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Handbook of Turbomachinery Second Edition Revised - Ventech!

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only, including pressure, temperature, and exhaust pressure. The actual or<br />

thermal efficiency <strong>of</strong> a power plant is the Rankine efficiency multiplied by<br />

the efficiency <strong>of</strong> the turbine. Because the overall unit efficiency is a function<br />

<strong>of</strong> both the thermodynamic and turbine efficiencies, an alternative<br />

performance criterion has been established in terms <strong>of</strong> heat rates. The<br />

heat rate is the ratio <strong>of</strong> the heat supplied to the turbine (or plant) divided by<br />

the output <strong>of</strong> the generator (or plant).<br />

The earliest plants used an open thermodynamic steam cycle, thus a<br />

primary problem was with contaminants in the feedwater supply. Control<br />

methods included water treatment and boiler blowdown. Evolution to a<br />

closed cycle led to easier and better control <strong>of</strong> water/steam purity.<br />

There were two other significant problems with the basic Rankine<br />

cycle. The moisture in the exhaust was high, which decreased the overall<br />

efficiency and led to significant blade erosion. The second drawback was the<br />

low efficiencies that were caused by the nonequilibrium heating <strong>of</strong> the<br />

compressed liquid along AB. The solution to these problems was to evolve<br />

to a regenerative (with reheat) Rankine cycle.<br />

Reheat<br />

The efficiency <strong>of</strong> the Rankine cycle is highest for the highest turbine inlet<br />

temperatures. However, there are limitations to how high a temperature can<br />

be used, such as material capability. Further, when pressures are raised to<br />

increase the work that can be produced, there is an increase in exhaust<br />

moisture that lowers the efficiency and increases the amount <strong>of</strong> liquid<br />

droplet erosion damage. To overcome these difficulties and also generate<br />

higher cycle efficiencies, the reheat cycle is used. The steam starts at a<br />

maximum temperature, then is partially expanded through a turbine section,<br />

and is subsequently returned to the reheater section <strong>of</strong> the boiler. There may<br />

be one or more reheats, the number being a matter <strong>of</strong> economics and<br />

efficiency. The use <strong>of</strong> reheat led to an increase in operating pressure.<br />

In a typical steam turbine unit, going to a single reheat Rankine cycle<br />

increases the efficiency (decreases the heat rate) by about 4–6%. Heat rate is<br />

a measure <strong>of</strong> fuel economy in a unit and can be defined as<br />

Plant heat rate ¼<br />

The plant efficiency is<br />

Z ¼<br />

3412:14 BTU<br />

kWhr<br />

Heat rate<br />

Heat equivalent <strong>of</strong> fuelburned<br />

Net useful kWhr generated<br />

ð8Þ<br />

6100% ð9Þ<br />

An additional advantage <strong>of</strong> reheat is that the steam going through later<br />

Copyright © 2003 Marcel Dekker, Inc.

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