Direct Method
The direct method computes efficiency as the ratio of heat usefully absorbed by steam to the total heat supplied by the fuel:
η = [ms × (hs − hf)] / [mf × GCV] × 100
Key inputs you need:
- ms — steam generation rate (kg/hr), from a flow meter
- hs — enthalpy of steam at your operating pressure and temperature (from steam tables)
- hf — enthalpy of feed water entering the boiler (from steam tables at feed water temperature)
- mf — fuel consumption rate (kg/hr)
- GCV — Gross Calorific Value of the fuel (coal ≈ 25,000 kJ/kg, heavy fuel oil ≈ 44,000 kJ/kg, natural gas ≈ 55,000 kJ/kg)
Derived outputs explained:
- Equivalent evaporation — kg of water evaporated from and at 100°C per kg of fuel, normalized using the latent heat of 2257 kJ/kg
- Factor of evaporation — how much harder the boiler works compared to standard conditions (value > 1 means superheated or high-pressure steam)
- Steam-to-fuel ratio — a quick operational metric for day-to-day monitoring
Limitations of the direct method: It doesn’t identify where losses occur. For detailed loss analysis (flue gas losses, radiation, blowdown, etc.), use the indirect method (heat loss method) as a complement.
Indirect Method
The indirect method (also called the heat loss method) computes efficiency by identifying and summing every individual loss, then subtracting from 100%. It is more accurate and diagnostically richer than the direct method because it pinpoints exactly where energy is being wasted.
The seven losses calculated:
L1 — Dry flue gas loss — usually the largest loss (5–15%). Driven by excess air and flue gas exit temperature. Reducing excess air and improving heat recovery directly cuts this.
L2 — Hydrogen in fuel loss — hydrogen in the fuel combusts to form steam, which carries latent heat out with the flue gas. Higher in oil and gas than coal.
L3 — Moisture in fuel loss — moisture evaporates and exits as superheated steam, wasting energy. High in biomass fuels.
L4 — Moisture in combustion air loss — humid inlet air contributes a small steam loss. Significant in tropical climates.
L5 — Incomplete combustion (CO) loss — CO in flue gas means unburned carbon is leaving. Even 0.5% CO represents a meaningful loss. Caused by poor air-fuel mixing or insufficient excess air.
L6 — Unburnt in ash — carbon particles carried out with fly ash or settled as bottom ash. Mainly relevant for coal-fired boilers.
L7 — Radiation, blowdown & miscellaneous — surface radiation and convection from the boiler shell, plus blowdown heat losses.
How to use it:
- Enter flue gas analyser readings (CO₂%, O₂%, CO%) and temperatures on the Flue gas tab
- Fill in fuel properties on the Fuel & moisture tab — use the fuel preset dropdown to auto-fill typical values
- Adjust additional losses on the Other losses tab
- Click Calculate — the Results tab shows the loss waterfall chart and full breakdown
