Blowdown is the controlled release of concentrated circulating water from the cooling tower basin to prevent excessive mineral and dissolved solids accumulation. As water evaporates, minerals remain behind, concentrating in the remaining water. Without blowdown removal, scaling, corrosion, and biological growth accelerate, reducing heat transfer efficiency and damaging equipment. Blowdown is typically the largest source of tower water loss after evaporation.
Technical Details
Blowdown volume is determined by the cycles of concentration, a ratio comparing dissolved solids in circulating water versus makeup water. A tower operating at 3 cycles of concentration removes one-third of the circulating water daily as blowdown. Typical systems operate between 2.5 and 5 cycles depending on water quality and treatment chemicals. For a 1,200 GPM tower, blowdown rates range from 8-20 GPM. Automated blowdown systems measure conductivity and adjust discharge volume proportionally.
Water Conservation Impact
Blowdown represents 30-50% of total cooling tower water consumption, making it the primary target for water conservation efforts. Higher cycles of concentration reduce blowdown but require better water treatment and corrosion inhibitors, increasing chemical costs. Reverse osmosis pre-treatment or other water recovery systems can reduce makeup and blowdown requirements by 40-60%. Regulatory restrictions in water-scarce regions increasingly limit allowable blowdown volumes, driving adoption of advanced water treatment technologies.