Note: Descriptions are shown in the official language in which they were submitted.
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Background of the Invention
Steam generators are used both in fossil-fuel and nuclear
power plants to generate working steam for driving turbines. ~later
is fed to heat-exchange surfaces connected directly or indirectly to
the nuclear-reactor core in the case of a nuclear system; in the case
of a fossil-fuel system, the surfaces are located in the furnace. A
steam-water ~ixture is thus produced, and this mixture is sent to a
'~ separator that returns the water to the heat exchange surfaces and
sends the steam ultimately to the turbine. Since the design of both
nuclear and fossil units are made with certain assumptions concerning
the thermal energy present in the coolant being fed to the heat-
- exchange sur~aces, it is desirable that the amoùnt of steam "carried
under" with the separated water be controlled. In general, the amount
of steam carried under should be minimized.
Summary of the Invention
The present invention is accordingly a means for reducing
carryunder, the amount of steam that accompanies the water returning
` to the heat-exchange surfaces in a steam generator. It is intended to
- be used in a steam generator that comprises means for separating
steam from water, means for transferring heat to water to create a
steam-water ~ixture, means for delivering the mixture to the separation
means, and means for returning the separated water along a flow path
from the separating means back to the heat-transfer means. The return
means in such a steam generator includes a plenum for collecting water
separated by the separation means so that water occupies the plenum up
; to a water level and steam occupies the plenum above the water level.
According to the present invention there is disposed in the
flow path a bubble rake having a surface facing generally down and in
the direction of the flow path and defining a generally downward-
opening channel extending longitudinally at an angle with the horizontal.The channel thereby has a high end, and the buoyancy of any steam
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entering the channel causes it to be trapped in the channel and to
migrate to the high end. The channel, according to the present
invention, forms an opening at its high end for releasing the trapped
steam. The steam is thereby released in a more concentrated state
and at a higher level, and this reduces the entrainment of steam by
the water.
Brief Description of the Drawings
These and further features and advantages of the invention
are described in connection with the attached drawings, in which:
lQ Figure 1 is a highly simplified diagrammatic view of a
typical fossil-fuel steam generator;
Figure 2 is a cross-sectional view of steam drum 12 of
Figure l;
Figure 3 is a view, partly in section,of separator 38 of
Figure 2, and
Figure 4 is a somewhat diagrammatic representation of the
rakes as seen at line 4-4 of Figure 3.
Detailed Description of the Preferred Embodiment
Figure 1 shows in a highly simplified manner the arra~gement
of a fossil-fuel steam generator. The steam generator 10 includes a
; furnace 11 in which heat-exchange surfaces, tubes 20 and 24, are
disposed. Fuel is burned in furnace 11, heating up tubes 20 and 24
and the fluid inside them. Means not shown supply feedwater to the
interior of a steam drum 12, and pump 16 forces this feedwater through
conduit 14 to tubes 20. As has been pointed out, this is a highly
simplified rendering, and there would normally be a plurality of tubes
20 covering the entire furnace wall. Accordingly, at least one
appropriate header 18 is provided for distribution of the water from
drum 12 to various tubes 20. Header 18 typically contains only water,
but during transit through tubes 20 this water absorbs enough thermal
energy to produce a certain amount of steam. Accordingly, by the time
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the fluid reaches header 28, a portion of it is steam. Steam separators
in the steam drum remove the water from the steam, and the removed
steam is sent out of the drum through conduit 32. This steam is then
distributed by header 30 to various superheater tubes 24, which
absorb further thermal energy to raise the steam temperature above
saturation. The superheated steam is collected by header 22 and
supplied to the turbine.
The water separated from the steam by the separators in drum
12 is returned, along with fresh feedwater, to the furnace through
the route previously described.
The steam drum is shown in further detail in Figure 2.
Figure 2 is a cross-sectional view of the steam drum, which is a
generally cylindrical drum containing two rows of separators 38 and 48.
~rum 34 is penetrated by pipes 2ga through 29d, which receive the steam-
water mixture from the furnace. A generally annular space 36 isprovided about the interior of the drum to lead the mixture to the
separators 38 and 48. Interior to the drum and radially inward from
annular space 36 is a water space 429 which defines a water level 74
and a steam space 50. In order to enter the plenum composed of steam
space 50 and water space 42, the mixture must pass through one of the
separators in row 38 or 48, and the water in the mixture is accordingly
separated from the steam. The water separated out by the separators
returns to the furnace through conduit 14, whereas steam in steam space 50
is passed through dryer 52 in order to remove the remaining water.
Z5 The steam is then sent to the superheater through tubes 32. (It is to
be understood that a row of tubes 32 is disposed behind the one shown,
just as ro~s of separators are disposed behind the two shown).
According to the present invention, bubble rakes 40 and 44,
to be described in more detail later, are arranged in the flow path
taken by water leaving the separator and returning to the furnace through
conduit 14. The steam removed by rakes 40 and 44 is conducted to steam
1~78~3
space 50 by appropriate tube 46, a means for conducting steam to the
part of the plenum above water line 74. As will be described in more
detail below, bubble rakes may also be included in separators 38 and 48.
Figure 3 is a cross-sectional view of one of the separators
shown in Figure 2. The separator is of the centrifugal variety and
comprises a can 58 that defines the separation zone 70 inside which
a stationary blade assembly 72 is mounted. Can 58 is mounted so as to
receive the steam-water mixture at its lower end and direct it past
blade assembly 72. A lip 56 is mounted at the top of can 58 in order
to skim off the liquid forced outward to the interior surface of the
can by the centrifugal action of the separator. A jacket 60 is mounted
around can 58 and, together with can 58, defines an annular downcomer
space 68 through which water skimmed by lip 56 is directed. A
secondary separator, consisting of corrugated plates, is mounted above
can 58 for further removal of water from the steam that leaves can 58
at its upper end. Unlike can 58 and jacket 60, the corrugated plates
of secondary separator 54 are not circular in cross-section, the water
separated by the plates cascades down to the right and left but does
not issue from the front (out of the page) or the back (into the page)
of the separator.
~ hen the steam generator is operating, the steam-water mixture
enters steam drum 34 (Figure 2) by way of pipes 29 and is conducted
through annular space 36 to separation space 70 (Figure 3) of separator
38. The fluid flows at a relatively high velocity, and blade assembly
72 imparts a circular motion to the fluid. The water droplets in the
fluid, being denser than the steam, tend to describe larger circles,
- and the liquid phase accordingly predominates at the outer edge of
separation space 70 when the fluid reaches the upper end of the can 58.
That part of the mixture that is skimmed by l;p 56 is predominantly
water, therefore, while the part of the mixture that reaches secondary
separa~or 54 has a higher concentration of steam. The skimmed-off
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fluid is directed by means of annular downcomer 68 to water space 42,
where it follows the ordinary flow path of the water from space 42 to
conduit 14 (Figure 2). The part of the mixture that passes through
secondary separator 54 (Figure 3) contacts the corrugated plates
5 therein, and much of the remaining liquid content deposits on the
corrugated plates and cascades down to water space 42 through the part
of steam space 50 exterior to jacket 60. This water also follows the
normal flow path back to the furnace. Most of the steam portion of the
mixture flowing through secondary separator 54 is released to steam
space 50, from which it passes through dryer 52 (Figure 2) and follows
pipe 32, ult;mately reaching the superheater. In summary, can 58,
together with blade assembly 72 and lip 56, constitu~es a means for
separating water from steam. The plenum consisting of steam space 50
and water space 42~ together with annular downcomer 68, is part of a
means for returning the separated water from the separation means to
the heat-transfer means. Thus, water keeps traversing the system
until it becomes steam.
As may be suspected, the separation is not perfect; some
steam remains in the water that is skimmed off into the annular
20 downcomer 68, and there is also steam that is entrained by water
cascading from secondary separator 54 into water space 42. The amouni
of steam carried into water space 42 is referred to as carryunder,
which it is the purpose of the present invention to reduce. According
to the present invention, the amount of s~eam that is carried into
water space 42 through downcomer 68 is reduced by means of bubble rakes
62, 64, and 66 and corresponding rakes positioned around the circumference
of annular downcomer 68. Bubble rakes 62, 64, and 66 are shown in
cross section in Figure 4, a section taken through line 4-4 of
Figure 3. Each rake e~tends longitudinally from can 58 to jacket 60
and is inclined upward in the direction of jacket 6Q. The rakes
have cross-sections in the shape of inverted U's, forming channels that
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open downward, generally facing the direction in which the liquid
water flows. At the upper ends of channels 76 and 78, jacket 60 is
penetrated by openings such as opening 63 in order to allow the channels
to open to the exterior of the separator.
It has been found that when water flows past discontinuities
in obstructions such as the rakes, a stall zone is formed immediately
downstream of them, and the stall zone is typically filled by steam.
If the rakes form channels as shown in order to trap the steam by its
buoyancy, then the rakes can be employed to remove the steam. Since
the rakes are inclined at an angle with the horizontal, the steam
trapped in the channels will tend to migrate to the high ends of the
channels, and the openings formed in the jacket will release the
steam to the steam space.
Water level 74 is shown in Figure 3 as being below the level
~ 15 of the rakes. As a practical matter, the water level in a steam
- - drum is subject to wide variations, and it may not be found desirable
or practical ~o locate the upper ends of the channels above the
highest level that the water space may reach. Though a lower location
of the rakes may contribute to a certain amount of carryunder, it is
thought that the overall effect of the rakes will still be carryunder
reduction. This is because the upper ends of the rakes can be
, expected to be above the water line at least some o~ the time, and
even when they are not, the steam at the outlet of the rakes will tend
` to be somewhat concentrated, reducing the amount of surface area per
pound of steam and thus the amount of entrainment. In other words,
steam in small bubbles is concentrated by the rakes into large bubbles,
. facilitating its release to steam space 50.
Bubble rakes can also be used in the positions shown in
Figure 2. Unlike the rakes illustrated in Figure 3, those shown in
Figure ~ reduce the carryunder caused both by the steam entering
through downcomer 6~ and that cascading from secondary separat-r 54.
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Such rakes can therefore be used whether or not rakes in the annular
downcomer region of the separator are also used. Rakes ~0 and 44 of
Figure 2 have cross-sections similar to those of rakes 62, 64, and 66.
(Of course, it is not absolutely necessary that the cross-section be
U-shaped. A V-shaped cross-section, or any cross-section that forms
a downward-opening channel, is appropriate for use in the present
invention.) Rakes 40 and 44, as well as a plurality of rakes
distributed behind them along the length of drum 34, are mounted on
ledges 39 and 41 formed by annular space 36. They are disposed in
; 10 the flow path of the water leaving separators 38 and 48 and flowing
to conduit 14, and they perform the same functions as rakes 62, 64,
and 66 of Figure 3. However, the steam that migrates to the upper
ends of rakes 40 and 44 is lead into steam space 50 by tube 46, and
it is practical to locate the upper end of tube 46 so that it is
always above water level 74. (Actually, it would be possible to
achieve this effect in the other arrangement by adding a second jacket
exterior to and concentric with jacket 60 that would be closed at the
bottom so as to prevent water from entering. This would correspond
in function to tube 46. However, this would require an extra fitting
20 on each of the separators, which in many cases would be impractical.)
It is to be expected that the combination of both types of
rakes would improve the reduction of carryunder over that afforded
by either method used by itself. If one or the other is to be used
alone, however, it is su~gested that rakes such as 40 and 44 be used,
not only because they affect the carryunder caused by cascading from
secondary separa~or 54, but also because their design tends to be
simpler. The reason for this simplicity has to do with the fact that
the rate of ~luid flow past the rakes as compared to their spacing
has an effect on their operation. If the rakes are spaced too widely,
the probability is too great ~hat steam in a given volume element of
the fluid will be too far from the discontinuity at the edge of a rake
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B33
to be drawn into a stall zone. On the other hand, if the rakes are
spaced so close together that the fluid velocity in the spaces
between them is too great, the pressure drop caused by the flow
velocity will lower the pressure in the channel below that of the
S steam space that the channel feeds. This would result in fluid
being drawn into the channel instead of being allowed to flow out.
Since the fluid-flow rate will normally not be designed
around the presence of the rakes, the spacing of the rakes between
each other as compared to their size must be optimized to afford a
trade-off between pressure drop and the number of rakes desired for
effective steam removal. This trade-off is a consideration in both
types of rakes, but it tends to be less difficult to take into account
in the type of rake exemplified by rakes 40 and 44. The fluid-flow
rate depends to some extent on the water level, which is non-uniform
as well as non-constant, so the fluid-flow rate in some of the
separators 38 and 48 may be high, causing steam to be drawn from the
steam space and re-entrained in the water, even though the average
flow rate is low. In addition, the water levels on different sides
of the same separator may be different. With rakes 40 and 44,
however, there will be a tendency for the rakes to redistribute the
water, reducing the non-uniformity and the possibility of re-entrainment.
Thus, the range of fluid-flow rates for which rakes 40 and 44 must be
designed can be expected to be narrower than that to which rakes
62, 64, and 66 must be designed.
~ 25 In further regard to re-entrainment, it is to be noted that
the rakes in Figure 4 are staggered and that the lowest row has no
openings at the ends of the channels. The staggered arrangement is
thought to reduce to an extent the pressure drop that results from
the increased flow rate caused by the rakes, the reduced pressure
drop will reduce the likelihood that steam will be drawn back in by
the low pressure caused by the fluid flow. The absence of openings
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on the lowest row indicates that it might be desirable that these
rakes be "blanks". Since there are no further rakes downstream of
the lowest raw to reduce the flow-induced pressure drop, the channels
may be closed, or the openings on their upper ends eliminated, in
order to prevent them from drawing steam in from the exterior of the
separator.
While the invention has been described in connection with
preferred embodiments, it is to be understood that many modifications
and variations will be apparent in light of the foregoing disclosure.
It is intended to include all such modifications and variations as
fall within the scope of the appended claims.
What is claimed is:
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