Improved cupola performance a major issue for operators.

In Cincinnati, nearly 200 individuals in the ferrous casting industry gathered October 7-9 for the [2.sup.nd] International Cupola Conference. Between speakers and attendees, eight countries were represented. Sponsored by AFS, the American Coke and Coal Chemicals Institute (ACCCI) and the Iron Casting Research Institute (ICRI), the conference featured 33 presentations covering current practices and proven technology to concepts for implementation in the future.

With each presentation, the speakers brought a fresh perspective to the attentive audience, offering them ideas and information to help them better utilize their cupola's melting performance.

Cupola Modeling

Because cupola performance is sensitive to several complex variables, the operation of a cupola is much more difficult than other melting processes, according to Seymour Katz, S. Katz Assoc., Inc. In order to eliminate the unwanted characteristics of the cupola's performance, a cupola process model that integrates the effects of numerous variables and guides operations toward optimal performance was developed by the Cupola Model Steering Committee, a subcommittee of the AFS Cupola Committee.

Katz said that the process model is a mathematical description of the important chemical and physical processes that govern cupola behavior. Given a set of input conditions, the model provides key output information such as melt rate, iron composition and temperature, and cost per ton of iron. These figures can be organized to develop control charts for predicting cupola performance, map broad ranges of output performance, study cost/benefits for current operations and potential major cupola modifications, develop an understanding of cupola behavior and solve problems.

The model describes heat transfer, fluid flow and chemical reactions, in terms of equations derived mainly from experimental data, and works in both Windows and DOS environments. To obtain a solution for a specific set of conditions requires less than 1 min with a 1998 state-of-the-art personal computer, Katz said.

Designing Your Melt Department

William Powell, Waupaca Foundry, Inc., stressed a melting engineer has to consider technical issues such as environmental compliance, iron types, scrap selection and melting costs, and softer issues like experience and comfort levels with a furnace type. These decisions fall mainly on the shoulders of the melting engineer when designing a new melt department.

He broke the department down into five areas: the cupola, material charging, air and water handling emission systems and the holding furnace. Cupola considerations start with defining required melting rate, diameter, height and number of tuyeres. When selecting tuyeres, size, number, angle of projection, height from the bed plate, manufacture method, water cooling pressure and volume all come into play, Powell said. Size, including both diameter and protrusion distance, is largely a function of achieving the accepted blast velocities of 6000-8000 scfm. For most moderately sized cupolas of 40-60 ton/hr, a 5-6-in. diameter tuyere is appropriate.

Powell noted that consideration must be given to charging rates and charging size, which is determined by cupola location, height, the estimated charges per hour and final melting rate. Allowance must be made for an increase in charges per hour for catch-up and melt rate increases above the planned maximum melting rate.

An over-sizing of the charge bucket and cycle rate by 25-35% is not excessive. The drive for the charging bucket also is important. A hydraulic drive can utilize smaller horsepower motors driving hydraulic pumps that drive hydraulic motors attached to the cable drum through planetary gearing. This results in a compact design with advantages such as multiple single speed, single direction motors, simplified alignment, smaller, low-cost gearing, easier maintenance, in-place back-up systems and virtually infinite speed capabilities.

Another vital consideration, according to Powell, is yard layout, which is often dictated by features such as railroad access, property lines, existing buildings and cupola locations. The least expensive form of weighing ferrous materials is the direct charging into a weigh hopper by crane magnet. This has a disadvantage, though, of requiring a highly skilled operator as well as a loss of charging time required for "dribbling" of the last few pounds required to make weight.

The largest system cost in the melt center is the emission system, which sometimes approaches 30% of total costs. This is due to new federal and state regulations and the requirements that "Best Available Current Technology" (BACT) for new installations be utilized. Powell suggested using the vertical heat exchanger when heating the blast air in the gas stream. He also mentioned the air-to-oil heat exchanger, which operates at higher temperatures than water, carrying more Btu's per gallon of oil than water.