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Why is the Carnot Cycle Impractical ?

The problem is that the

is not practical for

for the following reasons:

We have always used the

as our model for thermodynamic cycles.

Boiler

Isothermal heat transfer eliminates the possibility of using subcooled liquid boiler feed or producing superheated vapor in the boiler effluent.

Turbine

Adiabatic expansion yields low steam quality in the turbine feed.
This can result in damage to the turbine rotor.

Condenser

Isothermal heat transfer eliminates the possibility of using superheated vapor in the condenser feed.

Pump

Quality > 0 in the pump feed. This can result in damage to the pump rotor.

Lesson 9B: page 1 of 12

The Carnot Cycle is very useful. It has been a good tool for learning how different operating parameters influence the performance of thermodynamic cycles.
I have often pointed out that no real cycle can be reversible, for a variety of reasons.
But, there are other problems with the Carnot Cycle as well.
In this lesson we will consider the additional problems with the Carnot Vapor Cycle.
To begin with, the Carnot Vapor Cycle requires that the boiler operate isothermally.
This makes it easier to arrange for heat transfer through an infinitesimal temperature difference.
The problem is that this prohibits our boiler from producing superheated vapor.
Why is this important ?
Well, real turbines can be damaged if the quality of the effluent drops too low.
If we could superheat the turbine feed, we would be able to increase the quality of the turbine effluent.
The Carnot Cycle also requires that the condenser operate isothermally.
This means that we cannot allow the turbine effluent to be a superheated vapor.
This, again, puts our turbine rotor at risk of costly damage.
Finally, the most efficient pumps for vapor power cycles do not work well when any vapor enters them.
But, if the condenser is designed and operated to produce a saturated liquid effluent, our isentropic pump will yield a subcooled liquid at the pressure of the boiler.
This will make it impossible to operate the boiler isothermally.
The Rankine Cycle is a way to avoid all of these problems.