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Steam Turbine
Steam turbines are devices which convert the energy stored in steam into rotational
mechanical energy. These machines are widely used for the generation of electricity in a
number of different cycles, such as:
- Rankine cycle
- Reheat cycle
- Regenerative cycle
- Combined cycle
The steam turbine may consists of several stages. Each stage can be described by analyzing
the expansion of steam from a higher pressure to a lower pressure. The steam may be wet,
dry saturated or superheated.
Consider the steam turbine shown in the cycle above. The output power of the turbine at
steady flow condition is:
P = m (h1-h2)
where m is the mass flow of the steam through the turbine and h1 and h2 are specific
enthalpy of the steam at inlet and respective outlet of the turbine.
The efficiency of the steam turbines are often described by the isentropic efficiency for
expansion process. The presence of water droplets in the steam will reduce the efficiency
of the turbine and cause physical erosion of the blades. Therefore the dryness fraction of
the steam at the outlet of the turbine should not be less than 0.9.
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