Efficiency of a boiler is the ratio of heat absorbed by the feed water to the heat supply by the fuel in a given time. It is denoted by η.
Boiler efficiency = ME (H-HF)/C
ME = Mass of water actually evaporated in kg / kg of fuel = MS/MF
MS = Total mass of water evaporated in steam in kg.
MF = Mass of fuel used in kg.
H = Total heat of steam in kJ/kg of steam.
HF = Sensible heat of feeding water in kJ/kg of steam.
C = Calorific value of fuel in kJ/kg of fuel.