Purpose Of Excess Air

Perfect combustion is achieved when all the fuel is burned using only the theoretical amount of air. Perfect combustion cannot be achieved in a fired heater. Complete combustion is achieved when all the fuel is burned using the minimal amount of air above the theoretical amount of air needed to burn the fuel. With complete combustion, the fuel is burned at the highest combustion efficiency. Incomplete combustion occurs when all the fuel is not burned, which results in the formation of soot and smoke.

Oxygen for combustion is obtained from the atmosphere, which is about 21% oxygen by volume or 23% by weight. About 2000 cubic feet of air is required to burn one gallon of fuel oil at 80% efficiency at sea level. About 15 cubic feet of air is required to burn one cubic foot of natural gas at 75% at sea level. Most of the 79% of air that is not oxygen is nitrogen, with traces of other elements. Nitrogen is inert at ordinary flame temperature and forms few compounds as the result of combustion. Nitrogen is an unwanted "parasite" that must be accepted in order to obtain the oxygen. It contributes nothing to combustion, it increases the volume of combustion products to be vented, it steals heat from the reaction and now creates a growing environmental problem as well.

Air required in combustion is classified as:

Primary Air

Primary air controls the rate of combustion, which determines the amount of fuel that can be burned.

Secondary Air

Secondary air controls combustion efficiency by controlling how completely the fuel is burned.

Excess Air

Excess air is air supplied to the burner that exceeds the theoretical amount needed to burn the fuel.

Combustion air requirements are based on the composition of the fuel used and the design of the burner. Fuels commonly used contain nitrogen, ash, oxygen, sulfur, carbon and hydrogen. When a fuel has a large volume of nitrogen that must be accepted along with the desired oxygen, more excess air should be provided. That excess air has a chilling effect on the flame. Some fuel particles fail to combine with oxygen and pass out of the stack unburned.

Water vapor is a by-product of burning hydrogen. It too subtracts heat from the flame and becomes steam at flue gas temperature, passing out of the stack as vapor mixed with the combustion products.

Natural gas contains more hydrogen and less carbon per unit of heat content than oil and consequently its combustion produces a great deal more water vapor which withdraws a greater amount of heat from the flame. Therefore gas efficiency is always slightly less than oil efficiency.

Air requirements for combustion are generally expressed in cubic feet of air per gallon of oil or per cubic foot of gas for convenience because fans, ducts and other air moving devices are rated in cubic feet per minute or cubic feet per hour. The Fuel/Air Ratio for combustion is actually a weight ratio based on the required weight of oxygen for a given weight of fuel.

The good news about excess air is that it provides a measure of safety. The bad news is that it wastes fuel. The less excess air used results in the least amount of "waste".