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 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.
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 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.
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.
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.
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 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
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 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.
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
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.