Blowdown Heat Recovery: Flash Vessel and Heat Exchanger
Continuous blowdown leaves the boiler as water at full saturation temperature. Sent straight to drain, all that heat is lost. A flash vessel and a heat exchanger recover most of it, and it is one of the simplest heat recovery projects in a boiler house.
You'll learn
- Calculate the heat leaving in blowdown
- Explain flash recovery to the deaerator
- Explain heat exchange to the make-up water
- Estimate the fuel saved
Before this: TDS control and blowdown, Flash vessels
Hot water down the drain
At 10.54 kg/cm²(g), boiler water is at 185.5 °C and carries 787.4 kJ/kg. Blowdown leaves at that enthalpy. It was fed in as cold make-up, so every kilogram blown down takes the difference with it — along with water that was paid for, softened and dosed.
Recovery works in two stages. First, the blowdown is let down into a flash vessel at low pressure, where part of it flashes to clean steam that goes to the deaerator or feed tank. Second, the remaining hot water passes through a heat exchanger that warms incoming make-up before it goes to drain.
A worked example
- 5,000 kg/h of steam at 10.54 kg/cm²(g); continuous blowdown 4% (200 kg/h); flash vessel at 0.5 kg/cm²(g); make-up at 30 °C; exchanger cools the drain to 40 °C.
- Heat leaving in blowdown, against make-up: 200 × (787.4 − 125.8) = 1,32,308 kJ/h.
- Flash: 14.4% flashes, 28.8 kg/h of steam to the deaerator, recovering 73,818 kJ/h.
- Exchanger: 171.2 kg/h cooled from 111.4 to 40 °C recovers 51,333 kJ/h into the make-up.
- Recovered: 1,25,151 kJ/h, or 95% of the blowdown heat.
Design points
- Recovery suits continuous blowdown, which is steady. Intermittent bottom blowdown carries sludge and is better sent to a blowdown vessel.
- The flash vessel needs a float trap to drain the liquid and a safety valve; the flash steam line goes to a point that can always use it.
- Blowdown carries dissolved solids and some suspended matter: choose an exchanger that can be cleaned, and keep the hot side free-draining.
- Recovery is worth most where blowdown is high — hard or high-TDS make-up, little condensate return. Cutting the blowdown rate itself, through better feedwater, comes first.
Key takeaways
- Blowdown leaves at saturation temperature; all of that heat above make-up is lost if drained.
- A flash vessel recovers clean low-pressure steam; an exchanger recovers heat from the rest.
- Together they recover most of the blowdown heat.
- Reduce the blowdown rate first, then recover what remains.