Pure water is a rare commodity. Water, as we know it, contains many dissolved minerals. When evaporation occurs in a cooling tower, only the water evaporates; it exits the cooling tower as water vapor, but leaves the minerals behind to concentrate in the cooling tower’s water system.
The concentrations of these dissolved minerals gradually increase until a process called precipitation occurs. Precipitation happens when dissolved minerals such as calcium carbonate (limestone) reach a certain concentration and become solid, usually clinging to equipment and piping surfaces in the cooling tower. HVACR personnel refer to these solids as scale.