

Actually the reasons you don't see so many cooling towers at fossil plants are
1) fossil units (multiple units per plant) are generally smaller than
nuclear plants.  300 MWe seemed to be a very popular size when many
fossil plants were built.  The average nuclear plant is 1000 MWe.  2) many
fossil plants were grandfathered when water discharge regulations were
adopted ("why those old dirt burners can't harm anything, let 'em go.").  
3) powered draft cooling towers, low enough to the ground to be generally
not visible from off-site, are quite popular with fossil plants.  4) fossil
plants used to get much less regulatory attention than nuclears.



Actually the condensing environment is essentially the same for plants
of similar size.  The issues are the same regardless of where the 
heat comes from.  Condensers are run at as high a vacuum as possible in
order to reduce aerodynamic drag on the turbine.  The condenser pressure is
normally water's vapor pressure at the condensing temperature.  It is
desirable that the steam exhaust be free of water droplets because 
moisture in the steam causes severe erosion damage to the turbine 
low pressure blades and because entrained water moving at high velocity
causes erosion of the condenser tubes.  The coldest and thus lowest
pressure condensing environment is always the best.  

A related issue is that of pumping the condensate from the hotwell (where
the water ends up after dripping off the condenser tubes.)  Since the
condenser is at a very low pressure, the only force driving the 
condensate into the hotwell pumps is gravity.  If the condensate is too 
hot or the gravity head is too low, the condensate will reflash into
steam bubbles and cause the condensate pumps to cavitate.  This is a
particularly destructive form of cavitation that is to be avoided at all
costs.  

The hotwell pumps are located in the lowest point in the plant
in order to provide a gravity head to the pumps.  How much lower 
they must be is a function of how hot the water is allowed to get in
the hotwell.  Typically hotwell temperatures run between 100 and 120 
degrees depending on the temperature of the river water (this term is
used to describe the river grade water even when the cooling tower
system is operating in closed loop mode and essentially no river water
is pumped.)  When the river water temperature is high in the summer,
operators will typically allow the hotwell level to rise in order 
to provide more gravity head.  There is a tradeoff involved since higher
hotwell levels will encroach onto the condensing tubes and reduce the
condenser area.

At least in the East and elsewhere where moisture actually exists in the
air :-), the river water will almost always be cooler than the discharge
water from the cooling towers.  The temperature of the discharge water
from the cooling towers is set by the ambient air temperature and
humidity.  It is very rare in the East to hear of actual river water
temperatures exceeding 70 degrees.  A vast difference from the typical
"95-95" days (95 degrees, 95% humidity) we see routinely in the East.
It is not unusual, particularly where the econazis have been successful
in clamping rigid discharge water temperature limits on a plant, for the
plant to have to reduce the firing rate when the air temperature gets
too high and the condenser cannot handle the heat load without excessive
pressure.


True.  Actually secondary plant (the part that makes electricity and
feeds feedwater to the boiler) water chemistry has been the bastard 
stepchild until recently and has not gotten the respect it deserves.
The plant chemists have just in the past decade or so fully understood
the costs of impure water.  By "impure", I mean water with a few
dozen extra micromho of conductivity and/or a few PPM of dissolved
oxygen.  Secondary water is now typically the most pure one will 
find outside the laboratory.

John