Fouled Overall Coefficient

Fouling is the slow accumulation of scale, biofilm, corrosion product and process gunk on a heat transfer surface, and it is the single largest source of lost capacity in operating plants. TEMA tabulates it as a fouling factor $R_f$ in m²·K/W (or h·ft²·°F/BTU): 0.00018 for treated cooling-tower water, 0.00035 for river water, 0.0009 for untreated seawater, up to 0.002 for a heavy fuel oil. This page uses its reciprocal, the fouling conductance $h_f = 1/R_f$, so it lives in the same W/(m²·K) picker as the rest of the shard — 0.0002 m²·K/W is $h_f$ = 5000 W/(m²·K), and 0.001667 h·ft²·°F/BTU is 600 BTU/(h·ft²·°F).

The arithmetic is brutal in the direction people find surprising. A clean U of 1200 W/(m²·K) with a modest $R_f$ of 0.0002 falls to 1/(0.000833 + 0.0002) = 968 — a 19% loss from a deposit you could scrape off with a fingernail. On a high-U plate exchanger the same fouling factor can cost 40%, because the fouling resistance is now comparable to everything else in the stack. This is why shell-and-tube plants foul themselves into shutdowns: designers add fouling allowance as extra surface, the oversized unit runs at lower velocity than intended, low velocity deposits more solids, and the margin that was supposed to protect the exchanger is what killed it. Keep tube-side velocity above about 1 m/s and the chemistry in range, and the allowance stays an allowance.