Indirect Method for obtaining boiler efficiency
There are reference standards for Boiler Testing at Site using indirect method namely British Standard, BS 845: 1987 and USA Standard is ASME PTC-4-1 Power Test Code Steam Generating Units’.
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. A detailed procedure for calculating boiler efficiency by indirect method is given below. However, it may be noted that the practicing energy mangers in industries prefer simpler calculation procedures.
The principle losses that occur in a boiler are:
• Loss of heat due to dry fluegas
• Loss of heat due to moisture in fuel and combustion air
• Loss of heat due to combustion of hydrogen
• Loss of heat due to radiation
• Loss of heat due to unburnt
In the above, loss due to moisture in fuel and the loss due to combustion of hydrogen are dependent on the fuel, and cannot be controlled by design.
The data required for calculation of boiler efficiency using indirect method are:
• Ultimate analysis of fuel (H2, O2, S, C, moisture content, ash content)
• Percentage of Oxygen or CO2 in the flue gas
• Flue gas temperature in °C (Tf)
• Ambient temperature in °C (Ta) & humidity of air in kg/kg of dry air
• GCV of fuel in kCal/kg
• Percentage combustible in ash (in case of solid fuels)
• GCV of ash in kCal/kg (in case of solid fuels)
Theoretical air requirement