≈ 20 °C ≈ 1 pp
Stack drop per efficiency point
Boiler pressure parts · Heat recovery
Economizer in a boiler: the last heat-recovery surface before the stack
An economizer is a tube bank in the boiler back-pass that transfers residual flue-gas heat to the incoming feedwater. Every ~20 °C of stack-temperature reduction returns roughly 1 percentage point of boiler efficiency; a typical retrofit on a biomass unit recovers 3.1–4.0 pp. Arrow Energy Co., Ltd. fabricates economizer coils and banks to ASME I / EN 12952 practice.
+3.1–4.0 pp
Typical retrofit gain (biomass)
≥ 25–30 °C
Approach to saturation (steaming margin)
≤ 10–12 m/s
Gas velocity limit, bagasse fly ash
01 — Function and position
What the economizer does, and where it sits in the gas train
The cheapest megawatt in the plant is the one already paid for and leaving through the stack.
The economizer is the last water-cooled heat-transfer surface in the boiler back-pass. Flue gas that has finished its work in the furnace, superheater and generating bank still leaves at 180–250 °C on many biomass and sugar-mill units; the economizer intercepts that gas and transfers its heat to the incoming feedwater before the gas passes to the air preheater and dust collection train. Because the heat goes straight into feedwater that the boiler would otherwise have to heat with fresh fuel, the recovery converts almost one-for-one into fired-fuel savings: every ~20 °C of stack-temperature reduction returns roughly 1 percentage point of boiler efficiency.
Position matters for the whole gas train, which is why the economizer is specified as part of the boiler back-end, not as an isolated accessory — see the full scope of boiler pressure parts and heat-recovery equipment. Upstream sits the generating bank or superheater outlet, fixing the gas inlet temperature. Downstream, the economizer outlet temperature sets the operating point of the air preheater, the ESP or bag filter (which have their own temperature windows), and the ID fan, whose volumetric flow — and therefore absorbed power — falls as the gas is cooled. Cooling gas from 203 °C to 160 °C reduces actual volumetric flow by about 9 % ((203+273)/(160+273) = 1.099), which the ID fan sees directly as reduced draught demand.
02 — The savings arithmetic
How much a retrofit is worth, shown line by line
Not asserted — calculated, on a stated basis.
DESIGN BASIS — ILLUSTRATIVE CALCULATION, NOT A GUARANTEE · all concentrations @ 6 % O₂ dry
Take the recurring worked basis used across this site: a 60 t/h bagasse-fired boiler, flue gas ~117,000 Nm³/h (≈ 32.4 Nm³/s — about 51 m³/s actual at the collector's 160 °C design point, matching the design-basis ESP's 3,888 m² ÷ 75.7 s/m), stack temperature 203 °C before retrofit. An economizer sized to drop the stack to 160 °C removes:
- Gas-side heat recovered
- 32.4 Nm³/s × 1.38 kJ/Nm³·K × 43 K ≈ 1,920 kW ≈ 1.9 MW (≈ 6.9 GJ/h)
- Feedwater pickup
- 1,920 kW ÷ (16.7 kg/s × 4.35 kJ/kg·K) ≈ 26 °C — feedwater rises from 105 °C to ~132 °C
- Efficiency gain
- 43 °C ÷ ~20 °C per pp ≈ +2.1–2.2 pp (e.g. 61 % → ~63 % LHV basis)
- Fuel saved
- recovered heat displaces fired fuel at the boiler efficiency: 1,920 kW ÷ (0.61 × 7,350 kJ/kg) ≈ 0.43 kg/s ≈ 1.5 t/h bagasse
- Assumptions
- boiler efficiency ~61 % LHV basis; bagasse LHV ~7,350 kJ/kg (≈ 50 % moisture)
- Seasonal quantity
- ~1.5 t/h × 4,800 h crushing season ≈ 7,400 t bagasse per season
That ~7,400 t is bagasse freed for co-generation export or off-season firing; its monetary value depends on the mill's marginal use of fuel — CONFIRM: bagasse or steam valuation basis (THB/t or THB/t steam) to complete the payback line. A deeper retrofit toward the acid-dewpoint margin, typical of the units Arrow audits, recovers 3.1–4.0 pp. The energy audit service measures the actual stack temperature, excess air and unburnt loss before any surface is proposed, so the economizer is sized against measured duty rather than nameplate.
03 — Configuration selection
Bare-tube, finned or condensing — chosen by the ash, not the brochure
Fin density is an ash-handling decision before it is a heat-transfer decision.
Three configurations cover practical duty. Bare-tube banks are the default on high-ash biomass gas: lowest fouling propensity, fully cleanable by soot blowers, and tolerant of the abrasive, high-silica fly ash typical of bagasse and rice husk. Finned (extended-surface) banks pack 2–4 times the surface into the same casing, but on dusty gas the fin gap governs: 2–4 fins per inch is the workable ceiling for biomass ash, whereas clean gas (natural-gas exhaust, downstream of a high-efficiency collector) accepts 6+ fins per inch. Exceed that on bagasse gas and the bank bridges with ash within weeks, gas-side pressure drop climbs, and the recovered heat is lost to plugging. Condensing economizers deliberately cool the gas below the water dewpoint to capture latent heat; they demand corrosion-resistant surfaces (stainless or coated) and a low-temperature heat sink, and on sulphur-bearing fuels they must be designed for acid condensate handling — a special case, not a default.
Tube arrangement follows the same logic. Inline (aligned) tube layouts are preferred on dusty gas because the straight gas lanes stay cleanable by soot-blower jets and show markedly lower erosion peaks; staggered layouts give 10–20 % better heat-transfer coefficients but trap ash between rows and concentrate local velocity — acceptable only on clean gas. Coil-type (horizontal serpentine) elements suit compact retrofits into an existing second pass; loop-type (vertical hairpin) banks with headers outside the gas path simplify tube replacement on erosive fuels.
04 — Design limits
How close to saturation can an economizer run without steaming?
Keep the water outlet at least 25–30 °C below saturation temperature at drum pressure, checked at the worst case: lowest feedwater flow and highest gas inlet temperature. In the worked basis, feedwater leaves at ~132 °C against 257 °C saturation at 45 barg — a 125 °C approach, comfortably non-steaming.
Steaming — local boiling inside economizer tubes — causes water hammer, flow instability between parallel circuits, and tube-to-header weld fatigue. The 25–30 °C approach margin must survive turndown: at 50 % boiler load, feedwater flow halves while gas temperature falls much less, so the approach shrinks. A recirculation line or a bypass on the gas side protects the bank during start-up, when feedwater flow can be near zero while gas is already hot.
The cold end has the opposite constraint. Feedwater inlet temperature sets the coldest metal in the bank; if it sits below the flue-gas acid dewpoint — 115–140 °C for sulphur-bearing fuels, lower for low-sulphur biomass — sulphuric-acid condensate corrodes the inlet rows from the gas side. This is why deaerator operating temperature and any feedwater heating upstream of the economizer are part of the economizer specification, not separate decisions: raising feedwater inlet from 85 °C to 105 °C can be the difference between a 15-year bank and a 4-year one on the same fuel.
05 — Erosion
What gas velocity is safe on high-ash biomass fuels?
Keep design gas velocity through the bank at or below 10–12 m/s on bagasse fly ash, and lower still on rice husk, whose ash is 85–90 % amorphous silica and markedly more abrasive. Erosion rate scales with roughly the cube of local velocity, so a 20 % velocity overshoot near a wall gap can double metal loss.
Because wastage is driven by local, not average, velocity, the design work is in the details: uniform gas distribution at the bank inlet, sealed side-wall and casing gaps that would otherwise become high-velocity bypass lanes, and sacrificial protection where impingement is unavoidable. Arrow fits bolted or clamped tube shields (wear cassettes) on the leading rows and at soot-blower lanes, so wear is taken by replaceable hardware rather than pressure-part wall. Wall-thickness surveys at each outage — ultrasonic mapping of the leading two rows and shield zones — turn erosion from a failure mode into a maintenance line item.
06 — Materials and code
Materials, design code and typical duty envelope
Carbon steel does the work; the code and the corrosion assessment set the wall.
Economizer metal runs close to water temperature, so plain carbon steel covers nearly every duty: seamless tube to SA 210 Gr A1 or SA 192, headers to SA 106 Gr B or equivalent EN grades. Pressure design, weld qualification and hydrotest follow ASME Section I (with ASME IX welding qualification) or EN 12952-3; JIS practice is applied where the plant's existing boiler is Japanese-code. Code-stamp scope is stated per project.
| Boiler capacity | Design pressure | Gas inlet temp. | Tube material | Configuration |
|---|---|---|---|---|
| 30–80 t/h (sugar / biomass) | 25–68 barg | 250–380 °C | SA 210 Gr A1 / SA 192 | Bare inline, loop type |
| 80–170 t/h (bagasse co-gen) | 45–87 barg | 300–420 °C | SA 210 Gr A1 | Bare inline, shielded leading rows |
| 170–250 t/h (biomass power) | 65–110 barg | 320–450 °C | SA 210 Gr A1 / SA 210 Gr C | Bare or low-fin (2–4 fins/inch) |
| Clean-gas retrofit (post-collector) | 10–45 barg | 180–300 °C | SA 192, finned | Finned 6+ fins/inch, staggered permissible |
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.
07 — Scope
What Arrow supplies
Coils, banks and complete retrofits, fabricated in-house.
Arrow Energy Co., Ltd. designs and fabricates economizer coils, elements and complete banks with headers at its factory in Samut Sakhon, Thailand, to ASME/JIS fabrication practice with welder and procedure qualification to ASME IX. Scope runs from replacement coils matched to an existing bank drawing, through re-rated banks sized from a measured energy audit, to full back-pass retrofits coordinated with the air preheater and soot-blower layout so cleaning coverage and gas-side pressure drop are engineered together, not discovered at commissioning. ISO 9001:2015 certification (TÜV Rheinland) covers Arrow's spare parts, installation and maintenance services; pressure-part fabrication is to ASME/JIS practice with code-stamp scope stated per project. Guaranteed performance figures are stated per project after the technical assessment, on a stated basis (mg/Nm³, reference O₂, dry/wet, load range).
FAQ
Engineering questions, answered
What does an economizer do in a boiler?
It preheats boiler feedwater using flue gas leaving the last convective surface, dropping stack temperature before the air heater or dust collector. Each ~20 °C of stack reduction recovers about 1 percentage point of boiler efficiency, so a 40 °C drop is worth roughly 2 pp of fuel.
How much fuel does an economizer save?
On a 60 t/h bagasse boiler with ~117,000 Nm³/h of flue gas, cooling the gas from 203 °C to 160 °C recovers about 1.9 MW. At ~61 % boiler efficiency (LHV basis) and bagasse LHV ~7,350 kJ/kg, that displaces roughly 1.5 t/h of bagasse — about 7,400 t over a 4,800 h season. Figures are design-basis, not guarantees.
What is steaming in an economizer and how is it prevented?
Steaming occurs when feedwater leaving the economizer reaches saturation temperature, causing water hammer and tube vibration. Design keeps an approach margin of at least 25–30 °C below saturation at the lowest feedwater flow and highest gas-side load, verified across the full boiler load range.
Which tube materials are used for boiler economizers?
Carbon-steel tubes to SA 210 Gr A1 or SA 192 cover almost all economizer duties, since water-side metal temperature stays near feedwater temperature, typically below 250 °C. Design and construction follow ASME Section I or EN 12952-3; corrosion allowance and minimum wall are set from the acid-dewpoint and erosion assessment.
Can finned tubes be used on biomass or bagasse flue gas?
Yes, but fin density must be limited to about 2–4 fins per inch so abrasive, high-silica ash does not bridge and plug the gaps. Clean gases such as natural-gas exhaust accept 6 or more fins per inch. On heavy-ash fuels, bare tube banks with soot blowers remain the conservative choice.
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