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An enamel coated tube boiler air preheater (also written enamelled tube, enamel-lined tube or vitreous enamel tube air preheater) is a tubular, recuperative air preheater whose heat-exchange tubes are low-carbon steel coated inside and out with a dense, acid-resistant glass enamel fused on at high temperature. Hot boiler flue gas passes through the tubes; cold combustion air sweeps across the outside; heat moves through the enamelled tube wall and preheats the air. The enamel layer exists for one reason: to stop sulphuric acid, which condenses from the flue gas wherever the tube wall falls below the acid dew point, from corroding the steel. CSTHEATEXCHANGER manufactures enamel tubular air preheaters for coal-fired, biomass-fired, waste-incineration and circulating-fluidised-bed boilers, with double-sided enamel 0.3–0.5 mm thick, 100 % spark-tested tubes, and vertical or horizontal bundle layouts built to the boiler's flue-gas data and ductwork drawing.
Term | Meaning |
|---|---|
Air preheater (APH) | A heat exchanger at the boiler tail end that uses flue-gas waste heat to warm combustion air before it enters the furnace |
Tubular / recuperative | Flue gas and air are separated by a fixed tube wall; they never mix (as opposed to a rotary regenerative APH) |
Enamel / vitreous enamel / enamelled / enamel-lined / enamel coated | Interchangeable terms for a glass coating fused onto steel at high temperature — a continuous, non-porous, acid-resistant layer bonded to the metal |
Enamel coated air preheater tube | A 20# (low-carbon) seamless steel tube with sintered acid-resistant enamel on both the inner and outer surface, 0.3–0.5 mm thick, pinhole-free, used as the heat-exchange element |
So an "enamel coated tube boiler air preheater" is a conventional tubular air preheater whose tubes have been given a glass skin so they survive acid condensation.
From our product documentation, the heat-exchange flow is:
Hot flue gas (about 150–400 °C) from the boiler tail flows inside the enamelled tubes.
Cold ambient air flows across the outside of the tube bundle in cross-flow and absorbs heat through the tube wall.
Preheated air (typically raised by 100–260 °C, delivered at about 90–280 °C) goes to the burners or windbox; cooled flue gas continues to dust removal and desulphurisation.
The enamel layer isolates the steel from sulphur oxides in the flue gas, so even where the wall temperature drops below the acid dew point and sulphuric acid condenses, the acid touches glass, not steel.
The smooth enamel surface resists ash and tar adhesion, so heat transfer stays stable and soot blowing is needed less often.
The enamel adds almost nothing to the thermal resistance: our testing shows less than a 5 % drop in heat transfer coefficient compared with bare steel tubes.
Coal, biomass and waste fuels contain sulphur. Combustion produces SO₂, a fraction of which oxidises to SO₃; SO₃ combines with water vapour in the flue gas to form sulphuric acid vapour. Wherever a surface is colder than the acid dew point of that gas — which can be well above 100 °C — sulphuric acid condenses on it. The coldest surfaces in the whole boiler gas path are the air preheater tubes at the air-inlet end, because they are cooled by ambient air. Bare carbon steel there is eaten away; hence the enamel. (We cover this in depth in our acid dew point corrosion guide.)
Component | Specification |
|---|---|
Heat-exchange tube bundle | 20# seamless steel tubes, double-sided (inner and outer) high-temperature sintered acid-resistant enamel, coating 0.3–0.5 mm, pinhole-free; every tube passes a 100 % spark leakage test |
Tube sheets | Steel, drilled to the bundle pattern; sealed to the enamelled tubes |
Shell / casing | Steel frame and casing with flue-gas inlet/outlet ducts and hot-air hoods |
Expansion joints | Fitted to absorb boiler thermal expansion and contraction |
Soot-blowing and maintenance ports | Reserved on the casing |
Layouts | Vertical bundle (flue gas inside tubes, air cross-flow outside) — smaller footprint; Horizontal bundle — easier maintenance and soot cleaning on large boilers |
Modularity | Bundles are modular; single damaged tubes can be replaced without dismantling the unit |
Thermal cycling | Enamel layer fusion-bonded to the steel; resistant to frequent 0–400 °C temperature fluctuation |
Benefit | Evidence from our product data |
|---|---|
Service life | 3–5 times longer than a bare carbon-steel tubular APH under the same acid-corrosive flue gas |
Boiler efficiency | Overall thermal efficiency up 7–14 % |
Fuel saving | Coal/biomass consumption down 9–17 % |
Payback | Typically 8–16 months from fuel savings and reduced maintenance |
Fouling | Smooth enamel — little ash or tar adhesion; fewer soot-blowing cycles; lower ID/FD fan power |
Abrasion | Hard enamel resists fly-ash erosion |
Capital cost | Purchase price well below heat-pipe and all-stainless welded-plate air preheaters; no exotic alloys or vacuum sealing; mature mass production, short delivery |
Maintenance | Individual tube replacement; low labour and parts cost |
Downstream | Less heat loss from corrosion and air leakage; lower desulphurisation reagent consumption |
Our units are designed for boilers with corrosive, dusty flue gas:
Coal-fired industrial steam and hot-water boilers, 1–35 t/h
Biomass boilers — pellet, straw, wood chip (high alkali and sulphur)
Waste incineration boilers
Circulating fluidised bed (CFB) boilers
Small thermal power plants and district-heating coal boilers
Hazardous-waste incinerators
The enamel tubular air preheater is one of three gas-to-gas constructions CSTHEATEXCHANGER offers for waste-heat recovery; the others are heat-pipe units (for very dusty, high-temperature exhaust) and all-welded plate units (for clean flue gas). We recommend the enamel tubular type specifically where acid dew point corrosion is the governing problem — coal and biomass boilers above all.
A tubular air preheater whose steel tubes are glass-enamelled inside and out so they survive sulphuric acid condensation from boiler flue gas.
Yes. Vitreous enamel is the technical term for glass fused onto metal; the other words are common usage for the same coating.
Very little — less than 5 % compared with bare steel in our data — because the layer is thin (0.3–0.5 mm) and bonded directly to the tube.
Enamel is fused glass bonded to the steel; "glass-lined" tubes are related but differ in substrate and process. See our enamel-vs-glass-lined comparison.
Yes. Replacing bare carbon-steel bundles with enamelled bundles to the original tube-sheet pattern is a standard CSTHEATEXCHANGER retrofit — see our replacement guide.
Roughly 150–400 °C inlet, with the enamel resistant to frequent 0–400 °C cycling.
Any boiler whose flue gas carries sulphur and whose air preheater cold end falls below the acid dew point: coal, biomass, waste incineration, CFB.
Send CSTHEATEXCHANGER your boiler type and capacity, flue-gas flow, inlet temperature, sulphur and dust content, required hot-air temperature, available space and existing ductwork drawings. We will size the enamel tubular air preheater, choose vertical or horizontal layout, and return drawings for approval before manufacture.
Email: info@cstheatexchanger.com
International business: +86 0519 8878 2189
Website: https://www.cstheatexchanger.com/
CSTHEATEXCHANGER — enamel coated tube air preheaters that recover boiler heat without feeding the acid.
