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Injector-type burners have long been the choice in roof-fired tunnel kilns. Conventional injector technology is typically based on a "pipe within a pipe" style. High-pressure gas (3-6 psig) is delivered to the back of the injector, passed through a fixed orifice to set the injector fuel capacity, and then conveyed down the length of the injector. The initial pressure drop imposed by the orifice substantially reduces the pressure and the ability to drive the gas forward from the tip of the injector. Combustion air is delivered to the back of the injector, normally traveling through a pipe that surrounds the gas pipe described above. The fuel and air are then mixed near the end of the injector before entering the kiln from the roof. In some cases, a nozzle on the end of the air tube helps to direct these gases in the vertical plane.
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 Innovative AlternativeA new injector design* has recently been developed to overcome the limitations of current injector burners for roof-fired applications (see Figure 1). The gas orifice on the new injector is located at the "working end" of the gas tube, acting as a nozzle so that the 3-6 psig fuel pressure is used to help push the fuel‘and therefore the combustion energy‘to the bottom of the setting.
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:
- Improved temperature uniformity across the load from the "targeted" heat release.
- Reduced fuel consumption by saving the energy that had previously been overheating already hot portions of the load as the cooler portions came to minimum temperature.
- Increased push rates through the tunnel kiln as a result of reduced time spent waiting for cooler sections to catch up to the desired firing curve.
- Reduced heat loss through the car decks compared with those injectors that use a pulsed combustion system that concentrates fuel and heat delivery at the refractory deck. The appropriate setting of the injector will reduce the useful heat carried out of the kiln by the car deck.
- The potential to design new, shorter kilns or build conventional-length kilns with greater initial production capacities due to the faster firing schedules possible with improved heating uniformity.
Trial by FireThe new injector was recently field tested at Hanson Brick in Pleasant Garden, N.C., to demonstrate the ability to move the longitudinal (vertical) location of the peak flame temperature and the ability to improve temperature uniformity within the row, thereby decreasing the spread between the coldest and hottest portions of the load. A zone of the new injector burners was installed, and the testing zone, located at the second high-temperature heating zone on the kiln, comprised three rows of six injectors. The split between forward air (long flame mode) and spin air (short flame mode) was controlled with a single control motor and valve combination for all three rows. Data was collected by a 16-point traveling thermocouple unit.
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.
Authors' NoteThe new injector is designed for use in roof-fired tunnel kiln zones operating at temperatures above the auto-ignition temperature of the fuel. Like conventional injector burners, it has neither ignition nor flame supervision provisions. Appropriate safety codes, standards and operating practices should be observed.
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.