Industrial electric heating furnaces – Induction furnace device – Coreless
Reexamination Certificate
1999-01-12
2001-02-06
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Induction furnace device
Coreless
C373S142000
Reexamination Certificate
active
06185243
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to melting of insulating materials such as glass, by direct induction in a cold crucible.
PRIOR ART AND PROBLEMS CAUSED
Any materials may be made by melting in crucibles heated with induction. The most widely used and the simplest crucibles are made of refractory material, but this type of crucible is not compatible with all melting baths. Some materials are corrosive to refractory materials when they melt whereas others, and particularly glass, may be polluted by the refractory materials.
These materials then have to be prepared in special crucibles called “cold” crucibles which have non-polluting walls. A cold crucible is composed of metal sectors cooled by water circulation in which the material to be prepared is heated by a peripheral induction coil. Separating the crucible into sectors, or partitioning, limits temperature rises due to induction in the crucible wall, and enables direct induction heating of the material contained in the furnace.
The induction coil is composed of one or several turns usually wound around the crucible, since it is generally accepted that this arrangement gives the best performances.
German patent DE-C-33 16 546 describes a furnace with a cold metal crucible also comprising a lower induction coil placed under the bottom which is partitioned into sectors. However, this lower induction coil only forms a makeup heating element. A main induction coil is wound around the crucible which implies that the metal side walls of the crucible must also be partitioned into sectors.
British patent document No. 2 279 543 describes a pour crucible with non-partitioned side walls for smelting metal alloys, and not insulating materials, also comprising an induction coil placed under a partitioned bottom. The induction coil is designed to determine the pour temperature, this pour taking place in the middle of the induction coil, but it is not used and it is not designed to melt the entire quantity of metal contained in the crucible. The crucible is designed to be used with a main installation which melts the metal. Therefore, the induction coil is a makeup heating element. The main heating may be provided by induction, in which case either the side metal walls are partitioned, or the walls are refractory and introduce the incompatibility problems mentioned above.
German patent DE-C-564 693 describes a furnace with heating from the inside, comprising sleeves which penetrate into the melting bath through the bottom and containing induction turns. This type of furnace is complicated to make, particularly if it is to be made with cold walls.
It may be deduced from the state of the art described above that no-one has ever considered using an induction coil placed under the bottom of a crucible as the sole means of heating.
SUMMARY OF THE INVENTION
One purpose of this invention is to provide a melting furnace with a cold crucible particularly adapted to glass, which is particularly simple and inexpensive to make, while having guaranteed performances similar to those of conventional furnaces.
Another purpose of this invention is to provide such a furnace which, for a given capacity, requires less expensive components to supply the excitation voltage for the induction coil.
Another purpose of this invention is to provide such a furnace with a higher capacity making use of commercially available components to supply the induction coil excitation voltage.
Another purpose of this invention is to provide such a furnace with particularly good thermal insulation between the melting bath and the crucible walls.
This invention achieves these objectives by heating the glass only through an induction coil placed under the bottom. Since no induction coil acts through the side wall of the crucible, this side wall may be continuous, in other words not electrically partitioned, which makes it particularly easy to manufacture.
The average diameter of the induction coil is approximately half the diameter of an induction coil that would conventionally be wound around the crucible, which significantly reduces its inductance and therefore its excitation voltage. The components supplying the excitation voltage can therefore be less expensive for a given crucible diameter, or existing components may be used to make a crucible with a larger diameter.
Moving the periphery of the induction coil away from the crucible side walls means that a thick layer of solidified glass can be obtained on these side walls which reduces heat losses between the melting bath and the cooled crucible. This type of thermal insulation cannot be obtained for metals, which are good thermal conductors both in the liquid state and in the solid state.
For metals, the performances of this type of furnace are not very good compared with what can be obtained in conventional furnaces. However for glass, the performances of the furnace can be optimized by special sizing.
This invention is particularly aimed at a melting furnace for insulating materials comprising a cooled crucible with metal side walls, a partitioned and cooled bottom, and at least one induction coil placed under the bottom. The only means of heating is said induction coil and the metal side wall of the crucible is continuous.
According to one embodiment of this invention, the depth of the melting bath in the crucible and the excitation frequency of the induction coil are chosen such that said depth and said inside half-radius of the crucible are less than the thickness of the skin in the bath resulting from the excitation frequency of the induction coil.
According to one embodiment of this invention, the periphery of the induction coil is setback from the side wall of the crucible.
According to one embodiment OF this invention, the bottom is composed of tubes placed side by side.
According to one embodiment of this invention, the tubes are made of a material that is a good conductor or heat and a bad conductor of electricity.
According to one embodiment of this invention, the furnace comprises a metal base and lid which, together with the crucible to which they are connected, form a Faraday cage protecting the external environment from the radiation produced by the induction coil.
According to one embodiment of this invention, the furnace comprises several induction coils distributed under the bottom and controlled independently.
According to one embodiment of this invention, the furnace is rectangular shaped, and the induction coils are distributed along the long axis of the rectangle.
REFERENCES:
patent: 1645526 (1927-10-01), Gerth
patent: 4687646 (1987-08-01), Mateika et al.
patent: 5367532 (1994-11-01), Boen et al.
patent: 5479438 (1995-12-01), Blum et al.
patent: 564 693 (1932-11-01), None
patent: 608 476 (1936-03-01), None
patent: 938 263 (1956-01-01), None
patent: 33 16546 (1984-04-01), None
patent: 672 026 (1929-12-01), None
patent: 2 279543 (1995-01-01), None
patent: WO92/15531 (1992-09-01), None
Boen Roger
Bonnetier Armand
Ladirat Christian
Burns Doane Swecker & Mathis L.L.P.
Commissariat A l'Energie Atomique
Hoang Tu Ba
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