MCWILLIAMS, Kevin, Ronald (12 Broad Walk, Stratford-upon-Avon, Warwickshire CV37 6HS, GB)
CLAIMS
1. A radiant electric heater comprising: a base layer (3) of thermal and electrical insulation material; a peripheral wall (5) of thermal and electrical insulation material extending around a peripheral region of the base layer; a radiant electric heating element (11) mounted relative to the base layer; and a substantially planar temperature sensor extending from the peripheral wall at least partly across the base layer and above the heating element, characterised in that the thickness of the base layer is at least substantially 50 percent of the overall height of the heater.
2. A radiant electric heater as claimed in claim 1 , characterised in that the thickness of the base layer (3) is at least substantially 55 percent of the overall height of the heater.
3. A radiant electric heater as claimed in claim 2, characterised in that the thickness of the base layer (3) is at least substantially 55.8 percent of the overall height of the heater.
4. A radiant electric heater as claimed in claim 3, characterised in that the thickness of the base layer (3) is at least substantially 61.2 percent of the overall height of the heater.
5. A radiant electric heater as claimed in claim 4, characterised in that the thickness of the base layer (3) is at least substantially 80 percent of the overall height of the heater.
6. A radiant electric heater as claimed in any preceding claim, characterised in that the radiant heating element (11 ) comprises a coil of helically wound bare wire.
7. A radiant electric heater as claimed in any one of claims 1 to 5, characterised in that the radiant heating element (11 ) comprises a strip of corrugated ribbon.
8. A radiant electric heater as claimed in claim 7, characterised in that the strip of corrugated ribbon (11 ) is supported edgewise on the base layer (3) by partial embedding therein.
9. A radiant electric heater as claimed in any preceding claim, characterised in that the base layer (3) is mounted in a dish-like support (1 ).
10. A radiant electric heater as claimed in claim 9, characterised in that the dish-like support (1 ) is made of metal.
11. A radiant electric heater as claimed in any preceding claim, characterised in that the temperature sensor (21 ) comprises a substantially planar support of ceramic material provided thereon with an electrical component having an electrical parameter which changes as a function of temperature.
12. A radiant electric heater as claimed in claim 11 , characterised in that the electrical component is of film or foil form.
13. A radiant electric heater as claimed in claim 12, characterised in that the electrical component is an electrical resistance component.
14. A radiant electric heater as claimed in claim 13, characterised in that the electrical resistance component comprises platinum.
15. A radiant electric heater as claimed in any preceding claim, characterised in that the temperature sensor (21 ) has a thickness of substantially 1 mm.
16. A radiant electric heater as claimed in any preceding claim, characterised in that upstanding regions (23) are formed on the base layer (3) between adjacent turns of the heating element (11 ).
17. A radiant electric heater as claimed in claim 16, characterised in that the upstanding regions (23) have chamfered side edges.
18. A radiant electric heater as claimed in claim 16 or 17, characterised in that the upstanding regions (23) cover substantially half the surface area of the base layer (3).
19. A radiant electric heater as claimed in any preceding claim, characterised in that a scallop is formed in the base layer in the region beneath the temperature sensor.
20. A radiant electric heater as claimed in claim 19, characterised in that the scallop extends a short distance in the lateral direction and in the longitudinal direction beyond the periphery of the temperature sensor.
21. A radiant electric heater as claimed in claim 19 or 20, characterised in that the scallop has a depth in the range of substantially 1.0 mm to substantially 2.5 mm. |
RADIANT ELECTRIC HEATER
This invention relates to a radiant electric heater which is provided with a substantially planar temperature sensor.
It is well known, for example from GB-A-2 263 770, to provide a radiant electric heater with a temperature sensor in the form of a differential expansion device which extends at least partly across the heater. A temperature sensor within the heater is generally considered to be essential in order to prevent the cooking surface above the heater from overheating. The differential expansion device includes two elongate members of differing materials which have different coefficients of thermal expansion and the differential expansion is employed to operate a microswitch positioned externally of the heater. In practice, one of the elongate members is made of a metallic material and is generally in the form of a rod and the other of the elongate members is made of a ceramic or quartz glass material and is generally in the form of a tube surrounding the metal rod. The rod and tube are fastened together at the free end of the sensor, generally with one or more metal components.
It has been found that the size of the various components, at about 5 mm diameter, together with the fact that at least some of the components are of metal and are exposed within the heater above a bare wire or ribbon resistance heating element, imposes a number of constraints on the arrangement of components and the ability to provide thermal insulation within a standard overall heater depth which is generally some 32 to 33 mm.
If the depth of the temperature sensor could be reduced and if the exposure of metallic components of the temperature sensor within the heater could be reduced or eliminated, this would significantly increase flexibility in the manufacture and design of radiant electric heaters. In particular, it would be advantageous to increase the thickness of the insulation within the heater so as
to reduce the temperature in a cooking appliance so as to reduce the risk of overheating components, especially electronic components, of the appliance.
It is also known, for example from WO-A-03007660, to provide a temperature sensor in a radiant electric heater in the form of an electrical component, such as of film or foil, having an electrical parameter which changes as a function of temperature. However, the temperature sensor is mounted on an H or I shaped beam and no consideration is given to increasing the flexibility of manufacture and design of the heater.
It is therefore an object of the present invention to provide a radiant electric heater which overcomes, or at least ameliorates, the above disadvantages.
According to the present invention there is provided a radiant electric heater comprising: a base layer of thermal and electrical insulation material; a peripheral wall of thermal and electrical insulation material extending around a peripheral region of the base layer; a radiant electric heating element mounted relative to the base layer; and a substantially planar temperature sensor extending from the peripheral wall at least partly across the base layer and above the heating element, wherein the thickness of the base layer is at least substantially 50 percent of the overall height of the heater.
The thickness of the base layer may be at least substantially 55 percent of the overall height of the heater. Alternatively, the thickness of the base layer may be at least substantially 55.8 percent of the overall height of the heater. As a further alternative, the thickness of the base layer may be at least substantially 61.2 percent of the overall height of the heater. As another alternative, the thickness of the base layer may be at least substantially 80 percent of the overall height of the heater.
The radiant heating element may comprise a coil of helically wound bare wire. Alternatively, the radiant heating element may comprise a strip of corrugated
ribbon. The strip of corrugated ribbon may be supported edgewise on the base layer by partial embedding therein.
The base layer may be mounted in a dish-like support, for example of metal.
The temperature sensor may comprise a substantially planar support of ceramic material provided thereon with an electrical component having an electrical parameter which changes as a function of temperature. The electrical component may be of film or foil form, such as an electrical resistance component, for example of platinum.
The temperature sensor may have a thickness of substantially 1 mm.
Upstanding regions may be formed on the base layer between adjacent turns of the heating element. The upstanding regions may have chamfered side edges.
The upstanding regions may cover substantially half the surface area of the base layer.
A scallop may be formed in the base layer in the region beneath the temperature sensor. The scallop may extend a short distance in the lateral direction and in the longitudinal direction beyond the periphery of the temperature sensor. The scallop may have a depth in the range of substantially 1.0 mm to substantially 2.5 mm.
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:
Figure 1 is a cross-sectional view through part of a known radiant electric heater incorporating a temperature sensor in the form of a differential expansion device;
Figure 2 is a cross-sectional view through part of one embodiment of a radiant electric heater according to the present invention;
Figure 3 is a cross-sectional view through part of another embodiment of a radiant electric heater according to the present invention;
Figure 4 is a cross-sectional view through part of a modified radiant electric heater according to the present invention;
Figure 5 is a plan view of a further embodiment of a radiant electric heater according to the present invention;
Figure 6 is a partial cross-sectional view along the line Vl-Vl shown in Figure 5; and
Figure 7 is a partial cross-sectional view taken along the line VII-VII shown in Figure 5.
The radiant electric heater shown in part in Figure 1 comprises a metal dish-like support 1 having therein a base layer 3 of microporous thermal and electrical insulation material which is raised around the periphery thereof. A peripheral wall 5 of thermal and electrical insulation material, such as bound vermiculite, extends around the peripheral region of the base layer 3 and bears on the raised region of the base layer. The peripheral wall 5 is arranged to contact the underside 7 of a cooking plate 9, for example of glass-ceramic material.
A radiant heating element 11 is supported in the heater relative to the base layer 3. The heating element 11 can comprise any of the well-known forms of element such as a coil of helically wound bare wire and/or a strip of corrugated ribbon. In particular, the heating element 11 can comprise a corrugated ribbon heating element supported edgewise on the base layer by partial embedding therein.
A differential expansion device 13 of a temperature sensor extends through the peripheral wall 5 and at least partly across the heater above the heating element 11 and the base layer 3. In practice, the differential expansion device comprises a tube 15 of ceramic material which surrounds a rod 17 of metal, the tube and rod being secured together at the free ends thereof by means of a metal component 19.
The. dimensions of the known radiant electric heater shown in Figure 1 include an overall height of 32.25 mm, with a distance to the top of the heating element 11 of 18.25 mm, a distance from the top of the heating element to the underside of the of the differential expansion device 13 of 2.9 mm, a diameter of the differential expansion device of 4.7 mm and a distance from the top of the differential expansion device to the top of the peripheral wall 5 of 6.4 mm (making a total of 14 mm between the top of the heating element 11 and the top of the peripheral wall 5). The distance from the bottom of the metal dish 1 to the centre of the differential expansion device is 23.5 mm. The thickness of the metal dish is substantially 0.5 mm, so the thickness of the thermal insulation base layer 3 is 12.5 mm. The dimensions between the top of the heating element 11 and the underside of the differential expansion device 13 and between the top of the differential expansion device and the top of the peripheral wall 5 may vary slightly depending on the diameter of the tube of the differential expansion device, but the overall distance remains substantially constant.
Thus the distance between the top of the heating element 11 and the top of the peripheral wall 5 occupies some 43.4 percent of the overall height of the heater, while the distance from the bottom of the metal dish 1 to the top of the heating element 11 occupies some 56.6 percent of the overall height of the heater, with the thickness of the thermal insulation base layer 3 being some 38.8 percent of the height of the heater.
The radiant electric heater part shown in Figure 2 is similar to that shown in Figure 1 except that the differential expansion device has been replaced by a
temperature sensor 21 , which comprises a substantially planar support of ceramic material of generally rectangular shape when viewed from above, provided thereon with an electrical component of film or foil form and having an electrical parameter which changes as a function of the temperature of the cooking plate 9.
The electrical component of film or foil form may be, for example, an electrical resistance component, for example of platinum, whose electrical resistance changes as a function of temperature.
The dimensions of the radiant electric heater shown in Figure 2 include an overall height of 32.25 mm (i.e., the same as the height of the heater shown in Figure 1 ), with a distance to the top of the heating element 1 1 of 23.75 mm, a distance to the underside of the of the temperature sensor 21 of 4.5 mm, a thickness of the temperature sensor of 1 mm and a distance from the top of the temperature sensor to the top of the peripheral wall 5 of 3 mm (making a total of 8.5 mm between the top of the heating element 11 and the top of the peripheral wall 5). The distance from the bottom of the metal dish 1 to the centre of the temperature sensor 21 is 28.75 mm. Allowing for the metal dish, the thickness of the thermal insulation base layer 3 is 18 mm. The dimensions between the top of the heating element 11 and the underside of the temperature sensor 21 and between the top of the temperature sensor and the top of the peripheral wall 5 may vary slightly depending on the thickness of the temperature sensor, but the overall distance remains substantially constant.
Thus the distance between the top of the heating element 11 and the top of the peripheral wall 5 occupies only about 26.4 percent of the overall height of the heater, while the distance from the bottom of the metal dish 1 to the top of the heating element 11 occupies some 73.6 percent of the overall height of the heater, with the thickness of the thermal insulation base layer 3 being some 55.8 percent of the height of the heater.
The consequence of increasing the thickness of the thermal insulation base layer 3 from 12.5 mm to 18 mm is that less heat is transmitted through the base of the heater and the temperature of the metal dish 1 in the region of the centre of the base is reduced by some 30 to 40 degrees Celsius.
The radiant electric heater part shown in Figure 3 is similar to that shown in Figure 2 except that the height of the heating element is somewhat less at 3.5 mm instead of 5.25 mm. Consequently, the distance between the bottom of the heater and the top of the thermal insulation base layer 3 can be increased accordingly, from 18.5 mm to 20.25 mm, giving an insulation thickness of 19.75 mm.
Thus the distance between the top of the heating element 11 and the top of the peripheral wall 5 still occupies about 26.4 percent of the overall height of the heater, while the distance from the bottom of the metal dish 1 to the top of the heating element 11 still occupies some 73.6 percent of the overall height of the heater, but the thickness of the thermal insulation base layer 3 is now some 61.2 percent of the height of the heater.
The consequence of increasing the thickness of the thermal insulation base layer 3 from 12.5 mm to 19.75 mm is that less heat is transmitted through the base of the heater and the temperature of the metal dish 1 in the region of the centre of the base is reduced by some 40 to 50 degrees Celsius.
While the distance between the top of the heating element 11 and the top of the peripheral wall 5 is 8.5 mm in Figure 3, it has been found that this distance can be reduced even further and could be as little as 5 mm. For example, if the distance between the top of the heating element and the top of the peripheral wall, is 6 mm, the distance between the top of the heating element 11 and the underside of the temperature sensor 21 may be 2.5 mm, the thickness of the temperature sensor may be 1 mm and the distance from the top of the temperature sensor to the top of the peripheral wall may be 2.5 mm. In such a
configuration, the thickness of the thermal insulation base layer 3 is some 22.25 mm, the distance between the top of the heating element and the top of the peripheral wall occupies some 18.6 percent of the overall height of the heater and the thermal insulating base layer occupies some 69 percent of the overall height of the heater. If the distance between the top of the heating element and the top of the peripheral wall is 5 mm, the distance between the top of the heating element 11 and the underside of the temperature sensor 21 may be 2 mm, the thickness of the temperature sensor may be 1 mm and the distance from the top of the temperature sensor to the top of the peripheral wall may be 2 mm. In such a configuration, the thickness of the thermal insulation base layer
3 is some 23.25 mm, the distance between the top of the heating element and the top of the peripheral wall occupies some 15.5 percent of the overall height of the heater and the thermal insulating base layer occupies some 72.1 percent of the overall height of the heater. It should be noted, however, that the position ofthe temperature sensor 21 between the top of the heating element 11 and the top of the peripheral wall 5 may be varied.
The radiant electric heater according to the present invention and shown in either Figure 2 or Figure 3 can be modified in accordance with Figure 4 in order to further reduce heat loss through the base of the heater. This is accomplished by forming upstanding regions 23 of thermal insulation material on the base layer 3 between adjacent turns ofthe heating element 11. The upstanding regions 23 may have chamfered side edges, for example at 45 degrees to the horizontal and flattop surfaces and may cover about half the surface area ofthe base layer 3. That is, where the distance between adjacent turns ofthe heating element 11 is about 7 mm, the upstanding regions 23 may have a width of about 4 mm. This has the effect of increasing the thickness of the thermal insulation material by an average of up to about 1 mm.
As shown in Figures 5 to 7, a shallow scallop 25 may be formed in the thermal insulation base layer 3 in the region beneath the temperature sensor 21 , extending a short distance in the lateral direction and in the longitudinal direction
beyond the periphery of the temperature sensor. As can be seen from Figures 6 and 7, the depth of the scallop may vary and may be, for example, about 1 mm close to the peripheral wall 5 as shown in Figure 6 and about 2.5 mm at a distance further from the peripheral wall as shown in Figure 7.
The scallop 25 allows a distance of 8.5 mm to be maintained between the majority of the top of the heating element 11 and the top of the peripheral wall 5, but increases the distance to from 9.5 mm to 11.0 mm in the region of the temperature sensor 21.
The scallop 25 has a detrimental effect of the temperature of the metal dish 1. However, the temperature rise is small because about 80 percent of the thermal insulation base 3 is unaffected, while the remaining about 20 percent has an average reduction in thickness of about 1 to 1.5 mm, so the average reduction in thickness of the entire base layer 3 is significantly less than 1 mm.
In the illustrated example, the distance between the top of the heating element 11 and the top of the peripheral wall 5 occupies about 29.5 to about 34.1 percent of the overall height of the heater in the region of the scallop 25, while the distance from the bottom of the metal dish 1 to the top of the heating element occupies some 70.5 to 65.9 percent of the overall height of the heater in the region of the scallop, with the thickness of the thermal insulation base layer 3 being some 52.7 to 48.1 percent of the height of the heater in this region.
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