Calculation and design of glass melting tanks for glassmaking furnaces for container glass

Abstract

The procedure for calculating and designing melting tanks for glassmaking furnaces used in the manufacture of container glass is examined. It is shown that mathematical modeling of the internal heat and mass transfer makes it possible to increase substantially the quality of the design solutions, whose implementation creates objective prerequisites for attaining a high energy efficiency and productivity for glassmaking furnaces.Efficient use of the heat energy transferred to the tank as a result of the external heat transfer and intended for implementing the technological process is the main requirement for designing melting tanks. The results of mathematical simulation of the external and internal heat and mass transfer [1] show that in high-productivity glassmaking furnaces even with a flat flame of optimal length the maximum temperatures to which the surfaces of the molten glass and masonry are heated are comparable to their limiting values for refractories in the working space and melting tank. Hence it follows that it is practically impossible to increase the specific productivity of a furnace without intensifying internal heat and mass transfer and minimizing the heat losses through the masonry of the tank.Heat transfer from the high-temperature zone of the tank to its less heated parts is largely determined by the tank's hydrodynamics. In addition, because of the technical features of the glassmaking process it is best to redistribute the heat in the furnace loading direction. The external heating conditions limit the possibility of increasing the temperature of the tank's surface in the melting zone of the mix piles. For this reason, intensification of the glass-forming reactions in flame furnaces without additional electric heating becomes possible only if the temperature of the molten glass is increased in this part of the tank by intensifying heat transfer by means of the convection flow of the free-flow cycle.In taking account of production flow, the effect of the temperature gradient at the surface of the molten glass on the organization of the convection field of the melt is limited. Creating the conditions for a clearly defined convection flow of the free-flow cycle with counterclockwise rotation of the molten glass is a very important function of the melting tank. Increasing melt circulation in this loop is of fundamental importance for heat-transfer and the residence time of melt in the tank. Heat exchange on the right-hand side of the melting tank is determined not only by the boundary conditions on free and forced convection but also on the structural components of the tank which impede melt flow. Therefore, the design of the melting tank must take account of the organization of the convection flow of the process cycle. On the technological level, the evaluation of this flow can be based on two basic requirements. They concern the time that the melt resides in this part of the tank and the thermal uniformity of the molten glass extracted for production. Under the conditions being compared both ...

Heat transfer in ESR slags

“…This range of values is close to that found for the equivalent solid polycrystalline compositions [43]. Although the use of an 'effective' thermal conductivity which combines the phonon and photon contributions contains inherent flaws, measurements of the thermal conductivity of liquid oxide-based slags and glasses [39,40,49,[51][52][53][54][55][56][57] based on this premise have generated useful practical results. This assumption has been examined in relation to heat conduction in liquid glass and salts [37,39] and found only to hold if the liquid is quite opaque.…”

“…The values of optical parameters given for compositions related to ESR slags when inserted in the Rosseland equation give values of k rad which are reasonably close to the range used in Table 1 and the values for the refractive index, n, and the optical absorption coefficient, y, fall within the values reported for slags and glasses of intermediate transparency. It appears from the above that the ESR slag can be treated as a 'grey gas' for the purposes of thermal modeling as has been the case for studies of many computations of the temperature gradients in the very similar situation of the molten glass bath of self-heated electric glass-melting furnaces [38,[51][52][53][54][55][56]. Hubert [51] indicates examples of the use of this concept to successfully model the temperature distributions in glass melts contained in a self-heated electrical furnace, based on CFD software of the same general type as that used below in the 'MeltflowESR' modelling software, as do several additional authors [52][53][54][55][56].…”

“…It appears from the above that the ESR slag can be treated as a 'grey gas' for the purposes of thermal modeling as has been the case for studies of many computations of the temperature gradients in the very similar situation of the molten glass bath of self-heated electric glass-melting furnaces [38,[51][52][53][54][55][56]. Hubert [51] indicates examples of the use of this concept to successfully model the temperature distributions in glass melts contained in a self-heated electrical furnace, based on CFD software of the same general type as that used below in the 'MeltflowESR' modelling software, as do several additional authors [52][53][54][55][56]. Viskanta [56] comments specifically on the validity of using the grey gas assumptions together with an effective thermal conductivity representing the sum of phonon and photon conduction as derived from the Rosseland equation.…”

“…the dependence of the effective heat transfer coefficient on composition and on temperature. Many reports are available [51][52][53][54][55][56] describing the modeling of a closely analogous system, namely the liquid glass bath in an electric furnace, self-heated by its internal electrical resistance. The models demonstrate the practical validity of using an effective thermal conductivity representing the sum of photon and phonon mechanisms, in most cases based on the Rosseland equation (Equation (1) above).…”

“…The model structure and its component assumptions/equations are fully described by Kelkar [71]. The fundamental CFD algorithms in this model are essentially the same as those which have been used in modeling the electrically-self-heated glass furnace bath [51][52][53][54][55][56]. The two results illustrated in Figure 2 represent the limiting conditions of pure phonon conduction ( [23,[45][46][47][48], k = 0.7 W m -1 K -1 independent of temperature) and of photon conduction using the experimental value from Wadier [23], (Equation ( 2)) as the thermal conductivity of the slag.…”