Tunnel kilns account for the bulk of worldwide ceramic firing. For many products, a tunnel kiln offers the potential for better temperature uniformity and product quality—with an energy consumption that is typically half that of periodic firing systems. And tunnel kiln life is excellent; many kilns built more than 25 years ago are still in service today.
The Backbone of the Operation
significant variability that often exists in the rest of the manufacturing process, the value of a kiln that serves as a constant processing medium within the factory cannot be underestimated. When consistent kiln operations can be relied upon to produce good firing characteristics, then product defects can be rapidly evaluated independent of the firing process.
Alternatively, if the kilns are not accurate and consistent, they serve to confuse the resolution of day-to-day problems. In the worst case, poor kiln performance can be demoralizing and is sometimes viewed as an excuse to avoid process improvements in other areas.
Tunnel Kiln Firing Checklist
Many of the tunnel kilns in production today have excellent performance. Since so many older units are still operating, however, we often find less than optimum output. So how does your kiln stack up? Check the following list to evaluate potential problems—and solutions—to your firing challenges.The car movement should be stable and smooth inside the kiln. The kiln car pushing accuracy should consistently proceed within 2% of desired rate.Be sure the combustion system is accurate, with clean air input and automated control.Pressure control should be precise so that atmospheric conditions do not affect operations.The kiln needs to be of proper scale, with the correct proportion of heating and cooling sections.The kiln should have sufficient and effective controls, including multiple exhaust ports, air jets, kiln zoning, etc., to provide the ability to appropriately adjust the heating and cooling rates for the product being produced.In the case of dirt sensitive products, very fine filtration of all air and fuel inputs should be maintained to insure continuous dirt-free operation.All of the variables of input and exhaust should be monitored and accurately adjusted with adequate measurement devices.Your production mix should provide for reasonable stability of loading (within 10%) mass and consistent setting profile to avoid rapidly changing inputs of fuel and airflow.The instrumentation system should possess adequate, properly tuned controls to maintain desired setpoint values automatically without excessive input changes.
The Big Picture Of course, having the correct kiln is only one component of a successful operation. In his book, The 7 Habits of Highly Effective People,* Stephen Covey talks about the importance of maintaining the productive capacity—the “PC” component: “In our quest for short-term returns, or results, we often ruin a prized physical asset. Keeping P (production) and PC (productive capacity) in balance makes a tremendous difference in the effective use of physical assets.”
Taking care of the PC component, in this case, your tunnel kiln, is essential to quality long-term operation, with no unplanned interruptions or surprises. Some important elements to consider include:Well trained technicians help maintain consistency of operations, and their skills should be kept current.Kiln specialists need to share information on a company-wide basis (in the case of multiple production facilities).Kiln specialists should have the correct tools to monitor, maintain and continuously improve the operation.Important parts and materials in plants with multiple units should be compatible to insure minimal downtime and uninterrupted output.Traveling thermocouples should be run to determine actual product heating/cooling data—either through trailing wires (acceptable) or data acquisition systems (preferable).
The goal for tunnel kiln operations should be to maintain an unchanging firing environment that provides a consistent firing foundation for the manufacturing process. The kiln should be adjusted to provide the broadest possible firing “window” for the normal variations in product shape, size, formulation and raw materials.
Footnote *Covey, Stephen, The 7 Habits of Highly Effective People, Simon and Schuster, Fireside Division, 1989, pp. 54-55.