US3518351A - Heating element - Google Patents

Description

R. W. OHNSORG ET AL HEATING ELEMENT Filed Dec. 16, 1968 June 30, 1970 INVENTORS; ROGER W. OHNSORG RICHARD L. CASPER B wW W M H WNWUMNNHNNH ATTORNEY United States Patent US. Cl. 13-25 14 Claims ABSTRACT OF THE DISCLOSURE A heating element especially useful for connection to a source of three phase electric power. The element comprises three parallel hollow conductors mutually connected at one end by a connecting member having a resistance less than that of any one conductor. The connecting member is preferably of a material of low resistivity and preferably comprises a base portion having cylindrical projections that are individually received in the respective conductors to form a rigid, electrically conductive junction. Such heating elements are especially useful in the production of float glass wherein they are maintained at temperatures between 1000 and 1500 C. for long periods of time.

BACKGROUNDOF THE INVENTION Electrical heating elements such as those of silicon carbide which are capable of producing and withstanding temperatures in the range of 1000 to 1500 C. are wellknown in the art as described, for example, in Swiss Pat. 310,066 (1955), British Pat. 845,496 (1960) and British Pat. 1,049,082 (1966). These heating elements are used in a variety of applications but are especially useful in the production of float glass wherein they are inserted into a furnace through a hole in the furnace wall. These prior heating elements generally comprise three elongated conductors each having a low resistance segment and a high resistance segment wherein the high resistance segments are connected to one another by a connecting member in a Y. The connecting member is generally of a material having the same resistivity as the high resistance segments of the elongated conductors. The elongated conductors are generally in the form of hollow tubes in order to provide a high surface area for heat radiation while providing a relatively small cross section of material transverse to current flow to provide for electrical resistance heating. These elongated conductors are generally inserted in holes in the connecting member. When the elongated conductors are in the form of tubes, plugs are inserted in the ends of the conductors in order to eliminate oxidation of the tubes caused by hot gases passing therethrough, i.e. the so-called chimney effect which occurs when the tubes are in a vertical position.

Although such heating elements have found wide acceptance they suffer from a number of disadvantages. First the plugs are prone to fall out damaging the glass underneath and permitting the above-described undesirable chimney etfect to oxidize and degrade the heating element. Differences in resistance between the legs of the Y also frequently cause undesirably uneven heating. Since the elongated conductors are inserted in the connecting member the transverse dimension of the connecting member must be greater than that of the elongated conductors requiring a larger hole in the furnace wall.

SUMMARY OF THE INVENTION Accordingly it is an object of the present invention to provide novel heating elements substantially free of one or more of the disadvantages of prior elements.

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Another object is to provide novel heating elements which do not require the use of plugs in order to eliminate the chimney effect.

A further object is to provide novel heating elements having substantially equal resistances in each leg of the Y, giving even heating, and therefore being resistant to localized overheating.

A still further object is to provide novel heating elements having a transverse dimension no greater than the distance between extreme elongated conductors.

Yet another object is to provide novel heating elements of a compact structure which can be inserted into a furnace through a small opening in the furnace wall.

Still another object is to provide a novel connecting member useful in single and three phase heating elements such as those of silicon carbide.

The above and other objects are accomplished according to the present invention by providing a heating element having a plurality of conductors connected to a common connecting member of a material of lower resistivity than that of the conductor. According to another aspect of the present invention a connecting member is provided which has a number of projections designed to fit inside each conductor to fixedly attach the conductor to the connecting member.

DESCRIPTION OF THE DRAWINGS Several preferred embodiments of the invention are illustrated in the accompanying drawings wherein:

FIG. 1 is a side elevational view partially in cross section of a heating element according to this invention;

FIG. 2 is a side elevational view, partially in cross section of a modified form of the heating element according to this invention;

FIG. 3 is a perspective View of the connecting member of FIG. 1;

FIG. 4 is a cross-sectional view of the heating element along line 44 of FIG. 1;

FIG. 5 is a cross-sectional view along the line 5-5 of FIG. 1;

FIG. 6 is a cross-sectional view along the line 6-6 of FIG. 2;

FIG. 7 is a perspective view of a connector of the present invention designed to be used with a single phase heating element; and

FIG. 8 is a schematic diagram of the resistances of the various portions of the heating elements of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and in particular to FIGS. 1, 4 and 5 there is shown a heating element 10 of the present invention including three elongated tubular parallel conductors 11, 12 and 13 joined together at one end by a connecting member 14.

The conductor 11 has a low resistance segment 15 and a high resistance segment 16. In the end of the low resistance segment 15 is a transverse hole which receives a pin 17 which is employed in a known manner to keep the heating element 10 from slipping into the furnace. The end of the conductor 11 is provided with an aluminum coating 18 which constitutes means for attaching this end to a source of power not shown. The low resistance segment 15 is attached to the high resistance segment 16 by any convenient means such as welding or by use of the cementitious composition described in US. Pat. 2,907,- 972 and especially Example 2 thereof. The low resistance segment 15 can be of any suitable material which will withstand the temperatures to which the heating element is raised but is preferably a carbonaceous material which has been impregnated with silicon metal in order to reduce its total overall resistance and is preferably constructed of a material having a resistivity of 0.0005 to 0.010 and preferably 0.002 to 0.003 ohm-centimeters at room temperature. One such material found suitable for the purpose is that made according to U.S. Pat. No. 2,431,326. The tubular low resistance segment and the tubular high resistance segment 16 generally have equal outer diameters but the inside diameter of the low resistance segment is preferably less than that of the high resistance segment. The high resistance segment 16 can be any suitable material which will withstand the temperatures to which it is raised but is generally large grain silicon carbide having a resistivity of 0.001 to 10.0 and preferably 0.09 to 1.10 ohm-centimeters measured at the operating temperature and is preferably a silicon carbide body produced as described in U.S. Pat. No. 2,941,962. The conductors 12 and 13 are identical to conductor 11. The connecting member 14 fabricated in one piece is provided with a cylindrical projection 19 having an outside diameter substantially equal to the inside diameter of the high resistance segment 16 of the conductor 11. The cylindrical projection 19 is received in the conductor 11 to form a rigid, electrically conductive junction.

Referring now to FIG. 2 there is shown another embodiment of the present invention in the form of a heating element 20 comprising three conductors 21, 22 and 23 connected on one end to connecting member 24 which is identical to connecting member 14. The conductor 21 comprises a low resistance segment 25 and a high resistance segment 26 formed by cutting one parallel spiral slit 27 through the wall of the segment 26. The conductors 22 and 23 are identical to the conductor 21 and are connected to the connecting member 24 in the same manner as in the heating element 10. FIG. 2 illustrates the manner in which the heating elements 10 and 20 are inserted through a hole in the wall of the furnace. The conductors 21, 22 and 23 fit through holes in a frustoconical plug 29 which is seated in a correspondingly shaped hole in the upper wall of the furnace. When inserted in the hole the plug 29 occupies the illustrated position under its own weight and that of the heating element 20. The pin 17 which may be a refractory metal or ceramic material keeps the element 20 from slipping into the furnace.

As shown in FIG. 3 the connecting member 14 comprises a base portion 30 having attached to the upper surface thereof three cylindrical projections 19, 31 and 32 the axes of each of which are perpendicular to the upper surface 33 of the base member 30 and are parallel to one another. As is apparent the projections 19, 31 and 32 serve to close the ends of the conductors 11, 12 and 13 and therefore eliminate the chimney effect. Furthermore should one of the projections come loose the connecting member 14 will still be fixedly held by the other two projections and cannot fall into the furnace as can the plugs used in prior elements. The connecting member 14 is made of material of high density and low resistivity and preferably one having a density of 3.00 to 3.10 grams per cubic centimeter and a resistivity of 0.001 to 1.0, and preferably 001 to 0.10 ohm-centimeter at room temperature. The connecting member 14 is most preferably silicon carbide produced as described in U.S. Pats. Nos. 2,907, 972 and 2,938,807. The area, A, measured transverse to the direction of flow of current in the base portion 30 is greater and preferably at least three times greater than the area measured transverse to current flow of the segments 16 or 26. The length, l, of the connecting member 14 is preferably no greater than the maximum distance between extreme portions of the conductors 11 and 13. By means of this construction the heating elements 10 or 20 can be inserted through a hole in the furnace wall which is no bigger than the maximum distance between extreme portions of the conductors 11 and 13. Furthermore, in existing furnaces designed to accommodate heating elements employing the larger prior connecting members, heating elements of the present invention can be employed which heat radiating surfaces.

FIG. 7 shows a connecting member 34 similar in all respects to the connecting member 14 except having only two projections 35 and 36 making it suitable for use in a single phase system.

The particular advantages of the materials and structures of the heating elements of the present invention are illustrated schematically in FIG. 8 wherein R represents the resistance in ohms of the low resistance segment 15 of the conductor 11; R represents the resistance in ohms of the high resistance segment 16 of the conductor 11; R and R represent corresponding segments of the conductor 12; and R and R represent corresponding segments of the conductor 13; R represents the resistance in ohms of that portion of the connecting member 14 lying between the conductors 11 and 12; and R represents the resistance in ohms of that portion of the connecting member 14 lying between the conductors 12 and 13. In general R is smaller than R in order to confine the heat producing portion of the conductor 11 to that portion furthest from thefurnace wall. Since the conductors 11, 12 and 13 are manufactured by mass production techniques the resistances R R and R are substantially equal to one another as are the resistances R R and R In prior heating elements the connecting member was typically made of the same material as the conductors, the resistance of the first leg being equal to the sum of the resistances R R and R", that of the second leg being equal to R plus R and that of the third leg being equal to the sum of R R and R However in the present invention because of the large area A and the use of silicon carbide of low resistivity, R and R are substantially equal to zero resulting in an elegen having substantially equal resistances in all legs of Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described above and as defined in the appended claims.

We claim:

1. A three phase Y heating element for an electric furnace having three conductors at least a portion of each of which is silicon carbide, one end of each conductor being connected to a common connecting member of silicon carbide of lower resistivity than that of the conductors.

2. A three phase Y heating element of claim I having three conductors at least a portion of each of which is silicon carbide, each conductor having a low resistance segment and a high resistance segment connected to a common connecting member of silicon carbide material of lower resistivity than the material of the conductor and wherein the total resistance in ohms of that portion of the connecting member in series circuit with adjacent conductors is less than the resistance in ohms of the high resistance segment of each conductor.

3. A three phase heating element of claim 1 comprismg:

(a) three heating conductors each conductor comprising:

(1) a low resistance segment, and (2) a high resistance segment;

(b) a common connecting member having three projections adapted to fit inside each of the conductors to fixedly attach the conductors to the connecting member.

4. The heating element of claim 3 wherein the low resistance segment is constructed of a carbonaceous material impregnated with silicon.

5. The heating element of claim 3 wherein the high resistance segment is constructed of silicon carbide.

6. The heating element of claim 5 wherein the silicon carbide has a resistivity of 0.001 to 10.0 ohm-centimeters.

7. The heating element of claim 3 wherein the high resistance segment comprises a spiral portion.

8. The heating element of claim 4 wherein the connecting member is constructed of a material having a resistivity of 0.001 to 1.0 ohm-centimeter at room temperature.

9. The heating element of claim 4 wherein the connecting member is constructed of silicon carbide having a resistivity of 0.001 to 1.0 ohm-centimeter at room temperature.

10. A three phase heating element of claim 1 comprising:

(a) three elongated cylindrical conductors each conductor comprising:

(1) a first tube having means at one end to connect it to a source of power and connected at the other end to:

(2) a second tube whose outside diameter is substantially equal to the outside diameter of the first tube and whose inside diameter is substantially greater than the inside diameter of the first tube;

(b) a connecting member having:

(1) a base member whose area transverse to flow of current is greater than the area transverse to flow of current in the second tube,

(2) three cylindrical projections the outside diameter of each being substantially equal to the inside diameter of the second tube;

the axes of the cylindrical projections being substantially parallel to one another and substantially perpendicular to the upper surface of the base member; the length of the base member being no greater than the outside extreme distance between the first and third conductors, wherein the three cylindrical projections of the connecting member fixedly hold the conductors substantially parallel to one another.

11. A three phase heating element of claim 1 comprising:

(a) three elongated cylindrical conductors each conductor comprising:

(1) a first segment having means at one end to connect it to a source of power and (2) a second segment having spiral slits defining spiral portions of the tube,

(b) a connecting member having:

(1) a base member whose area transverse to flow of current is greater than the area transverse to flow of current in the conductors,

(2) three cylindrical projections the outside diameter of each being substantially equal to the inside diameter of the conductors,

the axes of the cylindrical projections being substantially parallel to one another and substantially perpendicular to the upper surface of the base member; the length of the base member being no greater than the outside extreme distance between the first and third conductors, wherein the three cylindrical projections of the connecting member fixedly hold the conductors substantially parallel to one another.

12. A connecting member which can be used in combination with three conductors to form a three phase heating element, said connecting member comprising a base member and three projections fixedly attached to the base member, the projections being adapted to fit inside each of the conductors to fixedly attach the conductors to the connecting member.

13. A connecting member which can be used in combination with three conductors to form a three phase heating element, the connecting member comprising a base member and three projections fixedly attached to the base member, the axes of the cylindrical projections being substantially parallel to one another.

14. A connecting member of claim 12 which can be used in combination with three conductors to form a three phase heating element, said connecting member comprisinga base member and three cylindrical projections fixedly attached to the base member, the axes of the cylindrical projections being substantially parallel to one another and being substantially perpendicular to the upper surface of the base member.

References Cited UNITED STATES PATENTS 2,268,691 1/1942 Brooke 13-20 2,725,032 11/1955 Mann 13--23 X 3,345,597 10/1967 Schrewelius 13-25 X FOREIGN PATENTS 594,530 6/1959 Italy.

H. B. GILSON, Primary Examiner US. Cl. X.R. 13-20

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