Field of Art
[0001] The present invention relates to an exhaust diffuser assembly for a low pressure
steam turbine and to a steam turbine comprising said exhaust diffuser.
Background Art
[0002] An exhaust system assembly is the last part of a steam turbine assembly, discharging
the exhaust steam from the steam turbine via an exhaust diffuser and through a discharge
opening, typically to a condenser to be condensed and brought back into the system.
A conventional exhaust system comprises an annular diffuser, which is positioned directly
behind the last row of blades of the steam turbine. The diffuser forms part of the
exhaust system of the steam turbine system and comprises an exhaust guiding channel,
formed by an inner guide and an outer guide, the outer guide being the guide that
is distant from a side wall of the exhaust casing and the inner guide being the guide
that is usually connected to the side wall of the exhaust casing. Once the steam has
passed the last row of blades of the steam turbine, it is directed through the guiding
channel and its outlet into the inner space of the exhaust system surrounded by the
exhaust casing. The exhaust system leads the steam into the condenser, heat exchanger,
or into an industrial process using the steam. The system may be oriented vertically
or horizontally, typically a vertical downward orientation is provided, in order to
ensure the uniform and efficient discharge of the expanded steam.
[0003] The configuration of the individual elements within the exhaust system influences
primarily overall efficiency of the steam flow and secondarily also complexity of
the construction and reduction of expenses, which follows from both the efficiency
improvement and reduction of complexity of construction. It is desirable to achieve
a smooth and acceptably uniform flow of the exhaust steam, i.e. uniform circumferential
and radial pressure distribution of the steam after passing through the last stage
blades, i.e. in the inlet area of the exhaust, and in the exhaust itself, while maintaining
the exhaust diffuser as axially short as possible with the effect of improved efficiency
with reduced expenses.
[0004] To achieve this goals,
US 2013/0243564 proposes a diffuser with the outer guide surface having a defined curvature - a first
angle with respect to the axis of the turbine and a defined ratio of a guide surface
axial length (SGL) to the active length of the last stage bucket (AL), which is between
about 0,45 to 0,70, preferably 0,55; the axial length being measured from the centerline
of the last stage bucket to the end of the steam guide surface, and the active length
of the last stage bucket.
[0005] Further according to
US 2013/0243564 the curvature of the outer guide may be given by several angles, starting with the
tip angle (angle A, defined at the blade tip), which is specified to be 0° or 25°,
and continuing with three further angles (B, C, D), corresponding to the range of
change in the angle over the previous 25%, 50%, and 75% meridional length of the steam
guide from the last stage bucket, i.e. to ranges of 0°-18°, 14°-32°, and 16°-32°.
[0006] It is also explained that the higher curvature of the outer guide surface may lead
to a flow separation from the outer guide surface, which in turn has negative effects
on static pressure recovery. In order to prevent or at least reduce the above-described
undesired effects,
US 2013/0243564 uses over tip leakage of the last stage blade.
[0007] There is no detailed information provided in
US 2013/0243564 with respect to the inner guide, however, it appears to have at least four inflection
points, the first inflection point being positioned relatively far from the blade
centerline, such that the first part of the inner guide between the blade and the
first inflection point is parallel with the longitudinal axis of the turbine.
[0008] EP 0418887 discloses an exhaust diffuser for steam turbines with the outer guide, which comprises
two parts and a small annular gap, through which a steam is tangentially blown into
the inner space of the diffuser. The inner guide appears to have at least three inflection
points.
Disclosure of the Invention
[0009] It is the aim of the present invention to provide an exhaust diffuser assembly for
a steam turbine with improved efficiency while keeping the axial length of the exhaust
as low as possible.
[0010] The exhaust diffuser assembly for a steam turbine, primarily for a low pressure steam
turbine, comprises a substantially annular diffuser with an inner guide and an outer
guide, forming a guiding channel. The inner guide is positioned closer to the annulus
center and part of the inner guide may form the side wall of the exhaust casing, while
the outer guide is positioned further from the annulus center and is also distant
from a side wall of the exhaust casing. The inlet starts directly behind the last
stage blades and the inlet area is given by the area of the last stage blades defined
between the base of the last stage blade and the stator body above the tip of the
blade. The outlet area is given as the area between the endpoint of the outer guide
and the side wall of the exhaust casing.
[0011] The inner guide comprises an inclined wall, which preferably comprises at least two,
but preferably not more than four inflection points that serve to adjust the shape
of the wall to guide the steam as smoothly as possible.
[0012] Preferably, the first inflection point is identical with the beginning of the inclined
wall of the inner guide and is positioned just after the base of the last stage blades
in the plane of the blade base, i.e. in a radial distance from the longitudinal axis
of the turbine, which corresponds to the radius of the rotor disc. The last inflection
point is the inflection point in the intersection of a side wall and the top of the
exhaust casing. Thus, the exhaust casing may form part of the exhaust diffuser.
[0013] The remaining inflection points - the second inflection point and optionally the
third inflection point, are positioned between the first inflection and the last inflection
point. The second inflection point may be positioned in the intersection of the inclined
wall of the inner guide and the side wall of the exhaust casing. Optionally, the third
inflection point may further be positioned at a distance of the side wall of the exhaust
casing, between the first inflection point and the second inflection point, i.e. side
wall of the exhaust casing.
[0014] Preferably, the inclined wall of the inner guide is not connected to the base of
the last stage blades. On contrary, due to a possible relative movement of individual
parts, a space between the base of the last stage blades and the first inflection
point, i.e. the inclined wall, is advantageous. The inclined wall of the inner guide
may be connected to the rotor casing sealing rather than to the base of the last stage
blades. The connection between the inclined wall and the rotor casing sealing may
be provided by any common means and at any point of the inclined wall. Preferably,
such a connection is provided in the first half of the inclined wall, i.e., in the
half that is closer to the base of the last stage blades.
[0015] The inflection points determine sections of the inclined wall with different inclination.
The inclination of the inclined wall of the inner guide is determined at the first
inflection point and may be equal to 10° - 20°, preferably about 15°, or 40°- 45°,
preferably about 42°, with respect to the longitudinal axis of the rotor disc, i.e.
the longitudinal axis of the steam turbine assembly. Preferably, the angle lies between
10° and 20°, when there are four inflection points in total, and between 40°and 45°
when there are three inflection points in total.
[0016] In an embodiment, where there are four inflection points provided, the third inflection
point is positioned between the first and the second inflection point and splits the
inclined wall of the inner guide in two parts. The first part of the inclined wall
is inclined by 10° to 20° with respect to the longitudinal axis of the steam turbine
assembly and the second part of the inclined wall is preferably inclined by 50° to
60°, preferably by 55°, with respect to the longitudinal axis of the steam turbine
assembly.
[0017] The second and the last inflection points form part of the side wall of the exhaust
casing, both are positioned at the distance from the centerline of the last stage
blades that determines the axial length of the exhaust diffuser, which is preferably
given so that the ratio of the axial length of the exhaust diffuser to the distance
between the base of the last stage blade and the stator body above the tip of the
blade is in the range of 1,25 to 1,5. Whereas the first inflection point is positioned
in the plane of the blade base, i.e. in a radial distance from the longitudinal axis
of the turbine that corresponds to the radius of the rotor disc, the second inflection
point is preferably positioned at a radial distance from the longitudinal axis of
the turbine, which is larger than the radius of the rotor disc but lower than the
length of the last stage blade.
[0018] The third inflection point may be positioned on the inclined wall between the first
and the second inflection point, preferably in the second third of the axial length
of the diffuser.
[0019] The outer guide of the diffusor comprises a curved wall, which may comprise one or
two parts, both attached to the stator body of the turbine above the tip of the last
stage blade. The curved wall of the outer guide has a starting point positioned above
the last stage blades, and an endpoint, which is located at a distance from the centerline
of the last stage blade, at a distance from the side wall of the exhaust diffuser
and at a radial distance from the longitudinal axis of the turbine assembly, the two
latter dimensions defining the outlet area of the exhaust diffuser.
[0020] The outer guide may comprise one part, or two parts, separated by a slit, through
which a steam may be blown in into the exhaust guiding channel in a controlled manner
to accelerate the steam passing close to the outer guide.
[0021] In order to ensure a smooth flow of the steam through the slit and into the exhaust,
the slit may be provided between the two parts of the outer guide in such a way, that
the end region of the first part of the outer guide and the starting region of the
second part of the outer guide overlap, creating an overlapping region and, in this
overlapping region, or at least in a part of it, the end region of the first part
of the outer guide and the starting region of the second part of the outer guide are
parallel or substantially parallel, such that the width of the slit is equal or substantially
equal along this part. Preferably, they may be positioned at a very small angle, the
angle being preferably less than 5°, more preferably less than 4°, even more preferably
less than 3°, even more preferably less than 2°, even more preferably less than 1°.
Most preferably, the two parts may be strictly parallel in the at least part of the
overlapping region.
[0022] The dimensions of the slit depend on various parameters and are, thus, to be computed
accordingly. Preferably, the length of the slit, or the length of the overlapping
region, respectively, is longer than the width of the slit, more preferably, the part
of the overlapping region, where the end region of the first part of the outer guide
and the starting region of the second part of the outer guide are parallel or substantially
parallel, is longer than the width of the slit, but preferably not longer than the
double width of the slit.
[0023] The shape of the outer guide may be given by the endpoint of the outer guide and
by the parallelism between the end region of the first part of the outer guide and
the starting region of the second part of the outer guide, if the outer guide has
two parts.
[0024] The curved wall of the outer guide starts above the tip of the last stage blade at
the connection of the outer guide to the stator body of the steam turbine assembly.
[0025] The endpoint of the outer guide at a distance from the centerline of the last stage
blade and at a distance from the side wall of the exhaust casing. The ratio of the
distance of the endpoint from the centerline of the last stage blade and the distance
between the base of the last stage blade and the stator body above the tip of the
blade is preferably between 0,45 and 0,6.
[0026] The position of the endpoint also defines the outlet area of the steam passing through
the diffuser. The ratio of the outlet area to the inlet area, which is determined
by the area of the last stage blades is preferably between 1,45 and 1,85.
[0027] In a preferred embodiment, the diffuser according to the present invention may form
part of the exhaust casing and may further be combined with a guiding rib. The guiding
rib may be attached to the upper wall of the exhaust casing, extending radially inwards,
and oriented perpendicularly to the longitudinal axis of the system. The guiding rib
serves as a guide for the steam exiting the outlet of the diffuser in the substantially
vertically upward direction to be turned in 180 degrees in order to flow downwards
to the outlet of the exhaust system.
[0028] The guiding rib may extend from the horizontal plane of the longitudinal axis of
the system and follow the cylindrical shape of the upper wall of the exhaust casing,
and it may have an I-shape or a T-shape in cross-section. The guiding rib may have
dimensions and may be positioned as defined in European patent application
EP 16189912.5, i.e. it may be positioned at a distance ranging between 1.2*
m and 1.9*
m from the inner surface of the side wall of the turbine casing, wherein the parameter
m is defined as a distance of an end point of an outer guide of a diffuser of the steam
turbine system from the inner surface of the side wall, and may have height that lies
within the range of 0.05*
H to 0.15*
H, wherein
H is the height of the steam turbine system between the horizontal plane of the longitudinal
axis and the highest point of the upper wall.
[0029] The application of the guiding rib leads to further efficiency improvement as it
serves as a guide for the steam exiting the outlet of the diffuser, thus reducing
vortices in the steam flow.
[0030] The above described configuration of the exhaust system leads to the efficiency improvement
of up to 6% of last stage blade efficiency in comparison with traditional shape of
low pressure diffuser.
Brief description of figures
[0031]
Fig. 1 shows the cross-sectional view of the steam turbine exhaust according to example
1, i.e. with the inner guide having four inflection points and with the outer guide
being formed of one part.
Fig. 2 shows the cross-sectional view of the steam turbine exhaust according to example
2, i.e. with the inner guide having four inflection points and with the outer guide
being formed of two parts, defining the slit for blowing in the steam from the steam-collecting
chamber, with a detailed view of the slit.
Fig. 3 shows the cross-sectional view of the steam turbine exhaust according to example
3, i.e. with the inner guide having three inflection points and with the outer guide
being formed of two parts, defining the slit for blowing in the steam from the steam-collecting
chamber.
Fig. 4 shows the efficiency improvement of the last stage of the turbine and the diffuser
with the geometry of the diffuser according to the present invention.
Examples
[0032] The present invention is further illustrated by way of examples of preferred embodiments
of several possible configurations of the outer guide and the inner guide for the
low pressure steam turbine system, described in the following with references to the
attached figures, which show vertical cross-section of a quadrant of the exhaust diffuser.
The examples should not be construed as limiting the claimed scope. The relevant parameters
of the exhaust diffuser geometry are summarized in the following Table 1.
Table 1: Relevant parameters of the exhaust diffuser geometry
Parameter |
Example 1 |
Example 2 |
Example 3 |
Dp |
1880 mm |
1880 mm |
2050 mm |
LLSB |
1220 mm |
1220 mm |
1370 mm |
L1 |
1250 mm |
1250 mm |
1390 mm |
LAL |
1820 - 1860 mm |
1600 - 1640 mm |
1890 - 1940 mm |
α |
15° |
15° |
42° |
β |
55° |
55° |
-- |
xE |
670 - 730 mm |
575 - 610 mm |
645 - 690 mm |
LAL / L1 |
1,45-1,49 |
1,28 - 1,31 |
1,36 - 1,40 |
xE / L1 |
0,54 - 0,58 |
0,46 - 0,49 |
0,46 - 0,5 |
S1 |
12,3 m2 |
12,3 m2 |
15,0 m2 |
S2 |
20,97 m2 |
18,2 m2 |
27,1 m2 |
S2 / S1 |
1,7 |
1,48 |
1,8 |
Example 1
[0033] In Example 1 and with the reference to Fig. 1 and Table 1, the length of the last
stage blade 3 is
LLSB =
1220 mm, the distance between the base 2 of the last stage blade and the stator body above
the tip of that blade is
L1=
1250 mm and the diameter of the rotor disc is
Dp = 1880 mm. The outer guide 6 comprises one part.
[0034] In this particular embodiment, the inner guide 7 has four inflection points - A,
B, C, D. The first inflection point A is positioned close to the base of the last
stage blades 3 and it is identical with the beginning of the inclined wall 71 of the
inner guide 7. The inclined wall 71 of the inner guide is connected to the rotor casing
sealing (not shown). There is no fixed connection between the inclined wall of the
inner guide and the base of the last stage blades. The last inflection point D is
positioned in the intersection of a side wall 42 and the top 41 of the exhaust casing
4. The second inflection point C is positioned in the intersection of the inclined
wall 72 of the inner guide and the side wall 42 of the exhaust casing 4, so that the
distance of the second inflection point (or the distance of the last inflection point)
from the centerline 31 of the last stage blades 3 determines the axial length
LAL of the exhaust diffuser. The ratio of the axial length
LAL to the distance
L1 between the base 2 of the last stage blade and the stator body above the tip of that
blade may be between 1,45-1,49.
[0035] In this particular embodiment, the inclined wall of the inner guide 7 is split in
two parts by the third inflection point B, positioned between the first inflection
point A and the second inflection point C. The first part 71 of the inclined wall
is inclined by
α =
15° with respect to the longitudinal axis of the steam turbine assembly and the second
part 72 of the inclined wall is inclined by
β =
55° with respect to the longitudinal axis of the steam turbine assembly.
[0036] The ratio of the distance of the endpoint E from the centerline 31 of the last stage
blade 3 and the distance
L1 between the base of the last stage blade and the stator body above the tip of that
blade lies between 0,54 - 0,58 and the ratio of the outlet area
S2 to the inlet area
S1, is approximately 1,7, wherein the inlet area is determined by the rotor diameter
DP and by distance
L1 between the base of the last stage blade and the stator body above the tip of that
blade and the outlet area is determined by the distance of the endpoint of the outer
guide from the side wall of the exhaust casing
m =
LAL -
xE (m not shown) and by the radial distance of the endpoint of the outer guide.
Example 2
[0037] In example 2 and with the reference to Fig. 2 and Table 1, the length of the last
stage blade is
LLSB =
1220 mm, the distance between the base 2 of the last stage blade and the stator body above
the tip of that blade is
L1=
1250 mm and the diameter of the rotor disc is
Dp = 1880 mm. Although these basic dimensions are not shown in Fig. 2, they are measured
in the same way as shown in Fig, 1.
[0038] The outer guide 6 comprises two parts 61 and 62 and a slit 63 between them, through
which a steam is tangentially blown into the inner space of the diffuser.
[0039] The first 61 and the second 62 parts are positioned in such a way that there is a
slit 63 between the first part 61 of the outer guide 6 and the second part 62 of the
outer guide 6 and the end region of the first part 61 of the outer guide 6 and the
starting region of the second part 62 of the outer guide 6 are parallel or substantially
parallel in the overlapping region
dparallel, or at least in a part of it, such that the width of the slit is equal or substantially
equal along this part, the end region of the first part and the starting region of
the second part of the outer guide may be positioned at a very small angle γ ≤
3° as can be seen in the detail insert in Fig. 2.
[0040] In this particular embodiment, the inner guide 7 has again four inflection points
- A, B, C, D. The inclination of the two parts of the inclined wall of the inner guide
is identical to the inclination given in example 1, i.e. the first part 71 of the
inclined wall is inclined by
α =
15° with respect to the longitudinal axis of the steam turbine assembly and the second
part 72 of the inclined wall is inclined by
β =55° with respect to the longitudinal axis of the steam turbine assembly, but the
second and the last inflection points are positioned at a distance from the centerline
31 of the last stage blade that is shorter than in example 1.
[0041] Thus, the ratio of the axial length
LAL to the length
L1 is between 1,28 - 1,31. The ratio of the distance
xE of the endpoint E from the centerline 31 of the last stage blade and
L1 is 0,46 - 0,49 and the ratio of the outlet area
S2 to the inlet area
S1, is approximately 1,48.
Example 3
[0042] In example 3 and with the reference to Fig. 3 and Table 1, the length of the last
stage blade is
LLSB =
1370 mm, the distance between the base 2 of the last stage blade and the stator body above
the tip of that blade is
L1=
1390 mm and the diameter of the rotor disc is
Dp = 2050 mm. Although these basic dimensions are not shown in Fig. 3, they are measured
in the same way as shown in Fig, 1.
[0043] The outer guide 6 comprises two parts 61 and 62 and a slit 63 between them, through
which a steam is tangentially blown into the inner space of the diffuser. The slit
63 is positioned as defined in example 2.
[0044] In this particular embodiment, the inner guide 7 comprises three inflection points
- A, C, D. The first inflection point A is positioned behind the base of the last
stage blades 3 and it is identical with the beginning of the inclined wall 70 of the
inner guide 7. The inclined wall 70 of the inner guide is connected to the rotor casing
sealing (not shown). There is no fixed connection between the inclined wall of the
inner guide and the base of the last stage blades. The last inflection point D is
positioned in the intersection of a side wall 42 and the top 41 of the exhaust casing
4. The second inflection point C is positioned in the intersection of the inclined
inner guide 7 and the side wall 42 of the exhaust casing 4.
[0045] In this particular embodiment, there is no third inflection point, so that the inclined
wall 70 of the inner guide 7 is only formed of one part, which is inclined by 42°
with respect to the longitudinal axis of the steam turbine assembly.
[0046] The ratio of the axial length
LAL to
L1 is between 1,36 and 1,40. The ratio of the distance
xE of the endpoint E from the centerline 31 of the last stage blade 3 and
L1 lies between 0,46 and 0,5 and the ratio of the outlet area
S2 to the inlet area
S1, is approximately 1,8.
Example 4
[0047] The diffuser according to the present invention may form part of the exhaust casing
and may further be combined with a guiding rib 8 as shown in all three examples 1,
2, and 3, as can be seen in Figs, 1, 2 and 3. The guiding rib 8 is attached to the
upper wall 41 of the exhaust casing 4, extending radially inwards and oriented perpendicularly
to the longitudinal axis of the system.
[0048] The guiding rib extends from the horizontal plane of the longitudinal axis of the
system and follows the cylindrical shape of the upper wall of the exhaust casing.
It may have an I-shape or a T-shape in cross-section.
[0049] The guiding rib 8 is positioned at a distance ranging between 1.2*
m and 1.9*
m from the inner surface of the side wall 42 of the turbine casing, wherein the parameter
m is defined as a distance of an end point of an outer guide of a diffuser of the
steam turbine system from the inner surface of the side wall, i.e.,
LAL -
xE.
[0050] The application of the guiding rib leads to further efficiency improvement as it
serves as a guide for the steam exiting the outlet 51 of the diffuser guiding channel
5, thus reducing collisions within the flow.
Example 5
[0051] The above described configurations of the exhaust system lead to the efficiency improvement
of up to 6% of last stage blade efficiency in comparison with traditional shape of
low pressure steam turbine diffuser. The efficiency improvement can be seen in Fig.
4. The efficiency of last stage of the steam turbine and the diffuser according to
the present invention is shown. It is given for a volumetric flow of a steam through
the last stage and the diffuser. It can also be seen that the efficiency improvement
is stable over a relatively wide range of the volumetric flow. Accordingly, the geometry
of the present invention can be applied widely and to various operation regimes.
1. An exhaust diffuser for a steam turbine comprising an inlet area (S1) and an outlet area (S2), an inner guide (7) and an outer guide (6) having an endpoint (E), wherein the inner
guide (7) comprises an inclined wall (70, 71, 72) with at least two inflection points
(A, B, C, D), wherein the inclination of the inclined wall (70, 71, 72) with respect
to the longitudinal axis of the steam turbine is given at the inflection points, wherein
the inner guide (7) is adapted so that a ratio of the axial length (LAL) of the exhaust diffuser to the distance (L1) between a base (2) of the last stage blade and a stator body above the tip of the
last stage blade (3) is between 1,25 and 1,5, and wherein the outer guide (6) is adapted
so that the ratio of the outlet area (S2) to the inlet area (S1) is between 1,45 and 1,85.
2. The exhaust diffuser according to claim 1, wherein the ratio of the distance (xE) of the endpoint (E) of the outer guide (6) of the exhaust diffuser to the distance
(L1) between the base (2) of the last stage blade and the stator body above the tip of
the last stage blade (3) is between 0,45 and 0,6.
3. The exhaust diffuser according to claim 1 or 2, wherein the inner guide (7) comprises
at least three inflection points (A, B, C, D) and / or the inclination of the inclined
wall (70) with respect to the longitudinal axis of the turbine is given at a first
inflection point (A) by an inclination angle (α) of 40° to 45°, preferably 42°.
4. The exhaust diffuser according to claim 3, wherein the ratio of the axial length (LAL) of the exhaust diffuser to the distance (L1) between the base (2) of the last stage blade and the stator body above the tip of
the last stage blade (3) is between 1,36 and 1,40, the ratio of the distance (xE) of the endpoint (E) of the outer guide (6) of the exhaust diffuser to the distance
(L1) between the base (2) of the last stage blade and the stator body above the tip of
the last stage blade (3) is between 0,46 and 0,5, and the ratio of the outlet area
(S2) to the inlet area (S1) is about 1,8.
5. The exhaust diffuser according to claim 1 or 2, wherein the inner guide (7) comprises
four inflection points (A, B, C, D) and / or the inclination of the inclined wall
(71, 72) with respect to the longitudinal axis of the turbine is given at a first
inflection point
(A) by an inclination angle (α) of 10° to 20°, preferably 15° and at the third inflection
(B) point by an inclination angle (β) of 50° to 60°, preferably 55°.
6. The exhaust diffuser according to anyone of the preceding claims, wherein the inner
(7) guide has no more than four inflection points.
7. The exhaust diffuser according to anyone of the preceding claims, wherein the first
inflection point (A) is identical with the beginning of the inclined wall (70, 71,
72).
8. The exhaust diffuser according to anyone of the preceding claims, wherein the inclined
wall (70, 71, 72) of the inner guide (7) is adapted to be connected to a rotor casing
sealing.
9. The exhaust diffuser according to anyone of the preceding claims, wherein the outer
guide (6) comprises one part.
10. The exhaust diffuser according to anyone of the preceding claims 1 to 8, wherein the
outer guide comprises a first part (61) having an end region and a second part (62)
having a starting region, the first (61) and the second (62) part being separated
by a slit (63) for blowing in a steam into the exhaust guiding channel (5) such that
the end region of the first part (61) of the outer guide (6) and a starting region
of the second part (62) of the outer guide (6) overlap to create an overlapping region
and, in this overlapping region, or at least in a part (dparallel) of it, the end region of the first part (61) of the outer guide (6) and the starting
region of the second part (62) of the outer guide (6) are substantially parallel.
11. The exhaust diffuser according to claim 10, wherein the end region of the first part
(61) of the outer guide (6) and the starting region of the second part (62) of the
outer guide (6) are positioned at an angle (γ) of less than 5°, preferably at an angle
of less than 3°.
12. The exhaust diffuser according to any of the preceding claims further comprising an
upper wall (41) and a guiding rib (8) attached to an upper wall (41) and extending
radially inwards.
13. The exhaust diffuser according to claim 12, wherein the guiding rib (8) is positioned
at a distance ranging between 1.2*m and 1.9*m from the inner surface of the side wall of the turbine casing, wherein the parameter
m is defined as a distance of the end point of an outer guide of a diffuser of the
steam turbine system from the inner surface of the side wall.
14. The exhaust diffuser according to claim 12 or 13, wherein the height of the guiding
rib (8) lies within the range of 0.05*H to 0.15*H, wherein H is the height of the steam turbine system between the horizontal plane of the longitudinal
axis and the highest point of the upper wall.
15. A turbine comprising an exhaust diffuser according to any one of claims 1 to 14.