Evaporator that is 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 last. The multiple-effect evaporation system combines two or more evaporator bodies in a series to conserve steam, which is condensed in the first-effect heat exchanger only. Water evaporated in the first-effect vapor body is condensed in the second-effect heat exchanger, which provides energy for evaporation in the second-effect vapor body (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 with a steam-jet air ejector or mechanical vacuum pump. The driving force is the pressure drop from the first to the last effect.

For the same overall pressure differential and process conditions, the evaporator with more effects will require less steam. For the same evaporation rate, a five-effect evaporator system will require about 25% of the steam required for a single-effect evaporator. 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.