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A kiln's primary task is to transfer heat uniformly and efficiently throughout a load. However, the way a kiln is designed can greatly affect its heat transfer capabilities. The wrong type and/or placement of burners, inadequate firing control and improper placement of the flue opening can all cause hot spots, cold spots and temperature differences between the top and bottom sections of the load, or from one end of the kiln to the other. A properly designed periodic kiln can help increase product quality and minimize product losses by maximizing heat transfer and uniformity throughout the load.
The Importance of WindWhenever a fuel is burned, products of combustion (flue gases) are released. In a kiln, these products of combustion are removed through a chimney or flue, and their movement from the fire to the flue creates a wind (see Figure 1). The strength of that wind depends on the strength of the burning mechanism, the amount of fuel being burned and the cross-sectional area the products of combustion must pass through. For example, a coal- or wood-fired kiln typically experiences a relatively low-velocity wind. Aspirator burners also create a relatively low-velocity wind because they draw air into the kiln around a burning flame much the same as burning coal or wood in a grate. Adding more wood to the grate or more gas to the aspirator burner will create more wind; however, at some point, the burning limit (the point at which the fuel begins to smother the fire) will be reached.
When air and gas are mixed at or before the burner, and the burner is sealed off from the air outside the kiln, a much greater wind velocity can be generated. Some forms of this type of burner create so much wind that they are called "jet" or "high-velocity" burners. In kilns fired by these burners, the strength of the wind depends largely on the force emanating from the burners.
In many such burners, the force is generated by causing the products of combustion to leave the burner through a small hole at its end. As the combustion gases go through this small hole, their velocity increases. The velocity can be calculated in this way: If a burner releases so many cubic feet of gases per minute, then the velocity (typically measured in feet per minute) is simply the number of cubic feet per minute divided by the number of square feet of burner opening. Fewer square feet in the cross section of the burner opening generates more cubic feet of wind velocity per minute.
In more sophisticated burners, the volume of gases leaving the burner is increased by burning inside the burner block to a higher temperature. If a burner is designed so that the temperature inside the block is as high as 3,000 degrees F, then the gases will expand 6.65 times, increasing the volume-and therefore the velocity of the gases leaving the burner-6.65 times. With a hot burner block, it is possible to obtain high velocities without reducing the outlet diameter of the burner block. However, higher combustion chamber temperatures promote higher NOx generation than burners that run cooler, so burners with cool blocks generally provide a cleaner atmosphere, while also having a longer lifespan.
Zones of ControlIn addition to using the right type of burners, temperature uniformity can also be improved by adding separate zones of control to each burner or group of burners. If the kiln contains more than one vertical layer of burners, a separate thermocouple and control instrument for each layer will create better temperature uniformity. Since the temperature naturally tends to be higher toward the top of the kiln chamber, this design allows burners in higher layers to burn with lower outputs, which helps keep the temperature even at each level.
In some cases, zones of control are also used at end walls in addition to central sections because the load seen by the burners is different on a wall than it is at the ware. For this reason, kiln designs in which each burner has its own control zone have been very successful.
Downdraft DesignsWhile the right burners and control technologies can help increase temperature uniformity in a kiln, they are only a part-albeit a big one-of temperature uniformity. The placement of the flue is also important. In designs where the flue is placed in the top of the kiln, it is unlikely that simply changing the burner type or control configuration will allow the kiln to burn evenly. The flue gases will tend to be drawn directly to the top of the kiln, rather than to the center and lower part of the load, which results in reduced temperature uniformity.
A kiln is improved when the wind is moved around the load through a "downdraft" design, in which the wind is forced to go through the load before it can escape the kiln. When the burners are placed in "fire lanes" outside the load, the wind is able to swirl around in the open space and even out the air temperature. The opening for the flue is placed in the center of the load, and the downdraft flue pulls the evenly heated hot air across the load, leading to a uniform distribution of radiating gases all over the kiln. The temperature will be lower at the point where the gases go into the flue but should be even around the outside of the load (see Figure 2).
Once the desired temperature is reached on the outside of the load, it is held there while the inside ware near the flue gradually heats up. Placing pyrometric cones or fire checks at various locations around the load should show that the heat treatment of the outside and inside of the load is the same at the end of the soak.
This design is so efficient that it is possible to make very tall kilns and still obtain even heat distribution from top to bottom. As the kiln becomes taller, more layers of burners are added. In this way, the temperature between the horizontal layers of burners remains quite uniform (see Figure 3).