What Causes Scale in Cooling Towers?

The mechanism: evaporation without removal

A cooling tower rejects heat by evaporating water. What evaporates is pure water; every dissolved mineral that came in with the make-up stays behind. Replace the evaporated volume with fresh make-up, and you have added another dose of the same minerals to a system that is already carrying the last one.

Concentration therefore rises continuously unless water is deliberately removed. That deliberate removal is blowdown, and the ratio between the concentration in the system and the concentration in the make-up is the cycles of concentration.

Scale forms when a dissolved salt exceeds its solubility at the conditions it finds itself in. Calcium carbonate is the usual culprit, because it is abundant in most supply waters and because it becomes *less* soluble as temperature rises.

Why it appears on the surfaces you care about

That inverse solubility is why cooling tower scale is such an expensive problem. The salt precipitates preferentially where the water is hottest — which is the heat-transfer surface, which is the one part of the system whose performance you are paying for.

Bulk water temperature is not the number that matters here. Skin temperature at the tube wall is higher than the bulk, and it is the skin temperature that determines whether precipitation occurs. A system can look comfortable on a bulk measurement and still be scaling at the tube.

The four things that decide how much scale you get

1. Make-up water chemistry. Calcium, magnesium, alkalinity and silica in the incoming supply set the starting point. Two identical towers on different supplies are not the same problem.
2. Cycles of concentration. Every cycle multiplies the dissolved concentration. Running higher cycles saves water and chemical, but it moves the system closer to the point where salts precipitate.
3. pH. Carbonate equilibrium shifts with pH. As pH rises, more of the dissolved carbon species is present as carbonate, and calcium carbonate becomes more likely to precipitate.
4. Surface temperature. The hottest surface in the circuit sets where and how fast deposition happens, regardless of what the bulk water reading says.

Indices such as the Langelier or Ryznar Saturation Index combine several of these into a single number. They are useful for direction — is this water scaling or corrosive — but they are indicators, not predictions, and they do not account for the inhibitor programme in the system.

How scale is actually controlled

Threshold inhibitors work sub-stoichiometrically: a small dose interferes with crystal growth so that the salt stays in solution above the point where it would normally precipitate. This is what makes higher cycles of concentration possible without depositing on the exchanger.

Dispersants handle what does form, keeping particles mobile so they leave with the blowdown rather than settling in low-velocity areas. Side-stream filtration removes the settled and suspended load mechanically, which reduces the demand placed on the chemical programme.

And control ties it together: conductivity-driven blowdown holds cycles at the value the chemistry supports, rather than at a conservative fixed setting that wastes water or an optimistic one that scales the plant.