Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).

Assertion (A): Counter flow heat exchanger is more effective than a parallel flow heat exchanger.

Reason (R): For same temperature limits of hot and cold fluids, the overall heat transfer coefficient of counter flow heat exchanger is more than 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:

Ask what actually changes between parallel and counter flow arrangements, the heat transfer coefficient or the temperature difference profile.

Answer 1

The Correct Option is C

Step 1: Check the assertion first. For the same inlet and outlet temperatures, a counter flow heat exchanger really does transfer more heat, or needs a smaller area for the same duty, than a parallel flow one. So Assertion (A) is true.

Step 2: Now check the reason. The overall heat transfer coefficient U depends on the individual film convective coefficients of the two fluids, the wall material and thickness, and any fouling on the surfaces. None of these change just because we switch the flow arrangement from parallel to counter flow. So U is not what makes counter flow better.

Step 3: The real reason counter flow is more effective is that it keeps the temperature difference between the hot and cold streams more uniform along the length of the exchanger. This gives a higher log mean temperature difference (LMTD) for the same inlet and outlet temperatures, which lets more heat pass for the same U and area.

Step 4: So Reason (R), as written, wrongly credits the improvement to a change in U. That statement is false.

Step 5: Since (A) is true and (R) is false, the correct choice is option 3.