GB2306284A

GB2306284A - Electric heating elements

Info

Publication number: GB2306284A
Application number: GB9514749A
Authority: GB
Inventors: Robert Andrew O'neill
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1995-07-19
Filing date: 1995-07-19
Publication date: 1997-04-30
Also published as: GB9514749D0

Abstract

An electric immersion heating element 1 of the elongate, metal-sheathed, electrically-insulated, resistance-heating type has no hot-return portion as is conventionally provided for enabling the element temperature to be sensed from the dry side of the element head. Instead, the cold termination 6 at one end of the heating element is formed of a high thermal conductivity material, e.g. copper or brass, and serves to conduct heat from the element 1 through the head 2 to the dry side thereof, and furthermore may have an end portion 7 shaped to nest with the bimetallic actuator 8 of a juxtaposed element protection control 9. For powering the element, a power supply lead runs from the high thermal conductivity cold termination 6 to a location adjacent to but spaced from the other cold termination 3 of the element, such locations corresponding to the locations of the cold terminations of a conventional heating element.

Description

IMPROVEMENTS RELATING TO ELECTRICAL HEATING ELEMENTS This invention concerns improvements relating to electrical heating elements and particularly, though not exclusively, electrically powered immersion heating elements for use with water heating vessels such as kettles, hot water jugs, pots, pans, urns and laboratory equipment for example.

A well known form of electrically powered immersion heating element which is widely used in electric kettles and hot water jugs comprises an elongate, metal sheathed, electrically insulated, resistance heating element coupled to an element head plate serving for mounting of the element in a wall of the kettle or hot water jug, the resistance heating element being curved back towards the element head plate to define a so-called hot return portion which is secured to the element head plate, generally at a position above the general plane of the element proper.

A control for switching off the supply of power to such a heating element in the event of the associated vessel being switched on dry (a so-called dry boil situation) or being allowed to boil dry commonly comprises a bimetallic element or other thermally responsive switch actuator which is held in close thermal contact with the rear of the element head plate in the region thereof where the hot return portion of the element attaches to the front of the element head plate. An example of such an element protection control is the X1 control manufactured by Otter Controls Limited which is substantially as described in GB-A-2 194 099 with reference to Figs.

3A, 3B and 3C thereof.

Whilst the aforementioned well known form of electrically powered immersion heating element has well withstood the test of time, it is nonetheless the case that the need to form the hot return portion of the element and affix it to the front of the element head plate is troublesome and costly. Proposals have been made, therefore, to form such elements without a hot return portion and instead to derive the heat necessary for actuating an element protection control directly from the element, rather than indirectly via the head plate, by forming the element cold tails, that is to say the terminations of the element proper which do not significantly heat up with the passage of electric current and thus remain relatively cold, from a thermally conductive material so that the cold tail temperature can be sensed as an indication of the element temperature.One such proposal is described in W090/09672 (Strix Ltd.) where the element cold tails directly mount a bimetallic switch actuator, and a similar proposal is described in GB-A-2 257 341 (Sheathed Heating Elements Limited). Yet another proposal is described in GB-A-2 265 071 (Otter Controls Limited) wherein, in addition to the provision of a heat transfer element enabling the element temperature to be sensed from the dry side of the element head, a tubular enclosure extends from the element head on the wet side thereof and surrounds part of the element for enabling automatic switch-off on boiling to be achieved by means of the element protection control.The function of the enclosure is to simulate a dry boil situation within the enclosure when steam is generated when water boils in an associated vessel, the steam expelling the water from the enclosure, so that the consequent rise in the local temperature of the element can be sensed via the heat transfer element and used to switch off the element.

The present invention derives from yet further efforts to design an electric heating element without a hot return portion and to adapt the resultant element for use with the aforementioned X1 element protection control. Described hereinafter with reference to the accompanying drawings is an exemplary electric heating element comprising an element head portion, preferably though not essentially formed of a synthetic plastics material, and an elongate metalsheathed electrically-insulated resistance-heating portion secured at its ends with respect to the head portion on one (wet) side thereof and having terminations accessible on the other (dry) side thereof, one of said terminations comprising a heat transfer element formed of a high thermal conductivity material providing for sensing of the element temperature at said other (dry) side of the head portion and including a portion shaped to provide a heat interface for the bimetallic actuator of an element protection control and a portion providing for connection of the respective termination to the electrical power supply by way of the element protection control.

As will become apparent from consideration of the following description of the arrangement abovementioned, the present invention provides an electric heating element which does not have a hot return portion and which provides for convenient interpositioning with the aforementioned X1 element protection control. Referring to the accompanying drawings: Fig. 1 is a schematic side elevation of an exemplary electrical heating element embodying the present invention and illustrating the interpositioning of an Xl control with the heating element; and Fig. 2 is a schematic end view of the heating element of Fig. 1 from the dry side of the element head portion.

Referring to Fig. 1, the electrically powered immersion heating element shown therein comprises an elongate metal-sheathed electrically-insulated resistance-heating portion 1 which, as is well known, is constructed so that the internal resistance heating wire of the element does not extend fully to the ends of the element. In consequence, the ends of the element, namely the element cold tails, remain relatively cool in operation of the element.The element cold tails are received in respective bores provided in a heat-resistant moulded plastics element head portion 2 so that electrical terminal portions 3 and 4 of the element extend through to the opposite (dry) side of the head, grommet seals 5 formed of a temperature-resistant material such as silicone rubber for example sealing the passage of the element cold tails through the head and serving furthermore to support the element with respect to the head. Note that the resistance heating portion 1 of the heating element does not have a hot return portion such as is commonly provided in conventional heating elements.

In the illustrated arrangement, the heating element cold tail 3 is designated for connection to the neutral side of an alternating mains supply, and the heating element cold tail 4 is designated for connection to the live side of the supply. As will be well understood by those possessed of relevant skills and knowledge, the neutral cold tail 3 comprises a low-resistance electrical termination which extends from the end of the metal-sheathed element through to the start 1' of the heating element proper 1 where it connects to an electrically-insulated resistanceheating coil. At the other end 1'' of the heating element proper, the resistance-heating coil is connected to a heat-transfer element in the form of a high thermal conductivity rod 6 of copper or brass for example which extends through the live coil tail of the element and, as is known from WO90/09672 and GB-A-2 257 341, enables the element operating temperature to be sensed at the rear (dry) side of the heating element head portion 2.

Affixed to the end of the high thermal conductivity rod 6, or integrally formed therewith, is a thermally-conductive generally-rectangular plate 7 which, as viewed from the left-hand side of the figure, is concavely-dished in order to nest with and provide a heat interface for the cold-condition convexly-dished snap-acting bimetallic blade 8 of a schematically shown X1 element protection control 9.

The plate 7 may be formed of copper or brass for example so as to be an effective conductor of heat conveyed to the plate 7 from the element via the heat transfer rod 6. As is well known the X1 control carries the bimetallic blade 8 at the ends of four legs of a thermally-deformable plastics material carrier 10, and the rectangular metal plate 7 is dimensioned to accommodate the feet of the carrier 10 as well as to nest with the bimetallic blade 8. In use, the four legs of the carrier 10 thus stand upon the plate 7 so that the function of the thermallydeformable carrier, namely to provide a fail-safe or secondary element protection function in the event that the primary protection provided by the bimetallic blade 8 fails to operate, is maintained.In the event of failure of the primary protection provided by the bimetallic blade 8, the element temperature will continue to rise and the increasing temperature will be transferred to the plate 7 via the rod 6. The plate 7 will eventually reach a temperature at which the plastics material of the carrier 10 will begin to soften and the carrier will then be collapsed by spring pressure from within the control 9 so as to cause a pair of electrical contacts within the control to open and thereby disconnect the power supply to the element.

The X1 control has a pair of spring contacts which, when the control is used with a conventional element, are adapted to be contacted by the cold tails of the element which are spaced apart from each other at the level of the neutral cold lead 3 in the heating element of accompanying Fig. 1. In use with the Fig.

1 heating element, one of the spring contacts of the X1 control is engaged by the neutral cold termination 3, and the other spring contact of the X1 control is contacted by a lead member 11 which extends from a suitable point of the plate 7 or from the thermallyconductive rod 6. It is immaterial that this arrangement results in the plate 7 and the bimetallic blade 8 being electrically live in operation, because the bimetallic blade 8 is well insulated by virtue of its mounting on the plastics material carrier 10. The lead member 11 could be formed integrally with the plate 7, or could be a separate part connected to the plate 7 or to the rod 6, and the lead member 11, the plate 7 and the rod 6 could even be integrally formed.

Fig. 2 shows the rear (dry) side of the heating element head portion 2. The neutral end of the element sheath 12 is supported by means of a stainless steel plate 13 which is clenched around the sheath, and an extension 14 of the plate 13 provides an earth (ground) connection to one of three fixing screws 15 that are provided on the element. The earth connection to the element sheath could alternatively (or additionally) be provided by means of a plate 16 (shown in phantom) clenched to the sheath at the live end 4 of the heating element.

Having thus described the present invention by reference to a specific embodiment, it is to be well appreciated that the described embodiment is exemplary only and that modifications and variations thereto may occur to those possessed of appropriate skills and knowledge without departure from the scope of the invention as set forth in the appended claims. For example, the element could be provided with an enclosure as described in GB-A-2 265 071 for the purpose of providing an automatic switch-off facility when water is boiled by the element.

Claims

CLAIMS:

1. An electric heating element comprising an elongate, metal-sheathed, electrically-insulated, resistance-heating portion engaged at its ends with an element head portion, and a heat transfer member formed of a high thermal conductivity material extending from a heated part of the element portion adjacent to one of its ends and through the element head to provide for sensing of the element temperature, said heat transfer member including a portion adapted to be juxtaposed with the thermal actuator of an element protection control in good heat transfer relationship therewith.

2. An electric heating element as claimed in claim 1 wherein the resistance heating portion of the heating element extends between its ends without any hot return portion making thermal contact with the element head portion.

3. An electric heating element as claimed in claim 1 or 2 wherein said portion of the heat transfer member comprises a plate adapted to be juxtaposed with a bimetallic blade actuator of an element protection control.

4. An electric element as claimed in claim 3 wherein said plate is concavely dished at the surface thereof which is adapted to be juxtaposed with said bimetallic blade actuator.

5. An electric heating element as claimed in any of the preceding claims including an electrical power supply conductor extending from said heat transfer member to a location of the heating element whereat, together with the other terminal end of the heating element, it is adapted to be contacted by power supply conductors of said element protection control.

6. An electric heating element as claimed in any of the preceding claims wherein the heating element head portion is formed of heat-resistant synthetic plastics material.

7. An electric heating element substantially as herein described with reference to the accompanying drawings.

8. An electric heating element as claimed in any of the preceding claims and further including an enclosure surrounding said heated part of the element portion for providing an automatic switch-off on boiling facility.

9. An electric heating element as claimed in any of the preceding claims in combination with an element protection control.

10. An electrically powered water heating vessel incorporating a combination as claimed in claim 9.

11. An electric heating element comprising an element head portion and an elongate, metal-sheathed, electrically-insulated, resistance heating portion secured at its ends with respect to the head portion on one side thereof and having terminations accessible on the other side thereof, one of said terminations comprising a heat transfer member formed of a high thermal conductivity material providing for sensing of the element temperature at said other side of the head portion and including a portion shaped to provide a heat interface for the thermally-responsive actuator of an element protection control when such control is operatively juxtaposed with said other side of the element head portion.

12. An electric heating element as claimed in claim 11 adapted for use with an element protection control providing power supply connections for the heating element terminations at first and second locations and providing for the sensing of the element temperature at a third location spaced from said first and second locations, said electric heating element including an electrical power supply conductor extending from a location on said other side of the heat portion of the element corresponding to one of said first and second locations to the location of said portion of said heat transfer member such last-mentioned location corresponding to said third location.

13. An electric heating element as claimed in claim 11 or 12 wherein the resistance heating portion of the heating element extends between its ends without any hot return portion making thermal contact with the element head portion.

14. An electric heating element comprising a head portion and, secured thereto, an elongate heating element proper having cold terminations extending through the head portion at first and second locations thereof and a resistance heating portion extending between said cold terminations without any hot return portion thermally contacting the head portion, one of said cold terminations including a heat transfer member enabling the temperature of the resistance heating portion of the heating element proper to be sensed at a respective one of said first and second locations, and a power supply conductor coupled to said heat transfer member and extending therefrom to a third location of the element head portion.