Patent Application
20230384037
The following invention relates to heat exchangers for transfer of heat between at least two circuits. More particularly, this invention relates to heat exchangers which route both circuits of fluids through multiple tubes within an array of tubes embedded within a solid matrix of heat transfer material.
Compact micro-channel heat exchangers have various deficiencies, and most notably the current state-of-the-art Printed Circuit Heat Exchanger (PCHE). This type of heat exchanger is also known as a Compact Platelet Heat Exchangers (CPHX), Diffusion Bonded Micro-Channel Heat Exchangers (MCHE), Diffusion Bonded Compact Heat Exchangers (DCHE) and others.
There are numerous types of heat exchangers (HX) on the market. The traditional HX design is the shell and tube configuration as epitomized by the ubiquitous drum shaped steam boiler found in many industrial and residential installations and in early locomotives and steam ships. The major issue with this type of HX is that it suffers from a catastrophic failure mode when run at off nominal conditions. There have been numerous reports of fatal explosions with shell and tube HXs, so much so that the ASME Boiler Pressure Vessel Code was originally written in response to these disasters. The compact heat exchanger in the form of a PCHE, addressed the problem of higher pressure containment by minimizing the flow passage size and does reduce the flow inventory and overall size, but have a number of other inherent shortcomings as will be pointed out. The present invention minimizes the overpressure failure mode by not only minimizing the fluid inventory but has double, or redundant pressure containing capability.
Cost-effective, high temperature and corrosion resistant heat exchangers are a key enabling component for next generation power systems including concentrating solar power (CSP) systems and nuclear power systems, among others. Current CSP systems utilize molten nitrate salts as both the primary heat transfer fluid (HTF) and as the thermal energy storage (TES) media, operating at temperatures less than 600° C. due to decomposition limits. Next generation CSP systems are expected to operate at temperatures in the range of 700-800° C. and take advantage of advanced power cycles such as supercritical carbon dioxide (sCO2) Brayton cycles. To support these higher temperatures, molten chloride salts have been identified as a probable next generation HTF. While possessing high decomposition temperatures and desirable thermal properties, these salts have highly corrosive properties, exacerbated by the high operating temperatures, thus producing an aggressive environment.
Advanced nuclear power systems are also expected to operate at temperatures in excess of 700° C. and take advantage of advanced power cycles such as supercritical carbon dioxide (sCO2) Brayton cycles. To support this, molten salts have been identified as the probable heat transfer fluid. These salts have highly corrosive properties, exacerbated by the high operating temperatures, producing an aggressive environment.
Aggravating the material problem is the fact that the salts used in the nuclear applications have differing corrosive properties. The current solution is to select a compromise material that tries to satisfy the corrosion problem for both circuits. Conventional HXs are of the shell and tube (S&T) configuration. These HXs are big, bulky, and expensive when employing the required materials and thicknesses necessary to support advanced high temperature, high pressure sCO2 power cycles.
The aggressive environments produced by molten fluoride/chloride salts are typically addressed by employing high temperature and/or high corrosion resistant alloys as materials of construction; examples include Inconel 600, 617 and 625, and Haynes 214, 230 and 242, and even alloys including refractory materials such as tantalum and tungsten. But these materials can be cost prohibitive, especially when coupled with the large volume required for traditional S&T type heat exchangers.
State-of-the-art compact diffusion bonded HXs, like the Printed Circuit Heat Exchangers (PCHE) or Compact Platelet Heat Exchangers (CPHXs), such as those provided by Clean Energy Systems, Inc. of Rancho Cordova, California, have been proposed but have yet to be fully developed and implemented for this application. Aggressive environments are typically addressed by adopting high temperature and/or high corrosion resistant alloys, which are traditionally high cost and may not be the best solution for either circuit. The significant disadvantage of such an approach, for either S&T or diffusion bonded HXs, is the large volume of high cost material required, much of which does not come in direct contact with the corrosive media.
Review of the designs of current HX types readily shows that very little of the material required for construction is in direct contact with the corrosive environment of a molten salts used in CSP or nuclear applications. This leads to the desire to make the majority of the structure from a cheaper, structurally adequate material, while using a corrosion resistant material for the wetted surfaces only. Others have sought to solve this problem with coatings and claddings that in themselves present additional problems of uniformity, assured coverage, and peeling or flaking after thermal cycling. The current invention proposes a radically different approach that will improve the compactness while significantly reducing the cost when compared to current diffusion bonded heat exchanger technology and shell and tube technologies in equivalent services.
The largest current market for PCHE type compact heat exchangers is the off-shore oil and gas industry. Their high pressure capability and compact size make them attractive for this application, especially when located shipboard or on an off shore platform. The major complaints and concerns from the users with this style of HX are the cost, lead time and what can be limited life. The inherent pressurized and non-inspectable parting line that exists in a PCHE provides a built in failure sight. On the manufacturing side, the processes used to produce a PCHE are inefficient, highly specialized and environmentally challenging. Approximately 40% or more of the material of construction is chemically dissolved during the process with reclamation difficult to impossible. The chemicals involved in the processing are highly toxic and dangerous, requiring special equipment and processing procedures. During diffusion bonding of a PCHE, applying the load evenly to the contact interface to form bonds of uniformly high quality can be a great challenge. The present invention addresses each of these concerns.
PCHEs, as discussed above, are the current state of the art. Some micro-tube HXs do exist but appear to have only one circuit constructed from tubes with the area external to the tube flooded with the second fluid circuit—ie. mini-shell and tube. The following patents were found that may be relevant.
Additive manufacturing, in the form of 3D printing, is being used to produce heat exchangers. Current metal 3D printing, which deposits materials in layers, does not readily allow for the selective materials that are available with the current invention. This is a significant limitation of 3D printing. From those in the industry that are using this process, we find that there are currently numerous issues that need to be addressed. They include:
Accordingly, a need exists for an improved micro-channel heat exchanger, especially when handling corrosive fluids at high temperatures and/or pressures. Such a heat exchanger would also benefit from a method of manufacture which is easy to operate and avoids waste, by being a largely additive process.
The present invention is generally in the category of a compact micro-channel heat exchanger. Its function is to efficiently transfer heat from one fluid to another in a highly compact assembly. Multi-fluid heat exchangers are used in virtually every industry including: space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing and aerospace. Compact micro-channel heat exchangers are especially desirable where high pressure and/or high temperatures are involved and/or where space is at a premium and/or where high effectiveness for improved cycle efficiency is desired. An example is heat exchangers used on off shore oil platforms.
The present invention may best be described as a “micro-tube metal matrix heat exchanger.” The micro-tube metal matrix heat exchanger concept is fabricated by a combination of processes in what is effectively an additive manufacturing approach.
The micro-tube metal matrix heat exchanger is characterized as having two or more fluid flow circuits formed by multiple small cross-section “micro-tubes” contained within a surrounding metal structure, or “metal matrix.” Though not limited to round cross-section, if they were round, the tubes would typically range in inside diameter from −0.010″ to −0.250″ although these values are not intended to indicate a limitation. The wall thicknesses of the tubes typically range from 0.002″ to 0.060″ although these values are not intended to indicate a limitation. Most any metal or metal alloy can be considered for the micro-tubes.
The micro-tubes are typically arranged in alternating layers of alternating flow circuits and may be organized in any number of arrangements including co-linear and at angles relative to the adjacent layer, up to and including 90 degrees. In heat exchanger parlance these arrangements provide for co-flow, counter flow and cross flow; combinations of all three are also possible within the micro-tube metal matrix heat exchanger. Woven patterns as exist in mesh screens, wire cloth and fabrics are also possible. These woven patterns are not limited to 2-D configurations but can also be 3-D.
The matrix is formed by filling the interstices between tubes with fine metal powder and then sintering and compacting the assembly in a hot isostatic pressing (HIP) process. This process joins the tubes together and to the matrix itself, producing a monolithic structure.
Accordingly, a primary object of the present invention is to provide a heat exchanger to exchange heat, especially with at least one high pressure, high temperature and/or highly corrosive fluid, over to a second fluid.
The novel HX of this invention has many favorable attributes which include:
The need for the novel HX is manifold as compact micro-channel heat exchangers (CMHE) have become prevalent and enabling devices for many industries. The current state-of-the-art in CMHEs is the printed circuit heat exchanger or PCHE as produced and marketed by a small number of companies around the globe. The major issues associated with this type of CMHE are the cost, lead time and limitations in materials of construction and operation. The novel micro-tube metal matrix heat exchanger addresses each of these issues and more.
Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims and detailed description of the invention.
Referring to the drawings, wherein like reference numerals represent like parts throughout the various drawing figures, reference numeral 10 (
In essence, and with particular reference to
To form the heat exchanger 10, in one embodiment, first tubes are arranged in a first layer 25, followed by second tubes 50 within a layer 45. Another layer 25 of first tubes 30 is then provided, followed by another layer 45 of second tubes 50. This process is repeated until a full array 100 of tubes 30, 50 have been provided. Space between the tubes 30, 50 is filled with metal powder 62 to fully embed the tubes 30, 50, except for at ends thereof. The assembly is then placed within a hot isostatic press (or other heat and/or temperature applying equipment) and the powder 62 is sintered or otherwise solidified into a solid matrix 60 within which the array 100 of tubes 30, 50 are embedded. Finally, manifolds 70, 72, 74, 76 are placed over ends of the tubes 30, 50 from similar circuits 20, 40 for supply and removal of fluids passing through the heat exchanger 10.
The novel HX of the present invention is described as a micro-tube metal matrix heat exchanger, a completed version of which is shown in
Although similar in outer appearance to other compact heat exchangers such as PCHEs/CPHXs, there are significant differences in the design and layout of the heat exchanger 10 internal passages as illustrated by
The layouts of
In the heat exchanger 10, primary structural integrity is provided by the tube 30, 50 walls 32, 52, with secondary structure provided by the metal matrix 60, intrinsically producing a more robust design than that available from the planer diffusion bonded PCHE/CPHX D, due to redundancy. The HIP processing of the heat exchanger 10 bonds the tubes 30, 50 and matrix 60 together, to produce a monolithic structure.
As mentioned, the tubes from which the heat exchanger 10 are produced are commercially available as either fractional or hypodermic tubing 30, 50. While both welded and seamless extruded tubing are available, seamless is the preferred embodiment from a structural standpoint, but with the supporting matrix 60, welded tubing 30, 50 will be acceptable for certain applications. Stainless steel, nickel based alloys and even refractory alloys are available in the sizes and wall 32, 52 thicknesses of interest and custom materials are readily produced. Tube 30, 50 configurations can also be varied to enhance heat transfer. Along with the wavy tubes 12 previously described, tubes 30, 50 can have internal protuberances and/or be twisted or otherwise configured to enhance heat transfer while minimizing pressure drop. Internal protuberances can be formed by simply dimpling the tube 30, 50 from the outside.
With the heat exchanger 10 construction, it is possible to mix materials to provide the optimal selection for corrosion and thermal compatibility and cost effectiveness with the respective circuit 20, 40 fluids at desired service temperatures. For example, for a fluoride molten salt HX that is coupled with H2O (Rankine cycle) or sCO2 (Brayton cycle) at temperatures in excess of 700° C., a high nickel alloy tube 30, 50 can be used on the salt circuit while lower cost stainless steel tubes 50, 30 can be applied to the opposite circuit, all contained within a low-cost stainless steel matrix 60. A significant advantage when compared to a typical PCHE/CPHX D that would require fabrication entirely from the high cost material. For another example, a type of molten salt reactor that utilizes two types of salts, e.g. fuel salts and chloride/fluoride salts, as a means of transferring heat from the reactor to secondary heat exchanger would inherently require two different types of corrosion resistant materials to combat the differing corrosion issues associated with the two types of salts.
An additional benefit of the heat exchanger 10 technology of this invention lies in the tolerance available from the use of tubing 30, 50 to form the passageways. The inside diameter of the micro-tubing used, in the sizes of interest, are expected to vary ˜+/−0.002″ or less. For an equivalent sized passage in a PCHE D or CPHX, the variation would be approximately double this amount. This tolerance control leads to a lower variation in pressure drop and improved performance.
The process for producing the matrix 60 is well-known, typically referred to as Powder Metallurgy—Hot Isostatic Pressing or PM-HIP. This process is used to make powder metallurgy billets and near-net components. Considerable interest and effort has been expended by the nuclear industry to investigate and allow PM-HIP components for pressure containing devices such as valve bodies and pump housings. The aerospace industry has also investigated and utilizes the process. As mentioned, the matrix 60 can be selected from any number of low cost alloys compatible with the process and application but it can also be a material such as copper or other high conductivity material to enhance heat transfer.
While a fully densified matrix 60 has been discussed so far, a compliant matrix 60 is also possible. One method to produce this would be low temperature sintering. This process would leave voids in the matrix 60 that would reduce its stiffness and aid compliance if required for the desired application.
Heat exchanger 10 core (
In summary, the multiple attributes of the heat exchanger 10 make it ideal for cost-effective, high temperature and corrosion resistant heat exchangers for next generation applications and existing applications requiring high performance at low cost. The approach makes efficient use of dedicated materials to address corrosion and cost issues while utilizing available technologies and manufacturing processes.
More specifically, and with particular reference to
Each of the first tubes 30 within the layer 25 extend from an inlet end 22 to an outlet end 24. Similarly, each of the second tubes 50 within the layer 45 including an entry 42 opposite an exit 44. The layers 25, 45 provide portions of separate circuits 20, 40 passing through the heat exchanger 10. These ends 22, 24 and entry/exit 42, 44 are grouped together at locations on the heat exchanger 10 separate from other groupings, so that manifolds 70, 72, 74, 76 can most conveniently be attached to the heat exchanger 10.
While in this one embodiment (
With particular reference to
Which particular reference to
Each of the first tubes 30 is made up of walls 32 having a wall thickness defined by a distance between an inner surface 34 and an outer surface 36. Inner surfaces 34 define that portion of the first tube 30 which are in direct contact with fluids passing through the tubes 30 of circuit A 40. The outer surface 36 comes into contact with second tubes 50 as well as with other first tubes 30 in typical embodiments, and into contact with the metal matrix 60 which fills gaps between the tubes 30, 50.
Thickness of these walls 32 can be made thinner than might otherwise be the case to handle pressures of fluids passing therethrough, because the walls 32 are supported on an exterior side thereof by the matrix 60, as well as by adjacent tubes 30, 50. Furthermore, thinner walls can save on exotic material costs which might be utilized to handle challenging fluid characteristics including corrosiveness, and extremes of temperature and pressure, when the heat exchanger 10 is operating as intended, or when experiencing off design states of operation.
While each of the first tubes 30 are shown with a circular cross-section, these first tubes 30 could alternatively have some other cross-section, such as a square cross-section or other cross-sectional form. In one embodiment, protuberances 35 extend inwardly from the inner surface 34 of the walls 32. Such protuberances can promote turbulence and improve heat transfer effectiveness. In one embodiment, such protuberances 35 are formed by forming dimples 37 on the outer surface 36, and with the walls 32 being sufficiently thin that the dimples 37 extend through the walls 32 to leave the protuberances 35 on the inner surface 34.
Similar to the first tubes 30, the second tubes 50 also include walls 52 bounded by an inner surface 54 opposite an outer surface 56, and with optional protuberances 55 and/or dimples 57 thereon. Other characteristics of the second tubes 50 can vary in a manner similar to that described above with respect the first tubes 30. Typically, the second tubes 50 are formed from a different material than that from which the first tubes 30 are formed. In one embodiment, one of the fluids passing through the heat exchanger 10 is non-corrosive, or less corrosive, or corrosive in a different way than the tubes 30 of the alternate circuit within heat exchanger 10. The tubes 50 can thus be optimized for the fluid they are containing, and to minimize cost. In one embodiment, the second tubes 50 can be formed of a similar material from which the matrix 60 is formed. As another alternative, the first tubes 30 and second tubes 50 can be formed of a common material which is different from that of the metal matrix 60.
While the tubes 30, 50 have been described as being within two circuits 20, 40, it is conceivable that more than two circuits 20, 40 might be provided in an alternative heat exchanger. For instance, a heat exchanger might be configured which exchanges heat between three different fluids passing through three different circuits, while each of the different circuits could be made of tubes formed of similar or different materials to the materials forming the other tubes of the other circuits and providing the matrix 60.
With particular reference to
With particular reference to
The heat exchanger 10 can then be removed from the press. Finally, ends of the first tubes 30 and second tubes 50 within the two circuits 20, 40 can be ground if necessary or otherwise cut to final size (if needed), and manifolds can be added. In particular, the first inlet manifold 70 can overlay inlet end 22 of circuit A 20. A first outlet manifold 72 can be placed over the outlet end 24 of tubes 30 of circuit A 20. A second inlet manifold 74 can be placed over entry end 42 of second tubes 50 within circuit B 40. A second outlet manifold 76 can be placed over exit 44 ends of the second tubes 50 within circuit B 40. The manifolds 70, 72, 74, 76 facilitate flow along the various circuits 20, 40 including circuit A flow along arrow A and circuit B flow along arrow B. While these arrows generally illustrate counterflow, other forms of flow could be provided by rearranging the tubes 30, 40 within the heat exchanger 10 or adjacent plumbing.
As an option when manufacturing the heat exchanger 10, the tubes 30, 50 (at least some of them) can be at least partially filled with support media. In one embodiment, the media is in the form of small pellets sized small enough that they can substantially fill interiors of the tubes 30, 50. Filling of the tubes 30, 50 with media allows for the HIP process to occur without deforming the tubes, 30, 50, especially by collapsing the tubes 30, 50 due to the pressure associated with the HIP process. Other related processes which involve pressure and/or heat could similarly minimize tube 30, 50 deformation by placing the media into the tubes, 30, 50. The media is typically in the form of beads of microscopic or near microscopic size. At a minimum, the media is a form of beads that are at least an order of magnitude smaller than a diameter of the tubes 30, 50.
The media is preferably formed of a material which can withstand high temperatures and high pressures without significant deformation. As one example, alumina ceramic beads of about 10 micron average size can be used as the media. A shape of the media in one embodiment is spherical.
With reference to
While it is generally desirable that the media does not collapse at all under pressures applied by the HIP or other process, some deformation of the media could occur, provided the media can still be removed after the matrix material has been sintered from a powder into a more solid form. While it is also generally desirable that the media does not melt under the temperatures associated with the HIP or other process to solidify the matrix, the media could, in at least one embodiment, melt into a liquid provided that the ends of the tubes 30, 50 are plugged to avoid such liquid escaping in an appreciable amount, or the process would need to be sufficiently rapid that the application of pressure would cease before an appreciable amount of such liquid escapes from the tubes 30, 50.
When such support media is utilized inside of the tubes 30, 50, a final step involves removing the media, typically before manifolds are added to the ends of the tubes 30, 50. Options for such a removal step can include application of suction, application of pressurized air, utilization of at least one elongate fiber placed in the end of the tube 30, 50 to push out the media, utilizing gravity, vibration and/or other appropriate media removal techniques (alone or in combination).
In an alternative embodiment, the media can be selected from a material which does deform slightly under pressure, or the media could less than entirely fill at least some of the tubes 30, 50 and the pressure associated with the HIP or other process would cause small amounts of tube 30, 50 deformation. Such small amounts of deformation could have the beneficial effect of roughening interior surfaces of the tubes 30, 50 to enhance heat transfer therealong, and could also facilitate some degree of compaction of the tubes 30, 50 together, which, in turn, could cause the heat exchanger 10 to have a more compact volume after completion of manufacture.
Utilizing the media also can beneficially allow for wall thicknesses of the tubes 30, 50 to be reduced to thicknesses less than would otherwise be able to withstand the pressure associated with the HIP or other matrix solidification process, so the thicknesses of the tubes 30, 50 can be optimized beyond what would otherwise be possible. Thus, in one embodiment, wall thicknesses for the tubes 30 and/or tubes 50 are less than a thickness which could withstand the pressure associated with the HIP process, without the presence of the media within the tubes 30, 50. One example pressure for one HIP process is about 15,000 psi (or greater).
This disclosure is provided to reveal a preferred embodiment of the invention and a best mode for practicing the invention. Having thus described the invention in this way, it should be apparent that various different modifications can be made to the preferred embodiment without departing from the scope and spirit of this invention disclosure. When embodiments are referred to as “exemplary” or “preferred” this term is meant to indicate one example of the invention, and does not exclude other possible embodiments. When structures are identified as a means to perform a function, the identification is intended to include all structures which can perform the function specified. When structures of this invention are identified as being coupled together, such language should be interpreted broadly to include the structures being coupled directly together or coupled together through intervening structures. Such coupling could be permanent or temporary and either in a rigid fashion or in a fashion which allows pivoting, sliding or other relative motion while still providing some form of attachment, unless specifically restricted.
This application is a continuation in part of U.S. patent application Ser. No. 17/947,938, filed on Sep. 19, 2022, which is a divisional of U.S. patent application Ser. No. 16/585,822, filed on Sep. 27, 2019 and issued as U.S. Pat. No. 11,448,467 on Sep. 20, 2022, which claims benefit under Title 35, United States Code § 119(e) of U.S. Provisional Application No. 62/738,123 filed on Sep. 28, 2018.
| Number | Date | Country | |
|---|---|---|---|
| 62738123 | Sep 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16585822 | Sep 2019 | US |
| Child | 17947938 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17947938 | Sep 2022 | US |
| Child | 18230307 | US |
