Abstrict
A radiant electric heater is provided with at least one covering
sheet (7) made of mica which is at least partially transparent.
The at least partially transparent mica may be in the form of natural
mica.
Claims
What is claimed is:
1. A radiant electric heater provided with only one single continuous
covering sheet consisting solely of mica which is at least partially
transparent.
2. A heater according to claim 1, wherein the at least partially
transparent mica comprises naturally occurring mica.
3. A heater according to claim 1, wherein there is provided at
least one electrical heating element selected from wire, ribbon
and lamp form.
4. A heater according to claim 3, wherein a base of insulation
material is provided having supported relative thereto the at least
one electrical heating element.
5. A heater according to claim 4, wherein the insulation material
comprises microporous thermal insulation material.
6. A heater according to claim 5, wherein the microporous insulation
material is compacted into a dish-like support.
7. A heater according to claim 6, wherein the dish-like support
comprises metal.
8. A heater according to claim 3, wherein the at least one covering
sheet of at least partially transparent mica is provided spaced
from the electrical heating element.
9. A heater according to claim 1, wherein a peripheral wall of
thermal insulation material is provided.
10. A heater according to claim 9, wherein the covering sheet of
at least partially transparent mica is arranged in contact with
an outer surface of the peripheral wall.
11. A heater according to claim 1 further including, an apertured
member overlying the covering sheet of at least partially transparent
mica.
12. A heater according to claim 11, wherein the apertured member
comprises a perforated sheet selected from metal, ceramic and other
suitable refractory material.
13. A radiant electric heater provided with only one single continuous
covering sheet, the covering sheet consisting solely of mica which
is at least partially transparent; and an apertured member, consisting
of metal, overlying the covering sheet.
14. A heater according to claim 13, wherein the at least partially
transparent mica comprises naturally-occurring mica.
15. A heater according to claim 13, wherein there is provided at
least one electrical heating element selected from wire, ribbon,
and lamp form.
16. A heater according to claim 15, wherein a base of insulation
material is provided having supported relative thereto the at least
one electrical heating element.
17. A heater according to claim 16, wherein the insulation material
comprises microporous thermal insulation material.
18. A heater according to claim 17, wherein the microporous insulation
material is compacted into a dish-like support.
19. A heater according to claim 18, wherein the dish-like support
comprises metal.
20. A heater according to claim 15, wherein the covering sheet
is provided spaced from the at least one electrical heating element.
21. A heater according to claim 13, wherein a peripheral wall of
thermal insulation material is provided.
22. A heater according to claim 21, wherein the covering sheet
is arranged in contact with an outer surface of the peripheral wall.
23. A heater according to claim 13, wherein the apertured member
is a perforated sheet.
Description This invention relates to radiant electric heaters which are provided
with a covering sheet. Such heaters may include one or more heating
elements which is or are supported on or adjacent to a base of thermal
and/or electrical insulation material.
DESCRIPTION OF PRIOR ART
Such heaters are well known, particularly in cooking appliances,
in which one or more heating elements such as of coiled wire form,
or of ribbon form, or of lamp form, is or are supported on or adjacent
to a base of insulation material which may be provided in a support
dish such as of metal. It is particularly well known and advantageous
to use, as a base, compacted particulate microporous thermal insulation
material.
Heaters of this kind have generally been used beneath a sheet of
glass-ceramic material in a smooth top cooking appliance or behind
such a sheet in oven or grill applications. One example of a radiant
heater incorporating a glass-ceramic sheet is known from GB-A-2
170 590.
It is well known that cooking operations are commonly accompanied
by issuance of steam and other vapours and spillages and that splattering
of liquids and food particulates occur on and in cooking appliances.
The sheet of glass-ceramic material normally prevents contact between
the heater and such vapours, liquids and particulates. However such
glass-ceramic material is expensive and also reduces the thermal
performance of the heater.
There are applications for heaters, such as in ovens, grills, deep
fat fryers, where it would be desirable to dispense with the use
of a glass-ceramic sheet. However it is known that microporous insulation
materials are seriously degraded when exposed to steam, water and
other liquids or vapours and can also be damaged by contact with
some particulate materials.
Furthermore, damage to the heating element may also occur by exposure
to some liquids, vapours or particulate materials.
It is also possible that, when a heater is located in an inverted
orientation in a cooking appliance, particles of insulation material
from the heater may fall into a cooking area of the appliance, which
may be disadvantageous.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or minimise
these problems.
SUMMARY OF THE INVENTION
The present invention provides a radiant electric heater provided
with at least one covering sheet, wherein the covering sheet comprises
mica which is at least partially transparent.
The at least partially transparent mica may comprise natural mica.
The heater may include at least one electrical heating element
of wire, ribbon or lamp form. A base of insulation material may
be provided, such as of microporous thermal insulation material,
having supported relative thereto (that is, on or adjacent to the
base) the at least one electrical heating element.
The microporous thermal insulation material may be compacted into
a dish-like support, such as of metal.
The at least one covering sheet of at least partially transparent
mica is suitably provided spaced from the at least one electrical
heating element.
A peripheral wall of thermal insulation material may be provided
for the heater. The at least one covering sheet of at least partially
transparent mica may be arranged in contact with an outer surface
of the peripheral wall.
An apertured member may be provided in contact with the at least
one covering sheet of at least partially transparent mica. The apertured
member may serve as a support for the at least one covering sheet
and/or as an electrical screen for the heater. Such apertured member
may comprise a perforated sheet of metal, ceramic or other suitable
refractory material.
The heater of the invention is particularly applicable to cooking
appliances such as grills, ovens, especially microwave ovens, deep
fat fryers.
The at least partially transparent mica applied in sheet form is
economically advantageous compared to high temperature withstanding
glass or glass-ceramic sheet form materials. Such transparent mica
can be applied as a thin sheet, for example of the order of 50 microns
in thickness, and exhibits low thermal mass. It also possesses good
transmission of visible and infra-red radiation and because of its
excellent electrical insulation properties, closer proximity between
the heating elements in the heater and adjacent metal components
of an appliance in which the heater is used can be tolerated compared
with the prior art.
Furthermore, the at least one sheet of at least partially transparent
mica serves as a barrier to vapours and particulate materials, such
as arising from cooking operations, and prevents such vapours and
particulate materials from contacting the one or more heating elements
in the heater and also insulation provided in the heater.
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 bottom plan view of a radiant electric heater according
to the invention; and
FIG. 2 is a cross-sectional view, in partly exploded form, of the
heater of FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to the drawings, a radiant electric heater 1 for a cooking
appliance (not shown) comprises a metal dish-like support 2 provided
with a base layer 3 of compacted microporous thermal insulation
material. A heating element 4 is provided secured to the base layer
3. As shown, the heating element 4 comprises a corrugated metal
ribbon mounted edgewise on the base layer 3 and secured by partial
embedding in the base layer 3. Such a heating element is well known
to the skilled person. The heating element 4 could instead be of
coiled wire or coiled ribbon form or of lamp form or of any other
form known to the skilled person. More than one form of heating
element could also be provided in the heater.
A terminal block 5 for the heating element is mounted on the heater
and allows the heater to be connected to a voltage supply for operation.
A peripheral wall 6 of thermal insulation material is provided
in the heater. As shown, this wall is integral with the base layer
3. However it could be provided as a separate member, as is well
known in the art.
In order to prevent contact with internal components of the heater,
such as the heating element 4 and insulation 3, by vapours and/or
particulate materials during operation of the heater, for example
in a cooking appliance, at least one sheet 7 of at least partially
transparent natural mica is provided. Such sheet 7 contacts the
outer surface 8 of the peripheral wall and covers the heater and
is suitably secured by bent over tags 9, such as of metal and which
may be integral with the metal support 2. If desired, there may
be more than one sheet 7. For maximum thermal efficiency the sheet
or sheets 7 should be as thin as is practically possible and may
typically be about 50 microns thickness overall. Such at least partially
transparent natural mica is sometimes referred to as clear natural
mica and allows adequate transmission therethrough of visible and
infra-red radiation from the heating element 4.
A perforated sheet 10 is optionally provided overlying the one
or more sheets 7 of substantially transparent mica. Such perforated
sheet 10 may serve a twofold purpose. It provides support for the
mica sheet or sheets 7, enabling the thickness of the sheet or sheets
7 to be minimised and can comprise a metal or a ceramic, or other
suitable refractory material. In applications of the heater such
as in microwave ovens, where electrical screening of the heater
is required in respect of electromagnetic radiation generated elsewhere
in the oven, the perforated sheet 10, formed of metal, also or alternatively
fulfils this screening function. |