Question

Given below are two statements, one labelled as Assertion (A) and the other labelled as Reason (R).
Assertion (A): Counter flow heat exchanger is more effective than a parallel flow heat exchanger.
Reason (R): For the same temperature limits of hot and cold fluids, the overall heat transfer coefficient of a counter flow heat exchanger is more than that of a parallel flow heat exchanger.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

Counter flow's advantage comes from a higher LMTD, not from a different overall heat transfer coefficient.

Answer 1

The Correct Option is C

Step 1: Assertion is true. For the same inlet and outlet temperatures of hot and cold fluids, a counter flow arrangement gives a higher log mean temperature difference (LMTD) than a parallel flow arrangement, so a counter flow heat exchanger needs less area for the same heat duty, that is, it is more effective.
Step 2: Reason is false. The overall heat transfer coefficient \(U\) depends on the individual film coefficients, fouling factors, and the wall material and thickness. It does not depend on whether the fluids flow parallel or counter to each other, so \(U\) is essentially the same for both arrangements with the same fluids and flow rates.
Step 3: Since (A) is true and (R) is false, the correct option is 3.