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Flash Steam Calculator

Calculate the quantity of flash steam produced when high-pressure condensate is released to a lower pressure. Uses IAPWS-IF97 steam properties.

Inputs
bar(g)

T_sat = 184.1 °C

bar(g)

T_sat = 100.0 °C

Results

Flash Steam Fraction

16.1%

Flash steam flow160.6 kg/h
Remaining condensate839.4 kg/h
Steam volume flow (at LP)268.8 m³/h
HP saturation temp184.1 °C
LP saturation temp100.0 °C
HP liquid enthalpy (h_f)781.4 kJ/kg
LP liquid enthalpy (h_f)419.0 kJ/kg
LP latent heat (h_fg)2256.5 kJ/kg
Energy in flash steam100.7 kW
Energy in condensate97.7 kW

About Flash Steam

Flash steam is produced when hot, high-pressure condensate is released to a lower pressure. Because the condensate holds more energy than saturated liquid at the lower pressure can contain, the excess energy causes some of the condensate to instantly evaporate (flash) into steam.

How It Works

The process is isenthalpic — the total enthalpy is conserved across the pressure reduction. The flash steam fraction is calculated from the enthalpy balance:

x=hf,HP−hf,LPhfg,LPx = \frac{h_{f,\text{HP}} - h_{f,\text{LP}}}{h_{fg,\text{LP}}}
  • hf,HPh_{f,\text{HP}} — enthalpy of saturated liquid at the high pressure
  • hf,LPh_{f,\text{LP}} — enthalpy of saturated liquid at the low pressure
  • hfg,LPh_{fg,\text{LP}} — latent heat of vaporisation at the low pressure

Energy Recovery

Flash steam carries significant energy and should be recovered where possible. Common recovery methods include using flash steam for low-pressure heating, feeding it to a deaerator, or using it in heat exchangers. Venting flash steam is a direct energy loss.

For detailed steam properties, see our steam tables calculator. For pipe heat loss through insulation, use the insulation calculator. To size steam traps and condensate return lines, use the condensate load calculator. For pipe network simulation with steam systems, try SimuPipe.

Frequently Asked Questions

What is flash steam?
Flash steam is steam that forms when high-pressure condensate is released to a lower pressure. At the higher pressure, the condensate is at saturation temperature. When the pressure drops, the saturation temperature also drops, and the excess energy in the condensate is released by evaporating a portion into steam. This flash steam contains valuable energy that can be recovered.
How much flash steam is produced?
The flash steam percentage depends on the pressure drop. A simple estimate is: flash % = (hf_high - hf_low) / hfg_low x 100, where hf is the liquid enthalpy and hfg is the latent heat at the low pressure. For example, condensate at 10 bar(g) flashing to atmospheric pressure produces about 15% flash steam by mass. This calculator uses IAPWS-IF97 for accurate results.
How can I recover flash steam energy?
Flash steam can be piped to a lower-pressure steam header, used in heat exchangers, or directed to a deaerator for feedwater heating. A flash vessel (flash tank) separates the flash steam from the remaining condensate. The economics are usually excellent — flash steam recovery systems often pay back in under a year because the energy in flash steam is essentially free.
Why is flash steam often wasted?
Many plants vent flash steam to atmosphere from condensate receivers, mistaking it for live steam leaks. The plume of steam from a condensate tank vent is usually flash steam, not a leak. Plants may lack a low-pressure steam header to receive it, or the flash vessel and piping required may not have been included in the original design. Awareness and relatively simple retrofits can recover this energy.
What is the difference between flash steam and live steam?
Flash steam is generated by releasing high-pressure condensate to a lower pressure — it comes from the stored energy in the hot condensate. Live steam is generated in a boiler by burning fuel. Both are real steam with the same thermodynamic properties at the same pressure, but flash steam costs nothing extra to produce since the energy was already paid for when the original high-pressure steam was generated.

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