Boiler Economisers: Recovering Flue-Gas Heat for Efficiency is the study of the heat exchanger fitted in a boiler's exhaust path that captures waste heat from hot flue gas and uses it to preheat incoming feedwater. By raising feedwater temperature before it reaches the boiler drum, the economiser reduces the fuel needed to bring that water to saturation, lifting overall thermal efficiency by several percentage points on a typical industrial or utility boiler.
Flue gas leaving the boiler's main convective banks still carries substantial sensible heat, often 200 to 350 degrees Celsius. The economiser is a finned or bare-tube bundle placed in this gas stream. Feedwater flows through the tubes while flue gas passes across them, transferring heat counterflow so the coldest water meets the coolest gas. This recovered energy would otherwise leave through the stack as a loss. Because the economiser sits at the cold end of the gas path, it operates at the lowest metal temperatures in the boiler, which is exactly why corrosion control governs its design.
A widely used field approximation states that boiler efficiency improves by roughly one per cent for every 20 degrees Celsius of flue-gas temperature reduction. On a boiler exhausting at 300 degrees Celsius, dropping stack temperature by 100 degrees can therefore return around five per cent in fuel savings. The relationship is not perfectly linear, but it is accurate enough for scoping a retrofit or judging whether a fouled economiser is worth cleaning.
| Flue-gas temperature reduction (deg C) | Approximate efficiency gain (%) |
|---|---|
| 20 | 1.0 |
| 40 | 2.0 |
| 60 | 3.0 |
| 100 | 5.0 |
The hard limit on how far flue-gas temperature can fall is the acid dew point. When fuels contain sulphur, combustion produces sulphur dioxide, a fraction of which oxidises to sulphur trioxide and combines with water vapour to form gaseous sulphuric acid. If any metal surface falls below the acid dew point, that acid condenses and attacks the steel. This is cold-end corrosion. The dew point rises steeply with fuel sulphur content, so heavy fuel oil forces much higher minimum metal temperatures than clean natural gas.
| Fuel | Typical sulphur | Approximate acid dew point (deg C) |
|---|---|---|
| Natural gas | Negligible | Below 100 (water dew point governs) |
| Light fuel oil | Low | 120 to 130 |
| Heavy fuel oil | High | 140 to 160 |
| Coal | Variable | 120 to 150 |
A non-condensing economiser is sized to keep every metal surface safely above the acid dew point, so it recovers only sensible heat and leaves the water vapour in the flue gas untouched. A condensing economiser deliberately drives flue-gas temperature below the water dew point to release the latent heat of vaporisation, which yields a further efficiency step but demands corrosion-resistant construction such as stainless steel or coated tubes and a drain for the acidic condensate. Condensing designs are most practical on low-sulphur fuels, above all natural gas, where the acid dew point is low and the recovered latent heat is large.
Two deposit problems dominate economiser maintenance:
Soot blowers, periodic offline gas-side cleaning and disciplined water treatment are the routine defences. Sootblowing also has to be balanced against acidic condensate washing across cold metal.
Economiser reliability is inseparable from feedwater chemistry. Dissolved oxygen accelerates cold-end corrosion, so proper deaeration is essential, as detailed in our guide to the deaerator and boiler feedwater. Hardness control keeps water-side scale from forming, and controlled boiler blowdown holds dissolved solids within limits so they do not deposit on economiser surfaces. Treat the economiser, the deaerator and the blowdown regime as one chemistry loop rather than isolated components.
Economiser condition is best tracked through trended data: stack temperature, gas-side pressure drop and feedwater outlet temperature together reveal fouling or scaling long before efficiency losses become obvious. Teams that log these readings against inspection findings in a CMMS such as Fabrico can schedule cleaning on evidence rather than on a fixed calendar, and can flag any drift toward the acid dew point before corrosion starts. Book a Fabrico demo to see how condition trends and work orders tie together.
It depends on how much heat is available in the flue gas and how far temperature can safely be lowered. Using the rule of about one per cent per 20 degrees Celsius of flue-gas temperature reduction, a well-applied economiser commonly returns four to seven per cent, with condensing designs on natural gas recovering more through latent heat.
Because the acid dew point sets a floor. With sulphur-bearing fuels, cooling any metal surface below that point condenses sulphuric acid and causes cold-end corrosion. Only condensing economisers built from corrosion-resistant materials, usually on low-sulphur fuels, can safely go below it.
Gas-side fouling from soot and ash, water-side scale from hardness in the feedwater, and cold-end corrosion when metal falls below the acid dew point. Sootblowing, offline cleaning and good feedwater treatment address all three.
Yes, and often more than on high-sulphur fuels. Natural gas has negligible sulphur, so the acid dew point is low and a condensing economiser can recover latent heat as well as sensible heat, giving a larger overall efficiency gain.
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