Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Inventi_
Burner systems for domestic convection ovens have provided
large combustion and/or mixing plenums due to the large burner
flame lengths. Also large amounts of excess air, are used at
normal baking temperatures since, when the oven is run at its
maximum temperatures such as for self-cleaning, the mass of air
circulated is reduced. It has been usual to vent the oven directly
and to use relatively high pressure blower systems to maintain
sufficient flow of excess air through the burner system at self
cleaning temperatures so that undesirable generation of noxious
fumes such as carbon monoxide does not occur.
In addition, it has been difficult to adequately monitor
actual flow of air into the burner system so that if the blower
fails the burner will reliably shut down.
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Summary of the Invention
In accordance with this invention there is provided a con-
vection oven having a burner and oven circulation blower system
in which the amount of the primary air-fuel mixture being drawn
into the combustion plenum through an elongated burner system is
; controlled by a convection vapor circulation system. More specif-
ically, the burner comprises a rnulti-section ribbon burner with
the two sections being spaced apart by a secondary air supply
region. The height of flame above the burner surface may be
less than the total width of the burner system being, for example t
on the order of an inch high, and the excess air may be in the
range of 65% to 150% of that required for complete combustion.
Preferably, the ribbon burner surface is positioned at a
level below the oven floor and behind the rear oven wall
containing the oven outlet through which vapor is drawn by the
oven vapor circulation system. By using two counter-rotating
blowers, positioned behind the rear oven wall, for the vapor
circulators, power to drive the blower may be a low value such
as 50 watts.
This invention provides a convection oven in which
a gas burner is positioned adjacent the input of a circulating
blower system for drawing vapor products out of the oven and
recirculating said products mixed with the combustion products
of the burner back through the oven. The burner comprises a
plurality of elongated ported sections for supplying a primary
fuel-air mixture to a combustion plenum separated by regions
through which secondary air is supplied to the combustion plenum.
More specifically, the burner comprises a plurality of metal
ribbons which are transversely corregated along their length so
that the corrugations act as ports. Preferably, the ribbons
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have a width extending from a plenum supplied with the primary
fuel-air mixture to the burner plenum with the ribbon width being
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 ribbons separated
from a source of secondary air by a distance which is less than
; twice the transverse dimension of the port and by using on the
order of 50% to 150% excess air, the height of the flame may be
reduced to a dimension which is substantially the same as or less
than the transverse distance across the burner face between
adjacent sources of secondary air.
Further in accordance with this invention the burner may be
run at high power continuously 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
front from the surfaces of the ports formed by the ribbons so that
heating of the burner port region is reduced thereby reducing
probability of flashback through the burner ports.
This invention further provides that by using a blower
system which draws the combustion products out of the combustion
plenum, the input to the air-fuel primary mixture plenum of the
burner may be fed from a gas regulator through an orifice.
Variations in blower speed or atmospheric pressure will then
cause corresponding variations in the flow of gaseous fuel from
the regulator so that greater uniformity of the fuel-air mixture
may be maintained and hence the excess air may be reduced from
the normal 300% or so found in domestic gas appliances to less
than 100~ while still preserving a sufficient safety margin of
excess air.
Further in accordance with this invention there is provided
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a control system for a convection oven in wh:Lch a circulatlng
blower system actuates a switch in response to movement of the
air or oven vapor which controls the burner so that if the blower
system fails, for example, due to motor burn out or the impeller
becoming loose on the shaft, the burner is disabled thereby
providing a fail-safe condition of operation.
In accordance with this invention it has been Eound
that by using a burner in which the excess air is substantially
reduced, the output temperature of the burner may be raised to
temperatures on the order of 1800F while still maintaining
sufficient oxygen in the combustion products from the burner
to readily pyrolize cooking vapors or vapor deposits on the oven.
In accordance with the present invention there is
provided a convection oven comprising an enclosure; a blower
system for circulating vapor through said enclosure; means for
supplying heat to said vapor in a region adjacent to the input
of said blower system comprising a burner positioned adjacent
the input of said blower system; said burner comprising at least
one secondary air port and means for supplying a primary fuel-
air mixture through a plurality of non-circular primary fuel-
air ports substantially adjacent said secondary air port; and
said vapor circulating ~lower system being adapted to control
air passing through said secondary air port.
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Brief Description of the Drawings
Other and further embodiments of the invention will become
; apparent as the description thereof 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 vertical partial sectional view of the
oven of FIG. 1 taken along line 2-2 of FIG. l;
FIG. 3 illustrates a transverse sectional view of the oven
of FIG. 1 taken along line 3-3 of FIG. l;
FIG. 4 illustrates a transverse sectional view of the oven
of FIG. 1 taken along line 4-4 of FIG. l;
FIG. 5 is an expanded view of the burner portion of FIG. 1
taken along line 5 5 of FIG. l; and
FIG. 6 is an expanded view of a burner of FIG. 4 taken along
line 6-6 of FIG. 4.
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Description of the Preferred Embodiment
Referring now to FIG.'s 1-6 there is shown a gas convection
stove 10 embodying the invention. Stove 10 comprises an oven
cavity 12 of metal such as 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 so that
the position of the rack 22 may be changed in accordance with
well-known oven practice.
The upper portions of the back wall of cavity 12 has an
elongated vapor inlet region 16 and the middle of-the back wall
has a pair of vapor outlet regions 18. Vapor is drawn out of
the enclosure 12 through regions 18 into a plenum 20.
A microwave radiator 26 is positioned below rack 22 and
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. Dish 30, as well as plate 28, are preferably substan-
tially 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.
Radiator 26 may comprise, for example, a plenum 36 whose upper
surface 38 contains a plurality of apertures 40 through which direc-
tive microwave energy patterns are radiated upwardly into oven
12. A 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 extension of conductor 42 is rotated by a motor
52 below waveguide 48. Microwave energy from a magnetron 54 is
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fed through waveguide 48 and coaxial line 44 to radiator 26. A
blower 56 blows air past 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 of oven 12
to cover radiator 26 and thereby prevent food juices or other
material fro~ being dropped on radiator 26.
Door 14 is preferably sealed to enclosure 12 by a high
temperature vapor seal with a microwave choke structure positioned
between said vapor seal and the interior of enclosure 12 so that
microwave energy radiated into oven 12 is largely prevented from
being absorbed from the high temperature vapor seal. However,
any microwave energy passing through said choke section is sub-
stantially absorbed by the high temperature vapor sçal. When
door 14 is closed, a latch is mechanically moved to lock 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.
It should be clearly understood that this invention may be used
without the foregoing microwave energy system.
In accordance with this invention a slight negative pressure,
such as .01 to .1 inches of water, is produced within plenum 20
by a blower system comprising two counter-rotating centrifugal
blowers 64 which draw vapor out of cavity 12 through regions 18
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
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air is mixed with the air blown by a second set of blowers 70.
Blowers 70 draw cool air in from the back of the stove 10 to cool
motors 72 driving blowers 6~ and 70 and to supply air to mix with
the output of duct 68 which then exits through a screened aperture
74 at the top of the stove 10 above the cooking surface.
As shown in FIG.'s 2 and 3, each of the apertured regions 18
supplies vapors from the oven t:o a separate blower 64, and
each blower 64 is driven along with one of the blowers 70, by a
common shaft of a separate motor 72 which is supported from a
back wall 78 of the stove 10. The heat from blowers 64 is thus
isolated from motors 72. A partition 80 between the two blowers
64 prevents tangential interaction of the vapor output of the
blowers 64, which rotate in opposite directions to cause the air
between the blowers to move upwardly adjacent 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 vapor through a plurality
of different 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 and reduce the
: blower power.
A burner system 90 is positioned at the rear of stove 10
behind and below enclosure 12. 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 vertical tubular member 94. The open lower end of member
94 is supplied with gas through a gas jet 96 from a solenoid
- operated valve 98 fed from a pressure regulator 100 through a gas
line 102.
As shown in FIG's 5 and 6, the ribbon burner 92 comprises two
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sections 104 and 106 each formed of seven ribbons of sheet metal
108 approximately one-half inch wide and forty thousandths of an
inch thick extending the length of the burner, said members 108
being corrugated, for example, by a die. The corrugations run
across the width of the members and are, for exa~ple, three
sixteenths of an inch from peak to peak. Interspersed between
members 108 are flat members 110 of the same width and extending
the lengths of the burner so that the spaces between the corruga-
tions act as ports through which a primary fuel-air mixture sup-
plied by pipe 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 the pipe 94.
The plenums 112 and 114 are made in the shape of rectangular
boxes 124 which are supported by bracket members 116 from the
bottom of a combustion plenum 118 communicating with the input
to the blowers 64 through plenum 20. The two sections 104 and
106 of the burner are separated by spacers 120 and the burner
assembly sections 104 and 106 are held together by rivets 122
extending through the spacers 120 and through all of the ribbon
members 108 and 110 and walls of boxes 124, as well as spacers
120. Air channel 140 between boxes 124 through which secondary
air is drawn into the combustion plenum 118 is provided by spacers
120 and by bumps 134 at the lower corners of boxes 124~ In
addition, secondary burner air is drawn through spaces 1~6
~ around the outer edges of boxes 124. As a result, none of the
; burner ports ~6 formed between ribbons 108 and 110 are spaced
from a source of secondary air by more than three intervening ports.
This invention discloses the discovery that by positioning
the secondary air close to the ports the flame height may be
made less than the total width of the burner section when sufficient
fuel-air mixture is drawn through the ports to cause the flame
to lift off the ports by a distance of, for example, a sixteenth
of an inch to an eighth of an inch. Such flame lifting reduces
the heating of the port 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 air-fuel
ratio is used. By thus reducing the length of the flame, the
combustion plenum may be positioned across the lower rear corner
of the oven immediately below the outlet regions 18 of the oven
and the combustion products drawn from the burner will still be
substantially deionized at the inputs to blowers 64.
In accordance with this invention a safety control circuit is
provided in which an air flow sensor 148, comprising a vane actuated
; switch, is positioned in the vent 68. A manual selector control
switch 168 energizes blower motors 72 when one of the convection
cooking modes is selected. The output in vent 68 of blowers 64
closes air sensor switch 148 to energize a conventional resistance
heater ignitor 132 extending into plenum 118. After a time delay
period of, for example, thirty seconds, solenoid valve 98 is
2Q energized to allow gas to be supplied to jet 96. When the resultant
fuel-air mixture reaches combustion plenum 118 through the ports
in burner 92 it is ignited by the ignitor 132 and the products
of combustion are drawn into the blower 64 and blown into the
enclosure 12 through the inlet 16. The portion of the output of
blowers 64 which is blown out vent 68 creates a slight negative
pressure in combustion chamber 118 and in enclosure 12 which
controls the amounts 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
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vapor temperature which vary the primary and secondary air drawn
into the plenum also cause some variations in the flow of gaseous
fuel through the regulator reducing the possibility of an over-rich
fuel-air mixture being burned to cause noxious fumes to emanate
from the screen 74 at the top of the stove.
The burner as shown herein can, for example, operate a
thermàl output of 20-30 thousand BTU's per hour. The thermal
output is selected by selecting the sizes of the pipe 94, and
jet 96 as well as the setting of the fuel pressure regulator 100.
The secondary air is selected by selecting the size of the space
140 between boxes 124 and the spaces 136 at the edges of the burner
sections 104 and 106 through which secondary air is drawn into
the combustion plenum 118.
The gas burner 92 may be controlled by turnlng a control
selector knob 168 to a section marked convection heating and by
setting a temperature control knob 170 to a convection vapor
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 regoin 16,
senses the temperature of the vapor circulated in the enclosure
12 and when the vapor is below the temperature set by control
knob 170, the burner system 90 is energized.
In accordance with this invention the burner plenum 118 and
the blower input plenum 20 are fabricated as a unit separable
from the enclosure walls of cavity 12. As shown in greater
detail in FIG.'s 3-6, the plenums 20 and 118 consist of a flat
wall member 180 containing holes corresponding to vapor inlet
and outlet regions 16 and 18 which is attached to the back of
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oven cavity 12, for example, by sheet metal screws 182 with a
vapor tight seal being produced by means of high temperature
gaskets 184 extending around regions 16 and 18 and providing
thermal insulation between the combustion plenum 118 and the
; rear wall of cavity 12. As a result, the interior temperature
of combustion plenum 118, which may be 1,500-2,000F is thermally
isolated from the interior surface wall of the cavity which may
be porcelainized and capable of withstanding temperatures of
around 1,000F. Sheet 180 extends downwardly below the bottom
of cavity 12 and has a lip 186 bent at right angles thereto to
form the edge of a plate spaced from box 124 of section 112 by
secondary air passage 136. Brackets 116 are attached by screws
188 to lip 186. Also attached to sheet 180, for example, by
welding is the shroud member 190 which is positioned directly in
front of the centrifugal blowers with holes concentric with
blower 64 and slightly smaller in diameter than blowers 64.
Blowers 64 are positioned in plenum 66 whose output is
: supplied to vent 68. The outer wall member 192 of plenum 66 is
also welded around its edges to sheet 180 with the lower portion
of member 192 defining, together with the lower portion of plate
180 the burner plenum 118. A recess in the lower wall portion
of member 192 holds a block of refractory material 194 to prevent
loss of heat outwardly from burner plenum 118. The edge of
member 190 below block 194 is bent at right angles to form a lip
196 at the same level as lip 186 extending toward lip 18~ to
form a space 136 with the edge of box 124 of burner plenum 114.
Support bracket 142 is also attached to lip 196 by a screw 188
so that the burner system 90 is rigidly attached to the lower
end of burner plenum 118 and the spaces 136 between the lips 186
and 196 and boxes 124 are accurately controlled to provide a
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uniform slot for the passage of secondary air into the combustion
plenum 118. Similar lips are positioned at the ends of the
burner plenums or the end spacings may be eliminated entirely.
Preferably, the spacings between the lips 186 and 196 and the
boxes 124 are made approximately one-half the spacing between
the boxes 124 so that each of the ribbon burner sections 104 and
106, fed by a separate plenum, is also supplied with a uniform
amount of secondary air along each edge thereof. As an example,
if the total port area in which ribbon burner is approximately 5
square inches by being about 14 inches long at about 3/4 of an
inch wide with the port area being about half the burner surface
area, then the spaces 136 may be about 3/16 of an inch wide and
the space 140 may be about 3/8 of an inch wide.
To improve the uniformity of fuel feed to the sections 104
and 106, plenums 112 and 114 have sloping bottoms 198 in boxes
124 which are attached to the bottom of boxes 124 and pipe 94 and
rise to points close to ribbons 108 and 110 at t~e close ends of
boxes 124 which are farthest from pipe 94.
During normal baking operation, the burner blower system of
this invention may operate with the primary air preferably being
; about 50 to 80% of that required for complete or stochiometric com-
bustion of the fuel, and with sufficient secondary air to pro-
vide 100% excess air in the combustion plenum 118 and the average
vapor temperature in ~he oven enclosure 12 is 300 F to 500 F.
When the burner first starts, 150% excess air may be drawn into
plenum 118 since blowers 64 will move a greater mass of the cooler
air whereas when the burner is operated continuously for self-
cleaning and the vapor temperature at blowers 64 approaches 1,000 F,
less air is drawn through into the plenum 118, for example, only
65% excess air, and the combustion products become hotter. ~owever,
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sufficient excess air is present to avoid production of noxious
fumes such as carbon monoxide.
This completes the description of the preferred embodiments
of the invention described herein. However, numerous modifications
thereof will be apparent to one having ordinary skill in the art
without departing from the spirit and scope of the invention. For
example, the vapor may be circulated through apertures located in
regions other than the back wall, any desired electrical and
mechanical control system for the burner may be used, and other
locations of the blowers and the burners may be used. Accor-
dingly, 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.
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