Document kDBL4qk36R8ymYRrMq0k84r10

1056 CHAPTER 42 1957 Guide Heating Systems In hot water heating systems or in steam heating boilers where all con densate is returned, troubles from sealing should not be severe. If neces sary, sodium phosphate or sodium carbonate may be added to the water to prevent the formation of adherent calcium sulphate scale. Boilers and High Temperature Equipment Where temperature exceeds 250 F, complete softening of the water is the only practical method for minimizing sludge formation. This is usually accomplished by artificial or natural- zeolites (called also ion-ex change materials) or by hot-process precipitation softeners. In boilers operating at pressures above 100 psig virtually all the calcium, magnesium, silica, iron, and manganese salts entering with the feed water are potential scale or sludge formers. In boilers operating at 100 to 250 psig, the formation of adherent cal cium sulphate (anhydrite) scales is most to be feared. Such deposits form on the hottest evaporative surfaces. The scale has a low heat con ductivity. Even a layer of egg shell thickness may so impede the rate of heat transfer as to bring about over-heating of the metal. The ortho-phosphates of sodium are most frequently used to prevent sulphate scales. The. concentration of phosphate required is such as to cause the precipitation of calcium phosphate as sludge, thus keeping the boiling water under-saturated with respect to calcium sulphate. To a lesser extent, sodium carbonate (called also soda ash and sal soda) is also used. Most of the effective boiler compounds contain either phosphates or soda ash, or both. Certain organic materials and colloids are some times found to minimize scale formation. Where chemicals are introduced directly into the boiler in amounts adequate to prevent scale, sludge is formed in amounts proportionate to the calcium and magnesium salts entering with the feed water. To prevent troublesome accumulation of this sludge, as well as soluble salts, as evaporation occurs, some blowdown of boiler water is necessary. CAUSES AND PREVENTION OF SLIMES A .water containing slime-producing organisms will produce prohibitive amounts of slime only when the conditions of use are such as to propagate their life processes. Whenever sufficient food material from normal water or from airborne dust. combines with optimum temperature conditions, such as exist on cooling surfaces and air washers, serious quantities of slime will be produced. Some natural well waters do not contain sufficient foods to support luxuriant slime growths. Algae, which require light for carrying on their life processes, are likely to cause difficulty in cooling towers and other areas where sunlight is abundant. The ordinary slime-forming bacteria are capable of using a wide variety of nitrogenous and cellulose material as food sources. These bacteria thrive best under dark conditions such as exist in condensers and other heat transfer surfaces. Other organisms capable of causing similar difficulties use such a wide variety of food material as algae,14 iron compounds,16 and inorganic sulphates.16 At present, the use of toxic chemicals and irradiation are the two general means employed .in slime control. The value of ultra-violet light, used so broadly in the beverage industry, is somewhat in dispute. Corrosion and Water Formed Deposits, Causes and Prevention 1057 Anti-fouling paints have been developed and are fairly satisfactory for the prevention of the growth of macro-organisms such as barnacles and mussels, but these paints must be renewed at frequent intervals, and are not applicable to inaccessible areas such as the inside of pipe lines aind cooling towers. Satisfactory anti-sliming paints have not been found. Names and other pertinent data relating to some of the more common chemicals used in slime control are shown in Table 5. Chlorine is the only chemical to which is attributed the ability to de stroy slime-forming organisms. The others are presumed to poison marine organisms, most of which recover when the chemical is not used regularly. While chlorine is the most generally used chemical, the use of others may occasionally prove to be more practicable. ; Choice of the chemical is conditioned largely by the design and operation of the system. Table 5. Common Chemicals Used for Slime Control Chemical Chlorine Hypochlorites Chlorinated Phenols Sodium-- Potassium Permanganate Copper Sulphate * As Shipped. Trade Name Chlorine Calcium Hypochlorites Sodium Hypochlorites Chlorophenylphenate Tetrachlorophenate Pentachlorophenate Permanganate of Potash Blue Vitriol Physical State* Gas Crystalline Briquettes Briquettes Briquettes Crystalline Crystalline 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 or suitable chlorine liberating compounds may be used. After being cleaned, the other chem icals may be used to prevent the reestablishment, of slime in the system. The removal of green algae from a cooling tower should never .be used 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 treat ment which completely eliminates algae. Copper sulphate must be used with care because it can cause serious corrosion of steel in a system. It is also ineffective in alkaline water be cause the copper is precipitated from the water.