I don't understand why in (Ideal) Rankine Cycle we assume that pressure remains constant at the boiler and the condenser. Shouldn't water pressure change, when water goes from liquid to vapor (boiler) and vice versa (condenser)?

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2 Answers 2


Shouldn't water pressure change, when water goes from liquid to vapor (boiler) and vice versa (condenser)?

No, it shouldn't. Because water is on a constant pressure line during the processes (from point $1$ to $2$ in the boiler and from point $3$ to $4$ in the condenser) for both cases. enter image description here

In addition, phase of a matter doesn't depend on the pressure only. For example, we can have water in gas, liquid and solid phases at the same pressure.

  • $\begingroup$ What about frictional losses within the boiler and condenser? Don't we ignore those? Otherwise, there could be a drop in pressure due to minor or major hydrodynamic losses. $\endgroup$
    – dearN
    Jul 11, 2017 at 16:24
  • $\begingroup$ The question says "Ideal" cycle, so we can assume perfect adiabatic lossless processes on pump and volumetric machine, and no pressure drops on pipes, boiler and condenser. $\endgroup$
    – Nillus
    Apr 25, 2018 at 23:22

Yes, pressure changes at evaporation or condensing. But it is nevertheless constant throughout the pipe.

If you force water through a long pipe, and then heat it up very much at the middle so the water evaporates at that point, then it will get harder for you to push the water through, since the expansion will work against your force.

So you must apply more force. That is, the pressure has to be bigger. It has to be bigger at any points in the pipe - not only at the point of evaporation.

It is true that the pressure will raise when you start evaporating water at a point of the pipe, but this new increased pressure (at the new flow-steady-state) will be the same throughout.


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