IL26761A - Process for homogenizing viscous liquid such as glass - Google Patents

Description

PROCESS FOR HOMOGENIZING VISCOUS LIQUID SUCH AS GLASS CASE ιΟΐ8 The present invention relates to a new method for creating and/or controlling flow in or homogenizing a liquid such as molten glass. More particularly, the present invention relates to a method in which a gas such as air is impinged against the surface of a mass of molten glass in a container to initiate flow patterns or control existing flow patterns within the glass container, pot or tank and to homogenize the molten glass within the container.

One of the most persistent problems inherent in the commercial production of quality glass is obtaining the desired degree of homogeneity. Different glass products require varying degrees of homogeneity, but all require a minimum amount. Many techniques have been employed with varying degrees of success to obtain the desired homogeneity. Optical glass tanks, for example, have incorporated mechanical stirrers. Plate, sheet and window glass tanks have utilized bubblers, mechanical stirrers, various strategically placed refractory members and varied tank designs.

Experience has taught large volume, flat glass manufacturers, for example, that to consistently produce good quality, homogeneous glass on a continuous basis, requires a relatively large, relatively long glass melting tank. Some tanks must also incorporate some means for retarding hot surface convective flow rates. Surface flow retardation is necessary to prevent a layer of hot surface glass from passing through the tank to the forming end at a speed too high for the glass to become homogenized and too high to condition the glass for forming. Typically, the means employed to retard the hot surface flow rates are surface skimmers and/or throats in the tank construction.

Because of the homogeneity requirement, the typical continuous glass melting tank design is expensive to construct and to operate. The sheer bulk of the tank refractories requires a considerable capital expenditure .

The cost of operating the tank is also high because the design requires a huge volume of glass to be continuously maintained at elevated temperature. Glass manufacturers have long tried to design a smaller glass melting tank which would permit a high volume of quality glass to be produced. This objective has always been unattainable because of the time required to allow the convection currents in the glass mass to develop the required degree of homogenization . The objective could be attained, however, if the glass manufacturer could develop some means other than reliance on convection currents to homogenize the glass.

The development of the present invention provides the necessary tool for designing the more efficient continuous glass melting tank. In the present invention, the need to rely on convection currents and prolonged residence time in the tank to homogenize the glass is minimized or eliminated.. For the first time, smaller glass tanks less costly to construct and operate are possible with capability of producing quality glass in large volume.

In the U. S. Patent No. 2,766,518, issued October 23, 1956 to A. W. Schmidt, there is disclosed a method for conditioning molten glass in the forehearth flow channel of a glass, melting tank. In this patent, glass is fed from the tank and passes under a skimmer refractory block into a trough where it is subjected to heat from a series of burners. The burners are used to maintain the glass temperature in the trough at a sufficiently high level to insure a constant continuous glass flow and to prevent devitrification of the flowing glass along the edges of the trough. This operation may, however, provide heat to the glass in the center of the trough in excess of that desired for forming purposes. The temperature of the glass across the width of the trough is equalized in the cited patent by blo.wing cool air vertically downward onto the surface of the glass in the center of the flowing glass mass while simultaneously applying heat to the edges thereof. No homogenization , nor creation or control of flow in he molten glass flow is taught, nor is such a result inherent therein.

In U. S. Patent No. 2,448,451, issued August 31, 1948 to John H. McKelvey et al, there is disclosed a technique for decreasing the wear of the sidewall refractories of a glass furnace using a stream of cool air. In the ordinary glass tank melting operation, the hottest molten glass is located at the surface and approximately equidistant from the sidewalls in the tank. This hot surface glass flows from the center of the tank to the cooler sidewalls. Upon reaching the sidewalls, the molten glass is partially cooled and then sinks along the sidewall down into the interior of the molten glass mass. This convective flow pattern of the glass causes the refractory sidewalls to wear predominantly at and just below the glass level in the tank. In the cited patent, air is blown into the glass tank through the openings in the sidewalls just above the glass level in the tank to cool the surface of the glass near the wall. The viscosity of the cooled glass is thus increased, causing a corresponding decrease in the rate of surface convective flow. The region of high convective flow rates of the molten glass is then shifted inward, away from the walls, decreasing the erosive action of the flowing glass upon the walls. The result is prolonged refractory wall life. No homogenization resulting from this technique is taught.

In U. S. Patent No. 2,771,711, issued November 27, 1956 to Bernard Long, there is disclosed a method of passing a stream of air across the surface of a flowing mass of molten glass to control the rate at which the molten glass arrives in the drawing chamber of a glass tank. The technique is also used to gradually decrease the temperature of the molten glass as it flows from the fining area to the drawing area, preventing the creation of additional seed in the glass. The technique is also taught to decrease the required drawing force by increasing the air pressure in the drawing chamber.

In U. S. Patent No. 1,667,145, issued April 24, 1928 to Hugh N. Diedericks, there is disclosed a method of conditioning molten glass in a sheet drawing operation. The method involves partially submerging a cooler in the path of the flowing glass to obstruct its passage and to force the glass to flow down beneath the cooler. The glass is thus cooled and properly conditioned for drawing into sheets prior to entering the drawing area of the tank. There is also disclosed a technique for raising the temperature of the molten glass if the cooler, during its operation, excessively chills the molten glass. This technique involves directing a plurality of gas flames upon the excessively cooled molten glass using a continuous gas pipe burner positioned parallel to and directly behind the cooler apparatus. No homogenization is taught as resulting from the operation of the burner while heating the excessively cooled glass.

Broadly, what has been discovered in the present invention is a method for homogenizing and/or creating or controlling the flows of a liquid such as molten glass in a glass melting tank or other container. The method consists of impinging a stream or streams of gas onto the surface of the molten glass with sufficient force to substantially change the surface flows. The impinging gas stream or streams, when of sufficient force, also create a visible disruption on the molten glass surface in the immediate area of gas impingement. By regulating such variables as the angle of gas impingement, the volume of gas, the gas pressure and the gas temperature, the flows in the glass mass can be initiated, increased, decreased or even reversed. These same variables can also be regulated to promote the development of an homogenizing circulation pattern from the surface to a controlled depth within the molten glass.

Various apparatus designs can accomplish the desirable objectives of the present invention. Any apparatus design which provides a means for conducting a gas under pressure into the interior of the glass tank, pot or container and means for directing the gas against the surface of the molten glass in a controlled stream or streams will suffice. The most frequently used design for the gas supply component proper is a tubular-shaped header chamber provided with a multiplicity of nozzles through which gas under pressure can be directed onto the surface of the molten glass in the glass tank. Although certain apparatus designs are suggested herein, many alternative designs will become apparent once the principles of the present invention are understood.

A principal advantage of the present invention is that reliance on convective flow to develop the necessary degree of homogenization is eliminated without invoking the need for mechanical stirrers. This permits the melting, fining, and homogenizing functions of the glass melting operation to be separated in both time and space. The separation of these glass manufacturing functions permits the design of more efficient tanks.

In addition to the new tank designs which are now made possible, the present invention is sufficiently flexible to be used to control glass flows and to homogenize molten glass in conventionally designed glass melting tanks or pots.

One advantage of practicing the present invention in a glass melting , tank is that it provides a substitute for various other commonly employed mechanical homogenizing means. Another advantage is that the present invention can also be operated to resist or enhance the naturally occurring convective flows in the glass tank. A third advantage is that the present invention provides a substitute for the refractory members presently incorporated in tank designs to control molten glass flow, such as skimmers or floaters. Eliminating the mechanical stirrers and the refractory skimmers or floaters eliminates the possibility of contamination, which is always present when solid members are immersed in molten glass. The present invention can also be used however in conjunction with skimmers, floaters, bubblers and stirrers if desired to combine the advantages of each.

Broadly, homogenization of the molten glass is accomplished by the present invention in the following manner. The apparatus directs a gas stream against the molten glass surface. The impinging gas stream thereby imparts kinetic energy to the molten glass. Depending upon the amount of kinetic energy supplied and the normal convective surface flow patterns in the immediate area of gas impingement, the surface glass flow rates may be initiated, accelerated, and retarded or even reversed in direction as a result. The impinging gas stream also produces a visible disruption or depression in the immediate area of gas impihge ment and creates a wave disturbance in the molten glass surface which increases the internal shear rate of the glass. The internal shear rate of the glass mass in the immediate area of the wave disturbance manifests itself as circulations about a transverse axis greater in depth than the depression in the molten glass. The differences in the velocity of the glass in the various zones of the circulating glass mass cause attenuation of the glass mass, which results in homogenization. The circulation takes place about an axis transverse to the major surface flow. During circulation, a portion of the circulating mass leaves the swirl and passes away from the influence of the impinging gas. During the time the molten glass is under the influence of the apparatus, the glass is attenuated and subjected to the increased shear rates which eliminate defects such as ream in the glass.

The size, width and depth of the visible glass surface depression, the size of the accompanying glass mound or mounds developing behind the depression, the depth of the homogenizing circulation pattern and the rate of rotation of the homogenizing circulation pattern are all dependent upon the quantity, the temperature, the velocity and the angle of gas stream impingement. Other factors which also influence the practice of the present invention are the viscosity of the molten glass and the driving forces of the natural convective flows in the molten glass mass.

Significant levels of resistance or surface flow retardation can be developed when the direction of gas stream impingement is in a direction opposing the natural convective flows in the system. (When used in this sense, "increased resistance'1 means increasing the ratio of glass throughput to convective flow in a continuous melting type of glass tank.) Resistance in the system contributes strongly to good operation in that it improves the thermal efficiency and minimum residence time which favors the production of homogeneous glass.

Operating an apparatus to direct a stream or streams of gas in a direction normal to the direction of convective surface flow or at a negative angle as hereinafter defined develops a unique flow pattern in the molten glass mass.

Figures 1, 2, 3 and 3A compare the velocity profiles of the convective flows developed in a typical flat glass tank, in which: Figure 1 represents a no-resistance, no-control velocity profile; Figure 2 represents a floater resistance controlled velocity profile; Figure 3 represents the controlled velocity profile produced by the apparatus of the present invention operated to impinge gas in a direction normal to surface flow; and Figure 3A represents the controlled velocity profile produced by impinging gas at a substantial negative angle to the direction of surface flow according to the present invention.

The angle of gas stream impingement in the present invention is defined to be the angle subtended by a line normal to the molten glass surface and a line parallel to the direction of gas stream flow. Positive angles are those which result when the direction of gas stream impingement is opposed to the natural surface flows in the tank. Positive angles of gas stream impingement retard existing surface flows. Negative angles are those which result when the direction of gas stream impingement is in the same direction as the natural direction of surface flow in the tank. Negative angles accelerate the surface flow rates.

In the ■ no-resistance system of Figure 1, the surface flows determined at point la are toward the forming end of the tank. The flows at lower levels in the glass mass at point 2a are toward the batch feed end of the tank. This convective flow pattern results in the no-resistance system because. heat is applied in area 3a of the tank to melt th batch materials. As the batch melts, the molten glass in the approximate center of the tank becomes the hottest glass in the tank melter. This hot glass exhibits a lower density then the cooler molten glass in other portions of the tank. The lower density hot glass floats on the surface of the cooler glass mass and flows from the hottest point, or spring zone, toward the forming end of the tank in one direction and toward the feeding end in the other direction. The no-resistance flow system is undesirable because the hot surface glass moving toward the forming end can spend too short a residence time in the tank to allow bubbles in the molten glass to fine out. One result of the no-resistance flow system can be a high seed count in the final glass produced. Another result of the no-resistance control system can be inadequate temperature conditioning for satisfactory forming.

The present commercial practice to increase the residence time of the glass in the tank is to introduce a refractory skimmer 4, as indicated in Figure 2. The residence time of the molten glass in the tank is increased because the glass must flow underneath the skimmer. The velocity profile in the glass is changed by the introduction of the skimmer which provides another surface of zero velocity lb. It is also displaced downward into the glass mass.

Figure 3 indicates the effect of operating the gas impingement apparatus 5 of the present invention at 0° angle, that is, normal to the glass surface. This arrangement is preferred for producing homogeneity without recourse to supplemental aids. As shown in Figure 3, the velocity profile at point lc is not greatly different from the no-resistance type of velocity profile, while the skimmer exerts its influence independently exactly as in Figure 2.

As shown, the impingement of gas normal to the surface creates a visible depression or trough with noticeable mounds up and downstream thereof. The thrust of the gas upon the surface creates a wave disturbance which, in turn, creates circulation in depth about a transverse axis in the glass mass. These actions introduce shear and produce homogeneous, well refined, high quality glass. The apparatus, since it does not directly contact the molten glass, also cannot introduce refractory stones or produce ream by contact, as is occasionally done by a refractory skimmer.

When all variables such as the as ressur v and angle of impingement are kept constant and only the height of the apparatus above the molten glass surface is varied, the following results are observed. As the height of the apparatus is increased, the effect on the surface flows is decreased. Decreasing the height of the apparatus causes a corresponding increase in the control over the surface flows.. In most applications of the present invention, the preferred height of the apparatus above the molten glass surface is about 3 inches or less.

Decreasing the height of the apparatus also results in an increase in the depth to which the homogenizing circulation is developed in the glass. A considerable degree of homogenization and control of surface flow can be achieved when the apparatus is operated as high as 6-1/2 inches ..above the molten glass surface. Limited mixing and control over surface flows can be achieved when the apparatus is operated even as high as 15 inches, but these higher heights of operation are not preferred.

The apparatus can also be operated at the same height as the surface of the molten glass or even below, provided the apparatus is started before being lowered into its final operation position.

Varying degrees of surface flow control and homogenization occur at angles of gas stream impingement between -90 and +90 degrees. A desirable angle for the surface flow retardation appears to be about +30 degrees. Optimum homogenization of the molten glass appears to occur at angles between about 0 and -30 degrees, but can be achieved to lesser extents up to about -80 degrees.

Working with 1/8 inch nozzles, the workable gas pressure range is between about 20 ounces per square inch and 120 ounces per square inch over atmospheric pressure. Obviously, the kinetic energy imparted to the glass is a function of flow and not pressure alone so that variation in orifice size will modify the usable working pressure range. Hereinafter, the abbreviation osig. (ounces per square inch, gauge) will be used to describe the pressure in ounces per square inch over atmospheric pressure. Excessively high gas flows, though they result in increased surface shear action, may result in excessive surface turbulence which causes bubbles and strands to be trapped in the molten glass. Furthermore, in a tank having relatively cold return flows beneath the surface flow of refined glass, too much force applied results in drawing unwanted glass from the return flow into the forward stream. Ream and seed or blister may be produced. Less than about 20 osig. of gas pressure ordinarily results in inadequate homogenization and control over the surface flows. The gas pressure can be., varied, however, for specific purposes other than homogenization and surface flow control, such as control of the tank atmosphere or to confine the batch and foam blanket. Between about 10 and 250 osig. can be employed if desired.

The preferred temperature of the impinging gas stream in the present invention is tied to the melting characteristics of the glass, the firing techniques and the geometry of the melting tank. For some purposes, the gas stream temperature should preferably be within about ° F. of the temperature of the molten glass. In commercial glass-making operations, this temperature is between approximately 2000°F. and 3500°F. The gas, however, can be introduced in many instances at room temperature (75°F.) if the heat sink developed by operating the apparatus at this relatively low temperature is not detrimental to the operation or is compensated for by other heating means in the tank. In this regard, gas burners or electric heaters may be installed to control the heat sink. In general, the higher the temperature of the gas utilized in the apparatus, the greater the overall efficiency of the operation, including increased homogenization and control over the final glass quality.

Figure 3a illustrates application of gas at a" substantial negative angle, say 30 degrees, and the effect on the velocity profile at point Id. For equal force applied, the mound illustrated is higher than that produced in Figure 3, the surface velocity is accelerated downstream and the depth of circulation is more pronounced, creating more shear.

The volume of gas required to practice the present invention depends upon the size of the glass tank, the size of the apparatus and the functions the apparatus is expected to perform. The size and type of glass tank also influence the size of the apparatus required. The size apparatus required in a pot, fiber glass tank or ophthalmic glass tank, for example, would be considerably smaller than one required in a plate, float or window glass tank.

The volume of gas required by an apparatus installed in a medium sized commercial window glass melting tank ranges from about 300 to 3000 cubic feet per minute depending on whether homogenization alone is desired or whether both control of the surface flows and homogenization of the glass is achieved. Control of the tank atmosphere may require even greater gas volumes. The volume of gas used by an apparatus installed in a fiber glass or ophthalmic glass tank is considerably smaller.

A feature of the invention is that when gas is impinged on the surface of the molten glass through an array of nozzles or through slots, the glass is visibly depressed. A trough is formed bordered by a mound on one or both sides of the trough depending on whether impingement is nearly normal to the surface or at an appreciable angle from normal. When gas at 100 osig. is introduced through 1/8 inch nozzles closely spaced together at an elevation of 3 inches above the glass surface and normal to the surface, molten window glass at fining temperature is locally depressed about 2 inches below the general level and the peak of the mound may be about 1 inch above the general level.

Local circulation of glass is induced in a pattern extending in depth many times the depth of the trough.

When treating the surface of molten glass in a continuous tank, care is exercised to impinge the gas upon the surface in a symmetrical pattern with reference to the center line between the side walls. Preferably, the arrangement is such that gas is impinged transversely of a major surface flow within the tank and essentially from wall to wall .

In continuous tanks having return flows of relatively cool glass in strata along the bottom and no physical separating means isolating the surface flows therefrom, care must be exercised to control the depth of surface circulation to avoid drawing the cool glass into the warmer surface flow. Otherwise, seed and ream may result.

In pot melts, care is exercised to traverse at least a radius of the pot and to impart a rotational movement around the center of the pot to insure all surface glass coming within the influence of the impinging gas.

The present invention will be more readily understood by making reference to the following examples.

EXAMPLE I To demonstrate the advantages of the present invention, the following study was conducted on a moderately shallow glass tank of the continuous melting type in which batch was introduced continuously at the feeder end and a glass ribbon rolled from the forming end. The tank was about 50 feet long and about 6 feet wide at the widest point. The depth of the molten glass was approximately 3 feet. The tank was gas fired using 24 firing ports located in the melting zone of the tank.

Figure 4 is a pictorial view of the apparatus used in conducting this study having various sections removed.

The apparatus was fabricated principally from two concentrically oriented stainless steel pipes. A cross section, taken above line V-V of Figure 4 is shown in Figure 5. The inner pipe 11 of the apparatus was 2-7/8 inches in outside diameter and the outer pipe 12 was 4 inches in outside diameter. The inner pipe served as the header and conduit for the gas and was provided with a multiplicity of nozzles 13, which allowed gas to pass from the interior 14 of the inner pipe through the structure to the exterior of the outer pipe. Water was supplied through pipe 16 and circulated between the walls of the two concentric pipes 15 to cool the apparatus during operation. The water was then returned through pipe 18 to its source. Spacers 17 were provided in the header portion of the apparatus to separate the inner 11, and outer 12, pipes. The overall length of the apparatus was 10 feet, of which only the center 54 inches was provided with the gas exit nozzles. The center effective length of the apparatus contained 107, one-eighth inch diameter nozzles, located 1/2 inch apart on center.

Figure 6 is a plan view of a portion of the tank indicating the location of the apparatus.

Figure 7 is a vertical section taken along line VII-VII of Figure 6 of the tank indicating the location of the apparatus.

In figure 6, the apparatus is indicated as 10, and the effective span of the apparatus as 20. The feed end of the tank is indicated as 21 and the drawing end of the tank 22.

The cross-section along line VII-VII of Figure 6, shown in Figure 7, shows the position of the apparatus with respect to the molten glass in the tank 23, and the tank crown refractory 24. The side and bottom refractories at the point of apparatus installation are indicated as 25 and 26 respectively.

The glass being prepared in the tank during this study had the following chemical composition: Component Si02 44.38 A1203 26.61 Li2o 5.04 Na2Q 11.00 P205 9.96 Znb 3.00 The melting temperature of this glass was about 2700°F.

The glass tank, when filled to operating capacity, contained approximately 50 tons of raw batch and molten glass. Glass was drawn from the forming end of the tank during operation of the apparatus at a rate of between 10 and 20 tons per day.

The gas used in this study was air at a tempera- o ture of approximately 80 F.

Table I below presents data concerning the amount of ream in the glass before operation of the apparatus and after. Ream was selected for investigation because it is a direct measure of the degree of homogeneity of the glass.

The apparatus was operated at the several different operating conditions indicated in Table I. All operating conditions resulted in a visible disruption of the molten glass surface in the immediate area of gas impingement.

TABLE I Apparatus Air Pressure Per Cent of Glass Height Above Glass (osiq . ) Rejectable for Ream Defect Before installation of air barrier 87 2-1/2 inches 20 ounces 40 2-1/2 inches 40 ounces 21 2-1/2 inches 60 ounces 7 This. study clearly demonstrated the effectiveness of the apparatus of the present invention in reducing ream.

It also illustrates use of the invention to assist in obtaining working temperatures with a shortened tank while increasing minimum residence time.

EXAMPLE II To more fully study the effect of operating the apparatus of the present invention on flows and mixing, a scale model of the glass tank of Example I was fabricated.

The scale model tank was made from sheets of polymethyl methacrylate (Plexiglas) and the molten glass was simulated by glycerin. The flow of the glycerin in the model tank was observed by introducing carbon black as a tracing dye.

The apparatus used in the model tank study was constructed of copper tubing and consisted of a 1/2 inch diameter header tube containing 47 holes, 0.025 inch in diameter, located 1/16 of an inch apart on center.

The natural uncontrolled flow pattern assumed in the model tank when 3 cubic centimeters of glycerin per minute was removed and added to what would correspond to the forming and feed ends of the tank respectively is comparable to that indicated in Figure 1. The glycerin was heated using infrared lamp heaters to a temperature of about 140°F.. in an area corresponding to 3a of Figure 1 to establish the natural convective flow pattern in the glycerin.

The following operating conditions were employed in the study: The height of the apparatus was 1/8 of an inch above the surface of the glycerin bath. The gas used was air at a pressure of 1.0 osig. The air was driven onto the glycerin at an angle of about +30 degrees.

. Figure 8 is a drawing indicating the circulation pattern developed in the glycerin by practicing the techniques of the present invention. As indicated in Figure 8, the direction of surface flow was reversed in the immediate area of gas impingement and an homogenizing swirl was developed to a depth of approximately 1.0 inch in the glycerin. The surface of the glycerin was visibly disrupted in the immediate area of gas impingement.

The carbon black tracer was visibly dispersed in the output glycerin. Normally in a tank, the working end of which has a configuration such as that illustrated in Figures 7 and 8, there would be a return flow along the slope of the floor. However, as illustrated, introduction of. gas at a positive angle has created a barrier or resistance such that flow in the conditioning chamber is virtually plug flow into the working chamber.

EXAMPLE III One type of commercially used window glass tank incorporates a neck or channel between the melting' and fining zones of the tank. Molten glass accumulated in the melting zone of the tank flows through the neck into the refining and working zones of the tank. Cooler glass flows in return below the forward flow.

In designing a window glass tank, some means is desirable to retard the flow of molten glass from the melting zone into the forming end of the tank. This is to afford sufficient time for the glass to adequately fine and be temperature conditioned. The method usually adopted in window glass tanks is to provide a floating refractory body across the neck. This resistance technique has been developed by window glass manufacturers to the point where acceptable window glass is produced fairly consistently. The technique does, however, have the disadvantages of occasionally introducing refractory stones and refractory streaks caused by the wearing away of the floater. Another disadvantage is that the floaters require frequent replacement. By installing an apparatus designed to perform the functions of the present invention in a window glass tank in place of the floater, all of these disadvantages are effectively overcome .

Figure 9 is a plan view of a window glass tank indicating the installation location of apparatus units of the present invention in the neck area.

V Two units are indicated at 40 and 40', respectively, and the effective span of the units is shown at 41. The skimmer dog houses and drawing bays are indicated as 42 and 43, respectively.

Figure 10 is a cross section taken along line X-X of Figure 9 indicating the location of upstream unit 40 and downstream unit 40' with respect to the crown refractory 45, and the molten glass 46. The side and bottom refractories are indicated as 47 and 48, respectively.

Extending transversely across the entire span 41 of the tank throat is a vane or baffle R located so as to isolate the forward flow from the return flow in the throat.

Preferably, the member R is constructed of platinum or platinum-rhodium alloy-coated ceramic. Ordinarily the vane R is canted slightly toward the feed end of the tank to insure that forward flowing glass remains in the superstratum and the return flow in the sub-stratum. With vane. R in position, the upstream gas impingement unit 40 is installed with nozzles arranged to direct gas at a substantial positive upstream angle, say 45 degrees. · Downstream unit 40' is installed with nozzles arranged at a substantial negative downstream angle, say 30 degrees.

One design of gas impingement unit suitable for use in the neck of window glass tanks can be fabricated of stainless steel pipes protected by a highly resistant alloy shell such as Inconel.

Figure 11 is a longitudinal cross-section of such a unit.

Figure 12 is a cross-section taken along line XII-XII of Figure 11, showing the arrangement of nozzles in the assembly and spokes for manual rotation.

Figure 13 is a cross-section taken along line XIII-XIII of Figure 11 showing the inlets and outlets for cooling water.

The gas supply component proper consists of two concentric pipes, the inner pipe 51 being a 3 inch internal diameter pipe, and the outer pipe 52 being a 5 inch internal diameter pipe. The inner pipe serves as a conduit for the gas used, preferably air at or near the temperature of the molten glass in the region undergoing treatment. The material of construction usually limits the upper temperature of the impinging gas, but we have worked with gas at temperatures as low as ambient room temperature, or about 75°F.

Separating the inner pipe 51 and outer pipe 52 is a refractory insulating material such as Carborundum Fiberfrax paper Type 970-J. Concentrically disposed about the gas supply pipes is an outer shell of Inconel 53 so arranged as to function as a jacket for cooling water. The space between pipe 52 and shell 53 is divided into four isolated compartments by water-tight welding of dividers 54 to the two concentric elements. Two of the compartments are connected to water inlets 55 and the remainder to water outlets 56.

As shown in Figures 11 and 12, a row of nozzles 57 in linear alignment are tapped into pipe 51 and extend into a recess in the walls of pipe 52 and shell 53 which is bordered by welded joint panels 58 · back up ring 59 is welded in water-tight relationship to the butt end of pipe 52 and shell 53» after which a circular flange 60 is welded to the circumference of pipe 51 about 1-1/2 inches inboard of the end.

Outer shell 53 extends beyond the extremities of pipes 52 and 51 as shown in Figure 11. Each is sealed by a welded plate at the extremity and buttressed with Fiber-frax or similar insulation.

Half couplings C,C are welded to shell 53 to facilitate in rotation as hereinafter described. Elements T are thermocouple extensions.

The unit just described is, in essence, an insu-lated manifold i+O suitable for supply of air or other gas under pressure through an array of nozzles such as to create a pattern of fine jets almost as one linear stream.

In application to the embodiment just described, such a unit is coupled to a source of hot air, in this instance a cylindrical excess air burner in which the output is fed into the inlet of the manifold 1+0. Such a burner, illustrated schematically as 62 in Figure 10, is available commercially. As shown, it is so mounted as to be concentric with manifold 1+0 and to be adjustably rotat-able so that the nozzles of the manifold can be aligned at any desired angle with respect to the horizontal. Such a burner-manifold combination is conveniently mounted on a roller frame, in turn guided by tracks for introduction and removal from a furnace .

In application to a furnace having a throat 10 feet wide, such a manifold unit is constructed 229 inches overall in length, with a 226 inch outer tube 52 and 221 inch inner tube 51. Nozzles 57 were disposed on 1/2 inch centers with the following grouping. In the center there were 189 nozzles of 1/8 inch bore, then progressing outwardly on each side therefrom four of 7/16 inch bore, four of 3/32 inch, four of 5/64 inch and four of 1/16 inch, making 221 nozzles in all, with a total span of 112 inches.

Figure 14 illustrates diagrammatically how the manifold behaves pressurewise at different distances from the nozzle extremities when air at 70 osig. is introduced to the inlet from a constant source. From the origin along the abscissa is the distance from the wall nearest the inlet to the various nozzles measured in inches and along the ordinate is the pressure in osig. at three different distances from the nozzles. The curves are generated from the peaks of pressure responses opposite the nozzles. It will be observed that there is an area of no pressure in the vicinity of the side walls and that, at a distance of about 4 inches from the walls, the pressure gradually increases to a maximum across the bath. This intentional control is designed to avoid agitating glass along the walls with the intention of avoiding basin wall erosion and entrapment of cold glass.

With two such units disposed as illustrated in Figure 9 and angled as above described in this example, air at 1700°F. was introduced to both units, the pressure of the upstream manifold being 100 osig. and the downstream 150 osig. After stabilizing the operation over a period of two days, the overall ream defect determined by standard procedures was improved by an average of 50 per cent over the pre-installation operation. Drawing single strength glass of standard quality for distortion, the overall drawing speed (one measure of throughput) was increased 10 per cent, which is a very significant gain in efficiency.

Figures 23 and 24 are schematics showing, in Figure 23, the velocity profiles incident to the arrangement just described, and in Figure 24, the change in flow characteristics .

When the gas impingement apparatus of the invention is installed in an operating tank, or for any other reason a "hot" application is made, it is desirable to. practice the invention without physical means to separate the forward flow from the return flow. In such instances, a conventional floater is usually already in place and the invention is utilized in conjunction therewith.

In a typical embodiment of the invention, a window glass tank such as illustrated in Figure 9 and of the dimensions described above was operated with a floater F in place as shown in dotted lines and the upstream manifold 40 removed. Manifold 40' was operated with nozzles directing air normal to the glass surface (0°) at a height of 3 inches above the surface, the air temperature being at 1700°F. and the pressure in the manifold being varied from 43 to 107 osig. This operation was performed before vane R was installed in the throat in consequence of which only gravity separated the surface forward flow from the lower return flow.

The tank and four machines were operated in this fashion for a period in excess of eighty days, during which the average tonnage drawn per day increased about per cent, while the ream expressed in terms of per cent of glass lites containing ream visible by shadowgraph at 0-3 inches was halved. Best results in terms of quality were obtained at pressures of 100 osig. at the outset of the trial when ream was one-fourth of the average before the trial.

In addition to the foregoing trials, we have operated the same apparatus under varying conditions of flow, temperature and angle of impingement. Summarizing the results, we have operated at -15 degrees with improvement of ream at pressures from 90 to 115 osig., and temperature of 1700°F., seed and blister being slightly higher at the higher flows. At -30 degrees and 1700°F. , ream was improved and pressure maximized around 70 osig.

At -45 degrees and 1700°F. , pressure was maximized between 50 and 60 osig. At -60 degrees and 1700°F., lower pressures are employed .

In general, in each instance, lower temperatures can be employed with diminishing returns in terms of quality. No absolute lower limit can be stated, but room temperature air can be employed, particularly when tank operating temperatures are raised.

The glass composition melted in the tank under these conditions follows, but any of the commercially popular soda-lime-silica compositions are suitable: Oxide Ingredient Per Cent by Weight Si02 73.0 Na20 13.2 CaO 8.5 MgO 3.5 Al203 1.2 Na2S0 0.4 NaCl 0.1 Fe203 0.1 EXAMPLE IV An apparatus can also be designed for use in an optical glass tank. Figure 15 is a plan view of an optical glass tank indicating two possible locations, 71 and 72, where such an apparatus could be installed.

Any optical glass can be homogenized regardless of composition. The present invention is particularly useful in the manufacture of optical glass because of the high degree of homogeneity required and because it permits the elimination of the mechanical metal stirrers presently used in the commercially used operation which occasionally introduces metal occlusions into the glass.

EXAMPLE V Figure 16 illustrates in schematics possible locations for manifolds applied according to the invention to a container-ware tank.

EXAMPLE VI The present invention can also be used for purposes of homogenization in the manufacture of fiber glass. Figure 17 is a plan view of a portion of a melting apparatus used to prepare fiber glass filaments indicating two possible installation locations, 76 and 77, for the apparatus.

All fiber glasses can be homogenized regardless of composition. The present invention, when used to homogenize fiber glass, has the additional advantage of permitting the glass to be prepared directly from batch rather than from previously prepared marbles of the fiber glass composition.

EXAMPLE VII The techniques of the present invention can also be used in the manufacturing process of water glass (sodium silicate ) .

The presently employed sodium silicate manufacturing process is surprisingly inefficient. Although the heat of fusion of sodium silicate, starting with sand and sodium carbonate as batch materials, is only 1.6 million BTU per ton, the commercially employed production process has been found to require approximately 8.0 million BTU per ton.

The principal reason for the high thermal input required is that, in the melting zone of the tank, the molten sodium silicate glass is covered by a relatively large batch and foam blanket. This large blanket of batch and foam impedes heat transfer to the melt, thus decreasing the efficiency of the melting operation. It has been calculated that if air is used as the gas at a temperature of 1900°F., according to the teachings of the present invention, the heat required to produce 1 ton of sodium silicate will be approximately 4.5 million BTU.

Improved continuous sodium silicate glass production can be obtained by several techniques using the basic teachings of the present invention. In the commercial production process for making sodium silicate, the batch materials sodium carbonate are mixed in a ratio of 1.0 part by weight Na2C03 to 2.46 parts by weight SiC^. This batch mixture produces a glass containing 1.0 part by weight Na2<-) and 3.25 parts by weight SiC^. The sodium silicate glass furnace is gas fired, 20 feet wide by 52 feet long, and of the regenerative type. The depth of the glass in the tank is about 3 feet, and firing is accomplished by 6 burners on each side of the tank.

Water cooled pipes are employed as skimmer bars in the tank to prevent unmelted batch from the large batch and foam blanket from passing through the tank without being melted. The batch and foam blanket normally extends from 2/3 to 3/4 of the length of the tank, or approximately 35 feet from the feed end tank wall. The production capacity of a tank of this type is rated at about 230 tons of sodium silicate per day.

To increase the efficiency of sodium silicate glass production, one or more apparatuses of the present invention can be installed in the tank as shown in Figure 18. By directing the impinging gas streams toward the batch or feed end of the tank at an angle, the batch and foam blanket 79 can be confined to about the first 15 to 18 feet of the tank. The water cooled skimmer bar pipes which are normally present can be left in the tank or removed as desired. The apparatus is indicated as 81.

A pair of apparatuses, 82 and 83, can also be installed in a V-shaped configuration as shown in Figure 19 and operated to direct the gas streams partly toward the feed end of the tank and partly toward each other. The batch and foam blanket 84 can thus be confined to the area shown in Figure 19. A series of two or more V-shaped configurations can be also employed in tandem or in any combination of V and straight configurations.

Two apparatuses , 85 and 86, can also be installed as shown in Figure 20, running the length of the sodium silicate tank and dividing the tank width roughly into thirds. In this modification, the nozzles of each apparatus providing the impinging gas streams may be directed partly toward the batch or feed end of the tank and partly toward the opposing apparatus or only toward the opposing apparatus. The batch and foam blanket 87 is thus confined to the central third of the tank width and to the approximate first third of the tank length. The flow pattern developed in the tank comprises a pair of helix spirals having their axes parallel to the longitudinal axis of the tank. This flow pattern provides resistance, homogeneity, thermal efficiency and reduces the wear rate of the tank refractory side walls.

The temperature of the impinging gas streams in all of the above modifications for use in a sodium silicate production tank is not critical. The higher the impinging gas temperature, however, the lower is the probability of developing an undesirable heat sink. Because bubbles and seeds in sodium silicate glass are not objectionable, relatively high gas pressures and gas velocities can be employed compared to those used in other types of glass melting operations. Gas pressures as high as 15 pounds per square inch, for example, are contemplated for use in sodium silicate melting tanks.

EXAMPLE VIII The homogenizing capability of the present invention can be separated from its resistance capability when the glass being, homogenized is melted in a crucible or pot. Pot melts from 3 pounds of a laser glass composition to 2100 pounds of a soda-lime-silica glass composition have been homogenized. In all cases, the degree of homogeneity obtained in the pot melt was greater than that previously obtained using conventional mechanical stirrers.

Figure 21 indicates one possible position for an apparatus 90, designed to homogenize glass melted in a pot.

The header element 91 of the apparatus is provided with at least one nozzle positioned to direct a stream of gas at an angle onto the surface of the melt. The apparatus should be provided with several such nozzles, however, for better results.

The impinging gas stream or streams cause the glass to rotate in a circular pattern about a vertical axis in the center of the pot. Simultaneously, the downward component of the force exerted by the impinging gas stream or streams and the centrifugal forces developed by the rotation of the molten glass mass about the vertical axis cause the surface molten glass to flow outwardly toward the walls of the melting container. Upon reaching the wall, the surface glass then flows downward along the wall to the bottom of the container, along the bottom to the center of the container, and back up through the center of the melt to the surface. The circulation pattern established then is a spiral flow pattern similar to a doughnut shaped helix. This circulation pattern promotes attenuation and thus homogenization of the glass in a short period of time.

To homogenize a pot melt of 2100 pounds of glass, an apparatus having an overall header element length of 45 inches and a span of 36 inches in the furnace was found suitable. The apparatus consisted of a set of concentric pipes in which the inner pipe (gas) was 1/2 inch diameter, and the outer pipe (water coolant) was 1 inch in diameter. The header was provided with 16 nozzles ' separated 1/2 inch on center, 1/8 of an inch in diameter and oriented to impinge gas streams at an angle of 30 degrees with respect to the glass surface. The gas used was dehydrated air at a temperature of about 200°F. The gas pressure at the start was 176 osig. which was gradually decreased as melting progressed to 48 osig. The apparatus was positioned and operated 2 inches above the surface of the molten glass.

To homogenize 3 pounds of a high phosphate laser glass composition in a crucible, an apparatus fabricated of graphite having a header 2-1/4 inches long and 5/8 of an inch in outside diameter was used. The header contained nozzles 1/4 of an inch apart on center and 0.040 inch in diameter. The angle of gas stream impingement with respect to the glass surface was 30 degrees. The gas used was forming gas (7 per cent H2 and 93 per cent at a temperature between 500 and 1000°F. The flow rate of the gas was 0.7 of a cubic foot per minute through the nozzles of the graphite header element. The apparatus was positioned and operated 3/4 of an inch above the surface of the molten glass.

EXAMPLE IX The techniques of the present invention can also be employed to enhance the rate and thermal efficiency of melting batch in a continuous glass melting tank. The apparatus or several apparatuses in tandem can be installed, for example, as indicated in Figure 22, between the spring zone location 93 and the batch blanket 94. The apparatus 95 is operated to direct streams of gas downward onto the glass surface and toward the feed end of the tank. This mode of operation prevents batch logs from traveling too far down the tank before being melted and also promotes the dissipation of any batch foam which may be present on the molten glass surface. The technique permits faster melting of batch and can be used in both gas fired and electric melting types of glass tanks. This technique also permits changing . - from one glass composition to another glass composition, in a continuous glass melting operation, to be possible in a shorter period of time and with a smaller production of intermediate composition glass.

By way of example, in a 300 pound per hour gas-fired fiber glass continuous melting tank having six firing ports on each side of the melter, and a fired refiner separated by a fridge wall, a water-cooled manifold was installed according to the invention. The unit consisted of "a 3-1/2 inch outside diameter pipe with a 3/16 inch Fiberfrax covering over which there is a .010 inch 90 per cent platinum. -10 per cent rhodium alloy cover. Such a unit reflects 70 per. cent of the heat reaching it. There are 121 nozzles, 1/2 inch apart, each with 1/16 inch internal diameter so that the distance between the two end nozzles is 5 feet." The tank was 14 feet, 6 inches in length and -1/2 feet wide, the firing ports being spaced approximately equally along the length. The gas impingement unit was installed through an existing peep hole 5 inches above the surface of the glass and rested on the wall in a corresponding peep hole on the opposite side. The location was approximately midway of the length, or 7 feet 6 inches from the back wall. The nozzles were directed at a 45 degree angle from the surface toward the filling doghouse so as to blow against the foam and encourage surface circulation from the spring zone toward the back wall. The vertical distance from nozzle to glass surface was 5-3/4 inches.

Air was introduced through the manifold at 150°F., 128 osig., and 73 cubic feet per minute.

Owing to the relatively low temperature of impinging gas, it was necessary to increase the firing rate slightly, but nevertheless, the throughput (efficiency) was improved. The foam line receded up-tank affording more effective transfer of heat to the melt. Obviously, one can elect to introduce heated gas through the impingement manifold and lower the firing rate, all with an improved end result.

Gas at a relatively high pressure may, if not heated to a sufficiently high temperature, cause chilling of the glass. As the glass is chilled, the viscosity of the glass increases. The mobility of the glass is then decreased, which destroys the desired viscous couple between the surface glass and the glass lying deeper in the tank necessary to develop the homogenizing swirl. Furthermore, as the glass surface begins to cool and stiffen, the time available for further chilling also increases. The glass may become so viscous that wave crests formed near the edge of the trough by the impinging gas streams may fold back to trap bubbles in the glass. The rate of travel of the wave crests may also decrease to the point where later formed wave crests can overtake those previously formed to develop another mechanism for entrapping gas bubbles and cause defects. These problems cannot be eliminated by increasing the pressure of the impinging gas. The effect of such a change is only to cause further chilling of the surface glass which compounds the problem. Increasing the temperature of the impinging gas is beneficial, but alone, may not be capable of attaining the desired results.

Stranding of the glass is another type of defect former which may occur if the velocity of the impinging gas is too high. Stranding is the blowing around or tearing up of the glass surface.

The preferred design of the apparatus of the present invention employs a single row of equally spaced, equally sized diameter, circular nozzles as the means for directing the gas onto the surface of the molten glass.

More than one row of holes can be used, however, and the size of the holes can be varied across the length of the apparatus to vary the amount of gas striking different parts of the glass surface spanned by the apparatus.

The nozzles need not have circular cross-sections, but can have oval, square, octagonal, triangular or any other shaped cross-section as desired. The nozzles can even vary in cross-section along the length of the nozzle.

In addition to a series of nozzles, the apparatus can be constructed using a single narrow slot of constant width or a series of short slots as the gas exit means.

The slot or slots can also vary in width along their lengths to impinge different amounts of gas on different portions of the molten glass surface.

The apparatus can also be fabricated with means for adjusting the size of the nozzles or slots, severally or individually. A typical adjustable design would consist of two concentric pipes having a common pattern of holes or slots which could be rotated with respect to each other to align the openings for maximum gas output or to disalign the openings to completely cut off the gas supply. At various positions in between, varying amounts of gas could be released.

The cross-section of the principal gas supply header component of the apparatus can also be circular, oval, square, rectangular or any other shape as desired.

The effective span of the apparatus can be straight, curved, V-shaped, wavy or even serrated. The apparatus can even be curved in both the horizontal or vertical planes.

The effective span can be arranged to cover the entire channel width or a portion thereof.

The advantages of the present invention are several and varied, depending upon the type of glass melting operation involved. One advantage is the possible elimination of refractory structural elements, such as the neck floater, in a window glass tank. In this regard, the elimination of this refractory element decreases the number of refractory stones found in the final glass..

Summarizing the benefits of the invention: In shallow tanks applied at a positive angle near the working zone, the gas stream can act as a resistance, even a dam, to isolate return flow from forward flow; it can increase the minimum residence time without reducing total throughput; at temperatures lower than ambient, it can assist in delivering refined glass to the working zone at proper working temperatures with diminished below normal tank length.

In pot melts it can be used to replace mechanical stirrers .

In all continuous tanks, it can be utilized to hold back foam in the melting zone, thus increasing the area of molten glass which "sees" the radiant heat and improving melting efficiency; it can be utilized to accelerate flow of glass below the batch in the melting area; it can be utilized at angles from 0 degrees to large negative angles to improve homogenization in the fining and working zones without resort to mechanical stirring.

In deep tanks, in conjunction with means to isolate forward flows from return flows, it can be utilized to replace floaters as a resistance and/or to subject the forward flow to energetic shear and homogenization without blending in glass from the return flow.

The present invention can also be adapted to control the degree of oxidation or reduction of the tank atmosphere by introducing various oxidizing or reducing gases. The invention even contemplates introducing inert gases into the tank when special glass compositions are being melted. It is also possible to use a combustible gas or mixture of gases to supply additional heating capabilities.

To further enhance the homogenization obtained in a glass melting operation, the techniques of the present invention can be combined with various other conventionally used mixing means, such as bubblers or mechanical stirrers. In this regard, the apparatus can be combined with conventional finger stirrers where a very high degree of homogenization in the final glass product is required, such as in the production of optical glass.

More than one apparatus of the present invention can also be used, arranged in tandem in a glass melting tank, if it is found that the degree of homogenization or control of surface flows developed by the operation of a single apparatus is inadequate.

The apparatus of the present invention can be modified in size and design to be useful in all types of glass melting tanks or pots, including, but not limited to, tanks used to produce window glass, plate glass, float glass, fiber glass, optical glass, water glass (sodium silicate) and various specialty glasses such as laser and phototropic glasses. The apparatus of the present invention can also be modified in design or in operation and installed in many different locations in these various types of glass melting containers.

An apparatus or a modification thereof can be placed in the approximate vicinity of the spring zone in a continuous glass melting tank to retard the convective surface flows away from the spring zone. This location can even be used to alter the exact position of the spring zone to a degree. By decreasing the convective flow rates from the spring zone, the average velocity of the circulating glass mass can be cut down to increase the residence the tank.

Finally, the apparatus of the present invention or modifications thereof can be incorporated anywhere in a glass tank or pot where the surface flow of the molten glass is desired to be controlled for any reason.

While the present invention has been discussed in terms of specific examples, the scope of the invention should be limited only by the language of the appended claims.

Claims (19)

CASE I1.OI8 Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. A method of homogenizing a viscous liquid such as molten glass in a container which comprises impinging at least one stream of gas against the surface of the viscous liquid at a location remote from the walls of the container with sufficient force to create a visible disruption of the viscous liquid surface in the immediate area of gas impingement .

2. A method according to claim 1, in which the impinging gas stream contacts the surface of the viscous liquid at an angle between +80 and -80 degrees with respect to the direction of surface flow of the liquid.

3. A method according to claim 2, in which the impinging gas stream contacts the surface of the viscous liquid at an angle between 0 and -80 degrees with respect to the direction of surface flow of the liquid.

4. Ι A method according to claim 3» in which the angle is between 0 and -30 degrees.

5. „ A method according to claim 1, in which the impinging gas stream contacts the surface of the viscous liquid at an angle approximately normal thereto.

6. A method according to any one of the preceding claims, in which the impinging gas causes a change in the surface flow so as to reverse the natural direction of surface convective flow of the viscous liquid in the container.

7. in which the impinging gas causes a change in the surface flow so as to accelerate flow in the same direction as the natural direction of surface convective flow of the viscous liquid in the container.

8. A method according to claim 7» in which a molten body of glass in an elongated container is homogenized, and which comprises feeding batch onto the surface of said body at one end of said container, firing the batch and body with a radiant heat source over the surface at said end, the gas being impinged against the surface of said glass toward said end with sufficient force to drive the resultant foam toward said end beyond the position it would occupy in the absence of such impingement, thus increasing the area of molten glass under the influence of direct radiation.

9. A method according to any one of the preceding claims, in which the stream of gas is impinged with sufficient force to create a circulation flow pattern in depth about an axis transverse to the major surface flow in the viscous liquid mass in the immediate area of gas impingement.

10. A method according to any one of the preceding claims, in which there is a subsurface flow in opposite direction to the surface flow and physical means is provided to isolate the surface flow from the subsurface flow.

11. A method according to any one of the preceding claims, in which the viscous liquid is homogenized as it flows through a channel.

12. A method according to any one of the preceding claims, in which a plurality of streams of the gas are impinged in a symmetrical pattern with respect to opposite sidewalls of the container.

13. of the gas are impinged from a central point between the opposite sidewalls to a location short of each wall,

14. lij.. A method according to claim 13» in which glass is conditioned in a deep. continuous tank having a broad melting region and a broad region separated by a relatively narrower neck region with forward flow at the surface and return flow below, the gas streams being impinged against the surface of the glass in the neck region in a symmetrical pattern from the longitudinal axis of the neck region toward each side boundary and the flow in said pattern being controlled from a maximum in the axial region to a minimum at the outer extremities,

15. 1 · A method according to any one of the preceding claims, in which the force and direction of gas impingement is sufficient to create a depression in excess of one inch below the normal surface level of the' viscous liquid.

16. A method accordin to any one of the preceding claims, in which the, gas is., air-.

17. method according to any one of the preceding . claims, in which the temperature of the gas is between .75°F. and 3500°F.

18. 0 A method of homogenizing, a viscous liquid such as molten glass substantially as herein described with reference to the accompanying drawings.

19. ' Glass treated by the method according to any one of the preceding claims. DATED the 25th October, 1966