Environmental Engineering Reference
In-Depth Information
air is drawn in through louvers in the bottom of the tower by natural draft. Heat is transferred from
the cooling water to the air directly, and the water is further cooled by evaporation of part of the
water. Cold water falls by gravity into the collecting basin from whence it is recirculated into the
condenser. A mist eliminator is placed above the fill. Nevertheless, a mist plume usually forms
above the tower, especially in cold weather. This white plume is often mistaken as pollution; in
fact it is just composed of water droplets or ice crystals.
7
A 1000-MW power plant working at 33% efficiency in a hot climate evaporates 0.63 m
3
s
−
1
water, which is 1.3% of the recirculating water. In a cold climate the same plant would evaporate
only 1% of the recirculating water. This amounts to 2E(7) or 1.5E(7) m
3
y
−
1
water that the
power plant must withdraw from a surface water, well, or municipal water in hot or cold climate,
respectively.
5.2.7.2 Dry Cooling Tower
In a dry cooling tower, the recirculating water flows through finned tubes over which cold air
is drawn. In a dry tower all heat rejection is to the atmosphere. The advantages are that no wa-
ter evaporates, and a dry tower is less expensive to maintain. The disadvantages are that it is
more expensive to build and the back pressure on the turbines is higher, causing plant efficiency
loss. Nevertheless, in arid areas where no make-up water is available, dry cooling towers are
employed.
5.2.8
Generator
The generator is the heart of the power plant. That is where electricity is generated. Compared to the
boiler, turbines, condenser, cooling tower, and other auxiliary equipment, the generator occupies a
small fraction of the total plant outlay. Its noise level is also negligible compared to the hum and
drum of the coal mills, burners, pumps, fans, and turbines.
The electromagnetic theory of the generator is described in Chapter 4. Briefly, the shaft of the
turbine turns conducting coils within a magnetic field. This induces an electric current to flow in
the coils. The electric power output of the generator equals the mechanical power input of the shaft,
minus minor resistive losses in the coils and frictional losses. In order to prevent the overheating
of the generator induced by these losses, generators are cooled by high conductivity gases, such as
hydrogen or helium.
The generator produces an alternating current (AC): 60 Hz in the United States and Canada,
50 Hz in most other countries. The shaft of the turbine must rotate at a precise speed in order
to produce the exact frequency of the AC. The voltage produced by the generator is enhanced
by step-up transformers and then transmitted into the grid. Because resistance losses are propor-
tional to the square of the current but linearly proportional to the voltage, it is advantageous to
transmit power at low current and high voltage. Long-distance transmission usually occurs in the
hundreds of kilovolt range. At the user, the voltage is reduced by step-down transformers to 110
or 220 V.
7
However, some solids dissolved in the make-up water may produce a particle plume downwind of the cooling
tower.
