A mere 20deg c deviation of flue gas temperatures from optimum value result in 1% excess fuel consumption – monitor flue gas temperatures to control stack loss.
During combustion heat is released when carbon combines with oxygen to form carbon dioxide. Theoretically if the right amount of air is supplied, fuel will consume the entire oxygen.
In oil and gas fired boilers, due to practical limitations of the burner, some excess air needs to be supplied to ensure complete combustion. If this excess air is higher than required, it carries away useful heat to the atmosphere via the chimney.
By rule of thumb, the maximum acceptable stack gas temperature is 30oC over the steam temperature. Every 20oC deviation from optimum flue gas temperatures increases the fuel consumption by 1%. Monitoring the flue gas temperatures helps to keep a continuous check on this loss.
The table below presents the average stack losses observed by us in the different boilers:
As seen, special attention must be paid to stack loss for solid fuel fired boilers.