Abstract: Air preheater cleaning removes the flue-gas side deposits that choke a boiler air preheater and push the exhaust gas temperature up. Three mechanisms dominate: fly-ash deposition bridging the passages between tubes or element baskets, ammonium bisulphate (ABS) condensation in the mid-temperature range typical of SCR-equipped units (roughly 120-230 °C), where the exact band varies with flue-gas composition and operating conditions, and low-temperature sulphuric acid dew-point corrosion in the cold end. Three online routes are in common use - steam soot blowing for routine housekeeping, online high-pressure water washing for soft ash, and online alkaline neutralisation for ABS - while a badly bridged unit needs offline basket-out chemical cleaning - the medium and corrosion control set by the equipment material, deposit type and operating conditions - with final rinsing by high-pressure water jetting. Danyang Blue Star Cleaning selects the route on two criteria: the deposit type, and whether a shutdown or a load reduction is acceptable. Effectiveness is then judged on three acceptance points - gas-side differential pressure, exhaust gas temperature at the same load, and air leakage rate - and written into a signed acceptance record. Online washing and jetting follow GB/T 26135-2020 and GB/T 26148-2025, and quality acceptance follows GB/T 25146-2010. For an assessment, call +86 18952832843.
1. Why an Air Preheater Chokes: Three Deposit Mechanisms
Air preheater cleaning is not routine housekeeping. The deposits that choke the gas side have three distinct origins, and each one calls for a different response - reading the deposit correctly is the whole basis for choosing a cleaning route.
(1) Flue-gas side fly-ash deposition. Pulverised coal and biomass ash settles in the narrow passages between tubes or element baskets and bridges across them. It is worst on units burning high-ash fuel, running at low load for long periods, or working with weak soot blowing. The clue is a slow, steady rise in gas-side differential pressure at the same load, with soft grey-brown ash that still brushes away.
(2) Ammonium bisulphate (ABS) condensation in the mid-temperature band. Downstream of an SCR denitrification system, ammonia slip reacts with sulphur trioxide in the flue gas to form ammonium bisulphate, which is sticky in roughly the 120-230 °C window and glues fly ash into a hard, brittle shell. It is worse with high ammonia slip, high-sulphur fuel and low-load running. The clue is a hard off-white to yellow-brown crust that does not respond to steam soot blowing at all and turns tacky once wetted.
(3) Low-temperature sulphuric acid dew-point corrosion. On high-sulphur fuel, where the metal temperature of the cold-end elements falls below the acid dew point, sulphuric acid condenses and reacts into sulphates. This deposit is wet, acidic and bonded tightly to the metal, and it comes with pitting and thinning of the cold-end elements. The clues are corrosion pits, an acid smell from the cold-end deposit and an unusually low exhaust gas temperature.
Two of the three mechanisms are temperature-driven and one is ash-driven. That single judgement - loose ash, a hard ABS shell, or an acid-bonded corrosion layer - decides everything that follows.
2. Three Direct Consequences
Absolute values vary with boiler type, fuel and load, so only the direction of the trend and the way to read it are given here.
Exhaust gas temperature rises, boiler efficiency falls. A fouled gas side cannot transfer heat to the combustion air, so more heat leaves with the flue gas. The exhaust gas temperature climbs at an unchanged load, boiler efficiency drops and coal consumption per unit of steam rises.
Gas-side resistance rises, fan power rises. Deposits narrow the passages, so the air preheater differential pressure climbs; the forced-draught and induced-draught fans have to work harder, power consumption rises, and the available air volume can fall short at full load.
Air leakage rises and heavy fouling forces load reduction. Cold-end corrosion and warped or blocked passages open up the leakage paths, so the air leakage rate rises; when the passages bridge badly enough the boiler is forced into load reduction and then into an unscheduled outage.
The way to read it is against a baseline: keep the same-load trend of differential pressure, exhaust gas temperature and air leakage in the operating log, and compare before and after. No single reading proves fouling; the three moving together do.
3. Tubular vs Rotary (Three-Sector) Air Preheaters: Cleaning Differences
The two designs hold their deposits in different places, and that changes what can be done while the boiler is still running.
| Item | Tubular air preheater | Rotary three-sector unit |
|---|---|---|
| Structure | Air flows inside the vertical tubes; flue gas passes across the outside of the tube bundle, with tube plates at both ends | A rotating rotor carries corrugated element baskets through the flue-gas, primary-air and secondary-air sectors |
| Where deposits sit | On the outside of the tubes and in the tube-to-tube passages, worst in the cold-end rows | On the element basket surfaces, worst in the cold-end layer and at the sector partitions |
| Online cleaning possible? | Limited: online washing reaches the cold end unevenly and cannot clear bridged tubes | Better: steam soot blowing and online water washing both reach the rotating element continuously |
| Cleaning methods | Mechanical punching of individual tubes, offline chemical cleaning, high-pressure water jetting | Steam soot blowing, online water washing, alkaline neutralisation for ABS, offline basket-out chemical cleaning |
| Shutdown needed? | Yes for a full clean; only light online washing is possible | No for soot blowing and washing; yes when the element baskets must come out |
| Key risks | Wall thinning and tube-end erosion if punching tools are misused; uneven cleaning across the tube plate | Cold-end corrosion, air leakage past the seals, basket warping, and hot-spots from unburnt carbon held in the elements |
Selection conclusion: where the deposit is soft ash on a rotary unit, stay online; where the passages are bridged, the tubes are blocked or an ABS shell has hardened, plan an offline clean instead of adding more soot-blower hours.
4. Choosing Among the Three Online Cleaning Routes
Online cleaning keeps the boiler in service. There are three routes, and they are not interchangeable.
(1) Steam soot blowing. The routine measure, run on a fixed schedule to stop loose fly ash from building up. It is effective on dry ash and largely ineffective on an ABS hard shell or an acid-bonded layer - more blowing hours only add steam consumption and wear.
(2) Online high-pressure water washing. Suitable where the deposit is soft or water-soluble and where the boiler can run at reduced load for the duration. Warm water lifts ash better than cold water, and the jetting pressure is set inside the machine's working range and trimmed at the nozzle so the elements are not deformed or eroded; water temperature and pressure are chosen for the deposit rather than fixed in advance. Wash water and the ash slurry it carries must be collected and routed to the plant drain or settling pit - never discharged uncontrolled, and never left to pool in the cold-end duct.
(3) Online alkaline washing for ABS. Aimed specifically at ammonium bisulphate: the bisulphate is neutralised into a water-soluble salt so it can be washed out rather than chipped off. The two parameters that decide success are the pH of the washing liquor and the volume of wash water used; both are controlled step by step, and the cold-end elements are protected against corrosion throughout. No fixed figure applies - the washing liquor is prepared and monitored on site for the deposit actually present.
Which route to take comes down to two criteria: what the deposit is (dry ash, an ABS shell or an acid-bonded layer), and whether the plant can accept a shutdown or a load reduction. Dry ash with room to run on: soot blowing. Soft ash with reduced-load operation available: online water washing. ABS: alkaline washing online, or offline chemical cleaning once the passages have bridged.
5. Offline Basket-Out (Full Unit) Chemical Cleaning Process
When the online routes can no longer clear the passages, the unit is taken out of service and cleaned in seven controlled steps. No step starts until the previous one has met its acceptance node.
(1) Shutdown, cool-down and flue-gas side isolation. Take the boiler off load, cool it down at the rate the drum and the structure allow, then close and blank the flue-gas dampers and lock and tag the drives. Isolation is verified before the casing is opened - this is a permit-controlled job.
(2) Basket-out, element removal or segmental isolation. On a rotary unit the element baskets are lifted out by crane; on a tubular unit the gas-side header or a section of the casing is opened and the work is split into segments. Removed parts are set down on a prepared, drained area.
(3) Mechanical loosening of the deposit layer. Hard crust and bridged passages are broken down with hand and rotary tools, with the tooling kept off the tube walls and element plates so nothing is thinned or deformed. This step decides how much chemical work remains.
(4) Chemical cleaning. The cleaning medium and the corrosion control are set by the equipment material, the deposit type and the operating conditions, with the agent selection and the corrosion-inhibitor regime written into the procedure for this unit; cleaning temperature and circulation time are controlled step by step and the metal is protected throughout. The acceptance node is a clean, wetted surface with no residual hard crust.
(5) High-pressure water jetting rinse and flush. Loose residue and spent liquor are flushed away with high-pressure water jetting, working to GB/T 26135-2020 for the machines and GB/T 26148-2025 for the safety of the operation. This is a jetting job, and the guardrails used for any high-pressure work apply in full.
(6) Neutralisation, passivation and drying. The flushed surfaces are neutralised, passivated and then dried; a wet gas side put straight back into service is an invitation to a fresh corrosion layer.
(7) Reassembly, sealing and air leakage check. Elements and baskets go back in, the seals and sector partitions are set, the casing is closed, and an air leakage check is made before the unit returns to service.
6. Effect Judgement and Acceptance
Three criteria decide whether the job worked, and all three are read against the same-load baseline recorded before the clean.
(1) Exhaust gas temperature. At the same load, the exhaust gas temperature should have fallen relative to the pre-clean reading.
(2) Gas-side differential pressure. At the same load, the air preheater differential pressure should have fallen relative to the pre-clean reading.
(3) Air leakage check. The leakage check after reassembly must show that the seals and the sector partitions are sound.
The job is closed out on the record: the before-and-after readings, the cleaning and jetting records, and the acceptance record signed by both parties. Waste liquor and ash water are not left on site - they are collected and handed over for commissioned disposal under the plant's environmental requirements, and the working area is left clean.
7. Frequently Asked Questions
What level of fouling means an air preheater must be cleaned?
Clean it once two readings worsen at the same load. The criterion is gas-side differential pressure climbing and exhaust gas temperature sitting well above its post-clean history at the same load while steam soot blowing no longer holds it down; a locally blocked passage or a fan that cannot deliver the air volume has to be dealt with in a shutdown.
Does online cleaning require a boiler shutdown?
No shutdown for online washing - but cut the load. A rotary three-sector unit can be washed and alkali-cleaned online with the cleaning zone isolated from the flue-gas system and the wash water and ash slurry collected along the route; most tubular units still have to come off load, and only some can be held by online soot blowing.
Do rotary and tubular air preheaters use the same cleaning method?
No - the two designs hold deposits in different places. A rotary unit fouls on the corrugated element plates and the rotor cold end, so it can take online high-pressure washing and alkaline neutralisation, with basket-out still needed for a hard shell; a tubular unit fouls on the tube outside surfaces and between the tubes, so it is normally taken off load and cleaned by mechanical punching plus chemical cleaning and high-pressure water jetting.
How many days does it take to clean one air preheater, and how is it priced?
Timing and price follow the deposit type and the workload. Online washing normally fits inside one maintenance window; offline basket-out chemical cleaning takes longer because of the lifting and the reassembly, and it also depends on the proportion of blocked tubes and the cleaning workload to be done. For pricing, call +86 18952832843 with the unit model, deposit photos and operating data.
How do we judge that the cleaning actually worked?
Two before-and-after readings at the same load confirm it. The exhaust gas temperature and the gas-side differential pressure must both fall; a rotary unit is also checked for air leakage rate, a tubular unit for casing sealing and performance on trial run. The job is only closed when the work records, the ash-water disposal records and the two-party signature are complete.
Related Reading
Further reading on comparable equipment and the same acceptance route:
- Reactor Cleaning Technology - comparable practice for confined space entry and material restrictions
- 304/316L Stainless Steel Tank and Reactor Surface Passivation
- Industrial Steam Boiler Chemical Descaling Technology
- Equipment Scaling Diagnosis Tool · Maintenance Cycle Calculator
The cleaning procedure in this article is set by the equipment material, the deposit type and the operating conditions; quality acceptance follows GB/T 25146-2010 Acceptance Standard for Chemical Cleaning Quality of Industrial Equipment; the high-pressure water jetting is carried out to GB/T 26148-2025 Safety Specification for High-pressure Water Jet Cleaning Operations and the equipment complies with GB/T 26135-2020 High-pressure Cleaning Machines.
Air Preheater Cleaning · Free Technical Consultation
Fouling Assessment | Online vs Offline Method Selection | Procedure and Safety Review | Acceptance Records
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Danyang Blue Star Anti-Corrosion Cleaning Co., Ltd. · Member of China Industrial Cleaning Association
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