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<br />being derived from steam, usually contains less scale-forming inorganic contamination <br /> <br />~ (hardness and alkalinity) than river water. It thus can be ~ore suitable for <br /> <br />::7:l <br />;.u <br /> <br />use in a cooling system than is river ~ater. <br /> <br />In the following sections we will first explain how water quality affects <br /> <br />the operation of a cooling tower and circulating cooling system, and show that <br /> <br />a cooling tower is a water treatment. Secondly we will mention the treatments <br /> <br />necessary to fit waste condensate for cooling system makeup. Finally we <br /> <br />consider the cost-quality relationships in disposal of the cooling tower <br /> <br />blowdown. <br /> <br />2.6 A Coolinq Tower as a Water Treatment <br /> <br />The problems inherent in control of a circulating cooling system can best <br /> <br />be summarized by quoting from Reference J: <br /> <br />'" <br />_~ I <br /> <br />A wet cooling tower is an evaporator, and salts dissolved in ~he makeup <br />water concentrate, often to the point of precipitation. T~e precipitate <br />tends to adhere to heat transfer surfaces formi~g a hard scale..... <br />Not only may the makeup waCer contain silt, but the circulating water in <br />its passage through the tower scrubs dust out of the air. Circulating <br />water t~us contains an ever-increasing amount of suspended matter, ~hich <br />will settle out in stagnant spots in the pipes and heat exchangers.... <br />The well-oxygenated cir=ulating water is very corrosive to heat t~~~sfe: <br />surfaces....Circulatinq cooling water is warm and well-oxygenated and is <br />an ideal habitat for microbial growth. The water is seldom sterile when <br />fee to t~e system and, in any case, receives a steady supply of air-borne <br />growth. Untreated cooling systems are subject to fungal rot of the <br />wooden parts of the tower, bacterial corrosion of iron and bacterial <br />production of sulfice, growth of algae in the sun-lit portions of ~~e <br />tower, anc suspended slo~ghed-off growth ~~at can lodge in the system anc <br />block the flow. Biccidal chemicals must be added to control growth. <br /> <br />Scaling is prevented by controlling the concentration of species which <br />form slightly soluble salts, particularly salts whose solubility decreases <br />with an increase in temperature so that precipitation tends to be on the <br />surface of t~e condenser, where hard, adherent scale is formed. The ~ost common <br />scales are calcium carbonate and calcium sulfate with additional problems from <br />silica, magnesium silicates and calcium phosphates. The species that must be <br />++ ++ = 3- <br />controlled are Ca ,Mq ,Si02, COJ and P04 . <br />Concentration of dissolved salts i~ circulating cooling water is usually <br />controlled by blowing down. There is always some drift of water droplets from <br />a cooling tower, which can be considered to be a blowcown. When the circulating <br />cooling water is very concentrated the drift may damage foliage and land upon <br />which it settl~s. For high concentration cooling systems, the best mode~ cri=~ <br />eliminators will be used. Slowdown cannct acceptably be prevented by allowing <br />a high drift. <br /> <br />30 <br />