Are you ready for a lesson about premix burners?
Welcome to Burner History 102. You all completed Burner History 101 [if you didn't, use the link at the bottom of the page], which should have familiarized you with atmospheric burners, injectors and proportional mixers. Before I dig more deeply into premix burners themselves, I'll look at one more device that can be used to supply them with a combustible air-gas mixture -- the fan mixer (figure 1).
A fan mixer is a fairly straightforward idea. If you're going to use a fan, blower or compressor to supply the air to a burner, why not pipe the gas to the fan's inlet, let it mix the gas with the air and then push everything to the burner?
Fan mixers are still used occasionally, but many people avoid them because of concerns about flashback. As you probably know, flashback occurs when the flame pops back through the premix burner nozzle, runs upstream in the mixture piping and finally stops where the air and gas are being mixed. Trouble is, flashback can occur so quickly it's an explosion, and explosions inside blowers and compressors aren't a good thing. They can be prevented by installing flame arrestors in the mixture lines, but the cost of the preventative measures may have you searching for a way to avoid the problem altogether.
Now, I'll move on to the premix burners themselves. All burners have unique performance envelopes, or the range of flows and air-gas ratios over which they're stable and operate reliably. At any combination of flow and ratio outside that envelope, the burner goes out.
The stable ratios are defined by the limits of flammability of the fuel. For example, the correct ratio for natural gas is about 10 volumes of air to one volume of gas, but it's possible to get mixtures to burn over a range as fuel-rich as five parts air to one part gas and as lean (excess air) as 15 to one. These limits are a property of the air-gas mixture -- beyond them, the combustion process doesn't generate enough heat to keep the chain reaction going, and the burner will go out. Actually, these limits can only be reached under closely controlled laboratory conditions. In real-life industrial situations, the limits are narrower because some of the flame's heat is being drawn away by the surrounding oven or furnace.
The flow rate of air-gas mixture through the burner also has an effect on its operating range. At low flows, premix burners usually have the widest range of stable ratios. As the mixture flow (firing rate) increases, its velocity through the burner increases, and this tends to push the flame off the burner. To keep it anchored on the burner nozzle, more heat has to be fed back to the incoming air-gas mixture. This calls for a hotter flame and a mixture closer to correct ratio. The result is a performance envelope that looks something like figure 2. Ultimately, the mixture velocity gets so high that the burner's rich and lean ratio limits converge on a single point. At that flow, the burner will be difficult to set up, and it will go out at the slightest provocation. At higher flows, it won't light at all. This is what defines the maximum firing rate of a premix burner.
The operating range of the burner can be improved by designing it to incorporate areas where the mixture velocity is low. Part of the air-gas mixture flows into these sheltered areas and burns, generating enough heat to reliably ignite the main body of gas-air mixture moving past it. Figure 3 shows several ways this is done.
- In tunnel burners, there's a sudden enlargement where the nozzle ends and the refractory block begins. This step provides a sheltered area where some of the mixture circulates at lower velocity, burning and acting as a piloting ring for the main mixture stream.
- Flame retention nozzles don't need a refractory block for stable operation -- they depend on a ring of small drilled ports that divert some of the air-gas mixture into a pocket around the perimeter of the nozzle. Small flames burn in this pocket, providing reliable ignition for the bulk of the mixture passing through the large center port.
- Line and infrared burners have a multitude of small ports. Some are stabilized by pockets or steps around the mixture openings, while others may depend on a reverberating screen that reflects some of the flame's heat back to the port area.
Before I close, I have one correction to make to the material in Burners 101. I said there were a couple of seats left in the back of the room. Anyone who has gone to college knows that the last open seats in a lecture room are always in the front row. Sorry `bout that!