Holding iron in the coreless furnace.
Modern day induction furnaces owe much to pioneer inventors, Dr. Edwin Northrup and Edward Colby. No matter what the capacity or power rating, all induction melting and holding systems operate under the basic principles these men developed.
Over the years, Colby's Ring Furnace became the channel furnace and has evolved into an efficient iron holding system in many foundry applications. Changes have been made in the geometry of the loop and in the shape of the furnace body. Multiple inductors and various frequencies have been applied to the inductors. Development of modern refractories has also had a significant influence on the performance and growth of the channel furnace. When the channel furnace's high electrical efficiency is combined with its flexible capacity and correct refractories, the channel furnace has proven itself to be an ideal unit for holding and duplexing iron.
During this same period of development, Dr. Northrup's high-frequency coreless furnace has become the modern coreless furnace. This too has seen many changes, particularly in the capability to change frequency and power control. Technology has advanced from sparkgap converters to motor generators, magnetic multipliers and mains frequency, all with fixed frequency outputs and options on manual or automatic controls. Today, these have evolved into fully automatic, variable frequency inverter power units, along with frequency matched melting and holding systems.
The modern inverter-powered coreless furnace is an unquestioned tool in many thousands of melt shops around the world. It, too, has benefited from the development of modern day refractories, which allows the melting of high-tech alloys required by today's more demanding casting requirements. Ranging in size from a few kilograms to over 100 tons with power inputs to 23,000 kW, the coreless furnace has, on many occasions, been used as a holding and duplexing unit, as well, but with some limitations.
Most coreless holding applications utilize furnaces from five tons upward with power units capable of meeting the superheating/melting requirements of molten iron, and with enough additional power for emergencies, such as when refractories cool down because the furnace lid is left open too long. When full, these furnaces perform flawlessly.
However, because in holding/duplexing operations the metal level in the furnace can vary greatly due to the need to tap metal for pouring or for refilling the furnace, and, in some cases, emptying it entirely, the coreless furnace can be inefficient as a holder. As the metal level within the power coil drops, its operational efficiency drops, significantly, and the power demand increases.
A number of variations to coil geometry have been developed in an effort to overcome these shortcomings. For example, with the early switchable coil design, the furnace coil is divided into three sections: an upper, middle and lower section. Power is applied to the upper and middle section for melting and alloy adjustments, and to the middle and lower section for holding and superheating.
This system, however, requires an increased number of water-cooled circuits and may need increased maintenance on the high-current changeover switch. The shorter coil design can also lead to lower electrical efficiency. And, once the metal level drops to low levels, the operating efficiency changes in much the same way as the single coil furnace.
Another solution has been the short-coil furnace. In this design, the power coil is restricted to the lower section of the furnace. This can result in difficulty in alloy adjustments because of the high static head of metal above the coil and the short coil configuration only provides cooling to the refractories retained within the coil. In addition, as the metal level drops below the top of the power coil the operational efficiency of the furnace drops.
Still another development is the result of a research program undertaken to efficiently couple power to the very shallow furnace used in the CLAS vacuum casting process. The resulting coil coupling design can be applied effectively to a furnace load 150 mm high (approx 6 in.) and with a diameter of 1000 mm (approx 40 in.)
This principle has been applied to the minimum heel coreless holding furnace. This design utilizes a full-length power coil which provides for adequate stirring of a full furnace for alloying of the metal and cooling for the refractory lining throughout its full length. With this coil design, operational efficiency remains down to 20% of its total capacity, and it will operate efficiently down to 10%.
While the channel furnace has proven to be an efficient holding/duplexing unit for cast iron, it, too, has its shortcomings. The basic principle on which this furnace operates requires that the loop be continuously full of molten metal. Because of this, the total capacity of the channel furnace cannot be used, and energy must be continuously applied to the loop to maintain the molten heel.
Some examples of how minimum heel coreless furnaces of various sizes and production requirements for holding iron stack up in terms of energy consumption with channel furnaces of various sizes and production requirements are offered in Fig. 1 and 2. The underlying assumptions in calculating energy consumption for both units also are shown. As indicated, the minimum heel coreless furnace can be an efficient holder of cast iron, because the furnace is to be emptied completely and turned off during off hours and shutdown periods. Changing alloy grades in this type of furnace is also a relatively easy task.
The coreless furnace also compares well when it comes to refractory usage. The development of modern refractories has enabled the channel furnace to grow into the foundry tool that it is today. These, however, tend to be sophisticated and costly materials which often require time-consuming and skilled installation.
In the case of the coreless furnace for melting and holding gray iron, cost-effective silica linings can be utilized. While these types of linings will not last anywhere near as long as those typically used in the channel furnace, modern push-out lining mechanisms and today's vibratory compaction methods can significantly reduce the time needed to reline the coreless furnace. [Figures 1 to 2 Omitted]
PHOTO : With proper coil design and attentive operating procedures, the coreless furnace can be an
PHOTO : efficient iron holding unit.
Graham Cooper Inductotherm Corp Rancocas, NJ
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|Date:||Mar 1, 1990|
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