Whether it’s flat glass or container glass, annealing lehrs are vital for improving the strength and durability of the final product. The process essentially involves heating up the glass to remove any stresses that might have formed as it cooled after the forming process. The glass is brought up to a consistent temperature throughout – usually around 600°C (1,100°F) – and then conveyed through the lehr so that it cools at a controlled, uniform rate until it reaches a more rigid form.
Annealing lehrs process in brief
The formed glass is brought up to a consistent temperature – usually around 600°C (1,100°F) – which is warm enough to remove stresses in the glass without altering its form. The glass then passes through the lehr so that it cools at a controlled, uniform rate to ensure microfractures are not introduced as it returns to a rigid form.
Robert Fouquette, Technical Marketing Manager, Kanthal.“Control is really important when it comes to heating,” says Robert Fouquette, Technical Marketing Manager, Kanthal. “You need the heat to be distributed evenly across the glass, and then you need to bring the temperature down uniformly as the glass heads toward the exit, so that you do not reintroduce the microfractures that you’ve just removed.”
A shift toward electric heating
Gas heaters – either in the form of open-flame or radiant systems – and electric heating are the two most common heating solutions for glass annealing lehrs. The choice varies from market to market depending on energy prices, although in recent years the trend toward decarbonization is seeing a switch to electric solutions.
It’s easier and safer to regulate an electric heating element than an open flame, and it is also quieter since electric heating is effectively silent.
“A lot of glass manufacturers are looking for greener alternatives and electric is the far cleaner option,” Fouquette says. “There are no local CO2 emissions, and you’re not introducing any contaminates or impurities into the furnace atmosphere either. This makes it better for workplace health and safety too. It’s easier and safer to regulate an electric heating element than an open flame, and it is also quieter since electric heating is effectively silent.”
Another benefit of electric heating is improved thermal efficiency, which is typically around 90 to 95 percent. By contrast, the efficiency of gas heaters is usually less than 50 percent and can be as low as 30 percent if open flame gas burners are used.
A wide portfolio of solutions
With decades of experience working with electric heating elements, Kanthal has a range of options depending on the glass being manufactured and the required temperatures. For narrower, thinner container glass, it has its Fibrothal® heating modules with rod overbend (ROB) meander elements. For higher power and temperatures, Globar® Silicon Carbide (SiC) heating elements, Kanthal® Super molybdenum disilicide (MoSi2) heating elements and Superthal® heating modules are available as options.
Four benefits of using Kanthal’s electric heating solutions in glass annealing lehrs
- Zero emissions: Switching from gas heating solutions to electric removes all harmful local emissions, and if the electricity is from a renewable source, the process is effectively emissions-free.
- A safer workplace: Electric heating means there are no harmful exhaust gases in the factory, and it is generally safer as it does not involve gas or open flames. Electric heating is much quieter too.
- Increased thermal efficiency: The thermal efficiency of electric heating is around 90 to 95 percent, whereas the thermal efficiency of gas heaters can be as low as 30 percent. This means that only 30 percent of the heat being generated is actually being used in the furnace.
- Broad range of solutions and extensive expertise: Kanthal has worked with electric heating solutions for decades and has developed a broad portfolio of solutions to suit every unique application.
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