USCG Exam PracticeThermodynamics and heat exchangers
The Carnot efficiency of a heat engine depends solely on which two quantities, and what design implication does this have for a marine steam plant?
- A. The mass flow rate of steam and the turbine speed; higher flow and faster rotation increase efficiency.
- ✓ The absolute temperatures of the heat source and heat sink; plants run boilers as hot as materials allow and condensers as cold as seawater permits to maximize the temperature difference.Correct
- C. The gauge pressures at the boiler and condenser; higher gauge pressure differential directly sets the Carnot limit.
- D. The enthalpy drop across the turbine and the feedwater pump work; minimizing pump work raises Carnot efficiency.
Why B is correct
DOE-HDBK-1012 §1-4 states that Carnot efficiency depends only on the two absolute temperatures of source and sink, and that the larger the temperature difference, the higher the possible efficiency — which is why plants run boilers as hot as materials allow and condensers as cold as the seawater permits. Pressure, mass flow, and enthalpy drop are not the direct determinants of the Carnot limit.
Cited:DOE-HDBK-1012 §1-4