Editing Flash evaporation (section)

==Flash evaporation of a single-component liquid==
The flash evaporation of a single-component liquid is an [[isenthalpic]] process and is often referred to as an '''[[Adiabatic process|adiabatic flash]]'''. The following equation, derived from a simple heat balance around the throttling valve or device, is used to predict how much of a single-component  liquid is vaporized.

:<math>X = \frac{H_u^L - H_d^L}{H_d^V - H_d^L}</math><ref>{{cite book|author1=Vic Marshall|author2=Steve Ruhemann|title=Fundamentals of Process Safety|date=2001|publisher=IChemE|page=46|isbn=9780852954317|url=https://books.google.com/books?id=Sn4f_N_2rsMC&q=flashing+fraction&pg=PA45}}</ref>

:{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right|<math>X</math> 
|align=left|= &nbsp; weight ratio of vaporized liquid / total mass
|-
!align=right|<math>H_u^L</math>
|align=left|= &nbsp;upstream liquid enthalpy  at upstream temperature and pressure, J/kg
|-
!align=right|<math>H_d^V</math><br>&nbsp; 
|align=left|= &nbsp;flashed vapor enthalpy at downstream pressure and corresponding saturation<br> &nbsp; &nbsp; temperature, J/kg
|-
!align=right|<math>H_d^L</math><br>&nbsp;
|align=left|= &nbsp;residual liquid enthalpy at downstream pressure and corresponding saturation<br> &nbsp; &nbsp; temperature, J/kg
|}

If the enthalpy data required for the above equation is unavailable, then the following equation may be used.

:<math>X = \frac{c_p ( T_u - T_d )}{H_v}</math>

:{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right|<math>X</math>
|align=left|= &nbsp;weight fraction vaporized
|-
!align=right|<math>c_p</math>
|align=left|= &nbsp;liquid [[specific heat capacity|specific heat]] at upstream temperature and pressure, J/(kg&nbsp;°C)
|-
!align=right|<math>T_u</math> 
|align=left|= &nbsp;upstream liquid temperature, °C
|-
!align=right|<math>T_d</math>
|align=left|= &nbsp;liquid [[saturation temperature]] corresponding to the downstream pressure, °C
|-
!align=right|<math>H_v</math><br>&nbsp;
|align=left|= &nbsp;liquid [[heat of vaporization]] at downstream pressure and corresponding saturation<br> &nbsp; &nbsp; temperature, J/kg
|}

Here, the words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.

This type of flash evaporation is used in the [[desalination]] of [[brackish]] water or ocean water by "[[Multi-Stage Flash]] Distillation." The water is heated and then routed into a reduced-pressure flash evaporation "stage" where some of the water flashes into steam. This steam is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure. More water is flashed into steam which is also subsequently condensed into more salt-free water. This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are met. A large part of the world's installed desalination capacity uses multi-stage flash distillation. Typically such plants have 24 or more sequential stages of flash evaporation.