How does a multiple-effect evaporator system improve steam economy?

Multiple-effect evaporation uses vapor from one evaporator as heating medium for the next effect operating at lower pressure/temperature. In a single effect, 1 kg steam evaporates approximately 1 kg water. In triple-effect, 1 kg steam evaporates approximately 2.5-2.8 kg water total across all effects. Steam economy = kg water evaporated / kg steam used. Trade-offs: more effects mean lower temperature driving force per effect, requiring more heat transfer area. Optimal number balances energy savings against capital cost.