In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last. The multiple-effect evaporation system combines two or more evaporators in a series to conserve steam. Water evaporated in the first-effect vapor body is condensed in the second-effect evaporator, which provides energy for evaporation in the second-effect(and so on for additional effects). Vapor from the last effect flows to a condenser. The last-effect vapour body is maintained at a high vacuum. The driving force is the pressure drop from the first to the last effect.
In actual practice, the steam economy can vary widely because of differences in feed temperature and other energy requirements such as heats of dilution and crystallization. The number of effects is limited by the total available temperature difference between the plant steam and the cooling water temperature. Process liquors with high boiling point elevations typically operate with fewer effects.
REVA develop state of art REVAPTM Technology which uses mechanical energy to drive the vapors and the steam consumption will be reduced further with REVAPTM. 30%-40% saving in stream is proven also the number of equipments (effects) required also got reduced lower foot print.