WO2005092159A2

WO2005092159A2 - Heaters for liquid heating vessels

Info

Publication number: WO2005092159A2
Application number: PCT/GB2005/001160
Authority: WO
Inventors: Michael James Scott; Martin Charles Critchley
Original assignee: Strix Limited
Priority date: 2004-03-22
Filing date: 2005-03-22
Publication date: 2005-10-06
Also published as: CN1933752A; WO2005092159A3; CN2738161Y; GB0406428D0

Abstract

A heater (2) for a liquid heating vessel comprises a base plate (4) for forming at least part of a base of the vessel. A generally arcuate sheathed heating element (6) is attached directly to the underside of the base plate (4). A heat diffusion member (10) is in good thermal contact with the base plate (4) and in good thermal contact with and arranged to extend only part way around a side wall of the element sheath (6). The heat diffusion member (10) further defines at least one region (12) for receiving a thermal sensor for detecting overheating of the element (6).

Description

Heaters for Liquid Heating Vessels This invention relates to heaters suitable for forming at least part of the base of a liquid heating vessel, particularly, but not exclusively a water boiling vessel. In recent years the cost of producing heated bases for liquid heating vessels has come down but there remains the need to reduce production costs even further. The Applicant has realised that an important element of the cost of such heaters is the aluminium diffuser plate which is normally provided between the base plate of the heater and the actual element . Such diffuser plates have traditionally been considered necessary in order to spread heat from the element across the base and in particular to ensure a sufficient flow of heat to thermal sensors to allow them to operate in the event that the heater overheats - e.g. by being operated dry. It is an object of the present invention to reduce the cost of producing plate heaters without significantly adversely effecting their performance of safety. When viewed from a first aspect the present invention provides a heater for a liquid heating vessel comprising a base plate for forming at least part of a base of said vessel, a generally arcuate sheathed heating element attached directly to the underside of said base plate and a heat diffusion member in good thermal contact with said base plate and in good thermal contact with and arranged to extend only part way around a side wall of the element sheath, said heat diffusion member further defining at least one region for receiving a thermal sensor for detecting overheating of the element . Thus it will be seen by those skilled in the art that in accordance with the invention a heat diffusion member is located only to the side of the element sheath and extending part way around it. The Applicant has discovered that such arrangements are sufficient to spread heat across the base to the required degree and to ensure proper operation of overheat sensors without requiring that the heat diffusion member extends across the whole of the heated portion of the base nor under the actual element . This means a reduction in the total area of the heat diffusion member and hence a corresponding reduction in material costs. The diffusion member need not be a single integral piece but could comprise two or more parts which may or may not touch one another. The heat diffusion member could be located partly or entirely radially outwardly of the sheathed element. In presently preferred embodiments however it is located inside the element such that it contacts the radially inner side wall thereof. This is the most convenient arrangement for the Applicant's current series of controls and enables its size to be minimised. The angular extent of the heat diffusion member is less than that of the actual element in accordance with the invention set out above. Typically, a sheathed heating element will have an angular extent of approximately 270° and thus it is preferred that the angular extent of the heat diffusion member in accordance with the present invention is less than 270°, preferably less than 240° and most preferably between approximately 200° and 220°. Any appropriate shape of heat diffusion member may be employed. For example, the heat diffusion member could be in the shape of a circle with a chord or an annular segment removed. Preferably, however, the heat diffusion member is broadly C-shaped. This has been found to be beneficial in allowing an optimum balance between the angle through which the element sheath and the heat diffusion member are in contact and the total surface area of the heat diffusion member. Indeed, the Applicant has appreciated that where a broadly C-shaped heat diffusion member is provided, i.e. where the heat diffusion member partly surrounds an area of the plate radially inwardly thereof which is left uncovered by the heat diffusion member, the advantages described above may still be achieved even if the angular extent of the heat diffusion member were to be increased to equal or exceed the angular extent of the sheathed element . Thus when viewed from a further aspect the invention provides a heater for a liquid heating vessel comprising a base plate for forming at least part of a base of said vessel, a sheathed heating element attached directly to the underside of said base plate and a heat diffusion member in good thermal contact with said base plate and arranged around and in good thermal contact with a side wall of the element sheath so as at least partially to surround an area of the base plate inwardly thereof in which no heat diffusion member is provided, said heat diffusion member further defining at least one region thereof for receiving an overheat sensor. It will be appreciated by those skilled in the art that this aspect of the invention covers both 0-shaped and C-shaped heat diffusion members. However, these descriptions should not be taken too literally as it is not essential for example that the radially inner uncovered area of the base plate includes the centre of the base plate, although this is preferred. As set out previously the heat diffusion member could be radially outward of the element in which case the recited area which is free of diffusion member might include the element. It is preferred however that the diffusion member is inside the element and so contacts the radially inner side wall of the element sheath. In preferred embodiments of the invention, the heat diffusion member is broadly C-shaped. In such embodiments, it is preferred that at least one and preferably both circumferentially opposite ends of the heat diffusion member are wider than the rest in order to accommodate overheat sensors, e.g. bimetallic actuators . In preferred embodiments of the invention the heat diffusion member is symmetrical about a centre line. This enables it to be formed of two separate parts of identical shape. This is beneficial since it means that smaller identical parts may be pressed, stamped or cut etc. which simplifies production of the heat diffusion member and therefore reduces the product cost further. In particularly preferred embodiments, the shape of each half of the heat diffusion member is such that they are at least partially tessellating. This allows usage of a greater proportion of a metal blank from which the diffusion member pieces are cut. In accordance with all of the aspects of the invention set out above, there is at least one area of the base plate inwardly of the element sheath that is not covered by the heat diffusion member. This uncovered area is preferably provided with one or more bosses for mounting control and/or connector components are located in this area. Preferably, all such bosses are located in this area. This is beneficial since it allows the element sheath, the heat diffusion member and the bosses all to be attached to the base plate, e.g. by brazing, at the same time although this is not essential . In some preferred embodiments the sheathed element is received in a recess (when viewed from the underside) formed in the base plate. This is advantageous in helping to locate the element prior to brazing. It also increases the area of contact between the sheathed element and the base plate which helps to improve thermal transfer from the element to the liquid during normal heating operation. This gives a corresponding reduction in the running temperature and hence thermal stress on the element . The heat diffusion member could be received in the recess too. Preferably however it is outside of the recess such that it contacts the element at a different vertical part of the wall of the sheath. For example, the heat diffusion member could be approximately flush with the element. In a particularly preferred embodiment the recess conforms to the shape of the element. This maximises the two advantages set out above but also the corresponding raised portion on the liquid-facing side of the heater facilitates the selective application of a coating thereto - e.g. a noise reduction coating as disclosed in GB-A-2386532 - without the need to use a mask. The selective application of a noise-reduction coating to the part of the heater above the element is beneficial in reducing dry boil interference as is explained in our co-pending application no. GB 0403954.1. A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 is a perspective view of the underside of a heater in accordance with the present invention; Fig. 2 is a plan view of a metal blank strip from which the heat diffusion member parts are stamped out; and Fig. 3 is a cross-sectional view, with enlarged detail portion, of another embodiment of the invention. Turning to Fig. 1, the heater comprises the Applicant's standard Sure Seal base plate 2 which is circular and which is provided with a peripheral channel 4 to allow it to be clamped to a vessel wall as is described in greater detail in WO 96/18331. The heater in Fig. 1 is shown upside down as compared to its orientation in normal use. An arcuate sheathed heating element 6 of a type well-known in the art is provided on the under side of the base plate 2 radially inwardly of the peripheral channel 4. The heated portion of the element 6 extends through approximately 270°, i.e. three quarters of a circle and is terminated at either end by electrical terminations 8 known as "cold tails". Also brazed to the underside of the base plate 2 is a heat diffusion member 10. The outer profile of the heat diffusion member 10 conforms to the inner profile of the sheathed element 6 and indeed the edge of the heat diffusion member 10 is brazed to the radially inner wall of the element sheath so as to be in good thermal contact with it as well as being in good thermal contact with the base plate 2. The overall shape of the heat diffusion member 10 is broadly C-shaped and it extends around approximately 210° of the sheathed element 6. The angularly opposite ends of the heat diffusion member 10 are enlarged to form pads 12 for receiving the two bimetallic actuators of the Applicant's standard TJ17 or U18 control unit. The shape of the heat diffusion member 10 is such that it partly surrounds an area of the base plate 14 inwardly thereof which is uncovered. Three mounting bosses 16 for the Applicant's standard controls are located in the uncovered area 14. Rather than being formed as a single piece, the heat diffusion member 10 is in fact formed in two halves 10a, 10b which are mirror images of one another. As may be seen from Fig. 2, the shape of the two halves 10a, 10b is such that when one of them 10a is inverted, it tessellates with the other part 10b. This allows the heat diffusion member halves 10a, 10b to be laid out in an efficient manner on the metal blank 16 from which they are stamped, thereby minimising wastage of material . To construct the heater shown in Fig. 1, the heat diffusion member halves 10a, 10b are stamped out from the aluminium sheet 16, one of the halves 10a is turned over and they are then placed on the underside of the previously formed heater base plate 2 along with the sheathed element 6 so that the two halves 10a, 10b of the heat diffusion member abut the inner wall of the element 6 and each other. Braze material is provided between the heat diffusion member 10 and the wall of the sheathed element 6 as well as between each of these and the base plate 2 and the element 6 and heat diffusion member 10 are then brazed onto the base plate 2. The bosses 16 may be brazed on at the same time or before or afterwards. Alternatively they could be spot-welded on. In normal use of the heater, heat will flow mainly from the element 6 directly through the base plate 2 and into the water on the other side. This will, however, be supplemented by lateral flow into the heat diffusion member 10 and then through the base plate 2 into the water. In the event that the heater is energised without being in contact with water, the majority of heat will instead flow laterally into the heat diffusion member 10 and so into the thermal sensor pads 12 thereof. This will cause the bimetallic actuators (not shown) in thermal contact with the pads 12 to operate and thereby disrupt power to the element to prevent damage. Thus, although the heat diffusion member 10 is significantly smaller and therefore less expensive to produce than conventional heat diffusion plates, safe and effective operation of the heater is not impaired. Fig 3 depicts another embodiment of the invention. This embodiment is the same as that of Fig. 1 except that the base plate 22 includes a recessed portion 24 which accommodates and is the same shape as the element 6. This aids location of the element 6 during manufacture and improves heat transfer from the element to the base plate 22 and hence to the water during operation. The corresponding raised portion on the water-side allows for the easy application of a noise- reduction coating 26 just in the region of the element 6. This limits production costs and helps to prevent dry-boil interference. It will be appreciated by those skilled in the art that the heat diffusion member depicted is only an example and there are many possible shapes which could be used to achieve a similar benefit.

Claims

Claims :

1. A heater for a liquid heating vessel comprising a base plate for forming at least part of a base of said vessel, a generally arcuate sheathed heating element attached directly to the underside of said base plate and a heat diffusion member in good thermal contact with said base plate and in good thermal contact with and arranged to extend only part way around a side wall of the element sheath, said heat diffusion member further defining at least one region for receiving a thermal sensor for detecting overheating of the element.

2. A heater for a liquid heating vessel comprising a base plate for forming at least part of a base of said vessel, a sheathed heating element attached directly to the underside of said base plate and a heat diffusion member in good thermal contact with said base plate and arranged around and in good thermal contact with a side wall of the element sheath so as at least partially to surround an area of the base plate inwardly thereof in which no heat diffusion member is provided, said heat diffusion member further defining at least one region thereof for receiving an overheat sensor.

3. A heater as claimed in claim 1 or 2 wherein the heat diffusion member is inside the element and so contacts the radially inner side wall of the element sheath.

4. A heater as claimed in any of claims 1 to 3 wherein the heat diffusion member has an angular extent of less than 270°.

5. A heater as claimed in claim 4 wherein the heat diffusion member has an angular extent of less than 240'

6. A heater as claimed in claim 5 wherein the heat diffusion member has an angular extent of between approximately 200° and 220°.

7. A heater as claimed in any preceding claim wherein the heat diffusion member is broadly C- shaped.

8. A heater as claimed in claim 7 wherein at least one and preferably both circumferentially opposite ends of the heat diffusion member are wider than the rest in order to accommodate overheat sensors .

9. A heater as claimed in any preceding claim wherein the heat diffusion member is symmetrical about a centre line.

10. A heater as claimed in claim 9 wherein the heat diffusion member is formed of two separate parts of identical shape.

11. A heater as claimed in claim 10 wherein the shape of each half of the heat diffusion member is such that they are at least partially tessellating.

12. A heater as claimed in any preceding claim wherein the sheathed element is received in a recess, when viewed from the underside, formed in the base plate.

13. A heater as claimed in claim 12 wherein the recess conforms to the shape of the element.

14. A heater as claimed in any preceding claim comprising at least one area of the base plate inwardly of the element sheath that is not covered by the heat diffusion member, said area being provided with one or more bosses for mounting control and/or connector components .