GB2262207A

GB2262207A - Thermal connection of heating element to temperature sensitive switch

Info

Publication number: GB2262207A
Application number: GB9223799A
Authority: GB
Inventors: Andrew Hunt; John Gordon Bamford
Original assignee: Sheathed Heating Elements Ltd
Current assignee: Sheathed Heating Elements Ltd
Priority date: 1991-11-26
Filing date: 1992-11-13
Publication date: 1993-06-09
Anticipated expiration: 2012-11-13
Also published as: GB9125038D0; GB2262207B; GB9223799D0

Abstract

The invention provides an improved heating element assembly in which a tubular heating element 12 is made of a material of relatively low thermal conductivity and is thermally connected to bi-metallic strip 20 acting as a temperature sensitive switch. In order to improve the thermal contact, between the element and bi-metal strip, a tubular copper insert 24 is provided within the element 12. The element is then held in a C-shaped clip 22 provided at one end of clamping arm 16 which extends through a vessel wall or plate 14 to contact the bi-metallic strip 20. Thermal contact is improved as the copper insert 24 serves to conduct heat to the clip 22 from a large axial length of element - the copper tube preferably has a longer axial length than that of the C-shaped clip 22. <IMAGE>

Description

IMPROVED HEATING ELEMENT ASSEMBLY This invention relates to an improved heating element assembly, especially of the type used in kettles or similar liquid heating vessels or appliances.

The use of heating elements in liquid heating'vessels is widespread and one problem with their performance is the necessity to provide a cut-out in the event of all the liquid in the vessel being boiled away. As would be readily understood in such circumstances the electric current to the heating element should be cut off as quickly as possible in order that damage to the element and/or the vessel does not occur.

One way of accomplishing this is to arrange for the heating element to be in thermal contact with a bi-metallic strip or other temperature sensitive switch. In such an arrangement when the heating element is no longer in contact with a liquid media the heat generated thereby will raise of the element's temperature which temperature increase will be noted by the bi-metallic strip and hence will cut off current to the element avoiding an overload and possible damage.

Usually such thermal contact has been provided by brazing the element to a supporting plate which is in turn in contact with the temperature sensitive switch. The relatively recent change to the use of stainless steel heating element has provided problems with such construction as steel is not easily brazed in comparison to the previously used copper heating elements. Additionally, stainless steel is a poorer heat conductor than copper and therefore better thermal contact is required between the heating element and the temperature sensitive switch in order to provide correct operation therefrom. Thus although it is possible to use the prior known constructions with a stainless steel element these constructions are more difficult to construct and expensive than heretofore known with copper elements.

It would be advantageous to provide an improved heating assembly element in which these problems are overcome.

An aspect of the invention therfore provides an improved heating element assembly adapted for location inside a heating vessel in which there is improved thermal contact between the heating element and a temperature sensitive switch.

A first aspect of the invention provides an improved heating element assembly comprising a tubular heating element adapted for location inside a liquid heating vessel and thermally contacting a temperature sensitive switch characterised in that the tubular heating element is made of a material having a relatively low thermal conductivity and is provided with a tubular insert at least in the region of the heating element which is thermally connected to the temperature sensitive switch, the tubular insert being constructed of a material having a relatively high thermal conductivity.

In this way the copper tube will serve to increase the effective area of thermal loading by conducting heat from other areas of stainless steel element into that which is connected indirectly to the temperature sensitive switch.

This construction is especially useful where the tubular heating element is constructed of stainless steel and the insert is of copper. Additionally this construction will improve the performance of known prior art heating element assemblies by increasing the effectiveness of thermal contact between the heating element and the temperature sensitive switch. Thus the construction thereof does not have to be so precise and the thermal connection between the element and the switch can be manufactured to greater tolerance.

Preferably the temperature sensitive switch is a bimetallic strip or similar device operating to cut power to the heating element in the event of the element itself having a substantially increased temperature.

In a preferred embodiment of the invention there is further provided a clamping arm to attached at one end to the element, which arm passes through a thermally insulating wall such that its other end contacts the temperature sensitive switch.

It will thus be understood that the clamping arm provides a thermal bridge between the heating element and the temperature sensitive switch, for example a bi-metallic strip.

Thus the arm may be made of a material having a relatively high thermal conductivity such that heat generated by the element will be readily conducted to the temperature sensitive switch for operation thereof.

In an advantageous modification of the invention the clamping arm has a C-shaped clip arrangement at one end into which the heating element is located. Advantageously the Cshaped clip's opening is located substantially in line with the axis of rotation of the arm.

It will be understood that provision of a C-shaped clip arrangement provides easy and positive location of the heating element. Additionally the intimate engagement with the Cshaped clip will provide good thermal contact therewith so that heat generated by the element will be readily conducted along the clamping arm. By providing the opening substantially in line with the arm the strength of the clamping arm is not impaired by the configuration and pressure exerted to locate the element is along the common axis.

Preferably the C-shaped clamp is arranged so that the clamping arm connects therewith relatively above the engagement with the element. this way the clamping arm will be uncovered by the fluid when the element is also uncovered thus providing a good heat conduction and limiting the cooling effect of the fluid on the clamping arm.

Advantageously a rubber seal is provided in the insulating wall and around the arm so as to thermally insulate the arm from the remainder of the wall. This wall will normally be formed of a metal or similar material and will usually be an integral part of the heating element construction.

The invention will be described further by way of example with reference to the accompanying drawings in which: Fig. 1 is a sectional side elevation of a preferred embodiment of the invention; and Fig. 2 is a sectional elevation along the line II-II of Fig. 1.

Referring firstly to Fig. 1, a first preferred embodiment of the invention provides an improved heating element assembly referred to generally by the reference numeral 10 of the type commonly used for a liquid heating vessel such as a kettle or a similar container. The heating element assembly 10 includes a tubular heating element 12 which is commonly of stainless steel and arranged in a coiled or looped configuration depending on the space available within the vessel and the required length of element. The heating element 12 is provided with electrical contacts (not shown) to an electric power source (not shown) which provides electric current to the element 12 causing an increase in its temperature. This temperature increase is transferred by conduction from the element to the surrounding liquid media in order to raise its temperature and/or boil the liquid.

One problem associated with the construction of such elements is the potential for damage to the element and to the container where electric current is supplied to the element causing an increase with temperature in the absence of a surrounding liquid media. In order to prevent such an occurrence the element 12 is commonly arranged to be thermally connected to a temperature sensitive switch, for example a bimetallic strip 20. Thus when the temperature increase in the element causes operation of the temperature sensitive switch the current to the heating element 12 will be cut off and further heating will be prevented.

In order to improve the thermal contact between a stainless steel element 12 and the associated bi-metallic strip 20 the invention provides a clamping arm 16, one end of which is in contact with the bi-metallic strip 20. The arm 16 passes through a wall 14 which can be a metal plate arranged in the vessel or formed as an integral part of the heating element assembly 10. In order to insulate the arm from the plate an annular rubber seal 18 is provided therearound.

The clamping arm 16 is provided at one end with a Cshaped clip 22 adapted to permit intimate engagement with the tubular heating element 12. This intimate engagement provides a good thermal contact between the clamping arm 16 which is preferably made of a material having a relatively high thermal conductivity and the stainless steel element 12.

However, as stainless steel has a relatively low thermal conductivity of the order of 3% of that of copper it is necessary to attach the bi-metallic strip to a relatively long length of conductor in order to provide an effective contact area.

In order to reduce this area required the invention provides a tubular copper insert 24 arranged within the stainless steel element 12. As can be seen more clearly in Fig. 2 the copper insert 24 has a substantially longer axial length than that of the C-shaped clip 22. Thus the clip 22 is effectively thermally coupled to a longer length of stainless steel element 12 than it is directly in contact with. The copper tube serves to conduct heat to the C-shaped clip from a large axial length of element.

The invention is not confined to the foregoing details and variations may be made thereto. For example the tubular copper insert can be used in conjunction with any stainless steel element and need not be used with the C-shaped clamping arm as described. Additionally the C-shaped clamping arm need not be of the configuration illustrated and variations are possible. For example, the arm could be substantially Ushaped with the element 12 located from relatively above the arm. Alternatively the arm could be of a two-part configuration fitting substantially entirely around the element at the point of contact with assembly required.

In order to provide improved sensitivity to heat build up it is advantageous to arrange the clamping arm so that it is uncovered by the liquid to be heated at the same time or further preferably before the element itself is uncovered. In this way the cooling effect of the liquid on the clamping arm which would reduce the heat conducted to the temperature sensor is removed. Other variations are also possible.

Claims

1. An improved heating element assembly comprising a tubular heating element adapted for location inside a liquid heating vessel and thermally contacting a temperature sensitive switch characterised in that the tubular heating element is made of a material having a relatively low thermal conductivity and is provided with a tubular insert at least in the region of the heating element which is thermally connected to the temperature sensitive switch, the tubular insert being constructed of a material having a relatively high thermal conductivity.

2. An assembly as claimed in claim 1 in which the insert is of copper.

3. An assembly as claimed in claim 1 or claim 2 in which the temperature sensitive switch is a bi-metallic strip operating to cut power to the heating element in the event of the element itself having a substantially increased temperature.

4. An assembly as claimed in any of claims 1 to 3 in which the element is attached to one end of a clamping arm, which arm passes through a thermally insulating wall such that its other end contacts the temperature sensitive switch.

5. An assembly as claimed in claim 4 in which the clamping arm has a C-shaped clip arrangement at one end into which the heating element is located.

6. An assembly as claimed in claim 5 in which the C-shaped clip's opening is located substantially on a common axis with that of the arm.

7. An assembly as claimed in claim 5 or claim 6 in which the C-shaped clamp is arranged so that the clamping arm connects relatively above the level of engagement with the element.

8. An assembly as claimed in any of claims 4 to 7 in which a rubber seal is provided in the insulating wall and around the arm so as to thermally insulate the arm from the remainder of the wall.

9. An assembly as claimed in any of claims 5 to 8 in which the tube of relatively high thermal conductivity material is provided within the heating element at least in the region which is to be connected to the clamping arm.

10. An improved heating element assembly substantially as hereinbefore described with reference to and as illustrated in Fig. 1 and in Fig. 2 of the accompanying drawing.