Boiler Blowdown & Cycles of Concentration (COC)
Cycles of concentration (COC) is how many times feedwater impurities are concentrated in the boiler: COC = boiler-water TDS ÷ feedwater TDS (or the chloride or silica ratio). Blowdown removes concentrated water to hold impurities within limits, and the blowdown rate ≈ 100 ÷ COC (%). Running higher COC means less blowdown — saving heat, water and chemicals — up to the ABMA/ASME limit.
What cycles of concentration means
Boiler water boils off as pure steam but leaves its dissolved solids behind, so impurities concentrate. Cycles of concentration (COC) is the ratio of impurity level in the boiler water to that in the feedwater — if the boiler water holds 10× the feedwater TDS, the boiler runs at 10 cycles. COC is limited by the maximum boiler-water TDS, alkalinity and silica the pressure band allows; exceeding it risks scale, carryover and foaming.
How to calculate COC
Use a species that stays in the water (does not evaporate or precipitate):
- TDS / conductivity: COC = boiler-water TDS ÷ feedwater TDS (quick and common).
- Chloride: COC = boiler-water chloride ÷ feedwater chloride — very accurate because chloride does not precipitate.
- Silica: also used, especially where silica carryover is the limiting factor.
Chloride and silica give the truest concentration factor; TDS/conductivity is convenient for continuous control.
Blowdown rate formula and a worked example
Blowdown rate as a fraction of feedwater is Blowdown % = 100 ÷ COC, or equivalently = feedwater TDS ÷ (max boiler TDS − feedwater TDS) × 100. Example: feedwater TDS 150 ppm, maximum boiler-water TDS 3000 ppm → COC = 3000 ÷ 150 = 20 cycles → blowdown = 100 ÷ 20 = 5.0% of feedwater. Halving feedwater TDS to 75 ppm allows 40 cycles and just 2.5% blowdown.
Why optimize blowdown
Every kilogram blown down is hot, treated water and energy sent to drain. Raising COC (by purifying feedwater) cuts blowdown, saving fuel, make-up water, chemicals and heat — often with heat-recovery on the blowdown as well. But COC must stay under the boiler-water TDS/alkalinity/silica limit, so the lever is better feedwater, not simply less blowdown.
How to control blowdown
- Continuous (surface/skimmer) blowdown — automatic control on conductivity/TDS holds cycles steady; add heat recovery.
- Bottom (mud) blowdown — short, periodic, to remove settled sludge.
- Automatic conductivity control — the most efficient way to hold COC at target without over-blowing.
COC vs blowdown (feedwater 150 ppm TDS example)
| Max boiler TDS | Cycles (COC) | Blowdown % | Note |
|---|---|---|---|
| 1500 ppm | 10 | 10.0% | High loss |
| 3000 ppm | 20 | 5.0% | Typical |
| 4500 ppm | 30 | 3.3% | Needs purer feed |
| 6000 ppm | 40 | 2.5% | Low-TDS feedwater |
Watch
Frequently asked questions
How do you calculate boiler cycles of concentration?
Divide a non-volatile impurity level in the boiler water by the same in the feedwater. The common methods are COC = boiler-water TDS ÷ feedwater TDS, or COC = boiler-water chloride ÷ feedwater chloride (most accurate, since chloride doesn't precipitate). Silica is also used where it is the limiting parameter.
What is the boiler blowdown rate formula?
Blowdown as a percentage of feedwater ≈ 100 ÷ cycles of concentration. Equivalently, blowdown % = feedwater TDS ÷ (maximum allowable boiler-water TDS − feedwater TDS) × 100. For example, 150 ppm feedwater and a 3000 ppm boiler limit give 20 cycles and 5% blowdown.
Why run higher cycles of concentration?
Higher COC means less blowdown, which saves hot water, make-up water, treatment chemicals and fuel energy. Because blowdown discharges hot, treated water, minimizing it (while staying within limits) is a major efficiency and cost saving — often combined with blowdown heat recovery.
What limits how high COC can go?
The maximum boiler-water TDS, alkalinity and silica allowed for the boiler's operating pressure (per ABMA/ASME) cap COC. Pushing past the limit risks scale, foaming and carryover into the steam. To raise COC safely you must purify the feedwater (softening/demineralization), not just reduce blowdown.
Does VCYCLETECH help optimize boiler blowdown and treatment?
Yes. VCYCLETECH supplies boiler scale/corrosion inhibitors, oxygen scavengers and amines, and provides a treatment and blowdown program based on your feedwater analysis and boiler limits to help you run higher cycles safely. Email sales@vcycletech.com with your water report.
About the manufacturer
VCYCLETECH is a China-based manufacturer of water treatment chemicals — boiler and cooling-water inhibitors, oxygen scavengers, amines, antiscalants, biocides, coagulants and flocculants — ISO 9001 / 14001 / 45001 certified, with a COA on every batch and OEM/ODM service. See our quality & certifications.
References
- Boiler blowdown — Wikipedia
- Boiler water — Wikipedia
- Minimize boiler blowdown (Steam Tip Sheet) — US DOE
Related: WT-503 · DEHA · Boiler water treatment · Program design by pressure · Oxygen scavengers