WO 2012/137161 PCT/IB2012/051686
t
GLASS FURNACE, IN PARTICULAR FOR CLEAR OR ULTRA-CLEAR
GLASS, WITH LATERAL SECONDARY RECIRCULATIONS
5 The invention•relates to a double recirculation current
glass furnace for heating, melting and fining materials
to be vitrified, this furnace being of the type of
those that comprise:
- an entrance for raw materials;
10 - a superstructure equipped with heating means;
- a tank containing a melt of molten glass on which a
"" '"blanket' of raw'materials floats from the entrance as
far as a certain distance, into the furnace; and
- an exit via which molten glass is removed.
15
The invention more particularly, but not exclusively,
relates to a furnace for clear or ultra-clear glass.
With reference to the schematic in figure 1 of the
20 appended drawings, a conventional float glass furnace
may be seen with an entrance E for raw materials, a
superstructure . R equipped with burners G, a tank M the
. bottom S of which supports a melt N of molten glass on
which a blanket T of raw materials floats from the
2 5 entrance, and an exit Y. Above the furnace, the
variation in the. temperature of the hot side of the
crown Tcrown of the superstructure R, along the length of
the furnace, is plotted on the y-axis in figure 1, and
is represented by the curve 1 the maximum of which is
30 located in the central zone I of the tank.
Two recirculation loops Bl, B2 of pool of glass, form in
the melt between a hotter central zone I of the furnace
and the entrance E and exit Y, respectively, which are
35 at a lower temperature. In figure 1, the recirculation
in the primary loop Bl takes place in the anticlockwise
direction: glass at the surface flows from the zone I
toward the entrance E, descends toward the bottom and
returns in the bottom part of the melt toward the
WO 2012/137161 . - 2 - PCT/IB2012/051686
central zone I before rising toward the surface. The
recirculation in the secondary loop B2 takes place in
the opposite direction, i.e. in the clockwise
direction. These two recirculation loops have an
5 influence on the principal flow of glass pulled from
the furnace. They modify the shape and the duration of
the travel of the principal flow depending on their
strength.
10 The shortest path the main flow can take, corresponding
to the shortest dwell time, which is critical to the
quality of the"" glass extracted" from the furnace, is
schematically shown by the dotted line 2, according to
which glass, near the entrance, moves to near the
15 bottom S, then rises along a relatively sinuous path 3
between the two recirculation loops in order then to
move along a trajectory 4, in the vicinity of the top
level of the melt, toward the exit Y. The upward
trajectory 3 corresponds to a central spring zone RC
20 comprised between the two loops Bl, B2 and their spring
zones Rl and R2. The turning point of the flow of glass
at the surface of the melt marks the point of
separation of the spring zones Rl and RC at the
surface. The distance between the entrance of the
25 furnace and this turning point defines the length C
shown in figure 1, which length is representative of
the extent of the loop Bl. It may be determined
experimentally or by numerical simulation. The fining
quality of the glass is determined ' by the initial
30 portion of the trajectory 4. In this initial portion,
the glass is kept at a temperature above the fining
temperature (about 1450°C for soda-lime glass) for a
certain length of time. The dwell time in the initial
portion of the trajectory 4 therefore determines the
35 quality of the glass produced. This dwell time is given
by the length L of the zone that is at a temperature of
above about 1450°C, for soda-lime glass, and by the
flow velocity of the glass. This•glass flow velocity is
WO 2012/137161 - 3 - PCT/IB2012/051686
related to the pull rate obtained at the exit of the
furnace and to the strength of the recirculation B2.
It is thus a target to maximize the "fining" dwell time
5 in order to improve the quality of the glass, or to
increase the pull rate of the furnace for a given
quality. The dwell time may be increased by slowing
down the secondary recirculation, thereby also allowing
furnace consumption to be decreased. Thus, a
10 restriction in furnace width, called a waist 5a, has,
for a number of years, been added to float glass
furnaces. In addition, use may bei™ made," in ""'this' waist
5a, of a water-cooled barrier 5b, which further slows
down the recirculation. Moreover, this recirculation
15 loop is essential for creating the spring zone in the
center of the tank on interaction with the first loop.
Cooling in the waist and in the working end ensures the
operation of the secondary loop by decreasing the
temperature of the glass.
20
With reference to the schematic in figure 2 of the
appended drawings, a schematic top view of the
conventional furnace shown in figure 1 may be seen.
25 In figure 2, the flow of glass at the. surface is shown
by parallel horizontal arrows 6a, 6b, 6c, 6d, 6e, 6f
that terminate on a continuous line 10a, 10b, 10c, lOd,
lOe, lOf. The length of the arrows 6a - 6f represents
the flow velocity. The position of the continuous lines
30 10a - lOf is representative of the direction of flow of
the glass: the glass flows from that end of the arrows
6a - 6f not making contact with the continuous line 10a
- lOf toward the other end that makes contact with the
line 10a - lOf. The flow of glass near the bottom of
35 the melting tank 9.1, for the loop B2, is shown by the
arrows 7a and 7b. The conventional zones used to cool
the glass, 8a and 8b in the waist and 8c in the working
end 9.2, are also shown in this figure.
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The arrows 6a show glass flowing at the surface toward
the entrance of the furnace, this flow being related to
the primary recirculation current. The arrows 6b show
5 glass flowing at the surface toward the exit of the
furnace, this flow being related to the secondary
recirculation current. The spring zone RC is located
between the two.
10 As the arrows 6b show, the velocity at which glass
moves at the surface is higher at the center of the
"furnace" and graduallydecreases toward Tihe edges of the
furnace. '
15 As the arrows 6c show, this effect progressively
increases as the waist 5a is approached. Thus, the
narrowing of the melting tank causes concentration of
the surface flow of the secondary loop before it enters
into the waist in. the center of said tank. Increasing
20 velocity in this zone decreases fining time.
As the arrows 7a and 7b show, the return flow of glass
. along the bottom of the melting tank is not at all
uniform over the width of the melting tank. In the
25 vicinity of the waist, in the corners of the tank,
there are therefore two "dead" zones 11 where the flow
of glass is very limited.
The aim of the invention, above all, is to provide a
30 double recirculation loop glass furnace that does not
have, or has to a lesser extent, the drawbacks recalled
above and that, in particular, allows a high fining
quality to be obtained, not only for ultra-clear glass
but also for clear and ordinary glass.
35
According to the invention, the glass furnace for
heating and melting materials to be vitrified,
especially, but not exclusively, comprises:
WO 2012/137161 - 5 - PCT/IB2012/051686
- an entrance E for raw materials;
- a superstructure R eguipped with heating means G;
- a tank M containing a melt of molten glass on which a
blanket . T of raw materials floats from the entrance as
5 far as a certain distance into the furnace; and
- an exit Y. via which molten glass is removed,
- two molten glass recirculation loops Bl, B2 forming
in the melt N between a hotter central zone I of the
furnace and the entrance and exit, respectively, which
10 are at a lower temperature;
and is characterized in that it comprises means for
cooling the ~~Tgl'ass","'"" 'which"''"'"mearis~'''a-re'""""Tocated in the
vicinity of the lateral sides of the furnace on either
side and upstream of a restriction, such as a waist, a
15 channel, or a overflow, so as to create or increase
lateral secondary recirculation currents of glass in
order to decrease the strength of the central secondary
loop.
20 Localized lateral cooling of the glass according to the
invention leads to a decrease in the temperature of the
glass and therefore an increase in its density. The
heavier glass descends toward the bottom then flows
toward the hotter central zone I of the furnace.
25
Preferably, the means for cooling the glass are located
in the vicinity of the entrance of the waist, in
particular in the corners of the tank.
30 Advantageously, the means for cooling the glass are
located near . the surface of the melt. They are
especially overhead coolers placed above the glass
melt, or coolers submerged in the melt and especially
cooled with water. .
35
In order to establish a spring zone in the center of
the furnace, the two recirculation loops must possess a
comparable driving force. This driving force is created
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on the one hand by . energy consumption by the bottom
side of the batch blanket. On the other hand, cooling
in the waist and working end combined creates the
driving force of the' secondary loop. According to the
5 invention, lateral secondary glass recirculation
currents contribute to the driving force of the
secondary loop.
According to the invention, conventional cooling is
10 partially or completely replaced by lateral cooling
before the entrance of the waist. Completely replacing
conventional cooling with lateral cooling is especially
advantageous for waist or overflow type furnaces with a
weak or absent return 7b of cold glass. Two lateral
15 loops B2La and B2Lb are created in this way, which
loops reinforce the driving force of the secondary
recirculation current B2. This reinforcement allows the
strength of the central loop B2C to be decreased and
thus the surface, flow velocity in the central zone,
20 before the entrance of the waist, to be decreased. This
results in an increase in the dwell time of the glass
.in the fining zone, and therefore a better fining
quality for the glass.
25 For an equivalent glass fining quality, this solution
allows the size of the working end 9.2 to be reduced,
this reduction being related to the decrease in cooling
required in the working end, or the pull rate from the
furnace to be increased.
30
The invention also allows the glass flow velocity to be
decreased at. the corners of the entrance of the waist,
thereby limiting the risk that • these corners will be
corroded.
35 . •
The invention consists, apart from the arrangements
described above, in a certain number of other
arrangements that will be discussed more explicitly
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below with regard to a completely nonlimiting
embodiment described with reference to the appended
drawings. In these drawings:
5 figure 1 is a schematic vertical cross section through
a conventional float glass furnace;
figure 2 is a schematic top view of the float glass
furnace in figure 1; and
10
figure 3 is a schematic top view, similar to that in
figure 2, of a "float glrass furnace according to the
• invention.
15 As figure 3 shows, the invention allows the position of
the spring zone to be maintained despite a decrease of
the central secondary recirculation B2C. This results
in a better distribution in the flow velocity of the
glass before the waist.
20
As the arrows 7a, 7b in figure 3 show, the existence of
the lateral loops B2La, B2Lb results in a flow of glass
along the bottom that is more uniform over the width of
the tank and in particular toward the edges 11 of the
2 5 furnace.
To obtain a notable lateral cooling effect, the heat
flux evacuated by the lateral coolers must be at least
5% of the flux consumed to melt the blanket of raw
30 materials. The energy required to melt the blanket is
in part delivered to the top surface of the blanket, by
radiation from the combustion chamber, and. in part to
the bottom side of the blanket, by convection from the
recirculation loop Bl. The contribution of each of
35 these two supplies of energy to melting the blanket
varies depending on the furnace design. It is typically
about 50/50%. To obtain a notable lateral cooling
effect, the energy flux evacuated by this lateral
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cooling must be at least 10% of the flux to the bottom
side of the blanket.
Operation of float 'glass furnaces, generally called
5 float furnaces, requires the exit of the furnace to be
kept at a constant temperature, typically 1100.°C. The
cooling in the waist and working end is adjusted to
maintain this temperature. The pull of the glass in
combination with the central recirculation of the loop
10 B2C constitutes the supply of heat to the working end.
As figure 3 shows, adding lateral cooling"" means 12a,
12b located in the vicinity of the lateral sides 13a,
13b of the furnace, on either side and upstream of the
15 waist, allows the cooling required in the waist, and
above all in the working end 9.2, to be reduced. The
cooling means 12a, 12b are preferably located in the
vicinity of the entrance of the waist, in particular in
the corners of the tank. The lateral cooling means 12a,
20 12b make it possible to create or strengthen the
lateral recirculation currents or loops B2La, B2Lb, in
which recirculation of molten glass takes place in the
same direction as for the central secondary loop B2C.
Implementing the invention makes it possible to
25 decrease the central recirculation strength of the loop
B2, for example by changing the depth of the barrier 5b
or the cross section of the waist. The temperature of
the glass at the exit of the furnace is maintained in
this way. Decreasing the cooling in the waist and in
30 the working end, and slowing down the central secondary
recirculation B2C are thus two associated actions. They
especially make it possible to increase the dwell time,
of the glass for fining and also for refining, for the
resorption of residual bubbles.
35
According to one embodiment of the invention, for a
float furnace with a small capacity of 200 tonnes of
soda-lime glass per day, with a raw material containing
WO 2012/137161 • - 9 - PCT/IB2012/051686
20% cullet requiring 5 MW of power to melt the batch,
the lateral cooling evacuates a power of 2 x 130 kW.
Reducing the central recirculation loop B2C leads to an
increase in the fining dwell time of 20%. For an
5 equivalent fining time, implementing the lateral
cooling according to the invention allows the pull rate
of glass from the furnace to be increased.
For a float furnace, the lateral recirculation currents
10 B2La and B2Lb make it possible to envision omitting the
fraction of the secondary recirculation in the waist
and "in the^working "end. "Nevertheless, the complete
suppression recirculation in the waist and in the
working end would prevent glass contaminated by the
15 walls from returning into the fining part of the
furnace. Depending on the quality of the glass required
and the refractory materials used, it may be
advantageous to maintain. a residual recirculation in
the waist and in the working end. The barrier device 5b
20 with its variable depth allows this recirculation to be
- easily adjusted. .
The absence of combustion at the end of the melting
tank in standard float furnaces and losses via the
25 walls create a certain lateral cooling of the glass at
the end of the melting tank before the waist, but the
energy evacuated in this way is substantially lower
than 5% of the flux consumed to melt the blanket of raw
materials. Increasing losses to the walls of the tank
30 via the glass allows an improvement to be obtained but
it remains very difficult to obtain enough losses to
activate or strengthen the lateral secondary
recirculation currents via the walls of the tank alone.
35 According to one embodiment of the invention, the
cooling devices 12a, 12b allowing the lateral secondary
recirculation currents to be created are overhead
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coolers. Such coolers may easily be introduced and
removed from the furnace.
The surface of the nielt may be cooled by an overhead
5 cooler via radiative heat exchange between the hot
surface of the melt•and the cold surface of the cooler.
It may also be cooled by convection, for example in the
case where the cooler ejects air onto a target area of
the melt. The temperature and the flow velocity of the
10 blown air are chosen . in order, to avoid any
devitrification risk.
In another embodiment of the invention, the cooling
devices 12a, 12b allowing the lateral secondary
15 recirculation currents B2La, B2Lb to be created are
coolers submerged in the vicinity of the surface of the
glass melt.
The coolers may especially be water cooled.
20
The cooling devices may be placed along the side wall
or, preferably, on the end wall, or both.
It is advantageous, according to the invention, to
25 place the cooling devices as close as possible to the
end wall in order to keep the surface glass hot for as
long as possible.
Advantageously, the cooling devices cover the entire
30 width of the end wall except for the exit width of the
glass, whether this is a waist, a channel or a
overflow.
It is advantageous for the cooling devices to partially
35 cover the exit width of the glass, so as to protect the
corners at the entrance of the device through which the
glass exits.
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Depending on the required cooling capacity, multiple
cooling devices may be provided. A plurality of types
of coolers, for example overhead and submerged coolers,
may also be combined."
The cooling devices may also consist in water-cooled
coolers placed, on the glass side, at the level of the
flux line of the glass.

WO 2012/137161 - 12 - PCT/IB2012/051686
CLAIMS
1. A glass furnace for heating and melting
5 materials to be. vitrified, especially, but not
exclusively, comprising:
- an entrance (E) for raw materials;
- a superstructure (R) equipped with heating means (G);
- a tank (M) containing a melt of molten glass on which
10 a blanket (T) of raw materials floats from the entrance
as far as a certain distance into the furnace; and
- an'exit "(Y) via whichT Molten glass" "is removed, "" '
- two molten glass recirculation loops (Bl, B2) forming
in the melt (N) between a hotter central zone (I) of
15 the furnace and the entrance and exit, respectively,
which are at a lower temperature;
- characterized in that it comprises means for cooling
the glass (12a, 12b), which means are located in the
vicinity of the lateral sides (13a, 13b) of the furnace
20 on either side and upstream of a restriction (5a), so
as to create or increase lateral secondary
recirculation currents (B2La), (B2Lb) of glass in order
to decrease the intensity of the central secondary loop
(B2C).
25
2. The furnace as claimed in claim 1,
. characterized in that the heat flux evacuated by the
lateral coolers is at least 5% of the flux consumed to
melt the blanket of raw materials.
30
3. The furnace as claimed in . claim 1,
characterized in that the means (12a, 12b) for cooling
the glass are located in the vicinity of the entrance
of the waist, in particular in the corners of the tank.
35
4. The furnace, as claimed in either of claims 1
and 2, characterized in that the cooling means (12a,
12b) are located near the surface of the melt.
§
5
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5. The furnace as claimed in.the preceding claims,
characterized in that the cooling means (12a, 12b) are
overhead coolers placed above the glass melt.
6. The furnace as claimed in either of claims 1
and 2, characterized in that the cooling means (12a,
12b) are copiers that are submerged in the glass melt.
10 7. The .furnace as claimed in either of claims 1
and.2, characterized in that the submerged coolers are
"cooXed 'with waterT