Question 5

The Correct Answer is D ### Explanation for Option D (Correct) **D) In a counterflow heat exchanger, the flow through the tubes and the flow through the shell are in opposite directions.** This statement accurately defines counterflow (or counter-current flow). In a single-pass shell-and-tube heat exchanger arranged for counterflow, the hot fluid enters one end, and the cold fluid enters the *opposite* end. They flow parallel to the axis of the shell but in diametrically opposed directions, maximizing the potential for heat transfer efficiency across the length of the exchanger. ### Explanation of Incorrect Options **A) In a counterflow heat exchanger, the flow through the tubes and the flow through the shell are in the same direction.** This is the definition of **parallel-flow** (or co-current flow), not counterflow. In counterflow, the flows are in opposite directions. **B) In a parallel-flow heat exchanger, the flow through the tubes and the flow through the shell are in opposite directions.** This is incorrect. In a parallel-flow heat exchanger, the two fluids enter at the same end and flow in the **same direction** through the length of the exchanger. **C) In a cross-flow heat exchanger, the flow through the tubes and the flow through the shell are in opposite directions.** This is incorrect. In a purely cross-flow arrangement, the fluid flows through the tubes (typically along the axis) while the fluid flowing through the shell (usually facilitated by baffles) moves **perpendicular** (at a 90-degree angle) to the tube flow, not opposite to it. Furthermore, shell-and-tube exchangers are classified primarily as parallel, counter, or mixed (multi-pass/baffled), though the baffle sections introduce a cross-flow component. For the primary directional definitions (parallel vs. counter), option C describes the condition for counterflow, not cross-flow.