What is counterflow in a heat exchanger?

• A. Flow in opposite directions
• B. Flow in same directions
• C. Flow with a 90-degree turn
• D. Flow when both sides are stagnant

Answer: (A)

Counterflow means the two fluids inside the heat exchanger move in opposite directions along the length of the exchanger. This arrangement keeps a larger and more uniform temperature difference between the hot and cold streams throughout the path, so heat can transfer more effectively from the hot side to the cold side. Because the hot inlet meets the cold outlet (and the hot outlet meets the cold inlet) at different points, you can achieve a greater overall temperature change than if the streams flowed in the same direction.

In contrast, when the streams flow in the same direction, the temperature difference between them diminishes along the length, reducing the driving force for heat transfer and overall efficiency. A 90-degree turn describes a change in path direction rather than the relative flow direction of the two streams, and stagnant flow means no movement and no heat transfer. So flowing in opposite directions best describes counterflow.