US3178665A

US3178665A - Electrical heating element

Info

Publication number: US3178665A
Application number: US219404A
Authority: US
Inventors: Wilfrid G Matheson; James P Clune; Theodore J Pricenski
Original assignee: Sylvania Electric Products Inc
Current assignee: GTE Sylvania Inc
Priority date: 1962-08-27
Filing date: 1962-08-27
Publication date: 1965-04-13
Anticipated expiration: 1982-04-13
Also published as: DE1515132B1; GB1015270A

Description

April 13, 1965 w. G. MATHEsoN ETAL 3,178,665

ELECTRICAL HEATING ELEMENT Filed Aug. 27, 1962 4 sheets-sheet 1' 1N VEN TORS ATTORNEY April 13 1965 w G. MATHEsoN ETAI. 3,178,665

ELECTRICAL HEATING ELEMENT 4 Sheets-Sheet 2 Filed Aug. 27. 1962 INVENTORS N O S E H T A M G. D R F H. W

JAMES P. CLUNE THEODORE J. PRICENSKI FIG. 3

ATTOR NEY April 13 1965 w. G. MATHEsoN ETAL 3,178,665

ELECTRICAL HEATING ELEMENT 4 Sheets-Sheet 3 Filed Aug. 27, 1962 MIM6 WILFRID G. MATHESON JAMES P. CLUNE THEODORE J. PRICENSKI INVENTORS ATTORNEY 4 Sheets-Sheet 4 INVENTORS ATTORNEY April 13, 1965 w. G. MATHEsoN ETAL ELECTRICAL HEATING ELEMENT Filed Aug. 2'?, 1962 (KQ 10") A WILFRID G. MATHESON JAMES F. CLUNE THEODORE J, PRICENSKI BY/av FIG.5

3,178,665 ELECTRlCAL HEATING ELEMENT Wilfrid G. Matheson, Marblehead, James P. Clune,

Danvers, and Theodore J. Pricenski, Ipswich, Mass.,

assignors to Sylvania Electric Products Inc., Salem,

Mass., a corporation of Delaware Filed Aug. 27, 1962, Ser. No. 219,404 31 Claims. (Cl. SSS-299) This invention relates to heating elements for electrical furnaces and more particularly to self-supporting heater elements suitable for use as cathodes and fabricated of a plexus of refractory metals such as molybdenum, tantalum,rhenium, columbium or preferably tungsten.

Electrical heating elements are known to the art and some types of elements have previously been fabricated of the above-mentioned refractory metals. Elements using refractorymetals, particularly tungsten however, have lbeen rfabricated yfrom sheet materials or machined from heavy stock `and the metals so fabricated do not possess crystal structures having optimum characteristics to resist breakage or maintain their geometrical form. But refractory metal wire does possess these characteristics and we have discovered that foraminous plexuses made rom wire may be used successfully as heating elements in vacuum and/ or inert or reducing atmosphere furnaces of the resistance or electronic types. This discoverey led to the achievement of rugged heating elements which can withstand temperatures as high as 2000o C. and often as high as 2500 to 3000 C. Such foraminous plexuses can be made to have electrical characteristics substantially identical to those known to the prior art, but yet be structurally far superior to their solid sheet counterparts. f

A foraminous plexus according to this invention cornprises a series of intertwisted, elongated, helical convolutions of refractory metal wires. To form the plexus, a number of helically convoluted wires each having similar electrical characteristics are intertwisted together in such a way that generally two convolutions are intertwisted in each other convolution except in the case of the rst and last convolutions of series, where only one convolution will be intertwisted. Thicknesses of Wire may vary depending upon the application 4and when desired, double or triple helical convolutions can he used advantageously to improve the strength of the plexus. For some applications, it may be desirable to insert straight support wires in the turn abutments to make the plexus more durable.

The use of a series of helical convolutions allows greater latitude in the application of the heater element. Primarily, even after heating and hence when each of the individual convolutions are quite brittle, the convolutions are movable in their adjacent convolutions and thus can withstand stresses which would ordinarily fracture solid sheets of similar refractory metals. Coupled with the inherent flexibility of the plexus is the increased strength of drawn wire due to the incorporation of long, fiber-like crystals as a result of processing. Thus, even though the individual convolutions of wire may become brittle after heating, the plexus itself is still very flexible and can be moved fairly freely in the furnace Without an inordinate danger of breakage. Further, the problem of thermal shock, usually resulting from heating too rapidly or cooling too quickly is materially reduced due to the construction and crystal structure provided by wire.

Accordingly, the primary object of this invention is the fabrication of heating elements having increased me- United States Patent O chanical strength and the ability to withstand fairly large thermal shocks.

Another object of this invention is the substantial elimination of distortion in the geometrical shape of heate ing elements after elevation to high temperatures in furnaces.

Another object of this invention is the extension of the life of electrical heating elements fabricated from refractory metals which can be heated to temperatures as high as about 3000 C.

A feature of this invention is the fabrication of a plexus of refractory metals from a series of intertwisted helical convolutions of refractory metal wire.

A further feature of this invention is the fabrication of a plexus in a foraminous form from a series of helical convolutions of refractory metal wire.

Another feature of this invention is the use of at least three plexuses connected together in the shape of a generally cylindrical cage so as to form a cathode for high temperature vacuum furnace.

And yet another feature of this invention is the joining together of a series of intertwisted helical convolutions by a conductor element which supports the series of helical convolutions in the furnace and conducts current thereto.

An advantage of this invention is that a heating element of a foraminous plexus can be stronger after heating and have a longer life than similar elements fabricated of sheet metal or machined from heavy stock.

Another advantage of this invention is the wider latitude afforded to the heating element designer in shaping heating elements of refractory metals so that many different geometrical configurations can be made.

Many other objects, features and advantages of this invention will become manifest to those conversant with the art, upon making reference to the detailed description which follows and the accompanying sheets of drawings in which preferred embodiments of heating elements of refractory metal, foraminous plexuses of different coniigurations are shown and described and wherein the principles of the present invention are incorporated by way of illustrative examples. Of these drawings:

FIGURE 1 is a perspective view of an electrical heating element particularly designed for use in a three phase electrical furnace of the resistance or electronic types.

FlGURE 2 is a top plan view of the heating element shown in FIGURE l with a portion of the upper surface cut away to illustrate the laminations forming the supporting structure for the plexuses. Furthermore, this ligure shows the Welds made to secure the laminations together, which were not shown in FIGURE 1.

FIGURE 3 is a fragmentary View of a portion of `a plexus which may be used in this invention.

FIGURE 4 is a perspective view of another modification of the electrical heater element which may be fabricated according to our invention.

FIGURE 5 is a top plan view of the heater element shown in FIGURE 4 with part of the Welds which hold the heating element together shown.

Referring now to FIGURE 1 of the drawing, the heating element which is contemplated by this invention can comprise three

foraminous plexuses

1, 3 and 5 each of which is formed of a series of intertwisted helical convolutions of refractory metal wires (shown in detail in FIGURE 3). As will be clearly seen in FIGURE 2, the plexuses can be assembled in substantially cylindrical form and can be connected together at one end by an outer conductor ring 15 and inner conductor ring (not envases shown) each of which can be prepared of tungsten when tungsten plexuses are used. Preferably, the attachment of the plexuses to the conductor rings is made by a heliarc welding of the rings together on their lowermost extremity, however other known techniques for joining and securing refractory metal parts may also be used. When welding on a tungsten plexus, care should be taken to weld only the lower portions of the conductor rings since welding directly on exposed portions of the plexus will tend to disrupt the crystal structure of the tungsten wire and make it fairly susceptible to breakage at the point where the welding heat is applied. Through a tight tit between conductor rings and welding the lower portion thereof, the free ends of the helical convolutions can be rigidly held together. Usually, a bridge 17 of intertwisted helical convolutions of refractory metal is disposed in the space between the inner and outer conductor rings and between adjacent plexuses to realize an effective electrical contact and add stability to the structure. When formed of helical convolutions, the bridge preferably is intertwisted into the outer convolutions of the adjacent plexus.

At the other end of the heating element are supporting electrical conducting arms, each of which is disposed upon an individual plexus and is electrically insulated from adjacent plexuses and conducting arms. Preferably, the radius of curvature of a portion of each of the arms is equivalent to the radius of curvature of the lower conductor ring 15 so that arcuate shapes may be easily formed. Since the heating element illustrated is particularly adapted to be used in a three-phase electrical circuit, we prefer to use three outwardly and radially extending conducting

arms

19, 21 and 23 disposed upon the plexuses and adapted to be tted into water cooled holders (not shown) which are connected to a power supply. These holders also serve to retain conducting

arms

19, 21 and 23 and support the downwardly depend ing heating element in the furnace. The

arms

19, 21 and 23 comprise outer

conductive segments

7, 9 and 11 and inner

conductive segments

39, 40 and 42 formed of shaped refractory metal stock and are preferably tied together by closely wrapped refractory metal wire disposed around the outside. The windings 25, 27 and 29 tie the outward extensions of the outer segments together and

windings

31, 33 and 35 tie the inner segments together as will be explained further in the description of FIGURE 2. It is apparent however, that other means may be used to retain the inner and outer segments together such as clips or possibly welding, however the windings shown are preferable since they afford radiant heat dissipation at the outward extremities and help to prevent overheating of water cooled holders (not shown) which support the heating element in the furnace.

As we have stated, the heating element according to our invention is particularly designed for resistance or electronic heating in a three-phase type of vacuum furnace. However, when other than three-phase heating is desired, the shape of the heating element and the number of plexuses can be appropriately changed such as for example, by reducing them to two. Many complex shapes can be fabricated by properly intertwisting the individual convolutions of refractory metals. For example, shapes such as rectangular, corrugated or square can easily be made and it is even possible in cases where the heater element is not to be subjected to extremely high temperatures to eliminate the lower conducting rings and form the plexuses into a conical shape with the apex serving as the conducting ring. In such cases, the plexuses should be insulated from each other throughout their length except at the apex, as is done with the cylindrical cage shown. Another modification is to fabricate the plexuses in a spheroidal shape by making loose windings and then appropriately bending the convolutions in a jig.

Although it is preferable in most cases to dispose the helical convolutions so that they vertically extend from the conducting

arms

19, 21 and 23 to the conducting rings 15, some requirements of furnace design may dictate the use of horizontally extending convolutions, in which cases the plexuses will have to be shaped in a jig to conform to the arcuate or other shapes desired. Of course, the vertical positioning is usually better since the plexuses can be more readily shaped and each convolution will be freer to turn in the adjacent convolutions. When rigidity of the distal ends of the plexus is a desirable feature, the ends may be turned back so as to form cylinders with the last turn of the plexus intertwisted into the body of the plexus.

We prefer to fabricate each plexus of a multiphase furnace in a way so that each has substantially identical electric characteristics. Prior art heating elements for multiphase type furnaces were difficult to fabricate with grids of identical electrical characteristics because each grid has to be individually rolled or machined. Slight variations in thickness from grid to grid for example, could lead to different electrical characteristics in each, since the resistivity of the grid depends upon among other things, cross section and length. With convolutions of wire however, this problem is materially reduced because wire may be drawn to very close tolerances by using techniques now common in the art. And then each of the plexuses for a given multiphase element can be formed by intertwisting the same number of helical wire convolutions for each. Calculation of the precise number of convolutions, their length and cross section can be performed routinely.

The individual wire helixes may be formed of any 0f the usual refractory metals such as molybdenum, columbium, tantalum, rhenium or preferably tungsten. Additionally, alloys of such materials having requisite melting points also have applicability in some cases. The wire diameter ordinarily should be about 0.010 to 0.125 inch since such wire sizes offer optimum characteristics in a furnace. Below about 0.010 inch, a heating element fabricated of these metals will volatilize too readily when heated due to the enlarged surface area and hence, life of the element will be drastically reduced. Above 0.125 inch the wire will be difficult to work and coil. The thickness of a plexus will vary depending upon the internal diameter of the helix together with the diameter of the wire. It is desirable for most applications of the plexus to form the helical convolutions on mandrels having diameters of about 0.025 to 0.500 inch. And while the upper limit may be increased to suit individual furnace design requirements, it is generally not feasible to go below the lower limits stated because the wire which will have to be used will be too tine to make an efficient heater having a reasonable life. Pitch of the individual convolutions can Vary from slightly above 200% (that is the spacing between the turns equaling slightly more diameter of the wire) to a l000% or even greater. lt is apparent however that at the upper limit the wires must have suflicient pitch to allow for intertwisting of several of the convolutions together. Preferably for most applications, we use a pitch of about 300% so that a tight foraminous plexus may be formed.

Referring now to FIGURE 2, the plan view of the upper end of our electrical heating element is shown. The welding flux has been partially cut away to reveal the laminations which support the plexus and conduct current from a power supply.

Particularly noting plexus 5 and the associated conducting arm 23, support band 37 is disposed on the inside of the cage at the uppermost extremities of the plexus. Each band of the element is preferably made of the same material as the plexus to which it is attached thereby reducing the possibility of breakage of the element due to the use of materials having dilferent coeicients of expansion. In the heating element depicted, each band is generally arcuately shaped and has a thickness of about 0.25 to 0.7 5 inch. However the dimensions can be varied to suit the individual needs of the Kfurnace in which the element will be installed together with the size and weight of the plexuses.

Disposed immediately upon the `support band 37 is the plexus 5. SinceV the individual coils are vertically aligned, the plexus 5 can be shaped easily to be compatible with the support band 37.

One the outside of each plexus we pre-fer to place a pair of inner segments 39 which are substantial mirror images of each other. Each of the inner segments 39 are of a size sutiicient to extend along about one half of the perimeter of the plexus and thence radially outward from the middle to abut against each other. We prefer to have the inner segments 39 extend outwardly at abou-t right .angles to the plexuses so that a minimum angle is made for each `segment thus minimizing localized overheating and possible ruptu-ring of the segments.

In order to facilitate a further reduction in heat transmission from the heating element to the water cooled holders (not shown) which support the depending heating element in the furnace, we prefer to use a tungsten wire wrapping 33 on the inner segments near the beginning of the outward extension. This wrapping 33 is quite advantageous because the necessity for welding elements together immediately near the bend can be eliminated. Because welding often changes the crystal structure of a refractory metal, such elimination tends to prevent the formation o-f crystals in the refractory metal which cannot withstand extremes in heat too well. The length of the outward extension of the inner `segment is a matter of choice and design depending upon the size of the furnace. Similarly as with the Vwrapping of wire around the inner segment 39, we prefer to form a wire wrapping 27 around the outer segment 11 to insure that it stays together and to increase heat dissipation. l

Generally we abut the outer surfaces of the inner segments 39 against the inner surfaces of the outer segments which also are mirror images of each other. Each of the outer ysegments extend along about one quarter of the perimeter of the plexuses measured from the distal edge. The outer segments 11 then branch oif from the inner segment 39 .and extend outwardly to abut against the outer surfaces of the outward extensions of the inner segments 39. In this manner, current is effectively ycarried to each and every portion of the plexus 5 since it passes directly from the power supply through the outer segments 11 to the outer portions of the plexus 5 and likewise, a generally equivalent amount of current will pass from therpower supply directly through the inner segment 39 to the inner half of the plexus.

A space suiciently wide to provide electrical insulation of the plexuses from each other is left between each plexus of the cage. The outer conducting ring 15 together with the inner ring 16 joins each of the plexuses together at their lower ends and the bridge 17 increases an electrical connection. As we have previously indicated, it is preferable that the `bridge 17 be yformed of intertwisted helical convolutions similar to those used to form the plexuses. And when using helical convolutions, the first and last of the bridge 17 can be easily intertwisted into the convolutions of the adjoining plexuses. Of course, in some cases it may be desirable to eliminate the bridge 17 or to substitute a solid sheet or possibly crimp the inner conductive ring 16 against the outer conductive ring 15.

Each of the

plexuses

1, 3 and 5 can be supported and connected to the source of power in a manner similar to that described with reference to plexus 5.

Usually, various elements ywhich make up the conductive supporting assembly for the heating element can be connected together by heli-arc welding along the top. We have found it best to divide each plexus into an imaginary four quadrants lfor welding purposes. At the outermost quadrants, both the inner and outer conducting segments are Welded to the upper edge of the plexus and the support band by welding over the top of the entire width of the assembly. At the inner quadrants, the inner segments alone are welded to the plexus and the support band. Care is particularly exercised to eliminate welding at the right angle bend o-f the inner conducting segment where the segment extends outwardly from the plexus. In this manner, excessive heat `from heli-arc welding on the inner segment is prevented together with elimination of the possibility of modication of the crystal structure which can make the plexus less resistive to shock and vibration.

In FIGURE 3, an enlarged fractional view of a typical assembly of helical convolutions of refractory metal which can make up a plexus is shown. For purposes of clarity of presentation, each of the four helical convolutions is shown with different shading although each is substantially the same in size, shape and material. Preferably, configurations for our invention are a series of helical convolutions having a wire diameter of about 0.035 inch and an internal :bore diameter of about 0.040 inch wrapped upon a mandrel at about 2.16 turns per inch. The pitch will thus be slightly greater than 300%. Now the parameters of the wire sizes can b e varied over a fairly wide range and still form workable plexuses. For example, wire of a diameter from 0.010 to 0.125 inch can be elfectively wrapped on a mandrel of .025 to 0.50 inch and even larger while the pitch can be varied from slightly greater than touching to 1000% and even greater. Using tight coils of slightly greater than 200% will have the advantage however of forming a fairly dense plexus while at 1000% the density can be reduced but ilexibili-ty increased. Although we show a configuration having two convolutions intertwisted in each other convolution, except Ifor the iirst and last of the series, many other variations can be used. For example, the single wire shown can -be doubled and possibly tripled and the multistranded wire wrapped into convolutions of the desired pitch on a mandrel. Other modifications include the insertion of straight or stranded wire into the turn abutments of .the convolutions to alford additional strength. And yet, another method of intertwisting the wire involves placing the convolutions of one strand into the interdental spaces between the convolutions of another strand and then joining the convolutions together by threading a straight wire through the two convolutions, which process can be repeated until a plexus of the desired size is attained. It is thus quite apparent that many variations can Ibe made in the method of intertwisting the convolutions and many plexuses of varying pitches and shapes can be readily fabricated.

The heating element herein described has been specilically designed to provide Ia high power output at a low voltage which is quite advantageous in high vacuum applications. Since high power output is more economically utilized by a three phase system, the heating element has been arranged ina cylindrical form so as to provide three equal resistances with one end of the cylindrically formed plexuses connected together by a conductor ring and with the other end thereof provided with outwardly extending conductive arms which are formed of conductive segments. As is manifest, there is one terminal for each phase, and hence, when an A.C. three-phase electrical current is fed to the arms, each member -of the heating element will heat to approximately the same temperature, thereby insuring a substantially uniform heating throughout. The uniformity of heating is achieved due to the -fact that the electrical resistance throughout every portion of the plexuses is substantially uniform. Since the plexuses, the arms and the conductive rings are constructed of relatively heavy material, they afford a low electrical resistance to the electric current passing therethrough, thereby enabling the described heat to be generated at relatively low voltages. In addition the rugged flexibility afforded, the use of forarninous plexuses enables the element to be selfsupporting and thus, no refractory or other electrical insulation need Ibe used for its support in the furnace.

Referring now to FIGURE 4, the heating element in this 'Z modification differs from that presented in FIGURE 1 by the use of a group of forarninous plexuses for each phase of the heating element. The construction is less dense than the previously described conguration however cost can be reduced. Additionally, the weight is minimized and since we use more loosely woven strands, the flexibility can be increased.

The group of

plexuses

2, 18, 20 and 26 from one phase for the heating element and each plexus in the entire group and the entire element can be fabricated and assembled identically. Plexus Z is supported at its lower end by a U-shaped bracket 6, which can be made of 0.025 inch tungsten sheet (when tungsten plexuses are used) 0.50 inch wide and 0.875 inch long. The ends of U-shaped bracket 6 are wel-ded to conductive ring 4 and pass around the sides and the front of plexus 2 to insure its retention in the heating element. It is generally quite important to use a single U-shaped bracket for each plexus instead of a continuous band of tungsten sheet since the former arrangement allows a more even distribution of current throughout the element.

When tungsten plexuses are used, they may conveniently be fabricated by continuously winding 0.035 inch tungsten wire upon a steel mandrel 0.040 inch in diameter at a pitch of 2.16 turns per inch. ln general, a number of feet of the coiling is made, for example 21/2 feet, which is then cut into a length desired for a plexus such as seven inches. The mandrel is then removed by techniques conventional in the art such as acid-leaching or other convenient processes and the individual helical convolutions are then ready -for intertwisting. For the plexus shown in FIGURE 4 we prefer to use seven helical convolutions which can be easily intertwisted in each other in a manner so that two helical convolutions are disposed in each other helical convolutions, except in the case of the rst and last convolution of the series. Of course other configurations and greater than two helical convolutions can be used when desired, as was described previous- 1y.

At the other end of the plexus 2, a U-shaped bracket 8, which can be identical to U-shaped bracket 6, is used for the upper support and to attach the plexus 2 to the conductive band 10. Welding each clip connecting each plexus separately to conductive band herein again helps to prevent an uneven distribution of current throughout the various plexuses in the element. When using a three phase element such as shown, it is generally preferable to use arcuate shaped conductive bands and ringlike conductors for joining the bottoms of' the plexuses together. Yet variations from these preferred embodiments will readily suggest themselves depending upon the Shape of the furnace in which the element is to be used and the heat to which it is to be subjected.

Extending radially from the middle of the conductive band 16 is a conductive arm 12 which is adapted to be supported in water-cooled holders (not shown) that are connected to a power supply. The structure and shape of the arm 12 will be described with greater specicity in the description of FIGURE 5.

The spacing of the plexuses around a given band 10 should be regular so that current is evenly distributed and thus to prevent a localized temperature increase on any lone plexus when the current passes through. Unless such even spacing is generally followed, one plexus may have a shorter life then another in a group because of unequal heating `and metal vaporization.

Referring now to FIGURE 5, the regular distribution of the plexuses is shown around the conductive band i0. Each plexus is joined into a group by U-shaped brackets which are welded at their ends to

arcuate portions

14 and 16 of conductive arm 12. Conductive arm 12 is formed of two

mirror image segments

14 and 16 which are arcuately shaped at one end to coincide with the radius of curvature of the conductive band 10 and then extend radially outward from the middle of the band 10 to abut against each other. The lower conductive ring 4 can conveniently be made of two conductive rings, the inner conductive ring 32 being inside of outer conductive ring 34. The use of two conductive rings, so disposed, increases the stability of the heating element.

In joining the plexuses together, we again prefer to heliarc weld over the entire upper and lower surfaces of the heating elements and when high temperatures are not to be reached, a weld may be made at the right angle bend of the inner and outer conducting segments.

lt is apparent that although we prefer to use conductive arms such as shown in FIGURES 4 and 5 for heating elements having phases formed of groups of plexuses, the arms described in FIGURES l to 3 also have applicability in many cases. And it will be seen from the foregoing that we have provided in accordance with the instant invention, an element for use in a vacuum furnace which has highly novel features, suggested flexibility and greatly improved element design.

The instant arrangement enables a high degree of heat to be obtained at low electrical potential and further results in a highly uniform heating arrangement. In addition, the heating element may be more efficiently baiiied which is a highly important feature. Also since the heating element is self-supporting, no refractory or other insulation need be employed for its support in the vacuum chamber. While there is shown and described herein certain specific structure embodying the invention, it will be manifest to those skilled in the art that various modications and rearrangements of parts may be made without departing from the spirit and scope of the underlying inventive concept and that the same is not limited to the particular forms herein shown and described, except insofar as indicated by the scope of the appended claims.

As our invention we claim:

1. An electrical heating element comprising: an elongated, foraminous plexus of an intertwisted series of helical convolutions of refractory metal wires, the turns of one of said wires being held by the turns of an adjacent wire; means for connecting said plexus to a power supply and supporting said intertwisted helical convolutions, said means transversely extending across both the upper and lower ends of said plexus.

2. The heating element according to claim 1 wherein t-he refractory metal is tungsten.

3. The heating element according to claim 1 wherein the refractory metal wire has a diameter of about 0.010 to 0.125 inch.

4. An electrical heating element comprising: an elongated, foraminous plexus of an intertwisted series of helical convolutions of refractory metal wires, wherein said helical convolutions are intertwisted into other helical convolutions, the turns of one of said wires being held by the turns of an adjacent wire; means for connecting said plexus to a power supply and supporting said intertwisted helical convolutions, said means transversely extending across the upper and lower ends of said plexus.

5. The heating element according to claim 4 wherein the refractory metal is tungsten.

6. The heating element according to claim 4 wherein each of said convolutions except the first and last of a series are intertwisted into two other convolutions.

7. The heating element according to claim 4 wherein the refractory metal wire has a diameter of about 0.010 to 0.125 inch.

8. An electrical heating element comprising: at least two spaced, elongated foraminous plexuses formed of a series of intertwisted, longitudinally extending, helical convolutions of refractory metal wires, the turns of one of -said wires being held by the turns of an adjacent wire; a conductor element disposed at one end of said plexuses, supporting said plexuses and joining them together, said plexuses thereby being insulated from each other throughout their length, except at their lower interconnected ends, at least two means for individually connecting each of said plexuses to a power supply and for supporting the upper ends, said means extending transversely across the other end of each of said plexuses.

9. An electrical heater element comprising: at least three spaced, elongated, foraminous plexus formed of a series of intertwi-sted, longitudinally extending, helical convolutions of refractory metal wires, each of said helical convolutions, except the first and last in the series, being intertwisted with two other helical convolutions, the turns of one of -said wires ,being held by the turns of an adjacent wire; a conductor element disposed at the lower ends of said plexuses, rigidly aixing the lower ends of the Ihelical convolutions together and joining said plexuses, said plexuses thereby being insulated from each other throughout their length, except at their lower interconnected ends; at least three means for individually connecting each of -said plexuses to a power supply and for rigidly aflxing together the upper ends of said helical convolutions, said means extending transversely across the other end of each of said plexuses.

10. The heating element according to claim 9 wherein the refractory metal is tungsten.

11. The heating element according to claim 9 wherein the refractory metal wire has a diameter of about 0.010 to 0.125 inch.

12. An electrical heater element comprising: at least three arcuate, spaced, elongated, foraminous plexuses formed of a series of intertwisted, longitudinally extending, helical convolutions of refractory metal wires, each of said helical convolutions, except the first and last in a series, being intertwisted with two other helical convolutions, the turns of one of said wires being held by the turns of an adjacent wire; a generally circular conductor element disposed at the lower end of said plexuses, rigidly aflixing the ends of the helical convolutions of each plexus and also joining the plexuses together, said plexuses thereby being formed into a generally cylindrical cage and being insulated from each other throughout their length. except at their lower interconnected ends; at least three means for individually connecting each of `said plexuses to a power supply and for rigidly afiixing together the upper ends of said helical convolutions, said means extending transversely across the upper end of each of said plexuses.

13. An electrical heater element comprising: at least three groups of foraminous plexuses, each group being formed of at least one elongated 4series of intertwisted helical convolutions of refractory metal wires, each of said helical convolutions, except the first and last in a series, being intertwisted with two other helical convolutions, the turns of one of said wires being held by the turns of an adjacent wire; a generally circular conductor disposed at the lower end of said groups, rigidly affixing the lower ends of the helical convolutions together and also joining each of the series of convolutions in a group together, said groups thereby being formed into a generally cylindrical cage and being insulated from each other throughout their length except at their lower ends; at least three means for individually connecting leach of said groups of plexuses to a power supply and for rigidly aflixing together the upper ends of each of the series of helical convolutions in a group.

14. An electrical element comprising: an elongated foraminous plexus formed of an intertwisted series of longitudinally extending, helical convolutions of refractory metal wires; means for connecting one end of said plexus to a power supply and rigidly aflixing said convolutions together, said means comprising a pair of inner segments extending transversely along the periphery of said plexus and thence outwardly at the middle at substantially right angles to said plexus to abut against each other and a pair of outer segments extending transversely along the periphery of the outer portions `of said plexus and thence outwardly from the middle to abut against the outer surfaces of said inner segments; means for securing the outward extensions of said inner and outer segments together; and means for joining said inner and outer segments to a power supply.

15. The heating element according to claim 14 wherein the refractory metal is tungsten.

16. The heating element according to claim 14 wherein the refractory metal wire has a diameter of about 0.010 to 0.125 inch.

17. An electrical heater element comprising: at least two arcuate, spaced, longitudinally elongated foraminous plexuses formed of a series of intertwisted, longitudinally extending, helical convolutions of refractory metal wires, each of said helical convolutions, except the rst and last in a series, being intertwisted with two other helical convolutions; a generally circular conductor element disposed at one end of said plexuses, rigidly aiiixing the lower ends of the helical convolutions of each plexus together, said plexuses thereby being formed into a generally cylindrical cage and being electrically insulated from each other throughout their length, except at their lower interconnected ends; at least two means individually connecting each of said plexuses to a power supply and for rigidly aliixing together the upper ends of said helical convolutions, each of said means comprising a pair of inner segments extending transversely along the periphery of each of said plexuses and thence outwardly at the middle at substantially right angles to each of said plexuses to abut against each other; a pair of outer segments extending transversely along the periphery of the distal ends of each of said plexuses and thence outwardly to abut against the outer surfaces of said inner segments; means for securing the outward extensions of said inner and outer segments together; and means for securing said inner and outer segments to -said plexuses.

1S. The heating element according to claim 17 wherein the refractory metal is tungsten.

19. The heating element according to claim 17 wherein the refractory metal wire has a diameter 0f about 0.010 to 0.125 inch.

20. A supporting arm for an electrical heating element comprising: a pair of inner segments extending transversely along the periphery of said heating element and thence outwardly at the middle thereof at substantially right angles to said heating element to abut against each other; a pair of outer segments extending transversely along the periphery of the distal ends of said heating and thence outwardly to abut against the outer surfaces of said inner segments; and means for securing the outward extensions of said inner and outer segments together.

21. An electrical heating element comprising: an elongated, foraminous plexus of an intertwisted series of helical convolutions of refractory metal Wires, the turns of one of said convolutions being held by the turns of an adjacent convolution; means for connecting the ends of said plexus to a powersupply.

22. The element according to claim 21 wherein there are at least three convolutions of refractory metal wires in the series.

23. An electrical heating element comprising: an elongated, foraminous plexus of an intertwisted series of helical convolutions of refractory metal wires, the turns of one of said wires being held by the turns of an adjacent wire; means for connecting said wires to a power source.

24. The element according to claim 23 wherein there are at least three convolutions of refractory metal wires in the series.

25. The element according to claim 24 wherein the refractory metal is tungsten.

26. The heating element according to claim 24 wherein the refractory metal wire has a diameter of about 0.010 to 0.125 inch.

27. An electrical heating element comprising: an elongated foraminous plexus of an intertwisted series of helical convolutions of refractory metal wires, the turns of one of said wires being held Iby the turns of an adjacent 1 1l wire; means for connecting said plexus to a power supply, said means comprising a pair of conductor elements, each of which is disposed at the distal ends of the wires forming said plexus.

28. The element according to claim 27 wherein the refractory metal is tungsten.

29. The element according to claim 27 wherein the refractory metal wire Ihas a diameter of about 0.010 to 0.125 inch.

30. The heating element according to claim 21 wherein each of said convolutions except the first and last of a series are intertwisted into two other convolutions.

31. The heating element according to claim 21 wherein each of said convolutions except the first and last of a series are intertwisted into two other convolutions.

References Cited by the Examiner UNITED STATES PATENTS Dreusike 5-189 X Howe 174-129 X Spong 174-129 X Colvin 57-144 Melson 338-295 X Heath 338-210 X Weitzel 219-211 Hill 219-553 X Richardson 245--5 RICHARD N. WGOD, Primary Examiner.

15 ANTHONY BARTIS, Examiner.

Claims

21. AN ELECTRICAL HEATING ELEMENT COMPRISING: AN ELONGATED, FORAMINOUS PLEXUS OF AN INTERTWISTED SERIES OF HELICAL CONVOLUTIONS OF REFRACTORY METAL WIRES, THE TURNS OF ONE OF SAID CONVOLUTIONS BEING HELD BY THE TURNS OF AND ADJACENT CONVOLUTION; MEANS FOR CONNECTING THE ENDS OF SAID PLEXUS TO A POWER SUPPLY.