Utility & Power Quality
A clean plate has almost no resistance of its own, so a thin scale layer takes over half the duty. Good exchangers foul worst.
— W/m²K
Clean coefficient with the scale resistance in series
Duty is computed at fixed terminal temperatures, which is the design case rather than what a fouled exchanger actually does — in service the outlet temperatures move instead and the true shortfall must come from measured temperatures on both sides. The load factor matters more than any other input for the cost figure and should be taken from flow logs, not assumed. Counter-current flow is assumed for the LMTD; a co-current or multi-pass arrangement needs its own correction factor. Fouling resistance itself grows over a cleaning cycle rather than sitting at one value, so run this at the end-of-cycle figure to size the cost of deferring a clean.
Dyehouse Heat Exchanger Fouling & Thermal Loss Predictor — free, with the formula and a worked example, at Textile School.