Method and apparatus for heating glass panels

Abstract

Glass panels are heated in a heating oven while supported on rolls. The glass panels are heated form above and below with convection air or with a combination of convection air and radiation heat. The convection air is heated by electric resistance elements and/or a combustible gas. The convection air passes through heat exchangers disposed in the oven en route to the glass panels.

Description

BACKGROUND

The present invention relates to a method for heating glass panels in a heating oven, wherein the glass panel is supported on top of rolls and said glass panel is heated from above and below with convection air or with a combination of convection air and radiation heat, said convection air being heated by electric resistance elements and/or a combustible gas.

In addition, the present invention relates to an apparatus for heating glass panels in a heating oven, comprising rolls for supporting a glass panel convection blast means or a combination of convection blast means and thermal radiators capable of heating the glass panel, and electric resistance elements or a gas burner for heating convection air.

This type of method and apparatus for heating a glass panel or sheet are prior known for example from the Applicant's patent application FI-20011923. In that document, disposed above and below a glass panel within a heating compartment are radiation heaters and convection air pipes, by which the convection air is supplied from outside the oven into the heating compartment and blasted to the surface of a glass sheet by way of nozzles included in the convection air pipes.

The Applicant's patent application EP 721922 discloses another prior known glass sheet heating method, based on convection blasting. The convection air is circulated onto the surface of a glass sheet through a fan and an electric resistance element fitted in the nozzle box. An oven applying a similar principle is known from Patent publication EP 910553. This comprises radiation panels heated by electrical resistance elements, the heat delivered thereby to a glass sheet providing a versatile oven configuration, regarding especially the development of a temperature profile. A principal function of the panels is the equalization of temperature differences caused by blasting at the surface of a glass sheet.

In the process of heating glass from room temperature to a tempering temperature of about 600-640° C., the temperature rise is consistent with a graph 100 shown in FIG. 1. Temperature rises as a function of time quickly at first and the rise becomes consistently slower, reaching its final tempering temperature little by little. FIG. 2 illustrates a graph 101 representing the rate of heat flow proceeding to a glass panel over the respective period. FIG. 3 illustrates the power (graph 102) of a prior art heating source, i.e., convection-air heating electric resistance elements, as a function of time, said power correlating with a heat flow captured by glass. In convection blast systems as described in the cited references, the electric resistance elements will have to be rated for power outputs according to the maximum heat flow (FIG. 2) captured by a glass panel.

It is an object of the present invention to provide a method and an apparatus, enabling a glass panel to be heated more efficiently than in prior known solutions and/or convection-air heating thermal sources to be rated for a power lower than before.

SUMMARY OF THE INVENTION

In order to achieve the above objective, a method of the invention is characterized in that the heating of convection air is effected by using a heat accumulator.

Furthermore, an apparatus applying the inventive method is characterized in that the heating oven is provided with a heat accumulator, which is capable of heating convection air.

This solution makes it possible that, in the beginning of a heating cycle, some of the heating effect captured by a glass panel can be claimed from the heat accumulator which had been heated during the treatment of a previous panel. A notable advantage of this is that heating sources can be rated for a power lower than what is feasible without a heat accumulator. A further advantage is gained by placing the heat accumulator in direct communication with heating sources which heat the convection air (e.g., electric resistance elements or gas burners).

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

FIG. 1 shows, in general, the temperature of a glass panel as a function of time in the prior art,

FIG. 2 shows, in general, the rate of a heat flow proceeding to a glass panel, as a function of time in the prior art,

FIG. 3 shows a prior art delivery of for power from heaters, as a function of time,

FIG. 4 shows the average heating effect as a function of time according to the invention, and

FIG. 5 shows one embodiment of an apparatus applying the inventive method.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

One exemplary embodiment for an apparatus applying the inventive method is shown in FIG. 5. The apparatus is a heating oven 1, inside whose walls 2 is provided a compartment 2a to be heated. A glass panel 4 is brought into the compartment 2a to be heated on a roll conveyor 3 constituting a substantially horizontal conveying track. The compartment 2a to be heated is provided with upper blast means 5a, 5b, 11 and lower blast means 6a, 6b, 12 for convection air. These include preferably ducts 5a, 5b and 6a, 6b, the horizontal duct sections or boxes 5b and 6b thereof being provided with nozzles for blasting air A to top and bottom surfaces of the glass panel 4. The blasting power of air A can be adjusted for example by means of fans 11 and 12 disposed in communication with the duct sections 5b and 6b. The oven 1 can be further provided with conventional radiation heaters (not shown), capable of heating a glass panel directly. The radiation heaters are mounted preferably above and below the glass panel 4, for example alongside the blast means.

In communication with duct sections 5a and 6a, on a suction side of the fans 11 and 12, are disposed heat accumulators 9 and 10 according to the present invention. Each heat accumulator comprises preferably a generally solid, but porous body, manufactured preferably of a heat accumulating material, such as metal, ceramics, silicon carbide or stone. The accumulators 9 and 10 define their own internal passages or flow paths, whereby the convection air A is adapted to proceed through the heat accumulators 9 and 10. The hot air A, blasted onto the glass panel's 4 surface, is circulated within the compartment 2a. Accordingly, the air A, blasted onto the glass panel 4, is guided (sucked) primarily from the glass panel 4 back to the heat accumulators 9 and 10. The accumulators 9 and 10 deliver heat, thus heating the air A passing through the accumulators.

Preferably, the means for heating the convection air (electric resistance elements and/or gas burners) are placed in direct communication with the accumulators. Thus, as shown, disposed in direct communication with the heat accumulators 9 and 10 are disposed respective electrical resistance elements 7 and 8 used for heating the heat accumulators 9 and 10. The electrical resistance elements can be replaced or supplemented for example with gas burners, the heat of which is generated by a combustible gas. Unlike the prior art, a primary function of the electrical resistance elements 7 and 8 is heating the heat accumulators 9 and 10, whereby the electrical resistance elements can be rated for top power outputs which are lower than the heating effect needed at the early stage of heating the glass panel 4. Furthermore, the electrical resistance elements can be optimized for such a power that the power output delivered thereby is substantially unchanged throughout the heating cycle. This unchanged power output, i.e., the average heating effect, is represented by a graph 103 shown in FIG. 4. In practice, of course, the electrical resistance elements 7 and 8 may have a power output hovering anywhere between the graphs 102 and 103, yet preferably closer to the graph 103. The achievable proximity to the graph 103 depends on the solidity of the heat accumulators 9 and 10 and the efficiency of heat transfer (heat transfer area) between the heat accumulators 9 and 10 and the convection air A. An example will now be described regarding such operation of me inventive apparatus.

At the initial stage of heating, a cold glass panel 4 is heated by means of the heat accumulators 9 and 10 or by a combined action of the heat accumulators 9 and 10 and the electrical resistance elements 7 and 8. Heat is delivered thereby to convection air A to be recirculated with a power which substantially matches the graph 101 of FIG. 2. The electrical resistance elements 7 and 8 strive to heat the heat accumulators 9 and 10 simultaneously with a given substantially unchanging power.

At the initial stage, the electrical resistance elements 7 and 8 need not provide a power sufficiently high to maintain the initial temperature of the heat accumulators 9 and 10 which have been heated during the treatment of a previous glass panel. When the heat flow proceeding to the glass panel 4 begins to decline over the final stage of heating, as depicted in FIG. 2, the power of the electrical resistance elements 7 and 8 reaches a limit at which some of the power delivered thereby is sufficient for heating the heat accumulators 9 and 10 and some of the power delivered thereby Is sufficient for generating a heat flow required by the glass panel 4 at the final stage of heating (in other words, for heating the convection air passing through the heat accumulator). This is in part enabled by recirculation of the convection air, the heating of which, especially at the final stage, only requires a small amount of power generated by the electrical resistance elements. This way, the heat accumulators can be heated to their initial temperature while completing the heating of a glass panel.

Furthermore, the electrical resistance elements 7 and 8 need not be adjusted during a heating cycle with respect to their power outputs or the adjustment demand is essentially lesser than in prior art solutions. In addition, the electrical resistance elements need not be rated to match a peak power output, which is needed for generating a high-power heat flow for the initial stage of heating the glass panel 4.

Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, modifications, substitutions, and deletions not specifically described may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for heating glass panels supported on rolls in a heating oven, comprising the steps of: A) directing convection air to the glass panels from above and below the glass panels; B) heating the convection air by a heat source; and C) passing the convection air in heat exchange relationship with a heat accumulator disposed in the oven.

2. The method according to claim 1 wherein step B is performed by a heat source comprising an electric resistance element.

3. The method according to claim 1 wherein step B is performed by a heat source comprising a gas burner.

4. The method according to claim 1 wherein the heat accumulator is directly heated by the heat source.

5. The method according to claim 4 wherein the heat source comprises an electric resistance element.

6. The method according to claim 4 wherein the heat source comprises a gas burner.

7. The method according to claim 1 wherein convection air directed to the glass panels from above, flows through a first heat exchanger, and convection air directed to the glass panels from below flows through a separate heat exchanger.

8. The method according to claim 1 wherein during an initial stage of heating of a glass panel the convection air is at least partially heated by residual heat from the heat accumulator, wherein the heat source operates at a maximum power rate less than heating power required during the initial stage of heating the glass panel.

9. The method according to claim 1 wherein the power source operates at a substantially constant power.

10. The method according to claim 1 wherein step A further includes heating the glass panels by radiant heat.

11. The method according to claim 1 wherein step C comprises passing the convection air through a porous heat exchanger body.

12. An oven for heating glass panels comprising: a heating chamber; rolls in the heating chamber for supporting glass panels; air directing structure in the heating chamber for directing convection air against the glass panels from above and below the glass panels; a heat source for heating the convection air; and a heat accumulator disposed in the heating chamber and arranged wherein the convection air passes in heat-exchange relationship therewith.

13. The oven according to claim 12 wherein the heat source comprises an electric resistance element.

14. The oven according to claim 12 wherein the heat source comprises a gas burner.

15. The oven according to claim 12 wherein the heat source is arranged to directly heat the heat exchanger.

16. The oven according to claim 15 wherein the heat source comprises an electric resistance element.

17. The oven according to claim 15 wherein the heat source comprises a gas burner.

18. The oven according to claim 1 wherein a first heat accumulator heats convection air directed against the glass panels from above, and a separate heat accumulator heats convection air directed against the glass panels from below.

19. The oven according to claim 12 further including radiant heaters in the heating chamber for heating the glass panels by radiant heat.

20. The oven according to claim 12 wherein the heat accumulator comprises a porous body through which convection air flows.