WO2021178447A1

WO2021178447A1 - System and method for manufacturing and operating a coaxial tube heat exchanger

Info

Publication number: WO2021178447A1
Application number: PCT/US2021/020536
Authority: WO
Inventors: Michael Wilson
Original assignee: Daikin Applied Americas, Inc.
Priority date: 2020-03-03
Filing date: 2021-03-02
Publication date: 2021-09-10
Also published as: US20210278137A1; CA3170432A1; MX2022010846A

Abstract

A coaxial heat exchanger is provided. Embodiments of the present disclosure relate to a coaxial heat exchanger for use in water source heat pumps or other applications involving fluid to fluid heat transfer. Embodiments of the present disclosure allow for the use of pre-existing engineered tubing with a textured or riffled interior surface and a folded fin intermediate member. Some methods of the present disclosure involve annealing and hydrostatically expanding the engineered tubing to increase contact and thermal transfer between the inner tube and the intermediate member. Additional systems, devices, and methods are also disclosed.

Description

System and Method for Manufacturing and Operating a Coaxial Tube Heat Exchanger

Cross Reference to Related Application

[0001] This application claims the benefit of U.S. Provisional Application No. 62/984,525, filed March 3, 2020. The entire disclosure of U.S. Provisional Application No. 62/984,525 is hereby incorporated herein by reference.

Field of the Invention

[0002] This invention relates generally to coaxial heat exchangers, and in particular, to manufacturing processes and equipment for producing coaxial heat exchangers, such as for HVAC systems and water source heat pumps.

Background

[0003] This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the presently described embodiments— to help facilitate a better understanding of various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.

[0004] Modern residential and industrial customers expect indoor spaces to be climate controlled. In general, heating, ventilation, and air-conditioning ("HVAC") systems circulate an indoor space's air over low-temperature (for cooling) or high-temperature (for heating) sources, thereby adjusting the indoor space's ambient air temperature. HVAC systems generate these low- and high-temperature sources by, among other techniques, taking advantage of a well-known physical principle: a fluid transitioning from gas to liquid releases heat, while a fluid transitioning from liquid to gas absorbs heat. [0005] In a typical system, a fluid refrigerant circulates through a closed loop of tubing that uses compressors and other flow-control devices to manipulate the refrigerant's flow and pressure, causing the refrigerant to cycle between the liquid and gas phases. These phase transitions generally occur within the HVAC's heat exchangers, which are part of the closed loop and designed to transfer heat between the circulating refrigerant and outside environment. This is the foundation of the refrigeration cycle. The heat exchanger where the refrigerant transitions from a gas to a liquid is called the "condenser," and the condensing refrigerant releases heat to the surrounding environment. The heat exchanger where the refrigerant transitions from liquid to gas is called the "evaporator," and the evaporating refrigerant absorbs heat from the surrounding environment.

[0006] A heat pump is a compression refrigeration system that is designed to reverse the flow of refrigerant to transition between heating and cooling modes. A reversing valve controls the direction of refrigerant flow through the refrigerant loop, thereby determining whether the heat pump is in heating mode or cooling mode. When the refrigerant flow is reversed, the potion of the refrigerant loop that previously functioned as an evaporator functions as a condenser and vice versa.

[0007] Water source heat pumps (WSHP) generally rely on two loops and one or more heat exchangers that transfer heat between the loops. Water source heat pumps utilize a water loop and a refrigerant loop. Depending on the mode of operation, heat is absorbed by one loop and transferred to the other. Heat is often transferred between the refrigeration loop and water loop using coaxial heat exchangers.

[0008] Coaxial heat exchanges, also called tube-in-tube heat exchangers, are used in numerous applications including various heating, ventilation, air conditioning, and refrigeration (HVACR) applications. Like other forms of heat exchangers, coaxial heat exchangers are used to transfer heat from one fluid to another. While coaxial heat exchangers are also referred to as tube-in-tube heat exchangers, neither the inner nor outer tube is required to be round. Either the inner or outer tube may be substantially any shape that allows a fluid to flowthrough the tube. The inner tube and outer tube are each designed to resist the working pressures associated with the fluids within the inner and outer tube respectively. In some coaxial heat exchangers, a tube may contain grooves, lobes, fins, projections, or other elements that increase the surface area of a tube or promote a higher heat transfer coefficient, thereby allowing a greater exchange of heat between a fluid on the interior of the tube and a fluid on the exterior of the tube.

[0009] In WSHPs, coaxial heat exchangers are used to transfer heat between the refrigerant within the refrigerant loop and the water, water/anti-freeze solution, or other fluid within the water loop. Water/anti-freeze solutions include, but are not limited to, water/methanol and water/glycol solutions. The water or other fluid in the water loop then transfers heat to the outside environment, such as the air or ground.

Summary

[0010] Certain aspects of some embodiments disclosed herein are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below.

[0011] Embodiments of the present disclosure generally relate to a heating, ventilation, air conditioning or refrigeration (HVACR) system adapted utilizing a coaxial heat exchanger that has been formed according to the disclosed methods. In some embodiments, engineered tubing may be hydrostatically expanded to achieve a higher degree of contact and/or a press fit with an intermediate member. It will be appreciated that expanding the engineered tubing may create a friction fit and/or interference fit with the intermediate sleeve. The engineered tubing and sleeve may be positioned within an outer jacket, thereby creating an inner fluid pathway and outer fluid pathway which may be used for fluid to fluid heat exchange.

[0012] Some embodiments of the present disclosure generally relate to a coaxial heat exchanger for water source heat pumps comprising an extruded aluminum fin member comprising an interior and an exterior. In some embodiments, the interior of the extruded aluminum fin member has a substantially circular interior diameter and the exterior of the extruded aluminum fin member comprises a plurality of projecting structures. Some embodiments further comprise an engineered copper tube with a rifled interior. In some embodiments, the engineered copper tube is positioned within the extruded aluminum fin member and expanded using hydrostatic pressure to increase contact between the exterior of the engineered copper tube and the interior of the aluminum fin member. Embodiments further comprise an outer jacket positioned outboard of the extruded aluminum fin member. [0013] Some embodiments of the present disclosure generally relate to a method for manufacturing a coaxial heat exchanger, the method comprising the steps of obtaining an engineered inner tube comprising an interior and an exterior, wherein the interior of the inner tube includes a rifled texture; annealing the engineered inner tube; positioning the engineered inner tube within the interior volume of an intermediate member comprising an interior and an exterior wherein the exterior of the intermediate member comprises projecting structures; deforming the engineered inner tube using a pressure to increase contact between the exterior of the engineered inner tube and the interior of the intermediate member; and positioning the intermediate member and engineered inner tube within an outer jacketing member.

[0014] Various refinements of the features noted above may exist in relation to various aspects of the present embodiments. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. Again, the brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of some embodiments without limitation to the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and other features, aspects, and advantages of certain embodiments will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

[0016] FIG. 1A illustrates schematically a heat pump in heating mode.

[0017] FIG. IB illustrates schematically a heat pump in cooling mode.

[0018] FIG. 2 illustrates schematically a cross section of a coaxial heat exchanger according to one embodiment.

[0019] FIG. 3 illustrates schematically a coaxial heat exchanger according to one embodiment.

[0020] FIG. 4 illustrates schematically a portion of a coaxial heat exchanger utilizing projecting structures according to one embodiment. [0021] FIG. 5 illustrates schematically a coaxial heat exchanger according to one embodiment.

[0022] FIG. 6 illustrates schematically a coaxial heat exchanger according to one embodiment.

[0023] FIG. 7 illustrates schematically a folded fin material according to one embodiment. [0024] FIG. 8 illustrates schematically a coaxial heat exchanger according to one embodiment.

[0025] FIG. 9 illustrates schematically a coaxial heat exchanger according to one embodiment.

[0026] FIG. 10 illustrates a data plot comparing the water side differential pressure of two coaxial heat exchangers

[0027] FIG. 11 illustrates a data plot comparing the heat transfer of two coaxial heat exchangers

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0028] One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0029] When introducing elements of various embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0030] Turning to the figures, FIG. 1A and FIG IB each illustrate schematically a heat pump. A heat pump operates on the principal that heat moves from a warmer material to a cooler material. A coil that is cooler than its surroundings will absorb head, and a coil that is warmer that its surroundings will release heat.

[0031] FIG. 1A illustrates a heat pump 100 in heating mode. In heating mode, the outdoor heat exchanger 110 serves as an evaporator and absorbs heat from its surroundings. Conversely, the indoor heat exchanger 120 serves as a condenser and releases heat to its surroundings. Low-pressure liquid refrigerant or a liquid-vapor mixture enters the outdoor heat exchanger 110, absorbs heat from the surrounding environment, and vaporizes. The low- pressure refrigerant vapor then enters the compressor 130 where it is compressed to a high- temperature and high-pressure vapor. The high-temperature, high-pressure vapor then enters the indoor heat exchanger 120 where it releases heat to the surrounding environment and condenses to a high-pressure liquid. The high-pressure liquid passes through a metering device 140, such as thermal expansion valve or a capillary tube, where it becomes a low- pressure liquid or liquid-vapor mixture and enters the outdoor heat exchanger and then repeats the process. It will be appreciated that the "indoor" heat exchanger is not required to be physically indoors. In certain FIVAC applications, the indoor heat exchanger may be in communication with the indoor air, a water loop, or another system that communicates heat, directly or indirectly, to the area to be climate controlled. It will similarly be appreciated that the "outdoor" heat exchanger is not required to be physically outdoors. In certain HVAC applications, the outdoor heat exchanger may be in communication with a water loop or other system that communicates heat, directly or indirectly, to the outside environment. [0032] FIG. IB illustrates a heat pump 101 in cooling mode. In cooling mode, the heat pump operates on the same underlying principal, but the outdoor heat exchanger 111 serves as a condenser and releases heat to its surroundings. Conversely, the indoor heat exchanger 121 serves as an evaporator and absorbs heat from its surroundings. The flow of refrigerant may be reversed when the reversing valve 151 changes position.

[0033] In water source heat pumps (WSHPs) the refrigerant transfers heat to and/or from a flowing stream of water or water-antifreeze mixture. This transfer of heat is typically performed using a coaxial heat exchanger. The water to refrigerant heat exchanger is generally referred to as the outdoor heat exchanger although it will be appreciated that the outdoor heat exchanger may be positioned indoors.

[0034] FIG. 2 schematically illustrates a cross section of coaxial heat exchanger 200 according to one embodiment. The inner tube 210 of the heat exchanger 200 is configured to allow water to flow through the inner tube. In some embodiments, the interior surface of the inner tube 210 is rifled, knurled, patterned, or otherwise textured in order to increase the heat transfer between water flowing through the inner tube and the material of the inner tube itself. In some embodiments, the inner tube 210 is engineered tubing or tech tube. In some embodiments, the inner tube 210 comprises copper, copper-nickel allow, titanium, and/or stainless steel. In some embodiments, the inner tube is an evaporator tube such as, for example, B4 or B5 tubes. In some embodiments, the inner tube is a condenser tube such as, for example, C+LW or C5 tubes. [0035] Coaxial heat exchanger 200 also includes an intermediate member 220 positioned outboard of the inner tube 210. The intermediate member 220 may be a configured as a sleeve with projecting members, folded fins, or a combination of the two. In some embodiments, a folded fin intermediate member may be arranged as a ruffled folded fin, plain folded fin, and/or a lanced and offset folded fin. In some embodiments, the projecting structures and/or fins of the intermediate tubing member may be textured, twisted, and/or axially rifled. In some embodiments, these surface features increase the mixing and/or turbulence of a flowing fluid, thereby enhancing the degree of heat transferred between the intermediate tubing member and the fluid.

[0036] As shown in FIG. 2, when intermediate member 220 is configured as a folded fin, portions of the folded fin are in contact with the inner tube 210, thereby allowing heat transfer from the inner tube 210 to the intermediate member 220. The increased surface area created by intermediate member allows for increased heat transfer.

[0037] In some embodiments, intermediate member 220 is initially in a generally planar configuration and is wrapped around the inner tube 210 to form a tubular intermediate member. In some embodiments, the intermediate member is brazed to itself to maintain a tubular configuration rather than a planar form. Depending on the respective length of the inner tube and the intermediate member, in some embodiments, multiple intermediate members sections may be wrapped around the inner tube or positioned axially around the inner tube. In some embodiments, an intermediate member section is between 4 to 6 inches long. In some embodiments, an intermediate member 220 may comprise one or more than one intermediate member sections. In some embodiments, the intermediate member sections may be in contact with each other. In some embodiments, the intermediate member sections are separated by a gap. In some embodiments, the gap between intermediate member sections is smaller at the first or last portion of the heat exchanger as compared to the middle portion of the heat exchanger. In some embodiments, an intermediate member comprises a plurality of rings axially aligned around the inner tube. In such embodiments, one or more than one of the rings includes axially projecting structures.

[0038] The intermediate member 220 is positioned within an outer jacket 230. The outer jacket 230 is outboard of the intermediate member 220 and the inner tube 210. In some embodiments, the outer jacket 230 is rigid. In some embodiments, the outer jacket 230 contains metal, such as, for example, steel, stainless steel, copper, or aluminum. The outer jacket 230 is sufficiently strong to resist deformation at the appropriate working pressures such as, for example, refrigerant pressures.

[0039] The volume contained within the interior of the inner tube 210 is referred to as the inner volume 240. The volume between the interior of the outer jacket 230 and the exterior of the inner tube 210 is referred to as the outer volume 250.

[0040] In some embodiments, the coaxial heat exchanger 200 allows a first fluid to flow through the inner volume 240 while a second fluid flows through the outervolume 250. Heat is exchanged between the first and second fluids through the inner tube 210 and the intermediate member 220. In some embodiments, the first fluid is water and the second fluid is a refrigerant which undergoes a phase change as heat is exchanged between the water and refrigerant.

[0041] In some embodiments, in order to create and/or increase contact between the inner tube 210 and the intermediate member 220, the inner tube is expanded or otherwise deformed. The inner tube 210 may be expanded using pressure, such as, for example, hydrostatic pressure, or using mechanical expansion processing. In some embodiments, it is advantageous to expand the inner tube 210 using hydrostatic pressure in order to avoid crushing or significantly deforming the texture and/or rifling on the interior surface of the inner tube 210.

[0042] In some embodiments, before the inner tube 210 is expanded, the inner tube 210 is annealed. The inner tube 210 is annealed by heating it to a predetermined temperature and allowing the inner tube 210 to cool at a controlled rate. Once the inner tube 210 is annealed, the inner tube material is generally softer and may be more easily expanded.

[0043] In some embodiments, the inner tube 210 is optimized to transfer heat between a water solution in the inner volume 240 and a refrigerant in the outer volume 250 when the refrigerant is in a two-phase mixture or is in the process of changing phases (either evaporation or condensation). In some embodiments, the exterior of the innertube contains ridges that are optimized to promote the evaporation of refrigerant by facilitating the formation of bubbles when the refrigerant evaporates. In some embodiments, the exterior of the inner tube contains ridges that are optimized to promote the condensation of refrigerant by facilitating the formation of droplets when the refrigerant condenses. It will be appreciated that embodiments that are optimized to facilitate evaporation of the refrigerant will also promote condensation of the refrigerant when compared to an inner tube with a generally smooth exterior surface.

[0044] In some embodiments, the first and/or last portions of a condenser or evaporator generally contain more single-phase refrigerant while the middle portion contains more liquid-vapor mixture. Accordingly, in some embodiments, engineered tubing with an enhanced textured exterior surface may be used for the middle portion of the innertube and tubing with a smooth or otherwise unenhanced exterior surface may be used for the first and/or last portion of the inner tube. In some embodiments, regardless of the state of the exterior surface of the inner tube, the interior surface of the inner tube will contain an engineered textured surface to take advantage of the increased turbulence and heat transfer with the liquid water solution flowing through the interior of the inner tube.

[0045] In some embodiments, the intermediate member increases the surface area in contact with a refrigerant, whether the refrigerant is in a single-phase (liquid or vapor) or in a two- phase mixture. In some embodiments, the intermediate member facilitates greater heat transfer when the refrigerant is in a single phrase as compared to when the refrigerant is in a two-phase mixture. In some embodiments, the intermediate member is only present at the first and last portions of the coaxial heat exchanger. In some embodiments, the intermediate member is not included in the portions of the heat exchanger that are expected to contain significantly two-phase mixtures of refrigerant. In some embodiments, the intermediate member is made of multiple intermediate member sections. In some embodiments, there is a gap between each intermediate member section. In some embodiments, the gap between intermediate member sections is larger in the middle portion of the coaxial heat exchanger as compared to the end portions of the heat exchanger. In some embodiments, the gap between intermediate member sections is larger in the four feet middle section of a ten feet long heat exchanger than in the three feet sections at either end of the ten feet long heat exchanger.

[0046] In some embodiments, the linear length of a coaxial heat exchanger is about ten feet. In some embodiments, about three-foot long sections closest to the ends of the heat exchanger contain inner tube members with a generally smooth or otherwise unenhanced exterior surface while the middle about four-foot section contains enhanced engineered inner tube with an exterior surface designed to promote evaporation or condensation of the refrigerant.. [0047] FIG. 3 schematically illustrates a coaxial heat exchanger 300 according to one embodiment. Heat exchanger 300 includes a copper innertube 310, an intermediate member 320 configured in a folded fin design, and a steel outer jacket 330. FIG. 3 also illustrates fluid inlet 340 that allows a second fluid to flow into the outer volume 360 to exchange heat with a first fluid in the inner volume 350. Not shown in FIG. 3 is an analogous fluid outlet that allows the second fluid to exit the outer volume of the coaxial heat exchanger 300.

[0048] FIG. 4 schematically illustrates a coaxial heat exchanger 400 according to one embodiment. FIG. 4 illustrates an inner tube 410 and an intermediate member 420. For clarity, FIG. 4 does not show an outer jacket. In some embodiments, innertube 400 comprises copper and has a textured or rifled interior surface. In some embodiments, the outer surface of inner tube 410 is generally smooth. In some embodiments, intermediate member 420 comprises extruded aluminum such as, for example, a single piece of extruded aluminum. Aluminum may be extruded to form particular cross-sectional designs. As shown in FIG. 4, intermediate member 420 may include a generally circular interior surface 423 with projecting structures 425 radiating therefrom. The generally circular interior surface 423 of intermediate tubing member 420 allows for a high degree of contact between the intermediate member 420 and the inner tube 410.

[0049] In some embodiments, contact between the inner tube 410 and the intermediate tubing member 420 is increased by annealing the inner tube, then axially inserting the annealed innertube into the intermediate tubing member and hydrostatically expanding the annealed inner tube within the intermediate tubing member. This process creates an increased degree of contact and facilitates heat transfer between the inner tube 410 and the intermediate tubing member 420. This arrangement allows the intermediate tubing member

420 to transfer heat between the first fluid, flowing within the inner volume within the inner tube 410 to or from a second fluid flowing in the outer volume between the intermediate tubing member 420 and the outer jacket (not shown) without the intermediate tubing member 420 being in contact with the first fluid. In some embodiments, the first fluid contains water and the second fluid contains a refrigerant such as, for example, R410A, R32, R454B, DR-55, R134a, R513A, R515A, R515B, HFO refrigerants such as HFO-1234ze, HFO- 1233zd, or FIFO-1234yf, or any number of combinations thereof. Expanding the annealed inner tube within the intermediate tubing member increases contact between the inner tube and intermediate member thereby facilitating thermal transfer. In some embodiments, expanding the inner tube within the intermediate member creates a press fit. This arrangement prevents the material of the intermediate tubing member from contacting the first fluid within the inner tube. This arrangement allows the intermediate member to contain materials that may not be suitable for sustained contact with the first fluid. Expanding the annealed inner tube using hydrostatic pressure allows the use of pre-existing engineered tubing or tech tube that has a rifled interior surface to be used with an extruded aluminum intermediate tubing member.

[0050] FIG. 5 schematically illustrates a coaxial heat exchanger 500 according to one embodiment. FIG. 5 illustrates an inner tube 510 and an intermediate member 520. Intermediate member 520 is an extruded member. In some embodiments, the extruded tube member 520 includes a generally circular inner surface 523 and a generally circular outer surface 525 with a plurality of channels 527 passing through the length of the tube member 520. This arrangement allows the tube member to form a high degree of contact and thermal transfer between the tube member and the inner tube, thereby facilitating a high degree of thermal transfer between the first fluid flowing through the inner tube and the second fluid flowing through the channels 527 of the tube member 520. In some embodiments, the outer surface 525 of the tube member 520 is connected to a coupling 550. Fluid inlet 540 allows fluid to enter the volume between the coupling 550 and the inner tube 510. This volume is in fluid communication with the channels 527 that pass through the tube member 520. The tube member 520 is positioned axially outboard of the inner tube 510. A coupling 550 seals the outer volume, causing the second fluid to flow through the fluid inlet/outlet 540 in order to pass through the outer volume. In some embodiments, no separate outer jacket is required as the second fluid is contained within the volume defined by the channels of the extruded tube member and the coupling 550.

[0051] In some embodiments, the coupling 550 is also in contact with the exterior surface of the inner tube 510. In some embodiments, the coupling is sealed, adhered, and/or brazed to the exterior surface of the inner tube 510 and exterior surface of the tube member 520 to prevent leakage of the second fluid. In such embodiments, no outer jacket is required. [0052] FIG. 6 schematically illustrates a coaxial heat exchanger 600 according to one embodiment. In some embodiments, the intermediate member 620 includes a fin portion 623 and a non-fin portion 625. In some embodiments, the intermediate member is formed as an aluminum extrusion with fins or other projecting structures along the length of the intermediate member. In some embodiments, the fins or projecting structures are machined off or otherwise removed from the intermediate member, thereby forming the non-fin portion of the intermediate member.

[0053] In some embodiments, the intermediate member serves as a double wall construction around the inner tube 610. This double wall construction prevents any mixing of the first and second fluids in the event that either the inner tube 610 or the intermediate member 620 corrodes or otherwise becomes damaged, resulting in a leak. In some embodiments, the outer jacket 630 is sealed around the non-fin portion 625 of the intermediate member 620. In some embodiments, the outer jacket 630 is arranged so that no separate coupling is required. As shown in FIG. 6, in some embodiments, the fluid inlet/outlet may be incorporated into the outer jacket 630, allowing fluid to flow into or out of the outer volume between the outer jacket 630 and the intermediate member 620 .

[0054] In some embodiments, the outer jacket 630 is brazed to the intermediate member 620 and any space between the intermediate member and inner tube is left open to the atmosphere. In such embodiments, if the intermediate member becomes damaged, any refrigerant flowing through the outer zone is released into the atmosphere rather than contaminating the circulating water solution within the inner tube. It will be appreciated that any space between the inner tube and intermediate member is very small and does not significantly reduce the thermal transfer between the first and second fluids.

[0055] FIG. 7 illustrates a folded fin structure 700 in a planar or flat form according to one embodiment. In some embodiments of the heat exchanger, the folded fin structure 700 is wrapped, rolled, or otherwise positioned around an inner tube to form the intermediate member.

[0056] FIG. 8 illustrates a heat exchanger 800 according to one embodiment. It will be appreciated that the components of heat exchanger 800 have been positioned for clarity. Engineered inner tube 810 includes a textured interior surface and a textured exterior surface. Intermediate member 820 is configured as a folded fin. Intermediate member 820 is positioned axially outboard of the engineered inner tube 810. In some embodiments, intermediate member 820 is initially a substantially flat or planar member and is wrapped around inner tube 810. The innertube 810 and intermediate member 820 are positioned within outer jacket 830. [0057] In some embodiments, the intermediate member is configured to create an axial gap portion 840. The exterior surface of the inner tube is generally not covered by the intermediate member in the axial gap portion. In some embodiments, the heat exchanger is coiled after it is assembled. In some embodiments, the axial gap portion allows the heat exchanger to be coiled while reducing the amount of crimping or crushing of the intermediate member. If the intermediate member were significantly crimped or crushed, the damaged portion of the intermediate member could restrict the flow of fluid through the volume between the inner tube and outer jacket. In some embodiments, the axial gap portion is positioned at the interior radius of the coiled coaxial heat exchanger.

[0058] In some embodiments, a spacer (not shown) may be positioned axially to the inner tube. In some embodiments, the spacer may be positioned in the axial gap portion. In some embodiments, the spacer comprises a flexible material that allows fluid to pass through the spacer such as, for example, copper wool.

[0059] FIG. 9 illustrates a heat exchanger 900 according to one embodiment. The inner tube 910 includes a rifled interior surface and a textured exterior surface. Intermediate member 920 is in contact with both the inner tube 910 and the outer jacket 930. In some embodiments, inner tube 910 has been hydrostatically expanded in order to increase contact between the inner tube 910 and the intermediate member 920 without deforming the texture on either the interior or exterior surfaces of the inner tube 910. In some embodiments, the outer jacket 930 or intermediate member 920 may be shrunk once the innertube 910 is positioned within the outer jacket 930 or intermediate member 920 in order to create a press fit or otherwise increase contact between the intermediate member 920 and the inner tube 910. [0060] FIG. 10 illustrates a data plot comparing the water side pressure drop of an example embodiment of the coaxial heat exchanger described herein (square data points) and a commercially available fluted tube coaxial heat exchanger (diamond data points). When water is pumped into a coaxial heat exchanger the water enters the heat exchanger at a higher pressure than the water exits the heat exchanger. This drop in pressure is due to liquid friction and turbulence created within the heat exchanger. The greater the drop in pressure, the more energy must be used to pump water into the heat exchanger.

[0061] As shown in FIG. 10, the coaxial heat exchanger according to one embodiment described herein had a lower water side pressure drop than the commercially available fluted tube heat exchanger at all flow rates. The difference in waterside pressure drop increases as the flow rate increases.

[0062] FIG. 11 illustrates a data plot comparing the heat transfer of an example embodiment of the coaxial heat exchanger described herein (square data points) and a commercially available fluted tube coaxial heat exchanger (diamond data points). As can be seen in FIG. 11, the coaxial heat exchanger according to one embodiment described herein had a greater heat transfer than the commercially available fluted tube heat exchanger at all flow rates. The difference in heat transfer increases as the flow rate increases.

[0063] While the aspects of the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. But it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.

Claims

1. A coaxial heat exchanger for water source heat pumps comprising: an inner tube, wherein the inner tube has a textured interior surface and a textured exterior surface; an intermediate member positioned outboard of the inner tube; and an outer jacket comprising an interior and exterior, wherein the outer jacket is positioned outboard of the intermediate member.

2. The coaxial heat exchanger of claim 1, wherein the inner tube comprises copper or a copper bearing alloy.

3. The coaxial heat exchanger of claim 1, wherein the exterior surface of the inner tube is configured to increase heat transfer to a condensing or evaporating fluid and the interior surface of the inner tube is configured to increase heat transfer to a single-phase fluid.

4. The coaxial heat exchanger of claim 1, wherein the intermediate member comprises a folded fin.

5. The coaxial heat exchanger of claim 1, wherein the outer jacket comprises steel and is brazed to the inner tube.

6. The coaxial heat exchanger of claim 1, wherein the intermediate member is configured to create an axial gap portion, wherein the exterior surface of the inner tube is not covered by the intermediate member in the axial gap portion.

7. The coaxial heat exchanger of claim 6, further comprising a spacer positioned axially parallel to the inner tube in the volume between the inner tube and the outer jacket.

8. The coaxial heat exchanger of claim 1, wherein the intermediate member comprises more than one intermediate member section.

9. The coaxial heat exchanger of claim 1, wherein the inner tube is annealed and hydrostatically expanded within the intermediate member.

10. The coaxial heat exchanger of claim 1, wherein the heat exchanger is coiled and has about a ten-inch diameter.

11. A method for manufacturing a coaxial heat exchanger, the method comprising: obtaining an engineered inner tube comprising an interior surface and an exterior surface, wherein the interior surface and exterior surface of the engineered inner tube are textured; positioning an intermediate member comprising an interior and an exterior axially outboard of the engineer inner tube; positioning the intermediate member and engineered inner tube within an outer jacket; and deforming at least one of the engineered inner tube, intermediate member, or outer jacket to increase thermal transfer between the engineered inner tube and the intermediate member.

12. The method of claim 11, further comprising annealing the engineered inner tube.

13. The method of claim 11, wherein the deforming comprises expanding the engineered inner tube using hydrostatic pressure.

14. The method of claim 11, wherein the deforming comprises shrinking the intermediate member or outer jacket.

15. The method of claim 11, wherein the interior surface of the engineered inner tube is rifled.

16. The method of claim 11, wherein the engineered inner tube comprises copper or copper bearing alloy.

17. The method of claim 11, wherein the intermediate member comprises extruded aluminum.

18. The method of claim 11, wherein the intermediate member comprises a folded fin.

19. The method of claim 11, further comprising brazing the outer jacket to the intermediate member.

20. A water source heat pump comprising: a compressor in fluid communication with a refrigerant line; a reversing valve configured to adjust the direction of refrigerant flowing in at least a portion of the refrigerant line; a water line wherein the water line is in thermal communication with the outside environment; and a coaxial heat exchanger comprising: an inner tube in fluid communication with the water line, wherein the inner tube has a textured interior and exterior surface; an intermediate member positioned outboard of the inner tube wherein the intermediate member has a folded fin structure; and an outer jacket positioned outboard of the intermediate member to define a volume between intermediate member and the outer jacket, the volume being in fluid communication with the refrigerant line.