92 Electrical Power Systems Technology
Table 4-1. Power Plant Terminology
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Boiler (steam generator) An enclosed unit that is used to produce steam by heat devel-
oped within the unit. Water that flows through it is converted to
steam as a result of the heat of combustion.
Superheater A grouping of tubes inside a boiler that absorbs heat from the
combustion gases to raise the temperature of the steam to a very
high temperature.
Feedwater Water that is supplied to the boiler to produce steam.
Coal Pulverizer A machine that reduces coal to a fineness suitable for burning in
suspension. Coal is ground to a talcum powder consistency.
Forced Draft (FD) Fan A fan to supply preheated combustion air under pressure to the
furnace, for mixture with the fuel stream.
Economizer A heat-absorbing section of the boiler that preheats incoming
cold boiler feedwater by transferring heat from the outgoing
combustion gases to the water.
Condenser A unit that converts steam into water.
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After steam has been produced, a rotary motion must be developed.
This rotary motion is produced by a steam turbine. A steam turbine, shown
in Figure 4-5 is typically made up of as many as 1500 blades. The rotor is
usually divided into two parts—the high pressure rotor and the low pressure
rotor. The low-pressure rotor is larger in diameter than the high-pressure
rotor. Steam is channeled to the high-pressure rotor, and it is then routed
to the low-pressure rotor.
Steam turbines ordinarily achieve a maximum efficiency of less than
30 percent, but only when run at very high speeds. Some turbines can pro-
duce as much as 160,000 horsepower. A speed of 3600 rpm is needed to
develop a 60-Hz electrical power output. The standard power frequency
in the U.S. is 60-Hz. Large three-phase AC generators are connected to DC
exciters. Generators are often cooled with hydrogen because hydrogen has
less than one-tenth the density of water. Therefore, much less energy is re-
quired to recirculate the hydrogen for cooling purposes.
The process just described summarizes the operation of a steam
power plant. There are several variations to the basic process; however,
most plants use similar methods. The individual parts of a steam-generat-
ing system will be discussed in more detail in the following sections.