FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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 sCO2power 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, Calif., 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.

1. U.S. Pat. No. 3,306,353 describes a porous sintered matrix that uses the matrix as the flow field for the second circuit, opposite the tubular circuit.

2. U.S. Pat. No. 6,668,912 describes a cast metal composite matrix around a tubular core, but again appears to have a single tubular circuit.

3. U.S. Pat. No. 8,177,932 describes a method for manufacturing a micro-tube heat exchanger.

4. U.S. Pat. Nos. 4,928,755 and 4,676,305 describe micro-tube stripe heat exchangers.

Additive manufacturing, in the form of3D printing, is being used to produce heat exchangers. Current metal3D 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 of3D 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:

1. Lack of dimensional control/configuration limitations; especially in horizontal channels.

2. Lack of consistency in deposited and processed material properties.

3. Size limitations.

4. Long fabrication time for large devices.

5. Residual stress, exacerbated by size.

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.

SUMMARY OF THE INVENTION

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 ˜. 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.

OBJECTS OF THE INVENTION

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:

1. The ability to segregate and contain exchanger fluids in a dedicated and minimized volume of material appropriate to the fluid contained, i.e. it allows for multi-material construction. For corrosive media, this minimized volume will be a high temperature and/or corrosion resistant material. For the opposite circuit, which may be a non-aggressive media, low cost material can be employed.

2. Three materials or components separate unlike flows (tube walls of the first circuit, tube walls of the second circuit and a metal matrix between the two circuits.

3. Utilizes proven, environmentally favorable manufacturing processes and materials which have either been adopted, or are in the process of being adopted, by ASME (American Society of Mechanical Engineers).

4. High pressure integrity, with both primary and secondary containment.

5. Compactness and effectiveness greater than PCHE (CPHX). Preliminary estimates are 15% to 25% greater.

6. Significantly lower cost than currently employed heat exchanger technologies. Preliminary estimates are 25% to 50% lower cost than PCHEs/CPHXs in equivalent service.

7. Shorter fabrication and lead times than currently employed heat exchanger technologies.

8. The ability to segregate and contain exchanger fluids in a dedicated and minimized volume of material appropriate to the fluid contained.

9. Allows the use of low cost materials where applicable.

10. Uses predominantly additive manufacturing processes.

11. Extreme compactness and higher effectiveness than current state-of-the-art compact heat exchangers such as Printed Circuit Heat Exchangers (PCHEs) or Compact Platelet Heat Exchangers (CPHX).

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.

