SCHMELZWANNE UND GLASSCHMELZANLAGE

Abstract

A melt tank for the production of a glass melt having a low portion of bubbles. The melt tank includes an inlet opening, an outlet opening, a floor, at least two side walls that adjoin the floor, a roof. The glass melt having a first bath depth in a melting segment, a second bath depth in a refining segment, and a third bath depth over a threshold between and smaller than the first and second bath depths. An electrical produced first heat energy is supplied via a multiplicity of electrodes that extend into the glass melt and a second heat energy is produced by the combustion of fossil fuel via at least one burner. Also, a method for producing a glass melt.

Claims

1. A melt tank for the production of a glass melt from at least one solid starting material, the melt tank comprising: an inlet opening for a supply of the at least one solid starting material, an outlet opening for an outflow of a molten glass, the melt tank being configured in such a way that the glass melt flows in a direction of flow from the inlet opening to the outlet opening during the melt process; a floor that limits the melt tank at a bottom; at least two side walls that adjoin the floor and that laterally limit the melt tank; and, a roof that is connected to the side walls and that limits the melt tank at a top, wherein the melt tank includes a melting segment and a refining segment, the glass melt first passing through the melting segment and subsequently through the refining segment when flowing in the direction of flow, the glass melt having a first bath depth B1 in the melting segment and having a second bath depth B2 in the refining segment, a threshold running transverse to the direction of flow being situated in the floor of the melt tank in a transition area of the melt tank between the melting segment and the refining segment, a third bath depth B5 over the threshold being smaller than the first bath depth B1 and smaller than the second bath depth B2, the melt tank being configured such that for the melting of the at least one starting material of the glass melt: an electrically produced, first heat energy portion (Q.sub.elec) is supplied via a multiplicity of electrodes that extend into the glass melt, and a second heat energy portion (Q.sub.fossil), produced by a combustion of fossil fuel, is supplied in at least one burner situated in a side wall, in the roof, or both, above the glass melt, a total supplied heat energy being made up of the first heat energy portion and the second heat energy portion, the first heat energy portion being at least 30% of the total supplied heat energy, and, wherein first row of electrodes, running transverse to the direction of flow, and including a multiplicity of electrodes, being situated in the floor of the melt tank on the threshold, in the direction of flow of the glass melt.

2. The melt tank as recited in claim 1, wherein a second row electrodes, running transverse to the direction of flow and including a multiplicity of electrodes, is situated in the floor of the melt tank at a distance L2 after the first row of electrodes in the direction of flow, and the second row of electrodes also being situated on the threshold.

3. The melt tank of claim 1, wherein the third bath depth B5 over the threshold is between 200 mm and 1000 mm.

4. The melt tank of claim 1, wherein the first bath depth B1 in the melting segment is between 1100 mm and 2000 mm, and the second bath depth B2 in the refining segment is between 700 mm and 2800 mm.

5. The melt tank of claim 1, wherein the multiplicity of electrodes situated on the threshold are rod electrodes and/or block electrodes.

6. The melt tank of claim 1, wherein the length L4 of the threshold is between 700 mm and 3000 mm in the direction of flow.

7. The melt tank as recited of claim 1, wherein the at least one burner for supplying the second heat energy portion is situated exclusively in the melting segment.

8. The melt tank of claim 1, wherein the at least one burner is controllable in such a way that the second heat energy portion (Q.sub.fossil) produced by the at least one burner corresponds to a loss of heat energy (Q.sub.wall) that is emitted to an outside via the melt tank.

9. The melt tank of claim 1, characterized in that a multiplicity of electrodes is situated in the melting segment in the floor of the melt tank, for example in at least one third row of electrodes running in the direction of flow.

10. The melt tank of claim 1, wherein a radiation wall is situated on the roof at the melting segment, at the transition area between the melting segment and the refining segment, or t both, the radiation wall running transverse to the direction of flow.

11. The melt tank of claim 1, wherein a further row of electrodes, running transverse to the direction of flow and including a multiplicity of electrodes, is situated in the floor of the melt tank, before the threshold in the floor of the melt tank in the direction of flow of the glass melt.

12. A glass melting plant comprising: a melt tank of claim 1, a constriction region following the melt tank, a conditioning region, and a channel.

13. The glass melting plant as recited in claim 12, wherein at least one cooling element is situated in the constriction region, on a cover of the constriction region.

14. A method for producing a glass melt in the melt tank of claim 1, the at least one solid starting material being supplied to the inlet opening, and the molten glass flowing out from the outlet opening, the glass melt flowing in the direction of flow from the inlet opening to the outlet opening during the melt process, the glass, the at least one starting material, or both first passing through the melting segment and subsequently the refining segment, as well as passing over the threshold in the floor of the melt tank running transverse to the direction of flow between the melting segment and the refining segment, the third bath depth B5 over the threshold being smaller than the first bath depth B1 and smaller than the second bath depth B2, such that for the melting of the at least one starting material the electrically produced, first heat energy portion (Q.sub.elec) is supplied to the glass melt by the multiplicity of electrodes extending into the glass melt, and the second heat energy portion (Q.sub.fossil), produced by the combustion of fossil fuel, is supplied to the glass melt by the at least one burner situated in a side wall, in the roof above the glass melt, or both the total quantity of heat energy supplied being made up of the first heat energy portion and the second heat energy portion, the first heat energy portion being at least 30% of the overall supplied heat energy, at least a part of the first heat energy portion being produced by a first row of electrodes, including a multiplicity of electrodes, on the threshold in the floor of the melt tank, running transverse to the direction of flow.

15. The method as recited in claim 14, wherein the at least one burner is regulated or controlled in such a way that the second heat energy portion produced by the at least one burner corresponds to the loss of heat energy (Q.sub.wall) that is emitted to the outside via the melt tank.

16. The melt tank of claim 2, wherein the distance L2 is between 500 mm and 1000 mm.

17. The melt tank of claim 4, wherein the first bath depth B1 in the melting segment (10) is between 1400 mm and 2000 mm.

18. The melt tank of claim 7, further comprising: a multiplicity of burners situated in a side wall running in the direction of flow.

Description

DESCRIPTION OF THE DRAWINGS

[0053] FIGS. 1A and 1B shows a first exemplary embodiment of a glass melting plant according to the present invention having a melt tank according to the present invention, in a vertical longitudinal section (1A)) and a horizontal cross-section (1B)),

[0054] FIGS. 2A and 2B shows a second exemplary embodiment of a glass melting plant according to the present invention having a melt tank according to the present invention, in a longitudinal section (2A)) and a horizontal cross-section (2B)),

[0055] FIG. 3 shows the exemplary embodiment of FIG. 2 in a vertical cross-section along the line A-A (see FIG. 2A)),

[0056] FIG. 4 shows a segment of FIG. 2A) with dimensions,

[0057] FIG. 5 shows, in a diagram, the functional dependence of the length of the melting segment (y axis, in meters) on the planned throughput of the glass melting plant (in tons/day) via the upper solid line, the functional dependence of the length of the refining segment (in meters) on the planned throughput of the glass melting plant (x axis, in tons/day) via the center, dashed line, and the functional dependence of the width of the melt tank in the melting segment and in the refining segment (in meters) on the planned throughput (in tons/day) via the lower dotted line,

[0058] FIG. 6 shows a third exemplary embodiment of a glass melting plant according to the present invention having a melt tank according to the present invention, in a longitudinal section,

[0059] FIGS. 7A to 7C shows the specific embodiment 7C) according to the present invention in direct comparison with conventional variants of the configuration of electrodes (7A) (document Mller) and 7B) (document Pieper), with regard to the step,

[0060] FIG. 8 shows a segment of a melt tank according to the present invention of a fourth exemplary embodiment of a glass melting plant according to the present invention,

[0061] FIGS. 9A to 9C shows a comparison of the temperature at the surface of the glass over the step for the specific embodiment 7A) according to the present invention with conventional variants of the configuration of electrodes (7B) and (7C), and a representation from the simulation, in a vertical longitudinal section, and

[0062] FIGS. 10A and 10B and 11A and 11B show a model calculation with the formation of bubbles in the feeder region, the bubbles in FIGS. 10A and 10B being formed at the right feeding side and in FIGS. 11A and 11B at the left feeding side of the melt tank, in each case in a horizontal cross-section (a) and in a vertical longitudinal section (b) through the melt tank.

DETAILED DESCRIPTION

[0063] FIGS. 1A and 1B shows a first exemplary embodiment of a glass melting plant according to the present invention having a melt tank according to the present invention, suitable for example for a daily output of 400 t of molten glass.

[0064] The melt tank includes the segments melting segment 10 and refining segment 20. The melt tank is followed by a constriction region 30, a conditioning region 40, and a channel 50. The starting materials for the production of the glass melt (primary raw materials and, possibly, shards) are continuously supplied at inlet opening 11 of the melt tank by a feed device (not shown). The starting materials are in particular melted in the melting segment 10 of the melt tank, and move (flow) together with glass melt 60 through refining segment 20, to outlet opening 12 of the melt tank, and further through constriction region 30, conditioning region 40, and channel 50, until they reach glass outlet opening 52. The direction of flow of glass melt 60 is indicated by an arrow 5 in FIGS. 1A) and 1B).

[0065] The melt tank has a floor 13, a roof 15 that is situated opposite floor 13, and side walls 16. In the exemplary embodiment shown in FIGS. 1A and 1B, lateral side walls 16 run parallel to the direction of flow of glass melt 60. In the context of the present invention (i.e. for all possible exemplary embodiments), the walls in the area of inlet opening 11 and outlet opening 12, which run transverse to the direction of flow, are also regarded as side walls.

[0066] Floor 13 of the melt tank has a rising step 17 and a falling step 28 that run transverse to the direction of flow of the glass melt, in particular perpendicular to the direction of flow, and are situated in the area of transition between melting segment 10 and refining segment 20. The two steps 17, 28 together form a threshold 27. Step 17 is in a sense the beginning of refining segment 20. Bath depth B1 in melting segment 10, which is the minimum of all bath depths in melting segment 10, is greater than bath depth B2 in the direction of flow after falling step 28, i.e. in refining segment 20. Bath depth B2 is the minimum of all bath depths in refining segment 20, the bath depths over threshold 27 and the electrodes that may be situated there not being taken into account in the ascertaining of the minima in the refining segment and in the melting segment.

[0067] In the direction of flow of the glass melt after first step 17, i.e. at threshold 27 in the transition area between melting segment 10 and refining segment 20, a first row of electrodes 21, having a multiplicity of electrodes situated alongside each other, and a second row of electrodes 22, also having a multiplicity of electrodes situated alongside each other, are provided. The electrodes, realized as rod electrodes, each go out perpendicularly upward from floor 13 of the melt tank and extend into glass melt 60. Alternatively, the electrodes of rows of electrodes 21, 22 can be realized at least partly as block electrodes; a block electrode is shown in section in FIG. 8. The row of block electrodes is designated by reference character 21a. Each row of electrodes 21, 22 runs transverse to the direction of flow. Because the electrodes of first row of electrodes 21 and of second row of electrodes 22 are situated after first step 17, seen in the direction of flow, they are situated in the region of floor 13, namely in the region of threshold 27, that has the smaller bath depth B5 in comparison with melting segment 10. The (minimum) bath depth B1 of melting segment 10 is between 1100 mm, preferably between 1400 mm, and 2000 mm, for example 1700 mm. The (minimum) bath depth B2 of refining segment 20 is between 700 mm and 1400 mm, for example 1100 mm. The electrodes situated one after the other in first row of electrodes 21 and in second row of electrodes 22 can be situated one after the other in the direction of flow (as in FIGS. 1A and 1B and 2A and 2B), or offset to one another.

[0068] For the heating of glass melt 60 using electrical energy, a third row of electrodes 19a and a fourth row of electrodes 19b are provided in melting segment 10, each also having a multiplicity of electrodes (e.g. rod electrodes) situated alongside one another, which extend from floor 13 into glass melt 16. Third row of electrodes 19a and fourth row of electrodes 19b each run in the direction of flow (arrow 5) of glass melt 60. Correspondingly, the orientation of third and fourth rows of electrodes 19a and 19b is perpendicular to the orientation of first and second rows of electrodes 21, 22. It is also possible for more than two rows of electrodes running in the direction of flow of the glass melt to be provided in melting segment 10.

[0069] Before step 17, in the direction of flow of the glass melt, there can be situated a further row of electrodes 23 having a multiplicity of electrodes (e.g. rod electrodes) situated alongside one another, transverse to the direction of flow, and extending upward from floor 13 of the melt tank into melt 60. Accordingly, the further row of electrodes 23 runs parallel to first row of electrodes 21 and to second row of electrodes 22. Further row of electrodes 23 is situated at a distance of for example 500 mm to 1500 mm, e.g. 800 mm, before the upper edge of the first step, i.e. still in the melting segment. The electrodes of further row of electrodes 23 are configured offset to the electrodes of the rows of electrodes 21, 22 provided on threshold 27.

[0070] In addition, in each of the two side walls 16 that run parallel to the direction of flow of glass melt 60, there are situated for example two burners 19c that supply heat energy to glass melt 60, the energy being produced by combustion of the fossil fuel gas using an oxidant in combustion chamber 18. The openings of burners 19c are situated in side walls 16 above the surface of glass melt 60, so that the burners heat the melt from above via combustion chamber 18. One burner 19c is situated above threshold 27.

[0071] In addition, a radiation wall 25 that runs downward from roof 15 in the direction of glass melt 60 can be provided in refining segment 20, above first row of electrodes 21 or second row of electrodes 22, or between these rows of electrodes 21, 22, as shown in FIGS. 2A through 4. Preferably, radiation wall 25 terminates above the glass melt as a straight (horizontal) arch. Such an arch is shown in FIG. 3. The apex of the arch has a height H1 above the surface of glass melt 60 that is at least 900 mm. As shown in FIG. 3, radiation wall 25 has a distance L1 from outlet opening 12 of the melt tank (see FIG. 2) that is at most 35% of the overall length of the melt tank between inlet opening 11 and outlet opening 12. Length L1 from radiation wall 25 to outlet opening 12 of the melt tank is for example 1500 mm to 3000 mm, for example 2150 mm.

[0072] In the region of constriction 30, in addition a cooling element 32 is situated that protrudes downward from the cover 15 of constriction region 30 and is immersed in glass melt 60. Cooling element 32 is for example cooled by water, and has the shape of a plate.

[0073] In conditioning region 40 and channel 50, bath depths B3 and B4, as shown in FIGS. 1A and 1B, can be further decreased.

[0074] FIGS. 2A and 2B shows a further exemplary embodiment of a glass melting plant according to the present invention having a melt tank according to the present invention that also differs from the exemplary embodiment shown in FIGS. 1A and 1B in that roof 15 is lower in the region after radiation wall 25. In the lowered region, the roof has a height H3 over the surface of glass melt 60 that is for example 50% to 80% of height H2. The vault height H3 in refining segment 20 after radiation wall 25 can for example be 65% of height H2 in melting segment 10, or in refining segment 20 before radiation wall 25. Here, height H2 is the distance of the lower service of roof 15 from the surface of glass melt 60 in melting segment 10. Length L1 of the lowered region from radiation wall 25 up to outlet opening 12 of the melt tank is for example 1500 mm to 3000 mm, for example 2150 mm.

[0075] In the specific embodiments shown in FIGS. 1A through 4 and 6 of a melt tank, or a glass melting plant, floor 13 of the melt tank in refining segment 20 forms a first threshold 27 that begins at first step 17 and has a length L4 in the direction of flow of glass melt 60 (see FIGS. 1A, 1B, 2A, 2B, and 4), the length L4 being between 700 mm and 3000 mm. In the area of first threshold 27, the floor of the melt tank has a bath depth B5 that is smaller than the bath depth B2 after first threshold 27 in refining segment 20. Bath depth B5 is for example between 200 mm and 1000 mm, for example 800 mm First row of electrodes 21 and second row of electrodes 22 are situated in the area of threshold 27, i.e. on threshold 27, as is shown in FIGS. 1A, 1B, 2A, 2B, and 4. At the end of threshold 27, in the direction of flow of the glass melt, the threshold has a second step 28 via which the bath depth in refining segment 20 again increases to the value B2. As explained above, threshold 27, and the electrodes situated thereon, bring about the rise of bubbles present in the glass melt, and thus bring about an improvement of the glass quality. The electrically heated threshold 27 forms the highest barrier in the melt tank between inlet opening 11 and outlet opening 12.

[0076] FIG. 6 shows the melt tank of a fifth exemplary embodiment of a glass melting plant for producing container glass. This exemplary embodiment largely corresponds to the exemplary embodiment shown in FIGS. 2A through 4, so that reference is made to that exemplary embodiment, the exemplary embodiments discussed in relation to FIGS. 1A through 4 being more suitable for the production of flat glass.

[0077] In the melt tank shown in FIG. 6, bath depth B2 of the refining segment after threshold 27 is greater than bath depth B1 of the melting segment, and bath depth B5 over threshold 27 is less than bath depth B1 of the melting segment. In addition, threshold 27 has only one row 21 of electrodes situated alongside one another, whereas at least two electrode rows are provided on the threshold in each of the exemplary embodiments according to FIGS. 1A through 4. In this exemplary embodiment, bath depth B1 is for example between 1100 mm and 2000 mm, for example 1400 mm Bath depth B2 is between 1200 mm and 2200 mm, for example 1800 mm Bath depth B5 over threshold 27 is for example between 200 mm and 1000 mm. In addition, no radiation wall extending from roof 15 is provided.

[0078] The design of the present invention has been tested in extensive modeling calculations, in particular for throughput quantities of from 300 t to 800 t glass melt per day. For high throughput quantities, the dimensions in melting segment 10 and in refining segment 20, (i.e. length and width of the melt tank) have to be adapted. In contrast, the bath depths and the width of threshold 27 in the direction of flow, and the height of threshold 27, are not changed. Of course, the width of threshold 27 transverse to the direction of flow is matched to the width of the melt tank in melting segment 10 and in refining segment 20. Correspondingly, the number of electrodes situated alongside one another in rows of electrodes 21, 22, 23 also increases.

[0079] In FIG. 5, the solid curve illustrates the relation between the length of melting segment 10 (in meters) as a function of the throughput (in tons/day) of the melt tank or of the glass melting plant; the dashed curve illustrates the relation between the length of refining segment 20 (in meters) as a function of the throughput (tons/day); the dotted curve illustrates the width of melting segment 10 and refining segment 20 transverse to the direction of flow (in meters) as a function of the throughput (tons/day). For example, for a throughput of 400 t glass melt per day, a length of melting segment 10 of approximately 12.5 m is required, whereas for a throughput of 700 t glass melt per day a length of melting segment 10 of approximately 17 m is required.

[0080] According to the present invention, it is decisive that the dimensioning of threshold 27, including the dimensioning of the electrodes situated on the threshold, ensures that bubbles that have a particular minimum diameter will move to the surface. In the following Table 1, suitable geometrical relations are shown for bubbles having a size >0.2 mm Here, the rise of the bubbles is calculated according to Stoke's Law. From Table 1, it can be seen that for all three calculated throughput quantities, bubbles having a standard diameter >0.23 mm had enough time to reach the surface of the glass melt. Here, the rise time is equal to the dwell time of the glass melt in refining segment 20, in the upper layer of the glass melt.

TABLE-US-00001 TABLE 1 Throughput kg/day 400,000 500,000 700,000 Melt density kg/m.sup.3 2400 2400 2400 Melt bath C. 1480 1480 1480 temperature Melt bath Pascal 6.4 6.4 6.4 viscosity (kg/m*s) Bubble mm 0.23 0.23 0.23 diameter Rise speed m/s 4.21E05 4.21E05 4.21E05 Refining zone Basin length m 6.2 7.2 8.2 Basin width m 7.4 8.45 9.5 Pre-flow layer m 0.135 0.135 0.135 thickness Melt dwell min 53.51 51.61 51.92 time in refining part Bubble rise min 53.5 51.6 52 time

[0081] The following Table 2 contains, in each column, three examples of the realization of a glass melting plant according to the present invention that corresponds to the exemplary embodiment according to FIG. 6. The first two columns contain glass melting plants for producing container glass, and the last column contains the data for a glass melting plant for producing flat glass. The second column contains the respective dimensional unit. Threshold 27 is designated electrode wall in the table. The abbreviation el. stands for electrodes, and portion of boosting in total energy refers to the ratio Q.sub.elec/(Q.sub.elec+Q.sub.fossil), i.e. the portion of the electrical energy in the total supplied fossil), heat energy, expressed in %. The portion of the electrical heating in the total energy is more than 60% in all three cases, as shown in the table. In all three examples, a large number of the bubbles that occur in the melt rise to the surface of the glass melt inside the melt tank, so that a good glass quality is achieved.

TABLE-US-00002 TABLE 2 Glass type/product Container Container Flat glass glass glass Melt output t glass/day 350.00 160.00 110.00 Melt surface m.sup.2 135.42 80.04 90.00 Melt tank length m 18.30 13.80 15.08 Melt tank width m 7.40 5.80 6.00 Length of the m 2.50 1.96 2.10 electrode wall Melting part (10) m 13.00 10.80 9.17 Refining part (20) m 5.40 4.00 5.91 Bath depth melting m 1.40 1.30 1.40 part (B1) Bath depth refining m 1.75 1.60 1.25 part (B2) Glass over electrode m 0.825 0.825 0.85 wall (B5) Number of 36 24 24 electrodes in the melting part Number of el. on 6 6 6 the electrode wall Portion of boosting % 80 64 76 in total energy Electrical energy kW 9330 5500 5250 Fossil energy m.sup.3/h 235 300 155 (natural gas Heat value natural kWh/m.sup.3 10 10.56 10 gas

[0082] FIGS. 7A to 7C shows a comparison of the constructions of the melt tanks according to the existing art (FIGS. 7A and 7B) with the solution according to the present invention (FIG. 7C) based on mathematical modeling calculations. FIG. 7C corresponds to the exemplary embodiment shown in FIG. 1, while FIG. 7A approximately shows the situation of the document by Mller, and FIG. 7B shows a realization of the document of Pieper. The FIGS. 7A to 7C each show the profile of the floor 13 of the melt tank in the region of the transition from the melting part of the refining part, the electrodes (black, thick, vertically running columns), as well as the surface of the glass melt 60. The calculated temperature at the surface of the glass melt over the highest barrier, or at the smallest bath depth, is indicated in the FIGS. 7A to 7C. In the model calculation, a portion of electrical energy greater than 60% in the total supplied heat energy was assumed. In the simulations, it turns out that the respectively indicated temperature at the surface of the glass melt (in FIGS. 7A and 7B, 1400 C. and 1360 C. respectively) is too low, and the bubbles, whose path in the glass melt is indicated by the arrows, sink again after crossing the barrier, because they have too little upward drive. Only in the solution according to the present invention, shown in FIG. 7C, is the surface temperature (1450 C.) due to the heating of threshold 27, and thus the rise of the bubbles, adequate for a significant portion of the bubbles to rise to the surface of the glass melt and for the gas to escape from the glass melt.

[0083] The same also results from the results of simulation calculations shown in FIGS. 9A to 9C. Here, the system of FIG. 9A corresponds to FIG. 7C, the system of FIG. 9B corresponds to FIG. 7A, and the system of FIG. 9C corresponds to FIG. 7B. The melt bath surface was 91 m.sup.2. For the simulations, bath depth B1 in melting part 10 was 1500 mm in all cases. The realization according to the present invention (FIG. 9A) had a threshold 27 having a length of 2100 mm and a bath depth B5 of 800 mm. The variant of FIG. 9B (corresponding to the document of Mller) had a highest barrier wall (without electrodes) having a bath depth of 300 mm in the area of the wall. The system according to FIG. 9C had a refining bank as highest barrier (without electrodes), having a length of 1800 mm and having a bath depth over the refining bank of 1000 mm Before the refining bank, electrodes are situated on a lower shoulder situated in front of the bank. In all cases, the throughput was 250 t/day of clear glass with 45% input of shards. The fossil energy input was 3340 kW and the electrical energy input was 5380 kW, i.e. about 60% of the total supplied heat energy.

[0084] The simulations showed that an adequately high temperature at the glass bath surface (see FIG. 9A) that significantly promotes the rise of the bubbles is achieved only with a threshold in which the glass flowing over the threshold is heated by electrodes.

[0085] The mathematical simulation makes it possible to also evaluate the refining performance. For this purpose, in the model bubbles having smaller size are defined in a layer below the glass melt surface in the melting region. These correspond to the bubbles that occur when the carbonates are decomposed during the melting of the raw materials. These bubbles move through the melt tank with the flow. The temperature increase along this path is the cause of the growth in size of the bubbles (expansion of the gas) and the increasing rise speed, in accordance with Stoke's Law; this is taken into account in the mathematical simulation calculation. In the model, it can be seen that the bubbles will grow and rise to such an extent that the glass melt is free of bubbles after the barrier only if a threshold is used having electrodes situated on the surface of the threshold. In the two other cases, a significant number of bubbles move across the barrier into the output glass flow, or into the product. The calculation has confirmed that adequate glass quality is ensured only given the use of a heated threshold with a large portion of electrical energy relative to the total supplied heat energyunder the constraint that the energy input from fossil fuels is significantly limited.

[0086] The above observations are illustrated in FIGS. 10A and 10B and 11A and 11B on the basis of the path shown there of the bubbles in glass melt 60. FIGS. 10A and 10B and 11A and 11B each show a model calculation with a formation of, in each case, 4000 bubbles, having a diameter of 0.2 mm, in the area of inlet opening 11; in FIGS. 10A and 10B, the bubbles are formed at the right inlet side, and in FIGS. 11A and 11B the bubbles are formed at the left inlet side. The parameters of the simulation match the parameters used with regard to FIGS. 9A to 9C. In FIGS. 10A and 10B and 11A and 11B, for reasons of clarity only a small portion of the calculated 4000 bubbles is shown. Moreover, in a real glass melting tank many times more than the calculated number of 4000 bubbles are present.

[0087] As explained above, the bubbles become larger along their path in circulating glass melt 60. It will be seen that all bubbles rise to the surface. Their escape at the surface is identified by a point at the end of the line of movement of each bubble. For all the bubbles calculated in the model, there is no path that extends up to outlet opening 12. The simulation shows the case of a model having a heated threshold and a high portion of electrical energy in the total supplied heat energy.

[0088] In the cases shown in FIGS. 7A and 7B, or 9B and 9C, (not shown in FIGS. 10A and 10B and 11A and 11B), in which only a simple barrier without heating is present, or a refining bank, i.e. a barrier having a pre-step that has electrodes, is present, in contrast a significantly large number of bubbles reach the outlet opening, and therefore remain in the glass melt, reducing its quality.