Electric resistance heating devices – Heating devices – Immersion heater details
Patent
1995-01-26
1998-01-06
Hoang, Tu B.
Electric resistance heating devices
Heating devices
Immersion heater details
392501, H05B 340
Patent
active
057063907
DESCRIPTION:
BRIEF SUMMARY
This Application is a 371 of PCT/GB93/00501 filed on Mar. 10, 1993.
FIELD OF THE INVENTION
This invention concerns improvements relating to electrically powered immersion heating elements and controls therefor and more particularly concerns immersion heating elements and controls for water boiling vessels such as kettles, hot water jugs, pots, pans, urns and laboratory equipment for example.
BACKGROUND OF THE INVENTION
A conventional immersion heating element for an electric kettle or hot water jug comprises an elongate, metal-sheathed, electrically insulated, resistance heating element coupled to an element head plate serving for mounting of the element in a wall of the kettle or hot water jug, the resistance heating element being curved back towards the element head plate to define a so-called hot return portion which is secured to the element head plate, generally at a level above the general plane of the element.
A control for switching off the supply of power to such a heating element in the event of the associated vessel being switched on dry (a so-called dry boil situation) or being allowed to boil dry commonly comprises a bimetallic element or other thermally responsive switch actuator which is held in close thermal contact with the rear of the element head in the region thereof where the hot return portion of the element attaches to the front of the element head plate. An example of such an element protection control is the X1 control manufactured by Otter Controls Limited which is substantially as described in GB-A-2194099 with reference to FIGS. 3A, 3B and 3C of the drawings thereof.
It is further well known to provide a control for switching off the heating element when water boils in the vessel, and this is conventionally accomplished by provision of a second bimetallic element or other thermally responsive switch actuator located so as to be subject to impingement of steam thereon when water boils in the vessel. An example of such a steam sensing control is the J1 control manufactured by Otter Controls Limited which is substantially as described in GB-A-2248519 with reference to FIGS. 5A, 5B and 6 of the drawings thereof.
A single sensor electronic control for a water boiling vessel is described in GB-A-2228634. This control utilizes a thermistor and senses element temperature as a function of the electrical resistance of the thermistor and the onset of boiling as a function of the rate of change of the thermistor resistance. However, even with the current state of modern electronics a circuit capable of switching a mains load cannot readily be manufactured at a price competitive with bimetallic controls.
A single sensor control is proposed in GB-A-1 143 834 (Matsushita) but has never been manufactured, so far as we are aware. According to this proposal, a heating element as above described has an apertured enclosure provided on the element head plate in the region where the element hot return portion attaches thereto. By enclosing the hot return part of the element within an enclosure provided with small openings sufficient to admit water to the interior of the enclosure when the kettle is cold, the proposal of GB-A-1 143 834 was that the steam generated within the enclosure on boiling would drive the water from the enclosure whereupon the hot return portion of the element would overheat and cause the bimetallic or other control to operate. In accordance with the arrangement proposed in GB-A-1 143 834, a single bimetallic or other control will operate both when a kettle boils and when a kettle is switched on dry. As mentioned above, the proposal of GB-A-1 143 834 has not to our knowledge ever been manufactured despite its apparent simplicity and the promised advantage of a single sensor bimetallic control providing both element overtemperature protection and boil sensing. An automatic kettle or hot water jug thus will conventionally have separate element protection and steam sensing controls, and furthermore will commonly have a secondary or back-up element protection function operable
REFERENCES:
patent: 2437262 (1948-03-01), Levitt et al.
patent: 3371192 (1968-02-01), Rosenel
patent: 4697069 (1987-09-01), Bleckmann
patent: 5020128 (1991-05-01), Bleckmann
patent: 5459812 (1995-10-01), Taylor
Hoang Tu B.
Otter Controls Limited
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