Ljungstrom air preheater

A Ljungstrom air preheater is a regenerative air heater that transfers heat from boiler flue gas to incoming combustion air using a rotating matrix of heat-transfer baskets. As the rotor turns slowly, each basket alternately passes through the hot flue-gas stream and the cooler air stream.

The design is widely used on utility and industrial boilers because it recovers heat efficiently in a compact casing. The trade-off is that the rotating seal system allows some leakage between gas and air sides, and the cold-end baskets are vulnerable to fouling and corrosion.

Basket arrangement

A typical rotary APH has several basket layers:

Hot end - high-temperature baskets with robust profiles and wider passages.
Intermediate layer - transition-duty baskets.
Cold end - high-surface-area baskets exposed to the lowest metal temperatures.

The cold end is usually the limiting fouling zone. It sees cooler metal, acid dew point risk, higher moisture sensitivity and, on SCR-equipped units, ammonium bisulphate deposition.

Failure modes

Cold-end plugging raises APH differential pressure and ID fan load.
ABS deposition creates sticky fouling after ammonia slip reacts with sulphur oxides.
Acid dew point corrosion attacks cold-end elements and casing.
Rotor seal leakage reduces efficiency and can increase fan load.
Basket erosion occurs where ash velocity is high or flow distribution is poor.
Fire risk can arise if combustible deposits accumulate in some services.

The usual symptoms are rising gas-side pressure drop, higher stack temperature, lower air preheat, fan margin loss, increased leakage and more frequent water washing.

Cleaning methods

Rotary air preheaters often use steam sootblowers, water washing during outages and sometimes online or semi-online washing systems. Washing removes hard deposits but creates wastewater, corrosion concerns and restart constraints.

Sonic horns mounted on the gas-side cold end can keep ash and early ABS deposits mobile between sootblower or wash campaigns. They are most useful when the deposit is still weakly bonded. They are less effective against thick, sticky ABS mats or corroded basket packs that already need replacement.

Design implications for acoustic cleaning

Horn placement should target the gas-side inlet to the fouling layer, avoid mechanical interference with rotor movement, and allow maintenance without opening major casing sections. Materials must match temperature, sulphuric-acid dew point conditions and any washdown exposure. Cycle timing should avoid unnecessary continuous noise while preventing deposit consolidation.

Operating and inspection detail

Ljungstrom performance is judged by heat recovery, leakage, pressure drop and cold-end cleanliness. Key variables include rotor speed, seal clearance, basket material, gas-inlet temperature, air-inlet temperature, sulphur level, ammonia slip, sootblower condition and wash quality. A small change in ammonia slip or cold-end metal temperature can shift deposits from dry ash to sticky ammonium-bisulphate fouling.

Operators usually see problems as higher fan load, lower air outlet temperature, increased gas outlet temperature, sector-plate rubbing, basket pluggage, ash carryover after sootblowing or corrosion found during basket replacement. Leakage matters because air in-leakage dilutes flue gas, changes oxygen readings and can increase fan duty. Severe pluggage can also disturb downstream ESP or baghouse loading.

Maintenance checks should include basket differential fouling by sector, seal wear, drive condition, sootblower nozzle condition, drain function, wash-water distribution and corrosion at the cold end. Acoustic cleaning is most useful when it keeps early deposits loose between sootblowing or washing. It should not be expected to rescue baskets that are already blinded by hard ABS or structurally weakened by corrosion.

Acoustic-cleaning limits

Acoustic cleaning on a Ljungstrom unit should be judged by cold-end fouling trend, not by whether ash is heard moving during a test. The rotor continuously carries baskets through gas and air sectors, so loosened material must have a discharge path and should not simply redeposit in the next sector.

If ammonium bisulphate is already sticky, horn cycles may reduce fresh accumulation but will not restore a blinded basket pack. In that condition, washing, basket replacement or process correction is needed. Acoustic cleaning is most valuable when installed before the cold end reaches that sticky, low-permeability state.