BACKGROUND OF THE INVENTION 1.0 Field of the Invention The present invention relates generally to environmental control systems, and more particularly, to an environmental control system incorporating a catalytic reactor for the decomposition of ozone in air. The environmental control system of the present invention has particular application for use in conditioning the air supplied to a passenger cabin of an aircraft. However, the environmental control system of the present invention may also be used in conjunction with other transportation machines such as military tanks.

2.0 Related Art Aircraft environmental control systems supply pressurized and conditioned air to the aircraft cabin. The temperature, pressure and relative humidity must be controlled to provide for the comfort of the flight crew and passengers within the aircraft.

Modern commercial jet aircraft are typically designed for fuel-efficient operation at relatively high altitudes of 25,000 feet or more above sea level where the ozone content of the air is relatively high. Accordingly, during operation at high altitudes the air supplied to the aircraft cabin from the environmental control system may contain ozone at a level of 1-3 ppmv. The presence of ozone in the air within the aircraft cabin can cause lung and eye irritation, headaches, fatigue and/or breathing discomfort. As a result, present Federal Aviation Agency (FAA) regulations limit ozone concentrations on commercial flights to 0.1 ppmv during a three hour time period and 0.25ppmv maximum at any time.

The use of catalytic converters to reduce or eliminate undesirable ozone in the air supplied to aircraft cabins, in situations where relatively high ozone leveis are expected, is known in the art. An example of a commonly known type of catalytic converter is illustrated in U. S. Patent 4,405,507 which discloses a ceramic (cordierite) monolithic support structure having a high surface area washcoat applied to the monolithic support, with the washcoat being used to carry the catalyst (in this case a platinum group metal and a non-precious Group VIII metal oxide or aluminate).

While catalytic converters, or reactors, of this type have found widespread use, they are subject to the following limitations. In the first instance, the washcoat is subject to attrition when exposed to continued vibration and thermal cycling, such as that which may be experienced when a converter is used in an aircraft application. Secondly, the washcoat and the catalyst carried by the washcoat may be washed off of the ceramic monolithic support structure during routine maintenance cleaning. Additionally, catalytic reactors of this type typically include a plurality of spaced flow channels which are formed in the support structure along an axially extending axis. and through which the ozone-containing air flows under laminar conditions. Catalytic reactors of this type are typically mass-transfer limited, i. e. the efficiency of the reactor may be limited by the ability of the ozone molecule to diffuse to the surface of the catalyst.

This requires laminar-flow reactors to be much larger, and consequently heavier, than those which may employ turbulent flow of the ozone-containing air through the reactor.

It is also known in the art to utilize catalytic converters for the decomposition of ozone in conjunction with aircraft environmental control systems as shown in U. S.

Patent 4,665,973 to Limberg, et al. issued May 19,1987 and assigned to the assignee of the present invention. Limberg, et al. discloses a"washcoat"type coating 58 which is applied to the plate elements 54 of the hot pass side of a primary heat exchanger 18 in an environmental control system 10 of the type utilized in aircraft, tanks, or other vehicles. Although the environmental control system of Limberg, et al. provides for ozone decomposition while minimizing weight and space penalties associated with the corresponding environmental control system, the catalytic coating 58 is of the type described previously and is therefore subject to attrition when exposed to continued vibration and thermal cycling, such as that which may be experienced in an aircraft environment.

Co-pending and commonly assigned U. S. Patent Applications 08/271,922 and 08/494, 656 each disclose improved catalytic ozone converters of compact size and lightweight construction. Each of the co-pending applications discloses an aluminum or aluminum alloy support structure which comprises one or more fin elements disposed within a converter housing. Each of the fin elements includes a plurality of fins arranged in an axial succession of off-set fin rows between the inlet and outlet ends of the housing. The configuration of the fin elements results in a relatively high mass transfer between the ozone-containing air and the ozone carried by the fin elements for purposes of ozone decomposition. In each disclosure the fin elements have an integral anodized surface layer at least two microns thick, with the catalyst disposed on and within the anodized surface layer. In Application Serial No.

08/271,922. one or more Group VIII noble metals and optionally base metals from

Groups VII, IIIA. and VIIA are disposed on and within the anodized surface layer of the fin elements. In one of the disclosed embodiments of Application Serial No.

08/494,656, silver is disposed on and within the anodized surface layer of the fin elements, with this embodiment being preferred for relatively low temperature applications. The catalytic converter disclosed in each of the referenced co-pending applications represent a significant improvement over the then-existing prior art since the integral anodized surface layer is not subject to attrition, unlike washcoats of prior ceramic monolithic converters, and the turbulent flow and resulting high mass transfer rate promoted by the fin element configuration (due to the tortuous paths created by the off-set fin rows), permits the converter to be more compact than prior designs.

The compactness of the disclosed converters, coupled with the use of aluminum or aluminum alloy supports results in a lighter converter as compared to those employing ceramic monoliths. However, notwithstanding the aforementioned advantages of the inventions disclosed in the referenced co-pending applications, the anodizing steps required in each disclosure are relatively expensive. Additionally, although the integral anodized surface layer is not subject to attrition, the catalysts which are disposed on and in the surface layer may be at least partially removed during routine maintenance cleaning.

The foregoing illustrates limitations known to exist in present catalytic converters, or reactors of the type which may be used in an environmental control systems for conditioning air supplied to a habitable space in a transportation machine such as the passenger cabin in aircraft. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.

SUMMARY OF THE INVENTION In one aspect of the present invention, this is accomplished by providing an environmental control system for use in a transportation machine, with the system being effective for receiving and conditioning a heated, ozone-containing airstream prior to delivering the airstream to a habitable space within the machine. According to a preferred embodiment, the environmental control system comprises a catalytic reactor effective for decomposing at least a portion of the ozone present in the heated, ozone-containing airstream. The catalytic reactor comprises a metallic housing having an inlet portion effective for receiving the heated, ozone-containing airstream, an outlet portion effective for discharging the airstream from the reactor, and a generally cylindrical portion disposed axially between and connected to the inlet and outlet portions. The catalytic reactor further comprises a thermally compliant core structure constructed from a catalytically-active metal alloy, and the thermally compliant core structure is disposed within and connected to the generally cylindrical portion of the housing. The catalytically-reactive metal alloy is effective for decomposing at least a portion of the ozone present in the heated. ozone-containing airstream as the airstream flows through the core structure.

According to other embodiments, the thermally compliant core structure comprises a plurality of fin assemblies, with each of the fin assemblies being configured as an annular ring. Alternatively, the core structure may comprise a single fin assembly wrapped upon itself in a spiral configuration. In the embodiments having a plurality of fin assemblies, the fin assemblies are generally concentrically disposed relative to one another about an axially extending centerline axis of the catalytic reactor. Each of the fin assemblies is brazed to radially adjacent ones of the fin assemblies so as to prevent nesting between radially adjacent pairs of the fin assemblies. Each of the fin assemblies includes a plurality of fins which are arranged in an axial succession of rows

of fins, with each of the rows of fins defining a plurality of axially extending flow channels. The fins and the flow channels of each of the rows of the fins are circumferentially off-set relative to the fins and the flow channels, respectively, of axially adjacent ones of the rows of the corresponding one of the fin assemblies so as to define a plurality of tortuous flowpaths through the flow channels of each of the fin assemblies for the flow of the heated, ozone-containing airstream through the thermally compliant core structure.

In other embodiments, the catalytically-active metal alloy comprises a silver- containing metal alloy, and may have a composition including but not limited to silver and copper. Even more particularly, the catalytically-active metal alloy may have a composition comprising, on a weight basis, about 55% silver, about 39% copper, about 5% zinc and about 1% nickel. The catalytically-active metal alloy may be thermally activated by heating the alloy to a temperature ranging from about 300°F to about 420°F for a period of time ranging from about 30 minutes to about 60 minutes.

In other embodiments, the thermally compliant core structure of the catalytic reactor may be brazed to the generally cylindrical portion of the housing.

In other embodiments, the environmental control system may include a first heat exchanger having an inlet and an outlet for the flow of the airstream therethrough, with the first heat exchanger being adapted to receive a flow of cooling fluid in heat exchange, non-mixing relationship with the airstream so as to cool the airstream. The inlet of the first heat exchanger is in fluid communication with the outlet portion of the housing of the catalytic reactor. The system may further include a compressor having an inlet in fluid communication with the outlet of the first heat exchanger and an outlet for discharging the airstream therefrom, with the compressor being effective for compressing the airstream. The system may further include a second heat exchanger having an inlet and an outlet for the flow of the airstream therethrough, with the inlet of the second heat exchanger being in fluid communication with the outlet of the

compressor and with the second heat exchanger being adapted to receive a flow of cooling fluid in heat exchange, non-mixing relationship with the airstream so as to provide cooling to the airstream. The system may further include a turbine rotatably coupled to the compressor, with the turbine having an inlet in fluid communication with the outlet of the second heat exchanger and an outlet for discharging the airstream therefrom. The turbine is effective for expanding and further cooling the airstream. The system may further include a third heat exchanger disposed upstream of the catalytic reactor, with the third heat exchanger having an inlet for receiving the heated, ozone-containing airstream and an outlet in fluid communication with the inlet portion of the housing of the catalytic reactor. The third heat exchanger is adapted to receive a flow of cooling fluid in heat exchange, non-mixing relationship with the airstream so as to cool the airstream. The system may further include a means for mixing the cooled airstream discharging from the turbine with a relative warmer airstream so as to produce a mixed airstream having a temperature which is higher than a temperature of the cooled airstream, prior to supplying the mixed airstream to the habitable space of the machine.

According to a second aspect of the present invention, an alternative to prior environmental control systems is accomplished by providing a method for conditioning the heated. ozone-containing airstream prior to delivering the airstream to a habitable space within the machine. According to a preferred embodiment, the method comprises the step of constructing a catalytic reactor having a housing and a thermally compliant core structure including a plurality of fin assemblies. The step of constructing comprises the steps of : manufacturing the fin assemblies from a single, catalytically-active metal alloy; configuring each of the fin assemblies as an annular ring; disposing the fin assemblies in generally concentric relationship with one another within a generally cylindrical portion of the housing; inserting braze foil between radially adjacent ones of the fin assemblies and between a radially outermost one of the

fin assemblies and the housing; and brazing the fin assemblies to one another so as to prevent nesting between radially adjacent pairs of the fin assemblies and brazing the radially outermost fin assembly to the housing. The method further comprises the step of flowing the heated, ozone-containing airstream through the thermally compliant core structure of the catalytic reactor so that at least a portion of the airstream is in direct contact with the catalytically-active metal alloy.

According to other embodiments, the method may further comprise the step of thermally activating the catalytically-active metal alloy, with the activating step comprising the step of heating the catalytically-active metal alloy to a temperature ranging from about 300°F to about 420°F for a period of time ranging from about 30 minutes to about 60 minutes. The catalytically-active metal alloy may be as described with respect to the first aspect of the invention. The method may further include the step of configuring each of the fin assemblies of the core structure so as to define a plurality of tortuous flowpaths for the flow of the heated, ozone-containing airstream through the core structure.

According to other embodiments, the method may further comprise the step of cooling the airstream after the airstream discharges from the catalytic reactor, with the step of cooling comprising the step of flowing the airstream through at least one heat exchanger after the airstream discharges from the catalytic reactor.

A main advantage of the apparatus and method of the present invention is the provision of an environmental control system which incorporates a light, compact and cost efficient catalytic reactor for the decomposition of ozone in air, with the core structure of the reactor being constructed from a catalytically-active metal alloy and the reactor being substantially mass-transfer limited.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other aspects of the present invention will become more apparent from the subsequent Detailed Description of the invention when considered in conjunction with the accompanying drawings wherein: Fig. 1 is a fragmentary isometric view illustrating a fin assembly, which is incorporated in the catalytic reactor of the present invention ; Fig. 2 is an elevational view taken along line 2-2 in Fig. 1; Fig. 3 is an ozone destruction curve for the catalytically-active metal alloy of the present invention and illustrates the effects of thermally activating the alloy; Fig. 4 is a series of graphs illustrating the effect of temperature on the surface composition of the catalytically-active metal alloy of the present invention; Fig. 5 is a photo micro-graph of the surface of the catalytically-active metal alloy of the present invention in an as-received condition ; Fig. 6 is a photo micro-graph of the surface of the catalytically-active metal alloy of the present invention after thermal activation ; Fig. 7 is an ozone destruction graph illustrating the ability of the catalytically- active metal alloy of the present invention to recover after temporary exposure to a sulfur dioxide contaminated, ozone-containing feed air ; Fig. 8 is a schematic view illustrating the environmental control system of the present invention; Fig. 9 is a perspective view illustrating the catalytic reactor included in the environmental control system of the present invention ; Fig. 10 is a cross-sectional view taken along line 10-10 in Fig. 9; Fig. 11 is an enlarged view of a portion of Fig. 10 illustrating the brazing of radially adjacent fin assemblies of one embodiment of the catalytic reactor of the present invention ;

Fig. 12 is a cross-sectional view, similar to Fig. 10, illustrating the catalytic reactor according to an alternative embodiment of the present invention.

DETAILED DESCRIPTION Referring now to the drawings, Fig. 8 is a schematic view illustrating an environmental control system 50 according to a preferred embodiment of the present invention. In the illustrative embodiment shown in Fig. 8, system 50 is used in an aircraft application. However, it should be understood that system 50 may also be used in conjunction with other transportation machines such as military tanks. The aircraft (not shown) may include a plurality of engines, one of which is indicated schematically at 52. The engine 52 receives ambient air 54 and processes the air 54 in a conventional manner for the purpose of producing propulsive power for propelling the aircraft. The environmental control system 50 receives the heated, ozone- containing airstream, indicated by flow arrows 16 in Figs. 1 and 9, via conduit 56. In the illustrative embodiment, the airstream 16 is supplied from a compressor stage, indicated at 58, of engine 52. Alternatively, the airstream 16 may be provided from other sources such as ram air and/or a combination of compressor bleed air from engine 52 and ram air. In typical aircraft applications, the temperature of the heated. ozone-containing airstream 16 may range from about 149°C to about 216°C.

However, in certain aircraft applications, the temperature of the heated, ozone- containing airstream 16 may be much cooler and lower temperature catalytic activity may be required to decompose the ozone present in airstream 16, which is achievable with the apparatus and method of the present invention as subsequently discussed in greater detail.

The heated, ozone containing airstream 16 is supplied via conduit 56 to a first inlet 60 of a heat exchanger 62 of system 50. Heat exchanger 62 may comprise a pre-

cooler mounted to engine 52 and may be optionally omitted from system 50. Heat exchanger 62 includes a first outlet 64 effective for discharging the airstream 16 from heat exchanger 62. Heat exchanger 62 is also adapted to receive a flow of a cooling fluid, indicated by flow arrows 66, in heat exchange, non-mixing relationship with the airstream 16 so as to cool the airstream 16. The cooling fluid 66 may comprise ambient air, or alternatively air supplied from engine 52 at a lower temperature than airstream 16, such as fan discharge air or air from a lower compressor stage of engine 52. The cooling fluid 66 is supplied to a second inlet 68 of heat exchanger 62 and discharges from heat exchanger 62 via a second outlet 70.

The first outlet 64 of heat exchanger 62 is in fluid communication with a catalytic reactor 74 which is effective for decomposing at least a portion of the ozone present in airstream 16. The reactor 74 includes a housing 73 having an inlet portion 72, an outlet portion 76, and a generally cylindrical portion 75 disposed axially between and connected to the inlet portion 72 and the outlet portion 76. Reactor 74 further includes a core structure 10 which is disposed within and attached to the generally cylindrical portion 75 of housing 73. The airstream 16 is supplied to the inlet portion 72 of housing 73 via a conduit 77 which establishes fluid communication between the inlet portion 72 and the first outlet 64 of heat exchanger 62. Additional features of reactor 74 are subsequently discussed in greater detail and the specific construction of reactor 74 comprises central features of the present invention. After the ozone within airstream 16 has been reduced to acceptable levels within reactor 74, the airstream 16 discharges from reactor 74 through the outlet portion 76 of housing 73 which is in fluid communication with a first inlet 78 of a heat exchanger 80. The airstream 16 is supplied from the outlet portion 76 of housing 73 to the first inlet 78 of heat exchanger 80 via a conduit 82. Heat exchanger 80 also includes a first outlet 84, effective for discharging airstream 16 therefrom. Heat exchanger 80 is also adapted to receive a flow of cooling fluid, indicated by flow arrows 86, in heat exchange, non-

mixing relationship with the airstream 16 so as to cool (or further cool, if heat exchanger 62 is employed) the airstream 16. The cooling fluid 86 may be supplied from the various sources discussed previously in conjunction with cooling fluid 66. It is further noted that the cooling fluids 66 and 86 may be supplied from a common source or from different sources. The cooling fluid 86 is supplied to a second inlet 88 of heat exchanger 80 and discharges from heat exchanger 80 via a second outlet 90.

System 50 further includes a rotatable compressor 92 having an inlet 94 which is in fluid communication with the first outlet 84 of heat exchanger 80, via a conduit 96. Compressor 92 is rotatably coupled to a turbine 98 via shaft 100. Compressor 92 is-effective for compressing the airstream 16, which discharges compressor 92 via an outlet 102. The airstream 16 is then supplied to a first inlet 104 of a heat exchanger 106, via a conduit 108. Heat exchanger 106 further includes a first outlet 110, effective for discharging airstream 16 from heat exchanger 106. Heat exchanger 106 is also adapted to receive a flow of cooling fluid, indicated by flow arrows 112, in heat exchange, non-mixing relationship with the airstream 16 so as to provide cooling to the airstream 16. Accordingly, heat exchanger 106 is effective for off-setting at least a portion of the temperature rise caused by the compression of the airstream 16 within compressor 92. The cooling fluid 112 is supplied to a second inlet 114 of heat exchanger 106 and discharges from heat exchanger 106 through a second outlet 116.

Cooling fluid 112 may be provided from the sources discussed previously in conjunction with cooling fluids 66 and 86, and may be provided from either common or different sources as those used to supply cooling fluids 66 and 86. After discharging from outlet 110 of heat exchanger 106, airstream 16 is supplied to an inlet 118 of turbine 98 via a conduit 120. Turbine 98 is effective for expanding the airstream 16 and accordingly, for further cooling the airstream 16. Airstream 16 provides the motive force for rotating turbine 98, which in turn drives compressor 92 via shaft 100. The airstream 16 which has been cooled and has an ozone content

within acceptable levels, discharges from turbine 98 through an outlet 122 and is supplied to a conditioner sub-assembly 124, which functions in a subsequently described manner, via a conduit 126. The airstream 16 is then supplied from the conditioner sub-assembly 124 to a habitable space 127, comprising a passenger compartment of the aircraft in the illustrative embodiment shown in Fig. 8, via a conduit 129.

During certain operating conditions, the temperature of the airstream 16 may be below ambient temperature as airstream 16 discharges from the outlet 122 of turbine 98 such that any water vapor present in airstream 16 may be condensed into liquid water. To prevent ice formation, the environmental control system 50 includes a means for mixing the cooled airstream 16 discharging from turbine 98 with a relatively warmer airstream prior to supplying the resultant mixed airstream to the habitable space 127. The means for mixing the cooled airstream 16 with a relatively warmer airstream includes a control valve 128 and conduits 130 and 132. A portion of the airstream 16 discharging from the catalytic reactor 74 is routed through conduits 82 and 130 to an inlet of control valve 128, which establishes the desired flow rate of this airstream, and is then supplied to conduit 126 via conduit 132 with this heated airstream mixing with the cooled airstream 16 so as to produce a mixed airstream having a temperature which is greater than the temperature of the cooled airstream 16 as it discharges from the outlet 122 of turbine 98. The mixed airstream is then supplied to the conditioner sub-assembly 124. Accordingly, by setting control valve 128 to a predetermined position, the temperature of the mixed airstream supplied to the conditioner sub-assembly 124 may be such that ice formation is prevented.

The conditioner sub-assembly 124 comprises a condenser (not shown), an air/water separator (not shown) and a re-heater (not shown). The conditioner sub- assembly 124 is effective for separating any water contained within the mixed airstream prior to delivering the mixed airstream to the habitable space 127 and is

further effective for increasing the temperature of the mixed airstream to an acceptable level prior to delivering the mixed airstream to the habitable space 127 of the aircraft.

The specific construction of the catalytic reactor 74 according to a first preferred embodiment of the present invention is illustrated in Figs. 1, 2 and 9-11. The inlet portion 72 of housing 73 includes a flanged, upstream end 134 which is attached to a flanged downstream end of conduit 77 via a pair of clamps 136 (one shown in Fig.

9). Similarly, the outlet portion 76 of housing 73 includes a flanged, downstream end 138 which is attached to a flanged upstream end of conduit 82 via a pair of clamps 140 (one shown in Fig. 9). Alternatively, inlet portion 72 and outlet portion 76 of housing 73 may be attached to conduits 77 and 82, respectively, by other conventional means such as mating pairs of bolted flanges. In a preferred embodiment, inlet portion 72, cylindrical portion 75 and outlet portion 76 of housing 73 comprise a unitary construction. Additionally, housing 73 is preferably made of stainless steel.

Alternatively, housing 73 may be made of other metals such as aluminum, aluminum alloys, titanium and titanium alloys. In the preferred embodiment the core structure 10 is directly attached, by brazing, to the stainless steel housing 73 as subsequently discussed in greater detail. In other embodiments which utilize a material of construction for housing 73 which does not permit brazing of the core structure 10 to housing 73, reactor 74 includes an intermediate ring (not shown) made of stainless steel which is brazed to the core structure 10. In these embodiments, the intermediate ring may be directly attached to housing 73 by conventional means such as rivets, or alternatively the intermediate ring may be attached to housing 73 with a resilient mount structure (not shown) which accommodates differential thermal growth between the housing 73 and the intermediate ring. The resilient mount structure may include one or more springs and may be made of an elastomeric material. The inlet portion 72 of housing 73 comprises a diffuser which increases in diameter from a location just aft of the flanged upstream end 134 to a downstream end which is

connected to the cylindrical portion 75 of housing 73. As shown in Fig. 9, the inlet portion, or diffuser 72 merges smoothly with the cylindrical portion 75 of housing 73.

The inlet portion, or diffuser 72 is effective for reducing the velocity and increasing the static pressure of airstream 16. This static pressure recovery may be important in the overall operation of the environmental control system 50 in certain applications since it is advantageous to limit the pressure drop through system 50 so as to avoid excessive use of power by engine 52 to provide the required airflow to the habitable space 127.

The core structure 10 of the catalytic reactor 74 includes at least one fin assembly 12. with a portion of a single fin assembly 12 being illustrated in Figs. 1 and 2. Each of the fin assemblies 12 of core structure 10 is constructed, or fabricated from a catalytically-active metal alloy which is effective for decomposing ozone in the airstream 16, which comprises a central feature of the present invention. The specific composition of the catalytically-active metal alloy is subsequently discussed in further detail. According to one preferred embodiment, which is illustrated in Fig. 10, the core structure 10 comprises a plurality of the fin assemblies 12, with each of the fin assemblies 12 being configured as an annular ring. In this embodiment, the fin assemblies 12 are generally concentrically disposed relative to one another about an axially extending centerline axis 142 (shown in Fig. 9) of reactor 74. Due to the physical limitations of configuring the fin assemblies 12 as annular rings, the center of the space defined by the housing 73 may be occupied by a small diameter support tube 146 having a finned strip 148. Alternatively, the support tube 146 may be capped at both ends. In another preferred embodiment, illustrated in Fig. 12, the core structure 10 of the catalytic reactor 74 comprises a single fin assembly 12 which is wrapped upon itself in a spiral, or generally helical configuration. It is noted that the views of the fin assembly shown in Figs. 1 and 2 are prior to the fin assembly being configured as shown in either Figs. 10 or 12. Each of the fin assemblies 12 includes an inlet end

14 which is effective for receiving the flow of the ozone-containing airstream 16 and an outlet end 18 effective for discharging the airstream 16 from the core structure 10.

Each of the fin assemblies 12 further includes a plurality of fins 20 which are arranged in an axial succession of rows 22 of the fins 20 between the inlet end 14 and the outlet end 18 of the corresponding fin assembly 12. Each of the fin assemblies 12 is made by shaping a substantially flat, relatively thin sheet of the catalytically-active metal alloy of the present invention by conventional means such as stamping so as to form the plurality of the fins 20. The number of the rows 22 of fins 20, which are arranged between the inlet end 14 and the outlet end 18 of each fin assembly 12, may vary with the particular application of the catalytic reactor 74. Three of the rows 22 of fins 20 are illustrated in Fig. 1 and designated as 22A, 22B, and 22C. Rows 22A and 22B are further illustrated in the elevation view shown in Fig. 2. Each of the rows 22 includes a plurality of the fins 20 and defines a plurality of axially extending flow channels 24. Each of the fins 20 includes a leading edge 25. The fins 20 and the flow channels 24 of each of the rows 22 are off-set by a distance x (Fig. 2) relative to the fins 20 and the flow channels 24, respectively, of axially adjacent ones of the rows 22.

For instance. the fins 20 of row 22B are off-set with respect to the fins 20 of rows 22A and 22C. The off-set x corresponds to a circumferential off-set after the fin assemblies 12. or the single fin assembly 12, have been configured as shown in Figs. 10 and 12, respectively. Consequently, each fin assembly 12 is configured so as to define a plurality of tortuous flowpaths through the axially extending flow channels 24, defined by each of the rows 22 of fins 20, for the flow of the heated, ozone-containing airstream 16 between the inlet end 14 and the outlet end 18 of the corresponding one of the fin assemblies 12. This may be illustrated with reference to Fig. 1, although it should be understood that in actual use the airstream 16 does not flow through the fin assemblies 12 until they have been configured either as shown in Fig. 10 or in Fig. 12.

The airstream 16 initially flows through the flow channels 24 of row 22A, depicted in

Fig. 1. After discharging from the flow channels 24 of row 22A, a portion of the airstream 16 impacts the leading edges, indicated at 25B, of the fins 20 of row 22B, prior to flowing through the flow channels 24 of row 22B. This process is repeated as the airstream 16 encounters each subsequent row 22 of the fins 20. For instance, after discharging from the flow channels 24 of row 22B, a portion of the airstream impacts the leading edges 25C of the fins 20 of row 22C. The impact of the airstream 16 with the leading edges 25 of the rows 22 of fins 20, adds turbulence to the airstream 16.

Consequently, the use of the off-set rows 22 of fins 20 within fin assembly 12 results in a substantially turbulent flow of the ozone-containing airstream 16 between the inlet end 14 and the outlet end 18 of each fin assembly 12. Since airstream 16 is substantially turbulent a relatively high mass transfer is achieved between the ozone- containing airstream 16 and the subsequently described catalyst, from which each fin assembly 12 is constructed. The overall mass transfer between airstream 16 and the catalyst of reactor 74 is further enhanced by the subsequently described methods of brazing radially adjacent ones of fin assemblies 12 to one another, or brazing radially adjacent spirals of an individual fin assembly 12 to one another (depending upon the particular configuration of reactor 74), as subsequently discussed in greater detail. As shown in Fig. 2, each of the fins 20 has a generally rectangular cross-section in a preferred embodiment, as seen in an axial view, and includes a height H, an axial depth D, a thickness t, a pair of flats F, and an axial depth. Each fin 20 approximates a full sine-wave shape as illustrated in brackets in Fig. 2. The off-set x, as well as the overall size of the fins 20 may be varied to achieve the desired flow characteristics through each fin assembly 12 for a particular application. Each fin row 22 has a fin density which may also be varied with application. In the illustrative embodiments shown in Figs. 1, 2 and 9-12. each fin row 22 has a fin density of about 28 fins/in. as measured in a lateral direction prior to configuring the fin assembly 12 as an annular ring as shown in Fig. 10, or wrapping the fin assembly 12 upon itself in a spiral configuration

as shown in Fig. 12. However, it should be understood that the fin density of each row 22 of fins 20 may vary with application. The fin density of each of the fin assemblies 12 configured as an annular ring as shown in Fig. 10, or of the single fin assembly 12 shown in Fig. 12, varies across the radial extent of the fins 20 due to the accordion effect caused by the particular configuration of the fin assembly 12, as subsequently discussed in further detail. The use of a plurality of fin assemblies 12, as shown in Fig. 10, or of a single spiral-wrapped fin assembly 12 as shown in Fig. 12, provides a relatively light weight and compact core structure 10 which is effective for reducing the ozone content within airstream 16, by decomposing the ozone, to acceptable levels so that the airstream 16 may be subsequently supplied to the habitable space 124. Although each fin 20 preferably has a generally rectangular shape, the fins 20 may alternatively have other geometric shapes such as a generally triangular cross-section, or a generally trapezoidal cross-section.

In the embodiment shown in Fig. 10, each fin assembly 12 is configured as an annular ring by conventional means such as rolling or forming, with the fins 20 of opposite ends of each fin assembly 12 being hooked together so as to maintain the desired configuration as an annular ring. The radially outermost one of the fin assemblies 12 is brazed to the generally cylindrical portion 75 of housing 73 by inserting a thin braze foil (not shown) between the radially outer flat F of each of the fins 20 of the radially outermost fin assembly 12 and portion 75 of housing 73, and then heating the structure to the required braze temperature. In this embodiment, each of the fin assemblies 12 is brazed to radially adjacent ones of the fin assemblies 12.

This brazing is illustrated in Fig. 11, which is an enlarged view of a portion of Fig. 10 and illustrates a pair of radially adjacent fin assemblies, denoted as 12A and 12B in Fig. 11. A thin foil of braze alloy, indicated at 144, is placed between the radially outer fin assembly 12A and the radially inner fin assembly 12B. In the embodiment shown in Fig. 10 which includes a plurality of fin assemblies 12, with each fin assembly

12 configured as an annular ring, each foil 144 of braze alloy is configured as a continuous concentric ring prior to brazing and is disposed between a radially adjacent pair of the fin assemblies 12. Upon exposure to the braze temperature the braze foils 144 melt and capillary into the areas where the flats F of aligned and radially adjacent fins 20 contact one another. Additionally, in the preferred embodiment where housing 73 is made of stainless steel, a braze foil 144 is disposed between housing 73 and the radially outermost one of the fin assemblies 12 so as to braze the core structure 10 to housing 73. In the embodiment shown in Fig. 12, which includes a single fin assembly 12 wrapped upon itself in a spiral configuration, each foil 144 of braze alloy comprises a long, continuous ribbon which is wound up at the same time that the single fin assembly 12 is wrapped upon itself in a spiral configuration. The braze foil 144 is given one additional wrap to cover the radially outer spiral of the fin assembly 12 which provides the necessary braze alloy to attach the single fin assembly 12 to the generally cylindrical portion 75 of housing 73 or to an intermediate ring (not shown), depending upon the material of construction of housing 73. As shown in Figs. 10 and 11, the individual fins 20 of radially adjacent ones of the fin assemblies may randomly align, partially align or be misaligned. Placement of the braze foil 144 between each radially adjacent pair of fin assemblies 12 causes the aligned ones of the fins 20 of the adjacent pair of fin assemblies 12 to be brazed to one another, after exposure to the required brazing temperature. More specifically, the lower flats F of the radially outer fin assembly 12A is brazed to aligned ones of the upper flats F of the radially inner fin assembly 12B. This brazing of radially adjacent ones of the fin assemblies 12 prevents nesting between the radially outer one and the radially inner one of each radially adjacent pair of fin assemblies 12 so as to maintain structural integrity of the core structure 10. In a similar manner. the radially adjacent spirals of the individual fin assembly 12 shown in Fig. 12, are brazed to one another. The method of brazing radially adjacent ones of the fin assemblies 12 shown in Fig. 10 to one another, or of

brazing the radially adjacent spirals of the single fin assembly 12 shown in Fig. 12 to one another, may be referred to as statistical brazing. Statistical methods predict the number of brazed fins 20 and the distribution of braze joints within the core structure 10. This information may then be used in determining the structural characteristics of the core structure 10 for predicting the service life of core structure 10. In conventional fin structures, such as those used in heat transfer devices, a solid separator plate which may be referred to as a"tube plate"or a"splitter plate"is typically placed between adjacent fin assemblies to prevent the individual fins of the adjacent assemblies from nesting within one another. As previously discussed the statistical brazing methods of the present invention avoids nesting between the radially adjacent ones of the fin assemblies 12 or the radially adjacent spirals of an individual fin assembly 12, without using tube plates or splitter plates. Accordingly, the use of the statistical brazing method of the present invention provides additional space for the fin assemblies 12, or the spirals of the individual fin assembly 12, within reactor 74 as compared to an otherwise similar device using splitter plates or tube plates between the radially adjacent ones of the fin assemblies 12 or the radially adjacent spirals of the individual fin assembly 12.

The quality and distribution of the braze contacts within the core structure 10 of reactor 74 is enhanced by the mismatch in circumferential fin densities which occurs at the interface between each radially adjacent pair of fin assemblies 12. In the embodiment shown in Fig. 10, the lateral width of each fin assembly 12, prior to configuration as an annular ring, is sized based on an average radius of the resultant annular ring. Due to the accordion effect, or"fanning out"of the individual fins 20 of a fin assembly 12, the circumferential fin density as measured at the radially outermost portion of each fin assembly (corresponding to the radially outer flats F) is less than the circumferential fin density as measured at the radially innermost portion of the fin assembly (corresponding to the radially inner ones of the flats F). Additionally, the fin

assemblies 12 are sized such that the circumferential fin density of the radially inner one of a pair of radially adjacent fin assemblies 12 is always less than the circumferential fin density of the radially outer one of the pair of fin assemblies 12 at the interface locations. For instance, the circumferential fin density of fin assembly 12B, as measured at location 146 is less than the circumferential fin density of fin assembly 12A as measured at location 148, further reducing the likelihood that radially adjacent fin assemblies 12 will nest with one another, thereby further enhancing the structural integrity of the core structure 10. As discussed previously, in conventional fin structures, such as those used in heat transfer devices, splitter plates are typically inserted between the adjacent ones of fin assemblies within a stack. The resultant matrix in these conventional structures becomes very rigid and is incapable of being thermally compliant to the surrounding environment when in service. In contrast, the core structure 10 of the present invention is thermally compliant which results from the ability of radially adjacent ones of the fin assemblies 12 shown in Fig. 10, or radially adjacent ones of the spirals of the individual fin assembly 12 shown in Fig. 12, to stretch or shrink, i. e. to accordion, when the core structure 10 is required to change diameter due to its service environment. This flexibility also permits direct attachment of the core structure 10 to the housing 73, for the preferred embodiment where housing 73 is made of stainless steel, by brazing the radially outermost one of the fin assemblies 12 shown in Fig. 10 or the radially outermost spiral of the individual fin assembly 12 shown in Fig. 12 to the housing 73 as discussed previously, without the use of an intervening thermally compliant support structure notwithstanding the incompatible coefficients of thermal expansion of the stainless steel housing 73 and that of the subsequently discussed catalytically-active metal alloy of the present invention. Another disadvantage of the use of splitter plates in conventional fin assembly stacks is that the use of such plates detracts from the overall performance of the device as previously discussed by reducing the average turbulation"T'factor of the

device. Also, the use of splitter plates reduces the space available for highly turbulent fin assemblies. The additional space available and the higher average turbulation combine to provide a distinct performance advantage to a non-splitter-plate device, such as the core structure 10 of the present invention.

The catalytically-active metal alloy used to manufacture the fin assemblies 12 comprises a silver-containing metal alloy and, in a preferred embodiment, has a composition including silver and copper as principle constituents. An example of an alloy which the inventors have determined to be acceptable for use in the present invention has a composition comprising, on a weight basis, about 55% silver, about 39% copper, about 5% zinc and about 1% nickel. This composition corresponds to the following atomic ratios: Ag: Cu: Zn: Ni: 30: 36: 4.5: 1. As subsequently discussed in greater detail, the inventors have identified silver as the active catalyst and accordingly, the catalytically-active metal alloy of the present invention must comprise a silver-containing metal alloy. However, it is considered to be within the scope of the present invention to utilize metals other than copper as a principle constituent in combination with silver.

During initial testing of a portion of one of the fin assemblies 12, which had been constructed using the catalytically-active metal alloy having the aforementioned atomic ratios of 30: 36: 4.5: 1, the inventors observed that the silver-containing metal alloy was not catalytically active at low temperatures, as shown in the"ramp up" portion of Fig. 3. However, upon further testing, the inventors determined that the silver-containing metal alloy became catalytically-active for ozone decomposition after being thermally activated, and remained catalytically active into the low temperature region as evidenced in the"ramp down"portion of Fig. 3. It is noted that the test results shown in Fig. 3 corresponded to an ozone concentration of 2. 36 ppmv and a flow rate of 1, 000, 000 GHSV (STP). As shown in Fig. 3, after the silver-containing metal alloy has been thermally activated, the alloy is extremely active for ozone

decomposition, providing mass-transfer-limited performance at temperatures as low as 100°C. Accordingly, for low temperature applications, i. e. temperatures in which the ozone-containing airstream 16 is less than about 149°C the silver-containing metal alloy must be activated by calcining, or heating the alloy in air to a temperature ranging from about 149°C to about 216°C for a period of time ranging from about 30 minutes to about 60 minutes. For higher temperature applications, i. e. those in which the ozone-containing airstream 16 is at least 149°C the thermal activation step may be omitted.

In an effort to gain an increased understanding of this phenomena, a detailed thermodynamic analysis of the surface composition of the fin assemblies 12 as a function of temperature was performed. Again, the composition of the silver- containing metal alloy corresponded to atomic ratios of : 30 : 36: 4.5: 1. The results of the thermodynamic analysis, illustrated graphically in Fig. 4, show that the surface composition of the silver-containing metal alloy (which is exposed to air) varies with temperature. At ambient temperature, the equilibrium surface composition is a mixture of CuO, Ag2O, ZnO and NiO. As shown in Fig. 4, this composition changes with temperature. As the temperature increases, Ag20 decomposes, forming Ag° and °2 There is a strong correlation between the activation temperature required to obtain high ozone destruction activity and the predicted change in equilibrium surface composition. The results shown in Fig. 4 confirm that Ag° metal is the species which is catalytically-active for ozone decomposition. These results were further verified in the subsequently discussed Examples and the associated examination of an as-received silver-containing metal alloy surface and an activated silver-containing metal alloy surface which was operated for greater than 300 hours in ozone air feeds at a variety of temperatures.

The following examples are provided to demonstrate some of the benefits which may be achieved by following the teachings of this invention.

EXAMPLE 1 A laboratory-scale reactor was assembled to include a section of a fin assembly having 2 rows of fins, with each row of fins including 8 fins. (Refer to Figs. 1 and 2).

Measured fin dimensions were approximately as follows: fin height (H in Fig. 2) was . 181 in. (4.60 mm) ; fin thickness was 0.0036 in. (. 091 mm); and fin axial depth (D in Fig. 1) was. 177 in. (4.50 mm). The lateral fin density in each row was 16 fins/in. The fin assembly section was constructed from a silver-containing metal alloy having a composition (D in Fig. 1) comprising about 55% Ag, about 39% Cu, about 5% Zn and about 1 % Ni. The section of the fin assembly was mounted in the laboratory-scale reactor and air containing 2.3 ppm by volume ozone was flowed through the fin assembly, so as to contact multiple surfaces of the silver-containing metal alloy, at 1 x 106 GHSV at STP and at the following five temperatures: 50°C, 100°C, 150°C, 200°C, and 250°C. The ozone conversion was measured, after 60 hours of operation using a PCI Ozone Monitor (Model LC) before and after the laboratory reactor. The results are presented in Table 1. Additionally, SEM-EDX analysis was performed on the surface of an"as-received"sample of the silver-containing metal alloy, as well as the surface of an aged sample. The low magnification (500x) SEM images of the"as received"and aged, or activated silver-containing fin surfaces are shown in Figs. 5 and 6, respectively.

TABLE 1 Ozone Destruction Performance 1,000,000 GHSV (STP); 2.3 ppmv Ozone Inlet Predicted Mass Initial Conversion After Temperature Transfer-Limited Conversion 60 hours Conversion Operation (HegedusModel) 250°C 46% 44% 44% 200°C 44% 40% 39% 150°C 42% 42% 45% 100°C 41% 40% 40% 50°C 38% 27% 17%

* note mass transfer-limited calculations may have an error as great as +/-3% conversion in this range The column labeled"Predicted Mass Transfer Limited Conversion"shown in Table 1 provides a reference value for comparison with the actual conversion as measured experimentally. The calculation assumes that the catalyst is able to convert any ozone which reaches it, that is, the chemical reaction is not limiting. The calculation then is based on the reaction rate which should be observed if mass transfer of the reactants and products to and from the catalyst is limiting (L. Hegedus, American Chemical Society, Chicago Meeting, August, 1973, pgs 487-502). This method is for straight channel monoliths and has been modified by the inventors to

account for the off-set fin design. If the actual conversion measured is the same as that which the mass transfer-limited calculation predicts, then it follows that mass transfer is limiting in fact and that the catalyst activity is not. Conversely, if the conversion is lower than the mass transfer limited conversion, then the catalyst activity is limiting. It may be seen from the results shown in Table 1 that mass transfer was limiting except at the temperature of 50°C.

The"as-received"surface of the silver-containing metal alloy had a"white- copper"sheen. As shown in Fig. 5, the"as-received"metal alloy surface was relatively smooth in appearance with sub-micron size surface striations (possibly due to milling of the alloy during manufacture). In contrast, the activated silver-containing metal alloy surface had a dull gray appearance which was observed immediately after thermal activation. As shown in Fig. 6, the surface of the activated silver-containing metal alloy is roughened with particles which are about 1-10 microns in diameter.

Further analysis of both the"as-received"and activated fin samples using EDX confirmed that the"particles"observed on the surface of the activated silver- containing metal alloy contained high concentrations of silver. In contrast, copper and oxygen were found in extremely low concentrations. This result supported the thermodynamic analysis discussed previously in conjunction with Fig. 4, verifying that silver metal is the active site for ozone decomposition.

EXAMPLE 2 A laboratory reactor was assembled to a test section of a fin assembly, having the same number of rows and fins, and made from the same alloy, as that described in Example 1. Durability testing was conducted to determine the ability of the silver- containing metal alloy to recover from a temporary poisoning with SO2, as follows.

Initially, a"clean"feed air containing 2.3 ppm by volume of ozone was flowed through

the fin assembly at 1 x 106 GHSV at STP and at the following temperatures: 100°C, 150°C, 200°C and 250°C. After 20 hours of testing with the"clean"feed, 1 ppm by volume SO2 was added to the ozone-containing air for a period of 5 hours, after which the aforementioned"clean"feed was used for an additional 75 hours, at each temperature. The ozone conversion was measured after 1 hour, 5 hours, 20 hours, and 60 hours of exposure to the ozone-containing air. The results of the test are shown in Table 2 and Fig. 7. SEM-EDX analysis of a poisoned alloy was conducted for comparison. Our spectroscopic analysis was in agreement with the hypothesis that the catalytically active sites were poisoned with sulfur. SEM-EDX of a poisoned catalyst showed that the sulfur was associated with regions which were high in silver content : in contrast. areas which were high in copper were relatively free of sulfur.

TABLE 2 Ozone Destruction Performance 1,000,000 GHSV (STP); 2.3 ppmv Ozone Durability in lppmv SO2 Recovery Inlet Predicted Initial After lhr 5hrs 20hrs 60hrs Temp Mass Transfer-Conversion 5 hours Limited ("clesn"SO2 Conversion feed) (Hegedus Model) 250°C 46% 49% 2% 29% 40% 44% 44% 200°C 44% 39% 0% 26% 37% 39% 40% 150°C 42% 39% 1% 32% 39% 39% -- 100°C 41% 43% 0% 36% 39% 39% 40%

It is well documented that sulfur can act as a catalyst poison, attenuating performance by either"masking"sites or by converting the active material into an inactive compound. Using the present silver-containing metal alloy, SO2 can either adsorb on the surface of the Ag particles ("masking"the sites) or the Ag and S02 can react in this oxidizing environment to form catalytically inactive Ag2 S04, which is extremely stable when formed. As illustrated in Table 2 and Fig. 7 (which corresponds to the 200°C test), a dramatic attenuation in performance was observed as soon as the SO was introduced. However, as shown in both Table 2 and Fig. 7, a rapid recovery to mass-transfer-limited performance was observed. This behavior was observed for-

each temperature range. Catalyst poisoning by surface adsorption of SO2 can be reversible depending on the strength of the catalyst-adsorbate bond. The rapid recovery in catalyst performance after SO2 removal, shown in Table 2 and Fig. 7 demonstrates that this poisoning was reversible over the temperature range tested.

Accordingly, the temporary performance attenuation observed was due to"masking" of the catalyst sites by SO2. These results demonstrate that the silver-containing metal alloy is not irreversibly poisoned by sulfur contaminates in ozone-containing air feeds.

The use of the silver containing metal alloy in the catalytic reactor of the present invention provides a cost efficient, and relatively maintenance free, catalytic reactor for the decomposition of ozone in air. The present reactor is cost reduced relative to prior reactors, due to the elimination of process steps, since it is not necessary to apply a washcoat to, or anodize, a metallic substrate. Additionally, since the alloy itself comprises the active catalyst, the catalyst will not be removed during routine maintenance cleaning. Furthermore, the catalyst will not attrit when exposed to the vibration and thermal cycling which may be experienced when the reactor is used in an aircraft application. Accordingly, the catalytic reactor 74 of the present invention has an increased service life relative to the known devices discussed in the Background section of this application. The use of the silver-containing metal alloy of the present invention in conjunction with the configuration of the core structure 10 which incorporates at least one fin assembly 12 and the statistical brazing methods of the present invention, provides relatively high mass transfer between the ozone and the silver catalyst, and accordingly permits the use of a compact, lightweight reactor.

Additionally, the statistical brazing methods of the present invention enhance the mass transfer of the catalytic reactor 74 of the present invention due to the avoidance of the use of splitter plates or tube plates which also minimizes the weight of the catalytic reactor 74 of the present invention which is particularly important in aircraft applications. Furthermore, the statistical brazing methods of the present invention

permit the core structure 10 of the catalytic reactor 74 to expand and contract with changes in environment temperature without the use of an intervening thermally compliant structure.

While the foregoing description has set forth the preferred embodiments of the invention in particular detail, it must be understood that numerous modifications, substitutions and changes can be undertaken without departing from the true spirit and scope of the present invention as defined by the ensuing claims. The invention is therefore not limited to specific preferred embodiments as described, but is only limited as defined by the following claims.