Abstrict
A radiant electric heater has first, second and third heating zones
(1, 2, 3) arranged substantially side-by-side. The first heating
zone (1) is provided with at least one first heating element (9).
The second heating zone (2) is arranged adjacent to the first heating
zone and is provided with at least one second heating element (10).
The third heating zone (3) is arranged adjacent to the second heating
zone and is provided with at least one third and one fourth heating
element (11, 12). A switch (15) is provided for switching between
first, second and third heating states. In the first heating state
the at least one first heating element (9) of the first heating
zone (1) is energised alone, the at least one second, third and
fourth heating elements (10, 11, 12) of the second and third heating
zones (2, 3) being de-energised. In the second heating state the
at least one first heating element (9) of the first heating zone
(1) is electrically connected and energised in parallel with the
at least one second heating element (10) of the second heating zone
(2), the at least one third and fourth heating elements (11, 12)
of the third heating zone (3) being de-energised.
Claims
I claim:
1. A radiant electric heater having multiple heating zones arranged
substantially side-by-side and comprising:
a first heating zone provided with at least one first heating element;
a second heating zone arranged adjacent to the first heating zone
and provided with at least one second heating element; a third heating
zone arranged adjacent to the second heating zone and provided with
at least one third and one fourth heating element; and switch means
for switching between first, second and third heating states; the
arrangement of the switch means being such that in the first heating
state the at least one first heating element of the first heating
zone is energized alone, the at least one second, third and fourth
heating elements of the second and third heating zones being de-energized;
and that in the second heating state the at least one first heating
element of the first heating zone is electrically connected and
energized in parallel with the at least one second heating element
of the second heating zone, the at least one third and fourth heating
elements of the third heating zone being de-energized; and that
in the third heating state the at least one first heating element
of the first heating zone is electrically connected in parallel
with the at least one second heating element of the second heating
zone and the parallel-connected at least one first and at least
one second heating elements are connected in series with the at
least one third heating element of the third heating zone to form
a combination, which combination is electrically connected in parallel
with the at least one fourth heating element of the third heating
zone and energized therewith.
2. A heater according to claim 1, wherein the first, second and
third heating zones are separated by walls of thermal insulating
material.
3. A heater according to claim 1, wherein the first heating zone
is circular and the second and third heating zones are annular,
the second heating zone surrounding the first heating zone and the
third heating zone surrounding the second heating zone.
4. A heater according to claim 1, wherein the heating elements
are selected from wire, ribbon and lamp forms and combinations thereof.
5. A cooking appliance provided with a radiant electric heater
according to claim 1.
Description This invention relates to a radiant electric heater having multiple
heating zones and which may be used, for example, in a cooking appliance
having a glass-ceramic cooking plate. More particularly the invention
relates to such a heater having three heating zones, which may be
concentrically arranged, and particularly, although not exclusively,
to such a heater having relatively large overall diameter, for example
of the order of 300 mm.
DESCRIPTION OF PRIOR ART
Radiant heaters having multiple, particularly two, concentric heating
zones are well known for use in glass-ceramic cooking appliances.
U.S. Pat. No. 4,158,127 describes a range top having a plurality
of main cooking stations with an auxiliary cooking station disposed
in the vicinity of at least one of the main cooking stations and
being conformed to and contiguous with the main cooking station
to define a substantially continuous enlarged cooking area. A further
auxiliary cooking station may be disposed between adjacent main
cooking stations. United Kingdom Patent Publication No. 2,114,829
describes a circuit arrangement for such a range top in which the
heating elements or cooking stations are controlled individually.
European Patent Publication No. 0,571,054 describes a radiant electric
heater which may incorporate two or three heating elements arranged
concentrically to permit selective heating of differently sized
areas. Two of the heating elements, in the form of an infrared lamp
and a bare wire heating element, may be permanently connected in
series to limit lamp inrush current, while a second bare wire heating
element may be energised together with the other elements for heating
larger size utensils.
Problems have been encountered with such heaters in that it is
desirable when heating a small utensil on, for example, an inner
heating zone of the heater, for the heating element in such inner
zone to be energised with a power level comparable to a single zone
heater of similar dimensions. This is in order to ensure rapid heating
of the utensil and its contents. However when a larger utensil is
being heated, covering both the inner zone and an annular heating
zone surrounding the inner zone, then, when the heating element
of the outer zone is energised in parallel with the element of the
inner zone, the high power of the inner zone results in the surface
power loading over the inner zone being greater than that over the
outer zone which is disadvantageous. The result of this arrangement
is a heater which is centre-weighted in power and it is generally
preferred for a heater to be edge-weighted in power.
A further problem is that with large multi-zone heaters, for example
of the order of 300 mm diameter, provision of heating elements to
provide a conventional surface power loading of, for example, about
7 watts per square centimeter results in an electric current level
in excess of the handling capabilities of the contacts in standard
thermal limiters and energy regulators used in the art.
The first problem is solved in part by European Patent Publication
No. 0,551,172 which describes a radiant heater having multiple heating
zones in which a heating element, or a combination of a lamp and
a coil of bare resistance wire permanently connected in series,
is provided in a first, circular heating zone and two heating elements
are provided in a second, outer heating zone. The heating power
of the first zone may be 1200 watts with a specific surface loading
of 0.073 watts per square millimeter, while, when the heating elements
in the second zone are also energised with one of the outer heating
elements in series with the heating element(s) of the first zone,
the specific surface loading of the first zone is reduced to 0.061
watts per square millimeter and the specific surface loading of
the second zone is 0.076 watts per square millimeter. Nevertheless,
this does not provide a solution where more than two heating zones
are present.
OBJECT OF THE INVENTION
It is an object of the present invention to overcome or minimise
these problems where three heating zones are present.
SUMMARY OF THE INVENTION
The present invention provides a radiant electric heater having
multiple heating zones arranged substantially side-by-side and comprising:
a first heating zone provided with at least one first heating element;
a second heating zone arranged adjacent to the first heating zone
and provided with at least one second heating element; a third heating
zone arranged adjacent to the second heating zone and provided with
at least one third and one fourth heating element; and switch means
for switching between first, second and third heating states; the
arrangement of the switch means being such that in the first heating
state the at least one first heating element of the first heating
zone is energised alone, the at least one second, third and fourth
heating elements of the second and third heating zones being de-energised;
and that in the second heating state the at least one first heating
element of the first heating zone is electrically connected and
energised in parallel with the at least one second heating element
of the second heating zone, the at least one third and fourth heating
elements of the third heating zone being de-energised; and that
in the third heating state the at least one first heating element
of the first heating zone is electrically connected in parallel
with the at least one second heating element of the second heating
zone and the parallel-connected at least one first and at least
one second heating elements are connected in series with the at
least one third heating element of the third heating zone to form
a combination, which combination is electrically connected in parallel
with the at least one fourth heating element of the third heating
zone and energised therewith.
The first, second and third heating zones may be separated by walls
of thermal insulating material.
The first heating zone may be circular and the second and third
heating zones may be annular, the second heating zone surrounding
the first heating zone and the third heating zone surrounding the
second heating zone.
The heating elements of the first, second and third heating zones
may comprise any of the known forms, such as wire, ribbon or lamp
forms, or combinations thereof.
By means of the invention, the heating elements of the first and
second heating zones, when the heater is energised in the first
and second heating states, provide relatively high surface power
loading (for example about 6 to 7 watts per square centimeter) over
the first and second heating zones. However, when the heater is
energised in the third heating state, the surface power loading
over the first and second heating zones is reduced, for example
to about 4 to 5 watts per square centimeter, while the surface power
loading over the third heating zone is high relative thereto at,
for example, about 6 watts per square centimeter. Advantageous edge-weighting
of power is thereby achieved when all three heating zones are energised,
whilst an advantageously high surface power loading is achieved
over the first and second heating zones when these are energised
without the third heating zone.
Furthermore the total power of the heater can be maintained at
such a level, for example 2700 watts, that excessively high currents
are avoided.
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 schematic representation of the connection of heating
elements in the heater of FIG. 1; and
FIG. 3 is a schematic representation of a known form of switch
for connecting the heater to a voltage supply.
DESCRIPTION OF PREFERRED EMBODIMENT
A radiant electric heater is constructed having three concentric
heating zones 1, 2, 3. A circular first heating zone 1 is surrounded
by an annular second heating zone 2 which is in turn surrounded
by an annular third heating zone 3.
The three heating zones are formed as follows. A metal supporting
dish 4 has provided therein a base layer 5 of insulation material,
such as well known microporous thermal and electrical insulation
material. A peripheral wall 6 of insulation material of well known
form is provided around the edge of the dish and two further annular
walls 7, 8 of similar form are provided in the dish to define the
heating zones 1, 2 and 3. The heater is intended for use in a glass-ceramic
cooking appliance with at least the peripheral wall 6 in contact
with the underside of a glass-ceramic cooking surface (not shown).
A first heating element 9, which may be of ribbon form although
any other forms could be considered, is provided in the first heating
zone 1.
A second heating element 10, of the same form as, or different
from, the first heating element 9, is provided in the second heating
zone 2.
A third heating element 11 and a fourth heating element 12 are
provided in the third heating zone 3 and may be of the same form
as, or different from, the first and second heating elements 9 and
10.
By way of example, the first heating zone may have a diameter of
about 145 mm, the first and second heating zones together may have
a diameter of about 210 mm and the first, second and third heating
zones together may have a diameter of about 275 mm.
A well-known form of thermal limiter 13 is provided having a sensor
rod thereof extending at least partly across the heater. As shown
in FIG. 1, the sensor rod of the limiter 13 is shielded by a block
14 of thermal insulation material from the influence of the second
heating element 10 in the second heating zone 2, but this is not
essential.
As shown in FIG. 3, the heater is arranged to be energised from
a voltage supply, such as a 230 volts supply, by way of a well known
form of switch (15) arranged to connect the heater in three heating
states as follows.
In a first heating state, the first heating element 9 in the first
heating zone 1 is energised alone by connecting terminals A and
F to the supply. By way of example, this results in a power dissipation
of about 1050 watts in the first heating element 9 and a surface
power loading over the first heating zone 1 of about 6.4 watts per
square centimeter.
In a second heating state, the first heating element 9 in the first
heating zone 1 remains energised as before, while the second heating
element 10 in the second heating zone 2 is energised in parallel
therewith by connecting terminals A and E to the supply. By way
of example, this results in a power dissipation of about 1150 watts
in the second heating element and a surface power loading over the
second heating zone 2 of about 7.6 watts per square centimeter.
A cooking utensil located over the first and/or second heating zones
1, 2 will be heated rapidly and efficiently as a result of the relatively
high surface power loadings over these heating zones.
In a third heating state, the first heating element 9 in the first
heating zone 1 is electrically connected in parallel with the second
heating element 10 in the second heating zone 2 and this parallel
arrangement of elements 9 and 10 is connected in series with the
third heating element 11 in the third heating zone 3 to form a resultant
combination of elements 9, 10 and 11, which combination is electrically
connected in parallel with the fourth heating element 12 in the
third heating zone 3 and energised therewith. For this purpose,
the voltage supply is connected to terminals A and D while terminal
E is connected to terminal F and terminal B is connected to terminal
C.
Again by way of example, this results in a power dissipation of
about 300 watts in the third heating element 11 and about 900 watts
in the fourth heating element 12, in the third heating zone. This
provides a surface power loading of about 5.6 watts per square centimeter
over the third heating zone. However, in this heating state the
power dissipated in the parallel combination of the first and second
heating elements 9, 10 reduces to about 1500 watts, compared with
a value of about 2200 watts in the second heating state, the total
heater power being about 2700 watts. This provides a surface power
loading of about 5.0 watts per square centimeter over the second
heating zone 2 and about 4.4 watts per square centimeter over the
first heating zone 1. In this heating state the heater is edge-weighted
in power and this, together with the associated reductions in surface
power loadings, is particularly advantageous for simmering operations
using large cooking utensils.
Since the total heater power in the third heating state is about
2700 watts, the resulting current of about 12 amps is well within
the handling capacity of the thermal limiter contacts, the switch
contacts and any associated energy regulator, where provided. |