Calcium carbonate scaling in a deluged dry cooling system Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ng451n19j

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  • Scaling characteristics of calcium carbonate on a heated surface in a deluged dry cooling system were investigated. Bulk temperature of the deluge water was maintained constant at 90°F. Air flow velocity was maintained constant at 181 cfm., and deluge water flow was maintained at the minimum level that provided thorough wetting on the heat transfer surface. The heat flow ranged from 540 Btu/hr-ft² to 1616 Btu/hr-ft². The wall temperatures ranged from 90°F with the deluge water on, to 120°F to 240°F with the deluge water off. Deluge water was supplied as city water fortified with dissolved calcium carbonate. The calcium hardness ranged from 144 ppm CaC0₃ to 177 ppm CaC0₃, and the pH ranged from 7.5 to 8.7. The deluge time ranged from 20 sec. to 8 min., and the drying time between deluges was maintained constant at 10 min. The amount of scale deposited on the horizontal cylindrical surface was a function of the radial position, with the greatest amount of fouling occurring at the bottom. The rate of growth of scale was a function of water chemistry, pH, number of deluge cycles, and duration of deluge cycle. The fouling resistances after 1500 cycles ranged from 0.2 x 10⁻⁴ hr-ft²-°F/Btu for Langelier Saturation Index (LSI) of -1.2 to 1.3 x 10⁻⁴ hr-ft²-°F/Btu for LSI of 1.0. The net deposition was dependent on both sensible and evaporative heat transfer effects. The deposition rate was constant for the first 1500 cycles, and it declined after that. The amount of data was insufficient to allow determination of the cause for the decline. It was not determined whether the fouling resistance had reached an asymptotic value. The heat flux appeared to have no effect on the fouling rate.
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