The tubing is then drawn over a line of support rollers by a drawing machine situated up to 120m away. The dimensions of the tubing are determined as the glass cools through its setting point at the catenarie or unsupported section between the blowpipe and the first line roller.
A given range of size is based on the diameter of the refractory sleeve, and variations within the range are obtained by adjusting the temperature of the glass, the rate of flow, the pressure of the blowing air and the speed of the drawing machine.
All together a very delicate process control job in which precise temperature, air pressure and speed control need to be coordinated. Invensys Eurotherm has an extraordinary amount of experience in glass tubing manufacturing process control applications.