Boiler Efficiency Testing: Direct and Indirect Methods
There are two ways to measure a boiler’s efficiency. The direct method divides heat in the steam by heat in the fuel. The indirect method adds up every loss and subtracts them from 100. The first gives a number; the second tells you where the fuel went.
You'll learn
- Run a direct-method efficiency calculation
- List the losses in an indirect-method test
- Calculate dry flue gas and moisture losses
- Know what makes a test trustworthy
Before this: Heat balance and boiler efficiency, Flue gas analysis
Two methods
| Symbol | Meaning | Unit |
|---|---|---|
| Steam generated | kg/h | |
| Enthalpy rise, steam minus feedwater | kcal/kg | |
| Fuel burnt | kg/h | |
| Gross calorific value of the fuel | kcal/kg |
The indirect method measures the losses instead — flue gas, moisture, unburnt fuel, radiation, blowdown — and computes efficiency as 100 minus their sum. It does not need the steam or fuel flow measured accurately, and it shows which loss to attack.
A direct-method test
- Over a steady test period: 5,000 kg/h of steam at 10.54 kg/cm²(g), feedwater 80 °C; 950 kg/h of coal at 4,000 kcal/kg GCV.
- From steam tables (IF97): = 2,781.8 kJ/kg, at 80 °C = 335.8 kJ/kg.
- Heat to steam: 5,000 × 2,446.0 ÷ 4.1868 = 29,21,066 kcal/h.
- Heat in fuel: 950 × 4,000 = 38,00,000 kcal/h.
The direct method is only as good as its measurements. A 3% error in the steam meter or the fuel weighing is a 3% error in efficiency — enough to hide or invent the improvement you are testing.
The indirect method, loss by loss
- Dry flue gas loss — the largest, from stack temperature and excess air.
- Loss from hydrogen in the fuel, which burns to water and leaves as vapor.
- Loss from moisture in the fuel and in the combustion air.
- Loss from carbon monoxide — incomplete combustion.
- Unburnt carbon in fly ash and bottom ash.
- Radiation and convection from the boiler casing, and blowdown.
| Symbol | Meaning | Unit |
|---|---|---|
| Dry flue gas per kg of fuel | kg/kg | |
| Specific heat of flue gas | kcal/kg·°C | |
| Stack gas minus ambient air temperature | °C |
- Stack 200 °C, ambient 30 °C; 7.5 kg dry flue gas per kg of coal, 0.23; fuel 3% hydrogen and 10% moisture (illustrative composition); GCV 4,000 kcal/kg.
- Dry flue gas loss:
- Water formed and carried per kg of fuel: 9 × 0.03 + 0.1 = 0.37 kg. Each kilogram takes 584 kcal to evaporate plus 0.45 kcal/kg·°C of superheat to stack temperature.
- Moisture and hydrogen loss:
Key takeaways
- Direct method: heat to steam ÷ heat in fuel. Simple, but needs accurate steam and fuel measurement.
- Indirect method: 100 minus the sum of losses. Shows where the fuel goes.
- Dry flue gas and moisture losses are usually the largest.
- State the basis, GCV or NCV, with every efficiency figure.