I was going through the $pV$ diagram of the ideal Rankine cycle and all of the images that I found on the internet and as opposed to the $TS$ Diagram, they tend to show the compression in the feedwater pump (that's between condenser and boiler to raise the water pressure to boiler pressure) to be an isochoric process while instead in an ideal Rankine cycle, it's an isentropic compression within the pump. For example, look at this diagram.
Process 1-2 which is the isentropic expansion in the turbine is depicted correctly but process 3-4 that's isentropic compression in the pump is in fact shown as isochoric.
Also in this article, it's said
Process 3-4: The water from the hot well or the surge tank which is at low pressure is pumped into the boiler at
high-pressure p1. Here pumping process 3-4 is isentropic.
While it's not as per the diagram. Can anyone provide some clues?
Best Answer
There's no contradiction here. A process can be both isochoric and isentropic.
In this case it's considered isochoric because liquid water is considered to be incompressible for the range of pressures involved. So the pump is not considered to decrease the volume of the water. It is providing the necessary pressure to force the water from the lower pressure condenser into the higher boiler pressure.
It is considered isentropic because the process is carried out both reversibly and adiabatically.
Hope this helps.