Note: Descriptions are shown in the official language in which they were submitted.
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KITCHEN EXHAUST SYSTEM WITH CATALYTIC CONVtK l t~
BACKGROUND OF THE INVENTION
The present fnvention relates to exhaust hoods for cooking
appliances. More specifically, the present invention relates to such hoods
designed for commercial/institutional cooking applications in which a clean
exhaust stream is desired.
Referring to Fig. 1, a kitchen exhaust hood according to the prior art
includes an intake hood 1, and an exhaust duct 2. A barbeque grill 3, located
beneath intake hood 1, has a gas heat source 4 and a grate 5 on which food
6 is placed. When grill 3 is heated, hot gases from grill 3 rise into intake hood
1 and ~ass into exhaust duct 2 via an inlet vent 9 in which a filter F is placed.
Air and hot gases pass into exhaust duct 2 urged by a natural-convection
draft or by forced-convection draft generated by a fan 21. When food 6 is
placed on grlll 3, the searing of food 6 generates gases and aerosols 7 which
are carried into the exhaust stream by the negative pressure flow in the
vicinity of intake hood 1. The gases and aerosols 7 generated by the cooking
of food 6 include oil/tar droplets and hydrocarbons (smoke), particularly when
cooking fatty meats at high temperature. These products tend to foul intake
hood 1 and exhaust duct 2 and are noxious pollutants. It is therefore desired
to remove them from the exhaust stream.
Most of the smoke from burning organic materials, such as occurs in
cooking operations (frying, barbe~uing, etc), apart from condensed moisture,
consists of materials that can be combusted by means of a catalytic
converter. So, for example, catalytic converters have been used to eliminate
smoke and release additional fuel energy in heating devices. For example,
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US Palent No. 4,373,507 (Schwartz, et. Al.~ describes a wood-burning stove
with a bed of catalyst through which the exhaust stream is forced by natural
convection. in this appfication, the catalyst is placed very close to the
location at which fuel is burning to keep it above the ignition temperature for
the catalyst.
Catalytic converters have also been applied in indoor ovens. ~or
example, JA 00~6839 (May 198~), JA 0157532 (.~un. 1990), JA 008~615
(Mar. 199Q), FR 2 705 76~ - A1 (Jun. 1993), and JA 0043380 (Mar. 1980)
describe various ways of applying catalytic converters to ovens. In ail of
these devices, the catalyst is located inside the oven to maintain a high
catalyst temperature. Adey (US Pat. No 2,933,080) proposes a barbeque
grill with a natural convection exhaust system that includes a catalytic
converter. In this application, as in the oven applications, the cooking space
~ is enclosed (access is through a front door) and the catalyst is located close
to the heat source to maintain high catalyst temperatures. The above
references do not address the problem of applying catalytic converters in the
exit duct of an exhaust hood, where substantial amounts of unheated air are
drawn into the effluent stream along with the smoke released from the
cooking process. The high catalyst temperatures required are difFicult to
maintain in such applications.
The problem of a low temperature exhaust is addressed in US Pat.
No. 4,138,220, to Davies which describes a portion of an exhaust system in
which cooking smoke is drawn by a fan through a gas-to-gas heat
exchanger. One side of the heat exchanger conducts untreated stream of air
and gas to a catalytic converter, the other side conducts the hotter heated
stream of gases to a suction fan. Heat from flameless combustion in the
catalyst is transferred from the treated stream to the untreated stream.
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Additional heat may be added, as required, to insure the catalyst is hot
enough to operate, from an electric or gas heater. In addition, there are
various references that described catalytic filter beds with built-in electric
heaters to pre-heat catalyst.
Another catalytic converter-based system is described in USP
~,~80,535. This patent, describes a method and apparatus for a broiler or
other source of cooking fumes. The specification describes embodiments in
which an effluent stream from a cooker, captured by a hood, is exposed to
a catalyst by running the effluent through one or more catalytic converters
drawn by a fan. The specification mentions that the catalyst or the effluent
stream may be heated if necessary to insure oxidation by the catalyst.
There remain a number of important problems associated with any
proposed application of catalytic converters to kitchen exhaust equipment.
First, the amount of energy lost by a system such as proposed by Davies
may be very high for several reasons: (1) the fan energy required to draw
cooking gases through the gas-to-gas heat exchanger, if high efficiency heat
transfer is to be obtained, is very high, (2) since extra air must be drawn in
from around the hood to prevent smoke from entering the kitchen, a great
deal of extra heat must be added to raise this air mixture temperature from
room temperature to the operating temperature of the catalyst. Second,
kitchen exhaust hoods are usually supplied in a modular package and
installed separately from the cooking equipment. In fact, they are usually
manufactured by different companies. This presents control problems for
new and retrofit installation that are not addressed by Davies and which are
avoided by the designs of the other references cited above because they are
combined heating and exhaust devices. According to the above prior art,
interconnections between the exhaust hood and the heat source must be
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made to control any source of additional heat required to preheat or maintain
temperatures of the catalytic converter. This is undesirable in the context of
separate cooking and exhaust systems.
There are also other problems not specifically related to the goai of
providing a separate exhaust system thàt is compatible with different cooking
equipment. For example~ ideal conditions can never be realistically
rnaintained and so it almost inevitable that the catalytic converter will at times
become fouled. This, as is well-known in the art, is fatai to its function. ~hus,
there is a need to keep the catalytic converter clean. For most cooking
equipment, the quantities of combustible contaminants emitted is very low
when no food is cooking and there is no residual food left on the cooking
surfaces. Even if heat is left on until more food is placed on the equipment,
which is common, there is no need to maintain the catalytic converter
temperature because there are nearly no combustibles left in the effluent
stream. Maintaining the catalytic converter temperatures under these
conditions wastes energy.
Still another important issues is the cost of the catalytic converter. The
fuel in grease laden air can be oxidized, but the amount of catalytic converter
surface required may be large. Catalytic converters are frequently very
expensive and it is desirable to minimize the load on it so that the size and
cost can be as low as possible.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to provide an exhaust system for kitchen
cooking equipment that is compatible with a wide range of cooking
2 5 equipment.
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It is another object of the invention to provide a kitchen exhaust hood
that automatically controls supplementary heat for a catalytic converter.
It is still another object of the invention to provide a kitchen exhaust
system that cleans an internal catalytic converter.
It is still another object of the invention to provide a kitchen exhaust
system that has an automatic cleaning cycle.
It is still another object of the invention to provide a kitchen exhaust
system with a catalytic converter for cleaning effluent stream from cooking
processes.
0 It is still another object of the invention to provide a kitchen exhaust
system with a catalytic converter that does not need to be removed for
normal rnaintenance.
It is still another object of the invention to provide a kitchen exhaust
system with a catalytic converter that incinerates and/or vaporizes aerosol
particles before an effluent stream passes through the catalytic converter.
It is still another object of the invention to provide a kitchen exhaust
system with a catalytic converter that augments the incineration and/or
vaporization of aerosol particles before an effluent stream passes through
the catalytic converter by inducing acceleration and/or straining of the main
2 o effluent flow.
Briefly, A kitchen exhaust system for a smoky cooking appliance, such
as a barbeque, includes a modular exhaust hood with a catalytic converter.
To maintain high catalyst temperatures with a minimum of auxiliary heat,
several features are provided. First, the exhaust hood is tapered for form a
2~ converging channel at the roof of the hood to guide fumes and air to an inlet
slot through the exhaust flow passes to the catalytic converter. Second, the
inlet slot is sized so that the flow through the inlet matches the average
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natural-convection plume velocity from the cooking appliance. Third, the inlet
slot is located over the middle of the cooking area. The above three features
minimize residence time of fumes in the hood and reduce large-eddy
turbulence in the hood. Fourth, a portion of the treated effluent stream is
recirculated to form a capture jet at the front of the hood to create a local
negative pressure that reduces potential for entrainment of fumes into the
surrounding area without increasing exhaust volume. Fifth, auxiliary burner
packs are provided to inject extra heat only as required. Sixth, a control
system provides for self-cleaning of the catalyst. Other features are also
1 0 described.
According to an embodiment of the invention, there is provided, an
exhaust device for a cooker, comprising: a hood partially enclosing a space
above said cooker and having a forward end where access is provided to
said cooker and a rear end opposite said forward end; said hood having an
exhaust duct with a catalytic converter; said hood having a fan to draw fumes
and air from a region around said cooker into said hood, through said
catalytic converter; a passage and a vent positioned to draw gas from a
treated stream and eject said gas from said forward end into a space above
said cooker, in a rearward direction, whereby a capture jet is generated; a
heater positioned to eject heat into a stream of untreated gas upstream of
said catalytic converter; and a controller configured to maintain a
temperature one of upstream and downstream of said catalytic converter at
a specified level effective to maintain said catalytic converter at a minimum
operating temperature at which said catalytic converter effectively burns fuel
in said fumes.
According to another embodirnent of the invention, there is provided,
an exhaust device for a cooker, comprising: a hood partially enclosing a
-
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space above said cooker and having a forward end where access is provided
to said cooker and a rear end opposite said forward end; said hood having
an exhaust duct with a catalytic converter; said hood having a fan to draw
fumes and air from a region around said cooker into said hood, through said
catalytic converter; said hood being shaped to form a converging passage
guiding fumes from said cooker and an inlet positioned substantially over a
middle of said cooker, whereby a length of travel of said fumes toward said
inlet is minimized, said inlet being sized so that an average velocity of
exhaust drawn through said inlet is substantially equal to a natural
convection plume velocity of said fumes rising from said cooker whereby said
fumes and outside ambient air drawing into said inlet pass smoothly into said
inlet and into said exhaust duct.
According to still another embodiment of the present invention, there is
provided, an exhaust system for a kitchen exhaust system for capturing and
treating an effluent stream consisting of aerosol particles and gas, comprising:an exhaust capture intake with a duct connected to convey the effluent stream
captured the capture intake, a catalytic converter connected to the duct such
that the effluent stream passes through the catalytic converter, the catalytic
converter having an ignition temperature, a source of hot gas connected to inject
hot gas into the effluent stream, carried by the duct, at an injection point
upstream of the catalytic converter, the hot gas being at a temperature selectedto incinerate the aerosol particles when the aerosol particles are exposed to the
hot gas at the temperature, the hot gas being injected at a rate sufficient to
- insure that the catalytic converter is raised to the ignition temperature.
2 5 According to still another embodiment of the present invention, there is
provided, an exhaust system, col"pri~-.ing: an exhaust capture intake with a duct
connected to convey the effluent stream captured the capture intake, a catalytic
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converter connected to the duct such that the effluent stream passes through
~ the catalytic converter, the catalytic converter having an ignition temperature, a
source of hot gas connected to inject hot gas into the effluent stream, carried by
the duct, at an injection point upstream of the catalytic converter, the hot gasbeing at a temperature selected to incinerate the aerosol particles when the
aerosol particles are exposed to the hot gas at the temperature, the hot gas
being injected at a rate sufficient to insure that the catalytic converter is raised
to the ignition temperature, the exhaust capture intake forming an opening with
an access and a blind end and a nozzle fed by a duct connected to recirculate
a portion of the effluent stream from, the nozzle being positioned at the accessand directed toward the blind end to generate a capture jet.
According to still another embodiment of the present invention, there is
provided, an exhaust system for a kitchen exhaust system for capturing and
treating an effluent stream consisting of aerosol particles and gas, comprising:an exhaust capture intake with a duct connected to convey the effluent stream
captured the capture intake, a catalytic converter connected to the duct such
that the effluent stream passes through the catalytic converter, the catalytic
converter having an ignition temperature, a source of hot gas connected to inject
hot gas into the effluent stream, carried by the duct, at an injection point
2 o upstream of the catalytic converter, the hot gas being at a temperature selected
to incinerate the aerosol particles when the aerosol particles are exposed to the
hot gas at the temperature, the hot gas being injected at a rate sufficient to
insure that the catalytic converter is raised to the ignition temperature, a
controller connected to regulate a thermal power rate of the source of hot gas,
2 5 the controller being programmed to regulate the thermal power rate such as to
insure that the catalytic converter is rnaintained at the ignition temperature, the
controller being programmed to enter an idle mode in which the power rate is
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lowered in response to one of a switch and a timer signal and an alarm
connected to indicate that the power rate is lower than required to maintain thecatalytic converter at the ignition temperature.
According to still another embodiment of the present invention, there is
S provided, an exhaust system for a kitchen exhaust system for capturing andtreating an effiuent stream consisting of aerosol particies and gas, comprising:an intake connected to a duct, the intake having an entrance into which an
effiuent stream may be captured and conveyed into the duct, a catalytic
converter in the duct and located such that the effluent stream passes through
the catalytic converter, the catalytic converter having an ignition temperature, a
heat source and a turbulence generator connected in such a way as to strain
the effluent stream to generate large-scale turbulence and associated local hot
regions in the effiuent stream at a point upstream of the catalytic converter, aregion of the duct downstream o~ the point and upstream of the catalytic
converter being sufficiently long to allow the large-scale turbulence to
substantially yield their turbulent energy to turbutence at scales at least an order
of magnitude smaller than the large-scale turbulence; the hot gas being at a
temperature selected to incinerate the aerosol particles when the aerosol
particles are exposed to the hot gas at the temperature and the hot gas being
2 0 injected at a rate sufFicient to insure that the catalytic converter is raised to the
ignition temperature.
The above, and other objects, features and advantages of the present
invention will become apparent from the foilowing description read in conjunction
with the accompanying drawings, in which like reference numerals designate the
2 5 same elements.
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LIST OF FIGURES
Fig 1A shows in partial section a cooking grill with an exhaust hood
according to an embodiment of the prior art.
Fig. 1 B shows in partial section the cooking grill and exhaust hood of Fig. 1A
with dimension lines indicating certain features of the prior art configuration.Fig. 2A shows in partial section a cooking grill with an exhaust hood
according to an embodiment of the invention.
Fig. 2B shows in partial section the cooking grill and exhaust hood of Fig. 2A
with dimension lines indicating certain features of an embodiment of the
1 0 invention.
Fig. 2C shows in partial section a cooking grill according to an embodiment
of the invention in which a capture jet is directed upwardly toward an inlet
slot.
Fig. 3 shows in partial section a cooking grill with an exhaust hood according
to another embodiment of the invention.
Fig. 4 shows in partial section the cooking grill of Fig. 2A with control
elements.
Fig. 5 shows in partial section the cooking grill of Fig. 3 with control
elements.
Fig. 6A shows in section a view down a duct section carrying effluent from
the cooking grill/exhaust systems of Figs. 1-5 into which hot exhaust from an
auxiliary burner is injected.
Fig. 613 shows in partial section from the side the duct section of Fig. 6A.
Fig. 6C shows a conceptual model of vortex generation and flow with the
associated inertial forces on an aerosol particle.
,
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Fig. 6D shows in section a view down a duct section similar to that of Fig. 6A,
but employing a different manner introducing the effluent stream into the duct
section into which hot exhaust is injected.
Fig 6E shows in partial section from the side the duct section of Fig. 6D.
Fig. 7 shows in section a side view of header injection system for adding
auxiliary heat to effluent from the cooking grill/exhaust systems of Figs. 1-5.
Fig. 8 shows in section an injection system employing baffles to accelerate
the flow for adding auxiliary heat to effluent from the cooking grill/exhaust
systems of Figs. 1 5.
Fig. 9A shows in partial section a helical duct into which the effluent stream
and hot gas are injected.
Fig. 9B shows in section the helical duct of Fig. 9A with a lead-in portion thatis not shown in Fig. 9A.
Fig. 10 shows in partial section an exhaust hood with a mixing portion in a
rising duct leading to the catalytic converter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 2A, a first embodiment of the invention includes an
intake hood 11, and an exhaust duct 12. A barbeque grill 3, located beneath
intake hood 11, has a gas heat source 4 and a grate 5 on which food 6 is
placed. When grill 3 is heated, a hot effluent stream from grill 3 rises into
intake hood 11, passing through an intake slot 19. The hot stream then
~ passes through a descending duct 35, into exhaust duct 12 impelled by
suction generated by a fan 21. A negative pressure in intake hood 11,
generated by fan 21, draws air 18 in the vicinity of intake hood 11, and
heated gases and aerosols 7 from grill 3, into exhaust duct 12. When food
-
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6 is placed on grill 3, the searing of food 6 generates additional gases and
aerosols 7 which are also carried into the exhaust stream by the negative
pressure flow in the vicinity of intake hood 11. The gases and aerosols 7
generated by the cooking of food 6 include oil/tar droplets and hydrocarbons
(smoke), particularly when cooking fatty meats at high temperature. Gases
and aerosols 7 are filtered from the effluent stream by a catalytic converter
22.
Catalytic converter 22 flamelessly burns combustible materials that
introduced in effluent stream 7 by the cooking of meat and by any incomplete
combustion (usually minimal or insignificant) of gas in gas heat source 4
These processes are well-known in the art and are not further described
here. In the process of flameless burning of combustibles in effluent stream
converts pollutants into relatively innocuous gases, as is well-known.
Therefore, a treated stream 27 leaving catalytic converter 22 is much cleaner
than an untreated stream 17 entering catalytic converter 22. As is also well-
known, the flametess burning in catalytic converter 22 generates a
substantial amount of heat, so that treated effluent stream 27 is also
substantially hotter than untreated effluent stream 17.
To operate properly, the temperature of a catalyst of catalytic
2 o converter 22 must be maintained at at least approximately 45QF or more.
Normal flue temperatures for a grill such as grill 3 are on the order of 300-
375F. A number of devices are added to the hood 11/du~t 12 system to
maintain the required catalyst temperature.
One device for helping to maintain the required high temperature of
2 5 the catalyst is a capture jet 24 generated by tapping part of treated stream
27 off, with take-off 25, to create a re-cycle stream 23. Recycle stream 23 is
discharged through a slot 29 (Slot 29 has a longitudinal dimension going into
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the page) to generated capture jet 24. Slot 29 runs along the entire length of
hood 11. Capture jet 24 causes entrainment of air near a forward edge 30
of hood 11. Entrainment is a phenomenon of turbulent jet flow. Jets generate
a local negative pressure near the jet where the velocity is relatively high.
Flow of surrounding air 18 into hood 11 is partly caused by negative pressure
in the hood generated by the natural convection stack effect in duct 12 and
fan 21, and partly caused by entrainment in capture jet 24 (also a local
negative pressure). Capture jet 24 is sufficiently effective to reduce
substantially the degree of negative pressure that must be maintained within
hood 11 to prevent gases and aerosols 7 from escaping into the kitchen
beyond forward edge 30. Capture jet 24 is a known device for reducing the
negative pressure required in hoods such as hood 11 and 1. The result of
reducing negative pressure in hood 11 is to raise the temperature of effluent
stream 7 by reducing the cooling effect of drawing in outside air 18. Aside
from reducing the quantity of cool room air drawn into untreated stream 17,
capture jet 24 heats untreated st.ream 17 because it is drawn from hot
treated stream 27. The prior art forms of capture jets are not drawn from flue
gas.
Another device for helping to maintain the required high temperature
of the catalyst is an auxiliary burner pack 28 mounted on either or both
side(s) of grill 3. Burner pack 28 generates additional heat which is added
to untreated stream 17 to raise its temperature directly.
One of the most important features of the invention which contributes
to reduction in the total quantity of room air required to be drawn into the
exhaust stream is the location of inlet slot 19 directly over the center of the
cooking surface. In comparison, the.prior art hood locates inlet vent toward
the rear of the exhaust hood. Also, inlet slot 19 is sized so that the average
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14
velocity of fluid entering inlet slot 19 is approximately equal to the velocity of
the plume of air and gases 7 rising upwardly toward exhaust hood 11. Also
note that the shape of the interior of exhaust hood 11 converges toward inlet
slot 19 forming a converging channel. The effect of these design features is
to minimize the tendency of infiltration of fumes into the kitchen carried by
large, slow-moving eddies which can be generated by drafts in the kitchen
or by turbulence generated by the rising plume of fumes 7. In the prior art
device of Figs. 1A and 1 B, the average velocity of fumes 7 passing into inlet
vent 9 is lower than the average plume velocity. The roof of hood 1 guides
fumes near the front of exhaust hood 1 over the substantially long length of
the exhaust hood 1 toward inlet vent g. Therefore, if exhaust volume rates
are reduced in the prior art device, fumes tend not to pass smoothly out of
the exhaust hood area. Rather, with low exhaust volume rates, the passage
of fumes is characterized by a relatively long residence time in the hood. In
addition, the mismatch between the plume velocity and the average \/elocity
at the inlet vent 9 causes fumes to swirl. Both these effects encourage
infiltration into the kitchen. The design of the hood according to the inventiontends to minimize these effects reducing inrillldlion and allowing the hood to
operate effectively, without inrill,~lion, with a low total exhaust volume.
2 o Still another device for increasing temperature of the exhaust is theprovision of a hot gas tap 31 to inject combustion products 26 directly from
gas heat source 4. This serves as an auxiliary source of heat to help raise
the temperature of the catalyst.
Comparing Figs. 1 B and 2B, a further reduction in the total quantity
of room air required to be drawn into the exhaust stream is in the shape of
hood 11 itself. According to prior art design, hood 1 has a wide access
indicated by dimension line A, a short lip indicated by dimension line B, and
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a negative overhang indicated by dimension line C. Exhaust hood 11
according to the present invention exhibits a narrower access indicated by
dimension line A', a much deeper lip indicated by dimension line B', and a
positive overhang indicated by dimension line C'. These features have
derlonstrated the ability to reduce the total exhaust volume required to
prevent gases and aerosois 7 from escaping into the kitchen. While each of
the above dimensional features contributes to the advantages of the
invention, no-single one of these features is required in the invention..The
specific choices made for these dimensional parameters ~as well as other
features described above) would involve the cooking process, that is, the
amount of "fuel" in the exhaust stream, the amount of additional heat
tolerable (for energy savings purposes), the presence or absence of
turbulence or drafts in the kitchen, the height of the access required, etc.
Referring to Fig. 2C, a variation on the design of capture jet 24' is
shown. In this embodiment, slot 29' is directed upwardly toward inlet slot 19
to form a capture jet 24' that flows upwardly rather than horizontally. Slot 29'is provided with turning vanes to adjust turbulence and velocity uniformity of
the jet.
Typically the volume flow rate of a commercial hood serving a cooking
2 o grill is about 280-300 cfm per lineal foot of grill. The capture jet flow rate is
typically on the order of 19 cfm per lineal foot of grill. With the use of a
capture jet, a hood can have flow rates of only 100-125 cfm. The use of a
capture jet composed of hot flue gas can potentially enable hood/duct
system 1 1/12 to operate with so little outside air 18 that the catalyst
temperature can be maintained above the operating temperature (~450F),
most of the time, without additional. heat from auxiliary burner pack 28. Of
course, this depends on the amount and nature of food cooking, the heat
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16
rate of grill 3, and whether surfaces of grill 3 and hood 11 have been heated
by continuous use. Any heat required when food is cooked at less than ideal
conditions are handled by a control system described below.
Referring to Fig. 3, two modifications of the configuration of Fig. 2A,
each of which could be made independently of the other, are shown in a
second embodiment. First, capture jet 24 is generated by tapping treated
stream 27 using a blower 51. In this case, take off 25 is not necessary. As
discussed later in connection with the control of the votume flowrate of
capture ~et 24, the control system can control the shaft speed of blower ~1
instead of using a damper as discussed in connection with the embodiment
of Fig. 2A. Second, burner pack 28 is arranged to vent hot products of
combustion into descending duct 35. Third, hot gas tap 31 is not used to
inject combustion products 26 directly from gas heat source 4. Instead, the
system relies, for auxiliary heat, on burner pack(s) 28. Note that alternatively,
burner pack(s) 28 could be omitted and the system could rely solely on hot
gas tap 31 for its auxiliary heat.
Controls
Referring to Fig. 4, a control system, for and connected to the
embodiment of Fig. 2A, includes a controller 53. Controller 53 is preferably
based on a programmable digital processor and includes internal switches
and analog voltage and/or current outputs (not shown, but known in the art)
to regulate various devices.
Controller 53 also includes input interfaces to allow it to determine
values of temperature and gas flowrate. Controller 53 is connected to control
fan 21 to control the flowrate through duct 12 by controlling the ~an motor
-
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17
speed through a motor controller 45. Note that the flowrate control could be
accomp~ished by numerous means, including using a blower that unloads
(e.g., a centrifugal blower) when flow is obstructed so that flow control couid
be accomplished with a damper.
Controller 53 is also connected to contro~ an alarm 46 which signals
a catalytic converter self-cleaning cycle. Alarm 46 could be a flashing light,
a horn, a bell or any of various different arrangements suitable for indicating
when the self-cleaning cycle is operating.
Controller 53 controls dampers 44 and 47 through damper drives 56
and ~7 respectively. Damper 4~ regulates the rate of flow treated gas 27 that
generates capture jet 24. Damper 47 closes off descending duct 35 during
the cleaning cycle, described below, and also may serve as a fire damper.
Burner pack 28 is regulated by controller 53. During the cleaning cycle
and during unsteady (start-up and cool-down) operation it is expected that
substantial amounts of heat must be added to prevent fouling of catalytic
converter 22 and maintain a clean treated stream 27. Burner pack(s) 28
is/are modulated to provide precisely the amount of additional heat required
to maintain the operating ternperature of the catalyst when required. The
rules for controlling this and the other elements controlled by controller 53
are described below after describing the sensor inputs to controller 53.
The temperatures entering and leaving catalytic converter 22 are
detected by temperature sensors 42 and 41, respectively. A corresponding
pair of temperature signals are generated by a transducer T, which might be
any of various interfaces for temperature measurement. For example, if
temperature sensors 41 and 42 were thermocouples, transducer T would
include a reference voltage and differential amplifiers to convert the small
.
-
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18
voitages generated by thermocouples. Also detected by controller 53 is
actuation of a switch 66 which turns the hood on.
Referring to Fig. 5, a control system for the embodiment of Fig. 3 is
shown. The controller 53 and the various controllers and sensors and their
connections are identical to what was described with reference to Fig. 4, with
the following differences: (1) Instead of controlling a damper 44, controller 53 controls blower 51. And (2)
The control system implements a number of operating modes as
defined below:
Staff-Up Mode
The start-up mode initiates a proving cycle in which the catalyst is
brought up to operating temperature and a base-line temperature difference
between the temperature upstream of catalytic converter 21 (given by
temperature sensor 42) and the temperature downstream of catalytic
converter 21 (given by temperature sensor 41 ) is measured. The baseline
temperature difference establishes two things: (1) it provides a measure of
the perforrnance of catalytic converter 21 during no-load operation (no food
is being cooked but the heat source is operating). (2) it serves as an
indicator that catalytic converter 21 is fouled.
The start-up mode is initiated when a user actuates switch 66. Fan 21
is started and run at a nominal rate, approximately 100-125 cfm per lineal
foot of grill 3 The flow rate of fan 21 is controlled by a digital control loop
based on the measured flow according to known techniques. (Of course,
other non-digital control techniques are also applicable.) Burner pack(s) 28
2 5 is/are run and operated initially at a full rate and then turned down
responsively to the temperature indicated by temperature sensor 42
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19
according to known control techniques. Damper 47 is held fully open and
damper 44 is controlled to fix the volume rate of capture jet 24 at
approximately 15 cfm.
Temperature sensors 41 and 42 are monitored during start-up and
once the difference between the two temperatures reaches a steady-state,
the temperature difference is recorded in an internal memory 67 (possibly,
but not necessarily a non-volatile memory device) of controller 53. If, after a
period of time established by experiment to be required to reach steady-state
under normal conditions (i.e., the condition where the catalyst is clean), a
steady-state temperature difference is not reached, a self-cleaning cycle is
initiated. The self-cleaning cycle is described beiow. In addition, if the steady-
state temperature difference is higher than a temperature difference
established by experiment to be normal for no-load operation, then the self-
cleaning cycle is also initiated. Also, if the steady-state temperature
difference is lower than a temperature difference established by experiment
to be normal for no-load operation, then the self-cleaning cycle is also
initiated.
The reason for initiating the cleaning cycle, and for establishing two
separate criteria for initiating it, is as follows. When a catalytic converter
2 0 becomes fouied, its ability to combust waste in the exhaust stream
diminishes. As a result, the temperature difference for a badly fouled catalyticconverter will approach ~ero, even though there is burnable waste in the
exhaust stream and the operating temperature has been reached. In this
case, a temperature difference that is below normal indicates a badly fouled
catalytic converter. For a converter that is not very badly fouled, some
portions of its surface may be operative and other portions inoperative, the
inoperative surfaces being so because they carry burnable waste.
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Alternatively all surfaces may be operative but still carrying accumulated
burnable waste. In this case! once the catalytic converter heats up, the
accumulated waste on the catalytic converter adds burnable waste to the
exhaust stream fueling an abnormal degree of catalytic combustion. Such a
condition is indicated as either a variable temperature difference (that is, thetemperature difference does not reach steady-state) or it is indicated by a
higher than expected temperature difference. Thus, only when the catalytic
converter sees a small amount of fuel in the exhaust stream, and therefore
registers a temperature difference Iying in a small band slightly above zero,
will the start-up operation bypass the self-cleaning cycle.
At the end of the start-up mode; which occurs after steady-state
temperature differences are reached or after the controller branches to a
self-cleaning mode, the controller goes into the steady-state cooking mode.
Sfeady-State Cooking Mode
When controller 53 is in the steady-state cooking mode, the input
temperature of catalytic converter 21 is controlled to be maintained at at leastthe minimum required operating temperature of the catalyst. Fan 21 is run
at the nominal rate, controlled by the digital control loop based on the
measured flow. Damper 47 is held fully open and damper 44 is controlled to
fix the volume rate of capture jet 24 at approximately 15 cfm per lineal foot.
During steady-state cooking mode, burner pack(s) 28 is/are run, if
required, at a rate required to maintain the catalyst temperature. The burners
are regulated responsively to the temperature indicated by temperature
sensor 42 according to known control techniques.
The temperature difference- between the temperature upstream of
catalytic converter 21 and the temperature downstream of catalytic converter
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21 is continuously monitored by controller 53. If the temperature difference
falls below a level that indicates the catalytic converter is fouled, controlter53 branches to the self-cleaning mode. If the temperature difference falls and
plateaus at a steady-state temperature that indicates no-load operation,
controller 53 branches to the steady-state idle mode.
Sfeady-Stafe Idle Mode
During steady-state idle mode, the conditions of steady-state co~king
mode are maintained. In the steady-state idle mode, the burner pack(s) 28
are turned off and the catalyst temperature is not maintained at the operating
temperature to conserve energy. The steady-state idle mode can be initiated
by a switch or by a timer or some other means. Alarm 46 is activated to alert
the user to the fact that catalyst temperatures are not being maintained. If
switch 66 is actuated again during steady-state idie mode, controller 53
branches to steady-state cooking mode, bringing the catalyst temperature
back up. Alternatively, the steady-state idle mode can be terminated by
sensing the upstream temperature sensed by temperature sensor 42. If the
upstream temperature falls precipitously, indicating a load has been placed
on grill 3, controller 53 can then branch to the steady-state cooking mode.
Self-Cleanmg Mode
2 o At the start of the self-cleaning mode, alarm 46 is activated to alert the
user to the status of controller 53. The user should cease cooking and turn
off the grill. Alternatively, an interlock could be connected to controller to
deactivate gas heat source 4. Burner pack(s~ 28 is/are activated and
operated at full. Fan 21 is regulated to operate at 25% Of norninal output.
Damper 47 is closed fully so that only heated gas from burner pack(s) 28,
-
CA 02229936 1998-02-19
WO 97148479 PCT~US97/1055
and minimal bypass air, are drawn by fan 21. During self-cleaning mode, the
difference between the temperature upstream of catalytic converter 21 and
the temperature downstream of catalytic converter 21 is continuously
monitored by controller 53. When the temperature difference reaches a
steady state level that indicates catalytic converter 21 is clean, control
returns to whatever mode initiated the self-cleaning mode. Alternatively, the
self-cleaning mode could be terminated based on a fixed time interval alone,
or, instead, a fixed time interval could establish an upper limit on the duration
of the self-cleaning mode otherwise terminated based on the temperature
difference. (In the latter case, alarm 46 could be activated if the self-cleaning
mode "time-out" before the expected "clean" temperature difference was
reached .)
Auxiliary Heat Addition
As discussed above, embodiments of the invention include the means
for adding auxiliary heat to maintain the necessary ignition temperatures. A
. goai of the invention is provide maximum cleansing of exhaust using a
minimum of energy. To achieve catalytic conversion, heat must be added.
That is, to elevate the catalyst to the ignition temperature, the input
temperature is elevated to at least, approximately, the ignition temperature.
Of course, the oxidation process produces heat so the temperature of the
gas entering the catalyst can be lower, depending on how much fuel is
supplied to the catalyst by the effluent stream. However, considering the cost
of the catalytic converter, and its efficiency, it is desirable, to minimize thesize of the catalytic converter.
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~3
According to a feature of the preferred embodiment of the invention,
auxiliary heat Is added by injecting hot exhaust from a burner directly into theeffiuent stream upstream of the catalytic converter. It has been found that
this can lead to significant incineration and vaporization of aerosol grease,
which increases the overall efficiency of a system which has a catalytic
converter that is not large enough to handle the total load alone, even if the
temperatures are high enough to achieve ignition by the cataiyst. That is, the
auxiliary heat is used to:
1. Heat the effluent stream as a whole to insure oxidation of
0 any combustibles by the catalytic converter.
2. Incinerate and/or vaporize as much of the aerosol material
in the effluent stream as possible prior to passing the effluent stream
to the catalytic converten
Thus, the input stream minimizes the burden on the catalytic converter
and helps to prevent fouling and other maintenance problems. Thus,
incineration/vaporization of the aerosol particles in the effluent stream
minimizes the amount of liquid that must be oxidized by the catalyst helping
to increase the effectiveness of the catalytic converter and reduce the
potential for fouling.
There is a problem with using the auxiliary heat for the above
purposes because the two goals require different amounts of heat. The
amount of heat that must be added to the effluent stream to raise it to the
temperature to incinerate the aerosol grease, (about 750F) is substantially
greater than the amount of heat required to raise the effluent stream to the
2 5 ignition temperature of the catalyst ~about 450F). This disparity exists
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24
because the effluent stream contains a large volume of room air drawn into
the hood and which carries the aerosol particles resulting in a relatively cool
temperature of the effluent stream. Actualiy, the amount of heat required to
incinerate the aerosol is very low because the mass of aerosol is very low.
Thus, in principle, the only heat required for incineration is that necessary toheat the mass of aerosolized grease plus sufficient air to provide oxidation.
The problem is that the carrying gas has such a large volume that it must
also be heated to heat the aerosol. The invention addresses this problem.
To minimize the amount of heat required to achieve the above two
goals, ideally, each particle of aerosol would be vigorously mixed with a
volume of hot gas from the auxiliary burnèr proportional to the fraction of the
total mass the particle represents relative to the total mass of aerosol with
just enough oxidizer from the carrying gas stream to oxidize the particle. This
means that most of the carrying gas is just excess oxidizer. The following
insights pertain:
a. That the auxiliary heat can (and should) be used to incinerate (or
vaporize) the aerosol,
b. That the auxiliary heat can (and should) be used to raise the input
temperature of the catalyst to the ignition temperature, and
c. That the chance that a given aerosol particle will be heated to a
temperature high enough to vaporize or incinerate it can be increased by
various mechanisms.
When a hot flow is injected into a two-phase flow stream comprising
a cool carrying gas and an aerosol, the hot flow mixes in a way that can only
be described statistically. Each aerosol particle will have a slightly different
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experience in going through the mixing zone created by injection. One
aerosol particle, carried by a cool volume of carrying gas might be strained
by (or strain) a hot volume of injected gas. The cool gas will dilute the hot
gas and, assuming sufficient mixing, sufficient oxygen, and a high enough
ratio of hot gas to cool gas in the mixed product, cause the aerosol to
incinerate. Insufficient oxygen in the mixed volume would cause the aerosol
to either vaporize, partially incinerate, or a combination of both. Some
particles will not 'Ssee" hot gas at ali or "see" very little. If there is a lot of.cool
carrying gas and not much hot gas, the chance that an aerosol particle will
be incinerated is low. One goal of the invention is shift this statistic toward
greater probability of incineration/vaporization. This is the idea behind item
c in the above list.
Item c of the above list, is addressed, according to the invention, by
several means. The preferred mechanism for selectively heating aerosol
(rather than simply convectively heating the entire stream) is to use inertial
forces strategically in two ways:
1. To create an aerosol-rich sub-flow and injecting hot gas locally into
that sub-flow.
2. To take advantage of the inertia of the aerosol particles to cause
them to migrate as much as possible relative to the flow carrying them
increasing the chance that during the particle's history it will be affected by
a volume of gas hot enough to cause incineration/vaporization. Another
mechanism is to use a radiant source to irradiate the aerosol in the
substantially transparent carrying gas stream. Still another possible
2 5 mechanism is to heat an impingement surface of an impingement separator
to burn the catalyst. Various embodiments implementing the preferred
mechanism are discussed in more detail below.
-
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~6
According to one embodiment of the invention, the inertia of the
aerosols is exploited in two ways: to cause the aerosol particies to migrate
relative to the flow carrying them as opposed to with the flow carrying them
and to create an aerosol rich flow-field sub-portion with which the injected hotgas can be mixed.
By generating a flow subportion that is aerosol-enriched, the cooling
effect of the aerosol-carrying gas is minimized allowing more aerosol to be
burned or vaporized by the minimat heat added from the auxiliary burner. Of
course, this aerosol-enriched flow-field subportion and the high temperature
of this subportion are not permanent. Turbulent and molecular diffusion
insure that the flow field properties averàge-out downstream. However, as
the flow evolves, turbulent diffusion dominates the mixing process. The
large-scale swirling flow generates smaller vortices which generate still
smaller vortices as the flow evolves. These vortices break subvolumes of hot
gas from the iniection region and move away from it. However, the smaller
vortices are still carried by the main flow and the larger vortices. The larger
scale vortical flows usually move faster than the eddies they generate
because they generate them through shearing forces that strain adjacent
volumes of fluid (except for vortex stretching which accelerates an existing
vortex by shrinking its diameter). Since larger vortices give rise to smaller
ones, and not the other way around, the larger scale vortices move faster
than the ones the smaller ones. This means the aerosols particles
experience centrifugal forces due not only to the smallest scale vortices in
which they are resident, but also they experience centrifugal forces due to
2 5 the larger scale vortices carrying those smallest scale vortices. These forces
cause the aerosol particles to migrate relative to the smaller scale vortices,
some of which are hot and some of which are cool in the evolving, not-yet-
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~7
fully-mixed portion of the flow. Thus, the chance that an aerosol particle,
migrating, due to its inertia, through a "sea" of regions (vortices) of varying
temperature, will be exposed to gas at a temperature high enough to cause
incineration/vaporization, is increased by the local acceleration of the flow.
This phenomenon will be further explained relative to the embodiment of
Figs. 6A and 6B, discussed below.
Referring to Figs. 6A and 6E~, the preferred embodiment of the
invention has a rising round duct section 101. Draft is induced by a fan (not
shown in Fig. 6, but similar to the embodiment shown in Fig. 5) augmented
by the natural convection, or so-called, stack-effect. Turning vanes 103 at the
inlet to duct section 101 may be used to cause the effluent stream contained
in duct section 101 to swiri (swirling flow indicated by helical arrow 114). A
nozzle 104 injects the exhaust from a flame-retaining power burner 105
which blows hot combustion products into nozzle 104 generating a jet 106.
Jet 106 is directed by nozzle 104 at a tangential angle increasing the swirling
effect of effluent stream. Aerosols in duct-section 101 migrate toward the
perimeter of duct section 101 as a result of inertial forces (Note spiral path
108 of hypothetical aerosol particle). The migration of the aerosols causes
the perimeter region 109 (perimeter region bounded by dotted line 1 10) to
become aerosol-enriched. In addition, the hot gases from power burner 105,
in the longitudinal region of duct section 101 near nozzle 104, coincide with
this aerosol-rich flow subportion, because these hot gases are injected at a
tangent, and therefore circulate with the swirling flow.
~ Referring to Figs. 6D and 6E, an alternative to using turning vanes to
impart an initial swirl to the effluent stream is to provide that the effluent
stream enters through a duct 401 ata tangent to the round duct section 101.
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28
This causes the effluent stream to begin swirling as indicated by helical
arrow 414.
Initially, the hot gases remain at the perimeter for at least a portion of
the longitudinal length of duct section 101 providing prolonged concentration
of the hot gases and aerosols in this area. The coincidence of the hot gases
from power burner 105 and the aerosol-enriched region helps to insure that
more hot gases are used to incinerate/vaporize aerosol material than just
used to heat the entirety of the effluent stream. That is, the hot gases are
injected tangentially into a swirling flow generating a local hot flow region
near the perimeter of duct section 101. The aerosol particles are
concentrated in the same region where the hot gases are concentrated,
resulting in a local flow region (the perimeter area) where
incineration/evaporation of aerosol particles can take place.
~ Further down duct section 101, the main swirling flow breaks down as
vortices are formed resulting from straining of the flow. The rate of break-
down depends on the length of duct section 101, the mean velocity of the
flow, the circular momentum of the flow, etc. turbulent vortex formation. The
vortex formation is a result of the continuous straining of both the hot and
cooler gases in both the vertical direction (owing to the curved velocity profile
which is high-valued in the center and low-valued at the perimeter) and to
strain caused by the swirling flow. Energy from the straining is converted into
turbulent velocity which breaks the flow into initially large subvolumes
(vortices) that have a component of velocity that is independent of the
straining flow that gives birth to the subvolume (vortex). This does not mean
2 5 that the daughter subvolume moves independently of the flow that generates
it, but that it foilows the mother flow imperfectly. The movement of these
subvolumes gives rise to further straining that generates smaller subvolumes
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29
which also move, to an extent, independently of the flow that gives rise to
them. However, the movement of the daughter subvolumes of gas is
dominated by the velocity of the mother volumes of gas so that the aerosols
carried by them are subjected to acceleration by their movement. The main
component of acceleration in a turbulent flow field is usually the result of themovement at the largest sc~les of the turbulence, which is where the vast
majority of the turbulent energy resides. This acceleration of the carrying flowcauses the aerosol particles to migrate in a flow that is made up of
subvolumes of hot and cool gas. A given aerosol particle, moving at least
partly independently of the gas surrounding it, is likely to come in contact
with gas at more than one temperature, since the gas surrounding is
continuously breaking down into increasingly smaller eddies of high
temperature gas and low temperature gas. So even though most of the
mixed flow is made of gas at a low temperature, the chance that an aerosol
particle will migrate into (or through~ a region of hot gas is increased by thismore-or-less independent movement of the aerosol particles. This effect
increases the chance that a given aerosol will be incinerated or vaporized.
Referring to Fig.6C, consider a few small eddies 120-122 composed of gas
at different temperatures. Movement of eddies such as cool eddy 120 or 121
("H" and "L" in the figure represent hot and cool gas respectively) could
transport adjacent volumes such as eddy 122 into position, as shown,
adjacent to other cool volumes of gas carrying aerosols, such as eddy 121.
The swirling flow (or it could just be a larger scale eddy) 126 carrying the
eddies 120-122 is responsible for most of the acceleration that an aerosol
particle 124 "feels" resulting in a net centrifugal force 12~ on aerosol particle
124. This force causes aerosol particle 124 to migrate relative to the local
flow field carrying it and causing it to be exposed to the high temperature of
CA 02229936 1998-02-19
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eddy 122. Note that Fig. 6A is a conceptual model only. In reality, vortices
will form due to strain in all axes to varying degrees (The maJor inputs being
~1) the strain that manifests as the axial velocity profile and (2) the rotational
(swirl) of the flow). Another effect that is known in the field of turbulence isvortex stretching which accelerates an existing vortex by stretching it
(conservation of angular momentum requires an increase in angular velocity
through an input in energy provided by the shearing forces that stretch and
shrink the vortex).
Considering the conceptual model of Fig. 6C it can be seen that as
the flow breaks into smaller and smaller voiumes of high and low
temperature gas and these volumes illler",i"gle due to turbulent convection,
there is an increasing chance that an aerosol particie, initially in the middle
of duct section 101, will come into contact with a hot volume of gas. That is,
hot volumes of gas will be transported into the center of duct by turbulent
convection shortening the migration path required for the aerosol particle to
be heated by a hot volume of gas. .~t the same time, the eddies carrying the
aerosol particles are still subject to acceleration by the swirling flow and by
larger-scale vortices fed by the strain of the swirling flow and the strain of the
axial flow. This acceleration causes the aerosol particles to migrate relative
2 o to the smaller eddies increasing the chance that a given particle will
encounter a volume (eddy) of hot gas.
It is also useful to note the approximate scales of the eddies that can
be created by a flow system such as that of Figs. 6A and 6B. The size of the
smallest vortices generated by a turbulent system is a function of the energy
~~3 ~1)
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input to the system. The smallest vortices are of a size at which viscosity and
momentum effects reach an approximate balance. The energy input rate can
be approximated as:
where ~ is the rate of energy fed into turbulent vorticity, I~ is the velocity of
the eddy and l is the size (length-scale) of the eddy as described in A First
Course in Turbulence. Tennekes, H. and Lumley, J.L., Mass. Inst. Tech.,
1972, the entirety of which is incorporated herein by reference. The velocity
of the vortices can be approximated by:
~ "~
c (I)
where v is the velocity of the largest vortices which can be taken as the
velocity of the swirling flow for the embodiment of Figs. ~A and 6B.This is an
estimate of the smallest turbulent scales (length microscales) generated. The
temperature microscales are of the same order of size as the length
microscales (assuming that kinematic viscosity and thermal diffusivity are
comparable in magnitude for the carrying gas). The temperature microscales
represent the size that the temperature fluctuations have to reach before
they dissipate due to molecular diffusion. Based on the above analysis and
some realistic operating conditions, the temperature and length scales are
of the order of 100 microns. In practice, the temperature fluctuations are very
- smeared out at this scale even though the dominant mixing mechanism isstill inertiai (i.e., turbulent diffusion rather than molecular diffusion). The
temperature scales associated with strong temperature differentials are
approximately ten times this amount, or 1 mm. A particle migrating a
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distance that is several times this distance would have a reasonable chance
of seeing significant fluctuations in temperature in the flow field substantially
remote frorn the injection point where the scale of the turbulence has
reached these micro-levels. In designing an injection system, it is desirable
for the downstream portion of the duct to be long enough to allow the
development of a relativeiy fine temperature field, but not fine enough that
molecular diffusion starts to dominate the diffusion of the temperature
parameter. Thus, downstream of the injection point, a mixing region, whose
size can be found by experiment to be the length such the temperature field
contains few local regions at a temperature hot enough to incinerate or
vaporize the aerosol. At the latter point, the hot-zones that are required for
incineration will have approached a fully-mixed temperature of the two gas
streams (the appropriately weighted average of the hot exhaust stream and
the cool carrier gas stream).
A rough calculation based on some approximate assumptions shows
that the inertial effects for even small particles can be enough in the above
system to cause significant migration relative to the scale of the temperature
fluctuations in the flow field. Assume a particle size of 2.5~. Assume the
vertical flow rate is about 1 m/s and we want the particle to migrate 10 times
2 o the temperature microscale in a half second. The earlier analysis shows that
a conservative estimate of the scale of significant temperature fluctuations
is about 1 mm. Assuming the time the particle is subjected to the fluctuating
temperature field upstream of the converter is 1 se~ond, then the tangential
velocity required to generate sufficient acceleration is about 6 m/s. This is a
reasonable number. It is based on the following calculation:
,2,~/RF"(~ ") ~2
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33
where .,~ ;s the tangentiai velocity, R is the radius of the duct, ~u~ is the
velocity of the particle relative to the gas, and ~,~ is given by:
P" P
CD = 24 ( I ~O.ISRe~.687)
~2~ -~1 ]D P
llg
where ~u, and P6 are the viscosity and density of the effluent gas (assumed
to have the properties of air), respectively, D" is the particle diameter. Now
2.5,u is at the low end of the particle size range. So that for larger particles,
significant concentration occurs in the perimeter region. For particles larger
than 2.5,u, the velocity drops off quickly - for a 1 O,u particle, the velocity is
only 1.5 mls.
Note that the acceleration of the flow can be performed by two means:
(1) creating a flow process in which a sustained large-scale acceleration of
the main flow is maintained or repeatedly generated or which is sufficiently
durable to persist for a long period and (2) generating strong strain
processes that inject a substantial amount of turbulent energy into the flow.
The latter pushes the temperature microscale down to smaller and smaller
levels. A combination of these two-is desirable and for most systems, the
acceleration of the main flow inevitably involves vigorous straining of the
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34
main flow so the two always occur. However, different systems can place
different degrees of emphasis on the two above processes. At a small scale,
both processes "look" the same because it makes no difference to the
aerosol particles whether they are induced to drift by motion of large scale
eddies or by an acceleration feature of the main flow.
In addition to the inertial concentration, tangential jet, eddy-formation
and other effects described above, the flow system generated by the
embodiment of ~igs 6A and 7A also has the following properties:
a. Aerosol movement is retarded in the upward direction by an
increase in drag forces increasing the mixing and residence time of
the aerosol particles in the hot region. This is a result of the fact that
aerosol particles are forced into the perimeter region where the axial
velocity is lower.
b. The hot gases are injected into and stay for a time in the
perimeter region, again where the axial velocity of the cool gas in duct
section 101 is lowest and, since the hot gas is injected with zero axial
momentum, it must be accelerated by shear forces by the adjacent
cooler flow. Both of these mean the movement of the hot flow
upwardly is delayed and thereby, that the duration of mixing with the
2 0 cooler carrier gas is increased.
c. The weight of aerosol particles also causes them to iag
slightly behind the flow increasing residence time of the aerosol in the
iniection area. This further increases the chance of
incineration/vaporization .
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The listed effects a, b, and c are desirable because they help to insure that
the aerosol particles are heated by the hot gases from power burner 105.
Referring to Fig. 7, another embodiment of an inertial concentrating
system employs a gas supply header 201 shaped as a cylinder in a
rectangular duct section 202. The flow configuration is a cylinder in cross-
flow. An entrance neck 203 to duct section 202 is shaped to neck the effluent
flow stream down. As the effluent stream expands into rectangular duct
section 202, aerosol particles, because of their inertia, follow a straighter
course than the fluid streamlines 209 which take a wider course around
supply header 201. The course 208 of a hypothetical aerosol particle is
shown in Fig. 7. Hot exhaust, from an auxiliary power burner 204, is injected
into the effluent stream from perforations in supply header 201 at the forward
stagnation point. The hot exhaust flows around supply header 201 forming
a hot boundary layer subportion 205 (shown bounded by a dotted line 21 1 )
around supply header2~1. Aerosol particles, which tend to follow a straighter
course than the effluent stream in the main, tend to concentrate in the vicinityof this hot boundary layer while a less concentrated flow bypasses the hot
boundary layer. The result is as described above, the hot gases are
combined with an aerosol-rich flow subregion helping to provide a greater
2 o chance that a given aerosol particle will be raised to a high enough
temperature to be incinerated or vaporized This means more aerosol will be
incinerated or vaporized. Note that the above description is simplified and
does not contemplate eddy generation, boundary layer separation, and other
aspects of the real-world flow system. These issues, however, do not affect
the ~asic ideas behind the embodiment shown in Fig. 7 and discussed
above.
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36
The above discussion relating to the turbulent convection and the
breakdown of the hot flow subportion applies to the device of Fig. 7 except
that acceleration of local flow will result from larger turbulent vortices formed
in the flow as it progresses. The advantage of having a relatively persistent
swirling flow, however, is not present in the embodiment of Fig. 7.
Referring to Fig. 8, a system, which insures continuous large-scale
acceieration of the carrying gas, employs baffle plates 50~. Hot exhaust 106
is injected through a header 501 into a rectangular duct section 507. The
repeated turning of the effluent flow (represented by arrows 502) has two
effects: (1) it strains the flow causing turbulent mixing and associated vortex
generation and (~, it sub3ects the main effluent flow to repeated
accelerations. The turbulent convection results in the hot exhaust being
broken up into many subvolumes of hot and cooler gas and the acceleration
caused by both turbulent vortices and by the baffles causes the aerosol
l 5 particles to cross between the subvolumes (owing to their inertia-that is, they
lag behind the accelerated carrying gas), increasing the odds that a given
particle will "see" a volume hot enough to incinerate/vaporize it.
Referring to Figs. 9A and 9B, still another embudiment designed to
accomplish a similar result as discussed above with respect to Figs. 6A and
6B, employs a helical duct section 601. The effluent stream 607 enters
helical duct section 601. At a point in helical duct section 601, hot exhaust
603 is injected through a nozzle 605 into the effluent stream 6Q7 generating
a jet 603. As discussed above, straining of the gases generates vortices
which create a heterogeneous mix of hot and cool regions which evolve
along the flow into ever smaller regions until full mixing may occur at some
point (i~ the duct is long enough). During this entire process, all the effluentstream 607 is subjected to constant acceleration by the turns in the helical
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duct section 601. Both the acceieration caused by vortices and that caused
by the shape of the duct (and gravitation) cause the aerosol particles to lag
the carrying gas resulting in an opportunities for aerosol particles to migrate
from a subregion of cool carrying gas arising from the effluent stream into a
subregion (eddy) of hot exhaust.
Referring to Fig. 10, the embodiments of Figs. 6A, 6B, 6D, 6E, 7, 8,
9A, and 9B can be incorporated in the embodiments of Figs. 2A, 2B, 2C, 3,
4, and 5 by inserting the former into the latter somewhere in the effluent
stream upstream of the catalytic converter. In the embodiment of Fig. 10, the
embodiment of Figs. 6~ and 6B forms a rising duct section 622. Any of the
embodiments of Figs. 6A, 6B, 6D, 6~, 7, 8, 9A can be incorporated using
appropriate duct transitions, etc according to known design techniques.
Other aerosol concentrators/ flow stream accelerators are possible
based on the above teachings. For example, an elbow-shaped duct without
turning vanes would concentrate aerosol against the far wall (right wall for a
left bend, left wall for a right bend) immediately following the elbow. Hot gas
would be injected along this far wall to create the hot, aerosol-rich, flow
subregion. The strain of the flow in the turn would generate vortices of the
width of the duct which would give rise to the acceleration and break-down
effects discussed above with reference to Fig. 6C.
The above technical calculations are based on the following
references, the entirety of each of which is incorporated herein by reference:
1 ) Dimitri Gidaspow, ''Multiphase flow and fluidization -- Continuum and
~ kinetic theory descriptions," Academic Press, 1994
2) O. Faltsi-saravelou, P. Wild, S.S. Sazhin and J.E. Michel, ''Detailed
modelling of a swirling coal flame," Combustion Science and Technology,
CA 02229936 1998-02-19
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Vol. 123, pp. 1-22, ( 1997)
3) S.A. Morsi, and A.J. Alexander, ''An investigation of particle trajectories
in two-phasae flow systems," Journal of Fluid Mechanics, Vol.55,
pp. 1 93-208
( 1~72) .
A refinement of the above systems adds a filtering stage upstream of
the zone in which gas is injected. This filtering stage removes the larger
aerosol particles from the emuent stream so that the effluent stream contains
only small aerosol particles. The smaller size increases the chance that the
aerosol will be completely incinerated or vaporized. If a larger particie is
exposed to hot gases and only surface burning takes place (burning of the
surface of the aerosol particle), ash formation could result, causing fouling
of the duct system and the catalytic converter. The filtering could be done
with any kind of filter, such as inertiai separator, impingement, or porous
filter. Preferably this filtering step wouid be performed with a grease
separator used in kitchen exhaust equipment.
Where the auxiliary heat source is a fired source such as natural gas,
or oil-fired heat source, it is preferable to insure that all combustion is
completed prior to introducing the hot combustion products into the effluent
stream. If less than stochiometric air is supplied in the burner, and the
system relies on oxygen in the effluent stream, the large volume of cool gas
(mostly air) may snuff the burning process of at least a portion of the still-
burning fuel mix emanating from the burner system into the duct. Thus,
sufficient oxidizer is preferably supplied by the burner to allow complete
combustion of the auxiliary fuel. In addition, preferably, all combustion of the
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39
auxiiiary fuel should be completed prior to the introduction of the hot gas intothe effluent stream. This will insure maximal utilization of the auxiliary fuel
(i.e., minimization of premature snuffing of the burning auxiliary fuel).
The effectiveness of the above embodiments, and other various
techniques for concentrating the aerosols, depend upon the size of the
aerosol particles, the speed of the main flow and subportions of the flow, the
drag forces on the aerosol particles, etc. In some cases, the inertial
concentration effect may not be sufficient to allow a desired percentage of
the aerosol to be incinerated or evaporated, for example, if the aerosol is
0 very light and smail in size. In such cases, the totai amount of heat supplied
by the auxiliary heat source may be increased. Or the hot gases may be
supplied into the stream in a way that enhances stability of the subflow ~or
example, by minimizing turbulent convection ~such as using a flow-
~ straightener in the injection nozzle in the embodiment of Fig. 6 to eliminate
the largest turbulent eddies). Thus, in some situations, desired conditions
can be met by adiusting a combination of the variables that affect the
effectiveness of the inertial concentration, the total quantity of heat, and thestability of the flow subregion. As discussed above, the embodiment of Fig.
6 provides two additional effects that help to increase the total amount of
incineration/vaporization: (1) it retards aerosol movement in the upward
direction by causing an increase in drag forces and (2) delays the movement
of the hot flow upwardly.
Although only a single or few exemplary embodiments of this
invention have been described in detail above, those skilled in the art will
readily appreciate that many modifications are possible in the exemplary
embodiment(s~ without materially departing from the novel teachings and
advantages of this invention. Accordingly, all such modifications are intended
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to be included within the scope of this invention as defined in the following
claims. In the claims, means-plus-function clauses are intended to cover the
structures described herein as performing the recited function and not only
structural equivalents but atso equivalent structures. Thus although a nail
and screw may not be structural equivalents in that a nail relies entirely on
friction between a wooden part and a cylindrical surface whereas a screw's
helical surface positively engages the wooden part, in the environment of
fastening wooden parts, a nail and a screw may be ec~uivalent structures.
