USCG Exam PracticeThermodynamics and heat exchangers
Why is a counter-flow heat exchanger thermally more efficient than a parallel-flow exchanger of the same surface area?
- A. Counter-flow allows both fluids to enter at the same end, maximizing the inlet temperature difference.
- ✓ Counter-flow maintains a more uniform temperature difference along the length of the exchanger, and the cold outlet can exceed the hot fluid outlet temperature.Correct
- C. Counter-flow eliminates fouling resistance by increasing shell-side turbulence.
- D. Counter-flow uses the log-mean temperature difference only at the inlet, where the gap is largest.
Why B is correct
DOE-HDBK-1018 Vol.1 §2-2 explains that in a counter-flow exchanger the temperature difference is more uniform along the length, the exchanger is thermally more efficient, and the cold fluid can leave hotter than the hot fluid's outlet — none of which is achievable in parallel flow.
Cited:DOE-HDBK-1018 Vol.1 §2-2