GB2258126A

GB2258126A - Cooling and ventilating oven heated with micrwaves and other means

Info

Publication number: GB2258126A
Application number: GB9115722A
Authority: GB
Inventors: Richard John Lane; John Rice
Original assignee: Creda Ltd
Current assignee: Creda Ltd
Priority date: 1991-07-20
Filing date: 1991-07-20
Publication date: 1993-01-27
Anticipated expiration: 2011-07-20
Also published as: GB9115722D0; GB2258126B

Abstract

An oven has combined microwave heating sources and other e.g. electrical resistance, heating sources. To prevent overheating of controls 7, 8 above the oven cavity, a fan 11 is provided. This fan is also used to set up an air flow through the oven when microwave heating is used in order to discourage the formation of condensation which is otherwise prone to occur. An actuator 20 lifts a flap 19 to the dotted position to divert the air from the fan through the oven. <IMAGE>

Description

Oven With Combined Microwave and Other Heating Sources This invention relates to ovens with combined microwave and other heating sources.

It has been proposed with ovens using only non-microwave heating sources e.g. electrical resistance heating sources, to provide a fan to direct an air flow onto controls for the oven, in order that the controls remain within their operating temperatures. Such controls are for temperature control of the oven or for controlling other functions, and are electronic or electromechanical in nature. Ventilation in such ovens is not a problem. A balance is chosen between a smaller vent opening corresponding to smaller heat losses, and a larger opening to remove more moisture and cooking products.

With microwave only ovens, on the other hand, there is less of a problem with controls becoming excessively hot, since the heating action in the oven cavity is by radiation on the water molecules, rather than by conduction or convection, but there is however a ventilation problem.

Because the oven walls are not heated, a larger opening than for an oven with conventional heating sources would be required to overcome condensation problems. However, this would complicate the construction because a mesh or other means would be needed to cover the larger opening to prevent the escape of microwave radiation. To overcome condensation problems, it has therefore been proposed to provide a fan to create a forced air flow through a smaller vent in the oven.

The invention provides an oven with combined microwave and other heating sources, which comprises a fan mounted outside the oven cavity for directing an air flow onto controls for the oven, and a flap which is movable to uncover an opening in the oven cavity when the microwave heating source is used, in order to permit a fan-assisted air flow through the oven to take place.

The movable flap enables one fan to perform the duty of keeping the controls cool as well as being used to discourage condensation on the oven walls during microwave heating.

The energisation of the fan motor and the means for moving the flap may be arranged to take place simultaneously when the microwave heating source is used. Separate means e.g thermostats may then be provided in order to de-energise the means for moving the flap and for energising the faun, if not already energised, when a predetermined temperature is reached, irrespective of whether microwave heating is taking place or not.

An oven with combined microwave and electrical resistance heating sources will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of the combination oven; Figure 2 is a schematic top view of the oven with the top plate and the controls removed; Figure 3 is a side view of the actuator of Figure 1 for moving the flap, on an enlarged scale compare to that of Figure 1; Figure 4 is a side view of an alternative form of an actuator for moving the flap of Figure 1, again on an enlarged scale compared to Figure 1; Figure 5 is a perspective view of a part of the actuator shown in Figure 4; and Figure 6 is a circuit diagram of parts of the combination oven.

Referring to Figures 1 and 2, the combination oven is built-in below an upper panel 1 and a lower panel 2, and has a cavity 3, closed by a door 5 pivotable at its lower end.

The rear wall of the oven bears a sheathed resistance heating element mounted behind a cover (shown schematically at 4), and a fan (not shown) is provided to provide circulation of the hot air. Above the oven cavity 3 is a control panel 6 having push button controls connected by means not shown to control units 7, 8 which control the temperature in the oven by means including semiconductors which cannot be allowed to overheat. The region above the oven cavity also houses a magnetron 9 (shown schematicallsy-) from which microwaves are guided into the oven cavity. The region above the oven cavity is closed by a top plate 10.

The oven can be operated in a conventional mode, with the resistance heating elements 4 heating the food, in a microwave mode, with microwaves from the magnetron 9 heating the food, or in a combination mode with both the elements 4 and the microwaves heating the food.

When the oven is operating in the conventional mode, the walls and the roof of the oven are heated, and the control units 7, 8 need to be cooled when the temperature in the upper region of the cooker exceeds a certain value. For this reason, a fan 11 driven by a motor 12 (Figure 6) is provided. Fan 11 is a tangential fan, i.e. the fan blades are spaced around the surface of the cylinder. The fan draws in air at its centre and expels it through opening 12a in housing 13. Thermostat 14 (Figure 6) in the upper region above the oven cavity, (and attached to a box containing a mode stirrer) is normally open but closes when its temperature exceeds 600C. Thus, when the oven is operating in a conventional mode, fan 11 is set into action at full speed when the temperature in the upper region above the oven cavity rises above 600C.Opportunity is taken of the fact that the oven is a combination oven, since cooling of the controls is assisted by a fan 17 driven by a motor 18 driven at half speed via resistor 26. The fan 17 and motor 18 are provided for cooling the magnetron, and are normally energised at full speed when microwave operation is selected. Energisation at half speed in the conventional mode of operation is via thermostat 14. In addition to assisting cooling of the controls, fan 17 dilutes combustion products expelled from vent 16 in the roof of the oven by means of a cowl 29 which passes over the vent (Figure 2).

This prevents the combustion products from condensing and causing excessively high temperatures in the fascia region.

When the oven is operating in a microwave mode, the walls of the oven are not heated and water is prone to condense on them while it is being driven out of the food being heated by energisation of the water molecules in the food by the microwave action.

In accordance with the invention, a flap 19 movable by an actuator 20 from the solid line to the dotted line position to uncover an intake 21 into the oven cavity diverts the output of the fan 11 which is now energised into the oven to set up an air flow through the oven. The air flow emerges at vent 16 in its roof.

The way in which this is achieved is as follows.

Control logic 22 connects one pole of a motor 23 for the mode stirrer (not shown) to live when microwave operation has been selected, either alone or in combination with conventional heating. This energises the coil 24 of a relay to hold contacts 25 closed: thus the motors 12, 18, 23 remain connected until the control logic goes open circuit.

The control logic 22 serves to energise the fan motors 12, 18, when microwave operation has been selected, but fan 11 rotates at half its normal speed since the motor 12 has been energised via resistor 26. Thermostat 27 is normally closed and only opens when its temperature exceeds 600C, so that, below this temperature, thermoactuator 28 is energised in order to open the flap, and divert the air flow from the fan 11 through the oven. Thermostat 27 is positioned, with thermostat 14, on the mode stirrer box. Thus, oven ventilation results from the air flow from half-speed fan 11 being diverted from flap 19 into the oven cavity, and the full speed air flow from magnetron cooling fan 17 serves to dilute the emerging combustion products to discourage condensation on, and prevent excessive temperatures of, the controls.

Referring to Figure 3, in one form the actuator comprises a piston 29 connected to the flap 19 which is movable, upwardly as seen in Figure 3, by the expansion against a spring of a wax-like pellet in a cylinder 30 mounted on an arm 31 which pellet expands on being heated by an electrical heating element (not shown). Thus, provided the thermostat 27 temperature is below 600C, the flap 19 is opened when the microwave mode is selected, and only closed when the non-microwave conventional mode is selected.

If the thermostat 14 reaches 600C in the microwave mode or in the combination microwave/conventional mode (it is thought that this would be unlikely to happen until the oven has been on for at least an hour), the thermostat closes so that the fan motor 12 is now run at full speed and, at the same time, the thermostat 27 opens thus de-energising thermoactuator 28 and closing the flap. While less likely to happen, it is still remotely possible for the controls 7,8 to overheat when microwave heat is being applied, and the action of the thermostats 14 and 27 serve to override the fan flow through the oven and concentrate the air flow on cooling the controls.

As an alternative, the flap actuator may consist of bimetal strip 32 (Figures 5 and 6) which deflects an arm 19a on flap 19 to selected the "controls cooling" and "oven ventilating" modes. The bimetal strip 32 has an associates electrical heater e.g. a heating wire 33 wound around a ceramic former, the heating wire being connected where the thermoactuator 28 is connected in Figure 6. If desired, the bimetal strip could be changed in order that lack of heating of it caused the flap 19 to open and heat by conduction and convection from the oven allowed the flap to close.

Although the conventional electrical heating source 4 as illustrated heat the whole oven, if desired they could be supplemented or replaced by electrical heating elements arranged in the form of a grill.

Various modifications may of course be made without departing from the scope of the invention. Thus, the invention is also applicable to combined microwave and non-microwave heating sources of any type e.g. gas heating sources. Equally, while in the described arrangement the selection of the microwave mode sets the fan 11 into action as well as opens the flap 19, the fan 11 may be activated when any oven mode is selected, and the flap 19 may just be actuated when the microwave mode is selected. Also, the relay 24, 25 could be a thermal relay e.g. the contacts 25 could include a bimetallic element, and the coil 24 could be a resistance heating element. This would have the advantage that the controls cooling function cf the fan 1 and, above 600C, the fan 12, would take place either when the bimetallic contacts were heated by the resistance heating element 24 when a microwave mode was selected, or when the thermal relay, positioned at the back of the fascia plate 6, became too hot anyway even if the resistance heating element 24 was not energised.

Claims

CLAIMS 1. An oven with combined microwave and other heating sources, comprising a fan mounted outside the oven cavity for directing air flow onto controls for the oven, and a flap which is movable to uncover an opening in the oven cavity when the microwave heating source is used, in order to permit a fan-assisted air flow through the oven to take place. 2. An oven as claimed in claim 1, including electrically-operated means for moving the flap, the electrically-operated means being arranged to be energised in response to energisation of the microwave heating source. 3. An oven as claimed in claim 2, in which the fan is arranged to be energised in response to energisation of the microwave heating source. 4. An oven as claimed in claim 3, in which the energisation of the fan is via resistive means to provide operation at reduced speed for air flow through the oven. 5. An oven as claimed in any one of claims 1 to 4, including a first thermostat arranged so that the fan directs airflow onto the controls when the temperature of the thermostat exceeds a predetermined value. 6. An oven as claimed in any one of claims 1 to 5, including a second thermostat arranged so that the flap closes the opening when the microwave heating source is used, when the temperature of the second thermostat exceeds a predetermined value. 7. An oven as claimed in any one of claims 1 to 6, in which a fan for cooling the magnetron is also arranged to dilute cooking fumes emerging from a vent in the oven. 8. An oven as claimed in any one of claims 1 to 7, in which the flap is arranged between the fan and the controls. 9. An oven as claimed in claim 8, in which the flap is pivotable about an edge. 10. An oven as claimed in any one of claims 1 to 9, in which the other heating source is an electrical resistance heating source. 11. An oven substantially as herein defined with reference to the accompanying drawings. Amendments to the dawns have been filed as follows

1. An oven with combined microwave and other heating sources, comprising a first fan for cooling the means for generating the microwave radiation, a second fan mounted outside the oven cavity capable of directing air flow onto controls for the oven, and a flap which is movable to uncover an opening in the oven cavity when the microwave heating source is used, in order to direct into the oven an air flow generated by the second fan.

2. An oven as claimed in claim 1, including electrically-operated means for moving the flap, the electrically-operated means being arranged to be energised in response to energisation of the microwave heating source.

3. An oven as claimed in claim 2, in which the electrically-operated means is a thermo-actuator.

4. An oven as claimed in claim 2 or claim 3, in which the second fan is arranged to be energised in response to energisation of the microwave heating source.

5. An oven as claimed in claim 4, in which the energisation of the second fan is via resistive means to provide operation at reduced speed for air flow through the oven.

6. An oven as claimed in any one of claims 1 to 5, including a first thermostat positioned in the vicinity of the controls and arranged so that the second fan is energised when the temperature of the thermostat exceeds a predetermined value.

7. An oven as claimed in any one of claims 1 to 6, including a second thermostat positioned in the vicinity of the controls and arranged so that the flap closes the opening when the microwave heating source is used, when the temperature of the second thermostat exceeds a predetermined value.

8. An oven as claimed in any one of claims 1 to 7, in which the first fan is also arranged to dilute cooking fumes emerging from a vent in the oven.

9. An oven as claimed in any one of claims 1 to 8, in which the flap is arranged between the second fan and the controls.

10. An oven as claimed in claim 9, in which the flap is pivotable about an edge.

11. An oven as claimed in any one of claims 1 to 10, in which the other heating source is an electrical resistance heating source.

12. An oven substantially as herein defined with reference to the accompanying drawings.