- The thermal efficiency of a steam generator is defined as the ratio of the heat transferred
to the water (steam) to the heat input with the fuel. One of the goals associated with
the operation, maintenance, and control of a boiler is to maximize its thermal efficiency.
- The boiler efficiency is influenced by many factors. A fully loaded large boiler that is
clean and properly tuned is expected to have the following efficiencies:
On coal: 88%, with 4% excess oxygen; 89%, with 3% excess oxygen. On oil: 87%, with 3% excess oxygen; 87.5%, with 2% excess oxygen. On gas: 82%, with 1.5% excess oxygen; 82.5%, with 1% excess oxygen.
Efficiencies will also vary as a function of excess air, flue-gas temperature, and
- Efficiency can be computed by the direct or the indirect method.
- The direct method uses the ratio of the rate of heat transferred to the water (outlet
steam specific enthalpy × steam mass flow–feedwater specific enthalpy ×
feedwater mass flow) to the rate of heat input by the fuel (higher heating value × fuel
mass feed rate).
- The indirect method uses fuel, ash, and stack gas analysis to do a per-unit-basis
accounting of all heat losses, subtracting all losses from the higher heating value
of the fuel and dividing the result by the higher heating value.
- The indirect method is more accurate, because it does not rely on the relatively
inaccurate steam and fuel flow measurements.
- The major losses considered by the boiler indirect efficiency calculation equations are:
(i) Dry gas loss: sensible heat carried out of the stack with the combustion air and combustion
(ii) Moisture loss: loss due to vaporizing the moisture in the fuel and the moisture produced from combustion of the hydrogen in the fuel.
(iii) Incomplete combustion loss: loss due to combustion of carbon that results in carbon monoxide (CO), instead of the complete combustion product, carbon dioxide (CO2).
(iv) Unburned carbon loss: loss due to carbon that does not get combusted and ends up in the
(v) Moisture in the combustion air loss: loss due to heating up water vapor contained in the
(vi) Radiation loss: heat lost from the external furnace walls to the surrounding air and other