Why is the Carnot Cycle Impractical?

Ch 9, Lesson B, Page 1 - Why is the Carnot Cycle Impractical?

• 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.