一公斤蒸汽冷凝釋放出的熱量和一公斤水蒸發需要吸取的熱量，基本上是相同的，于是對單效蒸發而言，蒸發物料中一公斤水，需要消耗一公斤新蒸汽;如果把蒸發所得的二次蒸汽用作下一-效的加熱蒸汽，則二效蒸汽、新蒸汽的耗量比之單效就節省了50%。效數增加，用汽減少，但設備的投資增加很多，所以一般不超過5效。

A kilo of steam condensation releases heat and a kilogram of water evaporation need to absorb heat, is basically the same, so the single efet evaporation, evaporation material by one kilo of water, need to consume a kilo of firesh steam; if the evaporation of the two steam is used as a effective heating steam, two. Steam, steam consumption than single effect saves 50%. The effect increases, steam to reduce equipment investment, but increased a lot, so is generally not more than 5.