Melting More Efficiently

April 15, 2005
You might try melting faster, or hotter, or in higher volumes, but still you may not be certain youre melting more efficiently.

Ford Motor Co. recently awarded a contract to Kuttner L.L.C., Port Washington, WI, for two complete cupola melt centers — proving again the value of the process to the metalcasting industry. About two-thirds of all the liquid iron destined for cast products is produced in cupola systems, making it the dominant system used by foundries that melt scrap. This order for new equipment will replace existing melting systems at Ford Powertrain Operations’ Cleveland (OH) Casting Plant. Kuttner L.L.C. is the U.S. subsidiary of Kttner GmbH & Co., K.G., Essen, Germany.

“Cupola capacity will be 75 ton/hour each,” a Kuttner release states. “Gray iron and ductile iron will be melted. The melt systems are designed to comply with the new EPA mandate iron and steel foundry MACT environmental standards. Design elements include top-gas combustion, recuperative hot blast, dry flue cooling, and a Lhr baghouse. The melt systems feature state-of-the-art controls, scrap-feeder batching, and vertical-lift charging, employing a bottom-door bucket.”

Naturally, new installations will have available the latest designs to melt metal efficiently. But, what does that mean. You might try melting faster, or hotter, or in higher volumes, but still you may not be certain you’re melting more efficiently. “Efficiency” is a dangerous term to toss around: everyone seems to understand what it means — and yet there’s so many ways to apply the word.

In melting operations, there’s an additional problem. What is “efficient” for one sort of operation may be inefficient for another, based on the metal that’s produced, the furnace type in use there, or a series of other standards.

Examining data

Proponents say that cupolas’ advantages over electric-melting furnaces include lower electric-power and scrap costs, higher tolerance for impurities, and more latitude for iron-production rates. Even so, there are disadvantages.

Cupola furnaces burn coke with an air blast to melt scrap steel, cast iron, and alloys into a consistent grade of iron. The variations in charge composition and blast effectiveness, among other variables, make the grade of iron produced less predictable. Still, cupolas offer several competitive advantages relative to newer electric-melt furnaces, including lower energy and scrap costs, higher tolerance for harmful trace elements, and a wider allowable range of iron-production rates.

Researchers working with the U.S. Dept. of Energy’s Office of Industrial Technologies (OIT), with funding also from the American Foundry Society, identified the biggest problem as being the complexity of cupola melting. It may involve 40 or more chemical reactions and a range of physical processes, which combine for operating difficulties and inefficiencies. It may result in iron that is sub-standard in quality, and thus inferior cast products, or scrapped metal, or do-over costs, or remediation.

OIT launched a research effort several years ago to develop a computer model for cupola melting, to integrate operating variables of cupola processes so that melting can be more consistent and easier to carry out, and more economically efficient. “By mastering and controlling the many variables involved, the computer process model can optimize operations and reduce energy consumption and greenhouse-gas emissions better than other available technologies,” according to the OIT report.

The model was completed in 2001, with impressive forecasts for its energy-saving potential. OIT anticipated it might save 91 billion Btu of coal every year for each installation, meaning vast amounts of cost and resource savings over fairly short periods. Just as important, it would allow greater operator control over the melting process and supply valuable data on the operating performance and improvement needs of cupolas.

So, what happened? Not enough, is the short answer. The model that was developed is considered to be an off-line program, and the principal researcher of the model has continued the inquiry toward developing an on-line program. Dr. Mohamed Abdelrahman is now an associate professor of Electrical and Computer Engineering at Tennessee Tech University’s Center for Manufacturing Research. He has maintained the effort, but the response from the foundry industry has been less than enthusiastic.

There are reasons for this. Few operations of any kind want to spare investment capital in order to be the first one to apply a “research quality” technology. And, many foundries apparently do not agree that a cupola is in need of this sort of improvement.

“I believe our program has been successful in achieving its research goals and objectives,” according to Dr. Abdelrahman, “but commercialization efforts have lagged behind for lack of funding to get the first prototype implemented in a commercial facility.”

He believes a new, and reasonably modest, joint-funding effort by DOE, the American Foundry Society, and private industry, would make the model commercially viable. But, the research effort is continuing in other ways.

Abdelrahman now is exploring “an off-line expert system that can be interrogated by cupola operators for solutions of operational problems. This expert system builds on the bulk of work invested in the development of a model for the cupola furnace by the AFS. It might be a first step towards the implementation of an online, computer-controlled cupola.” A prototype was developed at Tennessee Tech and is available now, but a commercial version of this is also stalled by the lack of available funding.

The rest of the story

Of course, cupola furnaces are only part of the picture. The fast-growing aluminum-casting sector is largely powered by induction melting systems.

Advances in medium-frequency technology have increased induction melting rates, and the availability of static frequency converters has helped improve furnace availability, lower maintenance costs, and scale back capital-investment costs. As for efficiency, with induction melting it is largely a function of electric power.

One of the leading builders of these systems is ABB, which offers a melting process module as an extension of its ControlIT process-control platform and OperateIT panel.

The Melt-IT processor is described as having “total control of the melting process.” It has no hard disk or active ventilation system and is insensitive to vibration, so it’s suited to the melt shop setting. One processor will control four furnaces and exchange data via an OPC interface with data highway configurations; high-speed communication can be performed via TCP/IP. Operator prompting and remote override are offered, and data can be stored and managed on an external PC.

This processor controls energy supply for melting and holding operations, and if required, start-up of the cold furnace or the sintering process. Customers can monitor limits and conduct maintenance inspection. All operating data and conditions are displayed and documented - electronically and/or in print, and MS Office extension (e.g., MS Excel) are included.

ABB’s line of coreless induction systems is configured to meet foundries needs in terms of furnace size, melting volume, and energy supply, among other custom considerations. These include the FS series with capacities from 1,000 to 4,000 kg and connected loads of 750 to 2,800 kW; the IFM series for higher melting rates, with capacities from 3 to 23 metric tons and connected loads of 0.8 to 1 MW/mt.; and large coreless induction furnaces with capacities of 22.5 to 60 mt and connected loads from 6,500 to 17,000 kW.

A recent design from Inductotherm was developed to emphasize efficiency. The Acutrak electric melting system for aluminum and nonferrous metals applies an electrical current directly to the crucible holding molten metal, not via filaments and fuel burners. Otherwise, the furnace components are air-cooled so water-cooling systems are not needed. Except for the crucible, the furnace remains cooled and because water-cooling is not need for the induction coil the metal melts quickly and cost-effectively. Reportedly, over 92% of the system’s electrical energy is applied to the melt.

Inductotherm emphasizes that induction technology delivers metal of “exceptional purity and at the highest temperature.” Also, it promotes the Acutrak as a system that is easy to install, operates with precise temperature control, and designed with a large diameter for simple ladling and charging.

A more recent entry to the field of coreless induction systems is Inter-Power Corp. This developer of induction-heating systems and power supplies, particularly for forging operations, has a line of coreless induction melting systems that includes three furnace types for capacities from 5 to 8,000 lb. The power supplies it designs range from 5 KW upward in various frequencies in IGBT transistor and SCR technologies.