Vertical glass melting furnace

Abstract: A vertical glass melting furnace of the type having an inlet for raw materials at the top thereof, an outlet for molten glass at the bottom thereof, and a tabular electric resistance heating element for melting which is immersed at some level in molten glass, has at least one opening, and covers almost entirely the cross-section of the furnace at that level, the vertical glass melting furnace comprising a stirrer which extends from above the furnace and passes through the batch layer and the tabular heating element so that the stirrer brings about forced circulation for homogenization of the molten glass which has passed through the tabular heating element and stays in the region below the tabular heating element.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a vertical glass melting furnace of the type having a batch layer on the surface of the molten glass and having an inlet for raw materials at the top of the furnace, an outlet for molten glass at the bottom thereof, a tabular electric resistance heating element for melting which is immersed at some level in the molten glass, has at least one opening, and covers almost entirely the cross-section of the furnace at the said level, said vertical glass melting furnace is identical in its horizontal sectional shape from the top to the bottom thereof and comprises a stirrer which extends from above the furnace and passes through the batch layer said tabular heating element and having a plurality of blades. 2. Description of the Prior Art: US-A-2 780 890 discloses a glass melting furnace and, in particular, a furnace designed for a continuous melting operation, for example, the continuous production of glass filaments or fibres. As further described in this document, the glass melting furnace is provided with a flanged tube extending upwardly through the centre of the bottom block, wherein said tube can be provided with a screw that extends axially from just below the upper edge of the tube to just above the cone baffle. The screws 92 however act in the described furnace in order to control the rate of production of the molten glass by choosing the appropriate speed and direction of rotation of such means as a screw and the propellers. Furthermore, since the screw is only disposed in the tube at the bottom of the furnace it is impossible that the molten glass in the furnace will be stirred in order to homogenise the molten glass. Concerning unmelted substances in the molten glass, it should be noted that instead of using the stirrer in order to solve this problem the upper end of the tube extends above the upper surface of the chamber bottom block forming a baffle or weir to trap heavy impurities such as unmelted stones. Consequently the molten glass is not stirred by the screw in total since otherwise those unmelted stones could not be trapped only by gravity as described. Additionally, there is no disclosure on how bubbles can be avoided in the molten glass. GB-A-1 498 184 discloses an apparatus for processing glass, which is similar to the apparatus as described in US-A-2 780 890, provided with an impeller which extends into the tube like chamber. As outlined on page 9, lines 91 to 100 the rotation of the impeller exerts a constant pressure on the softened glass only in the chamber whereby the glass is extruded in streams through the orifice in the stream feeder plate. The downwardly directed pressure on the glass established by rotation of the impeller may be varied and controlled by regulating the speed of rotation of the impeller. The impeller does not homogenise the molten glass by stirring since it should be rotated slowly so as not to disturb a downwardly smooth flow of the glass. There are several methods for homogenizing molten glass in a glass melting furnace. They are based on mixing by convection cr gas bubbling. There is disclosed in Japanese Patent Laid-open No. 199030/1990 (JP-A-02199030) a method for homogenizing molten glass in a cold-top vertical glass melting furnace. This method is applicable to a vertical glass melting furnace having an inlet for raw materials at the top thereof, an outlet for molten glass at the bottom thereof, and at least one electric resistance heating element immersed in molten glass at some level thereof such that it covers almost entirely the cross-section of the furnace at that level. According to this method, homogenisation is accomplished by the convection of molten glass which is induced by a further heating element for homogenization installed in the furnace below the resistance heating element. In the case of homogenization by forced circulation with a stirrer, it is common practice to install a homogenizing tank separately from the melting furnace, thereby eliminating entrapment of bubbles and unmelted substances in molten glass by stirring. There is a different situation in a vertical glass melting furnace of the type in which raw materials cover the surface of molten glass (forming so-called cold top) so as to reduce heat loss from the top of the furnace and the melting of raw materials is accomplished by applying an electric current directly to the molten glass. In this case, efforts to promote homogenization of molten glass by vigorous convection end up with entrapment of bubbles and unmelted substances in molten glass. Thus, with such a glass melting furnace, it is difficult to produce homogenized glass free of entrapped bubbles and unmelted substances. The same difficulties as above are also encountered in the case of forced circulation by a stirrer. OBJECT AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a vertical glass melting furnace that affords homogeneous molten glass free of entrapped bubbles and unmelted substances. The present invention is embodied in a vertical glass melting furnace of the type suitable for having a batch layer covering the surface of the molten glass, having an inlet for raw materials at the top of the furnace, an outlet for molten glass at the bottom thereof, and a tabular electric resistance heating element for melting which is suitably positioned for immersion at some level in the molten glass, has at least one opening, and covers almost entirely the cross-section of the furnace at the said level, said vertical glass melting furnace is identical in its horizontal sectional shape from the top to the bottom thereof and comprises a stirrer which extends from above the furnace, passing through the batch layer in use, and passes through said tabular heating element and having a plurality of blades at least one above the other, characterised in that each of the blades has a rotating diameter d in a range such that a ratio of d/D is between 0.3 < d/D < 0.5, wherein D is the minimum width across the furnace and the upper most blade is located below the heating element by a length more than d/2, so that in use the stirrer brings about forced circulation for homogenisation of the molten glass which has passed through the tabular heating element preventing stagnant, unhomogenised molten glass in the region below the tabular heating element. The vertical glass melting furnace of the present invention permits molten glass to flow down through the heating element, without appreciable turbulence above the heating element. The molten glass below the heating element undergoes forced circulation by the stirrer for homogenization. Thus there is obtained homogeneous molten glass free of bubbles an unmelted substances. For the vertical glass melting furnace to work satisfactorily, the following requirements should be met. There should be no short pass along which molten glass flows toward the furnace outlet in a short time. There should be no stagnation of molten glass. Molten glass below the heating element should undergo sufficient shearing by the stirrer. The flow of molten glass toward the heating element should be very weak. The present inventors found that the foregoing requirements are met with the vertical glass melting furnace provided with a tabular electric resistance heating element, if proper selections are made of the shape, size, position, and rotary speed of the stirrer, and the configuration of the cross-section of that region of the furnace where stirring takes place. The electric resistance heating element uniformly melts the glass batch, and the resulting molten glass passes through the openings of the heating element. Then the molten glass is mixed efficiently by forced circulation induced by the stirrer in the region where the stirrer is positioned. The vertical glass melting furnace of the present invention is constructed such that a stirrer is immersed in molten glass below the electric resistance heating element for melting. This stirrer brings about forced circulation for efficient mixing in the molten glass without appreciable turbulence above the heating element. Japanese Patent Laid-open No. 199030/1990 discloses a method for melting glass by means of a tabular heating element in a glass melting furnace. This method has an advantage of eliminating defective molten glass entrapping bubbles and unmelted substances therein due to reduced convection and insufficient heating and also eliminating the uneven melting of the surface batch. With the present invention, it is possible to improve the quality of molten glass without losing the advantage of Japanese Patent Laid-open No. 199030/1990. In the vertical glass melting furnace of the present invention, the electric resistance heating element for molten glass functions also as a baffle plate for the flow of molten glass produced by the stirrer. This baffle plate permits the use of a strong stirring which would cause entrapment of bubbles and unmelted substances in a furnace so designed as to perform melting by the direct application of an electric current through electrodes immersed in molten glass. In the vertical glass melting furnace of the present invention, the region below the tabular heating element where the stirrer is immersed should have an adequate cross-sectional shape. An octagonal shape is desirable from the standpoint of furnace construction and glass homogeneity. A circular cross-section is more desirable for higher homogeneity, and a square cross-section may be acceptable for lower homogeneity. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view (taken along the line 1 ○-1 ○ in Fig. 3) of a preferred embodiment (glass melting furnace) of the present invention. Fig. 2 is a cross-sectional view taken along the line 2 ○-2 ○ in Figs. 1 and 3. Fig. 3 is a horizontal sectional view showing the relative positions of the stirrer and heating element. Fig. 4 is a horizontal sectional view showing the relative position of the stirrer and heating element. Fig. 5 is a horizontal sectional view showing the relative position of the stirrer and heating element. Fig. 6 is a horizontal sectional view (taken along the line 6 ○-6 ○ in Fig. 2) of that part of the furnace below the heating element. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment of the present invention will be described in more detail with reference to the accompanying drawings. Referring to Fig. 1, there is shown a furnace proper 2 made up of tank blocks. The furnace proper 2 has a square inlet 1 for raw materials at the top thereof and an outlet 6 for molten glass at the bottom thereof (which is a throat projecting downward in this embodiment). The furnace proper 2 holds molten glass G therein. On the molten glass G is formed a batch layer 3 of uniformly fed raw materials. At some level in the molten glass is installed an electric resistance heating element 4 (4'). This heating element has a tabular heating part 4a (4a') in which are formed openings 5. The tabular heating part 4a (4a') covers almost entirely the cross-section of the furnace at the level where it is positioned. The region of the furnace below the tabular heating part 4a (4a') has an octagonal cross-section. The resistance heating element 4 (4') for melting has a tabular heating part 4a (4a') in which are formed a multiplicity of uniformly spaced openings 5, and also has terminals 4b (4b') at both ends thereof which are connected to a power source 9. The openings 5 in the resistance heating element 4 should be of the same size and shape and be positioned at regular intervals so as to ensure uniform horizontal temperature distribution in the molten glass G. The resistance heating element 4 (4') for melting consists of paired parts, each covering approximately the half of the horizontal cross-section of the furnace, so that the shaft 7A of the stirrer 7 can be passed through it from above the furnace. The halves 4 and 4' of the resistance heating element for melting should be positioned a certain distance apart so that the shaft 7A of the stirrer 7 can be passed through the gap between them. The gap between 4 and 4' should be wide enough for the free passage of the shaft 7A of the stirrer 7 but be as narrow as possible so as to prevent the outflow of molten glass containing bubbles and unmelted substances due to insufficient heating. For example, it should be smaller than 60 mm for a shaft 30 mm in diameter. In addition to the foregoing, there is another method for installing the stirrer. For example, it is possible to make an opening 20 in the heating element through which the shaft of the stirrer is passed as shown in Figs. 4 and 5. The stirrer 7 is driven by a motor 7B placed at the top of the furnace proper 2. Proper selections should be made of the position of the stirrer and the shape, size, and rotary speed of the stirring blades so as to carry out sufficient homogenization while avoiding a strong flow of molten glass in the region 8 above the heating element 4 (4') and an outflow of molten glass containing bubbles and unmelted substances due to insufficient heating. Since the heating element 4 (4') functions as a baffle plate for the flow of molten glass below it and also as a separator for molten glass above it, the vertical glass melting furnace of the present invention permits more vigorous stirring than the furnace designed to melt glass by the direct application of electricity. The stirrer has upper blades 7a, middle blades 7b, and lower blades 7c. The upper blades 7a are inclined downward (with respect to the direction of rotation) so that they produce upward flows in molten glass as indicated by arrows B. The middle blades 7b are held vertical so that they stretch molten glass as they rotate. The lower blades 7c are inclined upward (with respect to the direction of rotation) so that they produce downward flows in molten glass as indicated by arrows C. The stirring blades should preferably be large for sufficient homogenization. However, an adequate size is defined by 0.3 < d/D < 0.5, where d is the length (or rotating diameter) of the blade and D is the minimum width across the furnace. The reason for this size is that the flow of molten glass near the heating element 4 (4') should be weak so that the molten glass above the heating element undergoes adequate thermal history. The stirrer 7 should not be positioned too close to the resistance heating element 4 (4') for melting so as to avoid entrapment of bubbles and unmelted substances by molten glass. The distance L between the upper blade 7a and the heating element 4 (4') should be greater than d/2. The rotary speed of the stirrer depends on the position, shape, and size of the stirrer blades. A speed of 5-20 rpm was found adequate in the case where the center of the middle blade 7b is in the middle between the heating element and the furnace bottom. A higher rotary speed than this will cause entrapment of bubbles and unmelted substances and make unstable the batch layer in the upper part of the furnace. As mentioned above, the present invention provides a glass melting furnace which homogenizes molten glass without entrapment of bubbles and unmelted substances on account of its stirrer to bring about forced circulation for uniform mixing. Thus, this glass melting furnace affords molten glass with high quality and homogeneity.

Claims: A vertical glass melting furnace of the type suitable for having a batch layer (3) covering the surface of the molten glass (G), having an inlet (1) for raw materials at the top of the furnace, an outlet (6) for molten glass at the bottom thereof, and a tabular electric resistance heating element (4, 4') for melting which is suitably positioned for immersion at some level in the molten glass, has at least one opening (5), and covers almost entirely the cross-section of the furnace at the said level, said vertical glass melting furnace is identical in its horizontal sectional shape from the top to the bottom thereof and comprises a stirrer which extends from above the furnace, passing through the batch layer (3) in use, and passes through said tabular heating element (4, 4') and having a plurality of blades (7a, 7b, 7c) at least one (7a) above the other (7b, 7c), characterised in that each of the blades has a rotating diameter d in a range such that a ratio of d/D is between 0.3 < d/D < 0.5, wherein D is the minimum width across the furnace and the upper most blade (7a) is located below the heating element (4) by a length more than d/2, so that in use the stirrer (7) brings about forced circulation for homogenisation of the molten glass which has passed through the tabular heating element preventing stagnant, unhomogenised molten glass in the region below the tabular heating element (4, 4'). A vertical glass melting furnace according to claim 1, which comprises a furnace proper (2) to hold molten glass and raw materials forming a batch layer (3) covering said molten glass, a shaft (7A) extending from above said furnace proper (2) through said batch layer (3), into said molten glass (G), and beyond said heating element (4, 4'), stirring blades attached to that part of said shaft under said heating element, and means (7B) to drive said shaft. A glass melting furnace as defined in claim 2, wherein said tabular resistance heating element (4, 4') consists of paired halves with a gap between them which is wide enough to permit said shaft (7A) to pass through without contact and narrow enough to prevent passage of bubbles and unmelted substances. A glass melting furnace as defined in claim 2, wherein said stirring blades consist of upper blades (7a), middle blades (7b), and lower blades (7c), said upper blades (7a) being inclined downward (with respect to the direction of rotation) so that they produce upward flows in molten glass, said middle blades (7b) being held vertical so that they stretch molten glass as they rotate and lower blades (7c) being inclined upward (with respect to the direction of rotation) so that they produce downward flows in molten glass. A glass melting furnace as defined in claim 2, wherein said furnace proper (2) has an octagonal cross-section under said heating element (4, 4'). A glass melting furnace as defined in claim 2, wherein there is a single heating element (4, 4') and said shaft (7A) is passed through one hole made in said heating element.

Inventors: INAKA YOSHIYUKI, MANABE SEIICHIRO, NAGASHIMA YUKIHITO, NAKAGAKI SHIGEKI, SAKAGUCHI KOICHI, SUNADA TAKASHI, TANAKA HIROYUKI, INAKA, YOSHIYUKI, MANABE, SEIICHIRO, NAGASHIMA, YUKIHITO, NAKAGAKI, SHIGEKI, SAKAGUCHI, KOICHI, SUNADA, TAKASHI, TANAKA, HIROYUKI, Inaka, Yoshiyuki, c/o Nippon Sheet Glass, Manabe, Seiichiro, c/o Nippon Sheet Glass, Nagashima, Yukihito, c/o Nippon Sheet Glass, Nakagaki, Shigeki, c/o Nippon Sheet Glass, Sakaguchi, Koichi, c/o Nippon Sheet Glass, Sunada, Takashi, c/o Nippon Sheet Glass, Tanaka, Hiroyuki, c/o Nippon Sheet Glass
Applicants: NIPPON SHEET GLASS CO LTD, NIPPON SHEET GLASS CO. LTD.
Application year: 1992
Main classification: C03B5
IPC codes: C03B5