GB2369037A

GB2369037A - Appliances for heating liquids and foodstuffs

Info

Publication number: GB2369037A
Application number: GB0028015A
Authority: GB
Inventors: John Crawshaw Taylor
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 2000-11-16
Filing date: 2000-11-16
Publication date: 2002-05-22
Also published as: GB0028015D0

Abstract

A heating appliance 2 for liquid or foodstuff, comprises a vessel 4 for receiving liquid or foodstuffs, a thick film heater 10 arranged under and in good thermal contact with the base 22 of the vessel, means 32 for resiliently biassing said heater against said vessel base, provided by a clamping member 40, a multi arm spring connector 26 for making electrical contact with the thick film heater, and electronic controlling means which is self protecting in the event of overheating. The heater may comprise of a ceramic substrate 24, and the resilient biassing means 32 may be metallic and may extend over a substantial part of the entire heating surface of the heater. The biassing means 32 may further comprise a plurality of spring means 36 provided thereon in a desired pattern, the spring means may have spring tongues 34 released from the plate and may be of rectangular or triangular in shape. An electrically insulated layer may be provided between the heater and the biassing means, and may comprise of a thermally resistant material, also retaining means may be provided attached to a region of the vessel base, to retain the heater in position. The clamping plate 40 may be welded 60 to the vessel base 22 to form a watertight seal around a heater compartment 64, and may be in the form of a dished plate accommodating the heater, biassing spring and other components. The heater compartment may be filled with an electrically insulating sealing material of low viscosity, and the appliance may be cordless.

Description

Appliances for Heating Liquids and Foodstuffs The present invention relates to appliances for heating liquids and foodstuffs, for example deep fat fryers, cooking pots and so on.

A typical deep fat fryer comprises a metal, eg a stainless steel vessel, with a heating element attached to the underside of the vessel. Typically the heater comprises a sheathed heating element which is attached to the vessel base via a heat diffuser plate.

More recently, however, the use of so-called thick film heaters has become more common in liquid heaters such as kettles. Such heaters comprise an insulated substrate, eg a glass coated steel substrate, on which is deposited a resistive heating track.

The present invention seeks to provide a novel manner in which such heaters can be mounted and used in liquid and foodstuff heating appliances.

From a first aspect the invention provides a liquid or foodstuff heating appliance comprising a vessel for receiving liquid or foodstuffs to be heated, a thick film heater arranged under and in good thermal contact with the base of the vessel, and means for resiliently biasing said heater against said vessel base.

From a second aspect, the invention provides a heater assembly in or for a liquid or foodstuff heating appliance comprising a vessel for receiving liquid or foodstuffs to be heated, said assembly comprising a thick film heater arranged under and in good thermal contact with a base part of the vessel, and means for resiliently biasing said heater against said vessel base part.

In accordance with the invention, therefore, a thick film heater is resiliently biased against the base of the liquid or foodstuff receiving vessel of the appliance. This arrangement avoids the need to directly affix the heater to the vessel base, which may be

difficult. It also allows for relative thermal expansion between the vessel and the heater. In effect, the heater can float on the vessel base thereby minimising any stresses which might arise due to differential rates of expansion between the heater and the vessel base.

The arrangement also means that materials other than metal, and which cannot easily be attached to say stainless steel, a typical vessel material, may be used as the substrate for the heater. Indeed in the preferred embodiment, the heater comprises a ceramic substrate. A ceramic substrate is advantageous in that it exhibits a very low coefficient of thermal expansion and has a reasonably good thermal conductivity (comparable to that of stainless steel), so that heat from the track may pass relatively easily into the vessel. It also has excellent electrical insulation properties.

In order to ensure good thermal contact with the vessel base, the heater is biased against the vessel base. Preferably, the resilient biasing means extends over a substantial part of, and most preferably substantially the entire, heating surface of the heater. This allows thermal contact to be maintained over a substantial portion of the heater avoiding localised hot spots on the heater which might otherwise occur due to poor thermal contact with the vessel base.

Preferably, therefore, the biasing means comprises an"area"spring, ie one which extends over an area of the heater. Preferably the spring comprises a plate with a plurality of spring means provided thereon in a desired pattern. The individual springs may advantageously comprise spring tongues released from the plate. In a simple embodiment, the spring tongues may be generally rectangular in shape. In an other embodiment, they may be generally triangular in shape, and arranged in groups with the apices of the tongues

arranged together to form a"circular"spring arrangement. For example four or more springs may be provided in each group. Groups of springs may be arranged over the plate in the desired pattern.

The spring means is preferably metallic so as to be capable of withstanding the operating temperature of the heater, and so as to permit the springs to be easily formed by stamping and pressing.

Preferably an electrically insulating layer is provided between the heater and the spring means so as electrically to insulate the heater track from the spring. Although this could be formed as a deposited layer of the thick film heater, it is preferably a separate layer of a thermally resistant material such as mica eg reconstituted mica.

Retaining means should also be provided to retain the heater and other components in position. Preferably the retaining means is attached to the vessel, and most preferably to a region of the vessel base. In the preferred embodiment the retaining means comprises a clamping plate, which effectively sandwiches the heater and biasing means between it and the vessel base.

The clamping plate can be attached to the vessel in any suitable manner, but in the preferred embodiment, it is bonded, most preferably welded, to the vessel base. Preferably the clamping plate is in the form of a dished plate with the heater and biasing spring accommodated in the dish and the plate attached to the vessel around its rim, for example by a ring weld.

The clamping plate may also support other components in the base of the appliance vessel, for example a cordless electrical connector for the heater.

Preferably the connector is of the type which allows engagement between the appliance connector and the power base connector. The connector may also incorporate control connections between the appliance and its power stand base, as is shown for example in our co-pending

applications GB 9924026. 9 and 0002316. 8. Here the connector is a five pole connector which connects temperature sensors in the appliance to control means in the appliance base.

The clamping plate or dish may therefore be provided with an aperture for receiving the connector.

A peripheral flange may also be provided on the clamping plate to mount say a seating ring for the vessel.

Preferably the clamping plate forms a sealed watertight heater compartment under the base of the vessel such that the vessel can be disconnected from its power supply, eg removed from its power stand, and immersed in liquid for cleaning purposes without damaging the heater.

This is in itself a novel arrangement so from a further aspect, the invention provides a liquid or foodstuff heating appliance comprising a thick film heater mounted against the base of the liquid or foodstuff containing vessel of the appliance by a clamping member attached to the vessel, the clamping member defining with the vessel a watertight heater compartment under the base of the vessel.

In order to add a further level of electrical protection, the heater compartment so formed may be filled with a electrically insulating sealing material such as a silicone rubber. This may fill the interstices between the springs, but still allow sufficient flexibility to maintain the resilient biasing of the heater against the vessel base. Preferably the material has a relatively low viscosity so as to permit it to be easily flowed into the compartment after it is mounted in situ. The material may remain in a liquid or semi-liquid form.

The sealing material could, for example, be introduced through apertures provided in an electrical connector.

As stated above, the appliance preferably includes

a cordless electrical connector. The electrical connections to the heater track from the connector may be made by a sprung electrical connector. In order to spread the load applied to the heater by the connector, it is preferably formed with at least two arms for engaging respective terminals of the heater track. Most preferably, the connector is cruciform in shape. This is a new arrangement in itself, so from a further aspect, the invention provides a multi arm spring connector for making electrical contact with a thick film heater.

The control means for the heater may be entirely electronic, with suitable sensors provided in thermal contact with the heater so as monitor the temperature of the heater and the liquid or foodstuff being heated.

Such sensors may comprise one or more thermistors, for example. Such control means may function to supply energy to the heater in order to heat a liquid or foodstuff to a predetermined temperature and maintain that temperature, and/or to prevent overheating of the heater in the event that the vessel boils dry or is switched on dry.

In addition the thick film heater may be configured so as to be self protecting in the event of a serious overheat condition. In particular the track may be constructed in accordance with the principals discussed in W097/39855 and WOOO/30514.

Such an arrangement leads to a completely nonmechanical control, and from a further aspect, the invention provides a liquid or foodstuff heating appliance comprising a vessel for receiving liquid or foodstuffs to be heated, a thick film heater arranged under and in good thermal contact with the base of the vessel, and electronic control means for controlling the supply of electrical energy to the heater, said heater being constructed so as to be self protecting in the event of a serious overheat condition.

Some preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows an appliance in accordance with the invention; Figure 2 shows an enlarged cross sectional view of the base area of the appliance of Figure 1; Figure 3 shows an enlarged cross sectional view of the base area of Figure 2; Figure 4 shows a detail of a spring plate for use in the invention; Figure 5 is a section along the line 5-5 of Figure 4; and Figure 6 shows a plan view of a contact member for use in accordance with the invention.

With reference to Figure 1 there is shown a deep fat fryer 2. The fryer 2 comprises a fat-containing vessel 4 which is provided with a lockable lid 6. A removable basket 8 is mounted for a slidable movement within the vessel 4, as is typical in such appliances.

As will be described further below, the vessel 4 is provided with an electric heater 10 arranged under the base 22 of the vessel 4. Power is connected to the heating means through electrical connectors 12,14 arranged under the base of the vessel 4 and in a cordless power base 16 for receiving the vessel respectively, as will be described in further detail below. The vessel 4 is provided with a seating ring 18 which rests on the recessed upper surface 20 of the cordless power base 16 as shown in Figure 2.

Figure 3 shows the heater 10 of the fryer 2 and its mounting in greater detail.

As can be seen from Figure 3, the vessel 4 is provided with a laminated base 22. Typically this may comprise a stainless steel/aluminium/stainless laminate which gives good transfer of heat from the heating means 10 into the vessel 4. Such constructions are already

known in the cookware field. The heater 10 is arranged under and in good thermal contact with the base 22 of the vessel 4. The heater 10 is a thick film heater comprising a ceramic substrate 24 on which is deposited a resistive heating track (not shown) in a known manner.

The ceramic substrate 24 may also be provided with printed sensors for monitoring the temperature of the liquid in the vessel 4. Electrical connections are made to the heating track and sensors via spring connectors 26 which are connected to terminal members 28 of the connector 12 as will be described further below.

Arranged under the heater 10 is a layer 30 of mica e. g. of reconstituted mica. This serves electrically to electrically the heater 10 from a spring plate 32 which urges the heater 10 into close thermal engagement with the vessel base 22. The spring plate 32 may be regarded as an area spring since it applies force over a large area of the heater 10 so as to ensure good thermal contact between the heater 10 and the vessel base 22 over a substantial area of the heater 10.

The spring plate 32 can be made, for example, of 0.25 millimetre thick carbon steel, and individual spring tongues 34 formed, eg pressed, in the plate to give it its resilience. As can be seen in greater detail in Figures 4 and 5, the spring tongues 34 in this embodiment 34 are configured as triangles and arranged in groups e. g. of four such that the apices of the respective triangle lie adjacent to one another. As shown in Figure 4, the spring tongues are arranged in groups of four and might be considered as forming a generally circular spring arrangement 36. As shown further in Figure 4, the groups 36 of springs 34 may be arranged in a suitable pattern, for example a hexagonal pattern around a central group 36.

As shown in Figure 3, the spring plate 32 is positioned with its planar surface 38 arranged against the mica layer 30 so as to avoid localised abrasion of

the mica layer by the springs 34. The spring plate 32 is located by a clamping plate 40 which is shown in Figures 2 and 3.

The clamping plate 40 has a generally dish-shaped central region 42, an annular mounting region 44 and an annular peripheral flange 46. The mounting plate is also provided with a central aperture 48 for receiving the appliance electrical connector 12. The aperture 48 is provided with a flange 50, as shown.

The clamping plate 40 acts to clamp the heater 10 against the vessel base 22, to retain the seating ring 18 and to retain the electrical connector 12. To this end, the peripheral flange 46 of the clamping plate 40 traps an inwardly facing peripheral flange 54 of the seating ring 18 as shown in Figure 2, and the inner flange 50 of the clamping plate 40 retains the connector 12 by inter-engagement with a flange 56 provided on the outer wall of the connector 12. An 0-ring seal 58 is arranged between the respective flanges 50,56 so as to make this joint water tight.

The clamping plate 40 is mounted to the vessel base by a continuous ring weld 60 e. g. a laser weld. This one welding operation therefore connects together all the various components in the base of the vessel 4.

The weld 60 and the seal arrangement 58 are watertight, so as to create a is water tight heater compartment 64. This means that, if required, the whole of vessel 4, including its base 22 with attached heater 10, may be immersed in liquid for cleaning purposes without damaging the heater 10. To further prevent ingress of water or other liquid to the heater 10, the heater compartment 64 may be filled with a sealant, such as silicone rubber. This could be introduced via ports (not shown) in the connector pot 12.

In assembling the fryer 2, the electrical connector 12 can first be located in the central aperture 48 of the clamping plate 40, and the heater 10, mica layer 30

and spring plate 32 then mounted thereover in the dished region 42 of the clamp plate. Finally the seating ring 18 may be positioned over the peripheral flange 46 of the clamping plate 42 and the clamping plate 40 then be offered up to the base of the vessel and welded at location 60 as shown. This retains the connector 12 and the seating ring 18 in position and also compresses the spring plate 32 so as to bias the heater 10 up against the underside of the base 22 of the vessel. If required, a suitable sealant may then be introduced into the heater compartment 64.

With reference to the electrical connections to the heater track of heater 10, a terminal member 26 is shown in greater detail in Figure 6. The terminal member has a shape similar to that of a Maltese cross, with a plurality of arms 26 having contact portions 27 at each end. As shown in Figure 3, the end portions are folded over to ensure a smooth contact area with the heater track. The advantage of having a number of arms is to reduce the strain on the rivet arranged on the top of the terminal 28 which holds the connector onto the terminal 28. It also reduces the current through each leg of the connector thereby keeping the connectors as a whole cooler.

Electrical connections may also be made from the connector to terminals of electronic temperature sensors provided in thermal contact with the heater so as monitor the temperature of the heater and the liquid or foodstuff being heated. Such sensors may comprise one or more thermistors, for example. Control means provided eg in the cordless base of the appliance may function to supply energy to the heater in order to heat a liquid or foodstuff to a predetermined temperature and maintain that temperature, and/or to prevent overheating of the heater in the event that the vessel boils dry or is switched on dry. In addition the thick film heater may be configured so as to be self protecting in the

event of a serious overheat condition. In particular the track may be constructed in accordance with the principals discussed in W097/39855 and WOOO/30514.

It will be seen that the construction of the present invention allows a water tight heater compartment to be easily produced which allows the whole of the vessel 4 to be immersed in liquid for cleaning purposes. Furthermore, variations may be made to the particular embodiment shown without departing from the scope of the invention. For example, while the invention has been described in the context of a deep fat fryer, it could also be applied to other liquid heaters such as kettles, tea and coffee makers and so on and to other foodstuff heating or cooking appliances such as electric frying pans or cook pots.

Also, the base of the vessel may be an integral part of the appliance proper or a component mounted to the appliance.

Claims

Claims 1. A liquid or foodstuff heating appliance comprising a vessel for receiving liquid or foodstuffs to be heated, a thick film heater arranged under and in good thermal contact with the base of the vessel, and means for resiliently biasing said heater against said vessel base.
2. A heater assembly in or for a liquid or foodstuff heating appliance comprising a vessel for receiving liquid or foodstuffs to be heated, said assembly comprising a thick film heater arranged under and in good thermal contact with a base part of the vessel, and means for resiliently biasing said heater against said vessel base part.
3. An appliance or assembly as claimed in claim 1 or 2 wherein said heater comprises a ceramic substrate.
4. An appliance or assembly as claimed in claim 1,2 or 3 wherein the resilient biasing means extends over a substantial part of the heating surface of the heater.
5. An appliance or assembly as claimed in claim 4 wherein the resilient biasing means extends over substantially the entire heating surface of the heater.
6. An appliance or assembly as claimed in any preceding claim wherein the biasing means comprises a plate with a plurality of spring means provided thereon in a desired pattern.
7. An appliance or assembly as claimed in claim 6 wherein the spring means comprises spring tongues released from the plate.
8. An appliance or assembly as claimed in claim 7 wherein the spring tongues are generally rectangular in shape.
9. An appliance or assembly as claimed in claim 8 wherein the spring tongues are generally triangular in shape.
10. An appliance or assembly as claimed in claim 9 wherein the spring tongues are arranged in groups with the apices of the tongues arranged together.
11. An appliance or assembly as claimed in any preceding claim wherein the resilient biasing means is metallic.
12. An appliance or assembly as claimed in any preceding claim wheerin an electrically insulating layer is provided between the heater and the resilient biasing means.
13. An appliance or assembly as claimed in claim 12 wheerin said insulating layer is a separate layer of a thermally resistant material.
14. An appliance or assembly as claimed in any preceding claim further comprising retaining means for retaining the heater in position.
15. An appliance or assembly as claimed in claim 14 wherein the retaining means is attached to a region of the vessel base.
16. An appliance or assembly as claimed in claim 14 or 15 wherein the retaining means comprises a clamping plate, which sandwiches the heater and biasing means between it and the vessel base.
17. An appliance or assembly as claimed in claim 16 wherein the clamping plate is bonded, most preferably welded, to the vessel base.
18. An appliance or assembly as claimed in claim 16 or 17 wherein the clamping plate is in the form of a dished plate with the heater and biasing spring accommodated in the dish and the plate attached to the vessel around its rim.
19. An appliance or assembly as claimed in claim 16,17 or 18 wheerin the clamping plate also supports other components in the base of the appliance vessel.
20. An appliance or assembly as claimed in any of claims 16 to 19 wherein the clamping plate forms a sealed watertight heater compartment under the base of the vessel.
21. A liquid or foodstuff heating appliance comprising a thick film heater mounted against the base of the liquid or foodstuff containing vessel of the appliance by a clamping member attached to the vessel, the clamping member defining with the vessel a watertight heater compartment under the base of the vessel.
22. A heater assembly in or for a liquid or foodstuff heating appliance comprising a thick film heater mounted against the base of the liquid or foodstuff containing vessel of the appliance by a clamping member attached to the vessel, the clamping member defining with the vessel a watertight heater compartment under the base of the vessel.
23. An appliance or assembly as claimed in claim 20,21 or 22 wherein the heater compartment is filled with a electrically insulating sealing material.
24. An appliance or assembly as claimed in claim 23 wherein the material has a relatively low viscosity so as to permit it to be easily flowed into the compartment.
25. An appliance or assembly as claimed in any preceding claim wherein the appliance includes a cordless electrical connector and electrical connections to the heater track are made by a sprung electrical connector.
26. An appliance or assembly as claimed in claim 25 wheerin the connector is formed with at least two arms for engaging respective terminals of the heater track.
27. A multi arm spring connector for making electrical contact with a thick film heater.
28. A liquid or foodstuff heating appliance comprising a vessel for receiving liquid or foodstuffs to be heated, a thick film heater arranged under and in good thermal contact with the base of the vessel, and electronic control means for controlling the supply of electrical energy to the heater, said heater being constructed so as to be self protecting in the event of a serious overheat condition.