To those survivors of Burner History 102, welcome to Burner History 103. You've reached the final step in the course series on premix burners. Just in case you've forgotten everything you learned in Burner History 101 and Burner History 102, I'll do a quick overview of their high points.
Premix burners were the first purpose-designed burners, and they can be traced back more than 100 years to the Bunsen and similar laboratory burners. A premix burner system really consists of two key components, the burner head or nozzle, and the gas-air mixing device that feeds it. In some cases they're built as a single unit. The mixer uses the energy of a pressurized stream of air and/or gas to mix the two and present them to the burner nozzle, which provides an ignition and anchoring point for the flame and controls its shape.
The operating envelope of a premix burner is defined primarily by the limits of flammability of the air-gas mixture (roughly 5:1 to 15:1 for natural gas) and by the velocity of the mixture passing through the ports of the burner head or nozzle. The velocity, of course, is directly related to the mixture flow rate; in other words, the firing rate. As the firing rate is increased, the rich and lean limits of flammability tend to converge on a single point. At flows higher than that point, there's no mixture ratio that will burn stably. The flame lifts off the burner, so that becomes the maximum firing rate. The minimum firing rate is determined by how low you can turn the mixture flow (velocity) without risking flashback.
In the early days, ovens and furnaces tended to be relatively simple devices, and nobody had any great expectations about their performance. As the years went by, however, the demand for higher product quality, productivity and process flexibility began to squeeze premix burners. It wasn't that the burner designs were necessarily inferior -- they were just boxed in between the limits of flammability and the liftoff and flashback points. At low fire, the flames would become little concentrated fireballs right in front of the burners, and the flashback limitation often forced the burner to be operated at a higher firing rate than the process needed.
It was less of a problem in ovens that used secondary or dilution air -- that air could be drawn in around the burners (figure 1), cooling those little hot spots and churning up the oven air to create better uniformity. However, on furnaces operating at higher temperatures, the only way to avoid overheating at low fire was to periodically shut off the burner. Another problem arose on furnaces with multiple burners that operated for long periods at low firing rates. Temperature uniformity in the combustion chamber was terrible -- extremely hot right in front of the burners and cold elsewhere. Consequently, load temperatures varied from one spot to another, creating quality variations and adding an element of uncertainty about how long the load should remain in the furnace for proper heat processing.
Another limitation of the premix concept cropped up on direct-fired furnaces with large combustion chambers -- the fast-mixing premix flames often were too short and compact to project their heat uniformly over the length or width of the chamber. This led to poor temperature uniformity even at high firing rates. Bigger burners helped, but the flashback bugaboo raised its ugly head again. With mixture pipes larger than 6 or 8", flashback often occurred, even though the burners weren't running at minimum flow. This set the practical maximum firing rate for a single premix nozzle at around 8 to 10 million BTU/hr.
If this weren't enough, experience and testing had confirmed that luminous flames provided better heat transfer in some high temperature applications. With premix burners, luminous, yellow flames were difficult to conjure up.
These limitations led oven, furnace and burner designers to cook up a number of different workarounds. On multiple-burner furnaces plagued by poor temperature uniformity and insufficient turndown at low fire, designers installed air nozzles near the burners (figure 2). The air from these nozzles absorbed some of the excess temperature and heat of the flames, in addition to stirring up the stagnant gases in the combustion chamber.
Where longer, more luminous flames were needed, designers sometimes resorted to external gas or fuel oil jets, injecting the secondary fuel into the flame envelope (figure 3). This approach had a lot of shortcomings, including a tendency toward sooty, dirty flames, high fuel consumption and the production of excessive amounts of carbon monoxide, hydrogen and other combustibles in the flue gases.
As customers continued to raise the standards for furnace and oven performance, the inadequacy of these patchwork fixes became more apparent. Clearly, a better way was needed, and that better way would be the nozzle mix burner. The next course [Burner History 104] will begin our study of these burners, but first, you've got to pass the final exam. Good luck!