≈ 20 °C ≈ 1 pp
Stack drop per efficiency point
Boiler pressure parts · Heat recovery
Air preheater: combustion air heating and the cold-end problem
An air preheater (APH) transfers residual flue-gas heat to combustion air, returning roughly 1 percentage point of boiler efficiency per ~20 °C of stack-temperature drop and delivering the 150–180 °C+ hot air that ~50 %-moisture bagasse needs to burn stably. Selection is tubular vs regenerative; longevity is decided at the cold end, where metal temperature approaches the acid dewpoint.
150–180 °C+
Hot-air requirement, wet bagasse (~50 % moisture)
6–12 % of gas flow
Regenerative APH seal leakage
115–140 °C
Acid dewpoint, sulphur-bearing fuels
01 — Function and position
What the air preheater does, and why wet fuels cannot burn without it
The APH closes the heat-recovery train — and on bagasse it is a combustion device, not just an efficiency device.
The air preheater (APH) sits last in the flue-gas path, downstream of the economizer, and transfers the remaining recoverable gas heat to combustion air. The efficiency arithmetic is the same as for any back-end surface: every ~20 °C of stack-temperature reduction returns roughly 1 percentage point of boiler efficiency. But on high-moisture biomass the APH earns its place twice. Bagasse arrives at the grate at ~50 % moisture with an LHV of only 7.2–7.5 MJ/kg; hot air at 150–180 °C or more is what dries the fuel in suspension, anchors ignition on the grate, and keeps combustion stable when fuel moisture swings after rain or a mill stoppage. An undersized or leaking APH shows up first as unstable furnace conditions and rising unburnt carbon, and only second as lost stack heat. The APH is specified as part of the complete back-end alongside the other boiler pressure parts and heat-recovery surfaces, because its gas outlet temperature sets the inlet condition for the dust collector and ID fan.
This page also answers the common search phrasing "APH in boiler": APH is simply the standard abbreviation for the air preheater described here.
02 — Type selection
Tubular or regenerative air preheater — which type for which duty?
Tubular (recuperative) APHs have zero designed air-to-gas leakage and straight, cleanable gas passes, which suits dusty, erosive biomass gas; their penalty is size. Regenerative (rotary, Ljungström-type) APHs are compact for utility-scale duty but leak 6–12 % of gas flow across radial and axial seals — flow the fans must carry forever.
The leakage number deserves arithmetic rather than a shrug. Take the recurring 60 t/h design basis with ~117,000 Nm³/h of flue gas and an ID fan absorbing on the order of 300 kW. A 10 % seal leakage means the ID fan handles 10 % more volume through a fixed gas-path resistance: pressure drop rises with flow squared (×1.21) and fan power with flow cubed (×1.33) — roughly 100 kW of additional continuous absorbed power, plus the FD fan margin to push the same leaked air in the first place. Design basis — illustrative calculation, not a guarantee. On top of the power cost, leakage dilutes the flue gas ahead of the dust collector and distorts O₂-based combustion control. For the biomass and sugar-mill boiler class Arrow serves, the tubular type is the default; regenerative types are retained where they already exist and are re-sealed rather than replaced.
In a tubular APH the gas normally passes inside vertical tubes (cleanable by soot blowers and accessible for retubing) with air in crossflow outside, in one to three passes. On high-ash fuels the same erosion rules as any convective bank apply: bagasse and rice-husk fly ash is high in abrasive silica (rice-husk ash is 85–90 % amorphous silica), so tube-inlet velocities are limited, inlet ends are protected with ferrules or sleeves for the first 150–300 mm where jet impingement concentrates wear, and the top tubesheet is shielded.
03 — Cold-end metal temperature
Why does the cold end corrode when the gas is still above the acid dewpoint?
Because tube-metal temperature is not the average of gas and air temperatures — it is the film-coefficient-weighted mean, Tm ≈ (hgTg + haTa)/(hg + ha), and the air-side coefficient pulls metal temperature toward the cold air. Gas at 150 °C over air at 35 °C can put metal below a 120 °C acid dewpoint.
The mechanism: heat-transfer resistance on each side of the tube wall is unequal, and the wall temperature settles nearer the side with the stronger film coefficient. At the cold corner of an APH — coldest gas meeting coldest air — the air-side crossflow coefficient is comparable to or higher than the gas-side coefficient, so the weighting drags metal temperature well below the arithmetic mean. Evaluating cold-end metal from the mean of gas and air temperatures, a common shortcut, overestimates it by 15–30 °C and is the root cause of many "unexplained" cold-end failures.
Whether that metal temperature is safe depends on the flue-gas acid dewpoint, which is set by fuel sulphur: SO₃ formed in the furnace combines with water vapour to give sulphuric acid whose dewpoint sits around 115–140 °C for oil, coal and mixed firing, falling toward the water dewpoint (60–70 °C on wet biomass gas) for near-zero-sulphur fuels. Below the dewpoint, condensed acid wastes carbon steel at millimetres per year and glues ash into hard deposits. Countermeasures, in order of preference: hold cold-end metal above the dewpoint (SCAH, air bypass, or raising the design air inlet), use low-alloy weathering steel or coated tubes in the cold-end block, and make the cold-end block a bolted, sacrificial, separately replaceable module.
04 — Steam coil air preheater
SCAH: the small coil that fails first — and how to design one that does not
Most SCAH failures are drainage failures. The thermal design is the easy part.
A steam coil air preheater (SCAH) heats incoming cold air with auxiliary steam before the main APH, holding cold-end metal above the acid dewpoint during start-up, low load and cool-season operation. It is a small heat exchanger with a disproportionate failure record, and the failures are mechanical, not thermal.
The classic defective layout puts the steam inlet and the condensate outlet on the same header side, with tube passes that include a rising leg. Condensate formed in the coil cannot drain against the rise; it pools at the bottom of the loop. Each time steam is admitted — every start-up, every control-valve opening — steam collapses onto the trapped subcooled water and accelerates the slug until it strikes a header or return bend. The Joukowsky relation sizes the blow: Δp = ρ·a·Δv, so a condensate slug stopped from just 5 m/s (water, wave speed ~1,300 m/s) generates a spike of ~65 bar — an order of magnitude above the 6–16 barg rating of a typical coil. The repeated spikes plus the thermal cycling of partially flooded tubes produce low-cycle fatigue cracking exactly where the stress concentrates: at the tube-to-header stub welds. The coil then leaks steam into the air stream, and the plant learns about it as an unexplained loss of deaerator make-up.
Arrow's SCAH design principles remove the trap rather than armour against it: straight tubes with no rising legs; a continuous fall of 1:50 from steam inlet to condensate outlet so the coil is self-draining in any operating state; inlet and outlet on opposite headers; per-module steam control so each coil block sees full steam flow or none, instead of one throttled valve starving several modules into partial flooding; vacuum breakers so condensate is never held up by sub-atmospheric collapse after a trip; pump-trap condensate removal that works against back-pressure at low steam pressure; and an automatic warm-up sequence that admits steam slowly with drains open. These are layout decisions that cost nothing at the drawing stage and cannot be retrofitted cheaply afterwards.
05 — Duty envelope and materials
Typical duty envelope, materials and code basis
Selection figures are design-basis; guaranteed figures are stated per project on a stated basis.
| Unit | Boiler capacity | Design pressure | Temperature | Material grade |
|---|---|---|---|---|
| Tubular APH, hot block | 30–170 t/h | Gas/air draught, ±5 kPa casing | Gas in 250–400 °C | ERW carbon steel, SA 178 Gr A class |
| Tubular APH, cold-end block | 30–170 t/h | Gas/air draught, ±5 kPa casing | Metal 90–140 °C | Low-alloy weathering (Corten-type) or coated tube |
| Tubular APH, biomass power | 170–250 t/h | Gas/air draught, ±5 kPa casing | Air out 150–220 °C | Carbon steel, shielded tube inlets |
| SCAH modules | All classes | 6–16 barg steam | Steam ≤ 204 °C sat. | SA 192 / SA 106 Gr B headers |
Casings, tubesheets and expansion elements are designed for the draught system's transient pressures; SCAH coils are pressure parts and are designed, welded and tested to ASME Section I / ASME IX or EN 12952 practice, with JIS practice applied on Japanese-code plants. Reference classes for this duty range include a 170 t/h bagasse-fired sugar-mill boiler in Thailand, a 250 t/h biomass power unit in Thailand and a 230 t/h sugar-mill unit in Colombia — see the sugar and bagasse industry page for how the APH interacts with mill steam balance.
Arrow Energy Co., Ltd. fabricates tubular APH blocks, replacement tube bundles and SCAH modules at its Samut Sakhon factory. ISO 9001:2015 certification (TÜV Rheinland) covers spare parts, installation and maintenance services; pressure-part fabrication is to ASME/JIS practice with code-stamp scope stated per project. Retrofit scope typically pairs the APH with the economizer so the two surfaces split the available gas heat correctly: the economizer takes gas down toward the APH inlet temperature that keeps the APH cold end above dewpoint, instead of the two being sized in isolation and fighting over the same 40 °C.
FAQ
Engineering questions, answered
What is an APH in a boiler?
The air preheater (APH) is the last heat-recovery surface in the flue-gas path. It heats combustion air with flue gas leaving the economizer, recovering about 1 percentage point of boiler efficiency per ~20 °C of stack drop, and supplies the 150–180 °C+ hot air that high-moisture fuels like bagasse need for stable combustion.
Tubular or regenerative air preheater — which is better?
Tubular APHs have zero air-to-gas leakage and suit dusty, erosive biomass gas; they are larger per unit duty. Regenerative (rotary) types are compact for large duties but leak 6–12 % of gas flow across their seals, which the ID and FD fans must carry as extra flow and power.
Why do air preheater tubes corrode at the cold end?
Cold-end metal temperature is a film-coefficient-weighted mean of gas and air temperatures, not the arithmetic average — the air-side coefficient drags metal toward the cold air temperature. When metal falls below the 115–140 °C acid dewpoint of sulphur-bearing flue gas, sulphuric acid condenses and wastes the tubes.
What is a steam coil air preheater (SCAH) for?
A SCAH preheats cold combustion air with steam before it enters the main air preheater, lifting cold-end metal temperature above the acid dewpoint during start-up, low load and cold weather. Poorly drained SCAH coils trap condensate and fail by water hammer; Arrow's design uses straight tubes with a continuous 1:50 fall to a dedicated drain.
How hot must combustion air be for bagasse firing?
Bagasse fired at around 50 % moisture (LHV 7.2–7.5 MJ/kg) needs hot air of 150–180 °C or more to dry the fuel in suspension and hold stable ignition. Below that, combustion shifts down the grate, unburnt carbon rises, and the boiler becomes sensitive to fuel-moisture swings.
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