Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 78

In principle, the melting compartment (the tank) is a shallow basin whose typical dimensions (length L × width W × depth D) are 10 × 6 × 1 m³ for medium‐size container‐glass furnaces and 30 × 10 × 1.4 m³ for float‐glass furnaces. The required energy is delivered in a combustion space right above the melting compartment and transferred to the melt chiefly by black‐body radiation. Additional energy (5–20%) is delivered by direct electrical heating (boosting) of the melt.

The very melting process, i.e. steps M1–M3, takes place in one single box‐shaped compartment as sketched in Figure 2a, b. The sketches illustrate some essential features only (see Chapter 9.7 for details). Spatially, the individual process steps are separated – not in a rigorous but effective way – by two convective vortices. In furnaces with transversal flame direction (so‐called side port fired furnaces; flow pattern Figure 2a), these are mainly generated thermally by an appropriate distribution of fuel input to a series of burners arranged along the L axis above the melt. This results in two well‐developed vortices with a well‐localized hot spot at the position of maximum energy input. In furnaces with longitudinal flame direction (end port fired furnaces; flow pattern Figure 2b), the energy input via combustion along the L axis cannot be controlled at the same distinction as in a side port fired furnace. Here, the flow pattern is typically dominated by a single predominant vortex; the hot spot is shifted toward the end of the furnace. Local stabilization of the vortices is achieved by electrical heating from below, and – for the second type of furnaces – also mechanically by the action of air bubblers or by implementation of a solid barrier (a wall) at the bottom of the tank. As seen from Figure 2, the batch floats on the surface of the molten phase and melts continuously in the L direction, thereby typically covering an area whose length is about L/3, or more than 2L/3 for side and end port firing, respectively. Processes M2 (sand dissolution) and M3 (fining) then take place in the first and second vortices, respectively. Note that the first vortex conveys very hot melt right underneath the batch, helping to satisfy the very high intrinsic energy demand of melting. As for the second, it transports a very hot (hence low‐viscosity) melt along the surface area, thereby facilitating the escape of rising bubbles. This effect is especially pronounced in the pattern shown in Figure 2a while in Figure 2b, a large portion of the melt does not reach the surface at all.

Table 4 Scheme for final batch adjustment with sodium sulphate set to 4 kg/t glass for a targeted redox number R of −24.

Raw material i	mI(i)a	mII(i)b	mIII(i)c	R(i)	R(i)·mIII(i)
	kg/t glass	kg/t glass	kg/2000 kg sand
Sand	674.28	674.28	2000.00
Feldspar	44.02	44.02	130.57
Calumite	44.02	44.02	130.57	−0.073	−9.53
Dolomite	123.39	123.39	365.99
Limestone	70.61	70.61	209.44
Soda ash	231.99	229.01	679.26
Sulfate		4.00	11.86	0.67	7.95
Carbon		1.13	3.35	−6.70	−22.46
Redox number R = ∑ R i ·m III (i)	−24.04

∑ R_i·m_III(i) matches the target value R = −24.

^a Batch composition as calculated in Table 2.

^b Batch composition with 4 kg of sulfate added, soda ash reduced accordingly.

^c Batch composition normalized to 2000 kg of sand; amount of carbon varied until the sum.

Figure 2 Convection cells (vortices) in the melting tank of a glass furnace (vertical projection): (a) float glass furnace (side port firing), transit to the refining zone indicated by the dotted vertical line on the right‐hand side; (b) end port fired container glass furnace, transit to the refiner through a narrow opening at the lower right (the “throat”); D = depth of the tank.

At a given pull rate p in t/h, the nominal overall dwell time of the melt in the furnace is

(1)

where ρ is the density of the melt and L·W·D the volume of the melting tank. Depending on the size of the furnace and the targeted glass quality (in terms of residual bubbles), τ_NOM ranges from 20 to 40 hours. During this time, the average volume element circles 2–6 times in vortex 1, and about twice in vortex 2. For a detailed analysis of the role of the flow pattern on melting and fining, see [3, 4]. The process of refining (M4) already starts at the descent of vortex 2; it is completed in a subsequent compartment termed refiner, which is thermally separated from the melter. Thermal separation is accomplished either with a vertical wall leaving an opening of about 0.5 × 0.3 m² cross section (the throat) at half width right above the bottom of container‐glass furnaces (Chapter 1.5), or by an area of moderately narrowed width (the waist) in float glass furnaces (Chapter 1.4). For the sake of glass quality (i.e. homogeneity), it is mandatory to keep the position of the hot spot constant at any pull rate.