Methods for calculating the thermal efficiency of steam generators. The thermal efficiency of a steam generator refers to the percentage of fuel heat input that is effectively utilized. Modern power plant boilers can achieve efficiencies of over 90%, while industrial boilers can reach efficiencies above 75%.

In addition to effective utilization, the fuel heat input into the steam generator is lost in various forms, including: heat loss carried away by flue gas; solid incomplete combustion loss due to unburned carbon in fly ash, slag, and leaked coal; and heat dissipation loss. To evaluate performance and improve design, steam generators often undergo thermal balance tests. The method of calculating boiler thermal efficiency directly from the effectively utilized energy is called the direct balance method, while the method of inversely calculating efficiency from various heat losses is called the indirect balance method. When considering the actual efficiency of the boiler room, it is necessary to consider not only the thermal efficiency of the steam generator but also the energy consumed by the auxiliary equipment of the steam generator.

The amount of air required for complete combustion of a unit mass or unit volume of fuel, calculated based on chemical reaction equations, is called the theoretical air quantity. To provide more opportunities for fuel to come into contact with oxygen and burn in the furnace, the actual amount of air sent into the furnace is always greater than the theoretical air quantity. The ratio of the actual air quantity sent into the furnace to the theoretical air quantity is called the excess air coefficient. The actual excess air coefficient at the furnace outlet mainly depends on the fuel properties and combustion method, generally ranging from 1.05 to 1.5. Although increasing the air supply can reduce incomplete combustion heat loss, it will increase flue gas heat loss and exacerbate sulfur oxide corrosion and nitrogen oxide formation. Therefore, efforts should be made to improve combustion technology to achieve complete combustion in the furnace with as small an excess air coefficient as possible. For example, the excess air coefficient of oil-fired boilers can already be less than 1.03. This combustion technology using a low excess air coefficient is called low-oxygen combustion.