EPRI Heat Transfer & Fluid Flow 2026 Challenge – Dive In and Master the Flow!

The main idea is the driving force for heat transfer, which is the temperature difference between the two streams along the exchanger. In parallel flow, both streams enter at similar temperatures and heat transfer drives them toward a common temperature as they move in the same direction; the temperature difference is largest at the inlet and then steadily falls toward the outlet, so the average driving force is relatively modest.

In counterflow, the streams move in opposite directions, so the hottest portion of one stream can meet the coldest portion of the other. This arrangement keeps the temperature difference larger over a greater length of the exchanger, which means a larger average driving force (often described by the higher log mean temperature difference). With a higher average driving force, more heat can be transferred for the same area and overall heat transfer coefficient.

Therefore, counterflow generally provides a higher average driving temperature difference than parallel flow.