Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to self-regulating heaters
comprising an elongate strip of a PTC conductive
polymer, two (or more) parallel electrodes in electri-
cal (and optionally also physical) contact with the
strip, and an electrically insulating jacket. Such
heaters are known and are described for example in U.S.
Patents Nos. 3,793,716, 3,823,217, 3,861,029, 4,177,376
and 4,188,276 and the Thermal Desgin Guide published by
the Chemelex Divison of Raychem Corporation (H 50190
10 505 B5 1/78).
In defining such heaters, reference is often made
to their "passive power output" and their "active power
output". The "passive power output" of a heat is the
value of the term V2/Ro, where Ro is the resistance of
a unit length (usually 1 foot or 1 metre) of the heater
at 21C and V is the voltage of the source of electri~
cal power connected to the heater, usually 120 or 240
volts AC. The "active power output" of a heater is the
measure power output of a unit length of the heater
(usually 1 foot or 1 metre) when connected to the
source of electrical power, with one major surface of
the heater in contact with a metal substrate which is
maintained at some temperature related to the intended
use of the heater, e.g. 7 to 65C, such as 10C for
heaters designed to prevent pipes from freezing.
Conventional strip heaters comprise two or more elec-
trodes in the form of wires in electrical contact with
a PTC conductive polymer element and have passive power
outputs of substantially less than 165 watts per metre
(50 watts per foot) at their intended operating vol-
tage; the use of such heaters having higher passive
power outputs has been avoided because they give
little or no useful increase in active power output
and have substantial disadvantages, in particular
shorter life.
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We have now discovered that if a PTC conductive
polymer heater is surrounded by an envelope of metal or
other material of good thermal conductivity, it is possible
to make use of the heater at voltages at which its passive
power output is greater than 165 watts per metre without
the disadvantages previously associated with Ruch use, and
that 8 very valuable increase in active power output can
thereby be obtained. ~or example the addition of such an
envelope surprisingly makes it possible to use, at a
voltage of 240 volts, a PTC heater which has a passive
power output at 120 volts of less than 165 watts/metre and
at 240 volts of more than 165 watts/metre, even though
such a heater, in the absence of the envelope, has an
unacceptably short service life at 240 volts.
In one aspect, the invention provides a method
of heating which comprises passing current through a
self-regulating heater assembly comprising
(a) a PTC heater which comprises
(i) an elongate strip of 8 conductive
polymer composition exhi~iting PTC
behavior,
(ii) two elongate parallel electrodes
in electrical contact with aaid strip,
and
(iii) an electrically insulating jacket
which surrounds said ~trip and said
electrodes; and
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(b) an envelope which surrounds said PTC heater and which
is composed of a material having a thermal conductivi-
ty of at least 0.1 Cal/cm C sec. preferably at least
0.3 Cal/cm C sec.;
the current being supplied by a source of electrical power having a
voltage V and the heater having a passive power output at said volt-
age V at least 165 watts/meter (50 watts/ft.), preferably at least
230 watts/meter (70 watts/ft.).
The heater assembly can be used to heat a solid substrate,
in which case the envelope is preferably shaped to conform to the
substrate; or it can be used as a space heater, in which case the
assembly is preferably in the form of a coil, with space between
the coils for air or other liquid to circulate.
In another aspect the invention provides a self-regulating
heater assembly comprising a PTC heater and an envelope as defined
above, the heater having a passive power output at 120 volts of at
least 165 watts/metre.
The PTC heater preferably comprises two ~or more) elect-
rodes in the form of wires which are embedded in and in physical
contact with the PTC strip. ~lowever, the invention includes other
types of PTC heater, for example those in which one or more of the
electrodes is separated from the PTC strip by a layer of another
conductive material, e.g. a conductive polymer composition exhibit-
ing ZTC behavior, and those which comprise a laminar PTC element
and two or more laminar electrodes.
The passive power output of a heater is of course dependent
on the resistivity of the PTC composition and the size and shape of
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tlle electrodes and the PTC element. For a conventional strip heater,
e.g. as described in the example below, the passive power output is
preferably 165 to 655 watts/metre (50 to 200 watts/ft~, preferably
230 to 655 watts/metre (70 to 200 watts/ft). For the broad range
of heaters contemplated by this invention, the preferred ranges of
passive power outputs are better expressed by a term which includes
the width of the PTC element over which heat is produced. Thus the
passive power output of the heaters used in this invention is prefer-
ably 217 Dito 862 D, especially 302 D to 862 D, watts/metre, where
D is the largest cross-sectional dimension ~in centimetres) of the
PTC strip which lies between the electrodes.
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The active power output of the PTC heater
is substantially increased by the presence of the envelope,
and preferred heater assemblies have an active power
output at 10C which is at least 1.5 times the active
power output at 10C of the PTC heater without the envelope.
The envelope is preferably formed by a pair of
elongate sheets with the heater sandwiched between them?
the sheets contacting each other either side of the
heater. The sheets are preferably 0.025 to 1.3 mm, e.g.
0.075 to 0.2 mm, thick and composed of a material having a
thermal conductivity of at least 0.3, e.g. aluminum. They
may be secured together and/or to the heater by means of
an adhesive, e.g. an epoxy adhesive.
It is important that the envelope and the PTC
heater should be in intimate thermal contact, and the
envelope preferably contacts (either directly or through
an adhesive) at least the areas of the insulating jacket
adjacent those parts of the heater in which heat is
generated. Preferably there are no voids between the
envelope and the heater. It is preferred that the envelope
should extend outwards from the PTC heater in the form of
fins, so that the exposed surface area of the envelope is
at least 1.5 times, e.g. at least 2 times, the surface
area of the :insulating jaoket of the PTC heater, especially
at least 2 times. The envelope may also serve to limit
access of oxygen to the PTC composition as taught by Canadian
Application Serial No. 340~963 filed November 30, 1979.
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The heater assemblies can comprise two or more PTC
heaters. The heaters may be spaced apart from each
other and connected by an envelope which surrounds each
of them e.g. a plurality of parallel strip heaters
sandwiched between a pair of metal sheets.
The invention is illustrated by example only in
the accompanying drawings, in which
Figure 1 is an isometric view, partly in cross-
section, of a heater assembly; and
Figure 2 is a graph showing the relationship
between the electrical current and the
substrate temperature in the tests described
in the Example.
Referring now to Figure 1, this shows a PTC heater
comprising electrodes 1 and 2 embedded in a strip 3 of
a PTC conductive polymer composition which is sur-
rounded by an insulating jacket 4. The heater is
sandwiched between a pair of aluminum sheets 5 and 6
which are bonded to each other and to the heater by
means of an adhesive (now shown).
The invention is further illustrated by the
following Exarnple.
EXAMPLE
_ _
The PTC heater used was a 61 cm length of a PTC
strip heater as shown in Figure l. The electrodes
were copper wires 1.3 mm in diameter, with a center-to-
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center spacing of 7.6 mm. The PTC element was 10 mm wide
and 1.8 mm thick and was composed of a composition com-
prising a dispersion of carbon black in a mixture of
polyethylene and an ethylene/ethyl acrylate copolymer.
The insulating jacket was composed of a polyurethane and
was about 0.25 mm thick. The heater had a passive power
output of about 250-275 watts/metre. The active power
output of this heater, alone or as part of a heater
assembly, was measured by securing the heater or heater
assembly to an aluminum plate 1.25 x 15.25 x 61 cm,
connecting the heater to a 120 volt AC power supply and
allowing the system to reach equilibrium while maintaining
the plate at a desired temperature. In the tests, the
heater alone (Sample A) or the heater sandwiched between
two identical aluminum sheets 61 cm long and 0.003 cm
thick, and having widths of 11.4, 6.3, 3.8 and 1.9 cm
(Samples B, C, D and E) was used. Figure 2 shows the
relationship between the temperature of the plate and the
current passing through the heater. The Table below shows
the calculated active power output (current x applied
voltage) of the heater when the plate is at 50F (lODC).
TABLE
Width of Active Power
SampleMetal Envelope (cm)at 10C (watts/metre)
A none 70
B 11.4 140
C 6.3 145
D 3.8 150
E 1~9 140
