Abstrict
A radiant electric heater is manufactured by a method which involves
providing a base of microporous thermal and electrical insulation
material having a substantially continuous surface, and providing
a heating element in the form of an elongate electrically conductive
strip. The strip is urged edgewise into the continuous surface of
the base of microporous thermal and electrical insulation material
so as to embed and support the strip edgewise in the insulation
material along substantially the entire length of the strip to a
depth corresponding to at least part of the height of the strip.
Claims
I claim:
1. A method of manufacturing a radiant electric heater comprising:
providing a base of microporous thermal and electrical insulation
material having a substantially continuous surface; providing a
heating element in the form of an elongate electrically conductive
strip having a predetermined height; and urging the strip edgewise
into the continuous surface of the base of microporous thermal and
electrical insulation material so as to embed and support the strip
edgewise therein along substantially the entire length of the strip
to a depth corresponding to part of the height of the strip.
2. A method according to claim 1, wherein the electrically conductive
strip is provided in corrugated form along its length.
3. A method according to claim 1, wherein the strip is embedded
such that a significant proportion of the height thereof protrudes
from the base of microporous insulation material.
4. A method according to claim 1, wherein the base of microporous
insulation material is provided as a compacted layer inside a supporting
dish.
5. A method according to claim 1, wherein the base of microporous
insulation material is formed with a surface of substantially planar
form into which the strip is urged.
6. A method according to claim 1, wherein the strip comprises a
metal or a metal alloy.
7. A method according to claim 6, wherein the alloy comprises an
iron-chromium-aluminium alloy.
8. A method of manufacturing a radiant electric heater comprising:
providing a supporting dish to receive a thermal insulation material;
compacting microporous thermal and electrical insulation material
in the dish to form a base having a substantially continuous surface
and a density less than a final compaction density thereof; providing
a heating element in the form of an elongate, electrically conductive
strip having a predetermined height; urging the strip edgewise into
the continuous surface of the base of microporous thermal and electrical
insulation material so as to embed and support the strip edgewise
therein along substantially the entire length of the strip to a
depth corresponding to part of the height of the strip; and further
compacting the microporous insulation material, during or subsequent
to urging the strip into the base, to obtain the final compaction
density.
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 molded
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. In the case of a thin sheet of board material, the
mounting tabs are 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. It will be
appreciated that a surface having a groove formed therein is not
a substantially continuous surface.
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.
SUMMARY OF THE INVENTION
According to the present invention there is provided a method of
manufacturing a radiant electric heater comprising: providing a
base of microporous thermal and electrical insulation material having
a substantially continuous surface; providing a heating element
in the form of an elongate electrically conductive strip having
a predetermined height; and urging the strip edgewise into the continuous
surface of the base of microporous thermal and electrical insulation
material so as to embed and support the strip edgewise therein along
substantially the entire length of the strip to a depth corresponding
to at least part of the height of the strip. Surprisingly, in view
of the particulate nature of the microporous insulation material,
the heating element when urged into the material remains securely
located during subsequent operation of the heater and no further
securing means or process is required.
Preferably, the electrically conductive strip is of corrugated
(also known as sinuous, serpentine or convoluted) form along its
length.
The strip is preferably embedded to the extent that a significant
proportion of the height thereof protrudes from 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 base of microporous insulation material may be formed with
a surface of substantially planar form into which the strip is urged.
The provision of the compacted layer may, if desired, involve more
than one process stage. In a first stage, the base may be formed
by compacting the layer of microporous insulation material in the
dish to less than its desired final compaction density; and then
in a second stage during or after urging the strip into the base,
further compaction of the microporous insulation material may be
effected to obtain the desired final compaction density for the
base.
The strip may comprise a metal, or 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 method according to
the present invention;
FIG. 2 iS a plan view of a base of a radiant electric heater for
use in the present invention, for receiving the heating element
of FIG. 1;
FIG. 3 is a plan view of a radiant electric heater comprising the
components of FIGS. 1 and 2 and made by the method according to
the present invention; and
FIG. 4 is a cross-sectional view of the radiant electric heater
of FIG. 3.
DESCRIPTION OF PREFERRED EMBODIMENTS
A radiant 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. However, if desired, the edge of the elongate
strip 5 may be profiled or provided with tabs for embedding in the
base layer 2 of compacted microporous thermal and electrical insulation
material. The strip 5 itself is provided in 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 resulting heating
element 4 is located in contact with the surface of the base 2 of
microporous thermal and electrical insulation material and pressure
is applied uniformly to the heating element 4 to urge the strip
material 5 thereof edgewise into the base 2 and thereby cause the
heating element 4 to become securely embedded in the base 2 to a
depth corresponding to at least part of the height h of the strip
5. The heating element 4 is preferably embedded in the base 2 of
microporous insulation material to not more than 50 per cent of
the height h of the strip 5. A terminal connector 6 is provided
for electrically connecting the heating element 4 to an electrical
supply, for operation thereof.
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 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.
The provision of the compacted layer may, if desired, involve more
than one process stage. In a first stage, the base 2 may be formed
by compacting the layer of microporous insulation material in the
dish 1 to less than its desired final compaction density; and then
in a second stage during or after urging the strip 5 into the base,
further compaction of the microporous insulation material may be
effected to obtain the desired final compaction density for the
base 2. |