Patent Publication Number: US-2016238193-A1

Title: Pressure vessel

Description:
FIELD OF THE INVENTION 
     The present invention relates to a pressure vessel. More particularly the invention relates to a pressure vessel comprising interconnected hollow spheroids. 
     BACKGROUND OF THE INVENTION 
     Pressure vessels are used extensively in many applications, including as fuel tanks in satellites and other space vehicles. They are also used in hydraulic applications on aircraft and other vehicles. The optimum shape for a pressure vessel is spherical due to the uniform tensile stress in the skin generated by this geometry. Cylinders and toroids are almost as efficient as a spheroidal shape due to the quasi-uniform skin stress in these shapes. A common problem is that standard shaped pressure vessels do not fit well into the space available within a structural assembly, and so there is generally a compromise required. Shapes which are far from spherical are very inefficient pressure vessels and often infeasible. 
     Pressure vessels for lightweight applications are typically either high strength. alloy construction, usually involving fabrication steps such as welding, or more recently are filament wound carbon fibre composite construction. These manufacturing processes constrain the design to relatively simple geometries due to the complexities of fabrication. These relatively simple geometries (spheres, toroids, cylinders) can be difficult to accommodate within the space available in a design. In many circumstances it would be preferable to have a pressure vessel which could be of an arbitrary shape, thereby using the space available within the product, and thereby providing the maximum possible volumetric efficiency for storage of pressurized media. 
     SUMMARY OF THE INVENTION 
     A first aspect of the invention provides a pressure vessel comprising a plurality of interconnected hollow spheroids, each hollow spheroid is truncated to form at least two ports which are fluidly connected to respective ports of adjacent hollow spheroids, and adjacent hollow spheroids have overlapping radii at the connected ports. 
     A pressure vessel is a closed container which holds fluids at a pressure substantially different from the ambient pressure external to the pressure vessel. 
     A spheroid in this context is an approximately spherical body. Each spheroid is incomplete due to the truncation which forms the ports. The spheroids may he truncated approximate spheroids, or may be truncated true spheroids (i.e. ellipsoids of revolution about one of its principal axes). The spheroids may be spheres, oblate spheroids or prolate spheroids. 
     The invention is advantageous in that interconnected hollow spheroids may form a unit cell that can be repeated to form a wide variety of shapes for the pressure vessel such that it can fit in environments unsuitable for typical spherical, toroidal or cylindrical shaped pressure vessels. The structural efficiency of the pressure vessel is largely uncompromised since it comprises a plurality of hollow spheroids which are each structurally efficient. Each hollow spheroid is preferably designed to provide optimum geometric conditions with minimal stress concentrations in its outer wall (or skin). 
     The hollow spheroids can he arranged such that they fill the available arbitrary space efficiently while maintaining surface stresses similar to those of a larger spheroidal pressure vessel which would be less space efficient within the arbitrary volume allocated. This improved packaging of pressure vessels within constrained spaces may lead to mass reduction of the pressure vessel, and/or may lead to improved high pressure capability for a given available space as compared with conventionally shaped pressure vessels. The invention also allows for automation of pressure vessel design by using standard unit cells sized for specific pressures and then propagating them through a design space by using a meshing algorithm (such as is typically used in mesh generation for finite element modelling and computational fluid dynamics). 
     Each hollow spheroid may have an outer wall which is contiguous with the outer wall of an adjacent hollow spheroid at the connected ports. Contiguous in this context means that the hollow spheroids share a common edge or boundary without a break. In other words the outer wall of any one of the hollow spheroids continues without any break in the outer wall of its adjacent hollow spheroid at the connected port between the two adjacent hollow spheroids. In a preferred embodiment the hollow spheroids are integrally formed with one another such that the connection is free from butted edges of the outer walls of the adjacent hollow spheroids. Whether integrally formed or not, the contiguous outer walls serve to maintain surface stresses between adjacent hollow spheroids so as to maximize possible volumetric efficiency for storage of compressed media. 
     The outer wall may have thickness which increases nearest the ports. 
     The contiguous walls at a plurality of adjacent connected ports may intersect to form a pillar at the intersection. The pillar may be solid, or may be hollow. 
     The hollow pillar may provide a fluid path, and a plurality of the hollow pillars fluidly connect to chambers around the interconnected hollow spheroids. 
     The hollow spheroids may each be of the same size, or alternatively the hollow spheroids may be of different sizes. 
     The hollow spheroids may be arranged as a finite 2-dimensional or a finite 3-dimensional lattice. The lattice may approximate to a Bravais lattice. 
     Outer walls of the hollow spheroids may comprise one of more of: metal, plastic, or fibre reinforced composite materials. 
     A further aspect of the invention provides a fuel container comprising the pressure vessel according to the first aspect. 
     A further aspect of the invention provides a heat exchanger comprising the pressure vessel according to the first aspect. 
     In the heat exchanger the pressure vessel may have a fluid inlet and a fluid outlet, and a first fluid medium may be arranged for passing through the interconnected hollow spheroids of the pressure vessel and a second fluid medium may be arranged for passing around the interconnected hollow spheroids. 
     A further aspect of the invention provides a reactor comprising the pressure vessel according to the first aspect. 
     In the reactor a catalyst may be coated on an exterior surface and/or an interior surface of the hollow spheroids of the pressure vessel. 
     A yet further aspect of the invention provides a method of manufacturing a pressure vessel according to the first aspect, the method comprising building the pressure vessel by an additive manufacturing technique. Additive manufacturing in this context means the laying down of successive layers of material under computer control to form a three dimensional object, also known as 3D printing. Depending on the material used various additive manufacturing techniques may be employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, in which: 
         FIGS. 1 a - d    illustrate a pyramidal unit cell of four interconnected hollow spheres, and in which  FIG. 1 a    shows a front view,  FIG. 1 b    shows a left side view,  FIG. 1 c    shows a section view on A-A, and  FIG. 1 d    shows a section on B-B; 
         FIGS. 2 a - d    illustrate an arrangement of interconnected hollow spheres utilising the pyramidal unit cell shown in  FIGS. 1 a - d   , and in which  FIG. 2 a    shows a front view,  FIG. 2 b    shows a bottom view,  FIG. 2 c    shows a section view on C-C,  FIG. 2 d    shows a section view on D-D, and  FIG. 2 e    illustrates a space craft including a pressurised fuel container/vessel comprising the arrangement of interconnected hollow spheres; 
         FIGS. 3 a - k    illustrate an arrangement of interconnected hollow spheres based on a hexagonal close packed unit cell, and in which  FIG. 3 a    shows a front view,  FIG. 3 b    shows a top view,  FIG. 3 c    shows a section view on E-E,  FIG. 3 d    shows a section view on F-F,  FIG. 3 e    shows a partial cut-away front view of the hexagonal close packed unit cell,  FIG. 3 f    shows a section view on G-G,  FIG. 3 g    shows a section view on H-H,  FIG. 3 h    shows an isometric view of the partial cut-away hexagonal close packed unit cell,  FIG. 3 i    shows another partial cut-away front view of the hexagonal close packed unit cell,  FIG. 3 j    shows a section view on I-I, and  FIG. 3 k    shows a section view on J-J; 
         FIG. 4  shows an arrangement of interconnected hollow spheres isometric based on a face centred cubic lattice; 
         FIG. 5  shows a portion of a heat exchanger comprising the arrangement of interconnected hollow spheres of  FIG. 4 ; 
         FIG. 6  shows a portion of a reactor vessel comprising the arrangement of interconnected hollow spheres of  FIG. 4 ; 
         FIGS. 7-9  illustrate schematically various interconnected spheroids for use in the pressure vessel of the invention, and in which  FIG. 7  shows interconnected spheres/approximate spheres,  FIG. 8  shows interconnected oblates spheroids, and  FIG. 9  shows interconnected prolate spheroids; and 
         FIG. 10  shows schematically a powder bed ALM processing system. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
       FIGS. 1 a - d    illustrate an exemplary pyramidal unit cell of four interconnected hollow spheres  2 . Each of the hollow spheres  2  are substantially identical and each is truncated to form three ports  4  which are connected to respective ports  4  of neighbouring hollow spheres  2 . Each port  4  is formed by truncating the sphere by a plane so as to remove a spherical cap of the complete sphere, as best shown in  FIG. 1   a.    
     As best shown in  FIG. 1 c   , which is a section view on A-A of  FIG. 1 b   , each port  4  has a substantially circular aperture interconnecting the interior volumes (or chambers) of adjacent hollow spheres  2 . Each hollow sphere  2  has an outer wall  6 . The outer wall  6  has a substantially constant wall thickness. The outer wall  6  is contiguous with the outer wall  6  of an adjacent hollow sphere  2  at the connected ports  4 . This contiguous outer wall at the connected ports  4  is concave when viewed external to the pyramidal unit cell such that the port  4  forms a throat between the interior volumes of the adjacent hollow spheres. The thickness of the outer wall  6  nearest the ports  4  may be different from that of the outer wall  6  of the sphere  2 . For example, the wall thickness may be increased or decreased as compared with the substantially uniform outer wall thickness of the hollow spheres  2 . Varying the wall thickness around the ports enables stress optimisation around the cut outs. 
       FIG. 1 d   , which is a section on B-B of  FIG. 1 b   , shows the geometry of the pyramidal unit cell of interconnected hollow spheres. Each sphere  2  has a sphere centre c. The sphere centres c define an inner radius Ri and an outer radius Ro of the hollow spheres  2 . The difference between the inner and outer radii is equal to the wall thickness t of the outer wall  6 . In the pyramidal unit cell shown in  FIGS. 1 a -1 d    each sphere  2  is of the same size such that the dimensions Ri and Ro are identical for all spheres  2 . The section view in  FIG. 1 d    is a plane through the sphere centres c of three adjacent hollow spheres  2 . The three sphere centres c form an equilateral triangle and the distance s between any two sphere centres c is identical. As can be seen, the distance s is selected such that both the inner Ri and the outer Ro of adjacent spheres  2  are overlapping thereby forming the circular apertures interconnecting the interior volumes of the adjacent hollow spheres  2  at the ports  4 . 
     The outer wall at the connected ports  4  has an outer radius ro and an inner radius ri. In the illustrated embodiment the parameters Ri, Ro, the centre distance s and the radius ro fit a special condition where the contiguous walls at the adjacent connected ports  4  intersect to form a hollow substantially circular pillar  8  at the intersection. As can be seen from  FIG. 1 d    this pillar  8  is centred at the centroid of the equilateral triangle of the sphere centres c. The centroid of the equilateral triangle lies at a point where the radius Ro+ro of a circle about respective sphere centres c intersect. By varying the centre distance s, the sphere radii Ro, Ri or the curvature of the port profile ro the pillar  8  will vary from a hollow aperture of circular section to a variety of other section shapes which may be either hollow or solid. In particular, the pillar may have a generally triangular shape. Where the pillar  8  is hollow this may provide a convenient fluid path around the interconnected spheres of the pyramidal unit cell. 
       FIGS. 2 a - d    illustrate an arrangement  19  of interconnected hollow spheres utilising the pyramidal unit cell shown in  FIGS. 1 a - d   . As can be seen in the arrangement  19  shown in  FIG. 2 a    the pyramidal unit cell of  FIG. 1 a - d    has been used to construct a chain of interconnected hollow spheres. This chain contains both generally linear and non-linear portions when viewed in the front view in  FIG. 2 a   . From the bottom view of  FIG. 2 b    it can be seen that the chain extends along a linear central axis along C-C. A section view on C-C is shown in  FIG. 2 c    from which the internal ports  4  become visible. A section view on D-D from  FIG. 2 b    is shown in  FIG. 2   d.    
       FIG. 2 e    illustrates schematically a spacecraft  20  including various modules  21  to  25 . As can be seen from  FIG. 2 e    the various modules  21  to  25  leave a somewhat difficult packaging space within which to house a pressurised fuel container/vessel that is totally unsuited to conventional shaped pressure vessels, such as cylinders and toroids. In the spacecraft  20  shown in  FIG. 2 e    the arrangement  19  of interconnected hollow spheres utilising the pyramidal unit cell is arranged as a fuel vessel which, as can be seen from  FIG. 2 e   , fits well the space available between the various modules  21  to  25 . The fuel vessel  26  is connected via pipework  27  to one of the modules  24 . 
     Since the fuel vessel  26  comprises an arrangement of interconnected hollow spheres, the chambers defined by the interior volume of the interconnected hollow spheres can be arranged to fill the available arbitrary space efficiently while maintaining surface stresses in the outer wall  6  of the hollow spheres similar to those of larger spheroidal pressure vessels which would be less space efficient within the space available in the craft  20 . Dependent upon the pressure rating of the fuel vessel the hollow chambers can be designed to provide optimal geometric conditions with minimal stress concentrations in the outer wall  6 . 
     It will be appreciated that whilst in the above described embodiment the arrangement of interconnected hollow spheres  19  is arranged as a fuel vessel for a spacecraft  20 , the arrangement of interconnected hollow spheres  19  may provide a multitude of pressure vessels for the storage of liquids or gasses under pressure in a wide variety of applications. For example, the pressure vessel may be used to protect subsea electronics, as a hydraulic accumulator in a variety of vehicles for land, sea, air or space, or as a frangible device arranged to separate into a plurality of individual spheres above a predetermined positive pressure differential inside the interconnected spheres. 
     Whilst the arrangement  19  of interconnected hollow spheres based on the pyramidal unit cell of  FIGS. 1 a - d    is shown as a (substantially one dimensional) linear chain it will be appreciated that the pyramidal unit itself is particularly well suited for forming substantially two dimensional arrangements and substantially three dimensional lattice structures. This invention therefore contemplates that the plurality of interconnected hollow spheroids making up the pressure vessel may be arranged as a lattice. 
     The pyramidal unit cell shown in  FIGS. 1 a - d    is but one example of an arrangement of interconnected hollow spheroids that can be arranged to form a lattice system.  FIGS. 3 a - k    illustrate an arrangement of interconnected hollow spheres based on a hexagonal close packed unit cell  30 .  FIG. 3 a    shows a front view of a plurality of hollow spheres  2  that are interconnected and truncated so as to form ports  4  in exactly the same way as described above with reference to the pyramidal unit cell of  FIGS. 1 a - d   . It is notable from  FIG. 3 a    that the “pillars”  6  formed at the intersection of the ports  4  have, in this embodiment, a substantially triangular hollow shape as different from the substantially circular hollow pillar formed in the pyramidal unit cell of  FIGS. 1 a - d   . The spheres of the hexagonal close packed unit cell of  FIG. 3 a    are again all of the same size in that they have the same inner radius and outer radius.  FIG. 3 b    shows a top view of the close packed hexagonal unit cell  30  from which a substantially square interstitial opening  12  is formed between four spheres having centres lying in a common plane. 
       FIG. 3 e    shows a partial cut-away front view of the hexagonal close packed unit cell  30 ′ and  FIGS. 3 f  and 3 g    show section views on G-G and H-H from  FIG. 3 e   , respectively.  FIG. 3 h    shows an isometric view of the partial cut away hexagonal close packed unit cell of  FIG. 3 e   , From these cut-away views, the substantially triangular tubular pillars  6  formed at the intersection of the ports  4  are visible as is the complex of interconnectivity between the various chambers formed by the interconnected hollow of even a relatively simple unit cell such as the hexagonal close packed unit cell of  FIG. 3 a   - k.    
       FIG. 3 i    shows another partial cut-away front view of the hexagonal close packed unit cell  30 ″ again showing the substantially triangular hollow tubular pillars formed at the intersection of the ports  4  and  FIGS. 3 j  and 3 k    illustrate section views on I-I and of  FIG. 3 i   , respectively. 
     The pyramidal unit cell and the hexagonal close packed unit cell are merely two examples of lattice structures which may be formed by interconnecting regularly sized hollow spheres and it will be appreciated that a much wider variety of finite two dimensional or finite three dimensional lattices may be constructed, including lattice structures having uniformly sized hollow spheres and arrangements of hollow spheres having two or more different sphere sizes. For example, the lattice structures may approximate to any of the Bravais lattices. 
       FIG. 4  shows an arrangement of interconnected hollow spheres based on a face centred cubic lattice. In  FIG. 4  the arrangement  40  is shown in an isometric view, which has eight spheres, each sphere being truncated on six cubic faces to form six ports by means of which each sphere can connect to the ports of its six nearest neighbours (not all of which are shown in  FIG. 4 ). Similar to the pyramidal and hexagonal unit cells described above, the ports  4  each provide a substantially circular aperture, the spheres are of a common size and wall thickness, and the outer wall of each hollow sphere  2  is contiguous with the outer wall of its adjacent hollow spheres at the connected ports  4 . 
       FIG. 5  shows a portion of a heat exchanger  50  comprising the arrangement of interconnected hollow spheres  40  shown in  FIG. 4 . The heat exchanger  50  has a fluid inlet and a fluid outlet, a first fluid flow  51  is passed through the interconnected hollow spheres via their respective ports  4  between the inlet and the outlet. A second fluid flow  52  is passed around the arrangement of interconnected hollow spheres  40  such that the first and second fluid flows do not mix. In this way a temperature difference between the first and second fluids may be used to transfer heat energy through the (relatively thin) walls  6  of the hollow spheres from the first fluid flow  51  to the second fluid flow  52  or vice versa. The thin walled structure of the hollow spheres and the relatively large contact surface area between both of the first and second fluids and the sphere walls make the structure particularly well suited to use as a heat exchanger. The size of the ports  4  may be optimised to provide the required flow rates of fluid through the heat exchanger  50 . An embodiment of the heat exchanger may, for example, be used to cool the thrust nozzle of a rocket engine by passing fuel through the heat exchanger which provides the added benefit of pre-heating the fuel prior to combustion. The heat exchanger may be formed as pipework or a self-supporting structure of the nozzle wall. 
       FIG. 6  shows a portion of a reactor  60  comprising the arrangement of interconnected hollow spheres  40  shown in  FIG. 4 . The interior surface of the hollow spheres is coated with a catalyst. The reactor  60  has a fluid inlet and a fluid outlet, a reactive fluid  61  is passed through the interconnected hollow spheres via their respective ports  4  between the inlet and the outlet. The reactive fluid  61  passed through the interconnected hollow spheres and over the catalyst is chemically reacted. The arrangement of interconnected hollow spheres  40  has a large surface area to volume ratio making the structure particularly well suited to use as a reactor. 
     In an alternative arrangement the catalyst may be coated on the exterior surface of the hollow spheres and a reactive fluid may be passed over the exterior surface of the hollow spheres. Yet further alternatively two catalytic coatings (which may be the same or different) may be provided on the interior and exterior surfaces respectively of the hollow spheres for reacting two separate reactive fluids, one passed through the interconnected hollow spheres and the other passed over the exterior surface of the hollow spheres. I highly compact reactor may thereby be provided. The temperature of the reaction inside the pressure vessel may be controlled with a fluid outside the pressure vessel. 
     In the above described embodiments the truncated hollow spheroids are perfect or approximate spherical bodies  71  that are truncated to form the interconnection, as shown in  FIG. 7 . In alternative arrangements that may readily be combined with the embodiments described above the interconnected spheroids are substantially oblate spheroids  72 , as shown in  FIG. 8 , or are substantially prolate spheroids  73 , as shown in  FIG. 9 . 
     In the design of pressure vessels according to the invention the sphere size may be selected so as to provide a minimum wall thickness for a given maximum pressure, so as to optimise the structure. In practice there are limits to the minimum wall thickness for manufacture, handling or corrosion. The wall thickness to sphere radius may be designed to obey hoop stress laws, or otherwise. 
     In the above described embodiments the hollow spheres may comprise one or more of metal, plastic, or fibre reinforced composite materials. In particular the heat exchanger may be constructed of metal or other material having a high thermal conductivity. Metal structures may also be fatigue hardened during manufacture, or during use if intended, to improve their structural performance. The hollow spheres may be overwrapped with carbon or other fibre windings. The fibre windings may pass through the “pillars” between the spheres. 
     Whilst the arrangement of interconnected hollow spheres may be formed by a variety of manufacturing techniques, such as casting for example, it is expected that additive manufacturing techniques may be most appropriate. 
       FIG. 10  illustrates a powder bed processing additive layer manufacturing (ALM) system, as an example of one method for manufacturing the pressure vessels described above. The system comprises a pair of feed containers  10 ,  11  containing powdered metallic material such as powdered Titanium. A roller  12  picks up powder from one of the feed containers (in the example of  FIG. 10 , the roller  12  is picking up powder from the right hand feed container) and rolls a continuous bed of powder over a substrate  13 . A laser head  14  then scans over the powder bed, and a laser beam from the head is turned on and off to melt the powder in a desired pattern. 
     The substrate  13  then moves down by a small distance (typically of the order of 0.1 mm) to prepare for growth of the next layer. After a pause for the melted powder to solidify, the roller  12  proceeds to roll another layer of powder over substrate  13  in preparation for sintering. Thus as the process proceeds, a sintered part  15  is constructed, supported by unconsolidated powder parts  16 . After the part has been completed, it is removed from substrate  13  and the unconsolidated powder  16  is recycled before being returned to the feed containers  10 ,  11 . 
     Movement of the laser head  14  and modulation of the laser beam is determined by a Computer Aided Design (CAD) model of the desired profile and layout of the part. 
     Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.