U.S. patent application number 12/575805 was filed with the patent office on 2011-04-14 for dual enhanced tube for vapor generator.
Invention is credited to Bashir I. Master, Bhushan Ranade.
Application Number | 20110083619 12/575805 |
Document ID | / |
Family ID | 43470298 |
Filed Date | 2011-04-14 |
United States Patent
Application |
20110083619 |
Kind Code |
A1 |
Master; Bashir I. ; et
al. |
April 14, 2011 |
DUAL ENHANCED TUBE FOR VAPOR GENERATOR
Abstract
A heat exchange assembly for generating steam from a second
medium to be used to drive a steam turbine for generating
electricity or for other process, includes a tube having a
longitudinal axis and an inner wall and an outer wall. The outer
wall includes a plurality of spaced fins oriented generally
perpendicular to the longitudinal axis. The inner wall defines a
preheat zone and a dual phase zone. The preheat zone defines a
helical rib configured to provide swirling motion and increase heat
transfer surface area to liquid entering the tube increasing heat
transfer from the tube to the liquid. The dual phase zone is spaced
from the preheat zone and defines a helical rib configured to
provide swirling motion to steam and liquid passing through the
dual phase zone increasing heat transfer from the tube to the steam
and liquid while preventing vapor stagnation and film boiling.
Inventors: |
Master; Bashir I.; (Wayne,
NJ) ; Ranade; Bhushan; (Hillsborough, NJ) |
Family ID: |
43470298 |
Appl. No.: |
12/575805 |
Filed: |
October 8, 2009 |
Current U.S.
Class: |
122/32 ; 165/157;
165/182 |
Current CPC
Class: |
F22B 21/30 20130101;
F28F 1/422 20130101; F28F 13/12 20130101; F22B 15/00 20130101; F28D
7/06 20130101; F28F 2215/04 20130101; F28F 1/42 20130101; F28F
13/187 20130101; F28F 13/14 20130101; F28D 7/16 20130101; F28F 1/40
20130101; F22B 1/16 20130101; F28F 1/26 20130101 |
Class at
Publication: |
122/32 ; 165/157;
165/182 |
International
Class: |
F22B 1/02 20060101
F22B001/02; F28D 1/04 20060101 F28D001/04; F28F 1/10 20060101
F28F001/10 |
Claims
1. A heat exchange assembly for generating steam from a second
medium comprising: a tube defining a longitudinal axis and having
an inner wall and an outer wall, said outer wall including a
plurality of spaced fins oriented generally perpendicular to said
longitudinal axis of said tube; said inner wall defining a preheat
zone and a dual phase zone, said preheat zone defining a helical
rib configured to provide swirling motion and increase heat
transfer surface area to liquid entering said tube thereby
increasing heat transfer from said tube to said liquid, and said
dual phase zone being spaced from said preheat zone and defining a
helical rib configured to provide swirling motion to steam and
liquid passing through said dual phase zone thereby increasing heat
transfer from said tube to said steam and liquid.
2. The assembly set forth in claim 1, wherein said assembly
includes a plurality of tubes configured in a bundle, each tube
converting liquid to steam.
3. The assembly set forth in claim 1, wherein said inner wall
includes a smooth wall section separating said preheat zone and
said dual phase zone.
4. The assembly set forth in claim 1, wherein said spaced fins
disposed upon said outer wall of said tube includes an average
height of between about 0.6 mm and 0.7 mm.
5. The assembly set forth in claim 1, wherein said spaced fins
disposed upon said outer wall of said tube includes an average
height of about 0.66 mm.
6. The assembly set forth in claim 1, wherein said spaced fins
disposed upon said outer wall of said tube include an average fin
thickens of between about 0.2 mm and 0.4 mm.
7. The assembly set forth in claim 1, wherein said spaced fins
disposed upon said outer wall of said tube includes an average
thickness of about 0.3 mm.
8. The assembly set forth in claim 1, wherein said spaced fins
include a density of between about 7 to 12 per centimeter.
9. The assembly set forth in claim 1, wherein said rib includes an
average height between about 0.5 mm and 1.0 mm.
10. The assembly set forth in claim 1, wherein said rib includes an
average rib thickness of between about 0.38 mm and 0.44 mm.
11. The assembly set forth in claim 1, wherein said helical rib
includes an apex angle with respect to tube radius of between about
zero and thirty degrees.
12. The assembly set forth in claim 1, wherein said helical rib
includes a helix angle to said tube axis of between about ten and
forty five degrees.
13. A method of converting liquid to vapor using a heating fluid,
comprising the steps of: providing a tube having a first zone, a
second zone, and a third zone defined by an inner surface of said
tube; heating an exterior surface of said tube with the heating
fluid; preheating the liquid passing through said tube in said
first zone while providing swirling motion the liquid thereby
improving heat transfer from said tube to the liquid; converting at
least a portion of the liquid to vapor in said second zone; and
providing swirling motion to the vapor and liquid passing through
said third zone thereby preventing film boiling from occurring on
the surface of said third zone.
14. The method set forth in claim 13, wherein said step of
providing swirling motion to the liquid passing through said first
zone is further defined by providing a helical rib to said inner
surface of said tube at said first zone.
15. The method set forth in claim 13, wherein said step of
providing a tube is further defined by providing a tube with a
second zone having a substantially smooth surface.
16. The method set forth in claim 13, wherein said step of
providing a tube is further defined by providing a tube having a
two phase zone comprising vapor and liquid.
17. The method set forth in claim 13, further including the step of
providing sufficient heat energy to said third zone from the
heating fluid to continuously maintain both vapor and liquid inside
said tube.
18. The method set forth in claim 13, wherein said step of
providing swirling motion to the liquid passing through said third
zone is further defined by providing a helical rib to said inner
surface of said tube at said first zone.
19. The method set forth in claim 13, wherein said step of
providing a tube having a first zone, a second zone, and a third
zone is further defined by providing a tube having a first zone
comprising about 20% of a length of said tube, a second zone
comprising about 60% of a length of said tube and a third zone
comprising about 20% of a length of said tube.
20. The method set forth in claim 13, further including the step of
providing a different swirling motion to the liquid passing through
said first zone than the swirling motion provided to the vapor and
liquid passing through said third zone.
21. The method set forth in claim 13, further including the step of
providing a helical rib to said inner surface of said tube thereby
increasing the surface area of said inner surface of said tube
providing improved heat transfer from the heating fluid to the
liquid passing through said tube.
22. The method set forth in claim 21, wherein said step of
providing a helical rib to said inner surface of said tube is
further defined by providing a helical rib to said first and said
second zones.
23. A heat exchange assembly for converting water to steam,
comprising: a housing having an inlet and an outlet; a plurality of
baffle plates oriented in a predetermined fixed relationship,
spaced apart creating a serpentine path inside said housing
providing a route for a heating medium flowing from said inlet to
said outlet; a plurality of tubes defining an inner wall and an
outer wall, said tubes being disposed in said housing in a
substantially vertical orientation, each of said plurality of tubes
being received by at least some of said baffle plates providing
contact to said outer wall with the heating medium flowing along
said serpentine path; said outer wall of said tubes defining fins
thereby increasing the surface area of said outer wall and said
inner wall defining helical ribbing, said helical ribbing providing
swirling motion to water entering said tube and swirling motion to
water and steam passing through said tube thereby reducing film
boiling associated with converting water to steam while increasing
heat transfer through increased surface area of said inner wall
associated with said helical ribbing.
24. The assembly set forth in claim 23, wherein said housing
includes an inlet plenum for delivering water to said plurality of
tubes and outlet plenum for evacuating steam from said plurality of
tubes.
25. The assembly set forth in claim 23, wherein said inner wall of
said tubes include a first zone, a second zone, and a third zone,
said first zone being positioned adjacent said inlet plenum, said
third zone being positioned adjacent said outlet plenum and said
second zone being positioned between said first zone and said third
zone.
26. The assembly set forth in claim 25, wherein said first, second,
and third zones each include helical ribbing thereby providing
swirling motion to the water and the steam passing through said
plurality of tubes.
27. The assembly set forth in claim 25, wherein said second zone
comprises a substantially smooth surface of said inner wall and
said first zone and said third zone comprise helical ribbing for
providing swirling motion to the water adjacent said inlet plenum
and swirling motion to the steam adjacent said exit plenum.
28. The assembly set forth in claim 23, wherein said fins are
oriented in a substantially perpendicular relationship to the
orientation of said tubes.
29. The assembly set forth in claim 23, wherein said fins disposed
upon said outer wall of said tube includes an average height of
between about 0.6 mm and 0.7 mm.
30. The assembly set forth in claim 23, wherein said fins disposed
upon said outer wall of said tube includes an average height of
about 0.66 mm.
31. The assembly set forth in claim 23, wherein said fins disposed
upon said outer wall of said tube include an average fin thickens
of between about 0.2 mm and 0.4 mm.
32. The assembly set forth in claim 23, wherein said fins disposed
upon said outer wall of said tube includes an average thickness of
about 0.3 mm.
33. The assembly set forth in claim 23, wherein said fins include a
density of between about 7 to 12 per centimeter.
34. The assembly set forth in claim 23, wherein said helical
ribbing includes an average height between about 0.5 mm and 1.0
mm.
35. The assembly set forth in claim 23, wherein said helical
ribbing includes an average rib thickness of between about 0.38 mm
and 0.44 mm.
36. The assembly set forth in claim 23, wherein said helical
ribbing defines an apex angle relative to a tube radius of between
about zero and thirty degrees.
37. The assembly set forth in claim 23, wherein said helical
ribbing includes a helix angle to said tube axis of between about
ten and forty five degrees.
Description
BACKGROUND OF THE INVENTION
[0001] The use of steam, for driving steam turbines for the purpose
of generating electricity or for other processes such as
desalination or enhanced mineral recovery, has been a common
practice for many years. Various methods of generating steam have
been employed making use of fossil fuels, nuclear fusion, and more
recently, using solar energy. Generally, a heat exchanger having a
primary liquid that is heated from the various sources set forth
above heats a secondary liquid to generate steam.
[0002] As the demands for steam for generating electricity and
other recovery processes increase, it has become necessary to
improve the heat transfer efficiency between primary and secondary
mediums passing through a heat exchanger. Additionally, for
improved solar plant economics, the immediate use of available
solar energy has become very important, requiring optimized
transfer of heat from the primary fluid to the secondary fluid
during periods of solar field start-up and normal operations.
During the start up when the primary fluid is being heated to a
temperature reaching the desirable range, and mid-day operation
when the primary fluid may be heated to a temperature a bit above
the optimum range it is important to maintain optimum heat transfer
between the two fluids.
[0003] Known steam generating heat exchangers have made use of
smooth tubes as a part of a shell and tube heat exchanger where the
primary heating medium flows on the outside of the tubes and the
secondary vapor generating medium flows on the inside of the tubes.
When the temperature of the primary medium is maintained within a
narrow window, the smooth tubes have been known to provide
satisfactory heat transfer. However, vapor blanketing inside the
tubes is known to occur in the transition phase tube where the two
phase flow with high vapor fraction flows over hot metal surface
resulting in scaling of deposits known to reduce the heat transfer
efficiency of the heat exchanger. Further problems arise when the
temperature of the primary medium cannot be controlled within a
narrow range, which is known to occur in solar fields. When a solar
field heats the primary medium to a temperature above optimum,
scaling occurs inside the transfer tubes at an advanced rate due to
the rapid transition of the phase of the secondary fluid from
liquid to vapor. During start ups, when the primary medium
temperature is still less than optimum, heat transfer is very poor
in parts of the heat exchanger and it takes a long period for
generating required vapor fraction in the heat exchanger tubes.
Furthermore, prior art methods of generating steam have
intentionally restricted the amount of steam generation to about 10
to 12% of the amount of liquid passing through a heat exchanger to
avoid the problems set forth above.
[0004] Therefore, to meet the demands of high-efficiency steam
generation processes, it has become necessary to improve the heat
transfer efficiency of the steam generating system and the ability
to generate higher percentages of steam to liquid. Furthermore, it
is also necessary to reduce the frequency of downtime during which
the steam generating system is not operating due to the required
cleaning of the inside of the steam generator tubes and by reducing
the period of time for requisite cleaning of the tubes.
SUMMARY OF THE INVENTION
[0005] A heat exchanger assembly is used to transfer heat from a
first medium to a second medium to convert the second medium from
liquid to vapor. The vapor, or steam, is used to drive a steam
turbine for generating electrical energy or for other processes
such as desalination or mineral recovery. A tube is disposed inside
the heat exchange assembly and includes an inner wall and an outer
wall the length of which define a longitudinal axis. The outer wall
includes a plurality of spaced fins oriented in a generally
perpendicular manner to the longitudinal axis of the tube. The
inner wall defines a preheat zone and a dual phase zone. The
preheat zone defines a helical rib configured to provide swirling
motion and increased heat transfer surface area to liquid entering
the tube. The dual phase zone is spaced from the preheat zone and
defines a helical rib configured to provide a swirling motion to
the steam and liquid passing through the dual phase zone. In each
instance, the helical rib improves the efficiency of heat transfer
from the first medium to the second medium to convert liquid to
steam.
[0006] The configuration of the inventive heat exchanger and dual
enhanced tube is believed to provide heat exchange benefit for the
specific purpose of converting liquid to vapor while meeting the
demands of high-efficiency solar heat collector assemblies. By
providing helical ribbing to the entry of a phase conversion tube,
the efficiency of the first zone of the tube is improved to provide
enhanced heat transfer from the first medium to the second medium
while the second medium is still primarily in the liquid phase. In
the zone before the outlet of the tube, where the second medium
consists of high vapor fraction, the helical rib provides swirling
turbulent motion to the vapor preventing vapor blanketing on the
tube wall, which is known to cause reduced heat transfer as well as
rapid scaling and fouling with deposits further resulting in the
decrease of the performance of the heat exchanger. The dual
enhanced tube provides the ability to increase the steam to liquid
percentage to 20% without causing scaling inside the tube. This
increased percentage over prior art tubes provides significant
efficiency benefits. Therefore, the dual enhanced tube not only
improves heat transfer efficiency, but also reduces the amount of
maintenance and cleaning required of known steam generating
systems.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Other advantages of the present invention will be readily
appreciated as the same becomes better understood by reference to
the following detailed description when considered in connection
with the accompanying drawings wherein:
[0008] FIG. 1 shows a cross-sectional view of a shell and tube heat
exchanger having the inventive dual enhanced tube;
[0009] FIG. 2 shows a perspective view of the inventive dual
enhanced tube;
[0010] FIG. 3 shows a cross-sectional view of the inventive dual
enhanced tube with geometric identifiers
[0011] FIG. 3A shows a cross-sectional view of a first embodiment
of the inventive dual enhanced tube;
[0012] FIG. 3B shows a cross-sectional view of an alternate
embodiment of the inventive dual enhanced tube;
[0013] FIG. 4 shows an alternative embodiment of the shell and tube
heat exchanger where the inventive dual enhanced tubes are bent
into a generally u-shaped configuration; and
[0014] FIG. 4A shows the alternate embodiment of the dual enhanced
tube.
DETAILED DESCRIPTION OF THE INVENTION
[0015] A heat exchanger assembly of the present invention is
generally shown at 10 of FIG. 1. A first, or primary heating
medium, enters the assembly 10 through heating medium inlet 12 and
exits the assembly 10 through a heating medium outlet 14. The
heating medium is contemplated by the inventors to be any liquid
medium that is heated by an external energy source, for example,
fossil fuel burning furnaces, nuclear energy reactors or solar
energy collector fields. It is also contemplated by the inventors
that the heating medium does not change phase, but remains in a
liquid state throughout the process.
[0016] The second medium, or liquid, enters the assembly 10 through
a liquid inlet 16 and exits the assembly 10 through a two phase
flow outlet 18 after having been converted partially to vapor. It
should be understood by those of skill in the art that the two
phase flow will exit the assembly through the outlet 18. The second
medium contemplated by the inventors is water that is converted to
steam in the assembly 10 to be delivered into an external steam
drum either by natural circulation or forced circulation by pumps.
It should be understood by those of skill in the art that other
mediums may be used, such as, for example, an ammonia water mixture
or the like. Steam from the steam drum is then superheated in a
heat exchanger and supplied to drive a steam turbine (not shown)
generating electrical energy or supply to processes such as
desalination or mineral recovery, in a known manner.
[0017] As best represented in FIG. 1, an inlet tube sheet 20 is
located proximate the liquid inlet 16 and an outlet tube sheet 22
is located proximate the vapor outlet 18. A plurality of dual
enhanced tubes 24 extend between the inlet tube sheet 20 and the
outlet tube sheet 22 creating a heat transfer chamber 26 defined by
an exterior of the dual enhanced tubes 24, an assembly housing or
shell 28, and the inlet and outlet tube sheets 20, 22. The
descriptive term, "phase transition" when associated with the dual
enhanced tubes 24 is used in reference to the phase transition that
occurs to the second medium between liquid and vapor. The dual
enhanced tubes 24 are supported by a plurality of spaced baffle
plates 25. As should be understood by one of ordinary skill in the
art, the heating medium flows into the assembly 10 through the
heating medium inlet 12, through the heat transfer chamber 26, and
out of the heating medium outlet 14. The segmentally cut baffle
plates 25 cause the heating medium to flow along a serpentine path
to maximize the contact between the heating medium and the dual
enhanced tubes 24. Also, the liquid flows through liquid inlet 16
into an inlet chamber 30 through dual enhanced tubes 24 to the
outlet chamber 32 and subsequently through vapor outlet 18. The
assembly 10 is sealed on opposing ends by endplates 34 a first of
which defines the inlet chamber 30 with the inlet tube sheet 20,
and a second of which defines the vapor chamber 32 with the outlet
tube sheet 22.
[0018] FIGS. 2 and 3 show a perspective view and a cross-sectional
view respectively of the dual enhanced tube of the present
invention at 24. The dual enhanced tube 24 is defined by a
substantially annular wall 36. The annular wall 36 includes an
outer surface 38 and inner surface 40. The outer surface 38
includes a plurality of spaced fins 42 that extend radially
outwardly from the annular wall 36. The fins 42 are contemplated by
the inventors to be perpendicular to an axis defined by the length
of the dual enhanced tube 24. However, it should be understood by
those of skill in the art that the fins 42 can take an alternative
orientation such as helical, or angled to the axis defined by the
length of the dual enhanced tube 24.
[0019] Referring to FIGS. 3, 3A and 3B, substantially helical ribs
44 extend from the inner surface 40 of the annular wall 36. The
helical ribs 44 have a first configuration at the inlet of the dual
enhanced tube 24 where the second medium is in liquid phase. The
helical ribs 44 are contemplated to take a second configuration at
the exit of the dual enhanced tube 24 where, primarily, two phase
flow exits the tube as shown in FIG. 3B. It is further contemplated
by the inventor that an inner section spaced between the ends of a
dual enhanced tube 24 includes a smooth inner surface 40 for
purpose of which will be explained further below. However, it is
also contemplated by the inventor that the helical ribs 44 defined
on the inner surface 40 of the dual enhanced tube 24 is consistent
from the inlet to the outlet of the dual enhanced tube 24 as shown
if FIG. 3A.
[0020] The dimensional aspects of the dual enhanced tube are shown
in FIG. 3. The height (do-dr)/2 of the external fins 42 is between
about 0.6 mm and 0.7 mm having a target height of about 0.66 mm.
The average external fin 42 thickness is between about 0.28 mm and
0.32 mm, with a target thickness of 0.3 mm. The fins 42 may also
take a pyramidal shape where the fin thickness 42 is less at its
apex than at its base. The fins also have a spacing, Wp, defined by
the number of fins per unit length of tube, for example, between
about 7 and 12 per centimeter length of tube.
[0021] Internally, the helical rib 44 has an average rib height
(di-dp)/2 of between about 0.5 mm to 1.0 mm. The average thickness
of the internal helical ribs 44 is between about 0.38 mm and 0.44
mm. The spacing, Ws of the helical ribs 44 at the inlet of the dual
enhanced tube, as defined by the number of ribs per unit length of
the tube is about 6 per centimeter length of tube at the inlet and
2 per centimeter length of tube at the outlet. The helical ribs 44
terminate in height at an apex having an apex angle of about ten to
thirty degrees while along the axis of the tube forming a helix
angle of ten to forty five degrees with respect to the tube
axis.
[0022] During operation, liquid entering the dual enhanced tube 24
is initially heated in a first zone 46 which extends the length
required to raise the temperature of a liquid to its boiling point.
This is believed to be between about 15% and 33% of the length of
the dual enhanced tube 24. Therefore, the actual length of the
first zone 46 could vary depending upon the temperature of the
heating medium. Furthermore, the length of the first zone 46 is
shortened by the inventive dual enhanced tube due to the swirling
turbulence generated by the helical ribs 44 and the improved heat
transfer between mediums by the extended surface area generated by
the outer fins 42. By shortening the length of the first zone 46,
the phase transition zones of the dual enhanced tube 24 are
lengthened providing enhanced phase transition to the liquid
entering the dual enhanced tube 24.
[0023] A second zone 48 is contemplated to be in the central region
of the dual enhanced tube 24 where the liquid, now raised to a
temperature of its boiling point begins a transition from liquid to
steam, or vapor. The flow velocity of the second medium increases
in the second zone 48 due to a lower density attributable to the
generation of some vapor. The lower density is believed to cause
flow turbulence in the middle zone. Therefore, it is not believed
that a substantial amount of vapor film is built up in this middle
zone 48 that would cause scaling reducing heat transfer efficiency
of the assembly 10. Therefore, a reduced cost dual enhanced tube is
contemplated to have a smooth inner surface in the central region
or middle zone while helical ribbing 44 is only swaged at the
opposing ends of the dual enhanced tube 24.
[0024] A third zone 50 is located at the opposite end of the dual
enhanced tube 24 from the first zone 46. The third zone 50 is a
location in which more liquid is converted to vapor, or steam,
known to result in vapor film coating the inner surface 40 of the
dual enhanced tube 24. As set forth above, vapor film is known to
reduce the heat transfer efficiency of the tube and also results in
a build-up of deposits requiring frequent cleaning of the assembly
10. The helical ribbing provides swirling and turbulent motion to
the vapor preventing a vapor film from occurring and subsequent
build-up of deposits on the inner surface 40.
[0025] An alternative embodiment is shown in FIG. 4 where an
alternative dual enhanced tube 124 is shown in a u-shaped
configuration. Each of the elements that are common to the first
embodiment shown in FIG. 1 is numbered in the 100 series in FIG. 4.
In the alternative embodiment, the assembly 110 includes a heating
medium inlet 112 a heating medium outlet 114, a liquid inlet 116
and a two phase flow outlet 118, the fluids of which are segregated
by tube sheet 120. The heating medium circulates through heat
transfer chamber or shell 126 and around segmentally cut baffle
plates 127 to provide heat to the dual enhanced tube 124 as
explained above. Liquid enters the assembly 110 through liquid
inlet 116 into inlet chamber 130 where the liquid enters the dual
enhanced tube 124 and the two phase flow exits the dual enhanced
tube 124 passing into an outlet chamber 132.
[0026] An alternate dual enhanced tube 124 is shown in FIG. 4A. It
should be understood by those of skill in the art that a plurality
of u-shaped dual enhanced tubes 124 could be used in a
commercialized embodiment. A single endplate 134 defines the inlet
chamber 130 and the vapor chamber 132 with tube sheet 120, the
chambers of which 130, 132 are separated by separation member 135.
The alternative assembly 110 provides the benefit of the inventive
dual enhanced tube 124 while reducing the amount of space necessary
for adequately converting liquid to vapor. It is anticipated that
the alternate dual enhanced tube 124 includes three zones, a first
zone 146, where the first medium is primarily in the liquid phase,
a second zone or middle zone 148, where steam or vapor begins to
emerge from the liquid phase, and a third zone 150, where a
substantive amount of steam or vapor exists with liquid as set
forth above. Also as set forth above, the first zone and the third
zone include a helical rib 144, the configuration of which might
differ between zones to maximize phase transition efficiency.
Further, the second or middle zone 148 may have a smooth inner
surface, or may include a helical rib 144.
[0027] While the invention has been described with reference to an
exemplary embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
* * * * *