Note: Descriptions are shown in the official language in which they were submitted.
6S~2~,
Background of the Invention
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With conventional domes~ic gas ovens, a blue flame atmo-
spheric burner is typically positioned in a chamber below the
oven cavity. The best efficiency has been achieved by providing
communicating apertures in the floor of the cavity so that the
combustion vapors can pass from the chamber directly into the
cavity by natural convection. Furthermore, it has been common
to position an additional burner such as a radiant burner in
the cavity for broiling. The burners in these described
environments are located in large volume atmospheric combustion
chambers. Accordingly, almost all conventional blue flame
atmospheric burners can be used with favorable results in these
applications.
The introduction into the oven of apparatus for energizing
the cavity with microwave enerqy so as to provide a combination
microwave gas oven alters the conventional gas technology
approach. More specifically, it was found desirable to posi-
tion the magnetron, power supply, and waveguide coupling under-
neath the oven cavity in the chamber previously occupied by the
gas burner in a conventional gas oven~ Therefore, the burner
was positioned back of and underneath the oven cavity. The
volume to be allocated to the burner in this configuration was
further limited by the requirements of isolating the microwave
components and oven exterior surfaces from high temperatures;
in addition to the damage that could be caused to the microwave
components, American National Standard Institute standards re-
garding fire prevention and burninq hazard had to be satisfied.
Furthermore, a system of forced convection was preferable
because among other reasons, the comb~lstion vapors from the
burner at the rear of the oven were to be transferred into the
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cavity for enhanced efficiency. Also, because of the microwave
energy within the cavity, it was not desirable to position a
radiant burner therein. The combination of the above described
design parameters meant that it was desirable to have a gas
burner that operated in a relatively small volume having an
induced draft. Also, with the induced draft or negative
pressure above the burner, it was desirable to restrict the
secondary com~ustion air so as to improve efficiency.
A variety of conventional atmospheric blue flame
burners were installed in the environment described above.
However, good flame stability in the negative pressure without
the sound of combustion noise was difficult to attain.
Infrared burners r such as, for example, one having a
very large port covered by perforated steel or wire mesh
layers, were tried as one approach. The flame characteristics
were improved over other conventional burners in the negative
pressure by the reduced port loading, but the infrared radiant
heat was not very efficient for a forced air convection system.
Furthermore, the mesh raised to extremely high temperatures in
the oven self-clean mode.
Another approach such as described in my United States
Patent No. 4,373,504 which issued on February 15, 1983 and is
assigned to the same assignee herein, is a ribbon burner. The
tight flame on the top of the ribbon burner and the secondary
air flow under induced draft parallel but spaced from the
direction of the gas mixture resulted in a relatively quiet
flame with good flame characteristics. A longitudinal gap was
provided between two ribbon sections to provide improved
secondary air entry to the combustion chamber. How~ver, the
ribbon burner was substantially more expensive
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to fabricate than conventional burners. Also, because of the
increased burner temperature caused by the tight flame, the
burner had to be fabricated of an expensive material such as
stainless steel.
, 2 2
The present invention ls a combination microwave gas
convection oven comprising: a microwave cavity; means for
energizing said cavity with microwave energy; a chamber
positioned adjacent to said cavity, said chamber having a
bottom opening; means for recirculating vapor between said
cavity and said chamber; and a tubular gas burner positioned
horizontally below said opening for providing heat to said
chamber through said opening, said burner having a plurality
of top ports, said burner having a port loading of less than
25,000 Btu per hr-sg.in. of port area at the maximum operating
Btu rate of said burner.
It may be preferable that the energizing means be a
magnetron and that it be positioned below the cavity. ~he
recirculating means may be a blower system and more
specifically may preferably be a pair of counter-rotating
centrifugal blowers. The vapor may be recirculated from the
cavity to the chamber by way of a plurality of perforations
in the wall therebetween. Furthermore, it may be preferable
that the blower system be positioned in a second chamber which
draws air from the beforementioned chamber and exhaust into
the cavity. A specific example of the port loading may be
20,000 Btu per hr-sq. in. of port area as compared to a
typical value of 30,000 Btu per hr-sq. in. of port area.
The invention may also be prac-ticed by a combination
microwave gas convection oven comprising: a microwave cavity
having a wall with a plurality of perforations; means for
energizing said cavity with microwave energy; a chamber
positioned adjacent said wall and communicating wikh said
cavity by said perforations, said chamber having a bottom
opening, means for recirculating air between said cavity and
said chamber; and a tubular gas burner positioned below said
3 2 2
chamber for supplying products of combustion to said chamber
through said opening, said burner having a ylurality of top
ports, said burner having a port loading of less than 25,000
Btu per hr-sq. in. of port area at the maximum operating Btu
rate of said burner.
The invention also discloses a combination microwave
gas convection oven comprising: a microwave cavity having an
aperture in the floor thereof; a magnetron positioned below
said cavity; a waveguide for coupling microwave energy from
said magnetron to said cavity through said aperature; a chamber
positioned behind the back wall of said cavity and communicating
therebetween by a plurality of perforations in said wall, said
chamber having a bottom opening; means for recirculating air
between said chamber and said cavity; a tubulax gas burner
positioned below said chamber for providing a gas air mixture
to said chamber through said bottom opening in said chamber;
and said burner having a plurality of top ports, said burner
having a port loading of less than 25,000 Btu per hr-sq. in.
of port area at the maximum operating Btu rate of said burner.
It may be preferable that the invention further
comprise means positioned in the cavity for coupling microwave
energy from the aperture into the cavity, the coupling means
comprising a rotating member forming a radial waveguide in
combination with portions of the floor of the cavity. Also,
it may be preferable that the top ports define pairs of
elongated slots perpendicular to the length of the tubular
burner. These elongated slots may preferably have dimensions
of approximately 0.5 inches by 0.03 inches.
The invention may be practiced by a combination
microwave gas convection oven comprising: a microwave cavity
having a wall with a plurality of holes; means for energizing
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5~2
said cavi~y with microwave energy; a first chamber positioned
adjacent to said cavity and communi.cating therewith through
said plurality of holes; a second chamber positioned adjacent
to said first chamber and communicating therewith through an
aperture in a wall therebetween; a duct communicating between
said second chamber and said cavity; a blower positioned in
said second chamber for forciny air from said second chamher
to said cavity, the input air for said blower coming from said
first chamber through said aperture; an opening in the ~loor
of said first chamber providing entrance of air; and a gas
tubular burner having top ports positioned in the flow of air
passing through said opening to the slight negative pressure
created in said first chamber, said burner having a port
loading of less than 25,000 Btu per hr-sq. in. of port area
at the maximum operating Btu rate of said burner.
The first chamber may comprise a combustion chamber
for introducing produc~s of combustion into the recirculation
system. Further, a second blower may be positioned in the
second charnber.
The invention may also disclose a combination micro-
wave gas convection oven comprising: a microwave cavity
having an aperture in the floor; a magnetron; a waveguide
coupled to the output of said magnetron; a coaxial conductor
for coupling microwave energy from said magnetron to said
aperture; a microwave energy coupling member positioned in
said cavity and connected to the center conductor of said
coaxial conductorl said member forming a radial waveguide
combination with portions of said floor of said cavity; means
for rotating said member; a chamber positioned adjacent to
said cavity and having an opening in the bottom; means for
recirculating air between said cavity and said chamber; and
1 6~22
a gas tubular burner haviny top ports positioned adjacent to
said opening, said burner having a port loading of less than
25,000 Btu per hr-sq. in. of port area at the maximum operating
Btu rate of said burner.
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3 2 2
Brief Description of the Drawinqs
.. . . .
The objects and advantages of the invention will be more
fully understood by a reading of the description of the pre-
ferred embodiment with reference to the drawings wherein:
Figure 1 is a partially cut away view of a combination
microwave gas convection oven embodying the invention;
Figure 2 is a front view of the oven of Figure l;
Figure 3 is a cut away view along line 3-3 of Figure l;
Figure 4 is a cut away view alonq line 4-4 of Fiqure l;
Figure 5 is an expanded view along line 5-5 of Figure l;
Fiqure 6 is a top view of the microwave coupllnq structure;
and
Fiqure 7 is a side view of the couplinq structure of
Figure 6 also including the center conductor.
I ~ 6.S~
Description of the Preferred Embodiment
Referring to Figures 1 and 2, partially cut away side and
front views, respectively, of a combination microwave and con-
vection gas stove 10 are shown. As will be described in detail
later herein, food positioned in oven cavity 12 can be cooked
simultaneously by microwave energy and gas convection or by
either individually. Ridqes 14 in the side walls of cavity 12
are provided at different levels to support racks (not shown)
or a low loss plate 16 upon which food may be placed. Access
to cavity 12 is provided through door 1~ which may be of con-
ventional microwave choke desi~n; the door is shown closed in
Figure 1 and open in Figure 2. Shown in Figure 1 is a quarter
wavelength slotted choke such as described in detail in U.S.
Patent No. 3,767,884.
Thermal gasket 20 surrounds the entire periphery of door
18 and overlaps at the bottom center thereof to substantially
form a vapor seal. During the gas convection self clean mode
when the temperature in the oven rises to the order of 1000F,
it is desirable to prevent the hot vapors from escapinq the
cavity around the door. Also, in the convection cook mode, the
vapor seal prevents hot vapors from escapinq the cavity where
they could condense on the cooler outer surfaces. This thermal
~asket confiquration is different than a microwave electric
oven without forced convection where it is desirable to provide
a qap in the ~asket at the bottom of the loor to permit air to
flow into the cavity in response to chimney effect within the
cavity during self clean. Gasket 20 consists of a rope-like
inner insulation material and an outer metallic shield to
suppress the leakage of out of hand harmonics as regulated by
a government agency. When door 18 is closed, a latch 22 is
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5 ~ 2 ~
mechanically moved to lock the door shut and to permit ener-
gization of microwave energy.
The source of microwave energy is magnetron 24 which may
be of conventional design and preferably provides microwave
energy having an approximate frequency of 2450 megacycles. A
power supply (not shown) is coupled to the magnetron. Also, a
fan (not shown) is used to blow air through the fins 26 of
magnetron 24 to cool it~ The output probe 28 of magnetron 24
is positioned in and excites waveguide 30 with microwave energy.
The distances from output probe 28 to waveguide wall 32 and
coaxial center conductor 34 to end termination 36 are selecte~
using well known principles to couple a maximum of energy to
coaxial conductor 38. Coaxial conductor 38 comprises center
conductor 34 and outer conductor 40. Transition structure 42
provides for maximum coupling into coaxial conductor 38 and
also functions as a microwave choke to suppress the leakage of
microwave energy from the waveguide along the center conduc-
tor to motor 44 which rotates the center conductor. A ~eflon
sleeve 46 is shrunk onto center conductor 34 and provides a
tight fitting for support and microwave suppression between
center conductor 34 and transition structure 42. The ~eflon
sleeve 46 provides low friction to the transition structure 42
when the center conductor 34 is rotated.
The microwave energy travels up coaxial conductor 38 and
is coupled into cavity 12 by couplin~ structure 48. The coupling
structure 48 shape, which is shown in detail in Figure 6,
provides two important functions. First, it provides a favor
able impedance match between coaxial conductor 3~ and cavity
12 so as to provide a maximum transfer of microwave energy.
Second, coupling structure 48 provides a desirable microwave
5 8 2 2
energy power distribution within cavity 12. The floor of
cavity 12 is raised to form a plurality of bumps 50 upon which
microwave transparent dish 52 is supported to provide isolation
of coupling struc~ure 48 from the environment of cavity 12.
More specifically, dish 52 prevents food spills from falling on
coupling structure 48. ~urther, the dish provides some thermal
insulation for the microwave feed structure as described herein.
Air is recirculated through cavity 12, combustion chamber
54 and plenum 56 by a blower system comprising two counter
rotating centrifugal blowers 60. Blowers 60 create a slight
negative pressure, such as .01 to .1 inches of water, in the
center of plenum 56 which draws air from combustion chamber 54
through a large aperture 61 communicating therebetween. The
slight negative pressure so produced in combustion chamber 54
draws air thereinto from cavity 12 throuqh a plurality o cir-
cular perforations 62 in the rear wall of cavity 12. Referring
specifically to Figure 2, perforations 62 are positioned in two
circular patterns 63 each centered on one of blowers 60. Each
pattern 63 consists of 1278 perforations 62 each of 0.156 inch
diameter and arranged with 0.188 inch staggered centers.
Accordingly, each pattern 63 is approximately 63~ open area.
Centrifugal blowers 60 create a positive air pressure around
the periphery of plenum 56, which pressure forces air through
duct 64 into cavity 12. The entrance into cavity 12 is through
712 perforations 65 which are arranged in rectanqular pattern
66. The size of perforations 65 is the same as perforations
62; this size is below cutoff for the microwave frequency so
that microwave enerqy does not escape cavity 12 therethrough.
~t the upper end of duct 64 is a small ope~inq 67 into
outlet vent 68 whereby a small percentage oE the recirculatinq
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convection air is vented out of the recirculation system. A
second pair of blowers 70 which are mounted on the same shafts
71 of blowers 60 are positioned behind the back wall 69 of
plenum 56 and function to draw cool air in from the back of
stove 10 to cool motors 73 which drive the shafts 71 for
blowers 60 and 70. Furthermore, centrifugal blowers 70 provide
positive pressure around the periphery of their chambers 74
which causes the air therein to exhaust throuqh duct 75. Out-
let vent 6g couples into duct 75 so that the hot recirculation
air from cavity 12 is mixed with and cooled by the air in duct
75 before its exhaust through screened aperture 76 at the top
of stove lO.
Referring to Fi~ure 3, an expanded view of the recircu-
lation convection system taken along line 3-3 of Figure l is
shown. Each of the circular pattern 63 exhaust reqions on the
rear wall of cavity 12 supplies recirculating convection air
from the cavity to a separate blower 60. As described earlier
herein, each blower 60 is driven along with a second blower 70
on a common shaft 71 by a separate motor 73 which is mounted
on the back wall of the stove in separate chambers 74. A
partition 78 between the two blowers 60 prevents tan~ential
interaction of the convection air output of the blowers 60
which rotate in opposite directions to cause the air between
the blowers to move upwardly ad~acent partition 78. The
invention could be practiced by a single blower instead of
the dual blowers described and a plurality of differen~ types
of ducting systems could also be used. However, it has been
determined that the dual counter rotatin~ blower system
described herein improves the uniformity of the convection
heating in the oven cavity.
I ~ ~5~22
As described in t.he sackground herein, the positioninq
of the microwave components such as magnetron 24 and waveguide
30 beneath cavity 12 meant that the burner 80 could not be
positioned in its conventional place directly below cavity 12.
Accordingly, as is shown in Figure 1, burner 80 is positioned
to the rear and below cavity 12 immediately below combustion
chamber 54. Insulation material 81 provides thermal insulation
for the microwave components. Furthermore, insuiation material
83 surrounds cavity 12 to thermally insulate the cavity which
is especially important during the self cleaning mode when there
may be temperatures hiqher than 1000F in cavity 12. With this
insulation, stove 10 meets all of the American National Standard
Institute standards with regard to fire prevention and burn
hazard.
As is well known, input gas to stove 10 passes through a
pressure re~ulator (not shown), low voltage valve 82 activated
by silicon carbide ignitor 84, gas line 86, and orifice carrier
88. Nozzle support 90 is welded to the vertical section 92 of
burner 80 and positions the burner in the proper fixed alignment
with the nozzle of orifice carrier 88. Nozzle support 90 is
open in the front and back as viewed in Fiqure 2 so that primary
combustion air is entrained into the burner to form the gas air
mixture. Althouqh it is an objective of the system described
herein to obtain an ideal gas combustion air mixture of approxi-
mately 1:10, the qas primary combustion air mixture is somewhat
greater than with most atmospheric burners, the difference
beinq compensated by mixing less secondary combustion air. It
is noted that the vertical section ~2 of burner 80 is tubular
rather than the typical venturi or narrow throat design;
the venturi effect is not required to create the negative
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1 :~ 65~3~2
pressure in the throat because the burner is operated in an
induced draft environment.
Referring to Figure 4, there is shown a cut away top view
looking down into combustion chamber 54 from line 4-4 of Figure
l. The horizontal section 94 of burner 80 is approximately 20
inches in length with the upper surface being substantially
tangential to the plane of the floor 112 of combustion chamber
54. Relatively low port loading which reduces the noise of
combustion is provided by 36 pairs of elon~ated ports 9~ perpen-
dicular to the length of the tubular horizontal section. The
ports, as shown, are positioned on the top of the burner. Each
port has a dimension of approximately .032 inches by .5 inches.
Other port configurations could be used but it is desirable
that they be on top and provide low port loading. Burner 80,
as described, has a port loading of approximately 20,000 Btu
per hr-sq. inc of port area as compared to a typical value of
30,000 Btu per hr~sq. in. of port area. Both the horizontal
and vertical sections of the burner have a diameter of approxi-
mately one inch.
Referring to Figure 5, there is an enlarged side view of
burner 80 as shown in Figure 1. Collar 97 forms a rectangular
tunnel 98 having an openinq 99 at the bottom and an openinq 100
at the top adjacent to the combustion chamber, the tunnel beinq
elonqated in wiclth so as to house burner 80. A substantial
part of opening 99 at the bottom is covered by plate 102 which
is spaced ~rom the bottom of collar 97 by .375 inches. Plate
102 does not extend the entire length o~ collar 97 leavinq area
104 as shown in Fiqure 2 for the vertical section 92 of burner
80 to enter tunnel 98. Accordingly, secondary combustion air
may enter tunnel 98 in the .375 inch gap 106 between collar 97
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I ~ 65~2~
and plate 102 or through area 104.
The end of horizontal section 94 of burner 80 is pressed
down and formed into a mount 108 which is attached by sheet
metal screw 110 to the under side of floor 112 of combustion
chamber 54. As stated earlier herein, the horizontal section
94 of burner 80 is substantially tangential to the under side
of floor 112 of combustion chamber 54. As shown best in
Figure 4, floor 112 of combustion chamber 54 has a rectangular
aperture 114. The width of aperture 114 is approximately 1.5
inches and the leng~h~ as shown, is slightly longer than the
length of the horizontal section having slots which may prefer-
ably be approximately lS inches. The relatively low port
loading described earlier herein must be provided in a rela-
tively small area of the burner because aperture 114 limits
to exposure area and it is preferable that the ports be on the
top of the burner. Aperture 114 is limited in size to restrict
the amount of secondary combustion air flowing therethrough
toward the negative pressure so as to increase efficiency. The
depth of combustion chamber 54 alonq floor 112 may preferably
be slightly larger than 2 inches tapering to a depth of approxi-
mately one inch at the top of the chamber immediately in front
of blowers 60.
When the ~as burner is to be activated, silicon carbide
ignitor 84 is electrically energized and heats to a temperature
which will ignite an air ~as mixture whereupon valve 82 opens,
thereby causing said mixture to emanate from slots 96. As
stated earlier, the primary combustion air enters at nozzle
support 90. The secondary air enters around .375 inch gap 106
and area 10~ and flows up through aperture 114 ad~acent to
burner 80 into combustion chamber 54. As described earlier
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I :J 6~2~
herein, blowers 60 create a slight negative pressure in com-
bustion chamber 54 which causes air to be drawn through per-
forations 62 from cavity 12. The sliqht negative pressure
also causes air to be drawn into combustion chamber 54 through
aperture 114 putting burner 80 in an induced draft environment.
It has been found that burner 80, as described earlier herein,
operates in this induced draft environment with good flame
characteristics and without the noise o~ combustion. The
combustion vapors from burner 80 add to and become part of the
recirculating convection air. Outlet vent 68 compensates for
the addition of combustion vapors into the recirculation system
through aperture 114. The structure defined herein provides a
desirable and ef~icient balance between recirculating air and
added combustion vapors. It is no~ed tha~ the blower system
so described and air recirculation is activated when the ma~ne-
tron is turned on, even if the gas burner is not simultaneously
activated; in this case, the recirculation is used to remove
water vapor from cavity 12 rather than to introduce heat.
Referring to Figures 6 and 7, top and side elevation
views respectively of microwave coupling structure 48 are shown.
As stated earlier herein, coupling structure 48 performs two
functions and its shape is selected to optimize with regard
thereto. First, it is important that couplinq structure 48
provide a favorable impedance match between coaxial conductor
38 and cavity 12 for a wide variety of food loads. A proper
impedance match results in a maximum power transfer and improved
efficiency. Second, it is desirable to transfer the microwave
eneryy into the cavity uniformly so as to eliminate hot spots
within food bodies. Furthermore, lt has been found desirable
to have coupling structure ~8 operate as a directive antenna
I 3 6~,22
whereby a substantial amount of the coupled microwave energy is
incident on the food body before being reflected from the walls
of the cavity setting up a complex standing wave pattern. Also,
it has been found that it is desirable to have a concentration
of power directly up from coupling structure 48 through the
center of cavity 12 rather than angled out towards the sides of
the cavity; this provides for more uniform cooking in many food
bodies such as cakes. Without this concentration or focusing
of energy in the center, cakes may exhibit a frin~ing effec~
whereby energy concentrates at the edges of the cake causing
the edges to he done while the center is still undone and sogqy~
Still referrinq to Figures 6 and 7, plate 132 functions
as one conductinq surface of a radial wavequide excited by
coaxial conductor 38. The other conducting surface of the
radial waveguide is the floor 134 of cavity 12. As described
earlier herein, motor 44 coupled to extension 136 of center
conductor 34 causes coupling structure 48 to rotate for improved
uniformity of the microwave radiation pattern in cavity 12.
Accordingly, one of the conductor surfaces, plate 132, of the
radial waveguide is in motion while the other conductor surface,
floor 134, is stationary.
Still referring to Figure 6, plate 132 has a slot 138
therein. From the center of rotation of plate 132 at hole 140
cut therein for mounting to center conductor 34, the inner and
outer radii of slot 138 may preferably be approximately .67
and 1.3 inches respectively. The lenqth of slot 138 may pre-
ferably be defined by a 180 arc from hole 140. So defined,
it may be preferable that slot 138 be resonant or one half
wavelength at its inner radial dimension so that there is a
maximum coupling of energy through it from the radial waveguide
- 16 -
-- 1 1 65~22
into cavity 12. Slot 134 provides for the concentration or
focusing of energy directly up from the coupling structure 48
previously described herein as being desirable.
Referring again to Figures 1 and 2, a plurality of top
gas burners 116 is provided; these burners operate as con-
ventional gas surface burners in accordance with well-known
practice and may be activated by controls 128. Many other
conventional features are also incorporated into stove 10.
For example, a temperature sensor (not shown) may preferably
be mounted within cavity 12 to provide an output used to
control the gas heating cycle so as to requlate the cooking
temperature in the cavity. Preferably, the positioning of
the temperature sensor is such that vapors from rectangular
pattern 66 do not impinge directly upon it. Also7 the micro-
wave energy power level and activation time may be controlled
by control~--panel 118.- Furthermore, a light bulb 120 posi-
tioned outside cavity 12 may provide light to the cavity
through a light transparent high temperature ceramic 122 and
microwave shield screen 124. Also, clock 126 may be used to
initiate heating operations at a preselected time.
A safety control circuit is provided in which air flow
sensor 130 comprising a vane actuated switch is positioned
in duct 75. It is used to prevent the supply of gas to burner
80 unless the air recirculation system comprising blowers 60
is activated. Accordingly, after the operator selects a temper-
ature for cavity 12 and activates convection heating, the auto-
matic sequence of events may be blowers 60 begin to recirculate
air, air flow sensor 130 switch closes as a result oE exhaust
air in duct 75, silicon carbide ignitor ~4 activates and then,
after a delay for the ignitor to heat up to a temperature
- 17 -
1 ~ 6~32~
sufficient to i~nite burner 80, low voltage value 82 opens and
the gas is supplied to orifice carrier 88.
This completes the description of the preferred embodiment
of the invention described herein. However, numerous modifi-
cations thereof will be apparent to one of ordinary skill in
the art without departing from the spirit and scope of the
invention. Accordingly, it is intended that the scope of
the invention be limited only by the appended claims.
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