GB2044590A

GB2044590A - An electrical element assembly for an electric kettle

Info

Publication number: GB2044590A
Application number: GB8004837A
Authority: GB
Original assignee: DH Haden Ltd
Current assignee: DH Haden Ltd
Priority date: 1979-02-28
Filing date: 1980-02-13
Publication date: 1980-10-15

Abstract

An element assembly, for an electric kettle, has a hot spot 21 which is heated rapidly in the event of the kettle boiling dry to actuate a switch quickly to prevent damage by overheating. The element sheath 14 is shaped to provide a flat abutment area 32 which is brazed face to face with the hot spot via a thin film of brazing material; the pitch of the element wire is reduced in the zone of the hot spot; the hot spot is raised from the head; and the element is cranked, for rapid heating of a bimetal device 24 for actuating the switch. <IMAGE>

Description

SPECIFICATION An electrical element assembly for an electric kettle An electrical element assembly for an electric kettle.

Electric kettles are known which comprise an electrical element assembly and an electrical control switch unit, which element assembly and switch unit are secured together whereby to clamp a wall of the kettle therebetween, and whereby to bring a switch actuating bimetal device of the switch unit into contact with a hot spot projection on a head or boss of the element assembly. The element assembly comprises a tubular metal sheat which surrounds a mineral insulated resistance heating element, and has cold tail portions which extend through and are secured to the head or boss. Cold tails of the element extend from said portions to contact the switch unit.An intermediate portion of the sheath is bent to about that face of the head or boss which faces away from the switch unit so as to bridge a depression formed in said face by the hot spot projection, and the sheath is connected to the hot spot projection by filling the depression with silver solder or a like brazing or soldering material.

In such known electric kettles it is necessary to provide a switch unit which responds over a very restricted temperature range to switch of the element in the event of the element assembly overheating. If the switch unit 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 unit acts at too high a temperature, then the switch unit and/or the element assembly can be damaged by overheating. The restricted nature of the temperature range requires the bimetal device, in particular, to be made with extreme accuracy; makes the switch units relatively expensive; an gives rise to problems in use if, in the long term, the performance of the bimetal device varies slightly.

An object of the invention is to enable said restricted temperature range to be increased.

According to the present invention there is provided an electrical element assembly, for use in an electric kettle or the vessel for heating liquids, comprising a tubular metal sheath which surrounds a mineral insulated electrical resistance heating element, and comprising a head or boss formed from sheet metal so as to have a depression in one face thereof and a corresponding raised projection on the opposite face thereof; wherein end portions of the sheath extend through and are secured to the head or boss; wherein cold tails of the element project from said end portions; wherein an intermediate portion of the sheath is shaped to lie across said one face; and wherein part of said intermediate portion is further shaped to enter into said depression, and is provided with a shaped abutment area which is bonded face to face to a floor portion of said depression by means of welding, soldering or brazing so as to contact said floor portion either directly or via a thin film of soldering or brazing material, so that the raised projection serves as a hot spot.

Said area is preferably of at least the same number of square millimetres as is the diameter of the sheath in millimetres.

The element is preferably of helical form within the sheath having a substantially constant number of turns per centimetre of length over most of the length of the element, with the number of turns per centimetre being increased at least along said part of the intermediate portion.

The invention also provides an electrical element assembly, for use in an electric kettle or the vessel for heating liquids, comprising a tubular metal sheath which surrounds a mineral insulated electrical resistance heating element, and comprising a head or boss formed from sheet metal; wherein end portions of the sheath extend through and are secured to the head or boss; wherein cold tails of the element project from said end portions; wherein an intermediate portion of the sheath is shaped to lie across one face of said head or boss; wherein part of said intermediate portion bonded to said one face to produce a hot spot on the head or boss; and wherein the element is of helical coil wire form having a constant number of turns per centimetre of length over most of the length of the element, and a greater number of turns per centrimetre along at least said part of the intermediate portion.

The invention further provides an electric kettle comprising the element assembly of the invention in combination with an electric control switch unit having a bimetal device held against said hot spot.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 shows in part vertical section, part of an electric kettle provided with an element assembly of the invention; Figure 2 is a part sectional plan view of an intermediate portion of an element sheath and a head of the element assembly; Figure 3 shows a vertical cross-section through the parts of the element assembly shown in Fig. 2; and Figure 4 shows a horizontal section through one of the parts of the element assembly shown in Fig. 2.

Referring to Figs. 1 to 3, the kettle has a body 10, a rear wall 11 of which is provided a large aperture. The element assembly 1 2 comprises a wire element 1 3 surrounded by a tubular metalsheath 14, and comprises a pressed sheet metal head 1 5. Fasteners 17, only one of which is shown, are brazed or welded to the head 1 5 to enable the head and a switch unit (not shown) disposed outside the kettle to be secured together to clamp a gasket 1 8 between a rim 1 6 of the head and an abutment of the switch unit so that the gasket 1 8 embraces the wall 11 around the large aperture.

The head 1 5 is pressed so as to have on the face which confronts the switch unit, namely the outside face 20, a raised projection 21, and in the opposite face 22 a corresponding depression. Two holes are provided in the head, and an earthing tongue 23 of stiff but flexible form in welded or brazed to the rim 1 6 to abut firmly the wall 11, A bimetal device 24 is secured to the switch unit so as to be held against the projection 21.

End portions 25 of the sheath 1 4 extend through the holes in the head, and are brazed or welded to the head, so that cold tails 26 of the element project outwardly from the head to contact the switch unit.

Referring particularly to Figs. 2 and 3, an intermediate portion 30 of the sheath 14 is bent to lie across the face 22 of the head, and a part 31 of the intermediate portion is further shaped so as to enter into the depression. The part 31 is partially flattened to produce a flat abutment area 32 which abuts a flat floor portion 33 of the depression, and the area 32 and portion 33 are secured face to face by brazing in such a manner that little, if any, of the brazing material 34 lies between said area 32 and the portion 33, so that the projection 21 serves as a hot spot.

As shown in Fig. 4 the element 1 3 is of helically coiled wire form having a constant pitch along most of its length of a finer increase the heating of the portion 30, and especially the area 32, relative to the rest of the sheath. This enables the temperature at which the switch unit reacts to be further increased. The pitch may be finest within the part 31 so that the number of turns per centimetre is greatest within this part 31.

Experiments have indicated that the period needed for a particular switch unit to react to a thermal overload on the element assembly is reduced by about two seconds, and thereby the switch unit switches the element off earlier to reduce the overheating of the element as compared with a known element assembly.

Furthermore, a switch unit modified to react at a higher temperature than said particular unit when used in combination with the element assembly of the invention still reacted with sufficient speed to restrict the overheating of the element to less than that of a known form of element assembly in combination with said particular switch unit.

The invention is not confined to the precise details of the foregoing example and many variations are possible. For example, the element 1 3 may be of helical form having a constant pitch.

Other modifications are possible. For example, the face 22 may be brazed to the intermediate portion 30; welding or silver soldering may be used instead of brazing; the area 32 and portion 33 may have any suitable complementary shapes and may be in direct contact or make contact via a layer or film of material having a high thermal conductivity and a maximum thickness of 1 mm and preferably less than 0.5mm. The intermediate portion 30 may extend completely across the face, and may be in contact therewith or proximal thereto. The hot spot may be coplanar with the rest of the outside face 20.

The area 32 may have an area which is at least 5 square millimetres per kilowatt at which the element is rated, and is preferably at least 10 square millimetres per kilowatt, for example 1 5 to 30 square millimetres per kilowatt.

Claims

1. An electrical element assembly, for use in an electric kettle or the vessel for heating liquids, comprising a tubular metal sheath which surrounds a mineral insulated electrical resistance heating element, and comprising a head or boss formed from sheet metal; wherein end portions of the sheath extend through and are secured to the head or boss; wherein cold tails of the element project from said end portions; wherein an intermediate portion of the sheath is shaped to lie across one face of said head or boss; wherein part of said intermediate portion is bonded to said one face to produce a hot spot on the head or boss; and wherein the element is of helical wire form having a constant number of turns per centimetre of length over most of the length of the element, and a greater number of turns per centimetre along at least said part of the intermediate portion.

2. An assembly as claimed in claim 1, wherein said part is flattened to produce a flat abutment area which abutment area is bonded face to face to said one face so as to be in contact therewith either directly or via a thin film of soldering or brazing material.

3. An electrical element assembly, for use in an electric kettle or the vessel for heating liquids, comprising a tubular metal sheath which surrounds a mineral insulated electrical resistance heating element, and comprising a head or boss formed from sheet metal; wherein end portions of the sheath extend through and are secured to the head or boss; wherein cold tails of the element project from said end portions; wherein an intermediate portion of the sheath is shaped to lie across one face of the head or boss; and wherein part of said intermediate portion is provided with a shaped abutment area which is bonded face to face to said one face by means of welding, soldering or brazing so as to contact said one face either directly or via a thin film of soldering or brazing material.

4. An electrical element assembly, for use in an electric kettle or the vessel for heating liquids, comprising a tubular metal sheath which surrounds a mineral insulated electrical resistance heating element, and comprising a head or boss formed from sheet metal so as to have a depression in one face thereof and a corresponding raised projection on the opposite face thereof; wherein end portions of the sheath extend through and are secured to the head or boss; wherein cold tails of the element project from said end portions; wherein an intermediate portion of the sheath is shaped to lie across said one face; and wherein part of said intermediate portion is further shaped to enter into said depression, and is provided with a shaped abutment area which is bonded face to face to a floor portion of said depression by means by welding, soldering or brazing so as to contact said floor portion either directly or via a thin film of soldering or brazing material, so that the raised projection serves as a hot spot.

5. An assembly as claimed in claim 3 or 4, wherein said abutment area is flat.

6. An assembly as claimed in claim 3 or 5, wherein the abutment area is of at least the same number of square millimetres in area as the diameter of the sheath in millimetres.

7. An assembly as claimed in claim 2, 5 or 6, wherein the abutment area has an area of at least 10 square millimetres per kilowatt rating of the element.

8. An assembly as claimed in claim 7, wherein said area is 1 5 to 30 square millimetres per kilowatt.

9. An assembly as claimed in claim 4 or any one of claims 4 to 7 as appended to claim, 5, wherein the element is of helical form within the sheath having a substantially constant number of turns per centimetre of length over most of the length of the element, with the number of turns per centimetre being increased at least along said part of the intermediate portion.

1 0. An assembly as claimed in any one of claims 2 to 9, wherein said thin film is less than 0.5 millimetres thick.

11. An electrical element assembly substantially as hereinbefore described with reference to Figs. 1 to 3, or Figs. 1 to 4, or Fig. 4 of the accompanying drawings.

1 2. An electrical kettle comprising an assembly as claimed in any preceding claim in combination with an electrical control switch unit having a bimetal device held against the hot spot.