Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
= 29.386 Na2O + 19.346 K2O – 17.867 Al2O3 – 10.824 CaO,
S = difference to 1000 kg.
Oxide amounts are to be inserted in wt %. For the components k, the shorthand notation hm = FeO·Fe2O3, F = Fe2O3, M = MgO, C = CaO, N = Na2O, K = K2O, S = SiO2 is used. Column m(k) in Table 5 lists the resulting amounts of the constitutional components of the glass. By this procedure, one finds that the standard enthalpies of formation of the glass and melt are 14 189.7 MJ/t at room temperature and 12 665.9 MJ/t at 1300 °C, respectively. The enthalpy physically stored in the melt at 1300 °C relative to the glass at 25 °C is thus 1523.8 MJ/t. By the weighted sum of the heat capacity of compounds k, the latter value can be adjusted to any other exit temperature of the melt. For the batch given in Table 4, column “mII(i)”, a chemical energy demand of ΔH°chem = 461.8 MJ/t is obtained. Fusion of the selected batch with 50% cullet (yCULLET = 0.5) thus requires an intrinsic energy demand of
(17)
A well‐constructed and operated melting furnace (end port, air‐gas fired) reaches an efficiency of heat exploitation ηex of 48%. Thus, the actual energy demand Hin of the melting process amounts to Hin = Hex/ηex = 3637 MJ/t. This result is very much in line with industrial experience. Calculations of this kind are of high importance for the evaluation of glass furnace performance [9], for furnace design, as well as for the energy optimization of batch and glass compositions.
7 Perspectives
Although the energetics of the fusion process may be considered as satisfactorily assessed, the kinetic aspects of fusion are not yet well enough understood. The efficiency of heat exploitation ηex of a furnace varies according to a hyperbolic law of the type ηex = 1/(A + B·p) with the production rate p (t/h). Thus, furnaces are preferentially operated at the highest achievable rates. The limits for p are determined by the rate of heat transfer or the time demand of the fusion process required to achieve an acceptable glass quality. As of now, however, one does not even known which of the above constraints controls the melting rate. As a matter of fact, the answer depends on both furnace and batch design.
Table 5 Calculation scheme for the energetics of a soda‐lime silicate glass (composition in wt %).a
| Oxide | wt % | Compound k | H°k,GL | H k,1300 | c P,k,L | m(k) | m(k)· H°k,GL | m(k)· Hk,1300 |
|---|---|---|---|---|---|---|---|---|
| MJ/kg | MJ/kg | kJ/kg·K | kg/t | MJ/kg | MJ/kg | |||
| SiO2 | 71.84 | hm | 4.4313 | 3.0196 | 0.9217 | 0.18 | 0.8 | 0.5 |
| Al2O3 | 1.50 | FS | 8.7888 | 7.3999 | 1.0589 | 0.22 | 1.9 | 1.6 |
| Fe2O3 | 0.03 | MS | 14.9599 | 13.2740 | 1.4582 | 74.47 | 1114.1 | 988.5 |
| MgO | 2.99 | NS2 | 13.4194 | 11.6862 | 1.4335 | 284.99 | 3824.4 | 3330.4 |
| CaO | 9.47 | NC3S6 | 14.0278 | 12.6137 | 1.3301 | 332.51 | 4664.4 | 4194.2 |
| Na2O | 13.96 | NAS6 | 14.7131 | 13.2234 | 1.2358 | 65.46 | 963.2 | 865.6 |
| K2O | 0.21 | KAS6 | 14.0258 | 12.5775 | 1.3755 | 12.41 | 174.1 | 156.1 |
| Sum | 100.00 | S | 15.0023 | 13.6179 | 1.4347 | 229.75 | 3446.9 | 3128.8 |
| Sum | H°GLASS | H1300,MELT | ||||||
| 1000.00 | 14 189.7 | 12 665.9 | ||||||
| ΔH1300 |