Open recirculating cooling water treatment deposition control

Main - Table of Contents - Company Website


Home Up


Cooling Towers Manual


Various forms of calcium sulfate scale are found in cooling water systems. The most important is gypsum which contains two waters of hydration. The hemihydrate and anhydrite forms are found to a much lesser extent. Figure III-a, which charted the solubility of CaC03, also shows the solubilities of these salts in relation to temperature.

The solubility of gypsum increases until the temperature reaches approximately 100F and then gradually decreases. This scale, however, has more than one hundred times the solubility of calcium carbonate at normal cooling water temperatures. Traditionally, this fact provided the basis for scale control. Specifically, sulfate ions replace alkalinity when sulfuric acid is fed to cooling water systems to control system pH. The result is that the system can be run at higher cycles of concentration without exceeding carbonate solubility limits. 

However, it is important to be reasonably sure that the addition of sulfuric acid does not cause the solubility of calcium sulfate to be exceeded. If it does, there are various other alternative acids that can be used, the benefits and drawbacks of which are discussed in more detail later in this section. Use the following solubility curve to predict the solubility of calcium sulfate for a given water. However, please note that you must remember to include the sulfate contribution from sulfuric acid when determining the total sulfate concentration of the recirculating water.



select a link to continue or go to the table of contents

Up ]

Water Services  2008 - All rights reserved