Heat-transfer surface fouling

Heat-transfer surface fouling is the build-up of unwanted material on tubes, plates or fins that are supposed to transfer heat between a gas, liquid or steam circuit. In boilers and HRSGs it usually refers to ash, salts, soot, slag, corrosion products or sticky condensates on gas-side surfaces.

Thermal mechanism

A deposit adds thermal resistance between hot gas and the tube metal. Even a thin layer can reduce the overall heat-transfer coefficient if it is porous, insulating or poorly bonded to the surface. The result is lower heat absorption in that section and higher gas temperature downstream. In a power plant this appears as heat-rate degradation, changed steam temperatures, higher stack losses or more attemperation.

Hydraulic mechanism

Fouling also blocks open area. Tube bridges, fin-pack deposits and air-heater plugging increase pressure drop, raise fan power and can force load reduction when draft fans reach their limits. Uneven fouling creates gas lanes that erode clean areas while leaving dead zones dirtier.

Failure modes

Deposits can start loose and become sintered, molten, wet, acidic or chemically cemented. They can hide tube corrosion, cause local overheating, trap chlorides or sulphates, and create differential expansion. In waste, biomass and recovery service, deposit chemistry can be as damaging as deposit thickness.

Measurement and maintenance

Operators infer fouling from pressure drop, temperature approach, stack temperature, steam temperature, fuel use, fan load and inspection. Cleaning decisions should distinguish between heat-transfer loss and flow blockage because the best response may differ.

Acoustic cleaning relevance

Sonic horns are preventive tools for heat-transfer fouling. Their value is highest when deposits are dry or weakly bonded and before thermal cycling or chemistry converts them into hard scale or slag. A successful acoustic cleaning programme should show up as slower pressure-drop rise, steadier outlet temperatures and reduced offline cleaning scope.

Evaluation notes

A fouled heat-transfer surface should be evaluated with both process data and deposit samples. Temperature loss alone does not reveal whether the cause is insulating ash, gas bypassing, tube-side scale, excess spray attemperation or instrumentation drift. Pressure drop adds important context. Deposit chemistry can reveal whether cleaning should be mechanical, acoustic, water-based, chemical or prevention-focused. The best cleaning programme is one that slows fouling rate, not one that only restores surfaces after efficiency has already been lost.

Diagnosis and operating variables

Heat-transfer fouling is usually diagnosed from a combination of thermal and hydraulic symptoms. A fouled surface may show higher gas outlet temperature, lower steam or water-side duty, increased fan load, rising pressure drop or local tube metal temperature changes. The same symptoms can also come from low flow, instrument error, bypass leakage or firing changes, so operators compare trends at similar load and fuel conditions.

Important variables include surface temperature, ash chemistry, vapour condensation, gas velocity, fin spacing, sootblower coverage, acoustic-cleaner coverage, tube material and whether the deposit is dry, sticky or sintered. Cleaning strategy should match the deposit. Steam sootblowing can remove some ash but erode tubes if overused. Water washing can dissolve salts but creates wastewater and corrosion risk. Acoustic cleaning works best as continuous prevention for dry deposits before they densify.

Maintenance evidence

Good fouling records include photographs, deposit samples, pressure-drop trends, heat-duty estimates and the cleaning method used. A deposit sample can reveal whether the problem is mostly fly ash, sulphate salt, chloride salt, catalyst fines, corrosion product or leaked process material. That evidence prevents repeated cleaning of a symptom while the chemistry or leakage source remains unchanged.