Air heater

An air heater, or air preheater, is the heat-recovery equipment that transfers remaining heat from flue gas to incoming combustion air before the gas leaves for particulate control or the stack. It improves boiler efficiency, stabilises ignition and reduces fuel consumption, but it is also one of the most failure-prone cold-end components in ash-bearing service.

The main industrial forms are rotary regenerative heaters, such as Ljungstrom units, and tubular recuperative heaters. Rotary heaters pass hot flue gas and cold air over slowly rotating heat-transfer baskets. Tubular heaters keep the two streams separated by tubes. Both types operate near the temperature range where acid condensation, ammonium bisulphate and sticky fly ash become serious.

Failure modes

Common air-heater problems include basket plugging, cold-end corrosion, erosion, seal leakage, rotor distortion, fires from unburned carbon, high differential pressure and declining heat transfer. SCR-equipped boilers add the risk of ammonium bisulphate, formed from ammonia slip and SO3. The result is often a rapid rise in fan power and a loss of load capability.

Operation and monitoring

Operators track gas and air outlet temperature, pressure drop, oxygen leakage, cold-end metal temperature, sootblower effectiveness and basket wash intervals. Minimum inlet-air temperature rules protect against the acid dew point, especially during low-load operation and start-up.

Acoustic-cleaning relevance

Sonic horns can help keep dry ash mobile on upstream surfaces and may reduce the rate at which baskets load with loose particulate. They cannot remove mature sticky ABS or reverse corrosion, so timing matters: acoustic cleaning works best as a preventive layer alongside temperature control, sootblowing, leakage management and periodic washing.

Design and operating variables

Air-heater behaviour is governed by gas flow, air flow, element depth, basket profile, leakage, sootblower coverage, inlet temperature, cold-end metal temperature and ash chemistry. Rotary units add seal condition, rotor speed and sector-plate adjustment. Tubular units add tube layout, erosion allowance and access for washing. Small changes in leakage can have large effects because extra oxygen and cold air alter both heat transfer and corrosion risk.

Operators follow gas outlet temperature, air outlet temperature, pressure drop, oxygen balance, fan amps, seal-air performance and sootblower effectiveness. A rising pressure drop with falling heat recovery suggests plugging. A stable pressure drop with falling heat transfer can suggest fouled surfaces, leakage or damaged elements. Low cold-end temperatures raise concern for acid dew point and ammonium bisulphate formation.

Inspection and cleaning context

Outage inspections look for plugged baskets, hardened deposits at the cold end, corrosion in low-temperature zones, erosion at high-velocity edges, failed seals, cracked support structure and wash-water drainage problems. Online sootblowing or acoustic cleaning is normally preventive; offline washing removes deposits that have already become sticky or cemented. If wash water is not drained and neutralised correctly, corrosion can accelerate after the unit returns to service.

Acoustic cleaners are placed to improve ash mobility and reduce the growth rate between sootblowing or wash intervals. They are most useful when deposits are still dry enough to fracture under pressure oscillation. If the air heater is dominated by wet acid salts, the cleaning plan must address SCR ammonia slip, SO3, cold-end temperature and seal leakage as well as mechanical deposit removal.

Trend interpretation

Air-heater problems should be trended by load because leakage, heat transfer and pressure drop all move with operating point. A pressure-drop increase at the same load is more meaningful than a raw daily value. Operators also compare left and right gas paths, hot-end and cold-end observations, sootblower timing and wash history. Asymmetry often points to seal leakage, flow maldistribution, local plugging or a failed cleaning device.

When acoustic cleaners are used, their firing history should be compared with pressure-drop slope rather than a single before-and-after reading. The expected benefit is a slower fouling rate, fewer rapid plugging events and better stability between planned washes.

Maintenance evidence

Basket samples and wash-water observations are useful because they show whether the deposit is loose ash, acid salt, corrosion product or unburned carbon. That evidence should guide the next operating change. More cleaning intensity is not the right response if the sample shows acid chemistry or seal leakage as the main cause.

Acoustic-cleaning nozzles and horns should be inspected during air-heater outages because plugged mouths, damaged supports or local erosion can remove protection from one sector without triggering a plant alarm.

Operating evidence

Air-heater health is visible in gas outlet temperature, air outlet temperature, leakage, differential pressure, fan power, acid-dew-point margin and wash frequency. A regenerative heater can lose performance through basket plugging, seal wear, corrosion, erosion or ash carry-over, and those causes require different maintenance. Sticky ammonium bisulphate deposits may need temperature and reagent control, while dry ash plugging may respond to sootblowing, acoustic cleaning or washing. In Sylio-style applications, horns are evaluated by whether they slow pressure-drop rise and keep ash friable between washes. They do not repair leaking seals or reverse cold-end corrosion. Inspection photos of baskets and hoppers are important acceptance evidence.

Related terms

• Acid dew point
• Ammonium bisulphate
• Ljungstrom air preheater