Combustion Air Requirements - continue.

To avoid problems such as those mentioned earlier, we provide the furnace with an amount of air which is in excess of the calculated theoretical air. The excess air increases the likelihood that all regions in the main furnace will have sufficient air to complete the combustion process.

To monitor the excess air, the oxygen content of the flue gas in the stack is monitored. Remember, under ideal conditions, there should be no oxygen in the stack (all oxygen should get used up by the combustion process). Due to usage of excess air, we end up with some residual oxygen in the flue gas. Excess oxygen can range anywhere from 2% to 6%, depending upon the nature of fuel and the furnace design. These excess oxygen numbers can easily be translated in terms of excess air. In terms of excess air, we can say that usually the excess air lies in the range of 10% to 30%. Remember, oxygen is a small component of air, therefore excess oxygen numbers are smaller than excess air numbers.

At this time you may ask the following question: Since we use excess air to protect against furnace explosions, why don’t we use a lot more excess air to ensure there is absolutely no chance of furnace explosion? This is a good question. Here's the answer.

Suppose we increase the excess air from 20% to 40%. This will certainly reduce the likelihood of furnace explosions. On the other hand, under this condition, a large amount of hot flue gas (which will include a lot of unused air) will exit through the furnace. This will result in a huge thermal and efficiency loss. To operate a furnace safely we strike a balance between efficiency and safety.