A METHOD FOR ADJUSTING AND DIRECTING HEAT EFFECTS IN A GLASS TEMPERING OVEN AND AN OVEN

Abstract: A method to adjust and focus heat effects inside a glass tempering furnace on different structural elements and an air space and parts of the air space, in which method the glass (1) to be tempered travels on a conveyor of horizontal rollers (4) through the furnace and the glass sheet is heated by means of heating elements (2, 3) placed on the upper and the lower side of the glass sheet. By intern convection started in the furnace lower part in the final stage of the heating cycle the heat effect from the furnace lower part heat elements (3) on rollers (4), carrying the glass sheet, is deducted, circulation of air in the lower part is stopped when a new cold replacement glass sheet has reached a temperature of approximately 100 DEG C and by means of adjustable blasters of circulation air (8) in the furnace upper part the hot air motion and the heat effect on glass sheet (1) is adjusted and intensified in the final stage of the heating cycle when the glass temperature exceeds 400 DEG C.

Description: A METHOD FOR ADJUSTING AND DIRECTING HEAT EFFECTS IN A GLASS TEMPERING OVEN AND AN OVEN The invention relates to a method for heating glass in a tempering furnace and to a tempering furnace, where a glass sheet is conveyed through the furnace on horizontal rollers, the glass sheet heated in the furnace by means of heating elements on both sides of the glass sheet and the glass sheet moved out from the furnace at tempering temperature. Previously known from publication print FI-62043 is a glass tempering method and furnace as per the introduction chapter of patent claim 1. By this method the aim is to intensify heat transference onto the glass sheet upper surface by blasting air on the glass sheet upper side in the early stage of the heating cycle in order to make the upper surface heating to correspond to the effect of heat conducted from the carrying rollers to the glass sheet lower surface. One wants to keep the effect of heat transfer equal on both sides of the glass sheet in order to avoid the glass arching phenomenon. To the top of the glass sheet preheated air is blasted from the furnace outside. By means of the above presented method only the glass bending problem is solved, when the glass sheet enters the furnace and during the first minute of heating, whereupon the blast brought from the furnace outside must be stopped due to harmful effects. Then, among others, heating of the glass upper surface would start to fall behind from that of the lower glass surface. Prior known from the Finnish publication print 80872 is a glass heating and bending furnace, where from the furnace outside air is taken into the furnace and blasted out over perforated tubes. Blasting is possible on both sides of the glass sheet. At this stage the arching of glass is not an avoidable issue, since one wants the glass sheet to bend. The blasting can be adjusted, however, moving the tubes locally. Yet, in the solution as per FI-80872 the carrying rollers are not cooled by circulation air and, as to timing, no cooling takes place in the final or in the early stage of the heating cycle in order to avoid the glass sheet from arching. No circulation air is blasted to the glass top side but air is brought in from the outside. The method and tempering furnace as per this invention are characterized in what is disclosed in the enclosed patent claims. By means of the above presented method and tempering furnace the glass arching problem is solved when a glass sheet enters the furnace, becomes heated in the early stage of the cycle, and the thermal effect of the hot rollers on the cold glass sheet brought into the furnace is lessened. With blasters above the glass sheet hot air in the furnace upper part is circulated in the final stage of the heating cycle, whereby hot air, even very close to the furnace ceiling, can be put into motion and to convect heat to the glass sheet. Forced heat transfer in the final stage of the heating cycle in the furnace upper part equalizes differences of temperature and improves heat transfer from the heating elements, whereby their surface temperature drops, stress becomes smaller and working life grows. Arranging several blasters of circulation air in the furnace upper part makes it possible to heat many glass sheets, even of different size, at a time, when with the adjustable blasters convection heat is focused on necessary spots. In the furnace lower part the circulation air blasting can be general intern convection of the furnace or concentrated by means of channels especially on the carrying rollers. In both cases, when blasting is started in the final stage of the heating cycle ab. 20 - 40 seconds before moving the glass sheet, the rollers are cooled by blasting. The temperature of the rollers is, due to the radiation heat from the heating elements under them, higher than the air temperature, whereby the temperature of the rollers can be reduced also only by blasting circulation air. In another embodiment even the heating effect of the lower part heating elements is reduced during blasting of circulation air arranged in the in the lower part, whereby cooling of the rollers becomes still more effective. In the following the invention is disclosed with reference to the enclosed drawing, where Fig. 1 shows a cross-section of the tempering furnace. Fig. 2 shows a cross-section of another furnace embodiment. Figure 1 is a one-chamber glass tempering furnace, where glass sheet 1 is heated into tempering temperature. The glass sheet is conveyed to and out of the furnace along a track formed of the cooling rollers 4. By means of rollers 4 also oscillation i.e. the continuous to-and-fro linear motion of the glass sheet takes place . In the upper part of furnace there are heating elements 2 arranged as several fields and, correspondingly, in the lower part elements 3. In the lower part there is a blaster 5, channelling 6 and nozzles 7 located at rollers 4. In the final stage of the glass heating cycle, ab. 20 - 40 seconds before moving glass sheet 1 over to cooling, blaster 5 is switched on for cooling the rollers, which due to their better radiation absorbtion are hotter than glass 1 and also because the rollers are closer to the lower surface radiation sources 3 than the glass sheet. Blasting does not, however, cool glass sheet 1 but may even heat it more, while, in addition to conduction, the rollers transfer heat even by air to the glass sheet. Blasting continues uninterruptedly when the glass sheet is being moved out and a new glass sheet enters as replacement and starts to warm up. On the the glass sheet upper side there are several blasters 8 of circulation air individually adjustable to switch on and also with adjustable speed of rotation either as a group or separately. The swithch-on of the blasters is timed to take place in the final stage of the heating cycle, when the glass sheet has reached a temperature of at least 400°C. It is advantageous to use forced convection in the final stage of the cycle by circulating hot air from the upper part of the furnace down onto one or several glass sheets on the rollers. In the upper part heating elements 2 are at a distance from one another allowing air circulation between them. With the disposition and adjustment of blasters 8 the circumstances in the furnace section can be made different by adjustment, whereat it is possible to heat in the furnace to tempering temperature different kinds of glass sheets side by side or one after another at the same time. Adjustment of the blasters is done by means of a control unit into which the operation times and speeds of rotation of each single unit, for instance, are stored separately. In figure 2 blasters 9 are axial blasters located on both sides of the furnace viewed from the glass sheet 1 direction of travel. No special channelling is arranged. Free recirculation of air, and the temperature between rollers and air becomes equalized. Heating is continued after exit of a glass sheet and enter of a new cold glass sheet in stead as shown in the example in figure 1. Due to blasting the temperature of rollers 4 drops during glass sheet replacement 20-50°C more than without blasting. This is of great importance already by avoidance of glass sheet arching. When blasting is focused by nozzles on the rollers (figure 1) cooling of the rollers is made ever more effective. When the heating elements 3 contain mass, their effect can be lessened or they can also be switched of totally 20-40 seconds before the heating cycle comes to its end. Air circulation in the lower part reduces the surface temperature of the elements, their radiation effect on the rollers stops but still there is hardly any change in the temperature of the circulation air, since the elements release heat into the air due do their mass and because they are at the moment the effect is switched off hundreds of grades hotter than the air. Thus hot air heats the glass sheet from its under side more than in prior known solutions and compensates the phenomenon that in the present solution the conduction heat from the rollers to the glass sheet becomes smaller. When a cold glass sheet enters the furnace, the upper part blasters are switched off and the lower part air circulation switched on as well as heating elements 2. When the glass sheet is warmed up to ab. 100 °C, the lower part heating elements are switched on, and approximately by then also blasting of circulation air in the lower part is switched off. From the lower part heating elements heat is transferred slightly better to the glass sheet than from the upper part elements and so the lower part elements can be switched off for some time during the process, and likewise is intensification of upper part heat convection by the blasters advantageous just in the final stage of the heating cycle in order to heat the glass sheet almost by a similar effect from its both sides. The sheet lower surface warms up better than the upper surface, since natural convection from the lower elements on the glass sheet is greater than from the upper elements on the glass sheet, if there is any of it directly from them at all. Transfer of radiation heat is, in its turn, of same rate on both glass sheet sides both from elements 2 and 3 per unit area.

Claims: PATENT CLAIMS 1. A method to adjust and focus heat effects inside a glass tempering furnace on different structural elements and an air space and parts of the air space, in which method the glass (1) to be tempered travels on a conveyor of horizontal rollers (4) through the furnace and the glass sheet is heated by means of heating elements (2) (3) placed on the upper and lower side of the glass sheet, characterized in that - by intern convection started in the furnace lower part in the final stage of the heating cycle the heat effect from the furnace lower part heat elements (3) on the glass sheet carrying rollers (4) is deducted, - recirculation of air in the lower part is stopped when a new cold replacement glass sheet has reached a temperature of appr. 100°C, - by means of adjustable blasters of circulation air (8) in the furnace upper part the hot air motion and the heat effect on glass sheet (1) is adjusted and intensified in the final stage of the heating cycle when the glass temperature exceeds 400°C. 2. A method according to patent claim 1 characterized in that in the final stage of the heating cycle the effect of heating elements 3 under glass sheet (1) is reduced or they are switched off totally. 3. A method according to patent claim 2 characterized in that by means of the lower part air circulation the reduction of heat effect from the carrying rollers to the glass sheet is compensated in the final stage of the heating cycle or when the glass is taken out. 4. A method according to any of patent claims 1 - 3 characterized in that the lower part heating elements ( 3 ) are switched on, at the latest, when glass sheet(l) moved over into the furnace reaches the temperature of 100°C. 5. A method according to any of patent claims 1 -4 characterized in that blasting blasting the lower part of circulation air is focused on rollers (4)by means of channelling (6) and nozzles (7) . 6. A method according to any of patent claims 1 -5 characterized in that by blasters (8) of circulation air in the upper part, air from the upper part is circulated onto the glass sheet (1) upper surface in one or several divided parts of the furnace or the furnace in whole. 7. A glass tempering furnace for realization of the method seet forth in patent claim 1 including a conveying track of rollers (4) and heating elements (2) ,(3) above and under the glass sheet in the furnace, characterized in that there are in the furnace upper part adjustable blasters (8) of circulation air, for circulation of the furnace upper part air in the final stage of the heating cycle, and in the lower part at least one blaster (5), (9) of circulation air, for circulation of the lower part air and that the furnace includes control devices to control the operation of blasters (8) and (5), (9). 8. A glass tempering furnace according to patent claim 7 characterized in that at least the upper part heating elements (2) are placed at a distance from one another in order to arrange air circulation also through the field of heating elements. 9. A method according to patent claim 8 characterized in that that there is in the lower part one or several blasters delivering circulation air to rollers (4). 10. A method according to patent claim 9 characterized in that that there is in the furnace lower part a channelling (6) for circulation air and nozzles (7) focused on rollers (4) for steering circulation air to the rollers.

Inventors: LINDROOS MARKO, LINDROOS, MARKO
Applicants: GLASSROBOTS OY, LINDROOS MARKO, LINDROOS, MARKO
Application year: 1996
Main classification: C03B27
IPC codes: C03B29