Boiler Efficiency : Thermal efficiency of boiler is defined as the percentage of heat input that is effectively utilised to generate steam. There are two methods of assessing boiler efficiency.
1) The Direct Method: Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
2) The Indirect Method: Where the efficiency is the difference between the losses and the energy input.
Boiler Efficiency by Indirect Method Indirect method is also called as heat loss method. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. The standards do not include blow down loss in the efficiency determination process. The principle losses that occur in a boiler are: i. Percentage heat loss due to dry flue gas ii. Percentage heat loss due to evaporation of water formed due to H2 in fuel iii. Percentage heat loss due to evaporation of moisture present in fuel iv. Percentage heat loss due to moisture present in air v. Percentage heat loss due to unburnt in fly ash vi. Percentage heat loss due to unburnt in bottom ash vii. Percentage heat loss due to radiation and other unaccounted loss Efficiency(%) of boiler by indirect method = 100 – (i + ii + iii + iv + v + vi + vii)