Why the stack is expensive
A boiler discharging flue gas far above the temperature needed for draught is venting fuel you already paid for. An economizer puts a heat exchanger in that stream and moves the heat into feedwater or a secondary loop, so the boiler needs less fuel for the same output.
The trade-off most projects get wrong
Recovering more heat means a lower gas outlet temperature. But sulphur-bearing fuels produce an acid dew point: below it, condensate forms on the cold end and corrodes tubes rapidly. The correct design target is not “lowest possible stack temperature” but the lowest temperature that stays safely above that limit for the fuel you actually burn.
What to check before a retrofit
- Fuel type and sulphur content — sets the dew point and material choice.
- Feedwater temperature — cold feedwater recovers more heat but increases cold-end risk.
- Flue gas flow and temperature at normal load, not just at peak.
- Allowable gas-side pressure drop and existing fan margin.
- Space and access near the stack for installation and cleaning.
Materials and construction
Seamless steel tube and finned tube constructions are common where gas temperature and pressure are higher. Tube and fin geometry is selected against dust load: too tight a fin pitch on a dirty stream leads to rapid fouling and rising pressure drop.
Estimating the benefit
Recovered duty is flue gas mass flow × heat capacity × usable temperature drop. Multiply by annual operating hours and fuel cost, then divide by boiler efficiency. Subtract cleaning and fan-power costs to get a realistic payback.
FAQ
Q: Will an economizer reduce boiler output? No — it recovers heat that was leaving the stack. It must, however, be sized within the available draught and pressure drop.
Q: How often does it need cleaning? It depends on fuel and dust load. Specify soot blowing or accessible cleaning at the design stage.