GB2164530A - Improvements relating to electric immersion heaters - Google Patents

Abstract

The ends of a heater element 1 are sealed into a housing 2 by non-metallic seals 34 thereby allowing the housing to be made of plastic. In order to allow brazing of the element 1 to a pocket 17 containing a thermostat cut-out in a kettle, the pocket is of sufficient length that the brazing of the element thereto will not damage the ends of the element sealed into the housing of the kettle. <IMAGE>

Description

SPECIFICATION Improvements relating to electric immersion heaters This invention relatesto electric immersion heaters ofthe kind in which a sheathed wire electric heating element is secured to a housing which accommodates a thermally actuated cut-out device connected between the element and an electrical supply. The device is arranged to be responsive to a predetermined rise in temperature of the heating elementandto respond and de-energise the heater and thus protect it against the effects of overheating. Such immersion heaters will hereinafter be referred to as the kind specified.

Immersion heaters ofthe kind specified are widely used in domestic electric kettles, the housing beingprovided with means by which the heater is sealably supported in an aperture in a wall ofthe kettle body.

immersion heaters utilized in this way are particularly prone to mis-use and it is, therefore, essential to arrange the thermally actuated cut-out device so that it becomes effective as soon as possible when the element overheats. This can be achieved by providing the closest possible thermal relationship between a thermally responsive part of the cut-out device and the elementorthe heated liquid, andtothis end itis convenient to accommodate the thermally responsive part in a pocket provided on one wall ofthe housing the pocket being arranged to extend in the same general direction as the element so asto contact a part ofthe element.

It is usual for a partofthe elementto be brazed to the outside ofthe pocket, and the ends ofthe element have also been brazed to the housing where they project through it so as to ensu a water-tight seal.

The brazing processes have, however, meantthat the sealing of the ends ofthe element against moisture penetration can only be satisfactorily effected afterthe element has been fixed to the housing, as the common sealing techniques employed hitherto cannotwithstand the temperatures attained during such processes.

This is inconvenient and it is thus an object of the present invention to provide an electric immersion heater of the kind specified in which the element can be securedto the housing in aquickand easy way.

Accordingly, the invention provides an electric immersion heater ofthe kind specified in which the ends ofthe element are sealed to the housing by non-metallic seals.

In a preferred embodiment of the invention, the thermally responsive part ofthe cut-out device is secured within a pocket which extends away from the housing in the direction of the element and the length ofthe pocket is sufficient to ensure that brazing the elementtheretowill notdamagetheseals.

The non-metallicseals may be eithersilicone rubber '0' rings or epoxy resin or possibly thermoplastic. By avoiding the need to braze the ends ofthe elementto the housing the latter may itself be made of plastic and the pocket, which may be made of brass or other suitably thermally conducting material, may also be sealed to the housing by a non-metallic seal.

Thus in accordance with another aspect of the invention in an immersion heaterofthekind specified, having a pocket ofthermally conducting material for accommodating the thermally responsive part ofthe iut-out device brazed to the heating element, both the pocket and the ends ofthe heating element are sealed to the housing by non-metallic seals. Moreover, the wall ofthe housing through which the ends ofthe element extend can in some cases itself be formed of a plastics material.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure lisa plan view of an immersion heater suitable for use in a kettle, Figure 2 is a sectional side view taken on the line ll-ll in Figure 1 but drawn to a largerscalethanthatfigure, and Figure 3 is a sectional viewtaken on the linelll-lll in Figure 2.

Referring to the drawings, the immersion heater comprises a sheathed wire element 1 mounted on a cylindrical housing 2, the two adjacent ends ofthe element entering the housing through bosses 9 in its end wall 3 and being secured thereto in a fluid-tight manner. Aterminal lead from one of the ends ofthe element 1 extends th rough the housing 2 and is connected to a terminal pin 4 provided on an end plate 5 ofthe housing 2 by means of a conductive strip 31, the end plate 5 being formed from a suitable electrically insulating material such as a plastics material and being formed integral with the sidewall 7,for example, by means of a suitable moulding process.The end wall 3 which is also of a plastics material is sealed to the sidewall 7 by a silicone rubber washer8 but may alternatively be moulded integral with the wall 7. Aterminal lead 32 from the other end of the element 1 is connected by welding to a switch contact button 10, which is connectable to a second terminal pin 12 on the end plate 5 byway of a leaf spring contact 11. The leaf spring contact 11 and the contact button 10 form a cut-outfor disconnecting an electrical supplyfrom the element 1.

The leaf spring contact 11 is engaged by an arm 14 of an insulating material which is formed integrally with a push rod 15 having one end slidably mounted in an aperture in the plate 5 and which at its opposite end engages a thermally responsive part ofthe cut-out device (shown generally at 16). The thermally responsive part ofthe cut-out device is housed in atubular pocket 17 which is sealed to the end wall 3 ofthe housing 2 by a silicone rubber'0' ring 32 and extends in the same general direction as the element 1 to contact an active portion of the element at its opposite end which is closed by a circular end wall 18. The end wall 18 ofthe pocket 17 is attached to the adjacent portion ofthe element 1 by brazing in order to ensure good thermal contact between the element and the pocket.

The thermally responsive part 16 ofthe cut-out comprises a U-shaped bimetallic element 19 comprising a base and two arms which move away from one another with rising temperature. The arms ofthe bimetallic element each have a D-shaped aperture 20 towards the end thereof, which apertures receive a head of the push rod 15 and hold it againsttheforce exerted by a helical compression spring 22, so that the spring contact 11 and contact button 10 (Figure 2), complete an electrical circuit to the element 1. The bimetallicelement19 issupported on a slotted cylindrical guide 23 into which the head of the push rod 15 is a sliding fit The cylindrical guide 23 is a free fitwithin the pocket 17 and is held in the pocket by means of a resilient member 24 of stainless steel .

Athird terminal pin 13 extendsthrough the end plate 5to the end wall 3 where it is riveted in place and a conducting strap 36 is connected between the pin 13 and the pocket 17 to provide an earth connection for the element.

The tubular pocket 17 is deep enoughtthatthe brazing ofthe element 1 to its end wall 18 will not damagethe'O' ring seal 33. Clearly, normal operation ofthe elementwill also not damage the seal 33 since the normal operating temperature is considerably below the temperature necessary for brazing.

This reduction in temperature atthe end wall 3 of the housing enables the ends ofthe element 1 which are secured to the housing through bosses 9 to be sealed thereto in a fluid4ight manner simply by providing silicone rubber'O' rings 34 in the bosses 9.

Accordinglysince brazing of the ends ofthe element to a metallic head, in accordance with standard practice hitherto, is no longer necessary, it is possible to seal the ends ofthe element before attachmentto the head. The ends of each element are shaped to provide a lead-in bush 35which has a groove therein for receiving the '0' ring 34 which is housed in a similar groove on the inside wall ofthe respective boss 9. Thus,the element may be secured in a fluid-tight mannerto the housing using non-metallic seals by simply pressing the ends ofthe element into the bosses 9.

Theelementcanthen be brazedtothe pocket17 without damaging the seals because the end wall 18 of the pocket is far enough awayfrom the end wall 3 of the housing forthe heatto be dissipated before it damages the seals.

Claims (9)

1. An electric immersion heater of the kind specified in which the ends ofthe element are sealed to the housing by non-metallic seals.

2. An electric immersion heateraccording to Claim 1, wherein ihethermally responsive part ofthe cut-out device is secured within a pocketwhich extends away from the housing in the direction ofthe element.

3. An electric immersion heater according to Claim 2, wherein the pocket is of metal and has the element brazed to its end,the length ofthe pocket being sufficientto ensure that brazing the element thereto will notdamagetheseals.

4. An electric immersion heater according to Claim 1,2 or 3, wherein the non-metallicseals are either silicone rubber '0' rings or epoxy resin orthermoplastic.

5. An electric immersion heater according to Claim 4, wherein the housing comprises a hollow cylindrical enclosure and an end wall through which the ends of the heating element extend, the enclosure being of plastics material.

6. An electric immersion heater according to Claims 2 or3, wherein the pocket is sealed to the housing by a non-metallic seal.

7. An electric immersion heater according to any preceding Claim, wherein the wall ofthe housing through which the ends ofthe element extend is formed of a plastics material.

8. An electric immersion heatersubstantiallyas herein described with reference to Figures 1 to 3 ofthe accompanying drawings.

9. A domestic kettle incorporating an electric immersion heater according to any previous claim.