Abstrict
A radiant electric heater (2) comprises a base (8) of insulation
material having a surface supporting at least one electric heating
ribbon element (10) supported on edge. A rod-like temperature-responsive
device (16) extends lengthwise partly across the heater (2) from
an edge and over the at least one electric heating element (10).
The surface of the base (8) is provided with an elongate recess
(22) with sloping sides (24, 26) extending beneath and along the
length of the temperature-responsive device (16). The ribbon (10)
is supported in and traversing the elongate recess (22) such that
an upper edge (28) of the ribbon (10) substantially follows a contour
of the surface of the recess. The upper edge (28) of the ribbon
(10) at a region (30) underlying the temperature-responsive device
(16) is provided at a predetermined distance from the temperature-responsive
device (16) and is at a lower level relative to the upper edge (28)
of the ribbon (10) at regions at either side of the elongate recess
(22).
Claims
1. A radiant electric heater (2) comprising: a base (8) of thermal
and electrical insulation material having a surface supporting at
least one electric heating element comprising at least one elongate
electrically conductive ribbon (10), the at least one electrically
conductive ribbon (10) being supported on edge; a rod-like temperature-responsive
device (16) extending lengthwise partly across the heater (2) from
an edge thereof and over the at least one electric heating element
(10); the surface of the base (8) being provided with an elongate
recess (22) with sloping sides (24, 26) extending beneath and along
the length of the rod-like temperature-responsive device (16), characterised
by the at least one electrically conductive ribbon (10) being supported
in and traversing the elongate recess (22) such that an upper edge
(28) of the at least one electrically conductive ribbon (10) substantially
follows a contour of the surface of the recess and whereby the upper
edge (28) of the at least one electrically conductive ribbon (10)
at a region (30) underlying the rod-like temperature-responsive
device (16) is provided at a predetermined distance from the rod-like
temperature-responsive device (16) and is at a lower level relative
to the upper edge (28) of the at least one electrically conductive
ribbon (10) at regions (32, 34) at either side of the elongate recess
(22) the at least one electrically conductive ribbon (10) in the
regions (32, 34) at either side of the elongate recess (22) being
provided on a substantially planar surface of the base.
2. A heater as claimed in claim 1, wherein the rod-like temperature-responsive
device (16) comprises metal.
3. A heater as claimed in claim 1, wherein the rod-like temperature-responsive
device (16) comprises a metal tube.
4. A heater as claimed in claim 1, wherein the rod-like temperature-responsive
device (16) has a first end supported at an edge region of the heater
(2) and a second end (18) substantially unsupported at an inner
region of the heater (2).
5. A heater as claimed in claim 1, wherein the elongate recess
(22) has a width which increases with increasing distance from the
edge of the heater (2).
6. A heater as claimed in claim 1, wherein the elongate recess
(22) has a depth which increases with increasing distance from the
edge of the heater (2).
7. A heater as claimed in claim 6, wherein a substantially constant
angle of the sloping sides (24, 26) of the elongate recess (22)
is maintained as the depth of the elongate recess (22) increases
with increasing distance from the edge of the heater (2).
8. A heater as claimed in claim 1, wherein the elongate recess
(22) is of a form selected from substantially shell and scallop
form.
9. A heater as claimed in claim 1, wherein the at least one electrically
conductive ribbon (10) is of corrugated form.
10. A heater as claimed in claim 1, wherein the at least one electrically
conductive ribbon (10) is provided with a plurality of spaced-apart
legs (12), the legs (12) extending edgewise from the at least one
electrically conductive ribbon (10) and being at least partially
embedded in the surface of the base (8).
11. A heater as claimed in claim 10, wherein the legs (12) are
integral with the at least one electrically conductive ribbon (10).
12. A heater as claimed in claim 10, wherein the legs (12) are
secured to the at least one electrically conductive ribbon (10).
13. A heater as claimed in claim 10, wherein the legs (12) are
partially embedded in the surface of the base (8) to substantially
the same depth in the elongate recess (22) as elsewhere on the base
(8).
14. A heater as claimed in claim 1, wherein the base (8) comprises
microporous thermal and electrical insulation material.
15. A heater as claimed in claim 1, wherein the base (8) is provided
in a dish-like support (6).
16. A heater as claimed in claim 15, wherein the dish-like support
(6) is of metal.
17. A heater as claimed in claim 1, wherein the heater is adapted
for location beneath a cooking surface (4).
18. A heater as claimed in claim 17, wherein the cooking surface
(4) is of glass-ceramic material.
Description [0001] This invention relates to radiant electric heaters, for
example for use in cooking appliances behind a sheet of material
such as of glass-ceramic.
[0002] Radiant electric heaters are well known for use behind glass-ceramic
cooking surfaces. Such heaters can comprise a dish-like support
having therein a base layer of insulation material, such as microporous
thermal and electrical insulation material. At least one electric
heating element is provided, supported on the surface of the base
layer. One form of heating element which has found wide acceptance
is an elongate electrically conductive ribbon which is supported
on edge and secured in the surface of the base layer.
[0003] It is known to provide a temperature-responsive device of
rod-like form extending at least partly across the heater from an
edge thereof and overlying the heating element or elements. Such
temperature-responsive device operates to control the operation
of the heater, such as to prevent the glass-ceramic material of
the cooking surface reaching a high temperature at which damage
would occur thereto.
[0004] It has been found advantageous to provide the temperature-responsive
device of rod-like form with an outer tube of metal. Such a metal
outer tube is more robust than outer tubes of quartz, fused silica
or ceramic materials which were previously commonly used for rod-like
temperature-responsive devices. However, a problem arises with the
use of a metal, and hence electrically conductive, tube in close
proximity to the heating element or elements. With outer tubes of
quartz, fused silica or ceramic materials a metal termination or
ferrule is generally known to be provided at a distal end thereof
inside the heater. With such an arrangement it is readily straightforward
to ensure adequate clearance between the metal termination or ferrule
and the heating element or elements by providing a local region
underlying the metal termination or ferrule which is free from heating
element material. However, this is not possible with a temperature-responsive
device having a full length metal outer tube and some means is therefore
required to maintain a specified minimum electrical clearance between
the metal tube and the one or more heating elements along the full
length of the metal tube. The problem is exacerbated when the rod-like
temperature-responsive device is only supported at a peripheral
edge region of the heater, a distal free end region of the rod-like
temperature-responsive device being unsupported at a central region
within the heater. This means that, when the heater is subjected
to mechanical shock, the free end of the rod-like temperature-responsive
device may undergo displacement towards the heating element.
[0005] As a result of manufacturing tolerances, the rod-like temperature-responsive
device when mounted on the heater may not be parallel to the base
of the heater and such that the free end of the device is closer
to the heating element than is the opposite end of the device.
[0006] Furthermore, in the case of a heater having a rod-like temperature-responsive
device overlying a heating element or elements, a local increase
in temperature may occur in those regions of the heating element
or elements which lie directly beneath the rod-like temperature-responsive
device. This can result in premature operation of the temperature-responsive
device and may also contribute to premature failure of the heating
element or elements. This problem has been addressed in GB-A-2340715
wherein the surface of the base of thermal insulation material supporting
the heating element or elements is provided with a shallow recess
beneath the rod-like temperature-responsive device. A ribbon-form
heating element, which is partially embedded in the base, is partially
embedded to a lesser extent in the shallow recess than in the surface
of the base at either side of the recess. This results in a heating
element which has a substantially planar upper surface and although
it results in a lower temperature of the portion or portions of
the heating element directly underlying the rod-like temperature-responsive
device, it does not deal with the problem of ensuring satisfactory
minimum clearance between the outer tube, particularly of metal,
of the rod-like temperature-responsive device and the underlying
heating element or elements.
[0007] It is an object of the present invention to overcome or
minimise this problem.
[0008] According to the present invention there is provided a radiant
electric heater comprising:
[0009] a base of thermal and electrical insulation material having
a surface supporting at least one electric heating element comprising
at least one elongate electrically conductive ribbon, the at least
one electrically conductive ribbon being supported on edge;
[0010] a rod-like temperature-responsive device extending lengthwise
partly across the heater from an edge thereof and over the at least
one electric heating element;
[0011] the surface of the base being provided with an elongate
recess with sloping sides extending beneath and along the length
of the rod-like temperature-responsive device, the at least one
electrically conductive ribbon being supported in and traversing
the elongate recess such that an upper edge of the at least one
electrically conductive ribbon substantially follows a contour of
the surface of the recess and whereby the upper edge of the at least
one electrically conductive ribbon at a region underlying the rod-like
temperature-responsive device is provided at a predetermined distance
from the rod-like temperature-responsive device and is at a lower
level relative to the upper edge of the at least one electrically
conductive ribbon at regions at either side of the elongate recess.
[0012] The rod-like temperature-responsive device may comprise
metal and may comprise a metal tube.
[0013] The rod-like temperature-responsive device may have a first
end supported at an edge region of the heater and a second end substantially
unsupported at an inner region of the heater.
[0014] The elongate recess may have a depth which increases with
increasing distance from the edge of the heater.
[0015] The elongate recess may also have a width which increases
with increasing distance from the edge of the heater and may be
such that a substantially constant angle of the sloping sides of
the elongate recess is maintained as the depth of the elongate recess
increases with increasing distance from the edge of the heater.
[0016] The elongate recess may be of substantially shell or scallop
form.
[0017] The at least one electrically conductive ribbon may be of
corrugated form and may be provided with a plurality of spaced-apart
legs, integral with the at least one electrically conductive ribbon
or secured thereto, the legs extending edgewise from the at least
one electrically conductive ribbon and being at least partially
embedded in the surface of the base. The legs may be partially embedded
in the surface of the base to substantially the same depth in the
elongate recess as elsewhere on the base.
[0018] The base may comprise microporous thermal and electrical
insulation material and may be provided in a dish-like support,
such as of metal.
[0019] The radiant electric heater may be adapted for location
beneath a cooking surface, such as of glass-ceramic material.
[0020] By means of the present invention, the at least one electric
heating element in the form of at least one electrically conductive
ribbon is secured to and supported on the surface of the base such
that it descends into the elongate recess on one sloping side thereof,
passes underneath the rod-like temperature-responsive device at
a predetermined distance therefrom, and then ascends from the elongate
recess on the opposite sloping side thereof. Thus a required minimum
electrical clearance is achieved between the at least one electrically
conductive ribbon and the rod-like temperature-responsive device,
which is essential when the device comprises metal. Furthermore,
provision of increasing depth of the elongate recess with increasing
distance from the edge of the heater enables the minimum required
electrical clearance to be maintained if the rod-like temperature-responsive
device is mounted on the heater sloping downwardly towards the base
of the heater, or if the rod-like temperature-responsive device
undergoes deflection in a direction towards the base of the heater,
such as if the heater is subjected to mechanical shock conditions.
Provision of increasing width of the elongate recess with increasing
distance from the edge of the heater allows a substantially constant
and suitably shallow angle of the sloping sides of the recess to
be maintained, with the accompanying increase in depth of the recess
with increasing distance from the edge of the heater, whereby descent
and ascent of the at least one electrically conductive ribbon into
and out of the recess is facilitated.
[0021] The arrangement of the present invention also minimises
or reduces local increase in temperature of the at least one electrically
conductive ribbon at a region or regions thereof directly underlying
the rod-like temperature-responsive device.
[0022] For a better understanding of the present invention and
to show more clearly how it may be carried into effect, reference
will now be made by way of example to the accompanying drawings
in which:
[0023] FIG. 1 is a plan view of an embodiment of a radiant electric
heater according to the present invention;
[0024] FIG. 2 is a section along line A-A of FIG. 1;
[0025] FIG. 3 is a section along line B-B of FIG. 1; and
[0026] FIG. 4 is a perspective view of the radiant electric heater
of FIG. 1.
[0027] A radiant electric heater 2, such as for use underneath
a cooking surface 4, for example of glass-ceramic material, comprises
a metal dish-like support 6 having therein a base layer 8 of thermal
and electrical insulation material, such as microporous thermal
and electrical insulation material. A radiant electric heating element
is provided comprising an elongate corrugated electrically conductive
ribbon 10, such as of iron-chromium-aluminium alloy, having spaced-apart
legs 12 integral with or secured thereto and extending edgewise
therefrom. The ribbon 10 is mounted edgewise on the base layer 8
and secured by at least partial embedding of the legs 12 in the
surface of the base layer 8.
[0028] A peripheral wall 14 of insulation material is provided
in the heater and has a top surface which may be arranged to contact
a rear side of the cooking surface 4 in a cooking appliance.
[0029] A rod-like temperature-responsive device 16 is provided,
comprising a metal outer tube and extending partly across the heater
from an edge thereof. The rod-like temperature-responsive device
16 is supported at the edge of the heater and has a free end 18
which is unsupported at an inner region of the heater.
[0030] The rod-like temperature-responsive device 16 may comprise
differentially-expanding components, which operate a switch means
20 provided at a periphery of the heater. Alternatively, the rod-like
temperature-responsive device 16 may comprise a metal tube enclosing
an electrical component (not shown) having an electrical parameter
which changes with temperature. Such electrical component may be
a resistance temperature detector, such as a platinum resistance
temperature detector, whose electrical resistance changes as a function
of temperature. The electrical component may be connected to external
processing circuitry by means of lead wires passing through the
metal tube of the device 16.
[0031] An elongate recess 22, with sloping sides 24, 26, is provided
in the surface of the base layer 8. The elongate recess 22 extends
radially from the edge of the heater and beneath and along the length
of the rod-like temperature-responsive device 16. The electrically
conductive ribbon 10 is arranged to be supported in and to traverse
the elongate recess 22 such that an upper edge 28 of the ribbon
10 substantially follows a contour of the surface of the recess
22. The upper edge 28 of the ribbon 10 at a region 30 underlying
the temperature-responsive device 16 is arranged at a predetermined
distance from the temperature-responsive device 16 and is provided
at a lower level relative to the upper edge 28 of the ribbon 10
at regions 32, 34 at either side of the recess 22.
[0032] The electrically conductive ribbon 10 is arranged with its
legs 12 embedded in the base layer 8 to substantially the same extent
in the elongate recess 22 as elsewhere in the base layer 8.
[0033] As shown particularly in FIGS. 2 and 3, the electrically
conductive ribbon 10, supported on and secured to the base layer
8, descends into the recess 22 on one sloping side 24 thereof, passes
with predetermined clearance underneath the rod-like temperature-responsive
device 16, and then ascends from the recess 22 on the opposite sloping
side 26 thereof. The angle and dimensions of the sloping sides 24,
26 of the recess 22 are selected in accordance with a depth required
for the recess 22 and to facilitate turning of the ribbon 10 into
and out of the recess 22.
[0034] The free end 18 of the rod-like temperature-responsive device
16 may undergo displacement in a direction towards the base layer
8 in the event that the heater 2 is subjected to mechanical shock
load conditions.
[0035] Alternatively, or additionally, the rod-like temperature-responsive
device 16 when mounted on the heater may not be parallel to the
surface of the base layer 8, due to manufacturing tolerances, and
such that it slopes downwardly towards the base 8. In order to ensure
that a predetermined minimum electrical clearance exists between
the temperature-responsive device 16 and the ribbon 10 at all times,
under all conditions, it is arranged for greater clearance to be
provided between the ribbon 10 and the rod-like temperature-responsive
device 16 at the free end region 18 of the temperature-responsive
device 16 than at the end thereof near the edge of the heater. For
this purpose, the elongate recess 22 is arranged, such as by being
ramped, to have a depth which gradually increases with increasing
distance from the edge of the heater. This is particularly illustrated
in FIGS. 2 and 3. FIG. 2 represents a cross-sectional view of the
heater 2 at a region near the edge of the heater, while FIG. 3 represents
a cross-sectional view of the heater 2 at an inner region of the
heater, near the free end 18 of the rod-like temperature-responsive
device 16. As shown in FIG. 2, a distance D1 is provided between
the region 30 of the upper edge 28 of the ribbon 10 and the rod-like
temperature-responsive device 16 at the region of the heater near
the peripheral edge thereof. As shown in FIG. 3, a distance D2,
which is greater than the distance D1 of FIG. 2, is provided between
the region 30 of the upper edge 28 of the ribbon 10 and the temperature-responsive
device 16, at the inner region of the heater near the free end 18
of the rod-like temperature-responsive device 16.
[0036] In order to maintain a substantially constant angle of slope
of the sloping sides 24, 26 of the recess 22, while accommodating
the gradually increasing depth of the recess 22 with increasing
distance from the edge of the heater, it is suitably arranged for
the recess 22 to gradually increase in width with increasing distance
from the edge of the heater. Accordingly, the width W1 of the recess
22 at a region near the edge of the heater is made less than the
width W2 at an inner region of the heater near the free end 18 of
the temperature-responsive device 16. The recess 22 is therefore
arranged to taper in width as well as being ramped in depth and
the resulting form thereof is substantially that of a shell or scallop.
[0037] An end region 36 of the elongate recess 22 may terminate
in any desired form, such as a wall form, or be of stepped or sloping
form.
[0038] The arrangement of the recess 22 and the rod-like temperature-responsive
device 16 is further advantageous in that it minimises or reduces
any tendency for the ribbon-form heating element 10 to operate with
a higher temperature in the region directly beneath the rod-like
temperature-responsive device 16, compared with regions elsewhere
in the heater. |