Writer: ANDERS HOLMQVIST, GLASSROBOTS OY

The growth in demand for architectural glass is boosted by the increased use of safety glass in construction.  Another factor in the background is the need created by climatic change to develop safer and more energy efficient constructions. On the other hand people are battling, for example, with an increase in hurricanes, on the other they are looking for ways to save energy.  Safety glass has several challenging properties that must be taken into account in the development of glass processing machines.

New types of glass and coatings are entering the market.  When combined with the continuing efforts of customers to achieve higher quality, greater productivity and extremely low energy consumption, they are opening the way for modern convection technology.

Thermal or infrared radiation is becoming an outdated method for heating glass. In practise, various coated glasses, especially the low-emissivity glass,  exclude the possibility of using tempering furnaces that operate with infrared. As the coating on the glass heats up, it reflects infrared.  Consequently, heating takes considerably longer and therefore capacity falls and optical quality suffers. With a convection heating system, the glass is heated with a stream of hot air without direct radiation.  This means it does not matter what coating is on the glass.

Advances in technology

RoboTemp™, the flat tempering product family launched by Finnish supplier Glassrobots, based on more than 25 years’ experience, has received positive feedback from the industry and is a good example in the use of convection technology.

In its convection system, air flow is large volume, low speed and inverter controlled. Air flow is uniform over the surface of the glass and temperature can be profiled automatically and precisely according to the type of glass and its size.

Glass has to be kept flat from the moment it arrives in the furnace. If the glass is deformed, especially from the heat conducted by the rollers as the glass starts to heat up in the furnace, straightening during final heating could be difficult or even impossible. The end result, in addition to other quality defects, is a loss of optical quality. The controls have to react extremely quickly and precisely.

This is made possible by Glassrobots’ FuzzyTemp™ control system. As its name implies, the system applies fuzzy logic, or a different, faster and more accurate way of calculating and processing signals compared to conventional control systems. This results in flexibility and efficiency.

Another factor that has an impact on process efficiency and on the quality of the end product is quenching which also has to be carried out while controlling different parameters precisely and continuously. Quenching has to be uniform and yet rapid.

Speed

A heating time of 33 sec/mm, or seconds of heating time per millimetre of glass thickness, has been reported as the world record for low-E glass (emissivity or ε=0.02-0.04). However, Glassrobots reports that its RoboTemp™machine has achieved heating rates of less than 30 sec/mm. With clear, uncoated glass it has achieved a heating rate of 25 sec/mm. However, we should also consider actual process values in continuous production (back-to-back) with sensible size mix, edgework and loading of 60-70 % (See tables 1 and 2).

Extra capacity from the faster heating rate is not the only significant factor from the practical viewpoint. Extremely high optical quality, flatness and consistency in the end product give the glass processor a major competitive edge. For example, 4 mm glass that has been tempered with the latest technology can be laminated with a 0.38 mm PVB film. Until now, a 0.76 mm or even thicker film was necessary. This has a big impact on the cost of the end product.

The structure of the furnace and the way it has been built, linked with effective recycling of the air within the furnace, enables low levels of energy consumption. The overall temperature of the furnace is kept low, at approx. 680°C and does not need to be adjusted for different types of glass.

Low-E glass

When the furnace is capable of faster heating times, you can save both energy and labour costs. This is especially enhanced with, for example, soft coated low-E glass. With faster heating times a certain amount of glass will be processed quicker, saving in kW/m2 of glass produced, as well as in working hours spent.

What is even more interesting is how much more profit you can be made if you process a lot of low-E glass on a 24/7 basis over the course of a year. Output can be made much higher. This profitability is noted with a furnace such as Glassrobots’ in comparison to a radiation furnace with compressor assisted convection. It is also significant compared to a furnace with high speed blowers and low recycling air volume.

Selection parameters

There are a number of things that should be considered when selecting a tempering furnace. For example, whether the furnace is capable of adjusting temperature distribution over large glass sheets during treatment (this is especially important when it comes to low-E glass).

The continued growth of high efficiency coatings requires a machine superior in design and controls which is capable of producing superior quality product. Glassmakers and processors should ask whether the furnace they choose is capable of this.

Furnaces should also be capable of sustained heavy loading without loss of power, keeping the same capacity with back-to-back loadings and maintain quality. Operating software for the furnace should be robust and companies should possess a good track record in glass processing and thus knowledge about developing its own software.

Other areas that the glass makers and processor should consider before selecting a
tempering furnace include the quality and type of components used within the
machine, whether you can save energy and labour costs with the furnace and
whether the furnace offers the potential for more profit compared to other furnaces on
the market.

Glassrobots has delivered RoboTemp™ flat tempering machines to leading glass processing companies worldwide. Its largest full convection flat tempering furnace, handling a maximum glass size of 3.2m x 8m, was delivered to a European customer for façade glasses. Glassrobots is continuing to look into the development of even larger furnaces. In fact, its furnaces are now ready to take 3.3 m wide glass.

Modern lines

A glass processor thinks about how to be more productive, how to save energy, how to improve the quality of the end product, and how to win more of the market share and grow. Existing technology has to be continuously developed.

The process for tempering glass is based on natural laws - on physics. The main process of heating and cooling glass is in principle the same no matter who manufactures the production line.

Applying these laws, while taking into account the needs arising from the market, is a continuous challenge. Responding to this, based on current machine building, measuring and control technology and the available materials, requires continuous efforts in research and development. Glassrobots believes it has set new standards for the quality of tempered glass and for productivity and energy efficiency in the modern tempering line.

Specifications

1. We assume a "sensible" size mix on the loading table
2. 60-70 % loading
3. 15 and 19 mm tempered glass would be dictated by cooling time and not heating cycle time.

Off-line coated Low-E, emissivity 0.02-0.04

Clear float glass