Cycles of Concentration (COC = M/B)

An evaporative cooling tower throws away pure water vapour and keeps every dissolved mineral behind, so the water in the basin steadily gets saltier. Cycles of concentration is simply how many times more concentrated that circulating water has become than the makeup that feeds it — and because every litre of makeup either evaporates or leaves as blowdown, the ratio of makeup to blowdown gives the same answer as any chemical ratio. A tower taking 100 gpm of makeup while bleeding 20 gpm to drain is running at 100/20 = 5 cycles.

Cycles is the single number that decides whether a tower saves water or eats itself. Before the 1970s most towers ran at two or three cycles because nobody trusted the chemistry to hold scale off; modern phosphonate and polymer programs routinely hold six to ten, and every cycle you add cuts blowdown sharply — going from 3 to 6 cycles halves the bleed. The trap is chasing cycles past what the makeup can support: once calcium × alkalinity crosses the saturation line the tower fills with carbonate scale, and a fouled condenser costs far more in compressor power than the water ever saved.