US3256082A

US3256082A - Heat exchanger for sheet glass drawing apparatus

Info

Publication number: US3256082A
Application number: US20690A
Authority: US
Inventors: Cecil R Ward
Original assignee: Pittsburgh Plate Glass Co
Current assignee: PPG Industries Inc
Priority date: 1960-04-07
Filing date: 1960-04-07
Publication date: 1966-06-14
Anticipated expiration: 1983-06-14

Description

June 14, 1966 c. R. WARD 6 Sheets-Sheet 1 Filed April 7, 1960 R 0 TD W e L m t u 5oz no -cbzouo c m m 538. E. u 23. 325.90 [.56 IES 1 x V I m F536 .6 ESE. .8 5.9; 55 E93 55* 1/ 4 Q g o 0 OE m A TT'OkNE Y c. R. WARD 3,256,032

HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS June 14, 1966 6 Sheets-Sheet 2 Filed April 7, 1960 FIG. 2

m v m w W k 0 v E m M W M m U D N .6 Y 1 :v M W n 5 Q g M m e a a m M a r w P ,v,vv a w r K w m /,N( m w w m w m EMlSSNITY OF MATERIAL INVENTOK.

CEC/l 2. H4480" A 7'TOR/VEY June 14, 1966 c. R. WARD 3,256,082

HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS Filed April 7, 1960 6 Sheets-Sheet 3 m Q r c u EMISSIVITY O AVITY veasus' E I mmo or DEPTH TO WIDTH 6 row. VARIOUS MATERIALS u I F WIDTH or mace? PLANE IS 0 L2 TIMES WIDTH OF CAVITY ms'rmca FROM-CAVITY TO I. 94 macs-r l5 12 INCHES Ill .L l '90 O 1 2 3 4 .5 6 7 wm'r OF PENIN RATIO DEPTH OF CAVH'Y TO H o c INVENTOR czc/z 42.144420 I I I C. R. WARD 3,256,082

HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS June 14, 1966 6 Sheets-Sheet 4 Filed April 7. 1960 FIQ? FIG. 6

' INVENTOR. (EC/L 1Q. IVAQD June 14, 1966 c. R. WARD 3,

HEAT EXCHANGER FOR SHEET GLASS DRAWINGAPPARATUS Filed April 7, 1960 6 Sheets-Sheet 5 F'IG.9

FIGJO \24 IN VEN TOR. 628/4 2. Wflkfi June 14, 1966 c. R. WARD 3,256,082

HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS Filed April 7, 1960 6 Sheets-Sheet a FIG. l 1

INVENTOR. Cit/L E. WAQD 'ATTOENE'Y 3,256,082 Patented June 14, 1966 3,256,082 HEAT EXCHANGER FOR SHEET GLASS DRAWING APPARATUS Cecil R. Ward, Gibsonia, Pa., assignor to Pittsburgh Plate Glass Company, a corporation of Pennsylvania Filed Apr. 7, 1960, Ser. No. 20,690 4 Claims. (Cl. 65-204) This application is a continuation-in-part of applications Serial Nos. 807,915 and 828,836 filed April 21, 1959 and July 22, 1959, respectively.

This application relates to Heat Exchange Apparatus and particularly to the employment of sets of special heat exchangers which substantially behave as black bodies in the thermal treatment of material, particularly in the form of a sheet or a ribbon. While the various embodiments described herein relate to apparatus promoting area heating or cooling of glass, it will be understood that the present invention is equally suit-able for use in the thermal treatment ofmaterials other than glass, such as metals, alloys, plastics, etc. In considering the design of heat exchange apparatus for use in the thermal treatment of sheet material, such as glass, three characteristics are important. The first characteristic is the uniformity of the radiant field produced by the heat exchange apparatus. In present glass heating apparatus, heating zones comprise coils mounted in generally rectangular or cylindrical channels in insulating brick which forms the wall structure of the fur! nace. The temperature distribution over such an area is nonuniform because the channels containing the coils are maintained at a considerably higher temperature than the spaces between the channels.

' Even though a distance separates the heating elements from the path of movement of the sheet material to be heated or cooled, the work piece is subjected to a nonuniform radiant fluxpattern by virtue of the non-uniform energy emittance over the area occupied by the heating elements. This non-uniform radiant flux distribution makes the selection of a control point to monitor a heatingsection verycri-ti-cal, because different points within the section are at different temperatures. Furthermore, different points within the non-uniform temperature area do not follow the eifective radiant level of the heating area.

A second characteristic required for a radiant heating assembly is its efiiciency for heating the material. This is particularly critical in the case of heating glass. When glass sheets are heated by heating coils mounted in generally rectangular or cylindrical channels cut in refractory material, the radiant energy output at any given temperature is determined by the emissivity of the bricks or refractory material used in the construction of the walls supporting the heating elements. This emissivity is usually considerably less than the emissivity of a black body at that temperature. Because of this low efficiency, heating coils must be maintained at a higher temperature than is necessary when the coils are employed efiiciently in order to heat a work piece at a desired rate.

In heat exchange involving glass, the spectral distribution of the energy emitted by the heating coils is shifted toward the visible region where glass is transparent upon increasing the control temperature. Therefore, the eifect of heating the coils to higher temperatures does not necessarily result in an improved heating of the glass.

The third characteristic important for heating sections is the speed of response of the heating elements to the requirement for different heat patterns. Present heating furnaces are constructed of solid brick construction. The solid brick has a large thermal capacity necessitating considerable time to change its temperature. On the one hand, much time is required for a heat soaking at the start of any operation or between operations when the new ope-ration involves a higher radiant level than the previous operation. Similarly, much time is consumed in cooling the solid brick when a change in operation involving a lower radiant level is desired. This slow response also makes it difficult to control precisely the radiant levels of heating sections. Operators cannot obtain the full advantage of recently developed precise control elements which respond very rapidly to variations in temperature from a desired temperature pattern.

The response time of glass lehrs and furnaces in use before the advent of the present invention was often so long that a change in the temperature distribution over the heating area occur-red between the start of an operation and later during its run. This resulted in a drift in the energy spectrum of the radiant field to which the glass was exposed and it was often necessary to change the control temperatures of the lehr after the operation was in progress for a period of time to compensate for such drift.

The lack of one or more of the three above characteristics combines to produce .an additional effect which makes it almost'impossible for any two lehrs or furnaces or heat treatment apparatus of refractory material to have identical operational performances. As a result, process data taken from one heating system cannot be utilized to provide a basis for making or using another system without considerable experimentation and modification.

The present invention in its broadest aspect covers the use of heat exchange apparatus for cooling as well as for heating. When a ribbon, a sheet or other shape of material is to be controllably cooled, it is especially impor-. tant that the cooler be as uniform and perfect a heat absorber as possible or else the cooling rate cannot be controlled properly or cannot be utilized at maximum etliciency, respectively.

Since the emissivity of a hot body relative to its surroundings is the same as its absorptivity when it is colder than its surroundings, the term emissivity as used in this disclosure covers both emissivity and absorptivity.

The present invention avoids the above undesired detects by employing heat exchange apparatus comprising a series of refractory structures including a plurality of smoothly surfaced walls of material having an emissivity of at least 50% and extending along parallel axes in planes oriented obliquely to each other, each wall terminating is sharply angled relation to each adjacent wall to form a series of adjacent angular cavities of predetermined width and depth extending in sideaby-side relationship, each cavity havingan acute apex angle. Additional walls are provided to form a hollow chamber with the surface opposite the wall surface forming the angular cavities. for solid refractory minimizes the thermal capacity of the heat exchanger and promotes rapid response when a change of radiant energy level is desired.

A uniform radiant field needed for heating sheets of glass is produced by utilizing in a heating section of a furnace a number of adjacent heat exchangers of the type described above, each approximating a black body cavity.

The heat exchangers are located to cover at least an.

tion containing 60% silica and the balance substantial- 1y clay is especially elfective as a radiator, although pure silica and compositions containing as little as 50% silica Substitution of the hollow chamber and 50% clay are effective. For radiant absorption, cold drawn steels have been employed successfully in the cooling of drawn sheet glass. However, other refractory materials may be employed depending upon the temperatures of the ambient atmosphere involved in the heat exchange operation.

The construction of the cavities, their depth, width and apex angles, is correlated with the minimum distance separating the radiant heat exchanger from the surface of the work piece to insure that the surface is exposed to a continuous, uniform field of black body radiation. This latter is accomplished by choosing such dimensions for the heat exchanger that a solid angle of black body radiation emanating from each cavity overlaps that emanating from adjacent cavities at the surface of the work piece.

In order to insure proper control for the boundaries of the solid angle of black body radiation, the walls of the cavity must be smoothly surfaced. Conventional furnace refractories are too rough and uneven at their surfaces to control the internal reflections within the angular cavity as precisely as desired. Similarly, in cooling apparatus, a series of members having approximate black body characteristics are constructed with reference to a surface of a work piece to be cooled to provide a uniform absorptivity.

In order to understand the present invention more completely, a number of embodiments will be described illustrating how the present invention may be employed in certain illustrative heat exchange operations involving glass ribbons or sheets.

In the drawings which form part of the present de- 'scription and wherein like reference numerals apply throughout, FIGURE '1 is a schematic ray diagram illustrating how the geometry of an angular cavity is determined.

FIGURE 2 is a chart showing how the emissivity of a cavity varies with the emissivity of the material used to form the refractory structure where radiant energy is emitted with -no reflection, one reflection, two reflections, three reflections and four reflections before reaching the target are-a. This chart is equally true for the absorption of radiant energy impinging on the cavity and emanat ing from the target area.

FIGURE 3 shows how the emissivity or absorptivity of an angular cavity varies with the ratio of depth of cavity to the width of its opening.

FIGURE 4 is an end view of one embodiment of heat exchanger employed as a heating element in a furnace, showing fragments of adjacent heat exchangers.

FIGURE 5 is a view similar to FIGURE 4 of an alternate embodiment heating element according to the present invention.

FIGURE 6 is a fragmentary view at right angles to FIGURE 5, disclosing how individual heating elements are arranged in sets.

FIGURE 7 is a schematic view of a so-called horizontal f'urnace employing heating elements according to the present invention, wherein the sets of heating elements are arranged in spaced horizontal planes.

FIGURE 8 is a schematic view of a so-called vertical furnace in which glass sheets are supported in vertical planes :by ton-gs for heat treatment, wherein the radiant heaters constructed according to the present invention are \disposed on opposite walls of the heating furnace.

FIGURE 9 is a schematic view of a sheet glass drawing machine employing radiant heat exchangers or en- .ergy absorbers approximating black body characteristics.

FIGURE 10 is a fragmentary elevation partly in section of a series of radiant heat exchangers or energy absorbers included in the structure depicted in FIGURE 9.

FIGURE 11 is a schematic of a sheet glass drawing .machine employing radiant heat exchangers or energy absorbers approximating black body characteristics and showing a foot as the lower pass; and

FIGURE 12 is a fragmentary elevation partly in section of a series of radiant heat exchangers or energy absorbers included in the structure of FIGURE 11.

Referring to the drawings, the first three figures explain the criteria involved in determining how closely a cavity forming part of the heat exchange apparatus of the present invention conforms to a black body assembly.

FIGURE 1 depicts half the apex angle of an angular cavity formed between smoothly surfaced walls. R O P depicts a line running through a target, such as a surface of a glass sheet to be heated by a radiant heater or to be cooled by a black body absorber, and represents an area intersected by a solid angle of black body radiation emanating from the cavity. L represents the depth of the angular cavity. w represents half the width of the cavity or half the base of an isosceles triangle formed by connecting the spaced ends of the two converging side walls forming the cavity. d represents the distance between the base of the isosceles triangle and the target surface.

If the emittance to or the adsorption from the target plane R O P is to be an unchanging maxium, relatively insensitive to variations in the material of the cavity, then the cavity formed by the smoothly surfaced walls must have an emissivity equal to that of a black body or unity. Since most materials have an emissivity less than one, it is necessary to construct and shape the cavity in such a way as to utilize reflective radiation to augment the emitted radiation.

In the present case, radiation emitted from point A on the cavity to point P in the target plane is composed of radiation emitted directly from point A, that radiated from point B and reflected at point A, that radiated from point C and reflected at points B and A toward point P, etc; If the emissivity of the material used for the walls of the cavity is at least 50%, the combination of emitted energy and the reflected energy approaches unity asymptotically.

FIGURE 2, which compares the emissivity of a cavity versus the emissivity of the material used to produce the cavity, discloses how the emissivity of a body approaches unity when utilizing multiple reflections. For a straight wall, the emissivity equals the emissivity of the material chosen. As the number of internal reflections is increased before the energy is radiated from the cavity, it will be seen that the emissivity of the cavity approaches unity very rapidly even with relatively low emissivity materials.

Where 98% efliciency is desired, FIGURE 2 discloses that material of 50% emissivity must be constructed to form an angular cavity providing at least four internal reflections. With material of 70% emissivity, only two internal reflections are needed to raise the emissivity of the cavity to 98%.

FIGURE 3 discloses how the emissivity of a cavity increases as the ratio of its depth to the width of its opening increases. This figure indicates that the smaller the apex angle of a cavity, the closer it approaches black body characteristics.

FIGURE 1 indicates that as the emissive power of point A incorporates a larger number of reflective components that reinforce the beam from point A on the cavity to point P in the target area, point A approaches a black body radiator with respect to point P in the target plane. The illustrated ray from point A to point P is an extreme ray in a solid angle of substantially black body radiation emanating from the cavity. The least possible number of reflected components is present in this ray. Thus, if the black body condition is obtained for the ray from point A to point P, then point A provides black body emittance to every other point in the target plane R O P. It follows that every other point in the cavity has maximum emittance to the target plane R O P. Therefore, the cavity is a black body emitter with respect to the target plane. Likewise, if it is desired to use the heat exchange apparatus for cooling purposes, radiation emitted from a target plane R O P is absorbed and the cavity behaves as a black body absorber.

In determining the apex angle of the cavity-for a given distance to a target plane, it is necessary first to determine the number of reflected components required to raise the emissivity of the cavity to approach unity. This depends upon the emissivity of the material used.

In FIGURE 2, the emissivity of the cavity has been plotted against the emissivity of the refractory material for various numbers of reflected components reinforcing the directly emitted rays. As an example, if the cavity is constructed from a refractory having an emissivity of 0.80 and with an angle such that at least twice reflected components are present, then the emissivity is at least 0.992.

Knowing the number of reflected component beams desired, the angle of the cavity required to give this number of reflections can be determined. Referring again to FIGURE 1, the angle a which is the angle of direct emission of radiation from point A to point P can be determined by the equation:

where w is the half width of the base of the cavity, h is the half width of the target plane, d is the distance from the target to the cavity base and x is half the apex triangle of the cavity.

Similarly, b, the angle of emission of radiation from point B which would add to the emissive power of point A by first order reflection, is given by:

and c, the angle of emission of radiation from point C which would add to the emissive power of point A by second order reflection, is given by:

From these equations, the apex angle of the cavity can be determined for any refractory material with a given emissivity in order that sufficient reflected rays are present to bring the emissivity of the wedge substantially to unity. 98% or 99% is sufliciently good for commercial purposes. The dimensions of the cavity can be obtained from the relationship (aretan Tangent x: w/ L where w is half the width of the base of the cavity and L is the depth of the wedge.

In FIGURE 3 the curves shown are for a distance from cavity base to target plane of 12 inches, wherein the target plane is 1.2 times as Wide as the base of the cavity. The curves are also valid for any cavity to target distance greater than 12 inches. The target area has been chosen wider than the base of the cavity so that when the cavities are placed side by side, their respective black body fields will overlap into a uniform pattern.

Use of present invention in radiant heating FIGURES 4 through 8 show the construction of individual heater elements and their arrangement in various furnaces to provide heat for glass sheets.

A heat exchange member according to the present invention comprises a hollow refractory structure depicted generally by reference number 10 of a material having an emissivity of at least 50%. Such member may be constructed by slip casting a silica-clay material to insure that its surfaces are smooth.

A typical procedure for slip casting hollow refractory structures 12 inches long, 6 inches wide, 3 /8 inches high with grooves 1% inches wide and 1% inches deep formed of walls inch thick involves mixing 180 pounds of mesh fused silica grog with 120 pounds of ordinary clay and adding the solid mixture to a solution containing 3,000 cc. Na P O in 54 pounds of distilled water to form a slip. The slip was poured into a plaster of Paris mold having inner walls shaped to the outer shape desired for the refractory structure. The slip solidified adjacent the walls of the plaster of Paris mold at the rate of 'inch thickness per 10 minutes. After 10 minutes, the excess slip was removed and the solidified slip was premitted to air dry for about 10 minutes. The mold was then removed from the slip and the slip fired at 2156 F. for 72 hours.

Care must be taken to limit the firing temperature, because the fused silica changes into a high expansion form when it is fired to substantially higher temperatures. Also, the fused silica used must be of a fine mesh to promote smooth surfaces.

Each refractory structure 10 is constructed to provide a series of smoothly surfaced, longitudinally extending walls 12, which extend along parallel axes longitudinally of the member 10 in planes oriented obliquely to each other where their smooth outer surfaces 14 form cavities 16 of V-shaped cross-section that extend in side-by-side relationship along the length of the member 10.

Additional walls

18, 20, and 22 are attached to the outermost walls 12 of the flanking cavities 16 to form a hollow chamber 24 with the inner surfaces 26 of the walls 12.

Walls

18 and 22 of each refractory structure are constructed to extend linearly in parallel planes normal to the planes in which wall 20 is disposed.

Walls

18 and 22 are recessed adjacent the corners they form with wall 20.. Part of each recess forms a groove 28 extending longitudinally of the refractory structure 10 along each

wall

18 and 22. A clip 30 having a base 32 secured to the furnace structure terminates in tongues 34 inserted into the grooves 28 to support the refractory structure 10 to a furnace structure.

The refractory structures 10 are arranged in sets aligned longitudinally and transversely of each other so that the series of refractory structures 10 presents continuous lines of cavities 16 arranged in side-by-side relation. This is accomplished by actuating wall 22 of one refractory structure 10 against Wall 18 of its neighbor.

FIGURES 5 and 6 disclose an alternate construction for the refractory structures 10 in order to insure that the refractory structures 10 of each set are aligned properly in a longitudinal direction. This is accomplished by threading rods 36 through the grooves 28 and shortening walls 20 and the recessed portions of

walls

18 and 22 sufficiently to receive apertured flanges 38 dependingfrom plates 40 secured to the furnace structure. The apertures of the flanges 38 are located for alignment with grooves 28 when the refractory structures 10 are properly placed. Therefore, the rods 36 extend through the grooves 28 and aligned apertures of the apertured flanges 38 to insure proper alignment of the refractory structures 10 and their V-shaped cavities 16.

When the cavities 16 are employed as black body radiators, a source of heat is required to be operatively associated with each refractory structure 10. This source of heat may be provdied with passing heated fluids, such as burning gases, through the hollow chamber 24 of the refractory structures 10. Since the walls 12 of the refactory structure 10 are thin, preferably on the order of magnitude of about A; inch to about inch in thickas possible within its cavity 16 and that it covers a maximum of about of the cross-sectional area of the aperture of said cavity. If these precautions are not taken, the electrical resistance heating element causes the cavity to lose its black body characteristics.

In FIGURE 7, refractory structures 10 are employed in a horizontal tunnel-type furnace or lehr 44 having a roof 46, a floor 48 and walls 50. Conveyor rolls 52 are rotatably mounted to the walls 50 and are driven by conventional motor and drive means, such as chains and sprockets (not shown). Glass sheets or glass support means are moved through the furnace tunnel as the rolls 52 rotate. A set of refractory structures 10 is attached to roof 46 and another set of refractory structures 10 is attached to the floor 48 of furnace 44. Thermosensitive control units 54 are mounted through the walls 50 and focused on areas of the emitting surfaces of the refractory structures 10 to monitor and control the thermal output of the electrical resistance heating elements 42 mounted in the apex of each cavity 16.

The electrical resistance elements are interconnected in suitable resistance circuits to 'lead wires 56, each of which is coupled to a different voltage source (not shown) through a control circuit responsive to the reading supplied by a thermosensitive control unit 54. As many control circuits are provided along the roof and the fioor as are required to control the pattern of radiant heat both along and across the path of glass travel through the furnace.

In horizontal furnace 44, a screen 58 of open mesh work configuration is supported above the lower set of heating elements to keep glass fragments from contacting the electrical resistance heating elements 42 disposed below the conveyor in the event glass is broken during its passage through the furnace. Otherwise, the resistance wires may be caused to burn out because of the presence of the glass fragments at the wires.

A vertical furnace 60 in which the refractory structures 10 are carried by vertical 'walls 62 is shown in FIGURE 8. In this illustration of another structure employing the present invention, glass sheets G are gripped by tongs 64 carried by tong carriages 66. The latter are transported through the furnace 60 by means of conveyor rolls 6S driven by conventional driving means (not shown). Hot gases are passed through the hollow chambers 24 to provide a radiant heat source for the cavities 16.

It is understood that either source of heat described may be employed in either of the furnaces illustrated. Also, in either case, electrical resistances can be located within hollow chambers 24 to provide a suitable heat source for cavities 16.

While the apparatus depicted in FIGURES 7 and 8 is particularly useful in heating glass sheets for soaking treatments such as required for annealing, tempering or coating, the refractory structures 10 are equally suitable for employment in lehrs for bending glass sheets. When glass sheets are bent in pairs preparatory to their lamination to form laminated safety glass Windshields, only the upper surface of an assmbly of aligned Windshields is exposed to radiant heaters and masses of metal are dis: posed below selected portions of the assembly to withdraw heat from those portions that are to remain relatively flat. Hence, heaters are located only above the path of movement of the glass sheet assemblies. The refractory structures 10 of the present invention, therefore, may be disposed on either one side only of the path of movement taken by glass sheets or on both sides of the path of movement.

In order to verify the benefits of the present invention to provide uniform heating of a glass sheet, the following experiment was performed. A furnace 24 inches long, 18 inches high and 16 inches wide was first provided with solid refractory walls including refractory channel member 1 /2 inches deep and 1%. inches wide of rectangular cross-section and separated from each other by 1 /2 inches to extend longitudinally of the furnace in side-by-side relation to each other along the opposite walls of the furnace. Heating coils of /2 inch diameter were carried in the channels and extended throughout their ength. After three hours of continuous heating, the temperature of the radiating surface of the solid walls varied between 1200 F. at the heating coils to 1110" F. intermediate the coils.

A glass sheet at room temperature and having dimensions inch thick and 10 inches by 12 inches was suspended in a vertical plane at the middle of the furnace. Introducing the glass cooled the furnace. Enough current was supplied to the heating coils to cause their temperature to recover to 1200 F. After 45 minutes during which time the furnace temperature stabilized, the surface temperature of the glass sheet reached a temperature patern varying between 1100 F. and 1125 F.

The same experiment was performed after removing the solid refractory walls and substituting cast sections of a silica-clay composition having smooth walls providing angular cavities 1% inch wide and 1% inch deep extending along the furnace walls in side-by-side relation for the solid refractory walls. Heating coil of /2 inch diameter were installed within the cavities and heated to 1200 F. The surface temperature of the cavities varied from 1195 F. at their apex to 1185 F. at the widest part of the opening after only one hour of heating.

A sheet of glass as identical as possible in length, width, thickness and chemical composition to the sheet heated by the furnace provided with solid refractory walls was placed in the middle of the heated furnace provided with heat exchange members conforming to the present invention. The glass sheet reached a surface temperature that varied between 1185 F. and 1190 F. This temperature range was reached from room temperature (about 75 F.) in about 15 minutes.

A 10 kilowatt power supply was used to heat the coils in both of the experiments described hereinabove. In other words, the same power input capacity was available for both furnace constructions.

From the results of these experiments, the response time of the empty furnace was reduced from 3 hours to 1 hour to heat the furnace to its equilibrium conditions with the heating coils set at 1200 F., and from 45 minutes to 15 minutes to heat a glass sheet to its equilibrium point.

Furthermore, the temperature gradient of the heater was reduced from F. to 15 F. and the glass surface temperature raised from a range of 85 F. to F. below the coil temperature to a range of 10 F. to 15 F. below the coil temperature by employing heat exchangers according to the present invention rather than the solid refractory structures of the prior art.

Use of present invention in radiant heat absorption As stated previously, the present invention is equally susceptible to use in the construction of radiant heat absorbers. A typical example of such use is in the manufacture of sheet or Window glass.

In the manufacture of drawn.sheet glass, glass is drawn generally upwardly in the form of a continuous ribbon from the surface of a bath of molten glass. The glass in its upward travel passes between various cooling means. The conventional cooling means employed are usually constructed of a refractory material, such as metal, usually in the form of a plurality of connected rectangular or square tubes for the passage of cooling fluid, such as water, therethrough and present a continuous plane surface to the glass. The high heat to which these conventional cooling means are subjected causes a non-uniform-' 1y distributed scale to form on their surfaces, thereby decreasing their heat absorbing efiiciency. Scaling of the cooling means is a particularly serious problem after they have been in use for some length of time. These cooling means also reflect heat back to the viscous glass, thereby further reducing their heat absorbing efliciency. The combination of the two described effects is a source of difiiculty in maintaining a uniform gauge or thickness of the sheet, and materially reduces the speed of drawing, so that a lesserquantity of glass is produced.

Various attempts have been made to increase the drawing speed by increasing the size of the cooling means. However, as will be obvious, the problem of scaling and reflection of heat back to the glass still exists. Also, attempts have been made to control gauge by varying the absorbing properties of the cooling means, as by placing pads of heat resisting material, such as transite or asbestos, along the surface of cooling means facing the glass. This requires a constant observation of the sheet and a constant changing of position of these various pads, and in addition, may discharge scale from the cooling means into the bath thus contaminating the molten glass. The changing of pad position thus adds to the non-uniform scale problem and may score or mark the cooling means thus decreasing their useful life.

The present invention has been utilized to provide a maximum heat transfer from the sheet to the heat absorbers per unit absorber area, to provide a uniform heat absorbing area resulting in a more uniform thickness of the sheet and to maintain a constant drawing speed over the kiln cycle, independent of uneven coating, scaling or marking of the heat absorbers.

Broadly, this aspect of the present invention includes the use of heat absorbers made up of a series of jutaposed connected hollow members mounted one in side-byside relation to another and interconnected, preferably in series, at their terminal ends, so that cooling fluid, such at Water, maybe fed therethrough. Each member has smoothly surfaced walls extending along parallel axes in planes oriented obliquely to each other and terminating in sharply angled relation to each adjacent wall. The juxtaposed walls of adjacent members form angular cavities of predetermined width and depth and extend in side-byside relationship. The assembly presents a surface of adjacent V-shaped cavities having acute apex angles, facing the rising glass ribbon. The angle of each cavity is such that any entering radiant energy from predetermined solid angles is more than 98% absorbed even though the absorptivity of the material of the heat absorbers is as low as 50%.

Turning now to 'FIGURES 9 and 11, there is shown a sheet of glass 100 being drawn from a bath 102 of molten glass in a drawing kiln generally indicated at 104. A draw bar 106 extending transversely of the kiln 104 is submerged in the bath 102. The glass sheet 100 in its viscous condition forms a base or meniscus 107 with the surface of the bath 102 and is drawn from the bath 102 and through the drawing chamber 108 of the kiln 104 by means of drawing rolls 110 of a conventional drawing machine generally indicated at 112. The drawing chamber 108 as depicted in the drawing, is defined by the bath 102, conventional L'blocks 114, ventilator water coolers 116, end walls 118 and catch pans 120. The ventilator coolers 116 are each positioned between an L block 114 and the base framework of the drawing machine 112 and extend substantially to the end walls 118 of the kiln 108. The base of the drawing machine 112 is substantially closed by means of the generally U-shaped catch pans 120, which are formed as coolers and are positioned so as to catch broken glass which may drop in the machine and thus prevent entry of fragments into the bath 102. These catch pans 120 also extend substantially to the end walls 118 of the kiln 108 and are constructed for the passage of cooling fluid, such as water. One leg of each catch pan 120 is disposed substantially parallel to and spaced from the sheet 100.

In the embodiment illustrated in FIGURES 9 and 10, heat absorbers 122 constructed in accordance with the teachings of this invention are provided for absorbing a maximum amount of radiant energy from each unit area of the sheet I100. These heat absorbers 122 are spaced above the surface of the bath 102 and are positioned on opposite sides of the sheet 100 to extend substantially the width of the sheet, transversely of the kiln 108.

As depicted in the drawings, each heat absorber 122 is constructed of juxtaposed connected hollow members 124 of parallelogram section, having smoothly surfaced

walls

126, 128 facing the surface of the ribbon of glass.

100. The

walls

126, 128 of each member extend along parallel axes in planes oriented obliquely to each other and terminate in sharply angled relationship, as at 130. The juxtaposed

walls

126, 128 of adjacent members 124 form cavities 132 which extend in side-by-side relationship, each cavity having an acute apex angle 134. The cross section of the members 124 may differ from that illustrated, so long as the cavities 132 are as described.

The members 124 are series connected at their terminal ends for the passage of a cooling fluid, such as water, therethrough, and to provide for the series connections and the passage of the cooling fluid manifold boxes 136 are provided. Conduits 138 adapted to be connected to a source of cooling fluid and to' a sump (both of which are not shown) are connected to the manifold boxes 136 for the inlet and outlet of cooling fluid to the members In the embodiment illustrated in FIGURES l1 and 12, heat absorbers 122' constructed in accordance with the teachings of this invention are provided for absorbing a maximum amount of radiant energy from each unit of the sheet 100. These heat absorbers 122' are spaced above the surface of the bath 102 and are positioned on opposite sides of the sheet to extend substantially the width of the sheet, transversely of the kiln 108. Each heat absorber 122 has a foot portion extending sub- 'stantially parallel to the surface of the bath and rearwardly of the sheet 100.

As depicted in the drawing, each heat absorber 122 is constructed of a plurality of juxtaposed connected hollow members 124 of parallelogram section, having smooth- 1y surfaced

walls

126, 128 facing the surface of the ribbon of glass 100. The

walls

126, 128 of adjacent members 124' form cavities 132 which extend in side-by-side relationship, each cavity having an acute apex angle 134. The cross-section of the members 124 may differ from that illustrated, so long as the cavities 132 are as described.

The members 124 of the embodiment illustrated in FIGURES 11 and 12 are series connected at their terminal ends for the passage of a cooling fluid, such as water, therethrough, and to provide for the series connections and the passage of the cooling fluid manifold boxes 136 are provided. Conduits 138 adapted to be connected to a source of cooling fluid and to a sump (both of which are not shown) are connected to the manifold boxes 136 for the inlet and outlet of cooling fluid to the members 124.

Attached to the lower hollow member 124 and in series therewith is a foot member 140, shown as constructed of a plurality of side-by-side hollow rectangular members 142. This foot presents a flat surface to the rising glass ribbon and a flat surface to a portion of the bath 102, conditioning the bath adjacent the base of the sheet. As illustrated, the depth of the foot 140 from the sheet 100 toward the L-block is larger than the members 124.

The following experiments were performed with a drawing machine to compare the effect of radiant heat absorbers constructed according to the present invention with cooling means constructed according to teachings of prior sheet glass drawing art.

A drawing machine produced a ribbon of double strength glass (0.125 inch nominal thickness) at a given drawing speed using conventional, plane faced cooling means of a predetermined height positioned in a kiln of the construction shown in FIGURE 9. Increasing the height of the conventional, plane faced cooling means by approximately 30%, With other conditions remaining constant, increased the drawing speed of the machine for the same thickness of glass by approximately 16%.

Using heat absorbers of approximately the same height as the first named conventional plane faced cooling means and constructed in accordance with FIGURES 9 and 10 of this invention, with other conditions still remaining constant, resulted in a 17% increase in drawing speed for the drawing machine producing a ribbon of the same width and thickness. In fact, its drawing speed with a shorter heat absorber was even greater than that produced using a higher cooling means of prior art construction.

Using heat absorbers of approximately the same height as in the example above for the construction of FIG- URES 9 and 10 and constructed in FIGURES 11 and 12 of this invention, and with footed portions having a horizontal dimension of 1.25 times the horizontal dimension of a member 124, with other conditions remaining constant resulted in a 6% increase in drawing speed for the drawing machine producing a ribbon of the same width and thickness. This is a 24% increase in speed over that produced using cnventional, plane faced cooling means with all other conditions remaining constant and producing a sheet of the same width and thickness.

Generally, using a conventional drawing construction, the glass ribbon is allowed to vary :0.006 inch in thickness across a predetermined width of ribbon of glass, and to maintain this thickness variation requires hourly or more frequent changing of positions of the aforementioned heat resisting pads. Using an arrangement, as shown in FIGURES 9 and 11, with other conditions remaining the same, it has been possible to hold the ribbon to a thickness variation of less than half the above figure across the same width ribbon with pads placed only over the manifolds 136 and closely adjacent structure. Over a considerable period of time, such pads were not moved transversely of the heat absorber.

What is claimed is:

1. Heat exchange apparatus comprising an integral refractory structure including a plurality of smoothly surfaced walls of material having an emissivity of at least 50% and extending along parallel axes in planes oriented obliquely to each other, each wall terminating in sharply angled relation to each adjacent wall to form a series of adjacent angular cavities of predetermined width and depth extending in side-by-side relationship, each cavity having an acute apex angle, and additional walls attached to the outermost wall of the outermost cavities to form a hollow chamber on the side of said smoothly surfaced walls opposite said angular cavities. A

2. A radiant heater comprising a refractory structure including a plurality of smoothly surfaced walls of a refractory material consisting essentially of a silica-clay composition containing at least 50% silica and the balance substantially entirely clay having an emissivity of at least 50%, and extending along parallel axes in planes oriented obliquely to each other, each wall terminating in sharply angled relation to each adjacent wall to form a series of adjacent angular cavities of predetermined width and depth extending in side-by-side relationship, each cavity having an acute apex angle, a radiant heat source cooperating with said radiant heater, and additional walls attached to the outermost wall of the outermost cavities to form a hollow chamber on the side of said smoothly surfaced walls opposite said angular cavities.

3. Heat exchange apparatus comprising a refractory structure including a plurality of juxtaposed connected hollow members having smoothly surfaced walls and of a material having an absorptivity of at least 50%, said walls of each member extending along parallel axes in planes oriented obliquely to each other and terminating in sharply angled relation to each adjacent wall, the juxtaposed walls of adjacent members forming angular cavities of predetermined width and depth and extending in side-by-side relationship, each cavity having an acute apex angle, and means to allow an inlet and an outlet for the passage of. cooling fluid to and from said hollow members.

4. Apparatus for improving the speed of manufacture of drawn sheet glass drawn generally upwardly from a bath of molten glass during a continued drawing cycle comprising a pair of radiant heat absorbing cooling means, each positioned adjacent one surface of the glass sheet and extending the width of the sheet, each cooling means including an assembly of hollow members of parallelogram section and joined together to present openended angular cavities of predetermined width and depth facing the surface of the sheet, each of said cavities having walls defining an acute apex angle therebetween, each assembly having a substantially rectangular foot joined at its lower terminus and presenting substantially plane surfaces facing the surface of the sheet and the bath of molten glass, each foot including an assembly of hollow members of rectangular section, said last-named assembly extending from the sheet a greater distance than said first-named assembly, all said hollow members being connected to one another for the continuous passage of cooling fluid therethrough, and means to allow the flow of cooling fluid to and from said connected hollow members.

References Cited by the Examiner UNITED STATES PATENTS 1,545,893 7/1925 Gregory 165115 1,584,241 5/1926 Mulholland 263-8 1,762,201 6/1930 Strong 1320 2,151,983 3/1939 Merrill 60 2,167,333 7/1939 Foss -75 2,176,999 10/1939 Miller 65-107 2,352,539 6/1944 Halbach 6584 2,607,168 8/1952 Drake 65-204 2,794,300 6/1957 Golightly 65-158 2,828,948 4/1958 Caldwell et al. 165104 FOREIGN PATENTS 87,927 8/1956 Norway.

OTHER REFERENCES Handbook of Chemistry and Physics, 34th edition, page 2512.

DONALL H. SYLVESTER, Primary Examiner.

CHARLES R. HODGES, Examiner.

C. I. LAICHE, D. CRUPAIN, Assistant Examiners.

Claims

1. HEAT EXCHANGE APPARATUS COMPRISING AN INTEGRAL REFRACTORY STRUCTURE INCLUDING A PLURALITY OF SMOOTHLY SURFACED WALLS OF MATERIAL HAVING AN EMISSIVITY OF AT LEAST 50% AND EXTENDING ALONG PARALLEL AXES IN PLANES ORIENTED OBLIQUELY TO EACH OTHER, EACH WALL TERMINATING IN SHARPLY ANGLED RELATION TO EACH ADJACENT WALL TO FORM A SERIES OF ADJACENT ANGULAR CAVITIES OF PREDETERMINED WIDTH AND DEPTH EXTENDING IN SIDE-BY-SIDE RELATIONSHIP, EACH CAVITY HAVING AN ACUTE APEX ANGLE, AND ADDITIONAL WALLS ATTACHED TO THE OUTERMOST WALL OF THE OUTERMOST CAVITIES TO FORM A HOLLOW CHAMBER ON THE SIDE OF SAID SMOOTHLY SURFACED WALLS OPPOSITE SAID ANGULAR CAVITIES.