The multiple effect configurations combine two or more evaporator bodies 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 vapor body is maintained at a high vacuum with a steam-jet air ejector or mechanical vacuum pump.
A rough “rule of thumb” for comparison purposes is that 1/N kg of steam is required for 1 kg of Water evaporation, where N is number of effects. e.g In 4 effects evaporator 0.25 Kg (1/4) steam is required for 1 kg of water evaporation.