Every kilogram of steam raised leaves its dissolved solids behind in the boiler water — blowdown is how that concentration is kept under the limit that causes scaling, carryover, and corrosion. A genuine daily calculation for boiler operators and water treatment chemists.
Verified against published standard
Steam Rate & Water Chemistry
Max. allowable boiler TDS depends on operating pressure and boiler design (typically 3000–3500 ppm for low-pressure fire-tube boilers, much lower for high-pressure utility boilers) — check the boiler manufacturer's limit, not a generic default.
Enter steam rate and TDS limits to calculate.
About This Calculation
Every kilogram of steam a boiler generates leaves behind whatever dissolved solids were in the feedwater, since steam itself is essentially pure — without continuous blowdown to remove some boiler water (and its accumulated solids) and replace it with fresh feedwater, dissolved solids concentration in the boiler would rise without limit, eventually causing scale formation, carryover into the steam, and corrosion. Blowdown rate is set by a straightforward mass balance on dissolved solids between what enters with feedwater and what must leave with blowdown to hold boiler water TDS at its allowable limit.
Frequently Asked Questions
Why does blowdown rate depend on the RATIO of feedwater to maximum boiler TDS, not just feedwater quality alone?
The mass balance is between solids entering (feedwater TDS × steam rate) and solids leaving (blowdown TDS, which equals the boiler’s maximum allowable TDS at steady state) — a wider gap between feedwater TDS and the allowable boiler limit means each unit of blowdown removes proportionally more solids, so less blowdown is needed for the same feedwater quality.
What is "cycles of concentration" and why does it matter for water treatment?
It’s simply the ratio of maximum allowable boiler TDS to feedwater TDS — it tells you how many times more concentrated boiler water solids are allowed to become relative to what enters, and it’s a genuinely useful single number for comparing blowdown efficiency across different feedwater treatment quality levels.
Does improving feedwater treatment (lower TDS) always reduce blowdown proportionally?
Yes, and often dramatically — since blowdown scales with feedwater TDS divided by the TDS gap to the boiler limit, halving feedwater TDS roughly halves required blowdown for the same boiler TDS limit, which is exactly why investment in feedwater treatment (demineralization, condensate polishing) pays back partly through reduced blowdown heat and water loss.
Why is recovering blowdown heat considered standard practice, not optional?
Continuous blowdown at even a few percent of steam rate represents meaningful thermal energy being discharged from the boiler at boiler pressure/temperature — a flash tank or heat exchanger recovering that heat back into the feedwater or deaerator system is a genuinely worthwhile efficiency gain at the blowdown rates a real boiler actually requires.