# How much fuel can I save by recovering flash steam from condensate?

Canonical: https://senseible.earth/answers/flash-steam-recovery-fuel-saving

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

1. Flash steam (kg/h) = condensate flow x flash fraction.
2. Heat recovered (kJ/h) = flash steam x hfg at the low pressure.
3. Fuel heat saved = heat recovered / boiler efficiency.
4. 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.

- [Condensate recovery fuel and CO2 savings](https://senseible.earth/answers/condensate-recovery-fuel-co2-savings)
- [Failed steam trap fuel loss](https://senseible.earth/answers/failed-steam-trap-fuel-loss)
- [CO2 per tonne of steam](https://senseible.earth/answers/co2-per-tonne-of-steam)

Sources: [BEE Guide Book 2, Energy Efficiency in Thermal Utilities, Chapter 3 Steam System, flash steam recovery (beeindia.gov.in)](https://beeindia.gov.in), [2006 IPCC Guidelines, Volume 2 Energy, Chapter 2 Stationary Combustion (natural gas CO2 default)](https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol2.html)

Last reviewed 2026-10-18.