US3427421A - Electrical heating elements - Google Patents

Description

w. a. MATHESON E AL 3,427,421

ELECTRICAL HEATING ELEMENTS Sheet FIG.I

INVENTORS N 0 S E H T A M 6 w R F W JAMES F. CLUNE THEODORE J. PRICENSKI FIG.3

RNEY

Feb. 11, 1969 Original Filed May 7 1965 ws. MATHESON ETAL 3,427,421

ELECTRICAL HEATING ELEMENTS Original Filed May '7, 1963 Feb. 11, 1969 Sheet FIG.4

FIG.5

.IS N Mm w m E Cm MWPm MLJ: D mam w F 0 wk WJT m ATTORNEY United States Patent 3,427,421 ELECTRICAL HEATING ELEMENTS Wilfrid G. Matheson, Marblehead, James P. Clune, Danvers, and Theodore J. Pricenski, Ipswich, Mass., assignors to Sylvania Electric Products Inc., a corporation of Delaware Original application May 7, 1963, Ser. No. 278,660, now Patent No. 3,274,374, dated Sept. 20, 1966. Divided and this application Feb. 25, 1966, Ser. No. 547,108 US. Cl. 219-1049 6 Claims Int. Cl. Hb 5/02 ABSTRACT OF THE DISCLOSURE An induction heating furnace including a water cooled helical coil disposed about a generally-helical plexus in the shape of a susceptor which is formed of a multiplicity of intertwisted helical convolutions of refractory metal WII'CS.

This case is a division of Ser. No. 278,660 filed May 7, 1963 now Patent No. 3,274,374.

This invention relates to refractory metal heating elements for electrical furnaces and more particularly to heating elements which are suitable for use as anodes or susceptors. Such elements are particularly useful in high temperature electric discharge furnaces or induction heating furnaces respectively.

Anodes or susceptors are well known to the art as heating elements and certain of them have been previously fabricated of refractory metals. In the past however, elements using refractory metals have been fabricated from sheet materials or machined from heavy stock and when so fabricated they do not possess crystal structures having optimum characteristics to resist breakage or maintain their geometric form. But we have discovered that refractory metal wire does possess these characteristics and when properly fabricated, the elements may be used successfully as anodes for high temperature discharge furnaces or susceptors for inducting heating furnaces utilizing a vacuum and/ or inert or reducing atmospheres. This discovery has led to the achievement of long-lived, rugged heating elements which can withstand temperatures as high as 2000 C. and often as high as 2500 to 3000 C. When using refractory metal wire, the elements can have substantially identical electrical characteristics as those known in the prior art, but yet be structurally far superior to their solid sheet counterparts.

High temperature electric discharge furnaces operate by forming a thermionic electron emission from a cathode which surrounds the anode of this invention. When adequate temperature is reached, the ions instead of being supplied from the heated surface can be supplied from a controlled flow of gas into the discharge region between the anode the cathode or from direct emission in a vacuum. In gas containing furnaces, the is ionized by collision with the electrons and by controlling the number of ions so released the temperature may be regulated. The positive ions liberated tend to neutralize the space charge between the cathode and the bombardment surface. They also act as current carriers, as do the released electrons, increasing the current, but the carriers added relatively few; by far the greater proportion of the current is carried by initial thermionic discharge. The latter case is substantially exclusively true in high vacuum furnaces. The discharge is diffuse and tends to spread over the entire surface of the bombardment anode, and since it is diffuse there is no concentration of the current on a single spot to form an arc crater, whereby the anode would be vaporized to form a true arc. Heating of the cathode is preferably initiated by passing current through it directly, although it may be indirectly heated when desired. Heating of the cathode also occurs due to positive ion bombardment and by radiation from the anode and hence after the discharge has been maintained for a long enough period for equilibrium temperatures to establish themselves, the cathode temperature is maintained partly by heat from these sources and the power supplied to it directly can be reduced.

Induction heating has been extensively applied in industry for melting, heat treating, brazing and soldering, hot forming and other processing operations. In such applications, the susceptor which is electrically conductive, is generally heated directly by induced current when it is placed in an electromagnetic field established by a high frequency current flowing through a surrounding induction heating coil. The article of work is placed within the susceptor and heated by radiation and in recent years the use of such susceptors has significantly extended the application of induction heating. In its simplest form, a susceptor can be a metal tube having magnetic characteristics interposed between an electromagnetic coil and the article of work to be heated. The susceptor is then heated by the magnetic field established by the induction coil but the former essentially shields the work within it so that the element being treated can be heated primarily by radiation and or conduction from the heated susceptor. Not only can electrically conductive elements be heated thusly, but also many nonconductive materials such as ceramics and plastics.

The susceptors or anodes according to our invention can take a number of different forms and shapes. In the preferred embodiment, a number of helically convoluted wires (helical coils) each having generally substantially similar diameters are intertwisted together in such a way that generally two convolutions are intertwisted in each other convolution. In this manner we continue the intertwisting until a foraminous plexus is formed. Preferably, the plexus is then bent about the axes of the helical coils into a cylinder and welded at its abutting ends to form a seam. The diameter of the cylinder can vary without limit, depending only upon the size of the furnace in which it is to be used. Large diameter wires can be used for larger cylinders and smaller diameter wires for smaller cylinders if desired.

The use of a series of intertwisted helical convolutions allows greater latitude in the application of the anode or susceptor. Even after heating and thus when each of the individual convolutions is quite brittle, they are each movable in their adjacent convolutions and hence can withstand stresses which would ordinarily fracture solid sheets of similar refractory metals. Coupled with the inherent flexibility of plexuses is the increased strength resulting from drawn wire over solid sheets due to the incorporation of long fiber-like crystals as a result of processing. Thus even though the individual convolutions of wire may become quite brittle after the element has been used once, the plexus itself is still fairly flexible and can be moved in the furnace without an inordinate danger of breaking. Furthermore, 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. When desired for greater stability, and when flexibility can be dispensed with as a criteria, our plexuses may serve as a base for a thicker coating of flame sprayed refractory metals which may be prepared according to conventional techniques. The use of flame sprayed coatings is not generally practical with solid sheet anodes or susceptors because flame sprayed material will not permanently adhere to such bases.

Accordingly, the primary object of this invention is the fabrication of heating elements having increased mechanical strength and the ability to withstand fairly large thermal shocks without breakage.

Another object of this invention is the substantial elimination of distortion in the geometrical shape of anodes and susceptors after their 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 susceptor or anode from a plexus of refractory metals, the plexuses comprising a series of intertwisted helical convolutions of the metal.

An advantage of this invention is that the anode or susceptor formed of a foraminous plexuses can have a longer life and be stronger after heating then similar elements fabricated of sheet metal or machined from heavy stock.

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 susceptors or anodes of a refractory metal, foraminous plexus 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 the preferred embodiment of the anode or susceptor.

FIGURE 2 is an enlarged fragmentary view of the helical intertwisted wires which form one embodiment of the anode or susceptor shown in FIGURE 1. These wires are joined together at their ends by a weld which is schematically illustrated.

FIGURE 3 is an enlarged fragmentary view of the helical intertwisted wires which form another embodiment of the anode or susceptor shown in FIGURE 1.

FIGURE 4 is a cross-sectional view of our susceptor disposed within an electromagnetic-type furnace.

FIGURE 5 is a cross-sectional view of our anode disposed within a high temperature electric discharge furnace.

FIGURE 6 is a cross-sectional view of an embodiment of the invention in which the plexus is embedded in a refractory metal coating.

In the preferred embodiment shown in FIG. 1, the anode or susceptor element according to our invention can easily be fabricated by forming a plexus 1 of a series of intertwisted, individual wire helixes. The helixes may be formed of any of the usual refractory metals such as molybdenum, columbium, tantalum, rhenium or preferably tungsten. Additionally, alloys of such metals having requisite melting points also have applicability in some cases. The wire diameter ordinarily should be about 0.010 to 0.125" since such wire sizes offer optimum characteristics in a furnace. Below about 0.10", the element fabricated of these metals will volatilize too readily when heated due to the large surface area and hence, the life of the element will be drastically reduced. Above about 0.125" the wire will be difficult to work and coil. The thickness of the 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 element to form the helical convolutions on mandrels having diameters of about 0.025 to 0.500". 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 fine to make an efiicient element having reasonable life. Pitch of the individual convolutions can vary from about 300% (that is, the spacing between the turns equaling slightly more than two times the diameter of the wire) to about 1000% or even greater. It is apparent however that at the upper limit, the wires must have sufiicient 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 plexus is formed. Now we have stated that fine Wire should not be used because it would reduce the life of the element, however it must be pointed out that when the plexus is flame sprayed with the suitable material, even finer wires may be used since internal strength in such cases is not such an important prerequisite.

Intertwisting of the wire helixes may be performed easily by disposing one wire in a jig and then rotating a second wire helix into the turns of the first. A third wire helix is then intertwisted in the turns of the second helix and the operation is repeated again and again until a plexus of the desired width and length is attained. Although we show a configuration having two convolutions intertwisted in each other convolution, many other variations can be used also. For example, the single wire shown can be doubled and possibly tripled and the multistranded wire wrapped into helical convolutions of the desired pitch on a mandrel. Other modifications include insertion of stranded or straight wire into the turn abutments of the convolutions to afford additional strength. And yet, another method of intertwisting wire involves placing the convolutions of one coil into the interdigital spaces between the convolutions of another coil and then joining the convolutions together by threading a straight wire through the two convolutions, which process can be repeated until a plexus of a desired size is obtained.

Because in the prepared embodiment, the plexus will have to be shaped after it is formed, it is generally desirable to use coils which have not been stress-relieved. Such coils have not yet been treated to set the crystal structure and hence may be bent and shaped as desired whereas stress-relieved coils are brittle. After the final configurations of the element are made, the entire unit may then be heated to set the crystal structure.

When a plexus of requisite size is fabricated, it is then ready to be formed into the desired shape. Preferably, the coils of the plexus are bent into a generally cylindrical shape on a plane substantially perpendicular to their axes. When the desired shape is obtained, and if a regular, continuous shape is desired, a weld is formed by conventional heliarc methods at the edges of the plexus and on the distal ends of the individual wire helixes.

In the applications of the anodes or susceptors, a generally cylindrical shape is most desirable. Such shapes are preferably cylindrical although rectangular, polygonal or irregular shapes may be fabricated easily. The latter rectangular, polygonal or irregular shapes have particular application where the article which is to be heated has similar rectangular, polygonal or irregular shapes. As an example, the advantage of heating a rectangular article of work with a rectangular element is that uniform spacing between the article and the element is obtained. While a cylindrical shape might be practical for heating most articles or work which are small in comparison to the diameter of the element or for heating large cylindrical objects, the relationship is not always practical when heating large pieces of work of shapes different than the element. In these cases the other noncylindrical shapes should be used.

For some particular applications it may be desirable to heat one section of work to a higher temperature than the other adjacent sections. Such temperature differentials may be easily attained 'by shaping the anode or susceptor so that a section thereof would be closer to the article or work than the remainder of the element. Due to greater proximity to the work, greater heating will be attained at the point which is nearest to the element than at adjacent points which are more distant.

Referring now to the detailed view of FIGURE 2, it will be seen that the coils, 3, 4, 5, are intertwisted in such a manner that the upper coil 3 is intertwisted in the intermediate coil 4 which in turn is intertwisted in the lower coil 5. In the embodiment shown, the turns of the coils are spaced fairly closely together (300% pitch) in order to form a dense foraminous plexus.

In order to join the ends of the cylinder together and form a seam, we have previously indicated that a heliarc weld also may be used, however other means of securing the ends together such as mechanical clamping may also have applicability so long as such means can withstand the temperatures to which the element will be raised without appreciable warping, distortion or alteration of the electrical characteristics.

While a seam is the preferred method of forming the plexus into a generally cylindrical shape, we may also obtain this shape by intertwisting a helical convolution of refractory metal Wire similar to the adjacent helical convolutions 8 and 9 into the first and last coils 6 and 7 of the plexus as shown in FIGURE 3.

Sometimes it is desirable to use a basket-type element particularly where there are many small articles of work to be heated together. These elements may be easily formed by placing a bottom on the cylindrical element either 'by heliarc welding a flat, round plexus to the end thereof or using other suitable means of attachment. Of course, even sheet tungsten may be used, however sheets on the bottom suffer from the difficulties previously indicated. They will tend to warp after heating and become brittle.

Frequently when ruggedness is a critical factor, such as would be the case if the element is to be raised to elevated temperatures quite frequently and possibly for short durations of time, it may be desirable to heliarc weld a reinforcing rim (not shown) around the edges. The rim may be simply a tungsten flux if tungsten is used as the element material, or possibly, it may be fabricated of a ring of suitable refractory metal.

As we have indicated previously, the plexus 52 forming the anode or susceptor can be metalized by spraying a molten metal onto the surface to form a coating 51 as shown in FIG. 6. When using a tungsten element, it is generally preferred to coat tungsten thereupon. Other refractory metal plexuses can use similar refractory metal coatings. The process involves melting the metal in a flame and atomizing it by a blast of compressed air into a fine spray. This spray builds up onto the surface to form a solid metal coating. Because the molten metal is accompanied by a large blast of air, the object being sprayed is not heated sharply. When the element of our invention is metalized, the flexibility is lost, however strength is markedly increased. Even when a furnace temperature of 3000 C. is used, there is such a quantity of tungsten present that the life will be very long. Furthermore just as reinforced concrete has a greater strength than concrete without reinforcement rods, so too is a metalized heating element formed upon a foraminous plexus.

Referring now to FIGURE 4 of the drawing, a typical induction furnace is shown wherein the susceptor of our invention is illustrated. The principle equipment of an induction heater can be a coil or wire through which an alternating current flows. The alternating current induces eddy currents in any metal that is either located within the coil or closely surrounds the coil. The frequency ranges between 60 c.p.s. and 3 magacycles and even higher in certain special applications. Heat is generated in the metallic article or work by induction regardless of the composition of any non-metallic materials which are disposed between the article of work and the coil. The coils are generally wound from flattened copper pipes through which water is circulated.

In the furnace, a flow of inert and/ or reducing gas can be passed through the inlet port 10 over the members within the furnace which are to be heated and removed from an outlet port 12. In cases where a vacuum is to be maintained, the reduction of pressure can be attained by withdrawing gases through the outlet port 12 while the inlet port 10 is closed. The furnace 15 is lined with insulation 14 to reduce heat losses and when desired, a series of heat shields (not shown) can be disposed immediately inside of the insulation .14.

An internally water cooled helical coil 16 is attached to the source of current (not shown) by conventional techniques and the susceptor 18 according to our invention is disposed inside of the helical coils 16. As shown, the susceptor can have a generally cylindrical shape and can be fabricated according to the description indicated previously. Suitable supports (not shown) are provided within the furnace 15 so as to hold the susceptor 18 axially aligned within the helical, water cooled coil 16. An article of work (not shown) can be supported, suspended or disposed within the susceptor 18, as is conventional in the art.

While the preferred embodiment of our invention involves using the foraminous plexus which is bent on a plane perpendicular to the axes of the individual helical convolutions which are used to make it, it is apparent that other modifications may be used equally well. Frequently it is possible to use convolutions which need not be shaped until after the fabrication of the plexus. Such arrangement can easily be made by bending the plexus in a plane substantially parallel to the individual convolutions and then joining the ends together with a similar wire coil which will act as a retainer. In this manner an entirely cylindrical element can be formed without the necessity of welding.

The embodiment of our invention in which the heating element is used as an anode is shown in FIG. 5. In this embodiment, a furnace 20 which includes a layer of insulation 21 is used to surround the entire heating unit so as to contain the heat therein. Supported upon three buss bars, 22 and 23 (the third buss bar is not shown in this cross section) is a cathode which may be formed according to our co-pending application Ser. No. 219,404, filed Aug. 27, 1962 entitled Electrical Heating Element and assigned to the same assignee as the instant application, now United States Patent 3,178,665. As therein described the heating element can comprise three grates 24, 25, and 38. The anode is preferably assembled in a substantially cylindrical form and can be connected together at one end by an outer conductor ring 27 and an inner conductor ring 28. Preferably the attachment of the grates to the conductor rings is made by heliarc welding the rings together on their lowermost extremity.

At the other end of the cathode are supporting, electrically conducting arms 29 and 30 which extend radially upwardly from the heating element. These supporting arms 29 and 30 are adapted to be disposed within electrically conducting buss bars 22 and 23. Since we prefer to make the supporting arms of at least two portions and preferably four as described in the abovementioned application, we have found that it is frequently desirable to tie the portions together with windings 32 and 33. Such windings are preferable since they afford radiant heat dissipation at the outward extremities of the arms and help to prevent overheating of the buss bars. Disposed inside of the grates 24 and 25, at the upper end thereof, are inner support segments 34 and 35 which are joined to the grate 24 and 25 and in turn to the support arms 29 and 30 by a flux- type weld 36 and 37. The grate 38 and the rest of its structure is similar to that of either grates 24 and 25.

Surrounding the cathode are a series of heat shields 40 which may vary in number depending upon the heat to which the furnace is to be raised. These heat shields 40 are generally supported upon a rod or other suitable support device 42 which may be affixed to the refractory liner 43. A water cooling system 44 can be advantageously used to keep the temperature of the furnace within reasonable limits and to prevent localized overheating.

The anode '45 of our invention can be conveniently disposed within the cathode in such a manner that the axis thereof substantially coincides with the axes of the anode. Any articles of work which are to be heated can be supported, suspended or disposed within the anode 45 in a manner which is conventional in the art. The anode itself can be conveniently supported upon a stand 46 which is in turn supported upon refractory struts 47. The anode is placed in the electrical circuit -by passing current to the stand 46 through means well known in the art. Disposed beneath the anode are a series of heat shields 48 which radiate heat upwardly into the furnace.

While there is shown and described herein certain specific structure embodying our invention, it will be manifest to those skilled in the art that various modifications and rearrangements of the parts can be made without departing from the spirit and scope of the underlying concept and that the same is not to be limited to the particular fOlIIIlS of the invention herein shown and described, except insofar as indicated by the scope of the appended claims.

As our invention we claim:

1. An induction heating furnace having a water cooled helical coil disposed therein, said coil being attached to a source of current, the improvement which comprises: a susceptor disposed inside of said coil and spaced therefrom, said susceptor being a generally cylindrical plexus formed of a multiplicity of intertwisted helical convolutions of refractory metal wires.

2. The induction heating furnace according to claim 1 wherein the susceptor is formed in the generally cylindrical shape by joining together the distal ends of the multiplicity of intertwisted helical convolutions.

3. The induction heating furnace according to claim 1 wherein the generally cylindrical shape of the susceptor is formed by intertwisting a helical convolution of refractory metal wire similar to the helical convolutions in the plexus into the first and last convolutions thereof.

4. The induction heating furnace according to claim 1 wherein the refractory metal wires have a diameter of about 0.010 to- 0.125 inch.

5. The induction heating furnace according to claim 1 wherein a flame sprayed coating of a refractory metal is disposed upon the outer surfaces of the plexus.

6. The induction heating furnace according to claim 1 wherein the refractory metal is tungsten.

References Cited UNITED STATES PATENTS 223,262 10/ 1880 Wakeman 245-5 3,036,888 5/1962 Lowe 219-1049 X 3,172,002 3/1965 Johnson et al 313348 3,210,455 10/1965 Sedlatschek 1327 3,274,374 9/1966 Matheson et a1. 219426 3,350,494 10/1967 Kunitsky et a1 13-27 RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

US. Cl. X.R. 1327

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