What is the fuel-to-steam efficiency of an industrial boiler?

Fuel-to-steam efficiency is a measure of the overall efficiency of the boiler. It accounts for the effectiveness of the heat exchanger as well as the radiation and convection losses. It is an indication of the true boiler efficiency and should be the efficiency used in economic evaluations. As prescribed by the ASME Power Test Code, PTC 4.1, the fuel-to-steam efficiency of a boiler can be determined by two methods: the InputOutput Method and the Heat Loss Method.

Input-Output Method

The Input-Output efficiency measurement method is based on the ratio of the outputto-input of the boiler. It is calculated by dividing the boiler output (in BTUs) by the boiler input (in BTUs) and multiplying by 100. The actual input and output of the boiler are determined though instrumentation and the data is used in calculations that result in the fuel-to-steam efficiency.

Heat Loss Method

The Heat Balance efficiency measurement method is based on accounting for all the heat losses of the boiler. The actual measurement method consists of subtracting from 100 percent the total percent stack, radiation, and convection losses. The resulting value is the boiler's fuel-to-steam efficiency. The heat balance method accounts for stack losses and radiation and convection losses.

Stack Losses:

Stack temperature is a measure of the heat carried away by dry flue gases and the moisture loss. It is a good indicator of boiler efficiency. The stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler and reflects the energy that did not transfer from the fuel to the steam or hot water. The lower the stack temperature, the more effective the heat exchanger design, and the higher the fuel-to-steam efficiency.

Radiation and Convection Losses:

All boilers have radiation and convection losses. The losses represent heat radiating from the boiler (radiation losses) and heat lost due to air flowing across the boiler (convection losses). Radiation and convection losses, expressed in Btu/hr, are essentially constant throughout the firing range of a particular boiler, but vary between different boiler types, sizes, and operating pressures.