Tutorial to the basic physics behind flash steam generation
If the pressure of condensate - the saturated water at the boiling point at the actual pressure - is reduced, the heat energy in the water is reduced to a level appropriate to the final pressure. The relation pressure and the boiling temperature can be found in steam tables.
The energy - or enthalpy - made available when pressure is reduced, will evaporate a part of the water, producing flash steam.
Only a part of the condensate evaporates as flash steam. How much depends on the enthalpy in the condensate at the initial and the final pressures.
The amount of flash steam produced during the pressure reduction can be calculated:
w = (hil - hfl) / hfe (1)
w = ratio of flash steam generated (kg flash steam / kg condensate)
hil = initial liquid enthalpy (kJ/kg)
hfl = final liquid enthalpy (kJ/kg)
hfe = enthalpy of evaporation (kJ/kg)
Example - Flash Steam Generation
Condensate is produced inside an heat exchanger at pressure 5 bar gauge (6 bar absolute). The condensate contains 670.9 kJ/kg of heat energy - enthalpy - at saturation temperature 159 oC.
The pressure is reduced to atmospheric pressure - 0 bar gauge (1 bar absolute) through the steam trap and the maximum heat energy - enthalpy - water at atmospheric pressure and 100 oC can contain is 419.0 kJ/kg.
Evaporation energy of water at atmospheric pressure is 2257 kJ/kg.
The flash steam generated can be calculated:
w = ((670.9 kJ/kg)- (419.0 kJ/kg)) / (2257 kJ/kg)
= 0.11 (kg flash steam / kg condensate)
- en: flash steam generation
- es: generación de vapor de flash
- de: Flash-Dampferzeugung