Dmitri L. Kouznetsov

Rational Development of New Cooling Water Chemical Treatment Programs for Scale and Microbial Control

Crystal growth is part of a more extensive problem in supersaturated cooling waters known as scale formation. Scale usually refers to an intimate mixture of a multitude of sparingly soluble mineral salts. Depending on the pH, temperature and water chemistry, these salts include but are not limited to calcium carbonate, calcium phosphate, silica, silicate salts, calcium sulfate, corrosion products (iron (hydr)oxides) etc. Calcium carbonate is the predominant component of the hard and tenacious scale deposited from natural waters, especially in processes involving heat transfer. The main focus of this paper is on two major system stresses, high Ca2+ and oxidizing biocides, and under conditions common in Heating, Ventilation and Air Conditioning (HVAC) systems. The chief factor that promotes calcium carbonate scale formation is concentration of dissolved solids by repeated partial evaporation of the water.1,2 Therefore, a certain water that has no scale tendency will eventually become scale-forming when concentrated n times, where n = 2, 3, 4 and even higher. The number n is defined as cycles of concentration (COCs). Economic considerations to minimize water use require low blowdown and increased COCs. The severity of the scaling tendency of a certain water is related to the saturation index (SI), defined as follows: [Ca2+].[Ca32-] SI = (1) KspCaCO3