**5.1 Current status and energy consumption in desalination systems**

### *5.1.1 Multi-stage flash (MSF)*

The basis of working multi-stage flash distillation (MSF) is distills sea water by flashing a portion of the water into steam in multiple stages of what are essentially countercurrent heat exchangers [81]. In order to occur the flashing, the pressure in each stage must be lower than the vapor pressure of the heated liquid. by passing the cold feed from each stage, it be heated that is further heated in the brine heater. At the time of brine flows return, because of higher temperature than the boiling point in brine, in the normal pressure, a fraction of the brine boils to the steam. After this stage, the steam is starting to condensation on the external surface of heat exchanger tubes [82]. At the moment, two more well-known configurations of the MSF are the once-through MSF (MSF-OT) and brine mixing MSF (BM-MSF) [80].

At this moment, about 23% of all desalinated water in the world is produced by MSF plants, but due to the high energy consumption, their use is declining [83].

In the practical scale, for commercial MSF systems, a value of 8 to 12 kgdistillate/ kgsteam are typically reported [84]. Some parameters like as corrosion and pipe fouling, scale formation and etc. reduce the energy efficiency of MSF systems. In MSF plants the amount of energy that consume is between 23 and 27 kWh/m<sup>3</sup> [80, 85]. El-Naser [86] reported that in MSF plants the energy consumption is average 12–24 kWh/m<sup>3</sup> .
