Using multi effect evaporators in place of single effect evaporators reduces the steam consumption by about 1/3rd.
A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water.
In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the previous. Because the boiling point of water decreases as pressure decreases, the vapor boiled off in one vessel can be used to heat the next, and only the first vessel (at the highest pressure) requires an external source of heat.
Thus as the feed moves from one vessel to another its concentration keeps on increasing. However the steam energy input is only given to the first vessel.
It is typically observed that multiple effect evaporators have just 1/3rd steam of the steam requirement as compared to comparable independent evaporators.