Thermal behaviour of glass batch on batch heating

Abstract

The heating process of a barium-strontium glass batch has been studied in a 40 l pot furnace, using a multiple thermocouple assembly. The effect of several batch batch parameters on the heating process has been measured, including layer thickness, cullet fraction, water content and pellets. The results have been evaluated using a heat penetration batch model. In the model two heating stages, below and above a certain batch transition temperature ϑ_s, typically 800 to 900 °C, are distinguished. Values for the temperature-dependent thermal diffusivity of the batch have been derived from experimental temperature distributions in the batch during heating. Below the thermal diffusivity has an almost constant value of 0.4 · 10^-6 m²/s for a standard (powder) batch blanket; for ϑ > ϑ_s the net thermal diffusivity strongly increases with temperature, due to the formation of primary melt phases. For ϑ_s < ϑ < 1100 °C the average value is about 1.4 · 10^-6 m²/s. A 100 % cullet layer has a 50 % higher thermal diffusivity for ϑ < ϑ_s; pelletizing the batch is of little influence on the virtual thermal diffusivity and (extra) wetting has a retarding effect on batch heating due to extra heat absorption. As for the furnace temperatures it appears that increasing the temperature of the glass melt is more effective for improving the batch heating rate than increasing the temperature of the combustion chamber. Practical recommendations are given for batch preparation, charging and heating in industrial glass tanks.