Abstract:
A retention system for use within a molecular sieve unit includes a perforated plate having a top face and bottom face. The perforated plate is configured to be positioned atop a packed sieve bed proximate an outlet end cap of the molecular sieve unit. A skirt is coupled to the bottom face of the perforated plate and a biasing member is configured to engage the outlet end cap and the top face of the perforated plate. The biasing member urges the perforated plate against the packed sieve bed. The biasing member may be one or more wave springs thereby reducing the risk of losing sufficient biasing force against the perforated plate. In the event that a sufficient biasing force is lost, the skirt may operate as a failsafe so as to minimize or prevent tilting of the perforated plate within the housing.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to molecular sieve devices, and more particularly to a molecular sieve retention system to contain and pack particulate material within a molecular sieve unit, and still more particularly to a molecular sieve retention system to contain and pack the particulate material within the molecular sieve unit even after the molecular sieve unit experiences extreme environmental conditions. 
       BACKGROUND OF THE INVENTION 
       [0002]    Aircraft On-Board Oxygen Generation Systems/On-Board Inert Gas Generation Systems (OBOGS/OBIGGS) units are designed to separate gases from a pressurized air source in order to supply oxygen enriched air flow to the flight crew, while also supplying oxygen depleted air flow for inerting the fuel tank ullages. Such units typically employ molecular sieve gas separation processes, such as but not limited to pressure swing adsorption (PSA), wherein molecular sieve particulate material, such as a zeolite, is packed as a bed and retained within a molecular sieve unit. The sieve unit includes a housing, an inlet end cap, an outlet end cap and the sieve bed packed therein. A pressurized inlet gas may then enter through the inlet end cap, pass through the sieve bed wherein the inlet gas is separated by virtue of the sieve material such that unwanted components of the inlet air (e.g., N 2 ) are selectively adsorbed by the sieve material while a desired product gas (e.g., O 2 ) may pass through the sieve material and exit through the outlet end cap. 
         [0003]    To prevent unwanted movement of the particulate material of the sieve bed within the housing, traditional sieve housings include a plate and spring retention system where the spring provides a downward force upon the plate to maintain a compact bed. Such a force acts to inhibit the formation of gaps through which unwanted components of the inlet air may traverse the length of the housing rather than be adsorbed by the particulate material of the sieve bed. The challenge is to provide sufficient downward force upon the packed bed so that the sieve particles do not move relative to each other under severe application conditions while also avoiding force levels that crush the particles. The probability of achieving this condition is increased dramatically when the compaction force applied to the sieve bed is uniform across the cross sectional area that it is applied. With that in mind, previous sieve beds have utilized conical springs coupled to a perforated plate to provide the compaction force to spread the load to the packed bed. However, under extreme environmental conditions, the packed bed and its retention system may be subject to forces that can cause rocking and or tilting of the perforated plate, which can in turn result in movement of the packed bed and subsequent damage to the molecular sieve unit. The risk of this type of failure increases if there is even a slight misalignment of the perforated plate or spring during assembly. 
         [0004]    Therefore, a need remains for a molecular sieve retention system which can withstand the forces experienced due to extreme environmental conditions without risk of material bed damage and/or loss of sieve material retention. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is generally directed to a retention system for use within a molecular sieve unit. The molecular sieve unit includes a housing sealed at a first end via an inlet end cap having an inlet orifice defined therein and at a second end via an outlet end cap having an outlet orifice defined therein. The housing is further configured to retain a packed sieve bed of adsorptive material. The retention system may comprise a perforated plate having a top face and bottom face, wherein the perforated plate may be configured to be positioned atop the packed sieve bed proximate the outlet end cap; a skirt coupled to the bottom face of the perforated plate; and a biasing member configured to engage the outlet end cap and the top face of the perforated plate, wherein the biasing member urges the perforated plate against the packed sieve bed. The retention system may further comprise a felt filter and/or mesh screen configured to be interposed between the perforated plate and the packed sieve bed. At least a portion of the felt filter and/or mesh screen may also be interposed between the perforated plate and the skirt. In one aspect of the present invention, the biasing member may comprise a wave spring. In another aspect, the biasing member may comprise a wave spring assembly including two wave springs, wherein a first spring is centrally located within a second spring. 
         [0006]    In a further aspect of the present invention, the skirt may further include one or more alignment components configured to be interposed between the skirt and an internal wall of the housing. Each of the one or more alignment components may be an O-ring. An inner surface of the skirt may also be configured to direct the adsorptive material proximate the housing centrally inward toward a central axis of the housing. 
         [0007]    In another aspect of the present invention, a molecular sieve unit comprises a housing sealed at a first end via an inlet end cap having an inlet orifice defined therein and at a second end via an outlet end cap having an outlet orifice defined therein. A packed sieve bed of adsorptive material may be disposed within the housing and a retention system may be positioned between the packed sieve bed and the outlet end cap. The retention system may comprise a perforated plate having a top face and bottom face, wherein the perforated plate may be positioned atop the packed sieve bed proximate the outlet end cap; a skirt coupled to the bottom face of the perforated plate; and a biasing member configured to engage the outlet end cap and the top face of the perforated plate, wherein the biasing member urges the perforated plate against the packed sieve bed. 
         [0008]    In still a further aspect of the present invention, a retention system for use within a molecular sieve unit may comprise a perforated plate having a top face and bottom face, wherein the perforated plate may be configured to be positioned atop the packed sieve bed proximate the outlet end cap; a skirt coupled to the bottom face of the perforated plate, wherein the skirt includes one or more alignment components configured to be interposed between the skirt and an internal wall of the housing; a biasing member configured to engage the outlet end cap and the top face of the perforated plate, wherein the biasing member urges the perforated plate against the packed sieve bed; and a felt filter and/or mesh screen configured to be interposed between the perforated plate and the packed sieve bed. 
         [0009]    Additional objects, advantages and novel aspects of the present invention will be set forth in part in the description which follows, and will in part become apparent to those in the practice of the invention, when considered with the attached figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross section partial view of a prior art molecular sieve unit; 
           [0011]      FIG. 2  is a cross section partial view of a molecular sieve unit in accordance with an aspect of the present invention; 
           [0012]      FIG. 3  is an isolated view of a perforated plate and skirt that may be used within the molecular sieve unit shown in  FIG. 2 ; 
           [0013]      FIG. 4  is a perspective view of a concentric spring apparatus that may be used within the molecular sieve unit shown in  FIG. 2 ; and 
           [0014]      FIG. 5  is a graph depicting sieve compaction versus time during a vibration event comparing the prior art molecular sieve unit shown in  FIG. 1  with the molecular sieve unit in accordance with the present invention as shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Referring now to  FIG. 1  a prior art molecular sieve unit  10  generally comprises a tubular-shaped bed housing  12  capped by an inlet end cap at a first end (not shown) and opposing outlet end cap  14  at a second end  16 . Each end may be sealed with its respective end cap, such as through use of an O-ring seal  18 . Each end cap may further include a respective inlet or outlet orifice, such as outlet orifice  20  of outlet end cap  14 . A sieve bed  22  of adsorptive material (e.g., zeolite) may be located within bed housing  12  between the inlet end cap and outlet end cap  14 . In this manner, a pressurized air flow of inlet air may enter molecular sieve unit  10  through the inlet orifice of the inlet end cap, pass through the adsorptive material of sieve bed  22  whereby a desired product gas may be output through outlet orifice  20  of outlet end cap  14 . In one aspect of the invention, the adsorptive material is configured to selectively remove unwanted constituents (e.g., N 2 ) from an air supply so as to output a desired product gas (e.g., O 2 ). 
         [0016]    A bed retention system  24  may be positioned within housing  12  above bed  22  and be configured to cooperate with outlet end cap  14 . Bed retention system  24  may include a felt filter and/or mesh screen (hereinafter, felt screen)  26  layered atop sieve bed  22 , with a perforated plate  28  arranged to seat atop felt filter  26  along plate bottom face  30 . Felt filter  26  may prevent adsorptive material from escaping from bed  22  by traveling through perforations within perforated plate  28 . A conical spring  32  may be interposed between perforated plate  28  and outlet end cap  14  wherein conical spring  32  is biased to impart a downward force (indicated by arrow F) upon perforated plate  28  (toward the opposing inlet end cap) so as to maintain a compact sieve bed  22  within bed housing  12 . A compact sieve bed  22  is desired as any gaps within the adsorptive material may provide a fluid path through which the pressurized inlet air may flow without undergoing gas separation. As a result, unwanted components may remain in the product gas, thereby reducing air separation efficiency and potentially producing hazardous or deadly product gases. Conversely, packing a sieve bed  22  under too great a force may crush the adsorptive material, thereby reducing the number of, and availability to, active sights for gas separation on or within the adsorptive material (i.e., zeolite). Again, this situation may result in unwanted components remaining in the product gas. 
         [0017]    With continued reference to  FIG. 1 , adsorptive material of sieve bed  22  may settle or compact over time, and in certain environments, a molecular sieve unit, such as unit  10 , may be subject to extreme stresses, such as vibrations, particularly when installed within an OBOGS/OBIGGS unit of an aircraft, and more particularly when installed within a military aircraft. When subjected to such vibrations, the adsorptive material (i.e., zeolite) comprising sieve bed  22  may become further compacted thereby creating cracks and gaps within the bed. To prevent cracks and gaps from forming, molecular sieve unit  10  may include conical spring  32  configured to bias perforated plate  28  toward the inlet end cap while maintaining plate orientation and material containment. However, should sieve bed  22  continue to compact to such a degree that the spring force imparted by conical spring  32  is insufficient to maintain compaction of the bed via perforated plate  28 , bed retention system  24  may fail. In one instance, cracks and gaps may form which may decrease the air separation efficiency of the molecular sieve unit, potentially to an unsafe level. In a second instance, perforated plate  28  may tilt (i.e., no longer remain perpendicular to longitudinal axis A of bed housing  12 ) such that particulate adsorptive material of sieve bed  22  may be released from molecular sieve unit  10 , such as through outlet orifice  20 . Such a release would signal a catastrophic failure of molecular sieve unit  10 . 
         [0018]    Turning now to  FIG. 2 , a molecular sieve unit  34  in accordance with the present invention may generally include a housing  12  configured to receive a bed retention system  36 . Similar to molecular sieve unit  10  described above, molecular sieve unit  34  may generally comprise a tubular-shaped bed housing  12  capped by an inlet end cap  13  at a first end  15  and opposing outlet end cap  14  at a second end  16 . Each end may be sealed with its respective end cap, such as through use of an O-ring seal  18 . Each end cap may further include a respective inlet or outlet orifice, such as outlet orifice  20  of outlet end cap  14 . A sieve bed  22  of adsorptive material (e.g., zeolite) may located within bed housing  12  between the inlet end cap and outlet end cap  14 . In this manner, a prior art bed retention system  24  may be swapped with bed retention system  36  without requiring modification of housing  12  or either end cap. 
         [0019]    As shown in  FIGS. 2 and 3 , bed retention system  36  may include a felt filter  38  configured to be layered atop sieve bed  22 , with a perforated plate  40  arranged to seat atop felt filter  38  along plate bottom face  42 . Felt filter  38  may prevent adsorptive material from escaping from bed  22  by traveling through perforations within perforated plate  40 . Washer  44  may be arranged atop plate top face  46  of perforated plate  40  and opposite felt filter  38 . A biasing member, such as wave spring  48 , may be interposed between perforated plate  40  (or washer  44 , when included) and outlet end cap  14 . Wave spring  48  is biased to impart a downward force (indicated by arrow F′) upon perforated plate  40 /washer  44  toward the opposing inlet end cap so as to maintain a compact sieve bed  22  within bed housing  12 . 
         [0020]    Skirt  50  may be coupled to plate bottom face  42  by fasteners  52 , such as but not limited to screws, bolts or rivets. Skirt  50  may include an open bottom end  54  and open top end  56  defining a side wall  58  therebetween. Inner surface  60  of side wall  58  may be configured to form a cup-like shape while external surface  62  may also include one or more alignment components  64 . By way of example and without limitation thereto, alignment components  64  may include one or more O-rings. In accordance with an aspect of the present invention, skirt  50  along with alignment component(s)  64  may be configured to assist maintaining a perpendicular orientation of perforated plate  40  relative to the walls of housing  12 . Alignment component(s)  64  may further maintain an airtight seal between external surface  62  of skirt  50  and housing  12 . Without subscribing to any particular theory of operation, inner surface  60  may operate to capture adsorptive material of sieve bed  22  at the outer circumference of sieve bed  22  and direct the adsorptive material towards the center of perforated plate  40 . This movement may assist leveling of the absorptive material within sieve bed  22 , keep the adsorptive material from clumping or cracking, as well as alleviate any forces which may cause rocking or tilting of perforated plate  40 . 
         [0021]    In accordance with a further embodiment of the present invention, and as shown in  FIG. 4 , wave spring  48  of bed retention system  36  may be replaced with wave spring assembly  66 . Assembly  66  may incorporate first and second wave springs  68 ,  70  in which first wave spring  68  may be centrally located within second wave spring  70 . This configuration may promote uniformity of spring force F′ ( FIG. 2 ) applied to perforated plate  40  (and/or washer  44 ). The uniform spring force F′ may then be applied to sieve bed  22 , thereby further alleviating any potential rocking or tilting of perforated plate  40 . It should be understood by those skilled in the art that, while being shown and described as a pair of concentrically oriented wave springs, any number of wave springs may be included within wave spring assembly  66 , such as may be dictated by system requirements or housing capacities. It should be further appreciated that each of wave springs  48 ,  68 ,  70  may be manufactured from pre-hardened flat wire through an on-edge-coiling/edgewinding process. Moreover, while shown and described as wave springs, it should be appreciated by those skilled in the art that any suitable biasing member may be utilized, such as but not limited to coil springs or conical springs, and that such other and additional biasing members are to be considered within the scope of the present disclosure. With that being said, the exemplary wave spring  48  (or alternatively wave springs  68 ,  70 ) of molecular sieve unit  34  has less mass than corresponding conical or coil springs while also providing an increased free length for greater spring travel. 
         [0022]    Turning now to  FIG. 5 , sieve compaction  72  of a molecular sieve unit  34  in accordance with the present invention is compared with sieve compaction  74  of a prior art molecular sieve unit  10  as shown in  FIG. 1  when subject to random vibrations typically experienced when such molecular sieve units may be deployed within a military aircraft. It should be noted that one minute of test time corresponds generally to one hour of unit flight time. It should be further noted that use of a wave spring/wave spring assembly extends the spring limit  76  compared to spring limit  78  of conical springs (spring limits indicate the point where the spring force is inadequate to maintain a compact sieve bed). As can be seen within  FIG. 5 , prior art molecular sieve unit  10  has experienced sieve bed compaction greater than its spring limit at approximately 350 minutes of test time, thus indicating molecular sieve unit failure, as indicated at reference numeral  80 . In contrast, sieve compaction employing a bed retention system of the present invention, such as bed retention system  36 , failure of molecular sieve unit  34  was avoided even after nearly  700  minutes of test time. Accordingly, failure of molecular sieve unit  34  due to vibrational compaction of sieve bed  22  may be greatly reduced, if not eliminated, when employing a bed retention system in accordance with the present invention. 
         [0023]    The foregoing description of the invention has been presented for the purpose of illustration and description. It is not intended to be exhaustive nor is it intended to limit the invention to the precise form(s) disclosed. It will be apparent to those skilled in the art that the disclosed embodiments may be modified in light of the above teachings. The embodiments described are chosen to provide an illustration of principles of the invention and its practical application to enable thereby one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, the foregoing description is to be considered exemplary, rather than limiting, and the true scope of the invention is that described in the following claims.