Question

Which of the following statements is TRUE for counter-current heat exchangers when compared to parallel-flow exchangers?

• A. Lower heat transfer rate
• B. Lower LMTD
• C. Higher temperature approach possible
• D. Fluids enter at the same end

Hint:

Logic Tip: Counter-current is the "best" flow type. Therefore, it gives a higher heat transfer rate and a higher LMTD. By process of elimination, options A and B are false, and D defines parallel flow, leaving C as the only logical benefit.

Answer 1

The Correct Option is C

Concept:
The flow arrangement in a heat exchanger dictates its thermal efficiency. Counter-current flow is thermodynamically superior to parallel flow because it maintains a more consistent temperature gradient along the entire length of the exchanger.

Step 1: In parallel flow, both fluids enter at the same end. The cold fluid can never reach a temperature higher than the exit temperature of the hot fluid because the driving force drops to zero as their temperatures converge.

Step 2: For any given inlet and outlet temperatures, a counter-current exchanger always yields a mathematically higher Log Mean Temperature Difference (LMTD) than a parallel one, meaning it transfers heat more efficiently.

Step 3: In counter-current flow, the fluids enter at opposite ends. The exiting cold fluid is heated by the incoming, hottest hot fluid.

Step 4: Because of this opposing thermal profile, the cold fluid can be heated to a temperature that is actually higher than the exit temperature of the hot fluid.

Step 5: Therefore, counter-current exchangers allow for a "higher temperature approach" (the cold fluid can approach the highest temperature of the hot system), making option (C) the only true statement.