The quasi stationary changes in the conditions of state of the gas when flowing through a compressor stage can be represented in a T,s- diagram. Such a compressor stage can be either part of a multistage or a single stage compressor.
Gas enters the cylinder from the suction chamber in the condition S. During the flow into the cylinder throttling and heating up of the gas occurs.
The non stationary change of state in the working chamber is limited by points 1 and 2 of condition of state at the beginning and at the end of compression. For the quasi stationary evaluation of the stage the mean pressures during the intake and discharge are relevant.
The outflow of gas out of the working chamber is combined with a pressure loss and a cooling of the gas down to the conditions of the receiver D on the delivery side.
With the gas flowing through the interstage system its temperature will be reduced to the suction temperature T s+ of the next stage. The pressure is reduced due to friction losses to ps+. (The index + symbolises the next stage).
For the function of the stage the pressure ratios below are characteristic. (All pressure ratios correlate to a line in the T,s diagram.)
According to the first law of thermodynamics the specific work put into a stage equals the heat removed from the working chamber and the interstage system plus the mostly relatively small change in enthalpy of that stage.