Patent Publication Number: US-11661888-B2

Title: Heat exchangers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation and claims the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 14/296,603, filed on Jun. 5, 2014, which claims priority under 35 U.S.C. § 119(a) to United Kingdom Patent Application Nos. GB 1318098.9, GB 1318109.4, GB 1318100.3, GB 1318107.8, and GB 1318099.7, each filed on Oct. 11, 2013. Each of the aforementioned U.S. and United Kingdom patent applications is hereby incorporated by reference herein. 
    
    
     FIELD 
     The present invention relates to heat exchangers such as of the type which may be used in aerospace applications or in industrial or other applications. The invention also relates to engines such as aircraft or aerospace engines including such heat exchangers and to aircraft including such heat exchangers or engines. 
     BACKGROUND 
     GB-A-2241319 discloses a heat exchanger with inter-nested spiral tube sections. An inlet manifold is located on one wall and an outlet manifold is located on another wall at the end of the apparatus. However, it is difficult to build such a heat exchanger which will be subject to considerable temperature variations and in a form capable of enduring the numerous cycles that are desirable in certain heat exchangers such as when used in reusable vehicles. 
     Also, with a substantial airflow through the heat exchanger radially past the tubes, the drag of the airflow on the tubes produces a high radial force on the tubes and the tubes are difficult to support and make last a long time. Additionally, the heat exchanger has rather limited cycle functionality and it is difficult to configure heat exchangers, to accommodate them in an engine and provide a system with low weight and low pressure drop. Also, to achieve sufficient performance it has proven necessary to design a cycle which cools atmospheric air before it enters a compression section downstream of such a heat exchanger at a temperature below 0° C. This can unfortunately very easily result in a very rapid blocking of the heat exchanger with ice because there is often a considerable amount of water vapour in the lower atmosphere where the launch of a vehicle containing the heat exchanger for air cooling may take place, making the entire vehicle unviable. This has proven an exceptionally difficult problem which has been looked at for many years and has been a significant hurdle in being able to provide a reusable vehicle which can operate in a relatively economical horizontal take-off air-breathing mode, and either continue, such as the LAPCAT vehicle which is intended to be capable of transporting three hundred passengers from Brussels, Belgium to Sydney, Australia in about 4.6 hours in an approximately Mach 5 cruising mode, or switch, like the SKYLON vehicle, into a rocket mode for travel out of the atmosphere into orbit. 
     GB-A-2241537 discloses an air intake for aerospace propulsion which includes a first heat exchanger for cooling incoming air, a water separator downstream of the first heat exchanger, a liquid oxygen injector downstream of the water separator and a second heat exchanger downstream of the liquid oxygen injector. The injector reduces the airflow temperature so that water remaining in the airflow downstream of the water separator is converted to small dry ice crystals. The structure requires the use of two entirely separate heat exchangers and takes up a very considerable axial distance in a flow path along a duct between the two heat exchangers. Also, the liquid oxygen is used to reduce the temperature of the flow from between 5° and 13° C. to minus 50° C. or lower, such that a considerable amount of liquid oxygen needs to be used, to cool the air in a duct where there is already air containing oxygen which may be used downstream in the propulsion. 
     SUMMARY 
     According to a first aspect, there is provided a heat exchanger having at least one first conduit section for the flow of a first fluid in heat exchange with a second fluid in a flow path which passes the at least one first conduit section, and a support for the at least one first conduit section, wherein the at least one conduit section is mounted at a first location to the support, and in which the at least one conduit section at a second location thereon is movable relative to the support in response to thermal change. 
     Optionally, the at least one first conduit section includes a plurality of tubes for heat exchange. 
     Optionally, the tubes are connected at a first end thereof to an inlet header and at a second end thereof to an outlet header of the at least one first conduit section. 
     Optionally, said first location is at one of the headers, which is fixedly mounted to the support, the other of the headers being movable relative to the support in response to thermal change. 
     Optionally, the other of the headers is mounted to a movable support therefor which is movable relative to said support. 
     Optionally, each said first conduit section comprises a spiral section having a plurality of the tubes extending along in a spiral shape alongside and spaced from one another in rows. 
     Optionally, the tubes in said first conduit section are arranged in between 1 and 40 rows spaced from one another in a radial direction, for example 4 such rows. 
     Optionally, the tubes are arranged in about 10 to 1000 rows spaced from one another in an axial direction, for example about 70 to 100 such rows. 
     Optionally, the tubes are about 2 to 3 metres long and extend from a first header to a second header. 
     Optionally, the tubes have a diameter which is about 1 mm. 
     Optionally, the tubes have a wall thickness of about 20 to 40 microns. 
     Optionally, a plurality of said spiral sections are inter-nested with and oriented angularly spaced relative to one another. 
     Optionally, said spiral sections are configured in the shape of a generally cylindrical drum. 
     Optionally, the support includes at least one circular hoop to which said first conduit is secured. 
     Optionally, the support includes a plurality of said circular hoops which are configured spaced apart from one another in a generally cylindrical perforated drum structure and in which a header of said first conduit is fixed to a plurality of said circular hoops. 
     Optionally, the heat exchanger includes a support structure extending in an annular fashion and at least partly radially between said header and a further header of the respective at least one first conduit, the further header being supported by a guide member arranged to move circumferentially relative to the perforated drum structure in response to thermal change. 
     Optionally, said header is rigid or substantially rigid and is fluidly coupled to a flexible conduit. 
     Optionally, the flexible conduit is fluidly coupled to a rigid or substantially rigid manifold. 
     Optionally, the manifold is fixed axially in position relative to the support but free to move, e.g. grow, radially. 
     Optionally, the rows of tubes in a said first conduit section comprise a plurality of rows spaced from one another by spacers arranged to counter aerodynamic load applied to the tubes. 
     Optionally, a load element, such as a shim, is provided between tubes of two adjacent said first conduit sections for transmitting load therebetween while allowing relative sliding motion therebetween in response to thermal change. 
     Optionally, the load element is substantially aligned with the spacers to form a generally radially extending load path structure for reaction against aerodynamic load applied to the tubes while allowing relative movement between adjacent said first conduit sections in response to thermal change. 
     Optionally, said heat exchanger includes a plurality of said load path structures configured in a series in which they are spaced generally circumferentially from one another. 
     According to a second aspect, there is provided a vehicle engine including a combustion section and a heat exchanger according to the first aspect with or without any optional feature thereof, adapted to cool air (as the second fluid) in a flow path directed towards the combustion section. 
     Optionally, said vehicle engine includes a helium supply for providing helium as the first fluid, or another working fluid such as hydrogen. 
     According to a third aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes a heat exchanger according to the first aspect with or without any optional feature thereof. 
     According to a fourth aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes an engine according to the second aspect with or without any optional feature thereof. 
     According to a fifth aspect, there is provided a heat exchanger having a plurality of first conduit sections for the flow of a first fluid in heat exchange with a second fluid in a flow path which passes the first conduit sections, and a support for the plurality of first conduit sections, each of the first conduit sections comprising a plurality of tubes for heat exchange, each first conduit section comprising a spiral section having a plurality of the tubes extending along in a spiral shape alongside and spaced from one another in rows, wherein at least one load element is provided between tubes in mutually radially spaced rows for countering aerodynamic load applied to the tubes. 
     Optionally, said load element comprises a spacer fixing together, such as by brazing, tubes in radially spaced rows. 
     Optionally, said load element comprises an element, such as a shim, provided between tubes of two adjacent said first conduit sections for transmitting load therebetween while allowing relative sliding motion therebetween in response to thermal change. 
     Optionally, said element is fixed to a tube in one said first conduit section and slidably engages a further said first conduit section. 
     Optionally, said load element comprises at least one I-beam-shaped element. 
     Optionally, the tubes in a said first conduit section are arranged in between 1 and 40 rows spaced from one another in a radial direction, for example 4 such rows. 
     Optionally, said tubes are about 1 to 3 metres long from a first header to a second header. 
     Optionally, the tubes have a diameter which is about 1 mm. 
     Optionally, the tubes have a wall thickness of about 20 to 40 microns. 
     Optionally, the tubes are arranged in about 10 to 1000 rows spaced from one another in an axial direction, for example about 70 to 100 or 70 to 200 or 500 such rows. 
     In some embodiments tubes are made at least partly of alloy material, such as Nickel alloys or Aluminium alloys. Optionally, each tube comprising a first portion made of a first material and a second portion made of a second material. The first portion may terminate at one end of such a tube, such as at a header and the second portion may terminate at a second end of such a tube as at a further header. The first and second portions may be connected to one another such as by a sleeve. This may enable temperature capability to be increased by the use of a first material at a relatively hot region of the heat exchanger but a second material for reasons of density or lost to be used in a colder region. 
     For example, an aluminium alloy may be used in a colder region and a ceramic or heat resistant alloy in a hotter region. Each of the first and second portions may be connected at one end thereof to the intermediate header. 
     Optionally, the plurality of spiral sections are inter-nested with and oriented angularly spaced relative to one another. 
     Optionally, said spiral sections are configured in the shape of a cylindrical drum. 
     Optionally, the support includes at least one circular hoop to which a first said conduit is secured. 
     Optionally, the support includes a plurality of said circular hoops which are configured spaced apart from one another in a generally cylindrical perforated drum structure, and in which at least one longeron member is provided for engagingly supporting an adjacent said tube at a location substantially radially aligned with at least one said load element. 
     Optionally, a plurality of said load elements are provided in a generally radially extending load path structure for reaction against aerodynamic load applied to the tubes. 
     Optionally, the load path structure is adapted to permit relative movement between tubes of adjacent first said conduit sections in response to thermal change. 
     According to a sixth aspect, there is provided a vehicle engine including a combustion section and a heat exchanger according to the fifth aspect with or without any optional feature thereof, adapted to cool air, as the second fluid, in a flow path directed towards the combustion section. 
     Optionally, said vehicle engine includes a helium supply for providing helium as the first fluid. 
     According to a seventh aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes a heat exchanger according to the fifth aspect of the invention with or without any optional feature thereof. 
     According to an eight aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes an engine according to the sixth aspect with or without any optional feature thereof. 
     According to a ninth aspect, there is provided a heat exchanger having at least one first conduit section for the flow of a first fluid in heat exchange with a second fluid in a flow path which passes the at least one first conduit section, in which each first conduit section comprises a flow path via at least one tube from an inlet to an outlet, and in which an intermediate header is provided in the flow path between the inlet and outlet for flow communication with an intermediate fluid flow path. 
     Optionally, each of the inlet and outlet comprises a header tube. 
     Optionally, the header tubes are straight. 
     Optionally, the at least one first conduit section includes a plurality first flow tubes extending from the inlet to the intermediate header for flow therebetween and a plurality of second flow tubes extending from the intermediate header to the outlet for flow therebetween. 
     Optionally, the length of a first flow tube plus the length of a second flow tube is about 2 to 3 metres. 
     Optionally, the first flow tubes and/or the second flow tubes are about 1 mm in diameter. 
     Optionally, the first flow tubes and/or the second flow tubes have wall thickness of about 20 to 40 microns. 
     Optionally, said first conduit section comprises a spiral section having the first and second flow tubes extending in a spiral shape alongside and spaced from one another in rows. 
     Optionally, said heat exchanger includes a controller for controlling pressure in the intermediate flow path. 
     Optionally, the controller includes a flow valve. 
     Optionally, the intermediate header comprises an outer enclosure for enclosing first fluid and an injector for injecting intermediate fluid flow into the outer enclosure. 
     Optionally, the injector comprises a tube having a series of flow apertures spaced therealong for injecting fluid into the outer enclosure. 
     Optionally, each of the outer enclosure and the injector comprises a straight elongate tube. 
     Optionally, said rows include a plurality of rows spaced apart from one another along the longitudinal direction of the outer enclosure, the tubes in said spaced apart rows being fluidly coupled to the outer enclosure at respective spaced locations along the length thereof. 
     In some embodiments tubes are made at least partly of alloy material, such as Nickel alloys or Aluminium alloys. Optionally, each tube comprising a first portion made of a first material and a second portion made of a second material. The first portion may terminate at one end of such a tube, such as at a header and the second portion may terminate at a second end of such a tube as at a further header. The first and second portions may be connected to one another such as by a sleeve. This may enable temperature capability to be increased by the use of a first material at a relatively hot region of the heat exchanger but a second material for reasons of density or lost to be used in a colder region. 
     For example, an aluminium alloy may be used in a colder region and a ceramic or heat resistant alloy in a hotter region. Each of the first and second portions may be connected at one end thereof to the intermediate header. 
     A tenth aspect provides a heat exchanger having at least one first conduit section for the flow of a first fluid in heat exchange with a second fluid in a flow path which passes the at least one first conduit section, in which each first conduit comprises a flow path via at least one tube from an inlet to an outlet, wherein at least one of said tubes has first and second portions extending therealong which are formed of different materials to one another. 
     According to an eleventh aspect, there is provided a method of operating a heat exchanger according to the ninth aspect with or without any optional feature thereof, which comprises flowing helium through the at least one first conduit and through the intermediate fluid flow path. 
     Optionally, said method of operating a heat exchanger comprises flowing air as the second fluid in the flow path past the at least one first conduit. 
     According to a twelfth aspect, there is provided an engine, such as a vehicle engine, including a combustion section and a heat exchanger according to the ninth or tenth aspect with or without any optional feature thereof, adapted to cool air (as the second fluid) in a flow path directed towards the combustion section. 
     Optionally, said engine includes a helium supply for providing helium as the first fluid. 
     According to a thirteenth aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes a heat exchanger according to the ninth or tenth aspect with or without any optional feature thereof. 
     According to a fourteenth aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes an engine according to the twelfth aspect with or without any optional feature thereof. 
     According to a fifteenth aspect, there is provided a support structure for a heat exchanger having at least one first conduit section for the flow of a first fluid in heat exchange with a section fluid in a flow path which passes the at least one first conduit section, the support structure comprising a generally cylindrical perforated drum structure. 
     Optionally, said support structure includes a plurality of mutually axially spaced hoop supports. 
     Optionally, said support structure includes a plurality of mutually radially spaced longeron members which are adapted to supportingly engage the said first conduit sections at a generally radially aligned load path structure. 
     Optionally, the hoop supports are formed with bearers and/or attachment structures for locating header tubes of the first conduit sections on the hoop supports. 
     Optionally, the hoop supports and longeron members are configured with generally rectangular or square flow spaces therebetween. 
     Optionally, said support structure including at least one diagonally mounted bracing element extending across and within or adjacent at least one of the spaces, for example diagonally thereacross. 
     Optionally, each said space has two diagonally mounted said bracing elements configured in an X configuration thereby providing four substantially triangular flow apertures in the region of each said space. 
     According to a sixteenth aspect, there is provided a heat exchanger for cooling a fluid in a flow path and containing a component liable to phase change below a temperature thereof, the heat exchanger comprising a series of tubes for the passage of coolant and an injector for introducing an anti-freeze component into fluid in the flow path. 
     Optionally, the injector comprises an antifreeze injector, e.g. arranged to inject an alcohol, such as a methanol injector arranged for connection to a source of methanol. 
     Optionally, said heat exchanger includes a removal arrangement for removing liquid located within a body of moving fluid within the flow path. 
     Optionally, the removal arrangement is located downstream of the injector. 
     Optionally, the removal arrangement includes at least one catcher arrangement or element arranged to extend generally transverse to a general flow direction of fluid therepast. 
     Optionally, said heat exchanger includes at least one row of said catcher elements. 
     Optionally, the catcher elements are spaced apart in the row by a spacing distance which is less than the maximum cross-dimension of each catcher element in a direction of spacing thereof. 
     Optionally, the spacing distance is about one quarter, one third or half of said maximum cross-dimension. 
     Optionally, the catcher arrangement includes a section row of catcher elements, the second row of catcher elements being staggered (in a direction from one catcher element to another along a row) with the first row of catcher elements. 
     Optionally, the catcher elements in the first and second rows are spaced from the catcher elements in the same row substantially the same distance that they are spaced from the catcher element in the other row. 
     Optionally, each catcher element has a longitudinal extent and has at least one point therealong a substantially circular cross section. 
     Optionally, each catcher element comprises a hollow tube having at least one (such as a plurality of) liquid collection pocket located on an exterior surface thereof. 
     Optionally, the hollow tube is hydroformed of metallic material. 
     Optionally, each liquid collection pocket includes a scavenge aperture communicating from the pocket into an interior conduit of the hollow tube, the scavenge aperture area being of small size for limiting airflow drawn thereinto with liquid. 
     Optionally, said heat exchanger includes a mesh covering each pocket with a cavity formed between each pocket and the mesh. The mesh may be generally flat in the region of each pocket. The gap behind each such portion of mesh provides the cavity. The physical presence of the catcher element enables air to jig around the catcher element but for liquid to be intertially separated from the air. With the mesh in place, the liquid in the flow may travel into and be caught be the mesh but the air may continue flowing along around the catcher elements. 
     Optionally, the mesh is an approximately 25 micron to 100 micron mesh, for example a 50 micron mesh. 
     Optionally, the mesh is coated with a wetting agent. 
     Optionally, the mesh is coated with silica. 
     Optionally, said heat exchanger includes a suction system fluidly connected to the hollow tube for removing liquid from the collection pockets. 
     Optionally, said heat exchanger includes a second said removal arrangement which is located downstream of the said removal arrangement. Embodiments with three of more said removal arrangements are envisaged as well. 
     The antifreeze injector may include a plurality of injection portions and may be arranged to inject a more concentrated antifreeze at a first injection portion in a first region of air flow and a more diluted (with water) antifreeze at a second injection portion which is more upstream than the first injection portion at a second region of air flow that is warmer than the first region. The water is obtained by condensation out of the air passing through the heat exchanger due to humidity in the air. 
     The heat exchanger may include a recycle path for recycling antifreeze and liquid water removed from air flow at the second (or further) removal arrangement for re-injection, upstream of the first removal arrangement, at the second injection portion. Each such re-injection in a plural sequence of such re-injections optionally thus may be with the anti-freeze being more dilute, into warmer air and more upstream in the airflow than upon the previous injection, the anti-freeze thus being re-injectable along a sequence of re-injection points along a path that is counter to the direction of air flow through the heat exchanger. This may thus be considered a kind of “counterflow” of the antifreeze injection system even though the antifreeze, upon injection into the air flow, flows along with the air. The reason for the additional dilution is the condensation of water out of air passing through the heat exchanger. The caught or captured antifreeze (e.g. methanol) and water may then be re-injected (with the antifreeze thus more dilute) into the air flow at a point further upstream than the previous injection at a location where the air flow is warmer. This recycling of antifreeze enables sustained operation without having to use too much antifreeze since the same antifreeze may be injected then caught; and it may then be re-injected and caught again two or more times dependent upon the number of removal arrangements with catcher elements that are employed through the heat exchanger. This also allows the methanol content to become more and more concentrated as the flow becomes colder. 
     Optionally, said heat exchanger is adapted to cool air to below 0 degrees C. 
     Optionally, said heat exchanger is adapted to cool air to or to below about minus 100 degrees C., such as down to near minus 140 degrees C., or down to air liquefaction point—about −195 degrees C. 
     Optionally, said heat exchanger includes a control for controlling the environment in the vicinity of the coldest catcher element to be about 80 mol % or about 88 wt % methanol (on a water-methanol solid-liquid phase diagram) as temperature approaches about minus 100 degrees C. The environment in the vicinity of the coldest catcher element may as the temperature approaches or drops below about −100 degrees C. may in some embodiments be about 70 to 90 mol % methanol (on a water methanol solid-liquid phase diagram), such as about 75 to 85 mol % or 78 to 82 mol %. 
     Optionally, the catcher arrangements are configured and constructed with a sufficient number of catcher elements to remove over 90% of water content from air, such as over 95%, about 99% being one example. 
     According to a seventeenth aspect there is provided a heat exchanger assembly with a longitudinal extent in a longitudinal direction thereof and which comprises at least one generally annular heat exchanger module arranged to communicate with a longitudinally extending duct, wherein at least one guide vane is provided for turning flow between one and the other selected from (a) generally radial through the heat exchanger module and (b) generally longitudinal along the longitudinally extending duct. 
     The guide vane may adapted to turn flow from generally radial to generally longitudinal. 
     The guide vane may be annular or ring-like. 
     The guide vane may have a leading edge and a trailing edge and is optionally of substantially constant thickness between the leading edge and the trailing edge. 
     The leading edge may arranged at an angle of about 5 to 20 degrees relative to a radial direction, e.g. about 10 degrees. This may vary outside this range in some embodiments. 
     The trailing edge may be arranged at an angle of about 5 to 15, or about 8 to 12, degrees to the longitudinal direction. This may vary outside these ranges in some embodiments. 
     The guide vane may have a longitudinal extent in the longitudinal direction and the guide vane may comprise a curvedly-flaring leading section (which is optionally substantially an arc in cross section) and a substantially conical trailing section, the trailing section extending for about 50 to 85% of the longitudinal extent. 
     The heat exchanger assembly may include a plurality of said guide vanes which are optionally arranged in a mutually overlapped series along the longitudinally extending duct. 
     The vanes may be arranged with a narrowing therebetween so as to accelerate flow. In this way flow may be accelerated to substantially the same speed at each vane as a bulk exit speed from the longitudinally extending duct. This assists in maintaining pressure and mass flow rate uniform upstream of the vanes and through the heat exchanger module. 
     The heat exchanger assembly may include a plurality of said heat exchanger modules arranged in a series along and around said longitudinal duct and a series of said guide vanes may be provided extending adjacent and at least substantially the full longitudinal extent of at least one, optionally all, of said heat exchanger modules. This assists in maintaining pressure and mass flow rate uniform upstream of the vanes and through the heat exchanger modules. Thus two or more generally annular said heat exchanger modules may be configured around the longitudinally extending duct and arranged in a series therealong. Flow may pass overall and/or generally inwardly radially through each heat exchanger module into the longitudinally extending duct. The vanes enable the creation of a generally constant static pressure in the duct downstream of them and therefore uniform flow is drawn through the heat exchanger modules. 
     Although the heat exchanger modules may be the same size (including diameter or maximum cross-dimension thereof) as one another, they may be different sizes to one another in other embodiments, for example being a series of generally drum-like arrangements with different diameters to one another. The modules may nevertheless be adapted to operate with the same mass flux and pressure drop as one another and may include the same number of rows of heat exchanger tubes as one another, and may in some embodiments have the same difference outer diameter to inner diameter (of where tubes are in the module). 
     According to an eighteenth aspect there is provided an engine which includes a heat exchanger according to the seventeenth aspect, the heat exchanger being located upstream of an air compressor and/or combustion stage of the engine. 
     According to an nineteenth aspect, there is provided an engine, such as a vehicle engine, which includes a combustion section and a heat exchanger according to the sixteenth aspect with or without any optional feature thereof adapted to cool air in a flow path directed towards the combustion section. 
     Optionally, said engine includes a helium supply for providing helium as coolant flowable through the heat exchanger. 
     According to a twentieth aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle, which includes a heat exchanger according to the sixteenth aspect with or without any optional feature thereof. 
     According to an twenty-first aspect, there is provided a flying machine, such as an aircraft or orbital launch vehicle which includes an engine according to the eighteenth or nineteenth aspect with or without any optional feature thereof. 
     Thus, in some embodiments the heat exchanger is generally drum-shaped and includes a multitude of spiral sections each containing numerous small diameter metal alloy tubes. The spiral sections spiral inside one another. The tubes in some examples are each about 2 to 3 metres long and about 1 millimetre in diameter and have a wall thickness of about 20 to 40 microns or thereabouts. 
     Each spiral section has an axially-extending coolant inlet header tube which is fixedly attached to a central support sleeve. The tubes of each spiral section are sealingly connected at one end thereof, such as by brazing, to the inlet header tube and extend in a small plurality of rows radially (such as about 1 to 10 rows, 2, 3, 4, 5, 6 or 7 rows being some examples) and in a large number of rows axially such as about 75 to 100 rows or in excess of 100 rows. These various thin tubes all spiral out from the inlet header tube to an outlet header tube to which they are also sealingly connected, such as by brazing. In other embodiments, the flow direction could be reversed such that the inlet headers are at a radially outward location and the outlet headers at a radially inward location and air may flow through the heat exchanger in a radially outward direction in a generally counter-flow to the coolant in the tubes which may be helium. 
     Also, as temperature changes during operation of the heat exchanger or otherwise, the inlet header tubes remain fixed substantially in position relative to the internal support sleeve. However, due to the thermal expansion, the length of the small diameter tubes changes and the various spiral sections slide over one another in a generally circumferential fashion, the outlet header tubes also moving relative to the internal support sleeve. Therefore, the heat exchanger may be thermally cycled many times due to substantial alleviation of thermal stress, even though over 100 or up to 200 or more full flight cycles. 
     Also, at various points along their length, the tubes in each spiral section may be supportingly fixed together such as by brazing and each row of tubes which is adjacent to a row of an adjacent spiral section may be provided with a baffle plate, spacer or shim which extends fully along the tubes in the axial direction and slightly radially approximately 1 to 10 millimetres or so. The baffle plates or shims in adjacent spiral sections may slideably abut against one another or a baffle plate or shim of one spiral section may slideably engage upon adjacent fluid flow tubes of an adjacent spiral section. Due to the supporting fixing of the tubes in each section and the sliding abutment, not only are all of the spiral sections easily able to expand and contract as they desire during thermal cycling but the radial drag load of heatant such as air, passing through the heat exchanger outside and between the small diameter tubes may be transmitted inwardly (or outwardly) through all of these spiral sections in a strong substantially radial load path and may be reacted against by the internal support sleeve, or by an external support sleeve in the case of radially outward air flow. 
     With counter flow heat exchange between radially travelling air flow (heatant) and oppositely radially spiraling helium coolant flow in the narrow tubes, curved ducting may be provided radially outside the heat exchanger to curve approaching axial air flow into the heat exchanger or exiting axial air flow leaving the heat exchanger in the case of outwardly travelling airflow though the heat exchanger. Due to this type of construction, the heat exchanger tubes are relatively well protected and in the event of a problem such as bird strike or other ingestion causing an engine failure after take-off (EFATO), the helium tubes are unlikely to be damaged, the engine may easily be shut down and, with or without dumping fuel to come below maximum landing weight, the vehicle may have a good probability of performing a successful landing with the respective engine shut down. 
     The internal sleeve may be provided with a perforated or “bird cage” construction to enable the heatant, such as air, to pass therethrough without significant losses. The sleeve may comprise circular hoops at either end thereof, as series of mutually circumferentially spaced and axially extending longeron members, such as about 10 to 50 said longeron members, as well as one or more intermediate circumferential hoops space in a series along the support sleeve. The longeron members may be spaced from one another roughly the same distance that the hoops are spaced from one another to form generally square shapes or spaces which may be reinforced by x-shaped diagonal members such that all of the perforations through the internal support sleeve are triangles. This structure has been found to provide a lightweight form which is noticeably strong for resisting substantially radially inward loads which may be placed thereon without producing significant flow losses and also carries gravitational and inertial loads during flight. In the case of an arrangement with radially outwardly travelling air, a similar support sleeve or bird cage could be provided radially outside the coolant tubes. 
     It is possible to arrange one or more intermediate header tubes located part of the way along the route of the small diameter tubes from the inlet heads thereof to the outlet headers thereof. Thus, the tubes may conceptionally be thought of as “cut” part way along their length and with each of the cut ends attached to the intermediate header tube. An internal intermediate supply tube may be inserted within the intermediate header tube to extend substantially all of the way along its length, the intermediate supply tube having a series of outlet apertures formed in a series therethrough a long its length. During operation of the heat exchanger coolant may be fed into the heat exchanger not only at the inlet header tubes but also at the internal intermediate supply tubes such that further coolant may be added at the intermedial header tubes and different mass flow rates of coolant may flow through different parts of the heat exchanger, allowing the single heat exchanger construction to act as though it is a plurality of different heat exchangers but with roughly only the cost and complexity of building one. It will be appreciated that to add the further coolant at the intermediate header tubes, the pressure of the supply at the intermediate header tubes may be controlled to be similar or the same as the pressure of the nearby coolant inside the small diameter tubes. The use of the internal intermediate supply tubes with the series of outlet apertures formed in a series therethrough ensures that the uniform flow conditions can be achieved along the axial length of the heat exchanger. 
     The heat exchanger of the preferred embodiment to be described herein includes a frost control system and after an actual test carried out on a test heat exchanger in confidential conditions for the European Space Agency on behalf of the United Kingdom government, the European Space Agency confirm a successful demonstration of the frost control mechanism at laboratory scale. The tested heat exchanger included a total of over 40 kilometres of the small diameter tubes at a weight of less than 50 kilograms and the incoming airstream was cooled to minus 150° C. in under 20 milliseconds. No frost blockage was noted during low temperature operation and the European Space Agency stated they are now confident that a ground engine demonstrating test may now be performed. 
     The heat exchanger thus may be provided with one or more dogleg zones in each spiral section, each dogleg zone in including a short radially extending section. Thus, each spiral section is not fully spiral in shape but comprises a first substantially spiral portion then a short radial portion which is followed by another substantially spiral portion and so on. This construction causes the provision of a substantially arcuate box shape between the doglegs of adjacent spiral sections. 
     Within the arcuate box spaces are located a series of one or two radially spaced and circumferential staggered rows of catcher elements. Each catcher element may comprise a tube having an interior which can be connected to a suction system for sucking on the contents of a plurality of exterior pockets spaced along the capture element on the leading face thereof, the pockets being connected via through-apertures to the interior of the tube. The capture element leading face pockets may be covered with 50-micron silica-coated mesh to form a porous front face to the pocket. A wetting agent other than silica may be used in other embodiments as may meshes of different sizes. 
     In this embodiment, a methanol, or other anti-freeze substance, injection system is located upstream of the capture elements and as water condenses out of the flow it mixes with the methanol which maintains the water as a liquid rather than freezing and this mixture is separated from the air flow by inertial separation upon contact with the wet mesh of the capture elements, then being sucked by vacuum via the pockets and through-apertures and carried away along the capture elements. When there are a plurality of said dogleg zones in each spiral section, a further set of similar catcher elements may be employed in further-resulting arcuate box spaces. In this case, in an example when the air flow is radially inward and is being cooled, for example by helium in the narrow tubes, pure or relatively concentrated methanol may be injected at or just upstream of the radially innermost arcuate box section and then at least partially caught in a mixture with water by the respective catcher elements, and this more dilute mixture may then be recycled further radially outboard in the heat exchanger where the temperature is higher and caught again. Thus, the methanol may be considered in this embodiment to run in overall radial counterflow relative to the air which reduces the required methanol consumption to prevent freezing and blockage. 
     The methanol could be fully separated from the water after use but it may alternatively be allowed to pass with the engine supply air to a combustion section of the engine where it may contribute to thrust. While the thrust control system is running, the methanol&#39;s (combustion product) weight ejected backwards from the main engine rocket nozzles may in some embodiments add 2% to thrust. Also, the loss of the methanol&#39;s weight from the vehicle may be desirable in order to enable a higher vehicle velocity to be achieved in response to engine thrust later on during flight. 
     The frost control system described herein can remove typically 99% of the water content from air. 
     The conditions in the heat exchanger may be carefully controlled such that operation when considered on a water-methanol solid-liquid phase diagram the environment in the vicinity of the coldest catcher elements is in the region of about 80 mol % or about 88 wt % methanol as the temperature approaches about minus 100° C. 
     The two above-mentioned radially spaced and circumferentially staggered rows of catcher elements can be configured with the leading row elements circumferentially about half way between the adjacent elements of the trailing row. The leading capture elements may act as bluff bodies which deflect condensed fluid, which may have built up on the narrow coolant-containing tubes, in the flow generally towards the capture elements in the trailing row. This means that about 95% of the fluid removed by the two rows may be removed by the trailing row and only 5% by the leading row. Thus, in some embodiments, the leading row of capture elements may be replaced by a passive row of bluff body elements, which could be flat sheets with slots or slits opposite the capture elements. 
     The heat exchanger may be provided with deflector-shaped foil shims in the region of the short radially extending portions of the small diameter tubes in order to ensure that the liquid flow is directed towards the capture elements and does not easily take a short cut across the tubes in the regions of these radial sections. Also, the gaps between the tubes in the radially extending portions are preferably sealed. 
     Thus, it is anticipated that the frost control system may be used in the low atmosphere to remove water content from the air flow such that icing up and blockage of the heat exchanger does not occur. As the vehicle travels up towards the top of troposphere and towards the stratosphere, for example at an altitude of about 10 kilometres, thereabouts or somewhat higher, there is no longer enough water vapour present to cause troublesome icing and the frost control system can be switched off by shutting down methanol pumps and capture element suction pumps. 
     Even though the heat exchanger could cool the air down to its liquification point if necessary near the air outlet from the heat exchanger, i.e. near the internal bird cage when the flow is radially inward, the frost control system is set up so that the great majority of the water is removed with methanol at a higher temperature further back in the heat exchanger. At very low temperatures, down below about minus 50° C. and all of the way down to near minus 140° C. any remaining methanol-water liquid content will, if it solidifies, turn directly to mass ice rather than the feathery frost that can be formed at higher temperatures, by direct sublimation from the vapour, and so causes less of a blocking issue. 
     When a reusable vehicle in which the heat exchanger is used, such as a SKYLON vehicle or similar craft, is travelling back into the atmosphere at high speed, air inlet nacelles to the engine may be closed. Even with no air passing through the heat exchanger, helium may be cycled around the small diameter tubes in order to prevent overheating of the heat exchanger and potentially also nearby components due to aerodynamic heating via the outside body of the engine upon atmospheric re-entry. 
     In other embodiments, fluids other than helium may be flowed along inside the heat exchanger tubes, such as hydrogen. Instead of acting as a helium/air heat exchanger, the heat exchanger may act as a hydrogen/other fluid heat exchanger (for example). The heat exchanger tubes are generally circular in cross-section in the preferred embodiment although other shapes could be used in other embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention may be carried out in various ways and one preferred embodiment of a heat exchanger, an engine and an aircraft in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG.  1 A  is a side elevation of one preferred embodiment of an aircraft incorporating an engine with a heat exchanger in accordance with a preferred embodiment of the present invention; 
         FIG.  1 B  is a top plan view of the aircraft of  FIG.  1 A ; 
         FIG.  1 C  is a rear elevation of the aircraft of  FIG.  1 A ; 
         FIG.  2    is a schematic view of a prior art engine; 
         FIG.  3    is a schematic cycle diagram for the engine of  FIG.  2    which has been modified to include a heat exchanger in accordance with a preferred embodiment of the present invention; 
         FIG.  4 A  is an isometric view of a preferred embodiment of a heat exchanger in accordance with the invention as used as a modified heat exchanger in a modification of the engine of  FIG.  2    with a cycle as illustrated in  FIG.  3   ; 
         FIG.  4 B  is a detail view on area A of  FIG.  4 A ; 
         FIG.  5 A  is a section on Y-Y in  FIG.  5 B  of the heat exchanger; 
         FIG.  5 B  is a side elevation of the heat exchanger viewed in a radially inward direction; 
         FIG.  6    is an isometric view of a drum-shaped bird cage or perforated sleeve support for the heat exchanger; 
         FIG.  7 A  is a detail view of detail A in  FIG.  7 E ; 
         FIG.  7 B  is a detail view on detail B in  FIG.  7 E ; 
         FIG.  7 C  is a detail view on detail C in  FIG.  7 F ; 
         FIG.  7 D  is a detail view on detail D in  FIG.  7 F ; 
         FIG.  7 E  is a schematic section through the spiral section of the heat exchanger shown in  FIG.  7 F  as though looking up from below in  FIG.  7 F ; 
         FIG.  7 F  is an isometric view of a spiral section of heat exchanger tubes of the heat exchanger; 
         FIG.  8 A  is a view similar to  FIG.  7 F  but with a substantial proportion of the heat exchanger tubes of the spiral section removed for the purposes of clarity so as to show positions of baffle stations along the curved generally circumferential length of the heat exchanger section; 
         FIG.  8 B  is a detail view isometric on detail E in  FIG.  8 A ; 
         FIG.  8 C  is a detail view isometric on detail F in  FIG.  8 A ; 
         FIG.  9 A  is rear elevational view on the heat exchanger in the direction  9 A in  FIG.  9 B ; 
         FIG.  9 B  is a cross-sectional view of the heat exchanger on the two planes represented by the line A-A in  FIG.  9 A ; 
         FIG.  10 A  shows the support member of  FIG.  6    with optional longeron shoes fitted to longeron elements of the support and with the components of the spiral section shown in  FIG.  8 A  fitted in a spiraling fashion therearound; 
         FIG.  10 B  shows a detail isometric view on detail A in  FIG.  10 A ; 
         FIG.  11 A  is an end view of the support of  FIG.  6    with all 21 spiral sections fitted but with the capture elements and a methanol injection ring omitted for the purposes of clarity; 
         FIG.  11 B  shows an elevational detail view on detail A in  FIG.  11 A ; 
         FIG.  12 A  shows a front elevational view of the methanol injector assembly of the heat exchanger; 
         FIG.  12 B  shows a section on the plane A-A in  FIG.  12 A ; 
         FIG.  12 C  is a detail view on detail B in  FIG.  12 B ; 
         FIG.  12 D  is a detail view on detail C in  FIG.  12 B ; 
         FIG.  12 E  is a partly shortened cross-sectional view on the plane D-D in  FIG.  12 G  showing one of the injector tube assemblies of the injector assembly; 
         FIG.  12 F  is a detail view on detail E of  FIG.  12 E ; 
         FIG.  12 G  is an elevational view along the direction  12 G in  FIG.  12 E  of the injector tube assembly; 
         FIG.  12 H  is an isometric view of the injector assembly of  FIG.  12 A ; 
         FIG.  13 A  is an isometric view of part of an outer capture assembly for removing liquid from the heat exchanger in the frost control system; 
         FIG.  13 B  is a view in isometric of part of the components shown in  FIG.  13 A ; 
         FIG.  13 C  is an end view of the components shown in  FIG.  13 A ; 
         FIG.  13 D  is a section on the plane C-C in  FIG.  13 C ; 
         FIG.  13 E  is a view similar to  FIG.  13 C  but with a capture manifold plate of the capture assemble removed; 
         FIG.  13 F  is a detail view on detail D in  FIG.  13 E ; 
         FIG.  13 G  is an elevational view in a circumferential direction (around the drum center axis of the heat exchanger) of the parts shown in  FIG.  13 A ; 
         FIG.  13 H  is a section on the plane A-A in  FIG.  13 G ; 
         FIG.  13 I  is a detail view on the detail B in  FIG.  13 H ; 
         FIG.  14 A  is an elevational view in a circumferential direction of a capture tube of the capture assembly as hydroformed; 
         FIG.  14 B  shows the cross-sectional profile of the hydroformed capture tube of  FIG.  14 A  at a circular end spigot thereof; 
         FIG.  14 C  shows the profile section of the capture tube of  FIG.  14 A  at a raised point of the profile shown in  FIG.  14 A , at one end of each of two adjacent liquid capture pockets; 
         FIG.  14 D  shows the profile of the tube mid-way between two such pocket ends; 
         FIG.  14 E  shows a perspective view of the tube of  FIG.  14 A ; 
         FIG.  14 F  shows a sectioned perspective view of the tube of  FIG.  14 A  with a silica-coated mesh wrapped tightly therearound; 
         FIG.  15    is a schematic view of a modified version of the spiral sections including an intermediate header; 
         FIG.  16    is a schematic view showing the heat exchanger and intermediate header fluid flow path; 
         FIG.  17    shows a schematic view of part of the spiral section viewed in the direction  17  in  FIG.  15   ; 
         FIG.  18    shows schematically a modified injection system in which methanol (or other antifreeze) is recycled and injected more than once into the air flow; 
         FIG.  19 A  shows a shim plate used with catcher elements of the capture assemblies; 
         FIG.  19 B  shows the shim plate of  FIG.  19 A  positioned relative to the catcher elements; 
         FIG.  19 C  is a partial end view of the heat exchanger; 
         FIG.  19 D  is a detail view on detail E of  FIG.  19 C ; 
         FIG.  20 A  is a front view of the heat exchanger with circular catcher ring manifolds and S-shaped connection hoses thereof removed for the purposes of clarity; 
         FIG.  20 B  is a detail view on detail A of  FIG.  20 A ; 
         FIG.  21 A  is a schematic view of the heat exchanger placed around an outlet duct together with two passive units, the outlet duct leading in a longitudinal direction towards an air compressor of the engine; and 
         FIG.  21 B  is a detail view on detail B of  FIG.  21 A . 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS.  1 A,  1 B and  10   , an aircraft  10  with a retractable undercarriage  12 ,  14 ,  16  has a fuselage  18  with fuel and oxidant stores  20 ,  22  and payload region  24 . A moving tail fin arrangement  26  and all moving canard arrangement  28  are attached to the fuselage  18 . Main wings  34  with elevons  36  are attached to either side of the fuselage  18  and each wing  34  has an engine module  38  attached to a wingtip  40  thereof. As shown in  FIGS.  1 C and  2   , a rear of each engine module  38  is provided with four rocket nozzles  40  surrounded by various bypass burners  42 . 
     As shown in  FIG.  2   , the prior art engine module  38  includes an air inlet  43 , heat exchanger  44  in four parts, a turbo compressor  46  and cycle conduits  48 . In an air breathing mode of the engine module  38  inside the Earth&#39;s atmosphere, part of the incoming air passing through the air inlet  43  passes through the heat exchanger  44  to the turbo compressor  46  and part is bypassed along bypass duct  50  to the bypass burners  42 . 
     In the preferred embodiment, the prior art heat exchanger  44  is replaced with the heat exchanger or pre-cooler  52  or a plurality of said heat exchangers  52  operable in parallel. 
       FIG.  3    shows a schematic view of the cycle for the modified engine module  38 , which has been simplified for the purposes of clarity to show only one rocket nozzle  40  instead of four. 
     Thus, as shown in  FIG.  3   , in the air breathing mode, air enters the intake  43  and passes either to the heat exchanger  52  or via the bypass duct  50  to the bypass burners  42 . The air passing through the heat exchanger  52  is then compressed in a compressor  54  of the turbo compressor  46  before passing through valve  56  to rocket nozzle  40  and partly also to the pre-burner  58  before passing through heat exchanger  60  before passing with uncombusted hydrogen in pre-burner exhaust products for further combustion at the rocket  40 . 
     A liquid hydrogen pump  62  driven by a turbine  64  drives hydrogen through a heat exchanger  66  and the turbine  64  through a turbine  68  of a helium circulator  70  to the pre-burner  58  for partial pre-burner combustion, although some of the liquid hydrogen may be bypassed by valve  72  along the conduit  74  for combustion in the bypass burners  42 . 
     The helium circulator  70  includes compressor  76  which drives gaseous helium through the heat exchanger  52  in counter-flow heat exchange with the air passing in the opposite direction (considering radial components of the paths of the air and helium), the helium then passing through heat exchanger  60  before travelling through helium turbine  78  before being cooled by the hydrogen in heat exchanger  66  and then passing back to the helium compressor  76 . In this air breathing mode, the aircraft  10  is able to take off horizontally from stationary on the ground  80  ( FIG.  1 A ). 
     Once the aircraft  10  is travelling at a significant speed of approximately Mach 5, it may switch from the air breathing mode into a full rocket mode. In the full rocket mode, the air inlet  43  is closed; the path of hydrogen through the cycle conduit  48  is similar to that in the air breathing mode although no hydrogen is diverted by the valve  72  to the bypass burner  42 . The turbo compressor  46  is inactive. In the helium circuit, the gaseous helium now flows from the helium compressor  76  through heat exchanger  52  and heat exchanger  60  but then to turbine  82  before returning to the heat exchanger  66  and then to the helium compressor  76 . The helium turbine  82  drives liquid oxygen pump  84  which directs oxygen towards the rocket nozzle  40  as well as partly via the pre-burner  58  and then as a pre-burner exhaust product together with unburnt hydrogen to the rocket nozzle  40  for combustion therein. The valve  56  is closed in the full rocket mode. 
     In the full rocket mode, the aircraft  10  may accelerate up through high Mach numbers and into orbit. 
     As shown in  FIG.  6   , the heat exchanger  52  has a support in the form of a central bird cage or generally cylindrical perforated drum  84 . The drum  84  has two end support rings  86  spaced apart from one another and three intermediate support rings  88  which are spaced apart in an even series between the end support rings  88 . A series of 21 longeron members  90  extend along between the two end support rings  86  and are supported by all five of the support rings  86 ,  88 . The longeron members  90  comprise substantially flat radially aligned and thin plates. The longeron members  90  are attached by threaded bobbin members  92  to the two end support rings  86  and have a series of three slots  94  near radially inner edges  96  thereof. The slots  94  extend approximately one quarter to three quarters of the radial extent along each longeron member  90  and enabling supporting engagement between the longeron members  90  and the intermediate support rings  88 . The intermediate support rings  88  also have slots, which correspond, to enable the engagement shown in  FIG.  6   . 
     The support drum  84  also includes an internal stiffener tube  98  which comprises a perforated tubular element having longitudinal members engaging along the full length of each of the longeron members  90 , circumferentially extending members  102  extending along the full circumferential extent and engaging each of the support rings  86 ,  88 , as well as X-shaped bracing members  104  located adjacent to generally square spaces  106  formed between the various longeron members  90  and support rings  86 ,  88 . The stiffener tube  98  therefore forms a very stiff perforated lattice within the drum  84  which is designed to carry shear loads. The drum is capable of accepting a high radially inward load and the four triangles formed in the region of each X-shaped bracing member  104  enable substantial airflow to pass radially through the perforated drum  84  without a significant pressure drop. 
     As shown in  FIG.  10 A , 21 spiral conduit sections, only one of which is shown and which is only partly shown in this Figure, may be secured around the perforated drum with an inlet header  106  of each modular spiral section  108  secured to the support drum  84 , the spiral section  108  spiraling outwardly radially away from the central axis of the drum  84  as it extends circumferentially substantially 360° around the support drum  84  to an outlet header tube  110 . Each spiral section  108  may be modified in other embodiments to extend a greater or less angle around. As can be seen in  FIG.  10 A , the modular spiral section  108  includes first  112 ′ and second  114 ′ dogleg sections containing short radially extending sections of heat exchanger tubing as will be described below. In other embodiments more than two such dogleg sections may be used, such as three, four or five in each spiral section  108 . 
       FIG.  10 B  shows how the inlet header tube  106  includes a series of five mounting flanges  112  (only some of which are shown in  FIG.  10 B ) which have apertures (not shown) engaged by a locking rod  114  to lock the inlet header tube  106  in position to each of the support rings  86 ,  88 . As can be seen in  FIG.  10 B , an optional longeron shoe  116 ′ covers the radially outer edge of each longeron member  90  to protect the spiral sections  108  which may engage therewith. The longeron shoes  116 ′ are not used in other embodiments.  FIG.  8 A  also shows the location of baffle stations  116  which are regularly spaced along the spiral sections  108 . 
       FIG.  8 B  shows that the outlet header  110  (and the inlet header  106  is similar) has within it a series of 800 apertures  118  configured to accept 800 corresponding helium heat exchange tubes  120  only three of which are shown in  FIG.  8 B . Other embodiments may employ fewer or, indeed, more such tubes  120 . The helium tubes  120  are brazed into the apertures  118 . To prepare the helium tubes  120  for brazing, preferably the tubes are scanned for defects, wall thickness is measured (OD), a pressurization test is conducted, and the tubes are optionally electrochemically milled, washed, dried, and then cut and formed to shape. The brazing is preferably carried out under vacuum. 
     The helium tubes  120  are arranged in 200 rows spaced along the axial direction of the headers  106 ,  110  and four rows radially. The helium tubes  120  extend all of the way along from each inlet header  106  to each outlet header  110 . Since there are 21 spiral sections  108  and each tube  120  is approximately 2 to 3 meters long, the heat exchanger  52  contains approximately 40 kilometres of the tubes  120 . The tubes  120  are approximately 1 millimetre in diameter or somewhat more and have a wall thickness of about 20 to 40 microns. 
       FIG.  8 C  shows how the tubes  120  pass the doglegs  112 ′,  114 ′ with short radial portions  122  and J-shaped foils or joggles  124  located either side of the doglegs  112 ′,  114 ′ to direct flow away from the radial portions  122 . This is also shown in  FIG.  7 B . As can be seen in  FIG.  7 A , at the baffle stations  116 , race track section baffle elements  125  are brazed to the inner side of each of the four rows of helium tubes  120  in any given spiral section  108  and a flat shim plate  126  is based to the outer side of the radially outer tubes  120  in the four rows. The baffle elements  125  are made by flattening tube the same as the helium tubes  120 . The baffle elements and shim plates  126  extend axially the full length of all of the 200 rows of tubes  120  in the axial direction. As shown in  FIG.  11 B , the baffle stations  116  of the 21 spiral sections  108  are substantially aligned with one another wherever there is a longeron member  90  so as to be orthogonal to the matrix tubes  120 , although offset slightly angularly from the true radial direction R (see  FIG.  11 B ) from the central axis  128  of the heat exchanger  52  (see  FIG.  11 A ). Therefore, the shoe  116 ′ on the longeron member  90  engages the bottom baffle element  125  of a first spiral section  108  and radial force can be transmitted through the baffle station  116  to the shim plate  126  which engages the bottom of another radially inner baffle element of the next inter-nested spiral section. After the 11th spiral section  108 , a radially extending I-beam  130  may transmit the substantially radial load to the next spiral section  108 . After a further seven sets of baffle elements  125  and shims  126  of a further seven spiral sections  108 , there is a further substantially radially extending I-beam  130  for transmitting radial load and there are then a further three sections  108  that are passed in the radial direction until the spiral sections  108 , all twenty-one of them, have been fully counted. This configuration is illustrative only and may differ in other embodiments. 
     When the heat exchanger is operational, there is a substantial inward flow radially of air past all of the tubes  120  placing a substantial radially inward aerodynamic load on them. This load is countered by the substantially aligned shim plates  126 , baffle elements  125 , I-beams  130  and longeron members  90  which are aligned substantially in the radial direction. Accordingly, despite the very substantial aerodynamic loads, the tubes  120  may be securely supported. 
     In some cases, particularly at high Mach numbers, the air inlet temperature to the heat exchanger  52  at the radially outermost side in particular may be substantial, for example over 800 or even 1000° C. The temperature variation may cause a significant thermal change to the tubes  120  which, in particular, may grow in length with increasing temperature. Therefore, although the inlet header tube  106  is fixed in position to the perforated support drum  84 , the outlet headers  110  may move as the tubes  120  grow in length. The shim plates  126  of each spiral section  108  may therefore slide relative to the adjacent baffle elements  125  to enable sliding substantially circumferential motion of the spiral sections  108  relative to one another. With lengthening of the tubes  120  due to increased temperature, the line of thrust of the baffle stations  116  through the baffle elements  125 , shim plates  126  and I-beams  130  may rotate to be more in line with the truly radial direction of R from the centre axis  152  of the exchanger  52 . The line of the baffle elements  125  is maintained substantially orthogonal to the tubes  120 . Therefore, thermal expansion and contraction of the tubes in the circumferential (lengthwise) section of the tubes  120  may be allowed for. Naturally, the tubes  120  and other components may expand in the radial direction as they expand and contract with temperature and allowance for this is also provided. 
     As can be seen in  FIG.  7 D , each inlet header tube  106  is fitted with inlet header fittings  140  at either end thereof and the outlet header tubes  110  are fitted with outlet header fittings  142  at the respective two ends thereof. As shown in  FIGS.  4 A and  5 B , the inlet header fittings  140  at one end of the inlet headers  106  are fluidly connected by flexible hoses  144  to the inlet manifold  146  for the helium flow, and outlet header fittings  142  at the same end thereof are fluidly connected by flexible hoses  148  to helium outlet ring manifold  150  for the helium. 
     The inlet header fittings  140  at the other end of the header tubes  106  may be blocked off or may be fluidly connected via a ring manifold to the adjacent header fitting  140 . The same is so for the outlet header fittings  142  at the other end of the outlet header tubes  110 . 
     As can be seen for example in  FIGS.  11 A and  11 B , generally rectangular (but arcuate) pockets (or box sections)  160  are created extending axially all of the way along the heat exchanger  52  between the tubes  120  of adjacent spiral sections  108  in the regions of I-beams  130  and the respective doglegs  112 ′,  114 ′. These generally rectangular but arcuate pockets  160  contain capture elements  162  of a frost control system of the heat exchanger  52  which will now be described. 
     As shown in  FIGS.  4 A and  12 A , a methanol supply tube  170  feeds methanol to an annular gallery  172  of a methanol injection ring  174 . The parts  176  in the gallery  172  are sections through tubes  214  where these enter the gallery  172 . The methanol injection ring has a series of 180 injector tubes assemblies  182  ( FIG.  12 E ) (this arrangement could differ in other embodiments) located in a series spaced circumferentially around the heat exchanger  52 , each injector tube assembly  182  including an injector tube  184  extending axially between annular manifold  173  forming the annular gallery  172  and a further support ring  175  into which tapered ends  190  of end plugs  192  are fixed. Each of the injector tubes  184  includes a series of 22 injection holes  194  (one of which is shown in  FIG.  12 F ) spaced therealong in a series along the tube  184 , the diameter of each of the holes  194  being 0.2 mm. The end  196  of each tube opposite the tapered end  190  is secured to an injector fitting  198  having a non-circular flange  200  with a flat  202  for rotationally aligning the tube  184  by matching of the flat  202  with a corresponding formation in a support ring  177  attached to the ring  173 . A circular seal  204  is provided for sealing in an annular recess  206  of the fitting  198 . 
     When a methanol pump  210  ( FIG.  3   ) is operated to pump methanol along a conduit  212  which is connected to the methanol supply tube  170 , the methanol is forced through the gallery  172  and through supply channels  214  of the ring  173  and interior channels  216  of the fittings  198  to pump methanol into the interior of the tubes  184  and then out through the injection holes  194 . Accordingly, methanol is injected into airflow just about to enter into the volume defined by the spiral sections  108  of the tubes  120 . 
     The methanol is able to prevent the formation of ice in the heat exchanger  52  which would block the airflow through the same. The methanol lowers the freezing temperature of water droplets condensed out within the heat exchanger to do this and a substantial proportion of the methanol and water are together removed from the airflow by catcher elements as will now be described. 
     Each outer one of the two pockets  160  shown in  FIG.  11 B  has nine mutually spaced catcher elements located therein and each inner one of the pockets  160  has seven catcher elements  162  located therein. With reference to  FIGS.  13 A to  14 D  of the drawings, the catcher elements  162  will now be described. These Figures show one outer catcher assembly  240  having nine catcher elements. The inner catcher assemblies are similar apart from the number of catcher elements (since they only have seven). As shown in  FIGS.  13 A and  13 H , each catcher assembly  240  has an outer row  242  of four catcher elements  162  and an inner row  244  of five catcher elements  162 . Again, the numbers of catcher elements and the numbers of doglegs and rows (radially) of pockets may differ in other embodiments. Each catcher element  162  comprises a hydro-formed tube  246  with a varying cross-section along the length thereof as shown in  FIGS.  14 A to  14 D . In particular, the tube  246  is circular in section at respective ends  248 ,  250  thereof and has eight pocket depressions  252  formed therein between peaks  254  where the cross-section of the tube  246  is as shown in  FIG.  14 C , the cross-section a mid-point  256  between the peaks  254  being as shown in  FIG.  14 D  in which there are two outwardly concave portions  258 . As shown in  FIG.  13 I , the tubes  246  are covered with a stainless steel 50 micron filter mesh  258  (other meshes may be used in other embodiments) which is coated with a wetting agent such as silica. Each mesh  258  is generally cylindrical (it may vary in other embodiments) and forms a pair of cavities  260 ,  262  at each of the pocket depressions  252 , such that there are sixteen cavities  260 ,  262  in total per catcher tube  246 . Scavenge holes  264  are positioned through the wall of the tube  246  in each cavity  260 ,  262 —this number may differ in other embodiments. The catcher assembly  240  includes a control plate  270  and one end  248  of each catcher tube  246  is fitted to a non-rotational fitting  272  such that all nine catcher element tubes  246  communicate with the interior of a catcher suction manifold assembly  274  having a suction port  276  leading, via piping  278  ( FIG.  3   ), to suction source  280 . The catcher elements  162  are capable of operating upside down and also when used in any axially or otherwise accelerating flying vehicle. 
     As shown in  FIGS.  13 A and  13 D , the distal ends  282  of the catcher elements  246  are securely closed by end plugs  284 . 
     It will be seen that the catcher control plates  270  includes circular apertures  290  therethrough. The outlet header tubes  110  of the heat exchanger  52  pass through these apertures  290 . It will also be seen that the catcher plate  270  includes nine extra apertures  292 . The reason for this is that the adjacent catcher assembly  240  spaced one circumferentially around the heat exchanger  52  is arranged with the manifold  272  and end plugs  284  at opposite ends. Therefore, the apertures  292  serve to engage around the circular ends  250  of the catcher elements  162  of the adjacent catcher assembly  240  so that all of the catcher tubes  246  are supported at both ends. With a vacuum applied at the suction port  276 , there is suction at the scavenge holes  266 . With methanol and water in liquid form in the region of the catcher element  162 , when the liquid touches the silica coated mesh  258 , it becomes wetted onto the mesh  258  and is sucked through the mesh (the cavities  262  operating at a low pressure due to the small mesh pores), into the cavities  262 ,  260 /pockets  252 , then through the scavenge holes  266  and along inside the catcher tubes  246  to the respective manifold  274  and suction port  276 . The scavenge holes  266  control the air flow sucked through with the liquid to a low level. In this way, a substantial proportion of the water vapour in the incoming air may be removed from the flow such that the heat exchanger  52  does not block with ice. Whereas the mesh  258  is shown with a circular section for the purposes of clarity in  FIG.  13 I , as shown in  FIG.  14 F , the mesh in fact stretches out to a flatter configuration over the cavities  262 / 260 /pockets  252 . As shown in  FIG.  14 E  the peaks (or lands)  254  are configured generally flat. The lands  254  engage the mesh  258  so as to separate the pockets from one another. Thus, if one pocket should lose its integrity, for example if debris is ingested into the heat exchanger, the other pockets  252  will remain operational. 
     As shown in  FIG.  4 B , the outer catcher suction manifold assemblies  274  are connected by flexible S-shaped vacuum tubes  300 —the S-shape allows for manufacturing differences and for thermal expansion but need not be employed in other embodiments—to annular frost control catcher ring manifold  302  and the inner catcher element manifold plates  304  (corresponding to the manifold plates  274  but for only seven catcher elements) are connected by similar S-shaped flexible hoses  306  to frost control catcher ring manifold  308 . The ring  308  leads to a vacuum outlet conduit  309  and ring  306  to a vacuum outlet conduit  307  both leading via the piping  278  to vacuum source  280 . 
     Since there are similar vacuum manifold plates  272 ,  304  at the opposite axial end of the heat exchanger, a similar arrangement of S-shaped flexible hoses  300 ,  306  and frost control catcher ring manifolds is also provided at that end, as shown in  FIG.  5 B  with similar reference numerals denoting similar features. 
     It will be noted from  FIG.  13 H  that the spacing between the catcher elements  162  along the length of each of the rows  242 ,  244  is approximately one third of the diameter of the catcher elements  162 . It will also be noted that the distance between the catcher elements  162  in the row  242  and the catcher elements in the row  244  is approximately the same or slightly less. When liquid droplets  400  approach the outer row  242  they may travel and turn approximately as shown by arrows  402 . Droplets flow through the matrix of tubes  120  and whilst growing on the tubes are focused laterally by the airflow accelerating into the gaps between the catcher elements  162  of the outer row  242 . Therefore, the droplets  400  tend to be diverted by the catcher elements  162  in the first row  242  to be travelling almost straight towards the catcher elements  162  in the next row  244 . In practice, this means that the catcher elements  162  in the row  242  may act as bluff bodies and about 5% of the water extracted by the catcher assembly  240  may be extracted in the front row  242  (in the sense of airflow direction) and 95% in the next row  244 . In other embodiments, the front row catcher elements  162  of the row  242  may be replaced with solid bodies with no mesh or suction function. 
     With the heat exchanger  52  in operation, the temperature at the inner catcher elements and/or outer ones may be monitored by a temperature sensor  350  which may send data to a controller  352  ( FIG.  3   ) which may control a valve such as the valve  354  with a diverter part  356  for altering the helium flow through the heat exchanger  52 . In this way or in a similar way, the temperature at the catcher elements  162  may be controlled. The water and methanol removed from the airflow may be added back into the airflow approaching the rocket nozzle  40  and may in some embodiments add 2% to thrust. The loss of the methanol&#39;s weight from the vehicle may be desirable also in order to enable a higher vehicle velocity to be achieved in response to engine thrust later on during flight. The frost control system described herein can typically remove 99% of the water content from air. The control provided by the controller  352  just described may be such that on a water-methanol solid-liquid phase diagram the environment in the vicinity of the coldest catcher elements  162  is in the region of about 65 mole % or about 82 wt. % methanol as the temperature approaches about −100° C. Even though the heat exchanger could cool the air down to its liquification point if necessary near the air outlet from the heat exchanger, the frost control system is set up so that the great majority of the water is removed with methanol at a higher temperature further back in the heat exchanger. At very low temperatures down below about −50° C. and all of the way down to near −140° C. any remaining methanol/water liquid content will, if it solidifies, turn directly to mass ice rather than the feathery frost that can be formed at higher temperatures, by direct sublimation from the vapour, and so causes less of a blocking issue. 
     As shown in  FIG.  15   , the spiral sections  108  may be modified to incorporate an intermediate header  440 . The intermediate header  440  may have two sets of apertures, into one set of which all of the tubes  120  arriving from the inlet header  106  may be fixed by brazing and into which other set all of the tubes  120  leading to the outlet header  110  may be so fixed. The intermediate header  440  may have an outer tube  442  thus into which the tubes  120  communicate. The inlet header  440  may include an interior tube  444  having a series of injector apertures  446  located in a series at spaced locations therealong for injecting helium into the intermediate header  440 . Therefore, the tubes  120  downstream of the intermediate header  440  may carry more mass flow rate of helium than the tubes  120  upstream. This may be extremely useful in cycle design to be able to vary heat exchange characteristics substantially, for example to prevent exceeding allowable metal temperatures at high flight Mach, and in what is only one heat exchanger  52  essentially provides a system which may act like two different heat exchangers with different flows of coolant, helium or another coolant, flowing in them. This may be seen schematically in  FIG.  16    where air is seen flowing along one path  450 , and inlet header  106  may conceptually be thought of at an inlet point in the heat exchanger  52 , intermediate header  440  at an intermediate point and an outlet header  110  at an outlet point. A control valve  452  is schematically shown for controlling the flow of coolant into the intermediate header  440 . 
     As shown in  FIGS.  19 A to  19 B , a shim plate  500  is located in each pocket  160  located radially outside the catcher elements  162  of the rows  242 , 244 .  FIG.  19 A  shows the shim plate  500  for the outer pockets  160 , the inner pockets with seven catcher elements  162  having similar shim plates  500  (although the number of longitudinal slots  512 , 514 , 516  may be changed e.g. reduced). Each shim plate has a first side wall  502 , a top wall  504  and a second side wall  506 . Each side wall  502 , 506  comprises a downwardly slanted portion  508  joined to the top wall  504 , the downwardly slanted portion  508  being connected to a lower generally radially extending portion  510  of the side wall  502 . The top wall  504  defines generally all of the way therealong three longitudinally extending slots  512 , 514 , 516 , the slots  512 , 514 , 516  being interrupted by a series of small cross-connectors  518 . The three slots  512 , 514 , 516  are aligned along respective radial paths with the three middle-most catcher elements  126  of the second row  244  of catcher elements  126 . Therefore, as shown by the five air flow lines  520  shown in  FIG.  19 D , the slots  512 , 514 , 516  in the shim plates  500  tend to assist in directing the flow onto the catcher elements  126  in the second row  244 . The arrangement may be modified by omitting the first row  242  of catcher elements  126  and potentially by placing the top wall  504  of each shim plate  500  nearer to the remaining “second” row  244  with the slots  512 , 514 , 516  still being substantially aligned with the catcher elements  126  thereof. In some embodiments, the shim plates may have one slot (or slit) per catcher element  126  in this row. There are not necessarily three slots. There could be, for example five. 
     The shim plates  500  are optional. They locally increase air flow velocity so that aerodynamic forces are dominant over gravity. Gravity may tend to cause drops to move diagonally between catcher elements without hitting them and being caught. The shim plates  500  thus assist in the production of aerodynamic loads on droplets of water/anti-freeze which tend to direct them onto the catcher elements  126 ,  244 . Other structures than the shim plates  500  be used in other embodiments for a similar purpose. 
     As shown in  FIG.  20 A , and  FIG.  9 B , the heat exchanger is provided with a front bulkhead  530  and a corresponding rear bulkhead  532 , these bulkheads being essentially mirror images of one another. The bulkheads  530 ,  532  are secured with attachments to allow for movement due to thermal expansion. As shown in  FIG.  20 A , the front bulkhead  520  has an inner ring  534  sitting around and secured to the support drum  84 . The inner ring  534  is connected by a series of generally radially extending spokes  536  with an outer ring  538  of the bulkhead  530 . Outer ends  540  of the spokes  536  are connected to the methanol injection ring  174 . As shown in  FIG.  20 B , an outer header retainer spring  542  is spring-located to the ring  538  of the bulkhead  530  and to a tab washer  544  for the header tube  110  attached to each catcher assembly control plate  270 . A spring  542  applies a light radial clamping load to the matrix of tubes  120  whilst allowing thermal expansions. 
     Spokes  536  are slightly spiral in shape in this embodiment to provide a clearance but could be truly radial or have other configurations in other embodiments. 
       FIG.  18    shows a modification to the heat exchanger to incorporate recycling of the methanol or other antifreeze used to prevent blocking of the heat exchanger due to frost or ice formation. The air flow flows through the heat exchanger annularly inwardly, as before, shown schematically in the direction of the airflow arrows A,B with the air flow being cooled by the helium tubes  120  (not shown in  FIG.  18    for the purposes of clarity) and make it not shown in  FIG.  18    (for the purposes of clarity). Methanol (or other antifreeze) is fed from a source/pump  550  along a conduit  552  to a first injector manifold  554  located at a relatively cold downstream location in the air flow in the heat exchanger  52 . The methanol is then collected or a significant portion of it is collected with water in the air flow at a first downstream capture or removal arrangement  556 . This captured methanol, diluted with the water is then recycled by a pump  558  along a conduit  560  to a more upstream methanol/water injection manifold  562  and this mixture is then (at least partly) captured along with more water from the air flow at a further capture or removal arrangement  564 . This methanol (together with water) is then pumped by a further pump  566  along a further conduit  568  with the methanol further diluted to an upstream methanol/water injector manifold  570  where it is injected into the air flow. This further diluted methanol (diluted with water) is then collected or substantially all collected at an upstream catcher or removal arrangement  572  from where it is led away to an exit  574 , which may lead to a combustion or thrust producing section of the engine in order to be ejected from the engine to supply additional thrust. This recycling of the methanol in which it is re-injected each time at a location further upstream (and thus can conceptually be considered in counter flow to the air flow even though as shown by schematic arrows  577  the methanol flows along with the air while in the air flow) enables consumption of methanol or other antifreeze to be optimised to a minimum. 
     It is envisaged that in some embodiments the methanol and water may be lead away from the exit  574  to a methanol separator, such as a distillation system, for re-concentrating the methanol for re-use so as to reduce overall methanol consumption and the weight of methanol to be carried. 
     Instead of the arrangement shown in  FIG.  18    it would be possible to have two or more catcher sections (like catcher arrangement  572 ) in series (one after the other along the air flow path) after each injection manifold (like  562 ) and before the next ejection manifold. 
     As shown in  FIG.  21 A  the heat exchanger  52  (or pre-cooler) may be assembled in a test rig  580  together with a front passive heat exchanger simulator  582  and a rear passive heat exchanger simulator  584 . The simulators  582 , 584  have similar air flow characteristics to the heat exchanger  52 . The air flow simulators  582 , 584  and heat exchanger  52  are arranged annularly around an outlet duct  586  with a centre line or axis  588 . In practice for a flight-ready arrangement of three (or another multiple of) heat exchangers  52  next to one another in a similar configuration, the front and rear passive air flow simulators  582 , 584  may be replaced by additional heat exchangers  52  which may be substantially identical to the middle heat exchanger  52 . These three heat exchangers  52  may thus accept incoming air flows  590  from duct  592  leading from inlet  43  and the outlet duct  586  may lead to the air compressor  54  of the turbo compressor  46  shown in  FIG.  3   . 
     The airflow simulators  582 , 584  and heat exchanger  52  are each radially inwardly bounded by a series  594 , 596 , 598  of overlapping guide vanes  600 , each guide vane being an annular or ring-like element co-axial with the central axis  588 . Each guide vane has a longitudinal extent along the axis  588  and in cross-section as shown in  FIG.  21 B  has a curved generally radiused leading portion  602  extending approximately 20% of the longitudinal extent of the guide vane and a trailing portion  604  extending the remaining approximately 80% of the longitudinal extent of the guide vane  600  which is substantially conical—although  FIG.  21 B  shows the leading edge portion  614  to be several flattened portions it is in fact preferably smoothly curved. Due to the configuration of the vanes in which they are curved and inter-nested, and in which an exit area  606  between adjacent guide vanes  600  is considerably smaller than an entrance area  608  between or near leading edges  610  thereof (bearing in mind both a smaller slot length transverse to flow as shown in cross section and a smaller circumference at the exit area  606  too), the flow is turned and accelerated through the guide vane  600 . Each guide vane  600  is substantially the same width all the way along from its leading edge  610  to a trailing edge  612  thereof but this could be changed in other embodiments. The leading edge  610  (or leading edge portion  614 ) of each guide vane  600  is angled at approximately 10 degrees to radial, i.e. the angle A in  FIG.  21 B . The trailing edge  612  (or trailing edge portion  616 ) of the guide vane  600  is angled at approximately 10 degrees to the longitudinal axis  588  of the duct  586 , i.e. the angle B in  FIG.  21 B . 
     The guide vanes  600  serve to locally turn and accelerate the air flow up to the air velocity generally in the outlet duct  586  such that upstream velocity distribution, i.e. upstream of the front and rear passive air flow simulators  582 , 584  and the heat exchanger  52 , is forced to become more uniform such that the same or substantially the same air mass flow rate will flow through each even though they are different distances along the duct  586 . 
     Although it was originally assumed that a centre body inserted into the outlet duct  586  would eradicate the problem of static pressure distribution along the outlet duct reducing in the direction towards the engine, such a centre body unexpectedly did not when tested by the present applicants have the desired effect and forced even more of the flow to be drawn through the rear of the installation, the reason for this (the present applicants have worked out) being that the air entering the outlet duct enters in the radial direction but subsequently follows a curved path in order that it turns through 90 degrees to exit the outlet duct and whenever the fluid follows a curved path there is a pressure gradient perpendicular to the flow and the flow entering the outlet duct towards the rear of the installation (nearer duct exit  589 ) was found to follow a tighter radius of curvature than the air entering the duct from nearer the front  591  thereof, with the tighter radius of curvature and high velocity towards the rear causing larger pressure gradients and resulting in a higher pressure region underneath the front passive airflow simulator  582  compared to the rear passive airflow simulator  584 , thus causing higher velocities drawn across the rear passive air flow simulator  584  than the front passive airflow simulator  582 . 
     The turning vanes  600  alleviate this problem locally at the exit of the drum-like air flow simulators  582 , 584  and heat exchanger  52 . Although between neighbouring vanes  600  there will still be a pressure gradient in the axial direction, this is now constrained between boundaries of vane pairs. Hence by splitting the flow into a sufficient number of turning segments, the larger outlet duct pressure gradient can be removed or at least reduced. Not only are more even mass flow rates drawn through the three drums  582 , 584 , 52  but the streamlines across the heat exchanger  52  become almost radial (reducing a tendency without the vanes for more flow to pass through the heat exchanger  52  at one axial position than another), thereby helping to ensure a more uniform flow field through the heat exchanger  52  for the purposes of heat transfer. 
     With the turning vanes  600  installed, the pressure ratio, i.e. the ratio of pressure in the duct  586  in the region of the front airflow simulator  586  to the pressure in the region of the rear simulator  584  was seen to improve from a ratio of 72% without the guide vanes  600  to 89% with the guide vanes installed as shown in  FIG.  21 A . 
     The vanes  600  are thus shown to offer a solution to re-distributing the mass flow through a heat exchanger assembly having a longitudinal extent and leading to an axially-flowing duct and/or arrangements with multiple heat exchanger modules like the module  52  together with similar modules replacing the passive air flow simulators  582 , 584 . The vanes  600  provide more uniform radial velocity distributions through the heat exchanger  52 . 
     The blade outlet angles (B) may be varied along the axial length of the installation to increase uniformity of flow yet further and it is envisaged that a centre body  603  may be added as well such that a combined turning vane and centre body geometry can be utilised to provide a highly uniform flow distribution with minimal total pressure loss. With the outlet duct  586  generally cylindrical, the centre body  603  may be parabolic in cross-section, as schematically shown, in order to provide generally linear area increase per unit length (due to the 3D annular shape), enabling mass flux to remain substantially constant. 
     As shown in  FIG.  21 A , as well as inner guide vanes  600 , each heat exchanger  52  may also be provided with a series of outer guide vanes  601  extending fully along the axial length thereof, or substantially so. Only three such outer vanes  601  are shown in  FIG.  21 A  for the purposes of clarity. The outer vanes  601  are preferably each slotted as shown in  FIG.  21 A  since pressure increases and the flow velocity slows as flow passes the vanes  601  and the slotting stops flow stall. Slotting is not needed on the inner vanes  606  since pressure decreases and flow accelerates across them. With the flow reversed outwardly radial in other embodiments, the inner vanes may be slotted. In some embodiments, either the inner  600  or the outer  601  vanes may be omitted. 
     As shown, for example, in  FIG.  13 B , the catcher plates  270  have pegs  700  which protrude into the catcher pockets in the matrix of tubes  120  which allows them to follow the catcher pockets as the matrix moves under thermal and pressure displacements. It will be appreciated that the catch plates  270  overlap so as to prevent axial air leakage between them; and it is noted that and the catcher plates do not normally bear radially on each other or the front and rear bulkheads. 
     Although the embodiment shown in the majority of the drawings has only a single antifreeze/methanol injection point radially outside the matrix of tubes  120 , it is envisaged that a flight-ready engine may have at least two injection points at different radial locations or more radial locations as shown in  FIG.  18   . 
     The transition tubes  702  shown in  FIG.  9 B  may be useful in test rigs and may be removed in operational engines. 
     The concave dished pockets in the catcher elements  126  are approximately 50 mm long. The mesh  258  is wrapped tightly around the catcher tubes and secured to them. This forms 50 mm long pockets where each catcher tube  126  is dished, separated by the lands  254  where the mesh  258  contacts the tube so as to compartmentalise the suction cavities into separate pockets so that in case of screen damage remaining pockets remain operational. 
     The temperature control components including the temperature sensor  350  and controller  352  may be adapted or replaced in other embodiments with other apparatus known to the skilled person in the art form maintaining a constant airside temperature profile for frost control by providing the correct amount of methanol/water condensation at the particular catcher locations. The control in at least some embodiments is adapted to control the last (coldest) catcher row to be located at about −80 to −100 degrees C. air temperature where the methanol concentration should be about 80% mole fraction or 88% mass fraction to delay the freezing point to the lowest possible temperature. 
     The frost control system requires very little consumable materials compared to the previous publications mentioned above, i.e. the mass of methanol required is very low, translating into increased vehicle payload and improved economics. 
     The connection of the methanol manifold  174  to the spokes  536  is preferably via slotted holes (not shown) to allow for radial thermal expansion. 
     The methanol injector ring  174  shown in the drawings is composed of active injection tubes  710  with alternate plain non-fluid injecting tubes  712  of larger diameter. This arrangement provides increased air velocity close to the injection tubes  710  but the plain tubes  712  may be removed in other embodiments. 
     The light pressing of the outlet headers  110  against the matrix of tubes  120  using the springs  542  applies an initial preload which is augmented by the airside pressure drop when the engine is running and it also prevents the module spirals  108  from swinging open when the axis of the heat exchanger  52  is horizontal. The helium bypass controls,  350 , 352 , 354  may be replaced in other embodiments and the circuitry may be changed to include a recirculation loop and for alternative designs of catcher temperature control. 
     The heat exchanger may be used in other applications than with the engine shown and is not limited to use in the particular aerospace application described and may be used in various other aerospace and industrial applications. 
     Various features shown in the drawings may be varied to what is shown and described without departing from the scope of the invention. For example the end walls formed by the catcher plates  127  and bulkheads  530 , 532  may be in other embodiments formed in thin sheet material. 
     In embodiments with generally radially outward air flow (inside of radially inward), the tube support structure including the bird cage drum  84  and I-beams  130  may be reversed such that the drum  84  is positioned radially outside the spiral tubes  120  so as to resist outward loading thereon. 
     In situations where the fluid (such as air) being cooled does not contain water vapor, or if the fluid will not be cooled below 0 degrees, there is a relatively low likelihood that frost formation will impede the operation of the heat exchanger. In such situations, it may be advantageous to eliminate the frost control apparatus (e.g., the methanol injection system, the catcher assemblies  240 , the shims  500 , the doglegs  112 ′, 114 ′, I-beams  130  in the region of the spiral sections  108 , arcuate pockets  160 , radial portions  122  and foils/joggles  124 ) from the heat exchanger, to, for example, reduce weight when the heat exchanger is used with an engine (such as the engine disclosed in GB 1318111.0). 
     Various modifications may be made to the described embodiments without departing from the scope of the invention as defined by the accompanying claims.