When new installations of flat tempering machines suited for architectural glass are discussed today, the option of convention technology is certain to be raised. Furnaces based on traditional methods of radiation heating do not meet current market needs, but glass processors want the latest technology that can help boost productivity and improve the quality of production. This article looks at some of the problems that convection technology addresses and provides an overview of the technology currently available.

Flat tempering technology has advanced in leaps and bounds in recent years. Several machine manufacturers have improved and developed their traditional radiant furnaces with a view to better satisfying market demands. Factors driving this trend include the following:

• the tempering of coated glasses requires an increased use of convection
• advancing technology has meant ever stricter quality requirements for glass (optical properties, tempering marks)
• the lamination of tempered glass requires excellent flatness
• process output and yield requirements have increased
• new manufacturers breaking into the marketplace have brought along new innovations and forced traditional producers to develop their own technologies, also creating fiercer price competition
  

The single most significant technology trend today is represented by heating methods based on convection, which improve both quality and process speed, especially with new coated glasses.

A key factor with regard to glass quality and sufficient tempering is that the process of heating the glass in the furnace is as even as possible. Non-uniform heating will adversely affect the optical properties of glass as well as its flatness, not only in the furnace but also during quenching. These problems may adversely affect the shape or flatness of the end-product, its optical qualities or the surface of the glass.

Convection technology helps to eliminate many of the problems we see in traditional radiant furnaces, particularly in the production of coated glasses. The processing of coated glasses involves other problems as well. Some types of coated glass cannot be manufactured economically at all using traditional production methods.

Typical problems occurring during the heating process

It is well known that the quality of tempered glass is very much influenced by the heating process used in the furnace. Non-uniform heating causes deformation of the glass in the quenching process. The most typical problem is caused by rapid heating of the lower surface of the glass due to conduction of the heat from the hot ceramic rollers. The expansion of the lower surface bows the glass edges upwards and the glass moves on the rollers like a boat, resulting in damage called “centre line haze”. Sometimes optical distortions are also caused to the middle part of the glass. Other non-uniform heating results include overheated edges and an overheated middle part. Overheated edges cause what is known as bistable saddle, which may break the edges in the heating process, whereas an overheated middle part causes bistable bow, with the middle part of the glass being pushed side to side.

These problems are far more severe when processing coated low-E and reflective glasses. In addition to the problem of conductive heat from the rollers, the coating on the upper surface of the glass reflects the radiation from the upper heating elements, whereas the lower heating elements heat the glass twice (as the radiation from below penetrates the glass and is reflected back from the coated upper surface).

Non-uniform heating may also result in cold streaks in the direction of the glass. Here the uneven temperature caused by the resistance elements (or by convection air) gives rise to irridecence which is most clearly seen in a polarisation test, but may also be visible to the naked eye.

Uneven heat distribution may in turn occur when variable loads are run into the furnace one after another. When it enters the furnace, the cold glass absorbs the heat from the roller bend. Due to thermal inertia, the previous glass leaves the area where it has been oscillating cold, and consequently the next batch enters a roller bed which may have excess heat on the edges and a cold area in the middle. This can be partly compensated by adjusting the cross-sectional heat so that it only heats the loaded area.

The glass itself may also cause problems in heating. Radiant heat is differently absorbed in printed areas of the glass than in plain glass. The same applies to shaped glass lites.

No convection furnace can yield 100 %

Heat is transferred to the glass in three different ways: by radiation, conduction and convection. Regardless of the type of furnace, these three ways of heat transfer are always present. They can be further analysed into the following parts:

1. Radiation
a. Direct radiation from the heating elements (primary source of heat)
b. Indirect radiation from rollers and other internal parts of the furnace
2. Conduction from the ceramic rollers
3. Convection
a. Natural convection from the air without blowers or compressed air systems
b. Assisted convection by using compressed (cold) air to improve air flow
c. Forced convection from the hot air being blown onto the glass

The extent to which each of these contributes to the heating process depends upon the type of furnace, the type of glass and the phase of the heating process. In traditional furnaces the main source of heat transfer is conduction from the rollers (in the initial phases of heating) and then radiation. In full convection furnaces, the heat predominantly transfers through convection. Convection must play a major role if coated glasses are to be heated effectively.

Convection for coated glass and speed

In order to overcome these problems, furnace manufacturers are on the constant lookout for new solutions that are based on the use of convection. Indeed convection is seen as a must for any production line where coated glass is made. As well as helping to improve the quality of the end product, convectional heating has another important advantage over radiant systems, namely heating speed.

Tempering systems based on radiant furnaces heat up the float glass at speeds of about 40 sec/mm of thickness. With convectional heating, heating times can be reduced to 26-30 sec/mm of thickness, increasing output and productivity by up to 35%! As low-E and other coated glass types require much longer heating times in a radiant furnace, productivity is increased even more.

Different types of convection furnaces available

So what kinds of convectional systems are there available on the market? The first version that made use of convectional heating was a double chamber furnace, where the first chamber was a preheating chamber with nozzles blowing hot air (350-400°C) onto the glass surface. This offered a number of advantages over traditional systems: it was fast and produced excellent glass quality, and there was no heat shock or bending of the glass due roller conduction. The first single-chamber convection furnaces were brought to the marketplace in the late 1990s. Today, the following convectional furnace types are available:

1. Radiant furnaces, where convection has been increased by either:
a. compressed air (cold air blown into the furnace)
b. high pressure charger (hot air blown into the furnace)
2. True convection systems, where hot air is blown onto the glass through nozzles
a. Electric heated furnaces, heaters inside the nozzles
b. Gas fired systems

The greater the share of heat transfer that can be produced by convection, the better. To cope with the problem of varying loads and non-uniform heat distribution, the furnace must also allow for profiling of the heating. For this profiling to be effective, it is essential that there is immediate response to changing process conditions. Practically the only way to achieve effective heating control is to adjust the profile according to the furnace load.

The pros and cons of the different systems can be summarised as follows:

Radiant furnaces with convection through compressed air

This system usually consists of a basic furnace with electric heaters, either massive or free spirals. The convective system has tubes inside the furnace, through which the compressed air is blown in to increase the airflow. This is a relatively inexpensive system that can be added on to existing furnaces. However, it is far from the most effective system and increases energy consumption.

Charger-based systems offer the same advantages as compressed-based systems, but since the volume of hot air blow in is greater, they are also more effective. Both systems allow for profiling through control of radiant heating.

True convection systems

A true convection furnace (or forced convection furnace) is a system with air circulation where hot air is blown in through upper and lower nozzles onto the glass surfaces. Direct radiation has been eliminated either by encapsulating the heating elements or by heating the air somewhere else before it is blown back into the furnace. The system is relatively expensive to set up and cannot be added on to any existing furnace. It is far more efficient than the radiant furnace with aided convection. Some of the early designs failed to perform properly as they did not allow for any profiling of heating. Even these furnaces performed well only with even, repetitive loads.

Glassrobots is an experienced manufacturer of complete windscreen production lines, automatic bending furnaces for automotive and architectural glass industry, as well as curved tempering systems and flat laminating lines for architectural glass industry.