How to size boiler blowdown
Steam leaves a boiler as pure water, but every dissolved solid the feedwater carries in stays behind. Without blowdown the total dissolved solids (TDS) in the boiler water climb until scale, carryover and corrosion appear, so a planned share of boiler water is drained to hold concentration inside the allowable band.
The math comes from a solids mass balance: the solids entering with feedwater must equal the solids leaving with blowdown. That gives cycles of concentration = boiler water TDS ÷ feedwater TDS and blowdown = steam rate ÷ (cycles − 1). Required feedwater is simply steam production plus blowdown.
With 10,000 lb/hr of steam, 100 ppm feedwater TDS and a 3,000 ppm boiler water limit, the boiler runs at 30 cycles and blows down about 345 lb/hr, roughly 3.3% of feedwater. Raising cycles cuts that loss but demands tighter water treatment, so never exceed the TDS ceiling published for your boiler.
Treat the result as a design baseline for continuous blowdown. In service, an automatic TDS controller and routine water analysis fine-tune the rate, and on larger plants a flash tank or blowdown heat exchanger is usually worth recovering the lost heat.
Frequently asked questions
Feedwater keeps bringing dissolved solids in while the steam leaves as pure water, so boiler water concentration climbs without limit. That leads to scale, carryover and overheated heating surfaces.
It depends on operating pressure and water treatment. Low pressure firetube boilers often run 20 to 50 cycles, while higher pressure units have much lower allowable TDS and therefore fewer cycles. Follow the water chemistry limits published by your boiler maker.