Boilers
Air heater
Also known as air preheater, APH, air heaters.
An air heater — also called an air preheater (APH) — is the final heat-recovery device in a boiler's convective pass, recovering low-grade heat from cooling flue gas to preheat the combustion air. APHs lift overall boiler efficiency by 5–10 percentage points and are critical to heat-rate performance.
APH types
| Type | Description |
|---|---|
| Ljungström / regenerative | Rotating matrix of heat-exchange baskets cycling between gas and air sides |
| Tubular | Fixed tube bundle with flue gas through tubes, air around them |
| Plate-type | Cross-flow plate exchanger; smaller industrial duty |
The cold-end problem
The APH cold end is the coolest point in the flue-gas path before the ESP / baghouse. Two related failure modes dominate:
- Ammonium bisulphate (ABS) fouling on boilers with upstream SCR: sticky deposits plug Ljungström baskets and tubular APH tubes
- Cold-end corrosion below the acid dew point — sulphuric acid condenses and attacks baskets and tubes
Why sonic horns are routinely specified on APHs
ABS fouling is the single most common reason plants install sonic horns on the cold end. Continuous low-amplitude vibration prevents ABS from consolidating between water-wash campaigns, extending the campaign interval from quarterly to annual and avoiding capacity-derate excursions.
Related terms
Related terms
- BoilerA boiler is a vessel that converts fuel chemical energy into steam by heating water. Coal-fired, biomass, oil, gas and recovery boilers all foul; sonic horns clean heat-transfer surfaces.
- Ljungström air preheaterA Ljungström air preheater uses a rotating matrix of heat-exchange baskets that cycle between the flue-gas and combustion-air sides. The dominant utility APH design worldwide.
- Tubular air preheaterA tubular air preheater is a fixed tube bundle with flue gas through the tubes and combustion air around them. Common on smaller industrial boilers and on retrofit duty.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
- Cold-end corrosion and dew-point corrosionCold-end corrosion is the attack on air-heater and economiser surfaces below the acid dew point, where SO3 condenses as sulphuric acid. The leading cold-end failure mechanism.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.