A cooling tower works by evaporating water. Vapour leaves pure — it carries no calcium, no chloride, no silica — so every mineral that came in with the makeup stays behind in a shrinking amount of water. The basin gets saltier every hour it runs. That is the whole of tower chemistry in one sentence, and everything else is bookkeeping against it.

Cycles of concentration, written $\text{COC}$, is how many times more concentrated the tower water is than the water it was made from. It is a bare ratio: no units, and never below 1. Three measurements give it, and they should agree.

By flow: $\text{COC} = \dfrac{M}{B}$ — COC equals M over B, where $M$ is the makeup, the fresh water entering, and $B$ is the blowdown or bleed, the water deliberately sent to drain. Both are flows in the same unit — m³/h, L/s, gpm, it does not matter which, so long as it is the same one.

By conductivity: $\text{COC} = \dfrac{\sigma_t}{\sigma_m}$, where $\sigma$ — sigma — is electrical conductivity in µS/cm. The subscripts run the same way throughout this chapter: t is the tower water, the concentrated sample from the basin, and m is the makeup, the dilute one from the fill line. Concentrated on top, always.

By chloride: $\text{COC} = \dfrac{\mathrm{Cl}_t}{\mathrm{Cl}_m}$, the same two subscripts, with chloride in mg/L from a titration. This is the referee. Chloride is conservative — nothing in a treatment programme consumes it, precipitates it or adds it. Conductivity is quicker but it is not conservative: feed sulphuric acid for pH control and conductivity climbs without the cycles climbing, so the meter flatters you. Let hardness scale out onto the tubes and conductivity falls while the trouble rises.

The number matters because of what it costs. Running at 2 cycles wastes water; running at 8 saves it but concentrates the scale-formers until they come out of solution on the hottest surface in the plant. Every treatment programme is an argument about where between those two to sit.