Abstrict
A radiant electric heater is manufactured by a method which involves
providing a base of microporous thermal and electrical insulation
material having at least one groove formed in a surface thereof,
and providing an elongate electrically conductive strip to serve
as a heating element. The elongate electrically conductive strip
is located edgewise into the groove and surface pressure is applied
to the base of microporous insulation material in a region adjacent
to the strip to deform the base and to urge microporous material
of the base into contact with the strip so as to secure the strip
in the groove.
Claims
I claim:
1. A method of manufacturing a radiant electric heater comprising
the steps of: providing a base of microporous thermal and electrical
insulation material having at least one groove formed in a surface
thereof; providing an elongate electrically conductive strip to
serve as a heating element; locating the elongate electrically conductive
strip edgewise into the groove; and applying surface pressure to
the base of microporous insulation material in a region adjacent
to the strip to deform the base and to urge microporous material
of the base into contact with the strip so as to secure the strip
in the groove.
2. A method according to claim 1, wherein the surface pressure
is applied to cause controlled deformation of the base by effecting
compaction of the microporous material at selected locations thereof.
3. A method according to claim 1, wherein the surface pressure
is applied to cause controlled deformation of the base by effecting
compaction of the microporous material over substantially the entire
area thereof where the strip is located.
4. A method according to claim 1, wherein application of the pressure
is effected at opposite sides of the strip.
5. A method according to claim 4, wherein application of the pressure
at opposite sides of the strip is effected substantially simultaneously.
6. A method according to claim 1, wherein the pressure is applied
manually by means of one or more suitable press tools.
7. A method according to claim 1, wherein the pressure is applied
mechanically by means of one or more suitable press tools.
8. A method according to claim 1, wherein the groove is formed
of such a depth that the strip after securement protrudes from the
surface of the base of microporous insulation material.
9. A method according to claim 1, wherein the base of microporous
insulation material is provided as a compacted layer inside a supporting
dish.
10. A method according to claim 1, wherein the surface of the base
of microporous insulation material in which the groove is provided
is substantially planar.
11. A method according to claim 1, wherein the electrically conductive
strip is of corrugated form along its length.
12. A method according to claim 1, wherein the strip comprises
a metal or a metal alloy.
13. A method according to claim 12, wherein the metal alloy comprises
an iron-chromium-aluminium alloy.
Description This invention relates to a method of manufacturing a radiant electric
heater and, more particularly but not exclusively, relates to a
method of manufacturing a radiant electric heater for use with a
glass-ceramic smooth top cooker.
BACKGROUND TO THE INVENTION
Radiant electric heaters are known in which an element of coiled
bare electric resistance wire is supported on, and secured by staples
to, a layer of microporous thermal and electrical insulating material
compacted in a metal support dish. Such heaters are described, for
example, in GB-A-1 580 909 and are incorporated in glass-ceramic
smooth top cookers.
The term `microporous` is used herein to identify porous or cellular
materials in which the ultimate size of the cells or voids is less
than the mean free path of an air molecule at NTP, i.e. of the order
of 100 nm or smaller. A material which is microporous in this sense
will exhibit very low transfer of heat by air conduction (that is
collisions between air molecules). Such microporous materials include
aerogel, which is a gel in which the liquid phase has been replaced
by a gaseous phase in such a way as to avoid the shrinkage which
would occur if the gel were dried directly from a liquid. A substantially
identical structure can be obtained by controlled precipitation
from solution, the temperature and pH being controlled during precipitation
to obtain an open lattice precipitate. Other equivalent open lattice
structures include pyrogenic (fumed) and electro-thermal types in
which a substantial proportion of the particles have an ultimate
particle size less than 100 nm. Any of these particulate materials,
based for example on silica, alumina or other metal oxides, may
be used to prepare a composition which is microporous as defined
above.
The microporous insulation typically comprises a dry particulate
microporous material as defined hereinabove mixed with ceramic fibre
reinforcement, titanium dioxide opacifier and, for high temperature
use, a small quantity of alumina powder to resist shrinkage. Such
insulation material is described in GB-A-1 580 909.
Radiant electric heaters have also been proposed in which, instead
of an element of coiled resistance wire, an element comprising an
elongate electrically conductive strip of a metal or metal alloy
is provided, the element being supported on edge on an insulating
base. Arrangements of this kind are described, for example, in U.S.
Pat. Nos. 600 057, 3,612,829, 3,991,298, 4,161,648 and 4,292,504.
In U.S. Pat. No. 600,057, a conductor is mounted on a metal support,
or in a groove formed therein, by means of a coating of insulating
material such as a vitreous enamel. In U.S. Pat. No. 3,612,829,
a convoluted conductive strip element in the form of a spiral is
located in recesses pre-formed in the surface of a cast or moulded
fibrous ceramic refractory material. Staples are used to secure
the strip element to the supporting base. In U.S. Pat. No. 3,991,298,
the conductive strip element is in the form of a spiral and is loose
fitted in a pre-formed spiral groove in a rigid base of fire-resistant
mortar.
In U.S. Pat. No. 4,161,648, a convoluted strip element of spiral
form is provided with integral downwardly-extending mounting tabs
which penetrate an electrically insulating sheet of high-temperature-withstanding
board material and in the case of thin material may be bent over
at the back of the material. The board-like insulating sheet with
the element thereon is then located on top of a layer of microporous
thermal insulation material in a supporting dish. In the case of
a thick sheet of board material, a hardenable substance is used
and is hardened after the tabs have been urged into the material.
In U.S. Pat. No. 4,292,504, a heating element in the form of a
thin, foil-like strip of expanded metal is supported on edge substantially
along its entire length in a serpentine groove formed in the upper
surface of a ceramic fibreboard. The heating element is cemented
or held by friction in the groove formed in the board.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a method of
manufacturing a radiant heater in which an elongate electrically
conductive strip heater element is secured directly to a base of
thermal and electrical insulation material without the need for
mounting tabs or staples or any other additional securing means
or process.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of
manufacturing a radiant electric heater comprising the steps of:
providing a base of microporous thermal and electrical insulation
material having at least one groove formed in a surface thereof;
providing an elongate electrically conductive strip to serve as
a heating element; locating the elongate electrically conductive
strip edgewise into the groove; and applying surface pressure to
the base of microporous insulation material in a region adjacent
to the strip to deform the base and to urge microporous material
of the base into contact with the strip so as to secure the strip
in the groove.
The surface pressure may be applied to cause controlled deformation
of the base by effecting compaction of the microporous insulation
material either at selected locations thereof, or over substantially
the entire area thereof where the strip is located.
Application of the surface pressure is preferably effected at opposite
sides of said strip, preferably substantially simultaneously.
The pressure may be applied manually or mechanically by means of
one or more suitable press tools.
The groove may be formed of a depth selected according to the extent,
if any, to which the strip after securement protrudes from the surface
of the base of microporous insulation material.
The base of microporous insulation material is suitably provided
as a compacted layer inside a supporting dish, suitably of metal.
The surface of the base of microporous insulation material in which
the groove is provided is preferably substantially planar.
Preferably the said electrically conductive strip is of corrugated
(also known as sinuous, serpentine or convoluted) form along its
length.
The strip suitably comprises a metal or a metal alloy, such as
an iron-chromium-aluminium alloy.
Suitable microporous thermal and electrical insulation materials
are well-known in the art, for example as described in GB-A-1 580
909, a typical composition being:
______________________________________ Microporous pyrogenic silica
49 to 97% by weight Ceramic fibre reinforcement 0.5 to 20% by weight
Opacifier 2 to 50% by weight Alumina up to 12% by weight ______________________________________
The proportion of alumina is preferably in the range from 0.5 to
12 percent by weight.
The invention is now described by way of example with reference
to the accompanying drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a heating element comprising an
electrically conductive strip, for use in the manufacture of a radiant
electric heater in accordance with the present invention;
FIG. 2 is a plan view of a base for use in the manufacture of a
radiant electric heater in accordance with the present invention,
for receiving the heating element of FIG. 1;
FIG. 3 is a plan view of a radiant electric heater manufactured
in accordance with the present invention, comprising the components
of FIGS. 1 and 2;
FIG. 4 is a view of part IV of FIG. 3 on a larger scale;
FIG. 5 is a cross-sectional view of the radiant electric heater
of FIG. 3;
FIG. 6 is a view of part VI of FIG. 5 on a larger scale;
FIG. 7 is a perspective view of one embodiment of a metal rod for
applying surface pressure to the base of microporous insulation
material; and
FIG. 8 is a plan view of another radiant heater manufactured in
accordance with the present invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
A radiant electric heater is constructed comprising a metal dish
1 containing a base layer 2 of compacted microporous thermal and
electrical insulation material, having a substantially planar surface
and having a composition such as that described in GB-A-1 580 909.
A heating element 4 is provided from an elongate strip 5 of a metal
or metal alloy, such as an iron-chromium-aluminium alloy, having
a thickness of, for example, 0.05 to 0.2 mm and a height h of, for
example, 3 to 6 mm. The strip 5 is itself provided of corrugated
form (sometimes also known as sinuous, serpentine or convoluted
form) and is bent into the desired shape for the heating element
as shown in FIG. 1, using techniques well known in the art. It should
be noted, however, that the dimensions of thickness of the strip
quoted above are for the strip before making into corrugated form.
The surface of the base 2 of microporous insulation material is
provided with grooves 9 in a pattern corresponding to the shape
of the heating element 4. Such grooves 9 are suitably formed by
means of an appropriate molding tool during compaction of the microporous
insulation material into the dish 1 to form the base 2, or may be
machined into the surface of the base material after compaction.
The width of the grooves 9 is arranged to be at least as great as
the overall width (i.e. the `peak-to-peak` dimension) of the corrugated
strip 5.
The heating element 4 is then located with the base 2 so that the
strip 5 enters the matching grooves 9 edgewise. The depth of the
grooves 9 is selected such that, when inserted therein, the strip
5 protrudes from the base 2 to a required extent, such as, for example,
50 percent or more of the height h of the strip 5.
In order to secure the strip 5 in the grooves 9, controlled pressure
is applied locally to the surface of the base 2 in regions 11 adjacent
to the strip, on opposite sides thereof, to deform the base by compacting
the microporous material and urging the material into contact with
the strip 5. This is illustrated in FIG. 3 and, in more detail,
in FIG. 4 which shows on a larger scale that part of FIG. 3 identified
by the reference IV. One or more flat-ended metal rods, such as
the rod 12 illustrated in FIG. 7, could be used to apply the necessary
pressure, either manually or mechanically, and it may be preferable
to apply pressure simultaneously at opposite sides of the strip.
It will be apparent to the skilled person that a variety of techniques
could be used to apply the necessary pressure, either locally (as
shown in FIG. 3) or to the entire surface of the base 2 where the
strip is located (as shown in FIG. 8).
Against the side of the dish 1 is located a peripheral wall 3 of
thermal insulation material, such as a ceramic fibre material made
from aluminosilicate fibres, or alternatively microporous insulation
material.
A terminal connector 6 is provided for electrically connecting
the heating element 4 to an electrical supply.
A well-known form of thermal cut-out device 7 is provided, extending
over the heating element 4, to switch off the heating element in
the event of over-heating of the glass-ceramic cooking surface when
the heater is installed and operating in a cooking appliance having
such a glass-ceramic cooking surface.
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