GB2048024A

GB2048024A - Immersion heater assemblies, for electric kettles

Info

Publication number: GB2048024A
Application number: GB8004836A
Authority: GB
Original assignee: DH Haden Ltd
Current assignee: DH Haden Ltd
Priority date: 1979-02-28
Filing date: 1980-02-13
Publication date: 1980-12-03

Abstract

An immersion heater assembly incorporates an element assembly in which a sheathed element device is secured to a pressed sheet metal head which is indented to provide one recess or two recesses 25 in an otherwise planar outside face 20 and the ends 26 of the sheath are secured through the indented part of the head so that brazing material is confined to the recess or recesses to avoid contamination of a hot spot 15 on the planar part of the outside face. The recess or recesses are large enough to accommodate surplus sealing material used to seal the ends 26. The planar part of the outside face is used to locate a switch assembly of the immersion heater assembly. <IMAGE>

Description

SPECIFICATION Immersion heater assemblies, for electric kettles Immersion heater assemblies, for electric kettles, are known which comprise an element assembly and a switch assembly which is actuable by means of a bimetal device to switch off or interrupt the electrical power supply to the element assembly if the latter overheats. The bimetal device is disposed so as to be in thermally conductive contact with an outside face of a head of the element assembly at a hot spot.which is heated by a median or intermediate portion of a tubular metal sheathed mineral insulated electrical resistance heating element device of the element assembly, which median or intermediate portion is secured to an inside face of the head.The head is usually formed from sheet metal so as to have a main panel, on which said faces are provided, surrounded by an abutment portion which is adapted to be clamped against an inside face of a wall of a vessel around an aperture in said wall so that the inside face faces the inside of the vessel and the outside face faces towards said aperture.

A seal is usually interposed between the abutment portion and the vessel wall. The end portions of the element device are inserted and brazed or soldered into holes provided in the main panel so as to leave cold tails of the element device projecting outwardly from the head so as to be connectable to the switch assembly.

It is necessary to ensure that the switch assembly acts at a temperature within a restricted range to interrupt the current. If the switch assembly acts at too low a temperature, difficulties arise when the kettle is used in an arm having a hard water supply, because the water produces a coating of minerals on the sheath thereby causing the hot spot to reach said temperature before the water has boiled. If the switch assembly acts at too high a temperature, the switch assembly-and/or the element assembly can be damaged by overheating.

The restricted nature of the temperature range requires the switch assembly and the bimetal device to be made with considerable accuracy so as to have a performance which is precise and consistent for a long period of time; and gives rise to problems in use if the performance deteriorates slightly.

It has been found that the performance of pretested bimetal devices and thus the switch assemblies can be irregular. Whilst simple cleaning of the outside face and the use of a heat sink paste reduced said irregularities, those due to poor geometrical alignment of the bimetal giving poor thermal contact between the bimetal device and the hot spot could not be removed, thus giving rise to overheating if the bimetal were set to act other than at the lower end of said range.

What is needed is an improved immersion heater assembly, for heating a liquid in a vessel, which assembly is of the kind comprising an element assembly and a switch assembly actuable by a bimetal device, wherein the element assembly comprises an element device and a head, wherein the head comprises a main panel preferably surrounded by an abutment portion, wherein the element device comprises an electrical resistance heating element extending within a tubular metal sheath, wherein end portions of the sheath extend through the main panel and are bonded to the main panel, and wherein the intermediate or median portion of the sheath is secured to an inside face, i.e. that face which in use faces towards the inside of the vessel to which the immersion heater assembly is fitted,to produce a hot spot on the main panel.

Such an assembly is hereinafter referred to as an "immersion heater assembly of the kind described".

According to the present invention there is provided an immersion heater assembly of the kind described wherein a major proportion of the main panel is planar; wherein the major proportion includes the hot spot; wherein the main panel is indented so that a recess or recesses is or are in an outside face, on the opposite side of the main panel to the inside face; and wherein said end portions of the sheath extend into said recess or recesses.

It has been found that such indenting of the main panel substantially prevents bonding materials, used to secure the end portions to the head, from spreading onto the inside face at the hot spot; that the plane of the inside face of said major proportion can be used as a datum for locating the bimetal device acurately; and that because the inside face of the hot spot is located in an unworked, planar, uncontaminated and extensive area of the main panel, extensive face to face contact is possible between the bimetal device and the hot spot. The area of the recess is preferably larger than the internal cross-sectional area of one, and preferably both, of-the end portions of the sheath, so that the recess or recesses can accommodate a substantial amount of a sealing material used to close the end portions around the cold tails.

A depth of indentation of about 0.7 to 1.5 mm has been found to be effective, but said depth may be greater.

The intermediate or median portion of the sheath is preferably secured to the inside face so as to be in thermally conductive contact with an area of said inside face which extends substantially across the main panel so as to provide an elongate form of hot spot which has a length greater than half of the overall width or diameter of said major proportion. The sheath may be of known circular cross section, so that the thermally conductive contact is made in a direct manner along a line and in an indirect manner at each side of the line via fillets of bonding, e.g.

brazing or soldering, material.

In a preferred embodiment most of the sheath has a circular cross section, and the intermediate or median portion of the sheath is partially flattened to produce a flat or substantially flat abutment area which is bonded face to face to a flat or substantially flat portion of the head to make thermal contact with the latter either directly or via a thin film of bonding material.

The switch assembly is preferably clamped to and located by said major proportion of the main panel so as to hold the bimetal device against the hot spot.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein Figure 1 shows in end elevation an immersion heater assembly of the invention with a socket housing and cover plate removed; Figure 2 shows a vertical cross section through the immersion heater assembly on a line corresponding to the line li-Il in Figure 1; Figure 3 shows a modified fbrm of a median or intermediate part of a metal sheathed heating element in vertical cross-section; Figure 4 shows in end elevation a head of an element assembly included in the heater assembly shown in Figure 1;; Figure 5 shows a transverse cross section through the head and some parts of the heating element on a line corresponding to the line V-V -in Figure 4; Figures 6 and 7 are views similar to Figures 4 and 5 showing a modified element assembly in end elevation and in cross section on the line VIl-Vil in Figure 6; Figure 8 is-a part sectional view, on the line VIll-VIll in Figure 1, of parts of the assembly; Figure 9 is an exploded perspective view showing some of the parts shown in Figure 3; Figure 10 shows the internal side of a carrier moulding of a switch assembly of the immersion heater, and Figure 11 is a cross sectional view, on the line Xl-Xl in Figure 10, of the carrier moulding and parts of the switch assembly.

The immersion heater assembly shown in the drawings generally comprises an electric kettle element assembly, a switch assembly, and an earth pin 9.

The element assembly comprises an element device 10, a metal support member 11 and a metal head 12, secured together so that the member 11 and cold tails 13 of the element device 10 project from an outside face of the head. The element device 10 comprises, in all embodiments, an element 1 OA inside a metal sheath 10B with insulation therebetween. An intermediate portion 14 of the sheath is secured to an inside face of the head 12 to produce a hot spot 15.

The head 12 comprises a main panel 20 surrounded by an annular abutment portion 21.

The portion 21 has a cylindrical extension 22 which is externaily threaded to receive a clamping ring 23 (Figure 2) of known form for clamping the wall 24 of a kettle and a seal 25 between the abutment portion 21 and the clamping ring 23 in known manner.

The support member 11 is of elongate form, is secured by brazing and by rivetting to a central part of the main panel 20 and is formed to serve as a socket for a fastener 1 8.

Referring to Figures 2, 4 and 5,.the main panel 20 is indented to produce two shallow recesses 25 in the outside face. The recesses 25 have a combined area of about 30 to 40% of the total area of the outside face so that a major proportion of the outside face is left planar or substantially planar and said total area is more than twice, e.g.

is approximately three times, the combined internal cross-sectional areas of the ends 26 of the sheath in this embodiment. The major proportion includes the area of the hot spot 1 5 as indicated in broken lines in Figure 4. The recesses 25 are located around the ends 26 of the sheath 1 OB which protrude through the main panel 20 and serve to accommodate, and, more importantly, restrict the spread of brazing or soldering material used to secure the sheath to the main panel, and the spread of the sealing material used to seal the ends 26 around the cold tails 13. Some of the brazing material is indicated by reference numeral 27, and some of the sealing material is indicated by reference numeral 27B.

The main panel 20A of the modified head shown in Figures 6 and 7 is similar to that shown in Figures 4 and 5 except in that the indenting is performed to provide a single recess 28, and in that the support member 11 A is secured to the recessed portion of the main panel. The single recess 28 has an area which is between four and six times the combined internal cross-sectional areas of the ends 26. The support member 11 A has a preformed head 29A and is secured by brazing, whereas a head 29 (Figure 2) is produced on the member 11 by rivetting. In both forms of main panel 20 and 20A the major proportion includes an area 30 of the outside face which is diametrically opposite part of the hot spot 1 5.

The intermediate portion 14 in Figures 2, 5 and 7 is of constant circular cross section so that, in use, a proportion of the heat generated by the element 1 OA is conducted to the hot spot partly through substantial fillets 31 (Figure 2) of brazing or soldering material, which fillets 31 lie alongside a line of contact between the portion 14 and the panel 20 and 20A which contact is direct or via a layer of said material. In order to increase said proportion or to increase the rate of conduction of said proportion, the portion 14 may be flattened, as shown in Figure 3, to produce a flat or substantially flat contact area 32 which is secured in face to face contact, either direct or via a thin film or brazing or soldering material, to the inside face of the main panel 20 or 20A.

The switch assembly comprises a spacer moulding 40 of heat resistant material, a carrier moulding 41 of electrical insulating plastics material, a bimetal device 42, and electrically conductive components including live and neutral pins 43 and 44, fixed and movable switch contact members 45 and 46, and live and neutral terminals 47 and 48. The device 42 is provided with a thrust member 49 of electrical insulating material. The spacer moulding 40 is shaped to fit closely within the extension 22; is apertured to receive to provide two ways (not shown) through which the thrust member 49 extends (Figure 2): and is shaped to provide a mounting 50 for the bimetal device 42.A part 40A of the moulding 40 abuts the area 30, and other parts of the moulding 40 abut the major proportion of the outside face above and at both sides of the- hot spot 15 to hold the mounting parallel with the hot spot. The bimetal device 42 comprises a dished circular snap acting bimetal 51 peripherally located on the mounting 50 by a cylindrical flanged metal retainer 52 which is a push fit around a peripheral surface of the mounting. The retainer 52 has lugs (not shown) which can be bent to embrace parts of spacer'moulding 40 to retain the bimetal device in position on the spacer moulding so that the bimetal abuts the hot spot.

The carrier moulding 41 is shaped peripheraliy to fit closely into the extension 22; has an exterior side (Figure 1) from which the pins 43 and 44 project; and has an interior side (Figure 1 0) which is shaped to inter-engage with the spacer moulding 40 in a predetermined mutual orientation and to receive the members 45 and 46 and terminals 47 and 48. Stepped openings 54 are provided in the carrier moulding to receive the cold tails 13, part cylindrically curved portions of the terminals 47 and 48 and fasteners 55, as shown in Figure 8. The carrier moulding is moulded to include short pegs 56 upon which the contact members 45 and 46 are located during assembly, the pegs 56 being heated under- pressure thereafter to form heads thereon to retain the contact members in position.Further pegs 57 are provided which are engaged in sockets (not shown) in the spacer moulding, which pegs may be headed after assembly of the switch assembly to retain the mouldings together. The live and neutral pins 43 and 44 have stepped hollow ended extensions 58 which extend through the carrier moulding from the exterior side and are rivetted or swaged so as to be secured to the movable contact member 46 and the neutral terminal 48, whereby to secure the latter and the pins 43 and 44 in position as shown in Figures 8, 10 and 11.

The earth pin 9 has a foot 60 which is clamped rigidly to the support member 11 by the fastener 18, so that the foot bears against the bottom of a recess 59 in the carrier moulding to urge the latter and the spacer moulding towards the head 12 so as to thrust the bimetal device against the hot spot 15, as shown in Figure 2, and so as to ciamp the fixed contact between the mouldings.

The fasteners 55 are in the form of longitudinally split and flanged cylindrical collars as shown in Figure 9, which are pushed onto the cold tails and terminals to clamp the terminals tightly onto the cold tails. If desired, the collars may be pressed down to assume a substantially torroidal form shown in Figure 8.

After assembly, the openings 54 and the recess 59 may be filled or sealed with a suitable resinuous plastics, bitumastic or like material, prior to a plastics closure plate 70 being clamped to the exterior side of the carrier moulding 41 by a moulded socket housing 72 which shrouds the pins 9, 43 and 44. The housing 72 is clamped in position by the ring 23 as shown in Figure 2. The plate 70 fits closely around the pins 9, 43 and 44, and is otherwise un-apertured.

In use, if the hot spot reaches a predetermined temperature, the bimetal moves rapidly from a convex to a concave shape, thereby displacing the thrust member so that the latter moves the movable contact member 46 to separate the contacts 61,62 (Figure 6) and break the electrical circuit from the live pin to the element.

It will be readily appreciated that the contacts operate in a closed or substantially closed chamber so as to be protected from dirt, which chamber is separated from the hot spot by the spacer plate and the bimetal device; that the Fvitch assembly can be assembled as a unit and the element assembly separately assembled as a unit; and that said units can be fitted together either before or after the element assembly is inserted into the kettle.

The invention is not confined to the precise details of the foregoing examples, and many variations are possible, within the scope of the invention. For example, other forms of bimetal, e.g. rectangular or elongate, or snap or progressively acting, can be used, and the fasteners 35 may be in the form of spring clips.

A sealing ring 73 may be interposed between the ring 23 and the housing 72 further to urge the switch assembly towards the outside face of the head. The pin 9 and the member 11 may be integral, and formed as an elongate earth pin which extends through the mouldings 25 and 41 and is secured to the head so that the switch assembly is held in position in the head by the ring 23 and housing 72 via the ring 73 and plate 70, but further securing means may be provided.

Alternatively, the housing 72 may be secured by fasteners to the carrier moulding, and be merely surrounded, but not held or clamped, by the ring 73. The bimetal 51 may be of stressed form, and the fastener 55 may be split, and not flanged, flanged and not split, or plain cylinders.

Claims

1. An immersion heater assembly, of the kind described, wherein a major proportion of the main panel is planar; wherein the major proportion includes the hot spot; wherein the main panel is indented so that a recess or recesses is or are formed in an outside face, on the opposite side of the main panel to the inside face; and wherein said end portions of the sheath extend into said recess or recesses.

2. An assembly as claimed in claim 1, wherein the area of the recess or recesses is larger than the internal cross-sectional area of one of said end portions of the sheath.

3. An assembly as claimed in claim 1, wherein each end portion of the sheath is surrounded by a respective one of the recesses and wherein each recess has an area greater than the internal crosssectional area of said end portion.

4. An assembly as claimed in claim 1, wherein the end portions of the sheath are surrounded by the recess, and wherein said recess has an area between four and six times the total internal cross-sectional area of said end portions.

5. An assembly as claimed in any preceding claim, wherein the total area of the recess or recesses is between 30 and 40% of the area of said outside face of the main panel.

6. An assembly as claimed in any preceding claim, wherein the depth of indentation is between 0.1 and 1.5 millimetres.

7. An assembly as claimed in any preceding claim, wherein the intermediate or median portion of the sheath is secured to the inside face so as to be in thermally conductive contact with an area of said inside face which extends substantially across the main panel so as to provide an elongate form of hot spot which has a length greater than half of the overall width or diameter of said major proportion.

8. An assembly as claimed in any preceding claim, wherein most of the sheath has a circular cross section, and the intermediate or median portion of the sheath is partially flattened to produce a flat or substantially flat abutment area which is bonded face to face to a flat or substantially flat portion of the head to make thermal contact with the latter either directly or via a thin film of bonding material.

9. An assembly as claimed in any preceding claim, wherein the switch is clamped to and located by said major proportion of the main panel so as to hold the bimetal device against the hot spot.

10. An assembly as claimed in claim 1, wherein the main panel is substantially as hereinbefore described with reference to and as shown in Figures 4 and 5 of the accompanying drawings.

11. An assembly as claimed in claim 1, wherein the main panel is substantially as hereinbefore described with reference to and as shown in Figures 6 and 7 of the accompanying drawings.

12. An immersion heater assembly having an element assembly substantially as hereinbefore described with reference to Figures 2, 4 and 5; or Figures 2, 3, 4 and 5; or Figure 2 as modified by Figures 6 and 7; or Figures 2 and 3 as modified by Figures 6 and 7 of the accompanying drawings.