US2556679A

US2556679A - Heating element

Info

Publication number: US2556679A
Application number: US106880A
Authority: US
Inventors: Czepek Rudolf
Original assignee: Kanthal AB
Current assignee: Kanthal AB
Priority date: 1948-06-22
Filing date: 1949-07-26
Publication date: 1951-06-12
Anticipated expiration: 1968-06-12

Description

June 12, 1951 R. CZEPEK 2,556,679

HEATING ELEMENT Filed July 26, 1949 T 3 32225.10. Fgl]. Eff gi a if $5 fiuci ogj gape/ 2 Patented June 12, 1951 HEATING ELEMENT Rudolf Czepek, Hallstahammar, Sweden, assignor to Aktiebolaget Kanthal, Hallstahammar, Sweden, a corporation of Sweden Application July 26, 1949, Serial No. 106,880 In Sweden June 22, 1948 5 Claims.

"The present invention refers to a self-supporting electric heating element which can be used in. electric ovens and which is adapted to radiate freely into the oven chamber; such element being surrounded by loosely fitted insulating pieces and by an outer envelope in such a manner that neither electrical short-circuits between the individual turns or sections of the element nor detrimental mechanical stresses in the heating element or the envelope can arise.

Heating elements in the shape of helical coils or rings radiating freely into the oven chamber are already known. Generally, they are externally surrounded by a ceramic imbedding compound or solid ceramic form bodies which may be either smooth or provided with guide grooves. Since heating elements of all kinds, particularly those. for high temperatures, always expand and contract to a larger or lesser extent due to the heating and cooling involved and since their coefficients of expansion always are greater than those of the-imbedding compounds and form bodies, detrimental stresses often occur either in the heatingelement or in the surrounding form bodies.

Constructions are also known in which the form body surrounding the element consists of several parts and is internally provided with grooves keeping apart the individual turns or sections of the element winding. These known insulating bodies possess, it is true, the advantage that they are somewhat simpler to manufacture than a one-piece, internally profiled tube. But in principle and in operation they do not difier from the insulating tubes and form bodies above referred to. They therefore display the same drawbacks.

According to the invention these drawbacks are overcome by locating the heating element in a ceramic or metallic envelope having an internal dimension which will accommodate the heating element and its insulating spacers when such element is in its maximum expanded condition; such spacers being uniformly and loosely distributed about the periphery of the element for spacing the individual turns or sections of such heating element. Said insulating spacers are preferably made in T-shape or similar form and are inserted about the circumference of the heating element in such a manner that the longer intermediate portion of the spacer is between the turns or sections and the two shorter side portions of the spacer engage the external surface of the heating element. It is of importance that the length of the intermediate portion be greater than the space between the heating element and the envelope. In this manner, the insulating spacers being loosely inserted between the sections or convolutions of the heating element, are prevented from falling away or being displaced during the transportation or tipping of the oven. The space between the heating element and the envelope may be either empty or filled with a displaceable insulating material, for example a ceramic insulating powder. The heads of the T-shaped spacers prevent such insulating powder from coming down between the turns or sections of the winding into the oven chamber and at the same time the turns are kept uniformly spaced. Thus, the heating element in its entirety is heat insulated in the external metallic or ceramic envelope. Likewise it is possible. to put a layer of a ceramic imbedding composition over the heating element and insulating spacers. As the unavoidable expansion of the heating element occurs, the ceramic imbedding composition will develop ruptures or fractures of a greater or lesser magnitude. In the course of time, portions of such cover would break loose and tend to fall down between the turns or sections into the oven chamber; such falling is prevented by the arrangement of insulating spacers according to the invention.

The insulating spacers according to the invention furthermore possess the following properties by which advantages are gained not only from a constructional but also from a maintenance point of view. The same kind of insulating spacer may be used for all types and sizes of self-supporting heating elements. The diameter of the heating wire or the width of the heating strip respectively does not matter in that respect. In the embodiments hitherto known, on the other hand, it always was necessary to make special form bodies for wires or strips of different dimensions since the groove of the form body had to be exactly adapted to the strip dimension and to the spacing between the convolutions.

Furthermore, the spacer according to the in vention is not limited to a heating coil of predetermined diameter but has a fiat or straight head and is so short that all forms of elements used in practice regardless of diameter and curvature may be insulated with the same type of spacer.

Thus, the spacer according to the invention not only provides the technical advantages above referred to in respect of stress-free expansion of the heating element but also permits economi- 3 cal and simpler construction and maintenance as compared with prior art structures.

Some embodiments of the arrangement according to the invention are shown in the accompanying drawing. Figs. 1-6 show the heating element in longitudinal and cross section in different embodiments. Figs. 7 shows in section a portion of a heating element of round or differently profiled stock. Figs. 8-12 show various forms of the insulating spacers.

In Figures 1 and 2 the heating element 3 is formed as a circular coil of a metallic resistance strip. The space between the individual winding turns or sections is filled with the loosely fitting, T-shaped insulating spacers 2. The external envelope is designated by l and is illustrated as a ceramic tube having an internal diameter corresponding to the maximum expansion of the heating elements together with its insulating spacers. The envelope may also be made of metal if desired. It is of importance with this arrangement that the dimension n is somewhat larger than the dimension ml+m2. 4 denotes a displaceable filling of insulating material.

In Figures 3 and 4 the helical or ring shaped heating element has a rectangular cross section.

In Figures '5 and 6 the heating element is made in the formof a meandering or tortuous winding wherein the turns or sections do not extend through a full circle but reverse upon themselves.

I claim:

1. A self-supporting, freely radiating electric heating element comprising a convoluted resistance element having sections spaced from each other, an envelope spaced from and surrounding said element, and a plurality of juxtaposed spacers interposed between such spaced sections, such spacers each having a thin portion which is of a thickness throughout its length less than the spacing between sections and which freely extends into the space between adjacent sections toward the longitudinal center of the convoluted resistance element, and having at one end of said thin portion a relatively wide portion which engages the external circumference of the two adjacent sections and which is of a less extent longitudinally of the heating element than the pitch of the convolutions.

2. A heating element according to claim 1 wherein the spacers are imbedded in a ceramic composition which is disposed in the space between the envelope and the convolutions and spacers respectively and which allows the heating element to expand.

3. A self-supporting, freely radiating electric heating element comprising a convoluted resistance element having sections spaced from each other, an envelope spaced from and surrounding said element, and a plurality of juxtaposed T-shaped spacers interposed between said spaced sections, each of such T-shaped spacers having a leg which is of a thickness throughout its length less than the spacing between sections and which freely extends into the space between adjacent sections toward the longitudinal center of the resistance element, and having a relatively wide head which engages the external circumference of the two adjacent sections and which is of a less extent longitudinally of the resistance element than the pitch of the convolutions.

4. A heating element according to claim 3 wherein the length of the leg of the T-shaped spacer is greater than twice the distance between the interior surface of the envelope and the top surface of the head of the spacer when said resistance element is centrally positioned within said envelope.

5. A heating element according to claim 4 wherein the space between the envelope and the 'convolutions and spacers respectively is at least.

partially filled with a displaceable insulating material.

RUDOLF CZEPEK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,493,857 Greenwalt May 13, 1924 2,217,296 Shaw Oct. 8, 1940