Burner Operation – Operational Tips on Air
Don’t overfire any individual burner. One of the most common mistakes when firing oil is simply to put oil into a combination burner already firing at or near capacity on gas. This guarantees overfiring, local air starvation and long flames. Above the maximum design heat release per burner, at design draft, you can’t get enough air into the intended combustion zone even with the air register wide open.
It doesn’t matter if there is (overall) enough air in the firebox as indicated by flue gas oxygen. Sufficient air has to come across each burner register to burn its own fuel in its own intended combustion zone. Otherwise, the locally air-starved flame seeks out air elsewhere in the box, probably over among the radiant tubes or up in the convection section or stack. Even if you can increase the draft to get more air, the flame from an overfired burner may still be too large for the firebox.
Open each air register far enough to admit enough combustion air to each burner. Too little air at any firing rate gives long, billowy, yellow, smoky flames. The firebox must be tight. All the combustion air must come through the burner air register to produce a proper flame.
Best practice is to fire all burners at the same rate and take burners out of service only when the total firing load drops below minimum burner rates. Stagger out-ofservice burners to keep the heat input as uniform as possible. Make sure the air registers on out-of-service burners are closed.
Load changes are a problem, particularly on heaters with multiple burners at different levels which operate at different drafts. Slow, careful step-by-step adjustment of burner air registers and the stack damper must be done jointly. Every time there is a significant change in the firing rate, similar joint readjustment is needed to reach the desired draft at the top of the firebox (0.05 inch water column minimum) and excess air (10% gas, 15% oil). Unfortunately, good draft management is a lot of work. See Section 540 for further discussion.