Note: Descriptions are shown in the official language in which they were submitted.
129833~3
POSITIVE TEMPERATURE COEFFICIENT THERMISTOR HEATING PAD
SPECIFICATION
Background of the Invention
1. Field of the Invention
The present invention relates to electrical heating
devices that use positive temperature coefficient
thermistors as self-regulating heaters.
2. Description of the Prior Art
As exemplified in U.S. Patent No. 4,072,848,
electrical heating cables have been used commercially for
some time to provide heat to pipes and tanks in cold
environments.
Heating cables as disclosed in U.S. Patent No.
4,072,848 based their temperature control on the use of
variable resistance heating materials which provide a self-
regulating feature. The heating materials are generally
formed into chips made of barium titanate or solid
solutions of barium and strontium titanate which are made
semiconductive to the inclusion of various dopants. These
chips are referred to as positive temperature coefficient
thermistors and have a relatively low resistance at low
temperatures. As the temperature of the thermistor rises,
a sharp rise in the resistance occurs at a point termed the
"Curie point". The transition from low re~ to high
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resistivity occurs at a relatively sharp point as shown in
U.S. Patent No. 4,072,848. As these chips are well known
to those skilled in the art, no further discussion of
their construction is necessary.
As a voltage is applied to the thermistor, the
thermistor generates heat due to resistance effects. This
heat is then transferred to the environment, such as the
pipe to which the cable is attached. As the temperature
of the thermistor and the surrounding environment
increases, the thermistor temperature reaches the Curie
point, the heat producing capability of the thermistor is
reduced and the thermistor cools down. Thus the
thermistor temperature settles on or near the Curie point,
with the temperature of the surrounding environment being
based on the thermal conductivities of the various
materials in contact with the thermistor.
Prior art thermistor-based devices were cables and
other similar devices which covered only small lateral
areas, even though they could be extended for long
distances. While the prior art cables could be shaped in
serpentine patterns to cover larger lateral areas, this
often resulted in uneven temperature distributions over
the surface area and was hard to manufacture.
U.S. Patent No. 4,330,703 shows several examples of
prior art cables utilizing heat generating layers of
materials and having electrical conductors formed of metal
sheets, grid or meshes. The heat generating materials are
located over the entire area of the cable, not in discrete
and r-~arated areas as is the practice in thermistor-based
cables. Additionally, the electrical conductors are thin,
utilized only to supply electrical current to the heat
generating materials and not utilized to conduct
appreciable amounts of heat.
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Summary of the Invention
The heating pad of the present invention has
substantially flat, planar, flexible preferably woven,
electrical conductors disposed in overlying parallel
relationship and having a plurality of spaced thermistors
electrically connected thereto, wherein the electrical
conductors serve as the primary heat transfer means by
dissipating heat produced by the thermistors away from
them. The electrical conductors may be woven copper wire
cloth or other woven, electrically conductive fibres. The
thermistors have opposed planar surfaces in electrical and
thermal contact with the electrical conductors, preferably
by soldering, brazing or welding the thermistors to the
electrical conductors. The thermistors are spaced in a two
dimensional grid of substantially uniform pattern over the
area of the heating pad, with the total area of the
thermistors being less than the total area of each of the
electric conductors. A flexible insulator is disposed
2Q between the electrical conductors in the area not occupied
by the thermistors to prevent the electrical conductors
from contacting. Such construction results in an efficient
heat transfer between the conductors and the thermistors,
thus allowing heat to be removed from the thermistors.
Also such construction enables the thermistor to produce
high power levels with a given applied voltage before the
thermistor reaches the self-limiting temperature or Curie
point. A flexible insulating material is provided over the
external surfaces of the electrical conductors to reduce
the possibilities of grounding or shock. The entire
assembly is flexible or bendable for forming around vessels
or pipes. An optional metallic enclosure, such as
stainless steel, can be formed over the outer insulation
material for corrosion resistance or mechanical abrasion
resistance.
Such heat transfer using the electrical conductors
improves the temperature distribution over the surface of
the pad because the heat is transferred in all directions
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along the electrical conductors, which are good thermal
conductors, and away from the thermistors, limiting the
amount of local heat and improving the heat balance of the
pad. The construction of a heating pad of the present
invention allows ease of manufacture because complex
serpentine paths are not required.
The use of the woven electrical conductors
significantly decreases the thermal or mechanical stresses
which occur at the connections between the conductors and
thermistors because of the dispersed multidirectional
forces which are exerted because of the small size and
great number of wire strands in the material.
In a broad aspect, therefore, the present invention
relates to a flexible electric heating pad, comprising:
first and second flexible, planar electrical conductor
means extending substantially parallel to each other and
spaced from each other for conveying electrical current and
for conducting heat; heating means formed of variable
electrical resistance heating material electrically
connected between said first and second conductor means for
producing heat when current flows therethrough, said
variable resistance heating material substantially
increasing in resistance when a temperature limit is
reached to reduce the current flowing through said heating
means so as to control the heat output of the heating pad,
said heating means including a plurality of chips of said
variable resistance heating material, each of said chips
having opposed planar surfaces in electrical and thermal
contact with respective ones of said conductor means, said
chips arranged in a two dimensional array, said total chip
area being less than the total area of each of said
conductor means, said chips being held in electrical and
thermal contact with said conductor means by soldering,
brazing, or welding; flexible, electrical insulating means
disposed between those portions of said conductor means not
contacted by said heating means for preventing contact
between said first and second conductor means; flexible,
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1298338
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electrical insulating material disposed externally of and
covering the outer surfaces of said conductor means for
preventing short circuit or shock by contact with said
conductor means; wherein each of said conductor means
comprises a substantially flat sheet of electrically and
thermally conductive material having a planar thermal
conductance greater than the planar thermal conductance of
said electrical insulating means for preventing contact
between said conductor means; and wherein said assembly
including both said conductor means, said heating means and
both said insulating means is bendable.
Brief Description of the Drawings
Fig. 1 is a perspective view in partial cross-section
of a heating pad constructed according to the present
invention.
Fig. 2 is a graph illustrating the unit power
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produced at given temperatures and given voltages for the
heating pad of Fig. 1.
Fig. 3 is a graph representing the unit power
produced a~ given temperatures and given voltages for a
heating pad according to Fig. 1.
Description of the Preferred Embodiment
Referring to the drawings, the letter P generally
designates a heating pad according to the present
invention.
Fig. 1 illustrates the preferred embodiment of a
heating pad P constructed according to the present
invention. A plurality of thermistors 10 are inserted
into a separating dielectric insulator 12. The separating
dielectric 12 contains a series of holes or cavities 14 in
which the thermistors 10 are installed. The spacing
between the holes 14 is varied depending upon the specific
size of the thermistors 10 and the number of thermistors
10 required for a given desired thermal output of the
heating pad P. Preferably the holes 14 are slightly
smaller than the size of the thermistors 10 so that the
thermistors 10 are positively retained in the separating
dielectric 12. The thermistors 10 are shown as being
circular in cross-section, but any desired shape can be
used, with the holes 14 have corresponding shapes. The
dielectric material may be rubber, thermoplastic resins
such as polyethylene or polytetrafluoroethylene, asbestos
fiber, or any satisfactory material which is an electrical
insulating material and is capable of withstanding the
temperatures of the thermistors 10, while conducting
sufficient heat as desired and being flexible to allow the
heating pad P to be flexed as desired.
Conductive sheets 16, 18 are installed parallel to
each other and on opposite sides of the separating
dielectxic 12 to provide the source of electrical energy to be
converted by the thermistors 10 to heat. The conductive
sheets 16, 18 are attached to the thermistors 10 by
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soldering, brazing, welding or otherwise electrically and
mechanically connecting the conductive sheets 16, 18 to
the surfaces of the thermistors 10. Conductors 17, 19 are
attached to the conductive sheets 16, 18 and to the
voltage source (not shown) used to supply electrical
energy to the heating pad P. After the conductive sheets
16, 18 have been connected to the thermistors 10, an
insulating layer 20 is provided to protect the heating
pad P from the environment. In this way, short circuit
and potential shock conditions are prevented. If further
mechanical or corrosion resistant protection is desired or
where a more rigid surface is desired, a metallic sheath
22 can be formed over the insulating layer 20 of the
heating pad P. The metallic sheath 22 may be aluminum,
stainless steel, copper or any satisfactory metal or metal
alloy that can be formed about the pad.
Such construction, using conductive sheets 16, 18 of
adequate heat transfer capability, results in the
conductive sheets 16, 18 becoming the primary heat
transfer means. The use of the conductive sheets 16, 18
as the primary heat transfer means results in increased
heat removal from the thermistors 10 and a more even
temperature distribution over the surface of the heating
pad P. Thus, by reason of this invention, heat is removed
from the thermistors 10 and the heat is evenly distributed
over the area of the heating pad P.
The conductive sheets 16, 18 are preferably formed of
copper wire cloth approximately the same size and shape as
the -ating pad P. The conductive sheets 16, 18 can
alternately be formed of wire cloth made of aluminum,
stainless steel or other met~llic conductors.
Alternatively, carbon or graphite fibers, conductively
coated fiberglass yarn or other similar materials of known
construction as are commonly used in automotive ignition
cables and as disclosed in U.S. Patent No. 4,369,423 may
be used. The fibers can be electroplated with nickel to
further improve the conductivity of the fibers.
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Sufficient nu~lbers of the fibers are woven to provide a
conductive sheet which is capable of carrying the
necessary electrical and thermal loads. In yet another
alternative, the conductive sheets could be solid metallic
sheets of materials such as copper, aluminum or other
suitable materials. An exemplary copper cloth is
comprised of 0.011 inch diameter copper wire formed into a
mesh having 16 wires per inch in either direction. The
individual copper strands may be coated with a tin,
silver, aluminum or nickel plated finish.
The conductive sheet construction according to the
present invention is preferably formed with a large number
of smaller wires which are woven into sheets. The
increased number of contacts of smaller wire and the mesh
or woven pattern developed by the woven conductors
decreases the thermal and mechanical stresses which occur
at the connection between the conductive sheet 16, 18 and
the thermistor 10. The thermal stresses arise due to
differing expansion rates and other reasons and the
mechanical stresses occur due to the flexible nature of
the heating pad P. Because the woven wires are small and
are arranged in several different directions, the forces
exerted on each strand or wire are low, thereby increasing
the reliability of the heating pad P.
A heating pad P according to the present invention
can be cut or formed into almost any desired shape. The
exemplary embodiment shown in Fig. 1 is formed into a
square, but the heating pad P can be formed into circular
shap.--, irregular shapes or regular or irregular polygons
as desired. Because the thermistors 10 are relatively
small, and the other materials used in the present
invention are preferably flexible, the heating pad P i5
adapted to be flexed so as to substantially conform to an
item such as a vessel or pipe to be heated.
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Example
A heating pad P was constructed of copper wire cloth
according to Fig. 1 with Curie temperature 124-128 c.
thermistors 10. A one foot square separating dielectric
5 layer 12 of coated fiberglass having a thickness of 0.07
inches was used. Twelve thermistors 10 were placed in
openings 14 distributed evenly over the area of the
separating dielectric layer 12. Copper wire cloth having
a 16 by 16 mesh and formed of 0.011 inch diameter wires
10 was formed into sheets one foot square which were then
soldered to pre-tinned thermistors 10 with a silver
bearing, high temperature solder alloy. This heating
pad P was then insulated with high temperature RTV
silicone to form the insulating layer 20. The completed
15 heating pad P thus formed had a resistance of 90 ohms at
room temperature of approximately 77F.
This heating pad P was then placed in an
environmental chamber, and tested at equilibrium
temperatures of -35F, 0F., 50F., 100F., and 200F. and
20 energized at voltages ranging from 0 to 300 volts. The
power consumption at the various voltages and temperatures
was recorded and the results are shown in Figs. 2 and 3.
It can thus be seen that the present invention provides a
construction which produces high power levels with a given
25 applied voltage before the thermistors reach the
self-limiting temperature.
In another test, the same heating pad P was energized
by approximately 120 volts while the heating pad P was
sus~ ded in a free air environment having a temperature
30 of 76F. Temperature measurements were taken at a series
of locations on the surface of the heating pad P. The
: maximum and minimum temperatures at positions directly
over the thermistors 10 were 199F and 178F. The average
temperature directly over the termistors was approximately
i 35 183F. The outer edges of the heating pad P had
temperatures of 111F, 116F, 112 F and 102F. The
3 average temperature on the surface area at locations
between the thermistors 10 was approximately 121F, with a
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maximum of 134~F and a minimum of 108F. SUch results
indicate the efficient heat transfer from the thermistors
10 to the conductive sheets 16, 18 and the good thermal
conduction of the conductive sheets 16, 18.
It will be understood that because the heat is
generated initially at the thermistors, the pad may be
selectively formed or cut into any desired shape while
still retaining approximately the same watts per square
foot capability for the selected area, assuming an equal
area of remaining heating pad per thermistor.
The foregoing disclosure and description of the
invention are illustrative and exemplanatory thereof, and
various changes in the size, shape and materials as well
as in the details of the illustrated construction may be
made without departing from the spirit of the invention,
and all such changes being contemplated to fall within the
scope of the appended claims.
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