Defining Boiler Efficiency
How efficient is your water heating system?
There are many ways to measure boiler efficiency. Most are based on one part of the system and not the whole. This article by TEA explains the different ways efficiency is measured and defines the best way to determine true Boiler Efficiency.
Combustion Efficiency is an indication of the burner’s ability to burn fuel. The amount of unburned fuel and excess air in the exhaust, are used to assess a burner’s combustion efficiency. Burners resulting in low levels of unburned fuel, while operating at low excess air levels, are considered efficient. Well-designed burners firing gaseous and liquid fuels operate at excess air levels of 15% and result in negligible unburned fuel. By operating at only 15% excess air, less heat from the combustion process is being used to heat excess air, which increases the available heat from the load. Combustion efficiency is not the same for all fuels, and generally, gaseous and liquid fuels burn more efficiently than solid fuels.
Thermal Efficiency is a measure of the effectiveness of the heat exchanger of the boiler; it measures the ability of the exchanger to transfer heat from the combustion process to the water or steam in the boiler. Because the thermal efficiency is solely a measurement of the effectiveness of the heat exchanger of the boiler, it does not account for radiation and convection losses, it is not a true indication of the boiler fuel usage, and should not be used in the economic evaluation.
Boiler Efficiency is often a substituted term for thermal efficiency of fuel-to-steam efficiency. When the term “boiler efficiency” is used, it is important to know which type of efficiency is being represented. Thermal efficiency does not account for radiation and convection losses and is not an indication of the true boiler efficiency. Fuel-to-steam efficiency, which does account for radiation and convection losses, is a true indication of overall boiler efficiency. The term “boiler efficiency” should be defined by the boiler manufacturer before it is used in the economic evaluation.
Fuel-to-Steam is a measure of the overall efficiency of the entire boiler system. It accounts for the effectiveness of the heat exchanger as the radiation and convection losses and any additional devices that may be connected to the boiler system. It is very common in the laundry industry to have a fuel-to-steam efficiency of only 65-70%.
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