GB2379851A

GB2379851A - Improved protection against failure of planar heating elements

Info

Publication number: GB2379851A
Application number: GB0121970A
Authority: GB
Inventors: Robert Andrew O'neill
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2001-09-11
Filing date: 2001-09-11
Publication date: 2003-03-19
Anticipated expiration: 2021-09-11
Also published as: GB0121970D0; HK1054484B; HK1054484A1; GB2379851B

Abstract

Electric kettles, percolators and other water heating vessels, which may have bodies of plastics material, increasingly employ planar heating elements. The element may comprise a sheathed resistance heating element 1 brazed or soldered to an aluminium heat transfer plate 2, in turn fixed to a stainless steel plate 3 which forms the bottom of the vessel. There is a risk of the heating element 1 detaching from the heat transfer plate 2 and setting fire to the vessel body. This risk is not covered by the conventional practice of securing an element protector control to the heat transfer plate 2. To protect against this risk, an additional heat conducting plate 4 is fitted over the sheath of the element 1, and the centre 5 of the additional plate 4 is pressed against the centre of aluminium heat transfer plate 2. The additional plate 4 has a flat area 6, spaced from heat transfer plate 2 by an air gap 10 (fig.4). An element protector control (not shown), having bimetal actuators 7, is mounted in area 6, which normally sits at a temperature between those of the sheath 1 and the cooled central portion 5. By this means, the element protector control retains its responsiveness to a boil dry condition causing overheating of the heat transfer plate 2, and additionally is responsive to detachment of the heating element 1 from the heat transfer plate 2.

Description

IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS Field of the Invention: This invention concerns improvements relating to electric heating elements, particularly, though not exclusively, for use in electric kettles, hot water jugs, percolators and other water heating vessels.

Background of the Invention: Planar heating elements are increasingly popular as compared to the traditional immersion heating element and generally comprise two types, namely the thick film heating element which consists of a heater track or layer carried by an insulating substrate and the more conventional underfloor heating element which consists of a metal plate having a metal sheathed, mineral insulated, resistance wire heater affixed by one means or another, commonly by welding or brazing, to the underside of the metal plate.

Despite the hi-tech appeal of thick film heating elements, mechanical underfloor heating elements remain popular for electric kettles and hot water jugs for domestic use. Figure I of the accompanying drawings shows a typical form of underfloor heating element comprising an aluminiumsheathed resistance heating element 1 brazed or soldered to an aluminium heat transfer plate 2 which is in turn brazed or soldered to a stainless steel plate which, when the heating element is installed in a water boiling vessel, forms

the bottom of the internal water-containing compartment of the vessel.

Heating elements of this kind can have power outputs typically of 2 to 3KW. Other forms of underfloor heating element are known, such as for example that disclosed in EP-A-0 031 866 (Bleckmann).

We have extensively tested underfloor heating elements and have found that the element sheath can become detached locally from the heat transfer plate. this may be because of local contamination during the solder process, or subsequent mechanical damage. The initial length of detachment may be as small as a few millimetres. At the point where the sheath is no longer attached, the heat transfer is greatly reduced and the sheath temperature locally rises above that of the remainder of the sheath.

Differential expansion between the hot part of the sheath and the aluminium heat transfer plate, supported by the stainless steel, causes the sheath to bow away from the plate, which places the bond between the still-attached sheath and the plate under severe stress. As a result the bond may fail, bit by bit, extending the length of detachment. This effect can happen during normal use; each time the kettle is boiled the element locally overheats, placing further stress on the bond. Dry boil, namely switching on the heating element without first filling the vessel with water, also causes similar stress.

Over a period of time the length of detachment grows, and the temperature of the detached sheath, particularly in its central parts away from the surviving bond, rises until it reaches the melting temperature of the sheath.

At this point the element fails catastrophically, usually melting large parts of I I

the sheath (aluminium), which drips onto the appliance parts, which are normally polypropylene, setting them on fire. The resulting fire usually totally destroys a plastic bodied vessel and may also spread to the premises in which it is being used. Clearly undesirable.

Unfortunately conventional element protector controls such as our X4 and Strix's Ul7 controls cannot prevent this happening. They are normally mounted to the aluminium heat transfer plate and it is this that they are, in fact, protecting. Because the delamination of the element means that it no longer transmits as much heat into the heat transfer plate, so the element protector is unable to sense the local overheating of the sheath in the early stages of failure. In addition, during normal boiling the heat transfer plate is effectively cooled by the water in the vessel, running at about 105 C, and any local overheating in the region of the plate close to the failing bond is masked by this cooling effect in the parts of the plate between the point of failure and the bimetal actuator. As is well known, element protector controls have the function of switching off a heating element in the event of its temperature rising above a safe level.

Objects and Summary of the Invention: It is the principal object of the present invention to overcome or substantially reduce the abovementioned problem.

According to the present invention, an underfloor heating element comprising a resistance heating element affixed to a metal heat transfer plate

has affixed to the resistance heating element a thermally-conductive carrier for an element protector control, said carrier, between the control and the resistance heating element, being out of thermal contact with the heat transfer plate.

Differently stated, the present invention provides for an element protector control to be mounted in direct thermal contact with the resistance heating element of an underfloor heating element via heat transfer means which, as between the element protector and the resistance heating element, makes no direct thermal contact with the heat transfer plate of the heating element so as, in use of the heating element in a water boiling vessel for example, not to be subject to cooling by the water being heated.

Described hereinafter is an exemplary embodiment of the present invention wherein an underfloor heating element consists of a resistance heating element proper attached, with appropriate electrical insulation, to a heat transfer plate on one side thereof. According to the invention an additional heat transfer member, forming a mounting for an element protector control, is affixed to the resistance heating element proper on the side thereof opposite to the aforementioned heat transfer plate and as between the control mounting location and the resistance heating element proper, makes no direct contact with the heat transfer plate. By virtue of this arrangement, an element protector control affixed to the additional heat transfer member is responsive only to the temperature of the heating element proper and is insensitive to any

separation occurring between the heating element proper and the heat transfer I I

plate whereby the heat produced by the heating element proper is transferred to the water being heated.

Advantageously, for enhancing the responsiveness of the control to a dry boil situation, thermal contact is provided between the heat transfer plate and the additional heat transfer member on the side of the sensor of the control that is remote from the resistance heating element proper.

According to yet another aspect of the present invention, an underfloor heating element comprising a heating element proper and a heat transfer component thermally coupled thereto is combined with an element protector control such that, in use, the control is not subject to cooling of the heat transfer component by the liquid being heated in the thermal path between the heating element proper and the control.

The above and further features of the present invention are set forth in the appended claims and will become more clear from consideration of the following description of an exemplary embodiment given with reference to the accompanying drawings.

Description of the Drawings : Figure I is a perspective view of a conventional underfloor heating element described hereinbefore; Figure 2 is a perspective view, similar to Figure 1, showing an embodiment of the present invention;

Figure 3 is a top plan view of the underfloor heating element of Figure 2; and Figure 4 is a cross-sectional, perspective view of the underfloor heating element of Figures 2 and 3.

Detailed Description of the Embodiment: Figs 2,3 and 4 of the accompanying drawings illustrate the invention.

Figs 2 & 3 show a conventional sheathed underfloor heating element as described above with the invention added, and Fig 4 shows the same element in cross section. Features 1,2 and 3 are the same as on Fig 1. There is an additional heat conducting plate 4 which is fitted over the sheath of the heating element proper lover about half its length, roughly halfway between its ends, and which covers the area enclosed by the sheath. The centre 5 of the additional plate 4 is formed so that it is pressed against the centre portion of the aluminium heat transfer plate 2. Between this central portion of the additional plate 4 and the part which contacts the element sheath I is a flat area 6 to which an element protector such as the X4 may be mounted, with its bimetal actuators 7 in close thermal contact with this plate. This flat part 6 is spaced from the heat transfer plate 2 by an air gap 10 best seen in Fig 4. The additional plate 4 is secured to the element, for example by studs extending from the heat transfer plate as is conventional, so that the central part 5 is firmly pressed against the centre of the heat transfer plate 2, and the peripheral edge is firmly pressed agamst the top surface 8 of the element

sheath 1. Any type of suitable securement could be used, such as soldering or riveting. The figures do not show details of this securement for clarity. The periphery 9 of the additional plate 4 is preferably made resilient to ensure that a firm contact is made over a wide tolerance of element sheath forms.

In normal use the bimetal actuators sit at a temperature approximately halfway between the sheath temperature, typically 210 to 250 C for this type of element, and about 3000 for a Bleckmann type, and that of the cooled central portion (about 105 C). In the case of a dry boil, the central portion is no longer cooled, the element sheath temperature as a whole will rise, and the actuator can sense this and will disconnect the supply. In the case of a sheath bond failure, the temperature of the sheath and that of the additional plate 4 in close contact with it will rise. This temperature rise will be transmitted to the nearest of the bimetal actuators. As the failure progresses the local temperature becomes very high (in excess of 500 C), and the heated length extends, until the temperature sensed by the bimetal actuator is sufficient to trip it to disconnect the supply. This will happen even during normal boiling use of the kettle, since the central portion cooled by the water, is insufficient to mask the unsafe temperature rise. The response time in the case of this failure does not need to be as rapid as during a normal dry boil, so the delay in heat travelling from the failure point, through the additional plate 4 to the bimetal is acceptable. The normal effect of this protection would be that the kettle would start to switch off prematurely during boil. This effect would not be corrected by descaling (scale also causes premature switching). An

indicator could be provided to warn the user of this abnormal behaviour, so that the appliance can be discarded or returned to the shop. It is preferable that the element protector is of the non self resetting type, as allowing the kettle to cycle with this failure would ultimately cause the element to fail catastrophically.

The function has been described using a conventional X4 control, of which only the bimetal actuators are shown for clarity, and could be used in this way. Preferably the control would be of a special design which incorporated the additional plate as a form of chassis, providing a factory assembled heat transfer location between itself and the bimetal actuators, and requiring only simple direct assembly to the element at the appliance manufacturers. In this form the bimetal actuators would advantageously be secured and protected during transit by the additional plate. Alternative chasses could be provided to permit the use of the basic control on a range of different element types.

An important feature of this invention is the direct contact between the bimetal and a significant length of the element sheath via a heat transfer means which is not cooled, between the actuator and the sheath, by the water being boiled. The air space between the additional plate and the aluminium heat transfer plate gives thermal separation to allow the bimetal to sense when a part of the sheath has overheated. A contactstat directly in contact with one point on the sheath would only protect that part of the sheath, leaving the rest unprotected. The central portion of the additional plate being secured in a

heat transfer relationship with the water provides the necessary change of temperature to give a rapid response during dry boil. and allow the use of lower set bimetal actuators. Our experience shows that delamination is most likely at the hotter parts of the element, particularly at the apex of the horseshoe. The cooler parts near the cold connections are less stressed and unlikely to fail. Thus the additional plate 4 is preferably of a generally semicircular shape, and is arranged to cover the hot central portion of the horseshoe. The bimetal actuators of the X4 are arranged to one side of the control centreline, and conveniently sit on the additional plate at a position well suited to sense abnormal temperature rises on the plate's periphery. The thickness of the plate and the area of the centre of the plate arranged to be in contact with the heat transfer plate may be determined by experiment and thermal analysis to give the optimum combination of cost, dry boil performance and delamination protection.

Having described the invention in the foregoing by reference to a specific embodiment, it is to be well understood that the embodiment is exemplary only and that modifications and variations thereto will occur to the skilled reader and are possible without departure from the spirit and scope of the invention as set forth in the appended claims. For example, our X4 element protector control (described in W099/54903) and Strix's UI7 control (described in W095/34187) have been mentioned herein as appropriate controls for use in the practice of the present invention, but other suitable controls could be used in their place.

Claims

Claims:

1. An underfloor heating element having means providing for the mounting of an element protector control such that, in operation of the heating element, the control will be substantially insensitive to separation of the resistance heating element proper of the underfloor heating element from its heat transfer component.

2. An underfloor heating element as claimed in claim 1 wherein an additional heat transfer component is secured to the resistance heating element proper and serves as a mounting for the element protector control.

I-erein, as

3. An underfloor heating element as claimed in claim 2 wherein, as between the mounting location of the element protector control and the resistance heating element proper, no thermal contact exists between the firstmentioned and the additional heat transfer components.

4. An underfloor heating element as claimed in claim 2 or 3 wherein thermal contact is provided between the first-mentioned and the additional heat transfer components remotely from the heat transfer path between the resistance heating element proper and the mounting location of the element protector control.

5. An underfloor heating element substantially as herein described with I reference to Figures 2 to 4 of the accompanying drawings.

6. An underfloor heating element as claimed in any of the preceding claims in combination with an element protector control mounted thereto.

7. An electrically heated vessel incorporating a combination as claimed in claim 6.

8. An element protector control for use with an underfloor heating element, said control having affixed thereto a heat transfer component whereby said control may be affixed to the underfloor heating element with the heat transfer component in thermal contact with the heating element proper of the underfloor heating element.

9. An electrically heated vessel including an underfloor heating element in the form of a heating element proper secured to a heat transfer component, and an element protector control, and wherein, for protecting the vessel against detachment of the heating element proper from the heat transfer component, the element protector control is arranged to be responsive primarily to the temperature of the heating element proper rather than to that of the heat transfer component.

10. An electrically heated vessel including an underfloor heating element in the form of a heating element proper secured to a heat transfer component, an element protector control, and means defining a first heat transfer path between the heating element proper and a thermal sensor of the element protector control and a second heat transfer path between a thermal sensor of the element protector control and said heat transfer component.