U.S. patent application number 12/604873 was filed with the patent office on 2010-08-05 for system and method for controlling the environment around one or more vessels in a glass manufacturing system.
Invention is credited to Gilbert DeAngelis, Raymond E. Fraley, Jeffrey D. Girton, David M. Lineman, Rand A. Murnane, Robert R. Thomas.
Application Number | 20100192633 12/604873 |
Document ID | / |
Family ID | 36645640 |
Filed Date | 2010-08-05 |
United States Patent
Application |
20100192633 |
Kind Code |
A1 |
DeAngelis; Gilbert ; et
al. |
August 5, 2010 |
SYSTEM AND METHOD FOR CONTROLLING THE ENVIRONMENT AROUND ONE OR
MORE VESSELS IN A GLASS MANUFACTURING SYSTEM
Abstract
A system and method are described herein that control the
environment (e.g., oxygen, hydrogen, humidity, temperature, gas
flow rate, pressure) around one or more vessels in a glass
manufacturing system. In the preferred embodiment, the system
includes a closed-loop control system and a capsule that are used
to control the level of hydrogen around the exterior (non glass
contact surface) of the vessel(s) so as to suppress the formation
of gaseous inclusions and surface blisters in glass sheets. In
addition, the closed-loop control system and capsule can be used to
help cool molten glass while the molten glass travels from one
vessel to another vessel in the glass manufacturing system.
Moreover, the closed-loop control system and capsule can be used to
maintain an atmosphere with minimal oxygen around the vessel(s) so
as to reduce the oxidation of precious metals on the vessel(s).
Inventors: |
DeAngelis; Gilbert;
(Lindley, NY) ; Fraley; Raymond E.; (Waverly,
NY) ; Girton; Jeffrey D.; (Harrodsburg, KY) ;
Lineman; David M.; (Painted Post, NY) ; Murnane; Rand
A.; (Big Flats, NY) ; Thomas; Robert R.;
(Watkins Glen, NY) |
Correspondence
Address: |
CORNING INCORPORATED
SP-TI-3-1
CORNING
NY
14831
US
|
Family ID: |
36645640 |
Appl. No.: |
12/604873 |
Filed: |
October 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12366245 |
Feb 5, 2009 |
7628039 |
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12604873 |
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12362738 |
Jan 30, 2009 |
7628038 |
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12366245 |
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Current U.S.
Class: |
65/29.13 |
Current CPC
Class: |
C03B 5/23 20130101; C03B
5/225 20130101; C03B 5/163 20130101; C03B 5/1677 20130101 |
Class at
Publication: |
65/29.13 |
International
Class: |
C03B 18/20 20060101
C03B018/20 |
Claims
1. A method for producing a glass product, said method comprising
the steps of: forming and/or processing molten glass within at
least one of a melting, a fining, a delivery, and a forming vessel,
wherein the vessel has a wall comprising platinum directly in
contact with the molten glass; providing a capsule to enclose at
least one of the vessels having a wall comprising platinum; and
providing a H.sub.2O-containing atmosphere between an interior wall
of the capsule and around a non-glass contact surface of said at
least one vessel, wherein said step of providing an atmosphere
further includes: controlling a level of hydrogen within a mixture
of gases in the atmosphere so that a partial pressure of hydrogen
around the non-glass contact surface of said at least one vessel is
maintained sufficiently high to suppress the formation of
undesirable gaseous inclusions within the molten glass; and
controlling a level of oxygen within the gas mixture around the
non-glass contact surface of said at least one vessel to reduce
oxidation of platinum in the wall of the vessel of said at least
one vessel compared to in the air.
2. The method of claim 1, wherein said step of providing an
atmosphere further includes the step of controlling a cooling of
the molten glass while the molten glass travels from one of said at
least one vessel to another one of said at least one vessel.
3. The method of claim 1, wherein said gas mixture results in a
hydrogen partial pressure of at least a level determined by and
following equation and up to 38,000 ppm at 1700.degree. C. at the
non-glass contact surface of said at least one vessel: p H 2 ( ppm
) = 78000 .times. - 58900 + 13.1 T 1.987 T ##EQU00004## wherein T
is temperature in Kelvin of the non-glass contact surface of said
at least one vessel.
4. The method of claim 1, wherein said gas mixture is maintained at
a dew point temperature of 200.degree. F. or lower.
5. The method of claim 1, wherein said gas mixture has an oxygen
content with a level of less than 21% by volume.
6. The method of claim 1, wherein said gas mixture has an oxygen
level of 0.01% to 1% by volume and a water vapor level of 2% to 20%
by volume, with the balance being an inert gas.
7. The method of claim 1, wherein said gas mixture includes cracked
ammonia products.
8. The method of claim 1, wherein said non-glass contact surface of
said at least one vessel includes a metal selected from gold,
platinum, rhodium, iridium, molybdenum, palladium, rhenium,
tantalum, titanium, tungsten and alloys thereof.
9. The method of claim 1, further comprising a step of positioning
a constriction plate around the non-glass contact surface of one of
said at least one vessel that reduces a flow of the gas mixture
over that vessel and causes a laminar flow of the gas mixture
within said capsule.
10. The method of claim 1, wherein controlling a level of hydrogen
within the atmosphere includes using a closed-loop control system
to monitor and control the composition of the atmosphere.
11. The method of claim 10, where said closed-loop control system
includes: a controller; a plurality of sensors; a humidity feed
system; a heating/cooling control system; an air handler; and an
O.sub.2/N.sub.2 makeup system.
12. The method of claim 11, wherein the humidity feed system, the
heating/cooling control system, the air handler, and the
O.sub.2/N.sub.2 makeup system are connected to a network of pipes
which are connected to the capsule.
13. The method of claim 12, wherein the network of pipes includes a
main pipe attached to a first end of the capsule and the network of
pipes further includes an exit pipe attached to an opposite end of
the capsule.
14. The method of claim 13, wherein the network of pipes further
includes another pipe which has one end attached to the main pipe
and another end connected to an inlet of a tube used to connect a
pair of the at least one vessels where the gas mixture flows around
the tube which also has an outlet through which the gas mixture is
directed back into an atmosphere within the capsule.
15. The method of claim 11, wherein said controller processes
sensor measurements from the plurality of sensors and controls the
humidity feed system, the heating/cooling control system, the air
handler, and the O.sub.2/N.sub.2 makeup system.
16. The method of claim 11, wherein said controller obtains sensor
readings from the plurality of sensors including capsule supply
sensors, capsule sensors and capsule exit sensors.
17. The method of claim 16, wherein said capsule supply sensors
include a flow sensor, a dew point/humidity sensor, a temperature
sensor, an oxygen sensor, and a pressure sensor.
18. The method of claim 16, wherein said capsule sensors include a
flow sensor, a dew point/humidity sensor, a temperature sensor, an
oxygen sensor, and a pressure sensor.
19. The method of claim 16, wherein said capsule exit sensors
include a flow sensor, a dew point/humidity sensor, a temperature
sensor, an oxygen sensor, and a pressure sensor.
20. The method of claim 2, wherein said step of controlling the
cooling of the molten glass further includes using forced
convection to cool the molten glass while the molten glass travels
from one of said at least one vessel to another one of said at
least one vessel.
21. The method of claim 1, wherein the vessel enclosed by the
capsule is permeable to hydrogen gas at the operating temperature
thereof.
22. The method of claim 1, wherein the step of providing an
atmosphere includes controlling the partial pressure of oxygen in
the atmosphere to below 0.21 atmosphere.
23. The method of claim 1, wherein the step of providing an
atmosphere includes providing an atmosphere having a substantially
constant composition and temperature distribution during operation
of the vessel.
24. The method of claim 1, wherein the capsule and the non
glass-contact surface of the vessel enclosed by the capsule defines
a space to which human access is not needed during normal operation
of the capsule.
25. The method of claim 1, wherein the capsule is essentially
leak-tight.
26. The method of claim 25, wherein the total pressure of the
atmosphere inside the capsule is higher than the pressure of an
ambient atmosphere outside of the capsule.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application is a continuation of and claims the
benefit under 35 U.S.C. .sctn.120 of (i) U.S. patent application
Ser. No. 12/366,245, filed on Feb. 5, 2009 and entitled "SYSTEM AND
METHOD FOR CONTROLLING THE ENVIRONMENT AROUND ONE OR MORE VESSELS
IN A GLASS MANUFACTURING SYSTEM;" and (ii) U.S. patent application
Ser. No. 12/362,738, filed on Feb. 5, 2009 and entitled "SYSTEM AND
METHOD FOR CONTROLLING THE ENVIRONMENT AROUND ONE OR MORE VESSELS
IN A GLASS MANUFACTURING SYSTEM," both of which, in turn, claim the
benefit under 35 U.S.C. .sctn.120 of U.S. patent application Ser.
No. 11/116,669, filed on Apr. 27, 2005 and entitled "SYSTEM AND
METHOD FOR CONTROLLING THE ENVIRONMENT AROUND ONE OR MORE VESSELS
IN A GLASS MANUFACTURING SYSTEM," the contents of all three of
which are relied upon and incorporated by reference herein in their
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a system and method for
controlling the environment (e.g., oxygen, hydrogen, humidity,
temperature, gas flow rate) around one or more vessels in a glass
manufacturing system.
[0004] 2. Description of Related Art
[0005] Flat panel display devices like Liquid Crystal Displays
(LCDs) utilize flat glass sheets. A preferred technique for
manufacturing these glass sheets is the fusion process. In the
fusion process, the glass sheets are made by using vessels that
contain refractory/precious metals, e.g. platinum or platinum
alloys. The precious metals are generally considered to be inert in
relation to most glasses, and thus should not cause any inclusions
in the glass sheets.
[0006] However, this is not necessarily valid. There are oxidation
reactions that occur at the metal/glass interface inside the
vessels which lead to the generation of gaseous inclusions in the
glass melt and thus the glass sheet. One of the more common
oxidation reactions that occurs at the metal/glass interface is the
conversion of negatively charged oxygen ions to molecular oxygen
which is caused by the thermal breakdown of water and hydroxyl
species in the glass melt. This phenomenon occurs because at the
elevated temperatures of glass melting and delivery, a low partial
pressure of hydrogen exists in the glass melt. And, when hydrogen
comes in contact with the refractory/precious metal vessel
containing the glass melt, the hydrogen rapidly permeates out of
the vessel, depleting the metal/glass interface of hydrogen. Based
on the chemical balance, for every mole of hydrogen that leaves the
vessel, 1/2 mole of oxygen is left behind at the glass/metal
interface. Thus, as hydrogen leaves the vessel, the oxygen level or
partial pressure of oxygen at the metal/glass interface increases,
which leads to the generation of blisters or gaseous inclusions in
the glass melt. In addition, there are other reactions which
involve the catalyzing or oxidation of other species in the glass
melt such as halogens (Cl, F, Br) which can lead to the generation
of gaseous inclusions. Further, the oxidation reactions can occur
due to electrochemical reactions at the metal/glass interface.
These electrochemical reactions can be associated with thermal
cells, galvanic cells, high AC or DC current applications and
grounding situations.
[0007] Today, there are several known methods that can be used to
address these problematical oxidation reactions which cause the
formation of gaseous inclusions in the glass sheet. One known
method that can be used to help minimize the formation of gaseous
inclusions in glass sheets involves the use of arsenic as a fining
agent within the fusion process. Arsenic is among the highest
temperature fining agents known, and, when added to the molten
glass bath, it allows for O.sub.2 release from the glass melt at
high melting temperatures (e.g., above 1450.degree. C.). This high
temperature O.sub.2 release, which aids in the removal of O.sub.2
bubbles during the melting and fining stages of glass production
results in a glass sheet that is essentially free of gaseous
inclusions. Furthermore, any residual oxygen bubbles are reabsorbed
by the fining agent due to transition from the reduced to oxidized
state on cooling. However, from an environmental point of view it
is not desirable to use arsenic since it is considered a hazardous
material.
[0008] There are several other known methods that do not need
arsenic fining agents to mitigate oxidation reactions which lead to
the formation of gaseous inclusions in the glass sheets. One such
method is described in U.S. Pat. No. 5,785,726 which discloses a
humidity controlled enclosure that surrounds one or more
platinum-containing vessels and is used to control the partial
pressure of hydrogen outside the vessel(s) so as to reduce the
formation of gaseous inclusions in glass sheets. This humidity
controlled enclosure is discussed in more detail below. Although
the method disclosed in the patent mentioned above successfully
reduces the formation of gaseous inclusions in the glass sheets, it
would be desirable to provide an alternative method to prevent the
formation of gaseous inclusions in glass sheets. This need and
other needs are satisfied by the system and method of the present
invention.
BRIEF DESCRIPTION OF THE INVENTION
[0009] The present invention includes a system and method for
controlling the environment (e.g., oxygen, hydrogen, humidity,
temperature, gas flow rate) around one or more vessels in a glass
manufacturing system. In the preferred embodiment, the system
includes a closed-loop control system and a capsule that are used
to control the level of hydrogen around the exterior (non glass
contact surface) of the vessel(s) so as to suppress the formation
of gaseous inclusions and surface blisters in glass sheets. In
addition, the closed-loop control system and capsule can be used to
help cool molten glass while the molten glass travels from one
vessel to another vessel in the glass manufacturing system.
Moreover, the closed-loop control system and capsule can be used to
maintain an atmosphere with minimal oxygen around the vessel(s) so
as to reduce the oxidation of precious metals on the vessel(s).
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] A more complete understanding of the present invention may
be had by reference to the following detailed description when
taken in conjunction with the accompanying drawings wherein:
[0011] FIG. 1 is a block diagram that shows the components of a
glass manufacturing system in accordance with the present
invention;
[0012] FIG. 2 is a graph which shows the amount of blister
generation (measured in area coverage of blisters) versus hydrogen
level in the atmosphere on the exterior surface of a platinum glass
processing vessel that could be used in the exemplary glass
manufacturing system shown in FIG. 1;
[0013] FIG. 3 is a graph that is used to help explain the different
operating conditions in terms of ppm hydrogen vs. temperature that
are possible with known techniques and with the invention;
[0014] FIG. 4 is a photograph showing two glass samples that were
melted in an identical platinum glass processing vessel for about
ten minutes, one of the samples was processed in accordance with
known techniques, the other sample was processed in accordance with
the invention; and
[0015] FIG. 5 is a flowchart illustrating the basic steps of a
method for making a glass sheet in accordance with the present
invention.
DETAILED DESCRIPTION
[0016] Referring to FIG. 1, there is shown a schematic view of an
exemplary glass manufacturing system 100 that uses the fusion
process to make glass sheets 137 in accordance with the present
invention. The glass manufacturing system 100 includes a melting
vessel 110 in which batch materials are introduced as shown by
arrow 112 and then melted to form molten glass 114. The melting
vessel 110 is typically made from a refractory material. The glass
manufacturing system 100 further includes components that are
typically made from platinum or platinum-containing metals such as
Pt--Rh, Pt--Ir, etc, and combinations thereof. The
platinum-containing components include a premelt to finer
connection tube (PMFCT) 113, fining vessel 115 (e.g., finer tube
115), a mixing vessel 120 (e.g., stir chamber 120), a finer to stir
chamber connecting tube 122, a delivery vessel 125 (e.g., bowl
125), a stir chamber to bowl connecting tube 127, a downcomer 130
and an inlet 132. The inlet 132 is coupled to a forming vessel 135
(e.g., fusion pipe 135) which forms the glass sheet 137. Typically,
the forming vessel 135 is made from a refractory material.
[0017] In one embodiment of the present invention, the
melting/delivery system 141 which includes vessels 115, 120, 125
and tubes 122, 127 and 130 is encapsulated or encased within a
capsule 140. A jacket volume 142 is defined between the interior
walls of the capsule 140 and the exterior walls of the components
115, 120, 122, 125, 127 and 130 in the melting/delivery system 141.
The capsule 140 is preferably leak tight to the extent that it may
be used for maintaining a slightly more positive pressure of low
oxygen, moist atmosphere inside the jacket volume 142 that is
greater than the ambient conditions. As shown, the capsule 140 can
be made as one zone which encloses the entire length of the
melting/delivery system 141. Alternatively, multiple capsules 140
can be used as multiple zones where individual capsules 140 enclose
one or more of the vessels 115, 120, 125 and tubes 122, 127 and
130. An advantage of utilizing multiple capsules 140 is the ability
to independently control the atmosphere in a particular area of the
melting/delivery system 141.
[0018] The present invention also includes a closed-loop control
system 144 which controls the environment/atmosphere within the
capsule 140 and prevents the problematical oxidation reactions from
occurring at the metal/glass interface inside components 115, 120,
122, 125, 127 and 130. Again, the problematical oxidation reactions
lead to the formation of gaseous inclusions in the glass sheet 137.
In addition, problematical oxidation reactions with the precious
metal vessels and tubes can lead to the failure of the platinum (or
other precious metals) on the components 115, 120, 122, 125, 127
and 130.
[0019] In particular, the closed-loop control system 144 controls
the atmosphere inside the capsule 140 so as to suppress undesirable
oxidation reactions at the metal/glass interface by causing the
migration of hydrogen into the glass/metal interface. A controlled
level of hydrogen permeation into the glass/metal interface reduces
the production of undesirable species such as molecular oxygen, and
halogens, which in turn prevents the formation of undesirable
gaseous inclusions in the molten glass 114. The hydrogen permeation
into the glass/metal interface is achieved by supplying a higher
partial pressure of hydrogen to the exterior surfaces (non glass
contact surfaces) in the mixing/delivery system 141, relative to
the interior glass/metal interfaces. To accomplish this, a humid,
low oxygen atmosphere, which results in a controlled level of
hydrogen at the non-glass contact surface of the platinum system
that is preferably greater than 12 ppm at 1650.degree. C., is
maintained inside the capsule 140. It should be noted that the
hydrogen level in the atmosphere inside the capsule 140 has an
undetectable amount of hydrogen. However, hydrogen is generated
when the water breaks down at the high temperatures associated with
the molten glass 114. One gas system that could be used to create
this atmosphere would be a mixture of water vapor, oxygen and
nitrogen (or another inert gas like argon or helium). An exemplary
closed-loop control system 144 that uses this gas system to create
such an atmosphere inside the capsule 140 is described next.
[0020] The exemplary closed-loop control system 144 includes a
controller 150 that obtains sensor readings from one or more
locations within and outside the capsule 140. As shown, the
controller 150 can obtain sensor readings from capsule supply
sensors 152, capsule sensors 154 and capsule exit sensors 156 and
156'. In this example, the capsule supply sensors 152 include a
flow sensor 152a, a dew point/humidity sensor 152b, a temperature
sensor 152c, an oxygen sensor 152d, and a pressure sensor 152e. The
capsule sensors 154 include a flow sensor 154a, a dew
point/humidity sensor 154b, a temperature sensor 154c, an oxygen
sensor 154d, and a pressure sensor 154e. And, the capsule exit
sensors 156 and 156' each include a flow sensor 156a and 156a', a
dew point/humidity sensor 156b and 156b', a temperature sensor 156c
and 156c', an oxygen sensor 156d and 156d', and a pressure sensor
156e and 156e'
[0021] The controller 150 processes the sensor measurements and
controls different devices like a humidity feed system 158, a
heating/cooling control system 160, air handler(s) 162 and an
O.sub.2/N.sub.2 makeup system 164. The air handler(s) 162 have
access to air and steam. All of the devices 158, 160, 162 and 164
are connected to a network of pipes 166 which as shown is connected
to the capsule 140. In operation, the controller 150 controls the
devices 158, 160, 162 and 164 to create an environment/atmosphere
inside the capsule 140 where the hydrogen which is generated by the
decomposition of water vapor is done so at a rate that is equal to
or greater than the rate of hydrogen permeation through the metal
walls of components 115, 120, 122, 125, 127 and 130 that would be
occurring if an ambient atmosphere were present at the non-glass
contact surface of the components. And, when there is a higher
partial pressure of hydrogen, the reduction of undesirable species
such as molecular oxygen, and halogens within the molten glass 114
prevents the formation of undesirable gaseous inclusions in the
molten glass 114. Another advantage of having a higher pressure of
hydrogen is that the rate of oxidation of the platinum containing
components 115, 120, 122, 125, 127 and 130 is reduced or possible
eliminated due to the low level of oxygen inside the capsule
140.
[0022] To suppress the formation of inclusions in molten glass 114,
the level of hydrogen on the exterior surfaces of the platinum
containing components 115, 120, 122, 125, 127 and 130 needs to be
equal to or greater than the level of hydrogen on the inside
surfaces of the components 115, 120, 122, 125, 127 and 130. The
hydrogen level on the exterior surfaces of the platinum containing
components 115, 120, 122, 125, 127 and 130 is determined by the
thermodynamic equilibrium of the water decomposition reaction
H.sub.2O.fwdarw.H.sub.2+1/2O.sub.2.
In accordance with thermodynamic tables, the free energy (.DELTA.G)
for the water decomposition reaction is equal to 58,900-13.1 T,
where T is the temperature in degrees Kelvin and G is the free
energy in calories per mole. At a given temperature, the
equilibrium constant for the water reaction can be calculated by
using the relationship K.sub.eq=e.sup.-G/RT, where G and T are as
previously noted, and R is the universal gas constant. Once
K.sub.eq is known, the ratio of the partial pressures of the
various gases involved in the water breakdown can be calculated
where
K eq = ( p H 2 ) ( p O 2 ) 1 / 2 p ( H 2 O ) . ##EQU00001##
[0023] For example, at 1450.degree. C., K.sub.eq is equal to
2.47.times.10 .sup.-5. Thus, if a 75.degree. F. dew point air
environment (pH.sub.2O of 0.030 atmospheres) is heated to
1450.degree. C., then pH.sub.2 is calculated to be
1.59.times.10.sup.-6 atmospheres (1.59 ppm). In view of this
equilibrium, one can readily see that by lowering the partial
pressure of oxygen, while maintaining a constant dew point
(pH.sub.2O) one can substantially increase the hydrogen level in
the atmosphere. It should be noted that the presence of nitrogen
(or other inert gas) in the preferred gas mixture does not directly
participate in the water decomposition reaction. Instead, the
partial pressure of the inert gas affects the partial pressure of
oxygen in accordance with the ideal gas law. And, the change in
partial pressure of oxygen is what influences the equilibrium gases
formed, due to the water decomposition.
[0024] Table 1 shows the effect of water and oxygen level on the
level of hydrogen at various temperatures in the enclosed
environments of a traditional enclosure and the capsule 140.
TABLE-US-00001 TABLE 1 Traditional 1% 0.01% Capsule Enclosure
Oxygen Oxygen 140 Dew Point (.degree. F.) 80 80 80 140 Oxygen (%)
21 (air) 1 0.01 0.5 H.sub.2 concentration @ 0.2 0.9 9 8
1250.degree. C. (ppm) H.sub.2 concentration @ 2 9 88 77
1450.degree. C. (ppm) H.sub.2 concentration @ 11 52 524 463
1650.degree. C. (ppm)
[0025] The traditional enclosure is a room size enclosure that was
made in accordance with one embodiment of the invention in the
aforementioned U.S. Pat. No. 5,785,726. The traditional enclosure
ensures that the partial pressure of hydrogen outside components
115, 120, 122, 125, 127 and 130 in the melting/delivery system 141
is in an amount sufficient to prevent formation of oxygen blisters
in the glass that is adjacent to the vessel/glass interface.
Although the traditional enclosure successfully reduces the
formation of gaseous inclusions in glass sheets it still has some
drawbacks. First, the traditional enclosure is so large that it
makes it difficult if not impossible to maintain a uniform
environment around the components 115, 120, 122, 125, 127 and 130
in the melting/delivery system 141. Second, the traditional
enclosure is so large and the environment is so hot and humid that
it can be uncomfortable to people that walk into the enclosure.
[0026] The capsule 140 and closed-loop control system 144 of the
present invention addresses these drawbacks and other drawbacks
associated with the traditional enclosure. In the preferred
embodiment, the capsule 140 is a relatively small enclosure that
produces a small jacket volume 142 which facilitates better control
of the atmosphere. This is due to fact that a probe reading (such
as relative humidity or dew point temperature) for conditions
inside the capsule 140 is more likely to be representative of
conditions at the exterior metal surfaces of glass processing
equipment because the volume in the capsule 140 is smaller than the
volume in the traditional enclosure. In addition, if there is a
process instability or change in the water content of the molten
glass 114 that leads to an increase in hydrogen permeation
blistering, then there is often no way to respond to this problem
using the traditional enclosure since it may be operating at its
maximum dewpoint. The capsule 140 and closed-loop control system
144 has a better chance of solving this problem.
[0027] As can be seen, the capsule 140 and closed-loop control
system 144 of the present invention is essentially an enhanced
version of the traditional enclosure. Again, the traditional
enclosure uses a humidified air atmosphere around the metal portion
of the melting/delivery system 141. And, the capsule 140 and
closed-loop control system 114 create a low oxygen moist atmosphere
which allows hydrogen levels one to two or more orders of magnitude
greater than that which is possible with the use of a high dew
point air atmosphere in the traditional enclosure. The creation of
this low oxygen moist atmosphere also extends the range of glasses
that can be protected from hydrogen permeation blistering.
[0028] Referring to FIG. 2, there is a graph which shows the amount
of blister generation (area coverage of blisters on the glass
contact surface of platinum) vs. hydrogen level in the atmosphere
on the exterior (non glass contact surface) of a platinum
apparatus. As can be seen, the low hydrogen levels commonly
associated with the humid air atmosphere in the traditional
enclosure had unacceptable blistering over a wide range of
temperatures. And, the high hydrogen atmosphere associated with the
capsule 140 and the closed-loop control system 144 was very
effective in suppressing the blistering in glass. Again, the
traditional enclosure works well but the capsule 140 and the
closed-loop control system 144 of the present invention work even
better at allowing blister suppression over a broad range of
temperatures and glasses.
[0029] Referring to TABLE 1 and FIG. 2, it can be seen that it is
difficult for the traditional enclosure to maintain an atmosphere
with 12 ppm of hydrogen at 1650.degree. C. This is because with the
traditional enclosure it is not possible to create a low oxygen
atmosphere since people can and often enter and exit the enclosed
room. FIG. 3 is a graph that illustrates this difference and other
differences in terms of ppm hydrogen vs. temperature as they relate
to different operating conditions within the traditional enclosure
and the capsule 140. Typically, the area above the curve 302 is the
area where the capsule 140 can operate but it would be difficult to
operate the traditional enclosure. And, both the capsule 140 and
the traditional enclosure can effectively operate in the area below
the curve 302.
[0030] In view of FIG. 3, it can be seen that in order to improve
upon the traditional enclosure then the hydrogen level in the
atmosphere of the capsule 140 should be greater than or equal to
the hydrogen level calculated in the following equation which uses
the equilibrium relationship:
p H 2 ( ppm ) = 78000 .times. - G RT ( 1 ) ##EQU00002##
where G, R and T have been previously defined. This equation and
the graph shown in FIG. 3 were based on the pH.sub.20 and pO.sub.2
conditions in a traditional enclosure that tops out at a 80.degree.
F. dew point. In addition, this equation can re-written in
numerical form as:
p H 2 ( ppm ) = 78000 .times. - 58900 + 13.1 T 1.987 T ( 2 )
##EQU00003##
where temperature is in degrees Kelvin.
[0031] An example of the impact that a moist, low oxygen atmosphere
has on glass is shown in FIG. 4. The photograph in FIG. 4 shows two
glass samples that were melted in identical platinum vessels that
were 0.005'' thick for ten minutes at 1450.degree. C. The glass on
the right was melted using known techniques in a 20.degree. C. dew
point air atmosphere, while the glass on the left was melted in
accordance with the invention in a 20.degree. C. dew point
atmosphere containing 0.01% oxygen. To highlight the bubbles that
were generated at the platinum-glass interface, after testing, the
platinum was peeled from the glass and modeling clay was pressed
into the bubble areas. It is clear that the glass exposed to the
reduced oxygen atmosphere, which had a higher level of hydrogen,
had significantly less blisters than the glass tested in air.
[0032] As described above, the closed-loop control system 144
controls the moist, low oxygen atmosphere within the capsule 140 to
inhibit the generation of gaseous inclusions in the glass sheet
137. In the preferred embodiment, the closed-loop control system
144 accomplishes this by controlling a gas system that has a
mixture of water vapor, oxygen and nitrogen within the capsule 140.
The typical values of oxygen would be from 0.01% to 1%, water from
2 to 20%, with the balance of the gas being nitrogen (or another
inert gas like argon). The gas system could be run as high as 21%
oxygen and have a dew point as high as 200.degree. F. And, a gas
system with 0.01% oxygen and 20% water at a 200.degree. F. dew
point can give a range of hydrogen from 1 to 38,000 ppm at
1700.degree. C. Alternatively, the mixture of gases introduced into
the jacket volume 142 of the capsule 140 may include hydrocarbons
(and oxygen), ammonia, cracked ammonia products and/or combustion
products.
[0033] Referring again to FIG. 1, the glass manufacturing system
100 can also incorporate two optional enhancements which are
described next. The first enhancement involves the use of a
constriction plate 174 (or similar device) within the capsule 140
that constricts the flow of gas over a certain section or sections
of the mixing/delivery system 141. In the preferred embodiment, the
constriction plate 174 is located at an end of the fining vessel
115 such that 95% (for example) of the gas is diverted into pipe
166a and 5% (for example) of the gas flows over the fining vessel
155 and exits through pipe 166b. This configuration enables one to
design a laminar flow of gas within the capsule 140 which enhances
the ability to control of the environment. Alternatively, the
constriction plate 174 would not be needed if the capsule 141 was
shaped such that less volume is present in a particular area
between the mixing/delivery system 141 over which less gas flow
would be desired.
[0034] The second enhancement involves the use of pipe 166c which
provides one way to cool one or more specific components 115, 120,
122, 125, 127 and 130 in melting/delivery system 141. In this
example, the finer to stir chamber connecting tube 122 (FSC tube
122) is cooled. As shown, pipe 166c has one end 168a connected to
the main pipe 166 at a location prior to entering the capsule 140.
And, pipe 166c has another end 168b that is directly connected to
an inlet 170 that provides gas flow around the FSC tube 122. The
FSC tube 122 also has an outlet 172 through which the gas mixture
from pipe 166c is directed back into the atmosphere within the
capsule 140. The second enhancement is an important aspect of the
present invention in that it helps one to better control the heat
transfer in the glass manufacturing system 100. This heat transfer
control can be done at the same time the present invention is used
to control the atmosphere for hydrogen permeation control.
[0035] In another aspect of the present invention, the capsule 140
and the closed-loop control system 144 can be used to cool the
components 115, 120, 122, 125, 127 and 130 in the melting/delivery
system 141 even without the second enhancement. In particular, the
present invention can be used to help cool the molten glass 114
when it is moved from high temperature conditions suitable for
melting to lower temperature conditions suitable for forming.
Typically, the glass 114 needs to be cooled about 400.degree. C. To
help cool the molten glass 114, the capsule 140 and closed-loop
control system 144 uses forced convection which is related to the
gas flow outside the components 115, 120, 122, 125, 127 and 130.
Since, the capsule 140 is relatively small and has openings at
known locations which are used to connect to pipes 166, 166a, 166b
and 166c, the heat transfer can be carefully controlled, and
cooling performance can be replicated from one installation to the
next. Moreover, the laminar flow associated with the first
enhancement described above can also be used to help one better
control the heat transfer.
[0036] In contrast, the traditional enclosure has difficulty
controlling the heat transfer because the gas flow is uncontrolled
and depends on the different local temperatures and air flows
within the enclosed room. As a result, the local cooling rate on
the mixing/delivery system 141 in the traditional enclosure can
only be controlled by the raising or lowering of local zone
electrical heating devices. However, the local cooling rate can be
adjusted inside the capsule 140 by both heater power and gas flow
rate. This allows a larger range of cooling control capacity for
glass flow as indicated in TABLE 2.
TABLE-US-00002 TABLE 2 Glass Flow Capsule 140 Traditional Enclosure
Nominal X X Minimum 0.75X 0.85X Maximum 1.35X 1.2X
[0037] As shown in TABLE 2, the traditional enclosure has a minimum
glass flow boundary of 0.85.times. which occurs when the heater
power reaches its maximum. And, the traditional enclosure has a
maximum glass flow boundary of 1.2.times. which occurs when the
heater power is turned back far enough that some circuits are
turned off. If the circuits are turned off, then the effective
control of cooling is lost. The capsule 140 enables these
boundaries to be extended to 0.75.times.-1.35.times. (for example)
when air cooling is also variable. In addition, the flow boundaries
associated with the capsule 140 are dictated by cooling capacity,
not by head loss or other considerations. Thus, the capsule 140
enables one to control the cooling by forced convection. This type
of control is not possible in the traditional enclosure because the
enclosed room is so large and personnel can enter and leave the
enclosed room.
[0038] Referring to FIG. 5, there is a flowchart illustrating the
basic steps of a method 500 for making a glass sheet 137 in
accordance with the present invention. Beginning at step 502,
molten glass 114 is formed and processed within a melting vessel
110, a fining vessel 115, a mixing vessel 120, a delivery vessel
125, and a forming vessel 135. At step 504, the capsule 140 is used
to enclose one or more of these vessels 110, 115, 120, 125 and 135
and tubes 113, 122, 127 and 130. In the preferred embodiment, the
capsule 140 encloses the vessels 110, 115, 120 and 125 and tubes
122, 127 and 130 which are associated with the melting/delivery
system 141 (see FIG. 1). At step 506, the closed-loop control
system 144 is used to create, monitor and control a humid, low
oxygen atmosphere within the capsule 140. In the preferred
embodiment, the closed-loop control system 144 is used to control
the level of hydrogen within a mixture of gases so that at least 12
ppm of hydrogen at 1650.degree. C. (for example)(see TABLE 1, and
FIGS. 2-3) is present around the exteriors of the enclosed vessels
110, 115, 120 and 125 so as to reduce hydrogen permeation from the
molten glass 114 and effectively suppress the formation of
undesirable gaseous inclusions within the molten glass 114. In
addition, the closed-loop control system 144 can be used to control
the level of oxygen that is present around the exteriors of the
enclosed vessels 110, 115, 120 and 125 so as to reduce the
oxidation of the precious metal on the enclosed vessels 110, 115,
120 and 125 and tubes 122, 127 and 130. Moreover, the closed-loop
control system 144 can be used to control the cooling of the molten
glass 114 while it travels from one of the vessels (fining vessel
115) to another one of the vessels (mixing vessel 120).
[0039] Following are some advantages, features and uses of the
present invention:
[0040] The present invention could be used by any glass
manufacturer that melts, delivers or forms glass in a system in
which the glass contacts a precious metal device having one surface
in contact with the glass and the other surface being a non-contact
glass surface. This precious metal device does not need to be a
vessel but instead could be some other device like a thermocouple
sheath, stirrer or bowl liner (for example). In addition, the
present invention could be beneficial in the manufacturing of Vycor
tubing and sheet. Moreover, the present invention is beneficial in
the manufacturing any type of glass product.
[0041] The present invention reduces the oxidation of the external
surfaces of the platinum containing components. Current technology
relies on a coating, such as Rokide (aluminum oxide) that is placed
on the outer surface of platinum containing components to limit the
contact of air (oxygen) with the precious metal. This invention
provides a means of lowering the oxygen level, which is a key
driver in the undesirable oxidation reaction of platinum. There are
many advantages to using an inert or reducing atmosphere to prevent
platinum oxidation. First, the removal/reduction of oxygen
decreases the rate of oxidation by orders of magnitude. The best
coatings typically decrease oxidation by a factor of 2 to 4.times..
Secondly, the removal/reduction of oxygen eliminates the need to
use thicker sections of platinum in the vessels to stop failures
due to oxidation. As a result, the cost for vessels would be less
than a system designed with thicker sections for improved life.
Thirdly, the use of an inert or reduced gas coverage within the
capsule 140 makes it possible to protect all areas of the precious
metal system, even the areas that are intricate in shape which
would be difficult to coat.
[0042] The present invention can be used in any glass or melting
system in which glass comes in contact with precious metals such as
gold, platinum, rhodium, iridium, molybdenum, palladium, rhenium
tantalum, titanium, tungsten and alloys thereof. This contact could
be in the melting, delivery or forming phase of production.
[0043] This present invention eliminates the need for adding
multivalent species (fining agents) in the glass such as arsenic
and antimony oxides to buffer oxidation reactions at the Platinum
glass interface. And, if a multivalent specie is needed for fining,
its concentration can be minimized. In addition, multivalent
species that are less effective as fining agents can if needed be
used, that are not considered hazardous materials. This increases
the number of possible glass compositions and also allows for a
fully environmentally friendly glass to be produced.
[0044] The present invention requires no internal intervention into
the glass melting/delivery system 141, and can be applied anywhere
on the system from the external surface.
[0045] The capsule 140 can be a simple container or barrier that is
capable of maintaining a positive pressure of the low oxygen
environment. For instance, the capsule 140 can be something as
simple as a plastic or rubber bag or something that is more
permanent like an enclosure which is shown in FIG. 1.
[0046] It should be appreciated that the capsule 140 can also
enclose other components in the mixing/delivery system 141 in
addition to components 115, 120, 125 and tubes 122, 127 and 130.
For instance, the capsule 140 can also enclose components 113 and
132.
[0047] It should also be appreciated that the capsule 140 can have
more or less inlets and outlets than shown in FIG. 1.
[0048] A user of the present invention does not have to be
concerned about having too high a level of hydrogen around the
melting/delivery system 141 which can impact the integrity of the
precious metal vessels. Because, the present invention uses a
nitrogen, water, oxygen environment which makes it difficult if not
impossible to get hydrogen levels to the extent that glass
constituents (e.g., Fe, SN, As, Sb) would be reduced, causing the
metal of the system to be attacked and destroyed.
[0049] For more details about the aforementioned fusion process
reference is made to U.S. Pat. Nos. 3,338,696 and 3,682,609. The
contents of these two patents are incorporated herein by
reference.
[0050] Although one embodiment of the present invention has been
illustrated in the accompanying Drawings and described in the
foregoing Detailed Description, it should be understood that the
invention is not limited to the embodiment disclosed, but is
capable of numerous rearrangements, modifications and substitutions
without departing from the spirit of the invention as set forth and
defined by the following claims.
* * * * *