Document VG24e09B6QMrKmr3zj1Lqa9Zw

i 1054 CHAPTER 42 1957 Guided Corrosion and Water Formed Deposits, Causes and Prevention 1055 complished by the addition of polyphosphate in low concentrations." \ The use of polyphosphate in the ratio of two parts by weight to one part' To show the effect of concentration in a typical mechanical draft system assume: 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 ef 1. Recirculating rate = 100 gpm. fective stabilization of the iron is to occur. 2. Drift loss = 0.2 percent of recirculating rate = 0.2 gpm. Closed Recirculating Systems 3. Evaporation rate = 1 percent of recirculating rate = 1.0 gpm. 4. Calcium bicarbonate in make-up water = 100 ppm. i -- The closed recirculating system is one in which water circulates through one heat exchanger where it absorbs heat, has its temperature elevated, and circulates through another heat exchanger in which its temperature 5. Calcium bicarbonate in evaporated water = 0 ppm. Let K = concentration of calcium bicarbonate in recirculating water = concen tration of calcium bicarbonate in drift loss. is lowered. In cold water or chilled water systems scaling is seldom a problem. In hot water systems scaling is seldom a problem unless there is a large amount of makeup water.1* In this case surface-active agents The process of concentration in the system may be represented diagrammatically as follows: such as polyphosphate are used. i Evap. = 1 gpm Ca(HC0i)t = 0 ppm t_________ ijii ii /f Make up = 1.2 gpm Ca(HC0i)s = 100 ppm Recirculating Water System K = Ca(HC0,)i cone. Drift 0.2 gpm Ca(HCOt)t = K ppm iI !! i:i jl The concentration may be obtained from the equation *?/ (Make Up) [Ca(HCO,)t cone.] = (Evap.) [Ca(HCO,), cone.] + (Drift)[Ca(HCO,)s cone.] (1.2) (100) = (1) (0) + (0.2) (K) 1) , substituting, 751 ; TR >TV 25- ji HOS -t-n 120 = 0 + 0.2 K whence, 50 100 150 200 250 300 {! PPM ALKALINITY AS C* Co, OF MAKE - UP MATER K = 600 ppm Fig. 5. Relation op Bleed-Off Requirement to CaCOi in Make-Up Water Therefore the concentration of calcium bicarbonate in the recirculating |; Open Recirculating Systems water equals 600 ppm. This far exceeds the allowable concentration of 170 ppm at which scaling will occur. |; Where water from condensers and similar equipment is passed through To correct this situation, it is imperative to provide a continuous bleed- j; ;i a spray pond or cooling tower and then returned to the equipment, there is an increase in the concentration of solids because of the evaporation of off or blow down from the recirculating water circuit. Typical bleed-off requirements are shown in Fig. 5. Reference to the No Treatment curve f' some of the water into the cooling air, and, moreover, the aeration removes shows that a bleed-off rate of 1.2 times the evaporation rate is required ; carbon dioxide. Both factors promote the tendency to deposit scale. when a make-up water contains 100 ppm calcium bicarbonate (or alka : The corrosive properties of the water can also be increased through the linity) if scale is to be prevented. Calculation by a material balance will absorption of acid gases such as sulfur dioxide (S02), from the air.1' The show that the bleed-off plus drift loss will limit the calcium bicarbonate i absorption of gases from the air being processed and the concentration of to 171 ppm. |: solids due to evaporation are factors in air washer operation when the air It can be seen from Fig. 5 that very large amounts of bleed-off are re- 5 is humidified. qurred as a make-up water approaches or exceeds 150 ppm of alkalinity. The concentration of scale-forming minerals in open recirculated cooling When alkalinity exceeds 175 ppm, bleed-off alone is no longer effective, r water is limited by natural drift or windage loss. This is the loss of water the use of 5 ppm or less of polyphosphate is extremely effective in reducing | droplets from the system. Windage losses from typical systems may be the required bleed-off and making possible the use of make-up waters ; classified as follows, based on recirculating rates: which will not respond to bleed-off alone. Occasionally, waters are encountered which have exceptionally high Percent of Recirculat- alkalinities or which develop high pH values when recirculated. The use f? . in* Rate of sulfuric acid is common in these cases to maintain pH values between 1 Evaporative Condensers and AirWashers........................................0 to 0.1 < and 8. Polyphosphates are most always used when acid is required. } $ J Mechanical Draft Cooling Towers.......................... ..........................0-1 t ^ Atmospheric Cooling Towers............................................................. 0.3 to 1.0 Spray Ponds......................................................................................... 1.0 to 5.0 It is very important to control acid feeding carefully to avoid serious corrosion. Automatic pH controllers are often employed for such purpose.