Abstrict
A radiant electric heater is made by providing a base of microporous
thermal and electrical insulation material having formed in a surface
thereof at least one groove. The groove includes transverse webs
of the microporous insulation material spaced apart along the length
of the groove. An elongate electrically conductive strip is provided
to serve as a heating element and is located edgewise into the groove
and is urged into the webs so as to embed and secure the conductive
strip in the webs.
Claims
I claim:
1. A radiant electric heater comprising a base of microporous thermal
and electrical insulation material having a surface with at least
one groove formed therein, into which groove is located edgewise
an elongate electrically conductive strip of predetermined height
to serve as a heating element, the groove including transverse webs
of the microporous insulation material spaced apart along the length
of the groove and into which webs is embedded the conductive strip
so as to secure the conductive strip to the webs.
2. A radiant electric heater according to claim 1, wherein the
webs are provided substantially coplanar with that surface of the
base in which the groove is provided.
3. A radiant electric heater according to claim 1, wherein the
webs are provided below that surface of the base in which the groove
is provided.
4. A radiant electric heater according to claim 1, wherein the
located conductive strip protrudes from the webs in the base of
microporous insulation material such that the strip is not embedded
to its full height in the webs.
5. A radiant electric heater according to claim 1, wherein the
electrically conductive strip is of corrugated form along its length.
6. A radiant electric heater according to claim 1, wherein the
base of microporous insulation material is provided as a compacted
layer inside a supporting dish.
7. A radiant electric heater according to claim 1, wherein the
base of microporous insulation material has a surface of substantially
planar form in which the groove is provided.
8. A radiant electric heater according to claim 1, wherein the
strip comprises a metal or a metal alloy.
9. A radiant electric heater according to claim 8, wherein the
metal alloy comprises an iron-chromium-aluminium alloy.
Description This invention relates to a radiant electric heater for a cooker
and, more particularly but not exclusively, relates to a radiant
electric heater for use with glass-ceramic smooth top cookers. The
invention also relates to a method of manufacturing such a radiant
electric heater.
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. No. 600,057, U.S. Pat. No. 3,612,829, U.S. Pat. No. 3,991,298,
U.S. Pat. No. 4,161,648 and U.S. Pat. No. 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. 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.
In these known constructions, when the heating element is secured
in a base a two-stage fixing process is required, such as insertion
and subsequent stapling, bending over of tabs or cementing, or insertion
and hardening of the base.
OBJECT OF THE INVENTION
It is an object of the present invention to provide a radiant heater
and a method of manufacturing such 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 one aspect of the present invention there is provided
a radiant electric heater comprising a base of microporous thermal
and electrical insulation material having a surface with at least
one groove formed therein, into which groove is located edgewise
an elongate electrically conductive strip of predetermined height
to serve as a heating element, the groove including transverse webs
of the microporous insulation material spaced apart along the length
of the groove and into which webs is embedded the conductive strip
so as to secure the conductive strip to the webs.
According to another aspect of 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 formed in a surface thereof at least
one groove, the groove including transverse webs of the microporous
insulation material spaced apart along the length of the groove;
providing an elongate electrically conductive strip of predetermined
height to serve as a heating element; and locating the elongate
electrically conductive strip edgewise into the groove and urging
the strip into the webs so as to embed and secure the conductive
strip in the webs.
By means of the invention, the electrically conductive strip is
positively located by the groove(s) and securely fixed by embedding
in the webs.
Surprisingly, in view of the nature of the microporous insulation
material, the strip remains securely located during subsequent operation
of the heater.
The webs may be substantially coplanar with or below that surface
of the base in which the groove is provided. The located conductive
strip may protrude from the webs in the base of microporous insulation
material such that the strip is not embedded to its full height
in the webs.
Preferably the electrically conductive strip is of corrugated (also
know as sinuous, serpentine or convoluted) form along its length.
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 preferably has a surface
of substantially planar form in which the groove is provided.
The strip may comprise 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 a radiant electric heater
according to the present invention;
FIG. 2 is a plan view of a base of a radiant electric heater according
to the present invention, for receiving the heating element of FIG.
1;
FIG. 3 is a partial perspective view of a modification of the radiant
electric heater base shown in FIG. 2;
FIG. 4 is a plan view of a radiant electric heater according to
the present invention, comprising the components of FIGS. 1 and
2;
FIG. 5 is a cross-sectional view of the radiant electric heater
of FIG. 4; and
FIG. 6 shows part of FIG. 5 to a larger scale.
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 itself is provided of corrugated
form (sometimes also known as sinuous, serpentine or convoluted
form) and is bent into a 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. The grooves 9 are arranged to be at least
as wide as the overall width of the corrugated conductive strip
5. Transverse webs 10 of the same microporous material as base 2
are provided at spaced-apart locations along the grooves 9 and extending
from the bottom of the grooves 9. As shown in FIG. 2, the webs 10
are coplanar with the surface of the base 2, but they may alternatively
be provided below the surface of the base 2 as illustrated by the
webs 10' shown in FIG. 3. The grooves 9 and webs 10 are suitably
formed by means of an appropriate moulding tool during compacting
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 heating element 4 is then located on the base 2 and the heating
element strip 5 is urged edgewise into the grooves 9 as illustrated
in more detail in FIG. 6. At the same time, the strip 5 is urged
into and embedded in the webs 10 to a depth corresponding to at
least part of the height h of the strip 5 and becomes secured in
these webs.
Preferably the secured strip 5 protrudes from the base 2, for example,
by at least 50 per cent of the height h of the strip 5.
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, for operation thereof.
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.
|