Furnace and process for the production of high-transmission glass

Abstract

Float glass furnace (1) for melting materials to be vitrified, refining and conditioning high-transmission glass, comprising: - a first tank (3) intended to contain a bath of molten glass in which the melting and refining of the glass are carried out, - a frame (5) ensuring the connection between the first tank (3) and a second tank (6), - the second tank (6) intended to contain molten glass from the first tank (3) and in which the conditioning of the glass for forming is carried out, - a vault (8) arranged above the first tank (3), the furnace being equipped with first heating means (C1) configured to act at a first point (P1) located in a central zone (9) of the furnace in which a maximum temperature of the vault (8) is reached, - the vault (8) comprising a second point (P2) located substantially above the entrance of the frame (5),characterized in that second heating means (C2) are disposed in the furnace and are configured to act on the temperature of the vault at the second point (P2) such that the temperature difference between the first point (P1) and the second point (P2) is less than or equal to a predetermined value (dT). Figure for the abstract: 2,

Description

Title of the invention: OVEN AND METHOD FOR THE PRODUCTION OF HIGH TRANSMISSION GLASS Designation of the technical field concerned

[0001] The invention relates to continuous production furnaces for flat glass and more particularly to high transmission glass production furnaces, also called extra-clear glass, extra-transparent glass, ultra-white glass or even "low iron glass" in English, which means glass with a low iron content, the terminology used being different depending on the glass manufacturer. Technical problems that the invention addresses

[0002] In flat glass melting furnaces, raw materials are melted at high temperature to produce a glass paste which is then formed into a ribbon on a tin bath, according to the flotation process, or mechanically by a roller laminator.

[0003] Three main stages of processing are distinguished in the melting furnace:

[0004] Heating and melting of raw materials,

[0005] Degassing and homogenization at high temperature (approximately 1500°C),

[0006] . Final homogenization and heat conditioning at approximately 1100°C before forming.

[0007] For historical reasons of thermal efficiency and compactness of the refractory material enclosure, these three stages are carried out in single-chamber furnaces. Only a constriction called the corset creates a certain separation of the first two stages from the final homogenization and thermal conditioning in a section called the working basin.

[0008] The combination of the three stages in a single furnace induces significant thermal gradients in the molten glass. This leads to very complex convection currents. The main draft of the furnace between the feed side for the raw materials and the extraction of the glass paste, through an outlet called a channel, induces a forced convection component.

[0009] The thermal gradients between the different zones of the furnace induce thermoconvection.

[0010] Differences in density, for example between granular material and glass paste, induce other convection components, in particular the floating of raw materials on the molten glass paste which forms a more or less continuous mat.

[0011] The historical evolution of flat glass furnaces has resulted in a characteristic convection current field which is schematically presented in the diagram shown in [Fig.1].

[0012] The introduction of the raw materials at point 2, left side, causes both a drop in temperature in the combustion space C above the mat and also in the glass.

[0013] A first thermoconvection belt / roller Bl is established. The surface counter-current of this roller helps to keep the raw material mat in place and prevents its dispersion over the entire surface of the glass bath.

[0014] The lowering of the temperature of the glass, by various means of cooling before its exit from the oven at point 7, right side, induces a second thermoconvection roll B2.

[0015] The cooling of the glass which occurs upstream of the corset 5 contributes to conditioning the glass to reach the target outlet temperature in the channel.

[0016] The corset is equipped with a cooled barrier 12 which contributes to limiting the return current and cooling the forward current of the loop B2.

[0017] The combination of the two rollers creates a central RC resurgence zone in the glass. This resurgence zone proves particularly beneficial. It forces the freshly molten glass, still full of defects, to rise to the surface and reach high temperatures for refining.

[0018] The temperature differences between the central part and the ends thus induce thermoconvections in the glass with back-and-forth current flows far greater than the furnace draw.

[0019] The draw stream thus follows a trajectory T between the melting of the raw materials and the outlet of the furnace. The high quality of the flat glass is directly linked to this trajectory design, which ensures that all of the drawn glass passes through the central resurgence at the surface of the glass bath.

[0020] An operator of a flat glass furnace therefore always aims to enhance the central heat resurgence of the furnace. This is primarily achieved by adjusting the combustion power profile to create a central zone at a high temperature. This temperature is limited only by the strength of the roof material. Silica is the most commonly used material for flat glass furnace roofs and limits the hot-face temperature to approximately 1620°C max.

[0021] The vault temperatures on the corset side are often set at around 1400°C, or even lower. This temperature limitation aims to initiate the cooling of the incoming flow to support thermal conditioning and to limit corrosion of the walls at the corset inlet. Sometimes, additional devices are used to reinforce the central resurgence, such as large air bubble injectors or Electrodes for heating by current dissipation are used. However, these methods also have disadvantages, and the initial aim is to design and operate the furnace without them. Therefore, the first objective is to enhance the resurgence by natural convection.

[0022] This leads to the unusual situation where the time the glass spends on the main drawing path represents only 10-20% of the piston time in flat glass furnaces. Furthermore, the time spent at high temperature to ensure the refining and quality of the glass represents only 1-3% of the furnace's piston time (typically 50-70 hours).

[0023] However, it turns out that refining for 60 minutes above a temperature of approximately 1420°C for a soda-lime flat glass is sufficient to achieve high quality. These are typical values ​​that may vary depending on the type of glass and the required quality.

[0024] Any trajectory of the glass after the central resurgence must therefore reach a minimum temperature of 1420°C and a minimum duration of 60 minutes. The furnace operator / designer must therefore ensure these conditions along a trajectory L, in addition to the objective of ensuring the central resurgence.

[0025] A resurgence point is therefore planned as far upstream as possible to ensure the heating of the glass on the trajectory L for a sufficient period of time.

[0026] These criteria are therefore paramount in the design and operation of a flat glass furnace.

[0027] To ensure the central resurgence of the glass and prevent any passage of the glass to the hearth, the depth of the glass bath should be limited. However, a shallow hearth is exposed to higher temperatures, which reduces its lifespan.

[0028] A problem arises in particular if the temperature is approached or exceeded by 1400°C. The decrease in the viscosity of the glass facilitates its infiltration into the joints between the blocks and induces a risk of perforation of the base.

[0029] To ensure a high minimum processing time, and in particular a minimum ripening time of 60 minutes or more, the width and length of the oven could be increased. This is obviously costly in terms of investment and increases the oven's energy consumption due to increased heat loss.

[0030] In a more elaborate approach, one could aim for optimization of recirculation using geometric or thermal means. However, this approach encounters considerable complexity for the following reasons:

[0031] . Convection is determined by differential conservation equations of mass, energy and momentum. The coupling of these equations leads to non-linear relationships, which prevents finding a simple solution.

[0032] The properties of glass evolve with temperature and in a non-linear manner, including viscosity and effective conductivity (combining phononic and photonic conductivity).

[0033] The process of melting the raw material mat is very complex in its interaction with molten glass.

[0034] . The complexity of heat transfer, essentially by radiation, of the combustion energy towards the raw materials conveyor belt and the glass.

[0035] A very particular difficulty arises from the tint of the glass. The transparency of flat glass varies considerably between clear and tinted glass. Moreover, within the category of clear glass, there is a wide range from entry-level clear glass to highly transparent glass.

[0036] The terminology for the different qualities of clear glass is not standardized and varies depending on the manufacturer. However, transparency is mainly determined by residual iron oxide contamination and its redox state.

[0037] High transparency glasses are obtained with an iron contamination of less than 250 ppm compared to that for clear glasses with iron levels between 500-1500 ppm.

[0038] For comparison, a standard clear glass has an effective conductivity of approximately 100 W / mK at 1400°C. A very high transmission glass can reach 300 W / mK for the same temperature, therefore a conductivity three times higher.

[0039] Producing high-transmission glass has proven very problematic. Indeed, glassmakers are forced to reduce the draft of their furnaces by approximately 15% to maintain good refining quality. Despite this reduction in draft, the quality level often remains lower than that of standard clear glass production. The reduction in draft and quality naturally results in a significant financial disadvantage. Furthermore, the hearth temperature increases drastically, which reduces the furnace's lifespan.

[0040] For the production of a high-transmission glass, the aim is therefore:

[0041] A reduction in the temperature of the sole,

[0042] . An increase in the refining time (reduction of recirculation, temperature increase).

[0043] Isolated improvements lead to dead ends, as demonstrated in the examples below:

[0044] An increase in surface temperature at the end of the refining zone, by For example, improved insulation and additional heating in this area will prolong the glass's exposure to high temperatures. However, a higher temperature in this area further increases the already high core temperature of extra-transparent glass. Moreover, this increase reduces the glass's viscosity and promotes thermoconvection.

[0045] To better cool the sole, a higher flow rate of glass could be used cold air, coming from the working basin. This would lead to an intensification of the B2 recirculation and therefore a shortening of the time in zone L. Moreover, this increase in B2 recirculation would automatically result in an additional cost in terms of the furnace's energy consumption.

[0046] . A deeper oven will reduce the temperatures of the hearth. However, a significant glass height considerably promotes thermoconvection. An intensification of the B2 loop leads to an increase in glass velocity along the refining length L and reduces the refining time.

[0047] . Drastic cooling of the sole would reduce the temperatures and to increase viscosity. The corresponding reduction in recirculation B2 will prolong the refining process over length L. Conversely, the depth of refining will be reduced, as will its quality. A significant increase in consumption would be another consequence.

[0048] In a melting and refining furnace for the production of flat glass by a float process, the temperature of the glass near the surface is determined by adjusting the combustion temperature. Adjusting this temperature is crucial for proper refining of the glass.

[0049] However, the glassmaker has no simple means of adjusting the temperature of the glass at depth. Yet, excessively low temperatures at depth, and particularly at the hearth, lead to devitrification of the glass and pose a problem during changes in glass composition in the furnaces, due to the high viscosity of the glass which lengthens the duration of the transitions.

[0050] For standard clear glass, hearth temperatures in the refining zone are typically between 1250°C and 1350°C. The construction and refractory materials of the hearths allow for a very long lifespan, up to twenty years at these temperatures. The depths required to achieve these hearth temperatures are between 1.2 and 1.4 m. These values ​​have been determined experimentally. Increasing the depth beyond these values ​​results in excessively low hearth temperatures, particularly below the furnace loading point.

[0051] A significant increase in losses through the hearth allows for a lower temperature for high-transmission glass. However, this results in high furnace energy consumption. Furthermore, it requires a complex design if these losses must remain adjustable for different glass transparencies.

[0052] Correction of excessively low temperatures can be achieved by injecting air through a row of injectors (also called bubblers) in the furnace floor, or by electrical heating using electrodes immersed in the glass. However, these methods can only be implemented at the beginning of the refining zone to avoid disruption of the glass flow on the surface. However, for tinted glasses with low transparency, these methods of increasing surface temperatures are commonly used.

[0053] On the other hand, they are of no use for high transmission glass because the problem lies not in a temperature that is too low but in a temperature that is too high.

[0054] It is therefore highly desirable to have a method for determining an appropriate depth for a high transmission glass float furnace.

[0055] Furthermore, it would be particularly advantageous if a furnace sized for high transmission glass were also suitable for the production of standard clear glass.

[0056] It would also be highly desirable to limit the secondary recirculation of high transmission glass in a refining zone to prolong the refining time and to reduce consumption. Technical background

[0057] The method commonly used by glassmakers to lower the hearth temperature in a furnace for producing high-transmission glass is to reduce the hearth insulation. However, this is only possible if the reduction in insulation, or hearth ventilation, was planned during the furnace's construction. The effect on lowering the temperature of the glass at depth, through increased hearth heat loss, is often insufficient to reach acceptable temperatures again. Furthermore, increased heat loss through the hearth leads to an increase in the furnace's energy consumption and therefore its operating costs and CO2 emissions.

[0058] US patent 20200399164 addresses the problem of energy consumption in furnaces using low-iron glass, which therefore has high transmission. A reduction in transmission only at high temperatures is achieved with the addition of NiO oxide. However, the addition of nickel to the glass matrix has detrimental effects, particularly when the glass is tempered. This method is not applicable to architectural flat glass or PV applications.

[0059] The applicant's patent FR2950620 proposes a solution to the problem of excessively low hearth temperatures in the furnace loading zone. The glass depth is limited in the loading zone, which allows for optimization of the depth in the refining zone. Glassmakers could thus increase the depth in the refining section when designing a furnace for high-transmission glass, without risking excessively low temperatures during the production of less transparent glass.

[0060] However, FR2950620 does not offer any means of determining the required depth for high transmission glass, of obtaining correct core temperatures and not exceeding a critical temperature which would lead to deterioration of the core, nor of determining the core temperatures when manufacturing standard clear glass in the same furnace.

[0061] FR2950620 also does not propose any means against the increase in intensity secondary recirculation which results from an increase in the depth of the furnace.

[0062] In float furnace terminology, the so-called refining zone is often identified as the terminal zone after the last combustion port. However, in reality, the refining process begins much earlier, at the zero-velocity resurgence position, which is located approximately halfway along the furnace's length. This resurgence position coincides roughly with the maximum temperatures of the glass and the roof at the center of the furnace. We therefore consider, hereafter, that the refining zone begins at the position of maximum temperature at the center of the furnace. Summary of the invention

[0063] According to a first aspect of the invention, a float glass furnace is proposed for melting materials to be vitrified, refining and packaging high-transmission glass, comprising:

[0064] - an input for raw materials,

[0065] - a first tank intended to contain a bath of molten glass on which a mat of raw materials is able to float from the inlet to a certain distance inside the vat, in which the melting and refining of the glass are carried out,

[0066] - a corset ensuring the connection between the first vat and a second vat,

[0067] - the second tank intended to contain a bath of molten glass from the first tank, in which the glass is conditioned for forming,

[0068] - an outlet downstream of the second tank through which the conditioned glass is discharged,

[0069] - a vault arranged above the first vat, the oven being equipped with first heating means arranged above the bath and configured to act at a first point PI, located in a central area of ​​the furnace where a maximum vault temperature is reached,

[0070] - the vault comprising a second point P2 located substantially above the entrance of the corset,

[0071] characterized in that second heating means are arranged in the furnace, above the bath, and are configured to act on the temperature of the vault at the second point P2 so that the temperature difference between the first point PI and the second point P2 is less than or equal to a predetermined value dT.

[0072] Advantageously according to the invention, the predetermined dT value is a function of the targeted glass quality.

[0073] The first and second heating means can be arranged on the roof or on the sides of the furnace. The heating means can be burners, electric resistances, radiant tubes heated by combustion products or an electric element, or any other heating means suitable for the operating conditions in this section of the furnace and the power required.

[0074] Advantageously according to the invention, the intensity of the recirculation B2 is reduced, the length of the zone L for refining is increased, the flow velocity of the glass is limited and the temperature of the hearth is reduced.

[0075] This result is obtained by the combination:

[0076] . From a reduction in the thermal gradient of the vault after the central zone of resurgence due to an increase in temperature P2.

[0077] . By deepening the furnace in the refining zone (RC+L2), this being able to be extended over the extended area 10.

[0078] . Limiting the intensity of the glass returning from the working basin.

[0079] . Increased losses through the sole in the deep zone.

[0080] . The reduction of the peak vault temperature in the central resurgence zone.

[0081] This results in a relatively flat glass and dome surface temperature profile. This is counterintuitive for a person skilled in the art, who always seeks to reinforce the central resurgence in their float furnace.

[0082] Advantageously according to the invention, the first vat has a depth in the refining zone greater than the depth of the corset and the depth of the second vat.

[0083] The additional height of glass in the refining zone allows the temperature of the hearth to be lowered in this portion of the oven.

[0084] The depth of the first tank in the ripening area is at least 10% greater, and preferably at least 20% greater, than the depth of the corset and the depth of the second tank.

[0085] Renewing the refining depth of a high-transmission glass furnace in the corset and conditioning tank 6 would have the following disadvantages: • Large volumes in tank 6 with a risk of stagnation and devitrification of the glass, • Significant secondary recirculation, particularly for highly transparent lenses.

[0086] Significant secondary recirculation leads to increased energy consumption and a shorter glass refining time in the furnace. To reduce this recirculation to an acceptable level, a very deep dam would be necessary. Such deep dams pose mechanical and thermal problems. For these reasons, the depth of the furnace chamber and conditioning tank of a high-transmission glass furnace according to the invention is limited to values ​​closer to those of conventional furnaces, namely a depth between 0.8 and 1.3 m. Furnace refining according to the invention requires depths greater than 1.4 m, depending on the transparency of the low-iron glass. Depending on these depths, different values ​​result for the ratio of the depths between the furnace chamber and the furnace refining chamber. However, to achieve a sufficient effect according to the invention, a minimum increase of 10% is required.

[0087] Advantageously according to the invention, the thermal insulation of the portion of the floor of the first vat located in the ripening zone is reduced compared to that of the portion of the floor of the first vat located upstream, on the side of the oven entrance.

[0088] The heat losses through the portion of the hearth located in the ripening zone L2 are thus increased, compared to those of the portion of the hearth located upstream, on the side of the oven entrance.

[0089] The increase in these heat losses contributes to lowering the temperature of the portion of the floor located below the second heating means.

[0090] Advantageously according to the invention, the depth of the first tank in the refining zone is such that, for a given temperature Tbott of the glass at the bottom in the refining zone, the height hmeit of glass in the refining zone complies with the equation:

[0091]

[0092] Tbott: average temperature of the sole in the ripening zone

[0093] Tsurf: average surface temperature of the glass in the refining zone

[0094] hmeit: height of the glass in the refining zone

[0095] keff: effective conductivity of the glass including the contribution of radiation

[0096] qbott: heat flux density of the sole as a function of the insulation level

[0097] qsurf: heat flux density of the combustion space in the glass bath - value average of the free surface area of ​​the glass exposed to combustion radiation

[0098] With this relationship, the person skilled in the art has a simple means to determine the height of glass in the refining zone, as a function of the temperature of the glass at the hearth, or vice versa.

[0099] This relationship also allows, from the surface temperature of the glass, the heat input to the surface and the conductivity of the glass, the determination of the glass temperature at the bottom of the float furnace for different depths and levels of bottom insulation.

[0100] According to a second aspect of the invention, a glass manufacturing process is proposed using a high-transmission float glass furnace as described above, according to the first aspect of the invention, characterized in that the process includes adjusting the temperature at the first point PI, by means of the first heating means and / or adjusting the temperature at the second point P2 by means of the second heating means, so that the temperature difference between the first point and the second point is less than or equal to a value dT that is decisive for the quality of the glass.

[0101] Advantageously according to the invention, the value dT determining the quality of the glass is less than or equal to 150 °C and preferably less than or equal to 130 °C.

[0102] According to an embodiment of the invention, the process includes adjusting the temperature at the first point PI by means of the first heating means, so that the temperature difference between the first point PI and the second point P2 is less than or equal to the value dT determining the quality of the glass.

[0103] Compared to the usual setting of a float oven, the temperature at point PI is lowered.

[0104] According to another embodiment of the invention, the method comprises an adjustment of the temperature at the second point P2 by means of the second heating means, so that the temperature difference between the first point PI and the second point P2 is less than or equal to the value dT determining the quality of the glass.

[0105] Compared to the usual setting of a float oven, the temperature at point P2 is increased.

[0106] According to an embodiment of the invention, the process includes temperature control at the first point PI by means of the first heating means and temperature control at the second point P2 by means of the second heating means C2 so that the temperature difference between the first point PI and the second point P2 is less than or equal to the value T determining the quality of the glass.

[0107] Thus, compared with the usual setting of a float oven, the temperature at point PI is lowered and, at the same time, the temperature at point P2 is increased.

[0108] Analysis of data from numerous furnaces and the results of many models enabled the inventors to discover a surprising result. Despite the complexity of convection and the temperature field in a float glass furnace, the average hearth temperature in the refining zone can be estimated with a simple formula, which depends on very few key parameters.

[0109] Based on the surface temperature of the glass, the heat input to the surface and the conductivity of the glass, the temperature of the glass at the hearth of a float furnace can be estimated for different depths and levels of sole insulation, depending on key parameters.

[0110] Advantageously according to the process, for a given glass temperature Tbott at the hearth in the refining zone, the heat flux density of the hearth complies with equation (1) seen previously and recalled below: [YES] / \i( ! 7 p(l) Knelt ~ ' y^bott ' T surf} / [4^ " ' [4^ " ^surf}

[0112] TbOtt: average temperature of the sole in the ripening area

[0113] Tsurf: average surface temperature of the glass in the refining zone

[0114] hmeit: height of the glass in the refining zone

[0115] keffi effective conductivity of the glass including the contribution of radiation

[0116] qbott: heat flux density of the sole as a function of the insulation level

[0117] qsurf: heat flux density of the combustion space in the glass bath - value average of the free surface area of ​​the glass exposed to combustion radiation

[0118] According to a third aspect of the invention, a glass manufacturing installation is proposed comprising a furnace according to the first aspect of the invention. Brief description of the figures

[0119] Other features and advantages of the invention will become apparent upon reading the detailed description that follows, for understanding which reference should be made to the accompanying drawings in which:

[0120] [Fig. 1] is a schematically and partially represented vertical cross-sectional view of a float glass furnace according to the prior art.

[0121] [Fig.2] is a schematically and partially represented vertical cross-sectional view of a float glass furnace according to an embodiment of the invention.

[0122] [Fig.3] is a schematically and partially represented vertical cross-sectional view of a float glass furnace according to another embodiment of the invention.

[0123] [Fig.4] is a diagram illustrating the evolution of the effective conductivity of three qualities of glass with temperature.

[0124] [Fig.5] is a diagram illustrating the evolution of the surface temperature of the bath in the refining zone according to the iron content. Detailed description of the invention

[0125] We describe below a first example of application of the invention for a glass melting and refining furnace according to the invention, schematically represented in [Fig.2], enabling the production of clear glass and high-transmission glass whose main characteristics are:

[0126] . Output: 900 tonnes / day

[0127] . Length: 45.7 m

[0128] . Width: 12.5 m

[0129] . Area: 571 m2

[0130] . Surface area of ​​the glass under combustion: 375 m2

[0131] . Clear glass: 850 ppm Fe2O3 and 25% Redox

[0132] . High transmission glass: 130 ppm Fe2O3 and 30% Redox

[0133] In this example, a reduced depth of the hearth has been maintained in zone 11, on the loading side, in order to promote the advancement of the cold glass from the hearth towards the hot zone and increased the depth according to the invention in zone 10, towards the corset.

[0134] However, maintaining the same significant depth according to the invention over the entire length of the oven has the advantage of simplifying the construction of the oven.

[0135] The sizing data for this furnace are shown in the third column of the summary table below.

[0136] The furnace includes a set of complementary C2 burners arranged near the corset. These burners increase the furnace temperature near the corset, so that the glass is at a higher temperature near the corset, thus reducing the temperature difference between the glass and the surface of the bath along the length of the furnace.

[0137] The heat input from this set of complementary C2 burners makes it possible to lower the temperature of the furnace upstream, with less power delivered by the set of CL burners. This can also contribute to reducing the temperature difference of the glass on the surface of the bath.

[0138] The oven, according to this first embodiment of the invention, has a hearth of substantially constant thickness.

[0139] Average surface temperature of the glass in the refining zone (T surf):

[0140] The surface temperature of the glass changes more or less strongly between the point The glass is heated to a high temperature, midway through the furnace, until the end of the refining process. However, the average temperature between the hot spot and the end of the heating zone is found to be sufficiently representative for estimating the hearth temperature. In this embodiment of the invention, the average surface temperature of the glass is 1500-1550°C. Due to heat transfer and the need to maintain a specific refining temperature at a certain depth, this average temperature varies with the glass's transparency.

[0141] The diagram in [Fig.5] indicates the average surface temperatures of the glass (Tsurf avg.) in the refining zone, typically required for different iron contents.

[0142] This diagram also shows the maximum temperatures (Tsurf max) at the hot spot of the glass and the maximum temperatures (Tcrown max) of the crown. The glass surface temperatures are not always accessible on an oven without cameras or IR pyrometers. On the other hand, all ovens are equipped with thermocouples in the vault, which allow the hot face temperature of the vault to be determined.

[0143] High-transmission glasses develop a reduced temperature gradient with depth. To maintain a refining temperature at a sufficient depth, for example 400 mm, a high-transmission glass requires a lower surface temperature compared to that of a low-transmission glass.

[0144] Heat flux density of the combustion space in the glass bath (q surf)

[0145] The heat flux density from the combustion space to the material mat The heat flux density in the glass varies significantly depending on its position along the furnace. The reasons for these variations are numerous: changes in the temperatures of the conveyor belt and the glass, variations in the intensity of radiation from the combustion chamber depending on the presence of flames and the temperature of the refractories, etc. In the refining zone, beneath the combustion chamber, variations in heat flux density within the glass are observed, averaging around 70 kW / m². An average value for this heat flux density is required to calculate the hearth temperature. A simple method exists for estimating this value.

[0146] The heat balance of a float furnace allows us to extract the amount of heat transferred into the bath by the sum of the losses from the vessel and the energy of glass melting. This amount corresponds to the combustion power multiplied by the efficiency of the combustion system, which is on the order of 50-60% for a modern furnace.

[0147] In our example, the amount of heat required by the bath is approximately 31 MW. Part of this heat is directly absorbed by the surface of the mat. This amount transferred into the mat can be estimated as follows: 1. A float glass composition mat typically has a specific gravity of 4-5 tpd / m2 (tons per day per m2), with 4 tpd / m2 for clear glass and 5 tpd / m2 for high-transmission glass. With the draft of this furnace, a mat surface area of ​​approximately 180 m2 is required. 2. The surface of a carpet absorbs energy with an average flux density of approximately 100 kW / m2. A flux of approximately 18 MW is therefore directly transmitted to the carpet. 3. There remains a flux of 13 MW which is transmitted to the surface of the glass, mainly in the area covered by the combustion, outside the mat. 4. With a remaining surface area outside the conveyor belt of approximately 195 m2 in this example furnace, an average flux of approximately 70 kW / m2 is obtained.

[0148] Heat flux density of the sole as a function of the insulation level (q bott)

[0149] When designing the hearth of a glass furnace, layers of refractory materials are provided beneath the insulating layers, which are made of various materials resistant to heat, pressure, and glass seepage. Good insulation reduces furnace consumption. Poor insulation, which may require hearth ventilation, reduces the contact temperature between the hearth and the glass, thus reducing the risk of corrosion and glass seepage.

[0150] Good insulation of the hearth of a float furnace limits the heat flux density to less than 2 kW / m². A hearth with very low insulation allows a flux of 5 kW / m². Ventilation further increases this flux. Some hearth designs even allow for modulation of the heat flux density. However, these devices make the furnace more complex and expensive. When designing a furnace for high-transmission glass, the best compromise between reducing energy consumption and the risk of corrosion is sought.

[0151] Advantageously, according to our invention, in this embodiment, to transition from high-transmission glass to clear glass, the insulation in the refining zone is increased in order to reduce the average flux density from 3.5 kW / m² to 2.2 kW / m². Temperature profile of the vault

[0152] According to the invention, to produce high transmission glass, the furnace is operated with a small temperature difference between point PI located in a central area of ​​the furnace in which a maximum vault temperature is reached and point P2 located substantially above the corset inlet.

[0153] In this embodiment, for high-transmission glass, the arch temperature at point P1 is 1570°C and that at point P2 is 1470°C. This temperature P2 can be obtained and controlled by means of the additional heating means added according to the invention.

[0154] During the production of clear glass on this furnace according to the invention, the temperature at point P1 is typically about 30°C higher, i.e., 1600°C. The additional heating means is not used, which has the effect of lowering the temperature at point P2 to about 1430°C.

[0155] Effective conductivity of the glass including the contribution of radiation (k eff ) î

[0156] The iron concentration and the redox of the iron of the float glasses lead to the effective conductivities of the glass as a function of temperature according to the curves in [Fig.4] for three qualities of glass, a photovoltaic glass at 100 ppm of Iron and a Redox of 5%, a high transmission glass at 130 ppm of Iron and a Redox of 30%, and a clear glass at 850 ppm of Iron and a Redox of 25%.

[0157] The vertical temperature profiles of the glass in the resurgence zones are typically convex, with a significant temperature gradient at the surface and a reduced gradient towards the base. In the sizing estimates according to the invention, it proves more appropriate to apply the conductivity of the glass for the base temperature. Calculation result

[0158] The values ​​used in the calculation for high-transmission glass are summarized in the table below in the first column. A target average hearth temperature during refining (average hearth temperature during refining) of 1350°C is used. Simultaneously, insulation resulting in a hearth loss (hearth losses qbott) of 3.5 kW / m² is provided. With these target values ​​and the other characteristic values ​​of the design of this furnace, a depth of 1.7 m (glass depth) is obtained for the refining chamber depth. High-transmission glass Standard clear glass PV glass Iron content Fe2O3 ppm wt 130 850 100 Iron redox (FeO / Fe early) % 30 25 5 Draw tpd 900 900 800 Cullet % 20% 20% 15% Furnace length excluding doghouse m 45.7 45.7 39.5 Tank width m 12.5 12.5 12.0 Tank area under superstructure m2 571 571 474 Combustion end position m2 30 30 29 Area under combustion m2 375 375 348 keff at Tbott temperature W / m*K 220 120 320 Conveyor melting rate tpd / m2 5 4.5 5 Conveyor coverage area m2 180 200 160 Free glass area under combustion m2 195 175 188 Total flux required in the glass kW 31676 31476 29483 Flux density in the mat (average) kW / m2 100 100 100 Flux in the mat kW 18000 20000 16000 Flux in the glass kW 13676 11476 13483 Glass flux density qsurf kW / m² 70 66 72 Hot spot glass surface temperature °C 1530 1570 1525 Fine combustion glass surface temperature °C 1495 1540 1485 Medium glass surface temperature Tsurf °C 1513 1555 1505 Hearth losses qbott W / m² 3500 2200 4000 Medium refining hearth temperature Tbott °C 1350 1330 1375 Glass depth hmeit m 1.70 1.70 1.82 Arch temperature at point PI °C 1570 1600 1560 Arch temperature at point P2 °C 1470 1430 1470 Temperature difference dT °C 100 170 90

[0159] As can be seen in this table for standard clear glass, an average refining hearth temperature of 1330°C is targeted. At the same time, a fairly good insulation of 2.2 kW / m² is planned. With these targeted values ​​and the other characteristic values ​​of the design of this furnace, a refining chamber depth of 1.7 m is achieved.

[0160] It is observed that it is possible to produce high-transmission glass and clear glass in this furnace with:

[0161] . the same height of glass during refining,

[0162] . a limited adjustment of losses through the sole,

[0163] . a very small temperature difference at the sole between the two types of glass,

[0164] . the same draw, moreover a high one, particularly for a high-transmission lens,

[0165] . a slight adjustment of the operating parameters.

[0166] Limiting the hearth temperature for high-transmission glass through the furnace design also allows for better control of glass viscosity and convection. Reducing the temperature difference between the reflow zone and the terminal refining zone limits the recirculation intensity and maintains a suitable refining time. A reduction in furnace draft is no longer required, as is necessary with a prior art furnace.

[0167] The predictions according to the very simple method described here coincide well with the numerical and experimental results, which confirm that a prolonged refining time and good glass quality are obtained for high transmission glass and clear glass, with the same draw for these two types of glass.

[0168] New calculation example for a furnace dedicated to photovoltaic glass (PV glass)

[0169] We describe below a second example of application of the invention for a melting and refining furnace dedicated to PV glass, also as schematically represented in [Fig.2], allowing the production of only high-transmission glass for photovoltaic applications, the main characteristics of which are:

[0170] . Output: 800 tonnes per day

[0171] . Length: 39.5 m

[0172] . Width: 12.0 m

[0173] . Area: 474 m2

[0174] . Surface area of ​​the glass under combustion: 348 m2

[0175] High-transmission glass: 100 ppm Fe2O3 and 5% Redox

[0176] If a furnace is designed for PV glass with only 100 ppm Fe2O3 and a Redox potential of 5%, for a production rate of 800 tonnes per day, the question arises as to the choice of hearth insulation, temperature, and glass depth. An average hearth temperature of 1375°C is targeted for refining, sufficiently lower than the critical temperature of 1400°C. If a hearth heat loss of 4.0 kW / m² is applied simultaneously, a recommended depth of 1.82 m is obtained.

[0177] Compared to the first embodiment, the properties of PV glass lead to a greater depth of the furnace, while maintaining limited hearth and roof temperatures, allowing a good furnace life.

[0178] The maximum temperature of the vault at point PI is adjusted to 1560°C, the temperature before the corset at point P2 is 1470°C which leads to a dT of only 90°C. With this small temperature difference, B2 recirculation remains limited and a sufficient ripening time is ensured.

Claims

Demands

1. A float glass furnace (1) for melting materials to be vitrified, refining and conditioning high-transmission glass, comprising: - an inlet (2) for raw materials, - a first tank (3) for holding a bath of molten glass in which the melting and refining of the glass are carried out, - a corset (5) providing the connection between the first tank (3) and a second tank (6), - the second tank (6) for holding molten glass from the first tank (3), and in which the conditioning of the glass for forming is carried out, - an outlet (7) downstream of the second tank (6) through which the conditioned glass is discharged, - a vault (8) disposed above the first tank (3), the furnace being equipped with first heating means (Cl) configured to act at a first point (PI) located in a central zone (9) of the furnace in which a maximum temperature of the vault (8) is reached,- the vault (8) comprising a second point (P2) situated substantially above the entrance of the corset (5), characterized in that second heating means (C2) are arranged in the furnace and are configured to act on the temperature of the vault at the second point (P2) so that the temperature difference between the first point (P1) and the second point (P2) is less than or equal to a predetermined value (dT).

2. Oven (1) according to claim 1, characterized in that the first vat has a depth in the refining zone (L2) greater than the depth of the corset (5) and the depth of the second vat (6).

3. Oven (1) according to claim 2, characterized in that the depth of the first tank in the refining zone (L2) is greater by at least 10%, and preferably by at least 20%, than the depth of the corset (5) and the depth of the second tank (6).

4. Oven (1) according to claim 1, characterized in that the thermal insulation of the portion of the floor of the first vat located in the ripening zone is reduced compared to that of the portion of the floor of the first vat located upstream, on the side of the oven entrance.

5. Oven (1) according to any one of claims 1 to 4, characterized in that the depth of the first tank in the refining zone is such that, for Given a temperature Tbott of the glass at the hearth in the refining zone, the height hmeit of glass in the refining zone follows the equation: hmeit = • ^bott - 1 - - ^surf]

6. A glass manufacturing process using a high-transmission float glass furnace (1) according to any one of the preceding claims, characterized in that the process includes adjusting the temperature at the first point (PI) by means of the first heating means (Cl) and / or adjusting the temperature at the second point (P2) by means of the second heating means (C2), such that the temperature difference between the first point (PI) and the second point (P2) is less than or equal to a value (dT) that is critical for the quality of the glass.

7. A process according to claim 6, characterized in that the determining value (dT) for the quality of the glass is less than or equal to 150°C and preferably less than or equal to 130°C.

8. A method according to any one of claims 6 and 7, characterized in that, for a given glass temperature Tbott at the hearth in the refining zone, the heat flux density of the hearth complies with the equation: ^mell ~ ^eff ' bott ' ' ' ' ^surf]

9. Glass manufacturing installation comprising a furnace according to any one of claims 1 to 5.