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Boiler Efficiency

Efficiency is used to measure economic performance of any piece of equipment. In the boiler industry, there are three common definitions of efficiency, but only one true measurement. Following are the definitions and how to measure efficiency:

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.

Boiler Efficiency
The term "boiler efficiency" is often substituted for combustion or thermal efficiency. True boiler efficiency is the measure of fuel-to-steam efficiency.

Fuel-to-Steam Efficiency
Fuel-to-steam efficiency is the correct definition to use when determining boiler efficiency. Fuel-to-steam efficiency is calculated using either of two methods, as prescribed by the ASME Power Test Code, PTC 4.1. The first method is input-output, which is the ratio of Btu output divided by Btu 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 percent of stack loss and minus the percent of radiation and convection losses.

Stack Temperature and Losses
Stack temperature is the temperature of the combustion gases (dry and water vapor) 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 less 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. Higher excess air levels result in fuel being used to heat the air instead of transferring it to usable energy, increasing stack losses.

Radiation and Convection Losses
Radiation and convection losses will vary with boiler type, size, and operating pressure. The losses are typically considered constant in Btus/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 larger heating surface per boiler horsepower tend to be more efficient and operate with less thermal stress. Many packaged boilers are offered with five square feet of heating surface per boiler horsepower as an optimum design for peak efficiency.

Flue Gas Passes
The number of passes that the flue gas travels before exiting the boiler is also 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.

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 true packaged boiler design includes the burner, boiler, and controls as a single, engineered unit.