GB2369355A - Hot-air circulating means for uniform heating of glass sheet - Google Patents

Abstract

Apparatus for the tempering or heat strengthening of glass sheets comprises a heating zone incorporating one or more heating elements 14, means for passing the glass sheets 12 through the heating zone and circulating means 20 between the glass sheet 12 and one or more of the heating elements 14. The heat transfer rate from heating elements to the glass sheets is controlled by means of the circulating means which may be one or more fans.

Description

Heating of coated glass

The present invention is directed towards the heating of glass in, for example, tempering and heat strengthening processes. In particular, the present invention is directed towards the heating of coated glass.

Currently, sheets of glass are heated in a roller hearth furnace such as that shown in figure 1. Heating is by means of radiant heaters mounted above and below the glass sheet which is passed through the system on rollers. For uncoated glass, uniform heating is possible by controlling the temperature in a number of different zones.

Different temperature settings are necessary at any one time to make allowances for the difference in environment between the top and the bottom of the glass. The top of the glass will be subjected to free radiated heat whereas the bottom of the glass will be shielded by the rollers. In addition to the shielding of the glass from the radiated heat the rollers will subject the glass to extra heat transfer by conduction once the rollers become hot. Further, the edges of the glass heat differently from the middle of the glass.

Generally, uncoated glass may be processed satisfactorily by means of regular monitoring of the glass and setting temperature controllers at different temperatures. However, tempering or heat strengthening coated glass provides additional problems. The two surfaces on the glass (one coated and the other uncoated) will have different emissivity values. Generally, coated surfaces have a lower emissivity value than uncoated surfaces. The result of this is that the heat transfer rate to the coated surface at the same temperature is considerably lower than that to the uncoated surface.

In an attempt to overcome this problem a number of solutions have been tried. In order to overcome the low heat transfer rate, the temperature of the heaters over the top of the glass (heating the coated side which is normally kept away from the roller surface) has been set at a higher temperature than the lower heaters which

heat the uncoated side of the glass. This method has not achieved the required result as the higher temperatures employed have damaged the coating on the glass and/or slowed down the production rate to an unacceptable degree.

An alternative method for tempering coated glass sheets is to use a heating balance where heated compressed air is blown on to the upper surface. However, the production of hot compressed air is expensive and makes the use of this method uneconomical in practice. The regular discharge of hot compressed air is inefficient. In addition, the use of hot compressed air introduces a number of risks to the system, which require the provision of extra safety requirements to protect the operator from the hot pipes.

Another method employs the use of convected heating of the glass. The air circulation is arranged in such a way that the hot air is taken from the furnace and blown on the glass surface by electrically driven external hot resistant blowers.

The uniform distribution of hot air is achieved by complicated ductwork. The cost of the hot air blower, ductwork and setting and maintaining the system is very expensive.

Combinations of the methods mentioned above have also been tried without success. The plants and operating conditions required are expensive and do not provide a practical solution to the problem of tempering coated glass.

The object of the present invention is to provide a method and apparatus for the tempering or heat strengthening of glass, in particular coated glass.

According to the present invention there is provided apparatus for the tempering or heat strengthening of glass sheets, said apparatus comprising a heating zone incorporating one or more heating elements, means for passing the glass sheets through the heating zone and circulating means between the glass sheet and one or more of the heating elements, in which the heat transfer rate from the heating

elements to the glass sheets is controlled by means of the circulating means.

As indicated above, a problem with the system of the prior art is that the heat transfer rates to the two principal surfaces of the glass sheet are different. Controlling these has been difficult particularly in the case where the glass sheet has a coating on one surface.

Preferably, the glass sheets are coated on one side of the glass and the circulating means are between the coated face and the heating elements. Preferably the glass sheet is coated to an emissivity less than 0.2 (defined as low-E glass). Tempering or heat strengthening coated glass sheets is difficult in view of the increased difference between the heat transfer rates between the two surfaces. Controlling the heat transfer rate to each surface by means of circulating means enables the sheet to be tempered or heat strengthened economically and reliably. The use of circulating means rather than expensive compressed air considerably reduces the energy duty on the system.

The running costs as well as the manufacturing costs are therefore considerably reduced.

Preferably the means for passing the glass sheets through the heating zone comprise a series of rollers.

Preferably, the circulating means comprise one or more fans. Optionally, the fans are adapted to operate with a range of sizes of impeller. This provides a system with maximum flexibility, which can be easily adapted to treat a wide range of glass sheets.

Preferably, the impellers are attached to the body of the fan from inside the heating zone allowing heaters to be fixed to the entire top surface except for a small clearance for the fanshaft. If the impeller were to be attached on the outside of the furnace and then placed inside, it would be necessary to leave space in the roof of the furnace and thus reduce heating area.

Preferably, each fan is controlled by an individual electric motor. Of course, these motors may in turn be controlled centrally by an overall processing system. Preferably, the impeller diameter is in the range 30 to 75cm, more preferably 45 to 60 cm. Preferably the fan operates at a speed in the range 300 to 1500 rpm, more preferably 750 to 1000 rpm.

Preferably, the heating elements heat to a temperature of between 600 and 950oC, more preferably 700 to 800oC.

The invention also extends to a method of tempering or heat strengthening glass sheets in which a glass sheet is passed through a heating zone including heating elements and circulating means and is heated in a controlled manner, the circulating means controlling the rate of heat transfer from the heating elements to the surfaces of the glass sheet ; and the heated glass sheet is passed on to a cooling or quenching zone where it is cooled to room temperature.

The invention may be put into practice in various ways and a number of specific embodiments will be described by way of example to illustrate the invention with reference to the accompanying drawings, in which: Figure 1 shows a tempering/heat strengthening plant of the prior art; Figure 2 shows a furnace according to the present invention viewed from one end; Figure 3 shows a second embodiment of a furnace according to the present invention viewed from one end; and Figure 4 shows a furnace according to the present invention viewed from the side.

As indicated above, figure 1 shows a roller hearth furnace as used in the tempering and heat strengthening of glass sheets. The system may comprise four sections as shown in figure 1. In the first section the glass sheets 10 are loaded onto the rollers 12 for transportation in direction A to section 2, the heating zone or furnace 18. In this zone the glass sheets are heated from both above and below by means of radiant heaters 14 mounted above and below the glass sheet. The radiant heaters may be of any suitable form such as open coils. Above the bottom heater there are a number of heater protection trays 16 to prevent objects falling from the glass sheet into the heater. In particular, should the glass sheet crack, splinter or break, the heater protection trays will stop fragments of glass dropping into the heater and forcing the system to be shut down, cooled and cleaned.

After the heating zone, there is a third section, a quenching and cooling zone 3 in which the tempered glass is cooled to room temperature, before passing on to section 4 where the glass sheets are unloaded.

As indicated above, there are problems with plants of this type, which arise partly from the shielding effect of the rollers on the radiated heat to the bottom surface.

While this problem may be minimised in tempering uncoated glass sheets by setting temperature controllers at different temperatures, the problem is amplified when the glass sheet being tempered is coated on one surface. This coated surface is conventionally positioned away from the rollers. Attempts to overcome the differences in emissivity of the coated and uncoated sides of the glass by increasing the temperature of the heaters above the sheet have not achieved the desired result.

The higher temperatures employed have damaged the coating and/or slowed down the production rate of sheets passing through the heaters.

Figure 2 shows a first embodiment of the present invention. This end view shows a vertical cross section of the heating zone or furnace 18 without a glass sheet present.

The sheet again passes along the system on rollers 12 with the coated surface of the sheet away from the rollers. Above the glass sheet but below the upper heating

element 14 is at least one fan 20. The fan in figure 2 is shown in the middle of a narrow furnace. Figure 3 shows two fans 20, 21 arranged adjacent one another to substantially cover the width of the furnace. Of course, this can be extended to any number of fans as appropriate for the width of the furnace.

Figure 4 shows, from one side, a length of furnace (heaters not shown) including 4 fans 20,22, 24,26 spaced evenly along the length of the furnace. Once again, any number of fans may be employed along the length of the furnace as is appropriate for the furnace. By referring to figures 3 and 4 it will be clear that any combination of fans along the length and width of the furnace may be used to suit the situation.

The presence of the fan or fans above the coated surface of the glass helps to increase and control the heat transfer rate from the heater to the glass. The increase in heat transfer rate means that the difference in emmisities between the surfaces can be overcome without the need for increasing the temperature of the upper heater. There is therefore a reduced risk of damaging the coating through excessive heating.

The degree of increase of heat transfer rate can be controlled by the speed of the fan and can therefore be easily adjusted to accommodate different coatings, which may have different critical temperatures. In addition, the use of a fan still enables the system to use multizone heat control as has been used for treating uncoated glass to overcome differences caused by the rollers.

Of course, the apparatus of the present invention can also be used in the treatment of uncoated glass by employing the fan (s) to a lesser extent or not at all. It is therefore very easy to switch between the treatment of coated and uncoated glass without the need for significant changes to the apparatus, or even different sets of apparatus.

Referring again to figure 2, each fan 20 is a high temperature fan capable of operating at temperatures of up to 900oC. The impeller 30 for each fan 20 is fitted from the inside of the furnace 18 onto a fan shaft 32 which is already fitted through the roof of

the furnace 18. This arrangement means that the heaters 14 can cover nearly the whole of the inside face of the roof since there only needs to be a sufficient gap for the shaft 32 to pass through the insulation 34. Each fan is fitted with an electric motor 36 so that the speed of each fan is individually controlled to optimise the plant for each type of coated glass treated. These fans can, of course, be controlled centrally in an overall process control system.

By careful selection of the available variables, a whole range of glass sheets may be treated, both coated and uncoated. In particular the number and position of the fans within the furnace may be varied. Further, the size of the fan impeller and the speed of operation may be varied to take into account the glass sheet being treated. Typical dimensions of glass sheets being treated are 1m x 2m. These may be uncoated and hence treated with little or no use of the fans in the apparatus of the present invention or they may be coated. Typical coatings being low-E glass, i. e. being coated to an emissivity of less than 0.2.

In general, the fans are positioned every 60cm to optimise the heat transfer rate for a given heater element temperature. Each fan may employ an impeller blade of dimension 30 to 45 cm and may be rotated at a speed between 750 and l0OOrpm. The furnaces may operate at temperatures of between 700 and 780oC. Below in table 1 is listed an example of typical operating conditions for a specific coating.

Coating Temperature ( C) Impeller diameter (cm) Impeller speed (rpm) Pilkington"K"700 45 750 type low-E glass Table 1

Claims (14)

Claims :

1. Apparatus for the tempering or heat strengthening of glass sheets comprising a heating zone incorporating one or more heating elements, means for passing the glass sheets through the heating zone and circulating means between the glass sheet and one or more of the heating elements, in which the heat transfer rate from the heating elements to the glass sheets is controlled by means of the circulating means.

2. Apparatus as claimed in claim 1, in which the glass sheets are coated on one side of the glass and the circulating means are between the coated face and the heating elements.

3. Apparatus as claimed in claim 2, in which the glass sheet is coated to an emissivity less than 0.2

4. Apparatus as claimed in any preceding claim, in which the means for passing the glass sheets through the heating zone comprise a series of rollers.

5. Apparatus as claimed in any preceding claim, in which the circulating means comprise one or more fans.

6. Apparatus as claimed in claim 5, in which the fans are adapted to operate with a range of impellers.

7. Apparatus as claimed in claim 6, in which the impellers are attached to the body of the fan from inside the heating zone.

8. Apparatus as claimed in any one of claims 5 to 7, in which each fan is controlled by an individual electric motor.

9. Apparatus as claimed in claim 8, in which the motors are controlled centrally by an overall processing system.

10. Apparatus as claimed in any one of claims 6 to 9, in which the impeller diameter is in the range 30 to 60cm.

11. Apparatus as claimed in any one of claims 5 to 10, in which the fan operates at a speed in the range 300 to 1500rpm.

12. Apparatus as claimed in any preceding claim, in which the heating elements heat to a temperature of between 600 and 950oc.

13. Apparatus constructed and arranged substantially as herein specifically described with respect to and as shown in figures 2 to 4 of the accompanying drawings

14. A method of tempering or heat strengthening glass sheets in which a glass sheet is passed through a heating zone including heating elements and circulating means and is heated in a controlled manner, the circulating means controlling the rate of heat transfer from the heating elements to the surfaces of the glass sheet; and the heated glass sheet is passed on to a cooling or quenching zone where it is cooled to room temperature.