Combustion Efficiency

Combustion efficiency is the effectiveness of the burner only and relates to its ability to completely burn the fuel. The boiler has little bearing on combustion efficiency. A well-designed burner will operate with as little as 15% to 20% excess air, while converting all combustibles in the fuel to thermal energy.

Thermal Efficiency

Thermal efficiency is the effectiveness of the heat transfer in a boiler. It does not take into account boiler radiation and convection losses - for example, from the boiler shell, water column piping, etc.

Fuel-to-Steam Efficiency

The term "boiler efficiency" is often substituted for combustion or thermal efficiency. True boiler efficiency is the measure of fuel-to-steam efficiency. O’Brien Boiler Services guaranteed boiler efficiencies are fuel-to-steam efficiencies. Fuel-to-steam efficiency is calculated using either of two methods. The first method is input-output, which is the ratio of joules output divided by joules input x 100. The second method is heat balance, which considers stack temperature and losses, excess air levels, and radiation and convection losses. Therefore, the heat balance calculation for fuel-to-steam efficiency is 100 minus the total percentage stack loss and minus the percentage radiation and convection losses. Fuel-to-steam efficiency is important, but that efficiency calculation does not take into account cycling and purge losses, operating levels, and other variables.

In-Service Efficiency

Fuel-to-steam efficiency is important, but that efficiency calculation does not take into account cycling and purge losses, operating levels, and other variables. More importantly, in-service efficiency—the amount of fuel you have to use to get the needed output, may be a truer measurement of the value of your system. We supply the highest in-service efficiencies on the market. Whether modular, a single boiler, or a hybrid, our cutting-edge designs combined with advanced controls provide the absolute lowest operating cost available.

Stack Temperature and Losses

Stack temperature is the temperature of the combustion gases (dry and water vapour) leaving the boiler. A well-designed boiler removes as much heat as possible from the combustion gases. Thus, lower stack temperature represents more effective heat transfer and lower heat loss up the stack. The stack temperature reflects the energy that did not transfer from the fuel to steam or hot water. Stack temperature is a visible indicator of boiler efficiency. Any time efficiency is guaranteed, predicted stack temperatures should be verified.

Stack loss is a measure of the amount of heat carried away by dry flue gases (unused heat) and the moisture loss (product of combustion), based on the fuel analysis of the specific fuel being used, moisture in the combustion air, etc.

Excess Air

Excess air provides safe operation above stoichiometric conditions. A burner is typically set up with 15% to 20% excess air in higher firing ranges. Higher excess air levels result in fuel being used to heat the air instead of transferring it to usable energy, increasing stack losses and significantly decreasing efficiency. Boilers with lower excess air throughout the operating range have higher efficiencies.

Radiation and Convection Losses

Radiation and convection losses will vary with boiler type, size, and operating pressure. The losses are typically considered constant in BTU/hr, but become a larger percentage loss as the firing rate decreases. Boiler design factors that also impact efficiencies of the boiler are heating surface, flue gas passes, and design of the boiler and burner package.

Heating Surface

Heating surface is one criterion used when comparing boilers. Boilers with higher heating surface per boiler kW tend to be more efficient. Traditional packaged boilers are offered with .46 square metres of heating surface per boiler horsepower as an optimum design for peak efficiency, but new design breakthroughs allow Cleaver-Brooks EX boilers to have increased efficiency using more effective heating surface and reducing the size of the footprint.

Flue Gas Passes

The number of passes that the flue gas travels before exiting the boiler has been a good criterion when comparing boilers. As the flue gas travels through the boiler it cools, and therefore changes volume. Multiple pass boilers increase efficiency because the passes are designed to maximize flue gas velocities as the flue gas cools. Cleaver-Brooks has developed new design technologies in our EX boilers allowing for comparable efficiencies in fewer passes, resulting in smaller boiler systems that will fit in tighter quarters.

Integral Boiler/Burner Package

Ultimately, the performance of the boiler is based on the ability of the burner, the boiler, and the controls to work together. When specifying performance, efficiency, emissions, turndown, capacity, and excess air all must be evaluated together. The efficiency of the boiler is based, in part, on the burner being capable of operating at optimum excess air levels. Burners not properly designed will produce CO or soot at these excess air levels, foul the boiler, and substantially reduce efficiency. In addition to the boiler and burner, the controls included on the boiler (flame safeguard, oxygen trim, etc.) can enhance efficiency and reduce overall operating costs for the customer. A complete packaged boiler design includes the burner, boiler, and controls as a single, engineered unit, and it is this design that Cleaver-Brooks has focused on for more than 80 years.

For more information, refer to the ABMA Firetube Engineering Guide, the ASHRAE Handbook, or pick up the phone and call O’Brien Boiler Services.