In Burners 103, I covered the factors that define the operating range -- firing rates and gas-air ratios -- of premix burners. I also showed that as standards of oven performance and product quality increased, premix burners struggled to keep up, especially where wide operating ranges and uniform temperature distribution became priorities. Some add-on fixes were developed, but the fundamental problem was basic chemistry and physics, which wouldn't allow premix burners to perform the tricks expected of them.
What was needed was a burner free from the limitations of flashback, liftoff and flammability limits, and this could only be done by bringing the air and gas to the process as separate streams. This realization brought the nozzle mix burner back on the scene.
Strictly speaking, the nozzle mix principle dated back to the old furnaces with separate air and gas ports (see Burner History 101, link at bottom of page). However, these so-called burners were a part of the furnace structure and hard to service or replace, their cold startup characteristics were awful, and their combustion stability was poor to nonexistent at temperature less than 1,500oF (816oC). This made them difficult or impossible to use on low temperature ovens and furnaces, the applications that could benefit most from a nozzle mix burner's ability to operate with large amounts of excess air.
By the early 1920s, several models of stand-alone nozzle mix burners were being sold commercially. Most were pretty simple affairs -- essentially two concentric pipes, one for air and the other for gas (figure 1). The two streams were kept apart until it was time to burn them, so flashback was no longer a problem. Apart from this, though, these burners weren't much of an improvement over competing premix designs. At high firing rates, flames would lift off easily, ratio flexibility was still limited, and the velocities of the air and gas streams had to be carefully controlled to ensure a clean, stable flame.
Designers soon learned to stretch the range of firing rates and ratios by adding some sort of flame-stabilizing device -- spin vanes, a disc or tabs at the point where the air and gas first met -- to assist the mixing process. These stabilizers also provided the developing flame with an sheltered spot to anchor itself. This made a huge improvement in burner stability.
Over time, more sophisticated and complex flame stabilizers were designed, and burner performance envelopes expanded. They could be pushed to higher firing rates, turned down lower and operated over a somewhat wider range of gas-air ratios than premix burners.
World War II and the Korean War forced the development of new metals and materials, and along with them, process heating cycles that could bring out their best properties. This created the demand for burners that could operate over much wider ranges of fuel-air ratios (especially excess air) than before. Designers had responded with burners that could be operated from correct ratio to extremely lean mixtures, usually by holding the combustion airflow fixed at the full rate and throttling the gas flow to suit the temperature requirements of the process.
This seemed to defy the laws of physics -- burners operating at air-gas ratios of 50, 100, 200 to one, and higher. With the lean limit of natural gas and air at 15 volumes of air to one volume of gas, how could this be done?
The answer is staged mixing. Competing designs differ in how they do it, but the basic idea is to control the gas and air flows so their ratio doesn't go outside the flammability limits wherever combustion is taking place. Figure 2 illustrates the idea -- the burner's perforated flame stabilizer (or diffuser) breaks the airstream into several smaller streams. When the burner is at low fire, only a small amount of gas flows through it. The flame pulls back close to the burner nozzle, and only a small part of the combustion air is allowed to come into contact with it. The gas-air ratio at that point is near-stoichiometric, and the flame is stable and clean-burning.
As the products of completed combustion move farther out into the burner, they're met by the remainder of the combustion air, which mixes with them and lowers their average temperature to whatever value is appropriate for the process. It's like drawing bath water -- by itself, the hot water would scald you, so you blend in enough cold to make the temperature comfortable. That mixing is done at the faucet (nozzle), however -- not back at the water heater.
If more heat input and higher temperatures are needed, the control system increases the gas flow. The flame elongates as the gas searches for enough air to burn it, but at all points along the way, the amount of air actually mixing with the gas is no more than needed to complete combustion. Any excess air simply blends in farther on.
You've been a patient, attentive class. Just one more lesson [Burner History 105], and it will be graduation time.