1. Field of the Invention
The invention relates to a device for the production of a glass melt from a batch.
2. Description of the Related Art
Such a device for the production of a glass melt is mostly of rectangular ground plan. It has peripheral walls as well as a bottom. It is equipped, furthermore, with an electric heating system. Here there can also be provided several heating circuits. The heating-up occurs over electrodes. There takes place here a direct current flow through the conductive glass melt.
Under the bottom of the melt tank there is located a draw-off channel. The draw-off channel is fully open toward the melt bath. The channel runs essentially horizontally, and therewith in a plane parallel to the melt surface. It has an outlet which is located in the region of one of the side walls of the melt tank.
In operation the glass melt is always covered by a cold batch layer orxe2x80x9cbatch blanketxe2x80x9d. The tank, therefore, is also designated as axe2x80x9ccold top melt tankxe2x80x9d. There, heed is taken that the surface of the glass melt is always covered by batch. At the edges of the melt surface, i.e. on the peripheral walls, however, as a rule a so-called glow strip is left open that serves for the de-gassing of the melt. This can be controlled over an charging machine. The batch layer lying on the glass melt has, namely, an insulating effect. If the batch layer is not closed, there occur heat radiations, which means losses of energy. Mixture, accordingly, is always supplied in a corresponding amount.
For the achieving of a faultless quality it is absolutely indispensable that through the draw-off channel molten and refined (bubble-free) glass emerges and that still unmelted batch is not carried along. Even small quantities of unmelted or only initially molten batch are unacceptable in respect to the glass quality to be achieved.
All strivings are being undertaken in this direction. To this there belongs inter alia the correct arranging of the heating circuits and of the appertaining electrodes, as well as the bringing-in of a sufficiently high electric power. Despite these measures it does occur that unmolten or only insufficiently melted batch as well as incompletely refined glass passes through the draw-off channel with the flow of the glass melt, which is extremely troublesome in the further processing and results in an unsatisfactory quality of the glass.
DE 1 080740 B describes a device for the production of a glass melt from a batch, in which the heated melt tank has a draw-off channel the entry opening of which is arranged in the region of the bottom of the melt tank. The draw-off channel is located under the bottom of the melt tank and has an outlet opening in the zone of a peripheral wall. There the draw-off channel in at least two corners is brought directly onto the wall of the tank.
U.S. Pat. No. 4,410,997 A describes a device for the production of a glass melt from a mixture. Here too, the heated melt tank has a draw-off channel which runs in a central zone of the tank bottom. There, no covering is provided.
DE 29 17 386 A likewise describes a melt tank with a draw-off channel which proceeds from the bottom zone of the tank. A covering of the draw-off channel is not provided here.
Underlying the invention is the problem of constructing a melt tank of the type described in such manner that there is guaranteed a complete melting-up of the batch as well as a complete refining of the glass, and that not even the smallest batch components and bubble-infected glass pass into the draw-off channel and, together with the rest of the glass flow, pass through its outlet opening.
The inventors have perceived the following:
The draw-off channel mentioned, open toward the melt bath, is located outside of the main zone of the electric heating. There the flow velocities arising are comparatively low. Here the glass of the glass melt can dead-melt in certain zones. In the zone of the peripheral wall in which also the outlet is located, a predominantly downward directed flow builds up. Through this flow it is possible that incompletely melted-up mixture or incompletely refined glass will enter directly into the removal flow in the draw-off channel and will cause the disadvantageous consequences mentioned.
For the solution of the problem the inventors propose the four following courses of solution, which can be applied in each case alone or in combination.
The first course of solution consists in the following:
The draw-off channel is covered on a part of its extent. There remains only an opening that is located in the zone of action of the heating circuits. In general this entry opening into the draw-off channel will be arranged in a central zone of the tank bottom. The tank bottom, thereforexe2x80x94except for the entry opening into the draw-off channelxe2x80x94is simultaneously the covering of the draw-off channel. One could also say that the channel is largely covered. The covering extends from the zone of its outlet opening to a central zone of the tank bottom, purposefully, however, at least up to a tenth of the distance between two oppositely lying peripheral walls of the melt tank.
There is thereby prevented the possibility that incompletely melted-up batch components as well as bubble-infected glass will be able to pass out of the zone of the tank over the channel, directly into the channel and therewith into the removal flow. In this manner there can be achieved a perfect glass quality, provided that all the other parameters are in order.
As material for the covering mentionedxe2x80x94i.e. the partition between the glass melt and the channelxe2x80x94there come into consideration fireproof materials (metallic or nonmetallic) with low corrosion, high endurance, low costs and a low glass-fault potential. Here there offer themselves refractory metals such as molybdenum or tungsten as well as their alloys.
The second course of solution consists in the following:
a covering is provided, for example of the material of which also the tank consists. The covering covers off a part of the surface of the melt surface. The melt surface is therewith reduced. This leads to a favorable influencing of the glass flow, since the heat lead-off over the covering is less than over the mixture cover, and since therewith a thermal barrier is built up underneath the covering. Thereby there is prevented the possibility that incompletely melted-up batch of incompletely refined glass can be directly into the removal flow. In this manner incompletely melted-up batch constituents remain longer in the melt tank and are completely melted up and refined. The result is the same as with the first course of solution.
The third course of solution consists in providing an additional heating in the zone of the draw-off channel, and namely in the zone above the draw-off channel where the glass flow leaves the tank. The effect of this additional heating lies in the build-up of a thermal barrier, by which there is prevented the possibility that incompletely melted-up batch or incompletely refined glass can pass directly into the removal flow.
The fourth course of solution consists in installing a heated dome (heated by burners for example) and in not laying in any batch in this zone. There, accordingly, a blank glass surface is established through which energy can enter into the glass bath from the heated dome. As in the second and third courses of solution, therewith a thermal barrier is built up, by which there is prevented the possibility that incompletely melted-up batch or incompletely refined glass can pass directly into the removal flow. By all four solutions, accordingly, there is guaranteed a complete melting-up of the batch and a sufficient refining of the glass melt. There is prevented the possibility that any batch particles that are incompletely melted up, or any bubble-contaminated glass will pass into the draw-off and thus leave the melt tank with the glass flow.