Electric resistance heating devices – Heating devices – Immersion heater details
Patent
1993-06-24
1995-04-18
Walberg, Teresa J.
Electric resistance heating devices
Heating devices
Immersion heater details
219441, 392497, H05B 382
Patent
active
054085799
DESCRIPTION:
BRIEF SUMMARY
This invention concerns an electric element assembly for liquid heating purposes, particularly but not exclusively for an electric kettle.
Conventionally, an electric kettle element assembly, of the type with which the present invention is concerned, comprises a substantially flat metal backplate and a metal-sheathed resistance heating element which extends from and has its ends sealingly connected to the backplate, with a central portion of the sheath also in direct thermal transfer relation with the backplate. In use, the backplate is sealed relative to an aperture in the kettle wall, with the element extending into the liquid reception vessel.
The reason for having the central portion of the sheath in direct thermal transfer relation with the backplate is to effect rapid heat transfer through the backplate to a temperature sensitive cut-out (usually a bimetallic strip, but possibly a plastics strip) disposed at the other side in the event that the element is operated when not immersed in liquid, so as to cut off power supply to the element before too much damage is done.
The backplate and the element sheath are conventionally formed of copper or an alloy of copper, which are good heat conductors. However, stainless steel is increasingly popular since, despite having poorer conductivity, it has the advantage that it does not discolour over a period of use in boiling water.
Conventionally, the ends (legs) and central portion of the element sheath have been connected to the backplate by brazing. This involves applying a jointing or "brazing" material, such as a copper alloy, (in the manner of a glue) between the regions to be connected and then placing the entire subassembly in a furnace to heat all the material sufficiently to melt the brazing material, and form an effective joint. This would usually be done in two stages, firstly to connect the central element sheath portion, then to connect the legs. Brazing is imprecise, messy and costly and is best avoided as part of a manufacturing process. Moreover, in the case of stainless steel, higher temperatures are required to produce effective joints by brazing so a higher cost is involved in having to install, or adapt, and run a furnace for higher temperature operation in addition to the general difficulty of automating any brazing procedure.
In their earlier applications Nos. 91 02915 and 91 02883 the applicants set forth proposals for avoiding any brazing process in production of the relevant type of electric element assemblies by making special provision for welding a central region of the element sheath to the backplate and by connecting the ends of the element sheath to the backplate in various different ways.
The object of the present invention is to propose novel electric heating assemblies which avoid the need to connect a central portion of the element sheath to the backplate, by brazing or otherwise, by providing alternative means for actuation of a temperature sensitive cut-out.
According to a first aspect of the invention an electric element assembly for liquid heating purposes comprising an element, in the form of a resistance heating wire disposed within a metal sheath and connected to pins which project from respective ends of the sheath, is characterised by having temperature sensitive cut-out means mounted in thermal transfer relationship with the element sheath at one end thereof.
In practical embodiments in accordance with this first aspect of the invention the temperature sensitive cut-out means is conveniently connected to a sleeve or liner of thermally conductive material which is fitted into the element sheath in thermal transfer relationship with the inner surface of the sheath. Provision of such a sleeve or liner intermediate the sheath and the cut out means is especially desirable when the sheath itself is of steel and of relatively poor thermal conductivity.
Such a sleeve or liner may be a simple friction fit in the sheath or it may be spot welded to the sheath.
According to a second aspect of the invention an electric element ass
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patent: 2888547 (1959-05-01), Saper
patent: 3614387 (1971-10-01), Wrob
patent: 4044224 (1977-08-01), Jenkins
patent: 4697069 (1987-09-01), Bleckmann
patent: 4730099 (1988-03-01), Bleckman
patent: 5020128 (1991-05-01), Bleckmann
Sheathed Heating Elements Limited
Walberg Teresa J.
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