Abstrict
A radiant electric heater (1) includes a heating element (4) and
at least one covering sheet (7) spaced from the heating element.
The at least one covering sheet is in the form of a fabric comprising
glass filaments, ceramic filaments or metal filaments. Metal filaments,
such as of a high temperature withstanding alloy, may be woven in
the form of a fabric. Glass filaments, or ceramic filaments such
as of alumino-silicate material, alumino-boro-silicate material
or zirconia, may be of woven, knitted or mat form in the fabric.
An apertured member (10) may be provided in contact with, or adjacent,
to the covering sheet (7) of the fabric material.
Claims
Having thus described the invention, what is claimed is:
1. A radiant electric heater including an inner base wall and a
peripheral wall projecting outwardly therefrom to cooperatively
define an outwardly opening cavity; at least one heating element
contained in the cavity; and at least one covering fabric sheet
operatively supported by the peripheral wall in overlying relationship
to the cavity and separated from the heating element by an air space,
the base wall and peripheral wall being formed of thermal insulating
material, and the at least one fabric covering sheet being formed
of filaments selected from ceramic filaments and metal filaments.
2. A radiant electric heater according to claim 1, wherein the
metal filaments comprise a high temperature withstanding alloy.
3. A radiant electric heater according to claim 2, wherein the
alloy comprises a stainless steel.
4. A radiant electric heater according to claim 1, wherein the
ceramic filaments are selected from alumino-silicate material, alumino-boro-silicate
material and zirconia.
5. A radiant electric heater according to claim 1, wherein the
glass filaments comprise glass fibres selected from E glass, C glass,
R glass and S glass.
6. A radiant electric heater according to claim 1, wherein the
weight per unit area of the fabric is not greater than 1 kg/m.sup.2.
7. A radiant electric heater according to claim 6, wherein the
weight per unit area of the fabric is in the range from 50 to 300
g/m.sup.2.
8. A radiant electric heater according to claim 1 wherein the insulation
material comprises microporous thermal insulation material.
9. A radiant electric heater according to claim 1 wherein the insulation
material is provided in a dish-like support.
10. A radiant electric heater according to claim 9 wherein the
dish-like support comprises metal.
11. A radiant electric heater according to claim 1 wherein the
at least one covering sheet of the fabric is arranged to overlie
an outer surface of the peripheral wall.
12. A radiant electric heater according to claim 1, wherein the
at least one heating element is of ribbon form.
13. A radiant electric heater according to claim 1, wherein an
apertured member is provided in contact with the at least one covering
sheet of the fabric.
14. A radiant electric heater according to claim 13, characterised
in that the apertured member is selected from a perforated sheet
and a lattice arrangement of material selected from metal, ceramic
and refractory material.
15. A radiant electric heater according to claim 1, wherein an
apertured member is provided adjacent to the at least one covering
sheet of the fabric.
16. A radiant electric heater according to claim 15, characterised
in that the apertured member is selected from a perforated sheet
and a lattice arrangement of material selected from metal, ceramic
and refractory material.
17. A radiant electric heater including an inner base wall and
a peripheral wall projecting outwardly therefrom to cooperatively
define an outwardly opening cavity; at least one heating element
contained in the cavity; and at least one covering fabric sheet
operatively supported by the peripheral wall in overlying relationship
to the cavity and to an outer surface of the peripheral wall, and
separated from the heating element by an air space, the base wall
and peripheral wall being formed of a microporous thermal insulation
material, and the at least one fabric covering sheet being formed
of filaments selected from ceramic filaments and metal filaments.
18. A radiant electric heater according to claim 17, wherein the
heater further includes a metal, dish-like support for the insulation
material.
Description This invention relates to a radiant electric heater such as of
the type in which one or more radiant electric heating elements
is or are supported relative to (that is, on or adjacent to) a base
of thermal and/or electrical insulation and/or reflective material.
FIELD OF THE INVENTION
The invention is particularly, but not exclusively, applicable
to such heaters for use in cooking appliances, such as grills or
ovens, including microwave ovens. It is, however, also applicable
to such heaters for general applications, such as space heating
and industrial thermal processing.
DESCRIPTION OF PRIOR ART
It is well known, particularly in cooking appliances, to provide
radiant heaters in which one or more heating elements, such as of
wire form, or ribbon form, or lamp form, is or are supported on
or adjacent to a base of insulation material and/or reflective material,
which may be provided in a housing, such as a support dish, such
as of metal. It is particularly well known and advantageous to use,
as a base, compacted microporous thermal and electrical insulation
material.
For safe and satisfactory operation of such heaters it is required
that manual contact with energised heating elements therein should
be prevented. Furthermore, particulates such as of food materials
should be prevented from contacting the heating elements and any
insulating and/or reflecting materials provided in the heater. Such
particulates, and also liquids, may be splattered towards the heater
during cooking operations and vapours may also be emitted towards
the heater.
A further problem, particularly encountered with heaters for use
in ovens and grills, is that particulate materials in the heater,
such as particles of insulation material, may become dislodged and
must be prevented from falling into the oven or grill and contaminating
food being processed therein.
The aforementioned problems have hitherto been solved by operating
the heater behind a sheet of glass-ceramic material. Such glass-ceramic
material is expensive and also reduces the thermal performance of
the heater.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to provide a
radiant electric heater which eliminates or at least ameliorates
the above problems without the use of a sheet of glass-ceramic material.
SUMMARY OF THE INVENTION
According to the present invention there is provided a radiant
electric heater including a heating element, wherein at least one
covering sheet is spaced from the heating element, the at least
one covering sheet being in the form of a fabric comprising glass
filaments, ceramic filaments or metal filaments.
The metal filaments may be of woven form in the fabric.
The metal filaments may comprise a high temperature withstanding
alloy such as stainless steel.
The glass or ceramic filaments may be of woven, knitted or mat
form in the fabric.
The ceramic filaments may comprise alumino-silicate material or
alumino-boro-silicate material or zirconia.
The glass filaments may comprise glass fibres selected from E glass,
C glass, R glass, S glass and modifications thereof.
The weight per unit area of the fabric may be generally both greater
than 1 kg/m.sup.2 and may preferably be in the range from 50 to
300 g/m.sup.2.
The heating element may be supported on or adjacent to a base of
insulation material, such as of microporous thermal insulation material.
The insulation material may be provided in a dish-like support,
such as of metal.
A peripheral wall of thermal insulation material may be provided
for the heater. The at least one covering sheet of the fabric may
be arranged to overlie an outer surface of the peripheral wall.
The heater may include one or more heating elements of wire, ribbon
or lamp form.
An apertured member may be provided in contact with, or adjacent
to, the at least one covering sheet of the fabric. For example,
the apertured member may serve as a support for the at least one
covering sheet and/or as an electrical screen when formed of electrically
conductive material and/or as protection, for the heater. The apertured
member may comprise a perforated sheet or a lattice arrangement,
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. However it is also applicable in other applications,
such as in space heating and in industrial thermal processing operations.
The at least one sheet of the fabric serves as an efficient barrier
to particulate materials and liquid splashing, such as arising from
cooking operations. It also reduces passage into the heater of vapours.
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 plan view of a radiant electric heater according to
the present invention;
FIG. 2 is a cross-sectional view, in partly exploded form, of the
heater of FIG. 1.
FIG. 3 is a fragmentary plan view showing, as an alternative perforated
member, a lattice arrangement of material; and
FIG. 4 is a fragmentary sectional view of the material of FIG.
3.
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to the drawings, a radiant electric heater 1 comprises
a metal dish-like support 2 provided with a base layer 3 of compacted
microporous thermal and electrical insulation material. As shown
in FIG. 1, the dish-like support 2 is formed with a plurality of
depressed regions of arcuate form which permit the material of the
base layer 3 to engage around the arcuate portions to assist in
securing the base layer in the dish-like support.
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 particulate materials
and to reduce ingress of vapours, during operation of the heater,
for example in a cooking appliance, at least one sheet 7 of a fabric
comprising glass, ceramic or metal filaments is provided spaced
from the heating element. When metal filaments are employed, these
comprise a suitably high temperature withstanding alloy, such as
a stainless steel, and fine wire filaments, which may be as little
as 25 microns in diameter, are woven together to form the fabric.
When glass or ceramic filaments are employed, these may be woven,
knitted or matted to form the fabric.
The ceramic filaments may comprise alumino-silicate materials,
such as supplied by the Carborundum Company Ltd. under the trade
names Fibertex and Fibersil.
A fabric of woven alumino-boro-silicate filaments may be employed,
such as supplied by 3M Company under the trade name Nextel.
A zirconia cloth fabric may also be considered, for example as
supplied by Zircar Products Inc.
The glass filaments may comprise fibres of E glass, C glass, R
glass or S glass or modifications thereof. For example, the boron
oxide in E glass may be replaced by magnesium oxide, such a glass
being commercially available, under the trade name Advantex, from
OCF Corporation.
The weight per unit area of the fabric sheet 7 should be arranged
to be as small as is practicable to ensure optimum thermal efficiency
of the heater. In general the weight per unit area of the fabric
should be less than 1 kg/m.sup.2, with a typical weight per unit
area being in the range from 50 to 300 g/m.sup.2 for glass and ceramic
filament fabrics, but somewhat greater for metal filament fabrics.
The sheet or sheets 7 are at least partially translucent to allow
the transmission therethrough of visible and infra-red radiation
from the heating element 4. Additionally, especially in the case
of metal filament fabrics, the fabric may absorb and reradiate radiation
from the heating element.
The or each sheet 7 overlies the outer surface 8 of the peripheral
wall and covers the heater and is suitably secured by adhesive,
or staples, or bent over tags 9 which may be integral with the metal
support 2. It should be noted that in the partially exploded view
of
FIG. 2 the bent over tags 9 will in practice sandwich the or each
sheet 7 between the tags 9 and the peripheral wall. Alternatively
the sheet or sheets 7 may be secured at edges thereof to the outside
of the support 2 by adhesive or other means.
Thus the heating element 4 is spaced from the or each sheet 7 and
there is no contact therebetween. The spacing eliminates direct
contact between the heating element 4 and the sheet or sheets 7
so as to minimise the thermal capacity of the heating element and
to enable the same to reach full operating temperature as quickly
as possible. The spacing also protects the user from the electrical
potential of the heating element, especially where the sheet or
sheets are of metal filaments.
An apertured sheet 10 is optionally provided overlying the one
or more sheets 7 of the fabric. Such apertured sheet may serve more
than one purpose. It may provide support and/or protection for the
sheet or sheets 7 of the fabric, enabling the thickness of the sheet
or sheets to be minimised and also minimising the risk of manual
contact with internal components of the heater. The apertured sheet
can comprise perforations 10A in a metal or ceramic sheet or the
sheet can be of expanded metal form 10B, or other appropriate lattice-form.
The sheet 10 could alternatively comprise any suitably apertured
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 apertured sheet 10,
formed of metal, also or alternatively fulfils this screening function.
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