Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2089048
The present invention relates to the field of
electrical heating cable. In particular, the present
invention provides an improved parallel zone heating cable
with enhanced flexibility and shortened zone length.
Parallel zone heating cables are known ~E se and are
in common usage in the heat tracing industry~ In a typical
construction of a parallel zone cable, two or three
insulated bus wires ~also called electrode wlres) are
provided. They may be solid or stranded, and are typically
insulated with PCV, FEP, ~PR or any other known and
temperature rated conventional insulation. The insulated
bus wires are jacketed with a further layer of insulating
mate~ial, which is provided to maintain the bus wires in a
parallel, untwisted configuration, as is necessary for
further processing. The resulting jacketed bus wire
construction is referred to as a core. The insulation over
short, one to two inch sections of bus wire is then skinned
off, at alternating sites from one bus wire to the next
along the length of the core, to expose the metal bus wire.
A heater wire of known resistance, (measured in ohms/linear
foot) is then spirally wound around the core, making
electric contact at the alternative exposed sites, with the
bus wire. A layer of fibreglass ~ay then wound over the
heater wire, to secure and cushion the heater wire, and the
entire constr~ction is then jacketed with an electrically
2~ ins~lating layer.
~089048
The cable described above has been in common use for a
number of years and under most conditions will function
quite well. However, the heater wire that has traditionally
been utilized has been a monofilament wire, and under
conditions of rough handling or rapid heat cycling, it tends
to break, causing a zone (being the distance between two
alternative sites on the core where the insulation has been
skinned away) to lose electrical continuity and its heating
ability. A small number of zone failures is not considered
fatal to a cable, since a zone will be heated by the
preceding and following functioning zones, but a larger
number of zone failures will necessitate removal of the
affected cable.
It has also been observed in parallel zone cables of
the sort described above, that due to the thermal shock to
the heating wire during the application of an extruded outer
jacket, the installation of cable in curved configurations,
and rapid duty heat cycling, that there is a tendency for
the heater wire to form a V-shaped groove along the inner
curve of a cable, between the bus wires. This is referred
to as chevroning, and may result in heater wire kinking and
breakage.
The object of the present invention, in view of the
foregoing, is to provide a parallel zone electrical heating
cable that is very flexible, and able to withstand rough
handling and rapid heat cycling, with minimum zone failure.
2089048
A further object of the present invention is to provide such
a heating cable with a short zone length, since it is
desired to have a short zone length, as this will minimise
the impact of zone failure.
In a broad aspect, the present invention relates to a
heating cable, incl~ding: (a) at least a pair of elongated
electrode wires, each said wire being coated with a first
layer of insulating material, said first layer of insulating
material being at least partially stripped off said wires at
spaced, alternating locations; (b) a resistive heater wire
spirally wound around a core of insulating material, to form
an elongated resistor core, said elongated resistor core
being spirally wound around said electr~de wires whereby
said heater wire is brought into electrical contact with
said electrode wire at said alternating locations, to
electrically connect said alternating locations with said
resistive heater wire; (c) a second layer of an insulating
material over said resistor core and forming an outer
surface for said cable.
In drawings that illustrate the present invention by
way of example:
Figure 1 is a perspective view partially cut away of a
parallel zone heating cable typical of the prior art;
Figure 2 is a perspective view partially cut away of a
heating cable of a first embodiment of the present
invention;
20890~8
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Figure 2A is a detail view of the end of a heater wire
construction of the cable of Figure 2;
Figure 3 is a schematic of the manufacturing method for
manufacturing the prior art cable of Figure l;
Figure 4 is a schematic of the manufacturing method for
manufacturing the cable of Figure 2.
Referring now to Figures 1 and 3, it will be seen that
prior art parallel zone heating cables provide a pair of bus
wires l, coated with insulation 2. The pair of insulated
bus wires is then coated, while in a parallel state, with an
insulator coat 3. At alternating locations 4, typically 12-
36 inches apart, the insulating coats 2 and 3 are stripped
off of the bus wire, then the metal of the other bus wire,
and so on. A heater wire 5 is then wound around the
alternately stripped core to make electrical contact with
the bus wires 1, to create heating circuits between the bus
wires, corresponding to the distance between stripped
locations on the bus wires. A fibreglass layer 6, which may
be a woven braid or helically applied yarn, may then served
Zo over the heater wire. A final layer of insulation 7 is then
extruded over the fibreglass layer, yielding a finished
product.
The present invention, on the other hand, as can be
understood from Figures 2, 2A and 3, provides a different
construction to achieve an end result that shares many basic
2089048
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characteristics of known paralle~ zone heating cables, but
lS an improvement over same.
According to the present invention, a similar core of
parallel, untwisted and insulated 2 bus wires 1 is coated
with an insulating jacket 3, and stripped at alternating
locations 4. A comparison of Figures 3 and 4, however,
indicates that at this point, the present invention diverges
from the prior art. Whereas in the Figure 3 prior art
method of man~facture a heater wire 5 (see Figure 1) is then
wound directly over the bu~ wire core, in the method of the
present invention, a heater wire 9 (see Figure 2A) is wound
over a fibreglass or other resistive core 10, and then the
heater wire/fibreglass combination 9/10 is wound over the
bus wire core. Optionally, deren~ing on the desired use of
the product, a fibre~lass layer 6 may be wound over the
heater wire/fibreglass combination, but this is not strictly
necessary. It will be understood that the heater wire 9
utilized in the present invention may be of very much
smaller diameter than that of the prior art. This feature,
combined with the cushioning effect of the fibreglass core
10 provides a heating element combination that is very
flexible and supple. Moreover, it has been observed that
such a combination, because of the cushioning effect of
fibreglass core 10, is capable of withstanding mechanical
impacts associated with an individual installation
environment and rapid heat and cooling cycles without
breakage, unlike the heater wire of the prior art, that is
2089048
.
wound directly onto the fairly unyielding bus wire core.
Furthermore, because a greater length o~ heater wire 9 is
utilized, helically wrapped a fibreglass core lO, equivalent
heating characteristics, with much shorter zone lengths is
possible.
In a typical cable, according to the present invention,
the following materials are used:
bus wire l: stranded copper, AWG 18-lO
insulating material 2: PVC or similar
insulating material 3: PVC or similar
resistor core lO: fibreglass, stranded yarn
heater wire 9: 70% Ni, 30% Fe, AWG 30-48
insulating jacket 7: PVC or similar
This construction results in a cable having technical
specifications that meet or exceed industry standards, with
short zones and good impact resistance, as well as superior
ability to withstand rapid heating cycling without breaking
down.
It will be understood that the foregoing table is by no
means exhaustive. Bus wire 1 may be any desired, single or
multi strand wire, as will be obvious to one skilled in the
art. Insulating layers 2, 3, 7 may be FEP, PTFE, PFA, TPR,
PVC, fibreglass, ceramic fibre, or any other suitable
insulation.
2089048
.
Heater wire 9 may be AWG 30 to AWG 48, and resistor
core 10, as well as being fibreglass, may be polypropylene,
polyester, ceramic fibres, or other suita~le temperature
rated material. The selection of heater wire g wil~ depend
on the desired characteristics and the intended use of the
cable. Preferably, a heater wire exhibiting positive
temperature coefficient of resistance (PTC) is used, and in
this regard, a minimum 60% nickel wire is desirable. The
balance may be chrome, copper, or iron, or a combination
thereof. Preferably, 70% nickel to 99% nickel, remainder
iron, alloy is utilized.
It iæ to be understood that the examples described
above are not meant to limit the scope of the present
invention. It is expected that numerous variants will be
obvious to the person skilled in the heat tracing field art,
without any departure from the spirit of the present
invention. The appended claims, properly construed, form
the only limitation upon the scope of the present invention.
