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Low molecular weight substances or polymers (those whose chains are not long enough to link particles together via the bridging mechanism), tend to be absorbed on individual particles and on growing crystals of scale. The action of these substances in preventing scale can be classified as threshold inhibition, whereby sub-stoichiometric quantities of inhibitor agents, inhibit scale formation. Threshold inhibition can act through dispersion, sequestration, or scale crystal distortion, but the most common in water treatment are the substances that act through supersaturation. If we observe the solubility diagrams, we distinguish three zones. The zone of the solubility, the zone of unstability, and the heavily supersaturated area.
For the simple chemistry, every substance, dissolved into make-up water, enters the cooling system below the supersaturation limits, and most often at the “stable” region. Inside the system, due to evaporation which take place at the cooling tower, its concentration is increased. As long as the bulk water temperature into the system is higher than the temperature of the make-up, the solute takes a place onto solubility diagram positioned higher and right to its initial position. Speaking for scale forming solutes, like calcium Carbonate or Calcium Sulfate, this new position is inside the supersaturated area, and most probably inside the unstable region. The water, cooling the process, is heated at the heat exchangers and its temperature is increased. Taking under consideration the skin temperature of the critical heat exchangers, the final temperature is munch higher than the bulk water temperature, even higher than the boiling point of the water. This temperature increase, transfers the position of the specific water away the unstable region, inside the heavily supersaturated section. And deposition takes place. Deposition starts at the boundary of supersaturated and heavily supersaturated, and will be deposited a sufficient quantity of substance as to drop the concentration at the intersection of the undersaturation line to the specific surface skin temperature.
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