Field of the Invention

The present inventiuon is related with glass tempering furnace control system, especially improvement of the tempering furnace where heating are is devided into several independently controlled areas - matrixes (m) and control system is controlling temperatures and speed changes to set temperature of matrixes to the same value all the time when the glass is sent to the tempering.

Background of the invention

One of the most important parameters is the exit temperature of the glass from furnace into the tempering section. Exclusively used system is heating time. When the glass has been heated certain time, it reaches the tempering temperature and is sent from furnace into the tempering section. The heating time depends on glass thickness, heating method and furnace temperature and properties, (hot mass of furnace), heating power, heating methods, (radiation or convection and their efficiency glass properties, like coating, also somewhat whether the glass is colored or clear. For good glass quality temperatures should be high enough but not overheated. Glass temperature should be also homogenous all over the area, same on top and bottom sides of the glass. The same exit temperature of each glass load is necessary to obtain same quality glass from one load to another. This patent application improves and makes these important parameters easy to obtain. Especially glass exit temperature can be maintained automatically the same and practically without any additional cost.

Existing technology

The existing technology uses pyrometers and temperature scanners to measure the glass temperature when it exits from the furnace. If the temperature of the glass increases, the operator reduces heating time. If the glass temperature decreases, the operator increases the heating time. This requires continuous attendance of the operator and cannot really keep the glass exit temperature on the same level. Furthermore, pyrometers and temperature scanner are expensive and require continuous calibration and maintenance. Therefore, they are not good for glass quality, they do not make automatic operation possible and they costly for equipment and service. Also, glass mass loaded into the furnace affect heating speed and time. Nowadays practically all heating systems uses temperature control, which is so called matrix heating system, in which the furnace heating area is divided into rectangular areas, matrixes. Fig. 2 shows general arrangement. Each matrix is controlled normally by one thermocouple, (tc), which senses the temperature under each matrix. The control system tries to maintain matrix temperatures at the same level, because the matrix temperature is related to the glass temperature moving at location of each matrix. However, the response of glass temperature to the matrix temperature is not sufficient in spite of PID control system. This means, some parts of glass may be too cold, which causes breakage of glass. And the glass(es) is not sufficiently homogenously heated, which result in lower quality glass. Some areas of glass(es) are too cold and may break in tempering or do not meet safety standards.

Disclosure of the invention This novel idea is based on using the existing equipment and control systems in a new way.

Fig 1. shows, that cold glass heats very quickly when it enters into the hot furnace. Furnace temperatures is normally about 680 - 710 °C. Furnace temperature curve depends on mass, (thus also thickness) of glasses loaded into the furnace, hot furnace structures as they store heat during final heating period of previous load and also subjects as referred above. Glass heating speed for 6 mm glass is around 6 - 8 °C per second in the beginning of the heating period and decreases to well below 1 °C per second when the glass temperature reaches close to its exit, (tempering), temperature. Tempering temperature is abt. 610 - 640 °C depending mainly on glass thickness. Thicker glasses can be tempered in lower temperatures.

Thus, the furnace temperature and glass tempering temperature have certain, slowly changing temperature difference, ΔΤ, at the end of the heating period (see fig 4 and fig 3). For thin glasses the temperature difference between the glass and the furnace temperature is about 40 - 70 °C and for thicker glasses about 70 - 100 °C depending on furnace temperature. When this temperature difference is controlled individually for each matrix, matrix by matrix, the heating in these matrixes, which have reached the certain temperature difference, ΔΤ, can be completely switched off. This improves PID control accuracy. The correct temperature difference is known quite well and accuracy can be improved with simple tests.

The heating can be continued in those matrixes, which have not yet reached the required difference level, ΔΤ. When all matrixes, even the final one, has reached the required temperature difference, ΔΤ, the glasses can be sent into the tempering. In this way the heating time control parameter can be excluded, operation becomes more easy, glass quality is improved, all batches have the same quality and breakage glass breakage is reduced. Also heating speed of glass is related to the speed, in which thermocouple, (tc), sensed temperatures decrease, (tcTdec). That decrease of speed can also be used to send the glasses into the tempering with similar principle as the temperature difference.

This system also enables building process recipes for different glass types and thicknesses based on the principle explained. Also, speed changes of convection blowers can be activated by furnace average temperature. This additionally improves heating processes similarity from batch to batch.

In convection heating furnaces, where convection jets blow into return air flow and where thermocouples are affected by the return air flow of various matrixes, this system does not work ideally. Also, in radiation heating furnaces and most convection heating furnaces in which thermocouples are affected by the radiation of heaters, this does not work in the best way. However, in systems, where the air returns from each matrix back to the convection blower very directly, this control system works ideally when the thermocouples are installed in the return air flow. The document WO 2014/111622 Al describes such an ideal return air flow particularly in figures 3 and 5.

Fig. 2 shows the complete heating area of the furnace and typical matrix (M) arrangement. Thermocouples, (tc), are ideally arranged at each end of each matrix. This increases the accuracy of the control system. The average temperature value of the matrix (TavM) is then the sum of temperature readings of the two thermocouples divided by 2. This requires one extra row of thermocouples at the end of the furnace, but is worthwhile, since the furnace ends need even better temperature control than other parts of the furnace.

Useful heating parameter of the tempering furnace is average temperature, Tav, which is the sum of TavM divided by number of matrixes. This can be used until the glass(es) temperature start to reach their tempering temperature. PID control will reduce heating power of higher temperature matrixes and increase heating power of lower temperature matrixes trying to reach homogenous glass(es) temperature when heating time control parameter sends the glass(es) into the tempering. Fully homogenous temperature is not possible by using heating time control and especially this control system can include too cold areas in glass or glass batch. When this novel idea is used, the heating continues also very slowly in those areas, (matrixes), where heating has already been switched off. That is not very harmful, since little overheating reduces very little glass quality. Too low temperature in some area(s) of glass(es) is dangerous from the point of view of safety glass standards, as fragmentation would be too large. It would be also bad for profitability of operation, since glass breakage is often due to the too low glass temperature. Expensive and highly preprocessed glass breakage is always costly. This is why every part of glass load, in all matrixes, must be heated to the correct, sufficiently high tempering temperature before sending the glass(es) in to the tempering process.

Heating speed of convection heating furnaces is much faster that radiation heating furnaces. This applies even much more for low emissivity glasses, because radiation heating is reflected from coated surfaces. Also, pyrometers and temperature scanners do not work on low E-glasses. With convection heating furnaces, like WO 2014/111622 Al, the heating speed reduces very little, say around 10 - 15 % whether the glass is clear or low emissivity, even with emissivity 0,02. Thus, this novel control system works ideally for all glass types with very minor setting adjustment. Furthermore, the repeatability of the heating processes can be enhanced even more, when the glass batches enter into the furnace, when the furnace temperature is always the same, Tstart. This may require some waiting time between the exits and next glass batch entering into the furnace. This is often made by operators manually based on experience. This can be automated, too.

This novel idea can also be used for any heating process because similar idea works for all heating processes.