THERMAL EFFICIENCY
API 560 [2] gives a simple definition of thermal efficiency as follows:
Thermal efficiency total heat absorbed=total heat input
Total heat absorbed total heat input total heat loss
Heat loss can occur from the radiant section walls, convection section walls, and
from the stack gas (Figure 9.2). Stack losses are often exacerbated by allowing more air
into the firebox than is needed for safe and effective fuel combustion.
Since the purpose of boilers is to make steam, fuel-to-steam efficiency is sometimes
also referred to as boiler efficiency. Utility boiler efficiency is measured using ASME
Power test code 4, or ASME PTC 4, an industry standard prescribed by the American
Society of Mechanical Engineers.
When comparing efficiencies, one has to be careful in defining whether efficiency
is calculated using the lower or higher heating value of fuel. In the United States, boiler
efficiencies are usually calculated using higher heating value, whereas in Europe they
are usually calculated using lower heating value, which yields a higher numerical
value for the efficiency. The higher heating value is the thermodynamic heat of
combustion or the enthalpy difference between all combustion products, including
condensed water, and the fuel, at a standard temperature (commonly 77 °F). The lower
heating value subtracts the heat of vaporization needed to condense the water vapor
combustion product—basically assuming that energy cannot be recovered from
condensing water in the stack gas.