Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
21 9920n.
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STEAM CONDENSING APPARATUS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is generally related to steam condensers and
more particularly to steam condensers that combine the use of
air-cooled vacuum dry steam condensing technology with heat pipe
technology.
2. General Background
Air-cooled steam condensers used in the steam power-
generation cycle are typically arranged in an A-frame
construction with a fan at the base and inclined condenser tube
bundles on each side. Air flows through the fan and across
several sections of the steam condenser. The steam inlet is at
the top of each bundle and the vapor and condensate flow
concurrently downward. Typically, there are four rows of tubes
in each condenser bundle. As air flows through the four rows,
the air temperature increases and the temperature difference
between the condensing steam and air deceases. The lower
temperature difference for each successive tube row results in
less condensation. Since the condensate and steam flows are
lower for each successive tube row, the two-phase flow pressure
drop is also lo~er for each tube row. If the tube rows discharge
into a common rear header, the differences in tube row exit
pressures are resolved by steam and noncondensable gases in the
rear header entering the ends of the tube rows that have a lower
pressure. Since the lower tube rows have lower exit pressures,
they have steam entering both ends and over time noncondensable
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gases collect in the tubes. These pockets of noncondensable
gases block local steam flow, allowing condensate to freeze
during cold weather, which can result in tube rupture.
Noncondensable gases are normally vented from the rear header
with vacuum pumps or air ejectors. To overcome this problem, the
classical solution has been to design for excess steam flow
through each tube row. The excess steam prevents the
accumulation of noncondensable gases and maintains condensate
temperatures above freezing. This excess steam, typically twenty
to thirty-three percent of the total steam flow, is condensed in
a secondary or vent condenser. The typical vent condenser is a
dephlegmator (reflux condenser) which has steam flow up an
inclined tube, condensation on the tube walls, and drainage of
the condensate downward. The noncondensable gases flow upward
out of the tube and are removed by vacuum pumps or air ejectors.
Steam condenser freezing problems have also been overcome in the
past through the use of heat pipes. Heat pipes were used to
condense steam. The steam was passed over the evaporator side
of the heat pipes and condensed while ambient air was forced over
the condenser side of the heat pipes. The condensate was
collected at the bottom of the steam duct and returned to the
boiler for reuse. These approaches are subject to some
limitations and do not necessarily offer a simple approach to the
management of noncondensable gases.
SUMMARY OF THE INVENTION
The invention addresses the limitations of the prior art.
What is provided is an air-cooled steam condenser that also uses
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heat pipe technology so as to be freeze proof under any
ambient conditions and offer a simple approach to the
management of noncondensable gases. Steam flows through the
main condenser with concurrent steam and condensate flow
downward. The heat transfer surface area and fan air flow
are designed such that, over the range of operating
conditions, all of the steam does not completely condense
and vapor continuously exits each tube row. This continuous
flow of steam vapor purges these rows of noncondensable
gases. The excess steam flows into the lower header to a
secondary condenser section that utilizes heat pipes. In
the secondary condenser section, the excess steam condenses
on the evaporator side external surface of the heat pipes.
The noncondensable gases that remain in the lower header are
vented with an air removal system similar to conventional
condensers. Condensate in the lower header drains to a
condensate tank for reuse in the power generation cycle.
In a first aspect, the present invention provides steam
condensing apparatus comprising: (a) an upper steam header;
(b) a main condenser in fluid communication with the upper
steam header, the main condenser being designed such that
only a predetermined portion of the steam flow therethrough
is condensed therein; (c) a lower steam header in fluid
communication with the main condenser; (d) a secondary
condenser in fluid communication with the lower steam
header; and (e) a plurality of heat pipes received in the
secondary condenser that cause condensation of steam not
condensed in the main condenser.
In a second aspect, the present invention provides a
steam condensing apparatus comprising: (a) an upper steam
header; (b) a main condenser in fluid communication with the
upper steam header, the main condenser being designed such
to condense approximately twenty to eighty percent of the
steam flow therethrough; (c) a lower steam header in fluid
communication with the main condenser; (d) a second
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condenser in fluid communication with the lower steam header
in position in line with the main condenser; and (e) a
plurality of heat pipes received in the secondary condenser
that cause condensation of steam not condensed in the main
condenser.
In a third aspect, the present invention provides a
steam condensing apparatus comprising: (a) an upper steam
header; (b) a lower steam header; (c) a condenser adjacent
positioned between the upper and lower steam headers; (d) a
plurality of steam tubes positioned in the condenser and in
fluid communication with the upper steam header and the
lower steam header; and (e) a plurality of heat pipes
positioned in the condenser such that the evaporator section
of the heat pipes extend into the lower steam header.
BRIEF DESCRIPTION OF THE DRAWINGS
For a further understanding of the nature and objects
of the present invention reference should be had to the
following description, taken in conjunction with the
accompanying drawing in which like parts are given like
reference numerals, and wherein:
Fig. 1 illustrates a prior art air cooled steam condenser.
Fig. 2 illustrates a prior art air cooled steam condenser.
Fig. 3 illustrates a prior art air cooled steam condenser.
Fig. 4 illustrates the invention.
Fig. 5 illustrates an alternate embodiment of the invention.
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Fig. 6 illustrates a second alternate embodiment of the
invention.
Fig. 7 is a sectional view that illustrates one of the heat
pipes used in the invention.
Fig. 8 is a sectional view of an alternate embodiment of a
heat pipe that may be used with the invention.
Fig. 9 is a sectional view that illustrates an alternate
embodiment of the lower steam header of the invention.
Fig. 10 is a view taken along lines 10-10 in Fig. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
As seen in the prior art illustrations of Fig. 1, air-cooled
steam condensers are typically arranged in an A-Frame
construction with a fan 10 at the base and inclined condenser
tube bundles 12 on each side. Air flows through the fan across
several sections of the steam condenser. Steam from steam
turbine 14 is directed to an upper steam header 16 which provides
a steam inlet at the top of each bundle 12. The vapor and
condensate flow concurrently downward in the bundle to a lower
or rear header 18. An air ejector or vacuum pump 20 is used to
vent noncondensable gases from the rear header 18. The
condensate is collected in tank 22 and directed to condensate
pumps not shown for reuse.
Fig. 2 illustrates a prior art solution to prevent freezing
of condensate. The condenser tube bundle 12 is designed to cause
excess steam flow through each tube row. The excess steam
prevents the accumulation of noncondensable gases and maintains
condensate temperatures above freezing. This excess steam is
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condensed in a secondary or vent condenser 24. The typical vent
condenser 24 is a dephlegmator (reflux condenser) which has steam
flow up an inclined tube, condensation on the tube walls, and
drainage of the condensate downward. The noncondensable gases
flow upward out of the tube and are removed by vacuum pumps or
air ejectors.
Fig. 3 also illustrates a prior art solution to prevent
freezing of condensate. Heat pipes 26 are set up in a Y
configuration. The evaporator side of the heat pipes is enclosed
in a steam header 28. The steam is condensed as it passes across
the evaporator side of the heat pipes 26. The condensate is
collected at the bottom of the header 28 and returned to the
boiler for reuse. Fan 10 causes induced air flow across the
condenser sides of the heat pipes to cause cooling and
recondensation of the working fluid contained in the heat pipes.
The present invention is generally indicated by numeral 30
in Fig. 4. Steam condensing apparatus 30 is generally comprised
of main condenser 32, lower header 34, and secondary condenser
36.
Main condenser 32 is formed from upper steam header 38 and
one or more tube bundles 40. Upper steam header 38 receives
steam from steam turbine 42 via line 44 and then directs the
steam into tube bundles 40. Each tube bundle 40 is similar to
tube bundles generally known and used in the industry in that
several rows of tubes, usually four, are provided for receiving
and condensing steam. The main difference in the tube bundles
of the present invention from the prior art is that they are not
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designed to condense as much of the steam as possible. Instead,
the heat transfer surface area and fan air flow from fans 46 are
designed such that over the range of operating conditions, all
of the steam does not completely condense and steam vapor
continuously exits the bottom of each tube row into lower header
34. In the preferred embodiment, sixty-seven to eighty percent
of the available surface area is used in the tube bundles 40.
This surface area, combined with fan air flow, results in
approximately twenty to eighty percent of the steam being
condensed in main condenser 32. The continuous flow of steam
vapor purges the tube rows in main condenser 32 of noncondensable
gases. The excess uncondensed steam and noncondensable gases
flow into flow into lower header 34 and then to a secondary
condenser 36.
Secondary condenser 36 is in fluid communication with lower
header 34 and positioned in line with main condenser 32. Heat
pipes 48 are positioned in secondary condenser 36 such that the
evaporator side of each heat pipe is at the lower end of
secondary condenser 36 and extends into lower header 34. The
condenser side of each heat pipe is positioned toward the upper
end of secondary condenser 36. In this manner, the uncondensed
steam from main condenser 32 condenses on the evaporator side of
heat pipes 48 and flows out of lower header 34 through condensate
drain 50. Noncondensable gases are vented off to an ejector as
indicated by numeral 52.
Fig. 5 illustrates an alternate embodiment of the invention
wherein main condenser 32 and secondary condenser 36 are oriented
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in a W-shape configuration instead of an in-line configuration.
As above, the invention condenses the excess steam in secondary
condenser 36. Noncondensable gases are vented off via lines 54.
Fig. 6 illustrates another alternate embodiment of the
5invention wherein the main and secondary condensers described
above are consolidated into a single condenser 56. Single
condenser 56 includes conventional finned tubes 58 that direct
steam flow from top to bottom and heat pipes 26. As before, the
heat transfer surface area and fan air flow are designed such
10that over the range of operating conditions, all of the steam is
not condensed in tubes 58. The continuous flow of steam purges
tubes 58 of noncondensable gases. The remaining steam that exits
the bottom of tubes 58 is condensed by heat pipes 26 which have
their evaporator side extending below the exit end of tubes 58
15in lower header 34. Condensate drains from condensate drain 50
to be collected for reuse. Noncondensable gases are removed via
vent lines 54. Fig. 6 illustrates four rows of pipes, with heat
pipes 26 being the lower or first row. It should be understood
that heat pipes 26 may be positioned in any row of the tube
bundle.
Fig. 7 is a detailed sectional view of a heat pipe 26 and
lower header 34 as used in the invention. Heat pipes 26 may be
fabricated out of straight round, elliptical, or flat oval tubes
that may or may not contain an internal wick. Heat pipes 26 are
25sealed at both ends and contain a predetermined quantity of heat
transfer fluid 60 at a predetermined vapor pressure. The fluid
used will depend upon the application and conditions. Examples
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of heat transfer fluid used in different heat pipe applications
are, but not limited to, methanol, ammonia, and freon. Heat
transfer fluid 60 normally resides in evaporator section 62 of
heat pipe 26. When heat flows into evaporator section 62, heat
transfer fluid 60 vaporizes, removing heat from the steam and
causing condensation thereof, and travels upward into condenser
section 64 where the fluid is cooled and condensed, releasing the
fluid heat to the air flow. The heat transfer fluid condensate
returns to evaporator section 62 by gravity flow. Condenser
section 64 may be provided with fins 66 to provide a large heat
rejection surface area. Fins 66 may be extruded, embedded, or
wrapped aluminum or steel and can be solid or serrated depending
upon the pressure drop and heat transfer requirements. Heat
pipes 26 may be placed in inline or triangular tube pitches
depending upon the pressure drop and heat transfer requirements
of the system.
For improved heat transfer performance and corrosion
resistance, Fig. 8 illustrates a heat pipe 26 that has the outer
diameter of the evaporator section sleeved with a low friction
coating 68 such as polytetrafluoroethylene. The low friction
coating promotes drop wise condensation which improves the
condensing heat transfer rate by about one order of magnitude.
In addition, the coating provides a corrosion proof boundary that
allows the use of inexpensive carbon steel based tubes for heat
pipes 26.
Fig. 9 and 10 illustrate an embodiment of a lower header 34
that is provided with a plurality of thermowells or sleeves 70
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that are welded directly to lower header 34 to form a leak-proof
seal. Each sleeve 70 is sized to provide a small slip-fit
clearance between the inner diameter of sleeve 70 and the outer
diameter of the evaporator section of a heat pipe 26 to reduce
thermal resistance. If required to improve heat transport, a
thermally conductive substance such as grease or liquid may be
used to fill the annulus. Heat pipes 26 are held in place by
gravity and the tube supports commonly found in the condenser
bundle frame. This provides another means of eliminating
corrosive contact of heat pipes 26 with the steam. As referred
to above, the exterior of sleeves 70 may be coated with a low
friction coating to promote drop wise condensation, thus
improving the condensing heat transfer rate.
In operation, steam received in upper header 38 flows into
the tubes in tube bundles 40 where some of the steam is condensed
and flows into lower header 34. The remaining steam flowing out
of the tubes into lower header 34 purges the tubes of
noncondensable gases. The remaining steam is condensed on the
evaporator section 62 of heat pipes 26. Noncondensable gases are
removed via vent lines and/or vacuum pumps. The arrangement of
tubes and heat pipes causes a constant steam flow through the
tubes in the tube bundles to provide for freeze-proof tubes in
the tube bundles. The only freezing possible in the design of
the invention is on the outside of the heat pipe section located
in the lower header. Since this occurs on the exterior of the
heat pipes, it will not damage the heat pipes. The lower header
embodiment of Fig. 9 provides the advantage of being able to
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remove and install heat pipes in the field without the need to
cut and reweld the difficult seal weld between the heat pipe 26
and lower header 34.
Because many varying and differing embodiments may be made
within the scope of the inventive concept herein taught and
because many modifications may be made in the embodiment herein
detailed in accordance with the descriptive requirement of the
law, it is to be understood that the details herein are to be
interpreted as illustrative and not in a limiting sense.
