Increasing the inlet air temperature by 30-40degC increased the boiler efficiency by about 0.6%
For combustion to occur in a boiler the fuel needs to be supplied with adequate amount of oxygen i.e. air. This air that is added gets heated from ambient to a temperature of about 30degC above the steam temperature and exits the boiler. This steam of air called flue gas is discharged into the atmosphere via a chimney.
In the path of this flue gas a heat exchanger called air pre-heater (APH) is sometimes introduced. The purpose of the APH is to recover the heat from the boiler flue gas to the incoming fresh air to the boiler. Increasing the inlet air temperature would result in lower fuel consumption as now to attain the same flue gas temperature reduced amount of heat needs to be added.
Thus the thermal efficiency of the boiler is increased by extracting the useful heat lost in the flue gas. As an added benefit the flue gases are also sent to the chimney at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack to meet emissions regulations.
Typically an increase in the inlet air temperature by about 30-40degC will improve the thermal efficiency by about 0.6%. An APH can most conveniently be installed at the outlet of boilers using solid fuel (bagasse, coal, briquette, etc) and gas fired boilers. However in case of liquid fuel (FO, HSD, etc.) the opportunity needs to be carefully evaluated to avoid safety hazards resulting from sulphuric acid condensation.