What is the primary indication of fouling of a heat exchanger?

Correct Answer: Option D — An increased pressure drop associated with the side where the fouling exists and an increased temperature differential between the two fluids exchanging heat.

Fouling restricts flow, causing an increased pressure drop across the exchanger. Simultaneously, the insulating layer of scale or sludge reduces the heat transfer rate, resulting in a decreased temperature differential between the fluids. This combination is the primary diagnostic indicator of a degraded, fouled heat exchanger performance.