Retrofitting a boiler economiser
A boiler economiser retrofit fits a heat exchanger in the flue-gas path to recover heat from the exhaust and use it to preheat boiler feedwater. Because every degree of feedwater preheat is fuel the burner no longer has to supply, an economiser is one of the most reliable efficiency improvements available to a fired boiler.
What it is
Flue gas leaves a boiler still hot, carrying away a large share of the fuel's energy. An economiser is a finned-tube heat exchanger placed in that flue path through which feedwater passes, capturing heat that would otherwise go up the stack and returning it to the boiler as warmer feedwater. The retrofit adds this exchanger, its water connections and the necessary controls to an existing boiler.
Why it is done
Stack loss is typically the single largest efficiency loss on a fired boiler, and feedwater preheat attacks it directly. Recovered flue heat displaces fuel one-for-one, so the payback is driven by boiler running hours and fuel cost. The limit is the acid dew point of the flue gas: cool it too far and corrosive condensation forms, so the design balances recovery against metal temperature.
How it is done
Flue-gas temperature, flow and fuel sulphur content are characterised, and feedwater flow and temperature are checked to confirm a useful heat sink exists. An economiser is sized to recover heat down to a safe margin above the acid dew point, and integrated into the flue duct and feedwater line with bypass and controls. After installation, stack temperature and feedwater temperature are measured to confirm recovery, and the boiler efficiency baseline is updated.
- Characterise flue gas
- Confirm feedwater sink
- Size above dew point
- Integrate ducting & controls
- Commission & measure
- Update efficiency baseline
What to watch for
Cooling flue gas below its acid dew point to chase a little more heat corrodes the economiser and downstream ductwork. Fitting an economiser to a boiler that already returns very hot feedwater leaves little temperature gap to exploit, so the recovery disappoints.
Related practices
Changing over a boiler feedwater treatment programme
Adopting condensate recovery
Establishing a steam-trap survey programme
Related topics
How to Improve Boiler Efficiency: A Practical Guide · Economiser · Flue Gas Loss · Boiler Efficiency
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