Multiple-Effect Distillation (MED) evaporates seawater across a series of declining pressure vessels (effects), where vapor produced in one effect condenses inside horizontal tube bundles of the subsequent effect to generate further vapor. Thermal Vapor Compression (TVC) uses a supersonic steam jet ejector powered by high-pressure motive steam (10 to 25 bar) to suction a fraction of low-pressure vapor from a downstream effect and compress it into Effect 1. This thermo-compression loop dramatically elevates thermal efficiency, boosting the Gained Output Ratio (GOR) to 12-18 without external mechanical compressors.

The Gained Output Ratio (GOR) is the primary thermodynamic efficiency benchmark for thermal desalination processes. It is defined as the ratio of pure distillate water produced to the mass of high-pressure motive steam consumed: GOR = kg Distillate / kg Motive Steam. In a conventional MED plant without TVC, GOR is slightly less than the number of effects (e.g. GOR ~ 5.5 for a 6-effect plant). Adding a TVC doubles the effective GOR (e.g. GOR ~ 11 to 14 for 6 effects) by recycling latent heat.

Unlike Multi-Stage Flash (MSF) plants which operate up to 110°C, MED plants utilize horizontal falling-film evaporation over titanium or aluminum-brass tube bundles. Above 70°C, calcium sulfate (anhydrite and hemihydrate CaSO4) and magnesium hydroxide (brucite Mg(OH)2) reach retrograde solubility and precipitate directly onto outer tube surfaces as rock-hard scale that cannot be dissolved by acid cleaning. Limiting TBT below 68°C prevents alkaline scaling and allows the use of economical aluminum brass tubes.

High-pressure motive steam expands through a converging-diverging Laval nozzle to supersonic velocities (Mach 2 to 3), creating a low static pressure zone that draws in secondary low-pressure suction vapor from the evaporator. In the mixing section, momentum exchange compresses the mixture. The diffuser then converts kinetic energy back into static pressure, discharging steam into Effect 1 at a pressure and saturation temperature sufficient to drive evaporation.

Seawater contains dissolved atmospheric gases (nitrogen, oxygen) and carbonates that decompose into carbon dioxide when heated. In horizontal tube bundles, these non-condensable gases (NCGs) do not condense and accumulate inside tube dead zones. Even a 1% concentration of NCGs creates an insulating gas blanket that reduces the condensing heat transfer coefficient by over 50%. A continuous cascade vacuum venting system is essential to sweep NCGs toward the final condenser ejector.