Note: Descriptions are shown in the official language in which they were submitted.
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Description
Once-through evaporator
The invention relates to a once-through evaporator for a
horizontally constructed waste heat steam generator with a
first evaporator heating surface which incorporates a number
of first steam generation tubes, the arrangement of which is
essentially vertical and through which the flow is from the
bottom to the top, and another second evaporator heating
surface, which on the flow substance side is connected
downstream from the first evaporator heating surface, which
incorporates a further number of second steam generation tubes
the arrangement of which is essentially vertical and through
which the flow is from the bottom to the top.
In the case of a combined cycle gas turbine plant, the heat
contained in the expanded working substance or heating gas
from the gas turbine is utilized for the generation of steam
for the steam turbine. The heat transfer is effected in a
waste heat steam generator connected downstream from the gas
turbine, in which it is usual to arrange a number of heating
surfaces for the purpose of preheating water, for steam
generation and for superheating steam. The heating surfaces
are connected into the water-steam circuit of the steam
turbine. The water-steam circuit usually incorporates several,
e.g. three, pressure stages, where each of the pressure stages
can have an evaporator heating surface.
For the steam generator connected downstream on the heating
gas side from the gas turbine as a waste heat steam generator,
several alternative design concepts can be considered, namely
a design as a once-through steam generator, or a design as a
recirculatory steam generator. In the case of a once-through
steam generator the heating up of steam generation tubes,
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which are provided as evaporation tubes, results in the flow
substance being evaporated in a single pass through the steam
generation tubes. In contrast to this, in the case of a
natural or forced circulation steam generator, the water which
is fed around the circulation is only partially evaporated
during its passage through the evaporator tubes. After the
steam which has been generated has been separated off, the
water which has not yet been evaporated is then fed once more
to the same evaporator tubes for further evaporation.
Unlike a natural or forced circulation steam generator, a
once-through steam generator is not subject to any pressure
limitations. A high live steam pressure favors a high thermal
efficiency, and hence low CO2 emissions from a fossil-fuel
fired power station. In addition, a once-through steam
generator has, by comparison with a recirculatory steam
generator, a simple construction and can thus be manufactured
at particularly low cost. The use of a steam generator,
designed in accordance with the once-through principle, as the
waste heat steam generator for a combined cycle gas turbine
plant is therefore particularly favorable for the achievement
of a high overall efficiency for the combined cycle gas
turbine plant together with simple construction.
A once-through steam generator which is designed as a waste
heat steam generator can basically be engineered in one of two
alternative forms of construction, namely as a vertical
construction or as a horizontal construction. A once-through
steam generator with a horizontal construction is then
designed so that the heating substance or heating gas, for
example the exhaust gas from the gas turbine, flows through it
in an approximately horizontal direction, whereas a once-
through steam generator with a vertical construction is
designed so that the heating substance flows through it in an
approximately vertical direction.
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Unlike a once-through steam generator with a vertical
construction, a once-through steam generator with a horizontal
construction can be manufactured with particularly simple
facilities, and with particularly low manufacturing and
assembly costs. In this case, an uneven distribution of the
flow substance can arise across the steam generation tubes
located downstream on the flow substance side, in particular
within each individual row of tubes in the steam generation
tubes of the second evaporator heating surface, said tubes
being located downstream on the flow substance side, leading
to temperature imbalances and, because of different thermal
expansions, to mechanical stresses. For this reason expansion
bends, for example, have hitherto been incorporated to
compensate for these stresses, in order to avoid damage to the
waste heat steam generator. However, this measure can be
technically comparatively expensive in the case of a waste
heat steam generator with a horizontal construction.
The object underlying the invention is thus to specify a once-
through evaporator, for a waste heat steam generator of the
type identified above, which has a particularly long service
life while permitting a particularly simple construction.
This object is achieved in accordance with the invention in
that on the flow substance side an aperture system is
connected downstream from the second steam generation tubes.
The invention then starts from the consideration that it would
be possible to achieve a particularly simple construction for
the waste heat steam generator or once-through evaporator, as
applicable, by eliminating the previously-usual expansion
bends. In doing so however, the mechanical stresses caused by
the temperature imbalances in the steam generation tubes,
which are connected in parallel with one another in each
individual row, must be reduced in some other way. These
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occur, in particular, in the second evaporator heating
surface, to which is admitted a water-steam mixture. The
temperature imbalances are here caused by the different
proportions of water and steam at the flow side entry to the
individual tubes in a row of tubes, and the resulting
different through-flow through these tubes. It has been
recognized that this different through-flow in the tubes is
caused by a frictional pressure loss in the steam generation
tubes which is small by comparison with the geodetic pressure
loss. That is, a flow which has a high proportion of steam in
the flow substance flows through individual steam generation
tubes comparatively fast with a low frictional pressure loss,
whereas a flow with a high proportion of water is
disadvantaged by its greater geodetic pressure loss, caused by
its mass, and can tend towards stagnation. In order to even
out the through-flows, the frictional pressure loss should
therefore be increased. This can be achieved by connecting
into flow substance side downstream from the second steam
generation tubes an aperture system which causes an additional
frictional pressure loss of this type.
It is advantageous if the aperture system incorporates a
plurality of apertures, arranged in the individual second
steam generation tubes. Such a distributed arrangement of the
apertures ensures that separately in each steam generation
tube a sufficient additional frictional pressure loss arises
to provide a static stabilization of the flow, and thereby an
equalization of temperature imbalances.
This frictional pressure loss should be appropriately
determined by reference to the other operating parameters,
such as the pipe geometry, the dimensions of the heating gas
duct and the temperature conditions. Advantageously, the
aperture opening of each aperture should then be chosen in
such a way that the prescribed frictional pressure loss for
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the flow substance is established via the system of apertures.
This permits even better avoidance of temperature imbalances.
Advantageously, each aperture will have as the aperture
opening a bore with a diameter between 10 and 20 mm. Namely,
such a choice leads to a particularly good static
stabilization of the flow in the second steam generation
tubes, and thus to a particularly good equalization of the
temperatures in steam generation tubes which are connected in
parallel in the individual rows of tubes in the second heating
surface.
In order to permit an even more flexible structuring of the
aperture system, it should incorporate a plurality of
apertures connected one after another on the flow substance
side. By this means, an even more uniform temperature
distribution can be achieved.
In an advantageous embodiment, a number of first steam
generation tubes are connected one after another on the
heating gas side as rows of tubes. This makes it possible to
use as an evaporator heating surface a larger number of steam
generation tubes connected in parallel, which means a better
heat input from the enlarged surface. However, in this case
the steam generation tubes which are arranged one after
another in the direction of flow of the heating gas are
differently heated. Particularly in the steam generation tubes
on the heating gas entry side, the flow substance is
comparatively strongly heated. However, by the downstream
connection of an aperture system as described, a through-flow
which is matched to the heating can also be achieved in these
steam generation tubes. By this means, a particularly long
service life is achieved for the waste heat steam generator
together with a simple construction.
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In an advantageous embodiment, the first evaporator heating
surface is connected downstream from the second evaporator
heating surface on the heating gas side. This offers the
advantage that the second evaporator heating surface, which is
connected downstream on the flow substance side and is thus
designed to further heat up a flow substance which has already
been evaporated, also lies in a comparatively more strongly
heated region of the heating gas duct.
It is expedient to use a once-through evaporator of this type
in a waste heat steam generator, and the waste heat steam
generator is used in a combined cycle gas turbine plant. In
this case it is advantageous to connect the steam generator
downstream on the heating gas side from a gas turbine. With
this connection, a supplementary heat source can expediently
be arranged behind the gas turbine, to raise the heating gas
temperature.
The advantages achieved by the invention consist, in
particular, in the fact that connecting an aperture system
downstream achieves a static stabilization of the flow, and
thus a reduction in the temperature differences between second
steam generation tubes connected in parallel and in the
resulting mechanical stresses. This makes the service life of
the waste heat steam generator particularly long. An
appropriate arrangement of an aperture system makes further
expensive technical measures such as expansion bends
unnecessary, and thus at the same time permits a particularly
simple cost-saving construction for the waste heat steam
generator or a combined cycle gas turbine power station, as
applicable.
An exemplary embodiment of the invention is explained in more
detail by reference to a drawing. This shows:
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FIG 1 a simplified representation of a longitudinal section
through a steam generator with a horizontal
construction,
FIG 2 a graphical representation of the tube temperature
against its steam content at the entry to the heating
tube, with no aperture system arrangement, and
FIG 3 a graphical representation of the tube temperature
against its steam content at the entry to the heating
tube, with an aperture system arrangement.
The same parts have been given the same reference numbers in
all the figures.
The once-through evaporator 1 for the waste heat steam
generator 2 shown in FIG 1 is connected downstream from a gas
turbine, not shown here in more detail, on its exhaust gas
side. The waste heat steam generator 2 has a surrounding wall
3 which forms a heating gas duct 5 through which the exhaust
gas from the gas turbine can flow in an approximately
horizontal direction as heating gas, as indicated by the
arrows 4. Arranged in the heating gas duct 5 is a number of
evaporator heating surfaces 8, 10, designed according to a
once-through principle. In the exemplary embodiment shown in
FIG 1, each of two evaporator heating surfaces 8, 10 is shown,
but a larger number of evaporator heating surfaces could also
be provided.
Each of the evaporator heating surfaces 8, 10 shown in FIG 1
incorporates a number of rows of tubes, 11 and 12
respectively, each in the nature of a nest of tubes, arranged
one behind another in the direction of the heating gas. Each
row of tubes 11, 12 incorporates in turn a number of steam
generation tubes, 13 and 14 respectively, in each case
arranged beside each other in the direction of the heating
gas, of which in each case only one can be seen for each row
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of tubes 11, 12. The first steam generation tubes 13 of the
first evaporator heating surface 8, which are arranged
approximately vertically and connected in parallel so that a
flow substance W can flow through them, are here connected on
their output sides to an outlet collector 15 which is common
to them. The second steam generation tubes 14 of the second
evaporator heating surface 10, which are also arranged
approximately vertically and connected in parallel so that a
flow substance W can flow through them, are also connected on
their output sides to an outlet collector 16 which is common
to them. Here, a comparatively expensive collection system
could also be provided for both the evaporator heating
surfaces 8, 10. For flow purposes, the steam generation tubes
14 of the second evaporator heating surface 10 are connected
downstream from the steam generation tubes 13 of the first
evaporator heating surface 8, via a downpipe 17.
The evaporation system formed by the evaporator heating
surfaces 8, 10 can have admitted to it the flow substance W
which, in a single pass through the evaporation system, is
evaporated and after it emerges from the second evaporator
heating surface 10 is fed away as steam D. The evaporation
system formed by the evaporator heating surfaces 8, 10 is
connected into a steam turbine's water-steam circuit, which is
not shown in more detail. In addition to the evaporation
system which incorporates the evaporator heating surfaces 8,
10, the water-steam circuit of the steam turbine has connected
into it a number of other heating surfaces 20, indicated
schematically in FIG 1. The heating surfaces 20 could be, for
example, superheaters, medium-pressure evaporators, low-
pressure evaporators and/or preheaters.
An aperture system 22, which incorporates apertures 23
arranged in the individual steam generation tubes, is now
connected downstream from the second steam generation tubes
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14. The bore of the apertures 23 is chosen such that the
frictional pressure loss of the flow substance W in the steam
generation tubes 14 is appropriately high to ensure a uniform
through-flow within a row of tubes 11. By this means,
temperature imbalances are reduced. For this purpose, the
apertures 23 incorporate bores between 10 and 20 mm in
diameter.
The effect of the aperture system 22 on the temperature
differences is shown in FIGs 2 and 3. Each of these shows a
graphical representation of the mean tube wall temperature 25
and the tube exit wall temperature 27, plotted against the
proportion of steam DA in the flow substance. Here, FIG 2
shows the situation without a downstream aperture system 22.
In this case, the mean tube wall temperature 25 varies between
approx. 460 C and 360 C, the temperature of the tube exit
wall 27 between 480 C and 370 C, depending on the steam
content DA. FIG 3 shows that these variations are reduced to
approx. 440 C to 390 C or 470 C to 405 C respectively. The
temperature differences between tubes with a different steam
content are also clearly reduced.
The reduction in the temperature differences, between tubes
with differing steam content at the flow-side entry, reduces
the mechanical stress loads on the waste heat steam generator
2, and guarantees a particularly long service life and at the
same time a simple construction due to the elimination of the
previously usual expansion bends.
