The actual compression process in an air compressor is between the isothermal and the adiabatic and is referred to as Polytropic compression. The relationship between the pressure and the volume is Pvn = C, where n is a value of about  1.25 - 1.35. 

PV Diagram With Explanation

D-A: Intake
Assume there is no bumping clearance, initial volume when the piston at the top is zero. Thus as the piston moves from top to bottom, a volume of air V2 is drawn into the compressor from the atmosphere with pressure P1. The temperature of air is T1.

A-B: Compression
As the piston moves up, the air is compressed Polytropically (PV^n= C). The pressure of air increases from P1 to P2. The volume decreases from V2 to V1. The temperature increases from T1 to T2.

B-C: Delivery
The compressed air at the pressure P2, volume V2 and temperature T2 is delivered out of the compressor to the air receiver.


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  1. Both adiabatic and isothermal compression are idealized fictions. Real compression always follows fairly close to the polytropic model.

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