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High-velocity (HV) burners have been installed on many older kilns, typically with the goal of achieving better temperature uniformity and efficiency. HV burners offer several advantageous attributes. First, they develop gas stream velocities approaching 500 mph, which creates strong circulation inside the kiln and forces heat into the loading space. More importantly, as the high-speed gas stream exits the burner nozzle, it entrains up to 10 times the burner jet volume in kiln gases.
Although results have frequently been positive, disappointments do occur. Certain installation techniques can help improve performance.
Figure 1 shows the best installation technique. The burner is mounted so the outlet is flush with the hot face of the kiln. The HV jet is high in temperature from the entrainment of hot gases within the ware space. However, the cooling effect of these gases at kiln temperature results in an overall jet temperature that is very close to kiln temperature, which minimizes hot spots. In addition, the resulting jet is forced beneath the load—assuming the burners are firing below a setting deck of products—instead of being splashed on the product.
One problem remains, however: many installations have the burner mounted in thick walls (some as thick as 30 in.). In these installations, the entrainment does not occur. Instead, the burner velocity dissipates rapidly, and the hot gases often splash on the ware located at the lower part of the setting.
In these cases, products subjected to the hot gases are heated a few hundred degrees hotter than the kiln soaking temperature. This creates problems with absorption, glaze, etc. Figure 2 shows the burner pattern clearly; thermocouples are placed at the bottom, middle and top of the load, and the left and right bottom of the setting are being overheated in comparison to the product placed above the bottom level of the setting. When this occurs, it becomes necessary to run these burners with excess air, and the result is increased fuel consumption.
This problem has many possible solutions, and I have tried all of them. First, you can replace all of the burners in the kiln with special HV burners that are adjustable in length. These burners will place the outlet of the burner at the hot face and eliminate the problem. But at $2,500 per burner, the cost is considerable when your kiln may have 50 or more burners. The total installation cost can be as high as $3,500 per burner, with repiping, etc.
Second, some burner manufacturers suggest additional venturi blocks, which are placed directly in front of the burner within the firing tunnel where the HV burner is mounted. While these can improve the condition, they often fail in service, causing blockage of the HV jet and overheating of the kiln wall backup insulation.
A more economical, field-proven solution involves the installation of a ceramic tube in front of each burner (see Figure 3). These tubes ensure that kiln gas entrainment is restored, and the HV gases are directed beneath the load as desired.
The resultant DataPaq curve, which was run after the tubes were installed, shows the positive results (see Figure 4). Hot spots in the lower levels of the setting were eliminated, and the overall ware space became more uniform because of the circulation of gases due to entrainment.
System design is critical, and proper selection of the tube diameter and tube material is essential. The distance between the burner outlet and tube entrance is also important. If temperature non-uniformity in your kiln is creating defects, consider this simple and effective modification to make a large improvement.
Any views or opinions expressed in this column are those of the author and do not represent those of Ceramic Industry, its staff, Editorial Advisory Board or BNP Media.