WO1997038556A1 - Compaction of sheathed electric heating elements - Google Patents

Abstract

An electric heating element (10) having a circular sheath (11) and a resistance element (12) is filled with granular dielectric material (13) which is initially loosely packed by vibration only and then compacted by imparting a non-circular cross section to the sheath (11) so as to reduce the cross-sectional area without any significant change in the perimeter of the cross section of the sheath (11).

Description

COMPACTION OF SHEATHED ELECTRIC HEATING ELEMENTS FIFi n ΩF THE INVENTION

This invention relates to sheathed electrical heating elements and more particularly to an improved method for manufacturing such elements, together with a manner in which it may be performed.

BACKGROUND ART

Sheathed electrical heating elements are well known and usually consist of a resistive conductor or conductors embedded in a compacted mass of granular dielectric material within a tubular metallic sheath. Such heating elements are generally made by an initial step of filling round metal tubes on a multi-spindle filling machine with granular dielectric material, typically electrical grade magnesium oxide. This is accomplished in such a way as to keep the resistive conductor centrally disposed in the round tube, or, in the case of more than one conductor, appropriately spaced from the sheath and from each other. Such filling machines usually subject the element to mechanical vibration during the filling operation, so as to achieve a uniform column of the highest density obtainable from the granular dielectric material. However, in the case of magnesium oxide, which has a solid density of approximately 3.4 gms/cc, in granular form, filled densities typically achieved are only of the order of

2.36 + /- .04 gm/cc.

In order to increase the thermal conductivity of the dielectric and also lock the resistive conductor(s) into their respective disposition(s) within the sheath, it is then necessary to subject the granular dielectric material to further compaction. In the instance of nickel alloy sheath materials, typical compacted values of 2.9 to 3.0 gms/cc are adopted when using a granular magnesium oxide dielectric, these values decreasing for more malleable metal sheathing materials such as aluminium, copper and mild steel.

Hitherto, compaction was accomplished by hammer swaging of the sheath, wherein the diameter of the sheath of the filled element was progressively reduced by a series of blows from swaging blocks incorporating tapering semi-circular grooves. This diametral reduction is now more generally achieved by a roll reduction mill, wherein a series of driven pairs of rolls, typically 8 or more pairs, whose axes are alternately disposed at right angles to each other, and with generally semi-circular grooves of progressively decreasing radii, are used to achieve diametral reductions of up to 1 6%. For nickel alloy sheaths, reductions are typically of the order of 14% to 1 5%.

Although this round roll reduction process achieves compacted densities typically in the desired 2.9-3.0 gms/cc range for nickel alloy sheaths, it is accompanied by a number of significant side effects: a) The total element increases in length by typically 1 2-1 5%, depending on the amount of diametral reduction, type of nickel alloy sheathing and the selected wall thickness. (These values are considerably exceeded for more malleable sheath materials, with a corresponding decrease in compacted densities), b) For any given element specification, the mean rolled length can vary from batch to batch, due to minor variations in material batches, machine setups, machine wear, environmental conditions and the like. Variations in the mean rolled length of 0.5% over a series of batches are not uncommon, c) Within any particular batch of elements, this increase in length is not uniform, but varies typically by + /- 1 % in practice, d) These variations can give rise to major problems in subsequent processing, usually requiring an additional operation to trim, roll or stretch the elements to a more constant length. Even so, the finished elements may still have non uniform characteristics, such as the points at which the heated length commences, e) The sheath is extensively work hardened after having undergone both diametral reduction and elongation of this magnitude, so much so that an additional annealing operation is generally applied before the element can be formed to shape satisfactorily, f) The granular dielectric undergoes a significant degree of crushing during the process, which tends to reduce the mean grain size and hence reduce the thermal conductivity of the resultant dielectric body. Further, if silicone coated grains are used initially (to overcome hygroscopic properties in the case of magnesium oxide), this crushing can create uncoated surfaces on fractured grains and hence cause a decrease in electrical insulating properties, g) The amount of work to which the element is subjected during round roll reduction is such as to cause an increase in the diameter of the resistive conductor/s, typically causing resistances to decrease by 2% to 30%, depending on the initial diameter of the resistive conductor selected. Whilst this change can be allowed for, it tends to give a wider spread of resistance values and hence resultant wattages, than might otherwise be the case, h) Should an annealing operation over the total length of the element be required to offset work hardening after round rolled compaction, the temperature required is such as to burn out washers inserted during the filling operation, particularly if these are composed of synthetic materials as is frequently the case. Alternatively, such washers may be removed mechanically before annealing. As a consequence, new seals will need to be inserted after annealing, as is required in most applications, i) If the dielectric chosen has silicone coated grains, then the temperature required to anneal the sheath material over its total length causes depletion of the silicone coating from grains at open ends of the element. A further operation is usually required to replace the depleted silicone and restore uniform insulating properties to the granular dielectric, j) If silicone coated dielectric grains are employed, such an annealing operation can cause the dielectric to form a clogged cement-like mass, reducing slippage between grains when subsequently forming bends and hence causing sheath fracture. In addition, this clogging effect can give rise to formation of a partial vacuum inside the element during annealing, subsequently decreasing electrical breakdown values to the detriment of the electrical insulating properties.

It is therefore an object of this invention to provide a new method for achieving compaction of the granular dielectric material, without significant length variation, work hardening of the metallic sheath, or excessive crushing of the granular dielectric. SUMMARY OF INVENTION

According to the invention there is provided a method for the compaction of granular dielectric filled metal sheathed electric heating elements wherein a non-circular cross section is imparted to the element without any deliberate change in its perimeter, so as to reduce the cross sectional area and hence compact the granular dielectric material.

Preferably, the method is carried out in a single operation. The method of the invention minimises work hardening of the metallic sheath, eliminates the need for post annealing, greatly reduces any elongation of the element and any variations thereto, and at the same time preserving both the electrical and thermal properties of compacted granular dielectric material and permitting sealing mediums to be incorporated at the filling operation.

In one preferred form of the invention, the cross sectional shape generated is generally square in form, but with four concave sides of equal length, the four vertices comprising free formed blending radii, such that the perimeter of the cross section is substantially the same as the circular cross section prior to compaction, whereas the cross sectional area is reduced to provide the required degree of compaction. Although this is the preferred version for generally square forms, in that the concave sides impart rigidity, can provide positive locations and prevent buckling during subsequent forming operations, in another version the length of all or any of the four sides may be different to provide any desired general quadrilateral form. In another preferred form of the invention, the cross sectional shape generated is generally triangular, but with three slightly concave sides of equal length, the three vertices comprising free formed blending radii of comparatively generous size, such that the perimeter of the cross section is substantially the same as the circular cross section prior to compaction, whereas the cross sectional area is reduced to provide the required degree of compaction. Although this is the preferred version for generally triangular forms, in that the concave sides can impart rigidity, provide positive locations and prevent buckling during subsequent forming operations, the length of all or any of the three concave sides may be different so as to provide any generally triangular shape.

An advantage of the invention is that the amount of cold work imparted to the metal sheath during compaction is minimised, so that no subsequent annealing operations are necessary before forming the element to suitable configurations.

The invention may be applied to all metallic sheath materials generally used for heating element sheaths, for example high nickel alloys, or stainless steels in either black oxide annealed, bright annealed or as welded conditions; mild steels, copper, aluminium, brass and titanium, all diameters, all wall thicknesses and conditions generally employed. The invention is similarly applicable to elements containing single or multiple resistive conductors, or to elements with the metal sheath open at one or both ends. Further, roll-in or pull-out filling washers, frequently employed in the prior art, can continue to be employed with the invention. In general terms, the process of the invention achieves compaction of the granular dielectric by changing the circular cross section of the filled element to other shapes, having reduced cross sectional areas so as to provide that compaction, but without deliberately reducing the perimeter of the cross section as is the case in round rolled reduction.

In one example of the invention, the compacted cross section is generally square in shape, but modified in a number of important ways. It will be noted that if a true circular cross section is converted to a true square cross section of the same perimeter, then the reduction in cross sectional area is 21 .5%, sufficient to convert a filled density of 2.36 gms/cc to a compacted density of 3.0 gms/cc.

In practice, a small radius is required at each of the four corners to avoid localised work hardening of the sheath, as well as stress concentrations which could give rise to fracture points during a subsequent forming operation. Such corner radii will however increase the cross sectional area for a constant perimeter and hence slightly reduce compaction. It is further proposed to modify the four sides to a concave shape thereby giving a more rigid longitudinal structure but more particularly controlling any tendency for the four sides to buckle during any subsequent forming operations. For a constant perimeter, such concave sides will clearly reduce the cross sectional area and substantially increase compaction, offsetting any loss of compaction resulting from the four corner radii described above and hence providing nett compaction.

A further feature of the concave sides is to provide positive location if it is required to form the element around a bending arbor. A matching profile on the arbor will enable any required bend to be generated consistently in the required plane. Whilst this is the preferred form for a modified square shape, optional quadrilateral shapes can be employed. In another example of the invention, the compacted cross section is generally triangular in shape, but again with modifications. It will be noted that if a true circular cross section is converted to a true equilateral triangular cross section of the same perimeter, then the reduction in cross sectional areas is 39.5%. If this were applied to a filled element with a fill density of 2.36 gms/cc, then the theoretical compacted density would be 3.9 gms/cc, which is well above the solid density of a material such as magnesium oxide and hence clearly impossible. However, it is again necessary to allow corner radii, three in number for triangular forms, in order to avoid work hardening or stress concentration. These corner radii will likewise increase the cross sectional area and reduce the degree of compaction and must therefore be sufficiently large to reduce compaction to a practical level, typically 3.0 gms/cc or below.

As such generally triangular form elements are most suited to applications requiring either straight or U form elements when completed and as a consequence of having the sides shortened by the comparatively large corner radii, the form of the three sides is less crucial and may be only slightly concave to suit the particular requirement of the application. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cutaway sectional view of a filled metallic sheathed electrical heating element prior to compaction in accordance with the method of the invention, Fig. 2 is a view similar to Fig. 1 showing the element after compaction to a generally square form, Fig. 3 is a view similar to Fig. 1 showing the element after compaction to a generally triangular form, Fig. 4 is a schematic diagram of a roll former for changing the cross-sectional shape of the element according to one embodiment of the invention, Fig. 5 is a cross-sectional view of the former and element shown in Fig. 4, Fig. 6 is a schematic diagram of a roll former for changing the cross-sectional shape of the element according to another embodiment of the invention, and Fig. 7 is a cross-sectional view of the former and element shown in Fig. 6. MODE FOR CARRYING OUT THE INVENTION

The electrical heating element 1 0 shown in Fig. 1 consists of a tubular metal sheath 1 1 of circular cross section, a centrally disposed spiral of resistance wire 12 and granular refractory dielectric material 1 3 with which the intervening space has been filled, the only consolidation of the refractory at this stage resulting from vibration of the assembly. Hence at this stage of manufacture, the dielectric 1 3 is only loosely packed and provides a relatively poor thermally conductive body incapable of rapidly transferring heat from the resistance spiral 1 2 to the metal sheath 1 3.

After compaction in accordance with the invention, as shown in Fig. 2, the generally square form of the sheath 1 1 has substantially the same perimeter as the sheath 1 1 in Fig. 1 , but a considerably smaller cross sectional area. The granular dielectric 1 3 is now compacted to a hard dense mass with greatly enhanced thermally conductive properties. The four concave sides 14 and the four radiused vertices 1 5 referred to above are clearly shown.

In Fig. 3 the generally triangular form of the cross section of the sheath 1 1 has substantially the same perimeter as that shown in Fig. 1 and again the cross sectional area has been reduced, with the same effects on the dielectric as described in relation to Fig. 2. Slightly concave sides 1 6 and radiused vertices 1 7 are again evident.

Figs. 4 to 7 indicate two ways in which the cross sectional shapes and attendant characteristics of the process of the invention are achieved.

According to one embodiment, a preferred method for compacting elements to a generally square cross sectional shape in a single operation, is to pass the element between four driven rolls 20, 21 , 22 and 23, as shown in Figs. 4 and 5. These four disc like rolls 20, 21 , 22 and 23 are arranged in a right angled cruciform about the longitudinal element axis 24, all four being normal to the element 1 0 in the same plane, that plane being perpendicular to the longitudinal element axis 24.

The periphery of each roll has a convex workface 25 matching the concave radius being imparted to the element sides 14, each edge 26 of the rolls beyond the region of convexity being bevelled, typically at 45°, to provide clearance between adjacent rolls.

All four rolls 20, 21 , 22 and 23 are driven by a gear train, typically bevel gears, so that their peripheral speeds are the same. All rolls 20, 21 , 22 and 23 preferably have the same outside diameter. The direction of rotation at the point of contract with the element is the same for each roll

20, 21 , 22 and 23.

Each roll 20, 21 , 22 and 23 is provided with a rigid but adjustable mounting so that its distance from the longitudinal axis 24 of the element 10 can be precisely varied, then locked in position. Decreasing clearances between the element axis 24 and rolls 20, 21 ,

22 and 23 reduces the element cross sectional area and increases compacted density of the magnesium oxide, accompanied by incremental increases in elongation, variations in elongation and work hardening of the sheath, thus enabling a particular set of related variables to be selected to suit a specific sheath material and element application. Close fitting run in and run out guide bushes may be provided before and after the roll set, so as to maintain alignment for the element 10 during compaction.

Although the preferred method is to employ only one set of four rolls 20, 21 , 22 and 23 to impart the generally square cross section to the element 10, so minimising work hardening of its surface, in another embodiment of the method two or more sets of four rolls may be employed, at least one set of which must be driven.

According to another embodiment, a preferred method for compacting elements employing a generally triangular cross sectional shape in a single operation, is to pass the element between three driven rolls 30, 31 and 32, as shown in Fig. 6 and 7. These three disc like rolls 30, 31 and 32 are disposed radially with respect to the longitudinal element axis 24, typically at 1 20° from each other. The three rolls 30, 31 and 32 are normal to the element in the same plane, that plane being perpendicular to the longitudinal element axis 24.

The periphery of each roll 30, 31 and 32 has a convex workface 34 matching the concave radius being imparted to the element sides, each edge 35 of the rolls 30, 31 and 32 beyond the region of convexity being bevelled, typically at 60°, to provide clearance between adjacent rolls.

All three rolls 30, 31 and 32 are driven by a gear train, typically level gears, so that their peripheral speeds are the same. The rolls 30, 31 and 32 preferably have the same outside diameter. The direction of rotation at the point of contact with the element is the same. Each roll 30, 31 and 32 is provided with a rigid but adjustable mounting, so that its distance from the longitudinal axis 24 of the element 1 0 can be precisely varied, then locked in position.

Decreasing clearances between the element axis 24 and rolls 30, 31 and 32 reduces the element cross sectional area and increases compacted density of the magnesium oxide, accompanied by incremental increases in elongation, variations in elongation and work hardening of the sheath, thus enabling a particular set of related variables to be selected to suit a specific sheath material and element application. Close fitting run in and run out guide bushes may be provided before and after the roll set, so as to maintain alignment for the element during compaction.

Although the preferred method is to employ only one set of three rolls 30, 31 and 32 to impart the generally triangular cross section to the element 1 0, so minimising work hardening of its surface, in another embodiment of the method two or more sets of three rolls may be employed, at least one set of which must be driven.

The invention will now be illustrated with reference to the following examples, which are not to be considered in any way limiting. Example 1 .

A batch of elements, designed for incorporation in a diecast heat sink and to be compacted by imparting a generally square cross section, was prepared and filled on a multi spindle filling machine as follows: a) Tubes of copper flashed seam welded mild steel tube with a carbon content less than 0.02%, having an outside diameter of 8.0 mm and wall thickness of 0.7 mm, were cut to a length of 824 mm, b) Single resistance spirals of 25 B & S gauge, with a nominal composition of 80% nickel and 20% chromium, were wound with an outside diameter of 2.6 mm and a nominal cold resistance of 35.7 ohms. These spirals were welded to nickel plated mild steel terminal pins at each end, such that the unheated length at each end of the element sheath would be 80 mm, c) Disposable type synthetic filling washers were applied a bottom and top ends of the elements, before and after filling respectively, d) The elements were filled with a commercially available electrical grade of granular magnesium oxide which had been treated with approximately 0.3% by weight of silicone fluid.

After filling, six of the elements were compacted by passing between a single set of four driven rolls, as in Figs. 4 and 5, the convex radius on each roll periphery being 10 mm. The gap between opposing rolls was such as to give a mean dimension of 6.6 mm, measured over the element sheath at the lowest point between opposite concave faces of compacted elements. The following observations were without the elements being incorporated in a heat sink: a) Compacted elements now had a mean length of 847.0 mm, giving an elongation of 2.8% versus the cut length, b) Variations from the mean compacted were + /- 1 mm, or + /- 0.1 2% versus the mean compacted length, c) Cold resistance of the elements, which had been found to have a mean value of 35.35 + /- 0.25 ohms after filling, now had a mean value of 31 .45 ohms after compaction. Variations from the mean resistance after compaction were still within + /- 0.25 ohms, or + /- 0.8%, d) When checked for insulation resistance between spiral and sheath at 500 VDC at ambient temperature, all elements measured infinity, e) All elements withstood a dielectric strength test of 2000 VAC, applied between spiral and sheath for 1 minute at ambient temperature, f) Compacted density of the granular magnesium oxide was 2.92 gms/cc, g) Bending trials were carried out using an arbor and follower rolls which had 10 mm radii convex profiles to nest into the corresponding concavity of the element sides. When bent through 1 90° with a 1 3.5 mm centreline radius, there were no signs of wrinkling, necking or breakage of the sheath, on either the inside or outside of bends. Example 2

In this example, the elements were designed to be operated in air. The batch of elements, to be compacted by imparting a generally square cross section, was prepared and filled on a multi spindle filling machine as follows: a) Tubes of black oxide annealed seam welded nickel alloy with a nominal composition including 1 3% nickel and 21 % chromium, having outside diameter of 7.5 mm and wall thickness of 0.5 mm, were cut to a length of 1 000 mm, b) Single resistance spirals of 26 B & S gauge, with a nominal composition of 80% nickel and 20% chromium, were wound with an outside diameter of 2.4 mm and a nominal cold resistance of 42.1 ohms. These spirals were welded to nickel plated mild steel terminal pins at each end, such that the unheated length at each end of the element sheath would be 80 mm. c) Roll in type synthetic filling washers with a 1 50°C temperature rating were applied at bottom and top ends of the elements, before and after filling respectively, d) The elements were filled with a commercially available electrical grade of granular magnesium oxide which ad been treated with approximately 0.3% by weight of the silicone fluid. After filling six of the elements were compacted by passing between a single set of four driven rolls, as in Figs. 4 and 5, the convex radius on each roll periphery being 10 mm. The gap between opposing rolls was such as to give a mean dimension of 6.1 mm, measured over the element sheath at the lowest point between opposite concave faces of compacted elements. The following observations were then made: a) Compacted elements now had a mean length of 1021 .2 mm, giving an elongation of some 2.1 % versus the cut length, b) Variations from the mean compacted length were within + /- 1 mm, or slightly less than + /- 0.1 % versus the mean compacted length, c) Cold resistance of the elements, which had been found to have a mean value of 42.1 + /- 0.1 ohms after filling, now had a mean value of 37.4 ohms after compaction. Variations from the mean resistance after compaction were within + /- 0.2 ohms, or + /- 0.5%, d) When checked for insulation resistance between spiral and sheath at 500 VDC at ambient temperature, all elements measured infinity, e) All elements withstood a dielectric strength test of 2000 VAC, applied between spiral and sheath for 1 minute at ambient temperature, f) Compacted density of the granular magnesium oxide was 2.81 gm/cc, g) Bending trials were carried out using an arbor and follower rolls which had 10 mm radii convex profiles to nest into the corresponding concavity of the element sides. When bent through 1 80 with 1 3.5 centreline radius, there were no signs of wrinkling, necking or breakage of the sheath, on either the inside or outside of bends, h) When energised at 240 VAC until the sheath temperature had stabilised, test elements generated a mean wattage of

1 380 Watts equivalent to a loading of 7.0 Watts/sq. cm on the heated portion of the element sheath. The leakage current whilst so energised was less than 0.1 mA between spiral and sheath on all samples, i) It was noted that the seals provided by the roll in filling washers were not hermetic seals, the four corner radii of the generally square cross section creating four small channels for the ingress and egress of air. Results of the above tests were found to be typical when further elements to the same specifications were similarly processed.

Four of these samples were then formed to a W shape with three 1 80 bends of 23 mm centreline radius, using forming rolls with 10 mm radii convex profiles.

These four elements were then subjected to a period of life testing, being connected to a 240 VAC supply and operated in free air, cycling 0.7 hours on and 0.3 hours off, until 1 300 cycles had been completed, the time energised totalled some 900 hours. At the end of this period further observations were made as follows: a) All four elements were still operational, b) Their mean cold resistance was now 37.9 ohms, with a variation of + /- 0.8% from that mean, c) When checked for insulation resistance at 500 VDC at ambient temperature, all elements again measured infinity, d) All elements again withstood a dielectric strength test of 2000 VAC for 1 minute at ambient temperature, e) When energised at 240 VAC until stable conditions had been reached, the leakage current between spiral and sheath was found to have a mean value of 0.1 3 mA, the maximum reading being 0.1 9 mA, f) Whilst still energised at 240 VAC, all elements withstood a dielectric strength test of 1 250 VAC between spiral and sheath for a period of 1 minute, g) When subsequently energised at 254 VAC, or 1 .06 times the rated voltage of 240 VAC, until stable conditions had been reached, the mean leakage current between spiral and sheath was now 0.26 mA with a maximum reading of 0.39 mA, and all elements passed a dielectric strength test of 1 250 VAC for a period of 1 minute. It is apparent from the above description that the method for producing sheathed electric heating elements according to this invention are completely different from conventional processes and methods. Various modifications may be made in details of the process and method without departing from the scope and ambit of the invention.

Claims

1 . A method for the compaction of granular dielectric filled metal sheathed electric heating elements wherein a non-circular cross section is imparted to the element so as to reduce the cross sectional area and hence compact the granular dielectric material to a predetermined value without any significant change in the perimeter of the cross section of the heating element.

2. A method according to claim 1 wherein the cross sectional shape generated is generally square in form, but with four concave sides of equal length, the four vertices comprising free formed blending radii, such that the perimeter of the cross section is substantially the same as the circular cross section prior to compaction, whereas the cross sectional area is reduced to provide the required degree of compaction.

3. A method according to claim 1 wherein the cross sectional shape generated is generally triangular, but with three slightly concave sides of equal length, the three vertices comprising free formed blending radii of comparatively generous size, such that the perimeter of the cross section is substantially the same as the circular cross section prior to compaction, whereas the cross sectional area is reduced to provide the required degree of compaction.

4. A method for the compaction of a metal sheathed electric heating element, filled and loosely packed by vibration only with granular dielectric material, the metal sheath being open at both ends and containing a single resistive conductor permanently connected to conductive terminal pins at each end, filling washers being fitted at each open end of the metal sheath to retain the granular dielectric and terminal pins, wherein the initially circular section of the tubular element is reduced and the element compacted by imparting a generally square shape, having four concave sides and four smaller corner radii, without any deliberate or significant change in the perimeter of the metallic sheath.

5. A method for the compaction of a metal sheathed electric heating element, according to Claim 4, wherein the element contains a plurality of resistive conductors permanently connected in the parallel to the terminal pins at each end of the element.

6. A method for the compaction of a metal sheathed electric heating element, according to Claim 4, having a multiplicity of terminal pins at each end of the element so as to provide a multiplicity of separate resistive circuits.

7. A method for the compaction of a metal sheathed electric heating element, according to Claim 4, wherein the metal sheath is permanently closed at one end and the conductive terminal pins terminate and are retained by a single filling washer at the open end of the metal sheath.

8. A method for the compaction of a metal sheathed electric heating element, according to Claim 4, wherein the element is passed between a single set of four driven disc like rolls, all aligned to the element axis and each disposed at right angles to adjacent rolls about that axis, all normal to the element axis in the same plane, that plane being perpendicular to the element axis, the periphery of each roll having a convex radius matching he required radii of the four concave sides being imparted to the elements, clearances between rolls and the element axis being set to achieve the required depth of indentation, the peripheral speed of each roll being the same and the direction of rotation incident to the element being the same.

9. A method for the compaction of a metal sheathed electric heating element, according to Claim 8, wherein two or more sets of said four rolls being employed, at least one set being driven.

1 0. A method for the compaction of a metal sheathed electric heating element, filled and loosely packed by vibration only with granular dielectric material, the metal sheath being open at both ends and containing a single resistive conductor permanently connected to conductive terminal pins at each end, filling washers being fitted at each open end of the metal sheath to retain the granular dielectric and terminal pins, wherein the initially circular section of the tubular element is reduced and the element compacted by imparting a generally triangular shape, having three concave sides and three smaller corner radii, without any deliberate or significant change in the perimeter of the metallic sheath.

1 1 . A method for the compaction of a metal sheathed electric heating element, according to Claim 10, wherein the element contains a multiplicity of resistive conductors permanently connected in the parallel to the terminal pins at each end of the element.

1 2. A method for the compaction of a metal sheathed electric heating element, according to Claim 10, having a multiplicity of terminal pins at each end of the element so as to provide a multiplicity of separate resistive circuits.

1 3. A method for the compaction of a metal sheathed electric heating element, according to Claim 10, wherein the metal sheath is permanently closed at one end and the conductive terminal pins terminate and are retained by a single filling washer at the open end of the metal sheath.

1 4. A method for the compaction of a metal sheathed heating element in accordance with Claim 1 0 wherein the element is passed between a single set of three driven disc like rolls, all aligned to the element axis and disposed at predetermined angles to each other about that axis, all normal to the element axis in the same plane, that plane being perpendicular to the element axis, the periphery of each roll having a convex radius matching the required radii of the three concave sides being imparted to the element, clearances between rolls and the element axis being set to achieve the required depth of indentation, the peripheral speed of each roll being the same and the direction of rotation incident to the element being the same.

1 5. A method for the compaction of a metal sheathed electric heating element in accordance with the method of Claim 1 4, wherein two or more sets of three rolls are employed, at least one set being driven.