US2102301A - Method of making electric heater elements - Google Patents

Description

@ea M, M3? E. E. ARNO@ 2J# METHOD OF MAKING ELECTRIC HEATER ELEMENTS Filed Jur-ie 15, 1935 M60/U ATTORN EY Patented Dece 14, 1937 METHOD OF MAKING ELECTRIC HEATER ELEMENTS Edwin E. Arnold, Pittsburgh, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 15, 1935, serial No. 26,847

` 2 ciaims. (ci. ani-c4) This invention has to do with the manufacture of heating elements for heaters of the type ordinarlly known as strip heaters. Such heating elements usually consist of a resistor surrounded by a body of insulating material, which in its turn, is enclosed in .an envelope or armor, usually of rlietal. Diillculty has been encountered hereto- .Tore in securing, in the production of such elements, suiliciently perfect thermal connection l@ between the resistor and the refractory material surrounding it; Again imperfect thermal contion between said material and the metallic felope has been a cause oi low or uneven heat whole compressed together, without requiringtiiat the resistor element be forced into the reiractory material as a separate step.

A better understanding of the nature ci the invention, a further knowledge of its objects and a fuller acquaintance with the details thereof may be obtained from the following description and the accompanying drawing, in which: Figure i is a plan view oi one form of theresistor element;

Fig. 2 is a similar view of another form; Fig. 3 is a sectional view of a press showing one stage in the manufacture of one form ci my heater element; v

Fig. 4 shows a later stage of said manufacture; Fig. 5 shows the next step therein; Fig. 5 is a sectional view of a press. illustrating one stage in the process of making another iorm of heater element; A i

Fig, '7 is a similar sectional view illustrating the action oi the press; l

Fig. 8 illustrates the next stage in said process method;

Fig. 9 illustrates the stage which occurs next in the making of the form shown in Figs. '6, '7 and 8 andalso the stage after that shown in Fig. 5 in the making oi' the mst-mentioned form;

Fig. 1o illustrates .the insertion of the cakeshown in Fig. 5 iutoits envelope;

transfer, resulting in lower eiclency and shorter y i life than can be obtained by the methods herein Fig. 11 illustrates the completion of the closing of the envelope of Fig. 10;

Fig. 12 illustrates the insertion of the cake shown in Fig. 8 into the envelope; and

13 illustrates the complete closure oi' the 5 envelope on the cake of Fig. 8.

A strip of conductive material of comparatively high resistance, such as is frequently used ior the resistor element in heating devices, is shown at l. This flat strip is slitted, as shown at 2 and 10 3, by slits extending alternately from opposite edges. The sinuous ribbon of resistor material thus formed comprises many members extending transversely of the strip l.

, The next step in the production of the resistor l5 element consists in bending each of the transverse members." As shown at 5 and E, the bends may be twists in opposite senses in adjacent strips or, as shown at l in Fig. 2, adjacent twists may be all in the same sense. The twists at the opposite ends of each member must be in opposite senses in order that the edges of the original strip may remain flat.

A shallow member or plate lll, shown near the bottom of the press in Fig. 3 is composed of any 25 suitable material, preferably sheet metal, of a thickness suitable te constitute a part of the envelope or cover of the nished element. IThe plate preferably has its edges 11p-turned as illustrated at lil. The lip-turned portions are-of de- 3@ cideclly less width than the thickness of the hnished heater element. The plate iB is placed in 'the bottom o a trough-like arrangement upon the platen of a press and the trough is partially iilled with granular, pulverant, insulating mate- 35 rial it. Upon this material the resistor l laid and then more insulating material is placed in the trough and the plunger of the press is caused to descend thereon until the material is compressed to the thickness illustrated in Fig. 4. 4o During such compression some slight lateral flow ci' the material occurs whereby it is compacted against the side members of the trough. `lecause the upturned edges il of the plate Il! do not extend very far into the thickness of the 45 insulating material, the lateral flow of the material during compression is greater in that portion of the insulating material immediately in contact with the resistor element than in the portion in close contact with the plate I 0. To 50 make this diiference in the degree of lateral movement greater if possible, the edges of the working face l2 of the press are extended slightly toward the material being compressed, as shown at i3. This results in a rounded edge ot I6 the compressed material, as shown at i5, which is nearly symmetrical to the shape produced by the up-turned portion il of the plate I0.

'I'he lateral now of the insulating material during compression takes place along the i'aces of the transverse portions of the resistor element. This insures a greater intimacy o contact between the insulating material and the resistor element than would occur if, instead of slight up-turned edges il, a receptacle with side walls extending the full thickness of the compressed body had been used.

The up-turned edges ii not only add to the stiness of the plate i9, thus rendering the whole composite cake stiier but they also aiord a binding action betweenV the upper edges of each up-turned portion H, and the insulating material. The compressed material adheres thereto and thereby increases the mechanical stength of the cake as a whole and also adds to the reliability with which it is attached to the plate It. Ihe plate I0 is made by ordinary sheetmetal working methods and its edge, therefore, contains more or less irregularities which are sumcient to insure good bonding between the insulating material and the edge` The trough-like receptacle in the press comprises a bottom member i@ resting directly upon the platen of the press and side members il adjacent to each longitudinal edge of the member It. In the method for producing the compressed cake of Fig. 5, no insulating material comes into contact with the member ri6. In the method illustrated in Figs. 6 to 8,`the bottom le may be protected by a coating of oil, a layer of oiled paper or any other suitable means for preventing the contained material sticking to it.

This material, shown at 2t, is any insulating material available in a `granular form and capable of forming a coherent mass under pressure y or pressure and heating. In the practice of my invention I have found zircon embedding cement convenient for this purpose. This material, like most of the insulating materials used in the manufacture of heaters, is ground to dierent degrees of iineness and is largely powdery in texture so if extremely dry the fine particles have a tendency to ily. I incorporate with it a sufficient amount of moisture to prevent it from iiying andl at the same time enough to aid in promoting load under pressure and therefore convenient handling. The moisture needed for this purpose depends upon such factors as the presence of small additions of clay bond in the mixtures and upon the neness of the granular material. I have found, under some circumstances, as little as 5% of moisture sumcient and, under some circumstances, considerably more moisture can be used to advantage but not enough moisture must be used to make the mixture liquid v or unworkable.

, to place it upon the surface of the portion of the embedding cement already in the trough.

When the resistor is in place and covered by the insulating material the plunger I2 of the press is caused to descend thereon. Suiiicient pressure is exerted by the plunger to compress the material into a cake 22 of approximately onearoaeoi third of the thickness of the material when ilrst introduced into the press, as is shown in Fig. '1.

In this compressed state thecake 22, not having a plate like lil of Fig. 5, has not sufficient mechanical strength to stand ordinary handling` by unskilled workmen. The most convenient way ofhandling the cake in this condition is to support it upon a base 23. This base is preferably vof iron or steel and is so thick that it will support a long cake without sagging during handling. It is much stiiier, even when hot, than the iinished heater element. Even the cake I8, made by the inst-described process, although mechanically stronger than the cake 22, is more conveniently handled if supported, as shown at 23 in Fig. 5. The most convenient way to transfer the cake 22 from the bottom it of the trough to the support 23 is to slide the cake end-wise out of the trough onto a base which has been placed end to end with the trough. If desired, the side members il may be made removable and the bottom i6 may be tipped to cause the cake 2t to slide onto the base placed beside it. Or the member it and the cake 22 may have the base 23 placed on top of them and the whole combination rotated through to bring the cake 22 onto the base 2t.

The cake 22 on its base 23 is then put into an oven for baking. Thecake I6, with or without a base 23, is put at this stage into a similar oven and the process from this point on is simtlar for either form of cake. The oven is shown in Fig. 9 and is preferably equipped with ribs 25 upon which the base 23 rests. This affords a clearance between the base 2t and the bottom of the oven so that a fork or other tool for handling the cake and base may be readily inserted under the base 2t.

The cake is baked in the oven until it acquires sumcient mechanical strength to be handled without the support of the base. I do not regard my invention as limited to any particular degree of baking. The degree of baking should be suiricient to remove the moisture. I have found that this baking may be at desiccating temperature to as high as a red heat depending upon the structure and composition of the embedding material, followed by a suiiiciently slow and even cooling operation to avoid cracking.

When the baking is nished, the cake and the base 23 are removed from the oven and the cake is inserted in its envelope. This may be done by depositing the cake 22 upon the at member 30 of the envelope, lig.` l2, but the cake is now strong enough so that it may be lowered or even dropped into a trough-shaped member of the envelope if there is any reason for preferring this procedure.

In the preferable method of inserting the cake Mi into the envelope the portion 3i of the envelope Fig. l0, whichI has arlepth exceeding the thickness of the cake, is provided with oblique margins at the free edges of the side portions. as shown at 32. The cake I4 is inserted into the member di by sliding it in from one end. The edges Ii of the shallow member it glideupon the oblique margin portions 32 oragainst the side portions of the deep member 3i during such sliding motion. When the cake is in place within the envelope, the portions 32 are pressed against the external face' of y the member Ill, thereby completing the closure of the envelope, Fig. li. This method of assembling .the device avoids the necessity of a curling@ or crimping action for closing the envelope, a simple, direct thrust be 7'5' `the cake Il, this having been sufficiently compressed in the operation..illustrated in Fig. 4.

With the form of cake illustrated in Fig. 8, it is necessaryto provide a plate 30 because no part of the envelope is associated with the cake 22 during the preceding parts of the process. If the cake 22 is not vdeposited onV the strip 30 as valready described, the cake may be slid endwise into the member 3i and the plate 30 may be put in place by a Separate sliding action or it may be deposited on the cake and the two slid in together. When the cake 22 is in position in the envelope, the edges 32 are closed upon the plate 30 in the same way as described in connection with Figs. l and 11. This actionis illustrated in Figs. 12

a and 1 3.

Although after the cake has been baked it has suiiicient mechanical strength to be handled in other ways, the endwise sliding of -the cake into this envelope is preferred, because it subjects the cake to fewer mechanical shocks and strains and results in a smaller loss by breakage in the manu` facture of the heater elements.

Openings are "provided at the end of the casing for the projection of terminals which are formed integral with the ends of the resistor strip I. Since these terminals form no part of the present invention, theyare not herein illustrated or more specifically described.

When the device has thus been manufactured itis submitted to a test. During this test theu resistor I is heated by electrical current therethrough and heats the surrounding material. Any moisture which is left in the cake after the baking in the oven is evaporated during this test and escapes, principally through the joints between the margins 32 and the shallow member l0 or 30. Some moisture probably also escapes Thegood bond which I have thus secured between the members 30, 3| and the insulating-material is productive of a higher rate of heat transfer and thereby increased eiiciency of operation.

The transverse members of the sinuous resistor element are short,v extending only the width of the strip. During this expansion and contraction in the use of the heater, the relative movement between them and the resistor material is not sufficient to break the bond. Moreover, expansion and contraction of the device as a whole is most evident in-the direction of the length of the strip vand the resistor element is capable of very considerable stretching or contraction in the direction of its length without introducing strain. Any force tending torproduce relative motion between the resistor and surrounding material in the direction ofthe length of the strip 'is resisted by pressure ofl the faces of each transverse member flat-wise against the insulation. The faces of the transverse members are normal to the line of action such forces throughout almost the entire Vlength of each of said members. The bonding action ofthe'transverse members is effective not only in preventing thermal expansion from causing separation but also in preventing such separation of resistor and insulation during bending of the heater element.

A heater element made in this Way can be bent to t cylinders or curved surfaces without detrimental eiect on the insulating qualities of' this structure. Many variations in'the process herein described will occur to lth'ose skilled in the art and I do not desire to be restricted to the exact materials mentioned herein or the exact details of the several steps in the process.

. I claim as my invention:

1. The process of making an element for an at the ends of the element where the openings@ electric heater comprising a resistor element,

for the terminals are provided.'

' I have not found it necessary to compress the j casing, although no harm results if the action of folding the margins l2 incidentally exerts some pressure on the cake. Ordinarily mechanical skill usually affords a connection between thev element i and the radiating surface of the casing having suiiic'ient thermal conductivity.

When better thermal connection between the metallic envelope and the cake is particularly desired, I havefound it practical to treat the sur face of the cake with a slurry. Any inorganic Ielectric insulating material capable of forming a good thermal joint-between the cake and the envelope will serve as a slurry. I prefer an aqueous suspension of material similar to that used to form the cake. Again it is practical to` paint the cake with such a slurry, place it in an open casing', such as a trough, without the acy 1 then dried by heat supplied either specially for the purpose or during the testing of the element.

5compressible electric-insulating material therearound and an enclosing metallic envelope having a shallow member and a deep member of greater depth than the thickness of the finished element, which includes the steps of depositing a bed of compressible electric-insulating material upon said shallow member, placing a resistor element on the surface of said bed, depositing additional insulating material on said element and bed, compressing the deposited material into good bonding contact with the ,resistor element and with the shallow member, baking the compressed bonded body, inserting the baked body into said deep member .and clamping said memberstogether.

2. The 'process of making a heater element comprising a resistive conductor surrounded by insulating material and enclosed in a metallic envelope, which includes the steps of making an elongated shallow envelope member, making an elongated envelope member of a depth greater than the thickness of the insulating material in the finished article with inwardly inclined margins at the edges of its sides, making a cake comprising the resistive conductor surrounded by insulating material and bonded to the shallow member, then inserting AWsaidcake ,with the l shallow member endwise into said deep member and ilattening the margin against the shallow member.

EDWIN E. ARNOLD.

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