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Handbook of Turbomachinery Second Edition Revised - Ventech!

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<strong>of</strong> 20%, which would cause excessive erosion <strong>of</strong> the last stages <strong>of</strong> the LP and<br />

large losses in efficiency. As a result, a moisture separator (in which the<br />

steam is dried or separated mechanically) and a one- or two-stage reheater<br />

are provided for the steam exhausted from the HP before returning it to the<br />

LP.<br />

Piping from the HP turbine exhaust to the moisture separator/reheater<br />

is termed cold reheat or crossunder piping; that from the reheater to the LP<br />

turbine called hot reheat or crossover piping. In nuclear nonreheat units,<br />

there is typically only a moisture separator between the HP and LP turbines.<br />

The LP turbines for nuclear reactors are subject to nearly identical<br />

conditions as those in fossil units and have led to the use <strong>of</strong> the designs that<br />

are <strong>of</strong>ten identical. However, as the steam volumetric flows are 60–70%<br />

larger, the exhaust annulus area must be increased or the number <strong>of</strong> LP<br />

turbines increased to reduce exhaust losses [23].<br />

In the United States, the use <strong>of</strong> larger-sized components needed to<br />

handle the high steam flow rates in nuclear units has led to the use <strong>of</strong> lower<br />

speeds such as 1800 rpm for 60-Hz units, compared to 3,600 rpm for fossil<br />

units. The stresses that result in the turbine blades <strong>of</strong> fossil and nuclear units<br />

are roughly the same order <strong>of</strong> magnitude, as the allowable blade lengths are<br />

longer in the slower-turning nuclear-powered turbines. For 50-Hz systems,<br />

1,500 rpm and 3,000 rpm machines have been used in nuclear units.<br />

Steam Turbines for Co-Generation, Combined Cycle, and<br />

Geothermal Plants<br />

Co-Generation Turbines<br />

Steam turbines for co-generation may be <strong>of</strong> standard designs or incorporate<br />

specific features to improve the delivery <strong>of</strong> district heating, such as extra<br />

cylinders to allow more freedom in automatic extraction [9].<br />

Combined Cycle Turbines<br />

Combined cycle units may consist <strong>of</strong> a single gas turbine and single steam<br />

turbine on a common shaft with the generator, or multiple gas turbines and<br />

a single steam turbine. Depending on the steam conditions and flow, the<br />

steam turbine for combined cycle units may be a single cylinder or multiple<br />

cylinders, such as with an HP and a double-flow LP turbine. Combined<br />

cycle turbines typically do not employ regeneration cycles employing<br />

feedwater heaters.<br />

Copyright © 2003 Marcel Dekker, Inc.

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