Flash Steam Recovery
When high-pressure condensate is discharged to a lower pressure, some of it re-evaporates as flash steam. Learn how to capture and use this free energy instead of venting it to atmosphere.
What is Flash Steam?
When condensate at high pressure is discharged to a lower pressure, its temperature exceeds the boiling point at the lower pressure. The excess heat causes a portion of the condensate to instantly re-evaporate. This is flash steam.
For example, condensate at 10 bar (184 degrees C) discharged to atmosphere (100 degrees C) will produce approximately 15% flash steam by mass. That means 15% of the condensate's energy content is released as usable steam, for free.
Flash Steam Calculation
The percentage of flash steam produced can be calculated from the enthalpy difference between the high and low pressure conditions.
| From Pressure | To Pressure | Flash Percentage |
|---|---|---|
| 10 bar | 0 bar (atm) | ~15% |
| 10 bar | 1 bar | ~9% |
| 7 bar | 0 bar (atm) | ~13% |
| 7 bar | 1 bar | ~7% |
| 4 bar | 0 bar (atm) | ~10% |
| 4 bar | 1 bar | ~5% |
Flash steam formula
Flash percentage = (hf1 minus hf2) divided by hfg2, multiplied by 100. Where hf1 is the enthalpy of condensate at high pressure, hf2 is the enthalpy at low pressure, and hfg2 is the latent heat at low pressure. Use steam tables for values.
How to Recover Flash Steam
- Flash vessel: A pressure vessel that separates flash steam from condensate. High-pressure condensate enters the top, flash steam exits the top outlet, and remaining condensate exits the bottom.
- Route to low-pressure steam main: Flash steam from the flash vessel is piped into the low-pressure steam distribution system, supplementing boiler steam supply.
- Direct use: Flash steam can be used directly for feedwater heating (in the deaerator), space heating, or low-temperature process heating.
- Steam injector: A device that injects flash steam into a low-pressure steam main, using a thermocompressor to boost pressure if needed.
Economics of Flash Recovery
A typical flash steam recovery project pays for itself in 6 to 18 months. The recovered steam displaces boiler steam that would otherwise need to be generated, saving fuel proportionally.
Example: A process producing 1,000 kg/hr of condensate at 7 bar, discharged to a 1 bar flash vessel, recovers approximately 70 kg/hr of flash steam. At $0.02/kg steam, that is $11,200/year in savings. The flash vessel and piping might cost $5,000 to $10,000.
Tip: If you see steam venting from your condensate receiver tank, you are wasting flash steam. Install a flash vessel and route that steam to a low-pressure application.