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

Look at what stays fixed and what changes when you swap a parallel flow exchanger for a counter flow one, keeping the same two fluids, same flow rates and same inlet and outlet temperatures.

The convective coefficients on each side, the wall properties and any fouling resistances do not depend on which way the streams flow relative to each other, so the overall coefficient \(U\) stays essentially the same in both arrangements.

What does change is the temperature difference between the hot and cold stream along the length of the exchanger. In counter flow this difference stays closer to constant, giving a larger log mean temperature difference, \(\Delta T_{lm}\), for the same terminal temperatures. Since \(Q = U A \Delta T_{lm}\), a larger \(\Delta T_{lm}\) means more heat transfer for the same \(U\) and \(A\), which is exactly why counter flow is more effective.

So the assertion about counter flow being more effective is correct, but crediting this to a higher \(U\) is wrong, the real cause is the higher LMTD.

\[\boxed{\text{(A) true, (R) false}}\]