nagle@netcom.com (John Nagle) writes:  >>Great Explaination, however you left off one detail, why do you always >>see them at nuclear plants, but not always at fossil fuel plants.  At >>nuclear plants it is prefered to run the water closed cycle, whereas >>fossil fuel plants can in some cases get away with dumping the hot >>water.  As I recall the water isn't as hot (thermodynamically) in many >>fossil fuel plants, and of course there is less danger of radioactive >>contamination.  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, fossil fuel plants run hotter than the usual  >boiling-water reactor nuclear plants.  (There's a gripe in the industry >that nuclear power uses 1900 vintage steam technology).  So it's >more important in nuclear plants to get the cold end of the system >as cold as possible.  Hence big cooling towers.    >       Oil and gas fired steam plants also have condensers, but they >usually are sized to get the steam back into hot water, not most of the >way down to ambient.  Some plants do cool the condensers with water, >rather than air; as one Canadian official, asked about "thermal  >pollution" de-icing a river, said, "Up here, we view heat as a resource".    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.  >       Everybody runs closed-cycle boilers.  The water used is  >purified of solids, which otherwise crud up the boiler plumbing when >the water boils.  Purifying water for boiler use is a bigger job than  >cooling it, so the boiler water is recycled.  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