How much fuel can I save by recovering flash steam from condensate?
When condensate at 10 bar(g) drops to atmospheric pressure, about 16% of it turns into flash steam. Piping that steam into a low-pressure user instead of venting it saves the boiler fuel needed to make the same steam. Work out the flash fraction from steam tables, then the heat, then the fuel.
The flash fraction formula
Flash fraction = (hf at the high pressure minus hf at the low pressure) / hfg at the low pressure, where hf is the enthalpy of saturated water and hfg the latent heat, both from steam tables. From 10 bar(g) (about 781 kJ/kg) to atmospheric (419 kJ/kg, hfg 2,257 kJ/kg): (781 minus 419) / 2,257 = 16%. Flashing to 1 bar(g) instead (hf 505 kJ/kg, hfg 2,202 kJ/kg) gives about 12.6%.
Where flash steam can go
- A low-pressure header that feeds heating, such as tank heating, deaerators, air heaters or wash water.
- The deaerator or feed water tank, as long as it does not overheat and vent the steam anyway.
- The flash vessel needs a low-pressure user with demand at the same times the high-pressure condensate is produced; otherwise the steam is vented and nothing is saved.
From flash steam to fuel
- Flash steam (kg/h) = condensate flow x flash fraction.
- Heat recovered (kJ/h) = flash steam x hfg at the low pressure.
- Fuel heat saved = heat recovered / boiler efficiency.
- CO2 saved = fuel energy saved x the fuel's CO2 factor per GJ, or fuel quantity saved x Senseible's per unit factor.
Do not count the saving twice
The residual condensate leaving the flash vessel can still be returned to the boiler; that saving belongs on the condensate return calculation. Count the flash steam's latent heat here and the hot water's heat there, not both on the same kilogram.
Worked example: 2,000 kg/h of condensate at 10 bar(g) flashed to 1 bar(g)
- Flash fraction: (781.1 minus 504.7) / 2,201.6 = 12.6%
- Flash steam: 2,000 x 0.1256 = 251.2 kg/h
- Heat recovered: 251.2 x 2,201.6 = 553,000 kJ/h, or 0.553 GJ/h
- Fuel heat at 80% boiler efficiency (assumed): 0.553 / 0.80 = 0.691 GJ/h
- Over 6,000 hours: 0.691 x 6,000 = 4,146 GJ of fuel a year
- If the boiler burns natural gas: 4,146 x 56.1 kg CO2 per GJ = 232,600 kg, about 233 t CO2 a year
Steam table values from IAPWS-IF97; the efficiency and hours are assumptions. Use the factor for the fuel your boiler burns.
What Senseible does here
Senseible converts fuel quantities from your invoices to CO2 with IPCC 2006 defaults, so lower boiler fuel purchases show up as lower Scope 1.
Sources: BEE Guide Book 2, Energy Efficiency in Thermal Utilities, Chapter 3 Steam System, flash steam recovery (beeindia.gov.in), 2006 IPCC Guidelines, Volume 2 Energy, Chapter 2 Stationary Combustion (natural gas CO2 default)
Last reviewed 2026-10-18.