US3469013A

US3469013A - Segmented mesh type heating element formed with reinforcing mesh structure

Info

Publication number: US3469013A
Application number: US592099A
Authority: US
Inventors: James S Hetherington; James R Skinner
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1966-11-04
Filing date: 1966-11-04
Publication date: 1969-09-23
Anticipated expiration: 1986-09-23

Description

p 3, 1969 J. 5 HETHERINGTON ET AL 3,469,013

SEGMENTED MESH TYPE HEATING ELEMENT FORMED WITH REINFORCING MESH STRUCTURE Filed Nov. 4, 1,966 3 Sheets-Sheet 1 ATTORNEY FIG. 2

INVENTORS JAMES $.HETHERlNGTON BY JAMES R. SKlNNER lllllllll fir/111111 157 4 FIG. I PRIOR ART p 23, 1969 J. s. HETHERINGTON ET AL 3,469,013

SEGMENTED MESH TYPE HEATING ELEMENT FORMED WITH REINFORCING MESH STRUCTURE Filed Nov. 4, 1966 5 Sheets$heet z FIG.3

FIG.6B 24 2e 28 2s 25 FIG. 6C

INVENTORS JAMES S. HETHERINGTON BY JAMES R. SKINNER ATTORNEY Sept. 23, 1969 .1. s HETHERINGTON ETAL 3,469,013

SEGMENTED MESH TYPE HEATING ELEMENT FORMED WITH REINFORCING MESH STRUCTURE 5 Sheets-Sheet 5 Filed NOV. 4, 1966 INVENTORS JAMES S.HETHERINGTON BY JAMES R. SKINNER ATTORNEY United States Patent US. Cl. 13-25 11 Claims ABSTRACT OF THE DISCLOSURE A mesh-type heating element for vacuum, inert gas, and hydrogen furnace use, in the temperature range from 25 0O C. to an excess of 3000 C. is disclosed. The mesh heating element includes a mesh formed of a multiplicity of intertwisted helical convolutions of refractory metal wire. The mesh includes at least a pair of adjacent leg segments conductively joined together at an adjacent end for passing the heating current in series through the heating element in opposite directions in adjacent legs. The conductive connection between adjacent legs includes a mesh gusset structure, preferably having a rounded V- shaped crotch portion to reduce sharp corners and to reduce the heating current density for prolonging the operating life of the heating element. In a preferred embodiment of the present invention, the side edge portions of the leg segments are formed of increased thickness to strengthen the mesh against inwardly directed magnetomotive forces. The gusset structure preferably extends above a ring structure serving to conductively join the two leg segments for strengthening the mesh and for reducing the current density'in the crotch portion. In addition, reinforcing ribs are provided in the leg portions to reduce flexure of the mesh structure in use.

Heretofore, segmented tungstenmesh heating elements have been built for use in'vacuum furnaces operating in the aforecited temperature range. Such a heating element is described in US. Patent 3,178,665, issued Apr. 13,

.1965. In such a prior art heating element, the adjacent corners, thereby locally elevating the operating temperature in the corner portions of the adjacent mesh segments. Moreover, the sharp corner portions of the mesh produce a concentration of the operating stresses at these corner portions. As a result, the mesh heating element oftenfractures and fails at the corners, thereby severely limiting the useful operating life of the heating element.

Also ithas been found that the adjacent edges of the separated mesh segments, which are supported from only one side by the body of the mesh segment and from the ends by support ring segments, tend to distort excessively in the middle region with continued use at elevated operating temperatures. The heating element is supplied with an AC. current on the order of 2000 amps. which flows down one mesh segment and back through the adjacent mesh segment. Thus, a very intense alternating magnetic field is set up in the spaces between adjacent current carrying mesh segments. This alternating magnetic field produces severe magnetic motoring forces on the adjacent "ice other each half cycle of the AC. heating current, typically, 60 cycle current. Therefore, the central edge regions of the mesh segments, which have least support, experience a pulsating deflection which, in time, overstresses the mesh and produces a fracture thereof with catastrophic failure of the heating element.

In the present invention, a rounded V-shaped gusset is provided above the lower support ring to form a portion of the crotch between adjacent mesh segments or legs. The gusset rounds off the corners of the adjacent mesh segments to strengthen the corners and to prevent setting up of undue stresses. In addition, the gusset provides an additional conductive path between the adjacent segments, thereby reducing the current density at the corners of the mesh segments to prevent localized overheating thereof.

In a preferred embodiment of the present invention, the edges of the adjacent mesh segments are folded over to provide increased strength for the edge portions of the mesh segments for resisting the motoring forces on the heating element. In addition, the mesh is provided with strengthening rib members formed by twisting additional helical members into the mesh. Mesh type heating elements of the present invention offer substantially extended operating life.

The principal object of the present invention is the provision of an improved mesh type heating element.

One feature of the present invention is the provision of rounded V-shaped gusset mesh structure at the crotch between adjacent mesh segments or legs of a segmented mesh type heating element, whereby the current density in the heating mesh is reduced at the joined ends and undue stresses prevented for extending the operating life of the element.

Another feature of the present invention is the provision of an increased thickness of mesh at the marginal side edges of the mesh segments of the heating element, whereby the heating element is strengthened against magnetic motoring forces produced by the heating current.

Another feature of the present invention is the same as the preceding feature wherein the increased thickness of mesh is provided by a folded over portion of the mesh.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of additional helical strands woven into the mesh to form reinforcing rib structures for strengthening the mesh against distortion in use.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a diagrammatic perspective view of a prior art mesh heating element,

FIG. 2 is a perspective photographic view of a mesh type heating element incorporating features of the present invention,

FIG. 3 is a side elevational photographic view of the structure of FIG. 2 taken along line 3-3 in the direction of the arrows,

FIG. 4 is a perspective photographic view of a mesh heating element of the present invention as mounted in mesh segments causing them to be pulled in toward each its arcuate supporting ring structure,

FIG. 5 is a diagrammatic plan view of the mesh heating element of the present invention, and

FIGS. 6A-6C are diagrammatic sectional views of the structure of FIG. 5 taken along lines A-A, B-B, etc. of FIG. 5.

Referring now to FIG. 1 there is shown the prior art mesh type heating element 1. The heating element 1 includes a tungsten wire mesh 2 separated into three leg segments 3, '4, and 5. The upper ends of the leg segments are conductively joined, as by welding, to three

separate ring segments

6, 7, and 8, as of tungsten. Each ring seg ment has a laterally directed

support arm

9, 11, and 12, as of tungsten, which is connected to an output electrical terminal of a three phase, low voltage, high current power supply, not shown.

The lower ends of the

leg segments

3, 4, and 5 are joined together by gusset portions of the mesh, not shown, which are sandwiched between a pair of concentric rings 13, only the outer one of which is shown. The lower end portion of the mesh 2 is welded to the rings 13, as of tungsten, to form a conductive connection between the lower ends of all the

leg segments

3, 4, and 5.

In operation, a device to be heated, not shown, is inserted within the center of the heating element 1, which is located within a series of surrounding heat reflectors of a vacuum furnace, not shown, and evacuated to a suitable pressure as of torr. Three phase current as of 2000 amps. at 25 volts is passed through the mesh 2. The current is supplied with one phase applied to each of the

separate support arms

9, 11, and 12. The heating current flows through the support arm, upper ring segment, thence down through a leg segment of the mesh to the lower support ring 13. Current then flows through the ring 13 and mesh gusset to one of the adjacent leg segments and thence to the upper ring segment and out the adjacent support arm. The heating mesh 2 reaches an operating temperature in excess of 2500 C. and in some cases in excess of 3000 C.

It has been found that the intense magnetic fields produced by the heating current flowing in opposite directions in

adjacent legs

3, 4, and 5 exerts strong deflecting forces (magnetic motoring) on the

leg segments

3, 4, and 5 tending to pull these leg segments inwardly toward each other. As a consequence of the high operating temperature and strong pulsating forces, it has been found that the mesh 2 fractures, after a relatively few hours of operation, along the fracture lines indicated at 14, 15, and 16, thereby producing catastrophic failure of the heating element 1.

Referring now to FIGS. 2-6 there is shown the improved mesh type heating element 21 of the present invention. The heating element 21 is similar to that of FIG. 1 except that it is designed for operation with single phase heating current rather than three phase current employed for the structure of FIG. 1. The heating element 21 of FIG. 2 could employ three mesh segments for three phase operation, but the device will be described, for simplicity of explanation, as designed for single phase heating current.

The heating mesh 2 is divided into two

semicyclindrical portions

22 and 23. The

mesh portions

22 and 23 are each bifurcated to form a total of four

leg segments

24, 24' and 25, 25'. The longitudinal side edges 26 of the leg segments are reinforced by weaving into the mesh additional helical strands of tungsten wire or, in a preferred embodiment, the mesh is folded over on itself to a Width of, for example, to /2" at the side edge regions 26. In case the mesh 2 is folded over, additional helical wire strands are preferably threaded (woven) into the mesh at the corners of the folds such as, for example, at 27 and 27' (see FIG. 6A). The reinforced side edges 26 serve to resist sagging of the mesh and to reduce the stress on the mesh under the forces produced by the alternating magnetic field associated with the heating current, thereby prolonging the operating life of the mesh by reducing the possibility of fracture along

lines

14, 15, and 16 of FIG. 1. The folded edge portions of the mesh preferably overlap the support ring segments at the upper ends of the mesh 2 and are preferably fixedly secured to the overlapped ring segments, as by welding.

A gusset structure 28, defining the crotch portion 29 of the mesh, which lies in between

adjacent leg segments

24 and 25, extends to the lower ring segment 13 to provide additional strength and to provide additional conductive paths between

adjacent leg segments

24 and 25. Moreover, the gusset 28 is woven to provide a rounded V- shaped crotch in order to reduce stress on the mesh in the crotch region. Thus, the gusset structure 28 reduces the current density at the crotch region, strengthens the crotch and, due to the rounded V-shape, reduces the stress on the crotch. As a result, the probability of fractures of the type indicated by lines 14 of FIG. 1 are substantially reduced, thereby extending the operating life of the heating element 21. In a preferred embodiment of the present invention, the gusset structure 28 (see FIG. 6B) is woven into the mesh 2 to a double thickness. More specifically, one layer of the gusset structure 28 is woven between the

leg segments

24 and 25 and a second layer of the gusset structure 28' is woven between the folded over layers of mesh material at the reinforced side edges 26 of the

leg segments

24 and 25.

Additional strength for the heating element is provided by a plurality of longitudinally directed reinforcing rib structures 31 (see FIG. 6C) formed by weaving into the mesh additional helical wire strands at intervals spaced about the periphery of the mesh element. The additional helical wire strands, as woven into the mesh, produce raised ridge portions which define the rib structures 31. These rib structures 31 further resist sagging of the heating element, in use, thereby extending its operating life by inhibiting fractures of the type indicated by lines 15 of FIG. 1.

The lower ring segments 13' are each semi-circular and each include a pair of semi-circular tungsten ring segments with the mesh 2 sandwiched therebetween and with a weld bead joining the two members and the mesh together at the bottom edge surface of the ring segments 13'. The upper ends of the four

leg segments

24, 24' and 25, 25' are each similarly joined to four sets of

ring segments

32 and 32. Four support arms 33, as of tungsten, are joined to the four

upper ring segments

32 and 32. The support arms 33 extend laterally away from the heating element 21 in pairs A and B. The A pair is connected to one output terminal of the single phase power supply and the B pair is connected to the other terminal of the single phase power supply, not shown. In this manner, the two semi-cylindrical heating elements are connected in parallel.

In operation, heating current I flows from the power supply through support arms A to ring segments 32 and thence down

leg segments

25 and 25, in parallel, to ring segments 13'. The current flows through the ring segments 13' and crotch structure 28 to the leg segments 24 and 24', thence up the leg segments 24 and 24' to ring segments 32 and out the B pair of support arms. Heating currents of on the order of 2000 amps. at 25 volts are employed to heat the element 21 to operating temperatures in excess of 3000" C.

In a typical example of the heating element 21 of the present invention the mesh 2 is made by twisting together a multitude of identical helical wire strands as of, for example, 0.035" diameter tungsten wire having a helix inside diameter of 0.053", an outside diameter of 0.123", and 7.15 turns per inch to define a mesh having a characteristic thickness of 0.123. In one example, the heating element had a diameter of 4" and a length of 10".

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A mesh heating element including; means forming a mesh of refractory material adapted to be heated to operating temperatures in excess of 2500 C. by passing an electrical current through said mesh; said mesh being formed of a multiplicity of interwoven refractory metal wires; said mesh having at least a pair of adjacent leg segments; means for conductively connecting together said adjacent leg segments at one of their adjacent end portions such that said adjacent leg segments are connected in series for passing the heating current in opposite directions in adjacent leg segments of said mesh; said conductive connecting means including, a mesh gusset structure defining a rounded V-shaped crotch portion between adjacent leg segments of said mesh, whereby sharp corners are eliminated in the crotch and the heating current density reduced in the crotch portion for extending the operating life of the heating element.

2. The apparatus of claim 1 wherein the elongated side edge portions of said leg segments of said mesh are formed of increased thickness compared to the characteristic thickness of said mesh in between said side edges for strengthening the side edges of said mesh against inwardly directed deflecting forces produced on said mesh during operation thereof.

3. The apparatus of claim 2 wherein said side edge portions of increased thickness are formed by a portion of said mesh folded over on an adjacent section of said mesh to provide said side edge portions with a double layer of mesh.

4. The apparatus of claim 3 wherein said V-shaped gusset structure includes a double layer of mesh with one layer interconnecting a first layer of said folded mesh at said side margins of said adjacent legs, and a second layer interconnecting a second layer of said folded mesh at said side margins of said adjacent legs, whereby said V-shaped crotch portion of the heating element is strengthened against stresses produced in operation of the element.

5. The apparatus of claim 1 including additional helical strands of material woven into said leg segments of said mesh to provide raised ridge portions of said mesh forming a strengthening rib structure.

6. The apparatus of claim 5 wherein said ridge portions are longitudinally directed of said leg segments of said mesh.

7. A mesh heating element including, means forming a mesh of refractory material adapted to be heated to operating temperatures in excess of 2500 C. by passing electrical current through said mesh, said mesh being formed of a multiplicity of interwoven refractory metal wires, said mesh having at least two elongated leg segments conductively joined at one end such that heating current flows down one leg in a first direction and up the other leg in the opposite direction, said leg segments of said mesh having elongated side edges, and means disposed along said elongated side edges of said leg segments for providing additional strength for said mesh at said side edges, whereby said mesh is reinforced against sagging and fracture in use.

8. The apparatus of claim 7 wherein said means for providing additional strength for said side edges of said mesh comprises forming said side edges of said mesh of increased thickness compared to the characteristic thickness of said mesh in between said side edges.

9. The apparatus of claim 8 wherein said side edges of increased thickness are formed by a portion of said mesh folded over on an adjacent section of said mesh to provide said side edges with a double layer of mesh.

10. The apparatus of claim 9 including, means forming additional helical strands of refractory material woven into said leg segments of mesh to provide raised ridge portions of said mesh forming a strengthening rib structure.

11. A mesh heating element including: means forming a mesh of refractory material adapted to be heated to operating temperatures in excess of 2500 C. by passing an electrical current through said mesh; said mesh being formed of a multiplicity of interwoven refractory metal wires; said mesh having at least a pair of adjacent leg segments; means for conductively connecting together said adjacent leg segments at one of their adjacent end portions such that said adjacent leg segments are connected in series for passing the heating current in opposite directions in adjacent leg segments of said mesh; said conductive connecting means including, a mesh gusset structure between adjacent leg segments of said mesh, and a refractory metal ring segment afiixed to and conductively and supportively joining said leg segments of said mesh at said conductively connected end of said leg segments, said mesh gusset structure extending above said ring segment to define a crotch portion above said ring segment, whereby the heating current density is reduced in the crotch portion for extending the operating life of the heating element.

References Cited UNITED STATES PATENTS 2,938,992 5/1960 Crump 219545 X 3,020,378 2/1962 Eisler 219-549 X 3,088,019 4/1963 Crump 219 3,178,665 4/ 1965 Matheson et a1 338--299 3,274,374 9/ 1966 Matheson et al. 2l9426 3,283,284 11/1966 Eisler 338--212 3,300,746 I/ 1967 Franz 338-208 X VOLODYMYR Y. MAYEWSKY, Primary Examiner US. Cl. X.R.