ELECTRICAL HEATING ELEMENTS

Abstract: A heating element assembly comprises a sheathed heating element (4) having a hot return portion (14) brazed onto a head (6). The head (6) has a rib (22) formed on the front face of a dimple (16) which acts as a mounting location (18) for a bimetallic actuator (20). The element (4) has an increased watts density in the hot return portion (14) relative to the rest of the element, and the rib (22) acts to offset the actuator mounting location (18) from the hot return (14).

Description: ELECTRICAL HEATING ELEMENTS This invention relates to electrical heating element assemblies for use particularly as immersion heaters in water heating vessels such as kettles and hot water jugs. Traditionally, electric heating element assemblies of this type comprise a tubular metal sheathed element which is secured to a metallic plate-like element head which is mounted to overlie, and is sealed with respect to, an opening in a wall or base of the vessel. The element sheath is usually of copper, and it is brazed to the head at both its end portions, where it extends through the head to project by a small amount on the dry side of the head. So-called cold tails extend from the ends of the sheath for connection to an electrical supply. The element is also formed so that a "hot return" portion, which is a portion intermediate the ends, is bent back against the head and brazed thereto. In use, a thermally-sensitive actuator of a control, normally a bimetallic actuator, is arranged in thermal contact with the side of the head opposite the hot return i.e. the dry side, so that should the element overheat, for example when the appliance is switched on dry or boils dry, the temperature rise in the hot return portion of the element is conducted through the head to the actuator which, at a predetermined temperature, operates the control to interrupt the power supply to the element and thus de-energises the element. When elements of the above type are used in hard water areas, mineral scale is deposited on the element sheath and over the element head. Because of the heat insulating effect of the scale on the element, if the operating temperature of the bimetallic actuator in the control is set too low, the control could nuisance trip in normal operation, since more heat will be transmitted through the head to the actuator and the operating temperature of the actuator may be reached before the water in the vessel boils. Conversely, however, the operating temperature of the actuator must not be set so high that in a situation where the element is switched on without any water in the vessel (a "dry switch on" situation) , the element overheats before that temperature is reached. Thus the operating temperature lies within an empirically determined range. The Applicants have discovered that a further factor must also be taken into consideration when setting the operating temperature of the actuator. Even when the water heating vessel is substantially empty, the scale formed on the head acts as a sponge, drawing up water from the bottom of the vessel. This acts to cool the head adjacent the hot return when the element is switched on dry, due to evaporation of the water in the scale. This "wet scaling" means that in a dry switch on condition the temperature of the head adjacent the actuator is lower than might be expected, which means that it will take longer for it to reach the nominal actuator operating temperature. In the existing copper sheathed elements, it is believed that wet scaling does not cause problems in practice, since scale is deposited over both the element sheath and the head, and since the element sheath material is highly thermally conductive. These factors mean that the element as a whole will heat up relatively uniformly, so that even as water is evaporated from the wet scale on the head, the rest of the element stays at about the same temperature. Thus while it might take longer for the head to reach the operating temperature of the actuator, it will reach that temperature before some other part of the element overheats dangerously. Overheating of the element is potentially very dangerous, since the element may explode or may cause the vessel to melt. However, it is now becoming more common to replace copper elements with stainless steel elements, since they do not require cosmetic plating, and are easier to clean. It has been found with these elements that scale does not form on the element sheath but still forms on the head. In a wet scale condition with such elements, due to the low thermal conductivity of the sheath, relatively little heat is conducted back into the head to evaporate the water in the wet scale. Accordingly, the head stays relatively cool, while the remainder of the element overheats which is, as mentioned above, potentially dangerous. It is not possible to overcome this problem simply by lowering the operating temperature of the actuator, since nuisance tripping will occur during normal operation. Nor is it possible simply to increase the power input to the head through the hot return, since again nuisance tripping will occur. The present invention provides, from a first aspect, an electric heating element assembly comprising a heating element having a heating coil contained within a tubular metallic sheath of relatively low thermal conductivity, said element having end portions which extend through and are secured to an element head and a hot return portion intermediate the said ends which is secured to a front face of said head, characterised in that the element is formed to have an increased watts density in its hot return portion adjacent the head, and in that on its rear side, opposite the hot return portion, the head defines a mounting location for a bimetallic actuator, which mounting location is offset from the portion of said head to which the return portion of the element is attached. Thus, in accordance with the invention the watts density of the return portion is increased relative to the rest of the heater sheath. By increasing the watts density of the hot return, the heating effect on the head is increased, so that in a dry switch on condition with wet scale, sufficient energy may be input to the head to evaporate completely the liquid in the scale around the hot return, so stopping the 'sponge' effect and avoid prolonged cooling of the head. Once the water around the hot return has been evaporated, the head is heated relatively quickly to operate the bimetallic actuator. However, to avoid this increased heating effect causing nuisance tripping during normal operation of the element, the mounting location for the bimetallic actuator is offset from the connection of the hot return to increase the length of the thermal path to the actuator. Thus the arrangement is such that there is a heat path between the hot return and the actuator which is exposed to water and/or wet scale which has a cooling effect in normal operation. When dry, however, this is dried out and heat is more directly transmitted to the actuator. Preferably this offset is achieved by providing a raised portion on the front face of the head to which the hot return is connected, the raised portion providing the aforementioned heat path to the mounting location. The raised portion on the front face of the head leaves a corresponding recess in the rear face of the head, around or adjacent to which is preferably defined the mounting location for the bimetallic actuator. This avoids face to face contact between the portion of the head to which the hot return is connected and the mounting location. Preferably the hot return is joined to the head by a thermally conductive braze or the like. In the preferred embodiment, the raised portion is constituted by a raised rib, preferably a straight rib, extending across at least a part of the head. The rib may curve across the width of the head, although, preferably, it is straight, as viewed from above. The element hot return may be configured to facilitate its positioning and location against the head, or rib. For example it may be flattened in its region facing the head. In another development, a portion of the element may be deformed to conform to the shape of a part of the head or rib to assist in location during manufacture. Most preferably the head is formed with a dimple which is preferably convex on the side remote from the hot return, and from which the raised portion or rib projects towards the hot return. The convex area of the dimple around the rib may then act as the mounting location for the bimetallic actuator. Preferably the element sheath is of stainless steel and preferably the head is also of stainless steel. In the context of the invention the term "relatively low thermal conductivity" is meant to mean low relative to say copper or brass, for example of the order of the conductivity of stainless steel. In some thermally sensitive controls, a back up protector is provided in the event that the bimetallic actuator described above fails to operate, thereby preventing the element overheating. Typically the back up protector comprises a spring loaded thermally deformable plastics member which is biased into contact with the element head at a location remote from the bimetallic actuator, and which deforms when the head overheats, to open a set of contacts in the control. When, as is preferred, the head of the heater of the invention is also of stainless steel or other low thermal conductivity material, heat is transferred only very slowly from the return portion of the element to this location will be slow. Preferably, therefore, a bridge or strip of thermally conductive material such as copper is mounted on the side of the head remote from the hot return, but in good thermal contact therewith, whereby if the bimetallic actuator fails to operate, heat will be conducted from the hot return to the back up protector to allow it to function. In the embodiment described above, the dimple may be formed with a relief slot to allow the strip to extend to the rear of the raised portion of the head. Means, for example dimples or 'pips', may be provided on the back of the head to locate the heat bridge thereon. As was stated above, the watts density of the element is increased in the region of the hot return. Typically a stainless steel sheathed element operates at a watts density of about 30 W cm" 2 , measured at the surface of the sheath. In accordance with the invention, the watts density of the hot return is preferably raised above 40 W cm "2 , more preferably to between 40 and 50 W cm "2 , most preferably about 45 W cm "2 . In practice this is achieved using an increased number of turns of heating wire within the sheath in the hot return region. The invention also extends to a water heating vessel comprising an electric heating element assembly in accordance with the invention mounted in or to an opening in a wall or base of the vessel and a thermally sensitive control with a bimetallic actuator thereof positioned against the mounting location therefor provided on the heater head. The thermally sensitive control may also comprise a back up protector arranged to engage a mounting location therefor on the head as described above. A preferred embodiment of the invention will now be described by way of example only with reference to the accompanying drawings, in which: Fig. 1 is a rear elevational view of a heater assembly embodying the invention,- Fig. 2 is a sectional view along line A-A of Fig. 1; Fig. 3 is a plan view of the heater of Fig. l ; Fig. 4 is a rear elevational view of the head of the heater assembly of Fig. 1; Fig. 5 is an sectional view along line V-V of Fig. 4; Fig. 6 is an enlarged section along line B-B of Fig. 4, but showing the element joined to the head; Fig. 7 is a view, corresponding to Fig. l of a second embodiment of the invention; Fig. 8 is a sectional view along line C-C of Fig. 7; and Fig. 9 is a sectional view along line D-D of Fig 8. With reference to Fig. 1, an immersion heater assembly 2 comprises a stainless steel sheathed heating element 4 mounted to a pressed stainless steel head 6. The element 4 had end portions 8 which extend through and are attached to the head 6, and respective cold tails 10 extending from the end portions 8 for connection to an electrical supply for the element 4. Studs 12 are welded to the head 6 for mounting a thermally sensitive control to the head and also for mounting the element in the wall of a water heating vessel, in a known manner. Intermediate its end portions 8, the element 4 is provided with a hot return portion 14 which is brazed to the head 6, as will be described further below, over a length L as shown in Fig. 3. The watts density of the element 4 is about 30 W cm "2 , except over the length L where this figure is increased to about 42.5 to 45 Wcm" 2 . The head 6 is formed with a generally semi-circular dimple 16 which is convex in the direction away from the hot return portion 14. The depth of the dimple 16 increases in the upward direction to form an inclined mounting location 18 for a bimetallic actuator 20 which in use is located, in a known manner, on the front face of a thermally sensitive control, of the type presently sold commercially by the Applicant and is described for example in GB 2181598. The actuator is of the type described in GB 1542252, and its position is indicated on Figs. 1 and 2 in dotted lines. The dimple 16 is formed with a rib 22 extending across a substantial portion of the dimple 16. The rib 22, as can be seen most clearly from Fig. 5 projects forwardly from the front face of the dimple 16, and as shown in Figs, l and 4, for example, leaves a recess 24 in the rear face of the dimple. In the embodiment the rib 22, on its rear face is about 13 mm long by l.l mm wide and about 1.4 mm deep. As shown most clearly in Fig. 6, the hot return 14 of the element 4 is brazed to the rib 22 by a copper or other thermally conductive braze 26 which acts as a thermal bridge into the head 6, taking up any space between the hot return 14 and the rib 22. The braze 26 extends along substantially the whole length of the rib 22. The head 6 is also formed with a pressed slot 28 in its rear surface which extends through the upper part of the dimple 16 into the recess 24 formed by the rib 22. This accommodates a thermally conductive strip 30 of copper or similar which is secured in the slot 28 at its lower end so as to be in close thermal contact with the rib 22. In use, a thermal fuse of a back up protector will abut the upper end of the strip 28, in a position 34 shown in dotted lines. Operation of the immersion heater described above will now be described. Due to the local increased watts density of the hot return 14, there is a high thermal input to the head 6 through the thermal bridge 26. However, due to the rib 22, the mounting location 18 for the bimetallic actuator of the control is offset from the hot return 14 thereby increasing the length of the thermal path to the actuator. In normal operation of the heater, water will surround the element 4 and cover the hot return 14. Although a significant amount of heat passes into the head 6 through the bridge 26 and rib 22, the cooling effect of the water on the head around the hot return 14 the thermal bridge 26 and the rib 22, together with the offset of the bimetal mounting location 18 means that the mounting location will be kept at a temperature below the operating temperature of the bimetallic actuator 20, thereby avoiding nuisance tripping of the control of the vessel. In soft water areas, in the situation where no scaling occurs on the head 6 around the hot return 14, if the element 4 is switched on without any water surrounding it, the heat transfer to the mounting location 18 will raise its temperature quickly to the operating temperature of the bimetallic actuator 20 due to the absence of the cooling effect of water around the element 4, hot return 14, thermal bridge 26 or head 6. In hard water areas, however, where scale will form around on the head the hot return portion 14 when the element is switched on without any water surrounding it, the heat flow into the head 6 from the hot return 14 is sufficient to evaporate completely any water in the scale in this region, thereby preventing prolonged evaporative cooling of the head which would otherwise prevent the mounting location 18 reaching the operating temperature of the bimetallic actuator 20 within a safe time period. Thus, the control will operate safely although somewhat slower than in a non-scale condition. In the particular embodiment described in a no-scale dry switch on condition, the control may typically take 7 to 10 seconds to operate and in a wet scale dry switch on condition between about 17 and 20 seconds, with say an actuator operating at a nominal temperature of say 135°C. A slightly modified version of the embodiment of Figs. 1 to 6 is shown in Figs. 7 to 9. For ease of reference, features corresponding to those in Figs. 1 to 6 are indicated by the original reference number suffixed with a 'prime' ('). The general arrangement of the second embodiment is the same as that in the first embodiment, the head 6' still being formed with a dimple 16' with a rib 22' formed on its front face. A shallow pressed groove or slot 28' is also provided in the head 6', extending into the dimple to receive a thermally conductive strip 30' . In this embodiment, however, the rib 22' stands proud of the front face of the dimple 16' across its whole width, and does not lie flush with the slot 28' as in the earlier embodiment. As shown in Figs. 8 and 9, the rib 22' is generally flat along its front face 38, and the element hot return 14' also has a generally flattened area 40 to engage against the front face 38 of the rib 22' . The lower edge 42 of the rear surface area 40 may even be formed with a lip to extend under the lower edge of the rib 22' to facilitate its location during brazing. Braze 26' is applied around the rib 22' to locate the hot return 14' to the rib 22', and to ensure good thermal contact between the hot return 14' and the rib 22', the braze 26' taking up any space between the face of the rib 22' and the hot return 14' . The thermally conductive strip 30' is welded or brazed to a flat area 44 formed in the dimple 16' above the rib 22' . The area of welding or brazing is indicated by shading 46 in Fig. 7. The strip 30' is located by three 'pips' 48,50,52 formed in the head. The uppermost pip 48 locates a V notch 54 formed in the upper end 56 of the strip, while the lower pips 50,52 locate the strip 30' face to face against the head 6' . The second embodiment functions in the same manner as the first embodiment as described above. It will be apparent that the various parameters of watts density of the hot return 14,14', the size and depth of the rib 22,22' and so as can be chosen to give a safe response time in a "worst case" condition. What is important, however, is balancing the increased heat input to the head 6, 6' due to the increased watts density of the hot return 14,14* and the offset of the actuator mounting location 18,18' so that a satisfactory response is obtained in a 'wet scale' condition without having nuisance tripping during normal operation.

Claims: Claims 1. An electric heating element assembly comprising a heating element having a heating coil contained within a tubular metallic sheath of relatively low thermal conductivity, said element having end portions which extend through and are secured to an element head and a hot return portion intermediate the said ends which is secured to a front face of said head, characterised in that the element is formed to have an increased watts density in its hot return portion adjacent the head, and in that on its rear side, opposite the hot return portion, the head defines a mounting location for a bimetallic actuator, which mounting location is offset from the portion of said head to which the return portion of the element is attached. 2. An assembly according to claim 1 wherein said offset is formed by a raised portion on the front face of the head to which the hot return is connected. 3. An assembly as claimed in claim 2 wherein said raised portion comprises a raised rib. 4. An assembly as claimed in claim 3 wherein said rib is generally straight. 5. An assembly as claimed in any preceding claim wherein a portion of said hot return is flattened locally to facilitate engagement with said raised portion or rib. 6. An assembly as claimed in claim 5 wherein said hot return is brazed to said raised portion or rib. 7. An assembly as claimed in claim 6 wherein said braze extends completely around said raised area or rib. 8. An assembly as claimed in any preceding claim wherein the element sheath and/or the element head is of stainless steel. 9. An assembly as claimed in any preceding claim wherein said head comprises a dimple acting as a mounting location for a thermally sensitive actuator and from which said raised portion or rib extends towards the element hot return. 10. An assembly as claimed in any preceding claim further comprising a bridge or strip of thermally conductive material such as copper mounted on the side of the head remote from the hot return, but in good thermal contact therewith. 11. An assembly as claimed in claim 9 and 10 wherein said dimple comprises a relief slot to receive an end of said strip or bridge. 12. An assembly as claimed in claim 10 or 11 wherein said head comprises means for locating said bridge or strip. 13. An assembly as claimed in claim 12 wherein said locating means comprises one or more pips formed in said head. 14. An assembly as claimed in any preceding claim wherein the watts density of the hot return is above 40 Wcm" 2 , preferably about 45 Wcm" 2 . 15. A water heating vessel comprising an assembly according to any of claims 1 to 14 mounted in or to an opening in a wall or base of the vessel and a thermally sensitive control with a bimetallic actuator thereof positioned against the mounting location therefor provided on the heater head. 16. An electric heating element assembly substantially as hereinbefore described with reference to Figs. l to 6 of the accompanying drawings. 17. An electric heating element assembly substantially as hereinbefore described with reference to Figs. 7 to 9 of the accompanying drawings.

Inventors: MOORE RAYMOND, TAYLOR JOHN CRAWSHAW, MOORE, RAYMOND, TAYLOR, JOHN, CRAWSHAW
Applicants: MOORE RAYMOND, STRIX LTD, TAYLOR JOHN C, MOORE, RAYMOND, STRIX LIMITED, TAYLOR, JOHN, CRAWSHAW
Application year: 1995
Main classification: H05B3
IPC codes: H05B3