Combustion, Fuels, and Burner Systems
Combustion items reward a clear mental model: fuel plus oxygen plus heat, mixed turbulently, held hot long enough to burn out. The three T's are time, temperature and turbulence; oxygen supply is the fourth requirement.
Concepts to master
- Stoichiometry. Theoretical air is roughly 10 cubic feet of air per cubic foot of natural gas, and about 14.4 pounds of air per pound of No. 2 oil. Anything above stoichiometric is excess air.
- Flue gas analysis. Oxygen, carbon dioxide, carbon monoxide and unburned hydrocarbon readings. Excess air raises O2 and lowers CO2 and stack temperature efficiency; too little air raises CO and smoke.
- Draft. Natural, forced, induced and balanced. Furnace draft is normally held slightly negative (around -0.05 to -0.1 in. w.c.) so products leave and the operator is not pushed out of the sight port.
- Fuel oil. Grades, viscosity and preheat, atomization by pressure, steam or air, and why unburned oil becomes coke on the burner tip.
- Burner safety sequence. Pre-purge (typically several air changes), pilot proving, main flame trial-for-ignition, flame scanner supervision, and shutdown on flame failure.
- Acid dew point. Keep stack temperature above the sulfur-bearing flue gas dew point to avoid cold-end corrosion.
Traps
High O2 is not proof of good combustion because air leaking into the flue after the furnace fools the analyzer. CO can be high from too much air as well as too little.
Worked rule
Approximate excess air from dry O2: EA% = O2 / (20.9 - O2) x 100. At 3 percent O2, EA = 3 / 17.9 x 100, about 17 percent excess air.