Document O13xam68QaxLvJ0DGgj4gzmme

1949 Guide s uo. i .2 i|2 * wa ' + z< Zo V 5f3it ^ftH1"O in o o o S Zo K5- -<8 H ^ *g .< Ii 5s.0*s" :s ' - a x .c9^.`.32 . < = : QU .S--2 p -ft. "0 s.3 < sa o &2h.:2=^ za< - S-i' *S s0m < PS O <1 E jf 6 =0 2c .50 -5 5 oi c -- ."2 w ss O *- *1 M Corrosion and Water Formed Deposits, Causes and Prevention 975 While chlorine is the most generally used chemical, others may occa sionally prove to be more practicable. Choice of the chemical is con ditioned largely by the design and operation of the system. Open Recirculating Systems In spray ponds and cooling towers of the open type, light-loving algae growths are likely to cause blocking of the distribution piping and troughs. These organisms are most troublesome in areas accessible to sunlight. Algae slimes are usually stringy in character. In open recirculating systems, continuous use of small quantities of chlorine is generally most satisfactory. In once-through systems, where large quantities of water are used, intermittent treatment a few times each day will usually result in satisfactory slime removal and chemical economies. Neither the phenols nor copper sulphate may be used for the removal of slime already formed. For this purpose, chlorine gas is used. After being cleaned, the other chemicals may be used to prevent the reestablish- Table 6. Common Chemicals Used foe Slime Contbol Chemical Trade Name Physical State* : Chlorine Hypochlorites Chlorine Calcium Hypochlorites Sodiuin Hypochlorites Gas. Crystalline Chlorinated Phenols Sodium-- Chlorophenylphenate Tetrachlorophenate Pentachlorophenate Briquettes Briquettes Briquettes Potassium Permanganate Permanganate of Potash . Crystalline Copper Sulphate Blue Vitriol Crystalline * As shipped; ment. of slime in the. system. The removal of green algae from a cooling tower should never bemused as an indication that the true slime-forming organisms .on heat exchanger surfaces have, been removed. The more resistant slime formers, which so materially reduce heat transfer efficiency, will often be unaffected by treatment which, completely eliminates algae. Closed Once-Through Systems , In equipment wherfe light is excluded, slime formations are due to fungi. Usually, they predominate on the heat exchange surfaces. Bacteria form thick, soft slime. Yeast and molds form tough rubbery slimes. Chlorine and hypochlorite solutions, fed intermittently, are used to prevent such slimes. UNDER-WATER CORROSION When deleterious substances are present in water, the corrosivity of the solution is increased in proportion to the amount of deleterious substances present, the temperature, and usually the rate of flow of the solution over the metal surfaces. There are other relevant factors, but their influence in general is subordinate to those mentioned. Dissolved oxygen, acid