The three basic types of burners are: nozzle mix, in which the gas and combustion air do not mix until they leave the ports; delayed mixing, in which the gas and air travel a considerable distance from the burner before complete mixing and burning; and pre-mix, in which the primary air and gas are mixed at some point upstream from the burner ports by an inspirator mixer, an aspirator mixer or a mechanical mixer. High velocity or super velocity burners, which are part of the nozzle mix group, are becoming a popular choice among many ceramic manufacturers. The burners are very stable and are capable of high excess air limits, Feese said. About 70% of the combustion takes place outside the tile, making this a “cool” running burner with less wear and tear on the tile and lower maintenance requirements. It also offers high turndown ratios and produces relatively low NOx. Numerous styles of high velocity burners have been developed to suit virtually any application.
Ralph Ruark from Ruark Engineering, Inc., and Ceramic Industry’s senior technical editor, discussed kiln firing strategies for tunnel kilns. The modern tunnel kiln comprises four zones: preheat, main firing, rapid cooling, and slow and final cooling. The rapid cooling zone is especially important when flashing, Ruark said, because it “freezes” the process and keeps it from continuing and causing unwanted effects. To avoid damaging the product, the kiln should have the capability to remove air as well as inject air. Hot air moving toward the entrance can be used to lower the fuel consumption of the kiln, said Ruark. Other important factors to consider in kiln design include the kiln scale, pressure control requirements and body/glaze development.
Ruark also discussed some basic design requirements of an accurate, fast firing kiln. The doorway should prevent the suction of cold air into the ware space, and there should be a sufficient number of exhaust offtakes extending far enough up the tunnel to draft the kiln. In the preheat zone, there should be enough burners and zones to accommodate the body firing requirements, and enough circulation to promote top-to-bottom temperature uniformity. The main firing zone should be designed for firing accuracy, and the rapid cooling zone should be engineered to provide uniform cooling throughout the entire settling area. In the slow cooling zone, good circulation and multiple exhaust locations are important.
Fred Fuhrman, controls engineer for Hauck, discussed pulse firing in more detail. The technology was developed about 20 years ago in Europe to make the best use of high velocity burners, Fuhrman said. During pulse firing, low fire gas input is set by a bypass in or around the pulse regulator. The burner is ignited at low fire. Each time the air solenoid is energized, the burner switches from low fire to high fire. This uses the entrainment and stirring action benefits of high velocity burners to their best advantage. Additionally, since the air and fuel piping to each burner is identical, changing the pulse sequence or even moving a burner from one zone to another is all done electronically.
Another guest speaker, Nathan Fields manager of technical services for Southern Color & Chemical Co., offered a solution to the troublesome process of flashing bricks—a new coating that simulates the flashing effect. (See “No More Flashing!” on pp. 3-4 in this issue.)
The seminar closed with burner demonstrations and a tour of the Glen Gery brick plant in nearby Shoemakersville, Pa.