U.S. patent application number 14/035868 was filed with the patent office on 2014-01-23 for cross reference to related applications.
This patent application is currently assigned to Framo Engineering AS. The applicant listed for this patent is Framo Engineering AS. Invention is credited to Stig Kaare Kanstad, Julien Rolland.
Application Number | 20140020876 14/035868 |
Document ID | / |
Family ID | 49945572 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140020876 |
Kind Code |
A1 |
Rolland; Julien ; et
al. |
January 23, 2014 |
Cross Reference to Related Applications
Abstract
A heat exchanger system is described that includes an inlet and
an outlet for a first fluid and a heat exchanger between the inlet
and the outlet wherein the first fluid circulates, wherein the heat
exchanger comprises at least one deflector to guide the flow of a
second fluid. A method is also described to exchange heat between a
first and a second fluid using free convection velocity field to
create forced convection in the heat exchanger of a heat exchanger
system. A method to exchange heat between a first and a second
fluid comprising providing a heat exchanger system between the
first and the second fluids, said heat exchanger system comprising
a heat exchanger wherein the first fluid circulates and increasing
the flow turbulences of a second fluid around the heat
exchanger.
Inventors: |
Rolland; Julien; (Sandsli,
NO) ; Kanstad; Stig Kaare; (Fana, NO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Framo Engineering AS |
Sandsli Bergen |
|
NO |
|
|
Assignee: |
Framo Engineering AS
Sandsli Bergen
NO
|
Family ID: |
49945572 |
Appl. No.: |
14/035868 |
Filed: |
September 24, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13259789 |
Dec 6, 2011 |
|
|
|
14035868 |
|
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|
|
61705368 |
Sep 25, 2012 |
|
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Current U.S.
Class: |
165/165 |
Current CPC
Class: |
F28F 13/12 20130101;
F28F 2009/226 20130101; E21B 41/0007 20130101; F28D 1/022 20130101;
F28F 9/22 20130101; F28F 9/0275 20130101; F28D 1/05308 20130101;
E21B 43/01 20130101; F28F 2009/228 20130101 |
Class at
Publication: |
165/165 |
International
Class: |
F28F 13/12 20060101
F28F013/12; F28D 1/053 20060101 F28D001/053 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2009 |
GB |
0905338.0 |
Claims
1. A heat exchanger system for transferring heat between a first
fluid and an ambient second fluid, the system comprising: an inlet
configured to accept the first fluid; an outlet configured to expel
the first fluid; a plurality of vertically oriented parallel
conduits positioned between the inlet and the outlet configured to
carry the first fluid therein, the conduits each having an exterior
surface that is exposed to an ambient second fluid when the system
is submersed in said second fluid, wherein heat is transferred
between the first fluid flowing through the conduits and said
second fluid flowing a vertical direction along the exterior
surfaces of and parallel to the conduits by free convection; and at
least one deflector fixedly mounted exterior to the conduits
configured to impart non-vertical momentum in said flowing second
fluid thereby enhancing heat transfer between said first fluid and
said second fluid.
2. A system according to claim 1 wherein said conduits are tubular
pipes grouped into one or more groups, the tubular pipes of each
group being arranged symmetrically about a central axis of the
group.
3. A system according to claim 2 wherein said at least one
deflector are two horizontally oriented disks.
4. A system according to claim 2 wherein said at least one
deflector are three or more horizontally oriented disks for each
group of tubular pipes.
5. A system according to claim 4 wherein said three or more
horizontally oriented disks are five horizontally oriented
disks.
6. A system according to claim 3 wherein said group of pipes and
said horizontally oriented disks have a large central opening
configured to allow free passage of the second therethrough, and
said horizontally oriented disks are configured to force the second
fluid into and out of the said central opening of the group of
pipes thereby enhancing heat transfer.
7. A system according to claim 2 wherein said at least on deflector
includes at least one non-horizontally oriented structures that are
asymmetric with respect to said central axis of the group.
8. A system according to claim 7 wherein said at least one
non-horizontally oriented structure is configured to impart
momentum in a tangential direction with respect to said central
axis of the group.
9. A system according to claim 8 wherein said at least one
non-horizontally oriented structures is helical in shape.
10. A system according to claim 1 where said conduits are tubular
pipes arranged into a rectangular pattern of columns and rows.
11. A system according to claim 10 wherein said one or more
deflectors includes at least one horizontally arranged baffle.
12. A system according to claim 10 wherein said one or more
deflectors includes at least one non-horizontally arranged
baffle.
13. A system according to claim 1 wherein said second fluid is
water.
14. A system according to claim 1 wherein said second fluid is
seawater.
15. A system according to claim 1 wherein said system does not use
powered equipment to force said second fluid to flow past said
conduits.
16. A system according to claim 14 wherein said first fluid
includes hydrocarbon gas produced from one or more wellbores
penetrating a subterranean rock formation.
17. A system according to claim 1 wherein the at least one
deflector is configured to increase turbulent flow in the second
fluid thereby enhancing heat transfer.
18. A system according to claim 1 wherein the heat exchanger is
configured to cool said first fluid by transferring heat by free
convection from the first fluid to the said second fluid.
19. A method of transferring heat between a first fluid and an
ambient second fluid surrounding a plurality of conduits through
which the first fluid flows, the method comprising: exposing
exterior surfaces of said plurality of conduits to said surrounding
second fluid; and flowing the first fluid through the plurality of
conduits, wherein the plurality of conduits are vertically oriented
and parallel to each other, and heat is transferred between the
first fluid and said second fluid flowing in a vertical direction
along the exterior surfaces of an parallel to the conduits by free
convection and wherein at least one stationary deflector is fixedly
mounted exterior to the conduits configured to impart non-vertical
momentum in said flowing second fluid thereby enhancing heat
transfer between said first fluid and said second.
20. A method according to claim 19 wherein said conduits are
tubular pipes grouped into one or more groups, the tubular pipes of
each group being arranged symmetrically about a central axis of the
group.
21. A method according to claim 20 wherein said at least one
deflector are two horizontally oriented disks.
22. A method according to claim 20 wherein said at least one
deflector are five horizontally oriented disks.
23. A method according to claim 21 wherein said group of pipes and
said horizontally oriented disks have a large central opening
configured to allow free passage of the second fluid therethrough,
and said horizontally oriented disks are configured to force the
second fluid into and out of the said central opening of the group
of pipes thereby enhancing heat transfer.
24. A method according to claim 20 wherein said at least one
deflector includes at least one non-horizontally oriented structure
that is asymmetric with respect to said central axis of the group,
said non-horizontally oriented structure being configured to impart
momentum in a tangential direction with respect to said central
axis of the group.
25. A method according to claim 24 wherein said at least one
non-horizontally oriented structures is helical in shape.
26. A method according to claim 19 wherein said second fluid is
seawater.
27. A method according to claim 19 wherein said first fluid
includes hydrocarbon gas produced from one or more wellbores
penetrating a subterranean rock formation.
28. A method according to claim 19 wherein said flowing includes
pumping the first fluid through said plurality of pipes.
29. A method according to claim 19 wherein heat is transferred from
said first fluid to said second fluid thereby cooling said first
fluid.
30. A method according to claim 19 wherein heat is transferred from
said second fluid to said first fluid thereby heating said first
fluid.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims the benefit of U.S. Prov.
Ser. No. 61/705,368 filed Sep. 25, 2012. This application is a
continuation-in-part of U.S. patent application Ser. No. 13/259,789
filed Dec. 6, 2011. Both of the above applications are incorporated
by reference herein.
FIELD
[0002] The present disclosure relates generally to subsea cooling
of fluids. More particularly, the present disclosure relates
cooling of fluids in a subsea environment using a heat exchanger
with free convection on the ambient seawater side.
BACKGROUND
[0003] Subsea compression is a relatively recent technology
developed to enhance the lifetime of existing fields. Installation
of subsea compression systems enables balancing of the reservoir
depletion over time. The production plateau is extended while also
increasing total recovery of fields. These systems face substantial
challenges both from a technical and commercial point of view. In
the absence of any upstream processing on the well stream, the
production fluid flow is multiphase containing mainly gas, but also
condensate, water and sand. Additionally, the equipment faces
various subsea constraints including availability, flexibility and
robustness.
[0004] Subsea process cooling is an important aspect for subsea
applications. Standard heat exchangers or coolers with forced
convection on ambient side requires additional equipment, such as
sea water circulation pumps, guiding piping, and/or shells to force
the coolant fluid towards the cooling area. Circulation pumps rely
on a power supply as well as control systems. Subsea constraints
make reliability and operation of forced convection equipment both
challenging and expensive.
[0005] A free convection velocity field generated by buoyancy force
is usually much lower than velocity intensity that can be obtained
in forced convection. Therefore, corresponding heat transfer is
less efficient with free convection when compared to forced
convection. Since the heat transfer performance is inherently
lower, the exchange surface needs to be increased in order to
maintain sufficiently high cooling.
SUMMARY
[0006] According to some embodiments a heat exchanger system is
described for transferring heat between a first fluid and a
surrounding ambient second fluid. The system includes: an inlet
configured to accept the first fluid; an outlet configured to expel
the first fluid; a plurality of vertically oriented parallel
conduits positioned between the inlet and the outlet configured to
carry the first fluid therein, the conduits each having an exterior
surface that is exposed to the surrounding second fluid when the
system is submersed in the second fluid, wherein heat is
transferred between the first fluid flowing through the conduits
and the second fluid flowing a vertical direction along the
exterior surfaces of and parallel to the conduits by free
convection; and at least one deflector fixedly mounted exterior to
the conduits configured to impart non-vertical momentum in the
flowing second fluid thereby enhancing heat transfer between the
first fluid and the second fluid.
[0007] According to some embodiments, the conduits are tubular
pipes grouped into one or more groups, with the tubular pipes of
each group being arranged symmetrically about a central axis of the
group. The deflectors can be a number of horizontally oriented
disks (such as two, three, four or five) for each group of tubular
pipes. The group of pipes and the horizontally oriented disks can
each have a large central opening configured to allow free passage
of the second fluid therethrough, and the horizontally oriented
disks can be configured to force the second fluid into and out of
the central opening of the group of pipes, thereby enhancing heat
transfer.
[0008] According to some embodiments, deflectors can be
non-horizontally oriented structures that are asymmetric with
respect to the central axis of the group. The structure can be
configured to impart momentum in a tangential direction with
respect to the central axis of the group. According to some
embodiments, the non-horizontally oriented structures can be
helical in shape.
[0009] According to some embodiments, the conduits can be tubular
pipes arranged into a rectangular pattern of columns and rows. The
deflectors can be one or more horizontally arranged baffles or,
according to some embodiments, they can be non-horizontally
arranged.
[0010] According to some embodiments, no powered equipment is used
to force the second fluid to flow past the conduits. According to
some embodiments, the coolant second fluid is seawater, and the
first fluid includes hydrocarbon gas produced from one or more
wellbores penetrating a subterranean rock formation.
[0011] According to some embodiments, a method is described for
transferring heat between a first fluid and an ambient second fluid
surrounding a plurality of conduits through which the first fluid
flows. The method includes: exposing exterior surfaces of the
plurality of conduits to the surrounding second fluid; and flowing
the first fluid through the plurality of conduits, wherein the
plurality of conduits are vertically oriented and parallel to each
other, and heat is transferred between the first fluid and the
second fluid flowing in a vertical direction along the exterior
surfaces of an parallel to the conduits by free convection and
wherein at least one stationary deflector is fixedly mounted
exterior to the conduits configured to impart non-vertical momentum
in the flowing second fluid thereby enhancing heat transfer between
the first fluid and the second fluid. According to some
embodiments, the flowing includes pumping the first fluid through
the plurality of pipes. According to some embodiments, heat is
transferred from the first fluid to the second fluid thereby
cooling the first fluid. According to some other embodiments, heat
is transferred from the second fluid to the first fluid thereby
heating the first fluid.
[0012] According to some embodiments, a heat exchanger system is
described that includes: an inlet and an outlet for a first fluid,
and a heat exchanger between the inlet and the outlet where the
first fluid circulates. The heat exchanger includes at least one
deflector to guide the flow of an ambient second fluid. According
to some embodiments, the deflector guides the flow of the second
fluid to transfer the vertical momentum from the gravity induced
free convection flow of the second fluid to horizontal velocity.
According to some embodiments, the deflector is shaped depending
upon the heat exchanger design configuration. In one example, the
heat exchanger comprises tubular pipes and the at least one
deflector includes shapes surrounding the pipes. According to some
embodiments, the deflectors are shaped as a horizontal disk,
staggered plates or helical screw-like shapes.
[0013] According to some other embodiments, a method is described
for exchanging heat between a first and a second fluid using a free
convection velocity field to create a form of non-powered "forced"
convection in the heat exchanger without the use of a pump or other
powered equipment.
[0014] According to some other embodiments, a method is described
for exchanging heat between a first and a second fluid that
includes: increasing flow turbulences of the second fluid around
the heat exchanger wherein the first fluid is circulated. In one
example, the method includes increasing the velocity field, the
turbulence level and the flow mixing of the second fluid. In
another aspect the method includes breaking the thermal layer of
the second fluid by creating transverse flow and unsteady drag
effects within the second fluid.
[0015] According to some embodiments, the method includes
increasing the momentum of the ambient fluid around the heat
exchanger. In one embodiment, increasing the momentum around the
heat exchanger includes increasing the amount of the second fluid
participating in.
[0016] According to some embodiments, the method includes breaking
boundary layers of the second fluid so that layers of the second
fluid remote from the heat exchanger are dragged towards the heat
exchanger. It is known that free convection patterns generate a
thermal boundary layer that tends to act as insulation which
offsets the heat transfer increase due to the vertical momentum
increase of the second fluid.
[0017] These together with other aspects, features, and advantages
of the present disclosure, as well as the various features of
novelty, which characterize the invention, are pointed out with
particularity in the claims annexed to and forming a part of this
disclosure. The above aspects and advantages are neither exhaustive
nor individually or jointly critical to the spirit or practice of
the disclosure. Other aspects, features, and advantages of the
present disclosure will become readily apparent to those skilled in
the art from the following description of exemplary embodiments in
combination with the accompanying drawings. Accordingly, the
drawings and description are to be regarded as illustrative in
nature, and not restrictive.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] To assist those of ordinary skill in the relevant art in
making and using the subject matter hereof, reference is made to
the appended drawings, in which like reference numerals refer to
similar elements:
[0019] FIG. 1 is a schematic representation of a subsea production
setting with which some of the cooling systems and methods
described herein can be used, according to some embodiments;
[0020] FIG. 2 is a perspective view of a heat exchanger, according
to some embodiments;
[0021] FIGS. 3-1 and 3-2 illustrate further aspects of a heat
exchanger having horizontal baffles, according to some
embodiments;
[0022] FIG. 4 is a perspective view illustrating aspects of a heat
exchanger having helical-shaped baffles, according to some
embodiments;
[0023] FIG. 5 is a perspective view illustrating aspects of a heat
exchanger having a staggered combination of horizontal baffles,
according to some embodiments;
[0024] FIG. 6 is a graph showing the cooler group of pipes and
single pipe cooling performances versus the mass flow according to
the experimental results;
[0025] FIGS. 7, 8-1 and 8-2 illustrate flow patterns on the ambient
(sea water) side of the cooling group of pipes having horizontal
and helically-shaped baffles, according to some embodiments;
[0026] FIGS. 9-1, 9-2 and 9-3 illustrate further aspects of ambient
flow patterns and heat transfer for heat exchangers, according to
some embodiments; and
[0027] FIGS. 10-1, 10-2 and 10-3 illustrate using one or more
baffles in different configurations for a non-symmetrical
arrangement of cooling system pipes, according to some
embodiments.
[0028] It should be understood that the drawings are not to scale
and that the disclosed embodiments are sometimes illustrated
diagrammatically and in partial views. In certain instances,
details that are not necessary for an understanding of the
disclosed method and apparatus, or that would render other details
difficult to perceive may have been omitted. It should be
understood that this disclosure is not limited to the particular
embodiments illustrated herein.
DETAILED DESCRIPTION
[0029] Some embodiments will now be described with reference to the
figures. Like elements in the various figures may be referenced
with like numbers for consistency. In the following description,
numerous details are set forth to provide an understanding of
various embodiments and/or features. However, it will be understood
by those skilled in the art that some embodiments may be practiced
without many of these details and that numerous variations or
modifications from the described embodiments are possible. As used
here, the terms "above" and "below", "up" and "down", "upper" and
"lower", "upwardly" and "downwardly", "upstream and downstream",
and other like terms indicating relative positions above or below a
given point or element are used in this description to more clearly
describe certain embodiments. However, when applied to equipment
and methods for use in wells that are deviated or horizontal, such
terms may refer to a left to right, right to left, or diagonal
relationship, as appropriate.
[0030] It will also be understood that, although the terms first,
second, etc. may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another. For example, a first
object or step could be termed a second object or step, and,
similarly, a second object or step could be termed a first object
or step, without departing from the scope of the invention. The
first object or step and the second object or step are both objects
or steps, respectively, but they are not to be considered the same
object or step.
[0031] The terminology used in the description of the invention
herein is for the purpose of describing particular embodiments only
and is not intended to be limiting. As used in the description and
the appended claims, the singular forms "a", "an" and "the" are
intended to include the plural forms as well, unless the context
clearly indicates otherwise. It will also be understood that the
term "and/or" as used herein refers to and encompasses any and all
possible combinations of one or more of the associated listed
items. It will be further understood that the terms "includes,"
"including," "comprises," and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps, operations, elements, components, and/or groups thereof.
[0032] As used herein, the term "if" may be construed to mean
"when" or "upon" or "in response to determining" or "in response to
detecting," depending on the context. Similarly, the phrase "if it
is determined" or "if [a stated condition or event] is detected"
may be construed to mean "upon determining" or "in response to
determining" or "upon detecting [the stated condition or event]" or
"in response to detecting [the stated condition or event],"
depending on the context.
[0033] In the specification and appended claims, the terms/phrases
"connect", "connection", "connected", "in connection with", and
"connecting" are used to mean "in direct connection with" or "in
connection with via one or more elements", and the term "set" may
mean "one element" or "more than one element". Further, the terms
"couple", "coupling", "coupled", "coupled together", and "coupled
with" are used to mean "directly coupled together" or "coupled
together via one or more elements".
[0034] In the following detailed description of some embodiments,
reference is made to the accompanying drawings, which form a part
hereof, and within which are shown by way of illustration specific
embodiments by which the invention may be practiced. It is to be
understood that other embodiments may be utilized, and structural
changes may be made without departing from the scope of the
invention.
[0035] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to show structural
details of the present invention in more detail than is necessary
for the fundamental understanding of the present invention, the
description taken with the drawings making apparent to those
skilled in the art how the several forms of the present invention
may be embodied in practice. Further, like reference numbers and
designations in the various drawings indicate like elements.
[0036] The following abbreviations and relations shall be used
herein: [0037] CFD: Computational Fluid Dynamics; [0038] RANS:
Reynolds Averaged Navier-Stokes; [0039] SP: Single Pipe; [0040]
SST: Shear Stress Transport; [0041] WGC : Wet Gas Compressor;
[0042] amb: Ambient side of the cooler; [0043] ext: Cooler external
side; [0044] inl: Cooler inlet, process side [0045] int: Cooler
internal side; [0046] out: Cooler outlet, process side; [0047] A:
Area, [m.sup.2]; [0048] Cp: Heat Capacity, [J/kg/K]; [0049] D: Pipe
Diameter, [m]; [0050] e: Internal energy, [J/kg]; [0051] g:
Gravity, [m/s.sup.2]; [0052] HTC, h.sub.ext, h.sub.int: Heat
Transfer Coefficient, [W/m.sup.2/K]; [0053] k: thermal
conductivity, [W/m/K]; [0054] L: Pipe length, [m]; [0055] {dot over
(m)}: Massflow, [kg/s]; [0056] Q: Heat transfer, [W]; [0057] T:
Temperature, [K]; [0058] U: Global heat transfer coefficient,
[W/m.sup.2/K]; [0059] V: Velocity, [m/s]; [0060] .beta.: Thermal
expansion coefficient, [/K]; [0061] .mu.: Dynamic viscosity,
[Pa.s]; [0062] .rho.: Density, [kg/m.sup.3]; [0063] .tau.: Stress
tensor, [Pa];
[0063] LMTD = ( T inl - T amb ) - ( T out - T amb ) ln ( T inl - T
amb ) ( T out - T amb ) Log Mean Temperature Difference Nu = hL k
Nusselt number Pr = .mu. Cp k Prandtl number Ra = .rho. 2 g
.beta..DELTA. TL 3 Cp .mu. k Rayleigh number Re = .rho. VD .mu.
Reynolds number ##EQU00001##
[0064] FIG. 1 is a schematic representation of a subsea production
setting with which some of the cooling systems and methods
described herein can be used, according to some embodiments. Two
wells, 112 and 114, are being used to extract production fluid from
subterranean formation 100. The produced fluids from wells 112 and
114 move into manifold 118 from wellheads 122 and 124 via sea floor
pipelines 146 and 144 respectively. Manifold 118 includes a
compressor system 152 and according to some embodiments a subsea
heat exchanger system 150. The production fluid flows from manifold
118 upwards through flowline 132 to a surface production platform
130 on the sea surface 104 of seawater 102. As will be described in
greater detail infra, the heat exchanger system 150 has a plurality
of parallel pipes through which the production fluid is produced.
The external surface of the pipes is exposed to the seawater 102
which cools the production fluid as will be described in further
detail infra.
[0065] Note that although many embodiments are described herein as
being used in an exemplary application of subsea compression
systems, the methods and structures described herein are equally
applicable to many other types of heat exchangers. According to
some embodiments, the heat exchanger system components described
herein are used for nuclear power generation cooling (or heating)
applications. According to some other embodiments, the heat
exchanger components described herein are used for heating and/or
cooling applications in chemical processing applications.
[0066] Heat exchanger design. The cooling principle for heat
exchangers is to transfer heat from one fluid (the cooled fluid) to
another (the coolant fluid). Heat exchangers are commonly designed
using a forced convection heat transfer principle for both the
cooled and the coolant fluids. This is due to the higher heat
transfer rate that can be obtained using forced convection versus
free convection. As used herein, the term "free convection" refers
to a mechanism or type of heat transport in which the fluid motion
is not generated by any external power source (e.g. pump, fan,
suction device, etc.), but rather only by density differences in
the fluid occurring due to temperature gradients. Note that
although several embodiments are described herein with respect to
the application of cooling, the methods and structures described
herein are equally applicable to heating applications. In such
cases the heat is transferred from the ambient fluid (the heating
fluid) to the fluid flowing through the pipes (the heated
fluid).
[0067] The subsea environment is complex and aggressive. For
example, routine maintenance, inspection and cleaning possibilities
are both limited and challenging. In the case of forced convection,
difficulties are further increased by the nature and the
multiplicity of the equipment (pumps for example) used to generate
forced convection on the ambient side (the coolant fluid).
Conventional forced-forced convection (i.e. both the cooled fluid
and coolant fluid are pumped) heat exchanger technology is hence
not well suited for subsea applications. It has been found that a
passive design based on free convection on the ambient side (the
coolant fluid) is often more appropriate to face to challenges of
the subsea.
[0068] According to some embodiments, a heat exchanger system is
designed both to give a large turndown on thermal performance for
operation flexibility and, for subsea applications, to handle flow
assurance issues like sand accumulation, hydrates formation, wax
deposition, etc.
[0069] FIG. 2 is a perspective view of a heat exchanger, according
to some embodiments. Heat exchanger 150 includes a plurality of
vertical pipes 200 and distributing pipes 206, inlet manifolds,
outlet manifolds and collecting pipes 208 to distribute/collect
uniformly the flow of the cooled fluid. In particular, the vertical
pipes 200 are arranged in eight groups of pipes that are
symmetrically arranged about a central axis. The cooled fluid
enters through inlet 202 which leads to distribution pipes 206. The
distribution pipes 206 distribute the cooled fluid, as is shown by
the solid arrows to each of the eight groups of pipes. Each group
of pipes is fed by an inlet manifold. For example, group of pipes
210 includes an inlet manifold 212. The cooled fluid exits the
pipes 200 through outlet manifolds and collecting pipes 208, which
lead to outlet 204. According to one example, each group, such as
group 210, has 33 pipes arranged symmetrically about a central
longitudinal axis of the group. The manifolds, distribution and
collecting pipes are designed to uniformly distribute and collect
the multiphase flow to and from the cooling pipes while avoiding
excessive head loss. The heat exchanger 150 is arranged such that
it is symmetric and modular hence adaptable to different cooling
requirements. For further details of such symmetric heat
exchangers, see U.S. patent application Ser. No. 13/259,789, which
is incorporated by reference herein. Each of the groups of pipes
200 has 5 horizontal baffles mounted perpendicular to the
vertically oriented pipes. In group 210, the horizontal baffles
230, 232, 234, 236 and 238 are mounted around pipes 220. As will be
described in further detail, infra., the baffles arranged as shown
are effective in altering the predominantly vertical flow of the
free convecting coolant fluid so as to significantly enhance
cooling performance Note that although the coolant fluid is shown
in FIG. 2 as flowing from top to bottom, the techniques described
herein are also applicable to heat exchangers in which the cooled
(or heated in the case of heat exchangers operating in a heating
mode) fluid flow through the pipes in an upwards direction.
[0070] According to some embodiments, the free convection velocity
field is used to create a form of non-powered "forced" convection
on the ambient side (coolant fluid). According to some embodiments,
of described heat exchange systems, the heat exchanger includes one
or more external shapes to guide the coolant fluid flow and
transfer the vertical momentum of the freely convecting coolant
fluid from the gravity field to generate horizontal velocity.
Various shapes can be used to both generate either radial velocity
and circumferential velocity with respect to the longitudinal axis
of the pipes.
[0071] FIGS. 3-1 and 3-2 illustrate further aspects of a heat
exchanger having horizontal baffles, according to some embodiments.
FIG. 3-1 is a perspective view showing further detail of a single
cooler group 210, that may be combined with other cooler groups to
form a heat exchange system as shown for example in FIG. 2. Note
that although many of the embodiments described herein refer to the
heat exchanger as a "cooler" in which a "cooled fluid" within the
pipes is being cooled by an ambient "coolant fluid," all of the
methods and structures described herein can be equally applied to
heat exchangers operating as a "heater" in which a "heated fluid"
within the pipes is being heated by an ambient "heating fluid."
[0072] In this particular example, group 210 is made up of 33
vertically oriented pipes 200 that are symmetrically arranged in
two concentric rings about a central axis 310 such that there is a
large central open space within group 210. As can be seen, each of
the five horizontal baffles 230, 232, 234, 236 and 238 is a
disk-shaped piece mounted horizontally (i.e. perpendicular to the
vertically oriented pipes 200). FIG. 3-2 is a plan view of single
baffle. As can be seen each of the baffles (in this case baffle
238) has a large central opening 300 that is dimensioned to match
the central opening of the arrangement of pipes 200. Referring
again to FIG. 3-1, the direction of flow of the cooled fluid is
shown by the solid arrow 320 that in this example is downwards (the
negative z-direction). The coolant fluid generally moves in an
upward vertical direction (the positive z-direction) due to
buoyancy resulting from differences in fluid density. When the
vertically moving coolant fluid encounters a horizontal baffle, its
direction is altered as shown by the dotted arrows, and as will be
described in greater detail infra. Note that in the case of simple
horizontal baffles such as shown in FIGS. 3-1 and 3-2, the induced
velocity of the coolant fluid includes a substantial radial
component. As will be described in greater detail infra, an
unexpected result of the baffles is to: (1) force the ambient water
"inside the group" to flow out (out of area 300) to the area
outside of the group (into area 302) as illustrated by dotted arrow
304; and (2) force the ambient water "outside the group" (out of
area 302) to flow inside the group (into area 300) as illustrated
by dotted arrow 306.
[0073] FIG. 4 is a perspective view illustrating aspects of a heat
exchanger having helical-shaped baffles, according to some
embodiments. A single cooler group 400 of vertically oriented pipes
220 are shown as in the FIG. 3-1, that may be combined with other
cooler groups to form a heat exchange system as shown for example
in FIG. 2. In this particular example, two helical baffles 430 and
432 are attached to the exterior surface of the pipes 220 as shown.
It has been found that by arranging the baffle structures at a
non-horizontal angle to the pipes can aid in overall cooling
performance in many applications. In the case shown in FIG. 4, the
helical-shaped baffles 430 and 432 are effective in inducing
substantial velocity in the coolant fluid in both the radial and
circumferential directions with respect to the group central axis
310 as illustrated by the dotted arrows.
[0074] FIG. 5 is a perspective view illustrating aspects of a heat
exchanger having a staggered combination of horizontal baffles,
according to some embodiments. Many other shapes and arrangements
of baffles (such as horizontal, helical, and/or diagonal, etc.) or
other structures can be used to transfer momentum in the coolant
fluid from vertical directions to non-vertical directions. FIG. 5
shows one such example that is a combination of baffle sections
530, 532, 534, 536 and 538, which are provided to impart
non-vertical velocity on the coolant fluid. In this example, the
baffle sections impart a substantial radial velocity with respect
to the central axis 310 of the cooler group 500.
[0075] In order to evaluate the proposed heat exchanger system
performance and to compare it with empirical results for simple
geometries, the heat exchanger part of the heat exchanger system
and a single pipe were run in parallel. The single pipe had the
same characteristics (diameter, length, thickness, material . . . )
as the cooler pipes. The cooling pipe length was 4.3 meters. In an
example test, a slipstream of dry nitrogen was provided from an
existing compressor discharge and routed through the test pipes.
Flow/pressure through the pipes was controlled by means of the
chokes on the compressor outlet and control valves downstream test
pipes. The test operating conditions, taken in this example test,
are listed in Table 1 for a flow of process fluid to be cooled in a
subsea environment:
TABLE-US-00001 TABLE 1 example of the test operating conditions
Units Value Inlet temperature [.degree. C.] 60-90 Ambient
temperature [.degree. C.] 12 Inlet pressure [bara] 20-40 Massflow
(per pipe) [kg/s] 0.1-0.4
[0076] The process flow was measured by means of Coriolis mass flow
meters. Temperature was measured upstream and downstream the test
objects. Pressure was measured upstream the test objects. Six
individual temperature measurements were made in the seawater along
the test objects to determine ambient temperature and to check any
temperature layering in the pit. The head loss across the bundle of
pipes (or group of pipes) was in addition measured using a
differential pressure sensor.
[0077] Using pressure and temperature measurements at the test
pipes inlet, the gas thermodynamic properties (density and heat
capacity) are calculated and hence the amount of heat removed to
the process fluid (the cooled fluid) passing through the pipes was
obtained.
[0078] The thermal performances of the two test objects (the group
of cooler pipes and the single cooler pipe) were characterized for
different mass flow amounts. The global heat transfer coefficient U
can be calculated according to the following equations.
[0079] The heat transfer is defined by:
Q=UA.DELTA.T
with A the object area and .DELTA.T the temperature difference
between the process gas and the ambient water.
[0080] The heat transfer Q is directly related to the heat removed
to the gas:
Q={dot over (m)}.sub.gasCp.sub.gas(T.sub.outlet-T.sub.inlet)
[0081] Replacing the temperature difference between the process gas
and the ambient water by the Log Mean Temperature Difference (LMTD)
as the process gas is not constant all along the cooling pipes, the
global heat transfer coefficient is calculated using the following
formula:
U = m . gas Cp gas ( T outlet - T inlet ) A LMTD ##EQU00002##
[0082] It should be noted the global range from the tests is quite
uncommon due to the wide dimensions and cooling capacities of the
studied case (see Table 2: global results). The length scale, the
temperature difference and the total cooling load are likely to be
outside normal test conditions used to define the empirical
correlations. Comparison of Nusselt number based on 3 different
approaches which are experimental, analytical and numerical, for
such as high Rayleigh number range up to 10.sup.13 makes this study
very valuable.
TABLE-US-00002 TABLE 2 Global Results Units Min Value Max Value
Massflow (cooler) [kg/s] 3.1 12.3 Heat transfer (cooler) [kW] 163
436 Reynolds number (gas) [--] 1.2 10.sup.5 4.6 10.sup.5 Rayleigh
number (ambient) [--] 8.8 10.sup.12 2.9 10.sup.13
[0083] FIG. 6 is a graph showing the cooler group of pipes and
single pipe cooling performances versus the mass flow according to
the experimental results. In this case the group of pipes had two
horizontal baffles arranged perpendicular to the cooler pipes. The
cooling performance U is shown by curve 610 for the group of pipes
with the baffles and by curve 612 for the single pipe without any
baffles. Using 2.sup.nd order polynomial interpolation the
performance ratio is obtained (dotted line 614 using right scale).
The cooling performance of the group having baffles is about 23%
higher than the case of the single pipe without baffles. Note that
according to some embodiments, greater numbers of baffles, such 5
baffles as shown in FIGS. 2 and 3-1, will result in an even greater
increase in heat transfer performance.
[0084] The measured performance increase given in the FIG. 5 is
defined by:
.DELTA. perf cooler vs . SP = U cooler U SP ##EQU00003##
[0085] This relation is explained by the pipes interaction for the
cooler group with the baffles. The heat transfer from the pipe's
external surfaces to the ambient seawater is driven by free
convection. An external flow is generated due to the density
variation induced by the temperature increase caused by proximity
to the pipes. In the case of the group of cooler pipes, the pipes'
closeness causes the seawater flows to interact. The obtained
momentum is thus higher than the one obtained with a single pipe.
This phenomenon, called chimney effect, is described in further
detail infra.
[0086] Simulations. The heat transfer from the process gas (the
cooled fluid) to the ambient (coolant fluid) observed on the test
objects can be decomposed into the 3 following features: (1)
internal forced convection between the bulk gas and the internal
surfaces of the pipes; (2) conduction across the walls of the
pipes; and (3) external free convection between the external
surfaces of the pipes and the ambient water.
[0087] Based on the physical mechanisms as split above, the global
heat transfer coefficient can be defined by the following
formula:
U = 1 1 h int + D int 2 k steel ln ( D ext D int ) + D int D ext 1
h ext ##EQU00004##
[0088] The thermal performance difference obtained between the SP
(single pipe) and the cooler group of pipes is related to the free
convection pattern and intensity on the ambient side. In order to
characterize and to understand in detail the phenomena occurring
and the complex three-dimensional (3D) flow that develops around
the pipes, Computational Fluid Dynamics (CFD) simulations are
performed on both the cooler group and the single pipe. The case
with a mass flow of 0.22 kg/s has been studied using commercial CFD
software.
[0089] Numerical method. The geometry simulated consists of three
domains: (1) the gas flowing inside the pipes; (2) the solid pipe
walls; and (3) the ambient side of the pipes.
[0090] The two flows (ambient and gas) are described by the
Reynolds Averaged Navier Stokes (RANS) system coupled to the
internal energy equation for the ambient side by the buoyancy force
based on the Boussinesq assumption:
.differential. .rho. .differential. t + .gradient. ( .rho. u ) = 0
##EQU00005## .differential. ( .rho. u ) .differential. t +
.gradient. ( .rho. u .times. u ) = - .gradient. p + .gradient.
.tau. = + S M ##EQU00005.2## .differential. ( .rho. e )
.differential. t + .gradient. ( .rho. e u ) = .gradient. ( k
.gradient. T ) - p .gradient. u + .tau. = : .gradient. u .
##EQU00005.3##
[0091] The momentum source is hence for the water domain:
S.sub.M=-.rho..sub.ref.beta.(T-T.sub.ref)g
[0092] Turbulence is solved using the Shear Stress Transport (SST)
model. Both fluids, water and gas, are considered incompressible.
In the example simulations, the solid domain material is stainless
steel described by its thermal conductivity.
[0093] The boundary condition for the top and bottom faces of the
ambient domain is set to opening. In the proposed example, the
lateral face representing infinite is set to wall with free slip
condition and imposed temperature to 12.degree. C. For the gas
domain, standard incompressible boundary conditions are used, i.e.
mass flow imposed at the inlet (top) and static pressure imposed at
the outlet (bottom). The inlet temperature is fixed to 90.degree.
C.
[0094] Single pipe simulation. As shown in Table 3, the heat
transfer for the single pipe is very well simulated. The
discrepancy between test data and simulation is only about 5%. The
difference between test data and empirical correlation is about
10%, which is also acceptable regarding correlation accuracy.
Comparison of the CFD and test results for the single pipe is a
step that validates the numerical approach.
TABLE-US-00003 TABLE 3 Global HTC for the single pipe Method Units
Value Variation Experimental data W/m.sup.2/K 175 CFD 167 5%
Empirical correlation 192 10%
[0095] Comparison between numerical and analytical approaches with
test results
[0096] Cooler group simulation. Due to the compressed gas flowing
inside the pipes, the external pipe wall was warm and thus a
vertical flow in the ambient is generated due to free convection.
The gas flow continuously delivers heat to the ambient side and the
averaged gas temperature drops off from about 90.degree. C. down to
about 60.degree. C.
[0097] FIGS. 7 8-1 and 8-2 illustrate flow patterns on the ambient
(sea water) side of the cooling group of pipes having horizontal
and helically-shaped baffles, according to some embodiments. In the
case of the cooler group 210 with horizontal baffles shown in FIG.
7, on the ambient seawater side starting from the bottom, the water
free convection flow is routed from the ambient into the vicinity
of the pipes. What is remarkable and quite unexpected with the
cooler design having horizontal baffles is that the baffles block
the vertical flows and force the ambient water "inside the group"
(area 300) to flow out and in again. This phenomenon which
generates radial flows around the baffles was found to have a very
positive effect on the heat transfer as: (1) it increases the mean
velocity field and hence the heat transport; (2) it increases the
turbulence level and the flow mixing around the cooling pipes; and
(3) it breaks the thermal layer with transverse flow and unsteady
drag effects.
[0098] It has been found that the baffles generate secondary flows
of the ambient fluid around the baffles. The seawater is ejected
from the area in the center of the group of pipes just below each
baffle (positive radial velocity with respect to central axis 310)
and then just above each baffle the water is routed back into the
central area (negative radial velocity). This transverse flow
generated firstly increases the momentum level and by consequence
the heat removal is improved. A second aspect is that it increases
the turbulence level creating some turbulent structures.
[0099] FIG. 8-1 illustrates simulated streamlines for the cooler
group 400 shown in FIG. 4 having helical-shaped baffles. As can be
seen, a substantial circumferential velocity is imparted to the
ambient fluid flow that has been found to further increase the
effects described above with respect to the horizontal baffle
arrangement. It has thus been found that helical baffles are often
even more effective in increasing heat transfer than designs using
a horizontal baffle arrangement. FIG. 8-2 illustrates simulated
streamlines for cooler group 400 showing the substantial
circumferential velocity imparted to the ambient fluid flow using
helical baffles, according to some embodiments.
[0100] Test results and comparisons of the thermal performance
between the single pipe and the cooler group highlighted an
increase of the heat transfer for the cooler group. Due to the
similarity of the two test objects and the physical mechanisms
decomposition of heat transfer, the free convection on the ambient
side has been identified as the key phenomena to explain test
results deviation.
[0101] The CFD analyses performed on the two objects revealed a
complex 3D flow development for the cooler. This 3D flow is
responsible for the external HTC increase. FIGS. 9-1, 9-2 and 9-3
illustrate further aspects of ambient flow patterns and heat
transfer for heat exchangers, according to some embodiments. FIG.
9-1 shows a pipe 220 through which the cooled fluid is pumped. The
coolant fluid exposed to the exterior surface of the pipe 220 moves
upward due to buoyancy forces as shown by arrow 910. The moving
ambient fluid has an outer boundary 920 as shown. It has been found
that two effects interact and are coupled enhancing heat transfer,
according to some embodiments. First the pipes bundle generates the
high intensity vertical momentum due to free convection (as shown
FIG. 9-1), and secondly the baffles transfer this vertical flow
into radial and azimuthal (or circumferential) flows. FIG. 9-2
illustrates a radial flow direction of ambient fluid as described.
FIG. 9-3 shows the effect of transverse ambient fluid flow (e.g.
radial or circumferential) and the Von Karman vortex development on
the low-pressure (downstream) side of the cylindrical pipe. The
flow patterns created by the baffles increases the momentum in the
pipe vicinity, which amplifies the heat removal and likewise
generates Von Karman structures with high turbulence level.
[0102] Based on the analysis described supra, a new empirical
correlation can be described. According to some embodiments, the
heat exchanger is a cooler group with a passive design. That is, no
additional powered equipment is used to create forced flow on the
ambient side. Nevertheless the interaction between the pipes and
baffles is such that free convection flow generated by one pipe
tends to act similarly to a forced convection flow source for the
neighboring pipes. An expression of the following form can be used
describe the mixed convection:
Nu.sub.mixed.sup.n=Nu.sub.free.sup.n.+-.Nu.sub.forced.sup.n
[0103] It is important to note that analysis and results are
dependent of the configuration. Three special cases can be
identified: (1) buoyancy induced flow and forced flow parallel with
the same direction; (2) buoyancy induced flow and forced flow
parallel in opposite directions; and (3) buoyancy induced flow and
forced flow perpendicular, such as provided by several embodiments
described herein.
[0104] When analyzing a new heat exchanger design, challenges can
relate to the fact that there can be two different characteristic
lengths representative of the combined mechanism. It hence makes it
difficult to establish the combination of the characteristic
dimensionless numbers for the free and the forced convections. The
characteristic length to build the Nusselt number and quantify the
free convection intensity is the total vertical pipe length (L)
while the one representative of the forced convection in a
staggered bank is the pipe diameter (D). For this reason, the
formula proposed above is no longer well suited as the Nusselt
magnitude based on different length scale strongly deviates.
[0105] According to some embodiments, the following formula can be
used to correlate mixed convection heat transfer for the cooler
external heat transfer:
N u L cooler = Nu L SP + 0.3 L D Nu D S ##EQU00006##
The Nusselt number for a free convection vertical boundary layer
development is:
Nu.sub.L.sub.SP=0.183Ra.sub.L.sup.0.31
The value of 0.31 is appropriate for configurations using two
horizontal baffles. Other values, an be used for other baffle
configurations such as 5 horizontal baffles and/or helical baffles.
The Nusselt number for forced convection in staggered bank is:
Nu.sub.D.sub.S=1.13C.sub.1C.sub.2Re.sub.D.sup.mPr.sup.1/3
where the constant has the following values according to the
geometry configuration: C.sub.1=0.416, C.sub.2=0.75 and
m=0.568.
[0106] Therefore, the external heat transfer for the cooler
corresponds to that of the single pipe in addition to a component
representing the pseudo forced convection in the vicinity of the
baffle. According to the simulation analysis described supra, in
the case of a two-horizontal baffle configuration, only one third
of this component is included, as only one third of the total pipe
area is affected by this horizontal flow pattern. Other amounts of
the component should be included for other configurations and
numbers of baffles.
[0107] The test results presented in the FIG. 6 were compared with
the estimated global HTC obtained using the proposed empirical
correlation for the external HTC.
[0108] Based on these analyses, a new formula for combined free
convection and crossflow for a vertical pipe is proposed, supra.
The overall heat transfer coefficient obtained from these
correlations matches very well with the experimental data.
[0109] This described method enables optimization of heat exchanger
design and thus improves the heat exchange performances and
reducing the weight of the system for similar performances.
According to some embodiments, the heat exchange system includes an
inlet and an outlet for a first fluid, increases the velocity and
the turbulence intensity of a second fluid flow in neighborhood of
the heat exchanger with the first fluid. The heat exchange system
enables an increase of the heat transfer with the ambient second
fluid. According to some embodiments, the heat exchange surface
area is substantially reduced for equivalent performance and thus
resulting in a substantial decrease in cooling system
footprint.
[0110] Forced convection causes, as mentioned, higher heat transfer
rates than free convection. In some cases, currents and waves will
make the cooler acting as a forced convection cooler. These heat
transfer rate changes may be challenging when operating a system
while trying to maintain a constant cooling performance. It has
been found that according to some embodiments, the influence of sea
currents and waves on the heat transfer performance can be
decreased. The cooling performances will tend to be independent of
the horizontal velocity field from the sea current. This benefit
can be very useful on the system regulation point of view.
[0111] Although many embodiments have been described supra in the
context of a heat exchanger system in which parallel pipes are
arranged in symmetrical such as shown in FIG. 2, the techniques
described herein are applicable to many other types and
arrangements of heat exchangers. FIGS. 10-1, 10-2 and 10-3
illustrate using one or more baffles in different configurations
for a non-symmetrical arrangement of cooling system pipes,
according to some embodiments. In each case, the heat exchanger
1000 includes an inlet manifold 1002, and outlet manifold 1004 and
a plurality of parallel pipes 1020. In FIG. 10-1 a single
horizontally arranged baffle 1030 is positioned along the exterior
of pipes 1020 as shown. In FIG. 10-2 a series of four horizontally
arranged baffles 1032, 1034, 1036 and 1038 are positioned along the
exterior of pipes 1020 as shown. In the case of FIG. 10-3, two
baffles 1040 and 1042 are mounted in a slanted arrangement with
respect to the pipes 1020 of heat exchanger 1000. In each case the
baffles have the effect of disturbing the vertical flow of the
ambient coolant fluid (the water) and induce a substantial
non-vertical component (i.e. non parallel to the pipe axis) that
has been found to substantially increase the cooling performance of
the heat exchanger.
[0112] The particulars shown herein are by way of example and for
purposes of illustrative discussion of the embodiments of the
present invention only and are presented in the cause of providing
what is believed to be the most useful and readily understood
description of the principles and conceptual aspects of the present
invention. In this regard, no attempt is made to show structural
details of the present invention in more detail than is necessary
for the fundamental understanding of the present invention, the
description taken with the drawings making apparent to those
skilled in the art how the several forms of the present invention
may be embodied in practice. Further, like reference numbers and
designations in the various drawings indicated like elements.
[0113] Whereas many alterations and modifications of the present
invention will no doubt become apparent to a person of ordinary
skill in the art after having read the foregoing description, it is
to be understood that the particular embodiments shown and
described by way of illustration are in no way intended to be
considered limiting.
[0114] It is noted that the foregoing examples have been provided
merely for the purpose of explanation and are in no way to be
construed as limiting of the present invention. While the present
invention has been described with reference to exemplary
embodiments, it is understood that the words, which have been used
herein, are words of description and illustration, rather than
words of limitation. Changes may be made, within the purview of the
appended claims, as presently stated and as amended, without
departing from the scope and spirit of the present invention in its
aspects. Although the present invention has been described herein
with reference to particular means, materials and embodiments, the
present invention is not intended to be limited to the particulars
disclosed herein; rather, the present invention extends to all
functionally equivalent structures, methods and uses, such as are
within the scope of the appended claims.
* * * * *