Boiler Heat Balance and Efficiency: Direct and Indirect Methods
Every kilocalorie a boiler burns goes either into the steam or out as a loss. Accounting for both is the heat balance, and efficiency is the share that reaches the steam. It can be measured from the output side or from the loss side, and each method catches what the other misses.
You'll learn
- List where a boiler’s heat goes
- Calculate efficiency by the direct method
- Explain the indirect method and why it is more useful for improvement
- Read a published heat balance
Before this: Enthalpy, Laws of thermodynamics and entropy
Where the heat goes
Burn a kilogram of fuel and its heat leaves the boiler by several routes. Most of it should go into the water and steam. The rest is lost: carried up the chimney in hot flue gas, used to evaporate moisture and hydrogen-formed water from the fuel, left as unburnt fuel in the ash, lost as carbon monoxide, and radiated from the hot casing.
Efficiency is the share that reaches the steam. A boiler at 80% efficiency loses one fifth of everything it burns. Because fuel is most of the cost of steam, a few points of efficiency are worth more over a boiler’s life than almost any difference in purchase price.
The direct method
| Symbol | Meaning | Unit |
|---|---|---|
| Steam generated | kg/h | |
| Heat added per kilogram (convert to kcal if GCV is in kcal) | kJ/kg | |
| Fuel burned | kg/h | |
| GCV | Gross calorific value of the fuel | kcal/kg |
- 2,000 kg/h of dry saturated steam at 10 kg/cm²(g), feedwater at 80 °C.
- 400 kg/h of coal with a GCV of 4,000 kcal/kg (an indicative figure; use the lab analysis of your own coal).
- Heat to steam: 2,000 × (2,780.1 − 335.8) ÷ 4.1868 = 11,67,618 kcal/h.
- Heat in fuel: 400 × 4,000 = 16,00,000 kcal/h.
- Efficiency:
The indirect method
Here each loss is calculated from flue gas analysis, stack temperature, fuel analysis and ash tests, then subtracted from 100. It needs more measurements, but it says why efficiency is what it is, which the direct method cannot.
| Loss | % of fuel heat |
|---|---|
| Dry flue gas | 7.88 |
| Moisture in fuel | 5.91 |
| Hydrogen in fuel | 3.44 |
| Carbon lost as CO | 2.58 |
| Unburnt carbon in bottom ash | 1.77 |
| Other small losses | 0.65 |
| Efficiency | 77.77 |
Bureau of Energy Efficiency, Guide Book 4, ch. 1, §1.8. Losses total 22.23%.
Reading a heat balance like an auditor
Dry flue gas loss is the largest single loss in both of BEE’s examples, and it is the one an operator controls most directly: it rises with stack temperature and with excess air. Moisture and hydrogen losses are set by the fuel, which is why wet biomass and coal with high inherent moisture can never reach the efficiency of oil or gas, however well the boiler is run.
The basis matters. Indian practice states efficiency on gross calorific value; much European data is on net calorific value, which excludes the latent heat of the water formed from the fuel’s hydrogen. The same boiler reads several points higher on an NCV basis. Two efficiency figures can only be compared on the same basis.
Key takeaways
- Efficiency is the share of fuel heat that reaches the steam; the rest is losses.
- Direct method: output ÷ input. Quick, but sensitive to steam metering.
- Indirect method: 100 − losses. Slower, but shows where to improve.
- Dry flue gas loss is usually largest; compare efficiencies only on the same GCV/NCV basis.