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Handbook of Turbomachinery Second Edition Revised - Ventech!

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Figure 20 Thermodynamic diagram for combined cycle with three reheat cycles.<br />

Geothermal Plants<br />

Steam conditions (and as a result, efficiencies) in geothermal plants are<br />

usually low, typically below 7 MPa (1015 psi) and 170 8C (340 8F). The low<br />

available enthalpy means that very large flows are necessary. For example,<br />

in a typical 150 MW unit, the flow is 600 lbm/sec.<br />

Moisture Limitations<br />

A practical limit on the usable temperatures and pressures <strong>of</strong> steam is also<br />

imposed by the entrained moisture. Severe erosion, caused by excessive<br />

moisture levels at the inlet to the last few blade rows, was found in LP blades<br />

in early turbine designs and led to a limitation <strong>of</strong> about 12% on turbine exit<br />

wetness [21].<br />

Furthermore, the efficiency, Z, <strong>of</strong> the LP turbine decreases by<br />

approximately 1% for every 1% increase in wetness in the wet stages. The<br />

overall loss caused by moisture consists <strong>of</strong> a supersaturation loss and a<br />

moisture loss as illustrated in Fig. 21 for results obtained from a<br />

development laboratory LP turbine. The supersaturation loss occurs<br />

because the expansion in the turbine is more rapid than indicated by<br />

equilibrium expansion. Under these conditions, the temperature drops much<br />

more rapidly, and there is less available energy than equilibrium. Beyond the<br />

‘‘Wilson line,’’ condensation occurs and the stage efficiency drops by about<br />

0.76% per 1% increase in moisture. The overall loss introduced from zero<br />

Copyright © 2003 Marcel Dekker, Inc.

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