The cause of heat loss in flue gas is typical with the cause of heat loss in the stack. Air is required to make sure perfect combustion. Because of the mixing between air and fuel is not perfect to perform complete combustion in the steam boiler, excess air is totally needed. Amount of excess air in each type of fuel and type of furnace and burner are different. The amount of excess air must be made as small as possible because the heating which be needed to heat excess air does not has destination. The excess air which is not used in the process of combustion or firing will flow leave the steam boiler at temperature of stack, so it can be called heat loss.
The flue gas exit from the stack has temperature hotter will increase the heat loss then less efficiency of steam boiler. The reasons why the flue gas temperature is too hot may be caused by the burner or firing process generates more heat than specific load that is needed in the steam boiler. In addition, the temperature of flue gas is too hot because heat transfer in surfaces of pressure parts is not working correctly so it will impact heat is not transferred to the water.
Therefore, to prevent that, it need to perform maintenance and re-calibration of burner and its accessories, also clean the surfaces of pressure parts form slag. But, something must be considered that temperature of exit flue gas is maintained to don’t too much cooling, so temperature is not down until dew point that can cause of corrosion.