Electric glass melting furnace

A glass heating method and apparatus, such as a glass melting furnace or a forehearth, utilizing a refractory lining and electrically energized heating electrodes. The refractory lining is an erosion resistant material, preferably a chromic oxide refractory, having an electrical resistivity which is less than the resistivity of the molten glass, preferably an E glass, which is being heated. To avoid short-circuiting through the low resistance refractory, the refractory interposed between electrodes of opposite polarity is cooled to a temperature less than the temperature of the molten glass and at which the resistivity of the refractory is materially increased. Where the electrodes of opposite polarity are carried by opposing side walls, the end and/or side walls of the apparatus are cooled. Where the electrodes are all carried by a single wall, that wall is cooled.

BACKGROUND OF THE INVENTION 
In the electric heating of molten glass, it is conventional to confine the 
glass in a heating receptacle, such as a melting furnace or a forehearth, 
which is lined with a refractory. The heating electrodes project through 
the refractory walls, usually either the side walls or the bottom wall, 
into the pool of molten glass in contact with the refractory lining. The 
electrodes, of course, are of opposite polarity, and the glass is heated 
between the electrodes by the current flowing between the electrodes. 
Many different electrode arrangements have been proposed in the prior art 
to vary the electrode heating effects within the molten glass pool. One 
such arrangement utilizes electrodes carried by the opposing side walls of 
a melter or forehearth, the electrodes of each side wall being of the same 
polarity and the electrodes of the opposing walls being of opposite 
polarity. The resultant thermal current is used to heat the entire molten 
glass pool. Such an arrangement is disclosed in the pending U.S. 
application of Dunn et al, Ser. No. 405,851, filed Aug. 6, 1982. 
Another arrangement of heating electrodes involves the insertion of heating 
electrodes of different polarity through a single refractory lined wall, 
usually a bottom wall, for example, as shown in U.S. Pat. Nos. 3,757,020 
and 3,392,237. 
The refractory lining of such furnaces necessarily is electrically 
conductive to a greater or lesser degree, and the conventional electric 
furnace requires the utilization of a refractory which is less conductive 
than the molten glass. Expressed in terms of electrical resistivity, the 
effective electrical resistivity of the refractory must be sufficient 
relative to the resistivity of the molten glass at the operating 
temperature of the glass heating apparatus to avoid any appreciable 
short-circuiting of the heating current through the refractory rather than 
through the molten glass. For this reason, zircon-type refractories of 
high resistivity have been utilized in electrical glass heating apparatus. 
However, such zircon-type refractories are incompatible with certain 
glasses, such as C glass compositions, and are prone to erosion from such 
molten glass compositions flowing through the heating apparatus. Any 
electric glass heating apparatus utilizing such refractories with 
incompatible glass compositions has a notoriously short life. As a result, 
conventional electric glass heating apparatus has been limited to 
compatible, usually easily melted glasses, e.g., those glasses containing 
appreciable amounts of sodium oxide or the like, or to low throughout 
applications or to booster applications as a supplement to primary 
combustion heating. 
The utilization of refractories of higher erosion resistance, such as 
chromic oxide refractories, has not been practical because such 
refractories have an electrical resistivity that is appreciably less than 
the resistivity of the molten glass at the furnace operating temperatures. 
As a result, such refractories short-circuit, and the electric current 
flow through the refractory heats the refractory, so that the heating 
apparatus lining wears excessively and sloughs off into the molten glass 
causing stoning in the glass. Eventually the refractory melts from the 
heating current flowing through the refractory. 
Thus, the use of a chromic oxide refractory has not been practical although 
it has a service life which may be 7-10 times as great as the conventional 
zircon refractory when in contact with molten E glass, for example. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention now proposes a method and apparatus for electrically 
heating glass utilizing refractories which have high erosion resistance 
and low electrical resistivity by cooling the refractory to a temperature 
at which the resistivity of the refractory is increased and the tendency 
of short-circuiting through the refractory is appreciably reduced. 
The heating apparatus may be a melting furnace having a pool of molten 
glass surmounted by a layer of unmelted batch, the pool being confined by 
electrode-bearing side walls joined by end walls and a bottom wall with 
the electrodes of opposing side walls being of opposing polarity. The 
side, end and bottom walls are all lined with an erosion-resistant 
refractory, e.g., a chromic oxide refractory, the electrical resistance of 
which varies inversely with the operating temperature and which has an 
electrical resistivity, at the temperature of the molten glass, which is 
less than the resistivity of the molten glass. 
The heating apparatus, alternatively, may be a forehearth for conveying 
molten glass from a melting furnace to a forming apparatus. Here, the 
forehearth has side walls through which the electrodes extend into the 
molten glass stream to compensate for heat losses from the molten glass 
stream. Preferably, the side and bottom walls of the forehearth are lined 
with a similar erosion-resistant, low resistivity refractory. 
As a third alternative, the heating apparatus may be a melting furnace in 
which all of the electrodes project through a single wall, e.g., the 
bottom wall. Such bottom-entering electrodes of different polarity are 
energized electrically to heat the glass above the bottom wall, and the 
bottom wall as well as the side walls are lined with erosion-resistant, 
low resistivity refractory. 
The present invention proposes the cooling of that low resistivity 
refractory of the heating apparatus which is effectively interposed 
between electrodes of differing polarity to increase the electrical 
resistivity of the refractory and to reduce the tendency of the refractory 
to short-circuit in operation. 
In a melting furnace, as above described, where the electrodes are carried 
by the respective side walls, the refractory tends to short-circuit 
primarily through the end walls beneath the upper level of the molten 
glass. By cooling the end walls, the resistivity of the refractory of the 
end walls is materially increased, and short-circuiting is reduced. 
Similarly, the bottom wall may be cooled to reduce the tendency for 
short-circuiting through the bottom wall. 
In a bottom electrode melting furnace, the electrodes of the bottom wall 
are of different polarity, and cooling of the bottom wall will reduce the 
tendency toward short-circuiting through the bottom wall between 
electrodes of opposite polarity. 
In a forehearth, cooling of the bottom wall increases the resistivity of 
the refractory of the bottom wall and reduces short-circuiting by current 
flow through the bottom wall. 
By increasing the resistivity of the refractory, it becomes possible and 
practical to utilize chromic oxide refractories and similar refractories 
of enhanced erosion resistance, despite their inherent low electrical 
resistivity, so that the throughput of the heating apparatus can be 
increased, and the heating and melting efficiency of the apparatus is also 
increased. The amount of cooling of the side walls is sufficient to 
increase the electrical resistivity of the refractory to a value at which 
short-circuiting through the refractory is reduced, but not so great as to 
materially reduce the temperature of the molten glass being heated. 
This is particularly applicable to glass melting furnaces of the type 
having side wall entering electrodes since the molten glass is heated 
primarily above the electrodes by electrode-generated currents. In such a 
furnace, the heated molten glass circulates primarily vertically upwardly 
from the central space between the electrodes against the overlying batch 
blanket and then downwardly along the furnace side walls. Cooling of the 
end walls and the bottom wall for the purposes of the present invention 
does not materially chill the molten glass because of its rapid, 
circulatory motion well above the bottom wall and along the non-chilled 
side walls. 
Similarly, the cooling of the bottom wall of a forehearth or the bottom 
wall of a furnace having bottom entry electrodes does not materially 
decrease the temperature of the molten glass because the heated molten 
glass moves vertically upwardly from the electrode ends and away from the 
bottom wall. 
In any event, if the molten glass body is cooled undesirably, the operating 
temperature of the heating apparatus can be increased to compensate for 
any molten glass temperature reduction caused by the chilling of any wall 
for the purpose of increasing refractory resistivity. 
The cooling of the refractory lining is effective to reduce the 
short-circuit heating of the refractory to a level at which the refractory 
does not melt nor slough off into the molten glass body, but it may not 
prevent all short-circuiting through the refractory. However, the small 
amount of short-circuiting which does occur merely imparts a minor degree 
of heat to the refractory, and neither the refractory life nor the heating 
efficiency of the apparatus is materially affected. 
Further, the tendency to short-circuit through the refractory is directly, 
apparently linearly related to the distance through which the current must 
flow. The primary heating path is from one electrode tip to the opposing 
electrode tip of opposite polarity while the short-circuit path is 
peripheral to the pool of molten glass and through the refractory. Where 
the refractory path is from one electrode through an end wall to the other 
electrode or from one electrode through the bottom wall to the opposing 
electrode, the path for short-circuiting is always materially greater than 
the primary electrode-to-electrode heating path through the molten glass. 
As a result, it is not necessary to reduce the temperature of the 
refractory to the extent theoretically necessary to prevent any 
short-circuiting. 
Thus, the refractory need not be cooled to such an extent that its 
electrical resistivity is increased to the numerical value of the 
electrical resistivity of the glass. It is only necessary to cool the 
refractory to an extent such that its electrical resistivity is increased 
sufficiently to prevent substantial short-circuiting through the 
refractory path as compared to the electrode-to-electrode current path.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
As illustrated in FIGS. 1 through 3 of the drawings, the present invention 
is incorporated into a melting furnace 10 of the type illustrated and 
described in detail in the pending United States patent application of 
Charles S. Dunn et al, Ser. No. 405,851, filed Aug. 6, 1982 and assigned 
to the assignee of the present invention. 
More specifically, the furnace 10 comprises peripheral refractory side 
walls 11, end walls 12 and a bottom wall 13 formed of suitable refractory 
material and retained in position by appropriate supporting metal 
framework and foundations (not shown). The bottom wall is provided with a 
generally rectangular outlet opening 14. Preferably, the lining 11 is a 
conventional refractory identified in the art as a sintered zircon 
refractory having substantially the following composition: 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
ZrO.sub.2 65.5 
Al.sub.2 O.sub.3 
0.5 
Fe.sub.2 O.sub.3 
0.1 
TiO.sub.2 0.3 
______________________________________ 
The refractory side walls 11, end walls 12 and bottom wall 13, including 
the bottom wall opening 14 are lined with an erosion-resistant, but 
low-resistivity refractory, preferably a dense chromic oxide refractory, 
indicated generally at 20 and including side wall portions 21, end wall 
portions 22, bottom wall portions 23 and an opening lining portion 24 
which, in cooperation, form a complete lining for an interior space 25 for 
containing a body of molten glass. It will be noted that the side wall 
lining 21 and the end wall lining 22 extend vertically throughout the 
extent of the molten glass pool space 25 but terminate short of the upper 
ends of the side walls 11 and end walls 12. The pool of molten glass in 
the space 25 is surmounted by a blanket of unmelted, particulate glass 
batch 26. 
The side walls 11 and the side wall lining 21 are pierced by heating 
electrodes 30 which are connected to a power supply as indicated 
schematically in FIG. 2. The electrodes 30 piercing each side wall 11 and 
lining 21 are of the same relative polarity while the electrode 32 
piercing the opposite side wall 11 and lining 21 are of relatively 
reversed polarity. The power supplied from the power supply 31 supplies 
heating current to the electrodes 30 and 32 to heat the body of molten 
glass in the space 25 in the manner which is described in detail in the 
above-identified application of Dunn et al, and in the copending 
application of Eugene C. Varrasso, Ser. No. 342,869, filed Jan. 26, 1982, 
now U.S. Pat. No. 4,435,811, issued Mar. 6,1984, assignee of the present 
invention. 
As illustrated in FIGS. 1 and 2 of the drawings, an outlet opening 14 
circumscribed by the refractory portions 14 and 24 is provided in the 
bottom of the furnace, and this opening 14 communicates through a bushing 
block 27 with a lower forming apparatus 28, illustrated in the form of a 
bushing for forming filaments of fiberglass, which filaments are drawn 
downwardly about a collection roller 29 to a conventional winder (not 
shown). 
As explained in the above-identified Dunn et al application, the heating of 
the molten glass in the space 25 occurs primarily between the inboard ends 
of the electrodes 30 and 32, and heated molten glass between the 
electrodes rises vertically within the space 25 upwardly into contact with 
the undersurface of the batch blanket 26 due to the convection currents 
generated by the hottest glass between the electrode ends. The rising hot 
molten glass then flows outwardly along the undersurface of the batch 
blanket 26 and then downwardly along the outer wall linings 21, 22 back to 
the location of the electrodes 30, 32. 
Some of the downwardly flowing glass flows past the electrode location 
downwardly toward and through the outlet opening 14 and the bushing block 
27 into the forming apparatus 28. Due to the rising convection currents 
generated between the electrodes 30 and 32, the hottest glass of the 
molten glass pool within the space 25 is located generally above the 
location of the electrodes 30 and 32, and this glass is circulated and 
recirculated by convection from the electrodes 30 and 32 to melt the batch 
blanket 26. A minor amount of the thermally recirculated glass equal to 
the throughput of the bushing 28 flows downwardly past the electrodes. 
This quantity of glass is cooled in successive isothermal planes to the 
desired temperature for introduction into the bushing. Thus, the glass 
beneath the electrodes 30 and 32 is generally cooler than the glass above 
the electrodes 30 and 32, and this cooler glass flows through the 
substantially isothermal planes downwardly through the outlet opening 14 
and the bushing block 27 into the bushing 28. 
The present invention, while applicable to any glass composition, is 
particularly applicable to low flux content glasses, such as fiberglass 
compositions of relatively high melting point. Where the glass composition 
being melted is E glass, the hottest glass, i.e., that glass above the 
electrodes 30, is generally at a temperature on the order of 2700.degree. 
F. to 2800.degree. F. (1482.degree. C. to 1538.degree. C.) while the glass 
entering the bushing 28 is substantially cooler, generally at a 
temperature on the order of 2300.degree. F. to 2450.degree. F. 
(1260.degree. C. to 1343.degree. C.). A typical "E" glass composition is 
as follows: 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
SiO.sub.2 54.5 
Al.sub.2 O.sub.3 
14.5 
Fe.sub.2 O.sub.3 
0.4 
CaO 17.5 
Mgo 4.4 
Na.sub.2 O 0.5 
B.sub.2 O.sub.3 
6.5 
F.sub.2 0.3 
______________________________________ 
Glasses other than E glass can be suitably utilized in the present 
invention. For example, C glass can be formed into fibers in the bushing 
28. A typical C glass composition has the following composition: 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
SiO.sub.2 65.1 
Al.sub.2 O.sub.3 
3.7 
Fe.sub.2 O.sub.3 
0.4 
CaO 14.3 
Mgo 2.8 
Na.sub.2 O 8.1 
B.sub.2 O.sub.3 
5.5 
______________________________________ 
The refractory lining for the molten glass space 25 must be compatible with 
the glass being melted, i.e., the refractory must be inert to the glass 
composition at the operating temperature of the heating apparatus, and it 
must resist erosion by the glass particularly at the regions above the 
electrodes where the molten glass is rapidly circulated by the thermal 
convection currents generated by the heat imparted to the molten glass 
between the electrode tips. 
It has been found that a dense chromic oxide refractory, such as that 
manufactured by Corhart Refractories of Louisville, KY under the tradename 
"C-1215 Chromic Oxide Refractory" is compatible with E glass. This 
refractory has the composition: 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
TiO.sub.2 3.8 
Cr.sub.2 O.sub.3 
92.7 
Fe.sub.2 O.sub.3 
0.4 
Impurities 3.1 
______________________________________ 
The above refractory can also be used with C glass of the above 
composition. Also compatible with C glass, but not with E glass, is a 
refractory sold by The Carborundum Company of Falconer, N.Y., under the 
tradename "Monofrax E". This refractory has the composition: 
______________________________________ 
Ingredient % by Weight 
______________________________________ 
Cr.sub.2 O.sub.3 
79.7 
Mgo 8.1 
Fe.sub.2 O.sub.3 
6.1 
Al.sub.2 O.sub.3 
4.7 
SiO.sub.2 1.3 
Total Alkali 0.1 
______________________________________ 
As above explained, the above-defined refractories and other similar 
refractories are utilized as the heating apparatus linings 21, 22, 23 and 
24 because of their compatibility with the desired glass compositions and 
their high erosion resistance to the molten glass circulating within the 
space 25 and flowing through the outlet 14 and the bushing block 27 into 
the bushing 28, particularly at the elevated temperatures at which the 
glass is melted and conditioned within the furnace 10. However, the 
electrical resistivity of the refractories of the linings at the operating 
temperatures of the furnace 10 is less than the electrical resistivity of 
the molten glass body in the space 25 and flowing through the furnace 10 
into the bushing 28. 
As illustrated in FIGS. 7 and 8 of the drawings, these differences in 
electrical resistivity may be readily ascertained. In FIG. 7, the 
electrical resistivity of the high density chromic oxide refractory C-1215 
is plotted vertically against the temperature in both degrees Centigrade 
and degrees Fahrenheit which is plotted horizontally. Additionally, the 
electrical resistivity of E glass is plotted in FIG. 7. It will be seen 
from the chart of FIG. 7 that E glass has an electrical resistivity of 
about 12 ohm-centimeter at 2700.degree. F. (1482.degree. C.) while the 
refractory at the same temperature has an electrical resistivity of only 
about 2 ohm-centimeter. The refractory has an electrical resistivity of 
about 12 ohm-centimeter at a temperature of 2012.degree. F. (1100.degree. 
C.). Similarly, the Monofrax E refractory has an electrical resistivity 
which is less than that of E glass at the furnace operating temperature, 
as shown in FIG. 8. 
Since the electrical resistivity of the refractory lining portions 21-24 is 
less than the electrical resistivity of the molten glass in the space 25, 
the electrodes 30 and 32 will short-circuit through the lower resistivity 
refractory in preference to flowing through the molten glass of higher 
resistivity, and the current from the power supply 31 will heat the 
refractory rather than the molten glass. As a result, the refractory will 
be heated and, if not melted, will slough off into the molten glass within 
the space 25 forming stones or other solid discontinuities in the molten 
glass. 
The electrode current will not short-circuit through side walls 11 and the 
refractory 21 of the side walls since there is no appreciable flow of 
electricity between the electrodes of the same polarity carried by the 
side walls. 
To prevent short-circuiting through the refractory lining 22 of the end 
walls 12 and the refractory lining 23 of the bottom wall 13, the lining 24 
of the opening 14 and the lower bushing block 27, these areas preferably 
are water-cooled to a temperature at which the electrical resistivity of 
the refractory is increased appreciably. This is accomplished by means of 
heat exchangers 35 mounted on the exterior of the end walls 12 and heat 
exchangers 36 mounted on the exterior bottom surface of the bottom wall 
13. These heat exchangers are of any conventional design and preferably 
are of the type which provide labyrinthian passages through which cooling 
water is flowed as indicated by the appropriate directional arrows of 
FIGS. 1, 2 and 3. By so cooling the side walls 12 and the bottom wall 13, 
the chromium oxide refractory lining is cooled to an extent such that the 
electrical resistivity of the lining is appreciably increased and 
short-circuiting through the lining is minimized. 
As will be clear from the glass circulation diagram of FIGS. 2 and 3 and 
the above disclosure, the hottest glass and the most rapidly circulating 
glass are located above the plane of the electrodes 30 and 32. Thus, the 
most severe erosion and short-circuiting problems exist in the upper 
regions of the furnace 10, while the molten glass flows in cooler, 
essentially isothermal zones of relatively quiescent character along the 
furnace bottom 23 and through the outlet opening 14 and the bushing block 
27. Accordingly, it is possible to line these areas with a compatible 
zircon refractory or similar high resistivity, non-water cooled 
refractory, if desired. 
In that version of the invention illustrated in FIG. 4 of the drawings, the 
same principles as described in connection with FIGS. 1 through 3 are 
applied to a forehearth 40 which is simply a glass channel interconnecting 
a melting apparatus and a forming apparatus and through which molten glass 
flows. In FIG. 4, the forehearth side walls 41 and bottom wall 42 are 
formed of a suitable refractory, preferably a zircon refractory, as 
described in connection with FIGS. 1 through 3 and the side walls 41 and 
the bottom wall 42 are lined with a lining 43 and 44, respectively, formed 
of an erosion-resistant refractory of low electrical resistivity, also as 
described in connection with FIGS. 1-3, preferably a chromic oxide 
refractory such as those earlier herein disclosed. The side walls 41 and 
43 are pierced by opposing electrically energizable electrodes 45 by means 
of which the molten glass body 46 flowing through the forehearth is heated 
to compensate for any heat losses therein. 
In accordance with the principles of this invention and in order to prevent 
short-circuiting through the bottom wall refractory lining 44, a heat 
exchanger 50 is provided in full surface contact with the undersurface of 
the bottom wall 42, this heat exchanger being of the same type as those 
earlier disclosed in connection with FIGS. 1-3. The heat exchanger 50 
cools the bottom wall 42 and the bottom wall lining 44 to an extent such 
that the electrical resistivity of the lining 44 is substantially 
increased and the tendency for short-circuiting between the electrodes 45 
by electrical flow through the lining 44 is reduced. 
In that embodiment of the invention illustrated FIGS. 5 and 6, the 
principles of the present invention are embodied into a glass melting 
furnace having bottom entering electrodes. More specifically, the furnace 
60 comprises refractory side walls 61 and a refractory bottom wall 62, 
each of the side walls 61 and the bottom wall 62 being provided with an 
erosion-resistant lining 63 and 64, preferably of a chromic oxide 
refractory material as hereinbefore disclosed. One of the side walls 61 
and its lining 63 is provided with an exit port 65 through which molten 
glass flows from the pool of molten glass 66 confined by the side wall 
linings 63 and the bottom wall lining 64. This pool of molten glass 66 is 
surmounted by a layer of particulate, unmelted glass batch 67. 
Four electrodes 70 project upwardly through the bottom wall 62 through the 
bottom wall lining 64 into the molten glass pool 66, and these electrodes 
are energized by a power supply (not shown) effective to energize the 
electrodes with melting current of opposing polarity. The number of 
electrodes and their geometric arrangement, as illustrated in FIGS. 5 and 
6, is schematic and is intended merely as representative of any of the 
numerous well known, commercially available bottom entry glass heating 
electrode arrangements. Suitable electrode arrangements and suitable power 
supplies for such electrodes are well known in the art and are disclosed, 
for example, in the U.S. patents to Gell, No. 3,683,093; Orton, No. 
3,395,237; and Holler et al, No. 3,836,689, among others. 
Since the electrodes 70 are of opposite polarity and are carried by the 
common bottom wall 62 and 64 of the furnace 60, short-circuiting through 
the low resistivity bottom wall lining 64 is prevented by cooling the 
bottom wall 62 by means of a heat exchanger 75 in full face-to-face 
contact with the undersurface of the bottom wall 62 and receiving cooling 
water for circulation therethrough in the manner hereinbefore described. 
By cooling the bottom wall 62 and the lining 64 for the bottom wall, the 
resistivity of the bottom wall is increased to an extent such that 
substantial short-circuiting through the bottom wall 64 does not occur for 
the reasons and in the manner heretofore described. 
In the furnace of the type illustrated in FIGS. 5 and 6, the coolest glass 
in the furnace is that adjacent the bottom wall 62 and 64, since the 
heated glass from the electrodes 70 rises in the furnace and flows by 
convection away from the bottom wall. Further, glass supplied to the 
forming apparatus (not shown) through the aperture 65 is at a temperature 
which is substantially less than the temperature of the glass at the upper 
ends of the electrodes 70. Since the electrical resistivity of both the 
molten glass of the pool 66 and of the bottom wall lining 64 varies 
inversely and exponentially with temperature, it will be seen that the 
glass at the primary flow path between the electrodes is at a 
substantially higher temperature than the temperature of the lining 64. 
This temperature differential may range from about 300.degree. F. to about 
500.degree. F. If the heat exchanger 75 then cools the lining 64 to a 
greater extent, then this temperature differential increases even further 
and the relative resistivity of the lining 64 is increased to an extent 
such that short-circuiting through the lining 64 will be minimized.