Boiler Blowdown Rate from Steam Rate

Feedwater equals steam plus blowdown, and a solids balance says feedwater equals blowdown times the cycles: put the two together and B = S/(COC − 1). It is the same algebra as the cooling tower, with steam playing the role of evaporation, which is exactly right: both are pure water leaving and both are what drives the concentration. A 10,000 lb/h boiler held at 20 cycles must blow down 10,000/19 = 526 lb/h, about 1 gpm of scalding water.

Sizing that flow is what an orifice-plate or automatic surface blowdown valve does, and getting it wrong is expensive in both directions. Blow down too little and the drum climbs past its TDS limit until it foams and carries over; blow down too much and you are throwing away treated, deaerated, chemically dosed water at saturation temperature, which is why any boiler over roughly 500 lb/h of continuous blowdown deserves a flash tank and a blowdown heat exchanger, recovering 80% of that energy into the makeup. Note that the blowdown here is continuous surface blowdown only; intermittent bottom blows for sludge removal sit on top of this figure.