Formula
An energy balance across the pressure drop (an isenthalpic throttling to the flash pressure):
\(x = \frac{h_{f1} - h_{f2}}{h_{fg2}} \qquad \dot m_{fs} = x\,\dot m \qquad \dot m_c = (1 - x)\,\dot m \qquad \dot Q_{fs} = \dot m_{fs}\,h_{fg2} \qquad \dot V_{fs} = \dot m_{fs}\,v_{g2}\)
Variables
| Symbol | Meaning | SI unit |
|---|---|---|
| \(p_1\) | upstream absolute pressure of the saturated liquid | Pa |
| \(p_2\) | absolute pressure after the drop (flash vessel) | Pa |
| \(\dot m\) | liquid mass flow before the drop | kg/s |
| \(x\) | mass fraction flashed to steam | – |
| \(\dot m_{fs}\) | flash steam mass flow | kg/s |
| \(\dot m_{c}\) | remaining condensate mass flow | kg/s |
| \(T_1\), \(T_2\) | saturation temperatures at \(p_1\) and \(p_2\) | K |
| \(h_{f1}\) | saturated liquid enthalpy at \(p_1\) | J/kg |
| \(h_{f2}\) | saturated liquid enthalpy at \(p_2\) | J/kg |
| \(h_{fg2}\) | latent heat of vaporisation at \(p_2\) | J/kg |
| \(\dot Q_{fs}\) | latent heat carried by the flash steam | W |
| \(\dot V_{fs}\) | volume flow of the flash steam at \(p_2\) | m³/s |
Method and standards
Enthalpies from FireCAD's IAPWS-IF97 implementation on the saturation line (region 1 liquid, region 2 vapour, region 4 pressure–temperature relation). The throttling is treated as adiabatic with no kinetic energy change, the usual assumption for flash vessels and condensate receivers.
Assumptions and limits
- Saturated liquid upstream; sub-cooled liquid flashes less.
- Equilibrium at the flash pressure; real vessels carry over a little liquid.
- Both pressures absolute and below the critical pressure (220.64 bar).
References
- IAPWS R7-97(2012) IF97
Questions and answers
Why does condensate flash at all?
Saturated water at 40 bar holds 1 087 kJ/kg. At 2 bar liquid water can hold only 505 kJ/kg, so the surplus 582 kJ/kg evaporates about 26 % of the mass into steam at 2 bar.
What if the condensate is sub-cooled before the valve?
Use the actual liquid enthalpy instead of h_f1. The calculator assumes saturated liquid upstream, which is the case for steam traps and continuous blowdown; sub-cooling reduces the flash fraction.
What is the flash steam worth?
The heat shown as "heat carried by the flash steam" is the latent heat you recover if the flash steam is used, for example in a deaerator. For 2 500 kg/h of 40 bar blowdown to 2 bar it is about 400 kW.