Abstract:
There is provided a self cleaning filter assembly for use in an exhaust system, including: a casing for placement in the path of an air flow, the casing containing: a first filter; a second filter; a barrier for placement adjacent to the casing to prevent a cleaning fluid from dripping from the first filter and out from the casing; and a plurality of spray outlets for dispersion of fluid within the casing and onto the filters. The plurality of spray outlets create droplets of the cleaning fluid of a size able to combine with droplets of a contaminant to form combined droplets in the air flow and wherein the first filter and the second filter captures droplets of the contaminant and droplets from the spray outlets. A circulation system for use with the filter assembly is also disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a self-cleaning filter assembly and refers particularly, though not exclusively, to a self-cleaning filter assembly for use in domestic and industrial exhaust systems for exhausting vapours such as, for example, vapours produced by cooking in kitchens and chemical processes.  
       BACKGROUND TO THE INVENTION  
       [0002]     It is mandatory in many industries to employ exhaust systems for removing contaminated air from affected work areas. Examples of such industries are the manufacturing and chemical industries.  
         [0003]     Such exhaust systems are commonly employed in kitchens such as industrial and commercial kitchens in the food processing industry, and restaurants. Whenever food is cooked, it is common for the food to be cooked using oil. As such, cooking creates fumes which contain oil droplets.  
         [0004]     Filters that are installed within such exhaust systems usually comprise a steel mesh to capture the majority of oil droplets. Such filters may be located in the exhaust hood, the flue of the exhaust hood, or in the ducting. Droplets which are not captured by the filter may end up coating the internal surfaces of the exhaust hood, the flue, and the ducting. When these oil droplets cool and start to dry they become very sticky, and are hard to remove. Some of these droplets that do not coat the internal walls of the exhaust system may be expelled into the atmosphere, contributing to air pollution. Other problems that may arise are: shortened exhaust fan life span due to clogging by contaminants, a rapid build-up of bacteria in the internal walls of the exhaust system leading to a compromise of hygiene; increased fire risk from a dried grease layer along the internal walls; and workplace ventilation problems resulting from ineffective exhaust operation.  
         [0005]     The filters employed in such exhaust systems have fairly open mesh patterns as having too fine a mesh pattern would cause each filter to be easily clogged, leading to minimised system effectiveness, and increase in system downtime due to frequent filter maintenance and cleaning. Such maintenance sessions are time-consuming, tedious, and labour intensive task. As such, it is usually not a cheap process.  
         [0006]     There have been many proposals for self cleaning exhaust systems using water curtains, water baths, or sprays. However, most are one or more of: complex, large, expensive, ineffective, dangerous to use.  
       SUMMARY OF THE INVENTION  
       [0007]     There is provided a self cleaning filter assembly for use in an exhaust system, including: a casing for placement in the path of an air flow, the casing containing: a first filter; a second filter; a barrier for placement adjacent to the casing to prevent a cleaning fluid from dripping from the first filter and out from the casing; and a plurality of spray outlets for dispersion of fluid within the casing and onto the filters. The plurality of spray outlets create droplets of the cleaning fluid of a size able to combine with droplets of a contaminant to form combined droplets in the air flow and wherein the first filter and the second filter captures droplets of the contaminant and droplets from the spray outlets.  
         [0008]     The barrier may be a plurality of louvres that creates openings when the exhaust system is operational. The plurality of louvres may be fixed in a particular orientation. Alternatively, the barrier is slidable into the casing to create an opening when the exhaust system is operational.  
         [0009]     Advantageously, the barrier includes baffles to retard the air flow. Preferably, the barrier includes at least one chamber for the containment of fluid. It is preferable that there is a conduit for fluid drainage in the at least one chamber.  
         [0010]     It is preferable that each spray outlet disperses fluid with an arc of between sixty degrees and one hundred eighty degrees. It is more preferable that each spray outlet disperses fluid with an arc of ninety degrees. It is desirable for each spray outlet to be a nozzle.  
         [0011]     Preferably, the cleaning fluid includes water and a degreaser in a required ratio in the range 1:10 to 1:50. The plurality of spray outlets may be on an inlet side of a filter selected from the group consisting of the first filter and the second filter. The plurality of spray outlets may be located at each edge of a filter selected from the group consisting of the first filter and the second filter. The plurality of spray outlets may be located at the middle portion of each side of a filter selected from the group consisting of the first filter and the second filter. The plurality of spray outlets may also be located on opposed comers of a filter selected from the group consisting of the first filter and the second filter.  
         [0012]     It is preferable that the plurality of spray outlets are located within the casing. It is advantageous that the barrier comprises at least one chamber with at least one baffle therein.  
         [0013]     There is also provided a circulation system for fluids into at least one self cleaning filter assembly including: a circulation tank; and a circulation pump. Preferably, the circulation pump operates as a venturi pump to circulate fluids in the system. It is advantageous that there is at least one valve to control the flow of water into the circulation tank. There may also be a supplementary tank for the containment of a fluid constituant. A dispenser for the fluid constituant may also be included. The fluid constituant is preferably degreaser. It is preferable that the fluid is cleaning solution and the ratio of degreaser to water may be in the range of 1:10 to 1:50. It is most preferable that the ratio of degreaser to water is 1:20. A stream of air may preferably be introduced into the fluids in the system either before or after the circulation pump.  
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0014]     In order that the invention can be readily understood and put into practical effect, there shall now be described by way of non-limitative example only a preferred embodiment of the present invention, the description being with reference to the accompanying illustrative drawings in which:  
         [0015]      FIG. 1  shows a side view of the self deaning filter assembly of a preferred embodiment of the present invention with louvres at an “open” position;  
         [0016]      FIG. 2  shows a side view of an alternative embodiment of the self cleaning filter assembly of the present invention with louvres at an “open” position.  
         [0017]      FIG. 3  shows the front view of the nozzle arrangement in  FIG. 1 ;  
         [0018]      FIG. 4  shows a front view of a second filter of the self deaning filter assembly of the present invention with a second arrangement of spray outlets;  
         [0019]      FIG. 5  shows a front view of a second filter of the self cleaning filter assembly of the present invention with a third arrangement of spray outlets;  
         [0020]      FIG. 6  shows a front view of a second filter of the self cleaning filter assembly of the present invention with a fourth arrangement of spray outlets;  
         [0021]      FIG. 7  shows a side view of the self cleaning filter assembly of the present invention employed in an exhaust system;  
         [0022]      FIG. 8  shows the front view of the nozzle arrangement in  FIG. 2 ;  
         [0023]      FIG. 9  shows a front view of a first filter of the self cleaning filter assembly of the alternative embodiment of the present invention with a second arrangement of spray outlets;  
         [0024]      FIG. 10  shows a front view of a first filter of the self cleaning filter assembly of the alternative embodiment of the present invention with a third arrangement of spray outlets;  
         [0025]      FIG. 11  shows a front view of a first filter of the self cleaning filter assembly of the alternative embodiment of the present invention with a fourth arrangement of spray outlets;  
         [0026]      FIG. 12  shows a side view of the self cleaning filter assembly of the alternative embodiment of the present invention employed in an exhaust system;  
         [0027]      FIG. 13  shows a side view of the self cleaning filter assembly of the present invention with louvres at an “close” position;  
         [0028]      FIG. 14  shows a preferred embodiment of the present invention employed into an existing exhaust hood system;  
         [0029]      FIG. 15  shows a close up view of the edges of louvres in a fixed orientation in an alternative embodiment of the self cleaning filter assembly;  
         [0030]      FIG. 16  shows a system to circulate cleaning solution in an exhaust hood system; and  
         [0031]      FIG. 17  shows a flow chart for the operation of a system to circulate cleaning solution in an exhaust hood system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     To first refer to  FIG. 1 , there is shown an airflow path through an exhaust system in which air flows in the direction of arrow  50 . The air flowing in the direction of arrow  50  may contain contaminants such as, for example, oil droplets, dust, particulate matter and the like. Air is drawn in the direction of arrow  50  when an exhaust fan (not shown) of the exhaust system is in operation.  
         [0033]     A self cleaning filter assembly  52  is mounted in an airflow path in the exhaust system along the direction  50  of airflow. The self cleaning filter assembly may be adaptable to fit braces or slots of existing filter assemblies in existing exhaust systems. Existing baffle filters may be replaced by the self cleaning filter assembly  52 . This may enable a conventional kitchen exhaust system to be converted into an advanced grease removal kitchen exhaust system with minimal capital outlay. This is shown in  FIG. 14 . The self cleaning filter assembly  52  may be fitted in existing braces or slots in an existing exhaust system  120 .  
         [0034]     The self cleaning filter assembly  52  includes a casing  54 . The casing  54  may be a peripheral case bordering close to the internal walls of the pathways of the exhaust system. The filter assembly  52  should substantially block the pathway so that only a minimal volume of air does not pass through the filter assembly  52 . A first filter  56  and a second filter  62  are located within the casing  54  in the path of airflow for filtering contaminants such as, for example, oil droplets, dust, particulate matter and the like, from the air flowing in direction  50 .  
         [0035]     Self-cleaning may mean that cleaning is not complete. It may denote partial cleaning as well. However, there shall be some degree of cleansing involved.  
         [0036]     Located before a first end  53  of the casing  54  is an additional fixture  55  comprising a first chamber  57 , a second chamber  58 , baffles  59 , and an adjustable set of louvres  60 . The louvres  60  may be activated when the exhaust fan is in operation. The louvres  60  may also be activated when there is sufficiently fast air flow. The fixture  55  may be attached to the casing  54  or may be attached to the internal walls of the pathways of the exhaust system.  FIG. 1  shows the instance when the exhaust fan may be operational and the louvres  60  are in an “open” position. Air is then able to flow into openings  63  created by the open louvres  60 . When louvres  60  are in the closed position, openings  63  are also closed. However, the louvres  60  may be non-adjustable and locked in a fixed orientation. As such, openings  63  may always be present.  FIG. 13  shows the louvres  60  in a “closed” position.  
         [0037]     When the exhaust fan is not operational, cleaning of filters in the filter assembly  52  may still be carried out with the louvres  60  in a “closed” position. The edges of each louvre  60  may be lined with water-tight sealing material such as, for example, silicone, rubber and the like. As such, cleaning solution does not leak from the filter assembly  52  into other portions of the exhaust system and areas to be ventilated. This prevents leakage into areas to be ventilated, such as, for example, kitchens, clean rooms and the like. Besides employing louvres  60 , a shutter-door like assembly may be used to allow airflow into the filter assembly  52  and to block cleaning solution from exiting from the filter assembly  52 . The first chamber  57  and the second chamber  58  of the fixture  55  may also be employed to contain the cleaning solution with drainage in each chamber being drained by a conduit for fluid  64 , such as, for example, a drainage pipe.  
         [0038]     Located behind the first filter  56  is a second filter  62  preferably substantially the same as first filter  56 , so as to allow for inter-changeability. The mesh size of the second filter  62  may be the same as first filter  56 , or may be smaller. The first filter  56  may be a relatively coarse filter while the second filter  62  may be a relatively fine filter. Altematively, the first filter  56  may be relatively fine, and the second filter  62  may be relatively coarse. The baffles  59  in the fixture  55  may aid in retarding the speed of air flow through the filter assembly  52  and to prevent spray  68  from exiting through opening  63 , particularly when the exhaust fan is not in operation. Such an arrangement may allow for some contaminants to be removed by the first filter  56 , with the second filter  62  used for capturing even smaller contaminants. The first filter  56  may also be used to prevent spray  68  from splashing on baffles  59 . In such an arrangement, the risk of clogging both the first filter  56  and the second filter  62  is significantly reduced. The first filter  56  may prevent fluid sprayed from nozzles  66  and  70  from exiting the opening  63  of the filter assembly  52 . The second filter  62  may also prevent fluid sprayed from nozzles  66  and  70  from exiting the rear end  100  of the filter assembly  52 . This minimises the down-time of the exhaust system and may also lower maintenance costs. The filtration rating and type of each filter may be determined by the type of contaminant to be removed from the air flow.  
         [0039]     A cleaning solution basin or tank (not shown) may be operatively connected to the filter assembly  52 . The tank may be attachable to the exhaust system or may be incorporated in the exhaust system. If separate, appropriate connections such as by hoses, tubes, pipes, manifolds, and so forth will need to be provided. Located in or adjacent the tank may be a pump for optionally supplying a cleaning solution through pipes, tubes or hoses to a first nozzle  66  located in between the first filter  56  and the second filter  62 . Nozzle  66  may be a single nozzle, a plurality of nozzles in an array, an outlet manifold with a plurality of holes (as in a shower rose), a fan jet spray with spray concentrated over a small area, or the like.  
         [0040]     The purpose of nozzle  66  is to provide a fine spray  68  of water that may contain cleaning solution into the airflow onto the first filter  56  and onto a front surface  69  of the second filter  62  so that the air flow drawn by the exhaust fan draws the fine spray  68  onto substantially the front surface  69  of the second filter  62 . The nozzle  66  “atomizes” the cleaning solution to form a fine spray  68 . The nozzle  66  may disperse the cleaning solution over an arc of between sixty and one hundred eighty degrees. Preferably, the nozzle  66  disperses the cleaning solution with an arc of one hundred and ten degrees. A second nozzle  70  may also be located in the filter assembly  52  between the first filter  56  and the second filter  62  for the same purpose as the first nozzle  66 .  
         [0041]      FIG. 3  shows the front view of the nozzle arrangement as shown in  FIGS. 1 . The first nozzle  66  and a second nozzle  70  are located in front of front surface  69  of the second filter  62 . The second nozzle  70  may be the same as first nozzle  66 , or maybe different. The first nozzle  66  and second nozzle  70  each disperse cleaning solution in a fine spray  68  with an arc of ninety degrees, allowing coating of a substantial portion of the front surface  69  of second filter  62  with cleaning solution. Coating the front surface  69  of second filter  62  may improve the adhesive properties of the second filter  62  for capturing contaminants from the airflow  50 .  FIG. 4  shows an altemative arrangement of two nozzles that each disperse cleaning solution in a fine spray  68  with an arc of one hundred eighty degrees such that a substantial portion of the front surface  69  of second filter  62  is coated with cleaning solution.  FIGS. 5 and 6  show alternative arrangements of four nozzles.  FIG. 5  shows nozzles that each disperse cleaning solution in a fine spray  68  with an arc of ninety degrees while  FIG. 6  shows nozzles that each disperse cleaning solution in a fine spray  68  with an arc of one hundred eighty degrees. A single nozzle may also be sufficient for the task of coating a substantial portion of the front surface  69  of second filter  62  with cleaning solution; as is the deployment of a number of nozzles that are not specifically mentioned in the description. A separate pump may be used for each nozzle or, they may all be connected to a single pump.  
         [0042]     Referring to  FIG. 7 , the spray  68  should have a droplet size such that it will be carried by the airflow onto surface  69  of second filter  69 . A pressure of between two to three bars applied to each nozzle may affect the size of each droplet emerging from each nozzle. The pressure may also affect the speed at which each droplet emerges from each nozzle. By having filter assembly  52  in an angled position, any excess spray will tend to be captured in the chambers  57 ,  58  and drained by the conduit  64  for recycling. The conduit  64  may be a gutter that minimises the pipings required in and around the filter assembly. Consequently, the amount of cleaning solution required is also reduced. However, when louvres  60  are in “closed” position, the pressure of the spray  68  may be greater to force contaminants off the filters  56 ,  62 .  
         [0043]     In this way the second filter  62  is cleaned by spray  68 . The spray  68  will also combine with the contaminants in the airflow to form larger droplets. Louvres  60  may be coated with polytetrafluoroethylene (PTFE or commercially known as Teflon) for ease of removal of such hardened droplets during maintenance. The louvres  60  may also be devoid of any layer of coating. Hence, the louvres  60  may also aid in the filtration process.  
         [0044]     Also, the larger droplets will more likely be captured by second filter  62 . When captured by second filter  62 , as the oil droplets are still fluid as they have not yet cooled, they will be acted on by the degreaser in the cleaning solution of spray  68 , and will thus flow down second filter  62  under the influence of gravity to the bottom of casing  54 , and flow through an opening into chambers  57 ,  58 . Oil droplets acted on by the degreaser may form a precipitate and settle in the bottom of a waste tank.  
         [0045]     The spray  68  coats a substantial portion of the front surface  69  of the second filter  62 , thus enhancing the capturing of contaminants by the second filter  62 . Under the influence of the airflow, and the spray  68  simultaneously coats and flushes all surfaces of the second filter  68 .  
         [0046]     In this way the majority of contaminants and cleaning spray  68  is captured by filters  62 ,  56 . This prevents the majority of contaminants and the cleaning spray passing along subsequent portions of the exhaust system. This prevents damage to duct linings, exhaust fans and other components of the exhaust system.  
         [0047]      FIG. 2  shows an alternative embodiment of the present invention. There is shown an airflow path through an exhaust system in which air flows in the direction of arrow  20 . The air flowing in the direction of arrow  20  may contain contaminants such as for example, oil droplets, dust, particulate matter and the like. Air is drawn in the direction of arrow  20  when an exhaust fan (not shown) of the exhaust system is in operation.  
         [0048]     A self cleaning filter assembly  22  is mounted in an airflow path in the exhaust system along the direction  20  of airflow. The self cleaning filter assembly  22  may be adaptable to fit braces or slots of existing filter assemblies in existing exhaust systems. The self cleaning filter assembly includes a casing  24 . The casing  24  may be a peripheral case bordering dose to the internal walls of the pathways of the exhaust system. The filter assembly  22  should substantially block the pathway so that only a minimal volume of air does not pass through the filter assembly  22 . A first filter  26  is located within the casing  24  in the path of airflow for filtering contaminants such as, for example, oil droplets, dust, particulate matter and the like, from the air flowing in direction  20 .  
         [0049]     Located before a first end  23  of the casing  24  is an adjustable set of louvres  30  that are activated when the exhaust fan is in operation. The set of louvres  30  may be attached to the casing  24  or may be attached to the internal walls of the pathways of the exhaust system.  FIG. 2  shows the instance when the exhaust fan is operational and the louvres  30  are in an “open” position. The louvres  30  may also be activated when there is sufficiently fast air flow. Air is then able to flow into openings  21  created by the open louvres  30 . When the exhaust fan is not operational, cleaning of filters in the filter assembly  22  may still be carried out with the louvres  30  in a “closed” position. The edges of each louvre  30  are also lined with water-tight sealing material such as, for example, silicone, rubber and the like. As such, the cleaning solution does not leak from the filter assembly  22  into other portions of the exhaust system and areas to be ventilated. This prevents leakage into areas to be ventilated, such as, for example, kitchens, clean rooms and the like. Besides employing louvres  30 , a shutter-door like assembly may be used to allow airflow into the filter assembly  22  and to block cleaning solution from exiting from the filter assembly  22 . However, the louvres  30  may be non-adjustable and locked in a fixed orientation. As such, openings  21  may always be present.  FIG. 15  shows a close up of the louvre edges locked in a fixed orientation in fixed receptors  31 . Each louvre  30  may be easily removable for replacement, cleaning, or maintenance.  
         [0050]     Located behind the first filter  26  is a second filter  28  preferably substantially the same as first filter  26 , so as to allow for inter-changeability. Its mesh size may be the same as first filter  26 , or may preferably be smaller. The first filter  26  may be a relatively coarse filter while the second filter  28  may be a relatively fine filter. Such an arrangement may allow for the majority of contaminants to be removed by the first filter  26 , with the second filter  28  used for capturing even smaller contaminants. In such an arrangement, the risk of clogging both the first filter  26  and the second filter  28  is significantly reduced. This minimises the down-time of the exhaust system and may also lower maintenance costs. The filtration rating and type of each filter may be determined by the type of contaminant to be removed from the air flow.  
         [0051]     A cleaning solution basin or tank (not shown) may be operatively connected to the filter assembly  22 . The tank may be attachable to the exhaust system or may be incorporated in the exhaust system. If separate, appropriate connections such as by hoses, tubes, pipes, manifolds, and so forth will need to be provided. Located in or adjacent the tank may be a pump for optionally supplying a cleaning solution through pipes, tubes or hoses to a first nozzle  32  located in front of first filter  26 . Nozzle  32  may be a single nozzle, a plurality of nozzles in an array, an outlet manifold with a plurality of holes (as in a shower rose), or the like.  
         [0052]     The purpose of nozzle  32  is to provide a fine spray  34  of water that may contain cleaning solution into the airflow in front of front surface  36  of first filter  26  so that the air flow drawn by the exhaust fan draws the fine spray  34  onto substantially the complete front surface  36  of the first filter  26 . The nozzle  32  “atomizes” the cleaning solution to form a fine spray  34 . The nozzle  32  disperses the deaning solution over an arc of between sixty and one hundred eighty degrees. Preferably, the nozzle  32  disperses the cleaning solution with an arc of ninety degrees.  
         [0053]      FIG. 8  shows the front view of the nozzle arrangement as shown in  FIG. 2 . The first nozzle  32  and a second nozzle  38  are located in front of front surface  36  of the first filter  26 . The second nozzle  38  may be the same as first nozzle  32 , or maybe different. The first nozzle  32  and second nozzle  38  each disperse cleaning solution in a fine spray  34  with an arc of ninety degrees, allowing coating of a substantial portion of the front surface  36  of first filter  26  with cleaning solution. Coating the front surface  36  of first filter  26  may improve the adhesive properties of the first filter  26  for capturing contaminants from the airflow.  FIG. 9  shows an alternative arrangement of two nozzles that each disperse cleaning solution in a fine spray  34  with an arc of one hundred eighty degrees such that a substantial portion of the front surface  36  of first filter  26  is coated with cleaning solution.  FIGS. 10 and 11  show alternative arrangements of four nozzles.  FIG. 10  shows nozzles that each disperse cleaning solution in a fine spray  34  with an arc of ninety degrees while  FIG. 11  shows nozzles that each disperse cleaning solution in a fine spray  34  with an arc of one hundred eighty degrees. A single nozzle may also be sufficient for the task of coating a substantial portion of the front surface  36  of first filter  26  with cleaning solution; as is the deployment of a number of nozzles that are not specifically mentioned in the description. A separate pump may be used for each nozzle or, they may all be connected to a single pump.  
         [0054]     Referring to  FIG. 12 , the spray  34  should have a droplet size such that it will be carried by the airflow onto surface  36  of first filter  26 . However, the pressure applied to each nozzle should preferably not so great that spray  34  will be reflected off surface  36  with such force that it will be flow against the airflow and thus risk passing along the airpath into other portions of the exhaust system. By having filter assembly  22  in an angled position, any reflected spray will tend to be captured by the airflow and directed onto surface  36  of first filter  26 . The airflow is being used to assist the fine spray  34  onto surface  36  of first filter  26 , However, when louvres  30  are in “closed” position, the pressure of the spray  34  may be greater to force contaminants off the filter  26 .  
         [0055]     The cleaning solution may be water or, preferably, the cleaning solution is a mixture of water and a degreaser in a required ratio. The ratio may be any suitable ratio depending on the degreaser used, and the type of contaminant being filtered. For example, filtration of cooking fumes and chemical fumes would require different constituent percentage parts of the cleaning solution. Preferably the ratio is in the range of 1:10 to 1:50; more preferably 1:20. For example, if the contaminants are acidic the cleaning solution may be alkaline to not only capture and clean, but also to neutralize the contaminants. Similarly, for alkaline contaminants, the cleaning solution may be acidic. For gaseous contaminants, the cleaning solution may contain neutralizng solutions and/or gases.  
         [0056]     In this way the first filter  26  is cleaned by spray  34 . The spray  34  will also combine with the contaminants in the airflow to form larger droplets. The larger droplets will tend to fall from the airflow before contacting first filter  26  and will drop under the influence of gravity to the bottom of a pathway  110 . The larger droplets may also adhere to the surfaces of louvres  30 . Louvres  30  may be coated with polytetrafluoroethylene (PTFE or commercially known as Teflon) for ease of removal of such hardened droplets during maintenance. Hence, the louvres  30  may also aid in the filtration process.  
         [0057]     Also, the larger droplets will more likely be captured by first filter  26 . When captured by first filter  26 , as the oil droplets are still fluid as they have not yet cooled, they will be acted on by the degreaser in the cleaning solution of spray  34 , and will thus flow down first filter  26  under the influence of gravity to the bottom of casing  24 , and flow through an opening into a waste tank in the exhaust system. In this way the cleaning solution may be recycled as oil droplets in the waste tank will rise to the top facilitating separation of the cleaning solution while the oil may be collected for disposal or recycling.  
         [0058]     The spray  34  coats a substantial portion of the front surface  36  of the first filter  26 , thus enhancing the capturing of contaminants by the first filter  26 . By having the spray  34  in front of the first filter  26 , the spray  34  is continuously drawn to, into, and through the first filter  26  under the influence of the airflow, and thus simultaneously coats and flushes all surfaces of the first filter  26 .  
         [0059]     Droplets from spray  34 , particularly relatively fine droplets, and smaller than usual contaminants may pass through first filter  26 . They would then be carried by the airflow to second filter  28 . At least one nozzle may also be positioned between the first filter  26  and the second filter  28  to substantially coat a front surface  110  of the second filter  28  with cleaning solution for the identical purpose as first filter  26 .  
         [0060]     In this way the majority of contaminants and cleaning spray  34  is captured by filters  26 ,  28 . This prevents the majority of contaminants and the cleaning spray passing along subsequent portions of the exhaust system. This prevents damage to duct linings, exhaust fans and other components of the exhaust system.  
         [0061]     The louvres  30  may be in the same circuit as the exhaust fan for the airflow such that the louvres  30  are in the “closed” position when the exhaust fan is not operational. Hence, the spraying of cleaning solution will take place even when there is no airflow. In this way the cleaning solution cannot flow to the other parts of the exhaust system and the area to be ventilated. However, the louvres  30  should be able to be independently controlled, if desired. This may be required to enable cleaning of the louvres. The pump may also be in the same circuit as the exhaust fan such that sprays from the at least one nozzle may be drawn towards the filters and will not fall under the influence of gravity.  
         [0062]     The louvres  30  may also be operable dependent on the speed of the air flow in pathway  110  in the exhaust system. An anemometer (not shown) may be installed into the pathway  110  of the exhaust system to measure the speed of the air flow such that the louvres  30  may be automatically positioned to the “open” position if the speed of the air flow exceeds a specific amount. Similarly, the louvres  30  may be automatically positioned to the “closed” position if the speed of the air flow is of a negligible value, as this means that there is no activity in the area to be ventilated.  
         [0063]     By having filters  26 ,  28  continuously cleaned during the operation of the exhaust system, clogging is less likely and thus smaller mesh sizes may be used in filters  26 ,  28  to thus increase the effectiveness of their operation. The risk of fire from dried contaminants is also considerably minimised.  
         [0064]     Referring to  FIG. 16 , there is a cleaning solution circulation system  148  to circulate a degreaser/water cleaning solution in an exhaust hood system  120 . There is a circulation tank  150  for the containment of the degreaser/water cleaning solution. When the level of cleaning solution in the circulation tank  150  falls below a pre-determined level, a valve  152  opens and water from an external supply may flow into the circulation tank  150 . The valve  152  may be a ball float valve. There is a supplementary tank  154  for the containment of degreaser. There is a dispenser  156  that may dispense degreaser when flow of water towards the circulation tank  150  is detected in a pipe  158 . Alternatively, the dispenser  156  may also dispense degreaser when the valve  152  is opened. Preferably the ratio of the degreaser to water is in the range of 1:10 to 1:50; more preferably 1:20.  
         [0065]     Conversely, the dispenser  156  may cease dispensing degreaser when the valve  152  is closed. The degreaser and water then mixes in the pipe  158  before draining into circulation tank  150 . The valve  152  may be closed once the level of the cleaning solution in circulation tank  150  is filled to a predetermined level.  
         [0066]     When the exhaust system  120  is operational, cleaning solution may flow under the influence of gravity from the circulation tank  150  to the plurality of self cleaning filter assemblies  52  through a circulation pump  160 . The cleaning solution may be passed through a fine stainless steel wire mesh filter prior to entering the circulation pump  160 . As such, damage to the circulation pump  160  from particle clogging is minimised. The circulation pump  160  may operate like a venturi pump. The pump  160  may have a region of reduced cross-sectional area along the flow of the cleaning solution. Due to Bernoulli&#39;s principle, the velocity of the flow of the cleaning solution is increased after the region of reduced cross-sectional area. With an increased velocity, the cleaning solution may be propelled against gravity in a supply pipe  162  to the filter assemblies  52 . An increased velocity for the flow of the cleaning solution may also increase the velocity of the cleaning solution being spurted from nozzles in the filter assemblies  52 . A stream of air may be passed into the cleaning solution either prior or subsequent to the circulation pump  160  such that the pressure in the cleaning solution may be maintained. Aerating the cleaning solution may also aid in cooling the cleaning solution.  
         [0067]     After the cleaning solution is sprayed from the nozzles onto filters in the filter assemblies  52 , spent deaning solution is accumulated in chambers in each filter assembly  52 , before flowing back through a return pipe  164  under the influence of gravity to the circulation tank  150  when a predetermined level in each chamber of each filter assembly  52  is reached.  
         [0068]     The mixture of grease and other contaminants with cleaning solution would sink to the bottom of the circulation tank  150  as a precipitate. The circulation tank  150  may have an inclined base such that the precipitate accumulates at the apex  155  of the inclined base of the circulation tank  150 . A waste pipe  153  may be opened at a predetermined time to allow the accumulated precipitate to be drained away from the apex  155  of the inclined base of the circulation tank  150 .  
         [0069]     When the level of the cleaning solution in the circulation tank  150  exceeds a height of a drainage pipe  151 , cleaning solution in the tank  150  will flow out till it reaches the height of the drainage pipe  151 .  
         [0070]     In an alternative embodiment, the circulation system  148  may be positioned above the exhaust system  120 . The arrangement of the system  148  would then be to employ a circulation pump  160  where the flow of the cleaning solution goes against the force of gravity.  
         [0071]      FIG. 17  shows the process flow for the cleaning solution circulation system  148 . When cleaning solution in the circulation tank  150  falls below a predetermined level ( 200 ), valve  152  may be automatically opened to allow the flow of water towards the circulation tank  150  ( 202 ). Dispenser  156  may automatically dispense degreaser when water flowing towards circulation tank  150  is detected in pipe  158  ( 204 ). Water and degreaser then mixes in a preferred proportion to form the cleaning solution ( 206 ). The amount of cleaning solution remaining in the circulation tank  150  may then be topped-up with the newly-mixed cleaning solution ( 208 ).  
         [0072]     When cleaning solution is required in the filter assemblies  52  through supply pipe  162 , cleaning solution from the circulation tank  150  may flow into a circulation pump  160  ( 210 ). A stream of air may be passed into the cleaning solution either prior or subsequent to the circulation pump  160  such that the pressure in the cleaning solution may be maintained. Aerating the cleaning solution may also aid in cooling the cleaning solution. The cleaning solution may then be sprayed from nozzles in the filter assemblies  52  onto filters in the filter assemblies  52  to maintain the serviceability of the filters ( 212 ). Used deaning solution may be stored in chambers in each filter assembly  52  and may flow to the circulation tank  150  through return pipe  164  when the amount of cleaning solution exceeds a pre-determined amount in each chamber ( 214 ). Whenever a waste pipe  153  in the circulation tank  150  is opened, precipitate of grease/cleaning solution flows out through the waste pipe  153 , thus leaving re-usable cleaning solution for the circulation system  148  ( 216 ).  
         [0073]     All parts of each self cleaning filter assembly  52  may be dismantled for maintenance. Each self deaning filter assembly  52  may also be retrofitted into existing kitchen hoods of an appropriate size with the use of fasteners such as, for example, nuts and bolts.  
         [0074]     Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technologies concerned that variations in details of design or construction may be made without departing from the present invention. The present invention extends to all features disclosed either individually, or in all possible combinations and permutations.