---
title: "Ljungström air preheater"
description: "The Ljungström air preheater is a regenerative-type air heater using a rotating matrix of heat-exchange baskets that cycle between the flue-gas and combustion-air sides of the boiler. As the matrix rotates (typically at 1–3 rpm), each basket alternately absorbs heat from the hot flue gas and releases it to the cold combustion air. Patented by Frederik Ljungström in 1920, it is the dominant utility-scale APH design worldwide."
canonical_url: "https://sylio.co/glossary/ljungstrom-air-preheater"
last_updated: "2026-06-28T02:29:26.050Z"
---

The **Ljungström air preheater** is a regenerative-type [air heater](/glossary/air-heater) using a rotating matrix of heat-exchange baskets that cycle between the flue-gas and combustion-air sides of the boiler. As the matrix rotates (typically at 1–3 rpm), each basket alternately absorbs heat from the hot flue gas and releases it to the cold combustion air. Patented by Frederik Ljungström in 1920, it is the dominant utility-scale APH design worldwide.

## Basket arrangement

A typical Ljungström has hot-end, intermediate and cold-end basket layers, each chosen for its operating-temperature band:

- **Hot end** — flat, robust, large-pitch baskets
- **Intermediate** — moderate-pitch
- **Cold end** — small-pitch, high-surface-area baskets — *and the most fouling-prone*

## Cold-end fouling

The cold-end baskets are the smallest and most easily plugged. Two failure modes dominate: [ammonium-bisulphate (ABS)](/glossary/ammonium-bisulphate) deposition on SCR-equipped units, and [cold-end corrosion](/glossary/cold-end-corrosion-dew-point-corrosion) below the acid dew point. Fouled cold-end baskets raise APH ΔP, derate the ID fan, and ultimately force a full water-wash campaign.

## Cleaning

[Sonic horns](/glossary/sonic-horn) mounted on the cold-end gas side keep baskets clear between periodic steam-sootblower passes, extending the interval to full water-wash from quarterly to annual on many units.

## Related terms

- [Air heater](/glossary/air-heater)
- [Tubular air preheater](/glossary/tubular-air-preheater)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)
- [Sonic horn](/glossary/sonic-horn)