Boiler blowdown removes concentrated dissolved solids that remain after steam generation. A simple mass balance can estimate the blowdown flow when feedwater and boiler-water dissolved-solids levels are known.
Mass-balance formula
If steam is assumed to carry negligible dissolved solids:
Blowdown = Steam flow × Feedwater TDS ÷ (Boiler-water TDS − Feedwater TDS)
Example
Steam flow = 10,000 kg/hr, feedwater TDS = 100 mg/L and boiler-water TDS = 2,500 mg/L.
Blowdown = 10,000 × 100 ÷ (2,500 − 100) = 416.7 kg/hr, approximately 0.417 m³/hr for dilute water.
Blowdown as a percentage
Feedwater flow is steam plus blowdown, approximately 10,416.7 kg/hr in this example. Blowdown as a percentage of feedwater is about 4.0%.
Why the actual setpoint may differ
Boiler-water limits depend on pressure, treatment chemistry, steam purity requirements and OEM guidance. Conductivity is often used for automatic control, but silica, alkalinity or other limits can govern in some systems.
Energy consideration
Blowdown contains heat. If blowdown is excessive, the plant loses hot water and treatment chemicals. Heat-recovery systems can recover part of this energy where economically justified.
Check the calculation against operation
Compare estimated blowdown with measured valve flow, tank level change or flowmeter data where available. A correct theoretical result does not guarantee a valve or controller is operating correctly.
Use the Ramven Boiler Blowdown Calculator for a quick estimate.