Understanding Water Treatment Concerns With Air-cooled Condensers
By Andrew Howell
Dry cooling systems (air cooled condensers, ACCs) for condensing steam in power plants have been increasingly included in new unit design over the past couple of decades. Water scarcity, or the inability to use existing supplies for cooling, are the primary drivers in this direction. Various trade-offs are encountered in comparing wet and dry cooling, including turbine performance, capital cost, steam cycle chemistry and corrosion, and environmental impact. Certain design and operating options can be selected to mitigate some of these issues.
Steam-generating power plants require the removal of a significant amount of latent heat as low-pressure (LP) turbine exhaust steam is condensed to liquid water. This function has traditionally been accomplished by condensing the steam on the surface of tubes through which cooling water flows. Large volumes of water are required to cool this steam in a once-through cooling design— about 500 million gallons of water must be pumped daily through the condenser cooling tubes for a 750-megawatt (MW) steam turbine.
In 2004, Section 316(b) of the U.S. Clean Water Act was amended to require mitigation of potential damage to wildlife at the point of cooling water intake, for plants taking in more than 50 million gallons of water per day. In addition to water intake concerns, the temperature of this water is significantly elevated when it is returned to its source, thus raising the possibility of altering natural environmental conditions in the receiving water body. In a recirculating cooling tower design, much less water is required (about 10 million gallons per day for a 750-MW steam turbine), but 75% to 80% of this water is lost to evaporation. The water is concentrated in the cooling tower, perhaps from 5 to 15 times, and the concentration of various constituents may thus be unacceptably high for discharge into a receiving water body.
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