GB2042310A - A combination microwave and convection oven - Google Patents

Abstract

A microwave convection oven has a gas burner 90 positioned outside the oven cavity 12. Combustion products are circulated through the oven 12 through screens 16 and 18 for preventing escape of the microwave energy from the oven cavity 12. The burner 90 is the multi-portion ribbon type providing a high burner density and having secondary air closely adjacent to all portions of the burner, to reduce its incandescent flame length while permitting the burner to be positioned in a region between the screened vapour outlet region 18 and the inlet 66 of a blower system. The blower system 64 is used for mixing the output of the burner 90 with the vapour drawn from the oven 12; thus the temperature of the vapour outlet region 20 is maintained below that at which ionization of the vapour being drawn from the oven 12 might produce leakage of microwave energy through the screen 18. The burner is controlled by a temperature sensor 178 responsive to the oven vapour temperature. A temperature probe 150 stuck into the food body 32 signals when the body has reached a preset temperature and/or turns off the microwave generator 54. <IMAGE>

Description

SPECIFICATION A combination microwave and convection oven In combination microwave and convection ovens, such as shown in our copending U.K. Application 79 37415 (Serial No. ), combustion products from a gas burner may be circulated through an oven by mixing the combustion products with vapour drawn out of the oven into inlets of a circulating fan system.

As the capacity of the burner is increased, the flame length of the burner increases to a point where combustion is not completed before the fame enters a region between the inlet of the blower and a screened outlet of the oven. The vapour can thus become ionized and can act as a partial conductor at the screened region outlet which can permit microwave energy to be emitted from the interior of the oven into the region of the burner blower. This can result in absorption of the microwave energy and hence a reduction of the microwave energy cooking power in the oven. Furthermore the microwave energy can leak through the burner and/or blower structure out into the room.

According to the present invention, there is provided a combination microwave and convection oven comprising a conductive oven enclosure, a microwave source arranged to supply the enclosure with microwave energy, a blower arrangement outside the enclosure which, in operation, circulates heated vapour through the enclosure, entracting the vapour to a plenum through a microwave screen, and a ribbon gas burner which supplies products of combustion to the plenum, the burner comprising a plurality of rows of orifices for gas mixed with primary air.

More specifically, the burner may comprise a plurality of metal ribbons which are transversely corrugated along their length so that the corrugations act as ports. Preferably, the ribbons have a width, extending from plenums supplied with the primary fuel-air mixture to the combustion plenum, which is sufficient to prevent flashback into the supply plenum even when relatively high primary fuel-air mixtures such as 50-80% are used. In addition, by having each of the ports separated from a secondary air source by less than three intervening ports and by using excess air of the order of between 50 and 150%, the height of the flame may be substantially reduced so that for burner densities of greater than 40 watts for each square centimetre of burner surface, a flame height of less than 2.5 cm can be achieved.Thus, the combustion products can be mixed with vapour drawn from the enclosure in the region of the apertured outlet region without microwave energy in the enclosure leaking through the apertures.

Preferably the burner is run by using the negative pressure created by the inlet of a blower to draw the primary fuel-air mixture through the burner at a rate which lifts the burner flame from the surface of the burner ports so that heating of the burner port region is reduced thereby reducing the probability of flashback through the burner ports.

In a preferred embodiment a blower is used for drawing the combustion products from the burner out of the combustion plenum which is adjacent to both the blower system input and the vapour outlet of an enclosure supplied by microwave energy. A combustion burner system may be achieved which allows the major portion of the bottom of the oven to be used for the generation and feed of microwave energy to the oven.

According to the present invention there is also provided a combination microwave and convection oven comprising a conductive oven enclosure, a microwave source arranged to supply the enclosure with microwave energy, a blower arrangement outside the enclosure which, in operation, circulates heated vapour through the enclosure, entracting the vapour to a plenum through a microwave screen, and a temperature sensitive probe which is disposed inside the enclosure for insertion into a food body to sense the temperature of the food body and is coupled to a circuit outside the enclosure.

More specifically, the temperature probe may be used to indicate when the temperature of the food body reaches a predetermined temperature and a signal from the probe may be used to turn the microwave energy on and off to maintain the food body temperature, as sensed by the probe, at or above a temperature which may be preselected by a manual temperature selector. Simultaneously or alterna tively the temperature of vapour being circulated through the oven from a source of heat, such as a gas burner positioned outside the oven enclosure, may be sensed and this temperature used to turn the gas burner on and off to maintain the vapour in the enclosure at a temperature selected by a manual temperature selection control.

Preferably, the temperature probe is connected to the wall of the oven enclosure by a conventional cable such as a flexible coaxial line connected between the temperature probe and a jack which is insertable into a plug in the wall of the oven. The temperature probe is thereby removable from the oven so that the flexible cable and temperature probe may be fabricated of materials which need only withstand the convention oven vapour temperature used during normal cooking and may be removed from the oven when vapours of elevated temperatures of up to 540"C are circulated through the oven for self-cleaning purposes.

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 partially sectioned view of a detail of the oven of Figure 1 taken along line 2-2 of Figure 1; FIGURE 3 illustrates a front view of the oven of Figure 1; FIGURE 4 is an expanded view of the burner portion inside line 4-4 of Figure 1; and FIGURE 5 is an expanded view of a burner portion of Figure 4 taken along line 5-5 of Figure 4.

Referring now to Figures 1-5 there is shown a combination gas convection and microwave stove 10 comprising a metal oven cavity 12 of, for exam ple, porcelainized steel which is closed by a door 14 during operation.

A rack 22 made, for example, of steel rods is supported on bumps 24 formed in the side walls of the enclosure 12 sothatthe position of the rack 22 may be changed in accordance with well-known oven practice.

The upper portion of the back wall of the cavity 12 has an elongated vapour inlet region 16. The middle portion of the back wall has a pair of vapour outlet regions 18. Vapour is drawn out of the enclosure 12 through the regions 18 into a plenum 20. Regions 16 and 18 preferably have a plurality of apertures 34 whose maximum dimensions are substantially less than a half wavelength of the free space wavelength of the microwave energy radiated into the cavity 12.

Preferablythe apertures 34 have maximum dimensions of, for example, less than a tenth of the free space wavelength of the microwave energy radiated into the oven 12 by a radiator 26, so that microwave energy radiated into the enclosure 12 will not escape through the regions 16 or 18.

The radiator 26, which is positioned below the rack 22, is a rotatable microwave energy radiator which directs microwave energy up through the apertures in rack 22, 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 roast of meat. The dish 30, and the plate 28, are, preferably, substantially transparent to microwave energy so that the lower region of food body 32 and the interior portions thereof may be heated effectively by microwave energy.

The radiator 26 may comprise, for example, a plenum 36 whose upper surface 38 contains a plurality of apertures 40 through which microwave energy is radiated upwardly into the oven 12. A central conductor 42 of a coaxial line 44 supports the plenum 36 by being attached to the centre of the 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 buy a motor 52 positioned below the waveguide 48. Microwave energy from a magnetron 54 is fed through the waveguide 48 and coaxial line 44 to radiator 26. A blower 56 cools the magnetron 54 by blowing air past the fins 58. It should be noted that none of this air passes through the waveguide 48.

A cover 60 of microwave transparent material is supported overthe radiator 26 on centering bumps 62 located on the bottom of the oven 12 so as to cover radiator 26 and thereby prevent food juices or other material from being dropped on radiator 26.

The door 14 is preferably sealed to the enclosure 12 by a high temperature vapour seal with a microwave choke structure positioned between the vapour seal and the interior of the enclosure 12. In this way microwave energy radiated into the oven 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 the door 14 is closed, a latch is mechanically moved to lock the door 14 shut and to permit energization of the magnetron 54. Further details and advantages of such a microwave oven feed and directive energy rotating structure are disclosed in greater detail in the aforesaid copending application. However, any desired microwave feed structure, radiator, and/or door seal could be used.

Preferably a slight negative pressure is produced with the plenum 20 by a blower system comprising two centrifugal blowers 64 which draw vapour out of - the cavity 12, through the apertures 34, into the plenum 20 and blow it out into plenums 66 surrounding the blowers 64 and supplying the region 16. The upper ends of the plenum 66 are connected to an opening through which a small portion of the output of the blowers 64 passes through to an outlet vent 68 where the air 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 stove 10 to cool motors 72 driving blowers 64 and 70, and to supply air to mix with the output of the duct 68 which then exits through a screened aperture 74 at the top of the stove 10 above the cooking surface.

As shown in Figure 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 72 which is supported from a back wall 78 of the stove 10. 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 a partition 80.

It should be clearly understood that a single blower could be used in place of the dual blowers 64 and the plenum 66 could have separate ducting systems to direct the vapour through a plurality of regions 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 is positioned at the rear of the stove 10 behind and below the enclosure 12. The burner system 90 comprises a ribbon burner 92 extending across a major portion of the width of the oven and fed at one end with a primary fuel-air mixture through a vertical tubular member 94. The open lower end of the member 94 is supplied with gas through a gas jet 96 which is controlled by a solenoid operated valve 98 and fed from a pressure regulator through a gas line 102.

As shown in Figure 5, the ribbon burner 92 comprises two sections 104 and 106 each formed of seven ribbons of sheet metal 108 approximately 13mm and 1 mm extending the length of the burner, the members 108 being corrugated, for example, by; a die. The corrugations run across the width of the members and are, for example, 5mm from peak to peak. Interspersed between the members 108 are flat members 110 of the same width and extending the length of the burner so that the spaces between the corrugations act as ports through which a primary fuel-air mixture, supplied by pipe the 94, can flow.

The sections 104 and 106 are supplied from separate plenums 112 and 114 respectively which are both supplied at their ends from pipe 94.

The plenums 112 and 114 are made in the shape of rectangular boxes 124 which are supported by bracket member 116 extending from the bottom of a combustion plenum 118 communicating with the input to the blowers 64 through the plenum 20. The two sections 104 and 106 of the burner are separated by spacers 120 and are held together by rivets 122 extending through the spacers 120, all of the ribbon members 108 and 110, and the walls of boxes 124.

An air space 128, located between the boxes 124 and through with secondary air is drawn into the combustion plenum 118, is formed bythespacers 120 and the bumps 134 at the lower corners of the boxes 124. In addition, secondary burner air is drawn by the outer edges of the burner sections 104 and 106 through spaces 126. As a result, none of the burner ports, formed between the ribbons 108 and 110, are spaced from a source of secondary air by more than three intervening ports.

It has been found that by positioning the secondary air close to the burner ports and by utilizing excess air such as 100% excess air, the flame height may be made less than the total width of the burner section when sufficient fuel-air mixture is drawn through the burner ports to cause the flame to lift off the ports by a distance of, for example, 2mm-4mm.

This flame lifting reduces the heating of the port metal ribbons 108 and 110 so that flashback ignition of the primary fuel-air mixture in the plenums 112 and 114 is prevented even when a relatively high primary fuel-air mixture is used. By thus reducing the length of the flame, the combustion plenum 118 may be positioned across the lower rear corner of the oven 12 immediately below the outlet regions 18 of the oven. The vapour drawn from the oven 12 through the outlets 18 will still be substantially deionized so that microwave screens, produced by apertures 34, are effective to prevent leakage of microwave energy from the enclosure 12 into the plenum 20, even when both the microwave source 54 and the burner 92 are energized.

In the preferred embodiment a safety control circuit is provided in which an air flow sensor 148, comprising a vane actuated switch, is positioned in the output of blowers 70. A manual selector control switch 168 energizes the blower motors 72 when one of the convection cooking modes, which may include microwave heating, is selected. The blower output closes the air sensor switch 148 to energize a conventional resistance heater ignitor 132 extending into plenum 118. After solenoid valve 98 is energized to allow gas to be supplied to the jet 96. When the resultant fuel-air mixture reaches the combustion plenum 118 through the ports in the burner 92 it is ignited by the ignitor 132 and the products of com- bustion are drawn into the blower 64 and blown into the enclosure 12 through the inlet 16.A portion of the output of blower 64 is blown out of the vent 74 to create a slight negative pressure in the combustion chamber 118 and in the enclosure 12 to control the amount of primary air drawn in through the pipe 94 and secondary air drawn in around the edges of the burner sections 104 and 106. Since the tube 94 acts as an air restricting orifice for the primary fuel-air mixture, variations in blower speed and vapour temperature which vary the primary and secondary air drawn into the plenum also cause some variations in the low of gaseous fuel through the reg ulatorthereby reducing the possibility of an overrich fuel-air mixture being burned which would cause noxious fumes to emanate from the screen 74 at the top of the stove. The burner 92 as shown herein can, for example, operate a thermal output of 5.5-9 kW.The thermal output is selected by selecting the sizes of the pipe 94, and the jet 96 as well as the setting of the fuel pressure regulator. The secondary air is selected by selecting the size of the space between boxes 124 and the spaces at the edges of the burner sections 104 and 106, through which secondary air is drawn into the combustion plenum 118.

The foregoing described burner is preferably positioned in the lower rear portion of the stove 10 since if the outlet regions 18 are positioned less than a diameter of the individual blowers 64 from their inputs, then the burner system 90 comprising the ribbon burner 92 may be positioned just below the outlet regions 18 and any residual ionized portions of the combustion products will be drawn directly into the blower rather than eddying into the regions of the apertures 34. Thus, the burner system 90 may be positioned in the same general region as the vapour outlet and blower system thereby leaving available substantially the entire bottom of the oven enclosure 12 for the installation ofthe rotating microwave radiator 26 and transparent cover plate 60.

Since the enclosure vapour outlet region 18 is preferably positioned above the top of the microwave transparent cover 60, it is not necessary to cool the cover 60 by blowing air into the oven through the radiator 26. Even during self cleaning, when vapour having a temperature approaching 540"C is blown into the oven through the region 16 adjacent the top of enclosure 10, the vapour will be cool substantially prior to reaching the cover 60. Thus, the cover 60 may be made of conventional cookware materials such as Corningware.

In the preferred embodiment, a temperature probe 150, which may be inserted in the food body 32, is connected by means of an electric cord 152 to a jack 154 in the wall of enclosure 12, so that it may be removed therefrom. Atemperature indicator 156 is provided in the top panel of the stove 10 to indicate the temperature sensed by the probe 150. The temperature may, for example, be determined by setting a pointer 158 of the indicator 156 to the desired food body temperature. When the probe 150 is inserted in the food body and reaches the set temperature, a light 160 will light or an audible alarm will ring.

Alternatively, the temperature probe 150 can be used to control the application of microwave power to the oven, with microwave power being applied whenever the food body temperature, sensed by probe 150, is below the temperature set by the pointer 158 being turned off when the temperature is exceeded. Microwave power may be started at a preselected time which may be set by a knob 162 and the duration of the application of microwave power may be determined by the setting of a timer knob 164. The average microwave power level may be set by a knob 166 which varies the average microwave power supplied to the oven, for example, from 100 to 700 watts.

The gas burner 92 may be controlled by turning a control selector knob 168 to a section marked convection heating and by setting a temperature control knob 170 to a convection vapour temperature. A timer such as, for example, a digital clock 172, may also have on and off selector controls 174 and 176 for setting the time during which convection heat is supplied to the oven.

In operation, a temperature sensor bulb 178, mounted on a bracket in the enclosure 12 directly beneath inlet region 16, senses the temperature of the vapour circulated in the enclosure 12 and when the vapour is below the temperature set by the control knob 170, the burner system 90 is energized.

Preferably the microwave energy may be used to defrost the food body 32 or to rapidly bring the food body up to temperature. The convection heat is used to provide the predominant amount of heat for finishing the heating and/or cooking of the food body as well as for controlling the surface texture and colour thereof. Any desired combination of temperatures may be set by the separate control systems so that a wide variety of sizes, shapes, and types of food bodies may be heated with optimum program cycles to provide improved taste and speed of food preparation.

Browning of a food body such as a joint of meat can be controlled by blowing hot vapour through the inlet region 16 while radiating full power microwave energy into the oven. The velocity of the hot air dries moisture from the surface of the food body so that microwave energy is more readily absorbed by the dry food body surface causing it to brown. There fore, an added grilling or browning unit is not neces sary.

An example of the power levels required for effective food body browning may be microwave energy applied at a rate of 500-800 watts or around 2,000-3,000 BTU's per hour and a gas burner heating rate of about 1 -3-4 kW or around 4,000-15,000 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 electrical control systems and systems for supplying microwave energy to the oven may be used, and other types of circulating systems and burners may be used.

Claims (15)

1. A combination microwave and convection oven comprising a conductive oven enclosure, a microwave source arranged to supply the enclosure with microwave energy, a blower arrangement out side the enclosure which, in operation, circulates heated vapour through the enclosure, entracting the vapour to a plenum through a microwave screen, and a ribbon gas burner which supplies products of combustion to the plenum, the burner comprising a plurality of rows of orifices for gas mixed with prim ary air.

2. An oven according to claim 1, wherein the burner comprises two sections running the length of the burner with a channel for the supply of secondary air between the two sections.

3. An oven according to claim 2, wherein the burner sections are flanked by passages for the supply of secondary air.

4. An oven according to claim 1,2 or 3, wherein no orifice is spaced by more than three intervening orifices from a margin of the burnerwhereatsecondary air can flow into the flame.

5. An oven according to any of claims 1 to 4, wherein the orifices are formed by corrugated strips of metal sandwiched together to form the burner.

6. An oven according to claim 5, wherein the corrugated strips alternate with non-corrugated strips.

7. An oven according to any of claims 1 to 6, wherein the burner is below the microwave screen and has such a supply of secondary air as to prevent substantial ionization of vapour in the part of the plenum adjacent the microwave screen.

8. An oven according to any of claims 1 to 7, wherein the blower arrangement comprises two counter-rotating blowers.

9. An oven according to any of claims 1 to 8, comprising a rotating radiating structure inside the enclosure which launches microwaves into the enclosure from the microwave source.

10. A combination microwave and convection oven comprising a conductive oven enclosure, a microwave source arranged to supply the enclosure with microwave energy, a blower arrangement outside the enclosure which, in operation, circulates heated vapour through the enclosure, entracting the vapour to a plenum through a microwave screen, and a temperature sensitive probe which is disposed inside the enclosure for insertion into a food body to sense the temperature of the food body and is coupled to a circuit outside the enclosure.

11. An oven according to claim 10, wherein the circuit energises an indicator for indicating when the food body reaches a manually preselected temperature.

12. An oven according to claim 10 or 11, wherein the circuit controls the application of microwave energy to the enclosure as a function of the temperature of the food body.

13. An oven according to claim 10, 11 or 12, comprising a gas burner for introducing products of combustion into the recirculating vapour, and a sensor inside the enclosure responsive to the vapour temperature therein to control the gas burner.

14. An oven according to any of claims 10 to 13, comprising a rotating radiating structure inside the enclosure which launches microwaves into the enclosure from the microwave source.

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