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 method for designing a once-through
evaporator and 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
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steam generator the heating up of steam generation tubes,
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
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designed so that the heating substance flows through it in an
approximately vertical direction.
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, in
particular within each individual row of tubes in the steam
generation tubes of the second evaporator heating surface,
said tubes being connected 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
method for designing a once-through evaporator together with 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.
In respect of the method, this object is achieved in
accordance with the invention in that a minimum mass flow
density is prescribed and the second steam generation tubes
are designed in such a way that the mean mass flow density
which is established through the second steam generation tubes
when operating at full load does not fall below the prescribed
minimum mass flow density.
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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 the elimination of 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 occur,
in particular, in the second evaporator 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.
A static stabilization of the flow, and at the same time a
particularly simple construction for the waste heat steam
generator, can be achieved by direct modification of the
parameters of the steam generation tubes in the second
evaporator heating surface. Here, a reduction in the
temperature imbalances can be achieved by designing the second
steam generation tubes in such a way that the mean mass flow
density which is established through the second steam
generation tubes 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 180 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 static stabilization
of the flow in each individual row of tubes in the second
evaporator heating surface, and hence to a particularly good
equalization of the temperature in steam generation tubes
which are connected in parallel in each individual row of
tubes in the second evaporator heating surface.
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It has been recognized that this different mass flow density
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 in that the internal diameter of the second
steam generation tubes is advantageously chosen in such a way
that the mean mass flow density which is established when
operating at full load does not fall below the prescribed
minimum mass flow density.
The objective is further achieved by a once-through evaporator
designed in accordance with the method cited above.
A reduction in the internal diameter for ensuring a minimum
mass flow should not, however, be taken arbitrarily far. On
the basis of various operating parameters there can be a
minimum desirable diameter. So, for example, the surface of
the steam generation tubes must permit adequate heat input. In
this context, the steam generation tubes often also have
external ribbing, which in turn requires a certain minimum
diameter. A minimum thickness is also required on grounds of
rigidity and stability. Not least, if the internal diameter is
too small, the geodetic pressure loss of the water fraction of
the flow substance can be so low that a reversal of the
desired effect sets in, and a flow with a large proportion of
water reaches too high velocities in the parallel steam
generation tubes. For this reason, the internal diameter of
the second steam generation tubes should advantageously not be
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less than a minimum diameter, determined by reference to
prescribed operating parameters.
It is advantageous if the internal diameter of the second
steam generation tubes is then between 20 mm and 40 mm. That
is, a choice of internal diameter in this range determines the
mass flow density in the second steam generation tubes to be
such that the frictional pressure loss in the steam generation
tubes lies within a range for which a through-flow with a high
proportion of water and a through-flow with a high proportion
of steam lead to exit temperatures with comparatively small
temperature differences. Consequently, the temperature
differences within each row of tubes in the second evaporator
heating surface are minimized, whereby the other operating
prerequisites are satisfied at the same time.
In an advantageous embodiment, a number of second steam
generation tubes are connected one after another on the
heating gas side as rows of tubes. This makes it possible to
use for the evaporator heating surface a larger number of
steam generation tubes connected in parallel, which means a
better heat input from the enlarged surface. However, the
steam generation tubes which are arranged one after another in
the direction of flow of the heating gas are then differently
heated. Particularly in the steam generation tubes on the
heating gas entry side, the flow substance is comparatively
strongly heated. However, a through-flow which is matched to
the heating can also be achieved in these steam generation
tubes, by the design described for the steam generation tubes
such that the mass flow density at full load does not drop
below a minimum value. By this means, a particularly long
service life is achieved for the waste heat steam generator by
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.
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 second steam
generation tubes in such a way that the mean mass flow density
established through the second steam generation tubes when
operating at full load does not fall below a prescribed
minimum mass flow density achieves a static stabilization of
the flow, and thus a reduction in the temperature differences
between steam generation tubes connected in parallel and in
the mechanical stresses which result therefrom. This makes the
service life of the waste heat steam generator particularly
long. An appropriate design of steam generation tubes enables
further expensive technical measures such as expansion bends
to be foregone, and thus at the same time permits a
particularly simple cost-saving construction for the waste
heat steam generator or combined cycle gas turbine power
station, as applicable.
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An exemplary embodiment of the invention is explained in more
detail by reference to a drawing. The figure here 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
the FIG, 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
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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 the FIG. The heating surfaces 20 could be,
for example, superheaters, medium-pressure evaporators, low-
pressure evaporators and/or preheaters.
The second steam generation tubes 14 are now designed in such
a way that the mass flow density does not fall below a minimum
prescribed for full load as 180 kg/mzs. Here, their internal
diameter is between 20 mm and 40 mm so that, on the one hand,
the required operating parameters such as rigidity, heat input
etc. are satisfied and, on the other hand, temperature
imbalances within a row of tubes in the second evaporator
heating surface 10 are minimized. This reduces the mechanical
stress loadings on the waste heat steam generator 2,
guaranteeing a particularly long service life and at the same
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time a simple construction due to the elimination of the
previously usual expansion bends.
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