CA1065610A - Method and apparatus for making molten glass - Google Patents

Abstract

METHOD AND APPARATUS FOR MAKING
MOLTEN GLASS

Abstract of the Disclosure In a continuous, fuel-fired, glass melting furnace, a pair of substantially horizontal, longitudinally extending electrodes are provided in the molten glass closely adjacent to the sides of the floating batch blanket for supplying electri-cally generated booster heat to the underside of the batch blanket. The electrodes extend a substantial distance into the melting zone of the furnace through the fill end wall.
The electrodes also serve as physical barriers that prevent the batch blanket from drifting into contact with sidewalls of the furnace. The electrode arrangement boosts melting rates with efficient utilization of electrical energy, and avoids furnace wall erosion and unbalanced melting conditions.

Description

This is a divisional of Canadian Patent A?plication Serial No. 240,046 filed November 19, 1975.
20 Background of the Invention - This invention relates to the melting furnace of a glassmaking operation, and in particular to a method and appara-: tus in which electric heating is employed to boost the output of a furnace whose primary source of heat is the burning of liquid or gaseous hydrocarbon fuels.
The conventional continuous glass melting furnace is provided with an inlet and an outlet at opposite ends, raw, pulverulent batch material being introduced through the inlet, and molten glass being drawn off at the outlet. The heat for melting and reacting the batch material is furnished by large jets of flame projected across and above the pool of molten glass in the furnace. Since the melting rate of such a furnace . . .
,: -- 1 --': ' ' .~

.. : .

: .

. . .
. .
.~ ': .
!

is restricted by the limited ability of the walls to withstand high flame temperatures, various proposals have been made to speed the melting rate and boost total output by providing auxiliary electric heaters beneath the surface of the pool of molten glass. Such heaters are generally comprised of two or more electrodes inserted into the molten glass, between which alternating current is passed to heat the glass by the Joule effect. Typical prior art electric booster heating arrangements are shown in the following United States Patents: '

2,397,852. . .Gentil. . .April 2, 1946 2,749,378. . .Penberthy. . .June 5, 1956 2,767,235. . .Herrold et al. . .October 16, 1956 2,832,958. . .Penberthy. . .April 29, 1958 Although such arrangements may supply some extra heat to the melting operation, they do not provide the most efficient utilization of electrical energy, and they concen-trate the heating effect in portions of the molten glass that are closely adjacent to the walls of the furnace, thereby promoting erosion of the walls. This erosion is detrimental not ~ . . .
only because furnace life is shortened, but also because it causes greater numbers of particles from the walls to enter the molten glass, which, because they are of a different composition and difficult to melt, appear in the final product as inhomogeneities or defects known as "stones". Each of the above-cited patents shows a relatively large number of short : `
electrodes inserted through the furnace walls. Because current density will be greatest near the electrodes, all of these arrangements produce the hottest temperatures close to the walls, and thus promote erosion of the adjacent wall areas.
The Gentil patent also requires the batch material to be melted by the electrodes in small doghouses before entering the furnace.

: .

, , ~s~0 That arrangement places partially melted batch material, which - is even more corrosive than molten glass, into direct contact with the doghouse walls, and at the same time requires extremely high temperatures within the small space of the doghouses in order to effect complete melting there. Gentil's doghouse walls would therefore be subject to a high rate of erosion.
The erosion could be slowed by cooling the wall areas around each electrode in the prior art arrangements, but to do so would waste a significant portion of the thermal energy provided by the booster heating.
Another problem encountered in continuous glass melting furnaces is the directional instability of the layer of unmelted or partially melted batch material, known as the batch blanket, -~ which floats on the surface of the pool of molten glass. The end of the blanket farthest into the furnace often tends to drift against one of the sidewalls, which not only brings the corrosive batch material into contact with the sidewall, - but also establishes a persistent, unsymmetrical heating and circulation pattern in the furnace which is highly undesirable.
It is an object of this invention to overcome the draw-backs associated with electric booster heating in a glassmaking process by providing an arrangement that efficiently directs electrically generated heat to the zone where it is best utilized, while at the same time avoiding increased furnace wall erosion and improving the directional stability of the batch blanket.
These and other ojbects will become apparent from the following description of the invention.
.

- 3 --Summary of the Invention -~ The above-noted parent application Serial No. 240,046 describes and claims a continuous glass melting furnace with a pair of long electrodes that extend longitudinally and sub-stantially horizontally from the inlet end wall beneath the batch inlet opening along the sides of a substantial portion .
of the batch blanket. In the preferred embodiment, the electrodes ;
are submerged just beneath the surface of the molten glass, closely adjacent to the sides of the floating batch blanket and spaced from the sidewalls of the furnace. With the electrodes thus located, the passage of current through the molten glass generates heat in the coolest portion of the glass in the -- melting zone, the portion immediately beneath the blanket oE
unmelted batch, and yet does not appreciably increase the ;
temperature near the sidewalls. The only wall subjected to significant additional heating action is the inlet end wall, which in most cases will not be detrimentally effected since that wall is the coolest in the melting zone due to its proximity to the cold, newly-introduced batch material.
The electrodes are considerably longer than those pre-viously deployed in such a position in the prior art, thereby yielding a number of advantages. Because of the greater length, a given power input is distributed over a larger area, and thus - a large amount of thermal energy can be added to the furnace -without creating unduly high temperatures in the vicinity of the - electrodes. Because of this relatively low power density, erosion of the wall through which the electrodes are inserted will not be appreciably increased. Further advantages are realized from the present invention when it is desired or becomes necessary to addi,-onally impede erosion at the conjunction of the electrodes and the end wall by applying cooling means (such as water-cooled pads) to the wall.

- 4 ~

~ ;56 .
Since the electric booster heating of this invention is provided ;
by two large electrodes rather than many small electrodes, cooling can be limited to merely two areas on the end wall, thereby subtracting only a small fraction of the booster energy input. And because of the great length of the electrodes, a large majority of the heating takes place far removed from, - and unaffected by, any cooling of the wall.
Another major benefit derived from the inventive electrode arrangement is that the electrodes act as physical barriers - 10 that maintain the floating batch blanket spaced from the side-walls of the furnace, thereby maintaining symmetrical heating -conditions in the furnace and p,reventing the erosion that can occur when the batch comes into contact with the sidewalls.
Guiding the batch down the center of the furnace is also advantageous in that better melting conditions are encountered there than along the cooler regions near the sidewalls.
Accordingly, the length of the electrodes is selected not only to yield a low power density, but also to provide a lateral restraining effect sufficiently far downstream from the inlet that there is little or no chance of any part of the batch blanket drifting into contact with a sidewall. This guiding function of the electrode arrangement may be realized even when no electric current is-being passed through the electrodes.
Related to this aspect of the invention is Canadian application Serial No. 240i,012 of~ Wright M. Welton filed on November 11, - 1975, assigned- to the assignee of the present application, PPG Industries, Inc., and entitled "Method and Apparatus for Making Molten Glass with ~atch Guiding Means".
The present divisional is therefore directed to the guiding aspect of the rod-like electrodes and accordingly the invention herein claimed provides a continuous process for making molten glass wherein glass batch material is introduced s~

into a melting furnace through an inlet opening at an end - wall of the furnace to form a blanket of batch material on the surface of a pool of molten glass within the furnace, the blanket extending longitudinally into the furnace Erom the inlet end toward an outlet at an opposite end, the improvement comprising maintaining the batch blanket spaced from the side-walls of the furnace by means of rods that extend substantially horizontally through the inlet end wall beneath the inlet opening and through the molten glass along the sides of the batch blanket.
Description of the Drawings FIG. l is a vertical section along the length of a typical glass melting furnace incorporating a preferred embodiment of the present invention.
FIG. 2 îs a horizontal section through the same furnace shown in FIG. 1.
Description of the Preferred Embodiment This description refers specifically to the type of furnace conventionally employed in the manufacture of quality flat glass, but it should be apparent that the advantages of the invention render its inclusion in virtually any continuous ~ -glassmaking operation desirable.
Referring to FIGS. l and 2, there is shown in vertical and horizontal cross-sectional views, respectively, a convention-al, continuously fed, cross-tank fired, glass melting furnace having an enclosure formed by bottom ll, roof 12, and sidewalls 13 made of refractory materials. Overall progression of the glass is from left to right in the figures, toward an outletop~ng ~ 16 at a downstream location. Glass batch material 1~ is ; 30 introduced through inlet opening 15 in an extension 20 of the ;
furnace known as the fill doghouse. The batch may be introduced intermittently by feeding means (not shown) to form a ridged ~ ~' ~56~
blanket floating on the surface oE the molten glass 21 as shown, or it may be fed continuously to form a uniform blanket. Usually the most effective practice is to feed the batch across approximately the central two-thirds of the furnace width. Heat is provided by flames issuing from burner ports 22 spaced along the sidewalls, which are directed onto and across the molten glass. Although some heat for melting is obtained by conduction from the molten glass, the batch is melted primarily by radiant heat from above. Since unmelted batch acts as a relatively good heat insulator, melting takes place primarily from the top down and is thus not as rApid ~ -~
- as is sometimes desired. `
In order to supply more heat for melting, the present ~ -invention adds electrically generated heat closely adjacent to the underside of the batch blanket by providing a pair of electrodes 23 which extend into the furnace through holes drilled through the end wall 24 beneath the inlet opening.
The electrodes may be compr!ised of commercially available carbon rod electrodes having diameters of several inches and provided in sections that are several feet long and threaded on the ends to permit any number of sections to be connected. `~
Although not required, each electrode may be provided with a small axial bore along its length to permit gases generated at the glass/electrode interface to bleed through the carbon, into the electrode, and pass to the exterior of the furnace rather than generating bubbles in the molten glass. Although other materials are known as suitable for glass furnace elec-trodes, carbon rods (usually including a significant amount of graphite) are preferred here because of their rigidity at high temperatures and because any erosion of carbon from the electrodes does not contaminate the glass, since glass batch normally includes some carbon as an ingredient. The diameters .
.
' ' ' ' ~t6S~
of the rods will depend upon their lengths, in that sufficient strength must be provided to counteract the bending force produced by the buoyancy of the lighter carbon rods in the molten glass. For example, diameters on the order of about six to about nine inches (15 tio 23 centimeters) would be satisfactory ;
for electrodes that penetrate as much as about 20 feet (6 meters) into the molten glass, as would be typical in a large commercial melting furnace. Even longer electrode penetrations are contemplated, however, and can be readîly accommodated by utilizing rods with appropriately larger diameters, carbon electrodes as large as 24 inches (62 centimeters) in diameter being commercially available.
In order to direct the booster heat primarily toward the cool underside of the batch blanket, the electrodes are preferably located as close as possible to the sides of the batch blanket and submerged in the molten glass at a minimal depth sufficient to just cover the electrodes and prevent oxidation of the carbon in the atmosphere of the furnace. -~
Preferably, this depth may be on the order of one to two inches (2.5-5 centimeters) in a large scale commercial furnace. Because the batch extends several inches beneath the molten glass level, the electrodes act as physical barriers to lateral movement of the batch blanket, thus preventing batch material from drifting into contact with the sidewalls. The electrodes may be sub-merged below the molten glass level as deep as the unmelted batch extends (more than 12 inches (30 centimeters) in some cases), but a minimum depth is preferred for optimum guidance ;
of the blanket. One may, of course, forgo the benefits of batch blanket guidance by deeply submerging the electrodes and yet retain other advantages of the invention. In any case, the electrodes should be no closer to the bottom the furnace `~
than about half the depth of the molten glass to avoid eroding the bottom.

'' ~s~
The electrodes may generally be parallel to the sidewalls of the furnace and în contact with the outside edges of the widest portion of the batch blanket. Since the width of the - batch blanket is reduced as melting progresses, it may be advantageous for the electrodes to toe-in slightly to conform to the outline of the batch blanket, Placing the electrodes - as close as possible to the sides of the batch blanket concen-trates the electric heat beneath the batch blanket and main-tains a more stable, symmetrical melting pattern in the furnace, but it is permissible to space the electrodes farther apart than the width of the batch blanket so long as sufEicient spacing from the sidewalls is provided to avoid significant additional heating of the sidewalls, It is preferred that the electrodes be mounted as nearly horizontally as possible to simplify installation and to provide - more intimate contact of the underside of the batch blanket ` '`
with the electric heat. Since the depth of the batch blanket decreases as it moves downstream the electrodes may slope ``
upward a few degrees to follow the general contour of the ~0 underside of the blanket, but even in the most extreme cases it can be said that the electrodes would be substantially horizontal.
The specific optimum length for the electrodes will vary from furnace to furnace, and is determined largely by the dis-tance that the batch blanket extends into the furnace, which in turn depends upon the size and geometry of the furnace as well as various process para~eters (glass composition, furnace temperature, etc.). The electrodes should be long enough to distribute the electrical energy across a relatively large area closely adjacent to the underside of a substantial portion of the batch blanket and preferably to also impart sufficient lateral guidance to the blanket to insure that the portion of the blanket beyond the tips of the electrodes will not drift into contact with the sidewalls under normal circumstances.
In general, the electrodes should extend into the Eurnace at least 30% to 100%, preEerably 50% to 75%, of the distance `~
that the batch blanket extends into the furnace, measured from the point of batch introduction to the zone where the blanket begins separating into discrete, freely floating agglomerations of batch known as "logs" or "floaters". However, the electrodes may extend to virtually any distance beyond the end of the batch blanket if additional protection of the sidewalls from floating batch is desired. For example, the electrodes may reach to the region of the furnace where all the batch, including ~ `
the floaters, has been melted, or they may extend the entire ~ --length of the furnace. Lengths coincident with the major portion of the batch blanket length are preferred because the electric energy input is utili~ed most efficiently there while also obtaining significant improvements in batch guidance.
The melting pattern depicted in FIGS. 1 and 2 may be considered to~represent the fastest melting rate that would ordinarily be encountered in such a furnace in the flat glass industry. Such furnaces conventionally have about six to eight burner ports 22 on each side, only the first five being shown in FIGS. 1 and 2. With the primary sources of heat for ,,. :, melting thus spaced along the length of the furnace, the most effective melting temperature would not be encountered until the batch has passed a number of the burner ports at the inlet end. Typically the batch blanket does not begin to break up until it approaches the region of the third or fourth burner - port, nearly half the length of the furnace, but blankets reaching only the second burner port or as far as the fifth burner port are not exceptional. For a majority of the melting to have taken place when the batch is opposite the first of a series of burner ports, as shown in FIGS. 1 and 2, it would entail :.
-- 10 ~ ,:

.
~: ,, , . - . . : . :, - ~ .

extreme, but theoretically possible, furnace conditions.
Thus the desirable lengths for the electrodes may be structurally defined in terms of their relationship to the burner ports, viz., the electrodes should extend înto the furnace at least as far as the region approximately opposite the first burner port, In a furnace of the type shown in FIGS. 1 and 2 which includes a fill doghouse extension, conventional furnace geometry is such that the horizontal depth of the doghouse normally represents a small fraction of the distance from the doghouse end wall 24 to the end of the batch blanket. A
majority of the batch blanket in such a case would extend be-yond the doghouse, into the main body of the furnace.
Accordingly, it can be said that electrodes having a portion of their lengths extending through the doghouse should have at least an equal additional length extending into the main body of the furnace.
Another convenient rule of thumb for selecting the electrode length is that the electrodes should preferably extend through the molten glass a distance greater than about half the inside width of the furnace.
- As a specific example, consider a furnace of the type shown in FIGS. 1 and 2 whose size and proportions are typical of commercial flat glass manufacturing installations wherein:
~ the furnace is about 30 feet (9 meters) wide, with seven - burner ports on each side spaced 10 feet (3 meters) apart -from center to center and 10 feet (3 meters) from the ends, the fill doghouse extends about 5 feet (1.5 meters) beyond the back wall, the glass depth is approximately 4 feet (1.2 meters) and the batch blanket is about 20 feet (6 meters) wide and extends to a point between the third and fourth`burners.

In such a furnace, satisfactory results may be attained by ~;S6~ :
employing a pair of six-inch (15 centimeter) diameter electrodes that extend 15 feet (4.5 meters) from the doghouse end wall. A preferred embodiment in that case would utilize 8-1/2 inch (21.5 centimeter) diameter electrodes extending about 20 feet (6 meters). Extending the electrodes to 30 feet (9 meters) or more may yield additional protection of the walls from batch contact. The electrodes in each of these specific examples may be spaced about 5 feet (1.5 meter) or 1/6 of the total wîdth from the sidewalls.
i 10 Referring again to FIGS. 1 and 2 in general, it can - be seen that the preferred mounting arrangement for the elect-. . .
rodes locates a substantial length of the electrodes 23 exteriorly of the furnace, where electrical connections may be made by way of clamps 30 and cables 31. The portion of each electrode adjacent to the doghouse end wall 24 is received in a sheath 32 which is sealed at its outer end and pressurized with an inert gas and/or a reducing gas so as to prevent -oxidation of the carbon electrodes that would be caused by air seepage around the hot portions of the electrodes. Additional ~
oxidation prevention is provided by a water-cooled annular -chamber 33 around each sheath 32 at the wall end. Sheaths 32 and coolers 33 may be inserted into a counter bore at least part of the way into the thickness of wall 24 to protect the electrodes from oxygen migration through the refractory material of the wall. Cantilever support for each electrode is provided by two spaced brackets 34. The cantilever arrangement relieves ;
the bores and the various connections to the electrodes from the great leverage forces set up by the long, unsupported length of each electrode inside the furnace. The brackets 34 may be insul-ated from the electrical current carried by the electrodes by interposing an insulating refractory material between each bracket and sheath 32 at the points of contact. When extremely .. . . . . . . . . .

~06~

long electrodes are used, it may sometimes be desirable to provide additional vertical restraint with water-cooled hold-down rods e~tending from the furnace roof and engaging the dowstream ends of the electrodes.
The electrical current supplied to the electrodes is preferably single phase alternating current, although multi-phase current can be utilized if more than two electrodes are installed. Power consumption may vary widely, depending upon economic considerations, the size of the furnace, and the amount of booster heat desired, consumption of a few hundred ~..
to many thousand kilowatts being typical in large commercial furnaces. The current density is preferably limited to less than about 2 amps per square inch (0.31,amp per square centimeter) at the surface of the electrodes in order to avoid undue elevation of the glass temperature that would promote furnace w-all erosion.
It should be apparent that other modifications and variations as are known to those of skill in the art may be resorted to without departing from the spirit and scope of the invention as defined by the appended claims.

' ~ ' . ' . ' ' , . . . . .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a continuous process for making molten glass wherein glass batch material is introduced into a melting furnace through an inlet opening at an end wall of the furnace to form a blanket of batch material on the surface of a pool of molten glass within the furnace, the blanket extending longitudinally into the furnace from the inlet end toward an outlet at an opposite end, the improvement comprising main-taining the batch blanket spaced from the sidewalls of the furnace by means of rods that extend substantially horizontally through the inlet end wall beneath the inlet opening and through the molten glass along the sides of the batch blanket.