US2290901A - Electric heating element - Google Patents

Description

July 28, 1942. c. J. WEINHARD T ETAL 2,290,901

ELECTRIC HEATING ELEMENT Filed April 22, 1940 3 Sheets-Sheet l mm. W lllll' J/VL/E/VTOAS CARL J. hwmepnw Cccm h. GUNTHOPP,

l3) mm July 28, 1942. c. J. WEINHARDT ETAL gg ggg ELECTRIC HEATING ELEMENT 3 Sheets-Sheet 2 Filed April 22, 1940 ,JVVENTO/PS 64/91.. J h/ZEWVHA/PDT 1 6 CEcu. hf GU/VTHOPI? han/mm y 1942- c. J. WEINHARDT ET AL 2,290,901

ELECTR IC HEATING ELEMENT Filed April 22, 1940 3 Sheets-Sheet 3 Ina 47v 70/95 C424 J l'z a/wm/Pom CEC/L h. GU/VTHOPP, V mm Patented July 28, 1942 ELECTRIC HEATING ELEMENT Carl J. Weinhardt and Cecil H. Gunthorp, Indianapolis, Ind., assignors to Liberty Electric Company, Indianapolis, Ind., a corporation of Indiana Application April 22, 1940, Serial No. 330,935

3 Claims.

This invention relates to an electric heating element of the enclosed type wherein a resistance wire is embedded under a closed top heat radiating surface, and has for a primary object the formation of a unit that will give uniform heat distribution over its entire area to permit a high concentration of the heat within that area without endangering or reducing the life of the resistance wire, or causing warping or cracking of the enclosing cover.

The heat distribution above referred to applies particularly to the outer portion of the unit and the terminal regions from which heat is ordinarily radiated much faster than from other parts of the unit. A further primary object is to provide a greater degree of directly available heat from a given current input as compared to enclosed units heretofore employed. A still further important object of the invention is to pro vide a heating unit that will be extremely long lived and at the same time will lend itself to easy and low cost production.

These and many other objects, advantages and features, including the particularly new and useful structural combinations of details permitting achievement of those objects, will become apparent to those versed in the art in the following description of the invention as illustrated in the accompanying drawings, in which Fig. 1 is a top plan view of a heating unit embodying the invention;

Fig. 2, a side elevation;

Fig. 3, a bottom plan view of the unit top and resistance wire assembly before being filled;

Fig. 4, a side elevation of the unit in partial section and revolved ninety degrees from the position indicated in Fig. 2;

Fig. 5, a detail in section on the line -5 in Fig. 3;

Fig. 6, a further detail 6-6 in Fig. 5;

Fig. '7, a top plan view in partial section of a modified form of the invention;

Fig. 8, a central diametrical section of the modified form of unit; and

Fig. 9, a bottom plan view of that modified unit.

Like characters of reference indicate like parts throughout the several views in the drawings.

Referring first to that form of the invention as illustrated in Figs. 1-6 inclusive, a plate I0 is formed by any suitable means, such as casting, out of metal to have a relatively thin section through its top. An annular downturned rib or flange II extends integrally from the top of the in section on the line plate I0 from the underside thereof in a complete circle to be placed at or near the outer periphery of the top face of the plate.

A rib I2 leads inwardly from this outer rib II and around in spaced relation therefrom a short distance to divide into a pair of ribs I3 and It, one spaced from the other. The rib I4 continues around in a circle, the center of which is the center of the plate II], to join into a central circular rib I5, Fig. 3. The other rib I3 continues around concentrically of and between the ribs I I and I4 to terminate by the free end It in spaced relation from the end portion of the rib I4 which curves around to join that inner rib I5.

A single length of resistance wire I'I starts out from an outer terminal I8 and is laid around in the grooves formed between the spaced apart ribs II, I3 and I4 in the manner indicated in Fig. 3 to terminate by its other end at the terminal I9 which lies in the grooves between the ribs I3 and I4 adjacent their junction with the short rib I2.

Particular attention is directed to the wire I! in that that part of it which traverses the outer groove between the rib I I and the rib I3 is coiled to have the turns spaced closer one to another than are the turns within the next adjacent groove between the rib I3 and the rib M. Then there is a closer spacing of these turns in the grooves immediately surrounding the inner circular rib I5.

A third terminal 26 is placed to connect with the wire I! within the second groove removed outwardly from the circular rib I5 spaced around in that groove a short distance from the terminal end I6 from the rib I3, as indicated in Fig. 3.

Referring to Figs. 5 and 6, each of these terminals I8, I9 and 20 are identical and consists of a bolt 2| carried axially through a porcelain thimble 22 from the underside of the thimble. This thimble 22, in the form herein shown, has a bore 23 entering from its underside to receive the head of the bolt 2I therein and to form a cavity into which or through which the wire Il may enter or pass through slots formed between outwardly extending pairs of feet 24. These feet 24 project a short distance outwardly beyond the outer side of the thimble 22. The lower end of the thimble 22 is inserted between the opposing ribs in each case to rest against the underside face of the plate II] with the resistance wire ll connected to the respective bolts 2I, as above indicated. The thimble and bolt in each case extend well beyond the under edges of the ribs on the plate I 0.

As indicated in Figs. 4-6, the wire I! is completely embedded and surrounded to space it from contact with the plate I!) and the ribs thereon by a suitable electrical insulating material 25 in paste or mortar form. The exact material employed does not enter per se into the present invention but should be of that nature which will have a high heat transmission coefficient, such as alundum, magnesium oxide and zirconium oxides, in any event, to be capable of continual use at a high temperature and to be di-electric to such an extent that when the resistance wire is carried by the metal plate I0, the material will not conduct the electric current to form a circuit between the wire l1 and the plate It] and its ribs. The material is plastered, pressed, or poured over and into the grooves to have them completely filled and to be leveled off. The material thus plastered into the grooves is worked around the thimbles 22. A metal backing plate 26 having a diameter substantially equal to that of the circle of the rib II is applied against the underside of the plate I!) to be in intimate contact with the material 25 and the outer edges of the ribs ll, l3, l4 and I5, a sufficient amount of the material 25 being applied therebetween so as to prevent any air gaps at any place above the plate 28.

Then the plate l9 thus filled is placed in an oven and dried at a temperature sufficiently high in accordance with the material selected to bring the particles of that material into a fixed bonded condition adhering to the resistance wire to the surface of the plate Ill, its ribs and the backing plate 26. This material becomes quite hard, taking on a refractory nature and not only serves to insulate the resistance wire I! but also serves to fix in position the terminal thimbles 22. The unit thus formed becomes in effect an integral structure without any air spaces existing between the resistance wire and the plate It) and its depending ribs. In practice the material 25 will flow through the thimble slots to fill in the head cavity in each of the thimbles 22.

It is well to note that it is highly important to form not only the plate It with an extremely thin wall but also to form the various ribs ll, l3, I4 and I to have an equally thin cross section, the idea being to form the entire plate and ribs with uniform section throughout, the section to be as thin as casting methods will permit. In this connection a metal of principally gray iron serves admirably for making the plate It and its ribs. The space within the inner circular rib I5 is left free of the material and a boss 21 extends centrally downwardly therein from the underside of the plate 10. A cupped spacer plate 23 is brought up to bear by its outer periphery against the backing plate 26; and a terminal carrying reflector plate 29 is held against the plate 23, all by means of a screw which passes through the plates 29 and 28, and the plate 26, to screw-threadedly engage within the boss 21. As indicated in Fig. 4, the plate 28 provides an entrapped air space between it and the backing plate 26 and by means of a restricted downwardly projecting central portion spaces the plate 29 generally from the plate 28 to form thereby an additional reflecting surface on the plate 29.

A terminal block 3| is secured in position at a distance below the plate 29 by means of screws 32 passing therethrough and through spacing posts 33 to engage with the plate 29. Terminal bars 34 extend respectively from the terminals l8, l9 and 20 to terminal posts on the block 3|.

The plate 29 may carry any suitable unit positioning and retaining means, such as the springs 35 outturned therefrom, the exact method of mounting the unit not entering into the present invention. The terminals I8, l9 and 23 are sufficiently long to extend through holes provided in the plate 28 as indicated in Figs. 5 and 6 and also on down through the lower plate 29 so that the bars 34 may be connected to the termi nal bolts 2| from the underside of the plate 29.

By forming the plate l0 and its depending ribs out of extremely thin section metal and by reason of the spacing of the turns or coils of the resistance wire I! as above indicated, the peripheral portion of the plate l0 receives a greater degree of heat than does the intermediate portion of the plate. By thus varying the degree of heat supplied to different parts of the plate l9, the varying rate of heat radiation or dissipation from the plate is compensated for. For example, the outer part of the plate would normally radiate heat faster than would the central part of the plate. Moreover it is to be noted that the terminals l9 and 20 particularly are so positioned as to have the resistance wire I! pass in grooves on both sides thereof. The terminal l8 has the resistance wire carried in close proximity thereto from the inner side. Thus heat normally radiated from these terminals at a greater rate than from other parts of the plate It is compensated for by supplying more heat at those zones. The combined result is that the plate [9 maintains a very uniform temperature over its entire area and hence any tendency to crack or warp is avoided and the upper surface of the plate remains substantially in a plane throughout the entire life of the resistance wire H which is unusually long in comparison to units heretofore employed of different construction.

It is further to be seen that by the particular arrangement and spacing of the ribs on the plate In that the resistance wire I! may be quite easily put into position in a single length and yet different degrees of heat may be obtained by varying the current flow through the various circuit arrangements possible through the three terminals provided.

Referring now to the form of the invention as illustrated in Figs. 7-9, the modification therein shown is highly suitable for a lower cost unit although the same principle is employed as in the first form described. In this modified form,- a metal plate 40 having a flat or plane upper surface is provided with a downturned annular flange or rib 4! located adjacent to the peripheral edge of the plate 40. However, in this form, no further ribs are provided on the underside of the plate 40. Instead a porcelain plate 42 is formed to have an outer upwardly extending and encircling flange or rib 43; a central inner circular rib 44 and then a plurality of intervening and spaced apart ribs 45 formed and spaced in the nature of the ribs I3 and [4 of the first described form of the invention. These intervening ribs 45 terminate in an upper plane that has a lower elevation than that which includes the top edges of the ribs 43 and 44. The resistance wire I1 is placed within the grooves between the ribs on the porcelain plate 42and then these grooves are filled with the material 23. A backing plate 46 is formed to have an annular, outer upturned flange 41 of a diameter which will permit it to engage snugly within the rib 4| and the external diameter of the flange 43 is made to be such that it will engage snugly within this flange 41.

The plate 40 is positioned over the filled porcelain plate 42 and its under or backing plate 46 to have the plate flange 4| telescope thereover. The members are carried together with sufficient material 26 having been placed therebetween as will permit the upper edges of the ribs 4| and 44 to come into substantial abutment with the underside of the plate 40 and to have the material 26 completely fill all of the space therebetween so as to exclude the air and prevent formation of air pockets. The members thus assembled are mechanically secured in that relation by means of a central screw 43 which extends upwardly through the plate 46 and screwthreadedly engages within the boss 49 that extends downwardly within the open space left within the inner circular rib 44. The unit thus assembled is placed in an oven and heated to a sufficiently high temperature as will dry out the material 26 and effect a complete bond between the porcelain member 42 and the enclosin plate 40.

The porcelain member 42 thus in effect becomes an integral part of the top metal plate 40.

In the particular form of the unit shown in Figs. 7-9, this unit has additional resistance wire as compared to the form shown in Fig. 3 to produce a greater amount of heat. Advantage is taken of this additional wire by placing the outer terminal 18 back in the second groove from the periphery of the unit so that the resistance wire may pass on both sides of the terminal.

In the invention as described and shown in both of the forms, it is to be noted that there are no air passages through the unit nor are any such passages required in View of the unique design and the resistance wire may be continuously operated at high temperatures at the unit by reason of the fact that the refractory material employed is not only dielectric but is a very efficient heat conductor and therefore conducts heat away from the wire to prevent an over-heated condition.

In reference to the filling of the grooves in either the metal top plate or the porcelain under plate, this material is originally in a dampened state so as to permit plastering it as a plastic mass about the wires and in the grooves and then the moisture is driven off when the plate is fired.

While the invention has herein been shown in the best form as is now known, it is obvious that structural changes may be employed without departing from the spirit of the invention and it is, therefore, not intended that the invention be limited to that precise form beyond the limitations as may be imposed by the following claims.

We claim:

1. An electric heating unit comprising a flat,

closed metal plate, a refractory member under and intimately bonded to the plate, thin rectangular metallic ribs extending from said plate in laterally spaced apart relation into said refractory member with intimate bonding therewith, a backing plate applied under the first plate in intimate contact with the refractory member and the lower edges of the ribs and bonded with the refractory member and the ribs,

and electric resistance heating means dispersed within said refractory member between said ribs, porcelain posts depending from said refractory member, terminal posts connecting with said heating means and extending through said porcelain posts, a supporting reflector plate under said refractory member, and connecting means extending from said metal plate to said reflector plate.

2. An electric heating unit comprising a top flat, closed metal heat transfer plate having a planar under surface, a bottom metal plate, an electric resistance wire coiled around between said plates, metal ribs between the plates generally defining the position of said wire,-and a refractory filling between said plates and surrounding said wire to space and insulate the wire from said plates and said ribs, to completely fill in all the space between and around said ribs and between said plates, said filling further being intimately bonded to both top and bottom plates and to said ribs, whereby said unit is in effect an integral piece without openings therein or therethrough and free to expand and contract without warpage, said ribs being substantially rectangularly in cross-section and integral extensions of said top plate and bonded at their lower edges to the bottom metal plate, said top plate and said ribs being formed of metal as thin as casting operations permit.

3. An electric heating unit comprising a top fiat, closed metal heat transfer plate having a planar under surface, a bottom metal plate, an electric resistance wire coiled around between said plates, metal ribs between the plates generally defining the position of said Wire, and a refractory filling between said plates and surrounding said wire to space and insulate the wire from said plates and said ribs, to completely fill in all the space between and around said ribs and between said plates, said filling further being intimately bonded to both top and bottom plates and to said ribs, whereby said unit is in effect an integral piece without openings therein or therethrough and free to expand and contract without warpage, said ribs being substantially rectangularly in cross-section and integral extensions of said top plate and bonded at their lower edges to the bottom metal plate, said top plate and said ribs being formed of metal as thin as casting operations permit, the thickness of said top plate and said ribs all being equal and uniform.

CARL J. WEINHARDT. CECIL H. GUNTHORP.

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