HISTORY

The first device that may be classified as a reaction steam turbine was little more than a toy, the classic
Acolipile, described in the 1st century by Greek mathematician Hero of Alexandria in Roman Egypt. In
1551, Taqi al-Din in Ottoman Egypt described a steam turbine with the practical application of rotating a
spit. Steam turbines were also described by the Italian Giovanni Branca (1629) and John Wilkins in
England (1648). The devices described by Taqi al-Din and Wilkins are today known as steam jacks.

The modern steam turbine was invented in 1884 by Sir Charles Parsons, whose first model was
connected to a dynamo that generated 7.5 kW (10 hp) of electricity. The invention of Parsons' steam
turbine made cheap and plentiful electricity possible and revolutionized marine transport and naval
warfare. Parsons' design was a reaction type. His patent was licensed and the turbine scaled-up shortly
after by an American, George Westinghouse. The Parsons turbine also turned out to be easy to scale up.
Parsons had the satisfaction of seeing his invention adopted for all major world power stations, and the
size of generators had increased from his first 7.5 kW set up to units of 50,000 kW capacity. Within
Parson's lifetime, the generating capacity of a unit was scaled up by about 10,000 times, and the total
output from turbo-generators constructed by his firm C. A. Parsons and Company and by their licensees,
for land purposes alone, had exceeded thirty million horse-power.

A number of other variations of turbines have been developed that work effectively with steam. The de
Laval turbine (invented by Gustaf de Laval) accelerated the steam to full speed before running it against
a turbine blade. De Laval's impulse turbine is sipler, less expensive and does not need to be pressure-
proof. It can operate with any pressure of steam, but is considerably less efficient. fr:Auguste Rateau
developed a pressure compounded impulse turbine using the de Laval principle as early as 1900,
obtained a US patent in 1903, and applied the turbine to a French torpedo boat in 1904.

He taught at the École des mines de Saint-Étienne for a decade until 1897, and later founded a
successful company that was incorporated into the Alstom firm after his death. One of the founders of
the modern theory of steam and gas turbines was Aurel Stodola, a Slovak physicist and engineer and
professor at the Swiss Polytechnical Institute (now ETH) in Zurich. His work Die Dampfturbinen und ihre
Aussichten als Wärmekrafonaschinen (English: The Steam Turbine and its prospective use as a
Mechanical Engine) was published in Berlin in 1903. A further book Dampf und Gas-Turbinen (English:
Steam and Gas Turbines) was published in 1922.

The Brown-Curtis turbine, an impulse type, which had been originally developed and patented by the
U.S. company International Curtis Marine Turbine Company, was developed in the 1900s in conjunction
with John Brown & Company. It was used in John Brawn- engined merchant ships and warships,
including liners and Royal Navy warships.

Introduction:

A turbine is a device that converts chemical energy into mechanical energy, specifically when a rotor of
multiple blades or vanes is driven by the movement of a fluid or gas. In the case of a steam turbine, the
pressure and flow of newly condensed steam rapidly turns the rotor. This movement is possible because
the water to steam conversion results in a rapidly expanding gas.

As the turbine's rotor turns, the rotating shaft can work to accomplish numerous applications, often
electricity generation.
Working:

In a steam turbine, the steam's energy is extracted through the turbine and the steam leaves the turbine
at a lower energy state. High pressure and temperature fluid at the inlet of the turbine exit as lower
pressure and temperature fluid. The difference is energy converted by the turbine to mechanical
rotational energy, less any aerodynamic and mechanical inefficiencies incurred in the process. Since the
fluid is at a lower pressure at the exit of the turbine than at the inlet, it is common to say the fluid has
been "expanded" across the turbine. Because of the expanding flow, higher volumetric flow occurs at
the turbine exit (at least for compressible fluids) leading to the need for larger turbine exit areas than at
the inlet.

The generic symbol for a turbine used in a flow diagram is shown in Figure below. The symbol diverges
with a larger area at the exit than at the inlet. This is how one can tell a turbine symbol from a
compressor symbol. In Figure, the graphic is colored to indicate the general trend of temperature drop
through a turbine. In a turbine with a high inlet pressure, the turbine blades convert this pressure
energy into velocity or kinetic energy, which causes the blades to rotate.

Many green cycles use a turbine in this fashion, although the inlet conditions may not be the same as for
a conventional high pressure and temperature steam turbine. Bottoming cycles, for instance, extract
fluid energy that is at a lower pressure and temperature than a turbine in a conventional power plant. A
bottoming cycle might be used to extract energy from the exhaust gases of a large diesel engine, but the
fluid in a bottoming cycle still has sufficient energy to be extracted across a turbine, with the energy
converted into rotational energy.
Turbines also extract energy in fluid flow where the pressure is not high but where the fluid has
sufficient fluid kinetic energy. The classic example is a wind turbine, which converts the wind's kinetic
energy to rotational energy. This type of kinetic energy conversion is common in green energy cycles for
applications ranging from larger wind turbines to smaller hydrokinetic turbines currently being designed
for and demonstrated in river and tidal applications. Turbines can be designed to work well in a variety
of fluids, including ing a gases and liquids, where they are used not only to drive generators, but also to
drive compressors or pumps.

An additional use for turbines in industrial applications that may also be applicable in some green energy
systems is to cool a fluid. As previously mentioned, when a turbine extracts energy from a fluid, the fluid
temperture is reduced. Some industries, such as the gas processing industry, use turbines as sources of
refrigeration, dropping the temperature of the gas going through the turbine. In other words, the
primary purpose of the turbine is to reduce the temperature of the working fluid as opposed to
providing power.

Generally speaking, the higher the pressure ratio across a turbine, the greater the expansion and the
greater the temperature drop.

Even where turbines are used to cool fluids, the turbines still produce power and must be connected to
a power absorbing device that is part of an overall system.
An additional use for turbines in industrial applications that may also be applicable in some green energy
systems is to cool a fluid. As previously mentioned, when a turbine extracts energy from a fluid, the fluid
temperature is reduced. Some industries, such as the gas processing industry, use turbines as sources of
refrigeration, dropping the temperature of the gas going through the turbine. In other words, the
primary purpose of the turbine is to reduce the temperature of the working fluid as opposed to
providing power. Generally speaking, the higher the pressure ratio across a turbine, the greater the
expansion and the greater the temperature drop.

Also note that turbines in high inlet-pressure applications are sometimes called expanders. The terms
"turbine" and "expander" can be used interchangeably for most but expander is not used when referring
to kinetic energy applications, as the fluid does not go through significant expansion.

Advantages of Steam Turbines:

 Ability to use high Pressure & high Temperature.

 High Efficiency.

 High Rotational Speed.

 High capacity/weight ratio.

 Smooth nearly vibration free operation.

 No internal lubrication required.

 Oil free Exhaust System.

Disadvantages of steam Turbines:

For slow speed application reduction gears are required. The steam turbine cannot be made reversible.
The efficiency of small simple steam turbines is poor.

6. Types of Steam Turbines:

Steam turbines are made in a variety of sizes ranging from small <0.75 kW (<1 hp) units (rare) used as
mechanical drives for pumps, compressors and other shaft driven equipment, to 1 500 000 kW (1.5 GW:
2 000 000 hp) turbines used to generate electricity. There are several classifications for modern steam
turbines.

6.1.Blade & Stage Design

Turbine blades are of two basic types, blades and nozzles. Blades move entirely due to the impact of
steam on them and their profiles do not converge. This results in a steam velocity drop and essentially
no pressure drop as steam moves through the blades. A turbine composed of blades alternating with
fixed nozzles is called an impulse turbine, Curtis turbine, Rateau turbine, or Brown-Curtis turbine.
Nozzles appear simtar to blades, but their profiles converge near the exit. This results in a steam
pressure drop and velocity increase as steam moves through the nozzles. Nozzles move due to both the
impact of steam on them and the reaction due to the high-velocity steam at the exit. A turbine
composed of moving nozzles alternating with fixed nozzles is called a reaction turbine or Parsons
turbine.
Except for low-power applications, turbine blades are arranged in multiple stages in series, called
compounding, which greatly improves efficiency at low speeds. A reaction stage is a row of fixed nozzles
followed by a row of moving nozzles. Multiple reaction stages divide

the pressure drop between the steam inlet and exhaust into numerous small drops, resulting in a
pressure-compounded turbine. Impulse stages may be either pressure-compounded, velocity-
compounded, or pressure-velocity compounded. A pressure-compounded impulse stage is a row of
fixed nozzles followed by a row of moving blades, with multiple stages for compounding. This is also
known as a Rateau turbine, after its inventor. A velocity- compounded impulse stage (invented by Curtis
and also called a "Curtis wheel") is a row of fixed nozzles followed by two or more rows of moving
blades alternating with rows of fixed blades. This divides the velocity drop across the stage into several
smaller drops. A series of velocity-compounded impulse stages is called a pressure-velocity compounded
turbine.

6.2. Impulse turbines

In an impulse turbine, a fast-moving fluid is fired through a narrow nozzle at the turbine blades to make
them spin around. The blades of an impulse turbine are usually bucket-shaped so they catch the fluid
and direct it off at an angle or sometimes even back the way it came (because that gives the most
efficient transfer of energy from the fluid to the turbine). In an impulse turbine, the fluid is forced to hit
the turbine at high speed. Imagine trying to make a wheel like this turn around by kicking soccer balls
into its paddles.

You'd need the balls to hit hard and bounce back well to get the wheel spinning and those constant
energy impulses are the key to how it works. Water turbines are often based around an impulse turbine
(though some do work using reaction turbines).
6.3.Reaction turbines

In a reaction turbine, the blades sit in a much larger volume of fluid and turn around as the fluid flows
past them. A reaction turbine doesn't change the direction of the fluid flow as drastically as an impulse
turbine: it simply spins as the fluid pushes through and past its blades. Wind turbines are perhaps the
most familiar examples of reaction turbines.

If an impulse turbine is a bit like kicking soccer balls, a reaction turbine is more like swimming-in reverse.
Let me explain! Think of how you do freestyle (front crawl) by hauling your arms through the water,
starting with each hand as far in front as you can reach and ending with a "follow through" that throws
your arm well behind you. What you're trying to achieve is to keep your hand and forearm pushing
against the water for as long as possible, so you transfer as much energy as you can in each stroke. A
reaction turbine is using the same idea in reverse: imagine fast-flowing water moving past you so it
makes your arms and legs move and supplies energy to your body! With a reaction turbine, you want
the water to touch the blades smoothly, for as long as it can, so it gives up as much energy as possible.
The water isn't hitting the blades and bouncing off, as it does in an impulse turbine: instead, the blades
are moving more smoothly, "going with the flow."

7. Compounding in Steam Turbines:

Compounding of steam turbines is the method in which energy from the steam is extracted in a number
of stages rather than a single stage in a turbine. A compounded steam turbine has multiple stages i.e. it
has more than one set of nozzles and rotors, in series, keyed to the shaft or fixed to the casing, so that
either the steam pressure or the jet velocity is absorbed by the turbine in number of stages.

The steam produced in the boiler has very high enthalpy. In all turbines the blade velocity is directly
proportional to the velocity of the steam passing over the blade. Now, if the entire energy of the steam
is extracted in one stage, i.e. if the steam is expanded from the boiler pressure to the condenser
pressure in a single stage, then its velocity will be very high. Hence the velocity of the rotor (to which the
blades are keyed) can reach to about 30,000 rpm, which is pretty high for practical uses because of very
high vibration. Moreover at such high speeds the centrifugal forces are immense, which can damage the
structure. Hence, compounding is needed. The high velocity which is used for impulse turbine just
strikes on single ring of rotor that cause wastage of steam ranges 10% to 12%. To overcome the wastage
of steam compounding of steam turbine is used.

Types of Compounding

In an Impulse steam turbine compounding can be achieved in the following three ways: -

1. Velocity compounding

2. Pressure compounding

3. Pressure-Velocity Compounding

In a Reaction turbine compounding can be achieved only by Pressure compounding.

7.1.Velocity compounding of Impulse Turbine:

The velocity compounded Impulse turbine was first proposed by C G Curtis to solve the problem of
single stage Impulse turbine for use of high pressure and temperature steam.

The rings of moving blades are separated by rings of fixed blades. The moving blades are keyed to the
turbine shaft and the fixed blades are fixed to the casing. The high pressure steam coming from the
boiler is expanded in the nozzle first. The Nozzle converts the pressure energy of the steam into kinetic
energy. It is interesting to note that the total enthalpy drop and hence the pressure drop occurs in the
nozzle. Hence, the pressure thereafter remains constant.

This high velocity steam is directed on to the first set (ring) of moving blades. As the steam flows over
the blades, due the shape of the blades, it imparts some of its momentum to the blades and losses some
velocity. Only a part of the high kinetic energy is absorbed by these blades. The remainder is exhausted
on to the next ring of fixed blade. The function of the fixed blades is to redirect the steam leaving from
the first ring moving blades to the second ring of moving blades. There is no change in the velocity of the
steam as passes through the fixed blades. The steam then enters the next ring of moving blades; this
process is repeated until practically all the energy of the steam has been absorbed.
A schematic diagram of the Curtis stage impulse turbine, with two rings of moving blades one ring of
fixed blades is shown in above diagram. The Diagram also shows the changes in the pressure and the
absolute steam velocity as it passes through the stages.

where,

Pi= pressure of steam at inlet

Vi = velocity of steam at inlet

Po = pressure of steam at outlet

Vo = velocity of steam at outlet

In the above figure there are two rings of moving blades separated by a single of ring of fixed blades. As
discussed earlier the entire pressure drop occurs in the nozzle, and there are no subsequent pressure
losses in any of the following stages. Velocity drop occurs in the moving blades and not in fixed blades.

Optimum Velocity

It is the velocity of the blades at which maximum power output can be achieved▷ Hence, the optimum
blade velocity for this case is,
where n is the number of stages.

It is interesting to note that this value of optimum velocity is 1/n times that of the single stage turbine.
This means that maximum power can be produced at much low blade velocities.

However, the work produced in each stage is not the same. The ratio of work produced in a 2 stage
turbine is 3:1 as one move from higher to lower pressure. This ratio is 5:3:1 in three stage turbine and
changes to 7:5:3:1 in a four stage turbine.

Disadvantages of Velocity Compounding

 Due to the high steam velocity there are high friction losses

 Work produced in the low-pressure stages is much less.

 The designing and fabrication of blades which can withstand such high velocities is difficult.

7.2. Pressure compounding of Impulse Turbine

The pressure compounded Impulse turbine is also called as Rateau turbine, after its inventor. This is
used to solve the problem of high blade velocity in the single-stage impulse turbine.

It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing and the
blades are keyed to the turbine shaft.

In this type of compounding the steam is expanded in a number of stages, instead of just one (nozzle) in
the velocity compounding. It is done by the fixed blades which act as nozzles. The steam expands equally
in all rows of fixed blade. The steam coming from the boiler is fed to the first set of fixed blades i.e. the
nozzle ring. The steam is partially expanded in the nozzle ring. Hence, there is a partial decrease in
pressure of the incoming steam. This leads to an increase in the velocity of the steam. Therefore the
pressure decreases and velocity increases partially in the nozzle.

This is then passed over the set of moving blades. As the steam flows over the moving blades nearly all
its velocity is absorbed. However, the pressure remains constant during this process. After this it is
passed into the nozzle ring and is again partially expanded. Then it is fed into the next set of moving
blades, and this process is repeated until the condenser pressure is reached.
It is a three stage pressure compounded impulse turbine. Each stage consists of one ring of fixed blades,
which act as nozzles, and one ring of moving blades. As shown in the figure pressure drop takes place in
the nozzles and is distributed in many stages.

An important point to note here is that the inlet steam velocities to each stage of moving blades are
essentially equal. It is because the velocity corresponds to the lowering of the pressure. Since, in a
pressure compounded steam turbine only a part of the steam is expanded in each nozzle, the steam
velocity is lower than of the previous case. It can be explained mathematically from the following
formula i.e.
One can see from the formula that only a fraction of the enthalpy is converted into velocity in the fixed
blades. Hence, velocity is very less as compared to the previous case.

Disadvantages of Pressure Compounding

 The disadvantage is that since there is pressure drop in the nozzles, it has to be made air-tight.

 They are bigger and bulkier in size.

7.3. Pressure-Velocity compounded Impulse Turbine:

It is a combination of the above two types of compounding. The total pressure drop of the steam is
divided into a number of stages. Each stage consists of rings of fixed and moving blades. Each set of rings
of moving blades is separated by a single ring of fixed blades. In each stage there is one ring of fixed
blades and 3-4 rings of mo sing blades. Each stage acts as a velocity compounded impulse turbine.
The fixed blades act as nozzles. The steam coming from the boiler is passed to the first ring of fixed
blades, where it gets partially expanded. The pressure partially decreases and the velocity rises
correspondingly. The velocity is absorbed by the following rings of moving blades until it reaches the
next ring of fixed blades and the whole process is repeated once again.

7.4. Pressure compounding of Reaction Turbine:

As explained earlier a reaction turbine is one which there is pressure and velocity loss in the moving
blades. The moving blades have a converging steam nozzle. Hence when the steam passes over the fixed
blades, it expands with decrease in steam pressure and increase in kinetic energy.

This type of turbine has a number of rings of moving blades attached to the rotor and an equal number
of fixed blades attached to the casing. In this type of turbine the pressure drops take place in a number
of stages.
The steam passes over a series of alternate fixed and moving blades. The fixed blades act as nozzles i.e.
they change the direction of the steam and also expand it. Then steam is passed on the moving blades,
which further expand the steam and also absorb its velocity.

8. Steam Supply and Exhaust Conditions:

These types include condensing, non-condensing, reheat, extraction and induction.

Condensing turbines are most commonly found in electrical power plants. These turbines exhaust steam
from a boiler in a partially condensed state, typically of a quality near 90%, at a pressure well below
atmospheric to a condenser.

Non-condensing or back pressure turbines are most widely used for process steam applications. The
exhaust pressure is controlled by a regulating valve to suit the needs of the process steam pressure.
These are commonly found at refineries, district heating units, pulp and paper plants, and desalination
facilities where large amounts of low pressure process steam are needed.

turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits
from a high pressure section of the turbine and is returned to the boiler where additional superheat is
added. The steam then goes back into an intermediate pressure section of the turbine and continues its
expansion. Using reheat in a cycle increases the work output from the turbine and also the expansion
reaches conclusion before the steam condenses, there by minimizing the erosion of the blades in last
rows. In most of the cases, maximum number of reheats employed in a cycle is 2 as the cost of super-
heating the steam negates the increase in the work output from turbine.

Extracting type turbines are common in all applications. In an extracting type turbine, 4 steam is
released from various stages of the turbine, and used for industrial process needs or sent to boiler
feedwater heaters to improve overall cycle efficiency. Extraction flows may be controlled with a valve, or
left uncontrolled.

Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.

9. Casing or Shaft Arrangements:

These arrangements include single casing, tandem compound and cross compound turbines. Single
casing units are the most basic style where a single casing and shaft are coupled to a generator. Tandem
compound are used where two or more casings are directly coupled together to drive a single
generator. A cross compound turbine arrangement features two or more shafts not in line driving two or
more generators that often operate at different speeds. A cross compound turbine is typically used for
many large applications.

10. Two-flow rotors:

The moving steam imparts both a tangential and axial thrust on the turbine shaft, but the axial thrust in
a simple turbine is unopposed. To maintain the correct rotor position and balancing, this force must be
counteracted by an opposing force. Thrust bearings can be used for the shaft bearings, the rotor can use
dummy pistons, it can be double flow- the steam enters in the middle of the shaft and exits at both
ends, or a combination of any of these. In a double flow rotor, the blades in each half face opposite
ways, so that the axial forces negate each other but the tangential forces act together. This design of
rotor is also called two-flow, double-axial-flow, or double-exhaust. This arrangement is common in low-
pressure casings of a compound turbine.
11. Principle of Operation & Design:

An ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which
the entropy of the steam entering the turbine is equal to the entropy of the steam leaving the turbine.
No steam turbine is truly isentropic, however, with typical isentropic efficiencies ranging from 20-90%
based on the application of the turbine. The interior of a turbine comprises several sets of blades or
buckets. One set of stationary blades is connected to the casing and one set of rotating blades is
connected to the shaft. The sets intermesh with certain minimum clearances, with the size and
configuration of sets varying to efficiently exploit the expansion of steam at each stage.

11.1.Turbine efficiency

To maximize turbine efficiency the steam is expanded, doing work, in a number of stages. These stages
are characterized by how the energy is extracted from them and are known as either impulse or reaction
turbines. Most steam turbines use a mixture of the reaction and impulse designs: each stage behaves as
either one or the other, but the overall turbine uses both. Typically, higher pressure sections are
reaction type and lower pressure stages are impulse type.

11.2. Operation and maintenance:

Because of the high pressures used in the steam circuits and the materials used, steam turbines and
their casings have high thermal inertia. When warming up a steam turbine for use, the main steam stop
valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and
proceed to heat up the lines in the system along with the steam turbine. Also, a turning gear is engaged
when there is no steam to slowly rotate the turbine to ensure even heating to prevent uneven
expansion. After first rotating the turbine by the turning gear, allowing time for the rotor to assume a
straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine,
first to the astern blades then to the ahead blades slowly rotating the turbine at 10-15 RPM (0.17-0.25
Hz) to slowly warm the turbine. The warm up procedure for large steam turbines may exceed ten hours.

During normal operation, rotor imbalance can lead to vibration, which, because of the high rotation
velocities, could lead to a blade breaking away from the rotor and through the casing. To reduce this
risk, considerable efforts are spent to balance the turbine. Also, turbines are run with high quality
steam: either superheated (dry) steam, or saturated steam with a high dryness fraction. This prevents
the rapid impingement and erosion of the blades which occurs when condensed water is blasted onto
the blades (moisture carry over). Also, liquid water entering the blades may damage the thrust bearings
for the turbine shaft. To prevent this, along with controls and baffles in the boilers to ensure high quality
steam, condensate drains are installed in the steam piping leading to the turbine.

11.3. Speed Regulation in Steam Turbines

The control of a turbine with a governor is essential, as turbines need to be run up slowly to prevent
damage and some applications (such as the generation of alternating current electricity) require precise
speed control. Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which
causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the
turbine may continue accelerating until it breaks apart, often catastrophically. Turbines are expensive to
make, requiring precision manufacture and special quality materials.
During normal operation in synchronization with the electricity network, power plants are governed
with a five percent droop speed control. This means the full load speed is 100% and the no-load speed is
105%. This is required for the stable operation of the network without hunting and drop-outs of power
plants. Normally the changes in speed are minor. Adjustments in power output are made by slowly
raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a
basic system requirement for all power plants because the older and newer plants have to be
compatible in response to the instantaneous changes in frequency without depending on outside
communication.

12. Thermodynamics of Steam Turbines:

The steam turbine operates on basic principles of thermodynamics using the part 3-4 of the Rankine
cycle shown in the adjoining diagram. Superheated vapor (or dry saturated vapor, depending on
application) enters the turbine, after it having exited the boiler, at high temperature and high pressure.
The high heat/pressure steam is converted into kinetic energy using a nozzle (a fixed nozzle in an
impulse type turbine or the fixed blades in a reaction type turbine). Once the steam has exited the
nozzle it is moving at high velocity and is sent to the blades of the turbine. A force is created on the
blades due to the pressure of the vapor on the blades causing them to move. A generator or other such
device can be placed on the shaft, and the energy that was in the vapor can now be stored and used.
The gas exits the turbine as a saturated vapor (or liquid-vapor mix depending on application) at a lower
temperature and pressure than it entered with and is sent to the condenser to be cooled.[18] If we look
at the first law we can find an equation comparing the rate at which work is developed per unit mass.
Assuming there is no heat transfer to the surrounding environment and that the change in kinetic and
potential energy is negligible when compared to the change in specific enthalpy we come up with the
following equation

13. Isentropic efficiency:

To measure how well a turbine is performing we can look at its isentropic efficiency. This compares the
actual performance of the turbine with the performance that would be achieved by an ideal, isentropic,
turbine. When calculating this efficiency, heat lost to the surroundings is assumed to be zero. The
starting pressure and temperature is the same for both the actual and the ideal turbines, but at turbine
exit the energy content specific enthalpy for the actual turbine is greater than that for the ideal turbine
because of irreversibility in the actual turbine. The specific enthalpy is evaluated at the same pressure
for the actual and ideal turbines in order to give a good comparison between the two.

The isentropic efficiency is found by dividing the actual work by the ideal work.
14. Applications of Steam Turbines:

14.1. Direct drive

Electrical power stations use large steam turbines driving electric generators to produce most (about
80%) of the world's electricity. The advent of large steam turbines made central- station electricity
generation practical, since reciprocating steam engines of large rating became very bulky, and operated
at slow speeds. Most central stations are fossil fuel power plants and nuclear power plants; some
installations use geothermal steam, or use concentrated solar power (CSP) to create the steam. Steam
turbines can also be used directly to drive large centrifugal pumps, such as feed water pumps at a
thermal power plant.

The turbines used for electric power generation are most often directly coupled to their generators. As
the generators must rotate at constant synchronous speeds according to the frequency of the electric
power system, the most common speeds are 3,000 RPM for 50 Hz systems, and 3,600 RPM for 60 Hz
systems. Since nuclear reactors have lower temperature limits than fossil-fired plants, with lower steam
quality, the turbine generator sets may be arranged to operate at half these speeds, but with four-pole
generators, to reduce erosion of turbine blades.
14.2. Marine propulsion

In steam-powered ships, compelling advantages of steam turbines over reciprocating engines are
smaller size, lower maintenance, lighter weight, and lower vibration. A steam turbine is only efficient
when operating in the thousands of RPM, while the most effective propeller designs are for speeds less
than 300 RPM; consequently, precise (thus expensive) reduction gears are usually required, although
numerous early ships through World War I, such as Turbinia had direct drive from the steam turbines to
the propeller shafts. Another alternative is turbo-electric transmission, in which an electrical generator
run by the high- speed turbine is used to run one or more slow-speed electric motors connected to the
propeller shafts; precision gear cutting may be a production bottleneck during wartime. Turbo-electric
drive was most used in large US warships designed during World War I and in some fast liners, and was
used in some troop transports and mass-production destroyer escorts in World War II. The purchase
cost of turbines is offset by much lower fuel and maintenance requirements and the small size of a
turbine when compared to a reciprocating engine having an equivalent power. However, from the 1950s
diesel engines were capable of greater reliability and higher efficiencies: propulsion steam turbine cycle
efficiencies have yet to break 50%, yet diesel engines today routinely exceed 50%, especially in marine
applications.[21][22][23] Diesel power plants also have lower operating costs since fewer operators are
required. Thus, conventional steam power is used in very few new ships. An exception is LNG carriers
which often find it more efficient to use boil-off gas with a steam turbine than to re- liquify it.

Nuclear-powered ships and submarines use a nuclear reactor to create steam for turbines. Nuclear
power is often chosen where diesel power would be impractical (as in submarine applications) or the
logistics of refuelling pose significant problems (for example, icebreakers). It has been estimated that
the reactor fuel for the Royal Navy's Vanguard class submarine is sufficient to last 40 circumnavigations
of the globe - potentially sufficient for the vessel's entire service life. Nuclear propulsion has only been
applied to a very few commercial vessels due to the expense of maintenance and the regulatory controls
required on nuclear systems and fuel cycles.

14.3. Turbo Electric Drive System

Turbo-electric drive was introduced on the USS New Mexico (BB-40), launched in 1917. Over the next
eight years the US Navy launched five additional turbo-electric-powered battleships and two aircraft
carriers (initially ordered as Lexington-class battle cruisers). Ten more turbo-electric capital ships were
planned, but cancelled due to the limits imposed by the Washington Naval Treaty. Although New
Mexico was refitted with geared turbines in a 1931-33 refit, the remaining turbo-electric ships retained
the system throughout their careers. This system used two large steam turbine generators to drive an
electric motor on each of shafts. The system was less costly initially than reduction gears and made the
ships more maneuverable in port, with the shafts able to reverse rapidly and deliver more reverse
power than with most geared systems. Some ocean liners were also built with turbo-electric drive, as
were some troop transports and mass-production destroyer escorts in World War II. However, when the
US designed the "treaty cruisers", beginning with the USS Pensacola (CA-24) launched in 1927, geared
turbines were used for all fast steam-powered ships thereafter.