GB2253551A

GB2253551A - Heating element in cooking vessel wall

Info

Publication number: GB2253551A
Application number: GB9204875A
Authority: GB
Inventors: Peter William Worrall; Peter William Crossley
Original assignee: ELECTROLUX COOKERS
Current assignee: ELECTROLUX COOKERS
Priority date: 1991-03-05
Filing date: 1992-03-05
Publication date: 1992-09-09
Also published as: GB9104584D0; GB9204875D0

Abstract

An electrically heated cooking apparatus comprises a heating element 4 bonded to at least a portion of a surface of the cooking vessel to form intimate thermal contact with the vessel and means for controlling at least the temperature of the vessel contents. Preferably a metal vessel is glazed, a thick film heating element is applied to the outside of the vessel base and covered with a further glaze. The vessel may be secured by a bayonet fitting to an insulating base that connects the heating element to electric power. Portions of the element may be switched between series and parallel connection to control the power consumed. <IMAGE>

Description

Improvement in Electrically Heated Cooking Apparatus This invention relates to electrically heated cooking apparatus. Known electrically heated cooking apparatus in the form of hobs suffer from disadvantages.

Solid hot plate hobs, for example, are, both slow to heat-up and to respond to changes in control settings.

Radiant rings, however, whilst having better response times, are difficult to clean and glass ceramic hobs, whether powered by open wire, halogen lamp or induction heaters, are expensive to produce.

It is an object of the present invention to provide an electrically heated cooking apparatus which, whilst being of low cost construction, offers an easily cleanable surface and rapid response to changes in power input. It is a further object of the invention to provide improved thermal compling between the heating element and vessel to be heated thereby allowing greatly increased watts densities to be used when compared with those possible with conventional element configurations.

According to the present invention, an electrically heated cooking apparatus comprises a heating element screened on to at least a portion of a surface of the cooking vessel to form intimate thermal contact with the vessel and means for controlling at least the temperature of the vessel contents.

Preferably, the heating element is screened onto the said surface in the form of a thick film.~ The cooking vessel may be made from metallic, for example, stainless steel, alumium, copper, cast iron, or any suitable alloys or from non-metallic materials such as ceramics, glass-ceramics or any suitable temperature resistant synthetic material. Where the vessel is made from an electrically conductive material a layer of thermally conductive and electrically heat insulative material is interposed between the vessel and the heating element.

Thus, for example, where the cooking vessel is .r,ade from stainless steel, the thick film heating element 5 preferably, screened on to a protective underglaze applied to the vessel so as to prevent the vessel becoming electrically conductive during use.

Although the thick film heating element ,ay be applied to any part of the vessel, it is preferably applied to the base of the vessel.

According to one form of cooking apparatus of the present invention, the vessel is supported on a removable base made from electrically insulating material, for example a high temperature plastic such as polyphenylene sulphide or a ceramic material. rL removable base is clamped to the underside of the cooking vessel and trapping in place a seal, which prevents ingress of moisture in the area where electrical connection is to be made.

Conveniently, a heat sensor is also screened or otherwise applied onto the vessel thereby allowir.s through appropriate circuitry, monitoring and/or controlling the temperature of the vessel contents. Similar arrangements for the electrical connections to the sensor apply as for the element. The whole area of ohe element and sensor may be overglazed to prevent contact with the screened tracks, except in the connection area where protection is afforded by the base.

One embodiment of the invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is an sectional elevational through a cooking apparatus according to the invention: Figure 2 is an under plan of Figure 1, and Figure 3 is a circuit diagram of a heating element used with apparatus of Figures 1 and 2.

In Figure 1, a cooking vessel 1 is detachably supported on a base 2 with seal 3 trapped between c.

Thick film heating element and sensor tracks 4, are screened onto a protective underglaze applied to the vessel base to prevent it becoming electrically live.

Electrical conductor strips 5 run between the terminations of the heating element and sensor tracks to contact pads 6 housed in internally extending arms 7 of the base 2. The arms 7 also house magnets which form part of the control device.

Referring to Figure 2, a vessel location pad 8 is an upstanding area of the top of the base having recesses 9 in its top surface which connect with circumferentially extending slots 10, below and parallel to the surface. The base 2 of the vessel shown in Figure 1 fits around the periphery of pad 8 and the contact pad arms 7 pass through recesses 9. In a manner similar to a bayonet fitting, clockwise rotation of the vessel 1 locks the contact pad arms 7 into slots 10, such that contact pads 6 mesh with corresponding contact pads (not shown) within the body of location pad 8.

In order to ensure that the vessel and base as well as electrical contact may be correctly established as explained hereafter, the recesses 9 and associated slosts 10 are non-equiangularly disposed so that the vessel can only be attached to the base in one unique way. Figure 4 detaiLs the bayonet fitting.

Housed within the body of location pad 8 is a series of Reed switches (not shown) which are activated by the magnets housed in contact pad arms 7. Only when vessel 1 is correctly and uniquely positioned, are the Reed switches activated in the correct sequence and can power be connected to the element 4. This positive locking device has the added advantage that vessel 1 cannot easily by knocked or pulled over, lowering the danger of scalding etc.

A cooking apparatus according to the present invention is powered using high power levels of say 3-5 KW. Where, however, such high power levels are used on, for example, saucepans, power control when it is required to simmer or cook the vessel contents at a relatively low heat may lead to control difficulties since Power Supply Regulation IEC 555 specifies the maximum switching frequency. In order to satisfy the IEC 555 requirement, power control may be performed in three stages with the thick film screened heater track elements divided into a plurality of individual heating elements and various power levels may be obtained as follows: 1. High power - obtained by connecting the plurality of heating elements in parallel; 2. Intermediate or first turn-down - obtained by re-configuring the heating elements in a series and parallel conbination, or simply, in series, and 3. Fine control at low power settings - obtained by "mark" spacing serially connnected heating elements to provide, say a maximum power from the elements of 435 watts which is the limit below which regulation IEC 555 does not apply.

The amount of turn-down obtained from switching from parallel to series is for equal resistance track elements in accordance with the equation: P(series) = P(parallel) n2 where P = Power n = no of elements in heater track Values of 2, 3, 4 etc could be usefully used for n.

However, odd numbers are preferable having regard to the need for reversing polarity on switches is obviated. Dividing the heating element into three subsidiary heating elements is preferred because it: i. gives a high turn-down of 9:1; ii. requires only four wires to be brought out of the heater; iii. does not require changes in polarity on switches, and iv. gives the minimum number of switch poles to effect a change-over from series to parallel, namely two.

Although reference has been previously made to the use of a bayonet fitting for attaching the vessel 1 to the base 2, other forms of attachment may be used, for example, a single or multistart thread, or a conventional plug and socket arrangement.

Claims

1. A electrically heated cooking apparatus comprising a heating element screened on to at least a portion of a surface of the cooking vessel to form intimate thermal contact with the vessel and means for controlling at least the temperature of the vessel contents.

2. Apparatus according to claim 1 wherein the heating element is screened onto the said surface in the form of a thick film.

3. Apparatus according to claim 1 or claim 2 wherein the vessel is made from stainless steel, aluminium copper, cast iron, metallic alloys, non-metallic materials, ceramic materials or a mixture of metallic and non-metallic materials.

4. Apparatus according to any one of claims 1 or 2 wherein the vessel is made from an electrically conducted material and wherein a layer of electrically heat insulative material is interposed between the vessel and the heating element.

5. Apparatus according to claim 4 wherein the layer of electrically heat insulative material is a protective glaze applied to the vessel.

6. Apparatus according to any preceding claim wherein the vessel is supported on a removable base.

7. Apparatus according to claim 6 wherein the base is made from electrically insulating material.

8. Apparatus according to claim 6 or claim 7 including a sealing means disposed between the vessel and the base.

9. Apparatus according to any preceding claim including a heat sensor for monitoring and/or controlling the temperature of the vessel contents during cooking.