Abstract:
In one embodiment, the filter cartridge comprises a mesh structure forming a generally annular cylinder comprising a center opening, a plurality of compartments containing a filter medium, and spaces between the compartments. The spaces between the compartments comprise substantially unimpeded airflow passages that permit airflow even when the filter medium is fully loaded.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a filtration system releasably attachable to a blower wheel in an HVAC system, and in particular, to a filter cartridge having a plurality of compartments containing a filter medium and a plurality of substantially unimpeded air flow passages that maintain a high flow rate even when the filter medium is in a fully loaded state. 
     BACKGROUND OF THE INVENTION 
     With increased concern over environmental air quality, innovative solutions have been sought for adding filtration capacity to new and existing air circulation systems, such as heating, ventilation, and cooling systems (HVAC) for buildings and vehicles. For example, the HVAC systems in most vehicles do not include air filters. Minimal space is generally available for retrofitting a filter to the HVAC system. Moreover, it may be necessary to provide one filter for incoming air and a second filter for air recirculating within the passenger compartment. Even on new vehicles, space within the HVAC system is at a premium and it is difficult for some manufacturers to provide a location for an appropriate filter. 
     In addition to the difficulty of finding sufficient space for a filter, the failure mode of most filter media also raises concerns. Over time, environmental contaminants accumulate in filters, typically resulting in a reduced flow rate through the air circulation system. Failure to replace the filter media periodically can result in an increased static air pressure drop across the filter and reduced efficiency for the air circulation system. The reduced flow rate through a loaded filter can also create safety hazards, such as allowing insufficient airflow for operating the defrost system of an HVAC system. 
     One approach to retrofitting an air filter to an HVAC system of a vehicle is disclosed in U.S. Pat. No. 5,683,478 (Anonychuk). The air filter is sized and shaped to fit into a cavity located within a blower motor assembly. An outwardly extended lip is provided on the base of the air filter for rigid attachment to a rim located below the fan on the automobile. The fan in the blower motor assembly rotates around the stationary filter. Although the &#39;478 patent recognizes the need to provide filtration efficiency without impeding airflow, air flow will inevitably be reduced as the filter becomes loaded with environmental contaminants. The failure mode of the filter element may be an unacceptable reduction in airflow through the blower motor assembly. 
     U.S. Pat. No. 5,265,348 (Fleishman et al.) discloses the use of a rotating foam material on a rotary fan to reduce noise. 
     Various filters for blower wheels are disclosed in commonly assigned U.S. patent applications Ser. No. 09/126,189 (now U.S. Pat. No. 6,099,608, issued Aug. 8, 2000), entitled Filtration System for HVAC Applications; Ser. No. 09/126,190 (now U.S. Pat. No. 6,099,609, issued Aug. 8, 2000), entitled Moving Sorbent Filter Device; and Ser. No. 09/126,181 (now U.S. Pat. No. 6,102,988, issued Aug. 15, 2000), entitled Moving Filter Device having Filter Elements with Flow Passages and Method of Filtering Air, all filed on Jul. 30, 1998. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a filtration system attachable to a blower wheel in an HVAC system. The present filtration system is particulary useful to provide cabin air filtration to vehicles, which currently do not have a filter designed in the HVAC system. Movement of the filter cartridge with the blower wheel increases filtration efficiency during blower operation. The present moving filter cartridge can be retrofitted to most existing blower wheels. The filter cartridge releasably attaches to either the outside perimeter or the inside perimeter of the blower wheel. 
     An advantage of the present filtration system is that it is retrofittable into most existing vehicles. Since most cars have a blower wheel, a space exists to place the present filter cartridge. On the other hand, it is not feasible to easily retrofit a vehicle with a conventional cabin air filter since little additional space is available. Locating the filter cartridge at the blower wheel provides filtration of both outside air entering the HVAC system and air being recirculated within the system. Most cabin air filters only filter the air as it enters the vehicle. 
     The present filter cartridge includes both compartments containing a filter medium and flow passages of a size, density and shape such that a high flow rate is maintained even when the filter media is fully loaded. The spinning action of the filter cartridge causes the filter medium contained in the compartments to impact air as it passes through the flow passages. Some loss of filtration efficiency due to the flow passages is offset by increased efficiency due to the movement of the filter cartridge with the blower wheel. 
     The filtration system rotates in conjunction with a blower wheel. The blower wheel has a plurality of fan blades arranged in a spaced relationship radially around a blower cavity. The blower wheel defines a flow path extending radially outward from the blower cavity and through the fan blades when the blower wheel is rotating. The present filtration system can reduce the airflow through the blower wheel, thereby reducing the power consumption of the motor. The relationship between power and flow is a cubic function. By reducing the load (i.e., power consumption), the life of the motor is extended. 
     In one embodiment, the filter cartridge comprises a mesh structure forming a generally annular cylinder comprising a center opening, a plurality of compartments containing a filter medium, and spaces between the compartments. The spaces between the compartments comprise substantially unimpeded airflow passages that permit airflow even when the filter medium is fully loaded. 
     The compartments may extend radially into the center opening. In one embodiment, the compartments comprise one of corrugations or pleats in one or more of the mesh layers. The compartments can be shaped to operate as fan blades. The compartments are typically discrete pockets. The mesh typically comprises an expanded metal screen. In one embodiment, a mesh material extends across a top opening of the annular cylinder. The filter medium can be selected from a group consisting of electret charged medium, particulate medium, sorbents medium, agglomerated carbon or combinations thereof. 
     The filter cartridge comprises at least one end cap. An air flow passage can optionally extend through the at least one end cap. The end cap can also include weight balancing cavities and/or removable weight balancing tabs. 
     In another embodiment, the filter cartridge comprises a first mesh layer having a plurality of raised portions and a second mesh layer engaged with the first mesh layer such that the raised portions comprise compartments. A filter medium is retained in the compartments. Spaces between the compartments comprise substantially unimpeded airflow passages that permit airflow even when the filter medium is fully loaded. The filter medium is preferably agglomerated carbon or other filter medium such as activated carbon or other sorbent materials that remove odors and gases from the air, such as diesel exhaust, car exhaust, urban or farm smells, carbon monoxide, and ozone. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a perspective view of a filter cartridge in accordance with the present invention. 
     FIG. 1 a  is a sectional top view of the filter cartridge of FIG.  1 . 
     FIG. 1 b  is a sectional top view of an alternate filter cartridge in accordance with the present invention. 
     FIG. 2 a  is a sectional view of the filter cartridge of FIG.  1 . 
     FIG. 2 b  is a sectional view of an alternate filter cartridge in accordance with the present invention. 
     FIG. 2 c  is a sectional view of another alternate filter cartridge in accordance with the present invention. 
     FIG. 3 is a perspective view of a filter cartridge engaged with a blower wheel in accordance with the present invention. 
     FIG. 4 is a sectional view of a filter cartridge engaged with a blower wheel in accordance with the present invention. 
     FIG. 5 is a sectional view of an alternate filter cartridge engaged with a blower wheel in accordance with the present invention. 
     FIG. 6 is a sectional view of another alternate filter cartridge engaged with a blower wheel in accordance with the present invention. 
     FIG. 6 a  is a sectional view of a filter cartridge engaged with an outer perimeter of a blower wheel in accordance with the present invention. 
     FIG. 7 is a top view of a filter cartridge engaged with a blower wheel in accordance with the present invention. 
     FIG. 8 is a graphic representation of data relating to gas removal efficiency of the present filter cartridges and a conventional filter cartridge. 
     FIG. 9 is a graphic representation of data relating to shaped blade inclination on airflow performance. 
     FIG. 10 is a graphic representation of data relating to shaped blade orientation on airflow performance. 
     FIG. 11 is a graphic representation of data relating to permeability of the filter cartridge cap on airflow performance. 
     FIG. 12 is a graphic representation of data relating to permeability of the filter cartridge cap on gas removal efficiency. 
     FIG. 13 is a graphic representation of data relating to permeability of the filter cartridge cap on airflow performance. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1,  1   a  and  2   a  illustrate a filter cartridge  20  in accordance with the present invention. The filter cartridge  20  includes a first mesh layer  22  defining an inner filter surface  24  and a second mesh layer  26  defining an outer filter surface  28 . The first mesh layer  22  includes a plurality of raised portions  30  that protrude into center opening  32  of the filter cartridge  20 . Gaps or spaces  34  located between the raised portions  30  on the first mesh layer  22  are abutted against the second mesh layer  26  so that the raised portions  30  define a plurality of compartments  40 . The compartments  40  are preferably filled with a filter medium  42  (see e.g., FIG. 1 a ). In the illustrated embodiment, each of the compartments  40  define a discrete pocket that does not communicate with the adjacent compartments. In an alternate embodiment, the compartments  40  can be formed in a single mesh layer, obviating the second mesh layer. 
     The gaps or spaces  34  between the compartments  40  comprise air flow passages  44  that allow for high initial air flow through the filter cartridge  20 , that does not decrease over time. That is, the air flow passages  44  are substantially unimpeded when the filter medium  42  is fully loaded. 
     The filter cartridge  20  includes a top cap  50  and a bottom cap  52 . As best illustrated in FIG. 2 a , the caps  50 ,  52  serve to secure the first and second mesh layers  22 ,  26  together. The caps  50 ,  52  can also be used to retain the filter medium  42  in the filter cartridge  20 . The caps  50 ,  52  can be constructed from a variety of materials, such as plastisol. Flexible caps constructed from plastisol or other elastomeric material enhance conformability of the filter cartridge  20  and provide a friction fit with a blower wheel. 
     The first and second mesh layers  22 ,  26  can be constructed from a variety of polymeric or metal screens or scrims. In the embodiment illustrated in FIG. 1, the first and second mesh layers  22 ,  26  are constructed from expanded metal. The mesh layers  22 ,  26  have a substantial open area of about 70% or greater. The interface  33  between the gaps  34  on the first mesh layer  22  with the second mesh layer  26  may be held together by adhesives, spot welding or a variety of other techniques. 
     When using loose particle filter media  42 , the size or diameter of the openings in the mesh layers  22 ,  26  is preferably smaller than the smallest dimension of the particles. Alternatively, agglomerated carbon can be located in the compartments  40  and heat set to form a unitized sorbent mass. Consequently, the open area of the first and second mesh layers  22 ,  26  can be dramatically increased. Molded carbon particle agglomerate suitable for use in the present filter cartridge  20  are disclosed in U.S. Pat. No. 5,332,426 (Tang et al.). 
     FIG. 1 b  illustrates an alternate embodiment of the filter cartridge  20 ′ having a web  35 ′ that follows the contour of the first mesh layer  22 ′. The web  35 ′ may be a particle filter or a scrim to assist in retaining the filter medium  42 ′ in the compartments  40 ′. For example, the web  35 ′ may be useful to retain loose carbon in the compartments  40 ′. Preventing the filter medium  42 ′ from escaping from the compartments  40 ′ is important to prevent weight imbalances in the filter cartridge  20 ′ from forming during usage. In another embodiment, the web  35 ′ is a particle filter located generally in the center of the compartments  40 ′ and substantially surrounded by filter medium  42 ′. In yet another embodiment, the web  35 ′ is located on the outer surface of mesh layer  22 ′. 
     The filter cartridge  20 ′ can be constructed using a variety of techniques. For example, the web  35 ′ can be bonded or laminated to the first mesh layer  22 ′ either before or after formation of the raised portions  30 ′. Once the raised portions  30 ′ are formed, the portion of the web  35 ′ located along the gaps  34 ′ is removed using mechanical or thermal processes. Consequently, the second mesh layer  26 ′ is positioned directly against the first mesh layer  22 ′ so that the flow passages  44 ′ are substantially unimpeded by the web  35 ′. In another embodiment, the portions of the web  35 ′ along the gaps  34 ′ are not removed provided the web  35 ′ is sufficiently porous. That is, the web  35 ′ can form a continuous layer around the filter  20 ′. 
     FIG. 2 b  is a sectional view of an alternate filter cartridge  70  with a top cap  72  having one or more air flow passages  80 ,  81 . Air flow passage  80  extends through the top mesh material  74  and outer mesh layer  84  of the filter cartridge  70 . Air flow passage  81  extends through the inner mesh  77  and the outer mesh  84 . The top cap  72  is constructed from an elastomeric material that secures outer mesh layer  84  to top mesh material  74 . The top mesh material  74  includes a tab  75  that wraps around and is optionally attached to the inner mesh layer  77  to give the filter cartridge  70  additional structural integrity. 
     FIG. 2 c  is a sectional view of an alternate filter cartridge  70 ′ with a top cap  72 ′ having one or more air flow passages  80 ′,  81 ′, substantially as shown in FIG. 2 b . Air flow passage  80 ′ extends through the top mesh material  74 ′ and outer mesh layer  84 ′ of the filter cartridge  70 ′. The top cap  72 ′ secures outer mesh layer  84 ′ to top mesh material  74 ′. In the embodiment of FIG. 2 c , the top mesh material  74 ′ extends across the center opening  86  to prevent debris from collecting in the filter cartridge  70 ′. 
     FIG. 3 is an exploded view of a filter cartridge  100  in accordance with the present invention and schematic blower system  104 . The filter cartridge includes an inner mesh layer  112  and an outer mesh layer  114 , but no end caps. In the embodiment of FIG. 3, the filter medium (not shown) is retained in the compartments by friction, adhesives and/or a variety of other mechanisms. For example, the filter medium can be retained in a pouch or bag constructed from a scrim or other porous material. 
     The filter cartridge  100  can be located in blower cavity  106  so that the outer filter surface  108  forms a friction fit with fan blades  110  on the blower wheel  102 . Alternatively, the filter cartridge  100  may be retained to the blower wheel  102  by a variety of active removable fastening techniques including mechanical fasteners, such as clips, hooks and hook and loop fasteners, and/or retaining tabs. 
     As the blower wheel  102  and attached filter cartridge  100  rotate, the fan blades  110  generate a reduced pressure condition that draws air along flow path  116  into center opening  118 . The pressure differential draws air through the filter cartridge  100  and ejects it radially outward through the fan blades  110  into housing  120 . As pressure within the housing  120  increases, air continues along flow path  116 ′ through air outlet  122 . 
     The present filter cartridge  100  may be used as a conventional in-line filter for an HVAC system, such as disclosed in U.S. Pat. No. 5,683,478 (Anonychuk). Alternatively, the filter cartridge  100  may be attached to a blower wheel  102  such as illustrated in FIG.  3 . Blower wheel  102  refers generically to any squirrel cage rotors, centrifugal rotors and the like. Clips or fasteners may be used to attach the filter cartridge  100  to the blower wheel  102 , such as disclosed in commonly assigned U.S. Pat. Ser. No. 09/126,189, entitled Filtration System for HVAC Applications, filed Jun. 30, 1998. The filter cartridge  100  preferably has a height generally equal to the height of the blower wheel  102 . In an embodiment where the filter cartridge  100  has a height less than the height of the blower wheel  102 , the gap defines a flow passage that permits a portion of the air flowing through the blower system  104  to bypass the filter cartridge  100 . 
     FIG. 4 is a sectional view of a filter cartridge  130  engaged with a blower wheel  132  in accordance with the present invention. The arrow  131  shows the direction of rotation of the assembly  130 ,  132 . Shaped inner mesh layer  134  has a series of raised portions  136  separated by gaps  140 . Outer mesh layer  138  is positioned against the inner mesh layer  134  to form radial compartments  144 . Filter media  142  is deposited in the radial compartments  144  formed by the raised portions  136  abutted against the outer mesh layer  138 . The number of compartments  144  can be greater than, less than or equal to the number of blades on the blower wheel  132 . 
     The gaps  140  form air flow passages  146  through the filter cartridge  130 . Consequently, when the filter media  142  is fully loaded, the substantially unimpeded air flow passages  146  maintain adequate air flow through the filter cartridge  130 . The filter cartridge  130  releasably attaches to either the outside perimeter or the inside perimeter of the blower wheel  132  (see FIGS. 6 and 6 a ). 
     FIG. 5 is a sectional view of an alternate filter cartridge  160  engaged with a blower wheel  162  in accordance with the present invention. The arrow  161  shows the direction of rotation of the assembly  160 ,  162 . In the embodiment illustrated in FIG. 5, the raised portions  164  formed in the inner mesh layer  166  are canted or inclined so that the compartments  165  act as extensions of the fan blades  168 . The raised portions  164  can be canted or inclined at an angle of inclination  167  of less than about 45 degrees relative to an axis  163  normal to the filter cartridge  160 . A cant of −45 degrees relative to axis  163  is also possible for some applications. In an embodiment where the filter cartridge  160  is attached to the outside perimeter of the blower wheel  162 , the angle of inclination  167  is reversed (the filter cartridge  160  is flipped over). 
     Maintaining high airflow in the blower system is critical to effectively heat and cool the cabin of a vehicle. The shaping or inclination of the raised portions  164  increases the airflow through the filter cartridge  160  and blower wheel  162 . In the embodiment of FIG. 5, the raised portions  164  are aligned and in-sync with the fan blades  168 . As discussed in Example 4, locating the raised portions  164  out of sync with, or offset from, the fan blades  168  does not significantly diminish airflow. 
     FIG. 6 is a sectional view of an alternate filter cartridge  180  engaged with blower wheel  182  in accordance with the present invention. The embodiment of FIG. 6 corresponds generally with FIG. 4, except that the raised portions  184  are formed on the outer mesh layer  186 . The inner mesh layer  188  that encloses raised portions  184  to form compartments  189  is cylindrical. In an alternate embodiment, the inner mesh layer  188  may include raised portions that may or may not be aligned with the raised portions  184  on the outer mesh layer  186 . 
     FIG. 6 a  is a sectional view of an alternate filter cartridge  180 ′ engaged with an outer perimeter of blower wheel  182 ′ in accordance with the present invention. Raised portions  184 ′ are formed on outer mesh layer  186 ′. Inner mesh layer  188 ′ encloses raised portions  184 ′ to form compartments  189 ′ and engages with the outer perimeter of the blower wheel  182 ′. 
     FIG. 7 is a top view of a filter cartridge  190  engaged with a blower wheel  192  in accordance with the present invention. Top cap  194  includes a series of removable weight balancing tabs  196  that can be trimmed or removed entirely to balance the blower wheel/filter cartridge assembly. Alternatively, top cap  194  may include a plurality of gaps  198  of known volume. By depositing a material of known density in the gaps  198 , the same balancing procedure can be performed. 
     The filter media is preferably a material having a useful level of resistance to penetration or transfer of particles and/or aerosols while retaining a desirable level of gas transport through the material. Resistance to permeation or transfer of particles and/or aerosols may be measured by determining the retention (filtration) of particles and can be expressed as clean air delivery rate (CADR), as defined in ANSI Standard AC-1-1988. 
     The filter media may be paper, porous films of thermoplastic or thermoset materials, nonwoven webs of synthetic or natural fibers, scrims, woven or knitted materials, foams, or electret or electrostatically charged materials. The filter media may also include sorbents, catalysts, and/or activated carbon (granules, fibers, fabric, and molded shapes). Electret filter webs can be formed of the split fibrillated charged fibers as described in U.S. Pat. No. Re. 30,782. These charged fibers can be formed into a nonwoven web by conventional means and optionally joined to a supporting scrim such as disclosed in U.S. Pat. No. 5,230,800 forming an outer support layer. The support scrim can be a spunbond web, a netting, a Claf web, or the like. Alternatively, the nonwoven fibrous filter web can be a melt blown microfiber nonwoven web, such as disclosed in U.S. Pat. No. 4,817,942 which can be joined to a support layer during web formation as disclosed in that patent, or subsequently joined to a support web in any conventional manner. 
     EXAMPLES 
     Example 1 
     The performance of the present filter cartridge was tested and compared to a conventional stationary cabin air filter made by Minnesota Mining and Manufacturing Company. FIG. 8 shows a comparison of the gas removal efficiency of a conventional filter and two filter cartridges according to the present invention compared to a conventional cabin air filter. The conventional filter had about 82 grams of carbon. The blower filter with a diameter of about 12.95 centimeters (5.1-inches) had about 55 grams of carbon and the 11.4 centimeters (4.5-inches) diameter blower filter had about 20 grams of carbon. Each of the blower filters had the carbon generally equally divided among about 44 compartments (equal to the number of fan blades on the blower wheel). The compartments extended radially into the center opening of the blower wheel, as generally illustrated in FIG.  4 . 
     The test was carried out by vaporizing toluene in a vaporization chamber and delivering the vaporized toluene in a carrier stream to a blower system without a filter cartridge. The amount of toluene required to deliver a concentration of 80 parts per million at a flow rate of about 200 cubic meters/hour (117 cubic feet/minute) through the blower wheel was determined. After the concentration of toluene was determined, one of the filter cartridge described above was inserted into the blower wheel. 
     Each of the filter cartridges was subjected to a toluene challenge of about 80 parts per million concentration with a flow rate of about 200 cubic meters/hour (117 cubic feet/minute). In this accelerated test about 15 minutes is roughly equal to one year of use in a car. The initial airflow reduction due to the blower filter is roughly equal to the 22% flow reduction of the conventional cabin air filter. However, the conventional filter airflow decreases with time, while the blower filter did not due to the non-plugging airflow passages. That is, the failure mode of the conventional air filter is a reduction in airflow, whereas the present blower filters maintain airflow even when the filter media is saturated. 
     Example 2 
     The size of the filter and the amount of media in it largely effect filter performance. By using a slightly larger diameter blower filter, performance matching traditional cabin air filters can be achieved. However, during the design or optimization of the blower filter several other factors were found to effect performance, including the use of shaped pockets to improve airflow and an open top that improves airflow and gas removal efficiency. 
     A comparison of airflow using the radial shape carbon blades and inclined or canted carbon blades is shown in the FIG. 9 for various blower voltages. The angle of inclination for the shaped blades was about 25 degrees from an axis normal to the filter cartridge (radially inward along the flow passage). The Statistical bars showing the range of values are provided at the top of each bar. At all blower voltages, forward inclined shaped carbon blades resulted in greater airflow through the present filtration system than either the radial shaped carbon blades or backward curved shaped carbon blades. The greatest air flow was achieved using a filter cartridge having flow passages extending through the top cap, such as discussed in connection with FIGS. 2 and 2 a.    
     Example 3 
     FIG. 10 illustrates the result of a test to determine the effect of the incline or cant direction of the shaped carbon blades on airflow. The angle of inclination of the shaped blades was about 25 degrees from an axis normal to the filter cartridge. Statistical bars showing the range of values are provided at the top of each bar. The same filter was used for both tests. The filter was placed in the blower wheel such that the carbon pockets inclined in a backward curved (BC) configuration. The same filter was then turned over such that the carbon pockets formed a forward curved (FC) configuration. The forward curved configuration showed about an 18% improvement in airflow compared to the backward curved configuration (33% reduction vs. 27%). This test clearly shows the sensitivity of the airflow to the orientation of the carbon blades. 
     Example 4 
     A test to determine the sensitivity of the airflow to the alignment of the carbon blades with the blower wheel was carried out. The angle of inclination of the shaped blades was about 25 degrees from an axis normal to the filter cartridge. In one case the carbon blades of the filter were aligned and in-sync with the blades of the blower wheel. In the second case the blades of the filter were offset about {fraction (1/88)}′ th  of a revolution to the blower wheel blades. The aligned in-sync case improved the airflow by about 1%. This test established that it is not necessary to provide a feature to align the filter cartridge with the blower wheel blades. 
     Example 5 
     Although some type of top cap is typically required to locate the filter in the blower and to hold the carbon in place, it does not necessarily need to be a closed ring. FIG. 11 illustrates the respective airflow reduction using a perforated cap and a solid cap on the filter cartridge. The filter cartridge had a 11.4 centimeters (4.5-inches) diameter and contained about 20 grams of carbon. The angle of inclination of the shaped blades was about 25 degrees from an axis normal to the filter cartridge. Statistical bars showing the range of values are provided at the top of each bar. By using a perforated metal cap (about 70% open), the airflow of the filter was increased about 4% at various blower voltages. 
     Example 6 
     Example 6 is directed to evaluating the gas removal efficiency of the filter cartridges tested in Example 5. As illustrated in FIG. 12, the gas removal efficiency of the perforated cap design was about 4-7% higher than that of the solid cap design. The gas test used toluene at a concentration of about 80 parts per million and a flow rate of about 200 cubic meters/hour (117 cubic feet per minute) as described in Example 1. This test was carried out several times with the same result. 
     Example 7 
     One possible explanation for the lower efficiency of the solid cap design is that the carbon located directly beneath the solid cap is not subject to much airflow. To determine if this was true, a test was conducted on a 11.4 centimeters (4.5-inches) diameter filter with 20 grams of carbon. The angle of inclination of the shaped blades was about 25 degrees from an axis normal to the filter cartridge. The same filter was used for tests with and without top caps. The filter was first tested without a top cap. A series of masking tape strips, about 2.54 millimeters (0.10 inches) in width were wrapped around the circumference of the filter beginning at the top. The open top filter showed a linear decrease in flow as progressive widths of tape masked the filter. The results are presented in FIG.  13 . 
     The same filter was then capped and the test was repeated. This test showed that masking off about the top 5.1 millimeters (0.20 inches) of the filter with a cap did not effect the airflow, indicating that the cap was preventing air from passing through this region of the filter (see FIG.  13 ). Carbon located in the region below the cap filter was apparently not filtering due to the lack of flow. In a filter with about 20 grams of carbon, the cap may be blocking as much as about 2.5 grams (more than 10%) of the carbon. Consequently, it appears that the open cap design has both higher airflow and efficiency than the equivalent design with a solid cap. 
     The complete disclosures of all patents, patent applications, and publications are incorporated herein by reference as if individually incorporated. Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.