Note: Descriptions are shown in the official language in which they were submitted.
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21~8~87 3~ 3u~e 199~
ELECTRIC HEATER AND A HETHOD OF MA~ING A HEATER
This invention relates to an electric heater and
a method of making a heater.
According to a first aspect of the lnvention
there is provided a method of making an electric heater
comprising a heating element encapsulated in a rigid
plastics moulding and means for connecting the heating
element to a source of electricity externally of the
moulding, the method comprising the steps of:-
(a) placing the heating element between twosheets of a moulding compound, and
(b) subjecting the assembly comprising the
heating element and the two sheets of moulding compound to
a compression moulding step,
wherein
(c) part of the connecting means adjacent to,
and not disposed between, the two sheets of moulding
compound i5 protected from exposure to excessive heat
during the moulding step.
Preferably, the part of the connecting means
adjacent to, and not disposed between, the two sheets of
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mouldlng compound is contained within a metal retaining
device during the moulding step. In this case, the metal
retaining device is, preferably, positively located in the
mould during the moulding process.
Preferably, the heating element is in the form
of a flexible sheet.
Preferably, the heating element comprises an
electrically insulating substrate, such as a flexible
woven mat, coated with electrically resistive material,
such as a plastics (or other) material containing carbon
particles. In this latter case, the resistivity of the
electrically resistive material can be predetermined by
the amount of carbon in the plastics material.
Preferably, the heating element is disposed in
the rigid plastics moulding adjacent to a major external
surface of the moulding, and typically is spaced not
substantially more than about 2mm from said major surface
of the moulding.
In many applications it will be desirable to
arrange for the heating element to be substantially equi-
distantly spaced from each of two opposite major externalsurfaces of the moulding in order to radiate heat from
said two surfaces. However, in other applications it may
be desirable to arrange for the heating element to be
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closer one major surface than the other.
A metal plate may be secured to at least one of
the major external surfaces of the rigid plastics moulding
to increase the area over which heat is dispersed.
According to a second aspect of the invention
there is provided an electric heater made by a method
according to the first aspect of the invention.
According to a third aspect of the invention
there is provided a heating device comprising a casing, a
plurality of electric heaters according to the second
aspect of the invention mounted in spaced relationship in
the casing, and means for causing air to pass over the
electric heaters.
Preferably there are at least three of said
electric heaters and the spaces between adjacent pairs of
electric heaters are connected together in series.
According to a fourth aspect of the invention,
there is provided a heating device comprising a casing, at
least three electric heaters mounted in spaced
relationship in the casing, and means for causing air to
pass over the electric heaters, wherein each electric
heater comprises a heating element encapsulated in a rigid
plastics moulding and means for connecting the heating
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element to a source of electrlcity externally of the
moulding and wherein the spaces between adjacent pairs of
electric heaters are connected together in series.
The invention will now be more particularly
described, by way of example, with reference to the
accompanying drawing, in which:
Figure 1 is a partly cutaway perspective view of
a heater according to the first aspect of the present
invention,
Figure 2 is a perspective view illustrating one
step in the manufacture of a heating according to the
third aspect of the invention,
Figure 3 is a side view of part of a compression
moulding machine,
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Figures 4 to 6 show alternative embodiments of
a heater according to the present invention,
Figure 7 is a vertical section taken through
part of one embodiment of a heating device according to
the second aspect of the invention, and
Figure 8 is a sectional view taken along line
VIII - VIII of Figure 7.
Referring to Figure 1 of the drawings, the
heater shown therein is in the form of a panel and
comprises a heating element 10 encapsulated in a rigid
plastics moulding 11.
The heating element 10 is in the form of a
flexible sheet and comprises a woven glass-fibre mat 12
provided with a resistive coating of plastics material
containing carbon particles, and two copper ribbons 13
stitched or otherwise secured to opposite edges of the
Z0 mat 12.
The heating element 10 also has wire or cable
terminations 14 sheathed in rubber or pvc insulation and
soldered as at 15 to respective copper ribbons 13 for
connecting the heating element to a source of
electricity.
- In some circumstances, it may be desirable to
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provide four spaced copper ribbons 13 on the mat 12 to
avoid hot spots developing in the heating element 10.
In this case, alternate copper ribbons will be
electrically connected together.
The panel is typically about 4mm thick with
the heating element 10 substantially equi-~istantly
spaced from each major external surface of t~e panel.
It follows that the heating element 10 is within about
2mm of each major external surface of the panel. This
is important if heat is to be radiated by both major
surfaces as plastics material is a poor conductor of
heat. There may however be applications in which it is
desirable to radiate heat from only one major surface of
the panel or to radiate more heat from one than the
other surface. In this case, the heating element is
arranged closer to the one surface.
The encapsulating process may be carried out
by conventional moulding methods, including compression
moulding, transfer moulding, injection moulding,
casting, hand lay up G.R.P and extrusion moulding.
Typically, however, the encapsulating process is carried
out by compression moulding.
The encapsulating material may be any
appropriate heat resistant plastics material, including
both thermosetting and thermoplastics materials.
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Typically, the encapsulating material is a thermosetting
polyester but other plastics materials wili also
withstand temperatures of about 200 c without showing
signs of degradation. It is desirable that the plastics
material should also be impact resistant.
Certain polyester sheet moulding compounds
supplied by DSM Compounds UK Limited, of 5 Civ~c Way,
Ellesmere Port, South Wirral, have been found to be
suitable. In particular, the sheet moulding compound
sold under their grade no. 5520 has been found to be
suitable when a flame retardant material is required and
the sheet moulding compound sold under their grade no.
47-5710 has been found to be suitable when a chemically
resistant material is required.
The plastics material of the resistive coating
of the heating element 10 is typically a polyester based
compound and the resistivity of the coating can be
predetermined by the amount of carbon in the plastics
material in order to tailor the heating element 10 to
give specific watt densities. At given electrical input
values, this will determine the maximum attainable
temperature of a given heater and theoretically it is
possible to produce heating elements which will work on
any specific a.c. or d.c. voltage.
The maximum temperature attainable by the
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heater is dependent on the watt density of the heating
element 10, the power available and the temperature
characteristics of the encapsulating material.
A metal plate, typically of aluminium, may be
secured to at least one of the major external surfaces
of the moulding to increase the area over which heat is
dispersed.
One method of making the panel will now be
more particularly described with reference to Figures 2
and 3.
Firstly, referring to Figure 2, a sheet 16 of
polyester moulding compound is laid on a suitable non-
stick surface. The heating element 10 is then laid in
position directly on top of the sheet 16 and the cable
terminations 14 are located in a groove in the lower
half of a two part cable retaining device 17 supported
in a jig 18. A further sheet (not shown) of polyester
moulding compound is then laid directly over the heating
element 11 and pressed flat. The upper half (not shown)
of the cable retainlng device 17 is then located on the
lower half thereof and the two halves of the cable
retaining device are fastened together with sc~ews. The
purpose of the cable retaining device 17 is to prevent
the insulation on the cable terminations 14 rom
becoming damaged by heat during the moulding process.
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The device 17 may, therefore, be made of metal and it is
important that it should abut the edges of ~he two
sheets of moulding compound.
The assembly comprising the two sheets of
moulding compound, the heating element 10 and the cable
retaining device 17 is then placed into a compxession
moulding machine 20 (see Figure 3) and the mould is
closed. Two spigots 21, only one of which is shown, in
the lower part of the mould extend into holes 22 in the
cable retaining device 17 and a spring loaded plunger 23
in the upper part of the mould bears against the upper
surface of the retaining device 17, to positively
locate the retaining device 17 and ensure that the mould
cavity is closed.
After moulding, the aforesaid assembly is
removed from the mould 20 and cooled. The cable
retaining plate 17 is removed from the assembly and
used, after cooling in cold water, for a subsequent
moulding operation.
To ensure even flow of material in the mould,
it may prove necessary to provide additional moulding
material at appropriate positions on the outer surfaces
of sheets of moulding compound. This can be done by
laying strips of the moulding compound below the lower
sheet 16 and on top of the upper sheet.
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The panel need not be flat but could be
profiled, such as is shown in Figure 4, to suit a
particular application, this being made possible by the
flexible nature of the heating element 10.
The panel can be of any desired colour and, if
required, can have a decorative finish applied by
spraying or silk screen printing
The heater is fully insulated and water proof
enabling it to be immersed in a liquid or exposed to wet
or damp environments. Also, by ensuring that no air is
included in the panel, oxidation of the heating element
10 and the connections between the element 10 and the
terminations 14 will be prevented, thus ensuring a long
element life.
The heater described above can be used in any
application requiring a constant source of heat. For
example it has application in cabinet heaters (anti-
condensation/anti-frost panels), fishpond heaters, under
soil heaters, heated brewing pads, under desk heaters,
aquarium and vivarium heaters, ceiling heaterg, diesel
tank heaters (especially for motor vehicles),
incubators, plant and seed propagators, food heaters,
railway point heaters, and heated electrical insulators.
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In the case of ceiling heaters, these could
look like, and be substituted for, selected ceiling
tiles, particularly above work areas.
In the case of railway point heaters, these
could include a metal slipper attached to one surface of
the heater. The slipper could be coated in PTFE which
is chemically etched to the metal.
The heater need not be in the form of a
panel. It could be of any other appropriate shape, and
may, for example, be in the shape of a vessel, such as
is shown in Figure 5, or in the shape of a tube, such
as is shown in Figure 6.
Referring now to Figures 7 and 8 of the
drawings, the heating device shown therein comprises a
casing 30 and a heater unit 31, which comprises a
plurality of spaced apart heaters 32 (typically six in
number), mounted in the casing 30. The heating device
also comprises a fan 33 for drawing air in through an
air intake opening 34 at the lower end of one side of
the casing 30 and for causing that air to pass serially
between adjacent pairs of heaters 32 before exiting
through an air outlet 35 at the upper end of the casing
30.
Each heater 32 has a metal plate 36, typically
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of aluminium, secured to each of its two major external
surfaces to disperse heat over substantially the entire
surface area of the heater. The heaters 32 are
assembled into the unit 31 by threaded rods 37 which
extend through aligned apertures in the heaters 32, by
spacers 38 mounted on the threaded rods 37 between
adjacent pairs of heaters 32, and by nuts 39 which co-
operate with the ends of the threaded rods 37-to clamp
' the heaters 32 and the spacers 38 together.
Adjacent heaters 32 are staggered vertically
with respect to one another, as best shown in Figure 8,
and panels 40 of thermally insulating material are fixed
to top and bottom of the heater unit 31 so as to extend
over all but the foremost heater 32 at the bottom and
the rearmost heater 32 at the top so that the spaces
between each pair of adjacent heaters are connected
together in series. The front, rear and two sides of
the heater unit 31 are also preferably covered by panels
of thermally insulating material.
The casing 30 is formed of aluminium and
comprises an inner lower part 3Oa, which extends below
and serves to support the heater unit 31 and to house
the fan 33, and an outer part 30_ which houses the
heater unit 31 and which extends over and is secured to
the lower part 30a, typically by rivets.
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The top of the outer casing part 30_ may be in
the form of a h~nged lid 30c which can be pivoted
between a position as shown- in Figure 8 in which it
defines the air outlet 35 and a closed position when the
heating device is not being used.
The heating device also comprises an on/off
switch 42, an indicator light 43, a cable clamp 44, and
a thermal cut-out device 45.
The heating device described with reference to
Figures 7 and 8 is a space heater which will run off a
24 volt supply, typically drawing about 8 amps. It has
particular application as a heater for the cab of a
heavy goods vehicle which it can heat without
difficulty.
The embodiments described above are given by
way of example only and various modifications will be
apparent to persons skilled in the art without departing
from the scope of the invention as defined by the
appended claims. For example, whilst it is highly
desirable for the heating element to be flexible, it
need not necessarily comprise a woven mat coated with
resistive material. It could instead comprise any other
appropriate electrically insulating substrate supporting
resistive material or could be a resistive sheet such as
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a mat formed from a single aluminum strand. Also, the
copper ribbons could be replaced by any other
appropriate conductlve strips. Also, the heating
element could be made up of two or more parts connected
electrically in series.
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