Flash steam
When high-pressure condensate drops to a lower pressure it cannot hold all its enthalpy, so the surplus becomes vapour. That flash steam is energy that has already been paid for, and a flash vessel recovers it rather than venting it to atmosphere. The fraction depends only on the two pressures.
| Quantity | Value | Note |
|---|---|---|
| Condensate flow | 1000 kg/h | from the traps |
| Upstream pressure | 10.0 bar | saturated at 180 °C |
| Upstream h_f | 762.6 kJ/kg | liquid enthalpy |
| Downstream pressure | 1.0 bar | saturated at 100 °C |
| Downstream h_f | 417.5 kJ/kg | liquid enthalpy |
| Downstream h_fg | 2257.9 kJ/kg | latent heat |
| Enthalpy drop | 345.1 kJ/kg | the driving quantity |
| Flash fraction | 15.28% | (h_f1 − h_f2) / h_fg2 |
| Flash steam | 153 kg/h | recoverable vapour |
| Remaining liquid | 847 kg/h | to the return line |
| Energy per hour | 95.9 kWh | flash mass × h_fg |
| Flash line size | DN 80 | at 20 m/s, 68 mm bore |
Flash steam is not a loss mechanism — it is energy that was already paid for, and a flash vessel recovers it rather than venting it. The fraction rises with the size of the pressure drop and is highest when condensate returns to atmosphere.
(h_f1 − h_f2) / h_fg2 · Energy balance across the trap.
ṁ × fraction · Vapour available.
ṁ_flash × h_fg2 · What a flash vessel recovers.
kWh × fuel price · Annualised.
A clear path from inputs to results.
When high-pressure condensate drops to a lower pressure it cannot hold all its enthalpy, so the surplus becomes vapour. That flash steam is energy that has already been paid for, and a flash vessel recovers it rather than venting it to atmosphere.
How to use
- Enter the condensate flow.
- Set the upstream and return pressures.
- Add the running hours and the cost of steam.
- Read the flash fraction, the mass recovered and its annual value.
The formula
Reference: Energy balance across the trap; saturated properties from IAPWS-IF97.
Worked example
1,000 kg/h dropping from 10 bar to 1 bar
- 1,000 kg/h of condensate.
- 10 bar absolute upstream, 1 bar absolute return.
- 6,000 running hours, $0.04 per kWh.
- 1
At 10 bar h_f is 762.6 kJ/kg; at 1 bar it is 417.5 kJ/kg.
- 2
The enthalpy drop is 345.1 kJ/kg and the latent heat at 1 bar is 2,257.9 kJ/kg.
- 3
The flash fraction is 345.1 / 2,257.9 = 15.3%.
- 4
That is 153 kg/h of flash steam, leaving 847 kg/h as liquid.
- 5
At 2,257.9 kJ/kg the flash carries about 96 kWh an hour.
15.3% flashed · 153 kg/h recovered · about 96 kWh an hour.
Interpreting the result
Flash steam is not a loss mechanism — it is energy already paid for. The fraction rises with the size of the pressure drop, which is why condensate is returned at the lowest practical pressure and why a flash vessel pays for itself quickly on a large steam system.
Assumptions & limitations
- Saturated condensate at the upstream pressure.
- No heat loss across the trap.
- The downstream pressure is the return-line pressure.
- Fuel cost is expressed per kWh of steam energy.
Frequently asked questions
Is flash steam a problem or an opportunity?
Both. Venting it wastes energy; letting it into a return line without sizing for it causes water hammer and back pressure. A flash vessel turns it into an opportunity.
What is a flash vessel?
A small vertical separator that drops the condensate pressure, takes the vapour off the top to a low-pressure main and drains the liquid from the bottom.
How is the value calculated?
Flash mass times the latent heat gives the energy, which is priced at the cost of the steam that would otherwise have been raised.
Why does more pressure drop flash more?
Because the liquid enthalpy falls faster than the latent heat does, so a larger share of the energy cannot stay in the liquid phase.
Does subcooled condensate flash less?
Yes. Flashing assumes the condensate arrives at saturation; a subcooled trap discharge has already given up some enthalpy to the process.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.