TDS Control and Blowdown: How Much Water to Throw Away
A boiler concentrates the dissolved solids in its feedwater. Left alone, the concentration rises until the water foams and carries over into the steam. Blowdown removes a little concentrated water continuously or in bursts, replacing it with fresher feed. Getting the amount right is a balance between water quality and wasted heat.
You'll learn
- Explain why TDS rises and why it must be limited
- Calculate blowdown from feedwater and boiler TDS
- Work out the heat lost in blowdown
- Compare continuous and intermittent blowdown, and heat recovery
Before this: Raw water quality, Softeners and RO
Why water has to be thrown away
Steam leaves the boiler almost pure; the dissolved solids that came in with the feedwater stay. Every hour, the boiler water gets a little more concentrated. Above a certain level it starts to foam, the foam is carried out with the steam, and dissolved solids and water end up in the steam mains and on the process. Sludge also settles at the bottom.
Blowdown is the deliberate removal of some boiler water to keep the concentration below its limit. It is necessary, and it is also a loss: the water removed is at boiler temperature and has already been treated.
How much blowdown
| Symbol | Meaning | Unit |
|---|---|---|
| Dissolved solids in the feedwater | ppm | |
| Maximum allowed in the boiler water | ppm |
- Feed TDS 300 ppm; boiler water held at 3,000 ppm; 2,000 kg/h of steam at 10 kg/cm²(g); feedwater 80 °C.
- Blowdown percentage:
- Blowdown flow: 2,000 × 0.111 = 222 kg/h.
- Each kilogram leaves at 778.0 kJ/kg, having come in at 335.8 kJ/kg. Heat lost: 222 × 442.2 = 98,264 kJ/h (27 kW).
Continuous, intermittent and recovered
- Intermittent (bottom) blowdown: short, fast blows from the lowest point to remove sludge. Necessary, but a poor way to control TDS: concentration swings up and down.
- Continuous (surface) blowdown: a small, steady flow taken from just below the water surface, where concentration is highest. Holds TDS steady, close to its limit, which minimizes the water thrown away.
- Automatic TDS control: a conductivity probe measures boiler water TDS and opens a blowdown valve only when needed. It removes the guesswork that usually leads operators to blow down too much.
Because continuous blowdown is steady, its heat can be recovered. Passed into a flash vessel at about 0.2 kg/cm²(g), roughly 15% of it flashes to low-pressure steam that can feed the deaerator; the remaining hot water can preheat make-up through a heat exchanger before it goes to drain. Together they recover most of the blowdown heat.
Key takeaways
- Blowdown keeps boiler water TDS below the limit at which it foams and carries over.
- Blowdown % = feed TDS ÷ (max TDS − feed TDS) × 100.
- At 300 ppm feed and 3,000 ppm limit: 11.1% blowdown, about 2.0% of boiler heat.
- Continuous, automatic blowdown with flash and heat recovery minimizes the loss.