GB2284132A

GB2284132A - Thermal actuator controlled by relative expansion of parts of a heating element

Info

Publication number: GB2284132A
Application number: GB9422129A
Authority: GB
Inventors: John Crawshaw Taylor; Vincent Joseph Garvey
Original assignee: Strix Ltd
Current assignee: Strix Ltd
Priority date: 1993-11-02
Filing date: 1994-11-02
Publication date: 1995-05-24
Also published as: GB9422129D0; GB9322526D0

Abstract

An electric heater comprising an element 1 having substantially unheated end portions 4, 5 mounted to a head 7 and a heated return portion 14. The return portion 14 extends towards the head by a distance L beyond the ends of the end portions 4, 5, such that when the element is energised with no water around it, the return portion 14 moves towards the head 7. A movable push rod 16 cooperates with the return portion 14 to operate a switch 20 upon a predetermined movement of the return portion. <IMAGE>

Description

Electrical Heaters The present invention relates to electrical heaters and in particular but not exclusively to electrical immersion heaters of the type used for example in water heaters, kettles and the like.

It is customary for electrical immersion heaters to be provided with an overheat protector which interrupts or reduces the supply of electrical energy to the heater in the event that the heater overheats. This may occur, for example, in a kettle or hot water jug in the event of the failure of a steam control which normally operates to switch off the heater when water in the kettle or jug boils which results in the kettle boiling dry, or if the kettle or jug is turned on without water in it.

Typically such immersion heaters comprise an element comprising a tubular metallic sheath and a heating coil, arranged within the sheath and insulated therefrom by an insulating material such as magnesium oxide. The ends of the heating coil are attached, within the sheath, to respective terminal pins, known as "cold pins" which project from the ends of the sheath for connection to the electrical supply.

In most known arrangements, especially as used in kettles and hot water jugs, the element is mounted to a metallic mounting plate or "head" which is mounted to an opening in the side wall of the kettle or jug, and the element has a portion, intermediate its ends, known as the "hot return" portion which is brazed to the head. A control unit is arranged on the other side of the head, with a thermally-sensitive actuator, for example a snapacting bimetallic actuator, arranged in thermal contact with the head at a location directly opposite the point of attachment of the hot return portion to the head.

When the element overheats, for example if the kettle boils dry or is switched on dry, sufficient heat is transferred through the head to the actuator which operates to open a switch in the electrical supply to the element. Examples of such arrangements are disclosed for example in patent specifications GB 1470366, GB 2042269, GB 2097920 and GB 2181598.

More recently, it has been proposed, in WO 92/05675, to provide an immersion heater with a plastics head and a conductive heat link for transferring the heat generated at the return portion through the head to a control which is arranged as previously described.

Such controls require the careful choice of bimetallic actuators to ensure that they will operate to interrupt the power supply to the element in an overheat situation, but will not nuisance trip at lower temperatures, before the water in the kettle boils.

Furthermore, they are reliant upon thermal conduction, to the actuator, which may be affected by, for example, lime deposits formed on the head or element in hard water areas.

It has been proposed, in patent specifications GB 2138258, GB 1408388 and EP 172416, to use the expansion of an immersion heater element to operate a switch in the event that the element overheats. In the arrangements disclosed in the above patents, a reference member in the form of a transfer rod of a low thermal expansion coefficient such as Invar is connected to an end portion of an element remote from the element head, and extends the entire length of the element, and through the element head for cooperation with a switch unit mounted on the other side of the head. As the element heats and expands, its end remote from the head moves away from the head, pulling the transfer rod with it, the movement of the rod being used to open the switch when the expansion of the element, which will be directly linked to its rise in temperature, exceeds a given amount.

Such an arrangement has the considerable disadvantage, however, that the transfer rod is long and is exposed, which means that should it be disturbed for any reason (as might happen, for example, when a user is trying to scrape scale from the element in a kettle or hot water jug) the calibration of the device will be affected, thereby potentially rendering the element dangerous in an overheat condition. The provision of an elongate reference member also adds cost to the element.

The present invention seeks to avoid this problem, and from a first aspect, therefore provides an electrical heater comprising a heating element and actuating means, responsive to the differential expansion of portions of the element which are heated to different extents, for operating switch means to interrupt or reduce the supply of electrical energy to the element when the element reaches a predetermined temperature.

In accordance with the present invention, therefore the differential expansion of portions of a heater element heated to different extents, rather than the absolute expansion of the element relative to an elongate reference member is used to operate a switch.

This will, as described below, avoid the need for a separate elongate reference member as heretofore used, making the heater less prone to damage in use and less costly.

Most simply, the differential expansion may be between a heated and a substantially unheated portion of the element.

Preferably the element is a metal sheathed element the actuating means being responsive to the differential expansion of differentially heated portions of the sheath, for example an unheated end portion of the sheath and a heated return portion. From a further broad aspect the invention provides an electrical heater comprising a metal sheathed heating element mounted in or to a heater head and having end portions extending through said head, at least one of which end portions is substantially unheated and a heated, intermediate, return portion extending back towards said end portions, and wherein relative expansion of said heated and unheated portions of the element is used to interrupt or reduce the supply of electrical energy to the element in the event of the element overheating.

In the preferred embodiment, the element has a heating coil which is connected at both ends within the sheath to respective cold pins, which extend with their distal ends for a predetermined distance into the respective ends of the sheath, and also at their proximal ends outside the ends of the sheath for connection to an electrical supply.

The cold pins are typically made from mild steel and although heated to some extent by the current flowing through them to the heating coil (which typically may be formed of a high resistance material such as nichrome wire) will remain relatively cool.

Accordingly the end regions of the sheath, from which the cold pins are suitably electrically insulated, receive relatively little heat from the cold pins. Heat will be conducted from the heated portion of the sheath to the end portions, but at least initially in an overheat conditon the end portions remain relatively cool compared to the rest of the sheath. Accordingly, if a heated return portion is provided which extends towards the head beyond the distal end of an unheated end portion of the sheath, ie. beyond the distal end of the cold pin therein, a differential expansion will occur between these two portions, which can be used to actuate a switch.

In the preferred arrangement, the ends of the sheath are fixed to the head, for example by welding or brazing, and the return portion of the element sheath will then move relative te the end portions thereof and the actuating means will be responsive to that movement.

From a further aspect therefore, the invention provides an electric heater comprising a metal sheathed heating element formed to provide two end portions which are mounted to a heater head and an intermediate return portion extending towards the head, wherein respective cold pins extend into the ends of the element and the return portion extends towards the head beyond substantially the distal end of at least one cold pin, and further comprising actuating means co-operating with the return portion of the sheath and movable in response to relative expansion of said end portion(s) and return portion of said element and switch means operated by movement of said actuating means to interrupt or reduce the supply of electrical energy to the element.

The invention also seeks to provide an element assembly for use in a heater as above and from a yet further aspect therefore, the invention provides an electrical heating element assembly comprising a metal sheathed heating element formed to provide two end portions which are mounted to a heater head and an intermediate return portion extending towards the head, wherein respective cold pins extend into the ends of the element and the return portion extends towards the head beyond the end of at least one cold pin, and further comprising actuating means co-operating with the return portion of the sheath and movable in response to relative expansion of said end portions and return portions of said element.

Of course, it would be possible in an alternative embodiment for the return portion to be fixed to the head and the end portion or portions of the sheath to be movable to give an actuating movement, but this is not preferred.

The actuating means may take a number of forms. In one embodiment, it may comprise a member such as a push rod, for example a circular section rod, connected to or arranged to cooperate with the return portion of the element and passing through the head to operate a switch arranged on the other side of the head in which case suitable sealing means could be provided in the head to prevent water leaking therethrough.

In an alternative embodiment, the actuating means may comprise a diaphragm formed in the head, and against which the return portion of the element, or a part connected thereto, acts. Such an arrangement would have the advantage of not requiring sealing means to be provided in the head to allow for the passage of a push rod.

Preferably the return portion of the element extends closely adjacent the head. This maximises the relative expansion of the return portion with respect to the end portions of the sheath and thus the amount of actuating movement available. Furthermore, it minimises the exposed length of the actuating member, which reduces the possibility of the member being disturbed, thereby affecting the calibration of the switch.

The amount of expansion may further be maximised by the at least one cold pin extending a substantial distance into the end portion of the sheath. Preferably the cold pin extends into the end portion of the sheath by a distance of between 1S and 3 the length of the leg of the sheath, most preferably about M the length of the leg. Although this reduces the amount of the element actually heated, this can be compensated for by increasing the diameter of the element, for example from 6mm (which is standard in say kettle heating elements) to 8mm.

Although as mentioned above, the end regions of the sheath receive heat by conduction from the heated portions of the sheath, this effect may be reduced by, for example, making the sheath of a low thermal conductivity metal, such as stainless steel rather than of copper as is most commonly the case. The use of a stainless steel sheath also has the advantage of having an improved aesthetic appearance, and also gives greater strength to the element.

The element ends should be rigidly connected to the head to give good rigidity in the element (otherwise the element might buckle under loading) and also to protect the element against knocks which might de-calibrate the switch.

The heater also preferably comprises switch means operable by said actuating means, and preferably mounted to the heater head.

It will be appreciated that the actuating movement produced in the element will be relatively small.

Preferably, therefore the switch means comprises a microswitch which will be able to accommodate such small movements. Alternatively, the actuating means may operate a spring-loaded mechanism which is tripped after a given movement of the actuating means to open the switch. The switch may be such that it will reset automatically on cooling of the element, but if desired, it may be such that it must be manually reset after operation.

A preferred embodiment of the invention will now be described, by way of example only with reference to the accompanying Figure 1.

With reference to Figure 1, an immersion heater for use in a kettle or hot water jug comprises an element 1 stainless steel sheath 2 having parallel legs 3,4, respective end portions 5,6 which are TIG welded to a stainless steel head 7. A nichrome heating coil 8 is arranged within the sheath and electrically insulated therefrom in known manner by an MgO insulating powder which fills the space between the coil and the sheath.

The ends of the heating coil, only one end of which can be seen in the Figure, are attached in a known manner, to cold pins 10,11 which extend beyond the ends of the sheath 2 for connection to the electrical supply to the element as will be described later. The ends 5,6 of the sheath 2 are closed in known manner by respective insulating plugs 12,13, which retain the insulating powder in the sheath 2.

As can be clearly seen in the Figure, the cold pins extend into the legs 3,4 of the sheath 2 by a substantial amount. Both cold pins 10,11 extend into the end portions 5,6 by substantially the same amount.

The element sheath 2 is provided with a looped return portion 14, extending towards, and with its distal end 15 adjacent, the mounting plate 7.

The return portion 14 which is heated, extends beyond the distal ends of the cold pins 10,11 by a distance L.

Actuating means in the form of a push rod 15 abuts the distal end 15 of the return portion and extends through a hole 17 formed in the head 7. An O-ring seal 18 is provided in the hole 17 around the push rod 16 to prevent water escaping through the hole 17.

The push rod 16 is provided with an enlarged head 19 which operates a microswitch 20, which is mounted to the rear of the head 7.

The switch 20 comprises a base member 21 mounted to the head 7 by a screw 22. The base member 21 is provided with a forward facing projection 23 having a notch 24 which acts as a seat for the end of a C-spring 25 of an over-centre, snap-acting metallic blade 26.

One end 27 of the blade 26 is located in a collar 28 provided on the end of a calibration screw 29, mounted to the base member 21. The other end 30 of the blade 26 mounts an electrical contact 31, arranged opposite the end of the cold pin 10. One electrical connection of the power supply is made to the screw 29, and the other is made to the cold pin 11.

Operation of the control will now be described. In use the head 7 is mounted in an opening in a wall of a water heating vessel. When water is present around the element 1, the temperature of the sheath 2 corresponds largely to that of the water being heated, which means that the element as a whole expands evenly, and by a relatively small amount. The distal end 15 of the return portion 14 does not move noticeably, and the switch 20 remains closed, with the contact 31 abutting the end of the cold pin 10.

However, should the element be turned on without any water surrounding it, the temperature of the heated portion of the sheath 2 will rise rapidly, while the end regions thereof 4,5 having the cold pins 10,11 will remain relatively cool, being heated substantially only by thermal conduction back along the sheath. This heating effect will not be substantial, since the sheath material, stainless steel, has a relatively low thermal conductivity.As a result the end portions 4,5 will remain relatively cool, say under 100"C for a period of say 10 seconds, while in the same period the remaining parts of the sheath 2, including the return portion 14 will be heated rapidly to a very high temperature, for example, 700to. Thus the end portions 4,5 and the return portion 14 will expand by different amounts over the distance L, which will cause the distal end 15 of the return portion 14 to move towards the head 7. The push rod 16 will thus be moved towards the right in the sense of the Figure.The enlarged head 19 of the push rod will then push the spring blade 26 rearward to the point where the blade goes over-centre with respect to the C spring 25, at which point the contact 31 will be moved away from the end of the cold pin 10 with a snap action to the condition shown in the Figure, so disconnecting the power supply to the element.

As the element 1 cools, the return portion 14 will move away from the head 7 and the push rod 16 will move to the left, eventually to the point where the blade 26 moves over-centre in the opposite sense, thereby causing the contact 31 to move back against the end of cold pin 10 with a snap action to reconnect the power supply. If this is not desirable, a latch or similar mechanism may be provided which retains the blade 26 in its open position until it is reset manually.

The set point of the switch may be varied by moving the calibration screw 29 backwards or forwards, which changes the geometry of the blade 26, and thus the amount of movement of the push rod 16 needed to trip the switch 20.

It is expected that a movement of .1mum can be obtained from a 100"C rise in the temperature of the element. In order to turn the element off within a reasonable period of time, say within 10 seconds, the temperature rise of say 500"C may be achieved, in which case a total movement of .5mm will be all that is available to operate the switch. This is quite sufficient, however, to trip a microswitch or some other spring loaded mechanism. Furthermore, this movement will produce a considerable actuating force.

The immersion heater described above is particularly suitable for use in a kettle or hot water jug. However the invention is not limited to such and it could find application, for example in washing machine heaters.

Claims

1. An electrical heater comprising a heating element and actuating means, responsive to the differential expansion of portions of the element which are heated to different extents, for operating switch means to interrupt or reduce the supply of electrical energy to the element when the element reaches a predetermined temperature.

2. A heater as claimed in claim 1 wherein the respective said portions are, in use, heated and substantially unheated.

3. A heater as claimed in claim 1 or 2 wherein said heater element is a metal sheathed heating element.

4. An electrical heater comprising a metal sheathed heating element mounted in or to a heater head and having end portions extending through said head, at least one of which end portions is, in use, heated to a lesser extent than a heated, intermediate, return portion which extends back towards said end portions, and actuating means response to the relative thermal expansion of said heated and unheated portions for interrupting or reducing the supply of electrical energy to the element when the element reaches a predetermined temperature.

5. A heater as claimed in claim 4 wherein the element has a heating coil which is connected at both ends within the sheath to respective cold pins, which extend with their distal ends for a predetermined distance into the respective ends of the sheath, and also at their proximal ends outside the ends of the sheath for connection to an electrical supply, the return portion extending towards the head beyond the distal end of a said cold pin.

6. A heater as claimed in claim 4 or 5 wherein the ends of the sheath are fixed to the head.

7. An electric heater comprising a metal sheathed heating element formed to provide two end portions which are mounted to a heater head and an intermediate return portion extending towards the head, wherein respective cold pins extend into the ends of the element and the return portion extends towards the head beyond the end of at least one cold pin, and further comprising actuating means co-operating with the return portion of the sheath and movable in response to relative expansion of said end portions and return portions of said element.

8. A heater as claimed in any of claims 4 to 7 wherein said actuating means comprises a push rod arranged to cooperate with the element and passing through the head.

9. A heater as claimes in any of claims 4 to 7 wherein said actuating means comprises a diaphragm formed on said head.

10. A heater as claimed in any of claims 4 to 9 wherein said return portion of the element extends closely adjacent the head.

11. A heater as claimed in any of claims 5 to 10 wherein said at least one cold pin extends a substantial distance into the end portion of the sheath.

12. A heater as claimed in claim 11 wherein said cold pin extends into the end portion of the sheath by a distance of between 1S and 3A the length of the leg of the sheath, most preferably about M the length of the leg.

13. A heater as claimed in any of claims 4 to 12 wherein said metal sheath is of a metal having low thermal conductivity, such as stainless steel.

14. A heater as claimed in any of claims 4 to 13 further comprising switch means actuated by said actuating means.

15. A heater as claimed in claim 14 wherein said switch means is mounted to said head.

16. A heater as claimed in claim 14 or 15 wherein said switch means comprises a microswitch.

17. A heater as claimed in claim 14 or 15 comprising a spring loaded mechanism actuated by said actuating means to open said switch means.

18. A heater as claimed in any of claims 14 to 17 wherein said switch means resets automatically on cooling of the element.

19. A heater as claimed in any of claims 14 to 17 wherein said switch means is manually resettable.

20. A heater substantially as herein before described with reference to the accompanying drawings.