Encyclopedia of Glass Science, Technology, History, and Culture. Группа авторов
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Figure 8 Equilibrium thickness of floating glass on the molten tin when the gravitational forces and surface tensions are balanced [3].
Once poured onto the tin bath with a thickness of about 50 mm, the glass spreads out and thins to its equilibrium thickness in the upstream area in the float bath. As formed to the required thickness and width in the forming area (see Section 5.3), the glass ribbon is taken out from the bath either to receive appropriate reflective, low‐emissivity, solar‐control, self‐cleaning, or other specific coatings (Chapters 6.7 and 6.8) or to enter directly the annealing lehr at the temperature at which the viscosity is about 1010 Pa·s (i.e. about 600 °C for soda‐lime silicate). At the end of the lehr, whose length can reach 120 m, the ribbon is finally cooled down to room temperature and brought into the cutting area. Whereas both edges are cut out (to be recycled as cullet) because of the imprint left by the top rolls, the ribbon itself is cut either according to customers' specifications or as standard sheets, for instance, 6.0 × 3.21 m in Europe where tools used in the flat‐glass transportation industry have been fitted to this size (which, by the way, is too large to allow flat glass to be shipped in containers).
5.3 Thinner (Top‐Roll Process) and Thicker (Fender Process) Glass Ribbons
For forming thin sheets, the molten glass with its initial equilibrium thickness in the upstream area in the bath is subjected at the same time to longitudinal and lateral forces. The former are exerted by conveyor rolls that stretch the ribbon from the annealing lehr and pull it at a typical speed of up to 25 m per minute. The latter are exerted outwardly on the ribbon edges by pairs of top rolls, which are water‐cooled rotating gears, to reduce the narrowing of the glass ribbon because the imposed longitudinal stretching reduces not only its thickness but also its width (Figure 10a). In parallel, the glass ribbon is cooled down to prevent it from returning to its equilibrium thickness until its width is constant at the end of the forming area. In view of the fundamental influence of viscosity within the glass ribbon upon stretching and thinning, the temperature distribution and the top‐roll operations must be controlled very tightly to ensure a good forming quality. Besides, keeping the glass ribbon as wide as possible is important to maximize productivity.
For producing float glass thicker than the equilibrium thickness, a pair of water‐cooled carbon fenders serves as slipping guides to the flowing glass in the bath (Figure 10b). The glass thus proceeds with a restricted width and a large thickness. As it passes down the fender area, the effects of gravitational forces and surface tensions make both its upper and lower surfaces flat and thickness uniform. The glass is then cooled to an appropriate temperature in the downstream area of the fender where its viscosity is high enough not to allow width changes. In contrast to what is taking place in the top‐roll process, stretching is not significant at all and there is no drive to return to the equilibrium thickness because there is no glass–tin–atmosphere interface in the fender area.
Figure 9 Sketch of the tin bath part of the float process: (a) on vertical plane along centerline; (b) on horizontal plane. A reducing nitrogen–hydrogen gas mixture is supplied from above. Heaters and coolers are installed [10].
Figure 10 Sketch of the float process: (a) for sheets thinner than the equilibrium thickness, where the glass is stretched, from its edges by top rolls and from its downstream part by conveyor rolls; (b) for sheets thicker than the equilibrium thickness, where water‐cooled carbon fenders serve as slipping guides to glass flowing [7].
5.4 A Complex Industrial Problem
In spite of the simplicity of its principles, the float process is not readily implemented because flow in both tin and glass and heat transfer among tin, glass, and the radiative field are really complex processes. Glass forming is mainly determined by parameters such as the glass flow rate, conveyor speed, rotating speed and angle of top rolls, and by the viscosity distribution within the glass ribbon. It goes without saying that the viscosity of the glass strongly depends on temperature, but the temperature distribution in the bath is itself influenced by radiative heat transfer, the flow of molten tin, and the glass forming conditions. Radiative heat transfer is predominant in the bath at temperatures higher than 600 °C, but the flowing molten tin also contributes markedly to heat transfer as a result of its high heat capacity, high thermal conductivity (about 50 times higher than that of the glass), and low kinematic viscosity (about 8 times lower than that of water), whereas the glass flow carries a large amount of convective heat. On the other hand, molten tin flows by traction from the glass ribbon (i.e. velocity distribution) and buoyancy convection (i.e. temperature distribution in the bath). In order to understand forming conditions, one thus needs to understand the whole set of processes taking place in the bath since glass forming, heat transfer in the bath, and flow of molten tin affect one another in a very complex manner.
As summarized by C.K. Edge [4], the float bath thus is “a remarkable entity which, although first envisioned as a finisher of glass surfaces, also functions as a container, a conveyor, a forming unit, a chemical reactor and a heat exchanger.” In view of this complexity, valuable information has been drawn from mathematical simulations not only of the glass forming mechanisms, but also of the temperature field and the mutually related dynamics of the molten tin and glass ribbon. As examples, calculations with finite‐element methods of the thickness contour over the glass ribbon and of the lateral thickness distribution of the ribbon at the bath exit are shown in Figures 11 and 12 [10]. The rather good agreement of such model values with the temperatures,