---
title: "Air heater (APH)"
description: "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."
canonical_url: "https://sylio.co/glossary/air-heater"
last_updated: "2026-06-28T02:29:25.451Z"
---

An **air heater** — also called an **air preheater** (APH) — is the final heat-recovery device in a boiler's [convective pass](/glossary/convective-pass-backpass), 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

<table>
<thead>
  <tr>
    <th>
      Type
    </th>
    
    <th>
      Description
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      <a href="/glossary/ljungstrom-air-preheater">
        Ljungström / regenerative
      </a>
    </td>
    
    <td>
      Rotating matrix of heat-exchange baskets cycling between gas and air sides
    </td>
  </tr>
  
  <tr>
    <td>
      <a href="/glossary/tubular-air-preheater">
        Tubular
      </a>
    </td>
    
    <td>
      Fixed tube bundle with flue gas through tubes, air around them
    </td>
  </tr>
  
  <tr>
    <td>
      Plate-type
    </td>
    
    <td>
      Cross-flow plate exchanger; smaller industrial duty
    </td>
  </tr>
</tbody>
</table>

## The cold-end problem

The APH cold end is the coolest point in the flue-gas path before the [ESP](/glossary/electrostatic-precipitator) / [baghouse](/glossary/baghouse). Two related failure modes dominate:

- **Ammonium bisulphate (ABS)** fouling on boilers with upstream [SCR](/glossary/selective-catalytic-reduction): sticky deposits plug Ljungström baskets and tubular APH tubes
- **Cold-end corrosion** below the [acid dew point](/glossary/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](/glossary/sonic-horn) 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

- [Boiler](/glossary/boiler)
- [Ljungström air preheater](/glossary/ljungstrom-air-preheater)
- [Tubular air preheater](/glossary/tubular-air-preheater)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)
- [Cold-end corrosion / dew-point corrosion](/glossary/cold-end-corrosion-dew-point-corrosion)
- [Sonic horn](/glossary/sonic-horn)