In an MSF plant, pressurized seawater brine is preheated as it flows through condenser tubes across 16 to 24 flash stages, before receiving final heating in an external steam Brine Heater to the Top Brine Temperature (TBT, 90°C–112°C). The hot brine then enters the first flash stage, which is maintained under vacuum. Because the ambient pressure is lower than the saturation pressure of the brine, a fraction of the water flashes instantaneously into steam without boiling on heat transfer surfaces. The rising vapor passes through wire demisters and condenses on overhead tubes, dripping into distillate collection trays.

The Gained Output Ratio (GOR) is the primary energy efficiency metric of a thermal desalination plant, defined as the mass of fresh distillate produced per unit mass of heating steam consumed: GOR = M_distillate / M_steam. Alternatively, Performance Ratio is defined as kilograms of distillate per 2,326 kJ (1,000 BTU) of heat supplied. Modern MSF plants operate with GOR values between 8.0 and 11.5, depending on stage count and Top Brine Temperature.

TBT is limited by the inverse solubility of alkaline and non-alkaline scale compounds in seawater—primarily calcium carbonate (CaCO3), magnesium hydroxide (Mg(OH)2), and calcium sulfate anhydrite (CaSO4). While antiscalant chemical additives and acid dosing prevent carbonate scale up to 110°C, calcium sulfate exhibits a retrograde solubility cliff above 118°C. Once CaSO4 precipitates onto brine heater tubes, it forms a rock-hard glass-like scale that cannot be dissolved by acid washing and permanently ruins heat transfer.

Non-Equilibrium Allowance (typically 0.2°C to 0.6°C) is the thermodynamic temperature difference between the brine leaving a flash chamber and the true saturation temperature of the flashing vapor. Because brine flows rapidly through the chamber (with only 2 to 5 seconds of residence time), incomplete vapor release occurs. Deep brine pools, cold temperatures, and high brine velocities increase NEA, reducing flash vapor yield.

Once-through MSF plants (MSF-OT) dump the entire volume of warm reject brine to the ocean. Recirculation MSF plants (MSF-BR) mix a major fraction of the deaerated, chemically treated reject brine from the last stage with incoming seawater feed. Recirculation cuts expensive antiscalant and de-chlorination chemical consumption by up to 65%, allows stable operation through seasonal ocean temperature swings, and minimizes seawater intake civil works.