In a counterflow heat exchanger, how does the temperature driving force compare to a parallel-flow exchanger?

• A. The maximum temperature difference is the same in counterflow and parallel-flow, so LMTD is identical
• B. The maximum temperature difference is smaller in counterflow, leading to lower LMTD
• C. The maximum temperature difference is greater in parallel-flow, leading to higher LMTD
• D. The maximum temperature difference is greater in counterflow, leading to higher LMTD

Answer: (D)

In a counterflow arrangement, the two streams move in opposite directions, which keeps the temperature difference between them more favorable along a larger portion of the exchanger. This distribution means the driving force doesn’t fall off as quickly as in parallel flow, so the log mean temperature difference (LMTD) is higher for the same inlet temperatures and overall heat transfer, leading to more effective heat exchange.

So, the counterflow setup yields a higher LMTD than parallel flow, which is why it’s described as having a greater driving force in the sense that its effective gradient over the length is more favorable for heat transfer. In parallel flow, the driving force drops more rapidly along the length, giving a smaller LMTD and less heat transfer for the same area.