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Turbines.: The Pure Impulse Stage

The document discusses the basic theory behind steam turbines. It explains that in a single-stage turbine, steam expands through fixed nozzles which convert pressure energy to kinetic energy. The high-velocity steam then passes through moving blades which extract energy and produce torque. Arranging multiple single-stage turbines in series improves efficiency by compounding pressure, with each stage adding to the kinetic energy until only a small amount remains at the final stage.

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Meghali Borle
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23 views2 pages

Turbines.: The Pure Impulse Stage

The document discusses the basic theory behind steam turbines. It explains that in a single-stage turbine, steam expands through fixed nozzles which convert pressure energy to kinetic energy. The high-velocity steam then passes through moving blades which extract energy and produce torque. Arranging multiple single-stage turbines in series improves efficiency by compounding pressure, with each stage adding to the kinetic energy until only a small amount remains at the final stage.

Uploaded by

Meghali Borle
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOC, PDF, TXT or read online on Scribd
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Turbines.

Basic Steam Turbine Theory:

The Pure Impulse Stage: In a single stage, pure turbine the


steam pressure at entry to and exit from the moving blades
is equal, the whole expansion having taken place in the fixed
nozzle. Pressure energy in front of the nozzles is converted
to kinetic energy in the passage of steam through the nozzles.
The high velocity steam leaving the nozzles is then turned in
direction by the moving blades, and the change of
momentum of the steam produces a force on the blades, and
thus a torque on the shaft.
The passage of steam through the nozzles results in some
inefficiency due to friction, so not all the potential energy is
converted to kinetic energy. Similarly there is some loss due
to friction as the steam passes through the moving blades,
which results in reheating of the steam at constant pressure.
Finally, therefore, the gross stage efficiency is made up of
losses in both nozzles and blades.

Pressure Compounding: One of the disadvantages of the


pure, single stage, impulse turbine is the high velocity of the
steam leaving the moving blades, know as the leaving loss,
which can be as large as 11% of the initial kinetic energy.
By arranging for the pressure drop to occur over a number
of pure impulse stages in series, known as pressure
compounding, the efficiency can be improved. The velocity
of the steam leaving the fist stage carries over to the next
row of nozzles, augmenting the kinetic energy of expansion
in the nozzle of that stage, through to the final stage, where
again the steam leaves with high velocity but the leaving loss
is now a small part of the total available energy. The leaving
loss of such a turbine is usually about 2% and is called a
Rateau turbine.
Semi Curtis Wheel:
Tapered And Twisted Blade:
Rateau:

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