GB2344944A

GB2344944A - Liquid heating appliance with thick film heating element

Info

Publication number: GB2344944A
Application number: GB9826968A
Authority: GB
Inventors: Robert Andrew O'neill; Robert Henry Hadfield
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 1998-12-08
Filing date: 1998-12-08
Publication date: 2000-06-21
Also published as: GB9826968D0

Abstract

An electrically powered liquid heating appliance, particularly a water purifier, has a thick film heating element having a plurality of heating portions 1, 2 arranged to be selectively connected in circuit to provide different power output levels at different times during an operating sequence of the appliance. Heating portions 1, 2 may be initially connected in parallel to provide a power output of 1800 watts for bringing the water to the boil. A thermally responsive switch 3 may open at 95{C so that portion 2 is disconnected and the portion 1 alone then provides an output of 800 watts to complete the boil and distillation. A further thermally responsive switch 4 disconnects both portions 1 and 2 when the appliance boils dry. Switch 4 may be sited over a further portion 5 of the thick film heater track, the portion 5 either remaining energised or becoming energised when the switch 3 opens. Switches 3 and 4 may be constituted by a single device having two bimetals which operate at different temperatures. The appliance may be formed as a cordless appliance in which the switch 4 is reset in response to a vessel part being placed on a base. A printed circuit track of the heating element 1, 2, 5 may have terminals enabling power supply to the element and connections to switches 3, 4 to be made directly by spring connector parts, eliminating the need for much internal wiring. In an alternative arrangement, water is initially heated by element portion 1 alone until switch 3 opens to connect portions 1 and 2 in series (Fig.2). In this arrangement switch 3 may be positioned over the portion 2.

Description

IMPROVEMENTS RELATING TO ELECTRIC HEATING ELEMENTS FIELD OF THE INVENTION : This invention concerns improvements relating to electric heating elements, particularly for use in water heating appliances.

BACKGROUND OF THE INVENTION: Electric heating elements presently in use are commonly of the metal sheathed, mineral insulated resistance wire type and such heating elements are commonly employed in an immersion heating configuration, where the heating element is physically immersed in the volume of water that is to be heated, or in an underfloor heating configuration, where the heating element is clamped or clenched to the underside of a metal plate which, in some arrangements, constitutes the base wall of a liquid heating vessel.

More recently, so-called thick film heating elements have found favour with appliance manufacturers. Thick film heating elements commonly comprise a substrate, a stainless steel plate for example covered with a layer of electrical insulation such as of glass for example, having a resistance heating track or layer formed thereon for example by a printing technique employing electrically conductive ink paste. Thick film heating elements can be manufactured relatively economically and can provide a high watts density as compared to more conventional sheathed heating elements. Furthermore, when used in a domestic hot water jug or kettle for example, they provide a clean and uncluttered appearance within the appliance as compared to immersion type sheathed heating elements.

Objects and Summarv of the Invention: It is the principal object of the present invention to promote the utilization of thick film heating elements in areas where, so far as we are aware, they have not previously been utilized.

According to the present invention in one of its aspects a thicic film heating element is provided with a plurality of heating element portions arranged to enable the power output of the heating element to be switched between different levels by means of a thermally-sensitive control device.

Thermally-sensitive controls, commonly employing bimetal technology, are well known and one or more such controls associated with a thick film heating element according to the present invention having two or more different heating element portions could readily be arranged to short circuit respective heating element portions and/or interconnect respective heating element portions in order selectively to adjust the power output of the overall heating element. Depending upon the particular application, the thermally-sensitive controls can be arranged to be responsive to the temperature of water being heated in an appliance or can be arranged to be responsive to the temperature of the heating element track itself.

The present invention is seen as having particular application to situations in which it is desired to heat a volume of water quickly towards its boiling point and then to reduce the power output of the heating element so that boiling progresses in a less violent fashion than otherwise would be the case. An exemplary appliance which could take advantage of such an arrangement is a water purifier designed to boil a relatively large volume of water, condense the steam thus produced and collect the condensate as purified water. One such appliance that is known to us has a 1 gallon (4 litres) water heating container which supplies steam into a fan-cooled condenser coil, the condensed water from the condenser dripping into a receiving bottle.

The appliance is designed to operate from a 120 Volts electrical supply and has an 800 Watts immersion heater type sheathed heating element. The condenser is so designed that it can only cope with steam produced at the rate that it is generated by an 800 Watts heating element. The problem with the appliance is that it takes 30-40 minutes for the 800 Watts heating element to bring the full water heating container to the boil and it takes about 4 hours to purify the full contents of the water heating container. A further consideration is that the water in the receiving bottle must be allowed to cool before it is drinkable, and generation of steam at a faster rate, with the available condenser, would not give time for this.

According to the teachings of the present invention, the overall cycle time of a water purifying appliance as abovementioned can be reduced by shortening the time taken to bring the water up to its boiling temperature, this being achieved by provision of a high power heating element which is arranged to be switched off at, say, 95 C, the 800 Watt heating element thereafter being allowed to complete the boil and distillation. The appliance referred to is rated for a 15 Amp supply and, with a 120 Volt supply, could utilize a maximum heating element power of 1800 Watts to boil the water, thereby reducing that part of the cycle time by more than half. On a 240 Volt appliance, a 3000 Watt heating element could be provided, thereby further reducing the period required to boil the full water container.

The change in the heating element power output could be achieved by provision of two parallel heating element track portions and a thermostat arranged to disconnect one such track portion at the appropriate temperature.

Alternatively, the two track portions could be electrically in series and the thermostat normally closed in parallel with one of them so as to short circuit it, the thermostat going open circuit at the appropriate temperature so as to connect the resistance of the second track portion in circuit with the first and thereby reduce the power output. The latter of these two options may be preferable, since it means that during the distillation phase the whole of the heating element is in use, leading to a low power density and quieter, more efficient operation for the longer part of the cycle. However, conversely it also means that during the high power part of the cycle the power density is much greater, as the power output is from only a limited part of the heating element, and this may be impractical depending upon the available heating element area. Accordingly, the parallel track arrangement may be more practical, especially with 240 Volt appliances.

The above and further features of the present invention are set forth in the appended claims and, together with advantages thereof, will be well understood from consideration of the following detailed description given with reference to the accompanying drawings of embodiments of the invention.

Description of the Drawings : Figure 1 is a schematic circuit diagram showing an embodiment wherein first and second portions of a thick film heating element are connected in parallel and a thermostat is arranged to take one of the two heating element portions out of circuit at a predetermined temperature; Figure 2 is a schematic circuit diagram showing an alternative embodiment wherein first and second portions of a thick film heating element are connected in series and a thermostat having normally closed contacts is connected across one of the two heating element portions so as normally to short circuit it, the thermostat being arranged to open its contacts at a predetermined temperature so as to bring both heating element portions into operation; and Figure 3 is a schematic circuit diagram illustrating a modification of the Figure 2 embodiment.

Detailed Description of the Embodiments: Referring to Figure 1, in the first embodiment illustrated schematically therein the thick film heating element is designed for use in a water purifying appliance as aforementioned operating from a 120 Volts electrical supply and provides 800 Watt and 1800 Watt selectable power outputs. The thick film heating element track consists of a main portion 1 having a resistance of 18 Ohms and an auxiliary portion 2 having a resistance of 14.4 Ohms. The main and auxiliary track portions 1 and 2 are electrically connected in parallel, subject to the condition of a thermally-responsive device 3 designed to go open circuit at a temperature of 95 C. The device 3 can be a self-resetting contact thermostat, for example as described in our British Patent Application No. 9807924.7, arranged to sense the temperature of the water being heated through the heating element substrate and positioned spaced apart from both of the track portions 1 and 2, the device 3 operating to reduce the heating element output to its lower power rating for the distillation phase of the appliance operation.

A second thermally-responsive control device 4 is provided and functions to switch-off the power supply to both portions of the thick film heating element on completion of the appliance operation when the water heating container of the appliance boils dry and the heating element temperature begins to rise above its normal operating level with water boiling in the container. Element protector controls well adapted to perform this function are well known. The control device 4 should be of a kind which does not self-reset and rather has to be manually reset to ensure that the appliance is allowed to cool after its operating cycle has finished and before it is again operated.

Figure 2, wherein the same reference numerals as were used in Figure 1 designate like parts, is an alternative embodiment for use in the same water purifier and wherein the two heating element portions I and 2 are connected in series and the thermally-responsive device 3 is connected across the auxiliary heating element portion 2 and operates to short circuit heating element portion 2 so long as the sensed water temperature is below 95 C and thereafter to reduce the overall power output of the heating element by connecting the two portions 1 and 2 in series. Note that in this arrangement the main heating element portion 1 has a resistance of 8 Ohms and the auxiliary heating element portion 2 has a resistance of 10 Ohms. In this embodiment the control device 3 could advantageously be situated over the part of the heating element track which is energized only during the distillation phase of the appliance operation, namely over the auxiliary track portion 2, since heat from this track would ensure that inadvertent resetting of the device 3 during distillation could not occur.

As mentioned above, the thermally-sensitive device 4 should be of a kind that has to be manually reset after it has operated. Contact thermostats are known which have to be manually reset, this being a function of the temperature differential of the bimetal in the contact thermostat, namely the difference between the temperature at which the bimetal switches with rising temperature and the much lower temperature at which it will reset, and contact thermostats incorporating reset mechanisms are known. Altematively, by arranging the appliance as a cordless appliance comprising a vessel part incorporating the heating element and a separate base part for providing a power supply to the vessel part when the vessel part is seated on the base part, there being co-operating electrical connectors in the vessel and base parts, it could be arranged that the thermally-sensitive device 4 is arranged to be reset when the vessel part is replaced on its base after being removed therefrom for refilling. The control that is described in our British Patent Application No.

9808484.1 is capable of satisfying this function and accordingly could be utilized in the practice of the present invention.

Figure 3 shows a modification of the embodiment of Figure 2 wherein the thermally-sensitive device 4 is sited over a section of heater track 5 which either remains energized or becomes energized when the thermally-sensitive device 3 operates and the rest of the track cools. Such an arrangement provides a temperature maintained, non self-resetting operation of the thermally-sensitive device 4. Such an arrangement would reset either when the vessel part of the appliance was removed from its base part and filled with cold water, or after a short period of being switched off which would allow the track portion 5 underlying the device 4 to cool. The ability to add further heater circuits such as the track portion 5 for no or very small additional cost is a major advantage of thick film heating elements.

The two thermally-sensitive devices 3 and 4 could be constituted by a single device having two bimetals arranged to operate at different temperatures and such devices are known and even incorporate manual reset possibilities. For the arrangement of Figure 3, the temperature maintained non self-resetting device could be as described in our British Patent Application No. 9807924.7 and the direct spring connections to the heating element that such device provides would eliminate the internal wiring necessary with other devices. Since thick film heating elements may overheat locally if they are operated on a slope, any suitable means of combating this may be incorporated, for example by mounting the heating element at an inclination to the horizontal and locating a protective thermally-sensitive device at its highest part or by use of PTC/NTC slope protection as described in our British Patent Application No. 9817594.6.

The present invention as thus described hereinbefore provides a single thick film heating element having multiple heater output power levels configure and selectable to match closely the different heating requirements of an appliance operating cycle, and provides such advantages for little or no additional cost. The complexity of such a heating element is limited only by the availability of space on the heating element surface and the imagination of the heating element designer. The printed circuit track of the heating element can be provided with terminal portions enabling the power supply to the heating element and the connections to the thermally-sensitive devices 3 and 4 to be made directly by means of spring connector parts which eliminates the need for much of the appliance internal wiring, the heating element serving in this regard as a printed circuit board. This enables straightforward assembly methods to be utilized in which all necessary electrical connections are made before assembly of the heating element into the appliance. Furthermore, the assembly can be compact, thereby maximising the water capacity of the appliance in relation to its size.

Having described the present invention in the foregoing by reference to preferred embodiments, it is to be well appreciated that the described embodiments are exemplary and are capable of being modified or varied without departure from the spirit and scope of the invention as set forth in the appended claims.

Claims

CLAIMS: 1. An electrically powered liquid heating appliance having an operating sequence wherein different power output levels are required of the appliance heater at different times, the appliance heater comprising a thick film heating element having plural heating portions arranged to be selectively connected in circuit so as to provide said different power output levels.
2. An appliance as claimed in claim 1 wherein said plural heating portions comprise first and second portions arranged to be selectively connected in parallel.
3. An appliance as claimed in claim 1 wherein said plural heating portions comprise first and second portions arranged to be selectively connected in series.
4. An appliance as claimed in any preceding claim wherein the different power output levels are selected by means of a thermally-responsive device.
5. An appliance as claimed in claim 4 wherein said thermally-responsive device is non self-resetting.
6. An appliance as claimed in any preceding claim wherein the operating sequence of the appliance comprises a first period wherein the power output level of the appliance heater is at a first level and subsequently the power output level is reduced.
7. An appliance as claimed in claim 6 adapted and arranged to elevate the temperature of a volume of liquid towards its boiling point at a high power output level of the heater, and thereafter to reduce the power output level of the heater so that the appliance maintains the liquid at its boiling temperature.
8. An appliance as claimed in claim 7 adapted and arranged for operation as a water purifier, the appliance comprising a container for a volume of water, said heater being associated with said container for boiling water therein, and a condenser coupled to said container to receive steam therefrom and condense it into purified water.
9. An appliance as claimed in any preceding claim including a thermally-responsive device arranged to cut off the power supply to the heating element in response to the heating element temperature rising above a predetermined level.
10. An appliance as claimed in claim 9 wherein the thick film heating element includes a further heating portion adapted and arranged to be energised when said thermally responsive cut off device operates to cut off the power supply to the heating portions first mentioned, said further heating portion supplying heat to said thermally responsive device to prevent the same from resetting.
11. An appliance as claimed in claim 9 or 10 arranged as a cordless appliance comprising a vessel part, a base part and electrical connectors on said vessel part and said base part which co-operate to enable the supply of electrical power to the vessel part when it is seated on the base part and the connectors are operatively mated, the arrangement of said appliance being such that said thermally responsive cut off device is arranged to be reset when the vessel part is reseated on its base part after having been lifted therefrom and refilled.
12. An electrically powered liquid heating appliance substantially as herein described with reference to Figure 1 or Figure 2 or Figure 3 of the accompanying drawings.
13. A thick film heating element adapted and arranged for use in an appliance as claimed in any of the preceding claims.