GB2260070A

GB2260070A - Immersion element hot return mechanically mounted on head plate

Info

Publication number: GB2260070A
Application number: GB9120738A
Authority: GB
Inventors: Robert Andrew O'neill
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1991-09-30
Filing date: 1991-09-30
Publication date: 1993-03-31
Anticipated expiration: 2011-09-30
Also published as: GB9120738D0; GB2260070B

Abstract

A heating element 2, for use with e.g. jugs, pots, pans, kettles etc, is formed of stainless steel and, to avoid difficulties associated with the brazing of stainless steel, has a hot return portion 5 of the element proper secured to an element head portion 1 by virtue of the head portion being formed with two spaced-apart deep drawn pockets 3 and 4 defining therebetween a recess into which the hot return portion of the element proper is fitted in mechanical and close thermally coupled engagement with the element head portion. <IMAGE>

Description

IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS This invention concerns improvements relating to electrically powered heating elements, and more particularly concerns electrically powered heating elements for use with water heating appliances such as jugs, pots, pans, kettles, laboratory equipment and the like and which comprise a sheathed wire-wound element proper which is mounted to an element head portion serving for mounting the element in an appliance, the element proper having a looped configuration with a so-called hot return portion which engages the wet side of the element head to enable the element temperature to be sensed and controlled by an appropriate control provided on the dry side of the element head and further having socalled cold tails (or terminations) which extend sealingly through the element head to enable power to be supplied to the heating element from the dry side of the head.

Electrically powered heating elements of the above-described type are ,well knawn, particularly as used in electric kettles and hot water jugs. Controls for use with such heating elements are also well known and include element overtemperature protection controls arranged to sense the temperature of the hot return portion of the element proper, for example by provision of a bimetallic sensor at an appropriate location in thermal contact with the dry side of the element head, and to switch off the supply of power to the element in the event of the element overheating for example because of being switched on without sufficient water in the appliance or because of the appliance being allowed to boil dry.Other controls are known which serve to switch off or reduce the supply of power to the heating element when water boils in the appliance, such controls operating most commonly by sensing the generation of steam by means of a bimetal, though rate-sensitive controls responsive to the change in the rate of temperature rise which accompanies the achievement of boiling have also been proposed.

Heating elements of the subject type have commonly been made of brass, generally with an anticorrosive plating on the sheath of the element proper, mainly because brass is a relatively ductile material which can be formed easily into the necessary convoluted shapes and because no difficulties arise as regards ensuring effective thermal transfer from the hot return part of the element proper to and through the element head and to an accompanying element protector control as abovementioned. More particularly, brass has a good thermal conductivity and no difficulties arise in brazing the brass sheath of the element hot return portion to the brass head plate of the element.On the other hand, however, the good thermal conductivity characteristics of brass result in the seals that are conventionally provided between the element head plate and the appliance wall being subject to higher temperature stresses than are desirable from a viewpoint of seal cost and seal longevity. To overcome this problem, proposals have been made to form the element or at least the head portion of the element from stainless steel which, as is well known, has a significantly lesser thermal conductivity than brass. These proposals give rise to difficulties as regards the efficiency of heat transfer from the hot return portion of the element through the head plate to and accompanying element protector control and as regards the brazing of the hot return portion to the wet side of the element head.Attempts have been made to reduce these problems by the obvious expedients of using a thinner gauge of stainless steel for the element head than is conventionally used for a brass head and of using a composite head construction in which brass is used to achieve good thermal conduction and stainless steel is used to shield the seals, but these approaches are not particularly satisfactory. Other proposals have been to use plastics material for the element head with thermally conductive implants provided to ensure the requisite degree of thermal transfer through the element head, but again this is not regarded as satisfactory.

We have made strenuous efforts to find a solution to the well known problems of conventional stainless steel heating elements as described above and, faced with the apparently insurmountable problem of achieving a satisfactory brazed joint between the hot return portion of the element proper and the element head and being aware of the inadequacies of such proposals as have already been made, we have conceived the alternative solution of relying upon a wholly mechanical engagement of the hot return portion with the element head. According to the present invention, therefore, the head portion of an electrical heating element as above described has a formation adapted for mechanical and close thermally coupled engagement with the hot return portion, and the hot return portion is engaged with such formation.The invention is particularly applicable to heating elements formed of stainless steel, but could be used for heating elements formed of other materials such as brass for example.

In an embodiment of the invention which will be described in detail hereinafter, a stainless steel element head manufactured by a deep drawing process has two spaced-apart pockets which define between them a recess shaped for nesting engagement with the hot return portions of the element and advantageously arranged to receive the hot return portion in a snapfit manner. For sealing the cold tail ends of the heating element proper to the head portion, the embodiment in question has deep drawn tubular formations which can accommodate O-ring seals and can be mechanically crimped around the ends of the heating element proper to provide mechanical support. Tests that we have conducted to measure the rate of thermal transfer through a stainless steel heating element as thus constructed have found that it compares favourably with a conventional brazed element construction.

Conventional stainless steel elements have employed austenitic stainless steel and, in accordance with a further aspect of the present invention, it is proposed instead to utilise ferritic stainless steel which has similar corrosion resistance properties, a thermal conductivity some 50% greater, and may be deep drawn more easily to form the desired shapes.

The invention will best be understood from consideration of the following detailed description of an exemplary embodiment which is illustrated in the accompanying drawings wherein: Fig. 1 is a cross-sectional view vertically through an embodiment of kettle element according to the present invention; and Fig. 2 is a view from the underside of the kettle element of Fig. 1.

In the figures the element head is shown at 1, and the heating element proper at 2. The element head 1 is formed by means of a deep drawing process, similar to that used to form a conventional element head. In the present case, however, two pockets 3 and 4 are formed in the position in which it is required that the hot return 5 of the element 2 shall be located. The exact shapes of the pockets 3 and 4 in plan view is not critical, but on the wet side of the head 1, namely the side nearest to the element proper 2, the profile in side view desirably should be such as to fit closely around the hot return 5 for at least 40% of its periphery, and preferably at least 50%.

Figure 2 shows a suitable shape in plan view for the pocket 3.

On the dry side of the head 1, namely the side away from the element proper 2, the form of the head is arranged to conform to the thermally sensing features of the element protector control that is to be associated with the element. This is not shown on the figures, but the shape of the element head as shown in Figure 1 conforms to the shape of the bimetal and blade carrier of the X1 control manufactured and sold by us which is substantially as described in GB 2194099 with reference to Figs. 3A, 3B and 3C thereof.

In particular, the part 6 which is closest to the hot return 5, is arranged so that it will be in direct contact with the lower two of the four feet of the X1 bimetal carrier, in order to provide rapid response during the secondary operation of the X1, when an X1 control is fitted to the element head.

In order to provide mechanical support, and possibly also enhance thermal response, it is proposed to deform the pockets 3 and 4 around the hot return 5 after assembly, although it is possible that sufficient resilience can be achieved to allow a snapfit assembly.

Referring to the upper part of Figure 1, this shows the means by which the element cold leads 7 are sealed against water leakage and are mechanically supported. From the material of the head 1 a tube 8 is deep drawn which is of reduced diameter at the point 9 at which it joins the body of the head 1 as compared to its larger diameter at its other end 10.

The reduced diameter part 9 makes a close sliding fit on the cold tail sheath 7, while the larger diameter part 10 allows the fitting of a sealing O-ring 11 between the tube 8 and the cold tail 7. A mechanical deforming or crimping process may be applied to the larger diameter part 10 on assembly to retain the 0ring 11 in position and to mechanically support the cold tail 7.

While the foregoing description is intended for the simpler manufacture of stainless steel elements, the same approach could be applied to elements constructed of other materials as has been previously mentioned herein.

Having thus described the present invention by reference to a particular exemplary embodiment, it is to be well appreciated that the invention is not restricted to the embodiment described and that modifications and variations thereto will occur to those possessed of relevant skills without departure from the spirit and scope of the invention.

Claims

CLAIMS:

1. An electrical immersion heater element comprising an element head portion and an element proper and wherein the element proper has a hot return portion which is secured to the element head portion by virtue of the element head portion having a formation mechanically engaged with the hot return portion of the element proper.

2. An electrical immersion heater element as claimed in claim 1 wherein the element head portion has two spaced-apart formations defining therebetween a recess for nesting engagement with the hot return portion of the element proper.

3. An electrical immersion heater element as claimed in claim 2 wherein the two spaced-apart formations comprise pockets formed in the element head portion by a deep drawing process.

4. An electrical immersion heater element as claimed in claim 2 or 3 wherein the two spaced-apart formations define a recess arranged to receive the hot return portion of the element in a snap-fit manner.

5. An electrical immersion heater element as claimed in claim 2 or 3 or 4 wherein the two spaced-apart formations provided in the element head portion are deformed about the hot return portion of the element proper.

6. An electrical immersion heater element as claimed in any of the preceding claims wherein the hot return portion of the element proper fits closely to the element head portion for at least 40%, and preferably at least 50%, of its periphery.

7. An electrical immersion heater element as claimed in any of the preceding claims wherein the element proper has end portions which are sealed to the element head portion by virtue of the element head portion having tubular formations receiving therebetween the end portions of the element proper.

8. An electrical immersion heater element as claimed in claim 7 wherein the tubular formations incorporate seals.

9. An electrical immersion heater element as claimed in claim 7 or 8 wherein the tubular formations are crimped to the respective end portions of the heating element proper.

10. An electrical immersion heater element as claimed in any of the preceding claims wherein the element head portion is formed of stainless steel.

11. An electrical immersion heater element as claimed in claim 10 wherein the element head portion is formed of ferritic stainless steel.

12. An electrical immersion heater element substantially as herein described with reference to the accompanying drawings.