Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2174615
HEATING CABLE
R~CR~P~UND OF THE INVENTION
FIELD OF THE INVENTION
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.
DE8CRIPTION OF THE PRIOR ART
Parallel zone heating cables are known P~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 wires) are provided. They may
be solid or stranded, and are typically insulated with PCV,
FEP, TPR or any other known and temperature rated conventional
insulation. The insulated bus wires are jacketed with a
further layer of insulating material, 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
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contact at the alternating exposed sites, with the bus wire.
A layer of fibreglass may then be wound over the heater wire,
to secure and cushion the heater wire, and the entire
construction is then jacketed with an electrically insulating
layer.
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, there is a tendency for the heater
wire to form a V-shaped groove along the inner curve of a
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cable, between the bus wires. This is referred to as
chevroning, and may result in heater wire kinking and breakage.
8UMMARY OF THE INVEN$ION
The object of the present invention, in view of the
sforegoing, 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.
A further object of the present invention is to provide such
a heating cable with a short zone length, since it is desired
10to have a short zone length, as this will minimise the impact
of zone failure.
The objects of the present invention are substantially
met, and the defects of the prior art overcome, by utilizing
a different form of heating element, one that is less
15susceptible to kinking or breaking. To this end, the applicant
has designed a heating element in the form of an elongated
resistor core. A length of fibreglass or other insulating yarn
having good flexibility is provided, and a thin resistive wire
is helically wound around same, fairly tightly. The resulting
20elongated resistor core will exhibit a fairly high resistance
measured in ohms/linear foot, since it utilizes a much greater
length of heater wire, wrapped helically around the fibreglass
yearn, that the final length of resistor core, which will be
about equal to the length of fibreglass yarn utilized in the
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core. Moreover, the elongated resistor core, even though
tightly wrapped, will exhibit much more pronounced limpness
than a monofilament heating wire of necessarily thicker gauge.
This limpness serves to eliminate breakage due to kinking of
the heater wire, and also to eliminate chevroning.
Furthermore, the innovative design of the elongated
resistor core may be more rapidly cycled, without damage, than
previous designs. As the heater wire expands and contracts
against the fibreglass yarn core, the yarn core absorbs and
cushions the contraction of the heater wire. In a conventional
design, the heater wire's contraction is substantially
uncushioned, resulting in both breakage of the wire, and
stretching of the wire. Stretching of the wire causes both
chevroning, and looseness resulting in poor electrical contact
with the electrode wires.
In order to assure constant electrical contact between the
elongated heater core and the electrode wires at the stripped
portions of same, and to provide additional impact cushioning,
a fibreglass (or other insulating yarn) layer is braided over
the resistor core after it is wound around the electrode wires.
A final insulating layer is then applied.
In a broad aspect, the present invention relates to a
heating cable, including: (a) a pair of elongated electrode
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wires, each of said wires being coated with a first layer of
insulating material, said first layer of insulating material
being at least partially stripped off selected ones of said
wires at spaced, alternating locations; (b) a resistive heater
wire which together with a yarn of fibrous insulating material
is spirally wound around said electrode wires whereby said
heater wire is brought into electrical contact with said
selected ones of said electrode wires 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 resistive heater wire and
insulating material forming an outer surface for said cable.
BRIBF DESCRIPTION OF THB DRAWING~
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;
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 1;
Figure 4 is a schematic of the manufacturing method for
manufacturing the cable of the present invention; and
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Figure 5 is a perspective view, partially cut away, of a
second embodiment of the invention.
DE~PTPTION OF THE PREFERRED ENBODINENT
Referring now to Figures 1 and 3 ! it will be seen that
prior art parallel zone heating cables provide a pair of bus
wires 1, 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 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, 4 and 5, provides a different
construction to achieve an end result that shares many basic
characteristics of known parallel zone heating cables, but is
an improvement over same.
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_,
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 manufacture
a heater wire 5 (see Figure 1) is then wound directly over the
bus wire core, in the method of the present invention, a heater
wire 9 (see Figure 2A) is wound over a fibreglass or other
insulating core 10, and then the heater wire/fibreglass
combination 9/10 is wound over the bus wire core. Depending
on the desired use of the product, a fibreglass layer 11 may
be braided over the heater wire/fibreglass combination, as
shown in Figure 5. Use of a braided layer 11 provides an added
measure of assurance of good electrical contact between the
heater wire and the electrode wire. 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 me~-h~nical 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 wound directly onto the
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fairly unyielding bus wire core. Furthermore, because a
greater length of heater wire 9 is utilized, helically wrapped
around a fibreglass core 10, equivalent heating characteristics
with much shorter zone lengths are possible.
In a typical cable, according to the present invention,
the following materials are used:
bus wire 1: stranded copper, AWG 18-10
insulating material 2: PVC or similar
insulating material 3: PVC or similar
resistor core 10: fibreglass, stranded yarn
heater wire 9: 70% Ni, 30~ Fe, AWG 30-48
(up to 99% Ni wires with similar
PTC turn-down phenomena are suitable)
insulating jacket 7: PVC or similar
braid 11: fibreglass yarn
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,
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PVC, fibreglass, ceramic fibre, or any other suitable
insulation.
Heater wire 9 may be AWG 30 to AWG 48, and insulating core
10, as well as being fibreglass, may be polypropylene,
polyester, ceramic fibres, or other suitable temperature rated
material. The selection of heater wire g will 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 is 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.
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