Document KRvdyXggpV7oa9zMj98r5Da4w

738 CHAPTER 55 . 1959 Guide precipitation ia induced, the Langelier Index is a more re liable indication of wealing tendency. Prevention of seale in once-thiough systems is based pri marily on the adjustment of pH, the application of surfaceactive agents, or a combination of the two. Normally the addition of a few parts per million of a surface-active agent such as a polyphosphate" fed proportionately to a water with a scaling tendency is ail that is required to control scale. Where a given water does not respond satisfactorily to surface-active agents, reduction of pH with sulfuric acid or carbon dioxide is often employed. The pH is reduced only sufficiently to have it fall within the effective range of the surface-active agent. The use of acid or carbon dioxide is sometimes permitted without polyphosphates when corro sion will not be severe. In this case the pH is reduced so that the Langelier Index is less than +0.5. The add or car bon dioxide must be added with care. Use of automatic pH controllers is required if corrosion is to be prevented. A special case of scale from ferrous bicarbonate decompo sition was mentioned previously. Ferrous bicarbonate is con siderably less stable than calcium bicarbonate. The preven tion of this decomposition can be accomplished by the addition of polyphosphate in low concentrations.0 The use of polyphosphate in the ratio of two parts by weight to one part by weight of iron is usually most effective. The polyphosphate must be added to the water before exposure of the water to air or chlorine if effective stabilization of the iron is to occur. Closed Recirculating Systems The closed recirculating system is one in which water cir culates through one heat exchanger where it absorbs heat, has its temperature elevated, and circulates through an other heat exchanger in which its temperature is lowered. In cold water or chilled water systems wealing is seldom a problem. In hot water systems scaling is seldom a problem unless there is a large amount of make-up water" In this case surface-active agents such as polyphosphate are used. Open Recirculating Systems Where water from condensers and similar equipment is passed through a spray pond or cooling tower and then returned to the equipment, there is an increase in the con centration of solids because of the evaporation of some of the water into the cooling air, and, moreover, the aeration removes carbon dioxide. Both factors promote the tendency to deposit scale. The corrosive properties of the water can also be inftrewwd through the absorption of acid gases such as sulfur dioxide (50,), from the air." The absorption of gases from the air being processed and the concentration of solids due to evaporation are factors in air washer operation when the air is humidified. The concentration of scale-forming minerals in open re circulated cooling water is limited by natural drift or wind age loss. This is the loss of water droplets from the system. Windage losses from typical systems may be classified as folloWs, based on recirculating rates: Percent of Bearcalatinf Rato Evaporative Condensers and Air Washers..............0 to 0.1 MechanicalDraft Cooling Towers............................ 0.1 to 0.3 Atmospheric Cooling Towers....... ............................. 0.3 to 1.0 Spray Ponds.................................................................. 1.0 to 5.0 To show the effect of concentration in a typical mechani cal draft system assume: 1. Recirculating rate = 100 gpm. 2. Drift loss " 0.2 percent of recirculating rate " 0.2 gpm. 3- Evaporation rate 1 percent of recirculating rate -- 1.0 gpm. 4. Calcium bicarbonate in make-up water * 100 ppm. 5. Calcium bicarbonate in evaporated water = 0 ppm. Let K = concentration of calcium bicarbonate in recirculat ing water -- concentration of calcium bicarbonate in drift loss. The process of concentration in the system may be repre sented diagrammatically as follows: Evap. = 1 gpm Ca(HCOi)t " 0 ppm t____________ Recirculating Water System _K TM Ca(HCOi)i cone. Drift 02 gpm Ca(HCOt)t " K ppm Make-up 1.2 gpm Ca{HCOt)t " 100 ppm The concentration may be obtained from the equation (Make-Up)[Ca(HCOj)t cone.) = (Evap.)[Ca(HCO*) cone.) + (Drift)[Ca(HCO,), cone.) (1.2) (100) - (1) (0) + (0.2) (K) substituting, 120 - 0 + 02K whence, K = 600 ppm Therefore the concentration of calcium bicarbonate in the recirculating water equals 600 ppm. This far exceeds the allowable concentration of 170 ppm at which nealing will occur. To correct this situation, it is imperative to provide a continuous bleed-off or blowdown from the recirculating water circuit. Typical bleed-off requirements are shown in Fig. 5. Reference to the No Treatment curve shows that a PPM ALKALPfTYAS CiCt,-OF MAKC-UP WMTCR Fig. 5 .... Relation of Bleed-Off Requirement to CaCOj in Make-Up Water Corrosion and Water-Formed Deposits, Causes and Prevention 739 bleed-off rate of 12 times the evaporation rate is required when a make-up water contains 100 ppm calcium bicar bonate (or alkalinity) if seale is to be prevented. Calcula tion by a material balance will show that the bleed-off plus drift loss will limit the calcium bicarbonate to 171 ppm. It can be seen from Fig. 5 that very large amounts of bleed-off are required as a make-up water approaches or exceeds 150 ppm of alkalinity. When alkalinity exceeds 175 ppm, bleed-off alone is no longer effective. The use of 5 ppm or less of polyphosphate is extremely effective in re ducing the required bleed-off and making possible the use of make-up waters which will not respond to bleed-off alone. Occasionally, waters are encountered which have excep tionally high alkalinities or which develop high pH values when recirculated. The use of sulfuric acid is common in these cases to maintain pH values between 7 and 8. Poly phosphates are most always used when acid is required. It is very important to control Mid feeding carefully to avoid serious corrosion. Automatic pH controllers are often employed for such a purpose. Hearing Systems In hot water heating systems or in steam heating boilers where all condensate is returned, troubles from scaling should not be severe. If necessary, sodium phosphate or sodium carbonate may be added to the water to prevent the formation of adherent calcium sulfate 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-exchange materials) or by hot-process precipitation softeners. In boilers operating at pressures above 100 psig virtually all the calcium, magnesium, silica, iron, *nH 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 calcium sulfate (anhydrite) scales is most to be feared. Such deposits form on the hottest evaporative sur faces. The scale has a low heat conductivity. Even a layer of egg shell thickness may so impede the rate of heat trans fer as to bring about over-heating of the metal. The orthophosphates of sodium are most frequently used to prevent sulfate 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 sulfate. To a lesser ex tent, 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 ma terials and colloids are sometimes found to mmimizp. scale formation. Where chemicals are introduced directly into the boiler in amounts adequate to prevent scale, sludge is formed in amounts proportionate to the efrlrimn and mag nesium 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 pro duce 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 quanti ties of slime will be produced. Some natural well waters do not contain sufficient foods to support luxuriant dime growths. Algae, which require light for carrying on their life processes, are likely to cause difficulty m 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 beat transfer surfaces. Other organisms capable of causing similar difficulties use such a wide variety of food material as algae," iron compounds," and inorganic sul fates." At present, the use of toxic chemicals and irradiation are the two general means employed in dime control. The value of ultra-violet light, used so broadly in the beverage industry, -is somewhat in dispute. Anti-fouling paints have been developed and are fairly satisfactory for the prevention of the growth of macro- Table 5 .... Common Chemicals Used for Slime Control Chemical Trod Naop Phyacal Sloto* Chlorine Hypochlorites Chlorinated Phenols . Sodium-- Potassium Per manganate Copper Sulfate,, Chlorine Calcium Hypochlorites Sodium Hypochlorites Chlorophenylphenate Tetrachlorophenate Pentachlorophenate Permanganate of Potash Blue Vitriol Gas Crystalline Briquettes Briquettes Briquettes Crystalline Crystalline * A Shipped. organisms such as bamades and mussels, but these paints must be renewed at frequent intervals, and are not appli cable to inaccessible areas such as the inside of pipe lines and 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 destroy 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. 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-