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.