HEAT TRANSFER ELEMENT FOR THERMAL CONTROLS

Abstract: Thermal controls for electric kettles and the like commonly have a bimetallic switch-operating element (108) juxtaposed in heat transfer relationship with the heating element (222) of the kettle, and a heat sink compound in grease or paste form is commonly introduced between the bimetallic element and the heating element. The heat sink compound is relatively costly and is messy and difficult to apply with precision in an automatic manufacturing process. To overcome these problems which have plagued the industry for many years, the present invention proposes to use a pad or patch of self-adhesive heat transfer tape (130) rather than the heat sink compound, such heat transfer tapes previously having been used only in the electronics industry for heat sinking of power transistors and the like.

Description: HEAT TRANSFER ELEMENT FOR THERMAL CONTROLS Field of the Invention: This invention concerns improvements relating to thermal controls and more particularly concerns thermal controls for electrically powered liquid heating vessels such as kettles, jugs, pots, pans, urns, laboratory and industrial equipment and the like. Background of the Invention: It is well known to provide a thermal control for the electric heating elements of electric kettles, hot water jugs and the like, the control serving to protect the heating element and the appliance against the effects of the appliance being switched on inadvertently without sufficient liquid in the appliance or being allowed to boil dry. It is likewise well known to employ bimetallic elements as thermally-responsive actuators in such controls, the bimetallic elements being customarily arranged to open a εet of electrical contacts so as to disconnect the heating element from its power supply in response to a sensed overte perature condition. Exemplary thermal controls employing bimetallic elements as switch actuators are disclosed in GB 1401954, GB 1470366, GB 2117568, GB 2176055, GB 2194099 and WO 92/16003. Snap-acting bimetallic elements have been developed to overcome the problems of arcing between switch contacts and contact erosion which occurs when switch contacts are opened slowly. Many different forms of snap-acting bimetallic elements are known, but all are generally characterized by a dished configuration which can move into an opposite curvature with a snap-action. The dished configuration, however, gives rise to a problem as regards transferring heat efficiently from the heating element into the bimetallic element, and to meet this problem it has been common to provide a portion of the heating element, or more precisely the heating element head plate, with a curved configuration complementary to that of the bimetallic element so that the heating element and the bimetallic element nest together in relatively good thermal contact; such an arrangement is disclosed in GB 2176055 for example. In this connection, the foregoing reference to the heating element head plate will be understood to refer to the common heating element configuration in which the heating element proper, that is to say the part which heats up when the heating element is powered, is mounted to a head plate which serves for the mounting of the heating element in a vessel. The heating element proper is commonly of a tubular sheathed wire wound configuration with a mineral insulator packed into the tubular sheath, and commonly has a so-called hot return portion which is affixed to the heating element head plate on the wet side thereof, that is to say the side which will be in contact with the liquid being heated, the hot return portion defining on the opposite, dry side of the element head plate a location having a temperature approximating to that of the heating element proper which serves as the location whereat the bimetallic element senses the heating element temperature. In addition to configuring the element head plate to conform to the dished configuration of the snap- acting bimetallic element, it has furthermore been conventional to employ a heat sink compound as a heat transfer medium between the heating element head plate and the bimetallic element. The heat transfer compound is a grease, conventionally silicone based, with proprietary filling to enhance its thermal conductivity. The heat transfer compound is relatively costly and is difficult to handle economically or automatically. It is difficult to remove once applied and tends to lose its efficiency over time as the oils in the grease slowly dry out. The material has to be applied at the time when the heating element and its bimetallic protective control are assembled together, which means that it generally falls to the manufacturers of electric kettles, hot water jugs and the like, who have the task of assembling the heating element and its control to the vessel body, to apply the heat sink compound. Appliance manufacturers generally do not have the skill to apply optimum amounts of heat sink compound in optimum positions and this leads to excessive use of heat sink compound and impairment of the performance of the element protector control. Objects and Summary of the Invention: It is thus the principal object of the present invention to overcome or at least substantially reduce the abovementioned problems associated with the use of a grease heat sink compound. The present invention resides in the realization that the solution to the abovementioned problems arising out of the use of grease type heat sink compound, which problems have beset the industry for many years without any solution being forthcoming, is to use self-adhesive heat transfer tape, such as is commonly used in the electronics industry for assembling electronic components to heat sinks, instead of the grease compound. According to one aspect of the present invention, therefore, a control for a liquid heating apparatus which incorporates a bimetallic switch-actuating element arranged to be juxtaposed with the liquid heating apparatus in thermal transfer relationship therewith further includes a patch of heat transfer tape adhered to that surface (the facing surface) of the bimetallic element which is to face the liquid heating apparatus in use. The present invention also extends to a liquid heating apparatus provided with a control as aforesaid and with the patch of heat transfer tape sandwiched between a surface of the apparatus, particularly though not exclusively the dry side of a head plate of an electrically powered heating element of the apparatus, and the facing surface of the bimetallic switch-actuating element of the control. Furthermore, the invention also extends to a method of assembling a control as aforesaid or a liquid heating apparatus as aforesaid provided with such a control, the method including the step of adhering a patch of heat transfer tape to the facing surface of the bimetallic switch-actuating element of the control. Thermal controls for liquid heating apparatuses are also known in which the bimetallic switch- actuating element of the control is mounted behind a thermally-conductive mounting plate or cover plate, formed of aluminium for example, and one such control is disclosed in WO 92/16003. Further in accordance with the present invention, a modification of the control, the liquid heating apparatus and the method of assembly above recited, comprises the provision of a patch of heat transfer tape adhered to the bimetallic element of such a control as is just mentioned so as to enhance the conduction of heat from the mounting plate to the bimetallic element. Alternatively, or additionally, such a patch may be adhered to the facing surface of the mounting plate, that is to say that surface of the mounting plate which faces the liquid heating apparatus in use. Whilst the preference is to adhere the heat transfer tape to the bimetallic element of the control, and/or to the mounting plate of such a control as is discussed immediately above, it would be possible for the heat transfer tape to be adhered instead to the surface of the liquid heating apparatus, particularly but not exclusively to the dry side of the head plate of an electrically powered heating element of the apparatus, at a location where it will be effective to enhance the transfer of heat from the apparatus to the bimetallic element of the control. The present invention also extends to this modification, though such a modification is not presently preferred since it places the onus upon the appliance manufacturer to fit the patches of heat transfer tape, whereas it is considered to be preferable that they do not have this task. It is a relatively simple matter to modify the automatic production apparatuses that are used for the manufacture of bimetallic controls for liquid heating apparatuses so that a patch of heat transfer tape is automatically adhered to the facing surface of the bimetallic element. The control with the patch applied would then be shipped to the appliance manufacturer who would then assemble it into the appliance without the need to apply a heat transfer grease. As aforementioned, heat transfer tape is readily available, having been used heretofore in the assembly of electronic components such as power transistors to their heat sinks. It can be cut to shape to suit whatever configuration of bimetallic element is employed in the control. Heat transfer tape is available in a range of thicknesses and the selection of an appropriate tape thickness to suit a particular application is a simple matter. Suitable heat transfer tape is available from Warth International Ltd. of East Grinstead, Sussex, England whose products include Zemrex™ heat transfer material which has a 99% carbon composition and a thermal conductivity approaching that of silicone grease, and from Chomerics Europe Ltd. of Marlow, Buckinghamshire, England whose products include the Cho-Therm™ range of thermally-conductive electrically-isolating elastomer insulators which are silicone elastomer materials filled with boron nitride or aluminium oxide or ceramics particles and reinformed with glass fibre cloth. The present invention is particularly, though not exclusively, applicable to the XI control manufactured by us, this control being substantially as described in GB 2194099 with particular reference to Figures 3A, 3B and 3C of the drawings thereof. An exemplary embodiment of the present invention is such an XI control having a patch of Zemrex™ heat transfer tape of appropriate thickness adhered to the facing surface of the bimetallic element of the control. This and other embodiments of the invention will be described in more detail hereinafter with reference to the accompanying drawings. Description of the Drawings: Figures IA and IB are exploded perspective views of an XI control as described more fully in GB 2194099 abovementioned; Figure 2 is an exploded perspective view of a modified form of the control of Figures IA and IB as described more fully in GB 2283156: Figure 3 is a schematic sectional side elevation view of the control of Figure 2 assembled with a planar type of electric heating element. Detailed Description of the Embodiments: Referring to Figures IA and IB, the construction and operation of the XI element protector control shown therein will now be briefly described, it being appreciated that reference may be made to GB 2194099 for a more complete description. The element protector as shown comprises a sub-assembly 20 (the same reference numerals will be used herein and in Figures IA and IB as are used in GB 2194099 to denote like parts) comprising a collapsible carrier 21, a bimetal blade 22 and a push rod 23, the sub-assembly 20 being adapted to locate in use of the element protector between the rear face of the heating element head 24 and the outer side of an inner moulding 25 of the element protector. The inner moulding 25 is adapted to co-operate with a main moulding 26 defining a socket inlet 27 for a kettle connector plug so as to define within the assembled control a chamber for accommodating the L,N and E terminal pins 28, 29 and 30 of the control and for accommodating a contact carrying live (L) connecting spring 31, a contact carrying live leaf spring 32, a neutral (N) connecting spring 33 and an earth (E) connecting spring 34. The inner moulding 25 is adapted to be clipped to the main moulding 26 and has a pair of integrally-formed moulded spring clips 36 which co-operate with a pair of apertures 37 in the main moulding so that the inner moulding 25 makes a positive fit into the mouth of the main moulding 26 and is positively retained therein by the clips 36. Upstands are formed on the inner face of inner moulding 25 (that is the face that can be seen in Figure 4A) and co-operate with formations provided on the opposed face of the main moulding 26 (that is the face that can be seen in Figure 4B) for retaining the L,N and E terminal pins 28, 29 and 30 securely in the assembled control, the terminal pins extending through respective apertures provided within the socket inlet part 27 of the main moulding 26, and in similar fashion the live connecting spring 31, the live leaf spring 32 and the neutral connecting spring 33 are trapped between the inner moulding 25 and the main moulding 26 when the two are assembled together. The earth connecting εpring 34 is adapted to affix to the upstand 41 provided on the element head 24 when the element protector is fully asεembled and iε attached to the element head and, within the element protector, makes contact with the earth terminal pin 30. Referring now more particularly to the sub-assembly 20, the bimetallic blade 22 is generally rectangular with a central cut-out 42 of generally X-shaped configuration and is dished so as to be capable of moving with a snap action between two oppositely dished configurations, the X-shaped cut-out 42 ensuring a substantially greater operating movement at the centre of the bimetal than would be provided by a plain dished blade. The bimetallic blade 22 is retained in the sub-assembly 20 by means of the engagement of the push rod 23 both with the central cut-out 42 of the blade 22 and with the collapsible carrier 21, the push rod 23 being adapted to be inserted into a guide passage 45 formed in the carrier 21 from the side thereof which can be seen in Figure IA, but being incapable of pasεing completely through the guide passage 45, and having a nose portion adapted to be engaged in the centre of the cut-out 42 in the bimetal blade 22. As will be appreciated, the position of the opposite, pusher end of the push rod 23 relative to the carrier 21 will be dependent upon whether the bimetal 22 is its hot or its cold state. The carrier 21 is generally in the form of a four-legged table and has relatively large feet 46 for ensuring good thermal contact with the rear face of the heating element head 24 when the assembled element protector is affixed to the element head. The bimetallic blade 22 seatε at its corners on the ends of the four legs 47 of the carrier, flush with the soles of the feet 46. A pair of locating grooves 48 are provided in the carrier 21 and the inner moulding 25 is provided on its outer face with a pair of upstanding rails 49 adapted to slidingly engage in the grooves 48. On its upper surface, as shown in Figure IA, the carrier 21 has four upstanding poεts designed to project through an aperture 50 provided in the inner moulding 25, there being two small posts 51 and two larger posts 52 and 53. When the element protector as thus described is assembled together and to the element head 24 of an electrically heated water boiling vessel for example, the heating element cold tails 54 and 55 (that is the terminal ends of the heating element proper) extend through apertures 56 and 57 provided in the inner moulding and contact the upper ends of the live connecting spring 31 and the neutral connecting spring 33 respectively, the upper end portions of the springs 31 and 33 being curved as shown for accommodating such contacts. The heating element cold tails may be tipped with silver solder or may be provided with crimped ferrule terminations to enhance their connections with the leaf springs. The lower end of the live connecting spring 31 extends across the aperture 50 of the inner moulding 25 and carries a contact which constitutes the "moving" contact of the switching contacts set of the element protector. This lower limb of the live connecting spring 31 is arranged to be abutted by the pusher end of the push rod 23 for moving the moving contact in response to switching of the bimetallic blade 22 into its "hot" condition from its normal "cold" condition. The live leaf spring 32 is trapped between the inner and main mouldingε 25 and 26 when the two are assembled together, and has a first, relatively-substantial limb 58 which extends across the aperture 50 formed in the inner moulding 25 and is contacted and urged away from the inner moulding 25 and towards the main moulding 26 by the large post 52 upstanding from the carrier 21, which causes the limb 58 of the live leaf spring 32 to be biassed into contact with the live terminal pin 28. The live leaf spring 32 also has a second, less-substantial limb 59 which extends acrosε the aperture 50 in the inner moulding 25 and carrieε at itε free end a contact which cooperateε with the "moving" contact provided on the live connecting spring 31 and constitutes the "fixed" contact of the switching contacts set. The second limb 59 of the live leaf spring 32 is arranged to be contacted by the lowermost of the two small posts 51 provided on the carrier 21, such posts projecting through the aperture 50. This contact between the lower post 51 and the limb 59 of the live leaf spring 32 establishes the position of the contact carried by the live leaf spring 31 (the "fixed" contact of the switching contacts set) relative to the carrier and thus relative to the push-rod, the bimetallic blade and the element head and establishes the position of the "fixed" contact relative to the element head irrespective of variations in the dimensions of the element head. In similar manner, the neutral connecting spring 33 is trapped between the assembled mouldings 25 and 26 and has a relatively substantial limb 60 which extends acrosε the aperture 50 and is butted by the post 53 on the carrier 21 into contact with the neutral terminal pin 29. In operation of the element protector as thus described, an element head overtemperature condition will normally cause bimetal 22 to snap to its hot configuration thereby causing the moving contact carried by live connecting spring 31 to be pushed away from the fixed contact carried by live leaf spring 32 by the push rod 23. In the event of an abnormal overtemperature condition, such as might arise if the switching contacts were to weld themselves together for example, then a secondary protection mode comes into operation when the temperature of the element head reaches such a high level as to cause the carrier 21 to collapse towardε the element head by virtue of heat distortion of its legs under pressure from the spring parts 58 and 60 of the live and neutral leaf springs 32 and 33 respectively. As is more completely explained in GB 2194099, the collapse of the carrier 21 towards the element head 24 causes the leaf spring parts 58 and 60 to move out of contact with the live and neutral terminal pins 28 and 29 thereby disconnecting the heating element from its power supply. In use of the XI control of Figures IA and IB it would heretofore have been conventional to apply a small amount of silicone grease heat sink compound to the forward face of the bimetal blade 22 and/or to the rear face of the element head 24. In accordance with the present invention, however, a patch of self-adheεive heat tranεfer tape 200 iε adhered to the forward face of the bimetal, the tape patch being cut to fit the bimetal and having an internal cut-out 201 aε εhown which could for example match the generally X-εhaped cut-out 42 in the bimetal or extend generally around the cut-out 42 to ensure that the tape does not restrict the bimetal movement. Since the adhesive on the tape represents a heat transfer barrier, albeit small, it is preferably not provided over all of the surface area of the tape patch 100, but rather is provided in highly localized strips or spots. Whilst the conventional electrical heating element most commonly used in electric kettles, hot water jugs and the like comprises a wire-wound resistance heating element contained within a tubular metal sheath with a mineral insulating material packed around the resistance heating element within the sheath, planar electric heating elements are also known and the present invention is applicable also to such planar elements. One such planar electric heating element comprises a sheathed heating element as described above cast or clenched into an aluminium casting which may itself form the planar heating element or alternatively may be secured to a stainless steel plate which then forms the planar heating element. Another form of planar heating element is described in GB 2283156 and generally comprises a patterned resiεtance heating element formed on an electrically insulating subεtrate for example by use of l ithographic techniques using electrically-conductive ink or photolithographic techniques as are well known in the field of semiconductor device fabrication, and/or by other deposition techniques such as flame spraying of metals, plasma vapour deposition etc. which per se are well known. More particularly the planar heating element that is described in GB 2283156 comprises a thin sheet of stainleεε εteel, which forms the upper surface of the element, on the underside of which there is provided a thin layer of glass or other temperature-resiεtant electrically-insulating material. A reεiεtance heating element proper is formed on the surface of the glasε layer, for example by printing uεing an electrically conductive ink, and a further thin layer of glass or other temperature-resistantelectrically-insulatingmaterial is provided over the printed element. Hereinafter described with reference to Figures 2 and 3 of the accompanying drawings is a further embodiment of the present invention which comprises an element protection control adapted for use with a planar heating element as described in GB 2283156. Referring to Figure 2 , the element protector control illustrated therein is a modified form of the XI control described hereinbefore. The illustrated element protector control compriεeε firεt and εecond moulded plastics body parts 101 and 102 which are adapted to fit together and to capture therebetween first and second terminals 103 and 104 and spring metal conductors 105, 106 and 107. On the other side of body part 102 there is provided a bimetallic primary switch actuator 108 which, in similar manner to the XI control described in GB 2194099, is adapted to be mounted in a plasticε material carrier 109 which iε adapted in turn to locate in predetermined manner with the front (not visible) face of body part 102. The bimetallic primary switch actuator 108 is arranged, in a manner εubεtantially identical to that described in GB 2194099, to determine the status of a switch defined by the parts 110 and 111 of the spring metal conductors 105 and 106, a push-rod 112 being provided for this purpose. The parts 110 and 111 normally make electrical contact with each other and are opened by push-rod 112 when the bimetallic primary switch-actuator 108 responds, in use, to an element overtemperature condition. In the event that the primary switch actuator fails, for whatever reason, to open the contact between the parts 110 and 111, the heating element will continue to be powered so that its temperature will rise to a level whereat the secondary thermally-responsive actuator constituted by the plasticε material carrier 109 will become operative. The carrier 109 haε rearwardly-facing projections 113 and 114 which are designed to extend through the body part 102 and into contact with the parts 115 and 116 of the spring metal conductors 105 and 107 so that, when the control is operatively coupled to a planar heating element and the carrier 109 is urged rearwardly towards the body part 102, the spring metal parts 115 and 116 are urged by the projections 113 and 114 into contact with the terminals 103 and 104 respectively. The carrier 109 had four legs 117 with feet 118 which εit upon the rear surface of an associated heating element and, when the element temperature rises to such a level as to cause the legs 117 to begin to melt, the carrier 109 iε collapsed in a forwards direction towards the heating element by the spring action of the conductor parts 115 and 116 εo that the conductor parts 115 and 116 eventually move out of contact with the terminals 103 and 104 of the control. The element protector control of Figure 2 is adapted for use with a generally planar heating element and it will be εeen that the εpring metal conductors 106 and 107 have forwardly extending portionε 119 and 120 which are adapted to project forwardly of the control body part 102 and make electrical contact with terminal portions of the planar heating element for supplying electrical energy thereto through the control. The bimetallic blade 108 is adapted to be held in close thermal contact with the heating element and a schematically shown patch 130 of self-adhesive heat transfer tape is adhered to the forward face of the bimetal as in the embodiment of Figures IA and IB, the patch 130 having a cut-out 131 matching the cut-out in the bimetal. Referring now to Figure 3 this shows a sectional side elevation view of an exemplary form of planar heating element with which the control of Figure 2 can be used. The planar heating element comprises a stainless steel substrate or base plate 220 having an electrically-insulating first layer 221 of glass formed thereon in a central area thereof, a heater track 222 formed of electrically conductive ink formed on the glass layer 221, and an electrically-insulating second glaεs layer 223 formed over the heater track 222 and over the first glasε layer 221. An opening or window 224 iε provided in the second glass layer 223 so as to expose the heater track 222 beneath the opening, the opening 224 being sized to enable the bimetallic primary actuator 108 of the control of Figure 2 to be directly responsive to the temperature of the heater track 222, but to locate the feet 118 of the carrier 109 on the glass surface outside of and surrounding the opening. Additional openings (not shown) are provided in the second glass layer 223 at locations corresponding to terminal ends of the heater track 222 and these are arranged to be contacted by the forwardly projecting parts 119 and 120 of the control spring conductors 106 and 107 when the control is fitted in operative relationship with the heating element. The additional openings just mentioned are described in GB 2283156 together with other features of the planar heating element that are not described herein. In operation of the thus-described planar heating element with the thuε-described element protector control, the fact that the bimetallic primary actuator of the control is in good heat transfer relationship with the heater track of the element by way of the patch 130 of heat transfer tape ensures a rapid responεe of the control to an element overtemperature situation. At the same time, by thermally insulating the secondary protection means, namely the collapsible carrier 109, from the heater track 222, it is ensured that the thermal overshoot which normally accompanies the primary operation of the control, that is to say the dynamic temperature rise that normally occurs in the element after operation of the primary protection, will not result in actuation of the secondary protection. The close thermal contact of the bimetallic primary actuator with the heater track furthermore enables improved primary protection response times to be achieved which additionally reduces the temperature overshoot level. These factors facilitate the selection of appropriate materials for the carrier 109. The patch of heat transfer tape 130 can additionally serve the function of electrically insulating the bimetal 108 from the heater track 222 if an appropriate tape composition is utilized. Having thus described the preεent invention by reference to particular examples, it is to be appreciated that modifications and variations are posεible without departure from the εpirit and scope of the invention as set forth in the appended claims. For example, whereas the invention has been described in connection with the application of bimetallic controls for liquid heating veεεels, further application of the invention for example to controls incorporating PTCR (positive temperature coefficient of resistance) elements might be posεible.

Claims: CLAIMS : 1. The use of heat transfer tape, rather than a heat sink compound in grease or paste form, for enhancing the thermal transfer capability between an apparatus and a thermal control for the apparatus. 2. A control for a liquid heating apparatus, said control including a bimetallic element arranged to be juxtaposed with the liquid heating apparatus in thermal transfer relationship therewith, and a pad or patch of heat transfer tape adhered to that surface of the bimetallic element which is to face the liquid heating apparatus in use. 3. A control for a liquid heating apparatus, said control including a bimetallic element arranged to be juxtaposed with a mounting plate or cover plate which in turn is to be juxtaposed with the liquid heating apparatus in thermal transfer relationship therewith, and a pad or patch of heat transfer tape adhered to that surface of the mounting plate or cover plate which is to face the liquid heating apparatus in use and/or sandwiched between the bimetallic element and the mounting plate or cover plate. 4. A liquid heating apparatus provided with a control as claimed in claim 2 or 3. 5. A liquid heating apparatus provided with a control incorporating a thermal sensor in heat transfer relation with a surface of the apparatus, and a pad or patch of heat transfer tape provided between the thermal sensor and the said surface of the apparatus. 6. A liquid heating apparatus as claimed in claim 5 wherein the pad or patch of heat transfer tape is adhered to a surface of the thermal sensor. 7. A liquid heating apparatus as claimed in claim 5 wherein the pad or patch of heat transfer tape is adhered to a surface of the apparatus. 8. A liquid heating apparatus as claimed in any of claims 5 to 7 wherein said thermal sensor comprises a bimetallic element. 9. A liquid heating apparatus as claimed in any of claims 4 to 8 wherein the control is arranged so that the thermal sensor thereof is operatively juxtaposed with the head plate of an electric heating element of the apparatus. 10. A method of assembling a thermally-responsive control to a liquid heating apparatus, said method including the step of adhering a pad or patch of heat transfer tape to a surface of the apparatus which is to be juxtaposed with a thermally-responsive sensor of the control and/or to a surface of the control and/or of the sensor thereof which is to be subject to heat conducted from the liquid heating apparatus.

Inventors: JOHN KEVIN EDWIN, O'NEILL ROBERT ANDREW, JOHN, KEVIN, EDWIN, O'NEILL, ROBERT, ANDREW
Applicants: JOHN KEVIN EDWIN, ONEILL ROBERT ANDREW, OTTER CONTROLS LTD, JOHN, KEVIN, EDWIN, O'NEILL, ROBERT, ANDREW, OTTER CONTROLS LIMITED
Application year: 1996
Main classification: H01H37
IPC codes: H01H37, H05B1