Multi-effect evaporator

Multi-effect evaporator

Multi-effect evaporation is a process where the secondary steam from one effect is used as the heating steam for the next effect in a series of evaporation operations. In multi-effect evaporation, the operating pressure, corresponding heating steam temperature, and boiling point of the solution are sequentially reduced from one effect to the next.
In the process of evaporation production, the output of secondary steam is significant and contains a large amount of latent heat. Therefore, it should be recovered and utilized. If the secondary steam is introduced into the heating chamber of another evaporator, as long as the operating pressure and boiling point of the latter are lower than those of the original evaporator, the introduced secondary steam can still serve as a heating source. This method of operation is known as multiple-effect evaporation.
In multi-effect evaporation, each evaporator is referred to as a "stage." The purpose of using a multi-effect evaporator is to save on the consumption of heating steam. Theoretically, approximately 1kg of heating steam can evaporate about 1kg of water.
Due to heat loss and the fact that the latent heat of vaporization of water in the separator is greater than the latent heat of condensation in the heating chamber, the actual amount of heating steam required to evaporate 1 kg of water exceeds 1 kg. By experience, the economic efficiency of steam (U = W/D) is as follows: single-effect is 0.91; double-effect is 1.76; triple-effect is 2.5; quadruple-effect is 3.33; and quintuple-effect is 3.71. It can be seen that as the number of effects increases, the growth rate of W/D gradually decreases. For instance, switching from single-effect to double-effect can save about 50% of heating steam; however, changing from quadruple-effect to quintuple-effect saves only 10%. Nevertheless, as the number of effects increases, the temperature difference loss in heat transfer also increases, significantly reducing the production intensity of the evaporator and doubling the equipment costs. At a certain point, the cost savings from adding effects may not outweigh the additional equipment costs.