Abstract:
A planar omni-directional inlet assembly is provided for installing on a device, such as a dry filter unit, that draws a fluid medium, such as ambient air, to collect particulate matter suspended in the fluid medium onto a filter. The inlet assembly includes an annular platform removably positionable on the device; an annular nozzle to direct the air from the platform; an annular impactor disposed downstream of the nozzle; a housing that disposes the filter downstream of the impactor; and an exit for passing the air from the filter to the device. The impactor presents a flow obstacle for a portion of the particulate matter. The housing directs the air through the filter. The housing includes a base with a first cavity and a lid with a second cavity such that the filter is disposable between the first and second cavities.

Description:
STATEMENT OF GOVERNMENT INTEREST 
   The invention described herein was made in the performance of official duties by one or more employees of the Department of the Navy and/or under Contract Order N00178-05-M-3100, and thus, the invention herein may be manufactured, used or licensed by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 

   BACKGROUND 
   The invention relates generally to the sampling of moving fluids such as moving airflows, and more particularly to a sampling system that extracts samples of a moving fluid passing thereover. The moving fluid as a medium may represent a variety of gases. Matter to be sampled may represent solid particles or liquid droplets or other material physically or chemically distinguishable from the suspension medium. 
   As an example, airflows may be sampled in support of a variety of flow monitoring applications for determining aerosol content, or conforming microbiological presence, using standard microbiological techniques. The Dry Filter Unit (DFU) 1000 represents an exemplary air sampling device for collecting, for example, bio-particulates from suspension in ambient air for analysis. The DFU-1000 provides a pressure differential to draw air within, thereby enabling particles to be collected. Note that “ambient air” corresponds in this document to nominal temperature of 20° C. (degrees Celsius) and pressure of 101 kPa (kilopascals). 
   Particles smaller than a specific size may be categorized under an Environmental Protection Agency standard as specified under Title 40 of the Code of Federal Regulations (CFR). For example, particle measurement PM-10 under 40 CFR §50.6(c) corresponds to particles having effective diameters no larger than 10 μm (microns). Similarly, PM2.5 under 40 CFR §50.7(a)(1) corresponds to particles no larger than 2.5 μm.  FIG. 1A  shows an exemplary isometric view of the DFU-1000. 
   The DFU-1000 houses a pump/blower and is configured in a one-person portable carrying case for 40,000 hour-life continuous duty. The DFU-1000 may use commercially available power at 120/240 V AC  (volts) from alternating current at 50/60 Hz (hertz) or else direct current sources adaptable using an optional auxiliary power pack of 24 V DC . The DFU-1000 has dimensions of 15 in (inches) high, 13 in wide and deep, a mass of 42 lb m  (pounds-mass). The power requirements are 5.6 A (amps) at operation and 17 A starting at 120 V AC . 
   The DFU-1000 may be conventionally equipped with an open-top tandem-chamber inlet assembly  100  shown in  FIG. 1B  in isometric view as an exploded diagram. Air flows over a top-facing orifice  110  into which particulate matter descends entrained in an air stream at low velocity to an entrance chamber  120 . The air stream passes from the entrance chamber  120  through a forked pair of twin filter chambers  130  and rejoins in an exit chamber  140  to enter a top port of the DFU-1000, which draws in the air. 
   Each of the twin chambers  130  houses a cylindrical filter holder  150  that secures a polyester mesh filter  155  having a diameter of approximately 1.5 in. Each chamber  130  incorporates a foam seal  135  to avoid pressure drop losses between the inlet  110  and the exit  140 . The filter  155  traps particles larger than those that pass through its mesh. The resulting collection from the filter  155  may include particles of interest for further analysis as well as others larger than desired but collaterally entrapped nonetheless. 
   The holder  150  may comprise two halves  160 ,  165  pivotably connected to a molded hinge  170  and securable together when closed by a latch  175 . The holder  150  may be opened by disconnecting the latch  175  to remove and replace the filter  155 . Contents collected on the removed filter may then be analyzed at a separate facility. When the holder  150  is open, the filter  155  may be inserted into recesses  180 , one of which being equipped with baffles that prevent the filter  155  from being dislodged by the pressure differential produced by the DFU-1000. 
   Artisans of ordinary skill will recognize that the air intake described for the DFU-1000 represents an example application, and will recognize that other devices for drawing or otherwise moving fluid medium to enable particulate matter collection may be contemplated, accordingly. 
   SUMMARY 
   Practical limitations of the conventional inlet assembly include, but not limited to, reduced effectiveness in capturing particles within the target range with increasing wind speed (from interference with the pressure differential due to aspiration), as well as equipment degradation from operational handling, precipitation or other environmental conditions, such as dust or sea-spray. Moreover, the conventional inlet cannot inherently filter out particles larger than those of interest (e.g., dust). Such larger particles deposit onto the filter, which complicate subsequent analysis of the particulate matter. For example, biological sampling may produce erroneous results from introduction of organisms (e.g., small insects). 
   Various exemplary embodiments provide a planar omni-directional inlet assembly (within a horizontal airflow plane) a planar omni-directional inlet assembly for installing on a device that draws a fluid medium to collect particulate matter suspended in the fluid medium onto a filter. In particular, the inlet assembly includes a platform forming an annular inlet to receive the fluid medium positionable on the device; an annular nozzle to direct the fluid medium from the platform; an annular impactor disposed downstream of the annular nozzle; a housing that disposes the filter downstream of the impactor; and an exit for passing the fluid medium from the filter to the device, the exit being disposed downstream of the housing. The impactor presents a flow obstacle for a portion of the particulate matter. The housing directs the fluid medium through the filter and is removably disposable on the platform. 
   In a preferred embodiment, the inlet assembly operates wherein the fluid medium is ambient air and the device is a dry filter unit (DFU). Additional embodiments provide the housing as a filter holder for securing a particulate filter in an inlet assembly for installing on a DFU that draws in ambient air to collect its entrained particulate matter on the filter. In particular, the filter holder includes a base; a lid; and a hinge. The base has a first cavity that communicates with the DFU and is removably disposable on the inlet assembly along a sealable surface of the base. The lid has a second cavity that communicates with the inlet assembly and is disposable against the base opposite the sealable surface in a closed position. The filter is disposable between the first and second cavities. The hinge pivotably connects the base and the lid to enable an edge of the lid opposite the hinge to swing away from the base in an open position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and various other features and aspects of various exemplary embodiments will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which like or similar numbers are used throughout, and in which: 
       FIG. 1A  is an isometric view of a Dry Filter Unit (DFU-1000) equipped with a conventional inlet assembly; 
       FIG. 1B  is an isometric exploded-diagram view of the conventional inlet assembly; 
       FIG. 2  is an isometric view of the DFU-1000 with a planar omni-directional inlet; 
       FIGS. 3A and 3B  are plan and elevation diagrams of the planar omni-directional inlet assembly; 
       FIGS. 4A and 4B  are isometric detail views of the planar omni-directional inlet assembly; and 
       FIG. 5  is an elevation view diagram of particle impaction in conjunction with an inlet flowstream. 
   

   DETAILED DESCRIPTION 
   Service personnel periodically remove the filters in an inlet assembly to analyze the collected contents trapped thereon. Concurrently, the filters are replaced to ensure continued proper operation of the associated dry filter unit. Such duties may be performed under minimal illumination and/or while clad in cumbersome environmental protection gear. These conditions can inhibit expeditious filter replacement due to consequent sensory and/or manipulatory degradation. Various exemplary embodiments replace the conventional inlet assembly with a retrofitted planar omni-directional inlet to improve particle collection performance, enhance maintenance and minimize alteration to existing systems. Such systems may include any appropriate device suitable for drawing a fluid medium therethrough to enable collection of particles entrained therein. The DFU-1000 represents but a single example of such a device. 
     FIG. 2  shows an exemplary embodiment of an inlet unit  200  for the DFU-1000. In the configuration described herein, the inlet unit  200  may include opposing upper and lower circular plates  210 ,  220 . In  FIG. 2 , the plates  210 ,  220  are disposed parallel to each other in a closed position for operation with the DFU-1000. The plates  210 ,  220  facing each other may preferably be flat and circular in plan-form (i.e., from top view), although other shapes may be used without departing from the spirit of the invention. A mounting flange  230  may be attached to the lower plate  220 . The plates  210 ,  220  may be connected to each other by a pair of inlet hinges  240  and an inlet latch  250 . The flange  230  may connect to a tube  260  leading to the top port of the DFU-1000. 
     FIG. 3A  illustrates the exemplary embodiment of the inlet unit  200  as a plan (top) view from above. Similarly,  FIG. 3B  provides a complimentary elevation (side) view of the inlet unit  200  along section A-A. The plates  210 ,  220  may be dimensionally characterized by a plate diameter D P  and a plate thickness W P . An annulus or annular nozzle  300 , axisymmetric about an axis  305 , includes upper and lower portions  310 ,  320 . The annular opening  325  formed by the nozzle  300  may be characterized by a nozzle throat width W N  for the separation distance between the portions  310 ,  320 . 
   Air passing along the inlet unit  200  may flow along the plates  210 ,  220  and directed along the sloping ramps of the portions  310 ,  320  through the nozzle opening  325 . The upper and lower portions  310 ,  320  may be secured to their corresponding upper and lower plates  210 ,  220  by threaded screws or bolts, for example. An annular concentric impact ring  330  may be disposed inward and downstream of the nozzle  300  relative to an oncoming flow of the passing air. 
   The impact ring  330  may present a flat cross-section  335  as shown in  FIG. 3B . Alternatively, the cross-section may alternatively present concave or convex surfaces to the oncoming flow. The impact ring  330  presents an obstacle to particles (particulate matter) suspended in the passing air. Depending on the flow velocity through the nozzle  300  and the characteristic dimensions of the inlet unit  200 , large particles are more likely to strike the impact ring  330  than small particles as a consequence of differences in inertia and drag behavior. 
   Particles that avoid striking the impact ring  330  may impact upon the interior surfaces of one of the plates  210 ,  220  as the flow bends around the impaction plate  330  and flows between the supports  340  and  345 . The plates  210 ,  220  represent secondary impaction surfaces that may further restrict particles to those within a targeted size range to be collected on the filter  155 . 
   The impact ring  330  may be positioned relative to the plates  210 ,  220  by angularly distributed upper and lower legs  340 ,  345 . The cylindrical dimensions of the impact ring  330  may be characterized by a ring width W R  and a ring diameter D R , respectively. In particular, the drawings feature six complimentary pairs of legs  340 ,  345  distributed 60° apart, although other angular arrangements may be contemplated without departing from the inventive features disclosed herein. 
   The upper legs  340  provide separation distance between the impact ring  330  and the upper plate  210 . Similarly, the lower legs  345  provide separation distance between the impact ring  330  and the lower plate  220 . One of these sets of legs  340 ,  345  may be secured to their corresponding plates  210 ,  220  by bolts. 
   One or more filters  155  may be replacably contained within a removable filter housing  350 . The housing  350  may be removably disposed on the lower plate  220  above the flange  230 . To reduce flow leakage from the lower plate  220 , the housing  350  may include an O-ring in an annular groove  355  on its bottom surface and around the region containing the filters  155 . The housing  350  may be positioned between an opposing pair of alignment bolts  360  and detachably secured by an opposing pair of clamping block  370 . 
     FIGS. 4A and 4B  illustrate isometric views of the inlet unit  200  with the upper and lower plates  210 ,  220  unlatched and open to enable filter replacement. In particular, the inlet latch  250  is released and the upper plate  210  swings on the inlet hinges  240  substantially perpendicular to facilitate access to the housing  350 . As shown, the impact ring  330  may be secured to the upper plate  210  at the upper legs  340 , while the lower legs  345  may operationally separate from engagement with the lower plate  220 . 
   The housing  350 , disposed over the lower plate  220  over an orifice  400  in the flange  230 , represents a hinged rectangular container for a pair of filters  155 . The housing  350  includes a lid  410  and a base  420 . The lid  410  exhibits a pair of receiver openings  430 , complimented by corresponding egress openings  440  in the base  420 . One of these sets of openings  430 ,  440  may feature baffles  445  or containment grids, to secure the filter  155  from becoming entrained in the flow, as well as provide a tactile orientation reference for an operator that services under conditions of low illumination. In the configuration shown in  FIG. 4A , the baffles  445  are featured in conjunction with the egress openings  440 . 
   The lid  410  and the base  420  may be connected together by hinges  450 , shown as a tandem pair at corners of opposite sides, and by an opposing pair of permanent magnets  460 ,  465  recessed respectively within the lid  410  and the base  420  so as oppose or face each other. The hinges  450  may form a pivot axis substantially parallel to the lower plate  210 . The first magnet  460  is disposed on an inner surface of the lid  410  to present a first magnetized surface. The second magnet  465  is disposed on an inner surface of the base  420  to present a second magnetized surface. 
   The surfaces of the first and second magnets  460 ,  465  have opposite polarity so as to attract each other and secure the housing  350  to the closed position when shut. The strength of the magnetic field by the magnets  460 ,  465  may be gauged to an appropriate level to prevent the housing  350  from accidentally opening, but to enable an individual to resist the magnetic attraction by unaided hands. Use of magnets reduces the possibility of operator clothing and/or skin getting caught or torn while handling the holder and/or changing filters. Developing a small rip in protective gear may cause hazardous exposure, and manipulating small latches and hooks while wearing cumbersome protective gear can be cumbersome. 
   As will be apparent to skilled artisans, alternate configurations may be contemplated using two or more opposing pairs of magnets and/or positions along pivoting faces of the lid  410  and the base  420  (rather than along the sides) without departing from the spirit of the invention. Additionally, alternate configurations may be contemplated that employ alternate mechanisms to align and secure the lid  410  and the base  420  to each other, such as latches, hooks, threaded bolts and/or combinations thereof. 
   The housing  350  may be removable from the lower plate  220 , rather than be permanently fastened. Instead, the alignment bolts  360  enable an operator to position the housing  350  therebetween to cover the orifice  400 . Each clamping block  370  releasably secures the housing  350  by pivoting clamps  470 . The clamp  470  lifts away from its corresponding block  370  when releasing the housing  350 , and lowers onto the lid  410  when securing the housing  350  to the lower plate  220 . The clamp  470  may pivot along a hinge substantially parallel to the lower plate  210  so that a lever engages the housing  350  when lowered and releases the housing  350  when lifted. The bolts  360  and the block  370  may be discernable by tactile sensation. Skilled artisans will recognize that alternate configurations of alignment indicators are possible without departing from the scope of the invention. 
   The plates  210 ,  220 , nozzle  300  and the housing  350 , as fabricated by Applicants, are preferably composed substantially of anodized Al-6061 aluminum alloy suitable for harsh weather conditions, convenient portability and stringent reliability of service requirements. Further, Applicants provide simplicity in the above-described embodiments by eliminating items vulnerable to corrosion, such as screws and moving parts, and by configuring all surfaces to be readily reachable for cleaning. However, skilled artisans will recognize that the design principles disclosed herein are also applicable to alternate materials for different circumstances for applications for either milder conditions or shorter longevity requirements, such as more economically produced designs that use plastic, sheet steel or other suitable material produced by molding or casting. 
   Additionally, the exemplary dimensions described may most preferably include plate diameter D P =16 in (inches), ring diameter D R =10 in, nozzle width W N =0.313 in, ring width W R =1.000 in, plate thickness W P =0.090 in. However, skilled artisans will recognize that the design principles disclosed herein are also applicable to alternate geometries and dimensional constraints. 
     FIG. 5  is an elevation view diagram adapted from Marple, V. A.; Olson, B. A.; Rubow, K. L., “Inertial, Gravitational, Centrifugal, and Thermal Collection Techniques”, chap. 10 of  Aerosol Measurement: Principles, Techniques, and Applications,  2 nd  Edition, Baron, P. A.; Willeke, K. eds., Wiley, ©2001, incorporated herein by reference. An inlet  500  is comprised from left and right portions  510 ,  520  of an acceleration nozzle and an impaction plate  530 . The nozzle converges to a throat  540  having a gap width W and a thickness T. The impaction plate  530  is separated from the nozzle by a distance S. The geometry is important for the proper operation of the device by controlling the flow and aerosol impaction phenomena. 
   An air flow  550  approaches the nozzle and passes through the throat  540  towards the impaction plate  530  along streamlines  560 . Under subsonic adiabatic conditions, the air flow  550  accelerates as the path width decreases. Aerosol particles entrained in the air flow  550  possess drag characteristics that may differ from the ambient air. 
   A first particle with abundant inertia may impact the impaction plate  530  at the end of a first trajectory  570 . A second particle with much less inertia, such as one smaller than the first particle, may evade the impaction plate  530  along a second trajectory  580 . Because bio-agent particles to be analyzed are typically smaller than dust and sand commonly suspended in ambient air, the impaction plate  530  may bar the dust and sand so that filters  155  within a unit  200  trap the bio-agents as a larger fraction of the particulate matter collected. 
   The following example provides quantified values to demonstrate the impaction effects. Stokes number, St, represents a non-dimensional parameter to characterize aerodynamic properties (e.g., inertia) of the particles, with values approximately exceeding unity serving to predict particle impaction. Stokes number may be expressed from Marple as St=ρ p C c d p   2 U/(9ηW), where ρ p  is the particle density, C c  is the Cunningham slip correction factor, d p  is the effective particle diameter, U is the medium flow velocity, η is the medium dynamic viscosity and W is the nozzle gap width. Much of the technical literature labels viscosity by the symbol μ, but to reduce confusion from its incorporation for micro-units, η is used herein. 
   The slip correction factor C c  is an empirically derived parameter based on a dimensionless Knudsen number, Kn, that relates the gas molecular mean free path λ to the particle diameter by Kn=2λ/d p . For large particles in continuum flow (e.g., ambient air λ=0.0665 μm), Kn&lt;&lt;1. In contrast, for Kn≈1, particles may slip past the obstacles of air molecules. 
   Slip correction may be calculated based on particle elasticity characteristics and the medium physical properties. For solid particles, the slip correction factor may be approximated as C c ≈1+1.142 Kn. Reynolds number, Re, represents a dimensionless ratio between inertial and kinetic (or frictional) forces, as expressed by nozzle jet Re j =ρUW/η, where ρ is the medium density. Boundary-constrained flow regimes may be classified as laminar for Reynolds number below the transition value of ˜4000, or above for nozzle convergence. 
   For crystalline silica particulates from ground quartz as an example, the following quantities may be used: particle density ρ p =2660 kg/m 3  (kilograms-per-cubic-meter) and d p =2.8 μm. The corresponding slip correction factor may be approximated as C c ≈1.054. Based on the above-described geometry of the nozzle  300 , the throat width W=0.00795 m (meter). For the medium as ambient air, ρ=1.205 kg/m 3 , η=18.203×10 −6  Pa-s (pascal-second). Velocity through the nozzle may be estimated at 2 m/s (meters-per-second). From these values, St≈0.033, Re j =1050 and Kn≈0.05. These values predict that these fine quartz crystals would easily pass through the inlet  200  (St&lt;&lt;1) in laminar continuum flow (Re j &lt;4000, Kn&lt;&lt;1). 
   The distribution of particles according to size for high-bypass (i.e., small particle diameter) selection may be tailored by adjusting the flow characteristics of the air into the inlet unit  200 . Parameters to adjust may include flow rate and inlet geometry. Specifically, particles suspended in the air that pass through the nozzle  300  may be divided into three groups: (a) large particles that are substantially all collected on the impact ring  330 ; (b) small particles that are substantially detoured by the impact ring  330  towards the housing  350 ; and (c) intermediate particles of which a portion impinge and another portion deflect around the impact ring  330 . Minimization of the intermediate group may define the quality of a “cut” that segregates the high bypass particles intended for filter collection and subsequent analysis. 
   The “cut” may be refined by staging a series of concentric impact rings that may be augmented by associated intermediate annular nozzles disposed between the rings. This improves the “cut” by reducing the intermediate group of particles, but may yield loss of desired particles due to possible unintended impaction on additional surface area. Alternatively, the series may cascade slightly different rings and nozzles, each with a slightly smaller “cut”, as shown in Marple. By concatenating the interception of the particles, the fraction of intermediate particles may be reduced and the distribution of particle sizes that reach the filter housing  350  disposed in the center of the ring series may be tightened. In a similar fashion, to additionally exclude particles below a specified size, the housing  350  may be disposed at a position between selected concentric rings for high-and-low bypassing of particles that reach the filter  155  by impact as the remaining particles continue to the unit exit at the flange  230 . 
   The exemplary embodiments of the inlet unit  200  described herein provide several advantages to analysts assigned to evaluate the particles received from the filter  155 , thereby improving efficiency of the analysis by increasing the quantity of desired particle sizes within a given air flow. (Artisans of ordinary skill should note that the inlet unit  200  as described is designed to have a higher collection efficiency to thereby grab more particles out of the air than the conventional inlet, rather than act as a concentrator.) 
   Some of these advantages include: (a) a planar omni-directional inlet configuration using parallel plates  210 ,  220  with an omni-directional axisymmetric nozzle that allows wind-blown air to flow into the inlet unit  200  from any horizontal direction as well as avoid precipitation; (b) an impact ring  330  to selectively restrict particles by size for entry into the filter  155  to thereby reduce analysis error from large particle debris; (c) an O-ring seal within a groove  355  between the housing  350  and the lower plate  220  to reduce pressure drop degradation within the inlet unit  200 . In particular, the nozzle-impactor combination serve to route aerosol particles within a target size range more effectively to the filters, in addition to having greater resistance to intrusion of large objects, such as insects. Also, the improved seal improves flow stability, and inhibits particles of interest from leaking through an alternate exit. 
   The exemplary embodiments of the inlet unit  200  described herein provide several advantages to field operators assigned to replace filters  155  over conventional arrangements. These advantages are intended to facilitate and simplify manipulation while the operators wear cumbersome gear, lack a sophisticated suite of tools and perform duties under low illumination. Some of these advantages include: (a) absence of toggles to open; (b) magnetic fasteners  460 ,  465  to close the housing; tactile indicators to orient and align the housing  350  for disposing onto the lower plate  220 ; (c) schedule reduction in filter replacement frequency due to reduced non-target particulate matter from being collected on the filter  155  (i.e., improved selectivity of particles), as well as reduced filter degradation from precipitation; (d) no loose or moving parts; and (e) a replaceable filter housing  350  that can be loaded and unloaded in a laboratory environment separate from the inlet unit  200 . 
   While certain features of the embodiments of the invention have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments.