USCG Exam PracticeThermodynamics and heat exchangers
In a steam plant Rankine cycle, a regenerative feedwater heater uses bled steam from an intermediate turbine stage to preheat feedwater before it enters the boiler. Applying both the first and second laws, which statement correctly describes the thermodynamic benefit of this arrangement?
- A. It violates the second law because heat is transferred from a lower-pressure (cooler) steam extraction point to the feedwater, reducing entropy.
- ✓ It reduces the heat that must be added in the boiler and reduces the heat rejected to the condenser, raising cycle efficiency without violating either law because energy is recycled within the system rather than rejected to the environment.Correct
- C. It increases total entropy generation in the cycle because an additional heat-transfer process with a finite temperature difference is introduced.
- D. It improves efficiency solely by raising boiler pressure, consistent with the Carnot principle that higher source temperature increases maximum efficiency.
Why B is correct
DOE-HDBK-1012 §1-5 states that regenerative feedwater heating improves Rankine cycle efficiency by using bled steam to preheat feedwater. DOE-HDBK-1018 Vol.1 §2-2 confirms that a regenerative exchanger recycles energy within the system rather than rejecting it to the environment. The first law (§1-3) is satisfied because energy is conserved; the second law (§1-4) is not violated because heat still flows from the hotter extracted steam to the cooler feedwater.