What statement is true concerning the heat transfer rates through a heat exchanger boundary wall by conduction?

Correct Answer: Option D — Heat transfer rates are directly proportional to the temperature gradient across the wall and the area of the wall, and inversely proportional to the wall thickness.

Fourier's Law defines conduction heat transfer as directly proportional to the surface area and the temperature difference across the material, while being inversely proportional to the thickness of the material. Increasing the area or temperature gradient increases heat flow, whereas increasing wall thickness acts as a thermal resistance.