Tube fouling

Tube fouling is the build-up of ash, soot, salts, slag, catalyst dust or process solids on boiler and heat-exchanger tubes. It reduces heat transfer from gas to steam or water, changes gas-flow distribution and can create corrosion or erosion conditions under the deposit.

Fouling is not one problem. A soft powder layer on an economiser, a sticky ammonium-bisulphate film in an air heater and a fused slag layer on a furnace wall need different controls.

What fouling does

Tube fouling can:

• Lower steam generation or final steam temperature
• Increase boiler exit gas temperature and stack losses
• Raise induced-draught fan load through higher pressure drop
• Create gas lanes that erode selected tube rows
• Trap chlorides, sulphates or acids against tube metal
• Force more frequent sootblowing, washing or outage cleaning
• Increase fire risk where combustible dust accumulates

Operators often first see the problem through temperature trends, draft loss, fan amps, sootblower demand or thermal camera inspection rather than direct visual access.

Deposit drivers

Drivers include fuel ash chemistry, alkali and chlorine content, sulphur level, gas temperature, local velocity, excess air, additive use, load swings and cleaning frequency. Biomass, waste-derived fuels and black-liquor recovery boilers can create low-melting salts that turn a manageable dust layer into a sticky or corrosive deposit.

Cleaning toolkit

Cleaning options include sonic sootblowers, steam sootblowers, water cannons, manual lancing, online explosives, offline washing and fuel or combustion changes. The right choice depends on deposit strength and location. Acoustic cleaning is strongest as preventive maintenance for dry or weakly bonded deposits, especially in convective tube banks, economisers and air heaters. It is not a substitute for waterwall slag removal.

How fouling develops

Tube fouling begins when particles, vapours or reaction products reach a tube surface and remain there long enough to build a deposit. The first layer may be controlled by surface temperature, ash chemistry, condensation, electrostatic forces or sticky salts. Once a layer exists, it changes the local surface temperature and roughness, so later particles may attach more easily. In boilers and heat exchangers, this feedback can turn a light coating into bridging between tubes.

The variables include fuel ash, particle size, gas velocity, tube pitch, metal temperature, load cycling, excess air, ammonia slip, sulphur and chloride chemistry, sootblowing interval and whether the deposit can fall to a hopper. Dry, friable ash behaves very differently from sintered slag, ammonium bisulphate, tar, gypsum slurry or wet scrubber solids.

Operating consequences and cleaning

Fouling reduces heat transfer, raises gas-side pressure drop, changes steam temperatures, increases fan power and can create local erosion lanes. It may also hide corrosion under deposits or force operators to use more aggressive cleaning. Troubleshooting compares temperature approach, draft loss, tube-metal readings, inspection photos, deposit samples and cleaning response. A deposit that returns immediately after washing is usually driven by process chemistry or temperature, not by lack of cleaning effort alone.

Acoustic cleaning is most useful as preventive fouling control. Regular low frequency pulses can keep dry deposits from consolidating on tube banks or ledges, especially where direct sootblower coverage is limited. It is much less effective once deposits are fused, wet, chemically cemented or shielded behind dense tube rows. A strong application defines the deposit state, the path for released solids and the process trend that will prove success.

Related terms

• Superheater
• Economiser
• Tube erosion / tube wastage
• Sonic sootblower