Patent Publication Number: US-11022375-B2

Title: Apparatus and methods for additively manufacturing microtube heat exchangers

Description:
BACKGROUND 
     Field 
     The present disclosure relates generally to compact heat exchangers, and more specifically to microtube heat exchangers manufactured using additive manufacturing techniques. 
     Background 
     Heat exchangers transfer heat between fluids. For instance, an automobile radiator operates to transfer heat from circulating radiator fluid to forced air, a gaseous fluid. The automobile radiator is an example of a compact heat exchanger, designed to realize a large transfer surface area per unit volume. 
     Recently three-dimensional (3D) printing, also referred to as additive manufacturing, has presented new opportunities to efficiently build material structures including, but not limited to, automobiles, airplanes, boats, motorcycles, and the like. Additive manufacturing avails different complex geometries not previously available in traditional manufacturing processes; also, conventional techniques for joining parts, such as welding, may not be viable alternatives for use with some of these new materials. Accordingly, there is a need to discover and develop new ways to assemble components, including heat exchangers, using additively manufactured parts and techniques. 
     Further, applying additive manufacturing processes to industries that build material structures has proven to produce structurally more efficient products and also help the environment. As compared to traditional manufacturing processes, 3D printing does not significantly contribute to the burning of fossil fuels; therefore, applying 3D printing to other components, including heat exchangers, is an environmentally worthwhile pursuit. 
     SUMMARY 
     Several aspects of additively manufacturing heat exchangers will be described more fully hereinafter with reference to three-dimensional (3D) printing techniques. 
     In one aspect a method for manufacturing a heat exchanger comprises receiving a data model of the heat exchanger and additively manufacturing the heat exchanger based on the data model. The modelled heat exchanger comprises a microtube array and at least one header. The header is coupled to and integrated with the microtube array at a base plate. The at least one header is for directing a fluid through a plurality of tubes of the microtube array to transfer heat between the fluid and a medium external to the plurality of tubes. 
     The step of additively manufacturing the heat exchanger can comprise forming a continuous solid body comprising the microtube array and the at least one header. Additionally, the step of additively manufacturing the heat exchanger can further comprise additively manufacturing the microtube array and the at least one header integrated with the microtube array in a single three-dimensional (3D) rendering step. Also, a diameter of each of the plurality of tubes can be equal to or less than two millimeters (mm). 
     The step of additively manufacturing the heat exchanger can comprise additively manufacturing a first surface of the at least one header, the first surface including a microtube array interface disposed substantially parallel to the base plate. The step of additively manufacturing the heat exchanger can comprise additively manufacturing a second surface of the at least one header, the second surface being coupled to, and angled relative to, the first surface; and the step of additively manufacturing the heat exchanger can comprise additively manufacturing a third surface of the at least one header, the third surface connecting the first surface and second surface so as to form a cross section. The fluid can be configured to flow through the at least one header in a direction substantially perpendicular to the cross section. The cross-section can be substantially triangular. 
     The step of additively manufacturing the heat exchanger can further comprise depositing a plurality of layers including support material on the base plate. The support material can form an angled support structure configured to provide support to the second surface of the at least one header. The second surface of the at least one header can be additively manufactured atop the angled support structure. 
     The step of additively manufacturing the heat exchanger can further comprise depositing a plurality of layers including support material on the base plate. The support material can form a support structure having a plurality of angles relative to the base plate. The support structure can be configured to provide support to the second and third surfaces of the at least one header. The second and third surfaces can be additively manufactured atop different angled portions of the support structure. 
     The step of additively manufacturing the heat exchanger can further comprise adding a lattice structure between at least two surfaces of the at least one header. The lattice structure can be configured to provide structural support for the at least one header. 
     The step of additively manufacturing the heat exchanger can further comprise adding a lattice structure between the second and first surfaces of the at least one header. The lattice structure can be configured to provide structural support for the at least one header. 
     The step of additively manufacturing the heat exchanger can comprise forming a microtube array arranged in a curved or pleated pattern relative to a direction of incoming air flow. The step of additively manufacturing the heat exchanger can comprise forming a microtube array comprising a pleated arrangement of microtubes. Also, the step of additively manufacturing the heat exchanger can comprise forming fins between microtubes in one or more columns of microtubes. Each fin can connect a microtube to an adjacent microtube in each of the one or more columns. 
     The method for manufacturing the heat exchanger can further comprise additively manufacturing the heat exchanger at an angle relative to the base plate and adding to each microtube at least one fin connecting the microtube to another microtube. The fin can extend from the base plate to the top of the microtubes to which it is connected. 
     In another aspect an additively manufactured heat exchanger comprises a microtube array and a header. The microtube array comprises a plurality of microtubes forming a substantially parallel array extending from a base plate. The plurality of microtubes comprises a first plurality of microtubes and a second plurality of microtubes. The header comprises a plurality of header sections. The plurality of header sections comprises a first header section and a second header section. The first header section is integrated with the first plurality of microtubes at the base plate; and the second header section is integrated with the second plurality of microtubes at the base plate. The first header section is configured to direct a first fluid through the first plurality of microtubes so as to exchange heat with an external fluid; and the second header section is configured to direct a second fluid through the second plurality of microtubes so as to exchange heat with the external fluid. 
     The microtube array can have a non-planar configuration relative to a plane normal to the base plate. The external fluid can comprise a gas. The microtube array, the base plate, and the header collectively can form a solid and continuous body. A diameter of each of the plurality of microtubes can be equal to or less than two millimeters (mm). The external fluid can flow substantially orthogonal to a flow direction of the first and second fluids. 
     At least one of the plurality of microtubes can be curved. At least one of the plurality of microtubes can be pleated. 
     The header can comprise a top surface at the base plate or a section thereof, and at least one surface coupled longitudinally to the top surface at an angle relative to the top surface. Also, the header can comprise a channel having a cross-sectional surface. The cross-sectional surface can be substantially triangular. The channel can be substantially orthogonal to the microtube array. 
     The header can comprise a first surface, a second surface, and a third surface The first surface can be substantially parallel to the base plate. The second surface can be coupled longitudinally to the first surface at a first angle; and the third surface can be coupled longitudinally to the second surface at a second angle. The first, second, and third surfaces can define the channel. 
     The microtube array can have a non-planar configuration relative to a plane normal to the first fluid flow. The microtube array can comprise a curved configuration. Also, the microtube array can comprise a pleated configuration. 
     The heat exchanger can further comprise a fin disposed between each adjacent microtube in at least one column of microtubes. The fin can be disposed from the base plate to a surface of the connected microtubes, and the fin can be configured to provide structural support to the column of microtubes. 
     The heat exchanger can further comprise a fin disposed between each adjacent microtube in at least one row of microtubes in the array. 
     The heat exchanger can be additively manufactured at an angle relative to the base plate; and each of the plurality of microtubes can include at least one fin connected to an adjacent microtube. The at least one fin can be configured to provide the microtube array with structural support. 
     In another aspect an additively manufactured compact heat exchanger comprises a base plate, a first plurality of microtubes, and a first header section. The first plurality of microtubes form an array. The first plurality of microtubes are substantially parallel, and they extend from the base plate. The first header section is integrated with the first plurality of microtubes at the base plate and configured to direct a first fluid through the first plurality of microtubes so as to exchange heat with an external fluid. 
     The additively manufactured compact heat exchanger can further comprise a second plurality of microtubes and a second header section. The second plurality of microtubes can form an array. The second plurality of microtubes can be substantially parallel, and the second plurality of microtubes can extend from the base plate. The second header section can be integrated with the second plurality of microtubes at the base plate and configured to direct a second fluid through the second plurality of microtubes so as to exchange heat with the external fluid. 
     At least one of the first plurality of microtubes can be curved. At least one of the first plurality of microtubes can be pleated; and the external fluid can comprise a gas. 
     In another aspect a transport vehicle comprises an additively manufactured (AM) support structure. The AM support structure comprises an AM microtube lattice array; the AM microtube lattice array is configured to transfer heat between a first fluid and a second fluid. 
     The microtube lattice array can comprise a plurality of microtubes and a plurality of interstitial paths. The microtubes can be configured to carry the first fluid, and the interstitial paths can be configured to carry the second fluid. Heat can be transferred from the first fluid to the second fluid; also, heat can be transferred from the second fluid to the first fluid. The first fluid can be a liquid, and the liquid can be engine oil. The second fluid can be engine coolant. Also, the second fluid can be a gas, and the gas can be forced air. 
     In another aspect a method of manufacturing a heat exchanger comprises additively manufacturing a hollow support structure and additively manufacturing a microtube lattice array within the hollow support structure. 
     Different complex geometries and different microtube shapes may be used that were not previously available in traditional manufacturing processes. It will be understood that other aspects of additively manufacturing heat exchangers will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described in only several embodiments by way of illustration. As will be appreciated by those skilled in the art, the additive manufacturing of heat exchangers using 3D printed components can be realized with other embodiments without departing from the invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of apparatus and methods for additively manufactured heat exchangers will now be presented in the detailed description by way of example, and not by way of limitation, in the accompanying drawings, wherein: 
         FIG. 1  illustrates a side view of an additively manufactured microtube heat exchanger according to an embodiment. 
         FIG. 2  illustrates a cross-sectional view of a microtube array according to an embodiment. 
         FIG. 3  illustrates a side perspective view of a microtube array according to an embodiment. 
         FIG. 4A  illustrates a side perspective view of a microtube according to an embodiment. 
         FIG. 4B  illustrates a cross-section view of the microtube. 
         FIG. 5  is a plot of microtube heat exchanger effectiveness E ff  as a function of tube length times number of tubes. 
         FIG. 6  a side view of an additively manufactured microtube heat exchanger according to another embodiment. 
         FIG. 7  illustrates a cross-sectional side view of an additively manufactured microtube lattice array according to an embodiment. 
         FIG. 8  conceptually illustrates a process for additively manufacturing a heat exchanger according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the drawings is intended to provide a description of exemplary embodiments of manufacturing heat exchangers using additively manufacturing techniques, and it is not intended to represent the only embodiments in which the invention may be practiced. The term “exemplary” used throughout this disclosure means “serving as an example, instance, or illustration,” and should not necessarily be construed as preferred or advantageous over other embodiments presented in this disclosure. The detailed description includes specific details for the purpose of providing a thorough and complete disclosure that fully conveys the scope of the invention to those skilled in the art. However, the invention may be practiced without these specific details. In some instances, well-known structures and components may be shown in block diagram form, or omitted entirely, in order to avoid obscuring the various concepts presented throughout this disclosure. 
     A heat exchanger&#39;s purpose is to exchange heat between fluids. A fluid can include liquids or gases, such as air. For instance, when one fluid is a gas and another fluid is a liquid, the heat exchanger can be referred to as a gas-to-liquid heat exchanger. 
     More generally, heat exchangers can be classified according to a variety of attributes including, but not limited to, the fluid type, the number of fluids, the direction or configuration of fluid flow, physical construction, and surface compactness. For instance, one construction classification or type of a heat exchanger is a shell-and-tube heat exchanger. Shell-and-tube heat exchangers contain a large number of tubes arranged inside a shell. The axes of the tubes are arranged parallel to that of the shell, and heat transfer takes place as one fluid flows inside the tubes while the other fluid flows outside the tubes within the shell. However, despite their use in numerous industrial applications, shell-and-tube heat exchangers can be poorly suited for transport vehicles, such as automobiles or aircraft, because shell-and-tube heat exchangers typically have a large size and weight. 
     A type or classification of heat exchanger better suited for applications such as transport vehicles is the compact heat exchanger. The compact heat exchanger is designed to realize a large transfer surface area per unit volume, sometimes referred to as transfer area density or surface area density. For instance, a gas-to-liquid heat exchanger is referred to as a compact heat exchanger when it has a heat transfer surface having a surface area density above about 700 meters squared per meters cubed (m 2 /m 3 ). A liquid-to-liquid fluid heat exchanger can sometimes be referred to as a compact heat exchanger if its surface area density on any one fluid side is above about 400 m 2 /m 3 . By comparison, a typical shell-and-tube exchanger may only have a transfer surface area density of less than 100 m 2 /m 3  on one fluid side. And the human lungs, another form of gas-to-liquid heat exchanger, provide the supreme transfer surface area density of about 20,000 m 2 /m 3 . 
     In addition to surface area density, compact heat exchangers can also be characterized by heat transfer effectiveness E ff . Heat transfer effectiveness E ff  is a dimensionless figure of merit which can be expressed in terms of fluid temperature ratios. 
     Traditional compact heat exchangers are generally made of sheet metal construction, with fluid pipes and headers bonded and sealed to each other via adhesive or brazing. For heat exchange with low thermal capacity fluids, generally the heat exchange rate can be limited by the surface area exposed to that fluid so additional fins are added to conduct heat to the higher heat capacity fluid. Metal construction of compact heat exchangers is generally favored due to metal&#39;s high strength and thermal conductivity, allowing thinner wall sections, lower mass, and reduced cost. Compact heat exchangers can also use microtubes for improving performance. Microtubes are smaller and have smaller diameters than traditional tubes. For instance, a typical tube may have a diameter of greater than 0.5 inches while a microtube may have a diameter of 0.1 inches or less. Microtubes can advantageously hold very high pressures, be robust to damage, and provide a higher surface area density compared to traditional tubes. 
     Another construction type of compact heat exchanger is a printed circuit heat exchanger (PCHE). A PCHE can be constructed using a solid state process known as diffusion bonding. Due to manufacturing constraints, both the PCHE and traditional compact heat exchangers have only achieved transfer surface area densities of about one tenth the supreme value of 20,000 m 2 /m 3  with heat transfer effectiveness E ff  values of less than ninety percent. Accordingly, there is a need to design high performance compact heat exchangers capable of achieving higher values of transfer surface area density and higher values of heat transfer effectiveness E ff . 
     Apparatus and methods for additively manufacturing microtube heat exchangers are disclosed herein. A heat exchanger header is additively manufactured with high density microtube arrays to achieve an integrated structure having values of heat transfer effectiveness E ff  up to ninety percent and values of transfer surface area densities up to 20,000 m 2 /m 3 . The heat exchanger header can be printed with the high density microtube array to separate different types of fluids or liquids into different microtubes and to form a high quality seal. Additionally, microtubes and/or microtube arrays can be curved or pleated; and microtube lattice arrays can be compactly positioned within hollow support structures. 
       FIG. 1  illustrates a side view of an additively manufactured (AM) microtube heat exchanger  100  according to an embodiment. The AM microtube heat exchanger  100  includes a header  102  and a microtube array  103  integrated at an interface  105 . The header  102  has a header section  108  and a header section  110 . The header section  108  has an inlet/outlet port  104 , and the header section  108  has an inlet/outlet port  106 . A dividing wall or divider  107  can separate the header section  108  from the header section  110  to separate a fluid of the header section  108  from a fluid of the header section  110 . 
     Unlike conventional processes which use sheet metal construction metallic braze tubes to sheet and affix headers, additively manufacturing the header  102  with the divider  107  allows for fluid-tight division between the header sections  108  and  110 . Advantageously, this can allow dissimilar fluids to flow in the header sections  108  and  110 . Structures allowing fluids to be closely brought together in the same structure can have reduced mass, and hence realize improved heat transfer effectiveness E ff . 
     The microtube array  103  has a plurality of microtubes separated into a first microtube array  112  and a second microtube array  116 . Microtubes from the first microtube array  112  can carry a first liquid or fluid, and microtubes from the second microtube array  116  can carry a second liquid or fluid. For instance, microtubes including microtube  114  can carry a liquid such as oil, and microtubes including microtube  118  can carry a liquid such as a water-based coolant, although numerous other fluids are possible depending on the application. 
     As described herein, an AM microtube heat exchanger can be used to manufacture the header  102  so that it is integrated with the microtube array  103 . This can be accomplished without traditional brazing techniques, and the header  102  can be configured as a fluid distribution manifold for fluids. 
     The AM microtube heat exchanger can cool the fluids flowing within the microtubes of microtube array  102  by exchanging heat to air flowing perpendicular, or substantially perpendicular, to the microtubes. For instance, as shown in  FIG. 1 , air flow can be forced to flow via the inlet/outlet ports  106  and  108  in the direction indicated by air flow vector  113 . Alternatively, the direction of air flow can be opposite that of the air flow vector  113 . In this way the hot and cold fluids are separated (not mixed) in a cross-flow configuration. 
     Although the AM microtube heat exchanger  100  shows a header having two header sections  108 ,  110  and first and second microtube arrays  112 ,  116 , other configurations are possible. For instance, an AM microtube heat exchanger can be manufactured with a header having greater or fewer than two header sections; and more or fewer microtube arrays can be separated for carrying different types of liquids. Also, by using a 3D manufacturing process, the header  102  can have internal chambers (not shown) with any desirable high performance shape. For instance, the header  102  can be formed in the shape of a triangle or to have triangular features. 
     The header  102  can be additively manufactured from a base plate (not shown) at the interface  105 . In this way the header  102  can be additively manufactured with the microtube array  103  to be continuous. 
       FIG. 2  illustrates a cross-sectional view of a microtube array  200  according to an embodiment. Microtubes can be arranged in rows and columns including column  206 . As shown in  FIG. 2 , a microtube cross section  216  is located in the first row and sixth column and a microtube cross section  296  is located in the ninth row and sixth column. Additionally, fins can be formed between microtubes. For instance, as illustrated in  FIG. 2 , a fin  211  is formed between microtube cross sections  212  and  222 . In an exemplary embodiment, the fins  211  may extend from the base plate up to a surface of the microtubes  216 . The fins  211  may be used to transfer thermal energy more efficiently through the microtube array. The fins  211  may also be used to provide structural support for the microtubes  216  during the AM process. In certain embodiments, the fins  211  may be disposed diagonally between microtubes  216  in addition to or in lieu of the straight line configuration shown in column  6 . This alternative embodiment provides additional support for the microtube array in more complex 3-D printing arrangements, including where the heat exchanger is oriented at an angle relative to the build plate while being printed. 
     Microtubes can be arranged as a planar web connecting a separation distance between tubes arranged parallel to the flow path of liquids flowing inside. This can advantageously allow support material to connect to a row, referred to as a datum row, to provide support during the additive manufacturing process. 
     In some embodiments microtubes can be manufactured so that the microtube or tube axis is substantially parallel to the force due to gravity; in other embodiments microtubes can be manufactured so that a microtube or tube axis is substantially normal to the force due to gravity. When manufactured with a tube axis normal to the gravitational force, microtubes can be arranged so that they are secured or tied back to a datum (single supporting) row. When manufactured with a tube axis parallel to the gravitational force, microtubes can be free-standing. 
     In other embodiments rows and columns can be positioned using simple linear vectors allowing for a simplified analysis and design procedure. Configurations of rows and columns that provide an integral support structure can also be lighter than those that need additional material when constructed largely normal to the gravity vector. Also, hollow beam elements can be fluid passages, and appropriately designed unit cells can propagate and transport fluid across spans. 
     Advantageously, additively manufacturing a header, such as the header  102  of  FIG. 1 , can allow for the header to be a continuous part of the microtube array  200 . This can allow for improved fluid flow and improved sealing with the microtubes. For instance, unlike traditional brazing techniques to join headers with microtubes, an AM header-to-microtube seal can be uniform with less leakage. An AM process can be accomplished with a single-step process such as selective laser melting. This in turn can lead to lower tube blockage, lower tube fouling, and overall improved heat exchanger performance. 
       FIG. 3  illustrates a side perspective view of a microtube array  300  according to an embodiment. The microtube array  300  can include microtubes forming microtube arrays similar to the microtube array  103  of  FIG. 1  and/or the microtube array  201  of  FIG. 2 . The microtube array  300  can be secured with a base plate  302 , and can have a plurality of rows and columns of microtubes including microtubes  306  and  308 . 
     Advantageously, 3D printing allows for superior compact manufacturing of microtube arrays by creating an integrated seal. The integrated seal allows for the creation of an AM microtube array  300  having less mass than one created by traditional brazing techniques. This in turn can allow for improved heat exchanger effectiveness E ff  and improved surface transfer area. 
       FIG. 4A  illustrates a side perspective view of a microtube  400   a  according to an embodiment. The side perspective view shows the design parameter length L for modeling purposes. By modeling the microtube  400   a , a microtube array can be analyzed prior to 3D printing. 
       FIG. 4B  illustrates a cross-section view of a microtube  400   b . The microtube  400   b  shows a diameter d 1  and a diameter d 2  of an annulus  403  formed due to the thickness of the microtube. As shown in  FIG. 4B , the diameter d 2  of the annulus is larger than the diameter d 1 . Both the diameter d 1  and the diameter d 2  can be designed and additively manufactured to allow for improved heat exchanger effectiveness E ff . For instance, in some embodiments diameter d 1  can be less than or equal to 1 mm, diameter d 2  can be less than or equal to 1 mm, and/or both diameters d 1  and d 2  can be less than or equal to 1 mm in microtube  400   b . In some embodiments a tube length L to diameter (d 1  or d 2 ) ratio can be between three-hundred to one to one-thousand to one. 
     As described above, heat exchanger effectiveness E ff  can be a figure of merit for analyzing and designing a microtube heat exchanger. Effectiveness E ff , which is the generally accepted figure of merit for a heat exchanger, can also be defined as the fraction of minimum temperature difference of fluid outlet over fluid inlet. 
     In a two fluid heat exchanger such as a gas and liquid system where the liquid flows through a microtube array (hot fluid) exchanging heat to the gas (the cold fluid), a simplified analysis based on concepts of energy balance give Equation 1, below. Equation 1 relates the heat transfer effectiveness E ff  in terms of temperatures of the hot and cold fluids measured at heat exchanger inlet/outlet ports. In Equation 1 the port temperatures are as follows: T cold,out , the temperature of the cold fluid flowing out of the cold fluid outlet (port), T cold,in , the temperature of the cold fluid flowing into the cold fluid input inlet (port), T hot,out , the temperature of the hot fluid flowing out of the hot fluid outlet (port), and T hot,in , the temperature of the hot fluid flowing into the hot fluid input inlet (port). As expressed in Equation 1, the heat transfer effectiveness E ff  is a dimensionless figure of merit, and temperature can have dimensions of degrees (deg C.). 
     
       
         
           
             
               
                 
                   
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     In addition to Equation 1, a graphical procedure may be required to analyze a heat exchanger as a function of additional parameters including, but not limited to, the microtube length L as shown in  FIG. 4A  and the total number of microtubes (tubes). 
       FIG. 5  is a plot  500  of microtube heat effectiveness E ff  as a function of tube length times number of tubes. In  FIG. 5  the number of tubes can be the number of microtubes, and the tube length can be the microtube length L as shown in  FIG. 4A . As shown in  FIG. 5 , heat exchanger effectiveness E ff  scales with the product of number of fluid passages (microtubes) and their length (L) based on an engineering assumption that the fluids are exposed to each other across the fluid passage walls. This scaling can be a nonlinear function as effectiveness approaches one-hundred percent where the heat transfer is driven by temperature differentials. As shown in  FIG. 5 , this leads to the effectiveness trace  502  demanding an exponentially growing mass. 
     A high performance heat exchanger design can be achieved through 3D printing by preferentially transferring heat between hot and cold fluids using a reduced length scale. Because additive manufacturing can avail various geometries at a length scale of approximately 100 microns (0.1 mm), very high performance heat exchanger designs can be possible. This in turn can lead to heat exchanger surface area densities of up to the supreme value 20,000 m 2 /m 3 . Coincident with improved compactness is reduced mass which can avail performance heat capacity values of up to 10 Watts per kilogram per degree C. 
       FIG. 6  is a side view of an additively manufactured (AM) microtube heat exchanger  600  according to an embodiment. The AM microtube heat exchanger  600  is similar to the AM microtube heat exchanger  100  of  FIG. 1 , except the microtube array  603  includes microtubes which have a pleat (crease or bend)  630 . Also, similar to the AM microtube heat exchanger  100  of  FIG. 1 , the AM microtube heat exchanger  600  includes a header  602  and a microtube array  603 . The header  602  has a header section  608  and a header section  610 . 
     Similar to the microtube array  103  of  FIG. 1 , the microtube array  603  has a plurality of microtubes separated into a first microtube array  612  and a second microtube array  616 . Microtubes from the first microtube array  612  can carry a first liquid or fluid, and microtubes from the second microtube array  616  can carry a second liquid or fluid. For instance, microtubes including microtube  614  can carry a liquid such as oil, and microtubes including microtube  618  can carry a liquid such as a heating, ventilation and air conditioning (HVAC) refrigerant. 
     Although the embodiment of  FIG. 6  shows the microtube array  603  as having a pleat  630 , other construction types are possible. For instance, the microtube array  603  can include bends, additional pleats, also referred to as “zig-zags”, and curves which are not availed by traditional brazing processes. 
     Three-dimensional (3D) printing can advantageously avail the construction of a fluid-tight geometry. Fluid-tight arrays can be constructed in a non-planar arrangement. In some embodiments, fluid to air heat exchangers (alternatively called radiators) can be configured with radiators placed normal to flow. In other embodiments, a frontal area exposed to the flow can be negative to improve a transport vehicles performance. For instance, the use of zig-zags can be used to reduce an aerodynamic drag and thereby improve performance of a vehicle. 
       FIG. 7  illustrates a cross-sectional side view of an additively manufactured (AM) microtube lattice array  800  according to an embodiment. The AM microtube lattice array  800  can be a lattice of microtubes additively manufactured to fit within a hollow support structure of a vehicle; and unlike the microtube array  300  of  FIG. 3 , the AM microtube lattice array  800  can be additively manufactured to have a lattice structure capable of providing mechanical support and functioning as a heat exchanger. As shown by the cross-sectional view in  FIG. 7 , the AM microtube lattice array  800  includes microtubes  802   a - d  oriented in a first direction and microtubes  804   a - c  oriented in a second direction, perpendicular to the first direction. Additionally, the microtube lattice array  800  includes diagonal segments including microtube segments  806   a - c.    
     Also, as shown by the shaded regions in  FIG. 7 , the AM microtube lattice array  800  has interstitial regions forming interstitial paths or conduits between microtubes and microtube segments. For instance, an interstitial path  808   a  is formed between the microtube  802   a , the microtube  804   b , and the microtube segment  806   a . Similarly an interstitial path  808   b  is formed between the microtube  802   a , the microtube  804   b , and the microtube segment  806   b ; and an interstitial path  808   c  is formed between the microtube  802   b , the microtube  804   c , and the microtube segment  806   b.    
     The microtubes  802   a - d ,  804   a - c , and microtube segments  806   a - c  can advantageously provide mechanical lattice support to an interior, hollow structure of a transport vehicle while simultaneously functioning as microtubes to carry and circulate a first fluid. The first fluid can be a fluid such as engine oil. The flow direction of the first fluid flow can be along a microtube axis. For instance as shown in  FIG. 7 , within the microtube segment  806   c  the first fluid may flow in a direction parallel (or opposite) to flow vector  810   a . Similarly, within microtube  802   b  the first fluid may flow in a direction parallel (or opposite) to the flow vector  810   b.    
     The first fluid can exchange heat with a second fluid flowing in the interstitial paths. The flow direction of the second fluid can be along a direction perpendicular to the cross-sectional side view of  FIG. 7 . For instance, in interstitial path  808   a  the second fluid may flow in a direction into the page as represented by arrow tail vector  812   a ; similarly, in interstitial path  808   d  the second fluid may flow in a direction out of the page as represented by arrow tip vector  812   b . An example of a second fluid is engine coolant (antifreeze) or forced air. 
     The microtubes and interstitial paths of the AM microtube lattice array  800  can be sealed such that the first fluid and the second fluid do not mix. Heat can be exchanged between the first fluid and the second fluid via the microtubes. The microtubes can be additively manufactured to advantageously provide a high surface area density and to achieve high values of heat transfer effectiveness E ff . Additionally, although the embodiment shows one embodiment of an AM microtube lattice array, other embodiments having fewer or greater microtubes, microtube segments, and interstitial paths are possible. Additionally, a transport vehicle can advantageously have one or more support structures additively manufactured with AM microtube lattice arrays, thereby availing a large number of microtubes and transfer surface area for heat exchange. 
       FIG. 8  conceptually illustrates a process  900  for additively manufacturing a heat exchanger according to an embodiment. In step  902  a model can be received by an entity such as a 3D printer for the design and manufacture of an AM microtube heat exchanger. The model can be based on embodiments discussed above, and derivations of the model can be accomplished by analyzing and solving equations relating to microtube heat exchangers having microtube arrays. In some embodiments a software program can be used to solve differential equations describing fluid flow in a certain design configuration. In step  904  an additive manufacturing process can be used to realize and manufacture the heat exchanger modeled in step  902 . The additive manufacturing process can advantageously integrate a header and microtube array in a low mass compact structure. 
     The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Applications of additively manufactured microtube heat exchangers can include heat transfer in transport vehicles such as automobiles. By using additive manufacturing techniques, microtubes can be pleated and/or curved to compactly fit within hollow structures of a car. Various modifications to these exemplary embodiments presented throughout this disclosure will be readily apparent to those skilled in the art, and the concepts disclosed herein may be applied to other techniques for additively manufacturing heat exchangers, compact heat exchangers, and/or microtube heat exchangers. 
     Thus, the claims are not intended to be limited to the exemplary embodiments presented throughout the disclosure, but are to be accorded the full scope consistent with the language claims. All structural and functional equivalents to the elements of the exemplary embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112(f), or analogous law in applicable jurisdictions, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”