APPARATUS AND METHOD FOR PRODUCING GLASS RIBBONS

Abstract

An apparatus is provided for producing thin glass ribbons from molten glass. The apparatus includes a drawing tank, a direct heater, and an indirect heater. The drawing tank has a lower elongated nozzle opening through which the molten glass can exit downwards. The direct heater has one or more heating circuits operable to heat the drawing tank in a first heating zone. The direct heater has a power source for each on the heating circuits. Each heating circuit has connections to connected to a wall of the drawing tank so that current from the power source flows through at least a portion of the wall and heats the wall. Each heating circuit also includes current-carrying portions of the wall. The indirect heater has one or more heating elements to heat the drawing tank in a second heating zone.

Claims

1. An apparatus for producing thin glass ribbons from a molten glass, the apparatus comprising: a drawing tank having a wall and a nozzle opening through the wall, the nozzle opening being configured to allow the molten glass to exit the drawing tank; a direct heater having a heating circuit and a power source, the heating circuit defining a first heating zone to heat the drawing tank, the power source operatively connected to the heating circuit, the heating circuit having power connections that connect the heating circuit to the wall so that current from the power source flows through a current-carrying portion of the wall to heat the wall; and an indirect heater having a heating element defining a second heating zone to heat the drawing tank.

2. The apparatus of claim 1, wherein the heating circuit comprises four heating circuits defining four of the first heating zones, the four heating circuits being operatively connected to the power source so that the four heating circuits are separately controlled, the four heating circuit having different connections that connect the four heating circuit to different areas of the wall so that current from the power source flows through current-carrying portions of the wall to heat four different areas of the wall.

3. The apparatus of claim 1, wherein the heating element comprises three heating elements defining three of the second heating zones, the three heating elements being separately controllable and spatially distinct from one another.

4. The apparatus of claim 1, wherein the heating element comprises five heating elements defining five of the second heating zones, the five heating elements being separately controllable.

5. The apparatus of claim 1, wherein the heating circuit comprises a plurality of heating circuits defining a plurality of the first heating zones and the heating element comprises a plurality of heating elements defining a plurality of the second heating zones, wherein the plurality of the first and second heating zones are arranged on and around the drawing tank in a vertically and/or horizontally distributed manner.

6. The apparatus of claim 1, wherein the current is less than 2500 A.

7. The apparatus of claim 1, wherein the indirect heater contributes more than 50% of a total heating power output.

8. The apparatus of claim 1, wherein the drawing tank comprises flanges or collars that form the power connections.

9. The apparatus of claim 1, wherein the current-carrying portion of the wall is made from metal sheets having a thickness from 0.5 to 5 mm.

10. The apparatus of claim 1, further comprising a plurality of temperature measurement points positively connected to the drawing tank.

11. The apparatus of claim 11, further comprising a feedback control unit, wherein the plurality of temperature measurement points comprises at least two temperature feedback control measurement points in communication with the feedback control unit so that the feedback control unit regulates a heating power of the direct and/or indirect heater.

12. The apparatus of claim 1, wherein the heating element comprises heating tiles or meandering heaters.

13. The apparatus of claim 1, wherein the heating element comprises a resistance heating material selected from a group consisting of platinum, stainless steel, and SiC.

14. The apparatus of claim 1, wherein the drawing tank comprises a fine grain stabilized noble metal or metal alloy.

15. The apparatus of claim 14, wherein the fine grain stabilized noble metal or metal alloy is a metal selected from a group consisting of Pt, PtRh, PtAu, PtRhAu, PtIr, ZrO.sub.2 particles.

16. The apparatus of claim 1, further comprising an inlet upstream of the drawing tank, the inlet in communication with a manifold pipe of the drawing tank, the manifold in communication with a chamber that has a smaller cross section than the manifold pipe, wherein the nozzle opening is at a lower end of the chamber.

17. A method for producing thin glass ribbons from a molten glass, comprising: feeding the molten glass into a drawing tank via an inlet; causing the molten glass to exit the drawing tank from a nozzle opening; drawing the glass ribbon off the molten glass exiting the nozzle opening; and simultaneously heating the drawing tank using a direct heater and an indirect heater, the direct heater having a heating circuit connected to a wall of the drawing tank so that current flows through a current-carrying portion of the wall to heat the wall, and wherein the indirect heater has a heating element to heat the drawing tank.

18. The method of claim 17, further comprising: distributing the molten glass via the inlet to the drawing tank into a manifold pipe of the drawing tank; causing the molten glass to flow into a chamber having a smaller cross section than the manifold pipe; and adjusting a consistent linear throughput of the molten glass along the nozzle opening through a temperature profile in the drawing tank.

19. The method of claim 17, further comprising controlling the direct and indirect heaters to control a linear throughput through temperature adjustment and temperature distribution of the molten glass in the drawing tank such that a total throughput is kept constant.

20. The method of claim 17, further comprising controlling a temperature of the molten glass in the drawing tank such that a temperature-dependent viscosity thereof fulfills the following relationship: $48.Math.\frac{\stackrel{.}{v}}{B}.Math.{}_{H_L}.Math.\frac{}{{\left(D_S-D_L\right)}^3}.Math.d.Math.z+1.Math.2.Math.\frac{\stackrel{.}{v}}{B}.Math.{}_{H_S}.Math.\frac{}{D_S^3}.Math.d.Math.z>.Math.g.Math.h-p_u$ wherein {dot over (v)} is a volume flow of the molten glass, B is a width of the drawing tank in the direction along the nozzle opening (9), D.sub.S is a local width of a chamber in the drawing tank, D.sub.L is a local thickness of a guiding body in the nozzle opening, is a density of the molten glass, g is a gravitational acceleration, h is a height of the chamber, and p.sub.u is a pressure of 2000 Pa.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0054] The invention will now be explained in more detail with reference to the accompanying figures, wherein:

[0055] FIG. 1 is a cross-sectional view through a drawing tank with direct heater, indirect heater, and a guiding body;

[0056] FIG. 2 schematically shows a section of a drawing tank with direct heater in a cross-sectional view;

[0057] FIG. 3 schematically shows a section of a drawing tank with indirect heater in a cross-sectional view;

[0058] FIG. 4 is a graph showing the influence of the temperature profile at the drawing tank outlet on the thickness profile and the width of the glass ribbon;

[0059] FIG. 5 is a perspective view of a drawing tank with a single pair of collars as the power connections for direct heating;

[0060] FIG. 6 is a cross-sectional view of a drawing tank for illustrating parameters for calculating the pressure drop in the drawing tank.

DETAILED DESCRIPTION

[0061] FIG. 1 shows parts of an apparatus 1 for producing, in particular drawing, glass ribbons 3 from a molten glass 5, in particular with a thickness equal to or less than 3000 m, preferably from 15 to 1100 m. The apparatus 1 comprises a drawing tank 7 for holding molten glass 5, which has an elongated nozzle opening 9 at its lower end, through which the molten glass 5 can exit downwards, combined with direct heater 2 and indirect heater 4 according to the invention.

[0062] Preferably, a guiding body 11 may be arranged inside the drawing tank 7 and may protrude downward out of the nozzle opening 9 of the drawing tank 7. The guiding body 11 is preferably supported so as to be spaced apart from the edges 90, 92 of the nozzle opening 9, so that two nozzle slots 94, 96 are defined between the guiding body 11 and the edges 90, 92 of the nozzle opening 9. Therefore, the molten glass 5 preferably emerges from the nozzle slots 94, 96 in two substreams 50, 52. These substreams 50, 52 can run down along the guiding body 11 and combine at the lower end of the portion 100 of the guiding body 11, which preferably protrudes from the nozzle opening 9. This region in which the two substreams 50, 52 unite and from which the glass ribbon 3 is formed by being drawn off is referred to as a drawing onion 15. While being drawn off, the thickness of the glass ribbon 3 reduces by the drawing of the glass. At the same time, the glass becomes colder with growing distance from the nozzle opening 9, and accordingly becomes more viscous until it solidifies.

[0063] It is generally preferred for the guiding body 11 to protrude from the nozzle opening 9 by at least 30 mm, preferably at least 80 mm. This ensures good distribution of the molten glass 5 on the guiding body 11, so that variations in thickness of the glass ribbon 3 are prevented.

[0064] Without being limited to a specific exemplary embodiment, it is contemplated according to a further embodiment that the guiding body 11 has a greater thickness inside the drawing tank 7 than at the nozzle opening 9. The thickening is advantageous for mechanical reasons, but is not mandatory.

[0065] The guiding body 11 may comprise a resistance body 101 and a fin or blade 103 arranged below the resistance body 101, and the resistance body 101 preferably has a greater width than the blade 103 in order to restrict the flow cross section in the drawing tank 7.

[0066] Generally, the distance from the nozzle opening 9 to the lower edge of the flow resistance or resistance body 101 is preferably at least 3 mm, more preferably at least 8 mm.

[0067] What is not shown in FIG. 1, but technically conceivable as well is to heat the guiding body 11, in particular via dedicated separately controllable connections, so that at least one separate heating circuit independent of the drawing tank 7 is established and is grounded via one connection, for example.

[0068] For drawing off the glass ribbon 3, a drawing device 17 may be provided, which may comprise one or more pairs of driven rollers, for example.

[0069] FIG. 1 shows one heating circuit 206 of the direct heater 2, which has two connections 26, 27 through which the heating circuit 206 is connected to a wall 700 of the drawing tank 7, so that the current from power source 201 flows through at least a portion of the wall 700 and heats the wall 700, so that at least a portion of the drawing tank 7 can be heated, in the form of a first heating zone 34.

[0070] The direct heater 2 preferably comprise at least four separately controllable heating circuits 206, 207, 208, 209 which are operable to heat four different areas of the drawing tank 7 in the form of first heating zones 34, 35, 36, 37, as shown in FIG. 2, for example.

[0071] The direct heater 2 further comprise a respective power source 201, 202, 203, 204 for each of the preferably at least four heating circuits 206, 207, 208, 209, the heating circuits 206, 207, 208, 209 each having connections 26, 27 through which the heating circuits 206, 207, 208, 209 are connected to the wall 700 of drawing tank 7 so that the current from power sources 201, 202, 203, 204 flows through at least a portion of the wall 700 and heats the wall 700.

[0072] The heating currents of a heating circuit are preferably less than 2500 A, in particular less than 1000 A. However, power sources with a minimum power output of 200 A are preferred in order to be able to provide sufficient heating power when direct heating is using for temperature readjustment. More particularly, the heating circuits 206, 207, 208, 209 of the direct heating are established involving current-carrying metal sheets 78 of the drawing tank 7. Favorable thicknesses of the metal sheets 78 for direct heating are in a range from 0.5 to 5 millimeters.

[0073] In addition to heater 2, the apparatus 1 also comprises indirect heater 4 including heating elements for at least one second heating zone, as shown in FIG. 1. In particularly preferred implementations, the indirect heater 4 comprise heating elements for at least three or for five or more (FIG. 3) separately controllable spatially distinct second heating zones. In the exemplary embodiment of the apparatus 1 according to FIG. 1, a thermal insulation 14 to the external environment is furthermore provided around the indirect heater 4.

[0074] As schematically illustrated in FIG. 1, at least two temperature measurement points 18, 19 are provided in an advantageous embodiment, which can provide control measurement points for regulating the heating power output of the direct heater 2 and/or of the indirect heater 4 for a control unit 20 for feed-back control of the heating power. As indicated by the arrows, the control unit 20 accordingly derives the sensor values from temperature measurement points 18, 19 and controls the heating power of the heater 2, 4. For direct heating, current flow is adjusted for this purpose. The indirect heater 4 preferably also involve conductive heating, but may also comprise burners, for example. The temperature measurement points 18, 19 may be positively connected to the drawing tank 7, in particular welded thereto, for rapid feedback of temperature changes.

[0075] For reasons of better clarity, FIG. 2 shows a section of a drawing tank 7 with direct heater 2 comprising preferably at least four separately controllable heating circuits 206, 207, 208, 209 which are operable to heat four different areas of the drawing tank 7 in the form of first heating zones 34, 35, 36, 37. The direct heater 2 according to FIG. 2 comprise a respective power source 201, 202, 203, 204 associated with each of the heating circuits 206, 207, 208, 209, each one with connections 26, 27 through which the heating circuits 206, 207, 208, 209 are connected to the wall 700 of the drawing tank 7.

[0076] In an advantageous embodiment, the first heating zones 34, 35, 36, 37 of direct heater 2 are arranged on the drawing tank 7 in a vertically and horizontally distributed manner. This arrangement makes it possible to control the temperature in the drawing tank down to the nozzle opening 9 in a locally variable distinct way so that the viscosity of the molten glass 5 can be adjusted as desired.

[0077] In a particularly preferred embodiment of the direct heater 2 as shown in FIG. 2, the power sources 201, 202, 203, 204 are connected to the drawing tank 7 in such a way that at least three first heating zones 34, 35, 36 are preferably distributed in the upper portion of the drawing tank 7 transversely to the drawing direction of the glass ribbon 3, that is in particular along the longitudinal extension of the nozzle opening 9. One heating zone 35 of these three first heating zones 34, 35, 36 in the upper portion of the drawing tank 7 is preferably arranged centrally in the vicinity of an inlet 74 for the molten glass 5 to the drawing tank 7, and each of the two further heating zones 34, 36 is preferably located in the region of a manifold pipe 76 for the molten glass 5 downstream of inlet 74. As illustrated in FIG. 2, at least one further first heating zone 37 advantageously heats the lower portion of the drawing tank 7 including nozzle opening 9.

[0078] For reasons of better clarity, FIG. 3 shows a section of a drawing tank 7 with indirect heater 4.

[0079] Without being limited to the exemplary embodiments, it is contemplated according to one embodiment of the invention as shown in FIG. 3, that the indirect heater 4 preferably comprise six heating elements 41, 42, 43, 44, 45, 46 for six separately controllable second heating zones 61, 62, 63, 64, 65, 66, and the second heating zones 61, 62, 63, 64, 65, 66 of indirect heater 4 may extend vertically on the drawing tank 7, distributed next to one another, more preferably transversely to the drawing direction of the glass ribbon 5, that is in particular along the longitudinal extension of the nozzle opening 9.

[0080] Since the apparatus 1 of the invention for producing and in particular drawing glass ribbons 3 comprises direct heater 2 and at the same time indirect heater 4, it is possible to optimally adjust the temperature profile and linear throughput of the molten glass 5 in a spatially resolved manner, in particular due to the separation into distinct first and second heating zones, which especially promotes controllability and hence stability of the process performed with this apparatus. Such control makes it in particular possible to target a most consistent possible throughput along the nozzle opening 9.

[0081] Since the thickness of the glass ribbon is very sensitive to temperature deviations, optimization and fine adjustment of the temperature of the molten glass 5 is particularly important for a stable production process.

[0082] FIG. 4 is a graph showing the influence of the temperature profile at the drawing tank outlet on the thickness profile and the width of the glass ribbon 3.

[0083] The glass ribbons 3 comprise a central area 30 along which the thickness of the glass ribbon 3 does not vary at all or only slightly, and beads 31, 33 on the edges. The central area 30 is what is known as the quality area from which the glass products to be manufactured are produced. Typically, the beads 31, 33 are severed and the glass of the beads can be re-melted and returned into the drawing tank 7.

[0084] In order to obtain a glass ribbon 3 of particularly large width and at the same time uniform thickness in the useful quality area or central area 30, a temperature gradient between the edges 90, 92 of the nozzle opening 9 and the center of the nozzle opening 9 should ideally be adjusted to amount to 40 K, as shown in FIG. 4 by the thin middle line of the three lines.

[0085] If the center of the nozzle opening 9 is hotter, i.e. if the temperature gradient is 50 K, for example, the linear throughput of molten glass 5 will increase in the center, which is why the thickness profile of the glass ribbon 3 will assume a letter W shape, as indicated by the dashed line in FIG. 4, for example.

[0086] If the temperature differential is less than or equal to 20 K, as indicated by the bold line in FIG. 4, the width of the glass ribbon 3 will decrease and the thickness profile will become trough-shaped. The useful quality area or central area 30 will be reduced significantly thereby.

[0087] Therefore, in order to produce glass ribbons 3 of a largest possible width with consistent thickness, a temperature gradient of advantageously T.sub.grad=T.sub.centerT.sub.edge=0 to 50 K, preferably T.sub.grad=20 to 40 K is preferably adjusted in the drawing tank 7 along the nozzle opening 9 from the edges 90, 92 thereof to the center of the nozzle opening 9.

[0088] A further possibility for adjusting or controlling the thickness of the glass ribbon 3 by adjusting the temperature is implemented in an advantageous embodiment of the apparatus 1 comprising a drawing tank 7, in which the drawing tank 7 comprises flanges or collars 28 as the power connections 26, 27 for direct conductive heating.

[0089] FIG. 5 shows a section of a preferred embodiment, in which an inlet 74 for the molten glass is provided upstream of the drawing tank 7 and in which the upper portion of the drawing tank 7 is formed by a tubular section or manifold pipe 76 for the molten glass 5 which opens into a chamber 75. In this case, a single pair of collars 28 is fitted to the manifold 76 as the separate power connections 26, 27, for example. Each of the collars 28 preferably has a power connection tab 29.

[0090] According to a further embodiment of the invention, the pressure drop in the drawing tank 7 is adjusted in a specific way in order to improve the shape accuracy and dimensional consistency of the glass ribbon 3. This adjustment prevents a negative pressure from developing in the drawing tank 7. Such negative pressure might mechanically deform the drawing tank 7, which may also affect the glass thickness. Moreover, a local negative pressure may cause an unstable flow pattern of the molten glass 5 in the drawing tank 7, which may also lead to an inhomogeneous glass thickness or to glass defects.

[0091] The adjustment according to this embodiment will now be explained in more detail with reference to the schematic sectional view of a drawing tank 7 in FIG. 6. FIG. 6 shows a cross-sectional view of a drawing tank 7 indicating dimensions which are used to calculate the pressure drop in the drawing tank 7.

[0092] Here, the upper portion of drawing tank 7 is formed by a tubular section or manifold pipe 76 that opens into a chamber 75 which extends down to a bottom 70 of the drawing tank 7. Chamber 75 preferably has a smaller cross section than the manifold pipe 76. Accordingly, the width D.sub.A of the chamber 75 is smaller than the diameter of the manifold pipe 76. Due to the small cross section, pressure changes will primarily occur along the chamber 75. Particular contributions come from the sections in which preferably at least one guiding body 11 further narrows the chamber 75.

[0093] According to a preferred embodiment, the temperature in the drawing tank 7 is adjusted such that the condition as already mentioned above is met:

[00002]$48.Math.\frac{\stackrel{.}{v}}{B}.Math.{}_{H_L}.Math.\frac{}{{\left(D_S-D_L\right)}^3}.Math.d.Math.z+1.Math.2.Math.\frac{\stackrel{.}{v}}{B}.Math.{}_{H_S}.Math.\frac{}{D_S^3}.Math.d.Math.z>.Math.g.Math.h-p_u$

[0094] In the above relationship, 12 is the volume flow of the molten glass 5, B is the width of the drawing tank 7 in the direction along nozzle opening 9 or along the glass ribbon 3 perpendicular to the drawing direction, is the viscosity of the molten glass 5, D.sub.S is the local width of chamber 75, D.sub.L is the local thickness of guiding body 11, p is the density of the molten glass 5, g is the gravitational acceleration, and h is the height of chamber 75. The integration is made over sections H.sub.L and H.sub.S in the vertical direction z. The integration may also be made over two or more sub-sections, in which case the sub-integrals then having to be added. This is the case if further guiding bodies are provided in addition to guiding body 11, which are for instance separated apart from one another in the vertical direction.

[0095] The symbol p.sub.u denotes a pressure magnitude of 2000 Pa. This magnitude accounts for a negative pressure that is still tolerable. Thus, the right side of the relationship represents the hydrostatic pressure of the molten glass reduced by the still tolerable negative pressure p.sub.u. This term is a constant. The prefactor {dot over (v)}/B defines the thickness of the glass ribbon 3 which is predetermined so that the prefactor also represents a constant. On the other hand, what can be controlled through the temperature for a given thickness of the glass ribbon 3 is the strongly temperature-dependent viscosity .

[0096] As shown above by the implementations of direct heater 2 and indirect heater 4, the temperature can even be controlled in a locally differing manner. Also, the temperature in the drawing tank may vary along the vertical direction. Thus, the viscosity may be location-dependent, =(z). This dependency can accordingly also be taken into account in the integration.

[0097] It is thus contemplated according to one embodiment that the drawing tank 7 comprises a chamber 75 in which the guiding body 11 is arranged and which has the nozzle opening 9 at its lower end, and that the temperature of the molten glass 5 in the drawing tank 7 is adjusted such that the relationship given above is fulfilled with the temperature-dependent viscosity thereof.

[0098] It will be apparent to a person skilled in the art that the invention is not limited to the specific exemplary embodiments illustrated in the figures, but may be varied in various ways. Different embodiments may in particular also be combined with one another.

LIST OF REFERENCE NUMERALS

[0099]

TABLE-US-00001 1 Apparatus 50, 52 Substreams 2 Direct heater 41-6 Heating elements 3 Glass ribbon 61-66 Second heating zones 4 Indirect heater 70 Bottom 5 Molten glass 74 Inlet 7 Drawing tank 75 Chamber 9 Nozzle opening 76 Manifold pipe 11 Guiding body 78 Metal sheet 14 Thermal insulation 90, 92 Edges 15 Drawing onion 94, 96 Nozzle slot 17 Drawing device 100 Protruding guiding body portion 18, 19 Temperature 101 Resistance body measurement point 20 Control unit 103 Fin or blade 26, 27 Connections for 201-204 Power source heating circuit 28 Collar 206-209 Heating circuit 29 Power connection tab 700 Drawing tank wall 30 Central area of 3 31, 32 Beads of 3 34-37 First heating zones