Multi-Stage Flash (MSF) evaporation desalination plants are based on flash evaporation in a cascade of stages (effects) at different pressure levels. In the brine-recirculation mode, the brine is recirculated through the system.
In the last stage of an MSF plant, the heated feed water is led into the pressure vessel where it partially evaporates in a flashing process due to the pressure decline. The vapour is used to preheat the feed water. The part of the brine that does not evaporate is sucked into the next stage which has a lower pressure. There, the same flashing and evaporation procedure takes place again, but at lower pressure and thus lower saturation temperature.
In this way, the feed that is used to condense the steam at every stage from the lowest pressure at the first stage to the highest pressure at the last stage is heated up continuously. The result is a low mean temperature difference between the feed water and the condensing steam, although the heat transfer is of type latent-sensible. Thus, the exergy losses are low.
The circulation of the brine mainly reduces costs for pre-treatment, but also reaches higher operation flexibility and better thermal efficiency. At the same time complexity and costs for components, construction, and maintenance rise.