Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24C—DOMESTIC STOVES OR RANGES ; DETAILS OF DOMESTIC STOVES OR RANGES, OF GENERAL APPLICATION
  • F24C15/00—Details
  • F24C15/20—Removing cooking fumes
  • F24C15/2028—Removing cooking fumes using an air curtain
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/34—Chemical or biological purification of waste gases
  • B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
  • B01D53/86—Catalytic processes
  • B01D53/88—Handling or mounting catalysts
  • B01D53/885—Devices in general for catalytic purification of waste gases
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24C—DOMESTIC STOVES OR RANGES ; DETAILS OF DOMESTIC STOVES OR RANGES, OF GENERAL APPLICATION
  • F24C15/00—Details
  • F24C15/20—Removing cooking fumes
  • F24C15/2042—Devices for removing cooking fumes structurally associated with a cooking range e.g. downdraft
  • F24C15/205—Devices for removing cooking fumes structurally associated with a cooking range e.g. downdraft with means for oxidation of cooking fumes

Definitions

• 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
• a kitchen exhaust hood 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
• Catalytic converters have also been applied in indoor ovens
• JA 0096839 May 1985
• JA 0157532 Jun 1990
• JA 0085615 Mar 1990
• FR 2 705 766 - A1 Jun 1993
• JA 0043380 Mar 1980
• Adey US Pat. No. 2,933,080 proposes a barbeque grill with a natural convection exhaust system that includes a catalytic converter.
• a kitchen exhaust system for a smoky cooking appliance such as a barbeque, includes a modular exhaust hood with a catalytic converter
• 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
• the inlet slot is sized so that the flow through the inlet matches the average natural-convection plume velocity from the cooking appliance.
• 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.
• 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.
• auxiliary burner packs are provided to inject extra heat only as required.
• a control system provides for self-cleaning of the catalyst. Other features are also described.
• 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.
• 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
• 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
• an exhaust system comp ⁇ sing an exhaust capture intake with a duct connected to convey the effluent stream captured the capture intake, a catalytic converter connected to the
• 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
• 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 incine
• 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
• 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
• 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
• 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
• 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
• the temperature of a catalyst of catalytic converter 22 must be maintained at at least approximately 450F or more
• 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
• 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.
• Burner pack 28 Another device for helping to maintain the required high temperature of the catalyst is an auxiliary burner pack 28 mounted on either or both s ⁇ de(s) of grill 3. Burner pack 28 generates additional heat which is added to untreated stream 17 to raise its temperature directly
• inlet slot 19 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
• the prior art hood locates inlet vent toward the rear of the exhaust hood.
• 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
• shape of the interior of exhaust hood 11 converges toward inlet slot 19 forming a converging channel.
• the passage of fumes is characterized by a relatively long residence time in the hood.
• 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.
• 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
• 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
• 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
• 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
• 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.
• capture jet 24 is generated by tapping treated stream 27 using a blower 51. In this case, take off 25 is not necessary.
• 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.
• burner pack 28 is arranged to vent hot products of combustion into descending duct 35.
• 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.
• 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
• controller 53 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
• transducer T would include a reference voltage and differential amplifiers to convert the small voltages generated by thermocouples
• controller 53 is actuation of a switch 66 which turns the hood on
• controller 53 controls blower 51 And (2)
• the control system implements a number of operating modes as defined below
• 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
• 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
• a catalytic converter becomes fouled its ability to combust waste in the exhaust stream diminishes
• 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
• a temperature difference that is below normal indicates a badly fouled catalytic converter
• 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.
• the controller goes into the steady-state cooking mode.
• 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
• 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 the conditions of steady-state cooking mode are maintained.
• 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
• alarm 46 is activated to alert the user to the status of controller 53.
• the user should cease cooking and turn off the grill.
• 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
• 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 )
• 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
• 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
• 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
• auxiliary heat is added by injecting hot exhaust from a burner directly into the effluent stream upstream of the catalytic converter.
• auxiliary heat is used to:
• the input stream minimizes the burden on the catalytic converter 5 and helps to prevent fouling and other maintenance problems.
• 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.
• 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).
• 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.
• auxiliary heat can (and should) be used to incinerate (or vaporize) the aerosol
• auxiliary heat can (and should) be used to raise the input temperature of the catalyst to the ignition temperature
• 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.
• the inertia of the aerosol particles 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.
• 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.
• 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-
• 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.
• 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
• 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.
• 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.
• Movement of eddies such as cool eddy 120 or 121 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
• 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
• e is the rate of energy fed into turbulent vorticity
• n is the velocity of the eddy
• / 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
• 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
• 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
• the temperature and length scales are of the order of 100 microns
• 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
• ⁇ 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.
• 2.5 ⁇ 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 ⁇ , the velocity drops off quickly - for a 10 ⁇ particle, the velocity is only 1.5 m/s.
• 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
• 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
• the flow system generated by the embodiment of Figs 6A and 7A also has the following properties
• 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.
• 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 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
• 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
• 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.
• 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.
• 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.
• 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.
• 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
• this filtering step would be performed with a grease separator used in kitchen exhaust equipment
• auxiliary heat source is a fired source such as natural gas, or oil-fired heat source
• the large volume of cool gas mostly air
• the large volume of cool gas may snuff the burning process of at least a portion of the still- burning fuel mix emanating from the burner system into the duct
• sufficient oxidizer is preferably supplied by the burner to allow complete combustion of the auxiliary fuel
• 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.
• 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.
• the total amount of heat supplied by the auxiliary heat source may be increased.
• 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).
• 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.

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• 1997
  • 1997-06-18 EP EP97930096A patent/EP0881935A4/de not_active Withdrawn
  • 1997-06-18 CA CA002229936A patent/CA2229936A1/en not_active Abandoned
  • 1997-06-18 JP JP10503289A patent/JPH11514734A/ja active Pending
  • 1997-06-18 AU AU34006/97A patent/AU3400697A/en not_active Abandoned
  • 1997-06-18 WO PCT/US1997/010550 patent/WO1997048479A1/en not_active Application Discontinuation

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