During troubleshooting, which measurement helps identify fouling in a heat exchanger?

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Fouling adds insulation on the heat-transfer surface, which raises thermal resistance and lowers the rate at which heat moves from one fluid to the other. Because the heat-transfer rate is tied to how much the fluid temperatures change, measuring the temperature difference across the exchanger directly reflects its performance. By comparing the actual delta-T across the exchanger to what you’d expect under clean or design conditions, you can see if heat transfer is being hindered by fouling. A larger-than-expected delta-T indicates the exchanger isn’t transferring heat as efficiently as it should, pointing to fouling. The other measurements—ambient temperature, electrical resistance, or oil viscosity—don’t provide this direct link to the exchanger’s heat-transfer performance.