Note: Descriptions are shown in the official language in which they were submitted.
Z3S
GLASS-MELTING FUl~ACES
Background of the Invention
This invention relates to glass-melting furnaces. More
particularly, the invention relates to the use of electrodes
or other devices inserted at selected locations through the
batch of a vertical glass-melting furnace.
In an electric glass-melting furnace, electrodes are
coupled to a source of electrical power and placed in contact
with a bath of molten glass. Electrical energy flows between
the electrodes and dissipates energy in the form of Joule
heating in the molten glass for melting a blanket of glass-
forming batch materials deposited on and floating atop the
bath. Such electrodes may be inserted through openings in
- wall portions of the furnace as in conventional furnaces or
may be directly placed in contact with the molten glass from
above or through the layer of batch floating thereon as in
the case of a cold crown electric melter.
A significant characteristic of a cold crown vertical
furnace is its relatively great depth, e.g., 10'-15'. This
depth is required in order to produce a specific convection
pattern. An exemplary convection pattern comprises rapidly
moving glass in the upper 2/3 of the furnace, sometimes
hereinafter referred to as the active zone, and slower
moving glass in the lower 1/3 of the furnace, sometimes
hereinafter referred to as the quiescent zone. Such an
arrangement gives the furnace the ability to produce quality
glass at high melting rates. The present invention allows
for the use of a relatively shallow furnace structure.
In conventional vertical furnaces, electrodes are
located at the upper part of the walls near the batch
:1~122~S
blanket. Introduction of the power close to the wall causes
the hottest spot in the furnace to be at the wall. As a
result, the furnace suffers from high corrosion rates and a
short life.
Another problem with conventional vertical furnaces is
that the electrodes suffer from high corrosion and short
life. The electrodes project horizontally through the
furnace sidewall, and may consist of three rods with the
lateral surface area oriented perpendicular to the path of
electrical current flowing there between. Thus, corrosion is
concentrated at the tip of the electrode.
In prior art furnaces, the depth of the furnace must be
increased as one increases the diameter. This is partly the
result of the electrical power being dissipated close to the
walls so that the center of the furnace is much cooler and
produces a strong downward convection which in turn reduces
the thickness of the quiescent zone.
In general, electrodes positioned through the batch
have the advantage of being radially and vertically adjustable
within the batch blanket on the top surface of the furnace.
This adjustability allows optimization of furnace performance
for a particular output.
Batch electrodes are also more easily replaced than
electrodes which extend through openings in the furnace
sidewall. Consequently, the furnace is more reliable.
Also, the batch electrode rod is now vertically placed
within the furnace. With electrical current uniformly
placed over the side of the rod, the corrosion of the elect
trove is minimized and electrode life increased.
Batch electrodes can be placed in a wide variety of
positions. In general, these positions will coincide with
the electrical phases available in a manner that symmetry of
2~35
current flow from the electrodes it maintained. Symmetry of
electrical placement and firing are important and have been
found to favorably affect melting efficiency and enhance
furnace life.
In many glass-melting furnaces molybdenum Molly) it
used as the preferred electrode material. However, because
molt has a relatively low oxidation temperature of about
500~,C, complex protection devices are required to shield the
electrodes from deterioration by contact with oxygen trapped
in the glass-forming batch materials and/or other corrosive
agents therein. Such devices include conventional water-
cooled stainless steel sleeves or specially fabricated glass
contact refractory sleeves which surround the electrode.
These devices are expensive and somewhat short lived. For
example, water cooling tends to dissipate energy intended
for glass-melting purposes and has a deleterious effect on
melting efficiency and glass quality. Protection devices
tend to be heavy and cumbersome and are not easily adjusted
or replaced, thereby diminishing their versatility. Glass
quality may also be affected by contamination of the glass
by materials forming the protective devices which materials
eventually corrode and become mixed with the glass in the
furnace.
A preferred embodiment of the present invention utilizes
a relatively inexpensive and long-lived system for directly
immersing molt rods into a bath of thermoplastic material.
The molt rods are protected from oxidation without comply-
acted peripheral apparatus. The system requires no cooling,
and thus, energy utilization is enhanced. Further, the molt
rods are supported in a relatively simple holder thereby
facilitating adjustment and replacement.
--3--
It should be realized that the present invention is
also applicable to other devices which may be directly
immersed in a bath of molten glass as, for example, stirring
devices, oxygen sensors and thermocouples. Also other
oxidizable materials are contemplated ego. tungsten,
rhenium, columbium, etc.), as long as the oxidizable portions
thereof are protected in the manner set forth herein.
However, in order to simplify the disclosure herein, reference
will mainly be made to the advantages of the present invention
relative to molt electrodes. It is intended, however, that
such other alternatives are to be considered part of the
invention.
Summary of the Invention
A method and apparatus is set forth for operating a
glass-melting furnace having sidewall portions and a bottom
wall forming a relatively shallow vessel for containing a
bath of molten glass, wherein the furnace is electrically
fired by at least one group of oxidizable electrodes inserted
directly into the bath. The method includes the steps of
placing each group of electrodes at selected locations about
the furnace in a symmetrical circumferential pattern about a
geometric center thereof, adjusting each group of placed
electrodes to radial locations relative to said center which
locations are relatively uniformly spaced from the center
and at least a selected minimum distance from a sidewall
portion of said furnace. Each electrode in a group is
electrically fired in a symmetrical electrical pattern
relative to each electrode in the group and each other group
of electrodes such that heat energy within the furnace is
concentrated away from the sidewall portions of the furnace.
The number of electrodes and dimensions of the furnace are
chosen such that melting and refining of glass occurs within
respective relatively narrow band below an upper surface of
- the bath.
A method is also set forth for protecting or shielding
dyes adapted to be directly inserted within a bath of
molten glass. The method includes the steps of dipping the
device into the bath of molten glass along a selected axial
length thereof to a dipping level, allowing the molten glass
to adhere to the device over said selected length and with-
drawing the device and adhered molten glass from the bath at least to a selected operating level above the lower immersion
level, such that portions of the electrode experiencing
temperatures in excel s of an oxidation temperature thereof
are coated with a layer of highly viscous, partially ~olidif ted
glass .
The present invention also provides for a device for
immersion into a supply of glass in a molten state, said device
capable of operating in excess of an oxidation temperature thereof
comprising: an oxidizable rod of a selected length having a tip
end and extending axially therefrom to at least a support portion
thereof, said rod being oxidizable in the presence of oxygen and
at temperatures necessary to melt the glass, said rod being
locatable in said furnace and partially submergible in he
molten glass from the tip end to at least near the support
portion thereof, a relatively thin coating of vitreous
material adhered to a selected portion of the rod between
the tip end and the support portion, said vitreous coating
being relatively highly viscous at temperatures at least
near said oxidation temperature of said rod and remaining
adhered to said rod, and said rod being shielded from oxygen
-5-
by said Catalina where the temperature of said rod is in
excess of the oxidation temperature thereof..
A further aspect of the present invention is a method
of protecting or shielding a device from oxygen within a
glass-melting furnace containing a supply of molten glass,
the device being maintained at elevated temperatures above an
oxidizing temperature thereof comprising the steps of:
dipping the device into the molten glass to a selected
immersion level along a selected length thereof susceptible
to temperatures in excess of the oxidation temperature,
maintaining the device at the immersion level a sufficient
time in order to allow the molten glass to adhere to and
coat the device over said selected length, and withdrawing
the device and the adhered molten glass coating same from
the bath at least to a selected operating level above the
immersion level such that the device is shielded from
oxidizing agents along the selected length.
Description of the Drawing
Figure 1 it a schematic side sectional ill traction ox
a preferred embodiment of a glass-melting furnace of the
present invention including one exemplary batch electrode.
Figure 2 it a schematic top plan view of a typical
electrode layout for the furnace of Figure 1 including a
physics diagram superimposed thereon.
Figure 3 is a plot of furnace height versus melting
area for furnace with and without batch electrodes.
Figure 4 it a fragmented schematic idea sectional
illustration of an electrode (including a phantom view
thereof) immersed in a glass-melting furnace being orated
it accordance with the principle of the prevent invention.
Figure PA i schematic diagram illustrating an inclined
electrode.
-pa-
Figure 5 is a schematic illustration of an alternate
embodiment of an electrode constructed in accordance with
the principles of the present invention utilizing a graded
glass shielding device.
Figure 6 shows another embodiment of the present invent
toil wherein the electrode has an axial opening into which a
purge fluid is introduced.
Description of the Preferred Embodiments
Figure 1 shows a preferred embodiment of a vertical
electric glass-melting furnace 10 of the present invention
illustrated schematically in side section with cross hatching
eliminated for clarity. Preferably the furnace 10 is polyp
gonad or near circular having a geometric center C and
radius R (see Fig. 2). The furnace 10 includes an upstanding
sidewall 14 and a bottom wall 16 having an outlet opening 15
at center C. The furnace 10 contains a bath of molten
thermoplastic material such as glass 12. The bath of molten
glass 12 has a upper surface 18 upon which there it deposited
a quantity of glass-forming batch materials or batch 20.
The batch 20 is in the form of a floating blanket which
insulates the surface 18 of the bath 12 and retains heat
within the furnace 10. The molten glass 12 is initially
melted by conventional means including a gas burner (not
shown). Thereafter continuous melting takes place by means
of one or more groups of current-carrying electrodes 30
(subscripts sometimes omitted) inserted into the bath 12.
Electrodes 30 closes and the sidewall 14 are labeled with
the designation O for outer and the electrodes 30 closest
the center C are labeled I for inner.
Each electrode 30 may be carried by suitable means not
shown herein but clearly disclosed in any one of the above
9 'I
referred to US. patent applications. The electrodes 30 are
free to he moved vertically, radially, circumferential and
angularly. Radial positioning of the electrodes 30 is
especially important for maintaining proper heat distribution.
The outer electrodes 30-0 are placed no closer to the sidewall
14 than a selected minimum spacing or distance S. Heat
energy produced by outer electrodes 30-0 is removed from
sidewalls 14 rendering the same relatively cool in comparison
to prior art furnaces.
It is well known that a temperature gradient in a glass
melting furnace causes the glass to move in convective
rolls. In one embodiment of the present invention it is
preferred that the electrodes 30 produce heat directly under
the batch blanket 20. The outer electrodes 30-O are fired
with a greater power to produce more heat about the periphery
of the furnace 10. The glass 12 in the furnace tends to
move radially inwardly of the furnace and downwardly near
the center C which is relatively cooler. The glass 12 moves
in a convective roll pattern as hereafter described. The
convective roll TV (see arrows) circulates across upper part
of the furnace near the upper surface 18 radially inwardly
towards center C, thence downwardly near the center towards
an interface 21 separating upper active zone A from lower
quiescent zone Q. The glass 12 meets the boundary 21 and
tends to move radially outwardly from center C to sidewalls
14. Thereafter the glass 12 moves downwardly along sidewalls
14 towards bottom 16 and thence radially inwardly across
bottom 16 towards the center C and to outlet 15.
The convective roll TV shown represents the path taken
by freshly melted glass 12 having a minimum residence time
in the furnace 10 necessary to produce good quality product.
It should be clear that some of the glass 12 recirculates in
--7--
lZ~235
the furnace 10 and has a longer residence time. Also, other
patterns are possible. For example, in a furnace having a
refractory metal liner, the outer electrodes 30-0 may be run
cooler than the inner electrodes 30-I creating a "C" convection
pattern. The glass would move along the top of the furnace,
from the center C to sidewalls 14 and thence downwardly
towards bottom 16 and across inwardly to central outlet 15.
The "C" pattern provides for a shorter residence time.
However, in a lined furnace this may be compensated for by
running the furnace at a higher temperature such that high
quality glass may be produced.
The power applied to the outer electrodes 30-0 and
minimum spacing thereof from sidewalls 14 is important for
controlling the velocity and direction of the convective
roll TV. Hot glass 12 tends to remain high in the furnace
12 and cool glass 12 tends to descend. The relative dip-
furriness in glass temperature thus governs the raze at which
glass 12 rises or descends in the furnace 10. If the outer
electrodes 30-0 are overpowered or placed too close to the
sidewalls 14, heat energy concentrated at the electrodes 30-
0 will cause overheating and rapid corrosion of the sidewalls
14. Further, the flow of convective roll TV may be disrupted.
Thus, the glass 12 may follow a path to outlet 15 which does
not provide sufficient residence time to produce good quality
glass. If the outer electrodes 39-0 are far removed from
sidewalls 14 a fast downward flow may occur near said side-
walls causing reduced residence time and increased furnace
wear. Properly placed outer electrodes 30-0 control the
speed of the convective roll TV without overheating the
sidewalls 14.
The furnace 10 has two major vertical zones. Initial
melting of batch 20 takes place in the upper portion of the
235
furnace 10, herein before referred to as the active zone A.
Fining takes place in lower portion of the furnace referred
to as the quiescent zone Q. The respective active and
quiescent zones A and Q are shown schematically separated by
the dotted line 21.
In Figure 1, the sidewalls 14 are shown as extending
above the upper surface 18 of the glass 12. However, for
purposes of discussion herein, the furnace can be said to
have a height, depth or vertical dimension H as shown extend-
in across the respective active and quiescent zones A and. This dimension does not necessarily include a sup (not
shown) present in some furnaces.
Although the furnace may be constructed in various
shapes and sizes, for purposes of simplifying the discussion
and analysis herein, the furnace 10 may be considered to be
circular having radius R as lateral dimension measured from
the center to an interior surface 27 of sidewall 14. For
near circular shapes the lateral dimension should be considered
the shortest distance from the center line to the sidewall
(for example, assuming a regular polygon: the short per pen-
declare to a side.) In non circular arrangements the longer
dimension should control (for example one half the width of
a rectangle or the focal length of an ellipse). In the disk
cushion below, near circular shapes are emphasized because
they are believed to be most efficient.
Figure 3 illustrates that a furnace 10 having batch
electrodes may be significantly reduced in depth. Curve A
shows the relation of depth versus surface area in a vertical
refractory furnace with wall electrodes. Curve B show the
relation for the same type of furnace with batch electrodes.
The curves are relatively close together for small furnaces
twig. less than 25 ft2). However as the furnace size
s
increases the curves follow similar but offset path. For
example, in furnaces having a melting area of between about
100 and 300 ft2 the furnace with batch electrodes may be
about 2 ft. lower in depth. This is a significant reduction
in depth which results in lower construction cost. The
operating cost of such a furnace is also reduced due to
lower heat loss for the smaller sidewall surface area.
Notice that except for relatively small furnaces the depth
should exceed at least 4' overall. In the range 100 ft2-300
ft2 plus, the depth of a furnace without batch electrodes
increases to about 10 ft., including a 3' quiescent zone Q.
In the same range a furnace with batch electrodes has a
depth of about 8 ft. and a similar quiescent zone. The same
quiescent zone is needed to refine the glass but a shallower
active zone is needed for melting because of the improved
efficiency of batch electrodes.
In a typical furnace made and operated in accordance
with the present invention, an aspect ratio thereof Jay be
defined as the vertical dimension H divided by the lateral
dimension equal to the diameter D or twice the radius R. In
a small furnace where D is about 5 feet or less the aspect
ratio should not be less than about 1Ø As the diameter D
increases, the aspect ratio should follow curve B in Figure
3 to about 0.3. It should be understood however, that the
shallowest furnace is desired for the particular lateral
dimension chosen. Further, the dimensions should be chosen
to minimize energy losses as much as possible.
In Figure 2 there is shown a top plan view schematically
illustrating a typical electrode layout for the furnace 10
of Figure 1. In a furnace of the type herein described, two
sets of electrodes are set out. A first set or group of six
main electrodes TOM are located along radial lines at 60~
--10--
intervals or positions (lM-6M) about the center C of the
furnace 10. The main electrodes or mains 30M may be located
at some radial position RUM from the center C of furnace 10
The mains 30M (shown as dark circles) may be electrically
energized by a source ox power (not shown) in a cross fired
arrangement producing fussers PM. A second set of six pairs
of respective inner and outer staggered electrodes SUE,
SUE shown as open circles, are interspersed at six toga-
lions lS-6S circumferential half-way between the main
electrodes 30M. Similarly, the respective inner and outer
staggered electrodes SUE and SUE may be located at
respective radial positions RS-I and RHO Staggered elect
troves 30S-0 and SUE when energized produce a pair of
fuzzier PUS adjacent and in the same sense as each main fuzzier
PM. Other possible arrangement also include aligning inner
electrode SUE in line with mains 30M and cross fired.
Also inner electrode SUE could be placed intermediate
mains TOM and outer electrode SUE and independently fired.
In Figures 1 and 2, assuming a substantially circular
furnace 10 of radius R and depth H, the following are examples
of electrode positions for various nominally sized furnace:
Example I
Furnace Radius R = 10'
Furnace Depth H = 7.5'
No. Electrodes = 15-18
six (6) mains 30M
six (6) outer staggered SUE
three (3) to six (6) inner staggered SUE
Location Radius Angle between Position
E eastwards
30M RUM = 9' 60 lM-6M
30S-0 RHO = 9' 60~ lS-6S
(offset from
mains by 30)
SUE RSI = 3-5 120-60 on line with
outer staggered
electrodes
:l~lZ;~35
Spacing S from sidewall 14 = minimum 1' all electrodes
Example II
Furnace Radius R = 5'
Furnace Depth H = 5'
No. Electrodes = 9
six (6) mains 30M
three (3) staggered (inner)
Location Radius Ankle between Position
Electrodes
30M RUM = Al 60~ lM-6M
SUE RUM = 1.5-2 120 US, US, US
Spacing S from sidewall 14 = minimum 1' all electrodes
Example III
Furnace Radius R = 2.5'
Furnace Depth H = 3'
No. Electrodes = 3, 4 or 6
Position - RUM = US - 1.5-2.0
Angle - 120, 90, or 60
Spacing S from sidewall 14 - minimum 1' all electrodes
In the present invention batch electrodes 30 subscripts
sometimes hereinafter omitted ma be set up as in Example I
spaced from sidewalls 14 and placed along radial lines at
30 intervals. The radial position of each batch electrode
30 is a significant variable. Notice that batch electrodes
30 may be placed near the center C or near the sidewall 14
and that there may be more than one batch electrode 30 on
any radial line. Further it is possible to provide symmetric
eel placement locations, such that, no two electrodes lie on
the same radial line. By placing electrode 30 in these
positions, electrical symmetry of current flow is maintained.
-12-
Inner staggered electrodes SUE placed near the center
C of the furnace 10 (e.g., at RS-I = R/2 or Lucas, have two
advantages. First, by providing power in the center C of
the furnace 10, the melting rate in the center can be increased.
In conventional furnaces the center ordinarily has the
lowest melting rate since it is furthest from wall electrodes.
By placing electrodes 30-I near the center, either the
output of the furnace 10 can be increased or the wall them-
portray can be reduced.
A second advantage of placing inner staggered electrodes
SUE near the center C of the furnace 10 is that the furnace
10 need not be as deep. Power concentrates near the underside
20' of the batch blanket 20 in the active zone A where
melting is desired (see Figure 1). Concentrating power near
the batch blanket 20 tends to produce a relatively stable
quiescent zone Q in about the lower 1/3 to 1/2 of the furnace
10. Ideally, the glass 12 in the quiescent zone Q tend to
move slowly towards outlet 15 thereby providing sufficient
residence time for the glass 12 to fine.
The placement ox main electrodes 30M and outer staggered
electrodes SUE near, but spaced from the sidewall 14 of
the furnace 10 has significant advantages in addition to
those set forth above. The number of electrodes can b
greatly reduced since there is better utilization of elect
trove surface area. That is, significant current slows from
lateral spaces 30 of electrodes 30 rather than from tip
31. For example, in a conventional furnace having a radius
of 10', forty-eight I electrodes are used With the
present invention, electrode usage could be reduced to
between twelve (12) and eighteen lo electrodes.
In a large furnace having a diameter greater than about
5', batch electrodes 30 are placed around the periphery of
-13-
;235
the furnace 10 spaced from sidewall 14 by about 1-2 feet as
well as near the center C thereof. By eliminating convent
tonal wall electrodes and spacing electrodes 30M and SUE
I feet from the wall, the temperature of the sidewall 14
and hence corrosion of the refractory, can be greatly reduced.
In a small furnace 10, electrodes 30 should be placed closer
than 1' to the sidewall in order to produce the desired "S"
convection. If a "C" pattern is desired, the electrodes 30
could be placed at about R/2.
The invention operates as follows: At least one group
of electrodes 30 are arranged in a pattern, one each in a
selected position of the pattern relative to the geometric
center C of the furnace 10. The pattern is symmetrical in
radial and circumferential directions relative to the center
C. Except for small furnaces placement of the electrodes 30
near the sidewalls is restricted to not closer than about 1
foot. Each electrode 30 or groups of electrodes may be
carried separately by a dedicated support arm or other
suitable device (not shown). Likewise, different ones of
the various groups of electrodes 30 may be carried on a
common support (also not shown). Thereafter, the electrodes
30 are then lowered into the furnace 10 through the batch
blanket 20 and energized. Energization of the electrodes
should be symmetrical with each electrode in a group dissipating
substantially the same energy as other ones in the group.
The preferred embodiment seeks to produce uniform melting
across the furnace with relatively high heat near but spaced
from sidewalls 14 and somewhat lesser heat concentrated at
the center C. Of course, other arrangements of electrical
firing are possible and such should be tailored to the
idiosyncrasies of the furnace 10 to provide a heat distribution,
which while not totally uniform, produces good quality
-14-
Z~'~23~
glass.
The electrodes 30 may be operated with their tips 31 at
a selected operating depth DO below the upper surface 18 of
glass 12. Further, the depth ox one group of electrodes,
e.g., the mains 30M in Figure 2, may be different than the
depth of the staggered electrodes SUE and SUE. Also,
adjustments may be made to vary the depth of individual
electrodes if desired. However, for purposes of illustra-
lion herein, the operating depth DO of all the electrodes 30
is assumed to be the same and substantially constant once
determined.
The drawing of Figure 3 illustrates curves for relatively
clear glasses. Such glasses tend to require a relatively
thick active zone A because energy radiates toward the
bottom 16 preventing the thermal stratification that pro-
dupes a clearly defined quiescent zone Q. The temperature
difference between the upper surface 18 of the glass 12 and
the furnace bottom 16 may be as small as 25~C. The furnace
- must be deep enough to produce relatively distinct active
and quiescent zones. Other so-called dark glasses tend to
suppress radiation The active and quiescent zones are
probably more distinct and both Jay by somewhat narrower
than in a furnace melting clear glass. The drawing of
Figure 3 represents the case where active and passive zones
are broadest. It should be apparent that, except for small
furnaces, the overall height of furnaces operated in accord-
ante with the present invention may be reduced by about 2
feet.
For the clear glasses the tip 31 of the electrodes 30
should be placed as close to an underside 20' of the batch
20 without exceeding current density limits or creating hot
spots in the blanket. The operating depth DO of each elect
-15-
trove 30 may be changed by mean set forth in the above
noted patent application and are not detailed herein. It
can be readily appreciated that since adjustments to the
operating depth DO are easily accomplished, adjustment of
the operating characteristics of the furnace it facilitated.
More efficient melting can be achieved because the location
of the tip end 31 of each electrode 30 can be adjusted to
best suit melting characteristics of the particular glass
being melted.
Figure 4 illustrates another embodiment to the present
invention wherein a vertical electric glass-melting furnace
110 is illustrated in fragmented side section. The furnace
110 contains a bath of molten thermoplastic material such as
glass 112. The furnace 110 includes an upstanding side wall
114 and a bottom wall 116. The bath of molten glass 112 has
an upper free surface 118 upon which there is deposited a
quantity of glass-forming batch materials or batch 120. The
batch 120 is in the form of a floating blanket which insulates
the free surface 118 of the bath 112 and retains heat within
the furnace 110. The molten glass 112 is initially melted
by conventional means including a gas burner not shown.
Thereafter, continuous melting takes place by means of a
plurality of current-carrying electrodes 130 inserted into
the bath 112. Only on electrode 130 is shown in order to
simplify the drawing.
Each electrode 130 may be carried in a collar 134
secured to a support arm structure 132. The collar 134 has
an adjustment ring structure 136 for allowing the electrode
130 to slide up and down within a through opening 138 in
- 30 said collar 134. The support arm 132 is shown fragmented
and is suitably supported exterior of the furnace 110 by a
frame structure (not shown) which allows the support arm 132
-16-
SWISS
to move upwardly and downwardly in the direction of the
double headed arrow A in Figure 4. Support arm 132 may be
joined to collar 134 by sleeve 135 which allows individual
placement of electrode 130 (see curved double headed arrow
B). Also, support arm may be moved circumferential about
its frame structure by means not shown (in the direction
into and out of the page as illustrated by double headed
arrow C.)
Although other aspects and embodiments are described
herein, the present invention it primarily concerned with
protecting the electrode from its tip 131 to a point there-
along at 133 just below the collar 134. The electrode 130
is normally i~nersed so that its zip end 131 extend into
the bath 112 to a depth Do, referred to as the operating
level, as measured from the free surface 118. In the phantom
drawing, superimposed on the solid line drawing in Figure 4,
electrode 130 is shown with its tip 131 immersed to a
second or dipping level Do as measured from the tree surface
118 of the bath of glass 112.
The invention operates as follows: a portion of the
electrode 130 from the tip 131 to near the point 133 is
submerged or dipped into the bath 112. The electrode is
held submerged with its tip 131 at the depth Do for several
minutes until it becomes heated sufficiently, Such that, the
glass 123 becomes adhered to the electrode 130 at least
along a portion thereof submerged below the free surface 118
(i.e. from tip 131 to near point 133). After sufficient
time has elapsed for the molten glass 112 to adhere to the
electrode 130, it is partially withdrawn from the furnace
110 up to the operating level Do. Adhered glass shown at
reference numeral 140 forms a coating 140 having respective
upper and lower edges AHAB. The coating 140 covers or
coats a selected length L of the electrode 130 as a relatively
1~2~3~
thin film of thickness t thereby blocking oxygen inflator-
lion and protecting the electrode from deleterious oxidation.
The thickness t of the coating 140 is dependent upon the
temperature and viscosity characteristics of the glass 112.
The adhered glass coating 140 becomes partially solidified
or highly viscous due to the fact that the temperature of
the electrode 130 drops to near a solidification temperature
thereof as one moves away from the tip 131. Also, the batch
120 surrounding the electrode 130 is relatively cool and
insulates the coating 140 prom the high heat of furnace 110.
The depth at which the electrode 130 is operated may
vary about the depth of Do, but for purposes of illustration
herein, the operating level Do of the electrode 130 remain
substantially constant once it is determined. External
cooling of the electrode 130 is not generally necessary
since portions thereof above upper edge AYE which are
exposed to ambient oxygen are cooled by natural convection
to below the oxidation temperature of the molt. Portions ox
the electrode 130 below a-lower edge 140B of the coating 140
are protected from oxidation by immersion in the molter
glass 112.
The present invention has most significant applications
for batch electrodes or electrodes which penetrate a batch
blanket in cold crown vertical melters. In principle,
however, there is no reason why such an electrode could not
be utilized wherever electrodes are presently used in
furnaces (e.g. through the side walls 114 or bottom wall
116) as long as some form of protection it provided to
prevent furnace leaks.
The present invention affords considerable savings over
conventional protective devices. Further, since conventional
devices are typically water cooled, there are significant
-18-
energy savings available resulting in higher melting effi-
shanties.
For certain glasses the tip 131 of the electrode 130
should be placed as close as possible to an underside 120'
of the batch 120 near free surface 118. For other glasses
more efficient melting takes place when the tip 131 of the
electrode 130 is placed further down in the molten glass
112. It can be readily appreciated that these adjustment
are more easily accomplished by utilization of a bare rod
concept herein described. Since the electrode structure
formed of a cylindrical molt rod is significantly lighter
without the stainless steel water-cooled jacket of the prior
conventional furnaces, adjustment of operating level Do of
the electrode 130 is uncomplicated. Thus, more efficient
melting can be achieved because the location of the tip end
131 of the electrode 130 can be fine tuned to best suit
melting characteristics ox the particular glass being molted.
The operating level Do of the electrode 130 may be changed
by simply moving the support arm 13~ upwardly and/or downwardly
from exterior the furnace 110 or by moving the electrode in
support collar 134. Further, the electrode 130 may be
reciprocated between levels Do and Do to periodically replenish
the coating 140.
The outer surface of the electrode 130 may be treated
prior to immersion in the bath 112 in order to protect the
molt and to allow for more adequate adhesion of the glass
layer 140 to the electrode 130. A refractory substance such
as a flame sprayed aluminum oxide sold under the trademark
WRECKED appears to reduce oxygen contamination and has a
beneficial effect on adhesion of the glass layer 140 to the
electrode 130. coating of chromium oxide o'er the surface
of electrode 130 may also enhance adhesion of the glass
--19--
I
coating 140. It has been found that slight oxidation of the
electrode 130 itself may be helpful to glass adhesion. A
coating of molybdenum dieselized may also be used to protect
the electrode 130 from oxidation.
Sometimes gases are evolved during the glass melting
process (see Figure PA). If such gases come into contact
with the electrode 130, oxidation or corrosion thereof may
occur. As a further precaution against oxidation, there-
fore, the electrode 130 may be inclined about the vertical
by means of sleeve connection 135 lee double headed arrow
B). Gas bubbles evolved will tend to float vertically
upward and away from electrode 130.
In Figure 5 there is illustrated an alternative embody-
mint of the present invention wherein electrode 130' is pro-
shielded with a protective glass coating 140'. The electrode
130' has a molt collar 135 located near the tip end 131.
The collar 135 may be threaded, shrink fit or bolted onto
the electrode 130'. A plurality ox different glass-~orming
materials, in the form of unconsolidated gullet or solid
glass annular rings or annular cylinders EYE, may be
located axially of the electrode 130' along a selected
length L' to grade the protective coating 140' thereof. If
glass gullet is utilized for the rings AYE, an alumina
tube 139 of sufficient length may be joined at a lower end
141 to the collar 135 for containing the materials therein.
A fused silica material such as sold under the trademark
VICAR could be used for tube 139. At an upper end 143 of
the tube 139, an annular refractory cap or plug 145 may be
sleeved over the rod 130' and located within the tube 139 to
close a space containing the protective coating 140' therein.
The plug 145 may be a packing material such as FIBERFRAX3
Rope. Additionally, a readily available extrudable silicone
c
sealant 147 such as Dow Corning REV 732 could be placed over
refractory cap 145. A purge line 14~ may be fitted through
opening 149 in plug 145 and goal 147 for the introduction of
a purge gas P interior the tube 139. A purge gas P protects
the molt electrode during startup before the gullet rings
AYE melt. Thereafter, the molted material protects
electrode from oxygen contamination.
The electrode 130' illustrated in Figure 5 might be
suitably clamped to the support collar 134 and slowly lowered
lo through the batch 120 and into the molten glass 112. At
such time, the various layers of protective materials AYE
would become melted or softened and adhere to the electrode
130'. It should be realized that, as in the embodiment of
Figure 4, the protective layer 140 experiences a temperature
gradient when placed in service. The temperature of the
electrode 130' decreases as one moves axially thrilling
from the tip 131 to he point 133 near where it is supported
by collar 134. Different glass compositions may he used for
the rings AYE forming protective layer 140', each
having a different softening and annealing point. Each will
be susceptible to some viscous flow at various temperatures.
By tailoring the compositions of rings AYE prom relatively
hard glasses, for the lowest protective layer AYE near the
tip 131, to relatively soft glasses at the upper end of the
protective layer 133, each will exhibit the proper character-
is tics at its anticipated operating temperature. By grading
the glasses as hereinabove set forth, there is less likely-
hood of thermally shocking the protective layer 140' over
the temperature gradient thrilling. Further, because the
batch layer 120 acts as an insulator from the high heat
generated within the bath of molten glass 11~, the protective
coating 1401 will remain relatively intact even though it is
-21-
~Z235
softened.
The following Example is thought to set forth a suitable
embodiment of a graded protective coating 140' beginning
with the lower ring AYE or relatively softer glass and
progressing to the uppermost ring EYE of relatively harder
glass as follows:
AYE - Borosilicate (Corning Code 7740) (8" long)
137B Alkali Barium Borosilicate (Corning Code 7052)
(7" long)
137C - Borosilicate having a high boric oxide content
as set forth in US. Patent 2,106,744
DOW - Borosilicate glass as in 137C mixed with
increasing amounts of an hydrous boric oxide
from 20 to 40% respectively (I" long each)
Rings AYE - 100 mesh gullet
Tube 139 - VYCOR*brand tubing
Total length of coating - approximately 36"
The present invention also contemplates the use of a
borosilicate glass tube 139 such as Corning Code 7052 having
an expansion comparable with the molt. The tube 139 would
be sealed directly to the electrode 130' without the gullet
fill AYE. The electrode 130' should be preheated in
order to prevent thermal shock.
An advantage of the arrangement illustrated in Figure
5 is that electrode 130' may be prefabricated for quick
insertion into the furnace 110 without any other preparation.
The tube 139 not only contains there within the protective
layer 140' (if in granular form, but also provides or some
protection of the protective layer 140' at least until it is
consolidated during operation of the furnace 110~ The molt
collar 135 would normally be located below the level of the
free surface 118 of the bath 112 shown in Figure 4, and
thus, is protected from oxidation by its immersion in the
molten glass 112.
1;21~2~35
In Figure 6 there it illustrated yet another embodiment
of the present invention. An electrode 130" may have an
axial bore 150 drilled or formed therein. The bore 150
extends generally lengthwise thereof from an open upper end
151 to near tip 152 thereof. purge line 1~3 may be located
in the open end 151 and a purge fluid P introduced therein.
At elevated temperatures, hydrogen or other gases inert with
respect to molt will diffuse there through as shown by dotted
arrow Pd. This embodiment, when dipped, as shown in Figure 1
or otherwise protected from oxidation as elsewhere set forth
herein, is additionally protected from oxidation without
undue energy and materials costs.
While there has been described what are considered to
be the preferred embodiments of the present invention it
will be obvious to those skilled in the art that various
changes and modifications may be made therein without
departing from to invention, and it is intended in the
appended claims to cover all such changes and modifications
as fall within the true spirit and scope of the invention.
