Patent Publication Number: US-2022212522-A1

Title: Air filtration system

Description:
BACKGROUND 
     The present disclosure relates to an air filtration system for removing airborne solids from air that is being drawn into an interior space of a mobile unit, such as a recreational vehicle. 
     BACKGROUND OF THE RELATED ART 
     Mobile units, such as recreational vehicles, motorhomes, campers, and enclosed trailers, typically use one or more large vent fans to circulate air through an interior space for living, working, cooking or other activity. Typically, one or more vent fans are used to force large volumes of air out of the interior space through the ceiling, which causes a slight negative pressure that pulls air into the interior space through any of the available openings. These available openings may include screened windows and doors as well as unscreened gaps around slide outs and entry points for wiring and plumbing. If the mobile unit is located in an area having air with significant amounts of airborne solids, then the fans draw the airborne solids into the interior space along with the air. The airborne solids may include dust, bugs and pollen. Once inside the interior space, the airborne solids may be inhaled by individuals present within the interior space, perhaps causing allergies or other breathing difficulties. Furthermore, the airborne solids may accumulate on surfaces within the interior space. The interior space may be difficult to clean due a large number of nooks and crannies or other features suitable to the mobile unit. 
     BRIEF SUMMARY 
     Some embodiments provide an air filtration system for a mobile unit having a vent fan. The air filtration system comprises a rigid outer cover and a filter box. The rigid outer cover includes four rigid sidewalls, a rigid top and an open bottom, wherein each of the four rigid sidewalls have a bottom edge that is secureable to the mobile unit around a perimeter of the vent fan with the rigid top extending over the vent fan. At least one of the four rigid sidewalls includes an air inlet passageway. The filter box includes five rectangular air filters secured together in the shape of a box having four filter sidewalls and a filter top. The filter box has an open bottom so that the four filter sidewalls are positionable around the perimeter of the vent fan with the filter top extending over the vent fan, wherein the filter box is configured to avoid interfering with opening and closing of a vent fan cover that is coupled to the vent fan. The rigid outer cover can be opened for receiving the filter box and can be securely closed to retain the filter box in an operable position. The rigid outer cover is configured to extend over and around the filter box leaving an air flow space surrounding the filter box with the filter box in the operable position, with the rigid outer cover secured to the mobile unit, and with the rigid outer cover securely closed. 
     Some embodiments provide a kit for forming an air filtration system for a mobile unit having a vent fan. The kit comprises a foldable net including four sidewall panels of a rigid outer cover and four lower tabs, wherein the foldable net includes an aluminum composite panel having a plurality of grooves that establish fold lines for forming the rigid outer cover, and wherein at least one of the four sidewall panels includes an opening that forms an air inlet passageway. The kit further comprises a top panel securable to each of the four sidewall panels and a base plate having a central opening that is positionable around a perimeter of the vent fan. The kit still further comprises a first set of fasteners for securing the four lower tabs to the base plate and securing the base plate to the mobile unit, a second set of fasteners for securing the top panel to the four sidewall panels, and an elastomeric material that conforms to a surface of the mobile unit around the vent fan and the base plate. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating an interior space of a mobile unit having a vent fan in the ceiling of the mobile unit. 
         FIG. 2  is a perspective view of the vent fan. 
         FIG. 3  is a block diagram of electronic components of the vent fan. 
         FIG. 4A  is an exploded schematic side view of an air filtration system compatible with the vent fan 
         FIG. 4B  is a partially exploded schematic side view of the air filtration system of  FIG. 4A  with the rigid outer cover secured to the roof of the mobile unit. 
         FIG. 4C  is a schematic side view of the air filtration system of  FIG. 4B  with the filter box and top member in an operative condition. 
         FIG. 5A  is a schematic plan view of a net for forming four lateral sidewalls of the rigid outer cover. 
         FIG. 5B  is an exploded perspective view of the components of the rigid outer cover. 
         FIG. 6A  is a perspective view of a pleated air filter suitable for use in forming a filter box. 
         FIG. 6B  is a plan view of a filter box including five of the pleated air filters. 
         FIG. 6C  is a perspective view of the filter box of  FIG. 6B . 
         FIG. 7A  is a perspective view of an operational air filtration system including louvered screens extending across the side openings in the rigid outer cover. 
         FIG. 7B  is a schematic cross-sectional side view of the operational air filtration system of  FIG. 7A . 
         FIG. 8  is a schematic side view of an aluminum composite panel (ACP). 
         FIG. 9A  is a schematic side view of a sidewall panel and a top member secured together with screw. 
         FIG. 9B  is a schematic side view of a sidewall panel and a top member secured together with an L-shaped bracket and a pair of screws. 
         FIG. 9C  is a plan view of four lateral sidewall of a rigid outer cover illustrating placement of a plurality of L-shaped brackets according to some embodiments. 
         FIG. 10A  is a perspective view of a molded plastic embodiment of a rigid outer cover. 
         FIG. 10B  is a schematic side view of the rigid outer cover of  FIG. 10A  hingedly connected to the base plate and a secondary filter housing on the outlet side of the vent fan. 
         FIG. 11  is a schematic cross-sectional side view of an operational air filtration system having nested filter boxes. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments provide an air filtration system for a mobile unit having a vent fan. The air filtration system comprises a rigid outer cover and a filter box. The rigid outer cover includes four rigid sidewall panels, a rigid top panel and an open bottom, wherein each of the four rigid sidewall panels have a bottom edge that is secureable to the mobile unit around a perimeter of the vent fan with the rigid top panel extending over the vent fan. At least one of the four rigid sidewall panels includes an air inlet passageway. The filter box includes five rectangular air filters secured together in the shape of a box having four filter sidewalls and a filter top. The filter box has an open bottom so that the four filter sidewalls are positionable around the perimeter of the vent fan with the filter top extending over the vent fan, wherein the filter box is configured to avoid interfering with opening and closing of a vent fan cover that is coupled to the vent fan. The rigid outer cover can be opened for receiving the filter box and can be securely closed to retain the filter box in an operable position. The rigid outer cover is configured to extend over and around the filter box leaving an air flow space surrounding the filter box with the filter box in the operable position, with the rigid outer cover secured to the mobile unit, and with the rigid outer cover securely closed. 
     The mobile unit may be any type of vehicle, trailer, container or pod, without limitation, that may have, or be made to have, a vent fan that moves air between an interior space of the mobile unit and the surrounding environment. Specific, non-limiting examples of the mobile unit include a recreational vehicle, camping trailer, delivery truck or van, work truck, construction shed and food truck. 
     The vent fan may be a vent fan that is already installed for exhausting air from the interior space of the mobile unit. Alternatively, the vent fan may be newly installed specifically for the purpose of drawing air through the filters without regard to any previously installed vent fans. One advantage of installing a new vent fan for use in combination with the filter box is that the location of the vent fan may be selected for optimal installation, operation and maintenance of the filter box. However, mobile units may have any number of vent fans. For example, an enclosed trailer may have at least one vent fan, a medium sized motorhome or camper trailer may have two vent fans, and a large motorhome or camper may have 3 to 4 vent fans. One of the air filtering systems described herein may be installed for each of the vent fans or for at least one vent fan. 
     The vent fan may be a rotary fan that is operated so that the fan motor rotates one or more fan blades in a direction that draws outside air through the filter box and then into the interior space. If the vent fan is not prewired for rotation in this direction or is not equipped with a controller or switch for causing rotation in this direction, then the two wires supplying electrical power to the fan motor may be reversed. 
     In some embodiments, the air filtration system may be used with a vent fan having any of a wide variety of sizes, types and features. For example, a vent fan may include, or work in combination with, a rain sensor that causes a vent fan cover to close over the vent fan in response to detecting rain. Specifically, a hinged vent fan cover may be automatically actuated between an open position and a closed position in response to sensor input indicating the presence of rain. Some embodiments of the air filtration system may facilitate continued use of this feature or other automatic features. A standard vent fan on top of mobile unit may be designed to open the vent fan cover whenever the fan is turned on and to close the vent fan cover whenever the fan is turned off. In another example, a hinged vent fan cover may be automatically actuated between the open position and the closed position in response to sensor input indicating an air temperature within the living space that is greater than or less than a reference temperature. 
     The base plate is preferably flat and has a central opening that is positionable around a perimeter of the vent fan housing and over a top of the perimeter flange. The flat base plate is secureable to the mobile unit with screws having a length to extend through the flat base plate, through the perimeter flange, and into the roof of the mobile unit. An elastomeric material is preferably applied to the top of the perimeter flange before positioning the base plate over the perimeter flange, such that any gaps between the perimeter flange and the base plate are sealed. To the extent that the base plate may extend beyond the perimeter flange, the elastomeric material may also be applied to the roof of the mobile unit in a thickness sufficient to fill the gap between the roof and the base plate. The base plate preferably provides a flat upper surface about a perimeter of the vent fan for receiving the filter box and the rigid outer cover. In one option, a lower edge of the filter box may sit flush against the flat upper surface of the base plate. The base plate is preferably secured with screws having a flat countersunk head so that there are no screw heads extending upward from the flat upper surface of the base plate. 
     In some embodiments, the rigid outer cover or box may be hinged to the base plate and moveable between a closed position for protecting the filter box and an open position for replacing the filter box. The rigid outer cover or box may be secured to the base plate with one or more hinge along one of the edges of the rigid outer cover, preferably along a front edge of the rigid outer cover relative to the front of the mobile unit. The rigid outer cover may then be latched along another one of the edges, such as along an edge that is opposite to the hinged edge. Accordingly, the rigid outer cover may be secured in a close position to secure and protect the filter box, and the rigid outer cover may be selectively moved to an open position to install or replace the filter box. 
     The rigid outer cover may be made in various shapes, sizes and material. However, the rigid outer cover must be of sufficient size and shape to receive and secure the air filter in a manner that neither the rigid outer cover nor the air filter interfere with operation of the vent fan cover. The rigid outer cover may be made from any material or composition that is able to withstand high velocity wind and rain as would be experienced on a roof top during a wide range of weather conditions, either with the mobile unit in a stationary condition or driving at highway speeds. As non-limiting examples, the rigid outer cover may be made from a wide range of polymers, polymer mixture or composites, fiberglass, sheet metal, aluminum composite panels, and combinations of these materials. 
     In some embodiments, the rigid outer cover may be formed with an aluminum composite panel including a first aluminum sheet, a second aluminum sheet, and a polymeric core secured between the first and second aluminum sheets. An aluminum composite panel has less weight than a solid metal panel of similar thickness, yet is rigid, durable and resistant to sunlight and weather conditions. The aluminum composite panel (ACP), which may also be referred to as aluminum composite material (ACM), may be cut to various dimensions by cutting through all three layers (i.e., aluminum-core-aluminum) and may be cut to form fold lines by cutting a linear groove through one aluminum sheet/layer and a major portion of the thickness of polymeric core. These types of cuts, including interior cutouts of windows or air passageways, may be made with a computer numerical control (CNC) machine (also referred to as a CNC router) under the control of a computer. A computer-controlled CNC machine provides both high precision cutting and low manufacturing costs. The computer may execute program instructions to cause the CNC machine to form various parts, such as the net for the rigid outer cover sidewalls, lid and/or base plate. The CNC machine may make cuts through the entire thickness of the aluminum composite panel along the outline of each part and/or any cut-outs (i.e., holes, windows, openings, or ports) within each part. Fold lines may be formed by cutting a groove through a portion of the aluminum composite panel thickness, such as through a first aluminum sheet and a major portion of the polymeric core. Each fold line may be formed by cutting a groove in the interior face of the aluminum composite panel, either to enable an inside corner or an outside corner. 
     The foldable net is an arrangement of non-overlapping edge-joined polygons in a plane which can be folded along the linear grooves (which form an joined edge between the panels) to become faces of the rigid outer cover. In some embodiments, the foldable net includes only a portion of the faces that are needed to complete a five-sided rigid outer cover with an open bottom. For example, embodiments may include a foldable net that forms only the four lateral sidewall panels or faces of the rigid outer cover. In such embodiments, the top panel may be a separate component that requires connection to the four lateral sidewall panels. Forming a net of the some portion of the rigid outer cover allows multiple sides of the rigid outer cover to be made from a single sheet, such as a net that includes the four lateral sidewall panels of the rigid outer cover. 
     In some embodiments, the rigid top panel may be detachably secured to the four rigid sidewalls with a plurality of fasteners, such as screws. In one option, the four rigid sidewall panels may each include an upper tab, such that the fasteners may secure the rigid top panel to each of the upper tabs. In another option, the rigid outer cover may include a hinge coupled between the rigid top panel and a first rigid sidewall panel of the four rigid sidewall panels for moving the rigid top panel between a closed position and an open position, wherein the rigid outer cover includes a latch coupled between the rigid top panel and a second rigid sidewall panel of the four rigid sidewall panels for detachably securing the rigid top panel in the closed position. In a further option, the rigid top panel may be secured to the four rigid sidewalls using four angle bars. Each angle bar may be an elongate bar of metal or rigid plastic having a pair of legs that extend apart at a 90-degree angle to form an L-shaped cross-sectional profile. For each of the four rigid sidewalls, a first leg of an angle bar is secured to the rigid sidewall with the second leg flush with an upper edge of the rigid sidewall panel, such that the second leg forms an upward-facing ledge for receiving the top panel. The top panel may then be detachably attached to the angle bars, such as using a set of screws. An optional gasket or seal may be disposed over the upper edge of the four sidewall panels and/or the angle bars to prevent water intrusion into the rigid outer cover. 
     In some embodiments, the rigid outer cover may be formed by two foldable nets, wherein both foldable nets include multiple panels, and wherein the two foldable nets are used collectively to form the rigid outer cover. For example, a first foldable net may form a front sidewall panel, a left sidewall panel, and a right sidewall panel, while a second foldable net may form a top panel and a back sidewall panel. Optionally, the front, left and right sidewall panels of the first foldable net may include lower tabs for securing the front, left and right sidewall panels to the base plate, while the top and back sidewall panels of the second foldable net are detachably coupled to the front, left and right sidewall panels. One or both panels of the second foldable net may be detached in order to receive a filter box therein. In one option, the top panel of the second foldable net may be detached while the back sidewall panel of the second foldable net remains attached, such that the fold line between the top panel and the back sidewall panel may service as a hinge. 
     Some embodiments of the rigid outer cover include any number, size and location of air inlet passageways. In one option, the rigid outer cover has a back-facing side with an air inlet passageway with or without additional air inlet passageways on the lateral sides of the rigid outer cover. Accordingly, some embodiments will not include an air inlet passageway on the front side of the rigid outer cover relative to the front of the mobile unit. While the inclusion of a greater number of air inlet passageways may improve air flow into the rigid outer cover, air inlet passageways in the back-facing side and, to a lesser extent, the lateral sides of the rigid outer cover will be more protected from high velocity air flow and rain as the mobile unit is driven in a forward direction. Of course, a rigid outer cover having a complex, irregular or not rectangular shape may air inlet passageways that are directed at various angles and directions, but those facing more backward (rearward) and/or more downward may be preferred. 
     In some embodiments, the filter box may be configured to fit over the top of the vent fan without interfering with opening and closing of a vent fan cover. For example, the vent fan lid may be hingedly coupled to a vent fan housing and may be automatically or manually actuated between a closed position that prevents air circulation and/or rain entry into the interior space and an open position that enables air circulation into the interior space. Accordingly, the dimensions of the filter box may be affected by the width and length of the vent fan, as well as the height of the vent fan cover above the surface of the mobile unit when the vent fan cover is fully open. The rigid outer cover may be configured to fit over or enclose the filter box without closing off any of the surface area of the filter box. In other words, the rigid outer cover may have a greater height than the filter box, a greater width than the filter box, and a greater length than the filter box. 
     In some embodiments, the air filters may have pleated filter media, such that the effective surface area of the filter is greater than the planar dimensions of the air filter. The large amount of surface area provided by the filter box may enable a large volume of air to pass through the filter box and into the interior space. Furthermore, the large amount of surface area may a greater capacity for trapping particulate or solids and may, therefore, reduce the frequency at which the filter box must be replaced. Optionally, one or more of the air filters could be replaced with a cardboard blank so long as the filter box has at least one air filter with sufficient surface area to support continuous operation of the vent fan. Operating the vent fan in a reverse direction causes an inward air flow direction through the filter box and vent fan before the air enters the interior space of the mobile unit. Accordingly, the interior space receives filtered air that is placed in a “positive pressure” condition so that the filtered air then exits the interior space through screened and unscreened openings. As a result, airborne matter outside the mobile unit is generally prevented from gaining entry into the interior space through any of the screened or unscreened openings. 
     In some embodiments, the air filtration system may have a rigid outer cover in the form of a rigid outer box that supports the use of a filter box. The filter box may include five rectangular air filters, each air filter having a frame extending around the rectangular perimeter of the air filter, and an amount of double-sided tape for securing the frames of adjacent air filters arranged as a filter box having four lateral sides formed with four of the air filters, a top formed with one of the air filters, and an open bottom. Each of the five rectangular air filters are preferably oriented for an air flow direction from outside the filter box to inside the filter box. The air filters may be a standard household type of air filter, such as an air filter having a pleated filter media secured in a cardboard frame. The air filters preferably have the same nominal dimensions, model number and/or product identifier such as a stock keeping unit (SKU). An air filter with a cardboard frame may further include a cardboard lattice extending across one or both faces of the filter media to help retain the filter media and to provide additional structural integrity to the cardboard frame. Still further, an air filter may include a metal or plastic mesh positioned on a downstream side of the filter media to provide further support for the filter media to oppose the forces associated with airflow passing through the filter media. In some embodiments, the foregoing five rectangular air filters and fasteners, such as double-sided tape, may be included in a kit that is separate from the kit that forms the rigid outer cover. 
     In some embodiments, each of the air filters in a filter box may have a carboard frame that is secured to a cardboard frame of an adjacent one of the air filters. For example, the cardboard frame of each of the air filters may have a flat surface that is positioned flush with a flat surface of the carboard frame of each adjacent air filter, wherein the carboard frames may be secured together with double-sided tape and/or other fasteners. 
     In some embodiments, the air filtration system may accommodate and/or include multi-stage filter media such as two or more nested filter boxes, where each inner filter box fits within an adjacent outer filter box. An air gap may be provided between each of the nested filter boxes, but the presence of an air fap is not required. In one example, a first (outer) filter box fits inside the rigid outer cover and causes all of the air flow caused by the vent fan to pass through the first filter box. A second (inner) filter box fits inside the first (outer) filter box such that all of the air flow through the first (outer) filter box will then pass through the second (inner) filter box. An optional third (middle) filter box may fit between the first (outer) filter box and the second (inner) filter box, such that all of the air flow between the first and second filter boxes will pass through the third (middle) filter box. Each of the nested filter boxes may be made with the same or different air filters, such as the same or different filter media type and/or rating. For example, three nested filter boxes may include an outer filter box having a first filter type and/or rating, a middle filter box having a second filter type and/or rating, and an inner filter box having a third filter type and/or rating. Specifically, each filter box may have a plurality of parameters independently selected from disposable filters, reusable (washable) filters, electrostatic filters, antiviral filter media, pleated or fibrous media, various particle removal dimensions or ratings (i.e., MERV, HEPA, etc.), and the like. In one option, the outer-most filter box may have a low MERV rating and each successive inner filter box may have a higher MERV rating than the immediately adjacent filter box on the outside of the filter box. Furthermore, each of the nested filter boxes will have different dimensions, and the nested filter boxes may be made with rectangular air filters that are either square or non-square. 
     In some embodiments, the foldable net may include an end tab that is foldably connected to a first sidewall panel of the four sidewall panels for forming a lap joint with an edge of a fourth sidewall panel of the four sidewall panels, wherein the first sidewall panel and the fourth sidewall panel are the furthest apart of the four sidewall panels in the net when laying flat. The lap joint may include the end tab being folded to a 90 degree angle with the first sidewall panel to lay flat against a face of the fourth sidewall panel, where a third set of fasteners may be included in a kit for securing the end tab and fourth sidewall panel in a face-to-face connection. If the end tab is to be connected to an inner face of the fourth sidewall panel, then the first sidewall panel may be shorter than the second, third and fourth sidewall panels in order to form a more rectangular rigid outer cover. Alternatively, if the end tab is to be connected to an outer face of the fourth sidewall panel, then the first sidewall panel may be longer than the second, third and fourth sidewall panels in order to form more rectangular rigid outer cover. In one option, the end tab may include a first plurality of holes and the fourth sidewall panel may include a second plurality of holes alignable with the first plurality of holes with the net folded into the form of the rigid outer cover. Some embodiments of the kit may further include a plurality of fasteners for securing the end tab and the fourth sidewall panel together. For example, the plurality of fasteners may be a plurality of pop rivets, where each pop rivet has a first head and a shaft that is extendable through one of the first plurality of holes and an aligned one of the second plurality of holes. The pop rivet has a distal end of the shaft that is deformable to form a second head, such that the pop rivet secures the end tab to the fourth sidewall panel. 
     In some embodiments, the opening in the at least one of the four sidewall panels may include a louver. A louver is a set of horizontal slats that are angled downwardly and outwardly to admit air, but to keep out rain. The angle of the slats may be adjustable but is preferably fixed. Optionally, the louver may be those of a standard home soffit vent. 
     In some embodiments, each of the four lower tabs is connected to a separate one of the four sidewall panels. For each of the four lower tabs, one of the plurality of grooves forms a fold line between the lower tab and the separate one of the four sidewall panels to which the lower tab is connected. The grooves that form the fold line between each lower tab and a sidewall panel are preferably formed on the same side of the net as the grooves that form the fold line between each of the four sidewall panels and the end tab. Accordingly, each of the four lower tabs may be inwardly foldable to form an inwardly directed tab for face-to-face coupling to the base plate. Alternatively, the grooves could be formed on the opposing side of the net, such that the lower tabs each fold outwardly. 
     In some embodiments, the elastomeric material is provided in an amount sufficient for sealing the base plate to an exterior surface of the mobile unit even if the base plate is planar and the exterior surface of the mobile unit is convex. The elastomeric material may be in the form of a gasket or may be a spreadable material. A spreadable elastomeric material may be spread with a putty knife and dispensed from a cartridge using a caulk gun. The seal between the base plate and the exterior surface of the mobile unit is preferably a water-tight seal. A non-limiting example of the elastomeric material includes silicone rubber. 
     In some embodiments, the air filtration system may further include an air filter that is downstream of the vent fan. For example, the downstream air filter may positioned over the outlet of the vent fan, which is on an opposing side of the vent fan from the air filter box. In one option, the downstream air filter may be disposed within the interior space of the mobile unit. The downstream air filter may be secured in a molded plastic secondary filter housing that is aligned with the opening in the mobile unit and is attached to the ceiling of the mobile unit or a portion of the vent fan housing that is secured to the roof of the mobile unit. An air filter element may be replaceably secured within the secondary filter housing. For example, the secondary filter housing may have a slot with port, or a cover or door that hinges downwardly from the ceiling, so that a dirty air filter may be quickly removed and a replacement air filter may be quickly installed. 
     In some embodiments, the air filter box may have a first filtering effectiveness level and the downstream air filter may have a second filtering effectiveness level. While the first and second filtering effectiveness levels may be the same, one preferred implementation include the second filtering effectiveness level being greater than the first filtering effectiveness level. In this preferred implementation, the air filter box may serve as a prefilter to remove large particulate materials, such as dust, fibers, pollen and bugs, and the downstream air filter may serve as a secondary filter to remove fine particulate materials, such as mold spores, auto emission particulates, etc. Furthermore, the louvered screen prevents rain, large bugs, rocks, leaves and the like from even reaching the air filter box. 
     One suitable identifier of a filtering effectiveness level may be assigned to an air filter under the “Minimum Efficiency Reporting Value” (MERV) standard. The MERV scale was developed by the American Sociated of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). Another suitable identifier of a filtering effectiveness level may be assigned to an air filter under the “High-Efficiency Particulate Air” (HEPA) standard. A numerical or alpha-numerical rating under either and/or both of these standards, or other similar standards, may be used for comparison of filtering effectiveness levels of various air filters. 
     In some embodiments, the air filtration system may beneficially reduce the amount of airborne particulates or solids being drawn into an interior space of a mobile unit. Accordingly, some embodiments of the air filtration system may provide air with fewer airborne particulates, such that the interior space may stay cleaner and the air may cause fewer allergies or other symptoms for an individual spending time within the interior space. Without limitation, airborne particulates such as dust, mold and pollen may cause allergies when inhaled into the respiratory system of an individual or when coming into contact with the eyes of the individual. The air filtration system of some embodiments may enable a mobile unit to be used by individuals that would be unable to use the mobile unit without such an air filtering system. Similarly, a mobile unit that is equipped with the air filtration system of some embodiments may be more valuable than a similar mobile unit that is not equipped with such an air filtering system. 
     Some embodiments provide a kit for assembling and installing an air filtration system for a mobile unit having a vent fan. The kit may include a foldable net including four sidewall panels of a rigid outer cover and four lower tabs, wherein the foldable net includes an aluminum composite panel having a plurality of grooves that establish fold lines for forming the rigid outer cover, and wherein at least one of the four sidewall panels includes an opening that forms an air inlet passageway. The kit may further include a top panel securable to each of the four sidewall panels and a base plate having a central opening that is positionable around a perimeter of the vent fan. The kit may still further include an elastomeric material that conforms to a surface of the mobile unit around the vent fan and the base plate, a first set of fasteners for securing the four lower tabs to the base plate and securing the base plate to the mobile unit, and a second set of fasteners for securing the top panel to the four sidewall panels. 
     In some embodiments, the kit may further include five rectangular air filters, each air filter having a frame extending around the rectangular perimeter of the air filter, and an amount of double-sided tape for securing the frames of adjacent air filters arranged as a filter box having four lateral sides formed with four of the air filters, a top formed with one of the air filters, and an open bottom. 
     In some embodiments, the air filtration kit may be contained in packaging for storage and/or shipment to an end user in a flat or collapsed condition for the end user to fold out into an expanded condition for assembly, installation and use in conjunction with a vent fan of a mobile unit. The foldable net may be packaged and/or shipped in a flat configuration, either a single-layer flat net or a multilayer partially folded flat configuration rather than an operative open box configuration. An end user may then fold either the single-layer flat net or the multilayer partially folded flat configuration to form some or all of the operative open box configuration. In a further alternative, the end tab may be pre-secured to one of the side panels to form the four lateral sidewalls of the air box, yet the four sidewalls may be collapsed to a partially folded condition having the thickness of 2-3 panels. Storing and/or shipping the net in a flat unfolded condition and/or a partially folded condition with the end tab secured or not secured to one of the side panels may significantly reduce the cost of storage and/or shipment of small quantities of the net. 
     Some embodiments may further include printed instructions for assembling the parts of the kit into a component of the air filter system and/or installation of the component, or a uniform resource locator (URL) for obtaining equivalent instructions via a wide area network (WAN) such as the Internet. Optionally, any kit may further include instructions, or a link to instructions, for purchasing any parts or components that are not included in the kit or repurchasing any of the parts or components that have become worn out or consumed through use. For example, the filter box will become full of debris over time and will need to be periodically replaced, such that the kit may include instructions or links for repurchasing the air filters necessary to assemble a replacement filter box. 
     A mobile unit dealer or an individual owner of a mobile unit may assemble and install the air filtration system on the mobile unit. Some embodiments may require very few tools, such as a tool to use the fasteners. If a kit has fasteners in the form of rivets, then a rivet gun may be required. If a kit has fasteners in the form of screws, then a screwdriver may be required. If a kit has fasteners in the form of nuts and bolts, then a wrench may be required. Other types of fasteners may be used in assembling an air filtration system and may be included in a kit, such that a tool appropriate to the fastener may be required. Still further, some embodiments of a kit may include one or more fastener types and may or may not include the one or more tool necessary to utilize the one or more fastener types in the assembly and installation of the one or more components of the kit. 
     Some embodiments of the air filtration kit may include any component, feature or aspect of any one of the air filtration system embodiments described herein and/or any air filtration system embodiment may include any component, feature or aspect of any one of the air filtration kit embodiments described herein. 
       FIG. 1  is a schematic diagram illustrating an interior space  12  of a mobile unit  10  having a vent fan  20  on the roof  14  of the mobile unit. The vent fan  20  typically draws air from the interior space  12  through an opening  16 , before exhausting the air into the surrounding environment. Therefore, the vent fan  20  may produce a slight negative pressure in the interior space  12  which draws air into the interior space  12  through any available opening in the mobile unit  10 , such as through gaps around doors, windows, and penetrations for electrical, structural and/or plumbing systems. As a result of this air being drawn into the interior space  12 , airborne solids in the air surrounding the mobile unit  10  are also drawn into the interior space  12 . The air within the interior space  12  may therefore contain a variety of solids that can cause allergies, breathing difficulties, and solids deposition through the interior space  12 . 
       FIG. 2  is a perspective view of the vent fan  20  as viewed from above the mobile unit. The vent fan may include a housing  22  having a flange  24  with holes  26 . A set of screws may be run though the holes  26  and into the roof  14  to secure the vent fan  20  in place. The housing  22  has a central opening  28  where a fan motor  30  and fan blades  32  are supported and used. While the fan blades  32  are supported by a hub  34  that is coupled to the fan motor  30 . The fan motor  30  is secured by a set of mounting arms  36  (four shown) that extend inwardly from the housing  22  and support the fan motor  30  within the central opening  28  without blocking air flow through the central opening  28 . The vent fan  20  also include a vent fan cover  38  that is coupled to the housing  22  by hinges  39 . 
     The vent fan  20  further includes a controller  40  that receives electrical power from an electrical power system  19  (see  FIG. 3 ) of the mobile unit. Accordingly, the controller  40  may control the amount of power delivered to the fan motor  30  via a pair of wires  42 . The controller may also control the operation of an actuator motor  44  to cause a lever arm  46  to open the vent fan cover  38  when the fan motor  30  is to be used and to close the vent fan cover  38  under various other conditions. In one example, the vent fan  20  may also include a rain sensor  48  that is wired to the controller  40  (wiring not shown), such that the controller  40  may cause the actuator motor  44  to close the vent fan cover  38  in response to the rain sensor  48  detecting the presence of rain. 
       FIG. 3  is a block diagram of electronic components of the vent fan  20  according to some embodiments. The vent fan  20  includes the vent fan controller  40  that controls the operation of the fan motor  30 , the actuator motor  44 , and a wireless transceiver  50 . The vent fan controller  40  receives electrical current from the electrical power system  19  of the mobile unit and may use that electrical current for its own operation as well as supplying electrical power to the fan motor  30 , the actuator motor  44 , and a wireless transceiver  50 . In addition to electrical power, the vent fan controller  40  may also provide control signals and/or receive control signals from the fan motor  30 , the actuator motor  44 , and the wireless transceiver  50 . An on/off switch  52  may be used to turn on the vent fan controller  40 . Furthermore, a rain sensor  48  may provide a signal to the vent fan controller  40  in response to detecting rain. For example, the vent fan controller  40  may turn off the fan motor  30  and cause the actuator motor  44  to close the vent fan cover. Still further, a temperature sensor  56  may be provided, either as part of the vent fan  20  or merely in communication with the vent fan controller  40 , to enable some form of temperature control. For example, the vent fan controller  40  may operate the actuator motor  44  to open the vent fan cover and operate the fan motor  30  to circulate air through the interior space of the mobile unit in response to receiving a signal from the temperature sensor  56  indicating a measured air temperature. 
     The wireless transceiver  50  of the vent fan  20  may communicate with a remote-control unit  54 , which allows the user to interface with the vent fan controller  40 . The remote-control unit  54  may include various buttons that are each associated with a unique function of the vent fan  20 , such as a fan speed (i.e., low, medium, high), a vent fan cover position (i.e., open/close), and the like. However, the remote-control unit  54  may include various multipurpose buttons, such as a keypad (i.e., numbers 0-9), for entering numerical codes that are each associated with a particular function of the vent fan  20 . For example, the remote control unit  54  may be used to input a temperature setpoint to the vent fan controller  40 , and the vent fan controller  40  may operate the actuator motor  44  to open the vent fan cover and operate the fan motor  30  to circulate air through the interior space of the mobile unit in response to receiving a measured air temperature signal from the temperature sensor  56  indicating that an air temperature that is greater than the temperature setpoint. 
       FIG. 4A  is an exploded schematic side view of an air filtration system  60  that is compatible with the vent fan  20  of a mobile unit. For example, the vent fan  20  may be substantially as shown in  FIGS. 1-3 , including the housing  22  having the flange  24  secured to the roof  14  in alignment with the opening  16 . A set of screws  25  are shown to extend through the flange  24  and into the roof  14  of the mobile unit at any number of points around a perimeter of the flange. The housing  22  supports the mounting arms  36  that position the motor  30  in alignment with the opening  16 . A vent fan cover  38  is also coupled to the housing  22  by a hinge  39 , such that the vent fan cover  38  may be hingeably moved between a closed position relative to the housing  22  (shown in solid lines) and an open position relative to the housing  22  (shown in dashed lines) by an actuator motor  44  (see  FIGS. 2 and 3 ). In order to use the vent fan  20  in conjunction with the air filtration system  60 , the vent fan cover  38  should be in the open position and the fan motor  30  should be energized to rotate in a direction that causes the fan blades  32  to draw air from the surrounding environment outside the mobile unit (i.e., above the roof  14 ) through the opening  16  into the interior space (i.e., below the roof  14 ). 
     The air filtration system  60  includes a rigid outer cover  70  and a filter box  80 . The rigid outer cover  70  includes rigid sidewall panels  72  (two opposing sidewall panels shown), a rigid top panel  71  and an open bottom. Each of the sidewall panels  72  have a bottom edge, such as a lower tab  74  that is secureable to the roof  14  of the mobile unit around a perimeter of the vent fan  20  with the rigid top panel  71  extending over the vent fan  20 . For example, a set of screws  75  may be run through one of the lower tabs  74 , through a base plate  90 , through an elastomeric material  62  such as silicone, and into the roof  14 . At least one of four rigid sidewall panels  72  will include an air inlet passageway  73  that allows air to be drawn into the air filtration system from the surrounding environment. Optionally, the original screws  25  may be removed and the screws  75  may be used secured the flange  24 , base plate  90  and lower tabs  74  to the roof  14 . 
     The filter box  80  includes rectangular air filters  82  (three air filters shown) secured together in the shape of a box having four filter sidewalls and a filter top. The filter box  80  has an open bottom so that four filter sidewalls are positionable around the perimeter of the vent fan  20  with the filter top extending over the vent fan, wherein the filter box  80  is configured to avoid interfering with opening and closing of the vent fan cover  38  that is coupled to the vent fan  20 . The rigid outer cover can be opened (i.e., the top panel  71  may be opened or removed) for receiving the filter box  80  and can be securely closed to retain the filter box in an operable position. The rigid outer cover  70  is configured to extend over and around the filter box  80  leaving an air flow space surrounding the filter box with the filter box in the operable position, with the rigid outer cover secured to the mobile unit, and with the rigid outer cover securely closed. 
       FIG. 4B  is a partially exploded schematic side view of the air filtration system  60  of  FIG. 4A  with the rigid outer cover  70  secured to the roof  14  of the mobile unit. A set of screws  75  are made to penetrate the lower tabs  74 , the base plate  90 , the flange  24 , and the elastomeric material  62  before engaging the roof  14 . In one option, the screws  75  may be made to self-countersink into the lower tabs  74  in order to leave an even top surface of the lower tabs  74  upon which the filter box  80  may sit. An even top surface may improve the stability of the filter box  80  and reduce air gaps that might allow air to bypass the filter box  80 . 
       FIG. 4C  is a schematic side view of the air filtration system  60  of  FIG. 4B  in an operative condition. The filter box  80  has been positioned inside the rigid outer cover  70  and the top panel  71  has been secured to the sidewall panels of the rigid outer cover  70 . In the embodiment shown, the top panel  71  is secured to upper tabs  79  of each sidewall panel of rigid outer cover  70  using a set of screws  77  around the perimeter of the top panel  71 . Alternatively, the top panel  71  may be secured to the sidewall panels of the rigid outer cover  70  using an angle bar as shown in  FIG. 9B . Optionally, such an angle bar may be used in each of the four corners of the rigid outer cover  70  and at a midpoint of each of the four lateral sidewall panels. 
     In reference to  FIG. 4C , air is shown (illustrated as wavy lines) passing through the air inlet passageways  73  of the rigid outer cover  70 , through any or all of the filters of the filter box  80 , then through the vent fan  20  to the interior space of mobile unit below the roof  14 . Accordingly, all of the air entering the interior space has been filtered to remove solids. 
     It should be noted that the vent fan cover  38  is in the open position to allow air to pass through the vent fan  20 , yet the filter box  80  does not interfere with the movement of the vent fan cover  38  between the open position and the closed position. In addition, the filter box  80  covers the vent fan  20  such that substantially all of the air drawn through the vent fan  20  into the interior space must pass through the filter media of one of the air filters that form the filter box  80 . 
       FIG. 5A  is a schematic plan view of a net  100  for forming four lateral sidewall panels of the rigid outer cover  70  (see  FIG. 5B ). An inside face of the net  100  is shown, where a plurality of V-shaped grooves are illustrated as dashed lines. A cross-sectional profile of a V-shaped groove is shown in  FIG. 8 . Such V-shaped grooves each extend into a major portion of the thickness of the sheet-like material, such as an aluminum core panel, so that the sheet-like material will bend while remaining intact. Accordingly, each V-shaped groove (along the dashed lines) follows a straight line referred to as a “fold line.” 
     The net  100  includes four sidewall panels, including a first sidewall panel  102  having an air inlet passageway  73 , a second sidewall panel  104  having an air inlet passageway  73 , a third sidewall panel  106  without an air inlet passageway, and a fourth sidewall panel  108  having an air inlet passageway  73 . Each of the sidewall panels  102 ,  104 ,  106 ,  108  is delineated from each adjacent sidewall panel by a fold line therebetween. Specifically, the first sidewall panel  102  is foldably connected to the second sidewall panel  104  along a first (vertical) fold line  103 , the second sidewall panel  104  is foldably connected to the third sidewall panel  106  along a second (vertical) fold line  105 , and the third sidewall panel  106  is foldably connected to the fourth sidewall panel  108  along a third (vertical) fold line  107 . Still further, the net  100  may include an end tab  110  that is foldably connected to the fourth sidewall panel  108  by a fourth (vertical) fold line  109 . The end tab  110  and the first sidewall panel  102  have a set of three holes that are alignable for receiving fasteners, such as pop rivets, to secure the end tab  110  to the first sidewall panel  102 . 
     Each of the sidewall panels  102 ,  104 ,  106 ,  108  may have one or more tabs extending therefrom, such as an upper tab  102 A,  104 A,  106 A,  108 A, respectively, and/or a lower tab  102 B,  104 B,  106 B,  108 B, respectively. Each tab is foldably connected to a respective one of the sidewall panels  102 ,  104 ,  106 ,  108  along a (horizontal) fold line therebetween. 
       FIG. 5B  is an exploded perspective view of the components of the rigid outer cover  70  and a base plate  90 . The rigid outer cover  70  includes both the top panel  71  and the lateral sidewall panels form by folding the net  100  of  FIG. 5A . Each of the fold lines (both vertical and horizontal) shown in  FIG. 5A  have been folded to arrive at the configuration of the lateral sidewalls shown in  FIG. 5B . Accordingly, the net  100  may be described as being in an “expanded configuration” in  FIG. 5B . 
     The end tab  110  is in face-to-face contact with an edge of the first sidewall panel  102  with the holes in the end tab  110  (shown in  FIG. 5A ) aligned with the holes in the first sidewall panel  102 . The fourth sidewall panel  108  may be shorter that the other sidewall panels  102 ,  104 ,  106  by the thickness of the panel material, so that the end tab  110  will lay flat against the first sidewall panel  102  while allowing the four sidewall panels to form a rectangle (as viewed from the top). 
     A pop rivet  101  has been inserted into the aligned holes and deformed to secure the end tab  110  to the first sidewall panel  102  in the configuration shown. Furthermore, the upper tabs  102 A,  104 A,  106 A,  108 A have been folded to form upwardly facing surfaces that may be used to secure the top panel  71 . Similarly, the lower tabs  102 B,  104 B,  106 B,  108 B having folded to form downwardly facing surfaces that may be used to secure the sidewall panels  102 ,  104 ,  106 ,  108  to the base plate  90 . 
     One of the sidewall panels that is without an air inlet passageway, such as sidewall panel  106 , may be oriented to face the front of the mobile unit so that the filter box inside the rigid outer cover  70  is not subjected to intense wind and rain caused by driving of the mobile unit. 
     The top panel  71  may be a simple flat rectangle of the same dimensions as the outer perimeter of the expanded net of sidewall panels. The base plate may have outer dimensions that are greater than those of the expanded net of sidewall panels but has a central opening that fits around at least a portion of the vent fan housing  22  (see  FIG. 2 ). While the central opening  92  is illustrated with sharp internal corners, it may be preferable to provide rounded internal corners to match the shape and dimensions of the upright portion of the vent fan housing  22  or the vent fan flange  24 . 
       FIG. 6A  is a perspective view of a pleated air filter  82  suitable for use in forming a filter box. The air filter  82  includes filter media  84  that may be pleated to increase the surface area of the filter media. A cardboard frame  86  is formed around the edges of filter media  84  to provide some structural support for the filter media. Furthermore, a cardboard lattice  88  may be formed integrally with the cardboard frame  86  and may extend across one or both faces (front and back) of the filter media  84  to help retain the filter media and to provide additional structural integrity to the cardboard frame  86 . Still further, the air filter may include a metal or plastic mesh (not shown) positioned on a downstream side of the filter media  84  to provide further support for the filter media to oppose the forces associated with airflow passing through the filter media. The air filters  82  are rectangular, but may be square to facilitate formation of the filter box while avoiding the need for multiple unique air filter sizes. One non-limiting example of a suitable air filter has a width of 15 inches, a height of 15 inches, and a depth of 1 inch. The minimum efficiency reporting value (MERV) for a given air filter may be user-selected for the user&#39;s particular needs and does not require any change to the disclosed embodiments. 
       FIG. 6B  is a plan view (top view) of a filter box  80  including five of the pleated air filters  82 . An upper edge of the four lateral sidewall filters  82  are arranged in a rectangular pattern. In the specific option shown, the four lateral sidewall filters  82  are arranged so that their upper edges form a windmill pattern. The windmill pattern includes the rectangular upper edges of the air filter frames  86  forming a perimeter around a central rectangular opening, wherein a side edge of a frame of one air filter  82  abuts an end edge of a frame of an adjacent air filter  82 . In the windmill pattern, the resulting filter box  80  may use air filter of the same size to form a rectangular pattern having the same dimensions on each of the four sides of the rectangle. Furthermore, a fifth rectangle air filter  82  (shown in a dashed outline) of the same or other appropriate size may be positioned flat on top of the four lateral sidewall panels  82 , for example such that the fifth rectangular air filter  82  will overlap about half the width of the upper edge of each of the four lateral air filters  82 . Double-sided tape may be applied between adjacent edges of the air filters, preferably along the entire length of the interface between the air filters. A top edge of four strips of double-sided tape is shown at the four points  84  and four additional strips  86  of double-sided tape are used to secure the frame of the fifth air filter  82 . 
       FIG. 6C  is a perspective view of the filter box  80  of  FIG. 6B . While this view does not show each of the five air filters  82 , the filter box  80  should be considered to be symmetrical. Accordingly, a reverse view of the filter box  80  would look the same as the view shown. The bottom of the filter box is open (i.e., is not closed by an air filter). The air filters  82 , cardboard frames  86  and cardboard lattices  88  are labeled for consistency. 
       FIG. 7A  is a perspective view of an operational air filtration system  60  including a louvered screen  120  extending across an air inlet passageway  73  in the rigid outer cover  70 . A similar louvered screen  120  may extend across or cover each air inlet passageway  73  that is provided in any of the lateral sidewall panels  102 ,  104 ,  108  (See  FIG. 5B ). The sidewall panels  104 ,  106 , air inlet opening  73 , and fold lines  103 ,  105 ,  107  are labeled for consistency with  FIG. 5B . 
     A louver is a set of horizontal slats that are angled to admit air, but to keep out rain. The angle of the slats may be adjustable or fixed. The rigid outer cover  70  may have an air inlet passageway  73  in one or more of the sidewall panels, where optional screens, grills or other air passageway covers may be used to keep out large bugs, provide structural support to the rigid outer cover  70  and/or shelter the filter box  80  from rain or other physical damage. One example of such a louvered screen  120  is a standard home soffit vent cover. 
       FIG. 7B  is a schematic cross-sectional side view of the operational air filtration system  60  of  FIG. 7A , where like reference numbers are used to label like parts. Many of the parts are labeled for context. This cross-sectional side view illustrates the downwardly angled slats  122  (nine shown) of the louvered screen  120  having openings therebetween that allow the entry of air into the air filtration system  60 . The louvered screen  120  may be secured and/or sealed to an inside face of the sidewall panels  104 ,  108 . 
       FIG. 7B  also illustrates an air gap or space  124  that is provided between the rigid outer cover  70  and the filter box  80  disposed within the rigid outer cover  70 . This air gap  124  allow air to be drawn into the rigid outer cover  70  through the air inlet passageways  73  of the sidewall panels, yet the air may be distributed over the full area of the filter media of the filter box  80 . 
       FIG. 8  is a schematic side view of an aluminum composite panel (ACP)  130 . The aluminum composite panel  130  has a first aluminum sheet  132 , a second aluminum sheet  134 , and a polymeric core  136  disposed between the first and second aluminum sheets  132 ,  134  and bonded to the first and second aluminum sheets  132 ,  134  as well. In one option, the aluminum sheets having a thickness of about 3 millimeters, although the thickness may have any desired thickness. Non-limiting examples of the polymeric core include polyethylene and polyurethane. Furthermore, the polymeric core may be a dense solid, a solid foam, or a honeycomb structure oriented between the two aluminum sheets to provide a high crush strength. 
     A V-shaped groove  140  has been cut into one side of the aluminum composite panel  130  such that a portion of the first aluminum sheet  132  and the polymeric core  136  have been removed. However, care should be taken to avoid cutting into the second aluminum sheet  134  and it may be preferred to leave just a small amount of the polymeric core as a margin of safety. The V-shaped groove  140  may form a first exposed face  142  and a second exposed face  144 . A linear V-shaped groove  140  (in and out of the page) will form a fold line, such that the aluminum composite panel  130  may be folded at point  146  (i.e., an axis in and out of the page at point  146 ). The illustrated V-shaped groove  140  forms a right angle θ. A second V-shaped groove  140  was formed to the right, but is illustrated after having been folded about point  146  until the first and second exposed faces  142 ,  144 , are in contact or flush. Such a V-shaped groove  140  may be used to form each of the fold lines in  FIG. 5A  or each of the fold lines in other embodiments. 
       FIG. 9A  is a schematic side view of an upper edge of the sidewall panel  108  and a top panel  71  secured together with a screw  77 . The upper tab  108 A is folded over to form an upwardly-facing shoulder upon which the top panel  71  can be supported and secured. An optional tape  148  may be applied over the external interface between the top panel  71  and the sidewall panel  108  in order to weatherize against water intrusion into the rigid outer cover. 
       FIG. 9B  is a schematic side view of an upper edge of the sidewall panel  108  and a top panel  71  secured together with an L-shaped bracket  150  and screws  77 . The L-shaped bracket  150  may initially be secured to the sidewall panel  108  with screws  77  (only one shown) so that a first leg of the bracket  150  is held against the inside surface of the sidewall panel  108  along the upper edge of the sidewall panel  108 . As a result, the second leg of the bracket  150  is positioned horizontally and flush with the upper edge of the sidewall panel  108 . The top panel  71  may then be similarly connected to the second leg of the bracket  150  with a screw  77 . 
       FIG. 9C  is a plan view of the rigid outer cover  70  illustrating placement of a plurality of the L-shaped brackets  150  shown in  FIG. 9B  according to some embodiments. In addition to the L-shaped brackets  150 ,  FIG. 9C  illustrates the relative position of the base plate  90  extending under the rigid outer cover  70  and the filter box  80  (illustrated in dashed lines). The rigid outer cover  70  includes the four lateral sidewall panels  102 ,  104 ,  106 ,  108 . A plurality of the L-shaped brackets  150  are secured to the inside of the four lateral sidewall panels along an upper edge of each lateral sidewall panel consistent with  FIG. 9B . In the embodiment shown, there is one L-shaped bracket  150  located in each of the four corners of the rigid outer cover  70  and there is one L-shaped bracket  150  located at a mid-point along each of the four lateral sidewall panels  102 ,  104 ,  106 ,  108  of the rigid outer cover  70 . Accordingly, a top panel, such as the top panel  71  shown in  FIG. 7A , may be secured to the four lateral sidewall panels  102 ,  104 ,  106 ,  108  using eight (8) screws run through the top panel  71  and into the threaded hole in each L-shaped bracket  150 . It should be recognized that L-shaped brackets  150  may be used as a substitute for the upper tabs  102 A,  104 A,  106 A,  108 A, lower tabs  102 B,  104 B,  106 B,  108 B, and end tab  110  show in  FIG. 5B .  FIGS. 9A and 9B  illustrate how such a tab (i.e., tab  108 A in  FIG. 9A ) may be substituted with an L-shaped bracket (i.e., L-shaped bracket  150  in  FIG. 9B ). 
       FIG. 10A  is a perspective view of a molded plastic embodiment of a rigid outer cover  160 . The rigid outer cover  160  may have a variety of shapes and contours, but is shown with generally upright sides  162 ,  164  and a sloped front  166  having no air inlet passageways, and a rear face  168  that forms a louver  170 . The louver  170  includes a set of ten slats  172  arranged in series across a major portion of the rear face. The configuration and purpose of the louver  170  is similar to the louver  120  in  FIG. 7B , except that the louver  170  may be integrally formed as part of the molded plastic rigid outer cover  160 . 
       FIG. 10B  is a schematic side view of an air filtration system  200  in which the rigid outer cover  160  of  FIG. 10A  is connected to the base plate  90  and a secondary filter housing  190  on the outlet side of the vent fan  30 . A hinge  174  secures a front portion of the cover  160  to the base plate  180  and a latch  176 , such as a cotter pin coupling, secures a back portion of the cover  160 . Accordingly, the rigid outer cover  160  extends over the filter box  80  and protects the filter box  80  from rain and wind. The sloped front  166  will direct air and water up and over the cover  160  as a mobile unit with roof  14  travels forward down a roadway. 
     The rear face  168  include the louver  170  having upwardly and inwardly (or downwardly and outwardly) angled slats  172  that prevent most rainwater from getting inside the cover  160 . However, the louver  170  allow air to pass between the angled slats  172  and into the space under the cover  160 . When the vent fan  30  is being run to draw air from the surrounding environment, the outside air passed through the louver  170  and any large bugs, pieces of trash or leaves are prevented from entering under the cover  160 . After passing through the louver  170 , the outside air is filtered through the air filter  80  and drawn into the vent fan  30 . 
     In the embodiment shown, the air filtration system  200  further includes a secondary filter housing  190  on the outlet side of the vent fan  30 . The secondary filter housing  190  may be secured to the ceiling  191  of the mobile unit with a set of screws  192  (only one shown) around the perimeter of the housing  190 . Furthermore, the housing  190  may have a port  194  for inserting and removing a secondary air filter  196 . In a fully inserted and operable position (as shown), the secondary air filter  196  may be supported between an upper screen  197  and a lower screen or grill  98  that allows air to pass therethrough. 
     The filter box  80  may be replaced by releasing the latch  176  and rotating the rigid outer cover  160  upward and forward about the hinge  174 . After removing the old filter box and inserting a new filter box  80 , the rigid outer cover  160  may be rotated back and downward about the hinge  174  and secured in a closed position with the latch  176 . Even with the rigid outer cover  160  closed as shown, this embodiment provides an air flow gap between the cover  160  and the filter box  80  so that air may circulate around all sides of the filter box  80  and pass through the filter media in any of the five air filters. 
       FIG. 11  is a schematic cross-sectional side view of an operational air filtration system  210  having three nested filter boxes  220 ,  230 ,  240 . The outer filter box  220 , the middle filter box  230 , and the inner filter box  240  are nested, one inside the other so that air flows sequentially through the three filter boxes from the outside to the inside. In one non-limiting example, the outer filter box may be made with MERV  4  air filters, the middle filter box may be made with MERV  10  air filters, and the inner filter box may be made with HEPA air filters. As shown, the rigid outer cover or cover  70  has sufficient dimensions to accommodate the three filter boxes  220 ,  230 ,  240  therein. Optionally, the filter boxes have sufficient dimensions to avoid interfering with the operation of the vent fan  30  and the vent fan cover  38 . In such a configuration, it is unnecessary to remove and/or disable the vent fan cover, although this is an option. In a further option, any one or more of the filter boxes may be made in a manner described in reference to  FIGS. 6A-6C , although the individual filters  82  may differ in size along the sides and top to form any shape and/or size of rectangular filter box  220 ,  230 ,  240 . 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the claims. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components and/or groups, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the embodiment. 
     The corresponding structures, materials, acts, and equivalents of all means or steps plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. Embodiments have been presented for purposes of illustration and description, but it is not intended to be exhaustive or limited to the embodiments in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art after reading this disclosure. The disclosed embodiments were chosen and described as non-limiting examples to enable others of ordinary skill in the art to understand these embodiments and other embodiments involving modifications suited to a particular implementation.