Method of cooling molten glass

A method of cooling molten glass, which comprises slowly cooling the molten glass while the average temperature of the molten glass is within a certain range extending below and above the temperature at which the speed of absorbing bubbles is highest, and cooling it rapidly when the average temperature of the molten glass is outside said range.

This invention relates to a method of cooling molten glass, and to a glass 
melting furnace suitable for carrying out the cooling method. 
Conventional glass furnaces have a considerably large cooling tank which is 
about one-half of the melting tank because they are adapted to 
spontaneously cool the molten glass mainly by the dissipation of heat from 
the outer surface of the cooling tank. The cooling time is therefore long, 
and much heat is dissipated wastefully from the surface of the cooling 
tank. Furthermore, these glass melting furnaces have not employed any 
structure which is intended for effective utilization of the heat 
dissipated from the cooling tank. Furthermore, the large size of the 
cooling tank adds to the cost of equipment, and also causes the 
inconvenience that much time is required for changing the composition of 
molten glass in the furnace. 
It is an object of this invention to provide a method of cooling molten 
glass efficiently. 
Another object of this invention is to provide a method of cooling molten 
glass which enables shortening of the cooling time. 
Still another object of this invention is to provide a method of cooling 
molten glass, in which bubbles remaining in the molten glass can be 
effectively absorbed in the glass. 
Yet another object of this invention is to provide a method of cooling 
molten glass which enables reutilization of dissipated heat. 
A further object of this invention is to provide a method of cooling molten 
glass efficiently in which a cooling tank of a smaller size can be used. 
An additional object of this invention is to provide a glass melting 
furnace equipped with a cooling tank suitable for the practice of the 
method of this invention. 
Other objects and advantages of this invention will become apparent from 
the following description. 
According to this invention, the above objects and advantages are achieved 
by a method of cooling molten glass, which comprises slowly cooling the 
molten glass while the average temperature of the molten glass is within a 
certain range extending below and above the temperature at which the speed 
of absorbing bubbles by the molten glass is highest, and cooling it 
rapidly when the average temperature of the molten galss is outside said 
range.

The method of this invention is based on the new discovery that the 
temperature at which the bubble absorbing speed of the molten glass is 
highest appears very distinctly although it varies depending upon the 
types of the glass composition and the type of gas in the bubbles. For 
example, the highest speed of absorbing SO.sub.3 bubbles in soda lime 
silicate glasses is achieved at about 1390.degree. C. With borosilicate 
glasses, the maximum speed of absorbing SO.sub.3 bubbles is achieved at 
about 1400.degree. C. 
According to the method of this invention, the molten glass is slowly 
cooled when the average temperature of the molten glass is within a 
certain range extending below and above the temperature at which the speed 
of bubble absorption by the molten glass is highest, for example within a 
range of .+-.20.degree. C. of the temperature at which the speed of bubble 
absorption is highest. As a result, the molten glass rapidly absorbs the 
bubbles. When the molten glass is at a temperature outside the aforesaid 
range, it is rapidly cooled to expedite cooling. 
Desirably, the slow cooling of the molten glass is carried out at a cooling 
rate of not more than about 2.degree. C./min. The rapid cooling of the 
molten glass can be carried out at a cooling rate of at least about 
3.degree. C./min. 
The method of this invention can be conveniently carried out by a glass 
melting furnace provided by this invention, which comprises a melting tank 
for metling a glass batch, a cooling tank located downstream of the 
melting tank for cooling the molten glass, and a drawing section located 
further downstream of the cooling tank for drawing the molten glass, 
wherein within the cooling tank, at least a first rapid cooling zone, a 
slow cooling zone and a second rapid cooling zone in which the cooling 
rates are different are provided in this sequence from the upstream side, 
the temperature of the molten glass is controlled so that it is within a 
range extending below and above the temperature at which the speed of 
bubble absorption is highest, and the molten glass is slowly cooled while 
its average temperature is within said range. 
Accordingly, the method of this invention can be carried out, for example, 
by using a melting tank for melting a glass batch, a cooling tank located 
downstream of the melting tank for cooling the molten glass, and a drawing 
section located downstream of the cooling tank for drawing the molten 
glass. The method comprises providing within the cooling tank at least a 
first rapid cooling zone, a slow cooling zone and a second rapid cooling 
zone having different cooling rates in this sequence from the upstream 
side, controlling the temperature of the molten glass so that in the slow 
cooling zone, the temperature of the molten glass falls within the range 
extending below and above the temperature at which the speed of bubble 
absorption is highest, and slowly cooling the molten glass while its 
average temperature is within said range. 
Preferably, in the slow cooling zone, the average temperature of the molten 
glass is controlled so as to fall within the range of .+-.20.degree. C. of 
the temperature at which the speed of bubble absorption by the molten 
glass is highest, and the molten glass is slowly cooled while the average 
temperature of the molten glass is within this range. The molten glass is 
slowly cooled at a cooling rate of, for example, not more than 2.degree. 
C./min. in the slow cooling zone. In the first and second rapid cooling 
zones, it is rapidly cooled at a cooling rate of, for example, at least 
3.degree. C./min. 
One preferred embodiment of this invention will be described below with 
reference to the accompanying drawings. 
FIGS. 1 to 3 show one embodiment of a glass melting furnace 11 in 
accordance with this invention. FIG. 1 is a top plan view, FIG. 2 is a 
vertical sectional view, and FIG. 3 is a sectional view taken along line 
A--A of FIG. 2. 
The glass melting furnace 11 includes a melting tank 12 having a glass 
batch feeding opening 13. Regenerators 14 and 14' are provided on both 
sides of the melting tank 12, and each of the regenerators is connected to 
the melting tank by a plurality of blowoff openings 15. Flames generated 
by the burning of a heavy oil are directed to the glass batch through the 
blowoff openings 15. The glass batch is thus melted, and the molten glass 
16 is accumulated in the melting tank. The molten glass is subjected to 
heat at a temperature of at least 1500.degree. C. for more than 30 
minutes, and flows into a cooling tank 17 while it is at a temperature of 
1500.degree. to 1550.degree. C. 
The cooling tank 17 is shorter in width by about 40 to 60% than the melting 
tank 12, and has a depth of about 0.2 to 0.4 m. Its overall size is 
smaller than that of a conventional cooling tank. The amount of the molten 
glass contained in it is 1/5 to 1/6 of that in the conventional cooling 
tank. The inner space of the cooling tank 17 is divided into three zones 
17a, 17b and 17c. One end of the cooling tank 17 leads to the melting tank 
12, and the other end forms a drawing port 18. A partition 19 is disposed 
between zones 17a, 17b and 17c. 
The zones 17a and 17c are forced cooling zones, and the zone 17b is a 
spontaneous cooling zone. Accordingly, the molten glass is rapidly cooled 
in the zones 17a and 17c, and slowly cooled in the zone 17b. As forced 
cooling means for the zones 17a and 17c, a number of radiation cooling 
devices 20 are provided at the ceiling portion as shown in FIG. 3. Each of 
the radiation cooling devices 20 blows air into the cooling tank 17 
through a hole 20a. This air makes the inner surface of the devices colder 
than the molten glass, and thus the molten glass 16 is cooled by radiation 
and conduction. As shown in FIG. 3, the cooling tank 17 is constructed by 
attaching thermally insulative members 22 and 22' to the surface of the 
entire structure formed of bricks 21 so as to insulate the outside surface 
of the cooling tank by the thermally insulative members 22 and 22' and 
minimize heat dissipation from the cooling tank. 
The temperature of a molten soda lime silicate-type glass, for example, is 
preferably prescribed at its initial temperature of 1420.degree. C. in the 
zone 17a, at 1420.degree. to 1370.degree. C. in the zone 17b, and at less 
than 1370.degree. C. in the zone 17c. Preferably, the cooling rate is set 
at not less than 3.degree. C./min. in the zone 17a, not more than 
2.degree. C./min. in the zone 17b, and at least 3.degree. C./min. in the 
zone 17c. 
The reasons for setting the temperature range and the slow cooling rate as 
above will be stated below with reference to FIGS. 4 and 5. FIG. 4 shows 
changes in the number of bubbles in the glass at different cooling rates, 
the ordinate showing the number of remaining bubbles by ratios. In FIG. 4, 
the cooling rate is 2.degree. C./min. for curve a, 3.degree. C./min. for 
curve b, 4.degree. C./min. for curve c, and 6.degree. C./min. for curve d. 
FIG. 5 shows the relation between the temperature and the speed at which 
the bubbles decrease. In FIG. 5, the slow cooling rate is 2.degree. 
C./min. The glass used in obtaining these characteristics was of the soda 
lime silicate type and had the composition comprising 70.91% by weight of 
SiO.sub.2, 1.69% by weight of Al.sub.2 O.sub.3, 0.071% by weight of 
Fe.sub.2 O.sub.3, 8.76% by weight of CaO, 3.29% by weight of MgO, 13.45% 
by weight of Na.sub.2 O, 0.82% by weight of K.sub.2 O, and 0.29% by weight 
of SO.sub.3. 
As is clear from FIGS. 4 and 5, the speed of bubble absorption becomes 
highest in a temperature range around 1390.degree. C. If the molten glass 
is slowly cooled at a cooling rate of not more than 2.degree. C./min. 
within this temperature range, the efficiency of decreasing the bubbles 
becomes highest, and the number of bubbles in the molten glass decreases 
abruptly. Thus, according to the process of bubble decreasing in the 
molten glass during cooling, it is possible to slowly cool the glass at a 
cooling rate of not more than 2.degree. C./min. (preferably not more than 
1.degree. C./min.) only within the temperature range around 1390.degree. 
C. thereby decreasing the bubbles most effectively, and to rapidly cool it 
by forced cooling at a rate of, for example, at least 3.degree. C./min. at 
temperatures outside this range thereby accelerating cooling of the glass. 
The presence of a peak in bubble decreasing around 1390.degree. C. is 
believed to be because the remaining bubbles are SO.sub.3 bubbles and 
these bubbles rapidly dissolve in the molten glass at about 1390.degree. 
C. 
Since the cooling tank 17 is comprised of the first rapid cooling, slow 
cooling and second rapid cooling zones, it is no longer necessary to build 
it in a large size. In the zones 17a and 17c, the heat of the molten glass 
16 is absorbed by the air blown into these zones by the radiation cooling 
devices 20, and the hot air can be taken out of the tank to utilize it 
effectively. The depth of the cooling tank 17 is small as shown 
hereinabove. The provision of the shallow cooling tank makes it possible 
to inhibit the backflow of the molten glass from the drawing side of the 
cooling tank to the melting tank side, and to adjust the temperature 
difference in the direction of the tank width. This can also obviate the 
use of energy required for re-heating of the cooled molten glass that has 
been caused to flow back. 
The number of the zones in the cooling tank 17 may be more than three 
because the temperature of the absorption peak differs depending upon the 
type of the bubbles. The forced cooling means may also include water 
cooling performed by disposing a water-cooling tube within the molten 
glass. In this case, the partitions 19 are not necessary. 
FIG. 6 shows a top plan view of a conventional continuous glass melting 
furnace for producing a sheet glass, and FIG. 7 shows its side elevation 
in vertical section. A glass melting furnace 1 has a melting tank 2 and a 
cooling tank 3, and a neck 4 is provided between the two tanks. A batch 
feeding opening 5 is formed at one end portion of the melting tank 2, and 
regenerators 6 are disposed on both sides of the tank 2, A glass batch is 
fed continuously from the feed opening 5. Flames generated by burning a 
heavy oil are directed through blowoff openings 7 on both side walls of 
the melting tank 2, and the inside of the melting tank 2 is kept at about 
1600.degree. C. At this temperature, the glass batch within the melting 
tank 2 is melted. 
The glass which has been melted, homogenized and defoamed in the melting 
tank 2 flows out into the cooling tank 3 via the neck 4. In the cooling 
tank 3, the molten glass is cooled, and at the same time, fine bubbles 
remaining in the batch are dissolved and absorbed in the molten glass 
through the cooling process. This absorbing action decreases bubbles in 
the glass and increases the quality of the final product. The glass cooled 
in the cooling tank 3 is drawn from a drawing port 8. 
As shown in the drawings, the cooling tank 3 is considerably large and 
measures about one-half of the melting tank 2 in the conventional glass 
melting furnace, and because of this, the conventional cooling tank has 
the defects described at the beginning of this specification. 
As is clear from the foregoing description, according to the present 
invention, the molten glass is slowly cooled at a predetermined speed only 
when its temperature is within a specified range. It can be rapidly cooled 
outside this range. Hence, the cooling time can be shortened, and the 
cooling step can be controlled. 
Since in the present invention, forced cooling zones are provided in 
cooling the molten glass, the entire cooling tank can be made small-sized 
and shallow. This structure permits a drastic reduction in the loss of 
energy and a reduction in the cost of installing the cooling tank. 
The waste heat generated by the forced cooling means for the molten glass 
can be effectively utilized by taking it outside. Furthermore, since 
thermally insulative members are provided about the outer circumferential 
surface of the cooling tank, the loss of energy can be reduced further and 
the heat can be utilized effectively. 
Since the cooling tank can be built in a small size, the amount of the 
molten glass contained in it can be decreased. Accordingly, the 
composition of the molten glass can be rapidly and easily changed, and the 
temperature distribution of the cooling tank can be easily preset. Thus, 
the glass melting furnace is easy to control. 
The above embodiment is given with regard to SO.sub.3 bubbles in the soda 
lime silicate glass. The present inventors have found that a peak of 
SO.sub.3 bubble absorption is at about 1400.degree. C. in the case of a 
borosilicate glass. It should be understood therefore that the temperature 
of a peak of bubble absorption naturally varies depending upon the 
composition of the glass and the kind of bubbles (for example, O.sub.2 
bubbles or H.sub.2 O bubbles). 
The cooling method of this invention can be applied not only to the 
continuous glass melting furnace, but also to other glass melting methods, 
for example to the melting of glass in a crucible.