US 3859506 A
An electrical resistance heating element having a constant wattage per unit length comprises a conductor rod enclosed by a braided tubular insulator which in turn is enclosed by a braided tubular conductor. Strands of yarn having a high electrical conductivity and a high electrical resistance are interwoven between the inner and outer surfaces of the tubular insulator to form a plurality of parallel resistors between the conductor rod and tube.
Description (OCR text may contain errors)
United States Patent [1 1 Weckstein 1 CONSTANT WATTAGE HEATING ELEMENT  Inventor: Raymond A. Weckstein,
 Assignee: Sola Basic Industries, Inc.,
 Filed: June 15, 1973  Appl. No.: 370,414
 US. Cl 219/552, 174/110, 219/529,
219/545, 219/549, 338/214, 338/224  Int. Cl. 1105b 3/10  Field of Search 219/528, 529, 545, 549,
 References Cited UNlTED STATES PATENTS 2,264,285 12/1941 Bennett 338/214 X 2,322,773 6/1943 Peters 338/224 X Challemer 338/214 Buchanan et a1. 338/214 X 1 Jan. 7, 1975 2,905,919 9/1959 Lorch et a1. 338/224 3,309,643 3/1967 Ferretti et a1, 338/330 3,349,225 10/1967 Dubois 219/545 3,398,233 8/1968 De Lizasoain et a1. 174/110 3,454,746 7/1969 Dubois 219/549 3,501,619 3/1970 Buiting-et a1. 219/553 X 3,757,086 9/1973 lndoe 219/528 Primary Examiner-Volodymyr Y. Mayewsky Attorney, Agent, or FirmSmythe & Moore 1 1 ABSTRACT An electrical resistance heating element having a constant wattage per unit length comprises a conductor rod enclosed by a braided tubular insulator which in turn is enclosed by a braided tubular conductor. Strands of yarn having a high electrical conductivity and a high electrical resistance are interwoven between the inner and outer surfaces of the tubular insulator to form a plurality of parallel resistors between the conductor rod and tube.
3 Claims, 3 Drawing Figures Patented Jan. 7 1975 -CONDUCT|NG YARN 0N OUTER DIAMETER 0F BRAID F-IG.3
heater cables generally comprise a pair of conductors,
separated from each other by a semi-electrically conducting material or a material which is electrically conductive but has a high electrical resistance. The conventional heater cable constructed in the foregoing manner constitutes a series-connected resistance device with a total electrical resistance of the cable depending upon its length. Such heater cables have the disadvantage that they must be cut to a predetermined length depending upon their proposed use, and this length cannot be varied since to do so would vary the electrical resistance characteristics.
It has been proposed to construct an electrical heater cable having a constant wattage which would produce a given watt density or watts per unit length for a given voltage independent of the length of the cable. While several forms of such electrical heater cables have been proposed, they have had the disadvantage that the manufacturing thereof was complicated and expensive. Further, it has been difficult to construct such a cable having a desired electrical resistance per unit length for various lengths of the cable.
One of the objects of the present invention is to provide an improved electrical heater cable which has a constant wattage per unit length.
Another of the objects of the present invention is to provide such an electrical heater cable which is relatively inexpensive to manufacture and simple in construction.
According to one aspect of the present invention, an electrical resistance heating element may comprise a conductor rod enclosed by a tube of electrically insulating material. A tubular conductor encloses the tubular insulator. Means are provided in the tubular insulator to define a plurality of parallel discrete-like resistors between the conductor rod and tube so that the heating element has a constant wattage per unit length. The insulating tube may comprise a braid of insulating yarn within which are interwoven strands of a yarn having a high electrical conductivity and a high electrical resistance. These strands are woven between the outer and inner surfaces of the tubular insulator braid so as to form electrical conductors between the conductor rod and tube.
Other objects, advantages, and features of the present invention will be apparent from the accompanying description and drawings which are merely exemplary.
In the drawings:
FIG. 1 is a perspective view of a portion of an electrical heater cable according to the present invention with successive layers of the cable being cut away to show the details of construction.
FIG. 2 is a perspective diagrammatic view of the tubular insulating braid showing the manner in which a strand of electrically conductive material is interwoven therethrough; and,
FIG. 3 is a longitudinal sectional view diagrammatically showing the conducting yarn in the tubular insulator braid.
Proceeding next to the drawings wherein like reference symbols indicate the same parts throughout the various views, a specific embodiment of the present invention will be described in detail.
As may be seen in FIG. 1, there is indicated generally at 10 an electrical heater cable according to the present invention. The cable comprises a copper or suitable conductor rod or wire 11 surrounded by a coaxial tubular braid l2 woven from an electrically insulating yarn. One or more strands of an electrically conductive yarn 13 which has a high electrical resistance are interwoven in the insulating braid 12 so that portions of the strand 13 are exposed on the inner surface 14 and outer surface 15 of the tubular braid In this manner, a number of electrically conductive high resistance paths are formed through the insulator.
One example of electrically conductive strands is a strand of fiber glass or quartz which can be subjected to millimicron-size particles of a highly conductive material in a colloidal suspension. Materials such as graphite, silicon carbide and other semiconducting materials can be used. One colloidal suspension is Aquadag" E, Aquadag being a trademark of Acheson Colloids Co. for colloidal graphite in water with pH correction additives and other compounds that enhance adhesion to glass. Merely as an example, the material could have a resistance of 8 megohms per foot.
The insulating braid 12 is in turn surrounded by a coaxially positioned braid or conductor means 16 of electrically conductive material such as copper strands.
Surrounding the conductor braid 16 is a protective sheath formed of an electrically insulating material which in turn may be enclosed by an outer protective sheath of stainless steel or suitable material. The outer casing 17 could also comprise a stainless steel foil having stainless steel wire braid or seam welded stainless steel sheath.
The result of interposing a braid of insulating material having electrically conductive strands interwoven therein between a rod conductor and a tubular conductor is to form in effect a pair of conductors with a plurality of resistors connected between them in parallel. The result is a heater cable which a user can cut to any desired length in the field, and the resultant cut cable will maintain its given unit wattage or resistance.
It is to be understood that changes in various details of construction and arrangement of parts may be made without departing from the spirit of the invention ex- 7 cept as defined in the appended claims.
What is claimed is:
1. An electrical resistance heating cable comprising an electrical conductor rod, a coaxial tubular electrical insulating braid enclosing said conductor, a tubular electrically conductive braid enclosing said coaxial tubular insulating braid, and strands of electrically conductive yarn of high electrical resistance in said coaxial tubular braid exposed at spaced points on the inner and outer surfaces of said coaxial tubular braid defining a plurality of high resistance paths between said conductor rod and coaxial tubular braid whereby the heating element has a constant wattage per unit length.
2. An electrical resistance heating element as claimed in claim 1 wherein said coaxial tubular insulating braid comprises a braid of insulating yarn.
3. An electrical resistance heating element as claimed in claim 2 wherein strands of a yarn having a high electrical conductivity and a high electrical resistance are interwoven in said braid of insulating yarn.