Fouling characteristics of cooling tower water : pH effect

The fouling characteristics of simulated cooling tower water flowing over a heated metallic surface has been studied. The bulk temperature of the water was maintained around 95°F. Mainly three different water qualities with respect to calcium hardness level were investigated. The pH was controlled by the addition of hydrochloric acid. The flow velocities ranged from 1.5 ft/sec to 6.0 ft/sec, and surface temperatures averaged between 150°F and 200 °F. Runs 102, 103 and 104 were particularly made to investigate the relative fouling behavior of three different metallic surfaces exposed to cooling tower water of the same quality. For cooling tower water of pH of 7.5 and flow velocity of 4.0 ft/sec, the following equation relates the asymptotic fouling resistance to the surface temperature. R[subscript f]* = 3.6902 x10⁶ exp(-14308/T[subscript s]) Similarly for cooling tower of pH 8.5 and flow velocity of 5.0 ft/sec, the following arrhenius relationship was obtained. R[subscript f]* = 0.3264 exp(-4314.8/T[subscript s]) For both values of pH of 7.5 and 8.5 for cooling tower water, it was shown that asymptotic fouling resistance decreased with increase of flow velocity. For a flow velocity of 4.0 ft/sec, it was shown that a minimum value of R[subscript f]* exists in the neighbourhood of pH of 8.0. The results of the deposit analysis for runs made at an average pH of 8.5 indicate calcium, magnesium, silicon and carbonate as the major constituents. The chemical analysis of scales of runs made at an average pH of 7.5 indicate silicon, iron and copper as the chief constituents. Also it was found that inner layer of deposit contained higher amounts of magnesium, copper and silicon and the outer layer had higher percentage of calcium