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NFCRC Tutorial Steam Turbine

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A steam turbine based power plant consists of raising high pressure steam in a boiler from the thermal energy and expanding the steam in a turbine to generate shaft power which in turn is converted into electricity in the generator.

Axial flow steam turbines consist of circularly distributed stationary blades called nozzles which direct steam on to rotating blades or buckets mounted radially on a rotating wheel. Typically, the blades are short in proportion to the radius of the wheel, and the nozzles are approximately rectangular in cross section. Several stages of expansions are obtained by using a series of nozzles and buckets, with the exhaust from the buckets of one stage flowing directly into the nozzles of the following stage. A compact machine can be built economically with ten or more stages for optimum use of high pressure steam and vacuum exhaust by mounting the wheels of a number of stages on a single shaft, and supporting the nozzles of all stages from a continuous housing. Large axial turbines must be operated under such conditions that the exhaust steam does not contain more than 10 to 13% of liquid since condensate droplets could seriously erode the high velocity nozzles and blades. The moisture content of the exhaust is dependent upon the inlet steam pressure/temperature combination. Special moisture removal stages may be incorporated in the design when the steam superheat temperature is limited.

Steam may be utilized directly in the steam turbine without any superheat as may be done with low pressure steam, or superheated to increase the cycle efficiency. Reheat may also be included to further increase the ef ficiency of converting heat to power by superheating the steam after partial expansion and admitting the steam thus reheated back into the turbine.