GB2035767A - Microwave oven - Google Patents

Abstract

A blower 48 introduces heated vapour to the microwave oven chamber 12 through a microwave screen 16 which prevents microwave energy entering the region where the vapour is heated. The blower 48 draws vapour out of chamber 12 through an apertured microwave screen 18 where it is oxidized by the combustion products of a gas burner 60 before being reintroduced to the chamber 12 through the screened inlet 16. Some of the vapour bleeds off through a vent 52 with cooling air blown by a blower 54. The burner operates in a slight negative pressure and burns air rich. <IMAGE>

Description

SPECIFICATION Microwave oven In microwave ovens, such as shown in our copending U.K. application 7909337 (Serial No. 2 018 098) microwave heating is supplemented by auxiliary electric heating elements positioned in the oven.

However, if gas burners are positioned in the oven in place of the electric resistance heaters, it has been discovered that the microwave heating efficiency is very substantially reduced since the microwave energy is absorbed by the gas flame. In addition, for cooking pattern uniformity it is desirable that there be no auxiliary heating units in the cavity which can alter the pattern of the modes in the cavity to reduce microwave heating pattern uniformity.

According to the present invention, there is provided a microwave oven comprising a conductive enclosure; means for supplying the enclosure with microwave energy; and means for introducing a heated vapour into the enclosure through screen means preventing microwave energy in the enclosure from entering the region where the vapour is heated.

More particularly, the vapour may be the products of combustion from a gas fired burner. In the preferred embodiment, the products of combustion are circulated through the oven by a circulating blower through oven inlet and outlet regions. Both the inlet and outlet regions are formed with a plurality of apertures whose diameters are less than a half wavelength of the free space wavelength of the microwave energy and preferably less than a tenth of a wavelength of the microwave energy.

Not only is the burner in a separate region from the oven but the screening means prevent microwaves escaping into this region and being absorbed by the gas flame.

Preferably, microwave energy is fed into the enclosure from a different wall than that through which heated vapour is blown into the oven. More particularly in the embodiments to be described, the microwave energy is fed into the oven enclosure through the bottom wall. Preferably, the microwave energy is radiated through a rotating radiator in the enclosure, as described in the aforementioned application. The radiator may provide a plurality of separate, directional radiation patterns in the enclosure which are moved by rotation of the radiator and which are directed toward a body to be heated. The rotating radiator is preferably covered with a transparent ceramic cover and the body to be heated positioned on a support rack above the radiator.The vapour heated by the products of combustion is directed into the oven by the blower through a region either in the upper rear wall in one embodiment of the invention or through a region in the top wall in another embodiment of the invention and vapour is withdrawn from the oven through a region in the lower portion of the enclosure.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a partially broken away side elevation view of an oven embodying the invention; Figure 2 illustrates a transverse sectional view of the microwave radiator structure of Figure 1 taken along line 2-2 of Figure 1; Figure 3 illustrates a transverse sectional view of a blower portion ofthe oven of Figure 1 taken along line 3-3 of Figure 1; Figure 4 illustrates a sectional view of the burner portion of the oven of Figure 1 taken along line 3-3 of Figure 1 and showing burner details; Figure 5 illustrates a vertical sectional view of an oven illustrating an alternative embodiment of the invention; and Figure 6 is a transverse sectional view of the burner and blower portion of the oven of Figure 5 taken along line 6-6 of Figure 5.

Referring to Figure 1 to 4, there is shown a combination gas convection and microwave stove 10 comprising an oven cavity 12 closed by a door 14.

The cavity 12 has elongated inlet region 16 through which heated vapour is directed into the oven 12.

This inlet region 16 incorporates comprising apertures 20 for preventing the escape of microwave energy from cavity 12 and is preferably located in the back wall of the oven adjacent the top oven wall so that the hottest vapour enters adjacent to the top of the oven. The vapour outlet regions 18 in approximately the middle of the back wall of oven 12 contain apertures 20 for preventing the escape of microwave energy and through which the vapour is drawn out of the oven. The apertures 20 have maximum dimension which are less than a half wavelength of the free space wavelength of the microwave energy radiated into cavity 12, and are preferably holes whose diameters are less than a tenth of the free space wavelength of the microwave energy.

Microwave energy is fed into oven cavity 12, which is a conductive enclosure, by a rotating radiator 22 which may comprise, for example, a plenum 24 whose upper surface is a plate 26 which contains a plurality of apertures 28 through which microwave energy is radiated upwardly into enclosure 12. A central conductor 30 of a coaxial line 32 supports the pelenum 24 by being attached to the centre of plate 26. The conductor 30 extends downwardly from the lower end of coaxial line 32, through a waveguide 34, and through a microwave choke 36 to a bearing assembly 38. An extension of the conductor 30 is connected to a motor 40 below the waveguide 34 to rotate the radiator 22 through the central conductor 30.

Microwave energy from a magnetron 42 is fed through the waveguide 34, coaxial line 32 and plenum 24to be radiated upwardly from slots 28 into the enclosure 12 as a plurality of separate directional radiation patterns. A cover 44 of microwave transparent material is supported over the radiator 22 on bumps 46 on the bottom of enclosure 12 to prevent food juices or other material from being dropped on to the radiator 22.

The door 14 contains a microwave choke region surrounded by a high temperature vapour seal so that most of the microwave energy is trapped at the choke and prevented from being absorbed by the high temperature vapour seal. However, residual microwave energy passing through the choke section is substantially absorbed by the high temperature vapour seal, thereby preventing stray radiation from the oven. The details of such a directive microwave energy rotating radiator and door seal are disclosed in greater detail in the aforementioned U.K. patent specification.

Circulation of vapour through the enclosure 12 is produced by two centrifugal blowers 48 which draw vapour out of the cavity 12 through apertures 20 in the regions 18 and blow it into plenums 50 surrounding the blowers 48 supplying the top apertures 20 in region 16. The upper ends of the plenums 50 have an opening to a vent 52 through which a small portion of the output of the blowers 48 is blown and is mixed with the air blown by a second pair of blowers 54 driven (as are the blowers 48) by motors 56. The blowers 54 draw cool air in from the back of the oven 12 to cool the motors 56 and to mix with vapour from vent 52. The cooled vent vapour then leaves through a screened aperture 58 in the top of the oven.

A burner system 60 is positioned at the bottom of the oven 10 in a compartment adjacent the microwave generator and comprises a horizontal apertured tube 62 extending substantially the entire width of the oven 10. The tube 62 is fed a gas fuel and primary air mixture at one end by a vertical tubular member 64 whose lower end is supplied with air and with gas through a solenoid operated gas valve 66 and a pressure regulator 68. An electrically energized ignitor 70 ignites the mixture coming from the apertures in the tube 62 in the presence of secondary air to produce flue gas products in a burner plenum 72. The flue gas products are drawn into the region of the stove between the blowers 48 and the oven outlet regions 18 where the combustion products are mixed with vapour drawn from the enclosure 12.The burner is preferably run with excess air so that the temperature of the products of combustion is below 1650 C, for example, between 650"C and 1100 C, so that combustion is substantially complete. This heated oxidizing atmosphere contacting vapour drawn from the enclosure 12 substantially immediately oxidizes organic components of food vapours so that the vapours blown back into the enclosure through the region 16 or up the vent 52 into the room are substantially clean. The slight pressure differential between the inlet and outlet of the blowers 48 causes a slight negative pressure in the combustion plenum 72 which accurately regulates the excess air in the gas-air mixture drawn into the combustion plenum 72.

A rack 74 made, for example, of steel rods is supported on bumps formed in the side walls of the enclosure 12 so that the position of the rack may be changed in accordance with well-known oven practice. Positioned below the rack 74 is the rotatable microwave energy radiator 22 which directs microwave energy up through the apertures in the rack 74 and through a microwave transparent plate 76 positioned on the middle of the rack 74 and a microwave transparent dish 78 to a food body 80 such as a joint of meat.

A plurality of top gas burners 82 are provided on a hob in accordance with well-known practice. Thermal insulation 84 is provided around the oven 12 and around the burner plenum 72 to reduce loss of energy from the oven. The entire oven has a skin 86 of, for example, sheet metal surrounding the oven 12, the blower, burner, and microwave supply regions.

During operation, the temperature of the enclosure vapour is sensed by a temperature bulb 88 mounted, for example, in the enclosure 12 on a bracket below the vapour inlet region 16. The positioning of temperature sensor 88 is preferably chosen so that it is not directly in the entering hot vapour stream from region 16 but rather measures the temperature of the vapour eddying around in the enclosure 12. Its location may be so selected that the oven heating cycles have reasonable time periods for burner on and burner off and the temperature range fluctuates by large amounts. When the oven burner is lit and hot vapour is blown into the oven at velocities of 165 to 330 metres per minute is energized, heated is delivered in the form of hot vapour along the top surface of enclosure 12 and a substantially uniform convection heating pattern occurs.

The power level and time of the microwave energy may be controlled, for example, by a control panel 90 containing a manually set timer 92 and a manually set power level 94. A light 96 may be positioned outsidetheenclosure 12 illuminating enclosure 12 through a light transparent high temperture ceramic plate and microwave shielding screen 98.

In the alternative shown in Figures 5 and 6, a combination gas convection and microwave stove 100 comprises an oven cavity 102 of conductive metal closed by a door 104. An apertured convection vapour inlet region 106 is located in the top wall of the oven cavity 102 so that vapour is directed downwardly from the top of the oven. The oven vapour outlet region 108 is an apertured region in the bottom of the back wall of oven cavity 102. A rack 110 is provided made, for example, of steel rods supported on bumps 113 formed in the side walls of the oven.

Positioned below rack 110 is a rotatable microwave energy radiator 114 which directs a plurality of separate microwave energy patterns up through the apertures in rack 110, through a microwave transparent support plate 116 positioned in the middle of rack 110 and through a microwave transparent dish 118 of a food body 120. Apertures in the regions 106 and 108 have maximum dimensions substantially less than a half wavelength of the free space wavelength of the microwave energy which may have a frequency, for example, of 2.45 GHz and preferably the apertures are less than a tenth of the free space wavelength of the microwave energy. The door 104 has a microwave choke and vapour seal like that disclosed in the embodiment of Figures 1 to 4 and hence microwave energy is prevented from escaping from the oven 102. The radiator 114 is also as precisely described, with a cover 122 of microwave transparent material supported over the radiator.

Recirculation of vapour within the oven 102 is achieved by a motor 124 driving a centrifugal blower 126 through a belt 128. The blower 126 sucks vapour from the oven 102 through region 108 and blows the vapour out through ducts 130 into the oven 102 through the apertures in the top wall vapour inlet region 106. A small portion of the output of the blower 126 is blown outwardly through the outlet duct 132 to an exhaust duct where it is mixed with cool air drawn in through vents 134 in the back of the stove 100 to cool the air in the duct 132 which then leaves through a screened aperture 136 in the top back of the stove.

A burner system 138, positioned at the bottom of the stove 100, comprises a horizontal apertured tube 140 extending substantially the entire width of the oven and fed at one end by a vertical tubular member 142 whose lower end is supplied with air from vent 144 and with gas from a gas jet 146 controlled by a gas control valve 148 supplied with gas through a pressure regulator 150. An electrically energized ignitor 152 ignites the air-gas mixture emanating from the apertures in tube 140, causing a flame in a burner plenum 154 to produce flue gas products which are drawn through the blower 126 along with vapour drawn from the oven through vapour outlet apertured region 108 and the heated output of blower 126 is blown into the oven.

An example of power levels for effective food body cooking or browning may be microwave energy applied at a rate of 500-900 watts or around 2,000-3,000 BTU's per hour and a gas burner supplying at a rate of 1465 to 4400 watts (5,000-15,000 BTU's per hour). Such power levels will cook and brown a cake in a few minutes. Since microwave energy is prevented from entering the burner plenum 154, the burner can be operated while microwave energy is being supplied to the oven.

Among many possible modifications, the oven may be fed through apertures located in regions other than the top or back walls, the circulating blower may be inside the oven, and other systems for supplying microwave energy to the oven may be used. In addition, other types of burners such as ribbon or power burners may be used.

Claims (12)

1. A microwave oven comprising a conductive enclosure; means for supplying the enclosure with microwave energy; and means for introducing a heated vapour into the enclosure through screen means preventing microwave energy in the enclosure from entering the region where the vapour is heated.

2. An oven according to'claim 1, comprising a blower for blowing the vapour into the oven.

3. An oven according to claim 2, wherein the blower recirculates vapour through the oven and the heating region.

4. An oven according to claim 3, comprising a vent by way of which a proportion of the recirculating vapour is bled out of the system.

5. An oven according to any of claims 1 to 4, wherein the vapour heating region includes a burner.

6. An oven according to claim 5, wherein the burner is a gas burner.

7. An oven according to claim 6, insofar as dependent claim 3 or 4, wherein the blower establishes such pressure conditions that the gas burns with excess air and the vapour blown into the enclosure is an oxidising vapour.

8. An oven according to any of claims 1 to 7, wherein the screen means is formed by a plurality of apertures in a wall of the enclosure whose maximum dimensions are substantially less than half of a free space wavelength of the microwave energy.

9. An oven according to claim 8, wherein the apertures are circular holes with a diameter less than one tenth the free space wavelength.

10. An oven according to any of claims 1 to 9, wherein the oven has a door provided with a microwave choke seal and a vapour seal.

11. An oven according to claim 10, wherein the choke seal is between the microwave choke seal and the interior of the enclosure.

12. A microwave oven substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 4 or Figures 5 and 6 of the accompanying drawings.