How dissolved solids concentrate in a boiler, how much blowdown it takes to hold them at a limit, and how to turn that limit into a conductivity setpoint.
As in a cooling tower, cycles is how many times the feedwater has been concentrated:
Cycles = boiler water conductivity ÷ feedwater conductivity
At steady state, whatever the feedwater brings in, the blowdown must take out. That fixes the blowdown as a share of the feedwater:
Blowdown, as a share of feedwater = 1 ÷ cycles
Example. Feedwater at 300 µS/cm, boiler water held at 3000 µS/cm. The boiler runs at 3000 ÷ 300 = 10 cycles, so blowdown is 1 ÷ 10 = 10 % of the feedwater.
For a boiler producing 10,000 lb/h of steam:
Blowdown is boiler water at full temperature: every pound drained takes its heat, its treatment chemical, and its water with it. Running at more cycles means less blowdown.
| Cycles | Blowdown (% of feedwater) | Blowdown at 10,000 lb/h steam |
|---|---|---|
| 5 | 20 % | 2,500 lb/h |
| 10 | 10 % | 1,111 lb/h |
| 20 | 5 % | 526 lb/h |
| 50 | 2 % | 204 lb/h |
Returning more condensate is the cheapest way to more cycles. Condensate is nearly pure, so every gallon returned lowers the feedwater conductivity: at the same boiler limit, the boiler runs at more cycles and blows down less.
The boiler water limit comes from the boiler manufacturer and the treatment program — commonly based on ASME or ABMA guidance for the boiler’s type and operating pressure. Higher-pressure boilers need lower limits. Too high a limit causes:
Too low a limit wastes water, heat, and chemical with no benefit.
Limits are often written as TDS (ppm) rather than conductivity. Convert with the ratio for the water, typically TDS ≈ 0.5–0.7 × conductivity (µS/cm); measure it for the specific boiler water when accuracy matters.
High-alkalinity boiler water reads high. Hydroxide conducts far more strongly than the neutral salts, so an alkaline boiler water shows more conductivity than its TDS alone would suggest. Programs that run high alkalinity often specify neutralized conductivity — measured after neutralizing the sample — for setting limits. Follow the program’s method when converting.
The InteliHive’s Boiler profile creates the blowdown mode for you: a Setpoint Control mode, Rising, on the conductivity probe, driving the blowdown valve. See Sampling and Blowdown Control for how it samples.
Example. The program limit is 3000 µS/cm. Set the setpoint to 3000 µS/cm. The Boiler profile starts with a differential of 50 µS/cm, so blowdown stops once the reading falls to 2950.