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, by comparison with a
recirculatory steam generator, a once-through steam generator
has 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. However, in the case of a once-through steam
generator with a horizontal construction, the steam generation
tubes of an evaporator heating surface are exposed, depending
on their positioning, to greatly differing heating. It is
thereby possible that an unstable flow arises, in particular
in the steam generation tubes which are upstream on the flow
substance side, and this can endanger the operational
reliability of the waste heat steam generator. Hence, for the
purpose of dynamic stabilization there have previously been
proposals, for example, for restrictors at the entry to the
steam generation tubes, an enlargement of the pipe diameter
from the entry towards the exit, or the use of pressure
equalization lines and collectors. However, in the case of a
waste heat steam generator with a horizontal construction
these measures can either be ineffective or technically
impossible to implement.
The object underlying the invention is thus to specify a waste
heat steam generator of the type identified above which has a
particularly high operational reliability together with a
particularly simple construction.
This object is achieved in accordance with the invention in
that the first steam generation tubes are designed in such a
way that the mean mass flow density which is established in
the first steam generation tubes when operating at full load
does not fall below a prescribed minimum mass flow density.
The invention then starts from the consideration that it would
be possible to achieve a particularly high operational
reliability by a dynamic stabilization of the flow in the
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first steam generation tubes. In particular, a pulsating,
oscillatory type of flow is to be avoided. Here, it has been
recognized that a flow of this type arises in particular in
those first steam generation tubes which are located at the
heating gas side exit from the first evaporator heating
surface, and which experience comparatively limited heating.
These tubes contain a flow substance with a comparatively high
proportion of water. Because of the greater proportionate
weight of the flow substance in these tubes, the through-flow
in these tubes is reduced, partly to the point of stagnation.
For the purpose of avoiding this effect, it would be possible
to provide chokes or pressure equalization lines, but these
would mean a comparatively more expensive construction. Thus,
in order to avoid stagnation of the flow and at the same time
permit a particularly simple construction of the waste heat
steam generator, the parameters of the steam generation tubes
in the first evaporator heating surface should be directly
modified. This can be achieved by designing the first steam
generation tubes in such a way that the mean mass flow density
through the first steam generation tubes which is established
when operating at full load does not fall below a prescribed
minimum mass flow density.
It is advantageous in this case if the value of the prescribed
minimum mass flow density is 100 kg/m2s. That is, a design of
the steam generation tubes to achieve such a choice of mass
flow density leads to a particularly good dynamic
stabilization of the flow in the first steam generation tubes,
and hence to particularly reliable operation of the steam
generator.
It has been recognized that stagnation of the flow in the
tubes is caused by a comparatively large geodetic pressure
loss in the steam generation tubes. In order to stabilize the
mass flow density, the geodetic pressure loss should therefore
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be reduced as a proportion of the overall pressure loss. This
can be achieved in that the internal diameter of the first
steam generation tubes is advantageously chosen in such a way
that the mean mass flow density which is established in the
first steam generation tubes when operating at full load does
not fall below the prescribed minimum mass flow density, by
which means the overall pressure loss is increased by raising
the frictional pressure loss.
It is advantageous if the internal diameter of the first steam
generation tubes is then between 15 and 35 mm. That is, a
choice of internal diameter in this range determines the
volume of the first steam generation tubes to be such that the
geodetic pressure loss in the steam generation tubes is so low
that the mass flow density does not fall below prescribed
minimum, i.e. it is no longer possible for stagnation or
pulsation of the flow to occur. By this means, particularly
reliable operation of the steam generator is ensured.
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 exit side, the flow substance is
comparatively weakly heated. By the design described for the
steam generation tubes it is however also possible to avoid
stagnation of the flow in these steam generation tubes. By
this dynamic stabilization, particularly reliable operation 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 on the heating gas side from
the second evaporator heating surface. 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 strongly heated
region of the heating gas duct.
A once-through evaporator of this type can expediently be used
in a waste heat steam generator, and the waste heat steam
generator 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 designing the first steam
generation tubes in such a way that the mean mass flow density
established in the first steam generation tubes when operating
at full load does not fall below a prescribed minimum mass
flow density achieves a dynamic stabilization of the flow, and
thus particularly reliable operation of the waste heat steam
generator. By an appropriate design of the steam generation
tubes this effect is achieved even without further expensive
technical measures, and thus permits at the same time 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. The figure in the drawing
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shows a simplified representation of a longitudinal section
through a steam generator with a horizontal construction.
The once-through steam generator 1 for the waste heat steam
generator 2 shown in the FIG 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 the FIG
incorporates a number of rows of tubes, 11 and 12
respectively, each in the nature of a nest of tubes, arranged
behind each other 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
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
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to them. 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 the FIG. The heating surfaces 20 could be,
for example, superheaters, medium-pressure evaporators, low-
pressure evaporators and/or preheaters.
The first steam generation tubes 13 are now designed in such a
way that the mass flow density does not fall below a minimum
prescribed for full load operation as 100 kg/m2s. Here, their
internal diameter is between 15 mm and 35 mm. By this means,
stagnation of the flow in the first steam generation tubes 13
is avoided. A standing column of water with the formation of
steam bubbles, and a resulting oscillatory type of pulsating
through-flow, is prevented. By this means, the mechanical
loading on the waste heat steam generator 2 is reduced, and
particularly reliable operation is guaranteed at the same time
as a simple construction.
