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
Inputs and outputs explained
- High and low pressure. The pressure of the condensate before the pressure drop (the trap inlet or the vessel it drains from) and the pressure it is released into (flash vessel, receiver, or atmosphere at 0 bar(g)). Gauge units have the standard atmosphere, 1.01325 bar, added internally; the saturation temperatures shown are the IAPWS-IF97 values at those absolute pressures.
- Condensate mass flow. The total condensate entering the pressure drop. Flash is a mass fraction, so the flow only scales the steam, condensate, volume and energy outputs — the flash percentage itself depends on the two pressures alone.
- Flash steam fraction x. The mass fraction that evaporates: x = (h_f,HP − h_f,LP) / h_fg,LP, from an enthalpy balance across the pressure drop. It assumes the condensate arrives as saturated liquid at the high pressure — subcooled condensate carries less enthalpy and flashes less.
- Flash steam and remaining condensate. x and (1 − x) of the inlet flow. The remaining condensate is saturated liquid at the low pressure.
- Steam volume flow. The flash steam's mass flow times the saturated-vapour specific volume v_g at the low pressure. Because v_g is large at low pressure (1.67 m³/kg at atmospheric), a small mass fraction becomes a large volume — this is what sizes the flash vessel and vent line.
- Energy in flash steam and in condensate. Flash steam: mass flow × h_fg at the low pressure — the latent heat that becomes available when the flash steam is subsequently condensed in a heat exchanger or deaerator. Condensate: mass flow × h_f at the low pressure, i.e. sensible heat above the 0 °C datum of the steam tables — only the part above your actual return or feed temperature is recoverable.
- Flash steam table. The same calculation repeated for a range of low pressures below the chosen high pressure, to show how much the flash fraction, and therefore the vessel size, depends on the recovery pressure.
Worked example — 10 bar(g) condensate flashing to atmosphere
The calculator's defaults: 1,000 kg/h of saturated condensate from a 10 bar(g) system released to a vented receiver at 0 bar(g).
- High-pressure state: 10 bar(g) = 11.013 bar(a), T_sat = 184.1 °C, saturated-liquid enthalpy h_f,HP = 781.4 kJ/kg (IAPWS-IF97).
- Low-pressure state: 0 bar(g) = 1.013 bar(a), T_sat = 100.0 °C, h_f,LP = 419.0 kJ/kg, latent heat h_fg,LP = 2256.5 kJ/kg.
- Enthalpy balance: the condensate's surplus above the low-pressure liquid, 781.4 − 419.0 = 362.4 kJ/kg, evaporates a fraction x = 362.4 / 2256.5 = 16.1 % — the ≈ 16 % given in standard manufacturer flash tables for 10 bar(g) to atmosphere.
- Split: 160.6 kg/h of flash steam and 839.4 kg/h of condensate at 100 °C. At v_g = 1.673 m³/kg the steam occupies 268.8 m³/h — the vent-line and vessel sizing quantity.
- Energy: the flash steam carries 100.7 kW of latent heat; the remaining condensate holds 97.7 kW of sensible heat above 0 °C.
These figures are computed by the same function the calculator runs; the defaults reproduce them. Use the flash steam table above to see how a 1 bar(g) receiver instead of a vented one cuts the flash fraction — and the vessel size — by roughly a third.
Assumptions and limits
- Isenthalpic expansion: no heat loss, no work and negligible velocity change across the trap or orifice, so the enthalpy of the inlet condensate is conserved and redistributed between liquid and vapour at the low pressure.
- Thermodynamic equilibrium at the low pressure: the flash is complete and both phases leave at T_sat,LP. Real traps and flash vessels approach this closely; entrainment of liquid droplets in the steam is not modelled.
- Saturated condensate at the high pressure. Subcooled condensate (a trap with a fixed subcool, a long uninsulated line) arrives with lower h_f and flashes less — enter the pressure whose saturation enthalpy matches the actual condensate temperature, or expect the calculator to over-predict slightly.
- Pure water properties from IAPWS-IF97 (Regions 1, 2 and 4), valid to the critical point; boiler-treatment chemicals have a negligible effect on the flash fraction at normal dosages.
- Flash vessel sizing is not performed. The volume flow is the starting point; the vessel diameter follows from a maximum steam velocity (typically about 3 m/s for separation) and the vent or return line from the allowable pressure drop.
- The energy figures are thermodynamic quantities, not a recovery guarantee: what can actually be recovered depends on where the flash steam and condensate can be used and at what temperature they return.
The saturation properties come from the same IAPWS-IF97 engine as the steam-tables calculator, which reproduces the standard's published verification points; the SimuPipe solver's own accuracy is documented on the validation page.
References
Property source and engineering guidance behind this page:
- IAPWS R7-97(2012). Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam. iapws.org — the saturation-line and saturated-liquid / vapour properties used in the enthalpy balance.
- Spirax Sarco. Steam Engineering Tutorials — Flash Steam. spiraxsarco.com — the standard flash-steam derivation and tables against which the example is cross-checked.
- Armstrong International. Steam Conservation Guidelines for Condensate Drainage (Handbook N101) — flash tables and condensate-return practice from a trap manufacturer's handbook.
- Çengel, Y. A. and Boles, M. A. Thermodynamics: An Engineering Approach, McGraw-Hill — the throttling (isenthalpic) process and property-table method.
- US DOE, Advanced Manufacturing Office. Steam Tip Sheet #12: Flash High-Pressure Condensate to Regenerate Low-Pressure Steam. energy.gov — energy-recovery context and typical savings for flash-steam recovery.
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:
- — enthalpy of saturated liquid at the high pressure
- — enthalpy of saturated liquid at the low pressure
- — 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.
Design your pipe network with SimuPipe
Simulate flow, pressure drop, and sizing across your piping system.