"I don’t know what happened. We just opened the door, and the burner went out.”
It’s a common refrain. Oh, the circumstances might be a little different -- a sudden fluctuation in combustion chamber pressure as an exhaust damper moves, or a rumble from an immersion tube stack just before everything goes quiet, but it usually leads observers to conclude a momentary pressure change knocked the burner out.
Unfortunately they’re rarely right. Small pressure fluctuations do not pull flames off burners or snuff them out, unless the burner is set at the extreme limit of its stability or is damaged in some way. Modern burners simply have operating envelopes too wide to be bothered by small blips in combustion chamber pressures.
To understand, it’s helpful to look at what happens to air and gas flows and pressure drops through the burner. A typical packaged gas burner used on ovens, dryers and immersion heaters has a combustion air blower with a pressure rating between 2 and 10" w.c. (5 to 25 mbar), sometimes higher. I’ll use the 2" blower pressure to study the problem because, obviously, it will be most susceptible to backpressure fluctuations. Also, I’ll assume the oven or immersion tube runs at atmospheric (zero) pressure but can experience random pressure fluctuations up to +0.2" w.c. (0.5 mbar). Qualitatively, you know what will happen when one of those pressure pulses hits -- it will buffet the burner and cause a momentary decrease in air and gas flow.
But what about the quantitative effect? You can figure this using the Square Root Law of flow and pressure drop, which says the flow through a resistance is proportional to the square root of the pressure drop across it. Assume the burner is purring along at high fire, and the air pressure drop across its nozzle is nearly 2" w.c. (5 mbar) -- the full blower pressure, minus the chamber backpressure, which is zero. The pressure surge hits, throwing 0.2" w.c. against the burner. At that instant, the air pressure drop across the nozzle decreases to (2 - 0.2)" w.c., or 1.8" w.c. (4.5 mbar).
Does this momentarily stop the airflow, leading to a flame failure?
No, although it does cause the flow to decrease by the square root of the fraction (1.8 ÷ 2.0), or 0.95; in other words, a 5 percent loss for as long as the surge lasts. This decrease simply won’t stop a burner in its tracks. If the fluctuation were 0.2" w.c. negative, the airflow will increase 5 percent. Unless the burner is at the absolute limit of its high fire stability, it won’t lift off.
But doesn’t the surge affect gas flow, too? Couldn’t that put the burner out? It might, but if the gas pressure to the burner is the same as the air pressure (and on many modern burners, it is), the percentage flow change will be the same. If the burner operates with different air and gas pressures, the two flows will be affected differently, and this will cause the gas-air ratio to shift, on top of a flow change. Burners will go out if they’re too lean or rich, but chamber pressure fluctuations are rarely strong enough to drive them that far.
What if the burner’s at low fire? Air and gas pressures are lower, so they’re more likely to be upset by a sudden shift in backpressure. You’re onto something here. If the burner is turned down to air and/or gas pressures of 0.2" w.c. or less at low fire, that 0.2" w.c. pressure bounce will definitely stop the flows. The low fire rate has to be set a little higher. By contrast, a negative fluctuation will simply increase the low fire rate.
Realistically, sudden pressure changes as great as 0.2" w.c. are not that common, so for many installations, this is a hypothetical discussion. But this is no reassurance to anyone who has had an oven or furnace suddenly shut down. If the air and gas flows are relatively undisturbed, what’s shutting the burners off?
In my experience, the most likely culprit is a flow- or pressure-monitoring switch with a marginal setting. If it’s on the ragged edge, any little pressure blip that comes along may cause it to open. If it’s part of the safety limit circuit to the burner, the combustion system drops out. In fact, switches set too close to the limit sometimes trip due to mechanical vibration, like a forklift rumbling by.
There are other possibilities. On one installation I worked on years ago, an overheated damper motor seized. When called upon to open, it wouldn’t budge. Flow monitors in the oven’s recirculating system sensed the problem and shut everything down. On another system, the burner ran fine until I began to close the exhaust damper. After much digging into dark corners, I found the switch for the recirculating air fan had been piped to the wrong location. Another possibility is plugged or partially plugged tubing to the switches.
The bottom line is this -- when a burner system goes out after a chamber pressure fluctuation or mechanical jolt, don’t be in a hurry to blame the burner. More likely, you’ve lost a flow or pressure switch that’s misadjusted or has blocked or mis-located pressure lines.