Boilers
Ljungström air preheater
Also known as Ljungstrom APH, regenerative air preheater, rotary APH.
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.
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) deposition on SCR-equipped units, and cold-end 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 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
Related terms
- Air heaterAn air heater (also air preheater, APH) recovers low-grade heat from flue gas to preheat combustion air. Cold-end fouling and corrosion are the dominant operational challenges.
- 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.
- 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.