Formula
Dissolved solids enter only with the feedwater and leave only with the blowdown (steam carries none):
\(F\,c_{fw} = B\,c_{bd} \qquad F = S + B \quad\Rightarrow\quad B = \frac{S\,c_{fw}}{c_{bd} - c_{fw}}\)
\(N = \frac{c_{bd}}{c_{fw}} \qquad B/F = \frac{c_{fw}}{c_{bd}} \qquad \dot Q_{bd} = B\,(h_f - h_{fw})\)
Variables
| Symbol | Meaning | SI unit |
|---|---|---|
| \(S\) | steam generated | kg/s |
| \(c_{fw}\) | dissolved solids in feedwater (mass fraction) | – |
| \(c_{bd}\) | maximum dissolved solids in boiler water | – |
| \(p\) | drum absolute pressure | Pa |
| \(T_{fw}\) | feedwater temperature | K |
| \(B\) | blowdown flow | kg/s |
| \(B/S\) | blowdown as a share of steam | – |
| \(B/F\) | blowdown as a share of feedwater | – |
| \(F\) | feedwater flow | kg/s |
| \(N\) | cycles of concentration | – |
| \(\dot Q_{bd}\) | heat leaving with the blowdown, above feedwater | W |
| \(h_{f}\) | saturated liquid enthalpy at drum pressure | J/kg |
| \(h_{fw}\) | feedwater enthalpy | J/kg |
Method and standards
Steady-state mass balance on dissolved solids, the standard basis for continuous blowdown control. Enthalpies from FireCAD's IAPWS-IF97 implementation. Boiler-water limits are the user's input, taken from the boiler maker or a recognised water-chemistry guideline for the drum pressure.
Assumptions and limits
- Steady state; no solids carried over with the steam and none removed by intermittent (bottom) blowdown.
- Concentrations by mass (ppm = mg/kg); for dilute water mg/L is the same number.
- Feedwater enthalpy at drum pressure.
References
- IAPWS R7-97(2012) IF97
- ASME water chemistry guidelines (limits)
Questions and answers
How is blowdown calculated?
By a dissolved-solids balance. Solids enter with the feedwater and leave only with the blowdown, so B = S·c_fw/(c_bd − c_fw). For 10 t/h with 150 ppm feed and a 3 000 ppm limit, B = 526 kg/h, 5.3 % of the steam.
Where do the boiler-water limits come from?
From the boiler maker or a water-chemistry guideline for the drum pressure and the controlling species (TDS, silica, alkalinity, chloride). Use the species that gives the highest blowdown.
Why does the heat loss matter?
Blowdown leaves as saturated water at drum pressure. For a 50 t/h boiler at 45 bar(a) blowing down 3.4 % of steam, about 254 kW leaves above the feedwater enthalpy; a flash vessel and cooler can recover most of it.