The industrial glass melting process

The glass melting process starts from a granular mixture of natural and synthetic raw materials (the so-called batch) and yields a thermally and chemically homogeneous melt made available at a well defined temperature level. The consecutive steps of glass fabrication (forming, annealing, etc.) are not within the scope of interest of this paper. Figure 1 illustrates how a continuously working glass melting furnace functions in principle. Figure 2 presents in very general terms a heat balance of the glass melting process. It comprises amounts of heat related to the flow of matter led through the combustion space, amounts of heat related to the flow of matter led through the melting basin, and heat losses through the boundaries of the system. For simplicity, all quantities are referred to the standard temperature level T0 = 298 K of the environment and to isobaric conditions at p = 1 bar. The quantity of specific interest in this paper is the so-called exploited heat Hex. It comprises both the heat required to bring about the batch-to-melt conversion, and the heat stored in the homogeneous melt leaving the system boundary at a temperature Tex. For the quantification of the heat balance in figure 2, it is crucial to have an accurate account of Hex. It is true, the contributions Hin, Hoff, Hstack, and Hre related to the combustion space can be calculated in a most accurate way from measured process temperatures, and from the known amounts of fuel and air used. But for the wall losses Hwo and Hwu through the boundaries of the combustion space and the basin, respectively, rough estimates are available only. This is due to the complicated shape of a real glass furnace, to the presence of a considerable number of openings in the furnace walls, and – most of all, after some years of continuous furnace operation – to the unknown residual thickness of the refractory bricks of the furnace lining. Thus, if Hex is assessed at a high accuracy, then the balance in figure 2 can be completed, and even an accurate account of the wall losses may be obtained.