Liquid heating vessels

Abstract: A heated base (2) in or for a liquid heating vessel, comprises a base member (4), a sheathed electric heating element (12) bonded directly to the underside (10) of the base member (4) of the vessel for heating liquid contained in the vessel and a heat conduction member (14). The heat conduction member (14) extends over, and in close thermal contact with, a portion of the sheathed heating element (12), and is also arranged in close thermal contact with a portion of the base member (4).

Description: The present invention relates to liquid heating vessels, and in particular to liquid heating vessels having sheathed heating elements attached to the underside of the base of the vessel to heat liquid in the vessel. An example of such a liquid heating vessel is disclosed in EP-A-63665 (SEB) in the arrangement disclosed in that document, the heating element is mounted to the base of the liquid heating vessel via a heat diffusion plate of aluminium or a similar highly conductive metal. The heat diffusion plate acts to diffuse heat from the heating element over the base of the heating vessel. In typical liquid heating vessels a thermally sensitive control is provided so as to prevent the heater overheating in the event that the vessel is switched on dry or boils dry. In arrangements as described above, a thermally sensitive control is also mounted on the heat diffusion plate. GB 2 308 537 A discloses a heated base according to the preamble of claims 1 and 37. The Applicants have realised that, in fact, the effect of the heat diffusion plate is relatively limited, acting, in practice, to provide limited radial diffusion of heat from the heating element. Accordingly, it may effectively be dispensed with. However, means must be provided for permitting effective sensing of overheating of the heater in the event of a dry boil or switch on dry situation. The Applicants have sought to provide such an arrangement, and from a first aspect the present invention provides a heated base in or for a liquid heating vessel, comprising a base member, a sheathed electric heating element characterised in that the sheathed heating element is bonded directly to the underside of the base member of the vessel for heating liquid contained in the vessel and a heat conduction member extending over, and in close thermal contact with, a portion of the sheathed heating element, and also arranged in close thermal contact with a portion of the base member of the vessel. Thus in accordance with the invention heat is transferred directly into the base of the liquid heating vessel without the need for a heat diffusion plate. A heat conduction member arranged over and in good thermal contact with the heating element, and also in thermal contact with the vessel base, allows heat to be conducted to a thermally sensitive control which can sense when the heater is overheating. It is important that the heat conduction member is in thermal contact both with the heating element and with the base of the heating vessel since during normal operation of the heater, the vessel base will act to keep the temperature of the heat conduction member plate at a lower temperature thereby preventing accidental operation of a control, mounted thereon during heating of liquid in the vessel. Preferably, the heat conduction member comprises a strip or plate mounted over a portion of the heating element. The plate is preferably of a material having a high thermal conductivity, for example, of aluminium, copper, or alloys thereof. The conduction member plate need only engage a relatively small portion of the heating element in order to perform its function. Accordingly, in a preferred embodiment the heat conduction member need only engage a limited circumferential extent of the element. Typically, this will be less than 90°, preferably around 45° circumferentially of the heating element. The conduction member preferably extends over at least half the radial width of the portion of the heating element, and most preferably over the entire width of the portion. Indeed, in a preferred embodiment, the conduction member extends radially beyond the element and is attached to the vessel base beyond the element. The conduction member may be provided with suitable locating means for assisting its accurate location on the heater and vessel base. The locating means may engage with the heating element and/or the base of the vessel itself. In the preferred embodiment, the vessel base is dish shaped, and the conduction member extends at least partially up the side of the dish for location. A plurality of heat conduction members may be provided for receiving a plurality of thermal control means. In a preferred embodiment a single conduction member comprises mounting locations for a plurality of thermally sensitive control means. In the preferred embodiment, the arrangement is adapted for use with a thermally sensitive control having a pair of thermally sensitive actuators. An example of such a control is the Applicant's U17 or U18 control. In this embodiment, the thermal conduction member preferably extends across the base of the heating vessel and engage with respective opposed portions of the heating element. The respective mounting locations for the control actuators are, therefore, linked by a central bridge of material. The conduction member may be apertured both to save material costs and to manage the heat flow within the member. For example the conduction member may be provided with apertures, eg slots, extending around the actuator mounting locations so as to prevent heat transfer into the central region of the vessel base, which is unnecessary, and concentrate heat in the region of the actuator mounting locations. The apertures are preferably slots and in one embodiment may be arcuate. The heat conduction member of the invention may also be provided with means for mounting a control thereon. Accordingly, the heat conduction member may be provided with one or more tabs or other mounting locations for a control. Preferably the heat conduction member is brazed or soldered in position on the element and heating vessel base. In order to facilitate this procedure, the conduction plate is preferably provided with one or more apertures which allows the escape of fluxes etc. during the bonding process. In a preferred embodiment, the element and the conduction member may be coated with solder paste and located together on the vessel base and the assembly then heated to solder the plate and element in position. This is in itself a novel form of construction, so from a second aspect the invention provides a method of manufacturing a base for a liquid heating vessel characterised by comprising coating selected areas of a sheathed electric heating element and a thermal conduction member with a solder or braze material, locating the element and conduction member together on the base and heating the assembly to bond the element and the conduction member to the base. The element and the conduction member could be form locked together eg by pressing to assist in location. The element is preferably formed such that its height is less than its width. Preferably, it is generally triangular or trapezoidal in section and is provided with a flange at its base to increase its area of contact with the vessel base. Preferably the sheath of the heating element is of aluminium, and most preferably the wall thickness of the sheath is greater than 0.75mm, more preferably around 1mm to accommodate deformation in creating a flange as described above. From another aspect, the invention also provides a sheathed heating element for use in a base in accordance with the invention comprising an aluminium sheath whose wall is greater than 0.75mm in thickness and which is formed with a flanged base for mounting to the base. The invention also extends to a liquid heating vessel comprising a heated base in accordance with the invention. The base member may be a separate component mounted in the base of the vessel, which may be plastics. Alternatively the base member may be an integral part of the vessel body. The base member is preferably made from a material with a low thermal conductivity such as stainless steel. In certain embodiments of the invention, the heating element may be mounted to a generally planar portion of the base so as to protrude therefrom. In other embodiments, however, the base may be formed with a groove which receives the heating element, and the conduction member be arranged across the top of the heating element. Preferably the base is formed with a peripheral step which receives the heating element. Preferably the groove or step and the element are sized such that the conduction member lies flush with the adjacent region of the base. This allows a substantially planar conduction member to be used. In one embodiment, the conduction member extends substantially around the whole groove or step, although an opening may be provided to allow the cold tails of the heating element to project through. Some preferred embodiments of the invention will now be described, by wary of example only, with reference to the accompanying drawings in which: Figure 1 shows a plan view of a heated base in accordance with the invention; Figure 2 shows a sectional view along the line II-II of Figure 1; Figure 3 shows a perspective view of the heat conduction member shown in Figures 1 and 2; Figure 4 shows a plan view of the conduction member of Figure 3; Figure 5 shows a sectional view along the line V-V in Figure 4; Figure 6 shows a side view of a conduction member shown in Figure 4; Figure 7 shows schematically an alternative element configuration; Figure 8 shows a sectional view of a further embodiment of the invention; Figure 9 shows an underneath plan view of the arrangement of Figure 8, but with the control removed; and Figure 10 shows a sectional view of a yet further embodiment of the invention. With reference to Figures 1 and 2, a heated base 2 for mounting in the base of a plastics or metallic liquid heating vessel (not shown) comprises a stainless steel base member 4 having a mounted flange 6 for mounting in the vessel body. The underside 8 of the base member 4 is dished, having a flat bottom surface 10. A generally triangular cross-sectioned annular aluminium sheathed heating element 12 is mounted around the periphery of the underside of the base member 4. A generally rectangular heat conduction member 14 is also mounted to the undersurface 10, extending completely across the undersurface 10 and over and around opposed portions 16 of the heating element 12. The heating element 12 is generally triangular in section and is formed with an aluminium sheath arranged around a heating wire and electrical insulating material in a known manner. Both the element 12 and the heater conduction member 14 are soldered onto the flat undersurface 10 as will be described further below. Turning now to the heat conduction member 14, this is shown in greater detail in Figures 3 to 6. The heat conduction member 14 is formed as a strip of a thermally highly conductive material such as aluminium or copper or alloys thereof. In this particular embodiment it is formed of an aluminium strip about 1 mm in thickness. The conduction member comprises two end portions 20, 22 which engage over the heating element 12. The end portions 20, 22 are, therefore, formed into a shaped generally complementary to that of the heating element. The end regions 20, 22 are provided with respective locating lips 24, 26 which as can be seen in Figure 2 extend down over the side wall 28 of the underside 8 of the base member 4, thereby assisting in locating the conduction member 14 on the base member 4. The central region 30 of the heat conduction member 14 is provided with mounting locations 32, 34 for receiving bimetallic actuators 36 of a thermally sensitive control for the liquid heating vessel. In this particular embodiment, the arrangement is intended for use with one of the Applicants U17 or U18 series of controls, whose mode of operation is described in detail in W095/34187. Such controls comprise a pair of spaced apart bimetallic actuators 36 which sense the temperature of the base of a liquid heating vessel and operate in the event that they sense too high a temperature in order to interrupt the power supply to the heater of the vessel. The position of one such actuator 36 is shown in dotted line in figures for illustrative purposes. The mounting locations 32, 34 comprise respective apertures 38, 40 in the conduction member 14, surrounded by respective arcuate apertures 42, 44. The central apertures 38, 40 allow the satisfactory seating of the actuators 36 on the heat conduction member 14. The arcuate apertures 42, 44, serve to restrict the flow of heat from the mounting locations into the centre 30 of the conduction member 14 thereby assuring a faster response time in an overheat situation. It will also be seen that the thermally conductive member 14 is provided with a number of apertures 50 arranged over the rest of its surface. This acts to reduce material costs and, furthermore, will allow the escape of fluxes etc. during the manufacturing process as will be described further below. The conduction member 14 is also provided with mounting tabs 50 which may be used to mount the control to the conduction plate, thereby avoiding the need for any additional means of mounting for the control to the base member. The conduction member 14 and the element 12 are soldered to the underside 8 of the base member 4. In particular, the surface of the conduction plate 14 which faces the vessel base 4, and the element 14 may be coated with a suitable solder paste, the two components then assembled together onto the vessel base, which can then be heated to an appropriate temperature to melt the solder. Fluxes and excess solder can escape through the apertures 50 provided in the conduction plate 14 so as to provide a firm connection. Furthermore, the apertures 50 in the respective end regions 20, 22 of the heat conduction member allow fluxes to escape from between the heat conduction member 14 and the element 12. In use, the base 2 is mounted in the bottom of a liquid heating vessel via its mounting flange 6 in a conventional manner. A thermally sensitive control e.g. an Applicant's U17 or U18 series control is then mounted to the heat conduction plate via the mounting lugs 54. When liquid is being heated in the vessel, the temperature of the mounting locations 32, 34 for the bimetallic actuators 36 of the thermally sensitive control will be kept relatively cool due the thermal contact of the conduction member 14 with the base member 4 in that region. However, in the event that the liquid heating vessel boils dry or is switched on without any liquid in it, heat will be conducted from the heating element 12 into the mounting location for the bimetals through the heat conduction member 14, and there will be no cooling effect through the base member 4. In due course, therefore, the mounting locations will reach a temperature sufficiently high to operate the bimetallic actuators 36 which will then act to disconnect the power supply to the heater. It will be appreciated that various modifications can be made to the arrangement shown in the Figures 1 to 6 without departing from the scope of the invention. For example, the heat conduction member may be adapted merely to receive a single bimetallic actuator or other form of control. The invention is not limited to use with a U17 or U18 type control. Furthermore, the particular shape of the element may vary from that shown. For example, Figure 7 shows an alternative geometry for the heater. In this embodiment, the element 60 is formed with a mounting flange 62 at its base, to increase the surface area of contact with the base 8 of the vessel. Typically the sheath will be made of aluminium, again typically with walls about 1 mm thick which will accommodate the pressing necessary to produce the mounting flange 62. The shape of the base member may also be different to that shown in Figures 1 to 6. With reference to Figures 8 and 9, the base member 70 of a further embodiment of the invention is deeper than that of the earlier described embodiment, and is provided with a peripheral step 72. An annular sheathed heating element 74 is brazed or otherwise secured into the step 72, as shown, preferably along both contact surfaces. A planar conduction member 76 of high thermal conductivity material is mounted over the lower surface of the heating element 74 and is brazed to the lowermost surface 78 of the base member 70 and also to the element 74. The conduction member is provide with studs 80 for mounting a control unit 84. The control unit 18 illustrated in this embodiment is one of the Applicant's U18 control units. As mentioned above, the control unit comprises a pair of bimetallic actuators which, when the control is mounted in position, are arranged in good thermal contact with the conduction member 76. As can be seen most clearly from Figure 9, the conduction member 76 is circular and covers substantially the entire base 70 apart from a central aperture 84 and a peripheral cut-out 86. The cut-out 86 allows the cold tails 88 of the heating element 74 to project below the conduction member 76 for connection to terminals 90 on the control unit 82. This embodiment works in the same manner as the earlier embodiment, with heat being transferred to the actuators of the control unit 82 in an overheat situation through the conduction member 76, but heat entering the base 70 directly from the heating element 74 through the walls of the step 72 of the base 70. Figure 10 shows a further embodiment of the invention. This is similar in construction and operation to the embodiment of Figure 8 except that in this embodiment, two heating elements, 92, 94 are arranged in a step 96 of a base member 98. The upper element 92 is typically a higher power, main, heating element while the lower element 94 is a lower power, simmer, element. The two elements 92, 94 are brazed into the step 96 and preferably to each other. It will be understood that the conduction member 76 in the last two described embodiments may incorporate the various preferred features of the conduction member of the first embodiment. For example, they may extend only partially around the heating element(s). It may also incorporate features facilitating its attachment to the base member, for example small stand offs on its base facing surface to facilitate the distribution of braze during a brazing process. Also, the invention may be applied to all manner of liquid heating vessels, for example kettles, coffee makers, deep fat fryers and so on.

Claims: A heated base (2) in or for a liquid heating vessel, comprising a base member (4; 70; 98), a sheathed electric heating element (12; 74; 92, 94), characterised in that the sheatted heating element (12) is bonded directly to the underside (8) of the base member (4; 70; 98) of the vessel for heating liquid contained in the vessel and in that a heat conduction member (14; 76) is provided extending over, and in close thermal contact with, a portion of the sheathed heating element (12; 74; 92, 94), and is also arranged in close thermal contact with a portion of the base member (4; 70; 98) of the vessel. A base (2) as claimed in claim 1 wherein the heat conduction member (14; 76) comprises a strip or plate mounted over a portion of the heating element. A base (2) as claimed in claim 1 or 2 wherein the heat conduction member (14; 76) is a material having a high thermal conductivity such as aluminium, copper, or alloys thereof. A base (2) as claimed in any preceding claim wherein the heat conduction member (14; 76) plate engages only a relatively small length of the heating element. A base (2) as claimed in claim 4 wherein the heat conduction member (14; 76) engages less than 90°, preferably around 45°, circumferentially of the heating element (12; 74; 92, 94). A base (2) as claimed in any preceding claim wherein the heat conduction member (14; 76) extends over at least half the radial width of the heating element (12; 74; 92, 94), and most preferably over the entire width of the element. A base (2) as claimed in claim 6 wherein the heat conduction member (14; 76) extends radially beyond the element (12; 74; 92, 94) and is attached to the base member (74; 70) beyond the element (12, 74; 92, 94). A base (2) as claimed in any preceding claim wherein the heat conduction member (14, 76) is provided with locating means. A base (2) as claimed in claim 8 wherein the vessel base member (4; 70; 98) is dish shaped, and the location means of the heat conduction member (14; 76) extends at least partially up the side of the dish. A base (2) as claimed in any preceding claim wherein a single conduction member (14; 76) comprises mounting locations for a plurality of thermally sensitive control means. A base (2) as claimed in any preceding claim wherein said heat conduction member (14; 76) is adapted to receive one or more bimetallic actuators (36) of thermally sensitive control means. A base (2) as claimed in claim 11 wherein said conduction member (14; 76) comprises a pair of spaced apart locations (32, 34) for receiving a pair of thermally sensitive actuators (36), said being linked by a bridge (30) of material. A base (2) as claimed in any preceding claim wherein the heat conduction member (14; 76) is apertured. A base (2) as claimed in claim 13 wherein the heat conduction member (14; 76) is provided with apertures (38, 40) extending around the actuator mounting location (32, 34). A base (2) as claimed in any preceding claim wherein the heat conduction member (14; 76) also comprises means (50; 80) for mounting a control thereon. A base (2) as claimed in claim 15 wherein said mounting means comprises one or more tabs (50). A base (2) as claimed in any preceding claim wherein the heat conduction member (14; 76) is brazed or soldered in position on the element (12; 74; 92, 94) and base member (4; 70; 98). A base (2) as claimed in claim 17 wherein the element (12; 74; 92, 94) is also brazed or soldered onto the base member (4; 70; 98). A base (2) as claimed in any preceding claim wherein the element (12; 74; 92, 94) is generally annular, extending around the periphery of the base member (4; 70; 98). A base (2) as claimed in any preceding claim wherein the element (12; 74; 92, 94) is formed such that its height is less than its width. A base (2) as claimed in any preceding claim wherein the element (12; 74; 92, 94) is generally triangular or trapezoidal in section. A base (2) as claimed in any preceding claim wherein the element (12; 74; 92, 94) is provided with a flange at its base to increase its area of contact with the base member (4; 70; 98). A base (2) as claimed in any preceding claim wherein the sheath of the heating element (12; 74; 92, 94) is of aluminium. A base (2) as claimed in claim 23 wherein the sheath thickness is greater than 0.75mm, more preferably about lmm. A base (2) as claimed in any preceding claim wherein the base member (4; 70; 98) is generally planar. A base (2) as claimed in any of claims 1 to 24 wherein the base member (14; 70; 98) is formed wit a groove which receives the heating element. A base (2) as claimed in claim 25 or 26 wherein the base member (70; 98) is formed with a peripheral step (72; 98) which receives the heating element (74; 92, 94). A base (2) as claimed in claim 26 or 27 wherein the conduction member (76) is arranged across the top of the heating element (74; 92, 94). A base (2) as claimed in claim 28 wherein the groove or step (72; 96) and the element (74; 92, 94) are sized such that the conduction member (76) lies flush with the adjacent region of the base member (70; 98). A base (2) as claimed in any preceding claim wherein the base member (4; 70; 98) is of stainless steel. A base (2) as claimed in any preceding claim further comprising a sheathed heating element (12; 74; 92, 94) comprising an aluminium sheath whose wall is greater than 0.75mm in thickness and which is formed with a flanged base for mounting to the base. A liquid heating vessel comprising a heated base (2) as claimed in any of claims 1 to 30. A liquid heating vessel as claimed in claim 32 wherein the base member (4; 70; 98) is a separate component mounted in the base of the vessel. A liquid heating vessel as claimed in claim 33 wherein the body of the liquid heating vessel is of plastics. A liquid heating vessel as claimed in claim 32 wherein the base member is an integral part of the vessel body. A liquid heating vessel as claimed in any of claims claim 32 to 35 further comprising a thermally sensitive control (84) having at least one thermally sensitive actuator (32, 34) mounted in thermal contact with the conduction member (14; 76). A method of manufacturing a base (2) in or for a liquid heating vessel characterised by comprising coating selected areas of a sheathed electric heating element (12; 74; 92, 94) and a thermal conduction member (14; 76) with a solder or braze material, locating the element (12; 74; 92, 94) and conduction member (14; 76) together on the base (2) and heating the assembly to bond the element (12; 74; 92, 94) and the conduction member (14; 76) to the base (2).

Inventors: DONNELLY VERNON JAMES, SCOTT MICHAEL JAMES, TAYLOR JOHN CRAWSHAW, DONNELLY, VERNON JAMES, SCOTT, MICHAEL JAMES, TAYLOR, JOHN CRAWSHAW
Applicants: STRIX LTD, STRIX LIMITED
Application year: 2001
Main classification: H05B3
IPC codes: H05B3