GB2274915A

GB2274915A - An electrically heated oven

Info

Publication number: GB2274915A
Application number: GB9300387A
Authority: GB
Inventors: Michael Edward Dennis
Original assignee: IST Laboratories Ltd; Pifco Ltd
Current assignee: IST Laboratories Ltd; Pifco Ltd
Priority date: 1993-01-11
Filing date: 1993-01-11
Publication date: 1994-08-10
Anticipated expiration: 2013-01-11
Also published as: GB9300387D0; GB2274915B

Abstract

An oven comprises a lining (2) constituted by an assembly of panels (6, 7, 8, 9 and 10) which comprise thick-film printed circuits defining heating elements. A control panel (12) is edge connected to a top panel (6) and is also formed as a thick-film printed circuit formed on a metal substrate. The circuits forming the panels also comprise interconnecting conducting tracks (17) and temperature sensing tracks (15). A partitioning panel (33) is removably located at a position dividing the oven into an upper and lower chamber and is provided with an edge connector (34) for energising a thick-film printed circuit heating element formed thereon. <IMAGE>

Description

"AN OVEN" This invention relates to an oven and in particular but not exclusively to an oven of a domestic cooker.

It is known to provide electrically powered ovens for domestic cookers in which heating elements are contained within mineral based insulators. The heating elements are mounted on supporting brackets so as to be located between a lining which defines an oven chamber and an insulating jacket which encases the lining.

In order to achieve satisfactory heat transfer from the heating element to the contents of the oven chamber the element is typically run at a temperature in the range 10000C to 12000C. Electrical insulation to conductor wires supplying these elements must be able to withstand such temperatures and the insulating jacket must be adequate to provide sufficient thermal insulation to protect adjacent equipment. Typically the insulating jacket extends to two inches in thickness and this represents a substantial reduction in the available oven chamber size for an oven of given external dimensions.

According to the present invention there is disclosed an oven comprising a lining defining an oven chamber, an assembly of panels constituting the lining, and an electrical heating means operable to heat the lining, wherein one or more of the panels comprises a thick-film printed circuit formed on a metal substrate and having respectively one or more conductive tracks defining one or more heating elements constituting the heating means.

An advantage of such an oven is that, because the thermal transfer into the oven chamber is distributed over the oven lining rather than being localised between the lining and the insulating jacket, a satisfactory heat transfer can be achieved using a much lower operating temperature of the heating elements. It has been found that satisfactory results can be achieved using a heating element temperature which is only 50 0C above the maximum oven temperature, the maximum oven temperature typically being 2200C, thereby achieving a substantial reduction in the operating temperature of the heating element.

Consequently the degree of thermal insulation required to be provided by the jacket is substantially reduced. In practice it has been found that the thickness of the jacket can be reduced by 75% and typically will be one-half inch in thickness. For domestic ovens required of standard external size to fit into fitted kitchens and the like this is particularly important since it allows a greater volume of the oven chamber to be achieved for an oven of a given external size.

Preferably the metal substrates are formed of stainless steel.

Preferably the oven comprises a control circuit operable to supply electric current to the heating elements, the panels comprising a first panel to which the control circuit is connected and further panels electrically connected to the first panel by means of first edge connector means operable between respective edge portions of adjacent panels.

The use of edge connector means may thereby partially or completely obviate the need for a conventional wiring harness thereby simplifying connection between the control circuit and heating elements.

Advantageously the first panel comprises conductive tracks constituting power supply conductors providing connection between the control circuit and the heating elements of the further panels. The further panels are thereby placed in electrical communication with the control circuit without the need for a conventional wiring harness.

Preferably the control circuit comprises switch means operable to selectively energise the heating elements.

The heating elements of different panels of the oven lining can thereby be selectively energised. A single panel may also have more than one heating element provided by a corresponding number of conductive tracks and one or more of these heating elements may be selectively energised. The proportion of total available power dissipation can thereby be selected. During warm-up, when maximum total available power is required, all available elements may be selected, a reduced number of heating elements being selected once a required temperature has been attained so that only a small proportion of total available power dissipation is used to maintain constant temperature.

Preferably the thick-film printed circuits comprise further conductive tracks of measurable resistance and constituting temperature sensors connected to the control circuit.

In a preferred embodiment the planar panels include opposing left and right-hand vertical side walls, a rear wall, a bottom wall and a top wall defining the oven chamber. A top panel constituting the top wall may have a heating element connected to the control circuit so as to be operable independently of the heating elements of the remaining panels whereby the top wall panel constitutes an oven grill.

Conveniently the top wall panel constitutes the first panel to which the control circuit is connected.

Conveniently the control circuit comprises a thick-film printed circuit formed on a metal substrate and electrically connected to the thick-film printed circuit of the first panel by a second edge connector means operable between respective edge portions of the control circuit and the first panel.

Advantageously the substrate of the control circuit constitutes a control panel of the oven.

The oven may comprise a partitioning panel connected to the lining and located within the chamber so as to partition the chamber into upper and lower portions. The partitioning panel may comprise a thick-film printed circuit formed on a metal substrate and having one or more conductive tracks defining a heating element.

Conveniently the conductive tracks of the partitioning panel are electrically connected with conductive tracks of a panel of the liner by means of a third edge connector means operable between an edge portion of the partitioning panel and a planar portion of the panel of the liner.

Conveniently the partitioning panel is removably connected to the lining.

The partitioning panel may thereby be omitted or included as required.

The partitioning panel may alternatively comprise an insulating sheet.

Division of the oven chamber into upper and lower portions allows those portions to be operated independently.

Conveniently at least one of the panels constituting the lining comprises first and second heating elements located so as to heat the upper and lower portions of the chamber respectively and the first and second heating elements may be independently controllable to facilitate independent heating of the upper and lower portions.

One advantage of such an arrangement is that it provides the user with the option of operating only one portion of the chamber thereby achieving energy savings when heating small quantities of food.

The lining panels may be releasably connected to one another to facilitate dis-assembly and re-assembly.

Individual panels may thereby be removed for cleaning. The complete assembly of panels may alternatively be removable in its entirety.

Preferably the lining is encased in an insulating jacket having a thickness determined such that the internal oven volume defined by the oven chamber is more than or equal to 75% of the external oven volume defined by external surfaces of the oven.

A more efficient use of space in a cooking environment is thereby achieved.

A preferred embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings of which: Figure 1 is a partially exploded perspective view of an oven lining, control panel and door of an oven in accordance with the present invention; Figure 2 is an exploded perspective view of the oven of Figure 1 showing additionally an insulating jacket; Figure 3 is an exploded view of an edge connector of the oven of Figures 1 and 2; and Figure 4 is an assembled view of the edge connector of Figure 3.

An oven 1 as shown in Figure 2 consists of a lining 2 defining an oven chamber 3 and covered with an insulating jacket 4 which is held in place by a perforate metal cladding 5.

The lining 2 as shown in Figure 1 is formed by an assembly of planar stainless steel panels 6 to 10 forming an open fronted cube and comprising a top panel 6, a right-hand panel 7, left-hand panel 8, bottom panel 9 and a rear panel 10.

The oven chamber 3 is closed by a door 11 hinctedly mounted on the bottom panel 9.

A control panel 12 is connected to the top panel 6 so as to project upwardly from a forward edge portion 13 of the top panel.

Each of the panels 6 to 12 is formed as a thick-film printed circuit on a stainless substrate in a process in which the substrate is heat treated to form a chromium oxide surface layer, dielectric layers are applied to selected areas of the substrate and thick-film inks then applied to define conductive tracks as detailed below. In the case of the control panel 12, active and passive discrete components are surface mounted in contact with conductive tracks to form a hybrid circuit. The conductive tracks are encapsulated in a final insulating layer.

The top panel 6 has a heating element 14 formed by such a conductive track which extends over the surface of the panel in serpentine manner as shown in Figure 1. A temperature sensing track 15 is similarly formed on the top panel 6 and extends in proximity with the heating element 14. The heating element 14 and temperature sensing track 15 terminate in an edge connector 16 which provides electrical contact with corresponding tracks on the control panel 12. The heating element 14 is formed in a number of separate lengths to allow different lengths of the heating element to be selectively energised.

The top panel 6 is also provided with inter-connecting tracks 17 communicating between edge connectors 18, 19 and 20 co-operating with the control panel 12 and respective edge connectors 21, 22 and 23 co-operating with the left-hand panel 8, the rear panel 10 and the right-hand panel 7. The right-hand panel 7 also includes inter-connecting tracks 24 communicating between edge connector 23 and a further edge connector 25 co-operating with the bottom panel 9.

Each of the edge connectors 16, 18 to 23 and 25 is of a type shown in Figures 3 and 4 which illustrate connection between panels 6 and 7. Spring strips 35 surface mounted in contact with inter-connecting tracks 17 on panel 6 are engageable with contact pins 26 extending from a surface mounted conductor 27 on the panel 7. Panels 6 and 7 are mechanically assembled as shown in Figure 4 by means of a screw engaged mounting block 28.

The lining 2 is thereby assembled such that each of the panels 6 to 10 is electrically connected to the control panel 12 in a manner which facilitates control of heat dissipated in the panels and allows the temperature of the panels to be sensed.

The control panel 12 is provided with control potentiometers 29 each of which incorporate a resistive track printed on the control panel as part of the thick-film circuit and which is engaged by a sliding contact (not shown) of the potentiometer.

The control circuit 12 also incorporates triac power control components 30 and an integrated circuit chip 31 operable to control the switching function of the triac components.

The left and right-hand panels 8 and 7 are provided with welded supports 32 for supporting oven shelves (not shown) and for receiving a partitioning panel 33 which is formed as a thick-film circuit to provide both heating element and temperature sensing track in the manner described with reference to the above panels 6 to 10. The partitioning panel 33 includes a male edge connector 34 which is engageable with a female connector (not shown) surface mounted on the rear panel 10 such that the partitioning panel 33 when inserted into the chamber 3 can be heated in a controlled manner by the control panel 12.

Both the top panel 6 and the partitioning panel 33 may serve as respective oven grills to allow cooking by radiating heat downwardly on to food placed immediately beneath these respective panels. The panels 6 to 10 may be simultaneously energised to provide normal oven function in which the internal temperature of the chamber 3 is maintained at a controlled value.

The panels 6 to 10 are provided with respective heating elements 14 which are formed on external surfaces of the panels with respect to the oven chamber 3 so that heat generated by the heating elements is conducted through the substrate. The source of heat is therefore no longer localised as in the case of a conventional oven in which a relatively small element is supported within the oven chamber and this provides a number of important advantages for the present invention.

Since the heating elements 14 are printed directly on to the oven lining panels, the maximum temperature at which the heating element is required to run is at approximately 500C above the maximum oven temperature of 2200C i.e. less than 3000C.

Less thermal insulation is therefore required compared with a conventional 10000C element thereby enabling the insulator material to be 25% of the thickness of the conventional oven insulator. Taking for example a typical conventional oven chamber size of 1.87 cubic feet provided by 18 inch width and 12 inch height and 15 inch depth, this reduction in insulator thickness allows an oven in accordance with the present invention and having the same external dimensions to have an increased oven chamber volume of 3.28 cubic feet (i.e. internal dimensions of 21 inches, 15 inches and 18 inches).

Further advantages are that there is no longer a requirement to provide fixing brackets for the heating element and it is also no longer necessary to provide wiring with high temperature insulation to the heating element since in accordance with the present invention the heating elements and current carrying conductors are formed integrally with the lining.

By providing temperature sensing tracks of appropriate temperature co-efficient of resistance, the temperature of each panel can be sensed and controlled thereby giving improved thermal performance within the oven chamber.

Optionally the conducting tracks providing the heating elements may be provided in multiple sections having different proportions of the total available power dissipation for each panel so that different power inputs from each panel can be obtained by switching between different portions of the heating elements. If for example a panel is provided with three tracks each having 1/2, 3/8 and 1/8 of the total power dissipation, then all three tracks may be powered during an initial warm-up phase and the lower wattage element (1/8) could be powered when required to maintain a constant temperature. The available combinations of two out of three portions additionally enable intermediate stages of power to be selected.

This form of control allows changes of temperature within the oven chamber to be achieved with minimal overshoot and reduces the requirement for switching high currents when maintaining constant temperature.

As mentioned above, both the top panel and the partitioning panel may be used as oven grills in which case their operating temperature would be controlled 0 to be approximately 450 C when operating as a grill. A significant improvement in the quality of food grilled in this manner is obtained since the heat radiated on to the food is radiated from the panel in a far more evenly distributed manner than is associated with conventional electric heating grills thereby avoiding unevenness in the manner in which the food is cooked. Furthermore, the operating temperature of 4500C is considerably less than the typical operating temperature of a conventional grill providing the same energy input to the food and in consequence the radiation responsible for cooking tends to be concentrated at longer wave lengths.

This provides the advantage of improved quality of the cooked food with less surface charring.

The upper and lower portions of the oven chamber partitioned by the partitioning panel may be heated to different operating temperatures by selectively energising and temperature controlling heating elements of the panels forming the lining. The partitioning panel may include a heating element as described above or may alternatively be formed as a passive sheet which may be formed of an insulating material to facilitate independent temperature control of the upper and lower portions of the chamber. Such an arrangement allows the oven to operate if required in an energy saving mode in which only one portion of the oven chamber is heated.

The panels forming the lining may be removably connected to one another so as to be removable for cleaning.

Alternatively the entire assembly of panels may be removably mounted so as to be removable as a complete assembly.

The rear panel in an alternative configuration may be a passive panel (i.e. having no heating element) and may be permanently mounted so as to provide a supporting structure to which the remaining panels are releasably connected. Such a rear panel would require less cleaning since it would not be provided with a heating element.

Claims

CLAIMS:

1. An oven comprising a lining defining an oven chamber, an assembly of panels constituting the lining, and an electrical heating means operable to heat the lining, wherein one or more of the panels comprises a thick-film printed circuit formed on a metal substrate and having respectively one or more conductive tracks defining one or more heating elements constituting the heating means.

2. An oven as claimed in claim 1 wherein the metal substrates are formed of stainless steel.

3. An oven as claimed in any preceding claim comprising a control circuit operable to supply electric current to the heating elements, the panels comprising a first panel to which the control circuit is connected and further panels electrically connected to the first panel by means of first edge connector means operable between respective edge portions of adjacent panels.

4. An oven as claimed in claim 3 wherein the first panel comprises conductive tracks constituting power supply conductors providing connection between the control circuit and the heating elements of the further panels.

5. An oven as claimed in any of claims 3 and 4 wherein the control circuit comprises switch means operable to selectively energise the heating elements.

6. An oven as claimed in any of claims 3 to 5 wherein the thick-film printed circuits comprise further conductive tracks of measurable resistance and constituting temperature sensors connected to the control circuit.

7. An oven as claimed in any of claims 3 to 6 wherein the planar panels include opposing left and right-hand vertical side walls, a rear wall, a bottom wall and a top wall defining the oven chamber and wherein a top panel constituting the top wall has a heating element connected to the control circuit so as to be operable independently of the heating elements of the remaining panels whereby the top wall panel constitutes an oven grill.

8. An oven as claimed in claim 7 wherein the top wall panel constitutes the first panel to which the control circuit is connected.

9. An oven as claimed in any preceding claim comprising a partitioning panel connected to the lining and located within the chamber so as to partition the chamber into upper and lower portions.

10. An oven as claimed in claim 9 wherein the partitioning panel comprises a thick-film printed circuit formed on a metal substrate and having one or more conductive tracks defining a heating element.

11. An oven as claimed in claim 10 wherein the conductive tracks of the partitioning panel are electrically connected with conductive tracks of a panel. of the liner by means of a third edge connector means operable between an edge portion of the partitioning panel and a planar portion of the panel of the liner.

12. An oven as claimed in claim 9 wherein the partitioning panel comprises an insulating sheet.

13. An oven as claimed in any of claims 9 to 12 wherein the partitioning panel is removably connectable to the lining.

14. An oven as claimed in any of claims 9 to 13 wherein at least one of the panels constituting the lining comprises first and second heating elements located so as to heat the upper and lower portions of the chamber respectively and wherein the first and second heating elements are independently controllable to facilitate independent heating of the upper and lower portions.

15. An oven as claimed in any of claims 3 to 14 wherein the control circuit comprises a thick-film printed circuit formed on a metal substrate and electrically connected to the thick-film printed circuit of the first-panel by a second edge connector means operable between respective edge portions of the control circuit and the first panel.

16. An oven as claimed in claim 15 wherein the substrate of the control circuit constitutes a control panel of the oven.

17. An oven as claimed in any preceding claim wherein at least one panel comprises a plurality of heating elements of different power rating.

18. An oven as claimed in any preceding claim wherein the lining panels are releasably connected to one another to facilitate dis-assembly and re-assembly.

19. An oven as claimed in any preceding claim wherein the heating elements of respective panels of the lining are provided on surfaces of the panels external to the chamber.

20. An oven as claimed in any preceding claim wherein the lining is encased in an insulating jacket having a thickness determined such that the internal oven volume defined by the oven chamber is more than or equal to 75% of the external oven volume defined by the external surfaces of the oven.

21. An oven substantially as hereinbefore described with reference to and as shown in the accompanying drawings.