EXHAUST DIFFUSER FOR A STEAM TURBINE AND CORRESPONDING TURBINE

Abstract

 The present invention relates to 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 (L_AL) of the exhaust diffuser to the distance (L₁) between the base (2) of the last stage blades (3) and the 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. The exhaust diffuser with the geometry as defined above provides for the improvement of efficiency of the exhaust diffuser assembly, while it allows keeping the axial length of the diffuser as short as possible.

Description

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 L_LSB =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 D_p = 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 L_AL of the exhaust diffuser. The ratio of the axial length L_AL 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 D_P 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 = L_AL - x_E (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 L_LSB =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 D_p = 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 d_parallel, 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 L_AL to the length L₁ is between 1,28 - 1,31. The ratio of the distance x_E 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 L_LSB =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 D_p = 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 L_AL to L₁ is between 1,36 and 1,40. The ratio of the distance x_E 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., L_AL - x_E.

[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.

Claims

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 (L_AL) 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 (x_E) 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 (L_AL) 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 (x_E) 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 (d_parallel) 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.

Drawing