Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 98

The float bath contains several metric tons of molten glass. It is a large unit with a length of up to more than 50 m enclosed by a steel shell that is lined with thick insulating and nonreactive refractory materials and holds a pool of molten tin whose depth is 50–100 mm and total amount is up to more than 200 metric tons kept at temperatures decreasing from about 1000 to 600 °C from the hot to the cold end (Figure 9). A reducing gas mixture made up of 2–8% hydrogen and 98–92% nitrogen is supplied at a high rate of the order of 10³ m³/h from above to the bath to prevent oxidation of the molten tin and to maintain a positive pressure difference with the atmosphere at the bath exit where leakages are highest. The heaters, coolers, and other devices are installed and inserted in the bath. The molten glass is continuously supplied from the furnace conditioner via a canal where its flow rate is precisely controlled by an adjustable gate called a tweel. It arrives to a ceramic spout lip, which is an inlet of the float bath, through which it falls freely onto the molten tin. After many years of struggle at Pilkington Brothers to achieve excellent quality, the design and engineering of the inlet area were an outstanding invention to force the contaminated molten glass in contact with the refractory lip to flow outwardly so as to be brought forward at the outer edges of the ribbon [9].

Table 2 List of symbols regarding equilibrium thickness mechanism.

Symbol	Denotation
T e	Equilibrium thickness
ρ t	Density of molten tin
ρ g	Density of molten glass
S ga	Surface tension at glass–atmosphere interface
S gt	Surface tension at glass–molten tin interface
S ta	Surface tension at tin–atmosphere interface
g	Gravitational constant

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).