Abstract:
An electrostatic precipitator type filter is combined with replaceable, polarizable trapping media. In one aspect of the invention, the media is fitted between polarizing plates. In another aspect, the media is coated in sections to form a conductive surface which serve as the equivalent to the charged plates of the precipitator. These electrodes may be alternately displaced to provide a ready means to effect electrical connections.

Description:
FIELD OF THE INVENTION 
     This invention relates to air filters. In particular, it relates to electronically-enhanced filters that include a trapping medium. 
     BACKGROUND TO THE INVENTION 
     Precipitator-type air filters of the type depicted in U.S. Pat. No. 2,593,869 to Fruth (1952) operate by first ionizing particulate-carrying air to charge dust contained therein, and then pass the air between oppositely charged, end-on aligned parallel plates to which the dust adheres. Such precipitating air cleaners are highly efficient when the plates are initially clean. However, performance drops off as the plates become covered with collected dust. Hence, regular cleaning is required to maintain efficiency. This cleaning operation for precipitator-type air cleaners is awkward and costly to effect. 
     An advantage of filters of the trapping media type is that such media may be readily removed and replaced once they are filled with dust. 
     It is known that in trapping airborne particles in disposable filter media such as fibrous matrices of glass, wool and the like, the trapping capacity of such filter media can be enhanced by ionizing the air, and charging the dust therein, before it enters the filter medium. U.S. Pat. Nos. 3,706,182 to Sargent (1972), and 4,244,710 to Burger (1981) both depict such an arrangement. In both of these references, ions are introduced into the airflow stream by ion emitters positioned at an upstream location in the airflow, at a spaced distance from the filter medium that is intended to trap and remove charged particles from the airflow. Prior inventions by the present inventor also rely on the upstream release of ions into an air flow as presented in U.S. Pat. Nos. 5,518,531 (1996) and 6,077,334 (Jun. 20 2000). 
     It is also known that the trapping of dust particles, especially charged dust particles, can be enhanced by using as a trapping medium an air-permeable matrix of non-conducting, polarizable material. Local dipoles formed within such medium help trap and bind dust particles. An example of a prior art reference based on this principle is U.S. Pat. No. 4,549,887, by the present inventor. 
     The present invention makes use of the airflow-aligned, charged parallel plate principle and, optionally, the ionization principle in conjunction with polarized media to provide an improved performance air filter. 
     The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention a series of generally parallel, alternately charged metal electrodes, aligned to receive air-flow are edge-on, used as polarizing electrodes to polarize trapping media contained between electrodes. The trapping medium may be in the form of a fibrous dielectric pad and/or may comprise pleated panels of air permeable trapping material. The electrodes are preferably aligned parallel to the airflow (although this is optional, to provide a polarizing, transverse field though the trapping medium. The polarizing electrodes may be in the form of plates between which the trapping media is placed. Alternately, polarizing electrodes may be formed right on the trapping media surface as by sheets of conductive screening or fabric. This can also be effected by rendering surface segments of the trapping media conducting as well as by providing air permeable conductive layer laid over such surfaces. The electrodes and trapping media may conveniently be formatted as a cartridge for ready removal and replacement. 
     In all of these variants, ionization may be provided upstream in the arriving airflow by a series of ionizing needles or other ionizing elements such as fine wires or conducting strings (c.f. U.S. Pat. No. 5,573,577, Nov. 12, 1996 by the present inventor). Such ionization charges dust particles in the air flow, enhancing further the trapping efficiency of the media present in the polarizing field formed between the oppositely charged polarizing electrodes. 
     Conductive surface portions may be formed on alternating sections of trapping medium constructed as a continuous surface folded into pleated panels by coating the medium with a conductive material, such as fine carbon or aluminum, preferably mixed with a binder. Conductive surfaces may also be formed by transferring conductive panels of conductive, porous (air-permeable) media to the trapping media as by an adhesive. 
     With trapping media contained between polarizing electrodes, a high potential voltage source is connected to provide a polarizing potential difference between consecutive electrodes. This potential difference not only tends to polarize the intervening portions of the trapping medium but also creates an electrical potential field between the electrodes with a high field gradient. Dust particles, particularly charged dust particles, are drawn laterally in the air flow by this transverse field to contact and be retrained in the trapping medium. 
     By these arrangements an improved air filter of increased efficiency and cost effectiveness is provided. 
     The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross-sectional plan view of the air cleaner of the invention wherein polarizable, pleated filter media is disposed around charged polarizing plates; 
     FIG. 2 shows a cross-sectional plan view of an alternate format air cleaner wherein the pleated filter trapping medium is coated with conductive paint in strips and the strips are charged with high voltage of alternating polarity to form the polarizing electrodes; 
     FIG. 3 is a plan view of the stretched-out pleated media of FIG. 2 to demonstrate how the media is coated with conductive paint in strips; 
     FIG. 3A is a variant on FIG. 3 that allows electrical contact to be made on the leading face of the filter media; 
     FIG. 3B is a variant on FIG. 3 that allows electrical contact to be made on the top and bottom faces of the filer media; 
     FIG. 3C is a variant of FIG. 3 that includes an isolating strip to minimize electrical leakage; 
     FIG. 4 is a cross-sectional side view of the air cleaner assembly of FIG. 1 mounted in an air duct with ionizing elements placed in front of the air filter; 
     FIG. 5 is a cross-sectional rear end view of the pleated media of FIG. 2 compacted with glue-beads positioned to separate the folded pleats; 
     FIG. 5A is a pictorial depiction of the pleated media of FIG. 5 in transition as it is being folded to provide the compacted fitter assembly of FIG. 5; 
     FIG. 6 is a cross-sectional plan view of the media of FIG. 5 taken through the lines of glue beading showing the connection of the polarizing voltage source to the panel electrodes. 
     FIG. 7 depicts an alternate arrangement wherein multiple pieces of air-permeable, fibrous trapping media of dielectric material are sandwiched between conductive screens or plates; 
     FIG. 8 shows a cross-sectional top view of the arrangement of FIG. 7; 
     FIG. 9 shows two interrupted contacting bars for connecting the plates or screens of FIGS. 7 and 8 to a power supply; 
     FIG. 10 depicts a pair of continuous, non-interrupted electrical contacting bars applied along the leading face of the filter media of FIG. 3A; and 
     FIG. 11 is a depiction of continuous electrical contacting bars applied over the top and bottom faces of the filter media of FIG.  3 B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 a casing  1  or frame  1  contains the elements of the air cleaner. A permeable filter medium  2  of paper or the like which may be pleated is removably placed between and around a series of consecutive conductive plates  3  which serve as electrodes. While numerous plates are shown, the invention will work with two plates. It is highly preferable, however, to employ many plates. 
     Consecutive conductive plates  3  are respectively insulated from each other and are alternately connected to a high voltage power supply  4  which provides polarizing voltage of differential polarity between adjacent plates  3 . Permissibly, one set of plates  3  may be grounded. The object is to provide a strong electrostatic field with a steep gradient between the plates  3  and across the panels  12  of medium  2 . 
     A set of ionizing elements  5  charge the dust particles  10  arriving in front of the filter to increase its collecting efficiency. Ionizing elements  5  are supplied with high voltage from power supply  6 . 
     As an alternative to a single pleated sheet, a polarizable fiber matrix or the like may be inserted between the plates  3  as shown subsequently in FIG. 7, below. 
     FIG. 2 shows an alternate way of providing an electrostatic field across medium  2 . A conducting coating  7  such as graphite or aluminum powder with a binder is applied to the surface of filter medium  2  in non-contiguous panel sections as shown in FIG. 3 to provide the electrodes. Conductivity may also be imparted to the panels by applying an infiltrating conductive liquid that leaves a conductive deposit e.g. colloidal carbon in a solution; or an air-permeable, conductive layer may be transferred to the sections of surfaces of the trapping medium  2  and held in place by an adhesive. Examples of such a layer include conducive fabrics such as copper-treated polypropylene fabric, conductive plastic grids and wire mesh screens of aluminum or the like. Schedule A shows the amendments being made to this paragraph. 
     While every other panel  19  is shown as having a conductive surface  7  in FIG. 2, coating may also be effected intermittently so as to leave more than one intermediate panel  19  uncoated. 
     Metal rods  8  held by the frame  1  support medium  2  and at the same time make contact with the coated sections  7  on medium  2 . Adjacent metal rods  8  are insulated from each other and they are respectively connected to the high voltage power supply  9  so as to be alternately charged with differing potentials. The conductive coatings  7 , because they come in contact with metal rods  8 , become charged with differing electrical potentials and thus produce a strong electrostatic field between them. 
     FIG. 4 shows the air cleaner with its frame  1  installed into in a duct  11  of an air handling system. Ionizing elements  5  are optionally located upstream in the airflow  9 . The frame  1  is readily removeable to permit servicing, and replacement of the filter medium  2 . 
     FIGS. 5 and 6 show a pleated filter wherein the pleat panels  19  are separated by lengths of beads  13  of glue applied to the filter media  2  before it is pleated. The glue beads  13  keep the pleat panels  19  apart and at the same time make the filter self-supporting without any need for other structure, such as a screen. 
     The parts of the medium  2  that are coated, are charged to differential voltages as before by high voltage power supply  9 . This voltage can be applied, for example, by contacting fingers respectively carried on two contactor bars to every other conductive surface  7 . This type of filter can achieve efficiencies which are superior to a filter lacking the polarization feature. 
     In FIGS. 7 and 8 conductive plates  7  or screens  20  are positioned to serve as electrodes between sections of fibrous trapping media  21 . Electrode screens  20  are alternately charged by high voltage power supply  22  thus providing a strong electrostatic field between such screens  20  which, in turn, polarizes sections of media  21  placed between the plates  2 . The air-flow  9  enters the media  21  edge-on and flows through the body of the media  21 . The extent of this flow, and trapping efficiency, can be controlled by varying the depth of the media  21 . 
     The plates  7  or screens  20  need not be perfectly aligned, in parallel with the airflow  9 . Such plates  20  may be obliquely inclined to the direction of the entering airflow. In either case, the screens  20  receive the airflow  9  edge-on, as do the media sections  21 . And the airflow  9  between the screens  20  passes in a direction which is parallel to the surface of the electrode (in the colloquial sense, and not parallel to the mathematical direction of such surface). 
     FIG. 9 shows a method of connecting the plates or screens  20  to a high voltage power supply. Conductive rods  23  are insulated from the frame  1  of the filter and are connected to high voltage power supply  22 . These rods  23  carry insulator sleeves  24  which have cut-outs  25  to expose the rods  23  at alternating intervals. Thus, when the filter of FIGS. 7 and 8 is pressed against the rods  23 , one half of the screens  20  will make contact with one rod  23  and the other half with the other rod  23 . In this way, the screens  20  in the filter are connected to alternate polarities of the power supply. 
     Operation of the air cleaner is as follows. Air flow  9  coming into the device as shown in FIG. 4 first passes by the ionizing elements  5  whereby the dust particles  10  acquire a charge. Further down the duct  11 , the dust particles  10  encounter the strong, transverse polarizing electrostatic field present between the plates  3  or conducting surfaces  7  and are attracted towards such plates  3  or conducting surfaces  7  of the media  2 . As the dust particles  10  move towards the plates  3 , or surfaces  7 , they become deposited on the media  2 . To maintain the air cleaner in optimum operating condition, the media  2  is replaced with new, clean media  2  on a regular basis. 
     Optionally, the air cleaner may omit the ionizing elements  5  but the filter&#39;s efficiency will suffer. 
     In FIG. 2 the conductive surface  7  is depicted as being on the inside of the folds of the pleats  2 , extending around inside of the fold to contact rods  8  that are alternately charged to polarizing potentials. Such an arrangement requires installation of the folded pleats around the rods  8 . 
     In FIG. 6, the conductive surface  7  is depicted as being on the outside of the folds in the pleats  2 . Electrical contact with alternating conductive surfaces  7  has previously been proposed to be established by two contactor bars carrying a series of contacting fingers. 
     In FIG. 3A an alternate pattern for applying the conductive surface to the media  2  is provided. In FIG. 3A every alternate conductive surface  7 A is displaced upwardly and the intervening conductive surfaces  7 B are displaced downwardly. The result is that the extending portions  15 A, 15 B of the respective conductive surfaces  7 A, 7 B, along the upper and lower borders of the pleated media  2  are respectively aligned. This is shown in FIG. 10 wherein a pleated filter cartridge  16  of this type is shown, assembled with glue beads  13  as inter-panel spacers. 
     Based on the disclosed alignment for the conductive surfaces  7 A, 7 B, electrical contact can be made with the respective, alternate conductive surfaces  7 A, 7 B by placing conductive contacting bars  17 , 18  of differing potential along the upper and lower portions of the leading face of the pleated filter assembly  16  to make electrical connection with the extending portions  15 A, 15 B of the conductive surfaces  7 A, 7 B. This provides considerable convenience in installing a pleated filter cartridge  16  in an air flow  9 . The cartridge  16  need merely be slid into position against the contacting bars  17 , 18  to deliver polarizing potential to alternate conducting surfaces  7 A, 7 B. 
     In FIG. 10 the contacting bars  17 , 18  are shown as being placed along the upstream face of the filter cartridge  16  with respect to the air flow  9 . This positioning can be revised so that the contacting bars  17 , 18  are on the downstream side of the cartridge  16 , contacting protruding, exposed conducting surface portions  15 A, 15 B on a cartridge  16  that has been rotated 180 degrees from the orientation of FIG.  10 . 
     In FIG. 10 the contacting bars  17 , 18  are depicted as extending across the air-receiving face  19  of the cartridge  16 . In FIG. 3B, a placement pattern is shown for the conductive surfaces on a modified filter media substrate that allows electrical contact to be made with the conductive surfaces  7 A, 7 B along the top and bottom sides of the cartridge  16 A, outside the path of airflow  9  through the filter  16 A. 
     In FIG. 3B the conductive surface portions  7 A, 7 B extend alternately into tabs  20  which extend beyond the normal edge of the trapping media panel, at opposite sides of the media  2 . When this format of media  2  is assembled into a cartridge  16 A as in FIG. 9, the tabs  20  of alternate conductive surfaces  7 A, 7 B extend respectively above the top face and below the bottom face of the cartridge  16 A. Conveniently, they may be bent or inclined to overlie each other. As the extending tabs  20  alternate with consecutive conducting surfaces  7 A, 7 B, all the tabs  20  along the top face of the cartridge  16 A can be contacted by a single contacting bar  21  of a first electrical potential; and all of the tabs  20  along the bottom face can be contacted by a contacting bar  22  of a second, polarizing, electrical potential. Such a contacting arrangement is shown in FIG.  11 . 
     An advantage of the arrangement of FIG. 11 is that charged contacting bars  21 , 22  need not be present in the path of the air flow  9 . As well, due to the enlarged contacting surface accessible on the protruding tabs  20 , less voltage drop can be achieved in delivering potential to the adjacent areas of the conducting surfaces  7 A, 7 B. This is particularly convenient in the cases where the cartridges  16 A have deep pleats extending for an extended length along the direction of the air flow  9 . 
     A concern in preparing pleated trapping media  2  with alternately charged conductive surfaces  7  is the leakage of current that may arise between adjacent panels  19 . A significant source of current leakage may arise from moisture accumulating in the trapping media  2 . This may particularly occur when the trapping medium  2  is made of fine paper of the type used in other known HEPA filters. 
     To minimize current leakage when moisture is present in the air flow  9  to be filtered, the media  2  to be employed in the pleated filter cartridge may be treated in the manner shown in FIG.  3 C. In this Figure, similar to FIG. 3, the trapping media is modified by a series of narrow strips  24  extending transversely across the width of the developed trapping media surface. These strips  24  are impregnated with a sealant, such as wax. The purpose of this sealant is to exclude the infiltration of moisture into the matrix of the trapping media  2 . By providing impregnated strips  24  that extend entirely across the width of the developed media  2 , electrical isolation between adjacent conductive surfaces  7  can be maximized. 
     Test were conducted with an air flow volume of around 1000 cfm (cubic feet per minute) with a pleated filter of about 6 inches in depth and an area of 20×24 inches, installed as in FIG.  1 . The results of these tests are useful for the comparison of relative performances, and are not to be taken as accurate in absolute terms. Particle counts were taken in household air with an INNOVATION 5000 particle count meter by Climet Corporation of California. Efficiencies were alternately calculated in accordance with the following formulae, repeatedly applied to sets of measurement data:              Eff   =           us   _     -     ds   1         us   _       ×   100               where                   us   _       =           us   1     _     +     us   2       2                 Eff   =           us   2     -     ds   _         us   2       ×   100               where                   ds   _       =         ds   1     +     ds   2       2                                  
     On this basis, test results are shown in Tables 1 to 5 which now follow: 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Test with no ionizing elements and no voltage on the plates 
               
               
                 (Particle Counts = PC) 
               
             
          
           
               
                 PC at 
                 .3 mic 
                 % Eff 
                 .5 mic 
                 % Eff 
                 1 mic 
                 % Eff 
                 5 mic 
                 % Eff 
               
               
                   
               
             
          
           
               
                 us 1   
                 25096 
                 15.00(a) 
                 5462 
                 14.05 
                 586 
                 40.08 
                 20 
                 20.00 
               
               
                 ds 1   
                 21519 
                 15.45(b) 
                 4580 
                 10.21 
                 376 
                 37.67 
                 16 
                 40.00 
               
               
                 us 2   
                 25535 
                 15.47(a) 
                 5195 
                 15.32 
                 669 
                 31.13 
                 20 
                 61.90 
               
               
                 ds 2   
                 21660 
                   
                 4749 
                   
                 458 
                   
                  8 
               
               
                 us 
                 25713 
                   
                 6022 
                   
                 661 
                   
                 22 
               
             
          
           
               
                 Average Eff. 
                 15.31 
                   
                 13.19 
                   
                 36.29 
                   
                 40.63 
               
               
                   
               
               
                 us = upstream  
               
               
                 ds = downstream  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Test with negative ionizing elements and no voltage on the 
               
               
                 plates 
               
             
          
           
               
                 PC at 
                 .3 mic 
                 % Eff 
                 .5 mic 
                 % Eff 
                 1 mic 
                 % Eff 
                 5 mic 
                 % Eff 
               
               
                   
               
               
                 us 
                 26078 
                 26.39 
                  9450 
                 65.75 
                 1307 
                 68.28 
                 42 
                 86.52 
               
               
                 ds 
                 19827 
                 26.15 
                  3422 
                 65.95 
                  448 
                 70.19 
                  6 
                 81.91 
               
               
                 us 
                 27789 
                 23.37 
                 10530 
                 67.74 
                 1518 
                 72.84 
                 47 
                 81.51 
               
               
                 ds 
                 21215 
                 22.02 
                  3748 
                 68.60 
                  457 
                 73.58 
                 11 
                 77.78 
               
               
                 us 
                 27583 
                 26.05 
                 12707 
                 69.95 
                 1847 
                 75.12 
                 72 
                 73.08 
               
               
                 ds 
                 21804 
                   
                  4232 
                   
                  519 
                   
                 21 
               
               
                 us 
                 31390 
                   
                 15464 
                   
                 2325 
                   
                 84 
               
             
          
           
               
                 Average Eff. 
                 24.80 
                   
                 67.60 
                   
                 80.16 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Test on pleated filter without ionizing elements and positive 
               
               
                 8KV on alternate plates with other plates grounded 
               
             
          
           
               
                 PC at 
                 .3 mic 
                 % Eff 
                 .5 mic 
                 % Eff 
                 1 mic 
                 % Eff 
                 5 mic 
                 % Eff 
               
               
                   
               
               
                 ds 
                 1963 
                 51.20 
                 285 
                 52.91 
                 128 
                 75.57 
                 128 
                 75.57 
               
               
                 us 
                 4404 
                 49.21 
                 669 
                 53.38 
                 524 
                 78.40 
                 524 
                 78.40 
               
               
                 ds 
                 2335 
                 47.89 
                 345 
                 51.36 
                 128 
                 74.43 
                 128 
                 74.43 
               
               
                 us 
                 4791 
                 49.87 
                 811 
                 52.67 
                 661 
                 71.96 
                 661 
                 71.96 
               
               
                 ds 
                 2658 
                   
                 444 
                   
                 210 
                   
                 210 
               
               
                 us 
                 5813 
                   
                 1065  
                   
                 837 
                   
                 837 
               
             
          
           
               
                 Average Eff. 
                 49.54 
                   
                 52.58 
                   
                 75.09 
                   
                 75.09 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Test with negative ionizing elements and negative 8KV on 
               
               
                 plates 
               
             
          
           
               
                 PC at 
                 .3 mic 
                 % Eff 
                 .5 mic 
                 % Eff 
                 1 mic 
                 % Eff 
                 5 mic 
                 % Eff 
               
               
                   
               
               
                 ds 
                  771 
                 68.30 
                 114 
                 67.13 
                  72 
                 61.75 
                  6 
                 86.11 
               
               
                 us 
                 2711 
                 68.59 
                 432 
                 67.40 
                 332 
                 59.96 
                  54 
                 89.60 
               
               
                 ds 
                  938 
                   
                 170 
                   
                 182 
                   
                  9 
               
               
                 us 
                 3325 
                   
                 611 
                   
                 577 
                   
                 119 
               
             
          
           
               
                 Average Eff. 
                 68.44 
                   
                 67.27 
                   
                 60.85 
                   
                 87.85 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Second test with two negative ionizing elements and negative 
               
               
                 8KV on plates 
               
             
          
           
               
                 PC at 
                 .3 mic 
                 % Eff 
                 .5 mic 
                 % Eff 
                 1 mic 
                 % Eff 
                 5 mic 
                 % Eff 
               
               
                   
               
               
                 us 
                 14236 
                 66.61 
                 1284 
                 69.76 
                 106 
                 74.55 
                 18 
                 100.0  
               
               
                 ds 
                  4894 
                 69.75 
                  417 
                 68.42 
                  28 
                 74.12 
                  0 
                 96.43 
               
               
                 us 
                 16941 
                 70.44 
                 1474 
                 67.34 
                 114 
                 76.15 
                 14 
                 93.55 
               
               
                 ds 
                  5355 
                 68.65 
                  514 
                 66.04 
                  31 
                 76.71 
                  1 
                 88.24 
               
               
                 us 
                 19288 
                 67.34 
                 1674 
                 65.78 
                 146 
                 76.95 
                 17 
                 82.35 
               
               
                 ds 
                  6739 
                 67.10 
                  623 
                 66.57 
                  37 
                 74.00 
                  3 
                 88.35 
               
               
                 us 
                 21975 
                 67.48 
                 1967 
                 67.20 
                 175 
                 73.46 
                 17 
                 88.24 
               
               
                 ds 
                  7720 
                   
                  692 
                   
                  54 
               
               
                 us 
                 25509 
                   
                 2253 
                   
                 232 
               
             
          
           
               
                 Average Eff. 
                 68.48 
                   
                 67.30 
                   
                 75.14 
                   
                 90.16 
               
               
                   
               
             
          
         
       
     
     The progressive improvements in measured efficiency are apparent, with maximum efficiency arising with the combination of upstream ionization and charged, polarized plates. 
     CONCLUSION 
     The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow. 
     These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.