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Plants often have to curb the output of their kilns during economic slow-downs. This is no problem with periodic kilns, since they needn’t be fired as often and the fuel consumption in BTU/lb is normal. Continuous tunnel kilns do present challenges in efficiency when output is reduced, however.
We all know that the efficiency of tunnel kilns depends to a great extent on the cycle time of the ware processed. The fixed input to the kiln varies, but a reasonable value is around 50% of the fuel consumed at fast cycles. This fraction of the input doesn’t go away, even if the push cycle is slowed drastically. As a result, the input must be amortized over fewer and fewer pieces of ware, meaning the BTU/lb value increases significantly.
Efficiency can be improved, even at slower cycles, by making a few fundamental changes in the way the kiln is operated with reduced output. At a time when plants are struggling to achieve breakeven with low revenue, this technique can help brick manufacturers reduce fuel consumption.
Case in PointAs an example, consider a tunnel kiln that normally fires face brick on a rapid schedule of 24 hours cold-to-cold. Figure 1 illustrates a typical curve. In this cycle, the initial heating rate is around 140ºF/hour up to 1400ºF. Oxidation from 1400 to 1700ºF takes four hours, and 1700ºF to soak requires an additional four hours. Finally, the soaking time is an hour. Let’s assume that this is the fastest possible cycle.
When the kiln is slowed to half speed, the new cycle is 48 hours, and normally the only changes that are made would be to reduce the soaking temperature somewhat to maintain the proper brick absorption. There is nothing wrong with this method of operation, but knowing what the cycle must be to attain good properties allows for a different procedure that will save energy.
Simple AdjustmentsSince the optimum firing cycle is known, we can save a considerable amount of energy by reducing the preheating temperatures. Energy applied by the preheating burners is essential in fast cycles, but the burner gases in this part of the kiln can only pass through the load for a short time before the exhaust system removes them-and some of their energy-from the kiln.
We can turn off many of the preheating burners in our slow cycle and still stay below the heating maximum rates acceptable to the brick. This has the effect of reducing the exhaust temperature, therefore saving a significant amount of fuel. The new curve (in red) is shown in Figure 2.
Although the heating rates look different when drawn this way, they closely match the black curve that represents the standard cycle. The heating rates are similar for both curves, and the oxidation time is the same. The soaking temperature is slightly lower because we have twice as much time available at the depressed cycle. Most importantly, the fuel savings using this technique are significant.
When applying this technique, we need to be sure that we maintain the exhaust temperature at a level high enough to avoid condensation. We also need to keep an eye on heat uniformity to ensure that it doesn’t suffer too much without the benefit of preheat burner circulation. This viable technique will help reduce fuel consumption in times of decreased demand-which is just when we need to save as much money as possible.