Abstrict
A radiant electric heater comprises a base (3) of thermal insulation
material having supported thereon or adjacent thereto at least two
concentrically-arranged heating elements (7, 9) separated by a dividing
wall (5) of thermal insulation material to form an outer heating
zone (8) and an inner heating zone (6), the heater having a peripheral
wall (4) of thermal insulation material. A tunnel (11) formed of
thermal insulation material extends between the peripheral wall
and the dividing wall across the outer heating zone such that heating
element or elements of the outer heating zone are substantially
absent from an area occupied by the tunnel. A rod-like temperature-responsive
device (10) extends from a periphery of the heater through the tunnel
(11) and at least partly across the inner heating zone (6), through
an aperture provided in the dividing wall (5). One or more portions
of one or more heating elements (7) of the inner heating zone extend
into the tunnel. Ventilation means (18, 20, 21, 23, 24) is provided
for the tunnel (11) intermediate the ends thereof to reduce temperature
within the tunnel to a desired level.
Claims
I claim:
1. A radiant electric heater comprising:
a base of thermal insulation material having supported relative
thereto at least two concentrically-arranged heating elements separated
by a dividing wall of thermal insulation material to form an outer
heating zone and an inner heating zone, the heater having a peripheral
wall of thermal insulation material;
a tunnel formed of thermal insulation material extending between
the peripheral wall and the dividing wall across the outer heating
zone and such that at least one heating element of the outer heating
zone is substantially absent from an area occupied by the tunnel;
a rod-like temperature-responsive device extending from a periphery
of the heater through the tunnel and at least partly across the
inner heating zone, through an aperture provided in the dividing
wall;
at least one portion of at least one heating element of the inner
heating zone extending into the tunnel; and
ventilation means provided for the tunnel intermediate the ends
thereof to reduce temperature within the tunnel to a desired level.
2. A heater according to claim 1, wherein the ventilation means
comprises at least one aperture extending into the tunnel from the
outer heating zone.
3. A heater according to claim 2, wherein the at least one aperture
is provided at a base region of side walls of the tunnel.
4. A heater according to claim 1, wherein the tunnel is of substantially
inverted U-shaped cross section and supported on lower edges thereof.
5. A heater according to claim 4, wherein the tunnel is supported
such that at least one aperture is provided between the tunnel and
the base.
6. A heater according to claim 5, wherein the tunnel is supported
on spaced-apart protrusions provided on the base.
7. A heater according to claim 6, wherein the protrusions are integral
with the base.
8. A heater according to claim 5, wherein the tunnel has at least
one lower edge of substantially castellated form.
9. A heater according to claim 5, wherein at least one pedestal
region is formed by at least one lower region of the peripheral
wall, the tunnel being spaced from the base at an end thereof by
the at least one pedestal region.
10. A heater according to claim 5, wherein at least one pedestal
region is formed by at least one lower region of the dividing wall,
the tunnel being spaced from the base at an end thereof by the at
least one pedestal region.
11. A heater according to claim 4, wherein at least one recess
is provided in the base of thermal insulation material extending
beneath at least one lower edge of the tunnel.
12. A heater according to claim 1, wherein the tunnel is integral
with the dividing wall.
13. A heater according to claim 1, wherein the tunnel is integral
with the peripheral wall.
14. A heater according to claim 1, wherein the tunnel comprises
bound vermiculite.
15. A heater according to claim 1, wherein the at least one portion
of the at least one heating element of the inner heating zone extends
through the tunnel to a terminal region at the edge of the heater.
16. A heater according to claim 15, wherein the tunnel is provided
with a laterally-extending portion proximate the peripheral wall
to shield the at least one portion of the at least one heating element
in the vicinity of the terminal region.
17. A heater according to claim 1, wherein the at least one heating
element of the inner heating zone is energisable independently of
the at least one heating element of the outer heating zone.
18. A heater according to claim 1, wherein the heating elements
are of ribbon form.
19. A heater according to claim 18, wherein the heating elements
are supported edgewise on the base.
20. A heater according to claim 1, wherein the base comprises microporous
thermal and electrical insulation material.
Description The present invention relates to radiant electric heaters, such
as for use in cooking appliances and especially for use in cooking
appliances having glass-ceramic cooking surfaces.
DESCRIPTION OF PRIOR ART
It is well known to provide a radiant electric heater for use in
a cooking appliance beneath a glass-ceramic cooking surface and
in which at least two concentrically-arranged heating elements,
separated by a dividing wall of thermal insulation material, are
supported on or adjacent to a base of thermal insulation material,
such as microporous thermal and electrical insulation material.
An outer heating zone and an inner heating zone are thereby provided,
the inner heating zone having one or more heating elements which
are generally arranged to be energisable independently of the heating
element or elements of the outer heating zone.
A peripheral wall of thermal insulation material is generally arranged
around the edge of the heater and a rod-like temperature-responsive
device extends from a periphery of the heater across the heater
to a position at least partly across the inner heating zone.
In order to meet a requirement that the temperature-responsive
device should be responsive substantially only to heat from the
inner heating zone, even when both inner and outer heating zones
are energised, it is known to provide a tunnel of thermal insulation
material covering the rod-like temperature-responsive device in
the outer heating zone between the peripheral wall and the dividing
wall. The heating element in the outer heating zone is arranged
to stop short of the tunnel at either side thereof and hence do
not intrude into the tunnel. The temperature-responsive device is
thus thermally isolated from the heating element in the outer heating
zone.
It is further known to provide one or more portions of the heating
element of the inner heating zone extending into the tunnel through
an aperture in the dividing wall. It has been found expedient to
lead such one or more portions of the heating element through the
tunnel to a terminal region, or terminal block, at the edge of the
heater for electrically connecting the element of the inner heating
zone to a power supply. Although the provision of the portion or
portions of the heating element passing through the tunnel enables
the rod-like temperature-responsive device to be influenced by heat
from the element of the inner heating zone over substantially its
entire length, a problem can arise in that overheating can occur
in the portion or portions of the heating element inside the tunnel.
Although the problem applies to all forms of heating elements, it
has become particularly apparent with heating elements in the form
of corrugated ribbon material, with a risk of premature failure
of the portion or portions of an element of such material inside
the tunnel. The temperature of the portion or portions inside the
tunnel may reach a level about 20 to 50 degrees Celsius higher than
the highest temperature in the body of the heater.
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 insulation material having supported relative
(on or adjacent) thereto at least two concentrically-arranged heating
elements separated by a dividing wall of thermal insulation material
to form an outer heating zone and an inner heating zone, the heater
having a peripheral wall of thermal insulation material;
a tunnel formed of thermal insulation material extending between
the peripheral wall and the dividing wall across the outer heating
zone and such that the at least one heating element of the outer
heating zone is substantially absent from an area occupied by the
tunnel;
a rod-like temperature-responsive device extending from a periphery
of the heater through the tunnel and at least partly across the
inner heating zone, through an aperture provided in the dividing
wall;
at least one portion of at least one heating element of the inner
heating zone extending into the tunnel; and
ventilation means provided for the tunnel intermediate the ends
thereof to reduce temperature within the tunnel to a desired level.
The ventilation means may comprise one or more apertures extending
into the tunnel from the outer heating zone.
The one or more apertures may be provided at a base region of side
walls of the tunnel.
The tunnel may be of substantially inverted U-shaped cross section
and supported on lower edges thereof.
The tunnel may be supported such that one or more apertures is
or are provided between the tunnel and the base. The tunnel may
be supported on spaced-apart protrusions provided on the base and
which may be integral with the base. Alternatively, the tunnel may
have one or both lower edges of substantially castellated form.
As a further alternative, one or more pedestal regions may be formed
by one or more lower regions of the peripheral wall and/or the dividing
wall, the tunnel being spaced from the base at one or both ends
thereof by the one or more pedestal regions. As a still further
alternative, one or more recesses (grooves or holes) may be provided
in the base of thermal insulation material extending beneath one
or both lower edges of the tunnel.
The tunnel may be integral with the dividing wall and/or the peripheral
wall.
The tunnel may comprise bound vermiculite.
The one or more portions of the one or more heating elements of
the inner heating zone may extend through the tunnel to a terminal
region at the edge of the heater. In this respect, the tunnel may
be provided with a laterally-extending portion proximate the peripheral
wall to shield the one or more portions of the one or more heating
elements in the vicinity of the terminal region.
The one or more heating elements of the inner heating zone may
be energisable independently of the heating element or elements
of the outer heating zone.
The heating elements may be of ribbon form and may be supported
edgewise on the base.
The base may comprise microporous thermal and electrical insulation
material.
For a better understanding of the 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:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of one embodiment of a radiant electric heater
according to the present invention;
FIG. 2 is a cross-sectional view of the radiant electric heater
of FIG. 1;
FIG. 3 is a detail showing a part-sectional view of an embodiment
of tunnel ventilation means in the radiant electric heater of FIG.
1:
FIG. 4 is a detail showing a part-sectional view of an alternative
embodiment of tunnel ventilation means in the radiant electric heater
of FIG. 1;
FIG. 5 is a detail showing a part-sectional view of a further embodiment
of tunnel ventilation means in the radiant electric heater of FIG.
1; and
FIG. 6 is a detail showing a cross-sectional view of a still further
embodiment of tunnel ventilation means in the radiant electric heater
of FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, a radiant electric heater 1, for use
under a glass-ceramic surface (not shown) of a cooking appliance,
has a metal support dish 2 containing a base 3 of microporous thermal
and electrical insulation material. A peripheral wall 4 of thermal
insulation material, such as bound vermiculite, is provided around
the outside edge of the heater. Such vermiculite suitably comprises
exfoliated particles of vermiculite bound with, for example, potassium
silicate.
Two heating zones are provided, separated by a dividing wall 5
of thermal insulation material, such as bound vermiculite. An inner
heating zone 6 contains at least one heating element 7, supported
on the base 3 and suitably comprising one or more edgewise-mounted
corrugated ribbons. An outer heating zone 8 contains at least one
heating element 9, of similar form to the heating element 7.
A rod-like temperature-responsive device 10 of well-known form
extends across the heater from the periphery thereof and partly
across the inner heating zone 6, through an aperture provided in
the dividing wall 5.
In order that the temperature-responsive device 10 can be made
responsive to heat generated only by the one or more heating elements
7 in the inner heating zone 6, even when the one or more heating
elements 9 in the outer heating zone 8 is or are energised, a tunnel
11 of thermal insulation material, such as bound vermiculite, is
provided. The tunnel 11 is of substantially inverted U-shaped cross-section,
such as is shown in FIG. 6, and extends between the peripheral wall
4 and the dividing wall 5, covering the rod-like temperature-responsive
device 10 in this region. The one or more heating elements 9 in
the outer heating zone 8 is or are arranged to stop short of the
tunnel 11. The temperature-responsive device 10 is therefore thermally
isolated by the tunnel 11 from the heating element or elements 9
of the outer heating zone 8.
End portions 7A and 7B of the one or more heating elements 7 of
the inner heating zone 6 are arranged to pass directly or indirectly
through the tunnel 11 and are connected to terminals 12, 13 of a
terminal block 14 at the edge of the heater. Terminal regions of
the one or more heating elements 9 in the outer heating zone 8 are
connected to terminals 12 and 15 of the terminal block 14. By appropriate
connection of a power supply (not shown) to the terminals 12, 13,
and 15, the inner heating zone 6 can be energised alone, or can
be energised together with the outer heating zone 8.
As a result of a tendency for the end portions 7A, 7B of the one
or more heating elements 7 to operate at a higher temperature inside
the tunnel 11 than the remainder of the element or elements 7 outside
the tunnel in the inner heating zone 6, there is a risk of premature
failure occurring in these portions 7A, 7B.
In order to minimise or overcome this problem, ventilation means
is provided for the tunnel 11 intermediate the ends thereof, details
of which are shown in FIGS. 3 to 6.
Referring to FIG. 3, the tunnel 11, of inverted U-shaped cross
section, extends between the peripheral wall 4 and the dividing
wall 5. Lower edges 16 of the tunnel 11 are supported on spaced
apart protrusions 17 provided on the base 3 of thermal insulation
material so that apertures 18 are provided, between the lower edges
16 of the tunnel 11 and the base 3, through which air can circulate
to the interior of the tunnel 11 from the outer heating zone 9,
thereby preventing overheating of the portions 7A, 7B of heating
element inside the tunnel 11.
The protrusions 17 suitably result in a spacing between the lower
edges 16 of the tunnel 11 and the surface of the base 3 of between
about 0.3 and 5 mm, preferably between about 0.5 and 3 mm and most
preferably between about 1 and 2 mm. The temperature inside the
tunnel 11 suitably does not significantly exceed the highest temperature
in the body of the heater.
Referring to FIG. 4, the tunnel 11 is again shown extending between
the peripheral wall 4 and the dividing wall 5. Castellated lower
edges 19 are provided on the tunnel 11, enabling the tunnel to be
supported on the base 3 with resulting apertures 20 providing ventilation
of the interior of the tunnel 11. The apertures 20 are of similar
height to the apertures 18 of FIG. 3.
With the arrangements of FIGS. 3 and 4, the tunnel 11 may be formed
integrally with the peripheral wall 4 or the dividing wall 5, the
latter being generally preferred.
Referring now to FIG. 5, the peripheral wall 4 and dividing wall
5 provide pedestal regions 4A, 5A which result in a stand-off of
the tunnel 11 from the base 3. An aperture 21 is thereby formed
between the lower edges of the tunnel 11 and the base 3, providing
ventilation for the interior of the tunnel 11. The aperture 21 is
of a similar height to the apertures 18 of FIG. 3.
As shown in FIG. 5, the tunnel 11 is integral with the dividing
wall 5, pedestal 5A being effectively built into the integral construction.
A ledge is formed at the base of the peripheral wall 4 and constituting
the pedestal 4A which supports the outer end of the tunnel 11.
It will be appreciated by the skilled person in respect of any
of the above-described embodiments that the peripheral wall 4 could
be provided integral with the tunnel 11 additionally or alternatively
to the dividing wall 5 being formed integral with the tunnel 11.
Alternatively, the tunnel 11 could be separate from both the peripheral
wall 4 and the dividing wall 5 and both walls could be provided
with pedestals in the form of ledges, as for the pedestal 4A.
Referring to FIG. 6, the inverted U-shaped tunnel 11 is supported
on its lower edges 22 on the base 3. Ventilation for the interior
of the tunnel 11 is provided either by means of grooves 23 in the
surface of the base 3, extending under the lower edges 22 of the
tunnel, or holes 24 tunnelling through the base 3, under the lower
edges 22 of the tunnel 11. As explained above, the tunnel 11 could
be provided integral with either of the peripheral wall 4 or the
dividing wall 5, or could be separate from both the peripheral wall
4 and the dividing wall 5.
As shown in FIG. 1, the tunnel 11 is laterally extended in the
vicinity of the peripheral wall 4 to form a cover 25 for the end
portion 7A of the heating element 7 where it approaches the terminal
block 14. The cover 25 could also be extended, if required, to overlie
the other end portion 7B of the heating element 7. However this
is not required in the arrangement shown in FIG. 1, where a relatively
thick strip of metal 26 extends from the terminal block 14 to the
end portion 7B of the heating element 7. The extension of the cover
25 over the end portion 7A or end portions 7A and 7B or the heating
element 7 eliminates light from the end portion(s) of the heating
element being observable by a user and hence improves the aesthetic
appearance of the heater in use. |