Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the field of heating
cable.
There exists in the market two general types of parallel
type constructed heating cable that compete with one another
for maximum usage in the residential, commercial and process
industries. These cables incorporate heating elements that are
electrically connected in parallel, either continuously or in
zones, so that the watt density per lineal length is
maintained, irrespective of any change in length of the heating
cable. The parallel zone type of heating cable comprises two
or more parallel insulated electrode wires around which a
resistance wire is helically wrapped. The electrode wires are
alternately bared at discrete points or zones along their
length permitting current to flow through the resistance wire
which becomes hot due to its high resistance. The cable is
then covered with one or more insulating coatings. This form
of cable has advantages: it is relatively inexpensive and easy
to manufacture, and it will repeatedly reach a predetermined
heat output at a known voltage applied across the electrode
wires. It has some disadvantages though: it is susceptible to
damage and it has limited usage in situations of variable
ambient temperatures, such as those encountered in heating or
maintenance of temperature sensitive products.
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The second type of cable is known as a continuous self-
limiting or self-regulating cable. It comprises of two or more
parallel electrode wires which are embedded in an electrically
conductive polymer matrix. The primary cable construction is
then covered with one or more insulating coatings. Application
of voltage across the electrodes will cause current to flow
through the electrically resistive matrix and will develop
heat. Typically this type of heater cable must be cross-linked
to stabilize the operating characteristics of the finished
product. At an elevated operating temperature, the current
flow will diminish, due to increased resistance of the polymer
matrix. It will be understood, then, that this form of cable
finds favourable application in situations of variable ambient
temperature, because it will regulate its thermal output when
the combined effect of the ambient temperature and the heat
developed by the cable creates a change of resistance in the
matrix. As the conductive matrix is a compound, incremental
thermal output will vary which results in hot and cold spotting
along the length of the finished cable. This type of heater
cable is also characterized with a high start-up inrush which
demands the utilization of oversized distribution wiring and
circuit breaker design.
The object of the present invention is to provide an
inexpensive heating cable with the advantages possessed by each
F~
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of the foregoing heating cables and minimize or eliminate the
relative disadvantages.
In a broad aspect, the present invention relates to
a heating cable including at least a side by side pair of
insulated electrode wires, short circuited along their lengths
by heater wire wrapped helically around same and contacting
alternate ones of same at spaced apart locations along their
length at which the insulation on said wires has been removed;
a layer of fibreglass yarn wrapped around said heater wire
wrapped electrode wires; and a coating of insulating material
over said fibreglass yarn.
In drawings which illustrate the present invention by way
of example:
Figure 1 is a perspective view, partially cut away, of an
embodiment of the present invention having two or three
electrode wires;
Figure 2 is a chart of the performance curve in watt/ft
plotted against temperature (F) of the cable of the present
invention; and
Figure 3 is a schematic of a manufacturing process which
may be utilized to make the cable of the present invention.
Referring first to Figure l, the cable of the present
invention is provided with at least two electrode bus wires (1)
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which are covered with a temperature rated and electrically
insulating jacket (2). The electrode wires may be in a
parallel side by side or spiralled configuration. The
electrode insulation is stripped away at regular alternate
intervals (3) to expose the bus wires. A resistance wire or
element exhibiting PTC (positive temperature coefficient) is
helically wound over the electrode bus wire construction (4).
It will be observed, then, that the resistance wire (4) will
bridge the electrode from side to side where the insulation
layers are removed. When the electrodes are connected to line
voltage, current will flow in the resistance wire, causing it
to increase in temperature and give the cable its heating
capability.
The resistance wire (4) utilized by the present invention
has PTC characteristics, and is made from an alloy exhibiting
PTC properties, such as a 70 Ni, 30% Fe alloy, available as
KANTHAL 7 oTM from The Kanthal Corporation of Connecticut; or
BALCO ALLOY 400TM, available from Carpenter Technology of
Toronto, Ontario.
The present invention also provides a fibreglass layer
preferably helically wound, over the resistance wire. Fibre-
glass yarn (5) may be wound onto the wire using apparatus
utilized to wrap the resistance wire on a heater cable as shown
schematically in Figure 3. It has been found that in this way
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a fibreglass layer can be applied three to five times faster
to the cable construction than with conventional braiding
techniques. Moreover, advantages are associated with the
fibreglass layer. For instance, flexibility is imparted to the
cable while protecting the resistance wire from breakage in the
event of impact or repeated thermal cycling. To complete the
cable construction, a further temperature rated layer (6)
covers the fibreglass layer.
The present invention is further illustrated by way of
tests carried out on the cable of the present invention, as
follows:
Test Proce~ UL e:
A 10' (3m) section of double electrode VPC-501
(applicant's designation for cable produced in accordance with
the present invention) is placed on the power output
verification text fixture, which consists of a 2" (50mm) carbon
steel pipe connected to a circulating system. The pipe is
insulated with 1~" (36mm) fibreglass and covered with weather
barrier. The circulating system is turned on and the pipe
temperature is brought down to the desired temperature. The
cable is energized at the rated voltage and allowed to reach
equilibrium. The amperage is then recorded. This process is
repeated at several specified temperatures. The data is
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charted to determine the cable's characteristic performance
curve, the results being shown in Figure 2.
Test Results:
The pipe temperature was increased from 25F (14C) to
150F (66C). Amperage readings were taken and power outputs
calculated. The test results are illustrated graphically in
the form of a performance curve of power output (w/ft) versus
pipe temperature (F) [see Figure 3].
Conclusion:
The cable's power output at equilibrium decreased by
approximately 13.3% from 50F to 150F. From published
literature on KANTHAL 70TM (available from Kanthal Corporation
directly), a 25% change in resistance is expected over this
temperature range. The discrepancy arises from the Kanthal
literature being based on strand (heater element) temperature
and not pipe temperature. Put another way, equilibrium power
(strand resistance) is a function of actual strand temperature
and not necessarily the pipe/ambient temperature.
A 25% change would occur only if the strand temperature
varied from 50F (10C) to 150F (66C). In actuality the VPC
element is operating over a higher and narrower temperature
range.
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It should be noted, however, that due to the variations
of sheath heat transfer coefficient increasing with increasing
temperature and sheath to strand temperature differences
decreasing with decreasing wattage, variances in strand
temperatures between pipe temperatures of 10C and 66C are
much less.
The temperature versus power results can be seen below,
as well as in Figure 2.
VPC-501 Performance Curve
Percent Change
Temperature Wt/Ft W/N From 50F (10C)
50F (10C) 17.16 56.28 ---
75F (24C) 15.96 52.36 -7.0%
100F (38C) 15.48 50.78 -9.8%
125F (152C) 15.36 50.39 -10.5%
150F (166C) 14.88 43.81 -13.3%
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 design and manufacture of heating cable
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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