Calculate blowdown rate, makeup water requirement and drift losses from evaporation rate and cycles of concentration.
≈ 1.5–2% of circulation flow rate
Typical range 3–6 cycles
Enter evaporation rate and CoC (> 1) to calculate.
Blowdown = Evaporation ÷ (CoC − 1). Makeup = Evaporation + Blowdown + Drift. Drift estimated at 0.2% of evaporation.
Blowdown (or bleed-off) is the deliberate removal of a portion of the circulating water to reduce the concentration of dissolved solids. As water evaporates, minerals concentrate. Blowdown dilutes these minerals by replacing concentrated water with fresh makeup water.
Cycles of concentration (CoC) is the ratio of dissolved solids in the circulating water versus the makeup water. A CoC of 4 means the circulating water is 4 times more concentrated than the makeup supply. Higher CoC saves water but increases scaling and corrosion risk.
Most cooling towers operate best at 3–6 cycles of concentration. Lower cycles waste more water; higher cycles risk scale and biological growth. The optimal target depends on makeup water quality and the chemical treatment programme in use.