Tubular air preheater

A tubular air preheater (TAPH) is a fixed heat exchanger that transfers heat from boiler flue gas to incoming combustion air through banks of tubes. Gas normally flows on one side of the tubes and combustion air on the other, depending on the design. Unlike a regenerative rotary air heater, a tubular unit has no rotating heat-storage element.

Tubular air preheaters are common on industrial boilers, smaller utility boilers, biomass units, package boilers and process heat-recovery systems where simplicity and gas sealing are more important than the compactness of a rotary heater.

Where TAPHs are used

TAPHs are used to improve boiler efficiency by recovering low-grade heat from flue gas before the stack. They can preheat primary air, secondary air or process combustion air. They may be arranged vertically or horizontally, with multiple passes and sootblower or acoustic-cleaning access depending on fuel.

Because they sit near the cold end of the boiler, they are exposed to a difficult combination: lower metal temperature, fly ash, possible acid condensation and ammonium-salt deposition.

Fouling and corrosion

Common problems include ash plugging between tubes, ammonium bisulphate fouling when SCR ammonia slip and SO3 are present, sulphuric acid dew-point corrosion, chloride attack in biomass and waste service, tube leakage and cold-end condensation. Plugging raises draft loss and can starve the boiler of combustion air, while corrosion can perforate tubes and mix gas with air.

Design decisions include tube pitch, tube material, gas velocity, access doors, cleaning ports, drainability and whether the cold end needs enamel, alloy or corrosion-resistant coating.

Cleaning approach

Steam sootblowers, air lances, washing and sonic horns may all be used. Sonic horns are attractive where deposits are dry or sticky but still weak enough to move before hard plugging develops. They are less effective once ammonium bisulphate has baked into a dense mass or acid mud has formed. In Sylio-style duty reviews, the firing interval is set to prevent bridge formation between tubes rather than to recover a fully plugged bundle.

Operating variables

A tubular air preheater transfers heat from flue gas to combustion air through tube walls. Its performance depends on gas and air flow distribution, tube length, tube diameter, material, gas temperature, sulphur and moisture content, ash loading, leakage and the temperature margin above acid dew point. Unlike a rotary regenerative air heater, it has no rotating heat-storage matrix, but it can still foul, corrode and leak.

The cold end is the usual risk area. If metal temperatures fall below the acid dew point, sulphuric acid can condense and bind ash into sticky deposits. Biomass, waste and high-sulphur fuels add chloride or alkali concerns. High dust loading can plug tube entrances, while poor gas distribution can overheat or overload selected tube lanes. Leakage between air and gas sides reduces efficiency and can affect oxygen readings used for combustion control.

Maintenance and acoustic-cleaning relevance

Inspection focuses on tube plugging, cold-end corrosion, tube leaks, casing leaks, fouled inlet faces, expansion damage and blocked drains. Operators trend air temperature gain, flue-gas exit temperature, pressure drop and oxygen balance. Washing may restore pressure drop but can also create corrosion if water remains trapped, so drainage and drying are important.

Sonic horns can be effective on dry ash at inlet faces, tube mouths and gas-side plenums where deposits are still loose. They are less effective on acid-wetted cold-end deposits or hard scale inside tubes. A good installation gives the horn line of sight to the fouling area and confirms that dislodged ash will leave the preheater rather than settle deeper in the bundle.

Related terms

• Air heater
• Ammonium bisulphate
• Cold-end corrosion / dew-point corrosion
• Sonic horn