USCG Exam PracticeThermodynamics and heat exchangers
The Carnot efficiency of a heat engine depends solely on the temperatures of the heat source and heat sink. If a marine steam plant operates with a boiler supplying steam at an absolute temperature of 800 R and a condenser cooling water temperature of 500 R, which change would produce the greatest increase in theoretical maximum efficiency?
- A. Raising the condenser cooling water temperature from 500 R to 550 R.
- ✓ Lowering the condenser cooling water temperature from 500 R to 450 R.Correct
- C. Reducing the boiler steam temperature from 800 R to 750 R.
- D. Maintaining both temperatures constant while increasing steam flow rate.
Why B is correct
DOE-HDBK-1012 §1-4 states that Carnot efficiency depends only on the two absolute temperatures and that the larger the temperature difference between source and sink, the higher the possible efficiency. Lowering the sink temperature from 500 R to 450 R increases the temperature difference by 50 R, whereas raising it to 550 R decreases the difference. Reducing boiler temperature also decreases the difference, and flow rate does not affect Carnot efficiency.