US 2820076 A
Description (OCR text may contain errors)
Jan. 14, 1958v A., w. LILLIENBr-:RG 2,820,076
ELECTRICAL HEATING ASSEMBLY Filed May 2l, 1956 74 f IW l wf' 2.3i f6j ,MVL |14( 24 /6N n l l N VEN TOR.
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'@Und SWCS Patent 0f ELECTRICAL HEATING ASSEMBLY August W. Lillienberg, Chicago, Ill., assignor to Lindberg 5 Claims. (Cl. 13-25) This invention relates to electric heating furnaces and more particularly to improvements in electric heating elements for such furnaces.
It is known in the piror art to utilize elongated resistance heating elements comprised of relatively thin strips or sheets of highly resistive conductive material for electric heating furnaces and the like. This particular type of heating element construction has been found advantageous in meeting the problems of high voltage current leakage and unbalanced heat distribution encountered in many prior art types of heating furnaces.
` -Conventionally, these thin strip heating elements are disposed in a desired fashion adjacent the walls of the heating furnace and the ends thereof are welded to terminals of greater comparative thickness which extend through furnace walls for connection to a bus bar or some other source of power. It has been customary to make these terminals of large cross-section in order to minimize the IR drop, and hence the heat loss, therein. However, due to the existing diilerences in the relative thickness of the thin strip heating elements and the terminals, and, further, due to the relatively large amount of current which passes through these components during the normal operation of the furnace, there is a tendency for expansion strains to be developed at the welded joints. Consequently, the possibility of cleavage at the junction of a heating element and its associated terminal due to these expansion strains has been a cause of concern in the prior art.
.Accordingly, it is an object of this invention to provide an improved heating element assembly for use with an electric heating furnace.
It is another object of this invention to provide means for substantially reducing expansion strains which arise at the point of junction of two metal conductors having diierent cross-sections.
It is still another object of this invention to provide improved means for equalizing the cross-sectional differences on opposite sides of a weld between metal elements of dissimilar thicknesses.
' It is a further object of this invention to provide an improved technique for the fabrication and manufacture of heating element assemblies for electric heating furnaces.
It is a still further object of this invention to provide an improved and relatively trouble-free connection between electrical conductive members of different crosssections.
fIt is a still further object of this invention to provide a 'novel link member for eliminating splitting at the point of junction of metal members of dissimilar crosssection due to expansion strains set up therein when such members are heated.
These and other objects are realized in specific illustrative embodiments of the invention wherein a link member having a unique preshaped form is disposed between an end of a relatively thin heating element and an end of a terminal connected to a bus bar or other source of power.
2,820,076 Patented Jan. 14, 1958 ice l In accordance with one feature of this invention the link member has a thickness intermediate that of the heating element and the terminal. It is a further feature that the link member has a plurality of segments cut away at the end adapted to be welded to the heating element so that the cross-sectional area of this end is approximately equal to that of the end of the heating element welded thereto. By equalizing the cross-sectional areas of the welded ends of the link member and the heating element, these two elements heat at substantially the same rate. Thus expansion strains at the junction of the two elements due to the passage of high currents therethrough, and consequently the tendency of the welded joint to split, are substantially reduced or eliminated.
The above and other features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of this invention, however, its advantages and specic objects obtained by its use, reference is had to the accompanying drawing and descriptive matter in which is shown and described an illustrative embodiment of the invention.
In the drawing:
Figure 1 is a partial section through a heat treating furnace showing the heating element construction embodying the invention;
Figure 2 is a greatly enlarged view of a heating element assembly of Figure l;
Figure 3 is an elevation of the heating element assembly of Figure 2.
It will be appreciated that the improved heating element assembly may be used with many various types of electric heating furnaces but for the purposes of illustration a preferred form will be described wherein the invention is utilized with electric heating furnaces of the type disclosed in a copending application of I. R. Duiy et al., Serial No. 569,549, filed March 5, 1956, which generally are of conventional construction except for the heating elements employed therein.
As shown in Figure l of the drawing, a furnace embodying the invention may comprise a body 10 formed of heat resistant material such as refractory brick work or ceramic material, which may be covered if desired. Nor-I mally, the material to be treated is fed into the furnace chamber which is heated by electric resistance elements mounted in the chamber and energized from an external transformer or the like, not shown.
Advantageously, the heating elements are in the form of thin corrugated sheets or strips 14 of a high resistance material which extend along the sides of the furnace and which are connected at their opposite ends to link members 16. The latter, in turn, are connected to terminals 15 extending through the furnace wall and connected through bus bars 20 to a power transformer or other source of power. A
It has been found advantageous in the prior art to make terminals 15 of relatively large cross-sectional area for the purpose of reducing the IR drop therethrough, and consequently, heat losses therefrom. However, due to the differences in the cross-sectional areas of terminal 15 and the thin sheet or strip heating element 14 welded directly thereto in prior construction, expansion strains were created at the point of junction of these two elements. This problem was aggravated by the relatively' large amount of current which passes through these components during the normal operation of the furnace Therefore, in prior art devices, there has been a tendency for splitting to take place at the welded joint and for heating element 14 to be separated thereby from terminal 15.
In accordance with an important aspect of this inven- 'tion a link member 16 is connected between heating element 14 and terminal 15. Link member 16 is of intermediate thickness and is constructed so as to equalize the difference in cross-sectional area between the heating element and the terminal, thereby to reduce the above-described destructive expansion strains normally created therebetween. As shown most clearly in Figure 3 of the drawing link member 16 comprises a substantially rectangular metal member having a plurality of triangular segments removed therefrom. The resultant terminal,
therefore, includes a plurality of fingers 22, 24, 26 and 28 each of which has its greater width at the end of the link member adjacent terminal and which taper in the direction of heating element 14 so as to have their smallest width at the end connected to the latter.
Although link member 16 may be joined to heating element 14 and to terminal 15 in any suitable manner, preferably these elements are welded to each other. Thus, the cross-sectional area of the heating element end of link member 16, that is, the total cross-sectional area of the narrower portions of lingers 22, 24, 26 and 28, has been made substantially equal to the cross-sectional area of the conductive end of heating element 14 welded thereto.
By this unique arrangement the cross-sectional areas of these two components substantially have been equalized and, consequently, their rates of expansion due to heating resulting from the passage of current therethrough also substantially have been equalized. This equalization in expansion rates results in a virtual elimination of any tendency of heating element 14 and link member 16 to become separated at their point of junction, thereby providing an eiiicient yet reliable means of heating element construction.
It will be appreciated that link member 16 may be formed in any shaped pattern as desired, the only limitation being that the cross-sectional area of the heating element end of the link member be substantially equal to the cross-sectional area of the heating element welded thereto. Thus link member 16 may take the form of tapered fingers, rectangular segments, triangular segments, etc., or even may be of irregular shape. It further will be appreciated that the link member and the terminal may be integrally formed with the end thereof connected to the heating element suitably cut away for reducing expansion strains therebetween.
It is realized that there will be times during the operation of the heating furnace before the temperatures of the heating element 14 and link member 16 become more or less equalized. Perhaps the most critical period at which this condition obtains is when electric energy first is supplied to the electric furnace and the heating element and the link member both are cold. Due to the physical differences in material and construction between these components heating element 14 heats up more rapidly than link member 16 with the result that expansion strains are created at the welded joint 30 therebetween. However, even under these severe conditions, splitting of the welded joint is avoided by the unique construction of the link member. Due to the relatively narrow Widths of the fingers 22, 24, 26 and 28 at the welded joint 30 and the provision of the open spaces between these lingers, expansion strains created during the initial heating interval are accommodated and taken up without any undue strain upon the welded joint. Thus, in accordance with the invention the open spaces between the fingers of the linking members provide a flexibility which enables varying degrees of expansion to take place between the heating element and the link member during the starting' interval without placing an undue burden on the welded joints 30.
While a particular embodiment of the invention has been described in detail for purposes of teaching the principles of the invention it will be understood that this embodiment is illustrative only and is` not to be taken as a definition of the scope of the invention, reference being had for this purpose to the appended claims.
-I claim as my invention:
1. An electric heating furnace comprising a hollow body defining a chamber to receive material to be heated, a heating element of a relatively thin strip of conductive material mounted in said chamber adjacent a Wall thereof, a link connected to an end of said heating element and further connected to a terminal extending out through the wall of the chamber for connection to a source of power', said link being formed of a relatively thick, low resistance conductive material for reducing losses therein and having portions cut away such that the edge of the link which is connected to the heating element has substantially the same cross-sectional area as the end of said heating element.
2. An electric heating furnace comprising a hollow body defining a chamber to receive material to be heated, a heating element of a relatively thin strip of conductive material mounted in said chamber adjacent a wall thereof, said heating element being corrugated throughout its length, and a link member connected to the end of said heating element and further connected to a terminal extending out through the wall of the chamber for connection to a source of power, said link member being formed of a relatively thick conductive material and having portions cut away such that the edge of the link member' which is connected to the heating element has substantially the same cross-sectional area as the end of said heating element.
3. An electric heating furnace comprising a hollow body defining a chamber to receive material to be heated, a heating element of a relatively thin strip of conductive material mounted in said chamber, a link member connected to an end of said heating element and a terminal connected to said link member and extending out through a wall of the chamber for connection to a source of power, said link member being formed of low resistance conductive material for reducing losses therein and comprising a plurality of fingers, each of said lingers having a greater width at the end of said link member connected to said terminal and a smaller width at the end of said link member connected to said heating element.
4. An electric heating furnace comprising a hollow` body defining a chamber to receive material to be heated, a heating element of a relatively thin strip of conductive material mounted in said chamber, a source of power outside of said chamber adapted for energizing said heat,- ing element, and linking means connected between said heating element and said source of power, said linking means having a thickness greater than that of said heating element and having portions cut away therefrom adjacent said heating eleemnt for equalizing the cross-sectional areas of heating element and said linking means at their point of junction.
5. In an electric heating furnace of the type having a chamber for receiving material to be heated and a heating element of relatively thin strip conductive material mounted in said chamber, a terminal for connecting an end of said heating element to a source of power, said terminal being formed of a relatively thick low loss conductive material and having portions adjacent the end of the heating element cut away such that both the edge of the terminal and the edge of the heating element connected thereto have substantially the same cross-sectional area.
References Cited in the file of this patent UNITED STATES PATENTS 1,432,442 Collins Oct. 17, 1922 2,305,934 Steber Dec. 22, 1942 2,640,861 Kremers June 2, 1953 2,728,061 Bliss et al Dec. 2.0, 1955 i