Note: Descriptions are shown in the official language in which they were submitted.
~5~
This invention relates to a meth~d and appa~atus for the
manufacture of flat glass. More particularly the invention
relates to the manufacture of thin flat glass by the float
process, for example float glass of thickness in the range 1.5
mm to 5 mm and more especially in the range 1.5 mm to 3 mm.
In the float process for flat glass manufacture, molten
glass is delivered at a controlled rate on to one end, the hot
end, of a molten metal bath contained in an elongated tank
structure. Usually the molten metal bath is of molten tin or ~;
of a molten tin alloy in which tin predominates. The final
ribbon of glass is discharged from the bath by traction means~
usually driven traction ro:Llers, disposed beyond the outlet end
of the bath, which traction means applies tractive force to
advance the ribbon along the bath.
In some ways of operating the float process, regulation of
the applied tractive effort is effected along with regulation of
the thermal conditions to which the advanci~g ribbon of glass
is subjected so as to attenuate the ribbon to a desirea width
and thickness.
When operating under high load conditions, for example
at a rate of delivery of molten g]ass to the bath o~ 2,000
tonnes per week or more, a high speed oE discharge of the
ultimate ribbon of glass from the bath, for example greater
than 10 metres per minute, is necessary when attenuating the
glass to thic]~nesses below 3 mm. When the advancing ribbon
of glass is accelerating during attenuation to a uniform
high speed for discharge from the bath, it entrains an
appreciable quantity of the molten metal of the bath along
the bath surface towards the outlet end of the bath,
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which surface flow induces an upstream return flow of cooler
molten metal from the outlet end of the ba-th along the bo-ttom of
the bath towards the zone of the bath where the ribbon of glass
is being attenuated. In this zone the glass is at a viscosity
such that it is particularly susceptible to temperature
variations across the surface of the molten me-tal bath, and it
has been found that distortion introduced into the underface of
the ribbon of glass in this attenuation zone is present in the
ultimate ribbon.
Temperature variations across the surface of the bath can
result from a temperature gradient through the depth of the bath
and it is desirable to minimise such temperature gradients,
particularly in the attenuation zone. However although a
relatively small temperature gradient can be achieved by a
relatively shallow bath depth at low ribbon speeds, a high
ribbon speed over a shallow bath depth produces turbulence in
the molten metal from which distortion in the ribbon can result.
greater bath depth will reduce turbulence at high ribbon
speeds, but will inherently gi~e a grea-ter temperature gradient
through the bath depth which can introduce distortion into the
ribbon.
It has previously been proposed in Canadian
patent No. 1~054~370 to combat
the introduction of such distortion into the ribbon
of glass in the attenuation zone by employing a first
barrier at a first location in the region of the downstream
end of said attenuation zone to constraiin molten
metal flow at that location to forward flow of molten
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metal entrained beneath the ribbon and counterflow of
molten metal alongside the ribbon from downstream of
that location, and employing a second barxier at a
second location spaced upstream from said first loca-
tion and in the region of maxlmum acceleration of the
glass to constrain molten metal flow at that second
location to forward flow of molten metal entrained
beneath the accelerating glass and counterflow of
molten metal alongside the ribbon ~rom downstream of
the second location, there being established lateral
access into the region of the bath supporting the ribbon
between said first and second locations for said counter-
flow of molten metal at said first location to ensure
replenishment of the molten metal of the bath in the
attenuation zone between the first and second locations.
Such an arrangement reduces the temperature difference
between the surface molten metal and the molten metal
beneath the surface in the a~tenuation zone, thereby
reducing temperature variations in this zone which tend
to introduce distortion into the ribbon.
The invention seeks to provide simplified control of
the temperature of the counterflows oE molten metal replenish-
ing the molten metal bath in the attenuation zone.
According to the invention there is provided a
method of manufacturing flat glass comprising advancing
a ribbon of glass along a molten metal bath, applying
traction to the ultimate ribbon of glass to accelerate
the glass to a final discharge speed thereby causing, as
the glass accelerates, progressively increasing entrainment
~J ~
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of mol-ten metal of the ba-th over an upstream return flow of
cooler molten metal from -the outlet end of the bath, and, in the
region of the bath where the final discharge speed oE the ribbon
is achieved, receiving the upstream return flow of cooler molten
metal in a reserve region of greater bath dep-th than the bath
depth adjacent that reserve region, from which reserve region of
greater bath depth there are drawn upstream molten metal flows
to replenish the molten metal entrained by the acceleratil1g
ribbon.
The method of the invention may comprise containing the
molten metal bath in a tank structure haviny a floor provided by
abuttiny blocks of refractory material whose upper faces define
the level of the bottom of the molten metal bath, and defining
the reserve region of greater bath depth by blocks whose upper
faces are at a lcwer level than the upper faces of the blocks
defining the bath depth adjacent the reserve region.
Preferably the reserve region of greater bath depth exte~ds
for a predetermined distance downstream sufficient to ensure
mixing of the molten metal oE the re-turn flow with molten metal
constituting the reserve region of greater bath dep-th.
Further the method of the invention may comprise
constraining the upstream return flow of cooler molten metal to
a depth less than the depth of the reserve region of greater
bath depth, whereby the velocity of the return flow is reduced
as the return flow enters the reserve region of greater bath
depth and mixing bf the return flow with the molten metal in the
reserve region is enhanced.
The invention may also comprise a method of manufacturing
flat glass comprising advancing a ribbon of glass along a molt:en
metal bath, applying traction to the ultimate ribbon of glass to
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~L~95247
accelerate the glass to a final discharge speed thereby causing,
as the glass accelerates, progressively increasing entrainment
of molten metal of the bath over an upstream return flow of
cooler molten metal from the outlet end of the bath, in the
region of the bath where the inal discharge speed of the ribbon
is achieved, receiving the upstream return flow of cooler molten
~ . ,
metal in a reserve region of greater bath depth than the bath
depth adjacent that reserve region, constraining molten metal
flow at a location immediately upstream of the reserve region of
greater bath depth to forward flow entrained beneath the ribbon
and counterflows alongside the ribbon from the reserve region of
greater bath depth, and establishing lateral access to the
region of the bath supporting the rlbbon upstream of the
location for -the counterflows of molten metal which are drawn
from the reserve region of greater bath depth to replenish the
molten metal entrained by the accelerating ribbon.
Further the invention may provide regulating the applied
traction to attenuate the ribbon to a desired width and
thickness in an attenuation zone in which the glass accelerates
along the bath, and enforcing the step of constraining molten
metal flow at a location immediately upstream of the reserve
region, which location is in the region of the downstream end of
the attenuation zone.
Longitudin~l flow of molten metal along side regions of the
bath may be obstructed at a position upstream of the location.
The longitudinal flow may be obstructed at a plurality of spaced
positions upstream of the location.
The longitudinal flow may be obstructed at two spaced
positions.
Still further the invention may provide electromagnetically
inducing flows of molten metal through the lateral access to
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^~ the region of the bath supporting the ribbon upstream of
the location.
Flows of molten metal may be induced electromagnetically
from beneath the ribbon upstream of -the locaticn to mix
with the counterflow.
The counter~lows alongside the ribbon may be selectively
` heated.
Further the invention provides a method of manufacturing
float glass of thickness in the range 1.5 mm to 3 mm,
comprising applying marginal forces to the accelerating glass
at a series of oppositely disposed positions spaced along
the bath to control reduction in ribbon width and thickness,
and enforcing the constraint of molten metal flow, a-t a
loca-tion in the region of the downstream end of the at-tenua-
tion zone and spaced downstream from the furthest downstream
position at which marginal forces are applied to the ribbon.
Longitudinal flow of molten metal along side
regions of the bath may be obstructed at least at one
positi.on upstream from the location and spaced downstream
from the furthest` downstream position of application of
marginal forces to the glass.
The invention further provides a method of manufacturing
flat glass comprising advancing a r.ibbon of glass along a
molten me-tal bath, attenua-ting the ribbon to a desired width
and thickness in an attenuation zone in which the glass
accelerates along the bath, constraining molten metal flow
in the region of the downstream end of the attenuation zone
to forward flow entrained beneath the ribbon and counter-
flows alongside the accelçrating glass from downs-tream of
the attenuation zone, providing a deepened reserve zone of
molten metal downstream of said attenuation zone in which
return flows o~ molten metal from the outlet end of the bath
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quiesce and are heated, supplying said coun-terflows by means
of flows of molten metal from said reserve zone~ and from
said counterflows feeding la-teral flows of molten metal
drawn in-to -the molten metal flow entrained beneath the
accelerating glass.
Still further -the invention provides a method of
manufacturing flat glass comprising advancing a ribbon
of glass along a molten metal bath contained in a tank
structure having a floor of alumino~silicate refractory
blocks whose upper faces are at different depths in different ~ :
regions of the bath~ attenuating the ribbon to a desired
w~d-th and thickness in an attenuation zone in which the
glass accelerates along a relatively shallow region of ~ ~.
the bath, providing a reserve of molten metal just down-
stream of said attenuation zone in a relati~ely deep region
of the bath defined in a recessed part of said refractory
floor of the tank structure 3 receiving in said reserve
return flows of cooled molten metal from -the ou-tlet end
of the bath, which flows quiesce and are heated in said
reserve, and directing flows of molten metal from that
reserve alongside the accelerating glass to feed la-teral
flows of molten me-tal drawn into the molten me-tal flow
entrained in said relatively shallow region beneath -the
accelerating glass.
The invention fur-ther provides a method of manufacturing
~lat glass comprising advancing a ribbon of glass along a
molten metal bath contained in a tank structure having a
floor of alumino-silicate refractory blocks whose upper .
faces are at different depths in different regions of ~ .
the ba-th, attenuating the ribbon to a desired width and.
thickness in an attenuation zone in which the glass
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1~ ~ 5 ~ ~ ~
accelerates along a relatively shallow region of the bath,
cons-training molten metal flow in the region of the down-
stream end of the attenuation zone to forward flow entrained
beneath the ribbon and counterflows alongside the accelerating
glass from downstream of the a-ttenuation zone, providing
a reserve o~ molten metal just downstream of said
attenuation zone in a relatively deep region of the bath
defined in a recessed par-t of said refractory :Eloor of
the tank structure~ receiving in said reserve return flows
of cooled molten metal from the outlet end of the bath,
which flows quiesce and are heated in said reserve, and
directing flows of molten metal from said reserve into said
colmter~lows alongside -the accelerating glass to feed lateral
- flows of molten metal drawn in-to the molten metal flow
en-trained in said relatively shallow region beneath the
accelerating glass.
The invention alsp comprehends apparatus for manufacturing
M a-t glass comprising an elongated tank structure having end
walls) side walls, and a floor for containing a bath of
~0 molten metal, means for delivering glass to the bath at a
controlled rate ? and advancing the glass in ribbon form along
the ba-th, and means for applying traction to -the ul-timate
ribbon of glass -to accelerate the glass to a final discharge
speed9 and wherein~ in the region of the tank structure where
the ribbon achieves its final discharge speed, the floor of
the tank struc-ture is deepened to define a reserve zone for
receiving cooler molten metal ~low which is enforced in an
upstream direction over the floor by -the entrainment of hot~
ter molten metal by the advancing ribbon of glass.
A preferrecl embodiment of the apparatus comprises a
transverse barrier on -the floor of -the tank structure a-t
a location immediately upstream of said deepened tank floor ?
_ 9 _
which barriex extends beyond the position of the edges of
the ribbon with the top of the barrier positio~ed below ~ -
the level o~ the bath surface hy a distance which is
effective to constrain molten metal flow at that loca-
tion substantially to forward flow of molten metal entrained
beneath the ribbon and counterflow of molten metal alongside
the ribbon.
The barrier may extend beyond the position of the -~
edges of the ribbon but stops short of the tank side
walls.
The reserve zone defined in the floor of the tank
structure just downstream of said barrier may be of greater
depth than the bath upstream of said barrier.
The depth of the reserve zone may he approximately
twice the bath depth adjacent said zone.
Preferably the reserve zone extends across the full
width of the floor of the tank structure.
In a preferred embodiment the tank structure is
encased in a metal casing, the floor of the tan]c structure
comprises abutting blocks of refractory material which are
secured to the metal casing, and said reserve zone of ,
greater bath depth is defined by blocks whose upper faces
are at a lower level than the upper faces of the adjacent
blocks.
The upper faces of the blocks upstream and downstream
of the reserve zone may be at the same level. Preferably
the blocks are of alumino-silicate refractory.
In one embodiment the floor of the tank structure
downstream of said barrier is constructed to define,
considered in the downstream direction, said reserve zone
-- 10 --
., . : -. . , i
of ~reater depth than the b~th depth upstxeam of said
barrier, a reyion of lesser depth than the reserve
zone, and a further region of yreater depth than the
bath depth upstream o~ said barrier which further
region extends to the outle-t end of the tank structure.
An abrupt step may be provided where the floor
defines a change in bath depth.
In the preferred embodiment the elongated tank
structure has a shoulder region which joins an upstream
part of greater bath width to a downstream part of lesser
bath width~ said reserve zone of greater bath depth is
located at said shoulder region, and the barrier is
located just upstream of said shoulder re~ion.
Top rol]s may be arranged to enga~e the upper
surface of the ribbon margins at a series of oppositely
disposed positions along the bath to control the reduction
in width and thickness of the ribbon, the pair of top
rolls furthest downstream being at a position spaced
upstream from said barrier.
~t least one pair of baffles may be located adjacent `~
the bath side walls at oppositely disposed positions upstream
frvm said barrier and spaced downstream from the furthest
downstream pair of said top rolls to obstruct longitudinal
flows of molten metal alon~ the bath side walls at those
positions.
The apparatus may also include linear induction
motors mounted over the bath surface in the region of
the barrier to induce flows of molten metal electromag-
netically.
Heaters may be mounted adjacent the tank side walls
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upstream of the barrier to apply local heating to the
counterflows of molten metal.
The invention also comprehends flat glass produced
by the method of the invention.
Some embodiments of the invention will now be
described, by way of example, with reference to the
accompanying drawings in which:-
Figure 1 is a plan view of an elongated tank
structure containing a bath of molten
metal for use in the float process
for the manufacture of thin flat
glass by the method of the invention,
Figure 2 is a longitudinal section through the
floor of the tank structure of Figure 1,
Figure 3 is an enlarged view of part of Figure 2
, further showing a glass ribbon,
Figure 4 is an enlarged view of part of Figure 1,
Figure 5 is a view similar to Figure 4 illustrating
a modification of the apparatus of Figures
1 to 4,
Figure 6 is a view similar to Figure 5 illustrating
a further modification of the apparatus o:E
Figures 1 to 4,
Figure 7 is a view similar to Figure 5 illustrating
further modifications to the apparatus of
Figures 1 to 4,
and
Figure 8 is a longitudinal section through part of
the floor of the tank structure illustra-
ting another way of constructing the floor
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Referring to the drawin~ Figure l illustrates
in plan an elongated tank s-tructure for the manufacture
of flat glass by the float process. The tank structure
comprises an end wall 1 at its inlet end and an end
wall 2 at its outlet end, and parallel side walls 3
extending from the inlet end to a shoulder region
defined by inwardly inclined side wall portions ~
which connect the side walls 3 with further parallel
side walls 5 extending to the outlet end. The -tank
structure contains a bath of molten metal which is ~;
usually molten tin. The geometry of the tank structure
is such that it will accommodate between the sicle walls
3 of its wide part upstream of the shoulder regi.on the
maximum required glass layer on the bath surface, and
between the side walls 5 of its narrow part downstream
of the shoulder region the maximum required ultimate
ribbon width.
Molten soda-lime-silica glass is clelivered on to
the bath at the inlet end of the tan]~ structure by
pouring from a spout 6 which extends over the inlet
end wall 1. ~ regulating tweel 7 controls the rate
of flow of molten glass over the spout on to the bath
surface 8.
In manner well known in -the float process,
temperature regulators are provided in a roof structure .
not shown which is mounted over the tank structure, and
defines a headspace over the bath in which a protective
atmosphere is maintained. Temperature conditions at
the inlet end of the bath are such that the molten glass
9 arriving on the bath is permitted to flow freely,
4~
laterally unhinAered, during the firs-t part of its
advance along the bath. The temperature of the glass
is 990C when maximum spread is achieved and the
glass -thickness is of the order o f 7 mm. The layer
of mol-ten glass is advanced in ribbon form~ and the
ribbon is initially constitu-ted by low viscosity glass,
e.g. at a viscosity of 104-~ poises. This glass is
gradually cooled during its initial advance along -the
ba-th and ïts viscosi-ty slowly increases.
The temperature regulators in the roof s-tructure
set a temperature regime to which -the advancing glass
is subjec-ted, which regime maintains the glass in a
deformable state over a longitudinally ex-tending region
of the ribbon in which the glass is progressively atten-
uated as its velocity increases under the influence o
tractive effort applied to -the ultimate ribbon of glass
10 by driven rollers 11 located beyond the outlet end
wall 2 of the tank structure. As -the viscosi-ty of the
glass increases so does the influence of the longitu-
.~ ,
dinally directed tractive force, originating from the
rollers 11, in stretching the ribbon of glass. Gradual
and progressive reduc-tion in width and thickness of the
glass is controlled by the use of top rolls which engage
the upper surfaces of the margins of the glass. `-
. Initially while the glass is at a low viscosity
the margins of the ribbon are engaged by a palr of
inclined top rolls 12 mounted at oppositely disposed
positions on shaf-ts 13 which extend through -the -tank
side walls 3 and are driven by motors 14~ The top rolls
12 are knurled or too-thed graphite, s-tainless steel, or
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mild steel rol]s which are internally water cooled.
m e axes of the rolls are inclined at an angle to a
line at right angles to the direction of advance of
the ri~bon of glass along the bath~ Outwardly and
longitudlnally directed forces are thereby applied
to the margins of the nascent ribbon9 the outward
force componen-ts providing restrain-t against undue loss
in width. Slight attenuation o~ -the ribbon is begi~ning
to occur in -this region.
~urther similar pairs of top rolls 15, 16, 17, 1
and 19 are provided spaced along the tan~ structure, being
mounted on respective shafts 20, 2i, 22, 23 and 24 and
driven by motors 257 26, 27, 28 and 29, the -top rolls o~
each pair being at oppositely disposed positions. With
?5 such pairs of top rolls a-t a series of spaced positions
along -the bath control of -the progressive decrease in
ribbon width and thickness is achieved. As the glass
passes beyond the last pair of top rolls 19 its -tempera-ture
is cooling below 880C cor~esponding to a viscosi-ty of
105'2 poises.
A~ter the glass leaves the furthes-t do~ns-tream
pair of top rolls 19 its width and -thickness con-tinues
to reduce un-til a position at or near the shoulder
region of the tank struc-ture where its viscosity, under
2$ the applied temperature regime, is so high that the rîbbon
a,ssumes i-ts ~inal width and thickness and achieves its
~inal discharge speed which is the effective surface speed
of the rolls 11.
At or near the shoulder region the usual soda-
lime-silica glass has a viscosity of 107 poises~
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~5~7
correspondi.ng to a temperature of 750C and is in a
condition in which no further dimensional change can
take place under the influence of the applied traction~
The rib~on co`ols further during its t~avel between the
side walls 5 to the ou-tle-t end of the ba-th.
The g].ass is accelera-ted and the ribbon is
attenuated in a zone upstream of the shoulder region of-the tank struc-tureO The downs-tream end of this atten- :
ua-tion zone ls generally at or near the shoulder region,
and the position of maxim~ acceleration of the glass is
generally up~tream towards the last pair of top rolls
19. As the ribbon is accelerating in the attenuation
æone there is progressively increasing entrai~men-t of
. molten me-tal of the bath in a forward surface flow which
travels towards -the outlet end o~ the bath. This forward
surface flow is over an upstream return flow of cooler
molten me-tal from the outle-t end of the ba-th,and rnol-ten
metal is continuously being drawn under the ribbon to
compensate for that which is entrained. I-t is t~e
generalised return flow of cooler molten ~etal along the
bo-ttom of the bath ~hich produces -top to bot-tom temperature
gradients -through the depth of the ba ~ ~Irhich have been .:
shown to be-particularly -troublesome in the region of the
bath where -the rapidly accelerating ribbon is being
attenuated, and particularly in the region between the
las-t pair of top rolls 19 and the shoulder region. To
combat this effect the up.stream return flow of cooler mol-ten
me-tal is received in a reg~ion of greater bath depth than
the bath depth adjacent that deeper region~
.
.: . . !
~igure 2 shows the profile of -the floor FL of
the -tank structure which provides different bath depths
at different regions along the bath leng-th. At the inle-t
end o~ the bath the floor defines an initial region 30
of greater depth than -the shallower region 31 following
downstream, which latter region 31 provides the major
leng-th of the bath ups-tream of -the shoulders and under-
lies virtually -the entire attenuation zone~ The initial
region 30 may have a depth which is approximately one and
a half -times that of the downstream region 31. ~or
exam~le the region 30 may have a depth of 83 mm and the
region 31 a depth of 58 mm.
The region 31 extends downstream to a posi-tion
close -to the shoulder region, for example to a position
one or two metres upstream of a line joining the upstream
ends of the shoulder side walls 4.
At this position there begins a region of greater
bath dep-th than the bath dep-th adaacent that region. The
deepened floor of the tank structure which defines the
pocket region 32 is shaped as a recess in the floor
extending across the full width of the bath. This
pocket region 32 includes -the shoulder region and extends
downstream a distance of about 3 rnetres beyond a line
~oining the downstream ends of the shoulder side walls
4 The pocket region 32 extends, for example, length-
wise of the bath over a total distance of 7.5 metres,
and provides a reserve zone for receiving cooler molten
metal flow which is enforced in an upstream direction
over the floor by the entrainment of hotter molten metal
by the advancing rlbbon of glass. The depth of the
.
s~
region 32 is approxima-tely twice the bath depth in -the
adjacent upstream region ~1. For example, when the depth
of the regi.on 31 is 58 mm the bath depth in the reserve
zone 32 may be 108 ~n.
Downstream of the recess 32 the floor rises again9
for example o~er a len~th of 3 metres to provide a région
adjacent the reserve zone the same bath dep-th as that
of the region 31 upstream of the reserve zone. From the
region 33 to t-he outlet end of the bath the floor level
is such as to provide a region 34 having a bath depth
the same as that at the inlet end region 30 of the bath,
tha-t is, less than the depth of the reserve zone.
Where there is a change in floor level providing
a change in bath depth the step in the floor may be
chamfered as shown in Figure 2 or an abrupt step as
shown in Figure 8
The provision of a deepened reserve zone alone 5 ~'
such as the recessed pocket 32, in the region of the bath
where thè final discharge speed of the ribbon is achieved
has been found to be beneficial, since the pocket receives
the upstream return flow o~` cooler molten me-tal, and
mixes that cooler molten metal with molten metal hel.d in
the reserve zone, so that the cooler molten metal is
heated and there is minimal risk of the introduction of
thermal inhomogeneities beneath the accelerating glass
due to molten metal flows drawn upstream from the reserve
zone to replenish molten metal entrained by the accelera-
ting ribbonO
The ef~ect of the reserve zone is enhanced in the
embodiments illustrated by the provision at a location
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immediately upstream of the re~ion 32 o~ greater bath
depth, of a transverse barrier 35 which projects upward-
ly from the floor. The barrier 35 is a carbon bar of
upstanding rectangular cross-section and has a dove-
tail base 36 Figure 3, which is keyed into a matchingdove-tail groove 37 formed transversely of the bath tank
structure in the floor at the downstream end of the region
31 -that is, in the re~ion of the downskream end of the
attenuation ~one. The flat top of the bar is about 50 mm
long in the direction of ribbon advance and is spaced
from the level of the bath surface 8 by a sufficient
distance to constrain molten meta:L flow at that location
to forward flow 39 entrained beneath the ribbon and counter-
flows alongside the ribbon from the region 32 of greater
bath depth. The barrier 35 ensures that the lower layers
of entrained molten metal o~ the forward flow are directed
downwardly and then upstream as indicated by the arrow
38 in Figure 3. Usually the top surface of the barrier 35
is from 6 mm to 15 mm below the level of the bath surface,
the optimum distance dependin~ on the speed and accelera-
tion of the ribbon. In principle the top of the barrier
35 could be at a depth below the level of the bath surface
8 which is exactly such that all the entrained molten
metal of the forward flow 39 travels over the barrier but
no return flow o~ molten metal can pass over it. In
practice, however, such an exact setting is difficult to
achieve and the barrier height is therefore preferably
set as described above to direct the lower layers of
entrained molten metal of the forward flow as indicated
at 38.
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The molten metal flows ~ are ~ixected outwardly
and have a beneficial effect on the temperature of the
molten metal alongside the ribbon by intermingling or
mixing with cooler upstream counterflows from the reserve
zone as described below.
In the embodiments illustrated the barrier 35
extends transversely of the bath beyond the positions
of the edges of the ribbon but stops short of the side
walls 3. The ends of the barrier 35 are thus spaced
10 from the ~ide walls 3 to define channels for counter-
flows of molten metal indicated by the arrows 40 in
Figure 4, from the reserve zone 32 downstream of the
barrier, round its ends, and into the region upstream ~;
of the barrier.
The barrier 35 obstructs direct return flow of
molten metal along the bath bottom into the region
upstream of the barrier location, but permits counter-
flow round the ends of the barrier from the region of ;
greater bath depth thereby establishing lateral access
20 to the region of the bath supporting the ribbon as it
is being attenuated by acceleration of the glass upstream
of the barrier location.
The transverse barrier 35 is at a location
immediately upstream of the upstream end of the deepened
25 region of the bath. For example the barrier 35 may be
150 mm from the ups-tream end of the pocket region 32.
Upstream flow, indicated by arrows 41 in Fi~ures 3 and
4 of cooler molten metal travelling along the bath bottom
in an upstream direction towards the barrier location is
30 received in the pocket region 32 of greater depth just
- 20 -
., ~
downstream of the barrier 35. This reduces the velocity
of the cooler return flow, thereby giving time for mixing
the molten metal of said return flow with molten metal
constituting said region of greater bath depth, so that
there is time for heating of the molten metal of the
return Elow to occur. The molten metal in the pocket
region 32 effectively acting as a buEfer.
Replenishment of molten metal supporting the
accelerating glass occurs by the counterflows 40 of
molten metal from the pocket region 32 round the ends
of the barrier 35 and into the region upstream thereof,
the established lateral access enabling those counter-
flows to be drawn under the ribbon.
The provision of the region 32 of greater bath
depth in which the return cold flow of molten metal is
received, ensures that the counterflows of mol-ten metal
alongside the ribbon round the ends of the barrier 35
can have a relatively small temperature difference as
between the surface molten metal and the molten metal
below the surface. Such a small temperature difference
as between the top and the bottom of the molten metal
reduces the risk of local temperature variations in
the molten metal on which the ribbon oE glass is carried
as it is accelerated, thereby minimizing distortion in
the undersurface of the ribbon.
Examples of measured top and bottom molten metal
temperatures at positions just alongside the edge of
the ribbon are given below, the temperatures being
measured by thermocouples at a position A towards the
downstream end of the pocket 32, that is 6 metres
2~ -
~5~
downstream of the barrier 35; a posltion B around the
middle of the pocket 32 3 metres downstream of ~he
barrier 35; a position C just downstream of the barrier
35, that is at the ups-tream end of the pocket 32; a
position D just upstream of the barrier 35; a position
E 3 metres upstream of the harrier 35; and a position
F 6 metres upstream of the barrier 35, that is 2 metres
downstream of the last top rolls 19.
In one example of operation molten glass was
del.ivered to the bath at a rate of 3326 tonnes per
week to produce an ultimate ribbon 2.5 mm thickness
having a gross width of 3.74 metres travelling at a ~-~
speed of 865 metres per hour. The pairs of top rolls
12, and 15 to 19 were spaced alony the bath at 3 metre
intervals with the last top rolls 19 positioned 8.2
metres upstream of the barrier 35, and were disposed
with their axes at angles of slew to an axis at right
angles to the:direction of ribbon advance and were
driven at peripheral speeds as follows:-
20Top Rolls Slew Angle Speed ~,
,
12 2 165 m/hr
5 181 m/hr
16 7 201 m/hr
17 go 232 m/hr
1% 9 291 m/hr
19 go 3~0 m/hr ;~
The top and bottom tin temperatures at the above ~:~
mentioned positions just alongside the edge of the ribbon ~:
were measured as follows:-
~s~
Top Tin Temperature Bottom Tin
PO F. 1 tion ~ _ Tem _ rc~-turc (C)
A 797 734
. ~3 807 . 797
C . 818 812
D 836 836
E 826 822
F 341 826
It wil:l be seen that just ups-tream of the barrier 35,
at position D, -the top -to bo-t-tom bath -teMperature difference
was zero, and was less than 5C at a posi-tion 3 metres
further upstream at posi-tion E.
It has been :Eound -that -the -temperature uniformi-ty can
be fur-ther improved 9 i.e, -the top -to bo-ttom temperature
diference in the molten me-tal reduced further upstream
of the barrier, if longi-tudinal molten me-tal :Elo~s adja-
cent the ba-th side ~ralls are obs-truc-ted at a posi-tion
upstream ~rom the barrier location. To achieve this a
pair of carbon baffles or flags 42 ma~T be mounted
adjacent the side walls ~ respectively at opposed posi-
tions spaced upstream from the barrier 35 as sho~m in
Figure 5. The flags or baffles 42 ha~e a height greater
than the bath dep-th, are seated on -the floor and abut
agains-t -the side walls so as to obs-truc-t completely
longitudinal mol-ten metal flo~.~s adjacent the side ~alls.
lt is believed tha-t such obstruction of longitudinal side
lol~rs :improves in-termingling or mixing of out-rardly
direc-ted flows, indicated by arro~s 43, of relatively
hot surface molten meta]. from beneath the ribbon, ~ith
the co~lterflows 40 of cooler molten metal coMing from
- 2~ ~
~5~
the de~eper bath xegion downs-tream o~ the barrier, such
mixing~ or intermingling occurring alongside and not
under the ribbon.
The flags or baffles 42 are believed to prevent
the counterflows 40 from travelling along the bath
side walls and -then under the ribbon at an ups-tream
posi-tion without having mixed with the ~lows 43.
In an example o~ opera-tion carbon flags or
baf.~les 42 were mounted adjacent the side walls 3 at
opposed positions 3 metres upstream of -the barrier
location, the fiags projecting inwardly from the side
wall by a distance of 460 mm. Molten glass was delivered
to the bath at a rate o~ 3400 tonnes per week to produce
an ultimate ribbon o~ 2~5 mm thickness having a gross
wid-th of 3.62 metres and travelling at a speed of 865
metres per hour. The top roll positions were the same
as in -the previously desc~ibed example but the angles
of slew of -the last three pairs were altered, and the
speeds very slightly different as follows:- -
~ 3~ peed
: 12 2 165 m/hr
. 5 182 m/hr
16 7 202 m/hr
- 234 m/hr
18 ~o 292 m/hr
19 8 338 m/hr
With -this arrangement the top and bottom bath
temperatures at positions just alongside the ribbon edge
upstrearn of the barrier were measured 7 the actual
positions in th s case being posi^tion G 3 metres upstream ~:
-. - 24 - i
from -lhe barrier and 1 metre do~ls-tream of t~le carbon
flag or ba:`fle 42; posi-tion H 2,1 metres upstream of
the carbon flag or baffle 42; and posi.-tion I approx-
imately at -the po;sition of -the last top roll 19. The
measured temperatures were:-
Top Tin Tempera-ture Bo-ttom Tin
Position (C) _ Tem~erature (C)
G 840 842
H 837 828
1-0 I 851 839
It wil] be seen that at position G just downstrearn
of the carbon flag or baffle 42 the top tO bottom
temperature difference was only 2C, the bath bottom in
~act being hotter than the top and a-t posi-tion H ups-tream
of the flag or baffle the difference was only 9C, which
compares favourably with the 15 difference a-t roughly
the same posi-tion F in the previous example. Even at
the last top roll positlon I the top to bottom bath
temperature difference was only 12C.
Longi-tudinal molten metal flows acljacen-t the bath
side walls may be obstruc-ted a-t more than one position
upstream from -the barrier location. For example, as
ShOWIl in Figur~ 6, there may be provi.ded a further pair
of carbon flags or baffles 44 mounted adjacent the side
walls 3 at opposirtely disposed positions and spaced down~
stream from the flags or baffles 42 so as -to be located
close to, but slightly upstream of, the barrier location.
The spaces between the end of the barrier 35 and the inner
ends of the flags or baffles 44 iS sufficien-t to permit
counterflows 40 of rnolten rnetal therethrough. The
- 25 -
5~47
dimensions of the flags or baf~les 42 and 44, i.e, the
extent to which they project inwardly from the bath side
walls 3, are selec-ted to suit the particular requirements
of operation and -the upstream flags or baf~les 42 may
project inwardly a different distance from that of the
downstream flays or baffles 44. The effect of the addi-
tional pair of flags or baffles 44 as shown in Figure 6
is similar to that of the .~irst pair 42 in that they
are believed to cause outward flows 43 of hot molten
metal from beneathi the ribbon better to mix or inter~
mingle with the counterflows 40 at a position alongside
the ribhon, ancl to prevent the counterflows 40 :Erom
travelling along the bath side wall and then under the
r.ibbon at an upstream position without mix.ing.
Figure 6 also shows an additional pair of top
rol].s 45, moun-ted on shafts 46 driven by motors 47,
at oppositely disposed positions spaced downstream
of the top rolls 19. This fur-thest downstream pair of
top rolls 45 are useful when producing glass thinner
than that of the previous examples.
In one example of operation with an arrangement
as shown in Fi~ure 6 molten glass was delivered to the
bath at a rate o:E 3380 tonnes per week to produce an
ultimate ribbon of thickness 2.3 mm having a gross width
of 3.65 metres and travelling at a speed of 940 metres
per hour. The carbon flags or baffles 42 projected
inwardly 610 mm ~rom the side walls 3 and the carbon ;
Elags or baffles 44 projected inwardly 460 mm from the
side wails 3. The position of the top rolls 12 and 15
to 19 were as described in the previous examples and
- ~6 -
,
,.;. ~ . ~ .. ,
S2~
the addltional -top rolls 45 were at a position spaced
3 me-tres downstrearn from the top roll~s 19~ that is
5,2 me-tres upstream from -the barrier 35 and 2~2 me~res
upst.ream from the fla~s or baffles 42. The sle~ an~les
and peripheral speeds o~ the driven top rolls were as
.~ollo~s:-
olls Sle~ A~ S~eed
12 2 164 m/hr
3 182 m/hr
10. 16 5 202 m/hr
~7 7 234 m/hr
18 8 2g2 m/hr
19 8 338 m/hr
8 400 m/hr
,
The -top and bottom bath ternperatures were measured
ju~st alongside the ribbon edge at a position .J 3 me,-tres
downstrearn f`rom the barrier 35, that ls in the pocket
32; position K just do~rnstream of the barrier 35 a-t
the upstream end of -the pocket 32; position L just
upstream of the barrier 35 and flag 44; posi-tion M
approximately a-t -the position of the top roll 45; and
at position N approximately at -the position of the top
roll 19. The measured temperatures ~ere:-
Top Tin Temperature Bottom Tin
Po~sltion _ ~ _ T~m~:
J 811 799
K 813 797
- L ~42 842
M 854 837
3~ N 865 856
27 -
, -
~9~
.
It will be seen that -the top to bo,ttom temperature
difference JUSt upstream of the barrier 35 a-t position
L was again zero, as at positi.on D in the example described
above wi-th r'e,ference to Figure 4. The top to bottom
temperature difference at the position of the top roll 19s
posi-tion N, was only 9C. However, at -the position of the
last pair of top rolls 45, the top to bottom bath -tempera-
ture difference was somewhat higher being 17C a-t posi~
tion M.
The flags or baffles 42 and 44 were then changed -to
increase their length by 150 mm so that the ~lags or
baffles 42 had a length of inward projection from the
~ide walls 3 of 760 mm and the flags or baffles 44 had
a length of inward projection of 610 mm. The inner ends
15, f -the flags or baffles 42 were then only 155 mm from
the edges of -the ribbon.
In an example of operati.on wi-th this modified flag
or baf~le arrangement9 molten glass was delivered to the
bath at a ra-te of 3370 tonnes per week to produGe an
ultimate ribbon of 2.3 mm thickness havirlg a gross wid-th
o~ ~.58 metres travelling at a speed of 940 me-tres per
hour. The positions of the top rolls 45 were moved
610 mm upstream so as to be about 2. 45 me-tres from -the
top rolls 19. The top roll angles of slew and speeds
25, were:- .
Sle~ Ang~
12 2 162 m/hr ' ,~
3 180 m/hr
, 16 - 5 201 m/hr
3 . 17 6 232 m/hr
.
-- Z~3 --
~ 7
18 7 284 m/hr
19 7 3~0 m/hr
45 7 493 m/hr
With -this arrangement the top to bottom bath
temperature difference at the position o~ the las-t top
rolls 459 that is position M in Figure 67 was reduced
to 12C.
In another example of operation with an arrange-
ment as shown in Figure 6 a ribbon of thi~mer glass was
produced at a considerably increased ultimate ribbon
speed. Molten glass was delivered to the bath at a
rate of 3410 tonnes per week -to produce an ultimate
ribbon o~ thickness 1.8 mm having a gross width of
~.37 me-tres travelling at a speed of 1252 metres per
hour. The flags or ba~fles 42 and 44 were posi-tioned
as in the previous two examples, but the flags l~2 had
an inwardly projecting leng-th of 510 mm and the flags
44 a length of 610 mm. That is, in this example the
downstream flags 44 were slightly longer than the
upstream flags 42, The top rolls were positioned as
in the last previously described example and had slew
angles and speeds as follows:-
Top Rolls S:Le- A~ S~eed
,
12 2 163 m/hr
~o 180 m/hr
16 5 201 m/hr
17 6 232 m/hr
18 10 284 m/hr
19 10 ~24 m/hr
11 402 m/hr
- 2~ -
. .
~os~
Top alîd bo-ttom bath tempera-ture measuremen-ts just
alongside the edge of the ribbon ~/ere t~ken at the
previously described positions J, K, L~ M and N as
well as at further do~,ms-tream positi^.ns, narnely a
position 0 just downstream OI -the do~mstream end OI -the
pocket 32 and a position. P in the pocket 32 just upstream --
of its d~mstrearn end. I~he measured temperatures ~/ere:-
Top Tin Temperature Bottcm Tin
_si-tlon ~
0 748 729
P 77~ 75
J 783 772
K 775 765
L 831 830
15 ` ~ 837 818
N 844 830
It will be seen that the top to bottom temperature
difference, are 14C at position M at the last top rolls
45; and 19C at position N at the next to last top rolls
'19. However, even at this high ribbon speed which is
45% faster than in the first two e~:a.np1es described
above and 33~ faster than in the other examples, it will
be seen that the top to bottom bath temperature difference
juat ups-trearn of -the barrier at position L was only 1~
Further, the effectiveness OI the relatively deep pocket
region 32 is particularly apparent froTn this example i.n
that the top to bottom bath temperature difference at
the do~nstream. end of the pocket 32l posi-tions 0 and P,
was 20C, but was reduced to 10C at the upstream end
of -the pocket, positions J and K.
. . .
.
.
. . .
; . . ;
~!915Z~7
It wa.s also foun~ that the pocket 32 and barrier 35
arran~emen-t had an advantageous effect in reducing lateral
temperature variations across the bath and edge to centre
temperature varia-tions in -the ribbonO
The barrier 35 need no-t stop short of the side
.walls of -the -tank s-tructure as in -the embodimen-ts
illustrated, but rnay extend right up to -the side ~alls
3 with recesses in the top of the barri.er alongside the
ribbon to provide chalmels for the counterflows of mol-ten
~0 metal drawn ~rom the deepened region 32.
The region 33 of lesser b~-th dep-th immediately
downs-tream of the region 32, has the same depth as the
region 31 upstream of the barrier 35. The region 33
separa-tes the deepened region 32 ~rom the ou-tlet region
34 which has a bath depth less than -that in the deepened
region 32 but grea-ter than the depth of the regions 31
and 33. The upstanding region 33 provides some obstruc-
t.ion to return flow of cold molten me-tal along the very ~:
- bottom of the bath in the outlet region 34, whereby
the veloci-ty o~ the re-turn flow is reduced as the ti
return flow enters the region 32 o~ greater ba-th depth
and mixin~ of the return flow with the molten metal in
the region 32 is enhanced. The region 33 also provides
a reglon of relatively shallow bath depth at which ~near
mo-tors mounted over -the bath can be used particularly
effec-tively -to control molten metal flowsO
The bath depth may ho~ever be constant from -the
downstream end of the pocket region 32 to the ou-tlet
end of -the bath~ The provision of an increased bath ~.depth along -the outlet regi.on 34, relative to that in J
. , ', '
_ 3~
,.
t
,
~ O~i5
.
the region 31 upstream of the barrler 35j facilitates -the
effec-tive location of coolers in the outlet end of the
... ' !:: .
bath. - .
If desired, linear induction motors may be employed
to strengthen or control molten metal flows in the region
of the barrier 35. Fi~ure 7 ~hows a pair of such motors
48 mounted above the bath surface upstr~m of -the barrier
35 to induce electromagnetically flows of molten metal
from the counterflows 40 -to enter beneath the accelerating
ribbon. Alternatively, the motors L8 may induce molten
metal flow in an outward direction to streng-then the
outward flows 38 and/or 43 and assist mixing or inter~
mingling of those ou-tward flows with the coun-terflows
40. Linear induction motors may also be positioned as
indica-ted in broken line at 49 i~ Figure 7 to assist
movement o~ molten metal in the pocket 32 into the
counterflows 40. Further linear induction motors may
be posi-tioned as-indicated at 50 and 51 to direct the
counterflbws.
I~nersed or partially immersed heaters adap-ted to
effect selective local heating of molten metal flowing
under the heaters may also be employed to heat the
counterflows. For example, a pair of such heaters 52
may be located one adjacent each end of the barrier 35
to heat the coun-terflows 40. If necessary, small
extension pieces 5~ may be provided at each end of the
barrier to ensure that all the molten metal flow past
tha-t end of -the barriers travels under the respective
heater 52. Heate~s may be employed in conjunction l~ith
~0 or in place of the linear induction motors at positions
50 and 51.
,.: '
- ~ . .. .
~ig52~47
As sho~n in Figure 8, -the floor FL.o~ -the tank
structure may be formed by abutting blocks 54 of
refractory ma-terial, preferably alum.ino-si.licate
refractory, whlch are secured in kno~ln manner to a
metal shell or casing 55 which encases the tank
.structure. The upper faces of the blocks define -the
bottom of the molten metal bath. The reserve zone 32 cf
greater bath depth is defined by bloc7cs having a
height dimension less -than that of -the blocks in
~h~ adjacent regions 31 and 33 so that the upper
faces of the b1ocks in the zone 32 axe a-t a lower level -.
than -the upper .faces of the adjacent blocks.
However, as shown in Figures 2 and 3, in which
the vertical dimension is grea-tly exaggerated relative
to the horizon-tal dimension~ the blocks may be arranged
to provide a stepped bot-tom to the tank s-tructure cO
tha-t a-t the inle-t end of the -tank struc-ture bath blocks
of the same height dimension have -kheir upper f~ces at
different levels to provide different. bath dep-ths in
regions 30 and 31, and in the region of the ou-tlet end
of the bath blocks of differen-t height di.mensions have
-their upper surfaces at the same level to provide the
same ba-th depth in the region 3l~.
The me-thod and appara-tus of the present inven-tion
is especi.ally advantageolls for producing float glass of
thiclcness in the range 1.5 m~ -to 3 mm.. The inven-tion
can be used to advantage in producing float glass of
greater -thicl{ness ~Yhen the load and ribbon speed are
such that di.sa(lvantaOeolls molten me~al movemen-t occurs,
for example glass of thickness up to 5 mm or more.
... .. " ' ' ,: ~
- 33 ~
..
. . : , ~ -. . . ..
' `"
~LO~9:2~7
.
The method and apparatus of the invention c~n be used when
producln`g glass of even greater thicknesses~
Al-thouOh specifically described above .in relation
. to a bath having a shoulder region? ~he invention can be
: 5 applied to a tank struc~ure having parallel side ~allsextending at a constant spacing ~rom the inle~ end -to
^the ou~let end o~ the tank structureO
I~ desired an addltional barrier or addition barriers
may be located on the floor of the tank structure
. 10 ef~ec-tively to project upwardly into the bath at a posi-
tion or positlon.s spaced upstream from the barrier 35,
for example as described m the abovementioned patent
I applica-tion.
.-i Further the barrier ~5, althouGh conveniently
constructed and moun-ted in effectively ~ixed fashion in
.the floor as described abo~e, could take a different
- formg for example as described in the abovementioned
paten-t applicationi. and could, in par-tioular, be
cylindrical. Any addi.tional bar~ier or barriers may
also take any of the ~orms described in the above
mentioned patent application and may, if desired~
'~ I be mo~rable between different positions along the ..
\1 bath as therein descrlbed,.. ~
. . : ~ : . .
~.
.. ~: .
: . . , ,: , .
. : - .,. , , .
.
