a compact array of vertical rising film evaporators is disclosed in an evacuated multi - effect process , for desalination of seawater , using waste heat of thermal machines . vapor produced in one stage is the heat source for the next . this invention is based on a concentric disposition of three different evaporators , at which the first stage is a shell and tube evaporator built in a ring format , where is inserted inside the intermediate stage that is a bundle of vertical tubes in a ring format , and the last stage that is a bundle of vertical tubes placed inside the intermediate stage . by this arrangement , vapor pipelines are eliminated , dimensions are reduced , and heat transfer is improved per the rising film evaporators compared to falling film .