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Figure 17-3.—Energy relationships in the basic propulsion cycle of conventional steam-driven ships.
Since superheated steam is dry, it causes less
corrosion of piping and machinery.
For auxiliary purposes, some steam is
desuperheated by passing through the desuperheater
piping located in the steam drum. The superheated
steam is then ready for use to drive the turbine.
Expansion
In the expansion phase the thermal energy of
the steam is converted to mechanical energy in the
turbines. Turbines use nozzles to convert the higher
pressure of the steam into a high velocity. The kinetic
energy of the steam is then transferred to the turbine
blading, creating the mechanical energy of the
turbine rotor. That, in turn, through the reduction
gears, turns the propellers.
Condensation
As the steam leaves, or exhausts through, the
turbine, it is condensed so that the feedwater may be
reused. One boiler can generate 150,000 pounds of
steam per hour. If the feedwater were not recovered,
the system would require an enormously large
evaporator to produce the required feedwater.
As the steam exhausts into the main con-
denser, seawater passes through tubes in the
condenser. The cool seawater cools the steam to the
point of condensation. The condenser operates at a
vacuum, which helps this process and increases the
efficiency of the system.
The condensate pump takes a suction from
the main condenser hot well and delivers the
condensate (condensed steam) into the condensate
piping system and the air ejector condenser. The
17-7
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