Excess air is an expression of how much more air is used for the combustion than strictly necessary. The calculation is based on the measured level of oxygen in the flue gases and the known concentration of oxygen in the air. Excess air is generally expressed as a percentage in the form of 20 % excess air. This means that a combustion process has 120 % of the air needed for complete theoretical combustion.
In practice, a certain level of excess air is needed in all cases to ensure complete combustion. This excess air is around 3 - 5 % for liquid and gaseous fuels and approximately 11 % for solid fuels, depending on the preparation of the fuel. Excess air is basically a factor to avoid. The unneeded air simply carries heat up the stack away from the burner and is wasted. A fine line must always be drawn between too much excess air and incomplete combustion, which will manifest in high levels of carbon monoxide.
Excess air can either be calculated from the oxygen or carbon dioxide concentration and the fuel parameters (maximum carbon dioxide). In practice, it is easiest to use the oxygen concentration for two reasons:
The value of excess air calculated in this way will be accurate, but must also coincide with the manufacturer's recommendations. Particularly older burners may be designed for a higher level of excess air, which they require to achieve complete combustion.
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