re what do nuclear site s cooling towers do 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 fossil units multiple units per plant are generally smaller than nuclear plants mwe seemed to be a very popular size when many fossil plants were built the average nuclear plant is mwe many fossil plants were grandfathered when water discharge regulations were adopted why those old dirt burners can t harm anything let em go powered draft cooling towers low enough to the ground to be generally not visible from off site are quite popular with fossil plants 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 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 and 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 degrees a vast difference from the typical days degrees 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 john de armond wd oqc interested in high performance mobility performance engineering magazine tm interested in high tech and computers marietta ga send ur snail mail address to jgd dixie com perform dixie com for a free sample mag lee harvey oswald where are ya when we need ya
