Heat exchanger and fabrication

A method for making a heat exchanger assembly is described, involving generating a digital model of a heat exchanger assembly that comprises a heat exchanger core within a housing. The digital model is inputted into an additive manufacturing apparatus or system comprising an energy source. The additive manufacturing apparatus applies energy from the energy source to successively applied incremental quantities of a metal powder, which fuses the powder to form incremental portions of the heat exchanger core and housing according to the digital model. Unfused or partially fused metal powder is enclosed in a first region of the heat exchanger assembly between the heat exchanger core and the housing.

BACKGROUND

This disclosure relates to a heat exchanger, and specifically to methods of manufacturing a heat exchanger in a housing.

Heat exchangers are devices built for transferring heat from one fluid to another. Heat is typically transferred without mixing of the fluids, which can be separated by a solid wall or other divider. Heat exchangers can be used in various applications, including but not limited to aerospace, refrigeration, air conditioning, space heating, electricity generation, and chemical processing applications.

A fluid on a heat rejection side of a heat exchanger typically undergoes a drop in temperature between the heat rejection side inlet and the heat rejection side outlet. Similarly, a fluid on a heat absorption side of a heat exchanger typically undergoes an increase in temperature between the heat absorption side inlet and the heat absorption side outlet. Such temperature variations can subject heat exchanger components to thermally-induced stress. Such thermal stresses can be managed by incorporating robust structures in the heat exchanger itself or in external mounting components that are resistant to or tolerant of thermal stress, or that can transfer stress to non-critical stress-absorbing structures. However, such robust structures add complexity and expense to product designs, as well as requiring extra weight that is not desirable in weight-sensitive applications such as aerospace or automotive applications.

BRIEF DESCRIPTION

According to some aspects of this disclosure, a method for making a heat exchanger assembly comprises generating a digital model of a heat exchanger assembly that comprises a heat exchanger core within a housing. The digital model is inputted into an additive manufacturing apparatus or system comprising an energy source. The additive manufacturing apparatus applies energy from the energy source to successively applied incremental quantities of a metal powder, which fuses the powder to form incremental portions of the heat exchanger core and housing according to the digital model. Unfused or partially fused metal powder is enclosed in a first region of the heat exchanger assembly between the heat exchanger core and the housing.

According to some aspects of the disclosure, the above-described method further includes selective exposure of incremental quantities of metal powder in a layer of a powder bed over a support with a laser or electron beam to fuse the selectively exposed metal powder in a pattern over the support corresponding to a layer of the digital model of the heat exchanger assembly. This process is repeated by providing a layer of the powder over the selectively exposed layer and selectively exposing incremental quantities of metal powder in the layer to fuse the selectively exposed powder in a pattern corresponding to another layer of the digital model of the article. Metal powder is removed from fluid flow paths of the heat exchanger core without removing metal powder from the first region of the assembly between the heat exchanger core and the housing.

In some aspects of the disclosure, a heat exchanger assembly comprises a housing, a heat exchanger core within the housing, and unfused or partially fused metal powder in a first region of the heat exchanger assembly between the heat exchanger core and the housing.

DETAILED DESCRIPTION

Referring now to the Figures,FIG. 1depicts an example of a heat exchanger assembly100. The assembly100is shown in an isometric view with a cross-section along the front face to illustrate the inside of the assembly. As shown inFIG. 1, a plate-fin heater core102has heat absorption side flow paths104, and heat rejection side flow paths106contained in a heater core skin108. A hot fluid (e.g., air) enters the heat rejection side flow paths106through hot fluid inlet110and inlet header112, and exits through outlet header114and outlet116. It should be noted that althoughFIG. 1depicts a single pass heat rejection side flow path, that two-pass, multi-pass, or counter flow paths can also be used. A cold fluid (e.g., air) is fed through the heat absorption side flow paths104in a direction transverse to the flow through the heat rejection side flow paths106. The heater core102is housed within a housing118, which can be have features such as mounting brackets120,122, and opening124for the hot fluid inlet110(an opening in the housing118for outlet116is not shown). The region of the assembly between the heater core102and the housing118is filled with unfused or partially fused metal powder126.

Of course,FIG. 1is a specific example of a broader disclosure, and variations can be made by the skilled person. For example,FIG. 1depicts an embodiment where the entirety of the space between the heater core102and the housing118is filled with unfused or partially fused metal powder126, but smaller regions can be used as shown inFIG. 2.FIG. 2depicts a heat exchanger assembly100′ where the same numbering of identical features withFIG. 1is carried forward. As shown inFIG. 2, heat exchanger core102′ is disposed within housing118. Heater core102′ is depicted as a tube-shell heat exchanger core with the tubes105providing flow paths for one side of the heat exchanger (either heat rejection or heat absorption), and the space107outside the tubes105and inside of the heater core shell108providing flow paths for the opposite side of the heat exchanger (either heat rejection or heat absorption). For ease of illustration, inlets and outlets to both fluid flow path sides of the heat exchanger core through the housing118are not shown. Compared toFIG. 1where the entirety of the space between the heater core102and the housing118is filled with unfused or partially fused metal powder126,FIG. 2depicts an embodiment where the unfused or partially fused metal powder126is contained in regions around the corners of the heater core102′, which are separated by enclosure barriers130from regions132. In some examples of embodiments regions132can be free of metal powder, and in some examples of embodiments regions132can contain unfused or partially fused metal powder at a different density than the metal powder in the corner regions. Also, other configurations for the heater core, the housing, and the regions between the housing can be used by the skilled person. For example, unfused or partially fused metal powder could be disposed in the regions132while the corner regions are free of metal powder. Other types of heat exchanger cores (e.g., honeycomb) can also be used, as well as variations on the shape and configuration of the heat exchanger cores and housings.

The above-described heat exchanger assemblies can be made using an additive manufacturing process. Referring now toFIG. 3, an example of an additive manufacturing system or apparatus10includes energy source12that generates an energy beam14, a first wave guide or other optical guide16that is used to guide the energy beam, a second wave guide or optical guide18, a frame20, a powder supply22, a powder processing bed24, sintered powder material26, a spreader28, a powder supply support30, and a stack support32. Of course, the illustration in the Figure is schematic in nature, and many alternative designs of additive manufacturing devices are possible. Various types of additive manufacturing materials, energy sources, powder feed and storage, atmosphere control, and processes can be used to fabricate the heat exchanger and the individual features thereof that are described herein. The type of additive manufacturing process used depends in part on the type of material out of which it is desired to manufacture the component. In some embodiments, the heat exchanger is made of metal, and a metal-forming additive manufacturing process can be used. Such processes can include selective laser sintering (SLS), powder bed laser fusion (PBLF), or direct metal laser sintering (DMLS), in which a layer of metal or metal alloy powder is applied to the workpiece being fabricated and selectively sintered according to the digital model with heat energy from a directed laser beam. Another type of metal-forming process includes selective laser melting (SLM) or electron beam melting (EBM), in which heat energy provided by a directed laser or electron beam is used to selectively melt (instead of sinter) the metal powder so that it fuses as it cools and solidifies.FIG. 3merely illustrates one potential additive manufacturing system for creating an additively manufactured article.

Energy source12can be any source capable of creating focused energy. For example, energy source12can be a laser or an electron beam generator. Energy source12generates an energy beam14, which is a beam of focused or focusable energy, such as a laser beam or an electron beam. Optical guide16such as a mirror is present in some embodiments to deflect radiation in a desired direction. A second optical guide18, such as an optical head is present in some embodiments, and also directs energy in a desired direction. For example, optical guide18can include a mirror and be attached to an x-y positioning device. Frame20is used to contain powder material in powder supply22and in powder processing bed24. Powder supply22and powder processing bed24include powder material, such as or powdered metals. Powder processing bed24further includes fused powder26. Fused powder26is powder contained within powder processing bed24that has been at least partially sintered or melted. Spreader28is a spreading device such as an air knife using an inert gas instead of air, which can transfer powder material from powder supply22to powder processing bed24. The depiction of spreader28inFIG. 3is of course only schematic in nature, and does not depict specific features such as controllably directed air jet nozzles that could be used to remove metal powder from targeted portions of the assembly such as fluid flow passages in the heat exchanger core, without removing metal powder from the first region between the heat exchanger core and the housing. Powder supply support30and stack support32are used to raise and/or lower material thereon during additive manufacturing.

During operation, energy source12generates energy beam14, which is directed by the optical guides16and18to the powder processing bed24. The energy intensity and scanning rate and pattern of the energy beam14can be controlled to produce a desired result in the powder processing bed. In some aspects, the result can be partial melting of powder particles resulting in a fused structure after solidification such as a sintered powder metal structure having some degree of porosity derived from the gap spaces between fused powder particles. In some aspects, the result from exposure to the energy beam14can be complete localized melting and fluidization of the powder particles producing a metal article having a density approaching or equal to that of a cast metal article. In some aspects, the energy beam provides homogeneous melting such that an examination of the manufactured articles can detect no particle pattern from the original particles. After each layer of the additively manufactured article is completed, powder supply support30is moved to raise the height of powder material supply22with respect to frame. Similarly, stack support32is moved to lower the height of article with respect to frame20. Spreader28transfers a layer of powder from powder supply22to powder processing bed24. By repeating the process several times, an object may be constructed layer by layer. Components manufactured in this manner may be made as a single, solid component, and are generally stronger if they contain a smaller percentage of oxygen, hydrogen, or carbonaceous gases. In some embodiments, the quantity of impurities of, for example, oxygen, is reduced to less than 50 ppm, or even less than 20 ppm.

As mentioned above, a region between a heat exchanger core and a housing comprises unfused or partially fused metal powder. This powder around the core can allow thermal expansion of the core with reduced susceptibility to damage or failure. Unfused metal powder is metal powder that has not been fused with the additive manufacturing energy source. Partially fused metal powder is metal powder where particles have been fused together, but have not reached a state where the particles have melted and coalesced to form a solid metal of maximum density. Fully fused metal powder is metal powder where particles have fully fused together reaching its maximum density. Unfused metal can have an apparent density equal to about 50% of the wrought material density. Fully fused metal powder is metal powder that has reached its maximum density, typically at least 99.9% of wrought material with a porosity level of less than 0.1%. Partially fused metal powder can have a density between that of unfused metal powder and fully fused metal powder. In some examples of embodiments, the metal powder in the first region between the heat exchanger core and housing is unfused. In some examples of embodiments, the metal powder in the first region between the heat exchanger core and housing is partially fused. The structural components of the heat exchanger core and housing can be formed by fusing metal powder to form a solid metal of maximum density or sintering to form a solid metal having residual porosity from the particulate structure (e.g., having a density range between 99.2% and 99.9% of maximum density).

As mentioned above, unfused or partially fused metal powder is enclosed in a region between a heat exchanger core and housing. In the case of partially fused metal powder, the partial fusion of the powder will typically keep the metal powder in this region in place while fabrication of the assembly is completed and metal powder removed from open spaces such as heat exchanger core fluid flow paths or empty regions132(FIG. 2). In the case of unfused metal powder to be enclosed in this region, metal powder outside of the enclosure region must be removed without removing powder from the enclosure region. Depending on the design and configuration of the heat exchanger assembly, powder removal can be accomplished after completion of the structures enclosing the unfused metal powder (e.g., by blowing a fluid such as air through the fluid flow paths after completion of assembly) In cases where an enclosed region free of powder is called for or where powder removal after completion of the unfused powder enclosure structure is not feasible, powder can be selectively removed during the manufacturing process (without removing unfused powder from powder-containing region) using controllable air nozzles.

The digital models used in the practice of the disclosure are well-known in the art, and do not require further detailed description here. The digital model can be generated from various types of computer aided design (CAD) software, and various formats are known, including but not limited to SLT (standard tessellation language) files, AMF (additive manufacturing format) files, PLY files, wavefront (.obj) files, and others that can be open source or proprietary file formats.

As mentioned above, the powder used in the methods described herein comprises a metal powder. Various metals can be used, depending on the material and properties requirements for the application of the finished product. Various ferrous steel alloys can be used, including stainless and non-stainless steels, with optional inclusion of various alloying elements such as chromium or nickel for properties such as high-temperature performance. Other alloys such as aluminum alloys and titanium can be used as well. Metal powders can be formed using a gas atomized process. Examples of particle sizes for the metal powders can range from 5 μm to 150 μm. In some aspects, the alloy elements can be combined together before forming a powder having a homogeneous composition. In some aspects, one or more of the individual alloy elements can have its own powder particles that are mixed with particles of other elements in the alloy mixture, with formation of the actual alloy to occur during the fusion step of the additive manufacturing process. In some aspects, the powder is “neat”, i.e., it includes only particles of the alloy or alloy elements. In other aspects, the powder can include other components such as polymer powder particles. In selective sintering, polymer particles can help to temporarily bind metal powder particles together during processing, to be later removed by pyrolysis caused by the energy source or post-fabrication thermal processing.