
Except for their size, the cooling towers on nuclear power plants
are vertually identical in construction and operation to cooling
towers designed and built in the 1890's (a hundred years ago) for
coal fired power plants used for lighting and early electric railways.

Basicly, the cylindrical tower supports a rapid air draft when
its air is heated by hot water and/or steam circulating thru a network
of pipes that fill about the lower 1/3 of the tower. To assist cooling
and the draft, water misters are added that spray cold water over the
hot pipes. The cold water evaporates, removing the heat faster than
just air flow from the draft would and the resulting water vapor is
rapidly carried away by the draft. This produces the clouds frequently
seen rising out of these towers.

That slight pinch (maybe 2/3 of the way up the tower) is there because
it produces a very significant increase in the strength and rate of
the air draft produced, compared to a straight cylinder shape.

The towers are used to recondense the steam in the sealed steam
system of the power plant so that it can be recirculated back to the
boiler and used again. The wider the temperature difference across
the turbines used in the power plant the more effecient they are and
by recondensing the steam in the cooling towers before sending it
back to the boilers you maintain a very wide temperature difference
(sometimes as high as 1000 degrees or more from first stage "hot"
turbine to final stage "cold" turbine).

                                        R. Tim Coslet