A. Nature of flash evaporation

When a body of liquid, initially at saturation or slightly subcooled state, is flowing at high speed through a conduit or when a vessel containing saturated liquid is subjected to sudden depressurization so that the liquid pressure becomes lower than the saturation pressure corresponding to the local temperature of the liquid, a transient process termed flashing occurs. In this process, the liquid becomes superheated and starts to boil and a portion of the liquid flashes off to the vapor phase. Accordingly, the sensible heat of the liquid is changed to latent heat. Thereby the liquid cools itself and approaches a new thermodynamic equilibrium. The process is characterized by high deviation from both the thermal and mechanical equilibrium between the two phases. The vapor formation in the flashing process may take place in two ways. It may occur at the free surface of the metastable liquid and/or at freshly generated surfaces in the form of bubbles within the bulk of the liquid. The flashing process may start due to four reasons: wall boiling, release of dissolved gases, presence of impurities, and statistical fluctuations in the metastable liquid El-Dessouky (1995).

The flashing process differs from boiling in the following ways:

• Boiling takes place on heated surface, while flashing occurs in the liquid bulk.
• Boiling of pure liquids is isothermal, while flashing is associated with decrease in the liquid bulk temperature.
• The extent of flashing depends on fluid stagnation conditions, the location of flashing inception within the container or conduit, fluid properties, system geometry, and rate of depressurization. On the other hand, extent of boiling depends on the temperature difference between the hot surface and the liquid, the geometry of the heating surface, buoyancy forces, and surface tension between the liquid and vapor.
• Flashing is an adiabatic process similar to evaporative cooling, where sensible heat is changed into latent heat.
• Main advantage of liquid flashing is that evaporation occurs within the liquid body not on a heat transfer surface, which may lead to local increase in salt concentration and eventual formation of scale.
• Flashing is limited to low temperature applications.
• Both boiling and flashing are associated with high turbulence and non-equilibrium.

B. Industrial Applications