Economizers and Air Preheaters: Recovering Heat From Flue Gas
Flue gas leaving the boiler is still hot, and that heat is the largest loss in most boilers. An economizer uses it to warm the feedwater; an air preheater uses it to warm the combustion air. Both put heat back into the boiler that would otherwise go up the chimney.
You'll learn
- Explain where economizers and air preheaters sit and what they heat
- Calculate the heat an economizer recovers and the fuel it saves
- Use the sourced rules of thumb for flue gas, feedwater and air temperatures
- Know the corrosion limit on how far flue gas can be cooled
Before this: Heat balance and boiler efficiency, Sensible heat
Catching the heat before it leaves
Gas leaves the last boiler tubes well above the steam temperature, because heat only flows from hot to cold and the water inside is at saturation temperature. An economizer is a bank of finned or plain tubes in the flue duct after the boiler, through which the feedwater passes on its way in. The gas warms the water, the water enters the boiler hotter, and the boiler burns less fuel to make the same steam.
An air preheater does the same job for the combustion air: it takes heat from the outgoing gas and gives it to the incoming air. Hotter air also helps difficult fuels such as wet biomass and low-grade coal ignite and burn out.
How much an economizer saves
- 2,000 kg/h of feedwater at 10 kg/cm²(g), warmed from 80 °C to 110 °C.
- Water enthalpy rises from 335.8 to 462.0 kJ/kg.
- Heat recovered:
- As a share of the heat needed per kilogram of steam:
| Change | Worth about 1% | Source |
|---|---|---|
| Cooler flue gas | 22 °C | UNEP; US DOE says 40 °F (22 °C) |
| Hotter feedwater | 6 °C | UNEP |
| Hotter combustion air | 20 °C | UNEP |
UNEP, Energy Efficiency Guide for Industry in Asia (Boilers, §6.3); US DOE Steam Tip Sheet #3.
The rules agree with the arithmetic above. A 5.2% saving implies the flue gas leaves roughly 114 °C cooler, which is the kind of drop a well-sized economizer delivers.
The acid dew point, and what limits recovery
Flue gas from fuels containing sulfur carries sulfur trioxide, which combines with water vapor to form sulfuric acid at a temperature well above the ordinary water dew point. If any metal surface in the gas path is colder than that acid dew point, acid condenses on it and corrodes it quickly. In an economizer the coldest metal is at the feedwater inlet, so it is the feedwater inlet temperature, not the gas temperature, that has to be kept above the limit.
That is why economizers on oil- and coal-fired boilers are designed with a minimum feedwater inlet temperature and a minimum gas exit temperature, and why a deaerator upstream helps twice: it removes oxygen and it raises the feedwater temperature. Clean fuels like natural gas allow much colder exit gas, and condensing economizers deliberately cool below the water dew point to recover latent heat too, with corrosion-resistant materials.
Two other limits are mechanical. An economizer is a pressure part on the feedwater line, so it must handle full feed pump pressure and must not steam: if water boils inside it at low load, it can hammer and lose flow. And on solid fuels, ash fouls economizer and air preheater surfaces, so soot blowing and access for cleaning decide how much of the design saving is kept in service.
Key takeaways
- Economizers heat feedwater, air preheaters heat combustion air, both from outgoing flue gas.
- Raising feedwater from 80 to 110 °C at 10 kg/cm²(g) saves about 5% of fuel.
- Published rules: 22 °C cooler flue gas, 6 °C hotter feedwater or 20 °C hotter air ≈ 1% efficiency.
- Keep metal above the acid dew point on sulfur-bearing fuels; on economizers that means feedwater inlet temperature.