US3859500A

US3859500A - Heating element

Info

Publication number: US3859500A
Application number: US398185A
Authority: US
Inventors: Stanley J Matys
Original assignee: SQUARED R ELEMENT Co Inc
Current assignee: I SQUARED R ELEMENT COMPANY Inc; SQUARED R ELEMENT Co Inc
Priority date: 1973-09-17
Filing date: 1973-09-17
Publication date: 1975-01-07
Anticipated expiration: 1992-01-07

Abstract

A silicon carbide heating element having a spirally configurated hot zone section provided with a spiral slot for spacing adjacent windings away from each other. A ceramic insulating material, having substantially the same coefficient of thermal expansion as the material of the spirally configurated hot zone section, is deposited in the spiral slot for strengthening the heating element and for precluding the accumulation of contaminants in the slot between adjacent windings.

Description

United States Patent [1 1 Matys HEATING ELEMENT [75] Inventor: Stanley J. Matys, Depew, N.Y.

[73] Assignee: I Squared R Element Company,

Inc., Lancaster, N.Y.

[22] Filed: Sept. 17, 1973 [21] Appl. No.: 398,185

[52] U.S. Cl 219/553, 13/25, 317/98,

[56] References Cited UNITED STATES PATENTS 1,450,725 4/1923 Hodson 338/269 1,933,128 10/1933 Wiegand 338/269 X 2,556,679 6/1951 Czepek 338/296 X 3,248,679 4/1966 Ganci 338/269 X 3,336,431 8/l967 Biddulph. 13/25 3,360,760 12/1967 Johnson 338/296 Jjzg 22 l i Jan.7,1975

3,372,305 3/1968 Miculec 317/98 3,397,375 8/1968 Casper et al 338/330 3,467,812 9/1969 Terrell 317/98 X 3,518,351 6/1970 Ohnsorg et al. 13/25 3,688,007 8/1972 McKenna et a1. 13/20 FOREIGN PATENTS OR APPLICATIONS 310,066 12/1955 Switzerland 13/25 943,054 11/1963 Great Britain 13/25 Primary ExaminerV0l0dymyr Y. Mayewsky Attorney, Agent, or FirmChriste1 & Bean [57] ABSTRACT 7 Claims, 4 Drawing Figures 1 HEATING ELEMENT BACKGROUND OF THE INVENTION non-corrosive and are efficiently operable under ele- I vated temperature conditions, well above the operating limit of most'conventional metallic elements. One common form of a silicon carbide heating element comprises an elongated rod having a centrally located, spirally configurated portion, constituting the hot zone or heating section of the element and solid wall opposite end portions providing the cold end termination for the element, these cold ends being suitably connected to electrical connections. Location of the electrical terminals at the opposite ends of the heating element increases construction and wiring costs and, at least to some extent, limits the orientation of the heating element in the furnace.

In an effort to overcome these disadvantages, silicon carbide elements have been designed to provide both terminals at one end thereof by splitting such one end axially to form spaced legs connected to a double spirally configurated hot zone or heating section. While both such heating elements are admirably suited for theirintended purpose, they possess certain disadvantages. For example, the space between the spirals is susceptible to the ingress'of both electrically and nonelectrically conductive contaminants, which tend to accumulate and build up in the spaces provided by the slots. With electrically conductive contaminants, high temperature arcing can occur causing the element to fail. The build up of non-conductive contaminants in these slots also can cause failure of the element due to the differences in the coefficients of expansion of such contaminants and the silicon carbide material of which the element is formed. This is effective to generate stresses in the silicon carbide body upon temperature lation of contaminants in such slots to prolong the useful service life of the heating element.

In one aspect thereof, the silicon carbide heating element of this invention is characterized by the provision of a spirally configurated heating section having a spiral slot spacing adjacent windings of the spirals away from each other. A ceramic insulating material of a zircon base, having substantially the same coefficient of thermal expansion as the silicon carbide spirals, is deposited in the slot for strengthening the heating element and for eliminating the accumulation of contaminants in the slot.

The foregoing and other objects, advantages and characterizing features of the present invention will become clearly apparent from the ensuing detailed description of an illustrative embodiment thereof, taken together with the accompanying drawing wherein like reference characters denote like parts throughout the various views.

FIG. 1 is a side elevational view of one form of a heating element constructed in accordance with this invenvariations to cause fracture and premature failure of SUMMARY OF THE INVENTION Accordingly, a primary object of the present invention is to provide an improved silicon carbide heating element obviating the above disadvantages and having added strength offering versatility in design.

Another object of this invention is to provide the foregoing heating element with an insulating filler disposed in the slots between adjacent spiral windings for strengthening the element and eliminating the accumution, broken away to indicate an indeterminate length;

FIG. 2 is a cross sectional view, on an enlarged scale, taken about on line 2-2 of FIG. 1;

FIG. 3 is a side elevational view of another form of a heating element, incorporating the principles of this invention; and

FIG. 4 is a cross sectional view, on an enlarged scale, taken about on line 4-4 of FIG. 3.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT Referring now in detail to the illustrative embodiment depicted in the accompanying drawings, there is shown in FIG. 1 an electric resistance heating element, generally designated 10, constructed in accordance with this invention. Heating element 10 comprises an elongated, hollow, tubular resistance body 12 having a generally cylindrical outline and provided with a bore 14 (FIG. 2) extending axially therethrough.

Resistance body 12 is formed of a unitary, one-piece construction and is composed of a dense, self-bonded silicon carbide material, which is capable of withstanding elevated operating temperatures without disintegration or material deterioration, is non-corrosive and substantially inert to the deleterious effects of combustion products, and which changes in resistance very slightly, slowly and uniformly in use to provide stable heating over a long period of use.

Resistance body 12 includes a cold zone terminal section 16 comprising a pair of laterally

spaced legs

18 and 20, of generally semi-cylindrical, arcuately shaped cross sections, and which extend axially from one end 22 of body 12 inwardly to approximately midway of body 12, defining a pair of diametrically opposed, longitudinally extending slots 24 (only one of which is shown in FIG. 1). A coating of any suitable electrical conducting material 28, such as aluminum for example, can be sprayed or otherwise deposited on the outer surfaces of

legs

18 and 20 adjacent end 22 to provide electrical terminals for resistance body 12.

Resistance body 12 also includes a hot zone section 30 comprising a double helical spiral fonnation consisting of a pair of helices or

spirals

32 and 34 extending from the inner ends of

legs

18 and 20 toward the opposite end 36 of resistance body 12 and joined together at a common juncture 38 at such opposite end 36. The

double spirals

32 and 34 define a pair of parallel,

spiral slots

40 and 42 extending along hot zone section 30. These

slots

40 and 42 space adjacent windings or convolutions of the

spirals

32 and 34 away from each other. The

spaced legs

18 and 20, together with

spirals

32, 34 and juncture 38 form a continuous path for electric current via leg 18, spiral 32, juncture 38, spiral 34 and leg 20. Of course, the resistance of

spirals

32 and 34 is increased relative to

legs

18 and 20 because of the decreased cross sectional area of the former.

Various terminal assemblies can be affixed to the cold zone section 16 to provide the necessary electrical connections to conductors 44 and 46, in turn connected to a suitable source of electric power .(not shown). Since such terminal contact assemblies are well known and, per se, form no part of the present invention, no further amplification or illustration is believed necessary. If desired, insulated shims or spacers can be placed between

legs

18 and 20 to preclude shorting therebetween and maintain them in the properly spaced relation.

In accordance with the principles of this invention, the spaces defined by

spiral slots

40 and 42 between adjacent windings of

spirals

32 and 34 are filled with an impervious, insulating filler material 50 to eliminate the buildup of contaminants in such slots andto appreciably strengthen resistance body 12 to provide optimum design flexibility. It has been found that a zircon base, ceramic insulating material provided with a suitable binder was ideally suited as the filler material 50 because its coefficient of thermal expansion substantially approximates that of silicon carbide, thereby expanding and contracting to the same extent as the latter to avoid undesirable stresses in heating element 10. Also, the zircon base filler material 50 can withstand temperatures above 3,100F, well above the upper temperature limit for which heating element is designed, without decomposing or becoming electrically conductive. Moreoventhe zircon base filler material 50 will not plasticize nor drip, and remains stable in both reducing and oxidizing atmospheres over the entire temperature operating range of heating element 10, which is on the order of from about 1,000F to 3,100F.

Filler material 50 is deposited in the

spiral slots

40 and 42 in a generally heavy paste condition of high viscosity and allowed to dry under controlled temperature conditions. When dried, the filler material 50 is resistant to thermal shock and will not crack nor peel under the extreme conditions of intense radiant heat generated by the heating element 10 in use.

Serious design limitations, restricting the hot zone lengths and the spiral widths, were imposed on the prior known conventional heating elements of the spirally configurated hot zone type because of the inherent weakness in the hot zone induced by the spiral slots formed therein. For example, the length of the hot zone was dictated by the strength of the heating or hot zone section and had to be restrict within a predetermined linear dimension relative to the diameter of the heating element. Also, the widths of the spirals in the hot zone generally were restricted to the diameter of the heating element, thereby limiting the overall resistance of such known heating elements.

In contrast, it has been found that the zircon based filler material deposited in

spiral slots

40 and 42 of the heating element 10 of this invention more than doubles the strength thereof or strengthens such heating element 10 by at least a factor of percent. Such increased strength enables the length of hot zone section 30 to be materially increased well beyond the lengthdiameter ratios of known elements, thereby offering greater design flexibility. Also, because of the strengthening factor residing in the addition of filler material 50, smaller spiral widths can be designed to increase the overall resistance of the finished heating element 10. This also reduces costs of the auxiliary equipment by permitting higher design voltages with consequent reduced load currents.

. In addition to the design limitations imposed on the prior known heating elements by spiral slots formed therein, such slots formed passageways forthe ingress of contaminating vapors which could attack the inner surface of the elements and the opposed edges of the spirals defining such slots. Also, solid contaminants and other foreign particulates could build up in such exposed slots to reduce the useful life of the heating elements. For example, an accumulation of electrically conductive contaminants in the slots forms a conductive path thereacross which can generate high temperature arcs thereacross to cause premature failure of the heating element. A build up of non-conductive contaminants in the slots tends to generate stresses in the spiral elements because of the differences of the coefficients of expansion between the material of which the element is formed and the accumulated contaminants. These stresses can cause the heating element to fracture in the area of the hot zone to at least impair, if not destroy, the usefulness of the heating element. By filling

spiral slots

40 and 42 with the zircon base filler material 50 in accord with this invention, the problem of contaminant accumulation in such slots is completely avoided. Moreover, the surface area of the

spirals

32 and 34 exposed to contaminating or corrosive vapors is appreciably reduced, thereby prolonging the useful life of heating element 10.

Also, when heating air or other gaseous components, it often is desirable to pass the gas through a hollow heating element. Accordingly,

closing slots

40 and 42 in the manner described above in this invention forms a tubular conduit for the passage of gases entering one end and exiting at the other end to efficiently confine such gases within body 12 for precisely controlling the flow thereof in the proper direction to the desired destination without undesirable heat loss.

It should be understood that the principles of this invention are not restricted in use with heating elements of a double spiral configuration as shown in FIGS. 1 and 2, but has utility in other spirally configurated elements, such as that shown in FIGS. 3 and 4 for example. In this form of the invention, a heating element, generally designated 52, comprises an elongated, hollow, tubular resistance body 54 having a generally cylindrical outline and provided with a bore 56 (FIG. 4) extending axially therethrough. Resistance body 54 is composed of a very dense, self-bonded silicon carbide material similarly to the resistance body 12 of the heating element of FIG. 1. However, resistance body 54 differs from that first described by having a hot zone section 58 located centrally of body 54 and interposed between opposite cold

end terminal sections

60 and 62.

The outer ends of sections 60 an 62 are provided with a coating of a

suitable conducting material

64 and 66, such as aluminum for example, to provide electrical terminals for resistance body 54 and which are suitably connected to electrical terminal assemblies (not shown) in use, as is well known in the art.

The hot zone section 58 comprises a single helical spiral 68 joined at the opposite ends thereof to the inner ends of cold

end terminal sections

60 and 62 and defines a spiral slot 70 spacing adjacent windings from each other. The space defined by spiral slot 70 is completely filled with the impervious, insulating filler mate rial 50 in the same manner and for the same reasons advanced in connection with the description of heating element 10. Thus, it is apparent that the principles of this invention are applicable to variously shaped silicon carbide heating elements of the spirally configurated hot zone type or to any other heating elements having closely spaced high resistance legs or windings constituting the hot zone section.

From the foregoing, it is apparent that the objects of the present invention have been fully accomplished. By filling the spaces formed by the spiral slots between adjacent windings with a ceramic, zircon based insulating material, the strength of the heating element is materially increased to prolong the useful life of the element. In addition, this added strength offers more flexibility in design, enabling the hot zone section to be longer and the cross sectional areas of the spirals to be decreased for added resistance. Moreover, the insulating filler material closes the slots between adjacent spiral convolutions to preclude an accumulation of destructive contaminants therein and the passage of deleterious, contaminating vapors therethrough, further increasing the useful life of the heating element.

Illustrative embodiments of this invention having been disclosed in detail, it is to be understood that this has been done by way of illustration only.

I claim:

1. A heating element comprising: an elongated hollow body formed of a dense, self-bonded silicon carzone section having at least one continuous slot separating adjacent convolutions of said spiral from each other; and a zircon base ceramic insulating filler material permanently disposed in said slot between said spiral convolutions preventing accumulation of contaminants and for strengthening said element; said filler material having a coefficient of expansion substantially similar to the coefficient of expansion of said silicon carbide material, said filler material remaining in said slot and being capable of withstanding temperatures over the entire operating temperature range of said heating element from room temperature up to about 3,100F.

2. A heating element according to claim 1 wherein each of said terminal sections have a coating of electrical conducting material disposed on the outer surface thereof.

3. A heating element according to claim 1 wherein said hot zone section comprises a pair of parallel spirals extending generally longitudinally of said body and terminating at a common juncture at one end of said body, said hot zone section having a pair of continuous spiral slots separating adjacent convolutions of said spirals from each other; said slots being filled with said insulating filler material.

4. A heating element according to claim 3 wherein said terminal sections comprise a pair of laterally spaced, elongated members formed integral at the inner ends thereof with said spirals; said spirals having a lesser cross sectional area than the associated elongated members.

5. A heating element according to claim 4 including a coating of electrical conducting material deposited on the outer surfaces of said elongated members adjacent the ends thereof remote from said inner ends.

6. A heating element according to claim 1 wherein said elongated hollow body defines an empty region within the interior thereof, said filler material comprising the sole means for strengthening said heating element.

7. A heating element according to claim 1 wherein said elongated hollow body has an axial bore extending along the entire length thereof thereby forming a conduit for the flow of fluid therealong, said filler material comprising the sole means for strengthening said heating element.

Claims

1. A heating element comprising: an elongated hollow body formed of a dense, self-bonded silicon carbide material; said body having terminal sections and a hot zone section therebetween; each of said terminal sections being provided with electrical conducting material to provide electrical terminals for said body; said hot zone section being formed of at least one spiral extending generally longitudinally of said body; said hot zone section having at least one continuous slot separating adjacent convolutions of said spiral from each other; and a zircon base ceramic insulating filler material permanently disposed in said slot between said spiral convolutions preventing accumulation of contaminants and for strengthening said element; said filler material having a coefficient of expansion substantially similar to the coefficient of expansion of said silicon carbide material, said filler material remaining in said slot and being capable of withstanding temperatures over the entire operating temperature range of said heating element from room temperature up to about 3,100*F.