INSTALLATION FOR MEASURING THE TEMPERATURE OF THE RIBBON IN A FLAT GLASS ANNEALING LEHR, AND METHOD FOR
OPERATING A LEHR
The present invention relates to a device for measuring the surface temperatures of a glass ribbon in a flat glass annealing lehr.
A flat glass annealing lehr is a tunnel furnace equipped with heating and cooling means for making a glass ribbon undergo a controlled annealing/cooling thermal cycle. It consists of successive zones, generally denoted by AO, A, B, C, D, E and F, the zone AO being located on the ribbon entry side. In the zones AO, A, B and C of the lehr, the glass is cooled by radiative exchange with cool parts, commonly referred to as exchangers, or heating elements, whereas in the zones D, E and F the glass is cooled by convection using blown air. The zones AO to D are closed by insulating walls in order for the cooling of the glass to be controlled better. The lehr is placed downstream of the molten tin bath in the case of a production line using the float process, or downstream of the melting/conditioning furnace in the case of a laminated glass production line.
The first critical phase of the annealing/cooling cycle for the flat glass strip is in those zones of the lehr in which the glass is in a viscoelastic state. The cooling induces thermal gradients and stresses along the surface of the glass strip and in the core thereof. To limit the creation of residual stresses and to allow them to relax, the cooling starts at a reduced rate so as to allow the glass to undergo annealing. Too high a residual stress level causes problems in the subsequent treatment of the glass, such as the cutting operation. Once this annealing around the transition temperature has been completed, the second critical phase of the
cooling cycle starts, in which the aim is to cool the glass rapidly so as to limit the length of the lehr. Since the glass is present in the solid state, the thermal gradients during this cooling step induce what are called "temporary" stresses. Excessive temporary stresses over the width or in the thickness of the ribbon cause the glass to break. It is therefore important for the thermal profile in the longitudinal and transverse directions and in the thickness of the glass ribbon to be finely controlled.
The required temperature measurement precision for ensuring good control of the glass thermal cycle would be:
+ 5°C absolute in the annealing zones of the lehr ;
± 10°C absolute in the rapid cooling zones;
± 3°C relative in the case of the profiles over the width of the ribbon (all zones); and
± 3°C relative in the case of the upper and lower face temperatures of the ribbon (all zones).
The temperature measurement devices installed on lehrs according to the prior art result in practice in much larger errors. In addition, the measurement means generally available do not allow the temperature of the critical points to be measured, these being the edges of the ribbon. Likewise, the measurement of the temperature at a given point both on the upper face and the lower face of the ribbon is not generally available. Devices for measuring the temperature in the lehr according to the prior art will be described in greater detail below:
Temperature measurement carried out by thermocouples
Upper thermocouples are conventionally fitted into the roof and suspended above the ribbon. Lower thermocouples are fixed to a bar between the rollers
supporting the ribbon. These thermocouples placed in rigid tubes can be adjusted, but they often remain a few centimeters from the glass.
The thermocouples are always positioned at the end of the closed radiative cooling zones (A, B, C) . Thermocouples are not placed in the convective zones (D and F) because measurement is disturbed by convection in these zones.
Five thermocouples are generally installed on the top side over the width of the ribbon and three thermocouples on the underside. Since these thermocouples are placed at several centimeters from the ribbon, they receive the radiation emitted by the ribbon but are cooled by the exchangers. Consequently, they do not allow the temperature of the glass ribbon to be measured correctly.
Te/nperature /neasurement carried out by fixed optical pyrometers
These pyrometers are installed in the roof at several points over the length of the lehr and are directed at the upper surface of the ribbon.
In general, a single pyrometer is installed at the end of the zones. Sometimes, three pyrometers are installed over the width of the ribbon, thereby giving an indication of the transverse temperature profile. Increasing the number of these pyrometers is precluded owing to their prohibitive cost.
The emissivity of these pyrometers is not always controlled by the operators, thereby leading to measurement errors.
Temperature measurement carried out by portable pyrometers
in the limit of their measurement precision, give
In the open" sections, typically at the end of zone D and in zones E and F, the temperature of the upper face of the ribbon may be measured with portable pyrometers held by operators that have to be trained in their use. Because the position of the measurement over the width of the ribbon is not precise, because the size of the measurement area varies depending on the distance between the pyrometer and the ribbon and because the angle of sight is not constant, these measurements are not very precise and give merely very random tendencies.
Moreover, the emissivity to be regulated on the pyrometer is poorly understood by the operators. In addition, only certain places in the open sections are accessible via the presence of gangways over the width of the lehr, thereby limiting the number of measurement points over the length of the lehr.
Temperature measurement carried out hy scanners
Pyrometric scanners are installed in the roof and are directed over the entire width of the ribbon through a slot in the roof of the closed sections. The position of the measurement is necessarily at the end of a zone because of the obstacle that the heat exchange tubes present in the radiative zones represent.
It is not simple to exploit the measurements from the scanners because of the variation.in emissivity of the ribbon as a function of the measurement angle, because of heat reflections from the roof and because it is difficult to identify the position of the ribbon edge. In principle, it is conceivable to improve the scanners, but their cost remains prohibitive.
The various measurement methods described above could, in the limit of their measurement precision, give
comparable results. In fact, divergent values between the thermocouple measurements and the pyrometric measurements are observed.
An example of this divergence relates to the temperature measured over the length of the ribbon in a lehr.
In the slight cooling zones, the values measured by thermocouples and pyrometers are quite close. This can be explained by the measurement environment, which is almost isothermal. When the cooling increases, a large measurement difference occurs, which may reach 100°C. The thermocouples are heated by the ribbon but also cooled by the cooling devices. In lehrs without pyrometers or scanners, this measurement difference is taken into account by applying a correction factor, in order to correct the temperature delivered by the thermocouples and thus the true temperature of the ribbon is estimated. However, this approach is not very satisfactory because it does not allow the required precision in the temperature measurement to be achieved.
A second example of this divergence relates to the temperature measured over the width of the ribbon in a radiative zone B2 of a lehr. The difference between pyrometers and thermocouples is around 20 °C in this zone. The pyrometers may indicate an increase in the temperature of the ribbon from the left to the right, whereas the thermocouples indicate the opposite!
Measurement of the glass ribbon temperature in the lehr by thermocouples according to the prior art is unsatisfactory, the measurement obtained being disturbed by the radiation toward the walls of the lehr, the coolers and the rollers, and by the convective cooling in the convective zones. This
disturbance persists even if the thermocouple is in gentle mechanical contact with the glass.
The object of the invention is most particularly to solve this problem and to improve the measurement of the surface temperature of the glass ribbon.
The invention consists of a device for continuously measuring the surface temperature of a glass ribbon in a flat glass annealing lehr, characterized in that:
it comprises a subassembly placed on one of the two faces of the glass ribbon, which is flush with the surface of the glass ribbon and creates a thermally insulated space on that side of the surface of the ribbon where the subassembly is located; and
it comprises at least one temperature measurement member placed in the thermally insulated space,
and in that it includes a means for correcting the error in measuring the temperature of the thermally insulated space caused by the loss due to the radiation through the ribbon.
The means for correcting the measurement error may be formed by a thermal calculation means that takes into account the loss due to the radiation through the ribbon in order to correct the error in measuring the temperature of the thermally insulated space.
The absence of a subassembly opposite the measurement subassembly creates a radiative thermal loss because of the semitransparent properties of the glass. This is taken into account, in order to correct the temperature measurement carried out in the insulated space by a thermal calculation so as to obtain the true temperature of the ribbon.
In another arrangement, the means for correcting the measurement error may be formed by a second subassembly
located on the opposite side of the ribbon from the first subassembly, which is flush with the surface of the ribbon and creates an isothermal space. The device for continuously measuring the surface temperature of a glass ribbon in a flat glass lehr is then characterized in that:
it comprises two subassemblies placed respectively on either side of the glass ribbon and facing each other, each subassembly being flush with the surface of the glass ribbon;
the subassemblies create an isothermal space on either side of the glass ribbon by an optical thermal insulator; and
it comprises at least one temperature measurement member placed in at least one of the isothermal spaces and supported by at least one of the subassemblies, the other subassembly constituting the means for correcting the measurement error.
Advantageously, the isothermal space around each member for measuring the temperature of the ribbon is produced so as to limit the conductive, radiative and convective thermal losses. This isothermal space may be produced in the form of a hollow in that face of a sheet of an insulator which is flush with the surface of the glass ribbon.
Preferably, the two subassemblies placed respectively on either side of the glass ribbon are approximately symmetrical with respect to the glass ribbon so as not to generate a temperature difference between the two faces of the ribbon.
Advantageously, the device comprises several temperature measurement members placed at several points along a direction parallel to the width of the glass ribbon so as to determine the temperature profile over the width of the ribbon.
At least one temperature measurement member is placed on each of the faces of the ribbon in such a way that the measurement may be carried out on both faces of the ribbon, at one point or at several points along a direction parallel to the width of the glass ribbon, so as to determine the profile of the temperature difference between the two faces of the ribbon.
Preferably, the distance between the measurement points located along a direction parallel to the width of the ribbon is smaller on the edges of the ribbon than in the central zone so as to have more measurement points on the edges than in the center of the ribbon.
The temperature measurement member may for example be a thermocouple or a thermistor.
The temperature measurement member is placed close to the surface of the glass ribbon, without being in contact therewith. Advantageously, the temperature measurement member is placed at least one centimeter from the surface of the glass ribbon, without being in contact therewith.
The optical thermal insulator, which is flush with the surface of the glass ribbon, may be made of a flexible material having a low friction coefficient. Advantageously, the optical thermal insulator consists of a sheet of mineral wool or glass wool.
The device according to the invention includes mechanical protection members for protecting against damage caused by the glass ribbon breaking. These protection members may be fixed relative to the lehr. They may also be removable, such as the curtains used in lehrs to limit convection.
The device according to the invention also includes members for limiting the air convection between the ribbon and the temperature measurement member.
The invention also relates to a flat glass annealing lehr, characterized in that it is equipped with at least one device for measuring the temperature of the glass ribbon as defined above.
The invention also relates to a method of operating a flat glass annealing lehr, characterized in that measurement of the surface temperature of the glass ribbon is carried out continuously by a device
which comprises a subassembly placed on one of the two faces of the glass ribbon, which is flush with the surface of the glass ribbon and creates a thermally insulated space on that side of the surface of the ribbon in which the subassembly is located;
which ^ comprises at least one temperature measurement member placed in the thermally insulated space; and
which includes a means for correcting the error in measuring the temperature of the thermally insulated space caused by the loss due to the radiation through the ribbon,
this temperature measurement being used to automatically adjust the operating parameters of the lehr via a control loop.
The means for correcting the measurement error may consist of a thermal calculation means taking into account the loss due to the radiation through the ribbon in order to correct the error in measuring the temperature of the thermally insulated space.
According to another arrangement, the means for correcting the measurement error may be formed by a second subassembly located on the opposite side of the ribbon from the first subassembly, which is flush with
the surface of the ribbon and creates an isothermal space. The method of operating a flat glass annealing lehr is then characterized in that the measurement of the surface temperature of a glass ribbon being carried out continuously by a device that comprises two subassemblies placed respectively on either side of the glass ribbon and facing each other, each subassembly of which is flush with the surface of the glass ribbon, with an isothermal space produced around each member for measuring the temperature of the ribbon by an optical thermal insulator, which temperature measurement is used to automatically adjust the operating parameters of the lehr via a control loop.
According to the method of the invention, a combination of the system for controlling the lehr and of the temperature measurement equipment is advantageously provided so as to allow the operating parameters of the lehr to be rapidly adjusted so that the total stress level remains below a predetermined value, preventing the glass from breaking or preventing the ribbon from deforming perpendicular to the plane of the ribbon, and so that the residual stress level remains below a predetermined value enabling the glass to undergo the subsequent treatment.
The temperature measurements may be carried out over the width of the glass ribbon and may be used to adjust the heating distribution over the width of the ribbon and/or to adjust the cooling distribution over the width of the ribbon.
A mathematical model of the operation of the lehr may be established and used to define the optimum setpoints to be applied to the lehr according to the measurements made, so as to obtain the desired temperature and stress level.
The invention consists, apart from the arrangements explained above, of a number of other arrangements which will be explained in greater detail below with regard to exemplary embodiments described with reference to the appended drawings, which are in no way limiting. In these drawings:
figure 1 is a schematic longitudinal vertical section illustrating the principle of the device for continuously measuring the surface temperature of the glass ribbon in a lehr, according to the invention;
figure 2 is a schematic longitudinal vertical section of an exemplary embodiment of the device of figure 1;
figure 3 is a graph of the temperature profile through the thickness of the glass ribbon, the temperature being plotted on the y-axis and the thickness being plotted on the x-axis;
figures 4 and 5 are graphs illustrating the optical thickness plotted on the y-axis as a function of the wavelength plotted on the x-axis;
figure 6 is a graph with two curves, one for the optical thickness plotted on the y-axis with the left-hand scale and the other for the emission of the black body plotted on the y-axis on the right-hand scale as a function of the wavelength plotted on the X-axis,- and finally
figure 7 is a schematic top view of the installation of thermocouples according to the invention.
An example of a measurement device produced according to the invention is described below.
Figures 1 and 2 of the drawings show a thermocouple TC, preferably one which is jacketed and of low diameter, the diameter generally being equal to or less than 2 mm. The measurement point of the thermocouple TC is maintained at a point in the isothermal space and advantageously in the immediate vicinity of the surface
of the glass, while preventing any contact between the glass and the thermocouple TC. The expression "immediate vicinity" is understood to mean that the measurement point of a thermocouple is at a short distance from this ribbon, for example around 2 mm.
The absence of contact between the thermocouple TC and the glass G prevents the thermocouple TC from being heated up by the frictional heat that would result in a positive error in the measured temperature.
In order for the temperature of the ribbon to be correctly measured with this thermocouple TC, it is essential to establish thermal equilibrium between the ribbon G and the thermocouple TC. To prevent thermal losses from the thermocouple TC, an isothermal space 2 is created around the thermocouple using a flexible insulator 3 having a low friction coefficient, which is flush with the surface of the ribbon G. The expression "insulator having a low friction coefficient" is understood to mean an insulator that can touch the running glass without degrading the measurement device or the surface of the glass. The thermocouple TC is thus isolated, preventing heat loss to the outside.
Examples of suitable flexible insulators may include mineral wool and glass wool, which are two simple and inexpensive insulating materials suitable for the device, capable of withstanding temperatures well above those within a lehr. The use of a flexible insulator flush with the surface of the glass also prevents any flow of air between the surface of the ribbon G and the measurement device, thus preventing convective cooling of the temperature measurement member TC. This first part of the device enables the true temperature of the glass to be approximated.
The device produced according to the invention takes into account the semi-transparency of the glass. When
an isothermal space is created close to a black body on one face of the glass, some of the heat from this isothermal space is still lost through the ribbon by radiation within the spectral window in which the glass is transparent. Additional insulation 4 on the opposite face of the ribbon G enables the heat to be retained, by returning the radiation into the space created by the glass and the thermocouple. The thermocouple TC thus reaches a temperature very close to that of the glass.
The device according to the invention is thus characterized in that the isothermal space 2 around the member TC for measuring the temperature of the glass is obtained by limiting the conductive, radiative and convective heat losses.
The additional insulation 4 is advantageously produced in the same way as the insulation 3, using a flexible insulator, for example a sheet of mineral wool or glass wool.
This double insulation on the two sides of the ribbon also has another advantage. It allows a thermocouple to be placed on each of the two faces of the ribbon, namely the thermocouples TC already mentioned and a thermocouple TC2 placed in another isothermal space 5, which is in the form of a hollow in the insulation 4, so as to be in the immediate vicinity of that face of the ribbon on the opposite side from that corresponding to the first thermocouple TC. Thus, information about a possible temperature difference between the two faces caused by an imbalance in cooling is obtained, this being very useful in a lehr for adjusting the degrees of cooling on the top side and the underside.
Herein below is an example of improvement in the measurement made by a device produced according to the invention, installed at the exit of zone C of a lehr

having a production capacity of 600 tonnes/day for a ribbon width of 4 m, a glass thickness of 4 mm and a run speed of 10 m/min.
The theoretical temperature profile through the thickness of the ribbon is assumed to be that indicated in figure 3 . It shows that the temperature of a glass sheet is not homogeneous during cooling, with surfaces being cooler than the core. This is particularly so for a relatively thick glass, for example with a thickness greater than 8 mm. For a thinner glass, the temperature gradients in the glass during cooling remain limited.
Figure 3 shows a difference of 5°C between the center (i.e. at mid-thickness) and the surface of the ribbon for glass 4 mm in thickness and for a given degree of cooling. The error induced by a surface temperature measurement compared with the average temperature in the thickness is less than 2.5°C. For standard thin glass, the surface temperature measurement is sufficiently representative of the average temperature of the ribbon with regard to the intended precision. The situation is different for a thick glass. However, with information about the degree of cooling, the core temperature and the average temperature may be determined. The radiation exchange between the surface of the ribbon and the walls of the lehr, for a ribbon at 380°C having an emissivity of 0.85 and a lehr wall temperature of 170°C, is 7 kw/m'^ on each face. By creating an isothermal space for the temperature measurement with a fibrous insulator having a thermal conductivity of 0.06 W/m.K and a thickness of 50 mm, the thermal losses are limited to 0.24 kW/m^.
The calculation of the system taking into account the radiative exchange between the ribbon and the insulator, assuming an insulator emissivity e = 0.9, the conduction of a 5 mm thick air layer between the ribbon and the insulator and the conduction through the
insulator, gives a temperature of the insulator on the hot face of 375.9°C for a ribbon temperature of 380°C. The temperature of the thermocouple will be between these two values, hence a measurement error of less than 4°C with respect to the temperature of the glass ribbon.
The optical properties of the glass will now be taken into account.
A standard glass is opaque for wavelengths above 2 .7 /xm (i.e. 2.7 X 10''^ mm) and is transparent at lengths below this, whether for a standard thickness or for a thick glass. Figures 4 and 5 show the optical spectrum of the float glass for a thickness of 15 mm and 4 mm respectively.
Figures 4 and 5 show that the glass is transparent up to 2 .7 fjLva, it becoming opaque beyond this wavelength.
A partial device installed on only one face of the ribbon will create an isothermal space on one side of the ribbon that behaves as a black body with the spectral distribution of the corresponding radiation captured therein. However, owing to the optical properties of the glass, some of this radiation will escape through the thickness of the glass.
Figure 6 shows the optical spectrum of the 4 mm thick glass (curve LI) and the optical spectrum of a black body at 380°C (curve L2).
The integral of the curve L2 of the black body between 0 and 2 .7 jim represents a radiative flux of 0.36 kW/m^. It should be noted that for a temperature measured further upstream in the lehr, for example for a ribbon at 600°C, this flux would be even higher because of the offset of the black body curve.
Taking into account this additional loss in the above calculation, gives a temperature of the insulator on the hot face now of 369.6°C, still with a ribbon temperature of 380°C. The temperature difference in the isothermal space now rises to 10.4°C. This error is greater than the precision required for a lehr.
This radiative loss through the ribbon must therefore be eliminated.
To do this, the device produced according to the invention includes the second isothermal space 5 similar to the first one, enabling the losses within the optical window of the glass to be mutually eliminated.
Another method for eliminating the effect of the radiative loss through the ribbon is to correct the temperature measurement error using a thermal calculation means. This requires additional information about the optical properties of the glass and the exchanged radiation flux in a direction of the opposite face of the ribbon.
An example of a measurement device produced according to the invention will now be described more precisely with reference to figures 1 and 2.
The device is produced using the following: two jacketed thermocouples TC, TC2 with a diameter of 1 mm, for measuring the temperature of the two faces of the ribbon G; glass wool or mineral wool as insulator; and a fastening system 6, 7. It should be noted that the temperature in the isothermal space may be measured by thermocouples or also by thermistors or other-temperature measurement members.
The isothermal spaces 2, 5 have been shown only in figure 1, but similar spaces may be provided in the device of figure 2, although these have not been shown.
As shown in figure 2, the glass wool 3 is flush with the ribbon over a length of about 10 cm in the run direction of the ribbon. On each face of the ribbon, the thickness M of the glass wool in the form of a sheet, for example 5 0 mm in thickness, creates an isothermal space and prevents the convection of the air between the ribbon and the glass wool. The thermocouple TC is positioned slightly downstream of the middle of the covering zone, at the point where any ingress of air is reduced. The orientation of the thermocouple TC is preferably horizontal, parallel to the surface of the glass. In this way, any parasitic thermal conduction via the jacket and the wires of the thermocouple TC is eliminated at the measurement point. The jacket of the thermocouple TC is preferably introduced on the upstream side. In this way, any risk of the tip catching on the surface of the glass is avoided.
A description will now be given of the way in which the particular glass annealing process is taken into account in the definition of the measurement device according to the invention.
The design of the device according to the invention is such that it does not disturb the glass annealing process or disturbs it very little. In particular, the device is produced as follows.
The device consists of two subassemblies Dl, D2 placed respectively on either side of the ribbon G, these being sufficiently symmetrical so as not to generate a temperature difference between the two faces of the ribbon. The subassemblies Dl, D2 of the device have a minimum length in the run direction of the ribbon so as
to obtain the desired thermal and optical confinement. This confinement limits the surface cooling of the glass as it passes through the assembly. It will be readily understood that the absence of a subassembly on one face of the ribbon will result in a temperature difference between the two surfaces, the face not provided with a subassembly cooling normally. Therefore a device installed only on one face will result in an appreciable temperature difference between the two faces of the ribbon, hence a risk of creating additional stresses in the glass that are liable to pose breakage problems or problems when cutting the glass ribbon.
According to a first embodiment of the invention, the length of the device in the run direction of the ribbon is limited to that needed to obtain the desired thermal an optical confinement so as to reduce the length over which the cooling of the glass is disturbed. Preferably, the length L of the device is less than 200 mm. The thickness of the insulation is for example 5 0 mm.
The device must cause the least possible disturbance in the lehr. To do this, it is important for its dimensions to be small. Specifically, too large a dimension would result in the convective flow in the lehr being disturbed owing to a restriction in the flow cross section, especially in the convective zones. It is necessary not to disturb the exchange by radiation in the radiative zones between the glass and the various walls of the lehr because of the substantial obstacle to the radiation that too bulky a device would constitute.
To comply with this constraint, the thickness M of the insulator is limited according to the invention. Too small a thickness of the insulator may result in a large temperature gradient between its two faces.
because of greater thermal loss. This loss will be greater the higher the thermal conductivity of the insulator. To maintain high precision of the glass temperature measurement, the device according to the invention prevents this loss or takes it into account in order to correct the glass temperature measured.
According to a second embodiment of the invention, the thermal losses over the thickness of the insulator are eliminated by adding a heater on the face 3a, 4a of the insulator 3, 4, on the opposite side to that of the ribbon, for example, an electrical resistance element, so as to maintain the same temperature on both faces of the insulator 3 or 4.
According to a third embodiment of the invention, the losses are taken into account by calculating thermal losses in the insulator 3, 4 taking into account its thermal conductivity, the temperature measured on the hot face by the measurement member TC, TC2 provided in the device and that measured on the cold face 3a, 4a of the insulator by the addition of at least one temperature measurement member, for example a thermocouple. The temperature of the glass measured by the device is then corrected so as to take into account the influence of the thermal losses in the thickness of the insulator.
According to a fourth embodiment of the invention, the losses are taken into account by calculating the thermal losses in the insulator, taking into account its thermal conductivity on the basis of the temperature measured on the hot face by the measurement member TC, TC2 provided in the device and by estimating that on the cold face 3a, 4a of the insulator on the basis of the ambient temperature of the lehr at the point where the device is installed. The glass temperature measured by the device is then corrected in
order to take into account the influence of the thermal losses in the thickness of the insulator.
These methods, in combination with calculations of the temperatures in the ribbon, enable a precision to within 1°C to be achieved, this being largely sufficient for controlling the cooling in a lehr using suitable measurement members such as calibrated thermocouples.
An example of the device installed in a lehr will now be described with reference to figure 2.
Installation on the under side of the ribbon
The space between two support rollers Rl, R2 for supporting the ribbon G allows a bar 7 with several thermocouples TC2 over the width of the ribbon to be installed. A U-shaped bar 7 is used to hold the fibrous insulation 4 in place and to fix the thermocouples TC2. A means (not shown) for adjusting the height of the bar 7 enables the device to be adjusted close to the glass. The position behind a roller protects the system from glass fragments dropping on it in the event of the ribbon breaking.
Installation on the top side of the ribbon
The upper device is suspended from a bar 6 which laterally crosses the space over the ribbon G. The flexible sheet 3 of an appropriate length covers a short length of the surface of the ribbon. This sheet 3 is made up of a flexible fabric with its underside used for fixing the flexible thermocouples TC. Advantageously, a thin glass fiber fabric is used in which thermocouples TC are integrated. Laid on this fabric is a sheet of a glass fiber insulator with a thickness M of about 50 mm.
The diameter of the thermocouples is small, about 1 mm, so as to ensure that they are flexible. The contact zone 8 of the sheet 3 is adjusted to the area covered by the lower insulator 4. The upper sheet 3 can follow changes in thickness of the ribbon G without any problem, provided that the overlap with the lower insulator remains correct.
Installation of thermocouples over the width of the ribbon
The largest temperature variations over the width of the ribbon are close to the edges, especially if any thickness difference exists between the edges 9, 10 and the central part 11 of the ribbon G.
To be able to plot the temperature profile of the edge, which typically has a width of 150 mm for a thick ribbon, at least three thermocouples TC are required. The distance between two of these successive thermocouples TC will therefore be about 3 cm (30 mm) . Because the position of the edge typically varies by 3 00 mm depending on the width of the ribbon produced, in this example ten thermocouples TC are installed on both sides of the ribbon. The number ten corresponds to 3 00 mm divided by 3 0 mm.
Since the temperature variations in the central part 11 are much less, about six thermocouples TC are sufficient to plot the temperature profile. The total number of thermocouples TC per bar 6, 7 will for example therefore be twenty-six.
Figure 7 shows an example of the thermocouples located over the width of the ribbon.
Exploitation of the temperature measurements
The information delivered by the measurement equipment may be exploited by the operators of the device in order to adjust the lehr operating parameters manually.
According to another embodiment, the temperature measurements may be displayed for the information of the lehr operator, for example in the form of curves showing the temperature profile over the width of the ribbon, so as to enable him to confirm the settings of the heating and cooling distributions operated on the lehr. It is also possible to record these values, especially for monitoring the quality of the product.
Preferably, the information delivered by the measurement equipment is used by a installation control system for automatically adjusting the operating parameters of the lehr, by means of one or more control loops, in particular for adjusting the heating and cooling of the glass along the run direction of the ribbon and in its perpendicular direction.
Advantageously, the control loop may be supplemented with a physical model of the glass annealing which, on the basis of the measurements made in one section of the lehr, enables the settings for the various zones upstream and downstream of the measurement section to be calculated, both for heating and cooling the glass ribbon at each step of the glass annealing process. The physical model may advantageously exploit the temperature measurements delivered by the measurement equipment in order to estimate the stress levels in the glass and to define their distribution over the width of the ribbon, over its thickness or its length.

We Claims :-
1. A device for continuously measuring the surface
temperature of a glass ribbon (G) in a flat glass
annealing lehr, characterized in that:
it comprises a subassembly (Dl) placed on one of the two faces of the glass ribbon, which is flush with the surface of the glass ribbon (G) and creates a thermally insulated space on that side of the surface of the ribbon where the subassembly is located; and
it comprises at least one temperature measurement member (TC) placed in the thermally insulated space,
and in that it includes a means for correcting the error in measuring the temperature of the thermally insulated space caused by the loss due to the radiation through the ribbon.
2. The device as claimed in claim 1, characterized in that the means for correcting the measurement error is formed by a thermal calculation means that takes into account the loss due to the radiation through the ribbon in order to correct the error in measuring the temperature of the thermally insulated space.
3. The device as claimed in claim 1, characterized in that:
it comprises two subassemblies (Dl, D2) placed respectively on either side of the glass ribbon (G) and facing each other;
each subassembly (Dl, D2) is flush with the surface of the glass ribbon;
an isothermal space (2, 5) is produced around each member (TC, TC2) for measuring the temperature of the ribbon by an optical thermal insulator (3, 4); and
it comprises at least one temperature measurement member (TC, TC2) placed in at least one of the isothermal spaces (2, 5) and supported by at least

one of the subassemblies, the other subassembly constituting the means for correcting the measurement error.
4. The device as claimed in one of claims 1 to 3, characterized in that the isothermal space (2, 5) around each member (TC TC2) for measuring the temperature of the ribbon is produced so as to limit the conductive, radiative and convective thermal losses.
5. The device as claimed in claim 4, characterized in that the isothermal space (2, 5) around each temperature measurement member (TC, TC2) is produced in the form of a hollow in that face of a sheet of an insulator which is flush with the surface of the glass ribbon.
6. The device as claimed in claim 3, characterized in that the two subassemblies (Dl, D2) placed respectively on either side of the glass ribbon (G) are approximately symmetrical with respect to the glass ribbon so as not to generate a temperature difference between the two faces of the ribbon.
7. The device as claimed in any one of claims 1 to 6, characterized in that it comprises several temperature measurement members (TC) placed at several points along a direction parallel to the width of the glass ribbon so as to determine the temperature profile over the width of the ribbon.
8. The device as claimed in claim 3, characterized in that it comprises at least one temperature measurement member (TC, TC2) placed on each of the faces of the ribbon in such a way that the measurement may be carried out on both faces of the ribbon, at one point or at several points along a direction parallel to the width of the glass ribbon, so as to determine the
profile of the temperature difference between the two faces of the ribbon.
9. The device as claimed in claim 7, characterized in that the distance between the measurement points located along a direction parallel to the width of the ribbon is smaller on the edges (9, 10) of the ribbon than in the central zone (11) so as to have more measurement points on the edges (9, 10) than in the center of the ribbon.
10. The device as claimed in any one of the preceding claims, characterized in that the temperature measurement member is a thermocouple (TC, TC2).
11. The device as claimed in any one of claims 1 to 9, characterized in that the temperature measurement member is a thermistor.
12 . The device as claimed in either of claims 10 and 11, characterized in that the temperature measurement member is placed close to the surface of the glass ribbon, without being in contact therewith.
13. The device as claimed in claim 12, characterized in that the temperature measurement member is placed at least one centimeter from the surface of the glass ribbon, without being in contact therewith.
14. The device as claimed in any one of the preceding claims, characterized in that the optical thermal insulator, which is flush with the surface of the glass ribbon, is made of a flexible material having a low friction co-efficient.
15. The device as claimed in claim 14, characterized in that the optical thermal insulator consists of a sheet of mineral wool or glass wool.
16. A flat glass annealing lehr, characterized in that it is equipped with at least one device for measuring the temperature of the glass ribbon as claimed in any one of the preceding claims.
17. A method of operating a flat glass annealing lehr, characterized in that measurement of the surface temperature of the glass ribbon (G) is carried out continuously by a device
which comprises a subassembly (Dl) placed on one of the two faces of the glass ribbon, which is flush with the surface of the glass ribbon and creates a thermally insulated space on that side of the surface of the ribbon in which the subassembly is located;
which comprises at least one temperature measurement member (TC) placed in the thermally insulated space; and
which includes a means for correcting the error in measuring the temperature of the thermally insulated space caused by the loss due to the radiation through the ribbon,
the continuous temperature measurement being used to automatically adjust the operating parameters of the lehr via a control loop.
18. The method as claimed in claim 17, characterized in that the means for correcting the measurement error consists of a thermal calculation means taking into account the loss due to the radiation through the ribbon in order to correct the error in measuring the temperature of the thermally insulated space.
19. The method as claimed in claim 17, characterized in that the means for correcting the measurement error is formed by a second subassembly (D2) located on the opposite side of the ribbon from the first subassembly, which is flush with the surface of the ribbon and creates an isothermal space, the measurement of the surface temperature of the glass ribbon being carried
out continuously by a device that comprises two subassemblies (Dl, D2) placed respectively on either side of the glass ribbon (G) and facing each other, each subassembly (Dl, D2) of which is flush with the surface of the glass ribbon, with an isothermal space (2, 5) produced around each member (TC, TC2) for measuring the temperature of the ribbon by an optical thermal insulator (3, 4), which temperature measurement is used to automatically adjust the operating parameters of the lehr via a control loop.
20. The method as claimed in one of claims 17 to 19, characterized in that a combination of the system for controlling the lehr and of the temperature measurement equipment is provided so as to allow the operating parameters of the lehr to be rapidly adjusted so that the total stress level remains below a predetermined value, preventing the glass from breaking or preventing the_ ribbon from deforming perpendicular to the plane of the ribbon, and so that the residual stress level remains below a predetermined value enabling the glass to undergo the subsequent treatment.
21. The method as claimed in one of claims 17 to 20, characterized in that the temperature measurements are carried out over the width of the glass ribbon and are used for adjusting the heating distribution over the width of the ribbon and/or for adjusting the cooling distribution over the width of the ribbon.
22. The method as claimed in one of claims 17 to 21, characterized in that a mathematical model of the operation of the lehr is established and used to define the optimum setpoints to be applied to the lehr according to the measurements made, so as to obtain the desired temperature and stress level.