To determine the percent excess air or excess fuel at which a combustion system operates, you have to start with the stoichiometric air-fuel ratio. Also known as the perfect, correct or ideal fuel ratio, stoichiometric is the chemically correct mixing proportion. When burned, it consumes all the fuel and air without any excess of either left over.
Process heating equipment is rarely run that way, however. Even so-called "on-ratio" combustion, used in boilers and high temperature process furnaces, usually incorporates a modest amount of excess air -- about 10 to 20% beyond what is needed to burn the fuel completely. Why? To avoid the unintentional generation of flammable and toxic products of incomplete combustion, including carbon monoxide and hydrogen.
If a combustion system is operated exactly on ratio, there's a chance some of the oxygen in the combustion air won't get paired up with the fuel it's supposed to burn. In addition, as the ratio approaches stoichiometric, the temperature in the flame envelope rises rapidly, and at these high temperatures, a sort of reverse combustion, called dissociation, occurs. Combustion products like carbon dioxide and water vapor begin to break down in the intense heat, reverting to hydrogen and carbon monoxide. Adding a little excess air to the mix lowers the flame temperature enough to slow dissociation to a crawl.
Another reason for a little excess air is to provide a cushion against the ratio drifting over into the rich (excess fuel) range. With time, combustion systems tend to go richer. The big enemy is dirt, and it's more likely to accumulate in the air system than the fuel. The combustion and makeup air fans, in addition to everything else, are giant vacuum cleaners, sweeping in any dust, dirt and vapors that happen by. In time, the fan, air ducting, valves and burner can become partially obstructed, starving the system for air and causing the ratio to go rich. Filters help, but they have to be cleaned or changed on a regular schedule -- otherwise, they become the bottleneck in the air supply.
Okay, so how do you figure what percent excess air you're running at? First, you have to know the stoichiometric ratio for your fuel. Natural gas, being a blend of ingredients, varies from about 9.5 to 10.5. For most combustion work, use 10 as an average if you don't know the exact value. Propane is about 24 to 1 and butane is about 31 to 1 -- again, depending on whether the fuels contain small amounts of other ingredients.
Second, you need to know what percent excess air the system is running (or supposed to be running) at. Then you plug the numbers into this equation:
A = S x [1 + ( %XSA / 100 )]
where A is the airflow and S is the stoichiometric air/fuel ratio.
Let's try on some numbers for size. Say a system is burning natural gas (9.5 to 1 stoichiometric ratio) with 600% excess air.
A = 9.5 x [1 + ( 600 / 100 )] = 9.5 x [1 + 6] = 9.5 x 7 = 66.5, or a 66.5 to 1 ratio.
More likely, you'll have air and gas flow data and want to figure the % excess air. The rearranged equation looks like this:
%XSA = 100 x [(A/S) - 1]
If you plug in the same numbers, the equation looks like this:
%XSA = 100 x [(66.5 / 9.5) - 1] = 100 x [7 - 1]
= 100 x 6 = 600