Formula & Engineering Notes
Flash fraction (mass that re-evaporates):
h1 is the saturated-liquid enthalpy at the upstream pressure; hf2 and hfg2 are taken at the downstream (flash) pressure. Flash flow = fraction × condensate flow.
All steam properties are interpolated from the shared IAPWS-IF97 steam-table datasets — see the full steam tables and the Steam Property Calculator.
Flash steam looks like magic — condensate drops through a trap and suddenly there's steam on the other side — but it's just energy conservation. Hot saturated condensate at high pressure holds more enthalpy than saturated liquid can hold at the lower pressure. The leftover energy has nowhere to go but latent heat, so a slice of the condensate boils.
Don't vent it. That flash is free, clean low-pressure steam; piping it to a deaerator or a low-pressure header is one of the cheapest energy wins on a steam system. This number is where that business case starts.