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Irreversibilities in a Real Boiler

Rankine Cycle

Boiler

Heat added at constant pressure.

Process 1-2:

Turbine

Isentropic expansion.

Process 2-3:

Condenser

Heat rejected at constant pressure.

Process 3-4:

Pump

Isentropic compression.

Process 4-1:

Additional Sources of

Irreversibility :

Heat losses to the surroundings.
Frictional losses as the fluid moves through the boiler.

Process flow diagram of a Rankine Cycle with the boiler highlighted.

TS Diagram of a Rankine Cycle with a real boiler in which pressure drop due to fluid friction occurs.

Lesson
9D Blog

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Lesson 9D: page 2 of 11

In a real boiler we have heat losses to the surroundings and we have friction as the working fluid flows through the boiler.
These heat losses don’t effect the TS Diagram.
Friction leads to a pressure drop across the boiler.
Because of friction, the pressure must be higher at the boiler inlet than it is at the boiler outlet in order to get the fluid to flow.
As the pressure drops across the boiler, so does the temperature because we vapor-liquid equilibrium.
As a result, the upper right corner of the cycle path drops down along the saturation curve.
Point 2 is at a lower temperature and a higher entropy in the real cycle.
This is not a catastrophe, but all the little irreversibilities begin to add up by the time we complete the cycle.
Now, on to the turbine.

Irreversibilities in a Real Boiler

Rankine Cycle

Irreversibility :

Ch 9, Lesson D, Page 2 - Irreversibilities in a Real Boiler