Abstrict
A radiant electric heater includes a base (2) of thermal and electrical
insulation material having a surface supporting at least one heating
element (3) in the form of an elongate electrically conductive ribbon
(4). The ribbon (4) is supported on edge and is partially embedded
in the surface of the base (2). A rod-like temperature-responsive
device (7, 8) extends at least partly across the heater from an
edge of the heater and over the at least one heating element (3).
The surface of the base (2) is provided with a shallow recess (9)
beneath the rod-like temperature-responsive device (7, 8) and the
ribbon (4) is partially embedded in the shallow recess (9) to a
lesser extent than in the surface of the base (2) at either side
of the shallow recess.
Claims
I claim:
1. A radiant electric heater comprising a base of thermal and electrical
insulation material having a surface supporting at least one heating
element in the form of an elongate electrically conductive ribbon,
the ribbon being supported on edge and partially embedded in the
surface of the base, and a rod-like temperature-responsive device
extending at least partly across the heater from an edge thereof
and over the at least one heating element, wherein the surface of
the base is provided with a shallow recess beneath the rod-like
temperature-responsive device, the ribbon being partially embedded
in the shallow recess to a lesser extent than in the surface of
the base at either side of the shallow recess.
2. A heater according to claim 1, wherein the at least one heating
element, where partially embedded in the shallow recess, presents
an upper surface plane which is substantially coplanar with an upper
surface plane presented by the at least one heating element at either
side of the shallow recess.
3. A heater according to claim 1, wherein the shallow recess is
substantially in the form of a channel.
4. A heater according to claim 1, wherein the base comprises microporous
thermal and electrical insulation material.
5. A heater according to claim 1, wherein the at least one heating
element is generally in the form of a spiral.
6. A heater according to claim 5, wherein the spiral comprises
a double spiral.
7. A heater according to claim 1, wherein the at least one heating
element in the form of the elongate electrically conductive ribbon
incorporates a plurality of spaced-apart support members extending
edgewise from the remainder of the ribbon and being at least partially
embedded in the surface of the base.
8. A heater according to claim 7, wherein the spaced-apart support
members comprise legs.
9. A heater according to claim 8, wherein the legs are integral
with the ribbon.
Description This invention relates to radiant electric heaters, particularly
but not exclusively for use in cooking appliances behind a sheet
of a material such as glass-ceramic. In particular, the sheet of
material such as glass-ceramic may be a cooking surface.
BACKGROUND TO THE INVENTION
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 electrical
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 partially embedded in the surface of the base layer.
DESCRIPTION OF PRIOR ART
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.
It is important that the temperature-responsive device should primarily
respond to the temperature of the cooking surface rather than to
direct thermal radiation from the heating element or elements. It
is known to reduce the direct thermal radiation from the heating
element or elements on the temperature-responsive device by arranging
for a strip of the surface of the base layer directly beneath the
device to be free from the heating element or elements. This is
achievable when it is possible to provide the heating element or
elements in a pattern whereby suitable reversals are arranged in
the pattern such that there is no traversal of the element or elements
beneath the temperature-responsive device.
However there are situations where this cannot be achieved. One
in particular is where at least one heating element is provided
configured as a spiral, which may be a double spiral. With such
an arrangement it is unavoidable that passage of the element or
elements occurs beneath the temperature-responsive device. This
means that the full radiation power of the heating element or elements
is directly incident on the temperature-responsive device and furthermore
the heating element or elements reach a locally higher temperature
directly beneath the temperature-responsive device compared with
other regions of the heater. This may cause premature failure of
the element or elements as well as premature switching of the limiter.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or reduce
these problems.
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 heating element
in the form of an elongate electrically conductive ribbon, the ribbon
being supported on edge and partially embedded in the surface of
the base, and a rod-like temperature-responsive device extending
at least partly across the heater from an edge thereof and over
the at least one heating element, wherein the surface of the base
is provided with a shallow recess beneath the rod-like temperature-responsive
device, and wherein the ribbon is partially embedded in the shallow
recess to a lesser extent than in the surface of the base at either
side of the shallow recess.
As a result of the invention, the at least one heating element
operates at a reduced temperature beneath the rod-like temperature-responsive
device thus avoiding premature failure of the element or elements
and also providing a correspondingly reduced direct thermal influence
on the device.
The at least one heating element where partially embedded in the
shallow recess may present an upper surface plane which is substantially
coplanar with an upper surface plane presented by the at least one
heating element at either side of the shallow recess.
The shallow recess may be substantially in the form of a channel.
The base may comprise microporous thermal and electrical insulation
material.
The at least one heating element may be generally in the form of
a spiral which may be a double spiral.
The at least one heating element in the form of the elongate electrically
conductive ribbon may incorporate a plurality of spaced-apart support
members, such as legs which may be integral with the remainder of
the ribbon, extending edgewise from the remainder of the ribbon
and being at least partially embedded in the surface of the base.
For a better understanding of the present invention and to show
more clearly how it may be put into effect reference will now be
made, by way of example, 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 invention; and
FIG. 2 is a cross-sectional view of part of the heater of FIG.
1.
DESCRIPTION OF PREFERRED EMBODIMENT
A radiant electric heater comprises a metal dish 1 having provided
therein a base layer 2 of thermal and electrical insulation material,
preferably microporous thermal and electrical insulation material.
A radiant electric heating element 3 is provided comprising a corrugated
ribbon 4, such as of iron-chromium-aluminium alloy, incorporating
spaced-apart legs 5 integral therewith and extending edgewise therefrom.
Alternatively, the ribbon may incorporate separate legs which are
secured to the remainder of the ribbon. The heating element is mounted
edgewise on the base layer 2 and secured by partial embedding in
the surface of the base layer 2. This is achieved by pressing the
legs 5 on the corrugated ribbon 4 at least partly into the surface
of the base layer 2, the technique being known in the art. As shown,
the heating element is of double spiral form, but may be of another
suitable form.
A peripheral wall 6 of insulation material is provided in the heater
and has a top surface which may be arranged to contact the rear
side of a glass-ceramic cooking surface (not shown) in a cooking
appliance.
A temperature limiter of well known form is provided on the heater
and comprises a rod 7 extending over the heating element and operating
a switch arrangement 8 at the edge of the heater. Such temperature
limiter is operable at a predetermined temperature to de-energise
the heating element to prevent overheating particularly of a glass-ceramic
cooking surface when the heater is operated therewith.
In the absence of the present invention the heating element 3,
where it passes beneath the rod 7 of the temperature limiter, operates
at a locally higher temperature than the remainder of the element
and this, coupled with the close proximity of the element 3 to the
rod 7, results in premature failure of the element and also premature
switching of the temperature limiter.
In the present invention this disadvantage is minimised or reduced
by providing a shallow recess 9 in the surface of the base layer
2 directly beneath the rod 7 of the temperature limiter. The shallow
recess 9 is substantially in the form of a channel having a depth
of, for example, 1 to 2 mm. Where the heating element 3 crosses
the shallow recess 9, the legs 5 on the corrugated ribbon 4 are
embedded in the base layer 2 to a lesser extent than are those legs
on the ribbon at either side of the shallow recess 9. For example,
where the heating element crosses the shallow recess the legs 5
may be embedded to a depth of about 1 mm whereas elsewhere they
may be embedded to a depth of about 2 mm. This arrangement does
not alter the overall profile of the heating element, since the
element where partially embedded in the shallow recess presents
an upper surface plane 10 which is substantially coplanar with an
upper surface plane 11 presented by the heating element at either
side of the shallow recess 9.
As a result of the arrangement, the heating element has a greater
exposed surface area and consequently a reduced temperature where
it crosses the shallow recess 9 in the base layer 2 directly beneath
the rod 7 of the temperature limiter. This advantageously prevents
premature failure of the heating element 3 and also reduces the
direct influence of the heating element on the rod 7 of the temperature
limiter.
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