Boiler Blowdown & Cycles of Concentration Calculator
Optimize surface continuous blowdown rates, maintain target boiler water TDS levels, prevent mineral scale carryover, and quantify unrecovered thermal energy losses.
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Boiler Water Chemistry: Blowdown & Cycles of Concentration
As water is boiled into pure steam vapor inside an industrial boiler drum, non-volatile minerals (calcium, magnesium, silica, chlorides, and alkalinity) are left behind in the boiler liquid. Without regular blowdown, these total dissolved solids (TDS) concentrate until they precipitate out as insulating rock-hard scale on the boiler tubes, drastically reducing heat transfer, blistering tubes, and causing foaming and moisture carryover into the steam main.
Key Boiler Water Chemistry Formulas
- Cycles of Concentration (COC):
The number of times mineral salts concentrate relative to incoming feedwater:COC = Boiler Water TDS (PPM) / Feedwater TDS (PPM) - Blowdown Percentage (% BD):
The percentage of incoming feedwater that must be purged to maintain chemical equilibrium:Blowdown % = (Feedwater TDS / Boiler Water TDS) × 100 = 100 / COC - Mass Balance Flow Rates:
Feedwater Flow (lbs/hr) = Steam Flow / (1 - (Blowdown % / 100))Blowdown Flow (lbs/hr) = Feedwater Flow - Steam Flow - Blowdown Heat Loss:
Q_loss (BTU/hr) = Blowdown Flow × (h_boiler_water - h_makeup_water)
Installing a continuous blowdown flash tank and heat exchanger can recover 70% to 85% of this thermal energy into boiler makeup water.
Frequently Asked Questions
What are the ABMA recommended limits for boiler water TDS?
Per the American Boiler Manufacturers Association (ABMA): for operating pressures up to 300 psig, maximum allowable boiler water TDS is 2,500 to 3,500 PPM; for 301-450 psig, 3,000 PPM; for 451-600 psig, 2,500 PPM; and for high-pressure power boilers (>1,000 psig), under 100 PPM with demineralized water.
What is the difference between surface and bottom blowdown?
Surface blowdown operates continuously via an automatic conductivity controller skimming water from 2-3 inches below the steam-water interface where dissolved solids (TDS) and foam concentrate. Bottom blowdown is opened manually for 10-15 seconds per shift to blast out heavy precipitated sludge that settles in the bottom mud drum.
Why not run at very high cycles of concentration (e.g. 50+ cycles)?
While higher cycles reduce water waste, increasing cycles past 20-25 yields diminishing returns and creates severe scaling risks. If silica exceeds 150 PPM or hardness drops out of chemical suspension, glass-like silicate scale coats the burner fire tubes, leading to tube metal rupture within hours.
How does a blowdown heat recovery system save money?
Hot blowdown liquid (e.g. 353°F at 125 psig) flashes into low-pressure steam in a flash tank for use in the deaerator, while the remaining hot drain passes through a shell-and-tube heat exchanger to preheat cold municipal makeup water, cutting fuel bills by thousands of dollars annually.