HEATING CABLE

CABLE CHAUFFANT

English Abstract

A heating cable includes at least a pair of electrode
wires arranged in a parallel spaced apart relationship. A layer
of conductive polymeric material is extruded over said electrode
wires, portions of said polymeric material being stripped off
alternate ones of the wires, to expose same at spaced apart
locations. A heater wire is spirally wound over the polymeric
material, to electrically bridge the spaced apart locations of
exposed electrode wire; and an outer layer of insulating jacket
is applied over the heater wire.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. A heating cable including:
i) at least a pair of electrode wires arranged in a
parallel spaced apart relationship;
ii) a layer of conductive polymeric material extruded
over said electrode wires, portions of said
polymeric material being stripped off alternate
ones of said wires, to expose said wires at
spaced apart locations;
iii) a heater wire spirally wound over said polymeric
material, to electrically bridge said spaced
apart locations of exposed electrode wire; and
iv) an outer layer of insulating jacket over said
heater wire.
2. A cable as claimed in claim 1, further including an
inner layer of insulating material over said polymeric material,
and over which said heater wire is wound, said inner layer of
insulating material also being stripped off at said spaced apart
locations.
3. A cable as claimed in claim 2, further including a
layer of fibreglass over said heater wire, and under said outer
layer of insulating material.
4. A cable as claimed in claims 1, 2 or 3, wherein said
heater wire is selected from the group including nickel chromium


heater wires, iron nickel alloy heater wires exhibiting PTC
characteristics, and elongated resistor cores composed of a core
of insulating material around which is wound a layer of heater
wire.

Description

The present invention relates to the field of heating
cables.
It is known to manufacture heating cables for use in
electric heat tracing utilizing a construction .consisting of a
pair of spaced apart parallel electrode or 'bus wires, for
instance made of uninsulated stranded copper wire, over which is
extruded a polymer core interspersed with carbon black. When a
voltage is applied across the electrode wires, current will flow
through the polymer due to the inclusion of the carbon black
therein. The so-called conductive polymer shows significant
resistance, however, which increases as the temperature of the
cahle increases. The result is a self limiting cable, since at
a predetermined temperature (determined experimentally, and
variable according to the amount of carbon black ) resistance will
increase to infinity, and the flow of current through the polymer
will be interrupted. There has been a practical maximum heat
output of about 12 watts per foot of such cable.
Another principal type of electric heat tracing cable
is known as a zone heater. It comprises two or three parallel
insulated stranded copper (or other suitable) wires, over which
a jacket made from a plastics material may be extruded. At
regular spaced intervals, the insulation and any jacketting over
alternate ones of the electrode wires is stripped off to expose
the copper wire. High resistance heater wire is 'then helically
wound over the electrode wires, electrically bridging the exposed
portions of the wire. It will be understood, 'then, that when
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voltage is applied across the electrode wires, current will flow
through the resistance wire which will then produce heat. The
resistance wire may be a conventional heater wire, such as a
nichrome wire, or it may be one exhibiting positive temperature
coefficient of resistance behaviour (PTC) such as an iron-nickel
alloy wire.
The object of the present invention is to provide a
conductive polymer self limiting heating cable that has increased
thermal output.
In a broad aspect the present invention relates to a
heating cable including: i) at least a pair of electrode wires
arranged in a parallel spaced apart relationship; ii) a layer of
conductive polymeric material extruded over said electrode wires,
portions of said polymeric material being stripped off alternate
ones of said wires, to expose said wires at spaced apart
locations;
iii) a heater wire spirally wound over said polymeric material,
to electrically bridge said spaced apart locations of exposed
electrode wire; and iv) an outer layer of insulating jacket over
said heater wire.
Tn drawings that illustrate the present invention by
way of example:
Fig. 1 is a side view of a cable embodying the present
invention, with layers thereof successively exposed;
2

CA 02098154 2002-03-13
Fig. 2 is a cross sectional view through line II-II of
Fig. 1;
Fig. 3 is a side view of a cable of an alternative
embodiment of the present invention, with layers thereof
successively exposed;
Fig. 4 is a cross sectional view through line IV-IV of
Fig. 3.
Referring first to Figs. 1 and 2 a cable 1 is shown
0 comprised of the following layers:
i) stranded copper electrode wire 2;
ii) carbon black impregnated conductive polymer 3.
A suitable polymer for such use is a polyolefin
such as polyethylene or a fluoropolymer such as
5 teflon, with about a 12-16% carbon black particle
content;
iii) primary insulation jacket 4, that is stripped off
at alternate locations 5 which stripping extends
through conductive polymer layer 3 to expose
?0 electrode wires 2;
iv) heater wire 6, spirally wound around jacket 4 to
electrically bridge the stripped locations 5.
Wire 6 may be a conventional heater wire such as
a nickel-chromium wire, or a PTC exhibiting wire
25 such as a 70/30 iron/nickel alloy wire lake
Kanthol*or it may be an elongated resistor core
composed of a core of insulating material such as
* Denotes Tr. ade P!tark
3


CA 02098154 2002-03-13
fibreglass yarn over which is spirally wound a
conventional or PTC heater wire;
v) fibreglass insulation layer 7, which may be
either a braid or a spirally wound layer of yarn.
' Fibreglass layer 7 may be omitted if the cable
will not be subject to significant physical or
thermal stresses;
vi) exterior insulation jacket 8, which will be any
suitable insulating layer, such as polyethylene
0 or teflon.
Referring to Figs. 3 and 4, it will be 'observed that
a similar cable is shown, but without the inclusion of primary
insulating jacket 4, whereby heater wire 6 is wound directly over
5 the core of conductive polymer 3, and contacts electrode wires
2 at stripped off locations 5.
It has been observed, utilizing the cable. of the
present invention that there is an increase in thermal. output of
.'0 from about 20 to about 60~ over that obtainable utilizing the
conductive polymer core alone as_a heat generating means.
The most preferred embodiment of the present invention
is the embodiment utilizing a heater wire such as Kanthol* 70$
25 iron, 30~ nickel alloy, which possesses PTC characteristics. By
varying the guage of such heater wire, the spacing of the loops
over the core, and the length of the zone (i.e. distance between
stripped aff locations which will be typically 18" to 36"), one
* Denotes Trade mark
4



~0~~1~~
skilled in the art will be able to obtain a cable having suitable
characteristics for a desired application.
Numerous variations will be obvious to one skilled in
the art. It is to be understood that the invention is not meant
to be limited by the description of preferred embodiments
contained herein, the only limitations being those inherent in
a proper construction of the claims appended hereta.

Representative Drawing