Note: Descriptions are shown in the official language in which they were submitted.
:~3~7
Background of the Invention
Microwave ovens have had electric heaters positioned inside
the oven to ai~ in the cooking and/or browning process and have
had electrically heated air or air used to cool electrical com-
ponents such as magnetrons, power supplies, or magnetrons directed
through the oven to aid in controlling vapors produced by a food
body.
However, attempts to use the products of combustion in cooking
regions excited with microwave energy have been generally unsatis-
factory. Open gas flames in the oven can, among other things, act
as sources of ionization which can absorb large portions of the
microwave energy.
--1--
.
:; .
3'-~
Summary of the Invention
In accordance with this invention there is disclosed an
oven which can be heated by combustion products blown into
the oven by a blower system which draws heated vapor comprising
the combustion products from a burner plenum and controls
the air drawn through the burner, More specifically, the blower
system blows the mixture of vapor drawn from the oven and the
vapor comprising the burner products of combustion and second-
ary air back into the oven with a small portion of the mixture
being blown out an exhaust vent to create a slight negative
pressure in the burner plenum so a predetermined amount of air
will enter the burner.
This invention further discloses that the temperature of the
vapors blown into the oven can be controlled by a thermostatic
switch which controls an on/off cycle of the burner.
This invention further provides that the output of the oven
circulation blower enters the oven through the back wall adjacent
the upper oven surface. Such a direction of heated vapor into the
oven has been iEound to substantially improve heating pattern uni-
formity and to assist in browning the upper surfaces of food pro-
ducts such as pastries or cakes or meat products by microwave
energy while using less circulation blower power than other con-
convection ovens.
In accordance with this invention the burner heats a region
of a burner plenum during the time when the burner is energized
and the oven circulating blower system draws air through a gas
burner, and draws vapor from the oven to heat the vapor and to
circulate the heated vapor through the oven. However, when the
burner is off, for example, during the period when a thermostat
senses that the oven temperature has reached a value corresponding
~3~
to that which has been manually selected for a cooking temperature,
the circulating ~an continues to draw air through the burner plenum
where it is heated by the hea~ed region of the burner plenum.
In accordance with this invention it has been discovered 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 uni-
formity 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.
Further in accordance with this invention each motor driving
one of the oven recirculating blowers is separated from the re-
circulating blower by a second blower on the same shaft as the
recirculating blower. The second blower prevents thermal
energy leaking along the motor shaft by conduction from heating
the motor. Rather the second blower draws air over the motor to
cool the motor and supplies the air to an exhaust vent where it
is mixed with the portion of the vapor output from the oven circu-
lating blower to cool said portion prior to venting.
This invention is particularly useful in a microwave oven
wherein the burner is outside the microwave enclosure and the
vapor, 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.
-3-
~3~
In accordance with the present invention, there is provided in com--
bination: a conductive enclosure supplied with microwave energy; and a burner
and means for drawing air through said burner and blowing vapor comprising
said air and the products of combustion of said burner into said enclosure.
In accordance with the present invention, there is further provided
an oven system comprising: a conductive enclosure having an access opening
and a closure member, a seal for said closure member; and a burner and means
for recirculating heated vapor comprising the products of combustion from said
burner through said oven while exhausting from said system a portion of the
vapor passi.ng through said circulating means.
In accordance with the present invention, there is further provided
in combination: a conducti.ve enclosure supplied with microwave energy through
a rotating radiator; a gas burner; and means for drawing air through s~id
burner and for blowing a major portion of the vapor comprisi.ng said air and
the products of combustion of said burner through said enclosure while blowing
a sufficient portion of said vapor out a vent to control the amount of said
air drawn through saicl burner.
- 3a -
,~
~3~
Brief Description of the Drawings
Other and further embodiments of the invention will become
apparent as the description thereo:E progresses, reference being
had to the accompanying drawings wherein:
FIG. 1 illustrates a partially broken away side elevation
view of an oven embodying the invention;
FIG. 2 illustrates a front view of the oven illustrated
in FIG. l; and
FIG. 3 illustrates an enlarged sectional view o-E the micro-
wave radiator structure of FIG. I taken along line 3-3 oE FIG. 1.
--4--
~13~
Description of the Preferred Embodiment
Referring now to E~IG. 1 there is shown a combination gas
convection and microwave stove 10 embodying the invention. Stove
10 comprises an oven cavity 12 which is closed by a door 14 during
operation. Cavity 12 has elongated vapor inlet region 16 through
which heated vapor is directed into the oven 12, and a vapor outlet
region 18 through which vapor 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 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 roast of meat. Dish 30~ as well as plate 28, are preferably
substantially transparent to microwave energy so that the lower
region of food body 32 and the interior portions thereoE may be
heated effectively by microwave energy.
Regions 16 and 18 pre~erably 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 cavity 12. Preferably apertures 34 haviny maximum
dimensions of, for example, less than a tenth of the free space
wavelength of the microwave energy radiated into oven 12 by
radiator 26 so that microwave energy radiated into enclosure 12
will not escape through regions 16 or 18~
Radiator 26 may comprise, for e~ample, a plenum 36 whose
upper surface 38 contains a plurality of apertures 40 through
which microwave energy is radiated upwardly into oven 12. A
~L~3~
central conductor 42 of a coaxial line 44 supports plenum 36 by
being attached to the center of upper plate 38. Conductor 42
extends downwardly through the outer conductor 46 of coaxial line
44 and through a waveguide 48 to a microwave choke and bearing
assembly 50. An extensi.on of conductor 42 is rotated by a motor
52 below waveguide 48.
Microwave energy from a magnetron 54 is fed through wave-
guide 48 and coaxial line 44 to radiator 26. A blower 56 blows a:ir
passed the fins 58 of magnetron 54 to cool the magnetron, but
none of this air passes through waveguide 48. A cover 60 of
microwave transparent material is supported over radiator 26 on
centering bumps 62 on the bottom o:f oven 12 to cover radiator 26
and thereby prevent food juices or other material from being
dropped on radiator 26. 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 vapor seal with a microwave choke struc-
ture positioned between said vapor seal and the interior of enclo-
sure 12 50 t}-at microwave energy rad.iated into oven 12 -i~ 1argely
prevented from bci.ng absorl)cd Irom the higll tcml)cr.ltl:lrc v~po:r seal
Ilowever, any microwave energy passing through sa-id choke section is
substanti.ally absorbed by the high tempera-ture vapor seal. ~hen
door 14 is closed, a latch is mecllanically moved to lock door 14
shut and to permit energization of the magnetron 48. In accordance
wi.th this invention a slight negative pressure is produced within
plenum 20 by a blower system comprising two centrifugal blowers
~ i3~
64 which draw vapor out of cavity 12 through apertures 34 into
plenum 20 and blows it out into plenums 66 surrounding blowers
64 and supplying region 16. The upper ends of plenum 66 are
connected to an opening through which a small portion of the
output of blowers 64 pass through an outlet vent 68 where the
air is mixed with the air blown by a second set of blowers 70.
Blowers 70 draw cool ajr in from the back of the stove 10 to
cool the air in duct 72 which exits through a screened aperture
74 in the top of the stove above the cooking surface thereof.
As shown in FIG.'s 1 and 2, each of the apertured regions 18
supplies vapors from the oven sucked to a different blower 64
and each blower 64 is driven along with one of the blowers 70,
by a separate motor 76 which is supported from a bac~ wall 78 of
the oven. A partition 80 between the two blowers 64 prevents
tangential interaction of the vapor output of the blowers 64.
Blowers 64 preferably rotate in opposite directions to cause the
air between the blowers to move upwardly adjacent par-tition 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 ducting systems to direct the vapor through 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 overl.
A burner system 90 positioned at the bottom of stove 10 in
a compartment behind and below enclosure 12 comprises a horizontal
apertured tube 92 extending substantially the entire width of
the oven and fed through vertical tubular member 94 with a gas-
primary mixture supplied with gas through a gas valve 96. Second-
ary air is regulated by the spacing of a plate 98 from the bottom
of the entrance of the burner plenum. A pressure regulator 100
$~
connected to a gas line input 102 supplies gas valve 96. When
the oven is turned on, an igni~or 108 is electrically energized
and heats to a temperature which will ignite an air-gas mixture
whereupon valve 98 opens, thereby causing said mixture to emanate
from the apertures in tube 92 and producing a flame in burner
plenum 106. Flue gas products in plenum 106 are drawn into
plenum 20 along with excess secondary air by blowers 64 where
the combustion products and secondary air are mixed with vapor
drawn through apertures 34 from enclosure 12.
Burner 90, preferably run with excess secondary air
to reduce the temperature of the products of combustion, is below
3,000 F so that substantially no oxide products of nitrogen are
produced and combustion is substantially complete. Preferably
sufficient excess secondary air is drawn into plenum 106 by the
negative pressure in plenum 20 to produce a temperature in burner
plenum 106 of l,200 F to 2,000 F. Plenums 20 and 106 extend
; substantially the full width of the back of enclosure 12 so that
the burner plenum temperature is suhstantially uniform across
said width. Thus, since the blower speed is constant, the slight
negative pressure in combustion plenum 106 is substantially con-
stant and accurately regulates the burner primary and secondary
air drawn into the combustion chamber. The gas fed into the
chamber is also constant due to pressure regulator 100 so that the
excess air is accurately controlled.
A plurality of top gas burners llO is provided which operate
as convention 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 entire stove lO. 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 vapor is
sensed by a temperature sensor bulb 120 mounted, for example, in the
oven on a bracket 122 below the vapor inlet 16 from the plenum 66.
The positioning of sensor 120 is preferably chosen so that it
is not directly in the entering hot vapor stream from inlet 16
but rather senses the temperature of the vapor circu~
lating in the enclosure 12. The location of 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
fluctuate large amounts. When the oven is ener~ized, the recircu-
lating blowers 64 run continuously and when the burner 90 is
energized, heat is delivered in the form of hot vapor substantially
directly to the food body 32 in enclosure 12 and a substantially
uniform heating pattern occurs.
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 13~. In addition, a light
140 positionecl outside oven 12 but illuminating oven 12 through
a light transparent high temperature ceramic 142 and a microwave
shielding screen 144 may be provided. The microwave system may
be energized, for example, from a 110 volt 60 cycle AC supply.
In accordance with this invention there is disclosed the
discovery that browning of a Eood body such as roast 32 can be
controlled by blowing hot vapor through inlet region 16 while
radiating full power of microwave energy into the oven. This
effect appears to occur due to surface effects of the velocity
of hot air drying molsture from the surface of the food body and
can occur with microwave energy being more readily absorbed by
the dry food body surface to brown the food body surface, and
_g _
therefore, an added broiler unit is not necessaryO
An example of power levels 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 5,000-15,000 BTU's per hour. Such power levels will
bake and/or brown a cake in a few minutes.
This completes the description of the preferred embodiments
of the invention. Although preferred embodiments have been
described, it is believed that numerous modifications and altera-
tions thereto would be apparent to one having ordinary skill in
the art without departing from the spirit and scope of the inven-
tion. For example, 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. Accordingly, it
is intended that this invention be not limited to the particular
details of the embodiment illustrated herein except as defined by
the appended claims.
--10--
