A water coil generally should be piped to provide:

• A. Parallel flow
• B. Cross flow
• C. Counter flow
• D. None of the above

Answer: (C)

Heat transfer efficiency in a water coil depends on how the two fluids flow relative to each other. When water and air move in opposite directions along the coil—counterflow—the temperature difference between them stays relatively large across most of the coil length. That larger average driving force lets more heat be transferred for the same surface area and flow rates, making the coil more effective.

If the fluids run in the same direction (parallel flow), the temperature difference drops off quickly along the coil, so the overall heat transfer is reduced. Cross flow, where one fluid moves perpendicular to the other, can work in some exchanger designs but is generally less efficient than counterflow for a simple water coil in HVAC applications.

So, a water coil is generally piped to provide counterflow because it delivers the best heat transfer performance under typical conditions.