What happens to stack temperature when excess air is increased?

When excess air is increased during combustion, the stack temperature typically increases. This is because adding excess air can lead to incomplete combustion, which results in lower combustion efficiency. As more air is introduced, it absorbs additional heat from the combustion process but does not fully participate in the combustion due to the increased volume of air. Consequently, the products of combustion, which include unburned hydrocarbons and carbon monoxide, tend to carry more heat away with them, resulting in a higher stack temperature.

In addition, the presence of excess air can dilute the combustion gases, raising the overall temperature of the exhaust due to the additional energy from the combustion process not being transferred efficiently to the heating medium. Overall, since increased excess air leads to higher temperatures in the stack as a result of inefficiency and heat retention in the exhaust gases, this explains why the correct answer is an increase in stack temperature.