This invention relates to base assemblies and heater assemblies for liquid heating vessels, in particular to base assemblies and heater assemblies comprising an immersed heating element.

Heaters for domestic liquid heating vessels such as kettles, hot water jugs and the like conventionally take one of two main forms. A traditional design of a liquid heater takes the form of an immersed heating element mounted, together with an associated control unit, in an opening in a side wall of a vessel. The vessel may be connected directly to the mains power supply or it may be a cordless vessel that can be seated on a base unit to receive power via a cordless connector system such as P69 available from Strix Limited. A more modern design of a liquid heater is a so-called "underfloor heater" in which an electric heating element is arranged in good thermal contact with the underside of a metallic plate that is mounted in an opening in the base of a vessel. Typically, such an underfloor heater is connected to a control that is mounted directly below the plate in the base of the vessel and includes a cordless connector set allowing the vessel to be separated from a base unit with power being supplied to the heating element irrespective of the relative angular orientation of the connector parts i.e. a 360° connector set such P72 or P76 available from Strix Limited. The latter design has several advantages, including the element being hidden from view and the metallic plate in the base being easier to clean e.g. when a build-up of scale occurs in the vessel.

While underfloor heaters have become popular in domestic kettles, the material and manufacturing costs involved can be relatively high. The metallic plate in the base of the vessel must be resistant to corrosion and have good thermal conductivity but also be suitable for heating liquid for human consumption. Some doubt has been raised about the potential health risk of aluminium contamination so stainless steel is often preferred. However a stainless steel base plate is usually coupled to an aluminium diffuser plate brazed to its underside to increase heat conduction from the element. The aluminium diffuser plate is important for the accurate operation of overheat protection means, e.g. a bimetallic actuator mounted on the base plate of the vessel which operates to interrupt the electrical supply to the element. Both aluminium and stainless steel have been rising in cost in recent times.

It is an aim of the present invention to provide improvements in heaters for domestic liquid heating vessels.

When viewed from a first aspect the present invention provides a base assembly for a cordless liquid heating vessel, comprising an element head arranged to close an opening in the base of the liquid heating vessel, an electric heating element projecting from the element head so as to be immersed in liquid contained in the vessel in use, and a cordless electrical connector part for supplying power to the electric heating element, said cordless electrical connector part having multiple electrical terminals for engagement with corresponding electrical contacts in a base connector part, the connector parts being separable such that power is supplied to the electrical terminals of the connector part when it is engaged with the base connector part, in use, substantially irrespective of their relative angular orientation, wherein a heated portion of the electric heating element is arranged in good thermal contact with a sensing location for a thermally sensitive actuator that can operate to interrupt the power supplied to the electric heating element by the cordless electrical connector part.

It will be appreciated that the present invention has taken inspiration from traditional immersed heaters to re-design a new type of heater that is mounted in the base of a cordless liquid heating vessel. In such a base assembly the element head is arranged to close an opening in the base, rather than side wall, of a liquid heating vessel so as to facilitate engagement with a base connector part of the 360°-type e.g. on a corresponding power base unit. Locating the element head together with the cordless connector part in the base of the vessel also provides for a compact arrangement. The Applicant has recognised that a material saving can be made because the immersed element now heats liquid in the vessel directly rather than through a base plate and the aluminium diffuser plate usually involved in overheat protection can be removed by using the sensing location to directly detect overheating of the element instead. This is achieved by arranging a heated portion of the electric heating element in good thermal contact with a sensing location (e.g. on the element head) for a thermally sensitive actuator e.g. that can act when an overheat condition is detected. Overheat of the element can therefore be detected quickly without compromising safety. The element head may, for example, be made of stainless steel and the base assembly preferably does not include any aluminium (other than may be used for the heating element itself).

The invention also extends to a liquid heating appliance comprising a cordless liquid heating vessel and a base unit, in which: the base unit comprises a base connector part adapted to supply electrical power to a cordless electrical connector part in the vessel when the two parts are engaged, in use, substantially irrespective of their relative angular orientation; and the vessel comprises an element head arranged to close an opening in its base with an electric heating element connected to the cordless electrical connector part and projecting from the element head so as to be immersed in liquid contained in the vessel in use, wherein a heated portion of the electric heating element is arranged in good thermal contact with a sensing location and a thermally sensitive actuator is mounted to the sensing location so as to operate to interrupt the power supplied to the electric heating element by the cordless electrical connector part when an overheat condition is detected.

The sensing location may be provided separately from the element head, for example on a thermally conductive part of the base assembly such as a metallic mounting ring for the element head or for example on a metallic wall of the liquid heating vessel. However, in a preferred set of embodiments the sensing location is provided on the element head. This helps to make the base assembly compact and self-contained, with all essential components provided together in the base of the liquid heating vessel.

The sensing location that has a heated portion of the element in good thermal contact therewith may be termed a "hot return". Good thermal contact may be achieved, for example, by brazing a portion of the element to the sensing location (e.g. on the element head or elsewhere). Preferably a thermally sensitive actuator is mounted in good thermal contact with the sensing location e.g. on the dry side of the element head. As the immersed heating element is arranged in good thermal contact with the sensing location (e.g. on the wet side of the element head), this ensures that the actuator is sensitive to the element temperature and can detect an overheat condition. In a set of embodiments the thermally sensitive actuator is a bimetallic actuator.

It will be appreciated that the thermally sensitive actuator may be provided separately from the base assembly. For example, the base assembly may be mounted in an opening in the base of a cordless liquid heating vessel in one manufacturing step while a thermally sensitive actuator is mounted at (or in good thermal contact with) the sensing location in another manufacturing step. The thermally sensitive actuator may be electrically connected to the cordless electrical connector part by any suitable means so that it can act to interrupt the power supplied to the electric heating element e.g. when an overheat condition is detected. However, in at least some embodiments of the present invention the cordless electrical connector part is integrated with a control that also includes the thermally sensitive actuator. The control may be of the type described in WO 2012/164318 or WO 2012/164319. Preferably the control is arranged so that the thermally sensitive actuator operates when an element overheat condition is detected.

In embodiments in which the thermally sensitive actuator is provided by a control, the control may also be arranged to interrupt the power supplied to the electric heating element when the liquid heating vessel reaches another predetermined temperature condition. This could be that the liquid in the vessel reaches a predetermined temperature, e.g. boiling, and/or that the electric heating element reaches a predetermined temperature, e.g. to indicate boiling or extreme overheating. In another, not mutually exclusive, set of embodiments, the control may comprise a steam-sensitive actuator to detect boiling of liquid in the vessel. The control may comprise more than one actuator, e.g. one to sense boiling and at least one thermally sensitive actuator arranged in good thermal contact with the sensing location to sense overheating of the element. A further overheat back-up may also be provided, such as a thermal fuse, as part of the control or separate from the control.

Regardless of how the thermally sensitive actuator is provided, embodiments of the present invention provide a compact and convenient way of monitoring for overheat of an immersed heating element mounted in the base of a cordless liquid heating vessel. However it can be problematic if the temperature of the element is sensed only at a single location (e.g. on the element head or elsewhere). If, for example, the base unit of the appliance is accidentally placed on a work surface so that it is not horizontal, it is possible that as the vessel boils dry one part of the base (and hence of the element head) may still be covered by liquid while another is uncovered. This part of the base (e.g. element head) will therefore overheat first, but if the sensing location is still covered with liquid then an overheat condition will not be detected yet. In such a situation severe overheating of the element may occur locally, which is potentially very dangerous. This problem is particularly acute when an immersed heating element projects from an element head in the base of a vessel, where it may only be covered by a small amount of liquid with a large effect resulting from tilt. Whereas in conventional immersed heater arrangements any hot return at the side wall will tend to be uncovered if the vessel starts to boils dry even if it is tilted at an angle.

It is therefore preferable for the base assembly or element head to include two hot returns. In one set of embodiments the base assembly therefore comprises two separate sensing locations where heated portions of the heating element come into good thermal contact with the element head. Preferably the two sensing locations are spaced apart from each other such that they are thermally independent. This provides for more accurate detection of local overheating of the element e.g. in the event of the vessel being tilted while power is supplied to the heating element. In a preferred set of embodiments there are two separate heated portions of the immersed heating element in good thermal contact with the element head to form the two spaced apart sensing locations.

While the feature of two hot returns may provide most benefit for a cordless liquid heating vessel in which the immersed heating element is projecting from an element head mounted in the base, in any immersed element arrangement it can be advantageous to provide two independent sensing locations for thermally sensitive actuators so that there is a level of back-up in the event of failure of one of the actuators. This may be beneficial in both cordless and corded appliances.

This is considered novel and inventive in its own right, and thus when viewed from a further aspect the present invention provides a heater assembly for a liquid heating vessel comprising an electric heating element projecting from an element head so as to be immersed in liquid contained in the vessel in use, wherein two separate heated portions of said electric heating element are arranged in good thermal contact with two separate sensing locations for respective thermally sensitive actuators.

The Applicant has recognised that using the heating element to form two separate sensing locations can be advantageous over conventional immersed heater arrangements that provide a single hot return with a further back-up such as a thermal fuse. The dual hot return provided by the two separate sensing locations can be used to detect any local overheating of the element as soon as it occurs, e.g. greater sensitivity to angled boil dry situations or localised hot spots due to heating element manufacturing defects. One or both of the sensing locations may be provided separately from the element head, for example on any other thermally conductive part of the vessel such as a metallic side wall or a mounting ring for the element head. However, in a preferred set of embodiments the two separate sensing locations are provided on the element head. The heater assembly can then be made compact and self-contained.

Preferably the heating element is brazed to the element head (e.g. on the wet side) at the two separate sensing locations. In a further set of embodiments the heater assembly includes a pair of thermally sensitive actuators mounted on the dry side of the element head in good thermal contact with the two sensing locations. Preferably a thermally sensitive actuator is mounted at each of the two separate sensing locations e.g. on the element head. It will be appreciated that this provides independently operable thermally sensitive actuators at each location which improves the safety of the liquid heating vessel, e.g. in the event that one of the thermally sensitive actuators fails or the vessel is tilted so that only a portion of the heating element is exposed and overheats. In embodiments where the two sensing locations are provided on the element head, the element head may be thinned or even comprise cut-outs at the two separate locations to improve the thermal communication between the heating element and the respective thermally sensitive actuators. The two thermally sensitive actuators could have similar operating temperatures, e.g. both set to operate at 120 °C, 130 °C or 140 °C, or one of the thermally sensitive actuators may have a higher operating temperature than the other, e.g. 120 °C and 150 °C respectively. As such the thermally sensitive actuators could comprise the same type of actuator, e.g. bimetallic actuators, or they may be different, in particular one may comprise a bimetallic actuator, e.g. with an operating temperature of 120 °C, and the other may comprise a thermal fuse, e.g. with an operating temperature of 250 °C. It will be appreciated that the two thermally sensitive actuators may be provided separately from the heater assembly. For example, the heater assembly may be mounted in an opening in the base or side wall of a liquid heating vessel in one manufacturing step while the two thermally sensitive actuators are mounted at (or in good thermal contact with) the sensing locations (e.g. provided on the element head) in another manufacturing step. The two thermally sensitive actuators may be electrically connected to the cordless electrical connector part by any suitable means so as to act to interrupt the power supplied to the electric heating element e.g. when an overheat condition is detected. However, in at least some

embodiments the cordless electrical connector part is integrated with a control that also includes the two thermally sensitive actuators. The control may be of the type described in WO 2012/164318 or WO 2012/164319. Preferably the control is arranged so that one and/or the other of the two thermally sensitive actuators operate when an element overheat condition is detected. The two separate sensing locations (on the element head or elsewhere) which are contacted by heated portions of the element are preferably spaced from each other sufficiently such that they are thermally independent thus allowing independent overheat measurements to be made. In one set of embodiments the two separate locations are arranged either side of the points at which the ends of the heating element (termed "the cold tails") project through the element head.

This aspect of the invention also extends to a liquid heating vessel comprising the heater assembly described above mounted therein. As in the first aspect, the element head may be arranged to close an opening in the base of the liquid heating vessel. However this is not essential and the element head may alternatively be located in a side wall of the liquid heating vessel, e.g. adjacent to a handle of the vessel as is conventional.

The two separate sensing locations may be arranged (e.g. on the element head or elsewhere) such that, in use, they are generally covered by liquid contained in the vessel, i.e. at least part of the element is higher than the two sensing locations. This can allow even a small volume of liquid to be heated in the vessel without risk of activating the overheat sensors. In the embodiments in which the element head is arranged in a side wall of the liquid heating vessel this can be achieved by positioning the two separate locations on the element head below the point at which the ends of the heating element (i.e. the cold tails) project through the element head.

Alternatively, the two sensing locations may be arranged (e.g. on the element head or elsewhere) such that, in use, the portions of the element which contact the element head at the sensing locations are the first to become exposed, i.e. they are higher than at least some of the other parts of the element relative to the liquid level in the vessel. This helps to protect the heating element as the first part of the element to overheat is that at one or both of the hot returns, so it is not possible for the heating element to overheat without it being detected immediately. In the embodiments in which the element head is arranged to close an opening in the base of the liquid heating vessel, a trough could be provided around the element head with at least part of the heating element arranged in the trough at a lower level than the sensing locations. If the heating element initially projects vertically from the element head then it may need to be bent downwards into the trough. To help reduce the height of the heater assembly the ends of the heating element may be arranged to project substantially horizontally from the element head. This means that the heating element may be arranged to project directly into the trough surrounding the element head.

In the set of embodiments comprising a trough, the level of the element head will generally be higher than the level of the heating element in the trough. The heating element may be shaped such that two separate heated portions of the heating element pass over and contact the element head at the sensing locations, e.g. the heating element projects from the element head, and returns to contact the element head with two separate heated portions. The remaining portions of the heating element may project down into the trough, for example the sections between the ends projecting from the element head and the sensing locations and/or the sections between the sensing locations, though one or more of these sections may not project down into the trough. For the portion of the heating element located in the trough, preferably the heating element is spaced from the element head and the walls of the liquid heating vessel.

The trough may be formed by a peripheral channel formed in the vessel to at least partially surround the element head mounted in an opening in the base, e.g. the trough could be annular or toroidal in shape. The dimensions of the trough can be chosen so as to ensure that a minimum volume of liquid is always contained in the vessel when the hot returns (i.e. two sensing locations) are exposed due to a boil dry condition. However to keep the vessel compact the depth and/or volume of the trough may be kept to a minimum to reduce the minimum water level and/or volume necessary to satisfy this condition. Preferably the volume of the trough is less than 200 ml, e.g. less than 150 ml. This can be compared with the volume of water present in a vessel with a side entry immersed element when the hot return is exposed which is generally at least 250 ml.

The liquid heating vessel may be of the cordless type as in the first aspect of the invention, i.e. the heater assembly may comprise a cordless electrical connector part for supplying power to the electric heating element, said connector part having multiple electrical terminals for engagement with corresponding electrical contacts in a base connector part, the connector parts being separable such that power is supplied to the electrical terminals of the connector part when it is engaged, in use, with the base connector part. The connector parts may have a fixed orientation, but preferably the connector parts are of the 360°-type, allowing engagement substantially irrespective of their relative angular orientation. In other embodiments the liquid heating vessel may be corded, i.e. directly connectable to the mains power supply.

The element head, e.g. in the base of the vessel, may be integrated with the cordless electrical connector part by any suitable means. Preferably, the element head is integrated with a control, as mentioned above, for controlling the supply of power to the heating element. In the set of embodiments comprising a trough, it will be appreciated that it is advantageous to house these components, e.g. the element head, the cordless electrical connector part and/or the control, in the domed volume in the base of the vessel which is surrounded by the trough. This helps to make the appliance compact leading to material and transport savings thus reducing its ecological footprint.

As is mentioned above, the control may be of the type described in WO

2012/164318 or WO 2012/164319, and may contain one or more of the thermally sensitive actuators which operate when an element overheat condition is detected to interrupt the power supplied to the electric heating element. Preferably the control also contains a steam-sensitive actuator, where provided. This also helps to make the appliance compact, e.g. compared to a vessel using a steam switch which is remote from the control or heater assembly i.e. remote from the element head.

The element head may take any convenient shape, e.g. circular. However other geometries are envisaged, e.g. oval, especially if the element head is mounted in the base of the vessel. Advantageously, vessels with an oval footprint have a higher packing density than, for example, vessels with a circular footprint, leading to reduced transport and packaging costs, and a reduced impact on the environment. Conveniently the shape of the element head may mirror the shape of the vessel's footprint, for a base mounted element head, e.g. the element head could be concentric with the vessel's footprint.

The electric heating element may be rated at 1-1.5 kW for some domestic appliances, e.g. for use in the US or China, or it may be rated at approximately 3 kW e.g. for domestic appliances for use in Western Europe. The electric heating element could comprise any convenient arrangement, e.g. a thick film heating element, but in one set of embodiments the electric heating element comprises a sheathed heating element. Generally a sheathed electric heating element comprises a coiled length of thin resistance wire which is embedded in an electrically insulating material and encased in a hollow outer tube. Electrical terminal pins known in the art as "cold tails" are connected to the two ends of the resistance wire for making an electrical connection to it. The tube is sealed at either end with the respective cold tails projecting through the sealing plugs.

Thus a portion of each end of the sheathed heating element comprising the cold tail, which can be up to a few centimetres in length, is not heated, compared to the main central portion of the sheathed heating element which includes the resistance wire. In the embodiments which comprise a sheathed heating element it is these cold tails which preferably project through the element head, in contrast to the heated portions of the heating element. It is a heated portion of the element, i.e. somewhere along the heating element between the two cold tails, which comes into contact with a sensing location (on the element head or elsewhere).

The connector parts, i.e. the cordless electrical connector part and the base connector part, allow electrical connection to be made substantially irrespective of the relative angular orientation between them - i.e. a so-called 360° connector system. However it will be appreciated that the connector parts may not allow full 360°, but while still allowing electrical connection to be made substantially irrespective of this orientation, e.g. through a range of 345° or greater. The connector parts may have any of the features commonly known for 360° connectors. For example, the electrical terminals of the cordless connector part may comprise a central pin terminal (e.g. for connecting the live pole) and at least one annular terminal concentric therewith (e.g. for connecting the neutral pole and optionally also earth). The cordless connector part may comprise a plurality of concentric annular terminals to engage in respective annular openings in a base connector part, i.e. the connector part may be a three pole or five pole connector part. In a five pole connector part the two additional terminals that do not transmit power may be used to transmit electrical signals from and/or to components in the vessel, for example to transmit data signals from a thermal sensor such as a thermistor. The base connector part may be of the type described in WO

2012/164317 e.g. having a sub-assembly comprising a mains power supply cable and the electrical contacts, in which the electrical contacts are permanently attached to respective conductors in the mains power supply cable. The liquid heating appliance preferably comprises a domestic appliance such as a kettle, a jug, a rice cooker, a beverage maker, a coffee maker, etc.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 shows a perspective view of a kettle in accordance with an embodiment of the invention;

Fig. 2 shows a cut-away view of the kettle of Fig. 1 ; and

Fig. 3 shows a perspective view of the heater assembly of the kettle of Fig. 1.

Fig. 1 shows a perspective view of a kettle 1 in accordance with an embodiment of the invention comprising cordless a liquid heating vessel 2 standing on a base unit 4, with a power supply lead 6 connected to the base unit 4 to supply power to the liquid heating vessel 2 via a cordless connector set. The exterior of the liquid heating vessel 2 is of conventional type, with a lifting handle 7 attached to one side of the vessel 2, a lid 8 which closes an upper opening in the vessel 2 to allow filling of the vessel 2 with water, a spout 10 for pouring out the heated contents of the vessel 2, and an on-off switch 11. Fig. 2 shows a cut-away view of the kettle of Fig. 1 which reveals a sheathed electric heating element 12 that projects from an element head 14 which is mounted to close an opening in the base 16 of the vessel 2. The heater assembly 32 including the whole of the heating element 12 and the element head 14 can be seen in isolated detail in Fig. 3. The base 16 of the vessel 2 is shaped to provide an annular trough 18 surrounding the element head 14 which is secured to a central raised part of the base 16 by means of a clamping ring 20 and a sealing O-ring 22. The heating element 12 has two cold tails 24 (only one is visible in Fig. 2) which project through the element head 14 such that electrical connections can be made to the power supply below the base 16 of the vessel 2.

The annular trough 18 results in a domed volume being provided beneath the element head 14, in which there is provided a cordless connector part 26 integrated with a control 29 that is mounted to the element head 14. The cordless connector part 26 is arranged to engage with a base connector part 28 mounted on the base unit 4 such that electrical terminals provided in the cordless connector part 26 come into contact with electrical contacts provided in the base connector part 28. The control 29 may have the electrical terminals of the cordless connector part 26 mounted on moveable switch levers in the manner described in 2012/164318 or WO 2012/164319.

The main, heated part of the heating element 12 is arranged in a symmetrical loop which encircles the centre of the element head 14. After a reversal of direction in the heating element 12 close to the cold tails 24, the heating element 12 dips into the annular trough 18 before rising in height again to two separate sensing locations 30 on the element head 14 at which a heated portion of the heating element 12 is brazed onto the element head 14 to create good thermal contact between them. The remaining part of the heating element 12 dips back down into the trough 18 such that apart from the turns in the heating element 12 close to the cold tails 24, the heated portions of the heating element 12 at the sensing locations 30 are higher than the majority of the rest of the heating element 12. Arranged in the control 29, in good thermal contact with the sensing locations 30 on the underside of the element head 14, are two thermally sensitive actuators (not shown) arranged to be able to interrupt the power supplied to the heating element 12 upon sensing an overheat condition.

Operation of the kettle 1 will now be described, with reference to Figs. 1-3. Before power is supplied to the kettle 1 , the cordless vessel 2 can be lifted off the base unit 4 and filled with water, e.g. from a tap. The vessel 2 should ideally contain a minimum volume of water such that the heating element 12 is completely covered. Once the vessel 2 is returned to the base unit 4, the power supply lead 6 is plugged into an electrical socket and the power is supplied to the heating element 12 by depressing the on-off switch 1 1. The water in the vessel 2 is then heated by the heating element 12. In the event that the vessel 2 contains less than the ideal minimum volume of water, e.g. because the user did not put enough water into the vessel 2 or it was left to boil dry, at least part of the heating element 12 will no longer be covered by water and will become exposed. One of the first parts of the heating element 12 to become exposed will be one of the heated portions which come into contact with the two sensing locations 30 on the element head 14. This will be even more likely if the kettle 1 is placed on an uneven surface which causes the kettle 1 to be tilted, thus exposing one half of the heating element 12 more than the other. With the heating element 12 energised and part of it exposed, this exposed part will not benefit from being able to dissipate heat into the surrounding water and will increase in temperature rapidly.

The heated portions of the heating element 12 which come into contact with the two sensing locations 30 on the element head 14 are each in good thermal contact with two thermally sensitive actuators arranged on the underside of the element head 14. Therefore, once the exposed part of the heating element 12 increases in temperature above the actuation temperature of the thermally sensitive actuators, this will cause at least one of the thermally sensitive actuators to interrupt the power supplied to the heating element 12, thus minimising any damage to the heating element 12 and the kettle 1 from the heating element 12 overheating.