GLASS MELTING FURNACE

Abstract: A vertical glass melting furnace (10) is provided with a material entrance (1) at the uppermost portion and a molten glass exit (2) is provided at the lowermost portion. A resistance heating element 6 is provided over the substantially entire part of the horizontal section of the furnace at at least one level which is immersed in the molten glass.

Description: The present invention relates to a glass melting furnace and, more particularly, to a vertical electric furnace which is capable of producing high-quality glass with efficiency and stability. A conventional vertical electric glass melting furnace generally has a structure in which a material entrance 1 is provided at the upper most portion of the furnace, a glass material (batch) 3 supplied to the surface of a molten glass 4 in the furnace is melted by heating the molten glass 4 by electricity directly by passing an electric current through the glass from electrodes 5 which are immersed therein, and the molten glass after it is refined is conveyed to a working section from a glass exit 2 provided at the bottom portion of the furnace, as shown in Figs. 4 to 6. The rod electrodes 5 may be horizontally inserted from the side wall of the furnace, as shown in Fig. 4, vertically inserted from the bottom portion of the furnace, as shown in Fig. 5, or inserted from the upper portion of the furnace through the layer of the glass material 3, as shown in Fig. 6. In such an electric furnace, the surface of the molten glass 4 is covered with the layer of the glass material (batch) 3 so as to form what is called a "cold top" for the purpose of efficient utilization of thermal energy. The melting and refining of the glass proceeds in the direction of the depth of the furnace. In order to obtain high-quality glass by such an electric furnace, it is necessary that the glass supplied to the working section through the exit 2 at the bottom portion of the furnace has a sufficient thermal history and does not contain any bubble or unmelted substance. The thermal history of the glass reaching the exit 2 is determined by the temperature of the glass in the furnace and the flow of the molten glass. Since the flow of the molten glass is mainly subject to the convection current caused by the temperature distribution of the glass in the furnace, the most important thing in the operation of the furnace for producing high-quality glass is to maintain the furnace in a stable thermal equilibrium. It is difficult, however, to make the distribution of electric current uniform in the plane of the furnace in a conventional vertical electric furnace which utilizes the Joule heat produced on the glass by the rod electrodes for heating glass. Particularly, since the current density becomes high in the vicinity of the tip portions of the electrodes, the temperature of the glass between the tips or the electrodes is apt to be raised. Since the electric resistance of glass is reduced as the temperature thereof becomes higher, more and more current flows to the high temperature portion, thereby disadvantageously furthering the local heating. Such a non-uniform temperature distribution causes convection current of the molten glass in the furnace and, as a result, the molten glass having a short thermal history, namely, the molten glass containing bubbles and unmelted substances due to the insufficient heating flows out of the exit, which is supplied to the working section and may cause a defect in the product. In addition, the non-uniform temperature distribution and the convection current produced thereby on the molten glass lead to local melting of the batch on the surface of the molten glass, so that there produces some portions at which the molten glass is not covered with the batch layer, thereby increasing the heat loss and lowering the glass temperature. In a furnace for melting glass by heating it with Joule heat, the temperature control of the molten glass is very difficult for the above-described reasons. Thus, it is almost impossible to prevent the non-uniform temperature distribution of the molten glass. These problems are serious in the case of melting glass which does not contain any alkaline component and, hence, has a large electric resistance. That is, since such a non-alkaline glass has a large electric resistance, it is necessary to reduce the distance between the electrodes in order to provide the power necessary for melting the glass, so that the non-uniform temperature distribution becomes more prominent. The present invention is based on our finding that it is possible to achieve improved control of heating, more uniform temperature distribution with reduced levels of local convection, and thus the efficient production of high-quality glass, using a glass melting furnace which contains one or more essentially horizontally disposed resistance heating elements arranged as described hereafter. DE-C-958 149 describes the use of a horizontal array of parallel electric heating elements in a quartz melting furnace for the production of solid quartz particles. The quartz is fused during passage between said heating elements and passes to a perforated support plate, the holes in which are of such size, shape and number that only a slow flow of molten quartz passes through the plate, permitting equilibrium to occur with the quantity of quartz formed in the preceding fusion zone. The shape of the holes is also designed to promote breaking off of the molten quartz particles, which fall to a collection zone at the bottom of the furnace. The support plate may, if desired, be heated, e.g. by gas jets situated below it, to prevent it from being clogged by the quartz. It will be observed that the heating elements in this furnace are all situated in a zone of melting quartz and are thus in contact with a proportion of unmelted material. In US-A-4 026 689 there is described the use of electrically heated horizontal perforated plates for assisting in the temperature control and conditioning of molten glass in feeders or bushings from which streams of glass are drawn for attenuation into fibres. A number of other heat sources are also employed, however, the glass first being melted in a separate furnace, heated by gas jets in a forehearth channel, and further heated by electrical heating of the body members of the feeders/bushings. In contrast to these previously described types of apparatus the glass melting furnace of the invention relies on the resistance heating element or elements as the sole source of heat and is constructed so that in use such element or elements are immersed at at least one level in molten glass, thus permitting the particularly simple and efficient production of high-quality glass as described in greater detail hereinafter. Thus according to the present invention there is provided a glass melting furnace comprising a material entrance provided at the uppermost portion thereof, a molten glass exit at the lowermost portion thereof, and a chamber therebetween in which is positioned a resistance heating element which extends over substantially the entire horizontal cross-sectional area of the chamber while allowing material to pass downwardly therethrough, wherein that said heating element is positioned at a level such that in use it is immersed in molten glass. The resistance heating element may be a plate-like heating element having at least one opening. The resistance heating element may also be a plurality of rods horizontally arranged in parallel to each other. In a glass melting furnace of the present invention, since molten glass is heated by the resistance heating element, control of heating is facilitated. Since the heating element is provided over the substantially entire part of the horizontal section of the molten glass in the furnace, the temperature distribution in the plane of the molten glass becomes more uniform. In addition, since the heating element functions as the obstacle to the flow of convection, the convection is suppressed, and since the temperature is at its maximum at the surface portion of the heating element, and becomes lower toward the bottom of the furnace, convection of molten glass is reduced in the region lower than the heating element. Thus, in a glass melting furnace according to the present invention, it is possible to produce high-quality glass with efficiency. A glass melting furnace according to the present invention is applicable to any kind of glass under substantially the same conditions. In a conventional electric furnace, it is necessary to change the distance between the electrodes, the arrangement of the electrodes, the voltage applied to the electrodes, etc. in accordance with the electric resistance of the glass, namely, the kind of glass being melted, as described above. In contrast, a glass melting furnace according to the present invention can be universally used to any kind of glass. Embodiments of the invention are described below, by way of example, with reference to the accompanying drawings in which: Fig. 1 is a vertical sectional view of an embodiment of a glass melting furnace according to the present invention; Fig. 2 is a sectional view of the embodiment shown in Fig. 1, taken along the line II -II; Fig. 3 is a vertical sectional view of another embodiment of a glass melting furnace according to the present invention; and Figs. 4 to 6 are respectively vertical sectional views of conventional vertical glass melting furnaces. In a glass melting furnace shown in Figs. 1 and 2, a furnace body 10 made of brick has a square shape in a plan view and it is provided with a material entrance 1 at the upper portion thereof and a glass exit 2 (a throat protruding sideways in this embodiment) at the bottom portion thereof. The chamber of the furnace accommodates molten glass 4. A glass material (batch) 3 is uniformly supplied to the surface of the molten glass 4. A plate-like resistance heating element 6 is provided over the entire part of the horizontal section of the furnace at the level which is immersed in the molten glass 4. The plate-like resistance heating element 6 is provided with a plate-like heating portion 6a having a multiplicity of holes 7, and terminal portions 6b which rise from both ends of the heating portion 6a. The end portions of the resistance heating element 6 which are situated outside of the furnace are connected to a power source V. When electric power is supplied on the resistance heating element 6 in the glass melting furnace having the above-described structure, the glass is heated. Since the resistance value of the resistance heating element 6 is determined in one-to-one correspondence with the temperature, it is possible to exactly generate a predetermined calorific value by supplying a predetermined magnitude of electric power. Since the heating portion 6a of the resistance heating element 6 is uniformly provided over the entire part of the horizontal surface of the furnace, the molten glass 4 is uniformly heated over the entire area of the horizontal surface and the temperature distribution of the molten glass becomes conspicuously uniform. For these reasons, the convection of the molten glass due to the non-uniform temperature in the horizontal plane is reduced. In the molten glass 4 in the glass melting furnace, the temperature reaches its maximum at the portion at which the molten glass 4 comes into contact with the surface of the heating portion 6a, and becomes lower in proportion to the distance from the heating portion 6a. Therefore, almost no convection current of molten glass is produced on the underside of the heating portion 6a. On the upper side of the heating portion 6a, since the temperature becomes lower, the upper the portion is, convection current of molten glass 4 is produced. However, since the upper portion of the molten glass is separated from the lower portion thereof by the heating element 6a, the convection current of molten glass on the upper side of the heating portion 6a is hardly transferred to the underside of the heating portion 6a. Even the convection current of molten glass on the upper side of the heating portion 6a is very weak because the depth of the upper portion of the molten glass is made shallow by the partition of the heating portion 6a and the temperature distribution in the horizontal direction is uniform, thereby involving no fear of bringing about a problem such as local melting of the glass batch 3 and the exposure of the molten glass 4. Thus, the glass melting furnace of this embodiment is capable of uniformly melting the glass batch 3 and reliably producing molten glass containing no bubbles and unmelted substance. In the embodiment shown in Figs. 1 and 2, each of the holes 7 of the plate-like heating portion 6a is circular, but the shape of the hole 7 is not restricted. The shape, number, arrangement, etc. of the holes 7 may be modified so long as the calorific value is uniform over the entire surface of the heating portion 6a of the heating element 6 and the molten glass is not prevented from flowing to the bottom. In a glass melting furnace shown in Fig. 3, a plurality of rod-like heating members 8 are horizontally disposed in parallel to each other as a resistance heating element, and a slit 9 is provided between each heating member 8. In this embodiment, the heating members 8 are provided at vertically two stages in such a manner that the horizontal directions of arrangement of the heating members 8 are orthogonal to each other. The other structure is the same as in the embodiment shown in Figs. 1 and 2. This glass melting furnace shown in Fig. 3 also enables the reliable production of clear molten glass in the same way as in the glass melting furnace shown in Figs. 1 and 2. The preferable ratio of the area of the holes 7 to the area of the plate-like heating portion 6a of the heating element 6 in the glass melting furnace shown in Fig. 1, the diameter and the number of the rod-like heating members 8, the width of the slit 9 provided between every adjacent heating members 8 and the like in the glass melting furnace shown in Fig. 3 are determined by the amount of glass being melted, the calorific value necessary for melting glass, the electric resistance of the heating element, etc. in consideration of the fluidity of molten glass. If the area of the openings is too large, convection current of molten glass is produced. In such case, it is possible to suppress the convection of the molten glass by providing the heating elements 6 or 8 at plural stages, therby reducing the substantial opening area. The ratio T/S x 100 (%) of the area T of the holes 7 (the sum of the areas of the holes 7) to the total area S of the heating element 6 (the product of the length and the width of the heating element 6) is preferably 5 to 60 %, more preferably 10 to 40 %. When the heating elements 6 are provided at plural stages, the distance between each stage is preferably 20 to 350 mm, more preferably 50 to 200 mm. When the rod-like heating members 8 are horizontally arranged in parallel to each other at two stages, as shown in Fig. 3, the diameter of the heating member 8 is preferably 30 to 150 mm, more preferably 50 to 75 mm. The distance between each heating member 8 (the width of the slit 9) is preferably 30 to 300 mm, more preferably 30 to 150 mm. The distance between each stage is preferably 20 to 350 mm, more preferably 50 to 200 mm. In the case of horizontally arranging the rod-like heating members 8 in parallel to each other at only one stage, the diameter of the heating member 8 is preferably 30 to 150 mm, more preferably 50 to 75 mm. The distance between each heating member 8 (the width of the slit 9) is preferably 30 to 200 mm, more preferably 30 to 100 mm. When the rod-like heating members 8 are horizontally arranged in parallel to each other at three stages or more, the diameter, the horizontal distance and the vertical distance of each heating member 8 is substantially the same as in the case of providing the heating elements at two stages. The vertical position of the furnace at which the heating element is provided is the upper portion of the molten glass 4 and preferably in the vicinity of to the batch layer 3. However, if the heating element is too close to the batch layer 3, unmelted glass and/or unrefined glass passes through the openings of the heating element and flows to the bottom portion of the furnace. Accordingly, the heating element (when a plurality of heating elements are provided, the lowermost heating element) is provided at the portion at which the glass is completely molten and does not contain any unmelted substance and bubble. In ordinary cases, the heating element is preferably provided at a position 3 to 40 cm apart downwardly from the batch layer 3. A glass melting furnace provided with a heating element at a position deeper than 50 cm from the batch layer reduces the melting rate of a batch because the temperature of the molten glass in the vicinity of the batch layer 3 is lowered. On the other hand, a glass melting furnace provided with a heating element at a position 3 to 40 cm apart downwardly from the batch layer constantly supplies high-quality glass containing no unmelted substance and bubble. As the material of the resistance heating element in the present invention, platinum, platinum-rhodium alloy, molybdenum, tin oxide or molybdenum covered with platinum, etc. are usable. Among these, platinum and platinum-rhodium alloy are preferable. These materials are advantageous in that since they have much smaller temperature coefficients of the electric resistance as compared with those of glass, the temperature control is very easy. While there has been described what are at present considered to be preferred embodiments of the invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the scope of the invention.

Claims: A glass melting furnace (10) comprising a material entrance (1) provided at the uppermost portion thereof, a molten glass exit (2) at the lowermost portion thereof, and a chamber therebetween in which is positioned a resistance heating element (6,8) which extends over substantially the entire horizontal cross-sectional area of the chamber while allowing material to pass downwardly therethrough, wherein that said heating element (6,8) is positioned at a level such that in use it is immersed in molten glass (4). A glass melting furnace according to Claim 1, wherein said resistance heating element is a plate-like heating element (6) provided with at least one opening (7) for passing said molten glass therethrough. A glass melting furnace according to Claim 2, wherein said plate-like heating element (6) has a plurality of said openings (7). A glass melting furnace according to Claim 3, wherein said plate-like heating element (6) is provided at only one level. A glass melting furnace according to Claim 3, wherein one or more further such plate-like heating elements (6) are provided at a higher level or higher levels within the chamber. A glass melting furnace according to Claim 1, wherein said resistance heating element is composed of a plurality of rod-like heating members (8) which are horizontally arranged in parallel to each other with spaces (9) therebetween for passing said molten glass therethrough. A glass melting furnace according to Claim 6, wherein said rod-like heating members (8) are provided at only one level. A glass melting furnace according to Claim 6, wherein one or more pluralities of such rod-like heating members (8) are provided at a higher level or higher levels within the chamber. A glass melting furnace according to Claim 8, wherein the horizontal directions of arrangement of said pluralities of rod-like heating members (8) are the same at the different levels. A glass melting furnace according to Claim 8, wherein the horizontal directions of arrangement of said pluralities of rod-like heating members (8) are orthogonal to each other at adjacent levels. A glass melting furnace according to any preceding claim, wherein the material of said resistance heating element or elements (6,8) is selected from platinum, platinum-rhodium alloy, molybdenum, and tin oxide or molybdenum covered with platinum. A glass melting furnace according to any preceding claim, wherein said resistance heating element or elements (6,8) is provided at a position or positions which in use is 3 to 40 cm deep from the surface of said molten glass (4).

Inventors: MANABE, SEIICHIRO, NAGASHIMA, YUKIHITO
Applicants: NIPPON SHEET GLASS CO., LTD.
Application year: 1988
Main classification: C03B5
IPC codes: C03B5