GB2035768A - Combination microwave and convection oven - Google Patents

Abstract

A combination microwave and convection oven in which a gas burner 90, positioned outside the oven 12 provides heated combustion products which are drawn from the burner plenum 106 along with vapour from an outlet 18 of the oven by a blower system 64 and the combined output of the blower system is blown into the oven 12 through an oven inlet region 16 in the oven wall. Both the oven outlet 18 and the oven inlet 16 are formed of holes 34 substantially less than one-half wavelength in diameter to provide microwave shielding. A proportion of the blower output vapour is blown through an exhaust vent 68, thereby creating slight negative pressure in the oven 12 and in the burner plenum 106 to accurately control the air flow through the burner 90. An oxidising vapour is thus established which helps keep the oven clean and purifies the vented vapour.

Description

SPECIFICATION Combination microwave and convection oven In microwave ovens, such as shown in our copend ing U.K. application 7909337 (Serial No. 2018098), 'electric heaters are positioned inside the oven to aid in the cooking and/or browning process. Electrically heated air, or air used to cool electrical components such as magnetrons is directed through the oven to aid in controlling vapours produced by a food body.

However, attempts to use the products of combus tion, in cooking regions excited with microwave energy, have been unsatisfactory. If open gas flames are positioned in the oven they can, among other things, act as sources of ionization which absorb large portion of the microwave energy.

According to the present invention, there is pro vided, a combination microwave and convection oven comprising a conductive enclosure, first means for feeding microwave energy into the enclosure, a burner outside the enclosure, and second means arranged to introduce into the enclosure vapour containing products of combustion from the burner.

More particularly, a blower system can blow a mixture, comprising vapour drawn from the oven and vapour containing the products of combustion and secondary air, back into the oven; a small portion of the mixture being blown out of an exhaust vent to create a slight negative pressure in the burner plenum so that a predetermined amount of air will enter the burner.

In the preferred embodiment the temperature of the vapour, blown into the oven, can be controlled by a thermostatic switch which controls and on/off cycle of the burner. The output of the oven circula tion blower enters the oven through the back wall adjacent the upper oven surface. This is found to substantially improve the uniformity of the heating pattern and to assist in browning the upper surfaces of the food products such as pastries, cakes or meat products when using microwave energy, while at the same time using less circulation blower powerthan other convection ovens.

Preferably the burner heats a region of the burner plenum during the time when the burner is ener gized. The oven circulating blower system draws air and oven vapour through the gas burner causing it to heat up. The heated vapour is then circulated through the oven. However, when the burner is off, i.e. during the period when a thermostat senses that the oven temperature has reached a preset value corresponding to a particular cooking temperature -the circulating fan continues to draw air through the burner plenum where it is heated by the heated region of the burner plenum.

It has been found that a very rapid rate of cooking of a food body can occur even when the oven is first energized and the walls are still cool. the uniformity of the cooking pattern is believed to occur due to the velocity of air being directed into the top of the oven from two counter rotating fans spaced behind the oven wall so that the oven inlet duct has a uniform pattern of air issuing from all parts thereof.

Preferably each motor, driving one of the oven recirculating blowers, is separated from the recirculating blower by a second blower on the same shaft as the recirculating blower. The second blower prevents thermal energy, leaking along the motorshaft by conduction, forming heating the motor.

Furthermore the second blower draws air over the motor causing it to cool, and supplies the air to an exhaust vent where it is mixed with a portion of the vapour output from the oven circulating blower, thus cooling it prior to venting.

This invention is particularly useful in a microwave oven wherein the burner is outside the microwave enclosure and the vapour, drawn from the oven is blown back into the oven through the oven wall region having multiple apertures whose maximum dimensions are less than one-half of a free space wavelength of the microwave energy and hence prevent the escape of microwave energy into the blower system or burner.

The invention will be described in more detail, 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 front view of the oven illustrated in Figure 1; and Figure 3 illustrates an enlarged sectional view of the microwave radiator structure of Figure 1, taken along line 3-3 of Figure 1.

Referring now to Figure 1 there is shown a combination microwave and convection oven comprising an oven cavity 12 which is closed buy a door 14 during operation. The cavity 12 has elongated vapour inlet region 16 through which heated vapour is directed into the oven cavity 12, and a vapour outlet region 18 through which vapour is drawn out of the enclosure 12 into a blower input plenum 20. A rack 22 made, for example, of steel rods is supported on bumps 24 formed in the side walls of the enclosure 12 so that the position of the rack 22 may be changed in accordance with well-known oven practice.Positioned below rack 22 is a rotatable microwave energy radiator 26 which directs microwave energy up through the apertures in the rack 22 and through a support plate 28, positioned in the middle of rack 22, and through a dish 30 containing a food body 32 such as a joint of meat. Dish 30 and plate 28 are, preferably, substantially transparent to microwave energy so that the lower region of the food body 32 and the interior portions thereof may be heated effectively by microwave energy.

The regions 16 and 18 have a plurality of apertures 34, for preventing the escape of microwave energy, whose maximum dimensions are substantially 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.

The radiator 26 may comprise, for example, a plenum 36 whose upper surface 38 contains a plurality of aperture 40 through which microwave energy is radiated upwardly into oven cavity 12. A central conductor 42 of a coaxial line 44 supports the penum 36 by being attached to the centre of upper plate 38. The conductor 42 extends downwardly through an outer conductor 46, of the coaxial line 44, and through a waveguide 48 to a microwave choke and bearing assembly 50. An extension of the conductor 42 is rotated by a motor 52 positioned below the waveguide 48.

Microwave energy from a magnetron 54 is fed through the waveguide 54 and the coaxial line 44 to the radiator 26. A blower 56 cools the magnetron 54 by blowing air past fins 58. it should be noted that none of this air passes through waveguide 48. A cover 60 of microwave transparent material is supported over the radiator 26 on centering bumps 62 located on the bottom of the oven 12. This prevent food juices or other materials from being dropped on to the radiator 26. Further details and advantages of this type of microwave oven feed and directive energy rotating structure, as well as a switable door seal, are disclosed in greater detail in the aforesaid copending application. However, any desired microwave feed structure, radiator, and/or door seal could be used.

When closed, door 14 is preferably sealed to enclosure 12 by a high temperature vapour seal with a microwave choke structure positioned between the vapour seal and the interior of enclosure 12. In this way microwave energy radiated into the enclosure 12 is largely prevented from being absorbed by the high temperature vapour seal. However, any microwave energy passing through the choke section is substantially absorbed by the high temperature vapour seal. When door 14 is closed, a latch is mechanically moved to lock door 14 shut and to permit energization of the magnetron 48. Preferably a slight negative pressure is produced within the plenum 20 by a blower system comprising two centrifugal blowers 64 which draw vapour out of the cavity 12, through the apertures 34 and into the plenum 20 and blow it out into the plenums 66 surrounding the blowers 64 and the supplying region 16.The upper ends of the plenum 66 have an opening to an outlet vent 68 through which a small portion of the output of the blowers 64 is blown and is mixed with the air blown by a second set of blowers 70. The blowers 70 draw cool air in from the back of the oven to cool the air in duct 72 which then exits through a screened aperture 74 in the top of the oven.

As shown in Figures 1 and 2, each of the apertured regions 18 supplies vapours, from the oven, to the blowers 64. Each of the blowers 64 is driven, along with one of the blowers 70, by a separate motor 76 which is supported from a back wall 78 of the oven.

A partition 80 between the two blowers 64 prevents tangential interaction of the vapour output of the blowers 64. The blowers 64 preferably rotate in opposite directions to cause the air between the blowers to move upwardly, adjacent to partition 80.

It should be clearly understood that a single blower could be used in place of the dual blowers 64 and that the plenum 66 could have ducting systems to direct the vapour through the openings 16 into the oven. However, it has been found that the dual counter rotating blower system can improve the uniformity of convection heating in the oven.

A burner system 90 positioned at the bottom of oven in a compartment behind and below enclosure 12 comprises a horizontal apertured tube 92 extending substantially the entire width of the oven. The tube 92 is fed, through vertical tubular member 94, with a gas-primary mixture supplied with gas through a gas valve 96. Secondary air is regulated by the spacing of a plate 98 from the bottom of the entrance of the burner plenum. A pressure regular 100 connected to a gas line input 102 supplies the - gas valve 96. When the oven is turned on, an ignitor 108 is electrically energized and heats to a temperature which will ignite an air-gas mixture, whereupon the valve 98 opens. The mixture emanates from apertures in the tube 92 and produces a flame in a burner plenum 106.Flue gas products in the plenum 106 are drawn into the plenum 20, along with excess secondary air, by the blowers 64 where the combustion products and secondary air are mixed with vapour drawn through the apertures 34 from the enclosure 12.

The burner 90, preferably operated with excess secondary air to reduce the temperature of the products of combustion, is below 1650"C so that substantially no oxide products of nitrogen are produced and combustion is substantially complete.

Preferably sufficient excess secondary air is drawn into the plenum 106, by the the negative pressure in the plenum 20, to produce a temperature in the burner plenum 106 of 650into 1 0950C. The plenums 20 and 106 extend substantially the full width of the back of the enclosure 12 so that the burner plenum temperature is substantially uniform across the full width. Thus, since the blower speed is constant, the slight negative pressure in the combustion plenum 106 is substantially constant and accurately regulates the burner primary and secondary air drawn into the combustion chamber. The amount of gas fed into the chamber is also constant due to the pressure regulator 100 so that the excess air is accurately controlled.

A plurality of top gas burners 110 are provided which operate as conventional gas surface burners in accordance with well-known practice. Thermal insulation 112 is provided around the enclosure 12 and around the burner plenum 106 to reduce loss of thermal energy from the oven. A skin 114 of, for example, sheet metal surrounds the enclosure 12, the blower region and the burner and microwave supply region.

During operation, the temperature of the enclosure vapour is sensed by a temperature sensor bulb 120 mounted, for example, in the oven on a bracket 122 below the vapour inlet 16fromthe plenum 66 the positioning of the sensor 120 is preferably chosen so that it is not directly in the entering hot vapour stream from the inlet 16 but rather senses the temperature of the varpour circulating in the enclosure 12. The location of the sensor 120 may be selected so that the oven heating cycles have reasonable time periods for burner on and burner off and the temperature range fluctuates large amounts.

When the oven is energized, the recirculating blowers 64 run continuously and when the burner 90 is energized, heat is delivered in the form of hot vapour substantially directly to the food body 32 in the enclosure 12 and a substantially uniform heating pattern occurs.

The microwave energy power level and timer are controlled, for example, by a control panel 130 containing a timer 132 a power level 134, and on/off buttons 136 and 138. In addition, a light 140 may be positioned outside the enclosure 12 illuminating the enclosure 12 through a light transparent high tem- perature ceramic 142 and a microwave shielding screen 144. The microwave system may be energized,for example, from the AC mains supply.

It has been found that browning of a food body such as the joint 32 can be controlled by blowing hot vapour through the inlet region 16 while radiating full power of microwave energy into the oven. This effect appears to occur due to surface effects caused by the velocity of the hot air drying moisture from the surface of the food body. The microwave energy is more readily absorbed by the dry food body surface causing it to brown thus an added radiant browning element is not necessary.

An example of power levels needed for effective food body browning may be microwave energy applied at a rate of 500-800 watts or around 2,0003,000 BTU's per hour and a gas burner heating rate of about 1300 to 3950 watts or 5,000 - 15,000 BTU's per hour. Such power levels will bake and/or brown a cake in a few minutes.

Among many possible modifications the oven may be fed through apertures located in regions other than the back wall, other systems for supplying microwave energy to the oven may be used and other types of circulating systems and burners may be used.

Claims (14)

1. A combination microwave and convection oven comprising a conductive enclosure, first means for feeding microwave energy into the enclosure, a burner outside the enclosure, and second means arranged to introduce into the enclosure vapour containing products of combustion from the burner.

2. An oven according to claim 1, wherein the second means comprise a blower arranged to recirculate to the enclosure vapour drawn from the enclosure and to entrain the products of combustion in the recirculating vapour.

3. An oven according to claim 1 or 2, wherein the second means also draw into the vapour secondary airforthe burner.

4. An oven according to claim 3, wherein the amount of secondary air is less than 150 percent of that required for substantially complete combustion.

5. An oven according to claim 3, wherein the vapour contains secondary air in the range of 50 to 150 percent.

6. An oven according to any of claims 1 to 5, wherein the second means establishes such pressure conditions that the vapour introduced into the enclosure is an oxidising vapour.

7. An oven according to any of claims 1 to 6, comprising a vent by way of which a proportion of the vapour is bled out of the oven.

8. An oven according to any of claims 1 to 7, wherein the conductive enclosure has an access opening, a closure member, and a seal for the closure member comprising a microwave choice section and a vapour seal section.

9. An oven according to any of claims 1 to 8, wherein the microwave energy enters the enclosure through a rotating radiator.

10. An oven according to any of claims 1 to 9, wherein the burner is a gas burner.

11. An oven according to claim 10, wherein the gas burner is a ribbon type burner.

12. An oven according to any of claims 1 to 11, wherein the vapour enters the enclosure through a screen means which prevents the escape of the microwave energy from the enclosure.

13. An oven according to any of claims 1 to 12, wherein the microwave energy is fed to the enclosure via a coaxial line.

14. A combination microwave and convection oven substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.