Process for the adjustment of the glass temperature in a float glass lehr and float glass lehr in order to bring this process into operation

A float glass lehr for the heat treatment of a float glass ribbon, said lehr comprising:

• an elongated lehr tunnel;
• a multiplicity of rollers spaced apart longitudinally of said tunnel but extending transversely therein and forming a path along which said ribbon is displaced on said rollers;
• a lower array of longitudinally extending, mutually parallel coplanar heat-exchange tubes disposed below said path, and an upper array of mutually parallel coplanar transversely spaced heat-exchange tubes disposed above said path within said tunnel, each of said arrays being subdivided longitudinally into respective zones, each zone having an upstream side and a downstream side with respect to the direction of travel of said ribbon along said path;
• means connecting a plurality of tubes of each array and each zone in a respective group whereby, for each zone, each of said arrays comprises a plurality of such groups in succession transversely of said direction;
• a respective air heater communicating with the tubes of each group at an upstream end of the respective zone whereby heated air flows through said tubes in a direction opposite the direction of movement of said glass ribbon along said path;
• means at the downstream end of each of said zones for drawing air from said tubes;
• and
• means at the upstream end of each zone for measuring the temperature of the glass ribbon and controlling selectively the heating effect contributed by each group to said glass ribbon.

In a method of operating a float glass lehr in which a float glass ribbon is passed on rollers through a lehr tunnel and wherein ducts for heat-exchange with the float glass ribbon are provided above and below the glass ribbon and extend parallel to the direction of movement of the glass ribbon through the lehr, the improvement which comprises:

• (a) generating hot air at a relatively downstream location along said ducts with respect to the direction of movement of the glass ribbon through the lehr;
• (b) passing said hot air through said ducts from said downstream location in a direction opposite a direction of movement of the glass ribbon through the lehr, thereby heating said ducts to a temperature below that of the glass and thereby cooling the glass ribbon;
• and
• (c) collecting air from said ducts at a relatively upstream location thereof with respect to the direction of movement of the glass ribbon through the layer.