Patent Description:
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<NUM>) are selectively adsorbed by the sieve material while a desired product gas (e.g., O<NUM>) may pass through the sieve material and exit through the outlet end cap.

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.

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. From <CIT>, a device for cleaning and drying compressed gas is known to include a filter with a plate on top, the plate having an axially downward extending flange.

The above problems are solved by the subject-matter of the independent claims.

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 comprises 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.

The skirt further includes 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 is configured to direct the adsorptive material proximate the housing centrally inward toward a central axis of the housing.

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 is disposed within the housing and a retention system is positioned between the packed sieve bed and the outlet end cap. The retention system comprises the features defined in claim <NUM>.

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.

Referring now to <FIG> a prior art molecular sieve unit <NUM> generally comprises a tubular-shaped bed housing <NUM> capped by an inlet end cap at a first end (not shown) and opposing outlet end cap <NUM> at a second end <NUM>. Each end may be sealed with its respective end cap, such as through use of an O-ring seal <NUM>. Each end cap may further include a respective inlet or outlet orifice, such as outlet orifice <NUM> of outlet end cap <NUM>. A sieve bed <NUM> of adsorptive material (e.g., zeolite) may be located within bed housing <NUM> between the inlet end cap and outlet end cap <NUM>. In this manner, a pressurized air flow of inlet air may enter molecular sieve unit <NUM> through the inlet orifice of the inlet end cap, pass through the adsorptive material of sieve bed <NUM> whereby a desired product gas may be output through outlet orifice <NUM> of outlet end cap <NUM>. In one aspect of the invention, the adsorptive material is configured to selectively remove unwanted constituents (e.g., N<NUM>) from an air supply so as to output a desired product gas (e.g., O<NUM>).

A bed retention system <NUM> may be positioned within housing <NUM> above bed <NUM> and be configured to cooperate with outlet end cap <NUM>. Bed retention system <NUM> may include a felt filter and/or mesh screen (hereinafter, felt screen) <NUM> layered atop sieve bed <NUM>, with a perforated plate <NUM> arranged to seat atop felt filter <NUM> along plate bottom face <NUM>. Felt filter <NUM> may prevent adsorptive material from escaping from bed <NUM> by traveling through perforations within perforated plate <NUM>. A conical spring <NUM> may be interposed between perforated plate <NUM> and outlet end cap <NUM> wherein conical spring <NUM> is biased to impart a downward force (indicated by arrow F) upon perforated plate <NUM> (toward the opposing inlet end cap) so as to maintain a compact sieve bed <NUM> within bed housing <NUM>. A compact sieve bed <NUM> 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 <NUM> 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.

With continued reference to <FIG>, adsorptive material of sieve bed <NUM> may settle or compact over time, and in certain environments, a molecular sieve unit, such as unit <NUM>, 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 <NUM> may become further compacted thereby creating cracks and gaps within the bed. To prevent cracks and gaps from forming, molecular sieve unit <NUM> may include conical spring <NUM> configured to bias perforated plate <NUM> toward the inlet end cap while maintaining plate orientation and material containment. However, should sieve bed <NUM> continue to compact to such a degree that the spring force imparted by conical spring <NUM> is insufficient to maintain compaction of the bed via perforated plate <NUM>, bed retention system <NUM> 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 <NUM> may tilt (i.e., no longer remain perpendicular to longitudinal axis A of bed housing <NUM>) such that particulate adsorptive material of sieve bed <NUM> may be released from molecular sieve unit <NUM>, such as through outlet orifice <NUM>. Such a release would signal a catastrophic failure of molecular sieve unit <NUM>.

Turning now to <FIG>, a molecular sieve unit <NUM> in accordance with the present invention includes a housing <NUM> configured to receive a bed retention system <NUM>. Similar to molecular sieve unit <NUM> described above, molecular sieve unit <NUM> may generally comprise a tubular-shaped bed housing <NUM> capped by an inlet end cap <NUM> at a first end <NUM> and opposing outlet end cap <NUM> at a second end <NUM>. Each end may be sealed with its respective end cap, such as through use of an O-ring seal <NUM>. Each end cap further includes a respective inlet or outlet orifice, such as outlet orifice <NUM> of outlet end cap <NUM>. A sieve bed <NUM> of adsorptive material (e.g., zeolite) is located within bed housing <NUM> between the inlet end cap and outlet end cap <NUM>. In this manner, a prior art bed retention system <NUM> may be swapped with bed retention system <NUM> without requiring modification of housing <NUM> or either end cap.

As shown in <FIG> and <FIG>, bed retention system <NUM> may include a felt filter <NUM> configured to be layered atop sieve bed <NUM>, with a perforated plate <NUM> arranged to seat atop felt filter <NUM> along plate bottom face <NUM>. Felt filter <NUM> may prevent adsorptive material from escaping from bed <NUM> by traveling through perforations within perforated plate <NUM>. Washer <NUM> may be arranged atop plate top face <NUM> of perforated plate <NUM> and opposite felt filter <NUM>. A biasing member, such as wave spring <NUM>, may be interposed between perforated plate <NUM> (or washer <NUM>, when included) and outlet end cap <NUM>. Wave spring <NUM> is biased to impart a downward force (indicated by arrow F') upon perforated plate <NUM>/ washer <NUM> toward the opposing inlet end cap so as to maintain a compact sieve bed <NUM> within bed housing <NUM>.

Skirt <NUM> is coupled to plate bottom face <NUM> by fasteners <NUM>, such as but not limited to screws, bolts or rivets. Skirt <NUM> includes an open bottom end <NUM> and open top end <NUM> defining a side wall <NUM> therebetween. Inner surface <NUM> of side wall <NUM> is configured to form a cup-like shape while external surface <NUM> also includes one or more alignment components <NUM>. By way of example and without limitation thereto, alignment components <NUM> may include one or more O-rings. In accordance with an aspect of the present invention, skirt <NUM> along with alignment component(s) <NUM> may be configured to assist maintaining a perpendicular orientation of perforated plate <NUM> relative to the walls of housing <NUM>. Alignment component(s) <NUM> may further maintain an airtight seal between external surface <NUM> of skirt <NUM> and housing <NUM>. Without subscribing to any particular theory of operation, inner surface <NUM> may operate to capture adsorptive material of sieve bed <NUM> at the outer circumference of sieve bed <NUM> and direct the adsorptive material towards the center of perforated plate <NUM>. This movement may assist leveling of the absorptive material within sieve bed <NUM>, keep the adsorptive material from clumping or cracking, as well as alleviate any forces which may cause rocking or tilting of perforated plate <NUM>.

In accordance with a further embodiment of the present invention, and as shown in <FIG>, wave spring <NUM> of bed retention system <NUM> may be replaced with wave spring assembly <NUM>. Assembly <NUM> may incorporate first and second wave springs <NUM>, <NUM> in which first wave spring <NUM> may be centrally located within second wave spring <NUM>. This configuration may promote uniformity of spring force F' (<FIG>) applied to perforated plate <NUM> (and/or washer <NUM>). The uniform spring force F' may then be applied to sieve bed <NUM>, thereby further alleviating any potential rocking or tilting of perforated plate <NUM>. 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 <NUM>, such as may be dictated by system requirements or housing capacities. It should be further appreciated that each of wave springs <NUM>, <NUM>, <NUM> 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 <NUM> (or alternatively wave springs <NUM>, <NUM>) of molecular sieve unit <NUM> has less mass than corresponding conical or coil springs while also providing an increased free length for greater spring travel.

Turning now to <FIG>, sieve compaction <NUM> of a molecular sieve unit <NUM> in accordance with the present invention is compared with sieve compaction <NUM> of a prior art molecular sieve unit <NUM> as shown in <FIG> 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 <NUM> compared to spring limit <NUM> 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>, prior art molecular sieve unit <NUM> has experienced sieve bed compaction greater than its spring limit at approximately <NUM> minutes of test time, thus indicating molecular sieve unit failure, as indicated at reference numeral <NUM>. In contrast, sieve compaction employing a bed retention system of the present invention, such as bed retention system <NUM>, failure of molecular sieve unit <NUM> was avoided even after nearly <NUM> minutes of test time. Accordingly, failure of molecular sieve unit <NUM> due to vibrational compaction of sieve bed <NUM> may be greatly reduced, if not eliminated, when employing a bed retention system in accordance with the present invention.

Claim 1:
A retention system (<NUM>) for a molecular sieve unit (<NUM>), the molecular sieve unit (<NUM>) including a housing (<NUM>) sealed at a first end (<NUM>) via an inlet end cap (<NUM>) having an inlet orifice defined therein and at a second end (<NUM>) via an outlet end cap (<NUM>) having an outlet orifice (<NUM>) defined therein, the housing (<NUM>) configured to retain a packed sieve bed (<NUM>) of adsorptive material, the retention system (<NUM>) comprising:
a. a perforated plate (<NUM>) having a top face (<NUM>) and bottom face (<NUM>), wherein the perforated plate (<NUM>) is configured to be positioned atop the packed sieve bed (<NUM>) proximate the outlet end cap (<NUM>);
b. a skirt (<NUM>) coupled to the bottom face (<NUM>) of the perforated plate (<NUM>), the skirt (<NUM>) including a side wall (<NUM>) defined between an open bottom end (<NUM>) and an open top end (<NUM>) of the skirt (<NUM>), the side wall (<NUM>) having an inner surface (<NUM>) comprising a part which is, in a direction towards the open top end (<NUM>) of the skirt (<NUM>), curved towards a central axis of the housing (<NUM>) to form a cup-like shape such as to direct the adsorptive material proximate the housing (<NUM>) centrally inward toward the central axis of the housing (<NUM>), and an external surface (<NUM>) that includes one or more alignment components (<NUM>); and
c. a biasing member (<NUM>; <NUM>) configured to engage the outlet end cap (<NUM>) and the top face (<NUM>) of the perforated plate (<NUM>), wherein the biasing member (<NUM>;<NUM>) is configured to urge the perforated plate (<NUM>) against the packed sieve bed.