With regard to the relative flow patterns through the tubes and the shell of shell-and- tube heat exchangers, what statement is true?

Correct Answer: Option A — In a counter-flow heat exchanger, the flow through the tubes and the flow through the shell are in opposite directions.

Counter-flow heat exchangers maximize thermal efficiency by directing the two fluids in opposite directions. This arrangement maintains a more uniform temperature difference across the entire length of the heat transfer surface, allowing the cold fluid to be heated to a temperature higher than the outlet temperature of the hot fluid.