What is the effect of excess secondary air in a boiler?

Having excess secondary air in a boiler can lead to a decrease in overall efficiency. When secondary air is introduced beyond the optimal amount, it can cause incomplete combustion of the fuel. This incomplete combustion occurs because the excess air can dilute the combustible gases and lower the flame temperature, ultimately resulting in lower thermal efficiency. The extra air does not participate in the combustion process effectively, which means that more fuel is needed to achieve the desired energy output.

Furthermore, excess secondary air increases the volume of flue gases that must be heated, leading to more heat being lost out of the stack. This heat loss exacerbates the inefficiency because while additional air might theoretically improve combustion, in practice, the excess tends to cool the flame and reduce the energy extracted from the fuel.

Thus, the presence of excess secondary air disrupts the ideal combustion process and can lead to increased operational costs, showcasing why it is linked to a decrease in overall efficiency.