Once an injector is set in this manner, the vertical zone in which the energy from the fuel is released is fixed‘normally in the "middle third" of the vertical plane. As a result, the adjacent section of the brick hack receives considerably more heat treatment than the rest of the load. The upper portion of the load receives somewhat less heat treatment, while the lower portion of the load (at the deck) receives the least. The middle and upper part of the load thus receive considerably more energy than the bottom of the load, in order to fire the bottom brick to minimum specifications for physical and chemical properties.
Attempts have been made to improve this heat distribution by staggering the gas orifices and the air discharge cones on injectors across the zone. However, any adjustments require the burner to be removed from the kiln, and the orifices and cones must be physically changed. Since some of these parts are exposed to heat and corrosion from the kiln atmosphere, they might be impossible to change out after the burners have been in service for some time. Additionally, changes to accommodate different settings or products are difficult and time-consuming, so compromise configurations are normally used.
Finally, changing the gas orifice changes the input rating of the burner. If burner input is reduced, there may not be enough total energy available to maintain the required zone temperature.
An inner "spin air" tube is located around the gas tube. Using 16-20 osi air pressure and a series of spin vanes, the tube provides rapid spinning and mixing of the fuel and air, which produces a "short flame mode" that can heat the top of the load. An outer "forward air" tube is located around the spin air and gas tubes. It uses 16-20 osi air pressure with a nozzle located at the exit of the injector, which increases the forward air velocity and entrains both fuel and furnace gases along the vertical axis to heat the bottom of the load in a "long flame" mode.
*The Variable Heat Pattern Injector (VHPI), designed and manufactured by The North American Manufacturing Co., Ltd., Cleveland, Ohio (patent pending).
By dynamically changing the proportion of the air flow fed to the two air tubes, but keeping total air flow and fuel flow constant, the injector can be modulated between short and long flame modes without an appreciable change in flame diameter, thus moving the area of maximum heat release up and down in relation to the height of the load. This ability to change the heating pattern allows for an improvement in temperature uniformity within the load that cannot be matched by conventional injector burners. The movement of the maximum heat release can be programmed to the specific needs of the tunnel kiln, the product, the setting and the push rate.
The split between the spin air and forward air can be controlled for an entire row or an entire zone with a single control motor on linked valves. Injectors within a row or zone are balanced by means of metering orifices and valves in each of the individual air and fuel lines.
When the injector is used to replace existing injector burners where dynamic heat profiling is not required, the manual air valves can be set to provide a single, best location along the vertical dimension for the heat release. Profiling the firing zones with the injector is simple, since it is done exclusively external to the unit and does not require its removal from the tunnel kiln. Fuel input capacity is not affected by these adjustments.
Potential benefits of the injector, with its associated controls, include:
To highlight the temperature effects of shifting between forward and spin air, two cycles are shown in Figure 2. The upper thermocouple (located at approximately 70% of the height of the hack) was the hottest, and the lower thermocouple (located at approximately 5% of the height of the hack) was the coldest at times when the injector was in mostly spin air mode (short flame). The figure also shows that as the injector was shifted into a predominantly forward air mode (long flame), the temperature of the upper thermocouple decreased and that of the lower thermocouple increased. In fact, the temperature spread between the upper and lower thermocouples was reduced to less than 100ìF when firing in this long flame mode. Thus, the ability to move the location of peak heat release was successfully demonstrated.
Even though the coldest portion of the product started out nearly 40ºF lower for the demonstration testing than it was in the baseline case (with the old burners), the minimum temperature ended up almost 50ºF higher by using a portion of the cycle time in the long flame mode. Whereas the temperature spread in the baseline case remained about the same, the overall spread between minimum and maximum temperature was decreased by over 70ºF using the new injectors.
The burners have operated as designed for seven months to date and show no signs of deterioration from heat or corrosion. Although it is difficult to judge overall effects on kiln operation from just one zone, the benefits resulting from this single zone have included improved cold water absorption properties and evidence of reduced fuel usage.
The new injector has advanced the state of the art in roof-fired tunnel kilns by demonstrably allowing targeted heat delivery to the load. This technology gives brick manufacturers the potential to improve temperature and material uniformity in their product, reduce fuel consumption, and increase production.
For more information about the new injector, contact North American Manufacturing Co. Ltd., 4455 East 71st St., Cleveland, OH 44105; (800) 626-3477; fax (216) 641-7852; e-mail ; or visit http://www.namfg.com.
More information about Hanson Brick can be found at http://www.hasonbrick.com.