Volume 39, Issue 10, October 2004

A Look at Convection 
Furnace Tempering

by Juha Karisola

When the installations of new 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 may not meet current market needs, and glass fabricators want the latest technology that can help boost productivity and improve the quality of production. This article will look at some of the problems that convection technology addresses and provide an overview of the technology currently available.

Flat tempering technology has advanced by leaps and bounds in recent years. Several machine manufacturers have improved their traditional radiant furnaces to better satisfy market demands. Factors driving this trend include the following:
• Tempering coated glass requires an increased use of convection;
• Advancing technology has meant strict quality requirements for glass (optical properties, tempering marks, etc.)
• The use of laminated tempered glass is increasing steadily and lamination requires excellent flatness
• Process output and yield requirements have increased; and
• New manufacturers breaking into the marketplace have brought along new innovations, forcing traditional producers to develop their own technologies, while also creating fiercer price competition.

A significant technology trend today is represented by heating methods based on convection. Such technology is designed to 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 heating process in the furnace be 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 affect the shape or flatness of the end product, its optical qualities or the surface of the glass. The uniformity of the heating is achieved by compensating the temperature difference by profiled heating; therefore the possibility to profiled heating is of utmost importance.

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. In fact, some types of coated glass cannot be manufactured economically at all using traditional production methods.

Typical Problems 

It is well known that the quality of tempered glass is greatly influenced by the heating process used in the furnace. For example, non-uniform heating can cause deformation of the glass in the quenching process. The rapid heating of the lower surface of the glass is a typical problem caused by heat conduction from the hot ceramic rollers. The expansion of the lower surface bows the glass edges upwards, moving the glass on the rollers like a boat, resulting in damage called “centre line haze.” Sometimes optical distortions in the middle of the glass are also caused. Other non-uniform heating results include overheated edges and an overheated middle. Overheated edges cause what is known as bistable saddle, which may break the edges in the heating process, whereas an overheated middle causes bistable bow, with the middle of the glass being pushed from 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 iridescence, which is most clearly seen in a polarization test, but may also be visible to the naked eye.

Uneven heat distribution may occur, in turn, 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 that may have excess heat on the edges and a cold area in the middle. This can be partly compensated for 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 absorbed differently in printed areas of the glass than in plain glass. The same applies to shaped glass lites.

Full Convection Heating 
Heat is transferred to the glass in three different ways: radiation, conduction and convection. Regardless of the type of furnace, these three ways of heat transfer are always present. They can be further analyzed into the following parts:
• Direct radiation from the heating elements (primary source of heat); and
• Indirect radiation from rollers and other internal parts of the furnace.

Conduction from the ceramic rollers
• Natural convection from the air without blowers or compressed air systems;
• Assisted convection by using compressed (cold) air to improve air flow; and
• 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 constantly on the lookout for new solutions based on the use of convection. Indeed, convection is seen as a must for any production line where coated glass is made. In addition to improving 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 seconds/mm of thickness. With convectional heating, heating times can be reduced to 26-30 seconds/mm of thickness, increasing output and productivity by up to 35 percent. 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 
So what kinds of convectional systems are available on the market? The first version that made use of convectional heating was a double chamber furnace, in which 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 to 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:
• Radiant furnaces with assisted convection (where convection has been increased by either compressed air [cold air blown into the furnace] or high-pressure charger [hot air ventilated inside the furnace]);
• True convection systems, where ventilated hot air is blown onto the glass through nozzles (electric or gas fired systems with overall heating or electric heated furnaces with profiled heating [heaters inside the nozzles]).

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, there must be an 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.

Pros and Cons of Different Systems
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 may not be the most effective system for a given company and can also increase energy consumption. 

Charger-based systems: These offer the same advantages as compressed-based systems, but since the volume of hot air blown in is greater [than compressed-based systems], they are more effective. Both systems feature fast heating through assisted convection and allow for profiling through control of radiant heating.

As the profiling is achieved through radiant heating it performs well with float glass. However, since the new low-E glasses reflect almost all of the radiation from the glass surface here, understandably, the profiling through radiation fails. 

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. 

True convection furnaces can be divided into two groups:
• Electrical heaters inside the nozzles (allows profiling); and
• Electrical (or gas) heaters in channels or somewhere else (does not allow profiling).
Some of the early designs performed relatively well with limited glass sizes, but failed to perform properly with large sizes as they did not allow for any profiling of heating. The most recent technologies combine the benefits of the true convectional heating and profiling. 

the author
Juha Karisola is the director of sales and marketing for Glassrobots Oy of Tampere, Finland.


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