Title: Steam generator
Abstract: A steam generator has a continuous heating surface located in a fuel gas channel that can be traversed in an approximately horizontal fuel gas directions. The continuous heating surface has a plurality of steam generator pipes that are connected in parallel for the passage of a flow medium and is designed in such a way that a steam generator pipe, which is heated to a greater extent than another steam generator pipe of the same continuous heating surface, has a higher throughput of the flow medium than the other steam generator pipe. The aim is to produce a low-cost steam generator with a particularly high level of mechanical stability, even when subjected to different thermal stresses. To achieve this, the or each steam generator pipe has a respective downpipe section, which is approximately vertical and through which the flow medium can flow downwards and a respective riser pipe section connected downstream of the downpipe on the flow medium side, which is approximately vertical and through which the flow medium can flow upwards.
Patent Number: 6,868,807 Issued on 03/22/2005 to Franke, et al.
| Inventors:
|
Franke; Joachim (Altdorf, DE);
Kral; Rudolf (Stulln, DE)
|
| Assignee:
|
Siemens Aktiengesellschaft (Munich, DE)
|
| Appl. No.:
|
479994 |
| Filed:
|
December 8, 2003 |
| PCT Filed:
|
May 27, 2002
|
| PCT NO:
|
PCT/DE02/01936
|
| 371 Date:
|
December 8, 2003
|
| 102(e) Date:
|
December 8, 2003
|
| PCT PUB.NO.:
|
WO02/10129 |
| PCT PUB. Date:
|
December 19, 2002 |
Foreign Application Priority Data
| Jun 08, 2001[DE] | 101 27 830 |
| Current U.S. Class: |
122/406.4; 122/235.23; 122/258 |
| Intern'l Class: |
F22B 019//00 |
| Field of Search: |
122/1 B,406.4,235.11,235.23,451 S,7 R,258,136 R,138,140.2,152,153
|
References Cited [Referenced By]
U.S. Patent Documents
| 589553 | Sep., 1897 | Stiff | 122/40.
|
| 2699758 | Jan., 1955 | Dalin | 122/1.
|
| 4357907 | Nov., 1982 | Campbell et al. | 122/4.
|
| 4421065 | Dec., 1983 | Tillequin | 122/155.
|
| 4664067 | May., 1987 | Haneda et al. | 122/7.
|
| 4685426 | Aug., 1987 | Kidaloski et al.
| |
| 4858562 | Aug., 1989 | Arakawa et al. | 122/7.
|
| 5159897 | Nov., 1992 | Franke et al.
| |
| 5575244 | Nov., 1996 | Dethier | 122/406.
|
| 6019070 | Feb., 2000 | Duffy.
| |
| 6092490 | Jul., 2000 | Bairley et al. | 122/7.
|
| 6189491 | Feb., 2001 | Wittchow et al. | 122/1.
|
| 6557500 | May., 2003 | Schroeder | 122/7.
|
| 6588379 | Jul., 2003 | Bingham et al. | 122/235.
|
| Foreign Patent Documents |
| 1176155 | Aug., 1964 | DE.
| |
| 0425 717 | May., 1995 | DE.
| |
| 196 51 936 | Jul., 1998 | DE.
| |
| 197 00 350 | Jul., 1998 | DE.
| |
| 0 944 801 | Feb., 2001 | EP.
| |
Other References
Patent Abstracts of Japan, Publication No. 03221702 published on Sep. 30,
1991.
|
Primary Examiner: Wilson; Gregory A.
Attorney, Agent or Firm: Staas & Halsey LLP
Claims
What is claimed is:
1. A steam generator comprising:
a heating gas duct through which heating gas flows in an approximately
horizontal direction;
a once-through heating area arranged in the heating-gas duct, the
once-through heating area comprising a plurality of steam generator tubes
connected in parallel with respect to a flow of a flow medium such that a
first steam generator tube heated to a greater extent compared with a
second steam generator tube in the once-through heating area has a higher
rate of flow of the flow medium compared with the second steam generator
tube, at least one of the steam generator tubes having an approximately
vertically arranged down-corner section, through which the flow medium can
flow in a downward direction, and an approximately vertically arranged
riser section which is connected to said down-corner section at a location
downstream with respect to a flow direction of the flow-medium, the riser
section conveying the flow medium in an upward direction.
2. A steam generator as claimed in claim 1, wherein each steam generator
tube has a down-corner section approximately vertically arranged
down-corner section, through which the flow medium can flow in a downward
direction, and an approximately vertically arranged riser section which is
connected to said down-corner section at a location downstream with
respect to a flow direction of the flow-medium, the riser section
conveying the flow medium in an upward direction.
3. The steam generator as claimed in claim 2, wherein the down-corner
section of the steam generator tube is arranged in the heating-gas duct
downstream from the riser section with respect to a flow direction of the
heating gas.
4. The steam generator as claimed in claim 3, wherein the down-corner
section of the steam generator tube is connected to the riser section
through an overflow section.
5. The steam generator as claimed in claim 4, wherein the overflow section
is arranged inside the heating-gas duct.
6. The steam generator as claimed in claim 5, wherein at least one of the
steam generator tubes has a plurality of riser sections connected
downstream from a common down-corner section such that the riser sections
are connected in parallel with respect to the flow medium.
7. The steam generator as claimed in claim 6, wherein the riser and
down-corner sections of a plurality of steam generator tubes are
positioned relative to one another in the heating-gas duct to equalize
average position differences with respect to a flow direction of the
heating gas in such a way that:
if a first riser section is positioned further upstream than a second riser
section with respect to a flow direction of the heating-gas, and
if a first down-corner section is positioned further downstream than a
second down-comer section with respect to the flow direction of the
heating-gas,
then the first riser section is connected to the first down-corner section
and the second riser section is connected to the second down-corner
section.
8. The steam generator as claimed in claim 7, wherein at least one of steam
generator tubes has a plurality of down-comer sections and a plurality of
riser sections connected alternately such that each down-corner section is
followed on a downstream side thereof by a riser section.
9. The steam generator as claimed in claim 8, further comprising:
a main distributor;
a connecting line for each down-corner section, to connect the down-corner
section to the main distributor; and
a throttle device provided in each connecting line at a position upstream
from the downcomer section and downstream from the main distributor.
10. The steam generator as claimed in claim 9, further comprising a gas
turbine connected to the steam generator tubes of the once-through heating
area at a location upstream with respect to the flow direction of the
flow-medium.
11. The steam generator as claimed in claim 1, wherein the down-corner
section of the steam generator tube is arranged in the heating-gas duct
downstream from the riser section with respect to a flow direction of the
heating gas.
12. The steam generator as claimed in claim 1, wherein the down-corner
section of the steam generator tube is connected to the riser section
through an overflow section.
13. The steam generator as claimed in claim 12, wherein the overflow
section is arranged inside the heating-gas duct.
14. The steam generator as claimed in claim 1, wherein at least one of the
steam generator tubes has a plurality of riser sections connected
downstream from a common down-corner section such that the riser sections
are connected in parallel with respect to the flow medium.
15. The steam generator as claimed in claim 1, wherein the riser and
down-corner sections of a plurality of steam generator tubes are
positioned relative to one another in the heating-gas duct to equalize
average position differences with respect to a flow direction of the
heating gas in such a way that:
if a first riser section is positioned further upstream than a second riser
section with respect to a flow direction of the heating-gas, and
if a first down-corner section is positioned further downstream than a
second downcomer section with respect to the flow direction of the
heating-gas,
then the first riser section is connected to the first down-corner section
and the second riser section is connected to the second down-comer
section.
16. The steam generator as claimed in claim 1, wherein at least one of
steam generator tubes has a plurality of down-corner sections and a
plurality of riser sections connected alternately such that each
down-corner section is followed on a downstream side thereof by a riser
section.
17. The steam generator as claimed in claim 1, further comprising:
a main distributor;
a connecting line for each down-corner section, to connect the down-corner
section to the main distributor; and
a throttle device provided in each connecting line at a position upstream
from the downcomer section and downstream from the main distributor.
18. The steam generator as claimed in claim 1, further comprising a gas
turbine connected to the steam generator tubes of the once-through heating
area at a location upstream with respect to the flow direction of the
flow-medium.
19. A steam generator comprising:
a heating gas duct through which heating gas flows in an approximately
horizontal direction; and
a once-through heating area arranged in the heating-gas duct to fully
vaporize a flow medium in a single pass through the heating gas duct, the
heating area comprising a plurality of steam generator tubes connected in
parallel with respect to a flow of a flow medium such that a first steam
generator tube heated to a greater extent compared with a second steam
generator tube in the once-through heating area has a higher rate of flow
of the flow medium compared with the second steam generator tube, at least
one of the steam generator tubes having an approximately vertically
arranged down-corner section, through which the flow medium can flow in a
downward direction, and an approximately vertically arranged riser section
which is connected to said down-corner section at a location downstream
with respect to a flow direction of the flow-medium, the riser section
conveying the flow medium in an upward direction.
20. A steam generator comprising:
a heating gas duct through which heating gas flows in an approximately
horizontal direction;
a heating gas duct through which heating gas flows in an approximately
horizontal direction; and
a once-through heating area arranged in the heating-gas duct, the
once-through heating area comprising a plurality of steam generator tubes
connected in parallel with respect to a flow of a flow medium such that a
first steam generator tube heated to a greater extent compared with a
second steam generator tube in the once-through heating area has a higher
rate of flow of the flow medium compared with the second steam generator
tube, at least one of the steam generator tubes having an approximately
vertically arranged down-comer section, through which the flow medium can
flow in a downward direction, and an approximately vertically arranged
riser section which is connected to said down-corner section at a location
downstream with respect to a flow direction of the flow-medium, the riser
section conveying the flow medium in an upward direction,
wherein the down-corner section of the steam generator tube is connected to
the riser section through a connecting section.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is based on and hereby claims priority to German
Application No. 10127830.6 filed on Jun. 8, 2001, the contents of which
are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention relates to a steam generator in which a once-through heating
area is arranged in a heating-gas duct through which flow can occur in an
approximately horizontal heating-gas direction, which once-through heating
area comprises a plurality of steam generator tubes connected in parallel
to the throughflow of a flow medium and which is designed in such a way
that a steam generator tube heated to a greater extent compared with a
further steam generator tube of the same once-through heating area has a
higher rate of flow of the flow medium compared with the further steam
generator tube.
In a gas- and steam-turbine plant, the heat contained in the expanded
working medium or heating gas from the gas turbine is used for generating
steam for the steam turbine. The heat transfer is affected in a
heat-recovery steam generator which is connected downstream of the gas
turbine and in which a plurality of heating areas for water preheating,
for steam generation and for steam superheating are normally arranged. The
heating areas are connected in the water/steam circuit of the steam
turbine. The water/steam circuit normally comprises several, e.g. three,
pressure stages, in which case each pressure stage may have an evaporator
heating area.
For the steam generator connected as a heat-recovery steam generator
downstream of the gas turbine on the heating-gas side, a plurality of
alternative design concepts come into consideration, namely the design as
a once-through steam generator or the design as a circulation steam
generator. In a once-through steam generator, the heating of steam
generator tubes provided as evaporator tubes leads to an evaporation of
the flow medium in the steam generator tubes in a single pass. In contrast
thereto, in a natural- or forced-circulation steam generator, the
circulating water is only partly evaporated when passing through the
evaporator tubes. The water which is not evaporated in the process is fed
again to the same evaporator tubes for a further evaporation after
separation of the generated steam.
A once-through steam generator, in contrast to a natural- or
forced-circulation steam generator, is not subject to any pressure limit,
so that live-steam pressures are possible well above the critical pressure
of water (Pcri.congruent.221 bar)-- where there is only a slight
difference in density between a medium similar to a liquid and a medium
similar to steam. A high live-steam pressure promotes a high thermal
efficiency and thus low CO2 emissions of a fossil-fired power plant. In
addition, a once-through steam generator has a simple type of construction
compared with a circulation steam generator and can therefore be
manufactured at an especially low cost. The use of a steam generator
designed according to the once-through principle as a heat-recovery steam
generator of a gas- and steam-turbine plant is therefore especially
favorable for achieving a high overall efficiency of the gas- and
steam-turbine plant in a simple type of construction.
Particular advantages with regard to the cost of manufacture, but also with
regard to necessary maintenance work, are offered by a heat-recovery steam
generator in a horizontal type of construction, in which the heating
medium or heating gas, that is to say the exhaust gas from the gas
turbine, is conducted through the steam generator in an approximately
horizontal direction of flow. In a once-through steam generator in a
horizontal type of construction, however, the steam generator tubes of a
heating area, depending on their positioning, may be subjected to heating
that differs greatly. In particular in the case of steam generator tubes
connected on the outlet side to a common collector, different heating of
individual steam generator tubes may lead to a union of steam flows having
steam parameters differing greatly from one another and thus to
undesirable efficiency losses, in particular to comparatively reduced
effectiveness of the relevant heating area and consequently reduced steam
generation. In addition, different heating of adjacent steam generator
tubes, in particular in the region where they open into collectors, may
result in damage to the steam generator tubes or the collector. The use,
desirable per se, of a once-through steam generator of horizontal type of
construction as a heat-recovery steam generator for a gas turbine may
therefore entail considerable problems with regard to a sufficiently
stabilized flow guidance.
EP 0944 801 B1 discloses a steam generator which is suitable for being
designed in a horizontal type of construction and in addition has the
aforesaid advantages of a once-through steam generator. To this end, the
known steam generator is designed with regard to its once-through heating
area in such a way that a steam generator tube heated to a greater extent
compared with a further steam generator tube of the same once-through
heating area has a higher rate of flow of the flow medium compared with
the further steam generator tube. The once-through heating area of the
known steam generator therefore exhibits a self-stabilizing behavior like
the flow characteristic of a natural-circulation evaporator heating area
(natural-circulation characteristic) when individual steam generator tubes
are heated to a different extent, and this behavior, without the need for
exerting an external influence, leads to adaptation of the outlet-side
temperatures even on steam generator tubes heated to a different extent
and connected in parallel on the flow-medium side. However, the known
steam generator is comparatively complicated from a design point of view,
in particular with regard to the water- and/or steam-side distribution of
the flow medium. In addition, problematic differential expansions may
occur between adjacent evaporator tubes and may lead to inadmissible
thermal stresses and thus to damage to tubes and collectors.
SUMMARY OF THE INVENTION
One possible object of the invention is therefore to specify a steam
generator of the type mentioned at the beginning which can be manufactured
at an especially low cost and which has especially high mechanical
stability even during different thermal loading.
The inventors propose that one of or each of the steam generator tubes in
each case comprises an approximately vertically arranged downcomer
section, through which the flow medium can flow in the downward direction,
and an approximately vertically arranged riser section which is connected
downstream of the downcomer section on the flow-medium side and through
which the flow medium can flow in the upward direction.
In this case, a steam generator can be manufactured at an especially low
assembly and production cost, for an especially stable operating behavior
which is especially insensitive to differences in the thermal loading, the
design principle, applied in the known steam generator, of a
natural-circulation characteristic for a once-through heating area should
be logically developed and further improved. The once-through heating area
should in this case be designed for the application of a comparatively low
mass-flow density with comparatively low friction pressure loss.
In order to assist the natural-circulation characteristic of the
throughflow in this design, provision is made for dividing the steam
generator tubes of the once-through heating area into in each case at
least two segments (of parallel tubes), the first segment comprising all
downcomer sections and flow occurring through it in the downward
direction. Correspondingly, the second segment comprises all riser
sections and flow occurs through it in the upward direction. In the
downcomer sections of the first segment, the contribution of the geodetic
pressure, that is to say essentially the weight of the water column,
therefore acts in the direction of the intended throughflow and promotes
the latter by a positive contribution to the pressure change along the
flow path, that is to say by a gain in pressure. Only in the second
segment or riser section does the contribution of the geodetic pressure
act against the intended throughflow direction and therefore contribute to
the pressure loss. In total, however, the two geodetic pressure
contributions can virtually neutralize one another, it is even conceivable
for the throughflow-promoting geodetic pressure contribution in the first
segment or downcomer section to exceed the throughflow-inhibiting geodetic
pressure contribution in the second segment or riser section, so that, as
in natural-circulation systems, there is a flow-maintaining or
flow-promoting pressure contribution overall.
The downcomer section of each steam generator tube is expediently arranged
in the heating-gas duct downstream of the riser section assigned to it as
viewed in the heating-gas direction. In other words: the steam generator
tubes are expediently spatially arranged in the heating-gas duct in such a
way that the first segment or downcomer section as viewed on the
flow-medium side is arranged on the flue-gas side downstream of the second
segment or riser section as viewed on the flow-medium side. In such an
arrangement, each riser section is thus subjected to a comparatively more
intense heating by the heating gas than that downcomer section of the same
steam generator tube which is assigned to it. Thus, the relative steam
proportion of the flow medium in the riser section also markedly exceeds
the relative steam proportion of the flow medium in the downcomer section,
so that the geodetic pressure contribution, essentially given by the
weight of the water/steam column in the respective tube length, is
markedly higher in the downcomer section than in the riser section
assigned to it.
In a further or alternative advantageous configuration, an especially
simple construction of the once-through heating area, on the one hand, and
an especially low mechanical loading of the once-through heating area,
even during different thermal loading, on the other hand, can be achieved
by the downcomer section of one or each steam generator tube being
connected on the flow-medium side via an overflow section to the riser
section assigned to it. In such a configuration, the respective steam
generator tube therefore essentially has a u shape in which the legs are
provided by the riser section, on the one hand, and by the downcomer
section, on the other hand, and the bend is provided by the overflow
section connecting the riser section and downcomer section.
Such an arrangement is especially suitable for expansion compensation
during varying thermal loading; this is because the overflow section
connecting the downcomer section and the riser section serves in this case
as an expansion bend, which can readily compensate for relative changes in
length of the riser section and/or of the downcomer section. The overflow
section therefore ensures that the steam generator tubes are turned in the
bottom region of a first evaporator stage provided by the downcomer
sections and are directly continued and turned again in the bottom region
of a second evaporator stage formed by the riser sections.
The overflow section is advantageously arranged inside the heating-gas
duct. Alternatively, however, the overflow section may also be disposed
outside the heating-gas duct, in particular if a draining collector is to
be connected to the overflow section if the once-through heating area
possibly has to be drained.
In the event of the flow-promoting pressure contribution in the downcomer
section of a steam generator exceeding the flow-inhibiting pressure
contribution in the riser section assigned to it to an especially high
degree, the resulting outflow of flow medium from the downcomer section
into the riser section could exceed the inlet-side inflow of flow medium
into the downcomer section. Therefore the or each steam generator tube is
advantageously designed with regard to its overall pressure balance in
such a way that the flow-promoting pressure contribution occurring overall
in the downcomer section is only limited with regard to the
flow-inhibiting pressure contribution occurring in the riser section.
To this end, the downcomer section of one or of each steam generator tube
of the steam generator is advantageously designed for a sufficiently high
friction pressure loss of the flow medium flowing through. This may be
done, for example, by suitable dimensioning, in particular in cross
section, of the individual tube sections. In this case, one or each steam
generator tube, in a type of bifurcation, in each case also expediently
comprises a plurality of riser sections connected downstream of a common
downcomer section on the flow-medium side and mutually connected in
parallel to the throughflow of the flow medium. In an alternative or
further advantageous configuration, in each case a throttle device is
connected on the flow-medium side upstream of the downcomer section of the
or each steam generator tube, via which throttle device in particular the
individual rate of flow can be set during the feeding of the respective
downcomer section.
The steam generator tubes can be combined inside the heating-gas duct to
form tube rows, of which each in each case comprises a plurality of steam
generator tubes arranged next to one another perpendicularly to the
heating-gas direction. In such a configuration, the steam generator tubes
are preferably directed in such a way that the tube row of the downcomer
sections which is heated to the lowest degree or which is the last row as
viewed in the heating-gas direction is assigned to the riser sections
forming the tube row heated to the greatest degree, that is to say to the
first tube row as viewed in the heating-gas direction. To this end, the
riser and downcomer sections of a plurality of steam generator tubes are
expediently positioned relative to one another in the heating-gas duct in
such a way that a riser section lying comparatively far forward as viewed
in the heating-gas direction is assigned to a downcomer section lying
comparatively far back as viewed in the heating-gas direction. By such an
arrangement, which spatially corresponds essentially to a nested
arrangement of a plurality of u-shaped steam generator tubes, the riser
sections heated to a comparatively high degree are fed with flow medium
preheated to a comparatively low degree and flowing out of the downcomer
sections.
The geodetic pressure contribution, promoting the flow overall, through the
downcomer section connected upstream in each case is thus especially high
precisely in the riser sections heated to a comparatively high degree, so
that especially pronounced additional feeding with flow medium from the
assigned downcomer section is automatically effected. The automatic
additional feeding from the assigned downcomer section is therefore
effected in this case in such a way as to especially meet the requirements
precisely for tubes heated to a high degree, so that the desired
natural-circulation characteristic is intensified to an especially high
degree.
In order to provide the flow-promoting geodetic pressure contribution in
the respective steam generator tube, the respective steam generator tube
can be designed in such a way that it comprises merely one downcomer
section and merely one riser section connected downstream of the downcomer
section on the flow-medium side. However, especially high flexibility
during the adaptation of the heat adsorptivity of the flow medium flowing
through the steam generator tube to the temperature profile of the heating
gas flowing through the heating-gas duct can be achieved by a plurality of
steam generator tubes in each case comprising a plurality of downcomer and
riser sections connected alternately one behind the other on the
flow-medium side. In this case, each of these steam generator tubes, as
viewed in the direction of flow of the flow medium, has first of all a
first downcomer section, following which, after suitable turning,
preferably via an overflow section, is a first riser section designed for
throughflow of the flow medium in the upward direction. Connected
downstream of this riser section, preferably likewise after suitable
turning via an overflow section arranged inside the heating-gas duct, is a
second downcomer section designed for throughflow of the flow medium in
the downward direction. A second riser section then again follows the
second downcomer section. Furthermore, as and when required, a plurality
of downcomer and riser sections may also be connected downstream in an
alternating arrangement.
The steam generator is expediently used as a heat-recovery steam generator
of a gas- and steam-turbine plant. In this case, the steam generator is
advantageously connected downstream of a gas turbine on the heating-gas
side. In this circuit, supplementary firing for increasing the heating-gas
temperature may expediently be arranged downstream of the gas turbine.
The advantages achieved relate in particular to the fact that, by the
two-stage or multistage configuration of the steam generator tubes having
a downcomer section through which flow can occur in the downward direction
and a riser section which is connected downstream of the downcomer section
on the flow-medium side and through which flow can occur in the upward
direction, at least in the first segment of the steam generator tube, a
flow-promoting pressure contribution can be provided via the geodetic
pressure of the water column located therein.
It is certainly true that heated evaporator systems through which flow
occurs downward normally lead to flow instabilities which are not
tolerable precisely during use in once-through evaporators. However,
during feeding with comparatively low mass-flow density, a
natural-circulation characteristic of the steam generator tube can be
achieved in a reliable manner due to the comparatively low friction
pressure loss, which natural-circulation characteristic, when a steam
generator tube is heated to a greater extent compared with a further steam
generator tube, leads to a comparatively higher rate of flow of the flow
medium in the steam generator tube heated to a greater extent. This
natural-circulation characteristic, even when using the segments through
which flow occurs downwards, ensures a sufficiently stable and reliable
flow through the steam generator tubes.
In addition, such a characteristic can be achieved at an especially low
cost in terms of construction and assembly by the riser section being
directly connected downstream of the downcomer section assigned to it, and
without a complicated collector or distributor system being connected
therebetween. The steam generator therefore has a relatively low degree of
plant complexity in conjunction with an especially stable flow behavior.
Furthermore, both the downcomer section and the riser section, connected
downstream of it, of each steam generator tube can be fastened in each
case in a suspended type of construction in the region of the casing
ceiling of the heating-gas duct, free linear expansion being permitted in
each case in the bottom region. Such linear expansions caused by thermal
effects are now compensated for by the overflow section connecting the
respective riser section to the downcomer section, so that no distortions
occur on account of thermal effects.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the present invention will become
more apparent and more readily appreciated from the following description
of the preferred embodiments, taken in conjunction with the accompanying
drawings of which:
FIGS. 1, 2 and 3 each show in simplified representation a steam generator
in a horizontal type of construction in longitudinal section.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Reference will now be made in detail to the preferred embodiments of the
present invention, examples of which are illustrated in the accompanying
drawings, wherein like reference numerals refer to like elements
throughout.
The same parts are provided with the same designations in all the figures.
The steam generator 1, 1', 1" according to FIGS. 1, 2 and 3, respectively,
is arranged like a heat-recovery steam generator on the exhaust-gas side
downstream of a gas turbine (not shown in any more detail). The steam
generator 1, 1'. 1" has in each case an enclosing wall 2 which forms a
heating-gas duct 6 for the exhaust gas from the gas turbine, through which
heating-gas duct 6 flow can occur in an approximately horizontal
heating-gas direction x indicated by arrows 4. A plurality of heating
areas designed according to the once-through principle, and also referred
to as once-through heating areas 8, 10 and 12, respectively, are arranged
in each case in the heating-gas duct 6. In the exemplary embodiments
according to FIGS. 1, 2 and 3, in each case only one once-through heating
area 8, 10 or 12, respectively, is shown, but a larger number of
once-through heating areas may also be provided.
Flow medium W can in each case be admitted to the evaporator system formed
from the once-through heating areas 8, 10 and 12, respectively, and this
flow medium W, during a single pass, is evaporated by the respective
once-through heating area 8, 10 or 12 and, after the discharge from the
once-through heating area 8, 10 or 12, respectively, is drawn off as steam
D and is normally fed to superheater heating areas for further
superheating. The evaporator system formed from the respective
once-through heating area 8, 10 and 12, respectively, is in each case
connected in the water/steam circuit (not shown in any more detail) of a
steam turbine. In addition to the respective evaporator system, a
plurality of further heating areas 20, in each case indicated
schematically in FIGS. 1 to 3, are connected into the water/steam circuit
of the steam turbine. The heating areas 20 may be, for example,
superheaters, intermediate-pressure evaporators, low-pressure evaporators
and/or preheaters.
The once-through heating area 8 of the steam generator 1 according to FIG.
1, like a tube bundle, comprises a plurality of steam generator tubes 22
connected in parallel to the throughflow of the flow medium W. Here, a
plurality of steam generator tubes 22 are in each case arranged side by
side as viewed in the heating-gas direction x. In this arrangement, only
one of the steam generator tubes 22 arranged side by side in this way can
be seen in each case. Here, on the flow-medium side, a common distributor
26 is arranged in each case upstream of the steam generator tubes 22
arranged side by side in this way and a common discharge collector 28 is
arranged in each case downstream of the latter. In this case, the
distributors 26 are in turn connected on the inlet side to a main
distributor 30, the discharge collector 28 being connected on the outlet
side to a common main collector 32.
The once-through heating area 8 is designed in such a way that it is
suitable for feeding the steam generator tubes 22 with a comparatively low
mass-flow density, the steam generator tubes 22 having a
natural-circulation characteristic. In the case of this
natural-circulation characteristic, a steam generator tube 22 heated to a
greater extent compared with a further steam generator tube 22 of the same
once-through heating area 8 has a higher rate of flow of the flow medium W
compared with the further steam generator tube 22. In order to ensure this
with especially simple design means in an especially reliable manner, the
once-through heating area 8 comprises two segments connected in series on
the flow-medium side. In the first segment, each steam generator tube 22
of the once-through heating area 8 comprises in this case an approximately
vertically arranged downcomer section 34 through which the flow medium W
can flow in the downward direction. In the second segment, each steam
generator tube 22 comprises an approximately vertically arranged riser
section 36 which is connected downstream of the downcomer section 34 on
the flow-medium side and through which the flow medium W can flow in the
upward direction.
In this case, the riser section 36 is connected to the downcomer section 34
assigned to it via an overflow section 38. In the exemplary embodiment,
the overflow sections are directed inside the heating-gas duct 6 and, for
spatial fixing, through a perforated plate 40 arranged in the heating-gas
duct 6. Although this perforated plate 40 produces a local constriction of
the cross section of flow available for the heating gas in the heating-gas
duct 6, it has to be emphasized that the representation in FIG. 1 is not
to scale, so that the relative constriction of the cross section of flow
for the heating gas by the perforated plate 40 is only slight.
Alternatively, the overflow sections may also be directed outside, in
particular below, the heating-gas duct 6. This may be favorable in
particular for the case where draining of the once-through heating area 8
is to be provided for design or operational reasons. This draining, in the
case of overflow sections 38 directed outside the heating-gas duct 6, may
be effected by a draining collector connected to the overflow sections 38.
In this case, the draining collector is preferably arranged spatially in
the vicinity of the downcomer sections, so that the mobility of the
heating-tube sections with regard to thermal expansion is retained without
hindrance.
As can be seen in FIG. 1, each steam generator tube 22 of the once-through
heating area 8 virtually has a u-shape, the legs of the U being formed by
the downcomer section 34 and the riser section 36, and the connecting bend
being formed by the overflow section 38. In a steam generator tube 22 of
such a design, the geodetic pressure contribution of the flow medium W in
the region of the downcomer section 34-- in contrast to the region of the
riser section 36-- produces a flow-promoting and not a flow-inhibiting
pressure contribution. In other words: the water column of unevaporated
flow medium W located in the downcomer section 34 still "pushes" along the
flow through the respective steam generator tube 22 instead of hindering
it. As a result, the steam generator tube 22, considered as a whole, has a
comparatively low pressure loss.
In the approximately unshaped type of construction, each steam generator
tube 22 is suspended or fastened in the manner of a suspended construction
on the ceiling of the heating-gas duct 6 in each case in the inlet region
of its downcomer section 34 and in the outlet region of its riser section
36. On the other hand, the bottom ends, as viewed spatially, of the
respective downcomer section 34 and of the respective riser section 36,
which are connected to one another by their overflow section 38, are not
fixed directly spatially in the heating-gas duct 6. Linear expansions of
these segments of the steam generator tubes can therefore be tolerated
without the risk of damage, the respective overflow section 38 acting as
an expansion bend. This arrangement of the steam generator tubes 22 is
therefore especially flexible mechanically and, with regard to thermal
stresses, is insensitive to differential pressures which occur.
Heating of a steam generator tube 22 to a greater extent, in particular in
its riser section 36, in this case leads there first of all to an increase
in the evaporation rate, in the course of which, just on account of the
dimensioning of the steam generator tube 22, an increase in the rate of
flow through the steam generator tube 22 heated to a greater extent occurs
as a result of this heating to a greater extent.
The steam generator tubes 22 of various tube rows 24 of the once-through
heating area 8 are in addition arranged like U shapes nested one inside
the other. To this end, the riser sections 36 and the downcomer sections
34 of a plurality of steam generator tubes 22 are positioned relative to
one another in the heating-gas duct 6 in such a way that a riser section
36 lying relatively far forward as viewed in the heating-gas direction x
is assigned to a downcomer section 34 lying relatively at the rear as
viewed in the heating-gas direction x. By this arrangement, a riser
section 36 heated to a relatively high degree communicates with a
downcomer section 34 heated to a relatively low degree. A
self-compensating effect is also achieved between the tube rows 24 by this
relative positioning. This is because, precisely with a riser section 36
heated to a comparatively high degree and lying far forward, the heating
to a greater extent results in an especially pronounced production of
steam and thus in an especially high demand for additional feeding with
flow medium W. However, precisely a riser section 36 heated to such a high
degree is connected to a downcomer section 34 heated to a comparatively
low degree. The downcomer section 34, on account of the comparatively low
heat input into the flow medium W conducted in it, has an especially high
flow-promoting geodetic pressure contribution, so that precisely such a
downcomer section 34 heated to a comparatively low degree is suitable for
providing an additional feeding quantity of comparatively cool flow medium
W.
If the riser section 36 is heated to a greater extent then the
flow-promoting geodetic pressure in the downcomer section 34 will further
exceed the flow-inhibiting geodetic pressure in the corresponding riser
section 36. The relatively far distance of the downcomer section 34 from
the riser section 36 contributes to this effect. Greater heating results
in increased feeding to the riser section 36 with flow medium W. On
account of this therefore especially pronounced natural-circulation
characteristic of the steam generator tubes 22, the latter, to a special
degree, have a self-stabilizing behavior relative to locally different
heating: heating of a row of steam generator tubes 22 to a greater extent
leads in this case locally to the increased feeding of flow medium W into
this row of steam generator tubes 22, so that, on account of the
correspondingly increased cooling effect, an adaptation of the respective
temperature values automatically occurs. The live steam flowing into the
main collector 32 is therefore especially homogeneous with regard to its
steam parameters, irrespective of the tube row 24 through which flow
occurs individually.
Depending on the design point or intended operating point of the steam
generator 1, 1', 1", the flow-promoting geodetic pressure contribution
provided by an evaporator segment through which flow occurs downward may
markedly exceed the flow-inhibiting geodetic pressure contribution of the
second evaporator segment connected downstream. Therefore, it may be
advantageous as a function of the design point to design the first
evaporator segment for a comparatively high friction pressure loss. To
this end, a throttle device 42 is in each case connected upstream of the
tube rows of the steam generator 1 according to FIG. 1 between the main
distributor 30 and the distributors 26 assigned to them in each case,
which throttle device 42 can in particular also be designed to be
adjustable or controllable.
Alternatively, to this end, the steam generator 1' in the exemplary
embodiment according to FIG. 2 comprises a once-through heating area 10
whose steam generator tubes 50, in a first segment, in each case likewise
have a downcomer section 52, downstream of which, however, on the
flow-medium side, in each case a plurality of riser sections 54 mutually
connected in parallel to the throughflow of the flow medium W are
connected. In this case, in the exemplary embodiment, the overflow
sections 56, via which the downcomer sections 52 are each connected to the
plurality of riser sections 54 assigned to them, are again directed inside
the heating-gas duct 6 and are mounted in a perforated plate 58. As and
when required, however, they may also be laid outside the heating-gas duct
6. In the exemplary embodiment according to FIG. 2, in each case 2 riser
sections 54 connected in parallel on the flow-medium side are connected
downstream of each downcomer section 52. The tubes used here have
identical dimensioning, so that the free cross section of flow for the
flow medium W in the riser sections 54 connected in parallel is in each
case twice as large as the cross section of flow in the downcomer section
52 jointly connected upstream of them. Alternatively, such a limit of the
friction pressure loss in the downcomer sections 52, if required, can also
be achieved by suitable dimensioning, in particular by selecting a
comparatively small diameter.
The steam generator 1" in the exemplary embodiment according to FIG. 3
comprises a once-through heating area 12 which is likewise designed for a
comparatively low friction pressure loss and is therefore especially
suitable for ensuring a natural-circulation characteristic at a
comparatively low mass-flow density. In addition, however, with regard to
its heat absorptivity, the once-through heating area 12 of the steam
generator 1" is especially adapted to the temperature profile of the
heating gas flowing through the heating-gas duct 6. To this end, each of
the steam generator tubes 60 forming the once-through heating area 12 in
each case comprises a plurality--two in the exemplary embodiment--of
downcomer sections 62, 64 and riser sections 66, 68 connected alternately
one behind the other on the flow-medium side. Here, the first downcomer
section 62 as viewed in the flow direction of the flow medium W is in each
case connected via an overflow section 70 to the first riser section 66
connected downstream of it. The riser section 66 is in turn connected on
the outlet side via an overflow section 72 to the second downcomer section
64 connected downstream of it. The second downcomer section 64 is
connected to the second riser section 66 via an overflow section 74. The
overflow sections 70, 72, 74 are again disposed inside the heating-gas
duct 6 and are fastened in the base region and ceiling region,
respectively, of the heating-gas duct 6 via in each case a perforated
plate 76, 78 or 80, respectively.
The invention has been described in detail with particular reference to
preferred embodiments thereof and examples, but it will be understood that
variations and modifications can be effected within the spirit and scope
of the invention.
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