Abstrict
A radiant electric heater includes a base (2) of thermal and electrical
insulation material and at least one electrical heating element
(4) supported on a surface (3) of the base. A rod-like temperature-responsive
device (10) extends across the heater from a periphery and is spaced
from the electrical heating element (4). The rod-like temperature-responsive
device (10) extends only partly across the heater and the surface
(3) of the base (2) has a substantially continuous concave profile,
at least where the at least one heating element (4) is supported,
for accommodating an end region (12) of the device.
Claims
I claim:
1. A radiant electric heater comprising a base of thermal and electrical
insulation material having a surface supporting at least one electrical
heating element and a rod-like temperature-responsive device extending
across the heater from a periphery thereof and spaced from the at
least one electrical heating element, wherein the rod-like temperature-responsive
device extends only partly across the heater, wherein the surface
of the base has a substantially continuous concave profile, at least
where the at least one heating element is supported, for accommodating
an end region of the temperature-responsive device, and wherein
the lowest point of the concave profile substantially coincides
with a centre of a heated area of the heater.
2. A heater according to claim 1, wherein the concave profile is
substantially in the form of a section of an interior of a sphere.
3. A heater according to claim 1, wherein the concave profile is
substantially in the form of an inverted shallow cone.
4. A heater according to claim 3, wherein the shallow cone is truncated.
5. A heater according to claim 1, wherein a peripheral wall of
thermal insulation material is provided, the peripheral wall having
a surface, lying substantially on a plane for contacting a plate,
the rod-like temperature-responsive device being arranged substantially
parallel to the plane of the surface of the peripheral wall.
6. A heater according to claim 5, further including a glass-ceramic
plate disposed in contact with the surface of the peripheral wall.
7. A heater according to claim 1, wherein the end region of the
temperature-responsive device, remote from the periphery of the
heater, is located at a greater distance from the surface of the
base than a region of the device nearer the periphery of the heater.
8. A heater according to claim 1, wherein the end region of the
temperature-responsive device remote from the periphery of the heater
is located substantially in the vicinity of a region of maximum
concave depression of the surface of the base.
9. A heater according to claim 1, wherein at least the end region
of the temperature-responsive device remote from the periphery of
the heater comprises a metal.
10. A heater according to claim 1, wherein at least the end region
of the temperature-responsive device remote from the periphery of
the heater overlies the at least one electrical heating element.
11. A heater according to claim 1, wherein the at least one electrical
heating element is arranged substantially in the form of a spiral.
12. A heater according to claim 1, wherein a distance between the
rod-like temperature-responsive device and the at least one heating
element increases progressively with increasing distance from the
periphery of the heater towards a centre region thereof.
13. A radiant electric heater comprising a base of thermal and
electrical insulation material having a surface supporting at least
one electrical heating element and a rod-like temperature-responsive
device extending across the heater from a periphery thereof and
spaced from the at least one electrical heating element, wherein
the rod-like temperature-responsive device extends only partly across
the heater, wherein the surface of the base has a substantially
continuous concave profile, at least where the at least one heating
element is supported, for accommodating an end region of the device,
wherein the lowest point of the concave profile substantially coincides
with a centre of a heated area of the heater, and wherein a distance
between the rod-like temperature-responsive device and the at least
one heating element increases progressively with increasing distance
from the periphery of the heater towards a centre region thereof.
14. A heater according to claim 13, wherein the concave profile
is substantially in the form of a section of an interior of a sphere.
15. A heater according to claim 13, wherein the concave profile
is substantially in the form of an inverted shallow cone.
16. A heater according to claim 15, wherein the shallow cone is
truncated.
17. A heater according to claim 13, wherein a peripheral wall of
thermal insulation material is provided, the peripheral wall having
a surface, lying substantially on a plane, for contacting a plate,
the rod-like temperature-responsive device being arranged substantially
parallel to the plane of the surface of the peripheral wall.
18. A heater according to claim 17 further including a glass-ceramic
plate disposed in contact with the surface of the peripheral wall.
19. A heater according to claim 13, wherein the end region of the
temperature-responsive device remote from the periphery of the heater
is located substantially in the vicinity of a region of maximum
concave depression of the surface of the base.
20. A heater according to claim 13, wherein at least the end region
of the temperature-responsive device remote from the periphery of
the heater comprises a metal.
21. A heater according to claim 13, wherein at least the end region
of the temperature-responsive device remote from the periphery of
the heater overlies the at least one electrical heating element.
22. A heater according to claim 13, wherein the at least one electrical
heating element is arranged substantially in the form of a spiral.
Description This invention relates to radiant electric heaters, particularly
but not exclusively for use in cooking appliances, such as glass-ceramic
cooking appliances.
BACKGROUND TO THE INVENTION
Radiant electric heaters are well known comprising a base of thermal
and electrical insulation material, such as microporous insulation
material, having supported thereon at least one electric heating
element such as of bare wire or ribbon form. The base of insulation
material can be provided in a dish-like support, such as of metal.
The base has a flat surface and/or may be formed in discontinuous
sections. A wall of thermal insulation material is provided around
the periphery of the heater and an upper surface of the wall is
arranged to contact the underside of a glass-ceramic cooking plate.
It is usual to provide a rod-like temperature-responsive device
extending at least partly across the heater, between the heating
element or elements and the glass-ceramic plate and arranged substantially
parallel to the glass-ceramic plate. Such temperature-responsive
device is arranged to de-energise the heater at a predetermined
sensed temperature to prevent overheating of the glass-ceramic plate.
Increasing use is being made of rod-like temperature-responsive
devices which extend only partly across the heater and which have
an end located in a middle region of the heater. Such temperature-responsive
devices may comprise a metal rod of relatively high thermal expansion
inside a tube of relatively low thermal expansion. The tube is generally
of electrical insulating material, such as quartz or fused silica,
or a ceramic such as cordierite. The rod and tube are mechanically
connected at the end which is located in the middle region of the
heater and a metal component is generally exposed at this end location.
The heating element or elements is or are normally supported on
a flat surface of the base of insulation material, for example in
a plane parallel to that of the glass-ceramic plate.
With this arrangement the metal component at the end of the rod-like
temperature-responsive device may be in close proximity to the one
or more heating elements and this is particularly so when the element
or elements is or are provided directly underlying the end of the
device such as, for example, when the element or elements is or
are provided in spiral form from the periphery of the heater to
the middle of the heater.
A problem arises with such an arrangement in that electrical safety
legislation sets down limits for the minimum distance between the
heating element or elements, that may be of live electrical potential,
and neighbouring metal components, that are of zero electrical potential
or connected to earth, such as the metal component at the end of
the rod-like temperature-responsive device. Furthermore, heater
manufacturers are under pressure to provide heaters which are compact
and with as low a profile as possible, such that the distance between
the heating element and the glass-ceramic is as small as possible.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or minimise
this problem.
SUMMARY OF THE INVENTION
According to the present invention there is provided a radiant
electric heater comprising a base of thermal and electrical insulation
material having a surface supporting at least one electrical heating
element and a rod-like temperature-responsive device extending across
the heater from a periphery thereof and spaced from the at least
one electrical heating element, wherein the rod-like temperature-responsive
device extends only partly across the heater and the surface of
the base has a substantially continuous concave profile, at least
where the at least one heating element is supported, for accommodating
an end region of the device.
The concave profile may be substantially in the form of a section
of an interior of a sphere or substantially in the form of an inverted
shallow cone, preferably truncated.
The heater may be provided with a peripheral wall of thermal insulation
material having a surface for contacting a plate, such as of glass-ceramic
material, the rod-like temperature-responsive device being arranged
substantially parallel to the plane of the surface of the peripheral
wall.
The end region of the rod-like temperature-responsive device, remote
from the periphery of the heater, may be located at a greater distance
from the surface of the base than a region of the device nearer
the periphery of the heater.
The end region of the rod-like temperature-responsive device remote
from the periphery of the heater may be located substantially in
the vicinity of a region of maximum concave depression of the surface
of the base.
At least the end region of the temperature-responsive device remote
from the periphery of the heater may comprise a metal.
At least the end region of the temperature-responsive device remote
from the periphery of the heater may overlie the at least one electrical
heating element.
The at least one electrical heating element may be arranged substantially
in the form of a spiral.
The at least one electrical heating element may comprise a bare
wire or ribbon element.
When the at least one electrical heating element comprises a ribbon,
this may be supported on edge and secured by partial embedding in
the surface of the base.
The base may comprise microporous thermal and electrical insulation
material.
As a result of the concave surface profile of the base, the end
region of the temperature-responsive device remote from the periphery
of the heater can be at a greater distance from the underlying heating
element or elements than would be the case with a flat or planar
surface of the base. Any metal at the remote end region of the device
can therefore be located at a safe distance from the heating element
or elements.
It is to be understood that the expression "substantially
continuous concave profile" used herein is intended to cover
not only a profile corresponding to a shallow section of the interior
of a sphere but also other substantially similar shallow forms which
may, for example, be more practical to produce. Such other forms
include an inverted shallow cone, preferably truncated, which because
of its shallowness creates a similar profile to the section of a
sphere but is more readily provided at the surface of the base of
insulation material.
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 of the invention;
FIG. 2 is a cross-sectional view of the heater of FIG. 1; and
FIG. 3 is an alternative cross-sectional view of the heater of
FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENT
A radiant electric heater is constructed comprising a metal dish-like
support 1 having therein a base layer 2 of microporous thermal and
electrical insulation material, such material being well known to
the skilled person. The base layer 2 has a surface 3 supporting
an electrical heating element 4 providing a heated area within the
heater. The heating element 4 comprises a well known form of bare
corrugated ribbon supported edgewise on the surface 3 and secured
by partial embedding therein. Other forms of heating element could
be provided, such as bare wire, particularly in coiled form. A plurality
of heating elements could be provided if required.
The heating element 4 is arranged in the form of a spiral extending
from a middle region of the heater to a periphery thereof. An electrical
lead 5 is provided, connecting between the inner end of the spiral
form heating element 4 and a terminal block 6 at the edge of the
heater.
A peripheral wall 7 of thermal insulation material is provided
in the heater and has an upper surface 8 contacting the rear (lower
as illustrated in the figures) surface of a glass-ceramic cooking
plate 9.
A well known form of rod-like temperature-responsive device 10
is provided extending partly across the heater from the periphery
thereof. The device 10 operates as a temperature limiter to prevent
overheating of the glass-ceramic plate 9 and comprises a differentially-expanding
rod-in-tube assembly, operating a switch mechanism 11 at a predetermined
temperature to de-energise the heater. The rod and tube assembly
of the device 10 typically comprises a metal rod inside a tube of
quartz or fused silica or of a ceramic such as cordierite. The rod
and tube are joined together at the end remote from the switch 11
by means of a metal component 12.
It is important that the metal component 12 is maintained at a
specified safe distance from the bare live heating element 4. In
order to ensure this, the surface 3 of the base layer 2 of insulation
material is arranged to have a substantially continuous concave
profile, the heating element being partially embedded in this profiled
surface.
As seen in FIG. 2, the surface 3 is profiled in the form of a section
of an interior of a sphere of relatively large radius, resulting
in a substantially continuous shallow concave depression, the lowest
point of which substantially coincides with the centre of the heated
area of the heater. However, as such true spherical sections may
be difficult to achieve in practice, an arrangement as shown in
FIG. 3 may be preferred. In FIG. 3, the surface 3 is profiled in
the form of an inverted truncated shallow cone (the axis of which
substantially coincides with the centre of the heated area of the
heater), providing in effect a sloping shallow conical section 13
of shallow angle, truncated to provide a substantially flat base
portion 14. Because of its shallowness, the resulting effect is
of a substantially continuous concave profile similar to that of
the section of the interior of the sphere as shown in FIG. 2.
As a result of the concave surface profile 3 on which the heating
element 4 is supported, the distance between the metal component
12, at the end of the temperature-responsive device 10, and the
underlying heating element 4 is increased, compared with the use
of a flat, or a discontinuous, surface of the prior art. The end
region 12 of the temperature-responsive device is located substantially
in the vicinity of a region of maximum, albeit shallow, concave
depression of the surface of the base 2. In the arrangement of FIG.
3, this is readily ensured by appropriate selection of the dimensions
of the substantially flat base portion 14 of the profiled surface,
according to the length of the temperature-responsive device 10
being used. |