Lukacs also discussed the three different types of heat transfer—conductive, convective and radiation. In conduction, heat (energy) is passed from one side of the product to the other through the product’s molecules. Convective heat transfer—the only method that can go to the center of the product—occurs when hot gases come into contact with all surfaces of the product. Radiation is the most intense form of heat transfer but can only heat products that are in a direct line with the radiation source. Understanding the basics of combustion will help a kiln operator achieve safe, complete, efficient firing. (Lukacs’ presentation on combustion will be published in the May 2000 issue of Ceramic Industry.)
In any ceramic firing application, burners are required for effective heat transfer to the product. According to Fred McMann, southern regional manager of North American Manufacturing, the burner serves to balance the air/fuel mixing speed versus the flame speed, shape and direct the flame, keep the kiln within flammability limits, and possibly mix the fuel and oxygen. High velocity burners are the most commonly used and provide a short, compact, stable flame.
Blowers should be kept out of high-traffic areas. If air is taken from outside the plant, screens should be put in place to keep birds and other small animals out of the blower.
The setting pattern of the product is also important for efficient firing. To get a good, uniform product, manufacturers must spend time analyzing their setting patterns. The goal is to improve circulation and heat transfer. Saggers that are packed too tightly prevent heat from getting to the entire load.
Liquid fuels require onsite storage, pumping, heating, atomizing and pollution control. They might add sulfur or ash to the process or shorten refractory life. They, too, can be difficult to control in the combustion process.
Propane, which is often used as a standby fuel, requires vaporizers and mixing stations. Since it is heavier than air, it can collect on the ground level in storm drains and become an explosion hazard. If a propane system is used, be sure to check regularly for leaks.
Gaseous fuels require onsite storage, pumping, vaporizing and mixing. They are generally clean and easy to control.
No matter what type of fuel is used, various hardware is needed to measure and control pressure and fuel flow. Pressure reducing regulators, ball valves, limiting orifice valves, ratio/atmosphere regulators, vortex shedding devices, hot wire anemometers and turbine meters are some examples of the hardware available. Accurate readings of pressure are important—changes in pressure can have a big effect on the product in the kiln.
According to Lukacs, modern pulse-controlled firing systems provide improved uniformity and fuel efficiency with reduced power consumption. The systems rotate which burners are on high fire and which ones are on low fire to prevent hot and cold spots from occurring in the kiln. However, the benefits of a pulse-controlled firing system can only be fully realized with high velocity burners. Some companies using this type of temperature control have experienced as much as a 10% to 30% reduction in fuel usage, along with a one to four car per day increase in production. Because of these benefits, the systems typically pay for themselves in 18 months to two years.
Steve Ogonek, business group manager – integrated combustion systems for North American Manufacturing, discussed the use of loop controllers, PLCs and PCs to control kiln temperature. The job of a controller is to respond to changing conditions so that a desired setpoint is achieved and maintained, Ogonek said. When choosing a controller, the user should evaluate its accuracy, speed, maintenance requirements, expandability, connectivity to networks, and cost. Loop controllers are generally best for small installations since they offer limited control options. PLCs allow complex programming but are often proprietary and must be programmed by the manufacturer. According to Ogonek, PCs generally offer the best overall control, but the PC setup needs to be exact.
Accurate temperature measurement is important to ensuring temperature control. Kiln temperature is typically measured using thermocouples placed in the top or side walls of the kiln; however, they are only as good as their installation. A thermocouple is typically surrounded by a ceramic tube to protect it from the intense heat inside the kiln. In some plants, the thermocouples are surrounded by two or three protection tubes as an extra precaution—but the added layers can result in slow reaction time and inaccurate temperature measurement. A company should instead use the smallest protection tube possible for a given application. Lukacs recommended using a high alumina tube (98%) for most applications.
The thermocouple should extend into the kiln 10 times the diameter of the protection tube to obtain the most accurate temperature readings. For example, a 1/2-in. diameter tube would need to extend 5 in. into the kiln. The goal is repeatability. The thermocouples should be as close to the load as possible to measure what is happening to the product, rather than just the kiln atmosphere.
Location is also important—each thermocouple should be located in the mid 30% to 40% of the zone being measured. Since a thermocouple measures the air and the re-radiation effect from the refractory and product, a side wall installation typically provides the most accurate measurements.
McMann also provided tips on reducing NOx emissions through burner modification. High velocity burners can be made into low-NOx burners by staging the fuel, he said. Flue gas recirculation is a universal NOx reducer; however, it increases the system pressure requirements, blower horsepower, air pipe size and piping corrosion, and is an additional stream to be controlled. The best solution is to keep the base NOx low and minimize flue gas recirculation, McMann said.