Abstrict
A radiant electric heater for use in a smooth top cooker includes
a base of electrical and thermal insulating material and two heating
elements supported on the base. One of the heating elements is energisable
independently of the other element, for example to permit the heater
to be adapted to different size cooking utensils. A peripheral wall
of thermal insulating material extends laterally around the heating
elements and a dividing wall is arranged between the heating elements
such that, in use, separate and distinct heating zones are formed
on the smooth top of the cooker. The dividing wall is received in
the base of electrical and thermal insulating material and is retained
in the base by friction. The dividing wall may additionally be constructed
and arranged as a barrier effectively only to thermal convection
currents generated in the heater.
Claims
I claim:
1. A radiant electric heater for use in a smooth top cooker, the
heater comprising a base of electrical and thermal insulating material;
at least two heating elements supported on the base, at least one
of which heating elements is energisable independently of the other
element or elements; a peripheral wall of thermal insulating material
extending laterally around the heating elements; and at least one
dividing wall arranged between the heating elements such that, in
use, separate and distinct heating zones are formed on the smooth
top of the cooker, wherein the at least one dividing wall is received
in the base of electrical and thermal insulating material and is
retained therein by friction.
2. A radiant electric heater according to claim 1, wherein the
at least one dividing wall is constructed and arranged as a barrier
substantially only to thermal convection currents generated in the
heater.
3. A radiant electric heater according to claim 1, wherein the
at least one dividing wall is partly embedded in the base of electrical
and thermal insulation.
4. A radiant electric heater according to claim 1, wherein the
at least one dividing wall is received in a groove formed in the
base of electrical and thermal insulation.
5. A radiant electric heater according to claim 1, wherein the
at least one dividing wall comprises a metal strip.
6. A radiant electric heater according to claim 1, wherein the
at least one dividing wall comprises an electrically insulating
material.
7. A radiant electric heater according to claim 6, wherein the
electrically insulating material is selected from the group consisting
of fabrics incorporating glass fibers and/or ceramic fiber, ceramics,
mica and other micaceous materials.
8. A radiant electric heater according to claim 7, wherein the
electrically insulating material comprises a fabric, which fabric
is impregnated or coated with a stiffening medium.
9. A radiant electric heater according to claim 8, wherein the
stiffening medium is selected from the group consisting of a solution
of an alkali silicate, a solution of an alkaline earth silicate
and a silica sol, the stiffening medium being dried subsequent to
impregnating or coating the fabric.
10. A radiant electric heater according to claim 1, wherein the
thickness of the at least one dividing wall does not exceed 50 per
cent of the thickness of the peripheral wall of the heater.
11. A radiant electric heater according to claim 10, wherein the
thickness of the at least one dividing wall does not exceed 30 per
cent of the thickness of the peripheral wall of the heater.
12. A radiant electric heater according to claim 1, wherein the
thickness of the at least one dividing wall is less than 5 mm.
13. A radiant electric heater according to claim 1, wherein the
dividing wall is of substantially uniform thickness.
14. A radiant electric heater according to claim 1, wherein the
height of the at least one dividing wall is substantially the same
as the height of the peripheral wall.
15. A radiant electric heater according to claim 1, wherein the
height of the at least one dividing wall is less than the height
of the peripheral wall.
16. A radiant electric heater according to claim 1, wherein at
least two of the heating elements are concentrically disposed relative
to one another, a dividing wall being provided at each interface
between the heating elements.
17. A radiant electric heater according to claim 1, wherein at
least one of the heating elements is disposed laterally of the other
heating element or elements, the at least one dividing wall being
provided between the laterally disposed heating element and the
other heating element or elements.
18. A radiant electric heater according to claim 1, wherein the
at least one dividing wall comprises a thermal radiation reflecting
material.
19. A radiant electric heater according to claim 1, wherein the
at least one dividing wall is coated with a thermal radiation reflecting
material.
20. A radiant electric heater according to claim 1, wherein the
at least one dividing wall incorporates a thermal radiation reflecting
material.
Description This invention relates to a radiant electric heater for use in
a smooth top cooker and to a smooth top cooker incorporating such
a heater.
BACKGROUND TO THE INVENTION
In a smooth top cooker, a smooth top, normally of glass ceramic,
overlies one or more heaters comprising a metal dish, for example
of circular shape, in which is provided a base layer of thermal
and electrical insulating material. One or more electrical heating
elements is or are supported in the dish, such element or elements
comprising coiled wire and/or electrically conducting strip and/or
one or more infra-red radiating lamps, for example of tungsten-halogen
form. A peripheral wall of a thermally insulating material, such
as a ceramic fiber material or vermiculite, is normally provided
around the heater and, when the heater is installed, the peripheral
wall contacts the underside of the glass ceramic smooth top with
the heating element or elements being spaced from the smooth top.
DESCRIPTION OF PRIOR ART
In order to accommodate cooking utensils of different base areas
it is known to provide such a heater incorporating at least two
heating elements which are independently energisable and separated
by a dividing wall of thermal insulating material. In use, separate
and distinct heating zones are formed on the smooth top of the cooker
so as to enable the heated area of the cooker to be adapted substantially
to cooking utensils having different sizes and/or shapes. Such a
heater is described in GB-A-2 044 057.
With such an arrangement it is possible, for example, to provide
a circular heater including a central heating element and an annular
heating element surrounding the central heating element and separated
from it by a circular dividing wall of thermally insulating material.
For heating a cooking utensil of small base area the central heating
element is arranged to be energised alone. For heating a cooking
utensil of large base area, the central and annular heating elements
are both arranged to be energised together.
In the arrangement of GB-A-2 044 057, the dividing wall is selected
by way of its materials and dimensions to optimise thermal isolation
of the inner and outer heating zones from one another, the inner
heating zone incorporating the central heating element and the outer
heating zone incorporating the annular heating element. The dividing
wall, which is typically of ceramic fiber material, is arranged
to extend in height substantially up to the underside of the smooth
top of the cooker and has a typical thickness of about 10 to 15
mm, such a thickness being of a similar order of magnitude to the
peripheral wall of the heater. The material and dimensions of the
dividing wall provide an efficient thermal insulation barrier between
the inner and outer heating zones such that when only the central
heating element is energised, thermal transmission by conduction,
convection and radiation from the heated inner zone to the unheated
outer zone is minimised. In this way, concentration of heat in the
inner zone, that is within the confines of the dividing wall, is
maximised, as is the efficiency of heating a cooking utensil placed
on the smooth top and covering the area of the inner zone.
A disadvantage of such a known heater can be the substantial thickness
of the dividing wall which, as already stated, is typically 10 mm
or more. Such a thickness may, for example, be two, three, or more
times the typical distance between adjacent rows of wire coils in
a coiled wire heating element and from one aspect can result in
a non-uniform temperature across that area of the glass ceramic
surface of a cooker heated by the heater, when the elements in the
zones on both sides of the dividing wall are energised. From another
aspect, the thick dividing wall occupies a significant area of the
heater and results in a reduction in available area inside the heater
for the heating elements. This problem becomes severe when it is
required to provide a heater having more than two mutually isolated
heated zones, thereby necessitating two or more dividing walls.
OBJECT OF THE INVENTION
It is therefore an object of the present invention to provide a
radiant electric heater for use in a smooth top cooker with a dividing
wall which overcomes the above disadvantages.
SUMMARY OF THE INVENTION
According to the present invention there is provided a radiant
electric heater for use in a smooth top cooker, the heater comprising
a base of electrical and thermal insulating material; at least two
heating elements supported on the base, at least one of which heating
elements is energisable independently of the other element or elements;
a peripheral wall of thermal insulating material extending laterally
around the heating elements; and at least one dividing wall arranged
between the heating elements such that, in use, separate and distinct
heating zones are formed on the smooth top of the cooker, wherein
the at least one dividing wall is received in the base of electrical
and thermal insulating material and is retained therein by friction.
This permits the dividing wall to be considerably thinner than
hitherto and, in certain embodiments, to be constructed and arranged
as a barrier substantially only to thermal convection currents generated
in the heater. Thus, the dividing wall need not, for example, be
a barrier to thermal conduction. Consequently, the dividing wall
is only required to be of sufficient thickness so as to maintain
an upstanding, preferably self-supporting, position during operation
of the heater and to comprise a material which will withstand the
normal operating temperature range within the heater.
The dividing wall is maintained in its upstanding position by virtue
of the dividing wall being received in the base layer of electrical
and thermal insulating material and being retained therein by friction.
Thus, the dividing wall may be partly embedded in the base of electrical
and thermal insulation or may be received in a groove formed therein.
The dividing wall may be of a material selected without substantial
regard for its thermal conductivity or thermal radiation transmittance
or reflectance properties. Accordingly it is not required to comprise
a thermally insulating material in the sense thereof generally accepted
by the skilled person. The dividing wall may comprise a metal strip,
although for electrical safety reasons it may be preferred that
the wall comprises an electrically insulating material. Examples
of suitable electrically insulating materials for the dividing wall
are: fabrics incorporating glass fibers and/or ceramic fibers; ceramics;
mica and other micaceous materials. Such a fabric may, if required,
be impregnated or coated with a stiffening medium such as a solution
of an alkali or alkaline earth silicate solution, or a silica sol,
which is subsequently dried.
The thickness of the dividing wall should typically not exceed
50 percent, and preferably not exceed 30 percent, of the thickness
of the peripheral wall of the heater.
The thickness of the dividing wall is preferably less than 5 mm.
The dividing wall is preferably constructed of substantially uniform
thickness. Its height may be substantially the same as, or slightly
less than (for example by 1 to 2 mm) the height of the peripheral
wall of the heater.
In a particular embodiment, two or more heating elements may be
concentrically disposed relative to one another, a dividing wall
being provided at the or each interface between the heating elements.
In another embodiment, at least one of the heating elements may
be disposed laterally of the other heating element or elements,
a dividing wall being provided between the laterally disposed heating
element and the other heating element or elements.
If desired, the dividing wall may also comprise, or include, or
be coated with, a thermal radiation reflecting material, such as
alumina.
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 one embodiment of a radiant electric heater
according to the invention;
FIG. 2 is a section taken on line II--II of FIG. 1;
FIG. 3 is a plan view of another embodiment of a radiant electric
heater according to the invention;
FIG. 4 is a section taken along the line IV--IV of FIG. 3; and
FIG. 5 is a schematic sectional view of an arrangement for manufacturing
a radiant electric heater.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIGS. 1 and 2 show a radiant electric heater which comprises a
metal dish 1 containing a base 2 of electrical and thermal insulating
material. The insulating material is preferably microporous thermal
insulating material which is well known in the art and described,
for example, in GB-A-1 580 909. Against the side 3 of the dish 1
is located a peripheral wall 4 of thermal insulation material comprising,
for example, ceramic fiber material, vermiculite or microporous
thermal insulation material. A typical thickness of the peripheral
wall 4 is about 10 mm.
The heater is intended to be mounted in a smooth top cooker under
a glass-ceramic cooking surface 100, with the top surface of the
peripheral wall 4 in contact with the underside of the glass-ceramic
cooking surface.
Two concentric electric heating elements 5 and 6 in the form of
wire coils are located in grooves formed in the base 2. As an alternative
to wire coils, the heating elements may be in the form of elongate
electrically conductive strips, preferably corrugated, and partially
embedded edgewise in the base 2. The heating elements are separated
from each other by a circular dividing wall 7 having a height the
same as, or slightly less than (for example by 1 to 2 mm), the height
of the peripheral wall 4. The thickness of the dividing wall 7 is
such that it fits into a gap 12 between the heating elements 5 and
6 whose width is substantially the same as the distance d between
adjacent turns of the heating element 5. In practice, the thickness
of the dividing wall 7 will not exceed 50 percent of the typical
thickness of the peripheral wall 4 and preferably will not exceed
30 percent of the typical thickness of the peripheral wall 4. The
thickness of the dividing wall 7 is suitably less than 5 mm.
The dividing wall 7 separates the total heating area defined by
the peripheral wall 4 into a central zone and an annular zone. The
heating element 5 in the central zone is operable independently
of the heating element 6 in the annular zone, separate terminal
connectors 8 and 9 being provided for elements 5 and 6 respectively.
The material and thickness of the dividing wall 7 is selected primarily
with regard to preventing transmission of heat by thermal convection
from the central zone, heated by element 5, to the surrounding annular
zone in which is located the element 6. It is not therefore required
to be a thermally insulating material in the generally accepted
sense thereof and may comprise a metal. However, in the interest
of electrical safety it is preferred that the dividing wall should
comprise an electrical insulating material.
A particularly suitable material for the dividing wall 7 is a woven
glass fiber fabric, e.g. of thickness about 1 mm or less. Such a
fabric may, if desired, be stiffened by coating it, for example,
with a sodium silicate solution or aqueous silica sol which is subsequently
dried.
The dividing wall 7 could alternatively comprise a thin ring of
a ceramic, ceramic fiber material or mica or a micaceous material.
The dividing wall 7 is located in position by partially recessing
it into the surface of the base 2 so that the dividing wall is retained
in the base by friction. The dividing wall 7 may either be urged
into the surface of the base 2 or into a groove formed in the surface
of the base 2. I have found that to retain the dividing wall in
place by friction considerably facilitates manufacture and transportation
of the radiant heater.
A well-known form of thermal cut-out device 10 is provided to extend
across the heating element 5 in the central zone and is thermally
isolated from the heating element 6 in the annular outer zone by
means of a block 11 of thermal insulating material which surrounds
the cut-out device where it crosses the outer zone. The cut-out
device 10 is therefore responsive only to the heating element 5
in the central zone.
To assess the performance of the resulting heater a comparison
was made with a heater of the prior art which was of the same construction
apart from the dividing wall 7. In the prior art construction a
dividing wall was provided comprising ceramic fiber thermal insulating
material and having a thickness of about 10 mm. This thick dividing
wall of the prior art was intended to minimise thermal transfer
through it by conduction, convection and radiation. Both heaters
were located beneath a glass-ceramic cook top, and two identical
containers, each with one liter of water therein were placed on
the glass-ceramic surface overlying the central zone of the heaters.
The heating element 5 in the central zone of each heater was energised
and the time taken for the water in each container to reach boiling
point was measured. Very little difference in time was noted, the
heater according to the invention with the thin dividing wall taking
only a few seconds longer to bring the water to boiling point than
the heater of the prior art.
By contrast, in a further experiment a heater without any dividing
wall 7 was compared with a heater of the prior art. In this case
the heater without the dividing wall took about one minute longer
than the heater of the prior art to bring the water to boiling point.
Such experiments demonstrate that thermal convection is a prime
thermal parameter to be considered in such a heater and the thin
dividing wall of the present invention is an effective barrier to
thermal convection currents.
The use of the thin dividing wall in the heater of the invention
allows the spacing between the heating elements 5 and 6 in the central
and outer zones to be reduced compared with the prior art. Consequently,
when both heating elements are energised, a high degree of uniformity
of temperature across the entire heater can be achieved.
A further advantage of the thin dividing wall is that heaters with
more than two heating zones can readily be provided by incorporating
two or more such dividing walls, without the need to create additional
space for the dividing walls. This is primarily because the dividing
walls can be inserted in spaces having dimensions such as those
normally provided between adjacent rows of elements in a heating
coil or strip. Thus the dividing wall according to the present invention
gives considerably greater flexibility than has hitherto be available
in the design of radiant electric heaters for smooth top cookers.
In addition to heaters incorporating heating elements in the form
of wire coils or strips of metal or metal alloy, the invention is
also applicable to heaters incorporating one or more halogen lamps
instead of, or in addition to, one or more of the aforementioned
heating elements.
If desired, the thin dividing wall 7 may comprise, include, or
be coated with a thermal radiation reflecting material, such as,
for example, alumina.
The embodiment of FIGS. 3 and 4 is similar to that of FIGS. 1 and
2 and the same references are used to show the same or similar components.
In the embodiment of FIGS. 3 and 4, a radiant electric heater comprises
a metal dish 1 containing a base 2 of electrical and thermal insulating
material. Against the side 3 of the dish 1 is located a peripheral
wall 4 of thermal insulation material typically about 10 mm thick.
Two electric heating elements 13 and 14 in the form of wire coils
or strips of metal or metal alloy are located in grooves formed
in the base 2. The heating elements are separated from each other
by an arcuate dividing wall 15 having the same height as, or slightly
less than, the height of the peripheral wall 4.
The dividing wall 15 separates the total heating area defined by
the peripheral wall into a circular zone and a crescent-shaped zone.
The heating element 13 in the circular zone is operable independently
of the heating element 14 in the crescent-shaped zone so as to create
either a circular heating area or an oval heating area on the smooth
top 100 of the cooker. Separate terminal connectors 16 and 17 are
provided for heating elements 13 and 14 respectively.
The dividing wall 15 is partially recessed into the surface of
the base 2 by urging it into the surface of the base or into a groove
formed therein and is retained in the base by friction.
A thermal cut-out device 10 extends across the heating element
13 in the circular zone.
As an alternative to urging the dividing wall into the surface
of the base 2 or into a groove formed therein, there is shown in
FIG. 5 a press 18 for embedding the dividing wall into the base
at the time the base is formed. As illustrated in FIG. 5, the heating
elements can also be embedded in the base at the same time.
As can be seen from FIG. 5, the press 18 comprises a housing 19,
a cover 20, a plunger 21 and a press tool 22. The press tool 22
may conveniently be machined from a plastics material such as polytetrafluoroethylene
(PTFE) and has a stepped rim 23 and grooves 24, 25 and 26 formed
in its upper surface. Groove 24 is shaped in the illustrated embodiment
to correspond to the configuration of a generally circular dividing
wall, groove 25 is shaped in the illustrated embodiment to correspond
to the desired configuration of a generally circular heating element
to be positioned within the confines of the dividing wall, and groove
26 is shaped in the illustrated embodiment to correspond to the
desired configuration of a generally annular heating element to
be positioned around the dividing wall. It will be clear, however,
to the skilled person that numerous other configurations of dividing
wall and heating elements can be employed without departing from
the present invention. The depth of the grooves is selected to correspond
to whatever proportion of the height of the dividing wall or heating
element that is desired to be exposed in the resulting heater, i.e.
is required not to be embedded in the base of thermal insulation
material. Generally it will be desired that a major proportion of
the height of the dividing wall and of the heating elements will
be exposed.
Provision is made for air to escape from within the press 18, for
example by way of passageways 27 extending through the press tool
22 and the plunger 21. The upper end of the housing 19 is recessed
to receive the rim of a metal dish 28 which will form the support
for the heater.
Operation of the press 18 commences with retraction of the plunger
21 to the position shown in FIG. 5. A generally circular dividing
wall 29, for example of woven glass fiber material as hereinbefore
described, is placed edgewise in the groove 24. An inner, generally
circular, heating element 30, for example made from an elongate
electrically conductive strip of corrugated form, is placed edgewise
in the groove 25, and an outer, generally annular, heating element
31 similar to the heating element 30 is placed edgewise in the groove
26.
A predetermined quantity of powdery microporous insulation mixture
32 (shown in dashed line) is introduced into the press 18 on top
of the press tool 22, the dividing wall 29, the inner heating element
30 and the outer heating element 31. The metal dish 28 is then placed
in the recess in the upper end of the housing 19 and the cover 20
is closed and secured.
The powdery microporous thermal insulation material is described,
for example, in GB-A-1 580 909, a typical composition being:
49-97 percent by weight pyrogenic silica
0.5-20 percent by weight ceramic fiber reinforcement
2-50 percent by weight opacifier (such as titanium dioxide)
0.5-12 percent by weight alumina
The press 18 is operated, for example hydraulically, to urge the
plunger 21 and the press tool 22 towards the metal dish 28, thereby
compacting the insulation material 32 into the dish 28. The material
is compacted to a density of, typically, 300-400 kg/m.sup.3, and
the plunger 21 may be held in its final position for a dwell time
of several seconds to several minutes if necessary.
The cover 20 is opened and the dish 28 containing the compacted
insulation material 32, the dividing wall 29 and the heating elements
30 and 31 (shown in dashed lines in FIG. 5) is removed. The dividing
wall and the heating elements are found to be partially embedded
in the insulation material 32 and are retained in the insulation
material by friction. A major proportion of the height of the dividing
wall and of the heating elements is exposed above the surface of
the insulation material, this proportion corresponding to the depth
of the grooves 24, 25 and 26 in the press tool 22. The insulation
material is found to have been firmly compacted around the dividing
wall and the heating elements thereby securing the dividing wall
and the elements firmly in place in partial embedment in the insulation
material.
Assembly of the remaining components of the heater, for example
the terminal connectors, peripheral wall, thermal cut-out device
and a block of thermal insulation material if needed, as illustrated
in FIGS. 1 and 2, may then be effected.
If it is desired to secure the dividing wall more firmly in the
insulation material, apertures or recesses may be provided around
the periphery of the dividing wall so as to be embedded in the thermal
insulation material. It is found that insulation material enters
the apertures or recesses and becomes compacted therein so as to
secure the dividing wall more firmly.
If desired, the microporous thermal insulation material may comprise
more than one layer, with a main layer of silica-based material
being positioned adjacent the base of the metal dish and a surface
layer of alumina-based material. The surface layer is preferably
sufficiently thick for the embedded portions of the dividing wall
and of the heating elements to be accommodated entirely within it.
A suitable composition for the alumina-based material comprises:
55-65 percent by weight aluminum oxide
5-15 percent by weight silica
25-35 percent by weight titanium dioxide
1-5 percent by weight ceramic fiber
The aluminum oxide is in the form of a pyrogenic, or fume, material
such as that sold under the name Aluminum Oxide C by Degussa AG.
The layers may be formed in the metal dish in any suitable manner.
For example, the material for the silica-based insulation material
may first be introduced into the press and compacted using a first
press tool, the insulation material being compacted to less than
its final density. The dividing wall and heating elements may be
inserted into a second press tool and the alumina-based insulation
material may then be introduced into the press beneath the silica-based
material partially compacted into the metal dish. The alumina-based
material is then compressed onto the silica-based material with
the second press tool and the two layers compacted to their final
density while simultaneously securing the dividing wall and the
heating elements in position in the alumina-based material. Alternatively,
the two-layer arrangement can be manufactured in a single operation
by introducing the alumina-based material into the press on top
of the dividing wall and the heating elements and then introducing
the silica-based material on top of the alumina-based material.
The press is then operated to compact the two layers simultaneously
and to secure the dividing wall and the heating elements in position.
Various other modifications may be made to the methods described
above. Thus it is not essential for the heater to be manufactured
in an inverted position. It may be manufactured by placing the powdery
insulation material in the metal dish and then bringing the press
tool, with the dividing wall and the heating elements held therein,
downwardly onto the insulation material to compact it into the dish
and to secure the dividing wall and the heating elements in the
insulation material. Moreover, it is not necessary simultaneously
to secure the dividing wall and the heating elements in the insulation
material and the heating elements and/or the dividing wall may be
secured subsequently to forming a fully or partially compacted layer
of insulation material if desired.
The insulation material in one or more layers may be first compacted
in the dish, preferably to less than the final compaction density,
using the first press tool and the heating elements and dividing
wall may then be urged into the insulation material by means of
the second press tool, accompanied if necessary by final compaction
of the insulation material. |