GB2042857A - Immersion heater assemblies for electric kettles - Google Patents

Abstract

A heater assembly for an electric kettle comprises an element assembly 10 and a switch assembly held together by a fastener which also secures an earth pin 9 in position. The switch assembly comprises a moulded thermal and electrical insulating member 50 which is secured to an electrical insulating member 41 which carries all the electrical components. The member 50 serves to locate a bimetal 51 and a push rod 49 so that the bimetal is clamped against a hot spot 15 of the element head. The head is recessed to accommodate any surplus bonding or sealing material to leave the hot spot uncontaminated. If element assembly 10 overheats bimetal 51, via push rod 49, separates contacts 45, 46. <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 inside 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 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 very 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 area 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.

Some of the known switch assemblies are assembled progressively by hand to mount individual switch and bimetal component parts onto a moulded body or onto the element assembly. Such progressive assembly requires considerable amounts of skilled labour to make the components accurately to perform a relatively large number of fastening operations and to test and adjust the performance of the assemblies. In order to reduce the need for skilled labour and to reduce the time needed to assemble the switch assemblies, it has been proposed to utilise snap acting bimetal devices and modify many of the switch components so that they form inserts which can be loaded into dies, in which the moulded bodies are formed by a moulding machine, for producing bodies incorporating such components.However, the time taken to load the inserts into the dies severely limits the rate of production of the moulding machine; and many of the bimetals and switch assemblies are often found to operate outside the restricted temperature range and have to be rejected.

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 to interrupt afelectrical connection between the switch assembly and the element assembly, wherein the element assembly comprises an element device and a head, wherein the head comprises a main panel peripherally 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, andwherein an 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 the switch assembly comprises a spacer moulding and a carrier moulding; wherein electrically conductive switch components of the switch assembly are mounted on the carrier moulding; wherein the spacer moulding is clamped between the carrier moulding and an outside face of the main panel of the head so as to hold the bimetal device against the hot spot, and to provide a thermal shield between the main panel and said carrier moulding together with said switch components; and wherein a thrust member extending between the bimetal device and said switch components through the spacer moulding.

The immersion heater assembly preferably comprises an earth pin, and the element assembly preferably comprises an elongate support member which projects from the outside face; the earth pin being secured to the elongate support member so as to urge the carrier moulding and the spacer moulding towards the outside face so that the bimetal device abuts the hot spot.

Said electrically conductive switch components preferably comprise switch contact members located upon one side of the carrier moulding, live and neutral pins secured through the carrier moulding, and live and neutral terminals located upon the carrier moulding so that one of said terminals is connected to one of said live and neutral pins and the other of said terminals is connected to one of said switch contact members.

The connections between the live and neutral pins, the live and neutral terminals and the switch contact members are preferably made at said one side of the carrier moulding by rivetting or swaging to as to secure the live and neutral pins to the carrier moulding.

In a preferred form a major proportion of the main panel is planar and includes the hot spot; and the main panel is indented to that a recess or recesses is or are formed in the outside face, on the opposite side of the main panel to the inside face, and 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 of 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 accurately; 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 element preferably had cold tails which extend through the spacer moulding and are clamped to the live and neutral terminals.

The area of the recess or recesses is preferably larger than the internal cross sectional area of one of the end portions of the sheath, so that the recess or recesses are sufficient to accommodate a substantially amount of a sealing material used to close the end portions around the colt tails.

A depth of indentation of about 0.7 to 1 .5mm 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 particularly 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 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 Il-Il in Figure 1; Figure 3 shows a modified form 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 imersion 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 1 OB 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 1 5.

The head 1 2 comprises a main panel 20 surrounded by an annular abutment portion 21.

The portion 21 has a cylindrical extension 22 which is externally 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 particularly, restrict the spread of brazing or soldering material used to secure the sheath to the main panel, and 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 number 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 area of the ends 26. The support member 1 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 21 A 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 and thereby increase the upper limit of the aforementioned restricted temperature range, 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 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 abut the major proportion of the outside face above and at both sides of the hot spot 1 5 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 peripherally 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 10) which is shaped to interengage 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 riveted 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 1 2 so as to thrust the bimetal device against the hot spot 15, as shown in Figure 2, and so as to clamp 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 contact operate in a closed or substantially closed chamber so as to be protected from dirt and accidental damage, which chamber is separated from the hot spot by the spacer plate and the bimetal device; that the switch assembly can be assembled as a unit without requireing expensive tools or skilled labour in a simple and convenient manner from pre-tested or inspected components; that the units so formed are relatively robust so that the units can be easily handled and stored; that the units can be fitted to the element assemblies either before or after the element assemblies are fitted to the kettles; that the elements can be tested independently of the switches if a kettle fails, by removing the plate 70 to provide access to the cold tails; that it is made difficult for a user to remove or tamper with the unit without structurally damaging the assembly; and the spacer moulding protects the carrier moulding so that the switch assembly is resistant to damage by overheating and thereby allows the upper limit of the aforementioned restricted temperature range to be increased to broaden the range.

The invention is not confined to the precise details of the foregoing example, and many variation 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 fasteners may be split, and not flanged, flanged and not split, or a plain cylinder.

Other improvements may be incorporated in the immersion heater assembly of the invention.

For example, the element 1 OA may be of known constant pitch helical coil wire form so as to heat the sheath 1 OB evenly along its length between the ends 26. Alternatively the pitch of the element 1 OA may be constant except for a portion of finer or reduced pitch within the intermediate portion 14 so that the turns of the wire are closer together whereby to increase the rate of heating of the intermediate portion 14. This will increase the rate of heating of the hot spot so as to reduce the time needed for the bimetal device to actuate the switch in the event of the kettle boiling dry.

Claims (12)

1. An immersion heater assembly of the kind described wherein the switch assembly comprises a spacer moulding and a carrier moulding; wherein electrically conductive switch components of the switch assembly are mounted on the carrier moulding; wherein the spacer moulding is clamped between the carrier moulding and an outside face of the main panel of the head so as to hold the bimetal device against the hot spot, and to provide a thermal shield between the main panel and said carrier moulding together with said switch components, and wherein a thrust member extending between the bimetal device and said switch components through the spacer moulding.

2. An assembly as claimed in claim 1 including an earth pin, and wherein the element assembly compresses an elongate support member which projects from the outside face; the earth pin being secured to the elongate support member so as to urge the carrier moulding and the spacer moulding towards the outside face so that the bimetal device abuts the hot spot.

3. An assembly as claimed in claim 1 or 2, wherein said electrically conductive switch components comprise switch contact members located upon one side of the carrier moulding, live and neutral pins secured through the carrier moulding, and live and neutral terminals located upon the carrier moulding so that one of said terminals is connected to one of said live and neutral pins and the other of said terminals is connected to one of said switch contact members.

4. An assembly as claimed in claim 3, wherein the connections between the live and neutral pins, the live and neutral terminals and the switch contact members are preferably made at said one side of the carrier moulding by rivetting or swaging so as to secure the live and neutral pins to the carrier moulding.

5. An assembly as claimed in claim 1, 2, 3 or 4, wherein a major proportion of the main panel is planar and includes the hot spot; and the main panel is indented so that a recess or recesses is or are formed in the outside face, on the opposite side of the main panel to the inside face, and said end portions of the sheath extend into said recess or recesses.

6. An assembly as claimed in claim 5, wherein the area of the recess or recesses is larger than the internal cross sectional area of one of the end portions of the sheath.

7. An assembly as claimed in claim 5, wherein the recess or recesses are from 0.7 to 1.5mm deep.

8. An assembly as claimed in claims 3, 4, 5, 6 or 7, wherein the element has cold tails which extend through the spacer moulding and are clamped to the live and neutral terminals.

9. 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.

10. An assembly as claimed in any preceding claim wherein most of the sheath has a circular cross section, and wherein the intermediate or median portion of the sheath is particularly 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.

11. An assembly as claimed in any preceding claim, wherein the spacer and carrier mouldings are secured together to form, together with the bimetal device and switch components, a unit.

12. An assembly as claimed in any preceding claim, wherein the element is of helically coiled wire form, the pitch of the wire being reduced within said intermediate portion of the sheath.

1 3. An immersion heater assembly substantially as hereinbefore described with reference to Figures 1 and 2; Figures 1,2 and 3; Figures 1,2, 4 and 5; Figures 1, 2, 6 and 7; or Figures 1, 2, 8, 10 and 11 of the accompanying drawings.