A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently use to evaporate water from steam. In a multiple-effect evaporator, water is boiled in a sequence of vessels, each held at a lower pressure than the last one. As pressure decreases the boiling temperature of water decreases, the vapour boiled off in one vessel can be used to heat the next vessel, and only the first vessel (at the highest pressure) an external source of heat is required.
Multiple effect evaporators commonly use sensible heat in the condensate to preheat liquor to be flashed. In practice, the design liquid flow paths can be somehow complicated to extract the most recoverable heat and to obtain the highest evaporation rates from the types of equipment.
In any evaporation operations, the major process cost is the steam consumed. Therefore, methods of reducing steam consumption or of an increasing economy, (defined as the mass of vapour produced per unit mass of steam consumed) are a very important thing. The most common methods are to use the vapour generated in the first evaporator as the heating kept medium for a second evaporator. Ideally, this method produces almost 2 kg of vapour for every kg. of steam consumed. The method is feasible if the second evaporator is operated at a lower pressure than the first evaporator so that a positive value of ∆t is obtained across the steam-chest surface of the second evaporator. Several evaporators can be connected in series. In this way, the amount of vapour (kg) produced per kg of steam consumed equal to the number of evaporators bodies. The economy as the number of evaporators used is increased with the increase in latent heat with decreasing pressure and additional radiation losses affect. This method of evaporation in the series form is called multiple-effect evaporation, and each stage is called an effect.