WO1997048479A1

WO1997048479A1 - Kitchen exhaust system with catalytic converter

Info

Publication number: WO1997048479A1
Application number: PCT/US1997/010550
Authority: WO
Inventors: Rick A. Bagwell; Fuoad Parvin; Eliott Gordon; Mark A. Catan
Original assignee: Halton Company
Priority date: 1996-06-19
Filing date: 1997-06-18
Publication date: 1997-12-24
Also published as: JPH11514734A; EP0881935A1; AU3400697A; CA2229936A1; EP0881935A4

Abstract

A kitchen exhaust system for a smoky cooking appliance includes a modular exhaust hood (11) with a catalytic converter (22). High catalyst temperatures are obtained with a minimum of auxiliary heat. The exhaust hood (11) is tapered for forming a converging channel leading to an inlet slot (19), located over the middle of the cooking area, through which the exhaust flow passes to the catalytic converter (22). The slot (19) is sized to match the flow through the inlet and an average natural-convection plume velocity from the cooking appliance. A portion of the treated effluent stream (27) is recirculated to form a capture jet (24) at the front of the hood (11) to create a local negative pressure that reduces potential for entrainment of fumes into the surrounding area without increasing exhaust volume. A control system provides for self-cleaning of the catalyst. Auxiliary burners (28) are employed to insure catalyst inlet temperatures are sufficient for ignition of the catalyst. Heat is added to promote the incineration and/or vaporization of aerosol particles. To do this, hot exhaust from the burner (28) is injected directly into the effluent stream (17). Acceleration of the main flow is promoted during a turbulent mixing regime, characterized by a fluctuating temperature field, upstream of a phase in which molecular diffusion dominates. The acceleration causes the particles to migrate, relative to the carrying flow, through the varying temperature field, causing incineration/vaporization with less heating of the cool carrying gas.

Description

KITCHEN EXHAUST SYSTEM WITH CATALYTIC CONVERTER

BACKGROUND OF THE INVENTION

The present invention 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 pass 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 grill 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, barbequing, 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, US Patent 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 application, 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 For example, JA 0096839 (May 1985), JA 0157532 (Jun 1990), JA 0085615 (Mar 1990), FR 2 705 766 - A1 (Jun 1993), and JA 0043380 (Mar 1980) describe various ways of applying catalytic converters to ovens In all 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 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

5,580,535. 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 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 goal of providing a separate exhaust system that is compatible with different cooking equipment. For example, ideal conditions can never be realistically maintained and so it almost inevitable that the catalytic converter will at times become fouled. This, as is well-known in the art, is fatal to its function. Thus, 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 equipment. 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 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 maintenance

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 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 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 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 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 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; 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 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 According to still another embodiment of the present invention, there is provided, an exhaust system, compπsing 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 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 access and 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 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, the controller being programmed to regulate the thermal power rate such as to insure that the catalytic converter is maintained at the ignition temperature, the controller being programmed to enter an idle mode in which the power rate is 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 the catalytic converter at the ignition temperature.

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 intake connected to a duct, the intake having an entrance into which an effluent 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 effluent stream at a point upstream of the catalytic converter, a region of the duct downstream of the point and upstream of the catalytic converter being sufficiently long to allow the large-scale turbulence to substantially yield their turbulent energy to turbulence 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 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 following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 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. 1B 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 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. 6B 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. 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 Ftgs 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 that is 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 1 1 , 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 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 descπbed 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 flameless 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 converter 22 must be maintained at at least approximately 450F 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/duct 12 system to maintain the required catalyst temperature

One device for helping to maintain the required high temperature of 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 the page) to generated capture jet 24. Slot 29 runs along the entire length of hood 1 1. Capture jet 24 causes entrainment of air near a forward edge 30 of hood 1 1. 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 1 1 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 1 1 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 1 1 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 stream 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 sιde(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 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 1 1 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 1B, 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 9 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 velocity 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 invention tends to minimize these effects reducing infiltration and allowing the hood to operate effectively, without infiltration, with a low total exhaust volume. Still another device for increasing temperature of the exhaust is the provision 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 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 demonstrated the ability to reduce the total exhaust volume required to prevent gases and aerosols 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 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 11/12 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 7/48479 PC17US97/10550

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 volume flowrate of capture jet 24, the control system can control the shaft speed of blower 51 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 fan motor speed through a motor controller 45. Note that the flowrate control could be accomplished by numerous means, including using a blower that unloads (e.g , a centrifugal blower) when flow is obstructed so that flow control could be accomplished with a damper. Controller 53 is also connected to control 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 57 respectively Damper 44 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 temperature 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 voltages 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

Start-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 performance 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 is/are run and operated initially at a full rate and then turned down responsively to the temperature indicated by temperature sensor 42 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 below 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 becomes fouled, its ability to combust waste in the exhaust stream diminishes As a result, the temperature difference for a badly fouled catalytic converter will approach zero, 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 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, the temperature 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 lying 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.

Steady-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 least the 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 21 is continuously monitored by controller 53. If the temperature difference falls below a level that indicates the catalytic converter is fouled, controller 53 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.

Steady-State Idle Mode

During steady-state idle mode, the conditions of steady-state cooking 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 idle 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-Cleaning Mode 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 nominal output. Damper 47 is closed fully so that only heated gas from burner pack(s) 28, TT

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 goal 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 the size of the catalytic converter According to a feature of the preferred embodiment of the invention, auxiliary heat is added by injecting hot exhaust from a burner directly into the effluent stream upstream of the catalytic converter. It has been found that this can lead to significant incineration and vaporization of aerosol grease, 5 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 catalyst. That is, the auxiliary heat is used to:

1 . Heat the effluent stream as a whole to insure oxidation of o 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 converter.

Thus, the input stream minimizes the burden on the catalytic converter 5 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. 0 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 5 ignition temperature of the catalyst (about 450F). This disparity exists 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. Actually, 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 to heat 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 burner 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 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 "see" hot gas at all 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 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 According to one embodiment of the invention, the inertia of the aerosols is exploited in two ways: to cause the aerosol particles 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 hot gas 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 minimal 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 average-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 injection 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 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- 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 6B, 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 swirl (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 inerttal 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 110) 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 at a tangent to the round duct section 101 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 that the daughter subvolume moves independently of the flow that generates it, but that it follows the mother flow imperfectly The movement of these subvolumes gives rise to further straining that generates smaller subvolumes 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 the movement at the largest scales of the turbulence, which is where the vast majority of the turbulent energy resides. This acceleration of the carrying flow causes 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 this more-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 125 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 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 is vortex 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 volumes of high and low temperature gas and these volumes intermingle due to turbulent convection, there is an increasing chance that an aerosol particle, 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 At 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 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

( i )

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 e is the rate of energy fed into turbulent vorticity, n is the velocity of the eddy and / 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

v¹ 1/4

( I ) e

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 6A 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 is still inertial (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 distance that is several times this distance would have a reasonable chance of seeing significant fluctuations in temperature in the flow field substantially remote from 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 relatively 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 5μ Assume the vertical flow rate is about 1 m/s and we want the particle to migrate 10 times 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 second, 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

¥ VV ⁽²⁾ where u_θ is the tangential velocity, R is the radius of the duct, u_rιι_p is the velocity of the particle relative to the gas, and F_π is given by:

24

C₀=— (1 +0.15Λc°.687)

Re

[u -u ]D p

Re

^'g

where μ_g and p_g are the viscosity and density of the effluent gas (assumed to have the properties of air), respectively, D_p is the particle diameter. Now 2.5μ 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 2.5μ, the velocity drops off quickly - for a 10μ particle, the velocity is only 1.5 m/s.

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 mam 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 Figs 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 cooler carrier gas is increased c The weight of aerosol particles also causes them to lag slightly behind the flow increasing residence time of the aerosol in the injection area This further increases the chance of incineration/vaporization 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 211) around supply header 201 Aerosol particles, which tend to follow a straighter course than the effluent stream in the main, tend to concentrate in the vicinity of 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 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 basic ideas behind the embodiment shown in Fig 7 and discussed above 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 acceleration of the carrying gas, employs baffle plates 505 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 (2), it subjects 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 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 embodiment 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 607 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 (if the duct is long enough) During this entire process, all the effluent stream 607 is subjected to constant acceleration by the turns in the helical duct section 601. Both the acceleration caused by vortices and that caused by the shape of the duct (and gravitation) cause the aerosol particles to iag 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. 6A and 6B forms a rising duct section 622. Any of the embodiments of Figs. 6A, 6B, 6D, 6E, 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, 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 193-208 (1972).

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 effluent 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 particle 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 inertial separator, impingement, or porous filter Preferably this filtering step would 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 stochiometπc 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 auxiliary fuel should be completed prior to the introduction of the hot gas into the 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 very light and small in size. In such cases, the total 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 for 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 adjusting a combination of the variables that affect the effectiveness of the inertial concentration, the total quantity of heat, and the stability 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 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 also 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 equivalent structures

Claims

What is claimed is:

1. 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 said effluent stream captured said capture intake; a catalytic converter connected to said duct such that said effluent stream passes through said catalytic converter; said catalytic converter having an ignition temperature, a source of hot gas connected to inject hot gas into said effluent stream, carried by said duct, at an injection point upstream of said catalytic converter; said hot gas being at a temperature selected to incinerate said aerosol particles when said aerosol particles are exposed to said hot gas at said temperature; said hot gas being injected at a rate sufficient to insure that said catalytic converter is raised to said ignition temperature.

2. An exhaust system as in claim 1 , wherein said duct is shaped to cause said effluent stream to accelerate at a portion of said duct downstream of said injection point.

3. An exhaust system as in claim 1 , wherein said duct has a portion between said injection point and said catalytic converter to allow substantial turbulent mixing of said effluent stream and said hot gas to occur before a combined flow of said hot gas and said effluent stream reaches said catalytic converter.

4. An exhaust system as in claim 1 , further comprising: a nozzle through which said hot gas is injected into said duct; said nozzle being directed such as to urge said effluent stream to rotate in a first direction in said duct about an axis parallel to a flow direction of said effluent stream.

5. An exhaust system as in claim 4, wherein said duct has a round cross-section.

6. An exhaust system as in claim 5, further comprising: a swirl generator upstream of said injection point; said effluent stream entering said duct through said swirl generator, whereby said effluent stream is urged to rotated about said axis in said first direction.

7. An exhaust system as in claim 4, further comprising: a swirl generator upstream of said injection point; said effluent stream entering said duct through said swirl generator, whereby said effluent stream is urged to rotated about said axis in said first direction.

8. An exhaust system, comprising: an exhaust capture intake with a duct connected to convey said effluent stream captured said capture intake; a catalytic converter connected to said duct such that said effluent stream passes through said catalytic converter, said catalytic converter having an ignition temperature, a source of hot gas connected to inject hot gas into said effluent stream, carried by said duct, at an injection point upstream of said catalytic converter, said hot gas being at a temperature selected to incinerate said aerosol particles when said aerosol particles are exposed to said hot gas at said temperature, said hot gas being injected at a rate sufficient to insure that said catalytic converter is raised to said ignition temperature, said 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 said effluent stream from, said nozzle being positioned at said access and directed toward said blind end to generate a capture jet

9 An exhaust system as in claim 8, wherein said exhaust capture intake has an intake hood portion over a cooking area, said intake hood forms a converging channel at the top of which is an opening into which said effluent stream is drawn

10 An exhaust system as in claim 9, wherein said opening is shaped as a slot

11 An exhaust system as in claim 8, further comprising a controller connected to regulate a thermal power rate of said source of hot gas, said controller being programmed to regulate said thermal power rate such as to insure that said catalytic converter is maintained at said ignition temperature.

12 An exhaust system as in claim 8, further comprising a controller connected to regulate a thermal power rate of said source of hot gas, a temperature difference sensor connected to said controller and positioned to sense a temperature difference between a temperature of said effluent stream upstream of said catalytic converter and a temperature downstream of said catalytic converter, said temperature difference sensor having an output indicating said temperature difference, a start switch connected to said controller, said source of heat being capable of generating heat at a high rate, an intermediate rate, and a low rate, said controller being programmed to operate said source of heat responsively to said start switch, said controller being programmed to operate at a rate lower than said high rate after a specified period of time in response to said temperature difference being within a specified range for a period of time beginning a specified interval after said source of heat begins operating at said high rate

13. An exhaust system as in claim 12, wherein said controller is further programmed to continue operating at said high rate in response to said temperature difference being outside said specified range.

14. An exhaust system as in claim 8, further comprising: a controller connected to regulate a thermal power rate of said source of hot gas; a temperature difference sensor connected to said controller and positioned to sense a temperature difference between a temperature of said effluent stream upstream of said catalytic converter and a temperature downstream of said catalytic converter; said temperature difference sensor having an output indicating said temperature difference; a start switch connected to said controller; said source of heat being capable of generating heat at a high rate, an intermediate rate, and a low rate; said controller being programmed to operate said source of heat responsively to said start switch; said controller having a memory; said controller being programmed to store a value of said temperature difference in said memory in response to said temperature difference reaching a steady-state value in a specified range; said controller being programmed to operate at a rate lower than said high rate after a specified period of time in response to said temperature difference being within a specified range for a period of time beginning a specified interval after said source of heat begins operating at said high rate; and said specified range having a lower end equal to a value stored in said memory representing a stored temperature difference

15. 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 said effluent stream captured said capture intake; a catalytic converter connected to said duct such that said effluent stream passes through said catalytic converter, said catalytic converter having an ignition temperature; a source of hot gas connected to inject hot gas into said effluent stream, carried by said duct, at an injection point upstream of said catalytic converter; said hot gas being at a temperature selected to incinerate said aerosol particles when said aerosol particles are exposed to said hot gas at said temperature; said hot gas being injected at a rate sufficient to insure that said catalytic converter is raised to said ignition temperature, a controller connected to regulate a thermal power rate of said source of hot gas; said controller being programmed to regulate said thermal power rate such as to insure that said catalytic converter is maintained at said ignition temperature; said controller being programmed to enter an idle mode in which said power rate is lowered in response to one of a switch and a timer signal, and an alarm connected to indicate that said power rate is lower than required to maintain said catalytic converter at said ignition temperature

16 An exhaust system as in claim 15 further comprising a switch, said controller being programmed to discontinue said steady-state idle mode in response to an actuation of said switch

17 An exhaust system for a kitchen exhaust system for capturing and treating an effluent stream consisting of aerosol particles and gas, comprising an intake connected to a duct, said intake having an entrance into which an effluent stream may be captured and conveyed into said duct, a catalytic converter in said duct and located such that said effluent stream passes through said catalytic converter, said catalytic converter having an ignition temperature, a heat source and a turbulence generator connected in such a way as to strain said effluent stream to generate large-scale turbulence and associated local hot regions in said effluent stream at a point upstream of said catalytic converter, a region of said duct downstream of said point and upstream of said catalytic converter being sufficiently long to allow said large-scale turbulence to substantially yield their turbulent energy to turbulence at scales at least an order of magnitude smaller than said large-scale turbulence, said hot gas being at a temperature selected to incinerate said aerosol particles when said aerosol particles are exposed to said hot gas at said temperature, and said hot gas being injected at a rate sufficient to insure that said catalytic converter is raised to said ignition temperature

18. An exhaust system as in claim 17, wherein said duct is shaped to cause said effluent stream to accelerate at a portion of said duct downstream of said point.

19. An exhaust system as in claim 19, wherein said duct has a round cross-section.

20. An exhaust system as in claim 19, further comprising a swirl generator upstream of said injection point; said effluent stream entering said duct through said swirl generator, whereby said effluent stream is urged to rotated about said axis in said first direction.