ONLINE EXCLUSIVE: Improving Furnace Design Through Modeling
Graphite electrodes have been used in steel melting applications since the late 1930s. In the manufacture of graphite electrodes, carbonaceous fillers (calcined petroleum coke) are bonded with carbon-yielding binders (coal tar pitch), extruded into rods, placed in metal canisters and baked to temperatures of 1800 degrees F to drive off the volatiles from the binders. The rods are cooled in a reducing atmosphere, and then the electrode is reheated to temperatures in excess of 4500 degrees F in a subsequent graphitization process, which provides the material with the crystallinity required for high electrical and thermal conductivity.
In the initial baking phase, maintaining all of the products in the furnace at very close to the same temperature is crucial to producing a quality product. If the temperature is significantly higher or lower in certain areas, parts of the load might pass too quickly through critical temperature zones, causing the volatiles to escape too quickly and potentially causing cracking.
The normal approach to maintaining a uniform temperature during this stage is with recirculating fans attached to the roof or sides of the furnace that are designed to maintain airflow throughout the inside of the furnace and promote temperature uniformity through convection. However, fans also increase the initial cost of the furnace, consume a considerable amount of power while the furnace is operating, require frequent maintenance due to their high-temperature operating environment, and cannot efficiently provide intensive turbulence in the area adjacent to the furnace floor due to the limited space between the canisters. A better furnace design would maximize temperature uniformity without the use of fans—and developing the improved design would require the use of sophisticated modeling software.
Eliminating the FansWhen SGL Carbon, Morganton, N.C., approached kiln producer Swindell Dressler of Pittsburgh, Pa., for a new batch-type furnace for its graphite electrodes, Swindell Dressler engineers proposed eliminating the fans by using high velocity gas burners that improve temperature uniformity by generating intensive turbulence inside the furnace. Usually at the initial stage of the firing process, an automatic control system keeps these burners at their lowest capacity if the gas tracks the air in a proportional manner, which results in insignificant turbulence. If the burners work with gas-only control (the air stream is always kept at maximum rate), the temperature uniformity is good but the fuel consumption is very high. Improvements in control technology have made it possible to operate the burners in a pulsing mode, firing them for a few seconds at their maximum heat capacity, then cutting off the air and fuel for a period of time, then firing them again for a short period, and so on. This pulsing mode increases the level of turbulence inside the whole furnace space and promotes excellent temperature uniformity without the use of fans.
Although pulse firing is not a new concept, it has not been widely used in the graphite electrode industry. Additionally, SGL had rigorous specifications for the new furnace, including a required temperature uniformity of ±10.0 degrees C and the ability to safely handle the volatiles being released from the electrodes. The engineers were confident that they could meet SGL’s specifications using the pulse firing approach, but SGL wanted proof. Building the furnace to test the design concept would have been very expensive and risky. Building a scale model would have been less expensive but still quite costly, and would have run the risk that scaling difficulties might produce inaccurate results. The engineers had recently begun using computational fluid dynamics (CFD) to address problems like these. A CFD analysis provides fluid velocity, temperature, species concentrations and other variable solutions for problems with complex geometries and boundary conditions. As part of the analysis, a researcher can change the geometry of the system or the boundary conditions and view the effect on fluid flow patterns or concentration distributions. CFD also can provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop a device, thereby reducing design cycle times and costs.
Selecting CFD SoftwareTsvetan Ralchevski, Ph.D., development manager for Swindell Dressler, said that the company evaluated several leading CFD software packages and selected FLUENT from Fluent Inc., Lebanon, N.H., primarily for its strong combustion models. “FLUENT can handle any type of hydrocarbon fuel, which is important to us because our customers, especially those in developing countries, run a wide range of different fuels including natural gas, fuel oil and coal,” Ralchevski said. “Another advantage that was very critical in this application is that FLUENT models not only the fluid areas but also the solid areas inside the furnace. Since we had to prove the temperature distribution within the product load to validate this design, we couldn’t have solved this problem without this feature.”
Ralchevski used the program’s GAMBIT pre-processor to define the geometry of a 15-ft high section of the 58-ft long by 70.5-ft wide furnace that included the burners, product load and support area. The model included two North American Manufacturing Co. 4441-4A burners. He used the program’s time-dependant capability to enter the pulsing behavior of the burners and, through experimentation, determined boundary conditions that would generate the needed temperature profile. Ralchevski worked with William Clark from the Engineering Department, who is the designer of the furnace combustion system, to manipulate the input data regarding these conditions.
Since Swindell Dressler deals with large furnaces, each with many burners, the CFD program’s non-conformal mesh capability, along with the ability to read and write profile files, have helped the company significantly reduce computational costs. For the graphite electrode furnace, for example, Ralchevski and Clark modeled just one burner and used the resulting jet outlet profile as the inlet boundary conditions for all the burners connected to the furnace model.
Proving the New DesignThe simulation showed that the new combustion system design would provide excellent temperature uniformity. “The results of the computer simulation were exactly what was needed to validate the design and convince SGL of the value of the new approach,” Ralchevski said.
“The modeling program allowed us to be confident in asking for the furnace to be built to rigorous specifications, and it allowed Swindell Dressler to be confident in agreeing to those specifications,” said SGL’s Morganton, N.C. plant manager. “Without the computer simulation, we might not have purchased the new furnace, and we would have missed out on all of its related benefits.”
The new furnace was installed at SGL’s facility in early 2002 and has been in full operation for more than five months. While some optimization efforts are still under way, the furnace is reportedly running very close to the modeled results.
Beyond this application Ralchevski has also used the new software to solve a number of other design problems. In another application, he used CFD to validate a new design for a kiln that produces sanitaryware. The exhaust system in this type of kiln is normally attached to the kiln roof. The problem with this approach is that small particles that have eroded from the exhaust system may fall down on the product, where they can impact quality. Using CFD, Swindell Dresser validated a new design in which the exhaust system is attached to the sides of the kiln, so that particles are much less likely to fall on the product.
“CFD has helped us make significant design improvements by making it possible to evaluate our ideas without the expense of having to build a prototype,” Ralchevski said.
As a result, ceramic and related manufacturers are reaping the benefits of improved firing processes.
SIDEBAR: Other Applications of CFDThe benefits of CFD aren’t limited to kiln and furnace engineers—ceramic and glass manufacturers are also using this sophisticated modeling system to improve their own products.
For example, in float glass forming, the molten glass is conveyed on a molten tin bath and is stretched laterally by machines to enhance the ribbon width while it simultaneously cools and solidifies. The placement of overhead heating and cooling units is crucial to produce high optical quality (i.e., very uniform thickness). Dr. Rajiv Tiwary and Dr. C.K. Edge of PPG Industries, Inc. have successfully modeled the float glass forming process using CFD. The simulation results provided insight into the process, including width and thickness profiles, longitudinal and lateral stresses and residence times. Excellent agreement between the CFD results and plant measurements was obtained, and the model was subsequently used for several parametric design studies.
Modeling glass delivery systems is another important application. A glass delivery system (front-end system) must bridge the melter and forming to ensure that glass is conditioned to the requirements of the forming operations while maintaining the highest quality. An improperly designed front-end system can cause a number of problems, including poor glass quality and inhomogeneity in the glass thermal profiles. CFD has enabled glass manufacturers to guide and optimize such system designs.
CFD is also being used to model solid oxide fuel cells (SOFCs), enabling designers to optimize the efficiency of the fuel cells through detailed analyses of complex electrochemical and mass transport phenomena taking place inside the cells. As a result, manufacturing and operation costs are reduced, and the analysis turn-around time is considerably decreased.
By using CFD, ceramic and glass manufacturers can reduce the time and expense of developing new products, troubleshoot existing products and processes, decrease the number of prototypes needed and gain valuable physical insight into their problems.