Industrial electric heating furnaces – Induction furnace device – Coreless
Patent
1992-11-02
1994-11-22
Walberg, Teresa J.
Industrial electric heating furnaces
Induction furnace device
Coreless
373 35, 373142, 373144, 373509, 3731347, 3731356, H05B 622
Patent
active
053675320
DESCRIPTION:
BRIEF SUMMARY
The present invention relates in general terms to the continuous production of oxides melted in a furnace heated by high frequency, direct induction and having at its top an intake for the continuous distribution of the oxides to be melted and in its lower part a discharge nozzle fox the melted products.
More specifically, the present invention relates to such a direct induction melting furnace of the cold crucible type, i.e. whose side walls are cooled and therefore having a solid protective crust with respect to the oxide to be melted.
In most of the hitherto envisaged applications for such furnaces, materials based on oxides such as e.g. coloured glasses, glass-ceramics, crystal, etc., which were prepared had to undergo a prolonged refining, i.e. had to spend a long time in the furnace. This requirement and the design of the prior art furnaces has led to high specific electrical energy consumption levels, which are incompatible with the production of reduced refining melted oxides, such as are used in applications when the degree of refining is not determinative, such as e.g. enamels used for electrical household equipment or for the vitrification of waste.
A description will firstly be given of the present design of cold crucible direct induction furnaces by explaining their characteristics, the reasons for their high electrical power consumption and consequently their lack of suitability for producing reduced refining glasses of the types defined hereinbefore.
The direct induction furnaces at present in use in industry have a geometrical shape such that the melted oxide bath has a height roughly equal to its diameter. This shape has been chosen for three essential reasons. Firstly it makes it possible to ensure a good electrical transfer efficiency of the power between the induction coils and the melted material, because furnaces with a low bath height are considered to have a reduced efficiency. Secondly a significant melted product height ensures a long residence time for the same (roughly 10 hours) in the furnace and consequently allows advanced refining, which leads to the elimination of any bubbles and to the chemical and thermal homogenization of the complete liquid phase. Finally, the third reason for this dimensional choice results from problems linked with the furnace hearth because, to permit its cooling, it is made from metal and then constitutes a very prejudicial shield for the penetration into the molten bath of the electromagnetic field for heating the latter. Thus, in the case where the hearth is refractory, i.e. electrically insulating, most of the electromagnetic power emitted by the inductor is dissipated in the induced current penetration zone and this affects most of the mass of the bath. Conversely, if the hearth is made from metal cooled by water and this is a frequently encountered case, it constitutes a shield to the magnetic field and, by deforming the field lines, creates, as can be seen in FIG. 1 relative to the prior art, a zone 2 in which bath heating is reduced.
In FIG. 1, a furnace 1 has a cold crucible 3, an inductor 5 and a metal hearth 4. The curves 6 and 8 show the regions 10 where the induced currents penetrate the bath. In the annular zone 2 in the lower part of the bath directly surmounting the cooled metal hearth 4, there is a considerable restriction of the bath heating by the induced currents. Under these conditions, it can be readily gathered that an increase in the bath height decreases the significance of the phenomenon relative to the complete bath.
Unfortunately, the thus given extension to the furnace height, which correlatively conditions that of the lateral surface, in cold wall furnaces leads to heat losses through the vertical furnace walls, which are very high and with a flux density of e.g. 15 to 40 W/cm.sup.2, as a function of the melting temperature of the mixture and its physical characteristics. The losses through the furnace hearth are lower and generally correspond to flux densities of between 5 and 15 W/cm.sup.2. All these heat losses lead t
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Boen Roger
Gnilka Jean-Pierre
Ladirat Christian
Pilliol Henri
Commissariat a l''Energie Atomique
Hoang Tu
Walberg Teresa J.
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