Lehr is an essential part of the glass production line used for cooling the glass ribbon after forming. A uniform controlled cooling from about 600 °C to about 60 °C ensures an optimal distribution of residual stresses in the glass ribbon and thus facilitates easy glass workmanship.
During the cooling process mechanical stresses (lateral and plane) develop within the glass. These stresses can be permanent or temporary. Permanent stresses are formed at temperatures of 470 – 480 °C, whereas temporary stresses arise at lower temperatures. Excessive residual stress at the edges of the glass ribbon makes it difficult to cut the glass. On the other hand, if the residual stress at the edges is too low, then the glass becomes brittle and the edges will not cut cleanly.
We have utilised all of our experience gained over the recent years developing our Float Annealing Lehr. Since we know practically every weak point of the lehr, we’ve been working hard to improve on these and now present you the result.
Float Annealing Lehr is equipped with a drive mechanism that sets the rollers in motion. The rollers pull the glass ribbon from the tin bath into the lehr. The speed of the rollers depends on the glass thickness and can be monitored and controlled locally on site or remotely from the control room. PCs with appropriate software are readily available for this.
Float Annealing Lehr is divided into several zones:
In the insulated zones the glass ribbon is brought to the desired temperature by indirect cooling. These zones have different types of equipment: stainless steel and carbon steel heat exchangers as well as fans for continuous cooling, heating elements for edge heating, thermocouples and optical pyrometers for monitoring and controlling the cooling process. The heating elements’ area of application can be adjusted as required.
The thickness of the glass ribbon is measured in real-time at the lehr entrance (hot zone). For this purpose, 5 measuring points across the width of the glass ribbon are considered. Based on the measured thickness profile, the ribbon forming process can be controlled in near real-time. Such thickness measurement has a number of advantages, such as quick transition between glass thicknesses and improved product quality.
These zones provide a possibility to remove glass cullet.
In the non-insulated zones, the glass ribbon is cooled with preheated exhaust air and further with a mix of hot and fresh air. The air flow is generated by fans and distributed via the air nozzles located above and below the glass ribbon. The air flow can be adjusted manually as required. The proportion of hot exhaust air is regulated automatically depending on the required temperature.
HORN® manufactures the full range of equipment for the hot end in-house. This minimises the number of critical interfaces and eliminates potential problems that can arise when implementing adjustments made by different suppliers. Clarification and coordination of adjustments can cost a lot of time and money, and there’s a risk that some issues will still remain unsolved.
HORN® also produces all crucial lehr components in its own workshop in Plössberg. This means quality made in Germany.