Heating techniques for glass sheets

The invention relates to a method and a device for controlling the heating of a glass sheet heated in a horizontal furnace for heat treatment and / or bending.

It is well known to use for heating glass sheets of horizontal furnaces, said tunnels, through which the sheets conveyed on a roller bed. In these furnaces the glass sheet is heated gradually and present at the furnace exit an upper example by temperature to the glass softening point so that the sheet is then sufficiently plastic to be bent and / or tempered.

Such horizontal furnaces have a silk and a roof refractories and the heat input is obtained by means of electric resistances, mounted symmetrically on either side of the roller conveyor. These resistors are located in different longitudinal zones, whose number varies with the length of the oven, longitudinal areas themselves divided into horizontal bands of equal width and length. For information, a horizontal furnace with a length of 12 meters and width of 1.20 meters and can be divided into five longitudinal zones of 2.40 meters in length, each zone is itself divided into 3 zones 0.40 meter wide. Such dimensions are suitable for example for a heating furnace of automotive glass parts such as the side.

Although in the example cited by way of illustration, it has 15 resistance areas that can provide 15 different heating powers, we see that occurs in uneven heating of the glass sheets. As a corollary, it is very difficult to control this differential heating so warm for example strongest areas of glass undergoing the greatest deformation.

This non-uniformity of heating the glass or rather inability to regulate this is due to different reasons. First, it is explained by the influence of refractories used for the construction of the floor and roof of the furnace refractory which re-emit the heat received to the point that in some installations, barely 45% of the heat received by glass comes directly electrical resistances first heat source. This radiation refractory occurs in all directions of space and results in a leveling of heat emissions which counteracts the desired effect of differentiation heating zones. Moreover, certain areas of glazing is systematically less heated than others; indeed between zones resistors there are areas that do not contain resistors which determine the cross-sectional areas and longitudinal regions which do not contribute to the direct heating of the glass. As transverse regions are all of the same width, there is a continuous band parallel to the direction of glass to the scrolling, which does not contribute directly to the glass heating. These shadows, creating an uncontrollable differential heating which can be found in the following transactions with such lower quenching zone.

Finally, if by a very thorough control various phenomena we come to balance that, as we have indicated, have the opposite effect, the oven still remains unsatisfactory for certain types of windows. Let indeed the oven previously given example and imagine treating square panes of 50 cm. To be received correctly by the processing units placed downstream of the furnace, the glass must be centered which means systematically edge circulate in areas of shadow and can not be overheated. In summary, to mitigate some adverse effects due to the difficulty of maintaining the heating, can be played on the position of the glass sheet, but it creates problems for receiving said glass sheets from the oven.

It is also known from US-A 3,744,985 a furnace for forming the glass sheet by pressing in a vertical position. This furnace comprises a series of heating zones distributed along the oven length, each heating zone being itself divided into three vertical sub-areas of the same width, the temperature of said zones and sub-zones being regulated independently. In this type of furnace, the boundaries of subfields are aligned so that there is a continuous cold strip whose presence was difficult to detect due to the systematic use of dense refractory blocks whose re-emitted radiation is supposed to completely homogenize temperature in the furnace. Furthermore, this document relates to an oven said vertical which poses different problems of positioning of the glass sheets.

The present invention provides a horizontal furnace for heating sheets of glass such that it allows a precise control of the heating of the glass sheet.

According to the invention, this control of the glass heating is achieved firstly through a special distribution of electrical resistors placed in the oven. These resistors are divided into a lower layer, in the path of the glass sheets and an upper web. Heating method according to the invention is such that, for each ply, the heating resistors are divided into longitudinal zones and transverse sub-zones of different widths according to the longitudinal zones, the temperature of each longitudinal zone and more precisely of each transverse sub-zone being regulated independently of the temperature of the other zones, two adjacent transverse sub-zones belonging to different longitudinal zones being of different widths.

Such a distribution of heating elements allows precise control of the amount of heat supplied to the glass sheet and optionally a rapid modification thereof.

Indeed, the fact that the transverse sub-zones are different sizes from one another, there is more continuous gray areas parallel to the direction of travel of the glass sheets. Moreover, by modulating the temperature of each transverse sub-region of each longitudinal zone is obtained, with only a small number of transverse subzones, for example three by longitudinal zone, oven superior ovens to 5 or 7 same transverse sub-zones of equal size, as regards the problem of regulating the heating.

Further with a furnace according to the invention, the heating parameters can be easily modified, asymmetrically. Thus, if it is determined at the time of the quenching operation and / or bending one side of the glazing is not heated optimally, it is possible to remedy this by modifying the more often that the temperature a single transverse subarea.

The invention also relates to a heating furnace of the horizontal glass for carrying out the method according to the invention. The furnace, through which a conveyor of the type roller bed includes a roof and a floor supporting respectively a top layer and a lower layer of electrical resistors, divided into longitudinal zones and transverse sub-zones, each having different widths and with each means for regulating their temperature.

The sole and the arch are in a particularly preferred isolated by fibrous refractory low density that control the heating of each zone, and even each sub-area very precisely. An oven made of low density fibrous refractory has indeed a low thermal inertia that can change very quickly the heating conditions of the glass.

More preferably, the transportation of the glass sheets through the horizontal heating furnace is performed on hollow tubular elements in vitreous silica, covered ducting son woven silica. Such elements advantageously contribute to a better control of the heating of the glass sheets.

Other features and advantages of the invention appear in the following description with reference to the attached single board which is a typical pattern of distribution of heating elements in a furnace according to the invention. The characteristics of this oven are given only for illustration and should not be understood as limiting the invention.

The invention applies to furnaces for glass sheets of warming, the tunnel type furnace, wherein the glass sheets are loaded in horizontal position and are transported through the furnace by a horizontal conveyor generally constituted by a roller bed .

Such furnaces are particularly used for heating the glass sheets initially at room temperature to their softening temperature for the bulge and / or soaking. These thermal treatments are typically those most responsible for final defects and in particular optical defects. To minimize, it is particularly important to control furnace outlet temperature of the glass so as to have a temperature profile of the glass sheet very precise.

In many cases, if desired, a uniform tempering treatment for example, it is necessary to have a surface temperature of perfectly uniform glass. In other cases, if an area of ​​the glazing must undergo a particularly large deformation such as a bend, or if it must be at the level of quenching of the rest of the glazing, it is first necessary to have heated zones differently so well locate deformations.

This precision of the temperature profile of the glass sheet is obtained according to the invention by the combination of features developed below.

With such an oven, a relatively fine regulation of the heating zones is permitted. For that the furnace is divided by example 5 in longitudinal zones 1, 2, 3, 4, 5 of equal lengths and fed independently of each other. With oven 12 meters long in which the glass moves from zone 1 to zone 5, it will be successively an area 1 with a power of 130 kilowatts heating, zones 2 and 3 115 kilowatts areas 4 and 5 70 kilowatts.

Each of these longitudinal zones is divided into three transverse sub-zones of varying widths, ranging for example from 200 mm for cqntrales areas 1-2 and 5-2 at 540 mm to the area 3-2. For controlling the temperature of each of these subareas, the power supplied to each longitudinal zone is divided in the transverse subzones and is regulated by a PID controller which distributes power in each corresponding sub-zone through d a power distribution percentage potentiometer.

To order this regulation, preferably a microprocessor is used looping from temperatures sensed by thermocouples placed on the floor and on the roof of the furnace, the center of each longitudinal zone.

With a distribution of resistances as shown in the figure, that is placed such that the dashed lines separating the different transverse sub-zones constitute approximately diamond-shaped and providing relatively higher power powers in transverse fields marked with a cross on their diagonal in areas left blank in the figure, we get very good control of the glass heating. If the glazing dimensions correspond to those of the glazing shown schematically in dotted outline, is for example of the order of 0.4 meters long, preferably be chosen to maintain the zones resistors, 1.1, 1.3, 5.1 and 5.3 in a relatively low temperature, the heating obtained by means of the central zones resistor (1.2, 2.2, 3.2, 4.2 and 5.2) being sufficient. The operation of the furnace according to the invention is therefore particularly economical and all combinations of temperature zones of different resistors being possible it is avoided that portions of vi tration are insufficiently - or on the contrary - too strongly heated.

Such a furnace is especially useful if parts of different sizes must be subsequently processed by the same installation. Other distributions of the resistor areas can also be used, especially if the furnace is loaded with several pieces advancing front.

Successive changes of the heating parameters are shorter if, in a preferred manner, the oven is not built with refractory bricks of high density, but has a metallic structure with insulated inner walls by fibrous refractories. Such a furnace has a very low thermal inertia, and therefore its heating, and its heatsink, is very fast. Thus for example a tunnel oven 12 meters long and 1.20 meters wide for heat glass sheets to a temperature of 600-700_°C is cooled in less than two hours allowing rapid interventions for repairs and is reheated in about a half hour, whereas with dense refractory materials such operations necessarily lead to the interruption of production for at least a day.

Heating is provided for example by resistors wound around refractory tubes preferably alumino-silicate. Such assembly has the advantage of being light and therefore easily supportable by the metal frame of the oven. In addition, defective components can thus be replaced quickly and easily without waiting for the cooling of the furnace, due to good thermal shock resistance of such refractory tubes.

Transporting glass sheets through the tunnel kiln is effected by means of a roller conveyor, consisting of a row of tubes arranged in parallel and perpendicularly to the advancing direction of glass whose ends are glued pinions them -Same driven by chains. These tubes are preferably hollow tubes fused silica, minimizing the thermal exchanges between the glass and the roller bed, and again helps to improve the heat-Control & relative to the solid roller kilns.

Preferably the tubes of the conveyor are sheathed with a fabric of silica thread which a shock-absorbing and makes it softer contact between the glass and the roller, and on the other hand absorbs impurities such as any dust contained in the oven. In addition, this minimizes refractory fabric itself as the glass-roll heat exchange.

In a more particularly preferred manner, the glass sheets are loaded transversely, as indicated by the solid board that is to say they are arranged with their greatest length perpendicular to the traveling direction. Such an arrangement has the double advantage of minimizing optical defects, ie those found in the field of vision of the driver when the panes are mounted in an automobile and more equal rate, the furnace may have a lower length than that required in the case of a longitudinal load of glass sheets.

Thus a furnace according to the invention is a compact oven, easy to maintain and in which all elements contribute to good control of the glass heating temperature.