EP2276325A1

EP2276325A1 - Electrical heating elements

Info

Publication number: EP2276325A1
Application number: EP10251263A
Authority: EP
Inventors: Vincent Joseph Garvey
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 2009-07-14
Filing date: 2010-07-14
Publication date: 2011-01-19
Anticipated expiration: 2030-07-14
Also published as: CN101959337A; GB0912199D0; EP2276325B1; CN101959337B

Abstract

An electrical appliance comprises a sheathed electrical resistance heating element. The element comprises a length of resistance wire (20), a pair of electrical terminations (8, 12) connected to respective ends of the length of resistance wire (20) for connecting said element to a main electrical power supply, and a further electrical termination (14) connected to said resistance wire (20) intermediate the ends and providing an electrical power supply to another part of the appliance at a lower voltage than the main power supply.

Description

This invention relates to sheathed electrical heating elements, particularly those for heating water, and to appliances incorporating such elements.
The vast majority of small domestic electrical appliances for heating water, such as kettles, coffee makers, tea makers and the like, employ a sheathed resistance heating element in order to heat water therein. These elements typically comprise a coiled length of thin resistance wire, such as a nickel-chromium-aluminium alloy, which is embedded in an electrically insulating material such as magnesium oxide and encased in a hollow aluminium, stainless steel or copper tube (the latter two being used for elements immersed directly in water). 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 aluminium tube or sheath is sealed at either end with the respective cold tails projecting through the sealing plugs. The aluminium sheath can be straight but is more typically bent into an arcuate shape. Such elements are typically bonded to an aluminium diffuser plate which is in turn bonded to a metal, e.g. stainless steel, base plate, so that a chamber of water can be heated from beneath, or used as immersed elements.
The sheathed electrical resistance heating elements described above have remained virtually unchanged in their construction for decades and can now be made in high volumes without the need for skilled labour and at very low cost. This is part of the reason why it has been possible in recent years to satisfy a demand for very low cost domestic appliances such as kettles.
When viewed from a first aspect the present invention provides an electrical appliance comprising a sheathed electrical resistance heating element, said element comprising a length of resistance wire, a pair of electrical terminations connected to respective ends of said length of resistance wire for connecting said element to a main electrical power supply, and a further electrical termination connected to said resistance wire intermediate said ends and providing an electrical power supply to another part of the appliance at a lower voltage than the main power supply.
The invention also extends to a sheathed electrical resistance heating element comprising a length of resistance wire, a pair of electrical terminations connected to respective ends of said length of resistant wire for connecting said element to a main electrical power supply, and a further electrical termination connected to said resistance wire intermediate said ends, thereby providing an electrical power supply at a lower voltage than the main power supply.
Thus it will be seen by those skilled in the art that in accordance with the invention a heating element has an additional termination connected part-way along its length which provides a lower voltage than the voltage across the element as a whole and which can therefore be used (if necessary with appropriate rectification, smoothing and/or regulation) to power an auxiliary component or subsystem such as an electronic control or display circuit in the appliance, or a pump. Thus, in a preferred set of embodiments the appliance comprises an auxiliary component or subsystem powered by said lower voltage power supply. This could be one or more of an electronic circuit, a light source, a sound transmitter or a pump. The electronic circuit could be for control of the appliance and/or display - which could be simply a lamp or LED or a more complex display arrangement, e.g. incorporating a liquid crystal display. It will be appreciated however that the invention obviates the need to provide a transformer which represents a significant saving in cost and weight and so makes it more economically viable to provide an appliance with additional functionality.
In another set of embodiments the appliance comprises a connector electrically connected to said lower voltage power supply, said connector permitting connection of an external device to the appliance. In a particularly convenient embodiment envisaged by the applicant, this connector would allow the appliance to recharge a rechargeable battery either that had been removed from a device or by connection to a device incorporating such battery. Indeed, such an arrangement could be employed in reverse so that the battery from a typical cordless appliance such as an industrial cordless power tool could be used to power the heater. A particularly convenient example of this would be a small kettle which could be used on a building site to heat water using the battery from a cordless power tool.
In a set of embodiments the lower voltage power supply comprises means for converting an AC voltage to a DC voltage e.g. a rectifier and optionally a smoothing capacitor. Also optionally provided, depending upon the application, would be a voltage regulator. It is even envisaged that with appropriate circuitry in the lower voltage supply, a plurality or range of voltages could be supplied.
The heating element is preferably arranged in the appliance to heat a body of water. In a set of embodiments, the heating element is configured and arranged to heat water in the appliance to boiling. In a set of embodiments the heating element is directly immersed in water. In another set of embodiments the heating element is bonded to the underside of a metallic plate forming the base of a heating chamber. The heating element could be bonded directly to the base or with a heat diffuser sandwiched therebetween.
The intermediate connection to the resistance wire could be made in any convenient way. In a set of preferred embodiments, the resistance wire is coiled inside the sheath and a connection forming said intermediate termination is made using a terminal member extending longitudinally inside the resistance wire coil and connected to it at the appropriate place. Preferably, the terminal member extends out of one end of the element sheath. This could, for example, be parallel with a conductor forming the terminal connected to one end of the resistance wire. In a preferred set of embodiments, the intermediate terminal member and one of the end terminals (i.e. one of the cold tails) are coaxial. Most conveniently, the end terminal is of tubular form and has the intermediate terminal member passing through it. This arrangement facilitates manufacture of the element and the ease with which electrical connections can be made to such an element when incorporating it into an appliance.
In one set of embodiments, the intermediate terminal member comprises an enlarged head which makes contact with the coiled resistance wire from inside the coil. In another set of embodiments, the diameter of the resistance wire coil is locally reduced in order to make connection with the intermediate terminal member. This may mean that it is not necessary to provide the intermediate terminal member with an enlarged head - i.e. it could be in the form of a simple cylindrical pin.
In one set of embodiments the sheathed heating element comprises sealed end caps at either end of the sheath. Typically sheathed heating elements are packed with an electrical insulating medium, such as magnesium oxide powder. The parts inside the sheath are, in practice, embedded in the electrical insulating medium. In the set of embodiments in which sealed end caps and electrical insulating medium are provided, the sealed end caps therefore act to retain the electrical insulating medium inside the sheath.
In one set of embodiments the sheathed heating element is suitable for use in a liquid heating appliance. In these embodiments and where sealed end caps are provided, the sealed end caps are arranged to keep out water from the inside of the sheath. Conveniently, the end terminals and the intermediate terminal extend through the sealed end caps. Therefore as well as providing a seal to the sheath, the end caps form a seal around the terminals.
The voltage available at the intermediate terminal will of course be dependent upon both the voltage applied across the two ends of the element and the precise point along the length of the wire at which the intermediate terminal is connected. Typically, the thickness and resistivity of the wire is constant along its length and thus the resistance between the intermediate connection point and a given end is directly proportional to the length of wire between the connection point and that end. This allows an easy calculation of the point along the resistance wire at which to make the connection in order to give a required voltage. To give an example, if a heating element is powered with a mains AC supply of 230 V RMS, the peak voltage across the element will be higher by a factor of the square root of 2 - i.e. approximately 325 V. To provide a 12 V DC supply, the lower voltage power supply module will require an AC voltage of approximately 13 V peak (to account for a small voltage drop across the rectifier etc). This can be achieved by making the intermediate connection at 4% along the length of the resistance wire.
Although so far only one intermediate connection point has been mentioned, it should be clear that a plurality of such connections could be made to provide a range of different voltages.
In one set of embodiments, the sheathed heating element of the present invention is manufactured by first producing a sub-assembly of the resistance wire and terminals which is then placed in the element sheath. The sheath is filled with low density (e.g. about 2.2 g/cm³) magnesium oxide powder. The whole assembly is then rolled to reduce the diameter of the sheath from, e.g. about 8 mm to about 6.5 mm, which has the effect of increasing the density of the magnesium oxide powder, e.g. to about 3 g/cm³. This process also determines the resistance of the resistance heating wire as the rolling process increases the diameter of the heating wire slightly, thereby reducing the resistance value of the wire to its final value. A sheathed heating element manufactured according to this described method is capable of achieving power densities of greater than 30 W/cm², e.g. 30-50 W/cm².
In the set of embodiments with the sealed end caps, it can be seen that this method of manufacture produces a sheathed heating element which is arranged such that it is not possible to remove any of the resistance wire, the end terminations and the intermediate termination from the element sheath.
Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of a heating element embodying the invention;
Fig. 2 is a view of the heating element of Fig. 1 with the outer sheath and insulating material omitted for clarity;
Fig. 3 is a view of the electrical terminals at one end of the element;
Fig. 4 is a view similar to Fig. 2 but of a second embodiment of the invention; and
Fig. 5 is an exemplary schematic circuit diagram of an appliance in accordance with the invention.

Turning firstly to Fig. 1, there may be seen a sheathed electrical resistance heating element in accordance with the invention. For ease of understanding, this is shown as being straight, although typically in practice it would be bent round into an arc to fit onto a round heating plate, or into a tortuous loop bonded to an element head for use as an immersed element.
The element comprises an outer sheath 2 which is formed by a hollow aluminium tube of circular cross section. The ends of the tube are sealed by respective sealing plugs 4, 6 which are electrically insulating and able to withstand relatively high temperatures. An electrical terminal conductor 8 extends through the sealing plug 4 at one end of the element and a standard connection tab 10 is welded to it for allowing attachment of a connecting lead.
Extending through the sealing plug 6 at the other end of the element are two electrically conducting terminal members 12, 14. One of the terminal members 12 is cylindrical form with the other terminal member 14 being in the form of a solid pin which passes through the hollow core of the other terminal member 12. Connection tabs 16, 18 are welded to the two terminal members 12, 14 respectively.
Fig. 2 shows the element slightly enlarged and with the aluminium sheath 2 removed. Also omitted from this drawing is the compacted magnesium oxide powder in which the parts inside the tube are, in practice, embedded. This acts to provide an electrically insulating support to the resistance wire 20 whilst conducting heat away from it to the sheath 2.
The coiled length of resistance wire 20 typically comprises a nickel-chromium-aluminium wire of between 0.5 mm for a 3 kW element and 0.1 mm thickness for a very low power element. For a typical element rated at 2.2 kW at 230 V RMS the overall resistance of the wire 20 is approximately 24 Ohms.
One end of the resistance wire 20 is bonded to the single pin terminal member 8 which extends through the sealing plug 4 at one end of the element. The other end of the resistance wire 20 is bonded to the tubular terminal member 12 passing through the other sealing plug 6 at the other end of the element. As can be seen more clearly in Fig. 3, the terminal member 14 which passes coaxially through the tubular terminal member 12 has at the distal end thereof an enlarged head portion 22. The enlarged head portion 22 makes a further contact to the resistance wire 20 from inside the coil at a point part-way along its length. For example, the connection of the terminal conductor head portion 22 to the resistance wire 20 may be 4% along the length of the wire as measured from the contact point with a tubular conductor 12. For a 230 V RMS AC power supply across the two ends of the wire, i.e. across terminals 8 and 14, this would give a voltage across the two left end terminals 12, 14 of approximately 13 V peak (9.2 V RMS).
A second embodiment of the invention is shown in Fig. 4. This Figure is similar to Fig. 2 in that the outer aluminium sheath and the electrical insulation medium have both been omitted. The electrical connection tabs have also been omitted. This embodiment differs from that shown in Fig. 2 in that instead of the intermediate connection to the intermediate terminal member 14' being by means of an enlarged head, the intermediate terminal member 14' is a simple cylindrical pin and the coil diameter of the resistance wire 20' is locally reduced in the vicinity of the end of the intermediate terminal member 14' in order that contact can be made between them. Otherwise, the element of this embodiment functions in exactly the same way as the previous embodiment.
Fig. 5 shows a schematic circuit diagram of one possible implementation of a heating element in accordance with the invention in an appliance. The left hand of the diagram shows the live and neutral poles 24, 26 of the mains electrical supply to the appliance. This would typically be supplied by a flex terminated by a plug which plugs into a wall socket, or by a cordless electrical connector such as the applicant's P72 360° cordless connector (with the cordless base being connected to the wall socket). Two sets of switch contacts 28, 30 are provided, one in each respective pole of the supply in order to allow complete disconnection of the mains power. These could, for example, be provided by means of one of the Applicant's U18 series of underfloor heater controls which operate to disconnect the electrical power in both mains poles in the event that the associated heater overheats e.g. as a result of a water heater appliance being switched on dry or boiling dry.
The heating element 32, which could for example be as described above with reference to Figs. 1 to 4, although this is not essential, is connected across the live and neutral poles. For the element described above, this is achieved by means of the connector tabs 10, 16 connected to the respective ends of the resistance wire 20, 20'.
The intermediate terminal of the element 14 is connected to one node of a bridge rectifier arrangement 34. The opposite node is connected to the neutral pole 26 which means that the other two nodes of the bridge rectifier 34 provide a positive DC rail 36 and a 0 V rail 40 as is well known in the art. An electrolytic smoothing capacitor 42 is provided between the positive and 0 V rails 36, 40 in order to smooth the full wave rectified voltage across them as is also well known in the art. The bridge rectifier 34 and smoothing capacitor 42 together provide a DC power supply 38 which is at a lower voltage than the mains input voltage. As previously explained, the actual voltage across the positive and 0 V rails 36, 40 is dependent upon how far along the resistance wire 20 the intermediate terminal member 14 is connected. Circuit element 44 can be any module or subsystem which is powered by a DC power supply. This could, for example, be a display, light, alarm or other function associated with operation of the heating appliance.
This embodiment also shows by way of example how a circuit element such as a pump 46 which requires an AC power supply but at a lower voltage than mains can be employed. As illustrated here, a set of contacts 48 is used to switch the pump 46 on and off. The contacts 48 could, for example, be the contacts of a relay controlled as part of the DC circuit element 44. Of course, one or other of the DC power supply portion or the AC circuit element could be used on its own without the other depending upon the requirements of the particular application.
Thus it will be seen that in the embodiment of the invention described above, a power supply at lower than mains voltage is provided by the intermediate terminal on the heating element which allows an auxiliary system to be powered, but without the need for a bulky and heavy transformer as would normally be expected to be the case if a reduced voltage power supply were required. This allows for the compact and cost-efficient incorporation of further features and functionality into electrical appliances incorporating a sheathed resistance heating element.

Claims

An electrical appliance comprising a sheathed electrical resistance heating element, said element comprising a length of resistance wire, a pair of electrical terminations connected to respective ends of said length of resistance wire for connecting said element to a main electrical power supply, and a further electrical termination connected to said resistance wire intermediate said ends and providing an electrical power supply to another part of the appliance at a lower voltage than the main power supply.
An electrical appliance as claimed in claim 1 wherein the electrical appliance comprises an auxiliary component or subsystem powered by said lower voltage power supply.
An electrical appliance as claimed in claim 1 or 2 wherein the heating element is arranged in the appliance to heat a body of water.
An electrical appliance as claimed in claim 1, 2 or 3 wherein the heating element is configured and arranged to heat water in the appliance to boiling.
A sheathed electrical resistance heating element comprising a length of resistance wire, a pair of electrical terminations connected to respective ends of said length of resistant wire for connecting said element to a main electrical power supply, and a further electrical termination connected to said resistance wire intermediate said ends, thereby providing an electrical power supply at a lower voltage than the main power supply.
An electrical appliance or sheathed electrical resistance heating element as claimed in any preceding claim wherein the lower voltage power supply comprises means for converting an AC voltage to a DC voltage.
An electrical appliance or sheathed electrical resistance heating element as claimed in any preceding claim wherein the resistance wire is coiled inside the sheath and a connection forming said intermediate termination is made using a terminal member extending longitudinally inside the resistance wire coil and connected to it at the appropriate place.
An electrical appliance or sheathed electrical resistance heating element as claimed in claim 7 wherein the terminal member extends out of one end of the element sheath.
An electrical appliance or sheathed electrical resistance heating element as claimed in any preceding claim wherein the intermediate terminal member and one of the end terminals are coaxial.
An electrical appliance or sheathed electrical resistance heating element as claimed in claim 9 wherein the end terminal is of tubular form and has the intermediate terminal member passing through it.
An electrical appliance or sheathed electrical resistance heating element as claimed in any of claims 7 to 10 wherein the intermediate terminal member comprises an enlarged head which makes contact with the coiled resistance wire from inside the coil.
An electrical appliance or sheathed electrical resistance heating element as claimed in any of claims 7 to 10 wherein the diameter of the resistance wire coil is locally reduced in order to make connection with the intermediate terminal member.
An electrical appliance or sheathed electrical resistance heating element as claimed in any preceding claim wherein the sheathed heating element comprises sealed end caps at either end of the sheath.
An electrical appliance or sheathed electrical resistance heating element as claimed in any preceding claim wherein the sheathed heating element has a power density of greater than 30 W/cm².
An electrical appliance or sheathed electrical resistance heating element as claimed in any preceding claim wherein the sheathed heating element is arranged such that it is not possible to remove any of the resistance wire, the end terminations and the intermediate termination from the element sheath.