US2256706A - Electrical heating element - Google Patents

Description

Sept. 23, 1941. T. M. FOX

ELECTRICAL HEATING ELEMENT Filed March 20, 1939 2 Sheets-Sheet 1 k IA'IIIIIIIIIIII Thom Mi an ATTOQNiTC? Sept. 23, 1941. "r. M. FOX

ELECTRICAL HEATING ELEMENT Filed March 20, 1939 2 Sheets-Sheet 2 Thomas JNL POX T/ZW LILZI .F- ll... 1!!

Patented Sept. 23, 1941 UNITED STATES PATENT OFFICE.

ELECTRICAL HEATING ELEMENT Thomas M. Fox, Swissvaie, Pa., aaignor to Edwin L. Wiegand, Pittsburgh, Pa.

Application March 20, 1939, Serial No. 262,804

11 Claims.

several forms which my invention may assume,-

and in these drawings:

Figure 1 is a bottom view of a heating element embodying my invention, 1

Figures 2, 3, and 4 are sections taken, respectively, on the line 2--2, the line 3-4, and the line 44 of Figure 1, looking in the direction of the arrows, on an enlarged scale,

Figure 5 is a fragmentary view of portions of a heating element such as shown in Figure 1, on an enlarged scale, and

Figures 6, 7, and 8 are fragmentary plan views illustrating respectively other heating elements embodying my invention.

The heating element illustrated in Figures 1 through 5 comprises a metallic sheath III. The sheath is here shown as of annular or ring-form and as having disposed therein a resistor II suitably insulated from the sheath by insulating material I2.

The resistor II is here shown as made of helically wound wire, but it will be apparent to those skilled in the art that it may be made of other forms of electrical resistance material, wound or bent in other ways.

The insulating material I2, as is known in the art, may be a suitable refractory insulating material, for example, silica, or silicate, for example, zircon (zirconium silicate), or aluminum oxide, or any suitable refractory oxide or combination of refractory oxides, or any other: suitable refractory insulating material, desirably of a mineral character, and desirably having good heatconducting properties. The insulating material I2 may be mixed, if desired, with a bonding or cementing material, or mixed'with clay, or any other suitable binder.

The sheath I is here shown as comprising a sheet metal annular channel I3, providing an annular plane wall 20, and a sheet metal annular cover plate I4, the marginal portions l of the sides of ,the channel being bent or crimped over the cover plate I4. The resultant Joint or seam at a marginal portion I5 and the cover plate I4 need not be welded, or otherwise sealed, and, as far as certain aspects of the invention are concerned, these seams may be .welded or otherwise sealed, as will be pointed out hereinafter. The sheath may be provided with studs 3|, adapted to hold the element in position in an electric range.

The cover plate I4 is here shown as provided with an embossed portion I 6 of suflicient circumferential length to accommodate a pair of terminals Ila, [1b, in suitably circumferentially spaced relation. The terminals Ila, IIb are here shown as having threaded shanks I8, and as 1 having cylindrical heads I9, the latter being disposed, in this instance, at about the approximate plane of the cover plate I4, and, at any rate so that the end surfaces of the heads I! are suitably spaced from the adjacent limiting contours of the adjacent portions of the resistor II. The shanks I8 are disposed through and spaced from the margins of apertures 20 in the top of the embossed portion I6. Washers of mica 2|, or any other suitable insulating material, are here shown as surrounding the shanks I8, and disposed at the inside of the top (as viewed in the drawings) of the embossed portion I6. Similar washers 22 are provided about the shanks I8, at the outside of the embossed portion I6; and each shank is here shown as provided with a nut 23 bearing against a metallic washer 24 in turn bearing against the respective insulating washers 22.

The resistor II is here shown as having an end 25 connected to the terminal Na, and the other end, connected to the terminal IIb. The ends 2 26 are suitably mechanically and electrically connected to the respective terminals Ila, Nb, and in this instance they are disposed underneath metallic washers 21, held in place and caused to bear against the ends 25, 26 in any desired way. In the embodiments illustrated, a pre-formed button 28 of insulating material is interposed between the end surface of the head I9 of each terminal Ila, Nb and the adjacent portion of the resistor I I. These buttons 28 may be formed with a circumferential flange 29, so as to provide a recess in which the respective head I8 is disposed. The buttons 28 may be made of insulating material the same as or different from--that in which the major part of the resistor II is embedded, and may be made of any suitable refractory insulating material. The buttons '28 may be baked before assembly in the heating element, or they may be preformed but unbaked, or they may be omitted and instead thereof compactable comminuted or powdered insulating ma- From the point a concentric with the annular sheath l8.

. test of a standard element, 300

. a n, insulating material at all points that attain full terial may be utilized, the same as or difierent from the insulating material in which the major part of the resistor ii is embedded.

Considering, now, the form of the resistor ii in the embodiment of Figures 1 through 5, and tracing the resistor from the terminal ilb to the terminal i'io, the end 26 of the resistor extends downwardly slightly to the point a, in the general plane of the wound portion of the resistor ii. the general longitudinal locus or the portion of the resistor defines a circle The portion b merges with a tangential portion ti, which continues, clockwise in this instance, and crosses over to the portion 0. The longitudinal locus of the portion 0 defines a circle, concentric with the annular sheath, but of larger diameter than the portion b. The portion 0 merges with a tangent portion t2 which continues, clockwise, and crosses over to the portion d. The longitudinal locus of the portion d defines a circle concentric with the annular sheath, but of larger diameter than the portion c. The portion d merges, at c, with the end which is connected to the terminal lid.

As here shown, the portion c is disposed centrally oi the cross-section or the sheath IQ and the portions b and d are disposed symmetrically with respect to the portion 0.

It will be evident resistor ii, as a whole, in the hereinbeiore described embodiment, defines an endless loop.

In prior heating elements oi the type comprising an annular sheath with a resistor disposed therein, the sheath was not heated to substantially the same degree at the terminal zone as at other parts of the sheath spaced from. the terminal zone, because the general locus of the resistor defined a split ring, with a considerable gap at the terminal zone. The result was that at the terminal zone the heating element was at a much lower temperature than elsewhere, for example, by Fahrenheit lower.

It has been found,.in connection with prior heating elements, that under severe conditions of operation, the fact that the terminal zone was at a lower temperature, at times resulted in reducing the life of the heating element. It will be understood that, in operation, a heating element of the type herein illustrated, when used in an electric range, is disposed so that the wall 36 of the sheath is directed upwardly and the terminals iia, ilb are directed downwardly, and, though the seams at the marginalportions l5, and the terminals ila, iii) are therefore at the under, side of the element, it may happen that a particularly large amount of melted grease,'oil,

or like cooking material, or other possibly'delethat the general locus of the operating temperature, but, in the prior heating elements, at the terminal zone the material will terious material, may be spilled upon the heating element at the terminal zone and find its way either to the seams between the marginal portions it and the cover plate id, or to the terminals. This in itself might not be objection= able, but under a conjunction of certain circum stances the deleterious material might penetrate the seams, or penetrate the terminal openings 28, through the mica washers 22 or in some other way get into the insulating material it. Such penetration will occur principally, by way of example, if enough such material is present when the heating element is coolin perhaps because, as the element cools, a certain amount of suction is created. On heating the element the material 'will be burned out of the not be burned out but will be carbonized in the insulating material. If the cycle of heating and cooling is repeated often enough, and a certain amount of grease, oil, and the like happens to be present at a certain time during the cooling phase of the cycle, the carbonized material may accumulateto such an extent as may eventually result in failure of the heating element before its normal life is reached.

On the other hand, I have found that a heating element embodying my invention may be subjected to the severest such conditions of operation without occurrence of any penetration oi grease, oil, or other deleterious materials into the insulating material, and, consequently, the ele ment may be operated toits full, normal life in spite of such severe conditions. In tests of a heating element embodying my invention it was found that the terminals, at full operating tem perature, were at approximately the same tem= perature as the principal part of the sheath, about 800 or 850 Fahrenheit. Such temperatures are well below the burning or scaling temperature of Monel metal, for example. The mica washers were absolutely without signs of car= bonization or discoloration after operation of the heating element under the conditions hereinbefore referred to. The reason for this seems to be twofold. Ii the deleterious material does penetrate at a seam or at the terminals, the heating element is hot enough, not only at portions away from the terminal zone but also at the terminal zone, to burn out the material, or it is hot enough so that the material'is volatilized before it can penetrate.

Inasmuch as no harm is done in a heating element embodying my invention even if grease and the like do penetrate the insulating material in which the resistor is embedded, it is not only not necessary that the sheath be sealed, but the insulating material may even be exposed, as in partially sheathed heating elements, for example.

Aside from the foregoing advantages, the elimination of a relatively cold spot at and about the terminal zone or zones is of advantage where the heating element is used tor the purpose of fryingor in any other instances where an even temperature all over the element isdesirable. Furthermore, when the heating element is heated to a glowing temperature it will present an improved appearance because there will be no dark area, but on the contrary, the entire projected area will appear visibly uniformly heated. It

will be apparent that these advantages, among.

others hereinafter to he mentioned, will be present irrespective of whether a seamless sheath, or a sheath having sealed seams, or anyaother type of sheath is utilized.

While the advantages of the heating element for cooking purposes have been described, its use is of course not limited to that purpose.

One way of constructing the heating element illustrated in Figures 1 through 5 is as follows.

A suitable tool or form (not shown) analogous to the tool shown in the patent toflfWlegand- 1,133,-

347, March 30, 1915, may be provided, the tool,

that the resistor may be 7 figuration that it is desired it shall have in the Ilb, are also positioned by the supporting part in any suitable way. For convenience of insertion 01 the-buttons 28, portions of the resistor II at the terminal zone may be temporarily pulled aside and held by special pegs positioned laterally oi the terminal zone; then the buttons 28 may be put in place over the heads I8 of the terminals; and then the aforesaid portions of the resistor may be removed from the special pegsand brought back to the position in which they cross over the buttons. as the tangents tI and t2,

as shown in Figure 5 the drawings. The resistor, thus carried y the tool, is pressed into an open coverless channel, such as l3, before the margins I are crimped, the channel being partly filled to a predetermined level with comminuted or powdered insulating material, which, whether it is mixed with a binder or not, is in a plastic, or plastoid, or impressionable condition, and consequently theresistor ll becomes embedded in the insulating material. The pins. and then the supporting part, are then withdrawn, leaving the resistor II embedded in the insulating material. Additional insulating material is then introduced in the channel, over the embedded resistor, and the insulating material is then compressed to a thickness which is somewhat less than its final thickness. Additional insulating material may then be built up at the terminal zone. about the lower portions of the terminals Ila, Ila, so that, eventually, when the element is completed, this additional material will fill the embossed part Ii 01' the cover. This additional material is also compressed. The washers 2|, may then be applied, the incomplete element then appearing as indicated in Figure 5. The cover I4 is then applied, and the margins I5 are crimped over the cover. The element is then subjected to high pressure, dried, and baked.

Some examples of other forms of heating elements embodying my invention are shown in Figures 6 through '8, which also illustrate other ways in which the resistor may be disposed.

Referring now more particularly to Figure 6, there is here shown an electric heating element of the strip type comprising an elongated metallic sheath Illa which includes a channel-shaped portion I3a the margins l5a of which are bent, over a cover plate Ha as already-described in connection with the embodiment of Figures 1 through 5. If desired, the sheath I 0a may be extended at either end to provide mounting tabs 32 having apertures 33. The heating element is provided with a pair of terminals Illa, lllb located intermediate the ends of the element. A resistor III is disposed within and insulated from the sheath I Ila. The resistor III has its ends a, 26a connected to the terminals Illa, I Ilb respectively, the remainder of'the resistor being here shown in the form of a helix disposed back and forth within the sheath Ilia. Tracing the resistor Ill, and beginning at the end 25a connected to the terminal Illa, the end 25a merges with the portion j which extends to the lower end of the sheath lOa, as viewed in the drawings, where the resistor turns 180, continuing as the portion a. The portion 0 passes underneath the terminal Illb and crosses over to the portion 71, the latter continuing upwardly to the upper end of the sheath, as viewed in the drawings. The resistor then turns 180, continuing as the por- ,tion 2. The portion i'passes underneath the terminals Illa, Illb, crossing over to the portion 1 which continues to the lower end of the sheath.

There the resistor turns 180 and continues as the portion is to the upper end of the sheath, where it turns 180 and continues as the portion 1 which terminates in the end 26a connected to the terminal lllb.

Referring now to Figure 7, there is here shown an electrical heating element of the strip type comprising an elongated metallic sheath lib which includes a. channel portion lib and a cover plate Ilb. In this instance the heating element is provided with a pair 01' terminals 2Ila, zllb,

disposed at one end of the element. A resistor III is disposed within and insulated from the sheath Illb. The resistor 2 has its ends 25b, 26b connected to the terminals Illa, 2llb respectively, the remainder of the resistor 2 being here shown in the form or a helix disposed back and forth within the sheath. Tracing the resistor 2H, and beginning atthe end 25b connected to the terminal Illa, the end 25b merges with the portion m which extends from the upper end of the sheath. as viewed in the drawings, to the lower end of the sheath, where the resistor makes a turn continuing as the portion n. The portion a also extends the full length or the sheath, to the upper end, where the resistor makes a 180 turn, continuing as the portion 0. The portion 0 extends the full length of the sheath, to the lower end, where the resistor makes a 180 turn, continuing as the portion p. The portion 12 also extends the full length of the sheath and merges with the end 261) which is connected to the terminal Illb.

Referring now to Figure 8, there is here shown an electrical heating element of the strip type comprising an elongated metallic sheath Illc which includes a channel portion I3c and a cover plate Ilc. In this instance the heating element is provided with a pair of terminals Illa, Illb. which'are disposed at opposite ends of the element. A resistor 3 is disposed within and insulated from the sheath I00, and has its ends 250, 260 connected to the terminals 3Ila, lllb respectively, the remainder of the resistor being here shown as in the form of a helix disposed back and forth with the sheath. Tracing the resistor 3H, and beginning at the end 250. connected to the tenninal 3Ila the end 250 merges with the portion q which extends the full length of the sheath, to the lower end, where the resistor turns 180 continuing as the portion 1'. The portion r also extends the full length oi? the sheath, to the upper end of the sheath where the resistor turns' 180 continuing as theportion s. The portion 3 also extends the full length of the sheath, to the lower end, where it merges with the end 260 connected to the terminal 3| lb.

The terminals Illa, lb, and the terminals ma, 2IIb, and the terminals am, we, may be constructed and arranged similarly to the terminals Ila, Ilb of Figures 1 through 5, and it will be understood that at the terminal zones the respective resistors, in each of the embodiments shown in Figures 6 through 8, are disposed in a general'plane beneath the heads of therespective terminals (corresponding to the heads l0.

' of the terminals Ila, Ilb), analogously to the construction described in connection with Figor analogous type, the heat generated in the resistor may be also substantially uniformly distributed throughout the element, along the en tire length of the heating element, so that even the end portions are heated. The latter is of particular importance in cases where a heating element of relatively small dimensions, as for example a very short strip heater, is required and it is desired that the heating element shall provide desired heat close to or at the ends spaced from the terminal zone or zones is reduced thereby increasing the overload capacity of the element. Also, for a given capacity, since the normal load intensity or concentration is reduced and the average temperature is decreased, the possibility of failure by fusion, or otherwise, is decreased. I

It will be apparent from the illustrated'examples hereinbefore described, that a primary desideratum is to secure a substantially uniform distribution or energy over the entire projected area of the operating face of the heating element, and that another primary desideratum is to heat the terminal zone or zones of the heating element to a predetermined degree. it will be apparent from the disclosure that these desider atums may be realized, jointly or severally, in

ways other than in the particular embodiments illustrated, and in other forms or electrical re sistance heating elements.

It will he understood that I do not limit myself to such temperatures as hav beenzgiven hereinbefore merely by way of example. The sheath, and other metallicparts, such as the terminals, are of course made of suitable material, such as iron, or steel, or nickehor nickel chrome, or Monel metal, or other metal alloys, or any other suitable material capable of withstanding the particular temperatures involved.

rem the foregoing it will be apparent to those skilled in the art that each of the llustrated embodiments of my invention provides a new and improved electrical heating element, and accordingly, each accomplishes the principal ob ject of. my invention. On the other hand, it also will be obvious to those skilled in the art that the illustrated embodiments of my invention may be variously changed and modified. or features thereof, singly or collectively, embodied in other combinations than thos illustrated, without departing from the spirit of my invention, or sacrificing all of the advantages thereof, and that accordingly, the disclosure herein is illustrative only, and my invention is not limited thereto.

I claim:

1. An electric heating element, comprising: resistor means, arranged in a linearly extended formation; terminal means, disposed laterally thereof; electrical connection between said resistor means and said terminal means; and heatconducting electrical-insulating material enveloping said resistor means and said electrical conor heat generat-- aaseyoe nection; said element being so constructed, that when said element is in operation, the wattage in the vicinity of said terminal means is not materially less than the average wattage, per I said electrical connection; said element being so constructed, that when said element is in opera= tion, the wattage in the vicinity of said terminal means is not materially less than the average wattage, per unit of extent of said formation 3. An electric heating element, comprising: resistor means, arranged in a substantially circularly extended formation; terminal means, disposed laterally thereof; electrical connection between said resistor means and said terminal means; and heat-conducting electrical-insulat ing material enveloping said resistor means and said electrical connection; said element being so constructed, that when said, element is in operation, the wattage in the vicinity of said terminal means is not materially less than the average wattage, per unit of extent of said formation.

An electric heating element, comprising: resistor means, arranged in a linearly extended formation; terminal means, disposed laterally thereof; electrical connection between said resistor means and said terminal means; and heat conducting electrical-insulating material enveloping said resistor means and said electrical connection; sheet-metal meansproviding a recess in which said insulating material is disposed; said element being so constructed, that when said element is in operation, the wattage in the vi= cinlty of said terminal means is not materially less than the average wattage, per unit of extent of said formation.

5. An electric heating element, compris ng: resistor means, arranged in a substantially rectilinearly extended formation; terminal means, disposed laterally thereof; electrical connection between said resistor means and said terminal means; and heat-conducting electricalinsulating material enveloping said resistor means and said electrical connection; sheet metal means providing a substantially rectilinear recess in which said material is disposed; said element being constructed, that when said element is in operation, the wattage in'the vicinity of said terminal means is not materially less than the average wattage, per unit of extent of said formation.

6. An electric heating element, comprising: resistor means, arranged in an annularly emtended formation; terminal means, disposed laterally thereof; electrical connection between said resistor means and said terminal means; and heat-conducting electrical-insulating material enveloping said resistor means and said electrical connection; sheet-metal means pro= riding an annular recess in which saidinsulat lng material is disposed; said element being so constructed, that when said element is in operation, the wattage in the vicinity of said terminal I means is not materially less than the average wattage, per unit or extent of said formation.

7. an electrical heating element, comprising: a body or insulating material having a heat transfer surface; terminal studs having portions,

disposed in said body of insulating material, opposite and spaced from said heat transfer surface; and a resistor, for heating said heat transfer surface, disposed in said body of insulating material, and having its ends in said body of insulating material and connected to said terminal portions; said resistor being so constructed and arranged with respect to said heat transfer surface that its general longitudinal locus traced from a first one of said terminal studs to the other of said terminal studs is such that an in:- termediate portion of said resistor traverses the body of insulating material between the end of said first terminal stud portion and the opposite portion of said heat transfer surface; said body of insulating material including a portion comprising a pre-formed member of insulating material interposed between the transverse end surface of said end of said first terminal stud portion and the adjacent said intermediate portion of said resistor, said pre-formed member having a recess in which said transverse end surface is disposed.

8. An electrical heating element for use in electric cooking apparatus, comprising: a metallic sheath having an upper heating surface upon which cooking utensils to be heated are adapted to be placed; terminals having portions disposed within said sheath, and extending down- .wardly through an under wall portion of said sheath; means whereby said terminals are carried by and insulated from said sheath; a resistor, for heating said heating surface, disposed within and insulated from said sheath, and having its ends connected to said terminals; and said resistor being so constructed and arranged with respect to the regions of said terminals that said resistor is adapted to heat said heating element in the regions of said terminals to a temperature high enough to vaporize cooking materials spilled upon said heating element thereby to vaporize such materials at the outside of said sheath to prevent their entry into said sheath at the terminal regions.

9. An electrical heating element for use in electric cooking apparatus, comprising: a sheet metal sheath having an upper wall providing a heating surface upon which cooking utensils to be heated are adapted to be placed, and having a seamed lower wall; terminals having portions disposed within said sheath, and extending downwardly through a portion of said lower wall; means whereby said terminals are carried by and inwithin said sheath, and extending downwardly through an under wall portion of said sheath;

sulated from said sheath; a resistor, for heating said heating surface, disposed within and insulated from said sheath, and having its ends connected to said terminals; and said resistor being so constructed and arranged with respect to said sheath that said resistor is adapted to heat said heating element in the regions of said terminals and the seam of said lower wall to a temperature high enough to vaporize cooking materials spilled upon said heating element thereby to vaporize such materials at the outside of said sheath to prevent their entry into said sheath at the terminal regions or at said seam.

10. An electrical heating element for use in electric cooking apparatus, comprising: a metallic sheath having an upper heating surface upon which cooking utensils to be heated are adapted to be placed; terminals having portions disposed means whereby said terminals are carried by and insulated from said sheath; a resistor, for heating said heating surface, disposed within and insulated from said sheath, and having its ends connected to said terminals; and said resistor being so constructed and arranged with respect to the regions of said terminals that said resistor is adapted to heat said heating element in the regions of said terminals 'to a temperature high enough so that cooking materials spilled upon said heating element and which may have entered said sheath at the terminal regions are burned out.

11. An electrical heating element for use in electric cooking apparatus, comprising: a sheet metal sheath having an upper wall providing a heating surface upon which cooking utensils to be heated are adapted to be placed, and having a seamed lower wall; terminals having portions disposed within said sheath, and extending downwardly through a portion of said lower wall; means whereby said terminals are 'carried by and insulated from said sheath; a resistor, for heating said heating surface, disposed within and insulated from said sheath, and having its ends connected to said terminals; and said resistor being so constructed and arranged with respect to said sheath that said resistor is adapted to heat said heating element in the regions of said terminals and the seam of said lower wall to a temperature high enough so that cooking materials spilled upon said heating element and which may have entered said sheath at the terminal regions or at said seam are burned out.

THOMAS M. FOX.

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