Boiler efficiency is a measure of the total energy input to a boiler vs. its total energy output. In effect, it is an indication of the energy losses encountered during boiler operation through radiation, convection and inefficient combustion.
There are a number of factors that contribute to energy loss and boiler inefficiency. Some of these include:
1) Radiation and Convection Losses – These are energy losses in the form of heat emanating from the boiler. Although it is impossible to eliminate radiation and convention losses entirely, they can be reduced by insulating the boiler and its associated piping. Convection losses typically increase with flow of air over the surface of the boiler. As a result, efficiency can be improved by shielding the boiler from outdoor or indoor drafts.
2) Excess Air – Excess air in the combustion chamber will absorb energy from the fuel, which could have been used to raise steam. This is normally evident in a rise in flue gas temperature, indicating heat loss during combustion. One way to prevent such an occurrence is to install economizers that recover the heat in flue gases to reheat boiler feedwater. High flue gas temperature can also be an indication of inefficient heat transfer from fuel to boiler water. This can be avoided by cleaning heat transfer surfaces regularly.
3) Ambient Air Temperature – The temperature of the ambient air used for combustion can have a marked effect on boiler efficiency and stack temperature. However, flue gas temperature should not drop below dew point to prevent the accumulation of moisture and reduce the risk of corrosion in the boiler. Most boiler manufacturers assume an ambient temperature of 70 – 80°F for optimal boiler efficiency.
4) Steam Losses – Energy can be lost if steam or hot water is allowed to escape from the system through leaks in piping. Steam traps are frequently the source of steam loss. As a result, they should be checked and maintained regularly to eliminate energy loss.