US3420985A - Electric heating element - Google Patents

Description

Sheet Jan. 7, 1969 n. F. ANDERSEN ET AL ELECTRIC HEATING ELEMENT .filed July a, 196e mm... n "T EER Jah. 7, 1969 l, F, ANDERSEN ET AL 3,420,985

` ELECTRIC HEATING ELEMENT Filed July s, 196e United States Patent O 3,420,985 ELECTRIC HEATING ELEMENT Ingar F. Andersen and Richard D. Brew, Concord, N.H., assignors to Richard D. Brew Co., Inc., Concord, N.H., a corporation of New Hampshire Filed July 8, 1966, Ser. No. 563,890 U.S. Cl. 219-553 Int. Cl. H0511 3/10; H05b 3/66; H01c 3/00 8 Claims ABSTRACT OF THE DISCLOSURE This invention relates to heating elements and more particularly relates to electric resistance heating elements comprised of two or more heating panels electrically interconnected with one another for use in high temperature furnaces operating in regions of 3000 C.

Present day research and production techniques for producing various materials as for example, pure metals and alloys, require controlled temperature levels in the 2000 C. to 3000 C. range in vacuum or selecte-d positive or partial pressure environments. The source radiance of this thermal energy has conventionally been a generally annular configured single or multi-phase electric resistance device adapted to surround the specimen or load. These devices are typically represented in the prior art by U.S. Patent No. 3,057,936 and U.S. Patent No. 3,178,665. Though the design features of the herein invention are not restricted to any partic-ular configuration, this disclosure will be directed to annular shapes as typical of their applications. Once this disclosure is understood, these techniques can be employed for rectangular or cylindrical shaped element enclosures also.

Because the temperature environments in which these devices operate are so elevated, the materials from which they are made must necessarily have melting points higher than the maxim-urn temperatures achieved. In regions above 2000 C., these materials are limited to refractory metals which include, for example, tungsten, tantalum, molybdenum, col-umbium. and alloys thereof.

As exemplified by the prior art above noted, these heating elements generally assume two forms: (l) segments of sheet material whose radiant inner surface may be characterized as optically opaque and (2) a plexus of convoluted wire forms having their helixes intertwined and whose radiant structure may be characterized as optically transparent, i.e. it is visibly porous. Though such prior art devices generally have served their purpose, they exhibit certain deficiencies which limit their utility. In the case of sheet element configurations, when such elements are raised to operational temperatures of 2000 C. to 3000 C., internal stresses due to thermal expansion coupled with a loss of structural integrity and failure to support its own weight as temperatures approach the melting point of the material, creates a sagging condition which tends to distort the sheet element configuration. Further, when such sheet materials are raised to operational temperatures of 2000 C. to 3000u C., and then returned to ambient temperature environments, the crystalline grain structure -of the sheet becomes enlarged resulting in highly stressed fracture planes producing characteristics of extreme brittleness whereby even the ice slightest handling pressures fra-cture the entire sheet. When employed in vacuum environments, the opaque (Le. visibly non-porous) nature of sheet-type elements presents a further disadvantage in that it hinders out-gassing of molecular residue occurring from gaseous diffusion from within the work load region.

Heating elements of the plexus type construction, though they provide a degree of built-in flexibility because of their intertwined construction, have not adequately met the needs of the art because they use substantial 'amounts of expansive material to conform to their design criteria, and because of the porous, i.e. optically trans-parent, nature of their construction. they also compromise the essential requirement of an opaque radiant surface necessary for the most efficient radiance. Further, because of the mass of material used in their construction, these plexus type elements seriously lag in thermal response characteristics and because of their iuherent lack of lateral support continue to exhibit the prior art drawbacks of distortion and wraping due to reverse stresses induced during cycling from high operational ternperatures to ambient conditions.

We have -discovered that by providing a number of either wire or ribbon resistance filaments in contiguous side-by-side relation, and by having each such filament formed with coplanar deformations, we can virtually eliminate the deficiencies exhibited by the prior art while achieving a number of advantages, among which are that we can, in our construction, use lateral support means to substantially eliminate lateral deformation. Our invention further permits varying the temperature profile along the axial length of the heating element by control and variance of the amplitude of the coplanar deformations. Because of the iilamentary side-by-Side form of construction, our elements are not limited to conventional cylindrical shapes, but may be formed in many geometric forms, including compound curves. And because of the very substantial decrease in mass over the prior art, our construction does not tend to behave like a heat-sink, as do the prior art devices, hence, our structure is sensitive to electrical energy input variations, which in turn results in close temperature control. A further very important advantage is that because of the contiguous relation of each filamentary Imember, the composite structure is substantially opaque when viewed from any point along a line perpendicular to its radiant inner peripheral surface and thus assures a continued radiance of thermal energy against the load by generating a minimum watt density per unit area of heating element. By the same token, this construction, because of the coplanar deformations, provides an element which when employed in a high vacuum environment exhibits suicient porosity so that the molecular residue resulting from gaseous diffusion from within the work load region can be readily exhausted. This latter feature then necessarily means that the device when employed in vacuum environments is readily porous, that is, optically transparent, when viewed `from a location remote from a line of perpendicularity to its inner peripheral surface thereby permitting random moving Imolecules from out-gassing to escape through the heating ,element from within the inner confines thereof. This phenomenon of gaseous diffusion is a well known physics principle which metals characteristically exhibit and tend-s to irnpair the reaching of vacuum levels lower than 10-3 mm. Hg an-d into the ultra-vacuum regions in elevated temperature environments.

It is therefore among the various objects of this invention to produce an electrical resistance element for generating temperatures in excess of 2000 C. whose composite construction will not become embrittled or significantly warped with operative use.

It is a still further object of this invention to provide a heating element which can be used to produce and maintain a substantially isothermal temperature along its vertical length.

It is a further object of this invention to produce an electric resistance element having a high degree of optical opacity while concurrently exhibiting a correspondingly high degree of porosity and light weight without regard to its form of construction.

It is another object of this invention to provide an electric resistance element construction which will exhibit a high degree of thermal response and not behave as a heat-sink.

It is a still further object of this invention to produce an electrical resistance element for use in high temperature environments whose costs are significantly lower than what the current state of the art can produce, while maintaining a high operating efficiency.

A feature of this invention is the construction of a heating element from a plurality of wire or ribbon resistance elements having coplanar deformations. These deformations provide necessary porosity to the structure while at the same time providing a degree of freedom of movement in an axial direction resulting from heating and cooling stresses.

A further feature of this invention is the provision of lateral support means within adjacent ones of the deformations to resist lateral bending stresses.

Because of the columnar type construction, a still further feature is the freedom of design permitted by this disclosure in fabricating heating elements, particularly from refractory type metals.

With these and other objects in view as will hereinafter more fully appear and which will be more particularly pointed out in the appended claims, reference is now had to the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 illustrates a three-phase heating element typically fabricated in accordance with this invention;

FIGURE 2 illustrates an enlarged detailed view of the elemental construction employed taken on line 2-2 of FIGURE l;

FIGURE 3 shows a second embodiment of a threephase heating element manufactured in accordance with the herein disclosure;

FIGURE 4 shows a view in greater detail of the features of fabrication of the device of FIGURE 3; and

FIGURE 5 illustrates a typical construction for producing an isothermal condition along a vertically oriented heating element.

EMBODIMENT I Referring now to the details of FIGURE 1, there is shown a three-phase heating element comprised of three resistance segments 11 which are electrically interconnected only at their bottom peripheries by conductor member 12. Each segment 11 has rigidly xed to its upper peripheral edge an electrically conductive support member 13 comprised of a radially extending arm 14 joined to inner and outer peripheral segments 15 which are welded to the upper edge of segment 11.

Referring now to FIGURE 2, the segments 11 are comprised of a plurality of vertically oriented, correspondingly shaped sinuous iilamentary resistance wire members 16 in touching side-by-side relation. Adjacent ones of said conductor wire members (or filaments) are each vertically oriented relative to one another so as to be in substantial symmetry with respect to a common imaginary plane passing through each adjacent wire member thereof. In the illustrated embodiment, these wire members 16 may be made from any refractory metal, as for example, tungsten, and may range in size from .005 inch to .125 inch or more in diameter, depending on the design requirements for power output, size of heating element, and duration of operation at selected temperature levels. To impart structural rigidity to each segment 11 against lateral deflection forces, the illustrated embodiment depicts in FIGURE 2 the use of a pair of U-shaped pin members 17a and 17b also fabricated of refractory metal which are inserted in opposed relation into the same pair of adjacent passageways 18 and 19, formed by the plurality of contiguous wires 16, so as to be in frictional engagement with each other and with adjacent ones of wires 16.

It can now be seen that with this construction, the inner surface appears virtually opaque when viewed from any point along a line perpendicular to that surface. By the same token, this construction offers easy egress to the random moving molecules during an out-gassing operation because, when the inner surface is viewed from a location remote from a line of perpendicularity to its inner peripheral surface, it is visibly porous, that is, optically transparent.

In use, the heating element 10 is suspended within a furnace by its conductive support members 13. Threephase current flow, conventionally applied to each of these support members, is conducted to the peripheral segments 15, thence to the sinuous resistance wires 16 of each segment 11.

Test results have demonstrated that no perceptible distortion of the disclosed structure takes` place even after repeated cyclings from ambient conditions to elevated temperature levels of 3000 C. or more. This behavior is a clear departure from the manner in which the heating elements of the prior art perform with distortion and deformation occurring after only a few temperature cyclings. Further, the herein disclosed element, because of its construction, uses much less material since the developed length of the wire element 16 is significantly less than that of the prior art wire constructions and does not become so embrittled as elements of the prior art.

EMBODIMENT 1I This disclosure further contemplates a second embodiment of the invention which is illustrated in FIGURES 3 and 4. These drawings show a three-phase heating element 10 comprised of three resistance segments 11' which are electrically interconnected only at their bottom peripheries by conductor member 12'. Each segment has Welded to its upper peripheral edge an electrically conductive support member 13 comprised of a radially extending arm 14 joined to inner and outer peripheral segments 15' which are welded to the upper edge of segment 11.

The wire members previously described are replaced in this embodiment by narrow iilamentary strips or ribbons of resistance material 16', similarly configured in sinuous form. These ribbons may range in widths from .050 to .500 inch or more and have varying thickness depending on the size of the element and the operational characteristics desired.

This type of construction employing refractory metals as hereinbefore disclosed, is also useful where operating temperature levels vary from ambient to 3000 C. or

more.

This strip type construction is further advantageous in that it permits more design versatility in matching the electrical characteristics of a heating element construction to a given power supply. One important parameter of an element design is the electrical resistance of each sinuous member 16', which in turn is related to its crosssectional area. In the case of employing round wire members (as 16 in the rst embodiment) the electrical resistance of such a wire is related directly to its diameter. Therefore, whenever a a particular electric vacuum furnace system includes a given power supply, it is necessary to provide an element with a resistance which is determined by the voltage output of such a power supply. If an element is to be fabricated of round wire members,

eg. 16, then the wire cross-sectional area which is necessary is determined by its diameter, a singular dimension. On the other hand, if an element is to be fabricated of strip material, its resistance, as determined by its crosssectional area, is related not only to the thickness of the strip, but also to its width. Accordingly, if the heating element is to be fabricated of material in the form of sinuous strips or ribbons 16', then the resistance of any strip, a function of the cross-sectional area, is determined by varying the strip width of any given thickness of material. In this latter case then, the design characteristics and therefore the total resistance of each segment or panel 11 may be attained by controlling either one or both the thickness and/ or width of each strip member 16', and so long as the cumulative cross-sectional area of strips 16 remains constant for any given panel, the resistance will also remain constant, regardless of individual strip widths.

Lateral support for each segment 11' in this embodiment may also take the form of refractory metal pins 17a and 17b as hereinbefore disclosed, and shown in FIG- URE 4.

With the above disclosure of two basic forms of heating element construction, this invention further contemplates other novel heating element constructions. For example, there are circumstances where, in the generally annular element conguration, heat losses from the top and bottom regions cannot be suitably controlled. In this respect, an element comprised of wire or strip members configured as depicted in FIGURE 5 can be employed to produce an isothermal condition along its top to bottom dimension. As will be noted from this illustration, the coplanar deformations have a non-uniform period, i.e., the individual wire construction is somewhat more elongated at its center portion than it is toward its end portions. Accordingly, more yheat is radiated at the ends than at the center. This greater heat radiance at the end portions can thus be used advantageously to overcome heat losses in these areas. It should also be noted at this point that the amplitude of these coplanar deformations may also vary, so as to influence the radiant qualities.

Though this invention has been described in terms of using conductive resistance elements of sinuous symmetrical construction, this concept is also intended to broadly include any other geometric coplanar configuration of curves, lines, or combinations tof Wire elements or composite panel segments so long as the feature of symmetry of construction is present. For example, a square wave form, or triangular wave form may be employed, depending |on the thermal, electrical, and/ or structural characteristics desired.

It will be understood that various changes in the details, materials, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of this invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A non-woven electrical resistance heating element comprising:

(a) a plurality of refractory metal filamentary conductors in side-by-side contiguous relation, each conductor exhibiting a plurality of correspondingly similar coplanar deformations;

(b) adjacent ones of said contiguous conductors each being axially displaced relative to one another so as to have their corresponding adjacent deformations in an out-of-phase relation; and

(c) conductive means interconnecting said lamentary conductors across their adjacent terminal ends.

2. A non-woven electrical resistance heating element as set forth in claim 1 wherein:

(a) the axial displacement of adjacent ones of said contiguous conductors forms, between their adjacent out of phase deformation, a plurality of passageways of lateral extent across the element; and

(b) support means inserted into at least one pair of adjacent passageways.

3. The structure set forth in claim 1 wherein the coplanar deformations have a regular period.

4, The structure set forth in claim 1 wherein the coplanar deformations have a non-uniform period.

5. An electrical heating element comprising:

(a) a plurality iof elongated non-woven resistance segments insulated from one another along their longitudinal edges;

(b) each segment being comprised of a plurality of refractory-metal conductors in side-by-side contiguous relation;

(c) each conductor exhibiting a plurality of correspondingly coplanar deformations;

(d) adjacent ones of said conductors with coplanar deformations being vertically oriented relative to one another so as to be in substantial symmetry with respect to a common imaginary plane passing through adjacent conductors, and thereby forming passageways of lateral extent across the element;

(e) a continuous conductor means interconnecting each of the adjacent segments at the lower ends thereof; and

(t) conductive support means attached to each segment along its upper end portion.

6. An electrical heating element comprising:

(a) a plurality of non-woven elongated resistance segments vertically oriented in spaced apart relation forming an enclosure;

(b) electrically conductive means interconnecting only the adjacent bottom portions of said segments;

(c) conductive support means attached to each segment along its upper portion;

(d) said segments each being comprised of a .plurality of conductors in contiguous relation wherein each adjacent conductor exhibits a plurality of correspondingly similar coplanar deformations along its axial length;

(e) adjacent ones of said conductors each being oriented relative to one another so as to be in substantial symmetry with respect to a common imaginary plane passing through adjacent ones thereof, thereby forming a plurality of lateral passageways therewithin; and

(f) lateral support means frictionally carried within any adjacent pair of such passageways.

7. The structure set forth in claim 6 wherein said plurality of conductors in contiguous relation comprise tilamentary refractory wire.

8. The structure set forth in claim 6 wherein said plurality of conductors in contiguous relation comprise filamentary refractory ribbon strips.

References Cited UNITED STATES PATENTS 2,522,342 9/ 1950 Schaefer 338-208 X 2,533,409 12/1950 Tice 219--213 2,817,737 12/1957 Morris 338-208 2,884,509 4/ 1959 Heath 338--208 2,938,992 5/ 1960 Crump 219-528 3,178,665 4/ 1965 Matheson et al 338-299 3,218,436 11/ 1965 Edwards et al 219--544 VOLODYMY R Y. MAYEWSKY, Primary Examiner.

U.S. Cl. X.R.

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