GB2257341A - Heat transfer from resistance element to thermal switch. - Google Patents

Abstract

An electric element assembly for liquid heating purposes, e.g. in an electric kettle has a reliable and simple connection of a resistance heating wire 14 to a pin 12. The wire 14 can extend through a tubular sheath 16 and be electrically insulated therefrom by a packing 18 of magnesium oxide or other suitable material. The pin 12 is made of copper coated with, for example, nickel or chrome so as to conduct heat from the wire 14 to a bimetallic strip 10 of a thermal switch. A sleeve 24, of a material having a good thermal conductivity, is mounted inside the end margin of the sheath 16 to enhance the rate of heat transfer from the pin 12 to the bimetallic strip 10. <IMAGE>

Description

ELECTRIC ELEMENT ASSEMBLY This invention concerns an electric element assembly for liquid heating purposes, particularly but not exclusively for an electric kettle.

Ccnventionally, an electric kettle element assembly, of the type with which the present invention iS concerned, comprises a substantially flat metal backplate and a metal-sheathed resistance heating element which extends rrom ana Das s ends sealingly connected to the backplate, with a central portion of the sheath also in direct thermal transfer relation with the backplate. In use, the backplate is sealed relative to an aperture in the kettle wall, with the element extending into the liquid reception vessel.

The reason for having the central portion of the sheath in direct thermal transfer relation with the backplate is to effect rapid heat transfer through the backplate to a temperature sensitive cut-out (usually a bimetallic strip, but possibly a plastics strip) disposed at the other side in the event that the element is operated when not immersed in liquid, so as to cut off power supply to the element before too much damage is done.

The backplate and the element sheath are conventionally formed of copper or an alloy of copper, which are good heat conductors. However, stainless steel is increasingly popular since, despite having poorer conductivity, it has the advantage that it does not discolour over a period of use in boiling water.

Conventionally, the ends (legs) and central portion of the element sheath have been connected to the backplate by brazing. This involves applying a jointing or "brazing" material, such as a copper alloy, (in the manner of a glue) between the regions to be connected and then placing the entire subassembly in a furnace to heat all the material sufficiently to melt the brazing material, and form an effective joint. This would usually be done in two stages, firstly to connect the central element sheath portion, then to connect the legs. Brazing is imprecise, messy and costly and is best avoided as part of a manufacturing process.

Moreover, in the case of stainless steel, higher temperatures are required to produce an effective joint by brazing so that a higher cost is involved in having to install, or adapt, and operate a furnace at a higher temperature, in addition to the general difficulty of automating any brazing procedure.

In their earlier applications Nos. 91 02915 and 91 02883 the applicants set forth proposals for avoiding any brazing process in production of the relevant type of electric element assemblies by making special provision for welding a central region of the element sheath to the backplate and by connecting the ends of the element sheath to the backplate in various different ways.

The element more specifically comprises a resistance heating wire, generally in the form of a coil, and of any suitable alloy including any of nickel, chromium, iron and aluminum, disposed within the sheath with its respective ends connected to pins which project from the ends of the sheath. Both the wire and the pins are electrically insulated from the surrounding sheath, for example by a packing of magnesium oxide. The purpose of these pins, traditionally, has simply been to make the necessary electrical connection to the wire, for power supply thereto. In other words, they have merely been required as current carriers, and heat transfer therethrough has been undesirable. The traditional material for such pins has, therefore, been steel, which Is of relatively poor thermal conductivity.

An alternative approach to avoiding a brazing process, whereby the need to connect a central portion of the element sheath to the backplate may be avoided altogether, is to mount the temperature sensitive cutout means in direct thermal transfer relationship with one of the pins which project from the respective ends of the element sheath. The cut-out means can then form part of the power supply circuit to the element, and directly cut off the power supply upon thermally actuated deflection or disintegration, in the case of a bimetal or plastics cut-out respectively. Where the cut out is a bimetallic strip, which is most usual, it will conveniently carry a contact whereby the power supply may readily be broken or reconnected upon respective heating and cooling beyond a predetermined temperature threshold.

The concept of direct connection of a bimetal circuit breaker to one end of the central, electrically conductive portion of a heating element was disclosed in US. specification No. 3,135,860 dated 1963, and has recently been further elaborated in International specification No. WO 90/09672. In the latter specification, practical problems of ensuring an adequate rate of heat transfer from the element to the bimetal so that the latter attains the necessary temperature to effect cut-out after only a short period of dry operation, are appreciated and tackled in various ways.

A first, obvious measure is that the pin to which the bimetal is connected should be of high thermal conductivity, hence of copper instead of traditional steel. However, a further problem then arises in forming an adequate connection between a copper pin and the resistance heating wire of the element since conventional welding is not feasible. The proposals made in WO 90/09672 for laser welding or for use of an intermediate sleeve crimped or friction fitted to the pin are not believed to be satisfactory.

One object of the present invention is, therefore, to provide for reliable connection of the resistance heating wire of the element to a pin of high thermal conductivity in a manner which is relatively simple to accomplish.

Other measures proposed in WO 90/09672 for enhancing the rate of heat flow via the pin to the bimetal, included careful selection of the size and length of the pin (i.e. its thermal capacity), provision of insulating material of low thermal conductivity around the pin, and maximising the area of contact between the pin and the bimetal.

A further aspect of the present invention concerns an alternative proposal for achieving effective improvement in the rate of heat transfer from the resistance heating wire to the temperature sensitive cut-out means via the pin which interconnects these.

According to one aspect of the invention an electric element assembly for liquid heating purposes comprising an element in the form of a resistance heating wire, disposed within a metal sheath and connected to pins which project from respective ends of the sheath, and temperature sensitive cut-out means mounted in direct thermal transfer relationship with at least one of the pins, is characterised in that said pin or pins to which the cut-out means is connected is formed of copper coated with a material enabling a good electrical connection to be made between the resistance heating wire and the pin.

The coating may be of chrome or nickel or another similar material.

As previously stated, copper alone is not a suitable material for the pin or pins to which the cutout is connected because problems arise in forming a connection between a copper pin and the resistive heating wire, welding of copper to the alloy used for the wire not being feasible. A pin made of copper coated with nickel or chrome in accordance with the invention is advantageous in having suitable heat conductivity while also being connectable to the commonly used resistance heating wire by a straightforward weld connection.

The coating of the copper pin with nickel or chrome may be accomplished by electroplating or electroless plating techniques.

In order to further improve thermal transfer the electric element assembly preferably further includes a sleeve of thermally conductive material inserted into the end of the sheath so to as at least partially surround the pin to which the cut-out means is connected.

The rate of heat transfer to the temperature sensitive cut-out via the pin to which it is connected is enhanced by this measure because heat is then additionally transmitted from the wire to the sleeve, via the intermediate electric insulation, and from the sleeve back to the pin, again via the intermediate insulation, as well as by direct conduction from the wire to the pin.

A suitable material for such a sleeve or liner would be copper.

Such a sleeve may be fitted into the element sheath so as to be in thermal transfer relationship with the inner surface of the sheath in the manner of a liner to the sheath. In this respect, the sleeve may be connected to the sheath by spot welding, or be in direct contact therewith by being a friction fit therein.

Alternatively, such a sleeve may be separated from and spaced inwardly of the sheath with a layer of insulation therebetween.

In either case, the sleeve may advantageously extend into the sheath a sufficient distance to surround a portion of the resistance heating wire adjacent its connection to the inner end of the pin.

The temperature sensitive cut-out means is conveniently a conventional bi-metallic strip, which bends upon heating and thereby breaks the circuit whereby power is supplied to the element by way of the pins. The alternative of a plastics cut-out means is possible, but unlike a bimetallic strip it does not provide for reversible operation (it disintegrates upon overheating) so is only useful as an ultimate fail safe to prevent complete burn out of an appliance.

The invention will be explained further by reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of connection of a bimetal strip to one terminal pin of a heating element; Fig. 2 is an end view of the same arrangement; and Fig. 3 is a graph of temperature of the bimetal against time upon operation of the element when immersed and when not immersed in water, respectively.

Figs. 1 and 2 show a cut-out device in the form of a bimetal strip 10 directly connected, eg. by welding, to one pin 12 of an electric element assembly, eg. as used in a kettle. The pin 12 is connected to one end of a coiled resistance heating wire 14 of any conventional material, such as an alloy including any of nickel, chromium, iron and aluminum. The wire 14 extends through a tubular sheath 16 of stainless steel or copper but is electrically insulated therefrom, eg.

by a packing 18 of magnesium oxide or other suitable material. The pin 12 projects from the end of the sheath 16 and is similarly electrically insulated therefrom. A seal or plug (not shown) may also be provided outwardly of the insulating material.

At its other end the wire 14 is connected to another pin (not shown) which projects from the other end of the sheath 16 in similar manner.

The pin 12 is made of copper coated with nickel or chrome, e.g. by electroplating or electroless plating, so as to conduct heat at a sufficiently high rate from the wire 14 to the bimetal strip 10, as well as its normal function of conducting electricity.

An additional sleeve 24 is mounted inside the end margin of the sheath 16, for example as friction fit therein or by spot welding. The sleeve 24 is inserted into the sheath 16 a sufficient distance to surround an end portion of the wire 14, where the latter connects to the inner end of the pin 12. The connection between the sheath 16 and the sleeve 24 should allow good heat transfer, and naturally, the sleeve 24 must have good heat conductivity. It may be of copper or any suitable alloy. The sheath 16 is not a current carrier (diagrammatically represented by earth symbol) so there is no conduction of electricity to or through the sleeve 24.

Heat is conducted much faster when there is no water surrounding the element sheath 16 to take up the generated heat. Thus, under dry operation, the bimetal 10 rapidly reaches a temperature where it deflects and breaks the power supply circuit of which it forms part.

In this respect, with reference to Figs. 1 and 2, upon deflection of the bimetal 10, its contact 20 lifts off from a corresponding contact 22. The rapid temperature rise of the bimetal 10, to about 125'C, under dry conditions is shown in Fig. 3. While the element is immersed in water, the temperature rise is slower and the bimetal will not reach a temperature sufficient to cause breaking of the circuit.

As explained in the introduction, the purpose of the sleeve 24 is to enhance the rate of heat transfer via the pin 12 to the bimetal, by allowing for additional heat transfer from the wire 14 to the sleeve 24 at the inner end of the sleeve 24, then from the sleeve 24 back to the pin 12, where the latter is surrounded by the sleeve 24.

The foregoing is, of course, only an exemplary embodiment. In other arrangements the sleeve 24 may be spaced inwardly of the sheath 16, with a layer of insulating material 18 therebetween, and it need not extend so far into the sheath 16 as to encircle a part of the resistance heating wire 14 adjacent the inner end of the pin 12.

Respective cut-outs, such as bimetals 10, may of course be connected to the pins at both ends of the element, if so desired, as a fail safe measure, in which case thermally conductive sleeves, such as 24, are advantageously inserted into both ends of the sheath.

The terms sleeve or liner as used within this specification are intended to refer to members which, whilst not necessarily completely surrounding or encircling respective conductive pins, at a spacing therefrom, extend at least part way around said pins, usually around a major part. Thus the terms sleeve or liner include slotted or perforated members as well as simple cylindrical members. These need not be mounted concentrically of the element sheath or the respective pin, although they will most usually be so mounted.

Claims (13)

1. An electric element assembly for liquid heating purposes comprising an element in the form of a resistance heating wire, disposed within a metal sheath and connected to pins which project from respective ends of the sheath, and temperature sensitive cut-out means mounted in direct thermal transfer relationship with at least one of the pins, characterised in that the pin or pins to which the cut-out means is connected is formed of copper coated with a material enabling a good electrical connection to be made between the resistance heating wire and the pin.

2. An electric element assembly as claimed in claim 1 in which the material is nickel.

3. An electric element assembly as claimed in claim 1 in which the material is chrome.

4. An electric element assembly as claimed in claims 1, 2 and 3 in which the coating is formed by electroplating techniques.

5. An electric element assembly as claimed in claim 1, 2 or 3 in which the coating is formed by electroless plating techniques.

6. An electric element as claimed in any proceeding claim which further comprises a sleeve of thermally conductive material inserted into the end of the sheath so as at least partially to surround the pin to which the cut-out means is connected.

7. An electric element as claimed in claim 6 in which the sleeve or liner is of copper.

8. An electric element as claimed in claims 6 or 7 in which the sleeve is fitted into the element sheath so as to be in thermal transfer relationship with the inner surface of the sheath in the manner of a liner to the sheath.

9. An electric element as claimed in claim 8 in which the sleeve is connected to the sheath by spot welding.

10. An electric element as claimed in claim 8 in which the sleeve is in direct contact therewith by being a friction fit therein.

11. An electric element as claimed in claims 6 or 7 in which the sleeve is separated from and spaced inwardly of the sheath and a layer of insulation is provided therebetween.

12. An electric element as claimed in any of claims 6 to 11 in which the sleeve extends into the sheath a sufficient distance to surround a portion of the resistance heating wire adjacent its connection to the inner end of the pin.

13. An electric element assembly for liquid heating purposes substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.