Alstom Steam Turbine and Cycle Design with Integrated

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____________________________________________________________________________________________________________________ PowerGen International 2007 Klotz Pickering Brandt 11.-13. December 2007 © ALSTOM 2007. All rights reserved. Information contained in this document is provided without liability for information purposes only and is subject to change without notice. No representation or warranty is given or to be implied as to the completeness of information or fitness for any particular purpose.

Alstom Steam Turbine and Cycle Design with Integrated Boiler Feed Pump Drive for High

Efficiency Coal-Fired Power Plants

Heinrich Klotz1, Eric Pickering2, Christoph Brandt1

Alstom 1Mannheim, Germany

2Richmond, USA

Presented at the Power-Gen International Conference on 11-13 December 2007

ABSTRACT

The worldwide growing electricity demand increases the pressure to conserve both resources and

environment. Modern, high efficiency coal-fired power plants can significantly contribute to

fulfil these obligations. In this context, the steam cycle design and parameters play a major role.

In particular, increased turbine inlet pressures and temperatures contribute significantly to

improved efficiency of modern power plants.

The paper provides an overview of the Alstom steam turbine design for supercritical high

temperature (ultra-supercritical) applications as applied to the recently awarded projects in the

USA and Europe. Among them, there are the largest single shaft units worldwide. In addition to

sophisticated cycle parameters, advanced 3D blade design, effective sealing technologies, and

top performing large, last stage blades ensure outstanding efficiency levels. Furthermore, special

attention is given to operational aspects. Thermal elastic design enables short start-up times and

can cope with changing operation requirements over the lifetime of a power plant. Alstom’s

unique shrink ring high-pressure inner casing design and the welded rotor technology provide

maximum operational flexibility.

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For maximum output a turbine driven boiler feed water pump is used. The performance and

operational flexibility of this drive and its integration in the main turbine system contribute

significantly to the economics of the power plant. Alstom’s integrated solution ensures overall

optimization and uses synergies in planning and during the erection and commissioning phase.

To minimize the specific investment cost a maximum unit output accompanied by a compact

turbine layout is required. Alstom’s current high capacity steam turbine generator sets in

combination with a turbine driven feed-water pump provide maximum possible power outputs of

approximately 1’000 MW in 60 Hz grids and 1’200 MW in 50 Hz grids in a single shaft

configuration. Large low-pressure exhaust areas and the single bearing design lead to a cost

effective, compact turbine layout. Maximum possible power output and compact design

combined with top class performance and low maintenance requirements enable lower electricity

generation costs while saving resources and minimizing environmental impact.

INTRODUCTION

Today, coal plays the most important role as fuel in electricity generation worldwide.

Approximately 39% of the electricity generation is based on coal-fired power plants. Especially

in countries with large local coal deposits, the contribution can be much higher, e.g. 95% in

Poland, 90% in South Africa, 85% in Australia, 80% in China (all values are approx., 2002 [1]).

Also in the USA (55%) and in Germany (50%) the share of coal as fuel in electricity generation

is very significant [2].

From today until the 2030s the electricity consumption will increase by more than 50%

worldwide [3]. Although the share of renewable energy sources and gas-fired power plants will

increase, the most important fuel for electricity generation worldwide will remain coal with a

nearly unchanged share of 38% [4]. Therefore, there is already great demand for new coal-fired

power plants not only to meet the growing electricity demand in emerging markets like China

and India, but also to replace old, less efficient coal-fired power plants in the USA and Europe.

The advantages of coal as fuel compared to other fossil fuels like oil or gas are the relatively low

price level, the long-term availability, and the diversity of sources around the world. The most

significant disadvantage of coal is the CO2 emission per electric work unit which is much higher

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than for gas and renewable energy sources. Due to the latest discussions on climate change, the

necessity to reduce CO2 emissions of coal-fired power plants is of highest importance in order to

obtain regulatory and community acceptance. Because large scale CO2 capture technologies are

not yet available, increasing plant efficiency is today the most promising way to reduce the CO2

emissions of coal-fired power plants. Fortunately, in a certain range, efficiency gain correlates

well with economic gain, especially if the follow-up costs of the CO2 emissions are taken into

account. Today, coal-fired power plants with ultra-supercritical (USC) steam cycle parameters

and wet cooling tower can achieve efficiencies of approximately 46% for hard coal and 43% for

brown coal at typical Central European inland sites (efficiencies based on LHV). With direct

cooling, an additional percent improvement is possible.

To date, Alstom has received orders for five USC steam turbines for the 50 Hz market and

awards for further eight. Most of them are located in Germany. Among them, there are the

world’s largest single shaft turbines, which will work with the most sophisticated steam cycle

parameters, i.e. main steam pressure of 3990 psia (27.5 MPa), main steam temperature of 1110°F

(600°C), and reheat temperature of 1150°F (620°C) at the turbine inlets. The latter temperature is

Figure 1: Recent orders and awards of Alstom USC steam turbine generator sets in Europe and USA

Europe Output up to

Main steam pressure up to Main steam temperature up to

Reheat steam temperature up to USA

Output up to Main steam pressure up to Main steam temperature up to Reheat steam temperature up to

13 units 12.3 GW 1,114 MW 3,990 psia 1,112 °F 1,148 °F 2 units 1.6 GW 965 MW 3,510 psia 1,112 °F 1,125 °F

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close to the limit suitable for currently available ferritic/martensitic materials. In 2006, Alstom

received the first order for a 60 Hz USC steam turbine in the 700 MW class and expects further

orders to come soon for USC steam turbines up to 1’000 MW.

INCREASING STEAM CYCLE PARAMETERS

The increase of steam cycle

parameters is the most

promising way to improve the

efficiency of modern coal-fired

power plants. Raising the main

steam temperature by 18°F

(10 K) results in an plant

efficiency gain of

approximately 0.25%

(relative), whereas 18°F (10 K)

more in reheat temperature

leads to an approximately

0.2%-gain (relative) in plant

efficiency. On top of that, there

is a further efficiency gain by increasing the main steam pressure. Supercritical steam conditions

established at the end of last decade are a main pressure of approximately 3770 psia (26.0 MPa)

with temperatures of 1050°F (565°C) for the main steam and 1085°F (585°C) for the reheat.

Since the beginning of the new decade, these steam cycle parameters have been further

significantly raised due to the strongly increased pressure to serve both resources and

environment. Today, the temperatures have reached 1110°F (600°C) for the main steam and

1150°F (620°C) for the reheat steam. Together with the temperatures, the main steam pressure

has been increased up to 3990 psia (27.5 MPa) at turbine inlet and accordingly the reheat

pressure up to 870 psia (6.0 MPa). Steam cycle parameters with a main steam pressure above

3625 psia (25.0 MPa) and temperatures above 1050°F (565°C) are often called ultra-supercritical

(USC). Figure 2 shows the development of steam cycle parameters in the past and their impact

on the overall plant net efficiency.

Figure 2: Development of cycle parameters in the last years and their impact on the overall plant efficiency (relative)

2465 psia1000°F1000°F

+5.5%

3770 psia1050°F1085°F

3770 psia1110°F1110°F

3990 psia1110°F1150°F

5080 psia1290°F1330°F

subcriticalcycle

supercriticalcylce

USCcycle

1300°F/700°Ctechnology

state of the art

+6.2%

+11.5%

base line

+4.2%

chan

ge in

ove

rall

plan

t net

effi

cien

cy (r

elat

ive)

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The Alstom steam turbine is capable to work with USC steam cycle parameters up to highest

power outputs. An Alstom STF100 (Alstom's largest turbine frame) together with the GIGATOP

generator can generate a power output up to 1’000 MW for 60 Hz or up to 1’200 MW for 50 Hz

applications. These turbine generator sets are the world’s largest tandem compound units for

USC applications. They contribute to cost efficient electricity generation while saving resources

and minimizing environmental impact.

MODULAR STEAM TURBINE DESIGN

Alstom’s modular steam turbine design has been well established in the market for many years.

Each turbine section can be selected from a wide range of volumetric variants and, for the low-

pressure section, of last stage blade lengths in order to cover a spectrum of 100 MW up to

1’200 MW power output with different cold end designs. The steam path in the high-pressure

(HP) and intermediate-pressure (IP) section is specifically adapted and optimized for each

project. The low-pressure (LP) steam path design is standardized. Thanks to the modular

concept, the materials of turbine parts such as the inner and outer casing can be adapted easily to

match the requirements of steam cycle parameters. This specific material selection is also

possible for the rotor sections because of its welded design. The utmost thermal flexible design is

able to cope with all changing operation requirements (e.g., change from base load to cycling

operation) during the lifetime of the unit, even with USC steam cycle parameters. Figure 3 shows

an HP and an IP turbine module with its valve arrangement.

Figure 3: HP and IP turbine modules

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IMPROVING STEAM TURBINE PERFORMANCE

In parallel with increasing steam cycle parameters, it is necessary to improve the performance of

the steam turbine itself. There are at least three key features on which Alstom has worked in

recent years:

• advanced 3D blade design with minimized profile and secondary losses,

• improved sealing technologies to reduce sealing losses, and

• advanced top performing last stage blades in high strength steel and titanium to limit the

exhaust losses and/or to reduce the number of LP flows.

Advanced 3D blade design

Alstom uses reaction type blading with 3D state of the art profiles. This blading is adapted

specifically for the HP and IP turbine for each application. The blades are milled in one piece

from bar material with integrated shrouds and roots. They are assembled pre-twisted to form a

clearance-free ring with excellent vibration behaviour.

This well proven blade design has been

further improved in recent years,

particularly by minimizing secondary

losses, improving the aerodynamics at

the trailing edge and reducing the gap

and leakage flow interaction with the

main steam flow. To reduce the

secondary losses, special attention has

been given to the geometry at the

intersections of the airfoil with the

endwalls. Optimized compound radii

smooth these intersections in order to

reduce the endwall losses. A thin trailing edge improves the aerodynamics and reduces the

profile losses. However, the aerodynamic optimization is limited by the mechanics as well as by

the machining. The trailing edge thickness has been carefully optimized with respect to the

aerodynamics as well as with respect to the mechanical integrity and the manufacturability. The

gap and leakage flow interaction has been optimized by adaptations in the steam path geometry.

Figure 4: Alstom reaction type blading with 3D state of the art profiles

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The stage efficiency gain of all three mentioned improvement measures can accumulate to

approximately 0.7%, depending on the aspect ratio. All three measures require high

manufacturing and quality standards. Figure 4 shows the Alstom reaction type blading with 3D

state of the art profiles.

Improved sealing technologies

For USC applications, the sealing of glands and pistons becomes more important, especially for

the HP turbine. Due to the higher-pressure differences, the driving forces for the leakage flow

become stronger and the leakage flow would increase with standard sealing technology. In

addition to standard fin-strip-sealing, Alstom has developed two advanced sealing technologies

in recent years in order to reduce the leakage flows.

The brush seal consists of a flexible

super alloy bristle pack, held within a

welded front plate-backing ring

assembly, which replaces some of fin-

strip-pairs in a standard sealing section.

The flexible bristle pack allows the

reduction of the clearance nearly to zero.

Brush seals evaluate especially well on

the HP piston and in the HP glands for

USC applications, although its appliance

is neither limited to USC applications

nor to the HP turbine piston and glands.

The brush seal itself and its arrangement

in a sealing section of a HP turbine is

shown in Figure 5.

Abradable coatings can be used to reduce the clearances in standard fin-strip-seals without the

risk of metal to metal rubbing. The coating consists of a thin layer of metallo-ceramic material

applied to the static surfaces in the sealing sections.

Figure 5: Brush seal itself (top, left) and arranged in asealing section of a HP turbine piston (top, right andbottom)

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Advanced top performing last stage blades

The last stage blade (LSB) is of essential importance for the overall turbine efficiency. In

addition to the efficiency of the LSB itself, the exhaust area plays a central role because the

exhaust losses are much higher than for the HP and IP turbine and cannot be recovered.

Furthermore, the share of the LP turbine on the total turbine cost is superior. Therefore, a wide

spread of LSB with different exhaust areas is beneficial. It allows the optimization of the cold

end design with regard to both efficiency and economics. Using a larger exhaust area can reduce

the exhaust losses (improved efficiency) or eliminate the need for an extra LP casing (optimized

cost).

Currently, the Alstom LP turbine family includes

LSB from 25 in length up to 42 in for the 60 Hz

market and up to 49 in for the 50 Hz market. The

latest developments are the large steel LSB with a

length of 38 in for 60 Hz applications and 45 in for

50 Hz applications and the titanium LSB with a

length of 42 in and 49 in (Figure 6), respectively.

Today's high strength steel and lightweight titanium

give more freedom to design the airfoil, particularly

for longer blades. As a result, the stage efficiencies

of these new LSB have been increased remarkably.

In addition, the new LSB are equipped with

shrouds to reduce the leakage flow over the tip

compared to freestanding blades. At the same time

the shrouds, together with snubbers, enable a rigid

and robust blade ring with excellent vibration

behaviour. Following the latest LSB design, the

diffuser and the exhaust have been redesigned as

well in order to get best efficiencies over a wide

operating range. Naturally, the new LP turbines based on these LSB are also equipped with the

well-known 360° inlet scroll with radial-axial first stage. The LP turbines with advanced LSB

design have been well received by the market. The first units will be in operation in 2009.

Figure 6: 49 in titanium LSB (left) withdetailed shroud section (top, right) andintegrated optimization of diffuser andexhaust (bottom, right.

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OPERATIONAL ASPECTS OF STEAM TURBINES FOR USC APPLICATIONS

USC steam cycle parameters with temperatures up

to 1110°F (600°C) for the main steam and 1150°F

(620°C) for the reheat steam together with a main

steam pressure up to 3990 psia (27.5 MPa) increase

the requirements on the turbine with regard to

operational aspects. Both the increase of the

absolute level and the differences of temperatures

and pressures along or across the turbine modules

often result in longer turbine modules and thicker

casing walls, even with the application of new

materials. All the mentioned points can result in

stronger casing distortions with negative impact on

long-term efficiency and reliability.

Shrink ring design of the HP turbine

The unique shrink ring design of the Alstom HP

turbine was introduced in the 1960s. It eliminates

bolt flanges on the inner casing and results in a

radially symmetrical structure with best thermo-

elasticity. Therefore, the shrink ring design

prevents casing distortions almost completely,

which is of essential importance for USC

applications. The benefits are long-term stable

clearances and sustained efficiencies combined

with long-term reliability and operational

flexibility. HP steam extraction in order to increase

the final feed water temperature can simply be done with a shrunk-on extraction chamber. Due to

the double shell design the outer casing is exposed to the exhaust steam only, which allows

relatively small flanges at the outer casing. The assembly of the HP turbine with its shrink rings

is shown in Figure 7.

Figure 7: Shrink ring design of the HPturbine (assembly of inner casing, shrinkring, outer casing)

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Design features of the IP turbine

The standard Alstom IP turbine design is also of a double shell design. The lower pressure level

allows a horizontal split of the casings with conventional flanges. In order to minimize the casing

distortions for very high temperature applications the design of the inner casing has been

modified. This advanced design allows small and stable clearances also at highest temperatures,

which result in best and sustained efficiency levels. The material selection of the inner casing is

done accordingly to the requirements of pressure and temperature. Thanks to the inlet scrolls

with the integrated radial first stationary blade row and the welded rotor design a secondary

steam cooling with its negative impact on the efficiency is not required, not even for the highest

reheat temperatures of today.

Welded rotor design

The welded rotor design is a key feature of Alstom steam turbines since the 1930s. It allows the

material selection of each rotor section to match the respective temperature of the steam path.

Therefore, welded rotors can be easily adapted to USC steam cycle parameters. Because of the

cavity formed by the two welded rotor sections, the stress levels due to temperature differences

are lowered. This design allows a faster start-up and/or lower life consumption rates compared to

monoblock rotors. Further, the small forgings are easier to obtain on the market and to test than

big forgings for monoblock rotors.

COMPACT TURBINE LAYOUT TO SAVE SPECIFIC INVESTMENT COST

A compact turbine generator layout contributes to lower specific investment cost of the overall

power plant. A short length of the turbine generator set is especially important, because it

directly saves turbine hall length. The Alstom single bearing design, the large exhaust areas by

either long steel or titanium last stage blades, and the high capacity GIGATOP turbogenerator

enable a power output up to 1’000 MW in 60 Hz grids and up to 1’200 MW in 50 Hz grids with

top performance in a very compact layout. This compact turbine layout can be supported by

using a boiler feed pump turbine, which provides additional exhaust area.

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Single bearing concept

The single bearing concept of Alstom steam turbine generators is well proven and worldwide

established since many years. It significantly reduces the steam turbine generator shaft length.

Additionally, the single bearing concept ensures a long-term smooth and stable operation. Due to

fewer bearings, maintenance costs are reduced.

Large exhaust area

Alstom’s largest steel LSB for 60 Hz applications is 38 in long and offers an exhaust area of

about 80 ft2 (7.4 m2) per flow. The corresponding 50 Hz LSB is 45 in long and offers 115 ft2

(10.7 m2) per flow. Alstom’s largest titanium LSB has a length of 42 in for 60 Hz and 49 in for

50 Hz applications. The corresponding exhaust areas are 99 ft² (9.2 m²) and 142 ft² (13.2 m²),

respectively. These large LSB can be used not only to improve the efficiency by reducing the

exhaust losses, but also to save a

LP casing. Using the 49 in LSB

for typical Central European

inland site conditions with a wet

cooling tower allows Alstom to

offer a 1’100 MW steam turbine

generator with only four casings

and a very competitive

efficiency level. The optimal

solutions for the 60 Hz market

are a 6 LP flow arrangement for

the 1’000 MW output class and

4 LP flow for the 700 MW

output class (Figure 8). Alstom

is well experienced with 6 LP

flow arrangements and has

several references with extensive

operating experience.

Figure 8: Steam turbine generator set with well proven 6 LP flowand extremely compact 4 LP flow arrangement

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Boiler feed pump turbine

The compact turbine layout can be enhanced by a boiler feed pump turbine (BFPT). The BFPT

(Figure 9) extracts steam from the IP section of the main turbine and reduces the steam flow

through the LP turbine. The BFPT helps to keep the exhaust losses of the main turbine at a

satisfactory level by enlarging the total exhaust area and can therefore improve overall plant

efficiency. Since the BFPT drive does not draw power from the generator, the BFPT enables the

plant operator to provide the utmost power to the grid.

In contrast to the main turbine, the

BFPT has to cover a more or less

wide range of swallowing capacity.

For instance, at summer conditions

the condenser pressure of the BFTP

increases compared to average

conditions. The higher condenser

pressure reduces the length of the

expansion line whereas the needed

power output of the BFPT stays

constant. As a result, the BFPT

consumes more steam. If the

extraction point stays in the IP

section of the main turbine, the

needed swallowing capacity of the

BFPT increases. Therefore, the

summer conditions typically define

the design case for the swallowing capacity of the BFPT. In the best case (design case) of the

main turbine, which is normally at average conditions, the BFPT operates at part load and has to

be throttled to adapt its swallowing capacity. The efficiency of the BFPT decreases with

increasing throttling which can be more than 10%. As an alternative, the extraction point can be

switched to the cold reheat line. But the negative impact on the overall cycle efficiency is

enormous and normally overweighs the reduction of the BFPT efficiency by throttling.

Figure 9: 3D model and drawing of a BFPT

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To get the best efficiency for the BFPT over a more or less wide range of swallowing capacity,

Alstom can offer two inlet configurations for the BFPT: with and without control stage. Whereas

the first offers better efficiencies at partial loads, the second has the better efficiency at full load.

Both types can be provided with steam from either the IP section of the main turbine or the cold

reheat line. A combination of both (dual admission) is also possible. Depending on the operating

mode of the main turbine, the selection of the inlet configuration in combination with the steam

extraction point must be optimized specifically for each application. To adapt the exhaust area of

the BFPT to different cooling conditions and power output ranges, the BFTP can be a single- or

double-flow with two different length of LSB. Moreover, the preferred 100% BFPT and pump

solution reduces the overall cost. Therefore, this solution is the optimal one for the largest power

output classes from both an economical and efficiency point of view. Of the fourteen steam

turbine generator sets for USC applications ordered/awarded to Alstom, seven will be equipped

with an Alstom BFPT.

GIGATOP TURBOGENERATOR

For full speed applications with high

output, Alstom has developed and

optimized the GIGATOP turbogenerator

(Figure 10) over the last decades. The

GIGATOP modular design matches

power output requirements from

500 MVA up to 1’400 MVA. Special

design features contribute to the

excellent performance values of Alstom

steam turbine generator for USC

applications.

The previously mentioned single bearing concept together with the use of static excitation design

results in a very short overall lengths. Further unique design features like stainless steel hollow

conductors within the stator bars, Micadur® insulation of the bars, and laminated press plates at

both ends of the stator core lead to outstanding availability and efficiency. These performance

Figure 10: GIGATOP turbogenerator in operation

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figures are proven by the large operational experience of the Alstom GIGATOP fleet, which

includes the largest two-pole turbogenerators in operation worldwide (Lippendorf R&S). This

number one position is further extended with the 1’100 MW applications shown in Figure 1.

CONCLUSIONS

The worldwide growing electricity demand increases the pressure to conserve both resources and

environment. Modern high efficiency coal-fired power plants with USC steam cycle parameters

can significantly contribute to fulfil these obligations. Currently, the parameters have reached

3990 psia (27.5 MPa) for the main steam pressure, 1110°F (600°C) for the main steam

temperature, and 1150°F (620°C) for the reheat temperature. With these steam cycle parameters,

overall efficiencies of coal-fired power plants with wet cooling tower of approximately 46% for

hard coal and 43% for brown coal (both based on LHV) at typical Central Europe inland sites are

achievable. In parallel with increasing steam cycle parameters, Alstom has worked on three key

features to improve the steam turbine efficiency itself: advanced 3D blade and steam path design,

improved sealing technologies, and advanced top performing last stage blades with optimized

diffuser and exhaust. The higher requirements on steam turbines for USC applications

concerning operational aspects are met by well proven Alstom design features like the shrink

ring design of the HP inner casing and the welded rotor design. Additionally, some minor

adaptations of the standard design, e.g. as for the IP casing, have been implemented. The single

bearing concept together with large exhaust areas allow Alstom to offer a compact four-casing

1’100 MW steam turbine generator set for 50 Hz USC applications at a very competitive

efficiency level. An Alstom 100% boiler feed pump turbine can further improve such solutions.

Due to the reduced exhaust area available for LP turbines in 60 Hz applications, a 6 LP flow

arrangement is typically the superior solution for highest capacity units. This well proven

arrangement enables Alstom to build high efficient steam turbine generators set up to 1’000 MW

for 60 Hz grids in a tandem compound design. Such large steam turbine generator sets,

consisting of the Alstom steam turbine STF100 and the GIGATOP turbogenerator, allow low

electricity generation cost while saving resources and minimizing environmental impact.

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REFERENCES

[1] W. Reichel, Langfristige Kohleverstromung in Deutschland trotz CO2-Restriktionen? Glückauf 140 (2004), Nr. 1/2, pp. 48-53.

[2] World Energy Outlook 2006, International Energy Agency.

[3] International Energy Outlook 2007,

Energy Information Administration. [4] The Coal Resource – A Comprehensive Overview of Coal

World Coal Institute, 2005. [5] A. Tremmel and D. Hartmann,

Efficient steam turbine technology for fossil fuel power plants in economically and ecologically driven markets, VGB Power Tech, 11/2004, pp. 38-43.

[6] A. Tremmel, H. Mandel, U. Klauke, C. Brandt

Modernste Turbinentechnologie mit höchsten Dampftemperaturen für das Kraftwerk Boxberg, Block R, VGB Power Tech, 12/2006, pp. 71-75.

Documents

Alstom Steam Turbine and Cycle Design with Integrated