It is the ratio of heat actually utilised to general steam to the heat supplied by burning fuel in the same period.
where mw and mf are mass of steam and mass of fuel per hour reported and CV could be gross C . V or net C . V depending upon the flue gas emit temperature (kJ/kg) and h and hf are enthalpy or steam delivered and enthalpy of feed water respectively.
Generally is most performance calculations CCV is used to compute ηBoiler.
Heat Losses in a Boiler
Boiler efficiency of 100% is impossible to achieve. This is mainly due to the following heat losses observed in any boiler.
(a) Heat loss to fuel gases,
(b) Heat loss due to incomplete combustion,
(c) Heat loss due to un-burnt fuel, and
(d) Convection and radiation loss.
Heat Loss through Fuel Gases
This has two components, namely
(i) the dry flue gas loss, and
(ii) heat carried away by moisture.
The Dry Flue Gas Loses
where mg is mass flow rate of dry flue gas (or) mass of dry gases formed by kg and per unit time.
Tg is the exit temperature of the boiler gases oL
Ta is the ambient temperature of air oC and
Cpg is the specific heat of the dry gas, kJ/kg k
Heat Loss through Moisture
where ms is the mass of moisture produced per kg of fuel due to H2 burning
hf1 = enthalpy of liquid water at ambient condition, and
hf2 = enthalpy of moisture at partial pressure and at unit gas temperature ToC.
Heat Loss due to Incomplete Combustion
This is calculated using the measured volume (mole) fractions of CO in the flue gases and relating it with CO2 produced if combustion were complete.Heat loss due to incomplete combustion.
where CO, CO2 volume (mole) fractions expressed as percentage and C is the mass fraction of carbon in the fuel 23680 kJ/kg is the difference in heat
release due to incomplete combustion of C to CO instead of CO2.
Heat Loss due to Un-burnt Fuel
This is calculated by the following formula :
where mub is the un-burnt fuel mass per kg fuel supplied and GCV is the gross calorific value.