US3252692A - Woven heat shields - Google Patents

Description

May 24, 1966 E. J. LANGA 3,252,692

WOVEN HEAT SHIELDS Filed March 18. 1964 lnven t'ov: Edward J. Lang b aw/2 His Adv-to neg United States Patcnt O 3,252,602 WOVEN HEAT SHIELDS Edward J. Langa, Willoughby, Ohio, assignor to General Electric Company, a corporation of New York Filed Mar. 18, 1964, Ser. No. 352,826 6 Claims. (Cl. 26340) This invention relates to high temperature furnaces, and more particularly to refractory metal heat shields for use in furnaces operating at temperatures over 1800 F. and having non-oxidizing atmospheres.

In high temperature atmosphere and vacuum furnaces, radiation shielding is often used for minimizing heat losses from the furnace instead of, or in supplement to, conduction heat barrier type materials such as fire brick. In vacuum furnaces and in experimental furnaces with which it is highly desirable to avoid contaminating gases, radiation shielding by means of highly reflective metal sheet members is much preferred over the use of brick work due to gas absorption and evolution from the bricks. Furthermore, refractory metal heat shields can be used in close proximity to a very high temperature heat source, with lower temperature heat shielding provided by shields of other metals or by brick work around the periphery of the inner radiation shields. However, radiation heat shields made from refractory metal sheet have several drawbacks which make them relatively expensive to use. Aside from the initial material costs for relatively wide sheets of refractory metals, previously known refractory metal heat shields have been susceptible to buckling and other types of mechanical failure due to thermal cycling, static thermal stresses, localized chemical contamination due to gases given off by materials being heated, and other mechanisms. Such factors can lead to the necessity of replacing refractory metal heat shields rather often, particularly in furnaces which are cycled many times between operating temperature and room temperature and in those in which the material being heated gives off corrosive or reactive gases.

It is an object of the present invention to provide refractory metal heat shielding for high temperature furnaces having non-oxidizing atmospheres in a form that is much less susceptible to the above enumerated causes of failure in previously known refractory metal thermal radiation shields.

A further object is to provide elevated temperature furnaces for operation at temperatures over 1800 F. incor porating in their design refractory metal shielding of such a type.

Another object of the invention is to provide heat shields for elevated temperature furnaces which can be fabricated from relatively inexpensive narrow strips of refractory metal sheet rather than more expensive wider sheets. Such narrow strips can be made available either by purposeful manufacture as such, or as scrap material from the'manufacture of wider refractory metal sheet.

Still another object of the invention is to provide heat shields for high temperature furnaces which can be fabricated readily in a myriad of variable shapes and sizes, without limitations caused by the size of sheet metal material available or caused by any necessary intricate sheet stamping, forming and bending processes which may be applicable only with difficulty to large sheets of refractory metals.

Also included as an object of the invention is provision of heat shields one segment of which is physically independent from the other segments so that differential thermal expansion can be accommodated without placing undue thermal stresses and strains on the shield, and further so that one part of the shield may be replaced without the necessity for replacing the entire shield. Such ice heat shields have an ability to breathe on expansion and contraction without setting up excessive stresses.

Moreover, the invention includes the provision of heat shields which can be fabricated to include sight holes and other types of openings having square, rectangular or other geometrically shaped holes without the necessity of drilling or more complex machining operations.

In the drawing, FIG. 1 is a schematic representation partly in section, of a high temperature vacuum or atmosphere furnace having heat shields constructed according to the invention.

FIG. 2 is a view of a mandrel on which a heat shield is being woven.

FIG. 3 is a pictorial view of one type of heat shield according to the invention.

FIG. 4 is another embodiment of a heat shield according to the invention.

FIG. 5 illustrates a sight hole woven into a heat hield according to the invention.

FIG. 6 shows a variant type of weave that can be used for heat shields according to the invention.

Briefly stated, in one form, the invention comprises a heat shield having a basket-woven structure and being made of strips of refractory metal. The strips are preferably of a size between 0.001 inch and 0.015 inch thick,

. between 0.25 inch and 2 inches wide, and of a great enough length to facilitate weaving in and out to form a fabric-type body. In the specification, the term refractory metal is meant to include tungsten, molybdenum, tantalum, columbium, chromium, mixtures or alloys of two or more of these metals, and alloys based on these metals, An alloy is herein considered to be based on a certain metal when it contains at least 50% by weight of the metal.

As is readily apparent, a basket-weave-type structure can assume any of several different embodiments. One example of a basket weave structure is a simple one-over, one-under weave in which both the warp and the woof are strips of the same type of material having roughly the same dimensions. Another type of woven structure included within the invention is one in which the woof consists of relatively wide strips of refractory metal, placed closely together, and the warp consists of relatively narrow strips or wires or rods of refractory metals spaced widely apart, but still close enough together to give support to the woof and hold it together, thereby forming a fabric. The warp and woof are generally interchangeable. The term warp is used to indicate the strip mate rial which is laid out before weaving to form the skeleton of the weave. The woof is the strip material which is woven into the warp. Furthermore, heat shields of the invention can assume unlimited varieties of size and shape; for example, they can include cylinders, ovals, conical shapes, D shapes, flats, re-entrant closed or open bodies,

and any other shapes that can be fabricated from strip materials. Since the heat shields are to be made of numerous pieces of material, both the sizes and the numbers of pieces can be varied to produce heat shields of practically any size desirable. Splices can be made readily in the fabric by over-lapping the strips for a few weaves or by other means. 'The ends of a sheet of heat shield fabric can be made square by folding the ends back into the: fabric. Holes for sighting through into the hot zone and for other purposes can be made readily in various shapes including square, rectangular, and other shapes, by leaving an open space in the weave and by weaving in overlays of extra strips of material to shape the contours of the hole.

Turning now to the drawing, FIG. 1 shows schematically .and partly in section a high-temperature vacuum furnace having a port 1 connected to a conventional vacuum pumping system not shown, current inleads 2 connected to a refractory sheet metal heating element 3 which consists of two halves 3a and 3b surrounding the specimen location and joined together at the end opposite the end to which the current leads connect. The leads enter the vacuum tank 4 through insulators 2a and 2b. The heating element can be made of a metal such as tantalum or tungsten, for example. A specimen holder 5 is provided to allow the positioning of the specimen within the heating element 3. A typical heat shielding arrangement using woven strips of refractory metal is shown in this furnace generally at 6. The heat shield surrounding the heating element consists of three concentric cylinders of woven strip material of refractory metal 6a, 6b, 6c with spaced apart end members afiixed as at 7 to the permanent end of the heat shield through which the electrical inleads and the specimen holder enter. A removable lid is shown at 8. The lid consists of three woven strip flat pieces so constructed as to provide radiation shielding means for the tops of the three concentric sections of the heat shields 6a, 6b, 60. For purposes of optical sighting onto the specimen from above to determine its temperature by radiation measurement means, a sight hole 9 can be provided through the heat shield lid 8. Furthermore, other temperature control means could be provided such as by a thermocouple coming up along or through the specimen holder 5.

In such a furnace, the atmosphere can be controlled as desired. For instance, the vacuum pumping system could be used to evacuate air or contaminating gases from the chamber or tank 4, and then the tank could be back-filled with any desired gas such as argon, helium, nitrogen, or other gases. Depending on the materials of construction of the furnace, various gases could be used for purposes of reacting with the specimen. In like manner, the furnace could be used to remove various reactive gases from a specimen by high-temperature treatment. However, a major application of such a furnace would be in the use of a vacuum environment during heating.

Refractory metals are particularly sensitive to oxidation at elevated temperatures. For this reason, the invention is directed to furnaces having non-oxidizing atmospheres. Since most neutral and vacuum atmospheres contain some oxygen, the term non-oxidizing is used herein to mean atmospheres that are not so oxidizing as to destroy the commercial usefulness of the heat shields.

At temperatures over about 1800 F. the refractory metals become competitive for applications in such structures as resistance heating elements and heat shielding. Depending on the time of use, frequency of cycling between elevated temperature and room temperature, atmosphere, and other factors, refractory metals are preferred to other metals for such applications at elevated temperatures starting at about 1800 F.

Due to problems of fabrication, mechanical and thermal stability, chemical erosion, desirability of being able to replace part of a heat-shielding structure, and for other purposes, a basket weave structure is preferable in many refractory metal heat shield applications to previously known sheet heat shields. The problems of fabricating structures of refractory metals are peculiarly more difficult than with other metals useful at lower temperatures. Previously known heat shields were generally made of single sheets of metal rolled into a cylindrical form with the ends joined by riveting or by stapling or lacing with wire of similar metal. In some cases, welded structures have been used. Although refractory metal heaters have 'been produced in the form of metal wire cloth and cloth made of other materials such as graphite, heat shielding is an entirely different application due to the necessity for blocking radiation. Refractory metal wire cloth cannot serve the purpose of the total blocking of radiation because it cannot be commercially made with the extremely dense weave which would be necessary. On the other hand, strips of refractory metal are available both in manufactured form and as scrap, and according to the invention, can be used very effectively for thermal radiation shielding. Refractory metal strips can be made either "by slitting relatively wide sheet or by flattening wire or small diameter rod. Strip is also available in the form of scrap trimmings from the edges of Wider sheet. Such trimmings have a relatively low inherent value.

Sheet metal strip is adaptable enough to allow it to be formed into almost an infinite variety of sizes and shapes. In a closely woven one-over, one-under basket weave, each heat shield is in effect a double heat shield in that where a warp strip provides a reflective surface facing the heat source it is backed up by a woof strip which provides a second reflecting surface which reflects back toward the heat source the heat radiated by the back or cold side of the warp strip. In addition, the dead-air pocket or space between the front and back strips provides useful insulation in an atmosphere furnace. In another variety, such as shown in FIG. 6, the warp (or woof) can be made of a relatively narrow strip material, or even a wire or small diameter rod that would serve the purpose of holding the woof (or warp) together. As is apparent, such a structure still provides the advantages of the invention and comes within its inventive concepts. Moreover, other weaves more complicated than a simple oneover, one-under pattern can be useful for certain purposes. Any refractory metal woven structure which is somewhat self-supporting and in which a strip material provides line-of-sight blockage of substantially all thermal radiation for use in high temperature furnaces is considered to be within the limits of the invention.

FIG. 2 illustrates a practical method of weaving together refractory metal strips. Warp strips 11 are placed on the surface of a cardboard tube 12 parallel to the axis of the tube and are held in place by strips of tape 13. The woof strips or wire 14 are then woven in and out of the warp. Depending on the length of the strips, splices can be made wherever desired by lapping a new strip over the old one for a sufficient number of weaves. Edges can be made square by folding the ends back into the weave, as is well known in the art of basket making. When the heat shield is completely woven, the cardboard tube can be slipped out through the end. For more complicated shapes, collapsible mandrels may be preferred. Supporting members, such as rigid rings of refractory metal in the case of cylindrical heat shields, can be attached or woven into the shields if desired to aid in holding them in shape.

FIGS. 3 and 4 show two of the many variations of geometry in which heat shields can be made according to the invention. FIG. 5 illustrates a sight hole woven into a heat shield in which the strips which otherwise would block the hole have been folded back over the perpendicular strips in both directions and tucked back in on themselves to provide a hole as at 15. Other strips or sheet metal structures can be used to block parts of the perimeter of the hole to give a desirable shape and size. Sighting and access holes in such heat shields can also, of course, be constructed in any size and shape desired by appropriate folding back of strips on themselves or by other means.

I have constructed several cylindrical heat shields according to the principles of the invention ranging in size from about 3 inches high and one inch in diameter to about 30 inches high and 15 inches in diameter. For the latter heat shield, the material used was molybdenum strip about inch wide by 0.010 inch thick. Practical heat shields can be made from strip having a thickness of from 0.005 inch or less to 0.015 inch or more, although greater thicknesses are not usually necessary except, possibly, for applications requiring high mechanical strength for self support or other purposes. The preferred width of the strip ranges from 0.25 inch to 2 inches or greater. The lengths of these strips should be long enough to allow eflicient weaving. The length, width and thickness preferred for each application, of course, depends on the size and shape of the heat shield to be constructed.

The examples used in describing the present invention are presented merely for illustrative purposes and should not be considered to be limitative on the invention, the only limitations thereon being those established by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a high temperature furnace comprising walls defining an enclosure for a nonoxidizing atmosphere and containing heating means for generating temperatures in excess of 1800 F., a thermal radiation heat shield within said enclosure and substantially enclosing said heating means defining a hot zone, said heat shield comprising strips of metal selected from the group consisting of tungsten, molybdenum, tantalum, columbium, chromium, alloys based on at least one of said metals, and combinations of said metals and alloys, said strips being interwoven into a plexus which substantially blocks thermal radiation emanating from said hot zone.

2. In a high temperature furnace comprising walls defining an enclosure for a nonoxidizing atmosphere and containing electrical resistance heating means for generating temperatures in excess of 1800 F., a thermal radiation heat shield within said enclosure and substantially enclosing said heating means defining a hot zone, said heat shield comprising strips of metal selected from the group consisting of tungsten, molybdenum, tantalum, columbium, chromium, alloys based on at least one of said metals, and combinations of said metals and alloys, said strips being interwoven into a plexus which substantially blocks thermal radiation emanating from said hot zone.

3. In a high temperature furnace comprising walls defining an enclosure for a nonoxidizing atmosphere and containing electrical resistance heating means for generating temperatures in excess of 1800 F., a thermal radiation heat shield within said enclosure and substantially enclosing said heating means defining a hot zone, said heat shield comprising strips of metal selected from the group consisting of tungsten, molybdenum, tantalum, columbium, chromium, alloys based on at least one of said metals, and combinations of said metals, said strips being arranged in at least two generally perpendicular directions comprising a warp and a woof, with the Woof being woven into the warp in a generally one-over, oneunder pattern arranged so that the woof is also one-over, one-under in relation to the warp, said strips being woven together tightly enough to block substantially all thermal radiation emanating from said hot zone except where holes are left through said heat shield for purposes such as sighting for radiation pyrometry.

4. The heat shield of claim 3 in which the strips are of tungsten.

5. The heat shield of claim 3 in which the strips are of molybdenum.

6. The heat shield of claim 3 in which said strips have a thickness of from about 0.005 inch to about 0.015 inch and a width between about 0.25 inch and about 2 inches.

References Cited by the Examiner UNITED STATES PATENTS Re. 25,261 10/1962 Western 1331 X 610,555 9/1898 McNamee 126202 2,404,060 7/1946 Hall et a1 263 X I 2,641,456 6/1953 Schmertz 263-50 X 2,886,697 5/1959 Pomeroy 2078 X FOREIGN PATENTS 852,457 10/1960 Great Britain.

WILLIAM F. ODEA, Primary Examiner.

JOHN J. CAMBY, Examiner.

D. A. TAMBURRO, Assistant Examiner.

Claims (1)

1. IN A HIGH TEMPERATURE FURNACE COMPRISING WALLS DEFINING AN ENCLOSURE FOR A NONOXIDIZING ATMOSPHERE AND CONTAINING HEATING MEANS FOR GENERATING TEMPERATURES IN EXCESS OF 180*F., A THERMAL RADIATION HEAT SHIELD WITHIN SAID ENCLOSURE AND SUBSTANTIALLY ENCLOSING SAID HEATING MEANS DEFINING A HOT ZONE, SAID HEAT SHIELD COMPRISING STRIPS OF METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, TANTALUM, COLUMBIUM, CHROMIUM, ALLOYS BASED ON AT LEAST ONE OF SAID METALS, AND COMBINATIONS OF SAID METALS AND ALLOYS SAID STRIPS BEING INTERWOVEN INTO A PLEXUS WHICH SUBSTANTIALLY BLOCKS THERMAL RADIATION EMANATING FROM SAID HOT ZONE.

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