Which of the following statements is not a plausible and reasonable design constraint to impose when designing a new Rankine cycle heat engine?
A. The steam entering the turbine must have a quality of 0.95 or greater.
B. The boiler should be 35 K cooler than the hot reservoir.
C. The working fluid should be subcooled when it leaves the condenser.
D. The condenser needs to be 25 K warmer than the cold reservoir.
E. All of these are reasonable design constraints.
A. Incorrect. It is entirely plausible to require a particular quality in order to prevent the liquid eroding the turbine blades.
B. Incorrect. A significant temperature driving force is needed for heat transfer to the boiler. There is nothing “special” about the number 35, but specifying some ?T is logical.
C. Correct. If the working fluid is subcooled when leaving the condenser, this just means more energy must be added in the boiler to get it back to saturation.
D. Incorrect. The condenser needs to be warmer than the heat sink it is rejecting waste heat to or else there will be no (thermal) driving force for heat exchange. A large temperature difference is common to ensure a good heat removal rate.
E. Incorrect. One of these is a counter-productive design constraint.
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