U.S. patent application number 12/690373 was filed with the patent office on 2010-07-22 for plate-frame graphite-foam heat exchanger.
This patent application is currently assigned to LOCKHEED MARTIN CORPORATION. Invention is credited to Avram Bar-Cohen, Nicholas J. Nagurny, Kelvin D. Quarles.
Application Number | 20100181054 12/690373 |
Document ID | / |
Family ID | 42336022 |
Filed Date | 2010-07-22 |
United States Patent
Application |
20100181054 |
Kind Code |
A1 |
Nagurny; Nicholas J. ; et
al. |
July 22, 2010 |
Plate-Frame Graphite-Foam Heat Exchanger
Abstract
A heat exchanger for thermally coupling a first fluid and second
fluid is disclosed. The heat exchanger comprises plates comprising
cores of thermally conductive graphite foam. A plurality of
conduits for conveying the first fluid is formed in each core. Each
plate further comprises thermally conductive barriers that sandwich
the core, wherein the barriers are substantially impervious to r
the first fluid and second fluid. Plates are stacked in a frame
such that the frame and plates collectively define a plurality of
channels for conveying the second fluid. Heat is exchanged between
the primary fluid and the secondary fluid through the graphite-foam
cores and barriers. Heat exchangers in accordance with the present
invention can be lighter, have improved ratio of heat transfer
surface density to heat exchanger volume, be lower cost, and/or be
smaller for a given heat transfer capability than prior-art heat
exchangers.
Inventors: |
Nagurny; Nicholas J.;
(Manassas, VA) ; Quarles; Kelvin D.; (Woodbridge,
VA) ; Bar-Cohen; Avram; (College Park, MD) |
Correspondence
Address: |
Lockheed Martin c/o;DEMONT & BREYER, LLC
100 COMMONS WAY, Ste. 250
HOLMDEL
NJ
07733
US
|
Assignee: |
LOCKHEED MARTIN CORPORATION
Bethesda
MD
|
Family ID: |
42336022 |
Appl. No.: |
12/690373 |
Filed: |
January 20, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61145996 |
Jan 21, 2009 |
|
|
|
Current U.S.
Class: |
165/167 ;
29/890.03; 29/890.04 |
Current CPC
Class: |
F28D 1/022 20130101;
Y10T 29/49368 20150115; F28F 2255/143 20130101; F28F 13/08
20130101; F28F 21/084 20130101; F28D 9/0081 20130101; F28F 21/067
20130101; F28F 2275/025 20130101; Y02E 10/30 20130101; F28F
2275/062 20130101; Y02E 10/34 20130101; Y10T 29/4935 20150115; F28F
13/003 20130101; F28F 21/02 20130101; F28F 2245/00 20130101; F24V
50/00 20180501 |
Class at
Publication: |
165/167 ;
29/890.03; 29/890.04 |
International
Class: |
F28F 3/12 20060101
F28F003/12; B21D 53/02 20060101 B21D053/02 |
Claims
1. A heat exchanger for thermally coupling a first fluid and a
second fluid, wherein the heat exchanger comprises: a first frame,
wherein the first frame comprises; a first core, wherein the first
core comprises graphite foam comprising first conduits for
conveying the first fluid; and a first barrier for fluidically
isolating the first fluid and the second fluid; and a second frame,
wherein the second frame comprises; a second core, wherein the
second core comprises graphite foam comprising second conduits for
conveying the first fluid; and a second barrier for fluidically
isolating the first fluid and the second fluid; wherein the first
frame and the second frame collectively define a channel for
conveying the second fluid.
2. The heat exchanger of claim 1 wherein the second barrier
comprises a plate, and wherein the plate is mechanically rigid, and
wherein the plate is substantially impervious for each of the first
fluid and the second fluid.
3. The heat exchanger of claim 1 wherein the first barrier
comprises a metal.
4. The heat exchanger of claim 3 wherein the metal comprises
aluminum.
5. The heat exchanger of claim 1 further comprising: a frame,
wherein the frame consists of a first metal; wherein the second
barrier consists of the first metal; and wherein the second barrier
consists of the first metal; wherein the frame, first barrier, and
second barrier are joined with substantially galvanic
corrosion-free joints.
6. The heat exchanger of claim 5 wherein the galvanic
corrosion-free joints are friction-stir welding joints.
7. The heat exchanger of claim 1 wherein the first barrier
comprises a polymer that is substantially impervious to each of the
first fluid and the second fluid.
8. The heat exchanger of claim 7 wherein the polymer is a thermally
enhanced polymer.
9. The heat exchanger of claim 1 wherein the first core consists
substantially of graphite foam.
10. The heat exchanger of claim 1 wherein the first core consists
of graphite foam.
11. The heat exchanger of claim 1 wherein each of the first
conduits further comprises second conduits, and wherein the second
conduits and the first conduits are fluidically coupled.
12. A heat exchanger for thermally coupling a first fluid and a
second fluid, wherein the heat exchanger comprises: (1) a plurality
of plates, wherein each of the first plates comprises; (a) a core,
wherein the core comprises a plurality of conduits for conveying
the first fluid, and further wherein the core comprises graphite
foam; (b) a first barrier, wherein the first barrier is
substantially impervious to the first fluid and the second fluid;
and (c) a second barrier, wherein the second barrier is
substantially impervious to the first fluid and the second fluid;
wherein the first barrier is physically coupled with a first
surface of the core; wherein the second barrier is physically
coupled with a second surface of the core that is opposite the
first surface of the core; and wherein the first barrier and second
barrier are substantially thermally conductive; and (2) a frame for
locating each of the plurality of plates, wherein the frame locates
the plurality of plates such that the frame and the plurality of
plates collectively define a plurality of channels for conveying
the second fluid.
13. The heat exchanger of claim 12 wherein each of the frame, the
first barrier, and the second barrier comprises a first metal, and
wherein the frame and each of the first barriers and second
barriers are joined with friction-stir welding joints.
14. The heat exchanger of claim 13 wherein each of the first
barrier and second barrier comprises a polymer.
15. The heat exchanger of claim 14 wherein the polymer is a
thermally enhanced polymer.
16. A method for forming a heat exchanger for thermally coupling a
first fluid and a second fluid, wherein the method comprises:
providing a plurality of plates, wherein each of the plurality of
plates comprises a graphite-foam core and a barrier that is
substantially impervious to the first fluid and the second fluid,
wherein each of the graphite-foam cores comprises a plurality of
conduits for conveying the first fluid; providing a frame for
locating the plurality of plates; and mounting the plurality of
plates in the frame; wherein the frame and the plurality of plates
collectively define a plurality of channels for conveying the
second fluid.
17. The method of claim 16 further comprising fluidically coupling
a manifold and the plurality of conduits.
18. The method of claim 16 further comprising joining the plurality
of plates and the frame, wherein the plurality of plates and the
frame are joined with friction-stir welding joints.
19. The method of claim 16 wherein at least one of the plurality of
barriers is formed by operations comprising: locating a first core
in a mold of an injection molding system, wherein the first core is
the core of one of the plurality of plates; injecting a polymer
into the mold; and curing the polymer.
20. The method of claim 19 further comprising providing the polymer
as a thermally enhanced polymer.
21. The method of claim 16 wherein at least one of the plurality of
barriers is formed by operations comprising: bonding a first plate
to a first surface of a first core, wherein the first core is the
core of one of the plurality of plates; and bonding a second plate
to a second surface of the first core; wherein each of the first
plate and the second plate is substantially impervious for the
first fluid and the second fluid.
22. The method of claim 21 wherein the first plate is bonded to the
first surface by means of an adhesive.
23. The method of claim 21 wherein the first plate is bonded to the
first surface by operations comprising: disposing a first layer on
the first surface, wherein the first layer comprises a first metal;
and joining the first plate and the first layer.
24. The method of claim 23 wherein the first plate and first layer
are joined with a brazed joint.
25. The method of claim 23 wherein the first plate and first layer
are joined with a welding joint.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This case claims priority of U.S. Provisional Patent
Application U.S. 61/145,996, which was filed on Jan. 21, 2009
(Attorney Docket: 711-246US), and which is incorporated herein by
reference.
[0002] In addition, the underlying concepts, but not necessarily
the language, of the following case is incorporated by reference:
[0003] U.S. patent application Ser. No. 12/484,542, filed Jun. 5,
2009 (Attorney Docket: 711-231US).
[0004] If there are any contradictions or inconsistencies in
language between this application and one or more of the cases that
have been incorporated by reference that might affect the
interpretation of the claims in this case, the claims in this case
should be interpreted to be consistent with the language in this
case.
FIELD OF THE INVENTION
[0005] The present invention relates to energy conversion in
general, and, more particularly, to heat exchangers.
BACKGROUND OF THE INVENTION
[0006] Patent application Ser. No. 12/484,542, filed Jun. 5, 2009
(Attorney Docket: 711-231US)
[0007] Large, robust heat exchangers are needed to build a
closed-cycle Ocean Thermal Energy Conversion (OTEC) plant. OTEC
heat exchangers must withstand prolonged exposure to a primary
working fluid, typically ammonia, as well as a large secondary flow
of seawater. Further, it is highly desirable, if not necessary,
that such heat exchangers provide high overall heat transfer
coefficients, exhibit minimal mechanical pumping losses, and are
light weight. It is also important that the materials and
fabrication costs of these heat exchangers are not excessive.
[0008] In the prior art, heat exchangers are typically either
"shell and tube heat exchangers" or "plate-frame heat
exchangers."
[0009] Shell and tube heat exchangers commonly comprise a plurality
of tubes enclosed within an open volume of a surrounding shell.
Typically, the tubes and shell comprise conventional metallic
materials. In some cases, shell and tube heat exchangers comprise
shells made of more exotic materials, such as composites, graphite
foam, etc. Such materials enable a reduction in the weight of a
heat exchanger.
[0010] Shell and tube heat exchangers are widely used for liquid to
liquid, liquid to vapor, evaporator and condenser applications. In
operation, a primary fluid or vapor flows through the tubes. A
secondary fluid, typically air, flows around and through the space
that surrounds the tubes inside the shell. Heat is exchanged
between the fluids/vapors through the walls of the tubes.
[0011] Although shell and tube style heat exchangers are often used
for ship/submarine service, they are typically characterized by
relatively low heat transfer coefficients. As a result, shell and
tube heat exchangers are not well-suited for OTEC applications
since they would require undesirably large surface areas. This
drawback is further exacerbated by the fact that there is normally
only a small difference between seawater and process fluid
temperatures.
[0012] Plate-frame heat exchangers are typically constructed of
thin metal plates joined together to form a thermal transfer path
between the primary and secondary fluid streams. The plates are
normally joined using welding, brazing, epoxy, or mechanical
attachment. Plate-frame heat exchangers find use in a wide range of
liquid to liquid, liquid to vapor, evaporator and condenser
applications throughout the petro-chemical, pharmaceutical and
beverage processing industries. In some commercial plate-frame
style heat exchangers, the process fluid flows between alternate
plate pairs, and the secondary fluid flows in the intervening plate
pairs. Heat transfer is enhanced via chevron or other
turbulence-inducing stamped patterns. Plate-frame heat exchangers
have a high capital cost. For OTEC systems, wherein the heat
exchangers are in contact with sea water, materials such as
titanium or high alloy steel are normally required. As a result,
the cost associated with such heat exchangers would be increased
further.
[0013] Brazed aluminum plate frame heat exchangers are widely used
in gas processing and cryogenic applications such as nitrogen and
natural gas liquefaction and for LNG re-gassification. In cryogenic
and gas processing heat exchangers, brazed aluminum fins are used
on both the warm and cold fluid sides of the heat exchanger. Such
heat exchangers are not well-suited for OTEC applications, however,
since seawater is corrosive to brazed fin joints. Further, these
heat exchangers are typically characterized by small passages that
are prone to clogging due to biofouling.
[0014] In an attempt to overcome their corrosion problem in OTEC
applications, brazed aluminum fin heat exchangers have been
developed wherein aluminum extrusions are used in place of brazed
fins on their seawater sides. The brazed fins are retained on the
process fluid (e.g., ammonia) side of the heat exchanger, however.
Unfortunately, these modified brazed fin/extruded aluminum fin heat
exchangers require large braze furnaces for their fabrication. As a
result, they are also quite expensive to produce.
SUMMARY OF THE INVENTION
[0015] The present invention provides a heat exchanger that
overcomes some of the limitations and drawbacks of the prior art.
Embodiments of the present invention enable thermal coupling
between a primary fluid, such as a working fluid, and a secondary
fluid, such as seawater. Embodiments of the present invention are
particularly well-suited for OTEC applications.
[0016] An embodiment of the present invention comprises a
plate-frame heat exchanger that includes graphite-foam-based
plates, each of which comprises a plurality of conduits for
conveying the primary fluid through the heat exchanger. The
conduits are formed in the graphite foam itself. The plates are
arranged in a frame such that spaced between the plates define
channels for conveying the secondary fluid through the heat
exchanger.
[0017] Each plate comprises a "sandwich" structure of a
graphite-foam core that interposes a pair of barriers. The barriers
are substantially impervious to both the primary and secondary
fluids. As a result, the barriers inhibit cross-contamination of
the two fluids.
[0018] In some embodiments, the barriers are made of a thermally
enhanced polymer that is disposed on each of two opposing sides of
the graphite-foam core. The barriers are formed on these surfaces
in an injection molding process.
[0019] In some embodiments, the barriers are made of metal sheets
that are attached to the two surfaces of the graphite-foam core
using either an adhesive or by welding, soldering, or brazing the
sheets to a thin metal film disposed on the two surfaces.
[0020] In some embodiments, the conduits formed in the
graphite-foam core comprise open pores and capillaries that serve
to enhance heat transfer through the graphite foam.
[0021] In contrast to prior-art heat exchangers, embodiments of the
present invention provide: [0022] i. an improved ratio of heat
transfer surface density to heat exchanger volume; or [0023] ii.
reduced heat exchanger weight; or [0024] iii. improved thermal
transport rates or heat transfer coefficients between the primary
fluid and secondary fluid; or [0025] iv. improved surface area to
volume ratio for evaporative and condenser surfaces; or [0026] v.
smaller heat exchanger footprint for a given heat transfer
capability; or [0027] vi. reduced pressure drop for primary fluid
through the heat exchanger; or [0028] vii. any combination of i,
ii, iii, iv, v, and vi.
[0029] An embodiment of the present invention comprises a heat
exchanger for thermally coupling a first fluid and a second fluid,
wherein the heat exchanger comprises: (1) a first frame, wherein
the first frame comprises a first core, wherein the first core
comprises graphite foam comprising first conduits for conveying the
first fluid, and a first barrier for fluidically isolating the
first fluid and the second fluid; and (2) a second frame, wherein
the second frame comprises a second core, wherein the second core
comprises graphite foam comprising second conduits for conveying
the first fluid, and a second barrier for fluidically isolating the
first fluid and the second fluid; wherein the first frame and the
second frame collectively define a channel for conveying the second
fluid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 depicts a schematic diagram of an OTEC power
generation system in accordance with an illustrative embodiment of
the present invention.
[0031] FIG. 2 depicts a schematic drawing of a cross-sectional view
of a shell and tube heat exchanger in accordance with the prior
art.
[0032] FIG. 3 depicts a schematic drawing of a plate-frame heat
exchanger in accordance with the illustrative embodiment of the
present invention.
[0033] FIG. 4 depicts a schematic drawing of a heat exchanger core
in accordance with the illustrative embodiment of the present
invention.
[0034] FIG. 5 depicts operations of a method suitable for forming a
heat exchanger in accordance with the illustrative embodiment of
the present invention.
[0035] FIG. 6 depicts a schematic drawing of a plate in accordance
with the illustrative embodiment of the present invention.
[0036] FIG. 7 depicts sub-operations suitable for forming a plate
402 in accordance with the illustrative embodiment of the present
invention.
[0037] FIG. 8 depicts a schematic diagram of a manifold in
accordance with the illustrative embodiment of the present
invention.
[0038] FIG. 9 depicts a schematic drawing of a plate in accordance
with a first alternative embodiment of the present invention.
DETAILED DESCRIPTION
[0039] FIG. 1 depicts a schematic diagram of an OTEC power
generation system in accordance with an illustrative embodiment of
the present invention. OTEC system 100 comprises turbogenerator
104, closed-loop conduit 106, heat exchanger 110-1, heat exchanger
110-2, pumps 114, 116, and 124, and conduits 120, 122, 128, and
130.
[0040] Turbo-generator 104 is a conventional turbine-driven
generator. Turbogenerator 104 is mounted on floating platform 102,
which is a conventional floating energy-plant platform. Platform
102 is anchored to the ocean floor by mooring line 132 and anchor
134, which is embedded in the ocean floor. In some instances,
platform 102 is not anchored to the ocean floor but is allowed to
drift. Such a system is sometimes referred to as a "grazing
plant."
[0041] In typical operation, pump 114 pumps a primary fluid (i.e.,
working fluid 108), in liquid form, through closed-loop conduit 106
to heat exchanger 110-1. Ammonia is often used as working fluid 108
in OTEC systems; however, it will be clear to one skilled in the
art that any fluid that evaporates at the temperature of the water
in surface region 118 and condenses at the temperature of the water
in deep water region 126 is suitable for use as working fluid 108
(subject to material compatibility requirements).
[0042] Heat exchanger 110-1 and 110-2 are configured for operation
as an evaporator and condenser, respectively. One skilled in the
art will recognize that the operation of a heat exchanger as
evaporator or condenser is dependent upon the manner in which it is
configured within system 100. Heat exchanger 110 is described in
detail below and with respect to FIG. 3.
[0043] In order to enable its operation as an evaporator, pump 116
draws warm secondary fluid (i.e., seawater from surface region 118)
into heat exchanger 110-1 via conduit 120. In a typical OTEC
deployment, the water in surface region 118 is at a substantially
constant temperature of approximately 25 degrees centigrade
(subject to weather and sunlight conditions). At heat exchanger
110-1 heat from the warm water is absorbed by working fluid 108,
which induces working fluid 108 to vaporize. After passing through
heat exchanger 110-1, the now slightly cooler water is ejected back
into the body of water via conduit 122. The output of conduit 122
is typically located deeper than the depth of surface region 118 to
avoid reducing the average water temperature in the surface
region.
[0044] The expanding working fluid 108 vapor is forced through
turbogenerator 104, thereby driving the turbogenerator to generate
electrical energy. The generated electrical energy is provided on
output cable 112. Once it has passed through turbogenerator 104,
the vaporized working fluid enters heat exchanger 110-2.
[0045] At heat exchanger 110-2, pump 124 draws cold secondary fluid
(i.e., seawater from deep water region 126) into heat exchanger
110-2 via conduit 128. Typically deep water region 126 is
approximately 1000 meters below the surface of the body of water,
at which depth water is at a substantially constant temperature of
a few degrees centigrade. The cold water travels through heat
exchanger 110-2 where it absorbs heat from the vaporized working
fluid. As a result, working fluid 108 condenses back into liquid
form. After passing through heat exchanger 110-2, the now slightly
warmer water is ejected into the body of water via conduit 130. The
output of conduit 130 is typically located at a shallower depth
than that of deep-water region 126 to avoid increasing the average
water temperature in the deep-water region.
[0046] Pump 114 pumps the condensed working fluid 108 back into
heat exchanger 110-1 where it is again vaporized; thereby
continuing the Rankine cycle that drives turbogenerator 104.
[0047] FIG. 2 depicts a schematic drawing of a cross-sectional view
of a shell and tube heat exchanger in accordance with the prior
art. Heat exchanger 200 comprises folded conduit 202, shell 204,
inlet 208, and outlet 210.
[0048] Conduit 202 is a metal-based fluidic conduit for conveying a
primary fluid, such as ammonia, from inlet 208 to outlet 210.
Conduit 202 is typically folded into a serpentine shape to
substantially maximize the amount of its surface area that is in
contact with shell 204.
[0049] Shell 204 is a form of thermally conductive graphite foam.
Shell 204 includes recess 206 for locating conduit 204. Shell 204
further includes pass-throughs that facilitate fluidic connection
to inlet 208 and outlet 210. Shell 204 typically comprises two
halves that fit together in a clam shell configuration to
completely surround and sandwich conduit 202. Shell 204 also seals
conduit 202 from direct contact with secondary fluid during
operation of the heat exchanger.
[0050] In operation as an evaporator, relatively cooler primary
fluid flows into inlet 208 in liquid form. Heat exchanger 200 is
placed into a configuration wherein relatively warmer secondary
fluid flows along the outside surfaces of shell 204. Heat from the
secondary fluid is conducted through the graphite foam to the top
and bottom surfaces of conduit 202. The conducted heat causes the
primary fluid within conduit 202 to vaporize. Vaporized primary
fluid exits heat exchanger 200 at outlet 210.
[0051] In operation as a condenser, relatively warmer primary fluid
flows into inlet 208 in vapor form. Heat exchanger 200 is placed
into a configuration wherein relatively cooler secondary fluid
flows along the outside surfaces of shell 204. Heat from the
primary fluid is conducted through the graphite foam from the top
and bottom surfaces of conduit 202. This heat is conveyed into the
secondary fluid, which acts as a heat sink. The loss of heat causes
the primary fluid within conduit 202 to condense into a liquid.
Primary fluid exits heat exchanger 200 at outlet 210 in liquid
form.
[0052] One skilled in the art will recognize that heat exchanger
200 has several limitations, particularly with respect to OTEC
applications. For example, the efficiency of heat exchange between
conduit 202 and the graphite foam of shell 204 is a function of the
contact area between these elements. Since only the top and bottom
surfaces of conduit 202 are in direct contact with graphite foam,
the heat transfer coefficient heat exchanger 200 is low.
[0053] Further, for operation in an OTEC application, the
relatively small difference between the temperatures of the primary
and secondary fluids would require that heat exchanger 200 be
inordinately large in order to affect sufficient heat exchange.
[0054] Still further, conduit 202 is typically a heavy metallic
tube. As a result, the weight of conduit 202 is a significant
contributor to the overall weight of heat exchanger 200.
[0055] FIG. 3 depicts a schematic drawing of a plate-frame heat
exchanger in accordance with the illustrative embodiment of the
present invention. Heat exchanger 110 comprises heat exchanger core
302 and manifolds 304-1 and 304-2.
[0056] Heat exchanger core 302 (hereinafter referred to as "core
302") comprises a plurality of plates that are joined to a frame to
form a complete plate-frame heat exchanger core. Core 302 comprises
conduits through which working fluid 108 can flow. Core 302 also
includes channels through which seawater can flow. As they are
conveyed through core 302, heat is transferred between the two
fluids. Core 302 is described in more detail below and with respect
to FIGS. 4-7.
[0057] Manifolds 304-1 and 304-2 (referred to, collectively, as
manifolds 304) provide working fluid 108 and seawater to the
channels of core 302. Each of manifolds 304 comprises distributor
306 and ports 308. Manifolds 304-1 and 304-2 are described in
detail below and with respect to FIG. 8.
[0058] FIG. 4 depicts a schematic drawing of a heat exchanger core
in accordance with the illustrative embodiment of the present
invention. Core 302 comprises plates 402-1 and 402-2, and frame
404.
[0059] FIG. 5 depicts operations of a method suitable for forming a
heat exchanger in accordance with the illustrative embodiment of
the present invention. Method 500 begins with operation 501,
wherein plates 402-1 and 402-2 are provided. Although the
illustrative embodiment comprises two plates 402, it will be clear
to one skilled in the art, after reading this specification, how to
make and use alternative embodiments of the present invention that
comprise any practical number of plates 402.
[0060] FIG. 6 depicts a schematic drawing of a plate in accordance
with the illustrative embodiment of the present invention. Plate
402 comprises core 602, and barriers 604.
[0061] FIG. 7 depicts sub-operations suitable for forming a plate
402 in accordance with the illustrative embodiment of the present
invention. Operation 501 begins with sub-operation 701, wherein
core 602 is placed in the mold of an injection molding system.
[0062] Core 602 is a graphite-foam extrusion that comprises a
plurality of conduits 608 for conveying working fluid through plate
402. Conduits 608 are depicted as having a circular cross-section;
however, one skilled in the art will recognize that channels 608
can have any suitable cross-sectional shape (e.g., triangular,
square, rectangular, asymmetric, etc.). Conduits 608 comprise
graphite wall surfaces that comprise conduits 610. Conduits 610 are
open pores and/or capillaries that extend into the thickness of the
graphite foam. Conduits 610 enable evaporation and condensation to
occur over a much larger surface area than comparable conventional
heat exchangers. Heat exchangers in accordance with the present
invention, therefore, can be characterized by an improved ratio of
heat transfer surface density to heat exchanger volume, as compared
to prior-art heat exchangers. As a result, for a given heat
transfer duty, heat exchangers in accordance with the present
invention can be smaller than prior art shell and tube or
plate-frame heat exchangers. In some embodiments, conduits 610 are
not present or are too small to significantly enhance heat transfer
through core 602.
[0063] Typically, the graphite-foam composition used for core 602
has a specific gravity within the range of 0.6-0.7, and high
tensile strength. As a result, core 602 is both lightweight and
strong enough to withstand the fluid pressures and flow dynamics
that develop within heat exchanger 110. Heat exchangers in
accordance with the present invention, therefore, can be much
lighter than comparable conventional metal-based heat
exchangers.
[0064] Further, graphite foam has a bulk thermal conductivity of
approximately 180 W/M Deg C. range. This thermal conductivity is as
high as pure bulk aluminum, for example, and much higher than the
effective conductivity of most aluminum fin constructions.
[0065] At sub-operation 702, a thermally enhanced polymer is
injected into the mold to coat each of surfaces 606. The thermally
enhanced polymer material is selected such that it is substantially
impervious to both working fluid 108 and seawater once the polymer
is cured. For the purposes of this Specification, including the
appended claims, a thermally enhanced polymer is defined as a
polymer composition that comprises a thermally conductive filler
material, such as boron nitride, diamond composites, metals,
aluminum nitride, and the like. It will be clear to one skilled in
the art, after reading this specification, how to specify, make,
and use a thermally enhanced polymer suitable for forming barriers
604. In some embodiments, a non-thermally enhanced polymer is used
to coat each of surfaces 606; however, such embodiments are
typically characterized by a lower thermal transfer rate between
working fluid 108 and the seawater in the heat exchanger. In some
embodiments, the surface of the graphite foam comprises
graphite-foam fibers that protrude into the thickness of barriers
604. In such embodiments, the thermal transfer rate between the
working fluid and secondary fluid is enhanced. It should be noted,
however, that the integrity of the barrier, vis-a-vis the
mitigation of fluid cross-contamination, is retained.
[0066] At sub-operation 703, the thermally enhanced polymer is
cured to form barriers 604. As a result, barriers 604 are layers
that are substantially impervious to both working fluid 108 and
seawater. Barriers 604 are disposed on opposing surfaces 606 of
core 602. Barriers 604, therefore, inhibit cross-contamination of
the two fluids during operation of heat exchanger 110. In
embodiments wherein barriers 604 comprise a non-thermally enhanced
polymer, the thickness of the layers is typically reduced to reduce
their thermal resistance.
[0067] Since graphite foam is typically quite porous, the polymer
flows into the pores and other surface structure of surfaces 606.
As a result, the polymer forms a strong mechanical bond between
core 602 and barriers 604. In addition, this increases the surface
area of the interface between the graphite foam and the polymer,
which results in a highly thermally conductive interface between
core 602 and barriers 604.
[0068] Upon completion of the injection molding process, plate 402
emerges from the mold as a single unit.
[0069] At operation 502, plates 402-1 and 402-2 are mounted in
seats 408 of frame 404, which comprises sides 406, bottom 412, and
top 414.
[0070] Sides 406 are rigid aluminum alloy members that comprise
seats 410. Seats 410 are recessed regions of sides 406 for
accepting and locating plates 402-1 and 402-2.
[0071] Each of bottom 412 and top 414 is a rigid aluminum-alloy
plate that mates with sides 406 to complete frame 404.
[0072] Sides 406 and bottom 412 and top 414 are joined with
substantially galvanic corrosion-free joints 416 to mitigate the
effects of corrosion due to exposure to seawater.
[0073] In some embodiments, joints 416 are formed using
friction-stir welding. Friction-stir welding employs a rotating
probe, wherein a force is applied to the probe perpendicular to the
weld surface to join similar metals or alloys together. The immense
friction between the probe and materials causes material in the
immediate vicinity of the probe to heat up to temperatures below
its melting point. This softens the adjoining sections, but because
the material remains in a solid state, its original material
properties are retained. Movement of the probe along the weld line
forces the softened material from the two pieces towards the
trailing edge causing the adjacent regions to fuse, hence forming a
weld.
[0074] As opposed to other common joining techniques, including
other methods that produce galvanic corrosion-free joints,
friction-stir welding has several performance advantages. In
particular, the resultant weld is comprised of the same material as
the joined sections. As a result, galvanic corrosion due to contact
between dissimilar metals at the joint is reduced or eliminated.
Furthermore, the resultant weld retains the material properties of
the material of the joined sections. Friction-stir welding is
described in more detail in U.S. patent application Ser. No.
12/484,542, filed Jun. 5, 2009 (Attorney Docket: 711-231US), which
is included by reference herein.
[0075] Seats 410 are arranged so that bottom 412 and plate 402-1
collectively define channel 408-1, plates 402-1 and 402-1
collectively define channel 408-2, and plate 402-2 and top 414
collectively define channel 408-3 (Channels 408-1 through 408-3 are
collectively referred to as channels 408). Channels 408 are
suitable for conveying seawater through heat exchanger 110.
[0076] At operation 503, seals 418 are formed along each
intersection of a plate 402 and its respective seat 410. Seals 418
are lines of polymer adhesive applied to seal the spaces between
plates 402 and seats 410 so as to inhibit the flow of either
working fluid 108 or seawater through them. Seals also serve to
inhibit motion of plates 402 relative to frame 404.
[0077] At operation 504, manifolds 304-1 and 304-2 are joined with
core 302 using a substantially galvanic corrosion-free joining
technology, such as friction-stir welding.
[0078] FIG. 8 depicts a schematic diagram of a manifold in
accordance with the illustrative embodiment of the present
invention. Manifold 304 is representative of each of manifolds
304-1 and 304-2 and comprises distributor 306 and ports 308.
Manifold 304 consists of the same material as frame 404 to
facilitate friction-stir welding. Manifold 304 comprises two
substantially identical sides in order to mitigate deleterious
effects, such as pressure drops or turbulence, which typically
arise from transitioning fluid flow between a plurality of small
conduits and a large conduit.
[0079] During operation as an inlet manifold, distributor 306
receives working fluid from ports 308 and distributes it into
conduits 608 of core 302. During operation as an outlet manifold,
conduits 804 receive working fluid from conduits 608 of core 302
and provide it to ports 308.
[0080] Distributor 306 comprises region 802 which physically
expands the flow of primary fluid in order to introduce it into
conduits 804 with substantially uniform flow velocity. The number
of conduits 804 is based on the number of plates 402 in core 302.
Conduits 804 are interposed by access ports 806. Access ports 806
enable access for seawater to channels 408-1 through 408-3 of core
302. In some alternative embodiments, manifold 304 comprises a
second plurality of conduits for fluidically coupling seawater and
channels 408-1 through 408-3. Although the illustrative embodiment
of the present invention comprises a heat exchanger through which
primary and secondary fluids flow in a substantially parallel
manner, it will be clear to one skilled in the art, after reading
this specification, how to specify, make, and use alternative
embodiments of the present invention in which the primary and
secondary fluids flow in a non-parallel manner, such as a
cross-flow arrangement.
[0081] It should be noted that conduits 804 are tapered from wide
end 708 to narrow end 710. This tapering facilitates the flow of
primary fluid through conduits 608 with substantially uniform flow
velocity.
[0082] Although the illustrative embodiment comprises manifolds 304
that are made of the same material as frame 404, it will be clear
to one skilled in the art, after reading this specification, how to
specify, make, and use alternative embodiments that comprises
manifolds consisting of a different corrosion-resistant material
(e.g., a fiberglass composite), which can be joined to frame 404
with a substantially galvanic corrosion-free joint.
[0083] FIG. 9 depicts a schematic drawing of a plate in accordance
with a first alternative embodiment of the present invention. Plate
900 comprises core 602, plates 902, and adhesive 904.
[0084] Plates 902 are thin, thermally conductive sheets affixed to
surfaces 606 of core 602 by means of adhesive 904. Core 602 and
plates 902 collectively define a "sandwich" structure, wherein
plates 902 inhibit cross-contamination between working fluid 108
and seawater during heat exchanger operation. In some embodiments,
plates 902 are solid, mesh, or perforated sheets that augment the
mechanical strength of core 602 to enable plate 900 to withstand
greater pressures and forces inside heat exchanger 300. In
embodiments wherein plates 902 are not solid, however, an
additional barrier layer, such as barrier 604, is provided to
inhibit fluidic cross-contamination. In some embodiments, plates
902 are made of aluminum, aluminum alloy, or another metal that is
suitable for joining plate 900 to frame 404 by means of a galvanic
corrosion-free joining technology, such as friction-stir welding.
In such embodiments, therefore, joints 418 are substantially
galvanic corrosion-free joints.
[0085] In some embodiments, plates 902 are affixed to core 602 by
brazing or welding them to thin metal layers deposited on each of
surfaces 606. In such embodiments, adhesive 904 is not
required.
[0086] It is to be understood that the disclosure teaches just one
example of the illustrative embodiment and that many variations of
the invention can easily be devised by those skilled in the art
after reading this disclosure and that the scope of the present
invention is to be determined by the following claims.
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