Abstract:
An air scrubber for removing oils and contaminates from air has an underside inlet for the introduction of contaminated air. The contaminated air passes through a reduced aperture passageway which prevents retrograde flow of contaminates or cleaning fluids contained in the scrubber. The contaminated air is thereafter directed upwards through a scrub reservoir having a plurality of apertures, such that the contaminated air passing through the apertures forms jets which cause interaction between the cleaning fluid and the contaminates, thereby creating droplets containing cleaning fluid with entrapped contaminates, and cleaned air. The droplets and air are directed to a mist eliminator having a plurality of chevron V structures which capture the droplets and return them to the main reservoir. A circulating pump moves cleaning fluid from the main reservoir to an upper reservoir which spills into the cleaning reservoir.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority of U.S. Provisional Pat. No. 61/070,335 filed Mar. 24, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an air cleaner. More particularly, the present invention relates to an air scrubber and degreaser for use above a cooking surface, the air cleaner accepting contaminated air having a mixture of cooking vapors and oils in aerosol form, separating the contaminants by placing them in solution with a cleaning fluid which is returned to a main reservoir, thereby exhausting clean air. 
       BACKGROUND OF THE INVENTION 
       [0003]    Kitchen air cleaners are well known in the field of air handling equipment. A typical kitchen vent hood includes a grease trap such as a removable mesh for capturing airborne oils drawn through by a fan and delivered to an exhaust vent. Cooking vapors are drawn through the mesh which inefficiently captures a fraction of the airborne oils and grease, and the contained grease is extracted from the mesh by disassembling the vent hood and removing the mesh, using a degreaser to release the captured oils, thereafter reassembling the mesh back into the vent hood. Examples of such grease trapping hood systems are shown in U.S. Pat. Nos. 4,7388,244 by Welsh and 5,394,861 by Stegmaier. 
         [0004]    A specific risk for kitchen hoods is a hood fire, whereby combustible fats and oils which collect in the hood over time are ignited by a subsequent kitchen flare-up on the cooktop surface below, igniting the entrapped oils and fats. In this scenario, the vigorous fans of the draft hood provide a ready and continuous source of combustion air, and the fire may extend laterally through the vents of the hood, which usually contain depositions of fats and oils which have accumulated. Because of this risk, prior art kitchen hoods include significant structure related to fire suppression, such as U.S. Pat. Nos. 5,642,784 by Guay et al, 4,784,114 by Muckler et al, and 4,944,782 by Rajadran et al. 
         [0005]    Additionally, some municipalities may require vigorous control of emitted particulates and odors, further increasing the particulate filtering requirement, which may be satisfied using carbon filters, electrostatic filtering, and the like, which require large surface areas to prevent airflow restriction or otherwise reduce airflow for satisfactory operation. 
         [0006]    Industrial air scrubbers are well known in the art of pollution control. U.S. Pat. Nos. 4,388,090 and 5,938,820 describe the mixing of polluted air with a fluid to form a mixture which includes pollutants in solution, which solution is placed into a series of settling tanks for separation and isolation of the pollutants. U.S. Pat. Nos. 4,227,895, 5,085,673, 5,846,303 and 5,292,353 describe an air scrubber which operates by impinging the contaminated air onto a series of baffles which are sprayed with the contaminated solution. U.S. Pat. No. 5,641,338 describes a scrubber which includes a water tray for passing contaminated air through water. 
         [0007]    A kitchen hood/cleaner is desired which receives contaminated air from a cooktop surface, removes the contaminates such as particulates and aerosols, provides a mechanism for periodic self-cleaning of the hood which purges the contaminates that have been trapped, and provides fire suppression and containment for cooktop fires. 
       OBJECTS OF THE INVENTION 
       [0008]    A first object of the invention is a combination kitchen hood and air cleaner/scrubber which directs contaminated intake air containing micro particulates such as smoke, and macro particulates such as aerosol oils, from an underside rectangular air inlet to a reduced width passageway for increasing air velocity, thereby ensuring positive intake flow for micro and macro particles, adding cleaning fluid droplets to the contaminated air stream and directing the contaminated air to the bottom surface of a scrub reservoir which is wetted by the cleaning fluid droplets and subsequently passing the contaminated air through the apertures of the bottom surface of the scrub reservoir for the contaminated air to form jets through the scrub reservoir, thereby placing contaminates directly into solution or emulsion, and forming droplets containing contaminants in solution with cleaning fluid, and clean air, the droplets and air which are together directed to a mist eliminator comprising a plurality of chevron baffles for trapping the droplets and draining them back to the main reservoir, the main reservoir also having a recirculation outlet coupled to a circulating pump which delivers fluid into an upper reservoir which includes a spillway which empties into the scrub reservoir. 
         [0009]    A second object of the invention is an air cleaner/scrubber which collects contaminates in a solution or emulsion form into a main reservoir having a dam leading to an overflow drain, the main reservoir also having a fill mechanism which regulates the main reservoir level below the level of the dam leading to the overflow drain in an operational mode, and during a blowdown cycle, overfilling the main reservoir such that contaminates flow over the dam and to the overflow drain while the cleaner continues to remove contaminates from incoming air. 
         [0010]    A third object of the invention is an air cleaner/scrubber which provides fire suppression for combustible contaminated air by directing the combustible contaminated air through a scrub reservoir containing a non-combustible fluid such as water, the scrub reservoir further having a lower surface which contains a plurality of apertures for the passage of the combustible contaminated air, the apertures and passage through the non-combustible fluid resulting in the suppression of combustion and cooling of combustion products which pass through the scrub reservoir. 
         [0011]    A fourth object of the invention is a process for removing contaminants from air, the process having: 
         [0012]    a first step of directing contaminated air containing contaminates upward to a passageway for directing the contaminated air through a region for providing droplets of cleaning fluid to the contaminated air which is then directed to a scrub reservoir having a plurality of apertures and containing the cleaning fluid, the plurality of apertures for creating jets of contaminated air through the cleaning fluid and enhancing the interaction of the contaminated air and cleaning fluid, the jets resulting in the generation of droplets containing cleaning fluid mixed with contaminates, and clean air; 
         [0013]    a subsequent second step of directing the mixture of contaminate droplets mixed with clean air through a mist eliminator having a plurality of chevron impingement structures maximizing surface contact with the droplets, substantially trapping the droplets and draining them to the main reservoir; 
         [0014]    thereafter directing the cleaned air to an exhaust vent. 
         [0015]    A fifth object of the invention is cleaning contaminated air by passing the contaminated air through a scrub reservoir containing a mixture of water and a surfactant, the scrub reservoir having a plurality of apertures in a lower surface, the contaminated air passing through the scrub reservoir apertures and forming jets for increased interaction of the water and surfactant, thereby resulting in improved efficiency in removal of the contaminates, with the addition of surfactant to the water resulting in a reduced pressure drop across the scrub reservoir. 
       SUMMARY OF THE INVENTION 
       [0016]    A combination kitchen hood air cleaner and scrubber/degreaser accepts contaminated air containing contaminates such as oils and animal fats in vapor or aerosol form, and particulates such as smoke from a lower surface air inlet, thereafter increases the contaminated air velocity using a reduced area aperture to ensure positive intake, and directs the contaminated air through a pre-wet section which introduces droplets of cleaning fluid into the contaminated air stream, which is thereafter directed to the bottom surface of a scrub reservoir containing apertures, and the introduction of cleaning fluid into the contaminated air stream reduces the buildup of contamination in and around the apertures. The cleaning fluid is stored in, and pumped fluid drains back to, a main reservoir which contains the cleaning fluid such as water mixed with a non-foaming surfactant, or alternatively, the cleaning fluid can be any soluble solution for the contaminates to be removed from the contaminated air. The stream of contaminated air with cleaning fluid droplets is directed upward through apertures on the bottom surface of the scrub reservoir containing the cleaning fluid. Contaminated air which passes through the apertures of the scrub reservoir form jets through the cleaning fluid, the jets maximizing the interaction between the contaminated air and the cleaning fluid. Most of the contaminates are captured in solution by the jets, and remaining droplets of contaminates mixed with the cleaning fluid and cleaned air are thereafter directed through a mist eliminator which captures the droplets and returns them to the main reservoir. The cleaned air which exits the mist eliminator is directed over a dam which defines one edge of the main reservoir, and thereafter to an exhaust vent. The scrub reservoir additionally affords fire protection, acting as a water barrier for combustion products drawn into the cleaner. Additionally, a self-cleaning mode is provided by the main reservoir dam which is part of a blow-down cleaning mode whereby overfilling of the main reservoir causes oils which collect in the main reservoir (either at the surface or in emulsion with the cleaning fluid) to spill over the dam and to an overflow drain for removal from the hood simultaneously with the air cleaning functions. In one embodiment of the invention, the basic contaminated air stream entering the scrub reservoir is divided into numerous, very small jet streams as the contaminated air is directed through a perforated plate which forms the bottom of the scrub reservoir. This results in a large increase in the exposed surface contact between the streams of contaminated air and the flowing bed of liquid in the scrub reservoir, with a very high liquid-to-air interaction which is provided by a surfactant which is mixed with the water which forms the cleaning fluid of the scrub reservoir. The pollutants are absorbed on the liquid surfaces at the plate interface. A non foaming surfactant is an integral component of the cleaning fluid. The surfactant eliminates the surface tension between the gas stream and liquid which permits the efficient wetting of all surfaces. This reduction of surface tension permits the high collection efficiency of this collector and additionally greatly reduces the pressure drop through the scrub reservoir. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a side section view of a kitchen air hood. 
           [0018]      FIG. 2  shows a composite top section view of the scrubber of  FIG. 1 . 
           [0019]      FIG. 3  shows details of the level regulation, fill and surfactant system, and programmable logic controller. 
           [0020]      FIG. 4A  shows a flowchart for a background pump protection process. 
           [0021]      FIG. 4B  shows a flowchart for a reservoir fill/blowdown background process. 
           [0022]      FIG. 5  shows a flowchart for a start-up process of the PLC. 
           [0023]      FIG. 6  shows a flowchart for a blow-down sequence of the PLC. 
           [0024]      FIG. 7  shows a flowchart for the surfactant sequence of the PLC. 
           [0025]      FIG. 8  shows a diagram for a self-cleaning scrub reservoir. 
           [0026]      FIG. 9  shows a flowchart for a shutdown sequence of the PLC. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    The present invention is best understood with reference to side view  FIG. 1 , and section A-A of FIG.  1  as shown in composite  FIG. 2 , which also shows other structures (as dashed lines) projected into the section for reference. The kitchen scrubber hood  100  may have the dimension of a standard kitchen exhaust hood, such as 2′ high (z axis) by 4′ wide (x axis) by any length (y axis) required. In a vent-only service mode used when the scrub mode is not available such as during service or shutdown operations or during an emergency condition such as clearing smoke from a fire, a cleaning fluid may be present in a main reservoir  134 , but pump  150  is not turned on, and scrub reservoir  108  has drained back to main reservoir  134 , such that incoming air enters inlet duct  102 , passes through the apertures  106  of the lower surface of empty scrub reservoir  108 , through the structures of the mist eliminator  112 , and through the exhaust outlet. In the normal operational scrubbing mode, fan  120  is operative as well as recirculation pump  150  and flow sensor  154  measuring the output flow of pump  150 , and air inlet duct  102  directs incoming contaminated air containing airborne oil droplets and cooking odors with an perimeter of the edge of the hood inlet velocity of 75 to 100 feet per minute (FPM) towards a reduced aperture passageway  104  which accelerates the air flow velocity to 1100 fpm at aperture  104  to avoid retrograde flow of contaminants and cleaning fluid back to the inlet  102 , such as may otherwise occur from the turbulence from scrubbing interaction between the contaminated air and cleaning fluid. In one embodiment of the invention, the reduced aperture passageway  104  may comprise a snap-on strip for reducing the x-axis extent of the aperture  104 , thereby increasing the incoming air velocity in the reduced aperture region to a desired velocity as high as 1500 FPM, or any air velocity sufficient to prevent incoming contamination particles or water droplets from the scrub reservoir  106  from exiting through aperture  104  towards inlet  102 . The contaminated air draws cleaning fluid from pre-wet apertures  139  in the bottom of an upper reservoir  132  to scrub reservoir apertures  106 , which serves to keep the scrub reservoir  108  lower surface apertures  106  wetted, which minimizes the accumulation and plugging of the apertures  106 . The contaminated air is thereby directed upwards through scrub reservoir  108  having a porous bottom surface  106 , and the resulting efficiency of interaction between contaminates and cleaning fluid may approach or exceed 99% efficiency. The porous bottom surface  106  may be realized using a plurality of apertures which direct the contaminated air through the scrub reservoir  108  and form high velocity jets for interaction with a cleaning fluid such as water mixed with a non-foaming surfactant such as potassium pyrophosphate, and in one embodiment, the cleaning fluid is in the range of 0.1 part to 10 parts of potassium pyrophosphate surfactant in 500 parts water. In one embodiment of the invention, the apertures in the scrub reservoir bottom surface are in the range of 0.125 inch to 0.25 inch and the resulting scrub reservoir lower surface porosity is preferably in the range of 40% to 60%. The scrub reservoir  108  causes oils and particulates in the contaminated air passing through the scrub reservoir  108  to vigorously interact with and mix with the cleaning fluid, such that the majority of contaminates are transferred to the cleaning fluid of the scrub reservoir, and the output of the scrub reservoir  108  contains clean air and droplets  111  containing remaining contaminates are mixed and in solution with the cleaning fluid, or alternatively as an emulsion of contaminates and cleaning fluid. The function of the surfactant in the cleaning fluid of the scrub reservoir provides a reduced surface tension which results in highly efficient transfer of the contaminates to the liquid of the reservoir as the jets of contaminated air pass through the enhanced surface area of the cleaning fluid of the reservoir. In one standalone embodiment of the invention, contaminated air is passed through a scrub reservoir having a plurality of apertures, the contaminated air forming jets through the scrub reservoir which contains water and a surfactant, the contaminates remaining in solution with the cleaning fluid, and the output of the scrub reservoir containing cleaned air and droplets of cleaning fluid and contaminates which may be removed in any manner known in the prior art. Additionally, the use of a cleaning fluid which contains water and a surfactant increases the capture of contaminates of the contaminated air, and also reduces the resistance and associated pressure drop through the scrub reservoir. In the system of  FIG. 1 , the majority of droplets  111  are macro-sized droplets on the order of 0.125 inch to 0.25 inch, and are directed from a +Z axis movement to a −X axis movement through a 90 degree bend in flow to impinge on mist eliminator  112  which is formed from a series of chevrons (such as V shaped surfaces) which form a series of serpentine channels (seen in  FIG. 2 ) such that the macro-sized droplets impinge on the surface of the mist eliminator  112 , and drain back to the main reservoir  134 . The efficiency of droplet collection through the mist eliminator may approach or exceed 99%, exclusive of the vapor phase moisture which may exhaust. The clean air which remains after aggregation and removal of droplets is directed over the top of a dam  142  which forms one end of the main reservoir  134 , thereafter traveling upward to an exhaust vent  116  as directed by sloped surface  143  which leads to an overflow drain  140  used to capture overflow from main reservoir  134  which tops over dam  142  during a cleaning mode described later. For a long kitchen hood, such as a 10 foot (y axis) length, the exhaust vent  116  may be formed from a plurality of linearly arranged rectangular apertures 6″ wide by 36″ or 44″ long, which are aggregated together to form a single duct  118  leading to blower  120  in a remote location such as in an attic or on a facility roof. The overflow drain  140  typically leads to an external drain  144  such as a facility grease trap and drain for facility disposal and treatment of trapped grease and contaminates which drain out of the scrubber  100 . Sump drain  138  of the main reservoir is also coupled to the external drain  144  through a stop valve  152  of  FIG. 2  for manually emptying the main reservoir  134 . The main reservoir  134  also has an intake  136  which is delivered to recirculation pump  150  of  FIG. 2 , the output of which is directed to an upper reservoir supply  130  which fills upper reservoir  132  which overflows over the top of adjustable spillway  135  above the scrub reservoir  108  such that the upper reservoir  132  fills and uniformly tops over the extent of adjustable spillway  135  to the scrub reservoir  108 , which also separate the comparatively low pressure air (outlet) side of septum  141  from the air inlet side. Septum  141  also has a series of passageways  145  below the cleaning fluid surface which connects the main reservoir  134  extent adjacent to fill mechanism  300  with the main reservoir  134  under the scrub reservoir  108 . The adjustable spillway or weir  135  is disposed over a spillway support septum  137  which separates the upper reservoir  137  from the scrub reservoir  108 , and spillway  135  is capable of being adjusted after installation to create a horizontal spillway surface to uniformly drain from the upper reservoir  133  to the scrub reservoir  108 , thereby providing a uniform height spillway for water cascading from upper reservoir  133  to scrub reservoir  108 . 
         [0028]    The mist eliminator  112  may be formed from a plurality of substantially 2″×2″ right angle bent material of height Z of  FIG. 1 , which right angle bends are arranged in a series of offset chevron patterns, such that each successive mist eliminator row is offset by half of the distance from one mist eliminator chevron to the next as seen in  FIG. 2 . 
         [0029]    Evaporative and overflow losses are compensated using a fill housing  300  which senses an optimum cleaning fluid level in the main reservoir  134 , and allows the introduction of new water to replenish the main reservoir  134  cleaning fluid to a desired level. Typically, surfactant is introduced after the periodic blowdown cycle, where water is introduced which overfills the cleaning fluid from the main reservoir  134  over dam  142  and to the overflow drain  140 . During normal scrubbing operation, the main reservoir  134  level is typically below the dam  142  and above the level of recirculation pump inlet pipe  136 . The fill mechanism may use any fluid level sensing system of the prior art, including a float valve or conductivity probe. One example embodiment fill mechanism and programmable logic controller (PLC)  324  is shown in  FIG. 3 , which provides fill housing  300  with a liquid aperture  316  coupled below the surface of the main reservoir  134  and a vent aperture  314  above the main reservoir. The housing  300  may be located anywhere which isolates the level sensors  302  and  304  from water turbulence and short term variations in the main reservoir level, such that the sensors are exposed to average reservoir level  322 . A short sensor  302  has a conductive rod  306  which is a first distance L 1   318  below the dam  142  level, and a comparatively long sensor  304  has a conductive rod  308  a second distance L 2   320  below dam  142  level, with L 2 &gt;L 1 . Short sensor  302  is used to control the fill solenoid  326 , which when open, allows pressurized water source  328  to add to the cleaning fluid in reservoir  134 . Long sensor  308  is used to disable the circulation pump  150  of  FIG. 2  to protect against pump damage if the reservoir level falls below a minimum level established by long sensor  304 . The operation of the short and long sensors is described in flowchart  FIGS. 4A and 4B , which functionality is programmed into the programmable logic controller (PLC)  324  of  FIG. 3 . 
         [0030]    The controller PLC  324  thereby provides control of the main blower  120  of  FIG. 1  using control lines  330 , the circulation pump  150  of  FIG. 2  using control lines  332 , the fill solenoid  326 , control line  313  directed to the surfactant fluid pump  315  which is capable of metering known amounts of surfactant at controlled flow rates, and flow sensor  334  which provides input  311  which can be used to indicate the absence of surfactant flow during a surfactant demand request using pump  315 . PLC inputs  310  and  312  provide reservoir level measurement using short and long level sensors  302  and  304 . The PLC additionally accepts recirculating pump flow sensor input  317  from flow sensor  154  of  FIG. 2  which indicates flow through the recirculation pump  150  of  FIGS. 1 and 2 . The functions performed by the PLC upon external request such as from a control panel (not shown) may include the start up sequence shown in  FIG. 5 , the blow down cycle which performs periodic cleaning without draining the reservoir shown in  FIG. 6 , introduction of surfactant as shown in  FIG. 7 , and shutdown sequence shown in  FIG. 9 . 
         [0031]    The pump protection process of  FIG. 4A  provides continuous protection for the circulation pump against loss of fluid or pump circulation blockage. The process may operate as a background process in the PLC of  FIG. 3  and separate from any other process such as those described in  FIG. 4B ,  5 ,  6 ,  7 , or  9 . The pump protection process starts at step  401 , and step  402  detects the long sensor contact with the cleaning fluid of the main reservoir, such as by averaging, sensing and waiting, or any method which provides a reliable indication that the long sensor is in contact with the main reservoir cleaning fluid and accurately detecting a level. If the sensor is not in fluid contact, the circulation pump is disabled  404  until step  402  detects contact and proceeds to step  406  which enables the pump. Fluid flow through the pump is detected in step  407 , and if present, the process continues at step  402 , but if flow is not detected, the pump is disabled  409  and an error is reported  411  such as by indicator lamp or alarm bell at the front panel (not shown). Similarly for the reservoir fill operation, the process shown in  FIG. 4B  represents one possible embodiment of a program operative in the PLC  324  of  FIG. 3 , and once the process is started, steps  416 ,  418 ,  420 , and  422  operate as a continuous process for either maintaining fluid level, or overfilling the main reservoir for a blowdown mode which passes the excess cleaning fluid to the overflow drain. The reservoir fill process starts  414  and blowdown mode is tested in step  416  by checking to see if blowdown mode is enabled with a valid T 3  timer which indicates a blowdown cycle duration timer, as will be described in  FIG. 6 . If these two conditions are met, the fill solenoid is opened  420 . If the unit is not in blowdown mode, the short sensor is examined for fluid contact  418 , resulting in either the addition of water to the main reservoir by opening the fill solenoid  420 , or the short sensor makes contact indicating a full reservoir, and closing the fill solenoid in step  422 . 
         [0032]    The start-up sequence of  FIG. 5  starts  502  with the fill process of  FIG. 4B  which occurs in step  504 . Upon completion of the reservoir fill  506  as determined by the short sensor detecting a full reservoir  422  of  FIG. 4B , The circulation pump  150  starts  508  and causes the upper reservoir  132  to fill and spill over the adjustable weir into the scrubbing reservoir  108 , and the system fluid levels achieve equilibrium during delay T 1   510  and T 1  timer expiry  512 . As the main reservoir drops during the pumping of fluid to the upper reservoirs of the T 1  interval, the background process of  FIG. 4B  replaces the displaced cleaning fluid to maintain the main reservoir level. The T 1  pump start equilibrium interval represents the duration of time for the upper reservoir to overflow into and fill the scrub reservoir, and after the T 1  pump start equilibrium interval, the blower is started  514 , after which air scrubbing operation is in full effect. For purging of collected oils and contaminates while the system continues to operate, the PLC generates the optional blow-down cycle shown in  FIG. 6 , entering at step  601  which follows step  514  of  FIG. 5 , whereby the PLC  324  initializes a blowtime cycle interval timer T 2  in step  605  to the default value not_valid, indicating that no blowdown cycle is requested by asserting a not_valid value for the blowdown cycle interval timer T 2 . The operator settings for the blowdown cycle in step  602  include the programmable blowdown cycle interval time T 2  with a typical value of 30 to 60 minutes, and a blowdown cycle duration time T 3  with a typical value of 2-3 minutes. Upon application of the blowdown parameters in step  602 , the T 2  timer is started in step  604 , or if not initialized at all, is tested in step  606  which returns to test  602 . If the T 2  timer is valid but not expired, the process similarly returns to step  602 . When the T 2  timer is valid and has expired in step  607 , the T 3  blowdown cycle duration timer is started, during which time the fill sensor ignores level sensor input  310  and opens fill solenoid  326  in step  610  for the blow down cycle interval time T 3  which is tested in step  612 , during which T 3  interval the main reservoir  134  overflows over dam  142 , carrying oils and greases to overflow drain  140  for the blowdown cycle duration T 3 . During this time T 3  of step  610 , the fill solenoid  326  is maintained open, and after duration T 3 , the fill valve is closed in step  614  and normal reservoir fill operation of  FIG. 4B  resumes. 
         [0033]    Following the blowdown cycle at the end of step  614 ,  FIG. 7  shows a surfactant control process, which is advantageously performed when the surfactant concentration is known. After a settling time T 4   702  during which the excess cleaning fluid displaced by the fill water drains to the overflow drain, the surfactant pump is enabled in step  704 , surfactant flow is detected  706 , and the surfactant pump continues to operate for a surfactant injection duration time T 5   710 , after which the surfactant pump is turned off in step  712 . The failure to detect surfactant flow in step  706  causes an error condition  708 , such as the sounding of an alarm or the disabling of the circulation pump  150 . The objective of the surfactant introduction sequence is to maintain the concentration of surfactant in the main reservoir to an optimum range such as was described previously, and the introduction of surfactant after the blowdown cycle of  FIG. 6  is one way of accomplishing this objective, and is presented not to limit the invention to this method, but to aid in the understanding of the operation of the invention. 
         [0034]      FIG. 9  shows a shutdown sequence which may be entered upon user request from step  603  of  FIG. 6 . The shutdown sequence entry point of step  902  is followed by step  904  which shuts off the circulation pump, stops the reservoir fill process of  FIG. 4B , and turns off the blower in step  904 , finally waiting for a startup request in step  906 , upon which the process transfers to the startup cycle of  FIG. 5 . 
         [0035]      FIG. 8  shows another embodiment of the invention, where hot water spray nozzle  802  is added to the structures described in  FIG. 1 . Scrub reservoir apertures  106  may become plugged with congealed grease over time, for which a thorough cleaning mode may be provided by spraying lower surface  106  with hot water delivered to spray nozzle  802 , which operation may be done under manual control, or using a hot water solenoid under the control of PLC  324  described earlier. In another embodiment of the invention, multiple spray nozzles  802  are positioned over various inner surfaces of the air cleaner  100 , including the main reservoir  134 , the upper reservoir  132 , scrub reservoir  108 , and any other surfaces which may accumulate grease and oils, and after draining the main sump, the nozzles are charged with pressurized hot water and detergent with a temperature in excess of the 105° F. melting point of grease, and the sump drain  138  opened during the cleaning cycle. The PLC allows default values as well as independent programming of each of the time parameters T 1  (blower startup delay following circulation pump start, which fills the upper reservoir, spilling water into the scrub reservoir, thereby delaying operation of the blower until the scrub reservoir is filled), T 2  (blowdown cycle interval—the interval between blowdown cycles), T 3  (blowdown cycle duration timer—the duration of a blowdown cycle), T 4  (overflow settling time after shutoff of the circulation pump causes cleaning fluid from upper reservoir and scrub reservoir drain back into the main reservoir, over the spillway, and into the overflow drain), T 5  (surfactant injection time). Additionally, it is possible to change the order or manner of operation from the examples shown in  FIGS. 4A ,  4 B,  5 ,  6 ,  7 , and  9 . 
         [0036]    The hood scrubber thereby provides several advantages over the prior art. The scrub reservoir  108  contains cleaning fluid such as water mixed with surfactant, which is non-flammable, and acts as a flame barrier, extinguishing flames which enter the scrubber. The scrubber intrinsically satisfies the flame controls tests required under Underwriter Laboratories standard 710 (UL-710), which prior art devices satisfy using a separate flame control system apart from the vent mechanism. Another advantage is the ability of the kitchen hood to operate in a “normal” hood mode, whereby the blower  120  can be turned on without circulation pump  150  for ventilation without cleaning. The overflow drain  140  in conjunction with fill solenoid  326  of  FIG. 3  provides “blow-down” cleaning mode, whereby excess oils which collect at the surface and in emulsion with the fluid of the main reservoir are spilled over dam  142  and removed from the system. 
         [0037]    The present description of the vent hood and air scrubber is provided for understanding of the invention, and is not intended to limit the scope of the invention. Structures such as the scrub reservoir  108  and apertures  106  of the lower surface may be practiced any number of ways, including regular arrays of apertures, round, oval, or square apertures, or other structures such as meshes which provide interaction between the cleaning fluid and contaminated air. Certain other porous structures may be substituted in the scrub reservoir to improve cleaning fluid interaction and trapping, or to improve flame control. The drain valve is shown as a manual valve, but could be an automatic valve with cleaning functionality incorporated into the PLC. Any cleaning fluid which provides emulsification of oils, solution of fats, or solubility with particulates such as smoke may be used. The level sensors of  FIG. 3  may be practiced any number of ways, including float sensors, ultrasonic level sensors, or any prior art method for sensing the level of a fluid. The mist eliminators are shown as chevron structures which are suitable for capture of large droplets of contaminates, but may alternatively be any structure which captures droplets and returns them to the main reservoir for re-use.