Steam Boiler Surface Blowdown (Cycles of Concentration)
The continuous surface blowdown a steam boiler needs to hold its dissolved solids below the limit, by a TDS mass balance: the steam leaves TDS-free, so the feedwater solids must exit in the blowdown at the max boiler-water concentration. Cycles of concentration CoC = boiler-water TDS limit / feedwater TDS; blowdown rate = steam rate x feedwater TDS / (limit - feedwater TDS); as a share of feedwater it is 1/CoC. A 10,000 lb/hr boiler on 100 ppm feedwater held to 3,500 ppm runs at 35 cycles and blows down about 294 lb/hr (2.9% of feedwater); cleaner makeup raises the cycles and cuts the blowdown and its heat loss. Every pound blown down is hot treated water lost -- a flash vessel and heat exchanger recover some. Steady-state surface blowdown only; bottom (mud) blowdown, the treatment program, the ASME/manufacturer limits, and a licensed operator govern. Distinct from cooling-tower cooling-water-makeup.
Formula and source
cycles_of_concentration = max_boiler_tds / feedwater_tds; blowdown_rate_lb_hr = steam_rate x feedwater_tds / (max_boiler_tds - feedwater_tds); blowdown_pct_of_feedwater = 100 x feedwater_tds / max_boiler_tds = 100 / CoC.
Steam-boiler surface-blowdown TDS mass balance (cycles of concentration), by name; the ASME / manufacturer boiler-water limits and the treatment program govern the actual schedule.
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