Module 12 · Efficiency and Audits3 min read
Economizer and Air Preheater Payback
Flue gas leaving a boiler is still hot. An economizer puts some of that heat into the feedwater; an air preheater puts it into the combustion air. Either way, fuel falls — and the saving is calculated, not guessed.
You'll learn
- Calculate the fuel saved by heating feedwater
- Calculate simple payback
- Know the limit set by acid dew point
- Choose between economizer and air preheater
Before this: Economizers and air preheaters, Cost of Steam
BeginnerThe idea in plain words, and where you meet it. No maths.
Where the saving comes from
Every degree the feedwater gains in the economizer is a degree the furnace does not have to supply. Every degree the stack gas loses is heat that stays in the boiler. The two are the same heat, seen from either side.
Simple payback
| Symbol | Meaning | Unit |
|---|---|---|
| Equipment, erection, ducting and controls | ₹ | |
| Fuel saved × price × operating hours | ₹/year |
IntermediateThe formula, its units, and a worked example on real plant numbers.
A worked example
Worked example
An economizer on a 5 TPH coal-fired boiler
Given
- 5,000 kg/h at 10.54 kg/cm²(g); economizer raises feedwater from 80 to 120 °C; coal 4,000 kcal/kg, 82% efficiency, ₹6.5/kg (indicative); 7,200 h/year.
- Heat recovered: 5,000 × (504.4 − 335.8) = 8,43,143 kJ/h.
- Coal before: 890.6 kg/h; after: 829.2 kg/h — 61.4 kg/h saved, 6.9%.
- Per year: 61.4 × 7,200 × ₹6.5 = ₹28,73,365.
Answer: About ₹28,73,365 a year. Divide the installed cost by this figure for simple payback.
As a cross-check: 40 °C of feedwater rise saved 6.9% of the fuel — close to the familiar rule of about 1% for every 6 °C.
ExpertThe fine print: where the simple model breaks, and what it costs.
How far can recovery go?
- Acid dew point: sulfur in the fuel forms acid that condenses if gas or metal falls below its dew point. Keep the economizer’s coldest metal — the feedwater inlet — above it, especially on coal and furnace oil.
- Feedwater: the economizer outlet must stay below saturation at boiler pressure with a margin, or it will steam and hammer at low load.
- Fouling: ash-laden gas needs tube spacing and soot blowing that keep the surface clean; a fouled economizer saves little.
- Economizer or air preheater: an economizer is the usual first step on shell boilers; an air preheater suits solid-fuel firing where hot air helps dry and burn the fuel, and larger water-tube boilers often have both.
Key takeaways
- Saving = feedwater enthalpy gained × steam flow ÷ (efficiency × GCV).
- Rule of thumb: about 1% fuel per 6 °C of feedwater rise.
- Payback = installed cost ÷ annual saving, at real load and hours.
- Acid dew point and feedwater steaming set the limits.