MIL-HDBK-1003/7
Topping Cycle. A schematic diagram of a topping cycle is shown in Figure 14.
5.1.3.3
The topping cycle consists of a high pressure steam boiler and turbine generator with
the high pressure turbine exhausting steam to one or more low pressure steam turbine
generators. High pressure topping turbines are usually installed as an addition to an
existing lower pressure steam electric plant.
General Economic Rules. Maximum overall efficiency and economy of the steam
5.1.4
turbine power cycle are the objectives of a satisfactory design. Higher efficiency and
a lower heat rate require more complex cycles which are accompanied with higher initial
investment costs and higher operational and maintenance costs but lower fuel costs.
General rules to consider to improve the plant efficiency are listed hereinafter.
a) Higher steam pressures and temperatures increase the turbine
efficiencies, but temperatures above 750 degrees F (399 degrees C) usually require more
expensive alloy piping in the high pressure steam system.
b) Lower condensing pressures increase turbine efficiency. However, there
is a limit where lowering condensing (back) pressure will no longer be economical,
because the costs of lowering the exhaust pressure is more than the savings from the
more efficient turbine operation.
c) The use of stage or regenerative feedwater cycles improves heat rates,
with greater improvement corresponding to larger numbers of such heaters. In a
regenerative cycle, there is also a thermodynamic crossover point where lowering of an
extraction pressure causes less steam to flow through the extraction piping to the feed
water heaters, reducing the feedwater temperature. There is also a limit to the number
of stages of extraction/feedwater heating, which may be economically added to the cycle.
This occurs when additional cycle efficiency no longer justifies the increased capital
cost.
d)
Larger turbine generator units are generally more efficient than
smaller units.
e) Multi-stage and multi-valve turbines are more economical
than single stage or single valve machines.
f) Steam generators of more elaborate design and with heat saving
accessory equipment are more efficient.
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