Multi effect evaporator is a series evaporation operation in which the secondary steam of the former effect is used as the heating steam of the next effect. In multi effect evaporation, the operating pressure of each effect, the corresponding heating steam temperature and the boiling point of solution decrease in turn.
In evaporation production, the output of secondary steam is large and contains a lot of latent heat, so it should be recycled for use. If the secondary steam is introduced into the heating chamber of another evaporator, as long as the operating pressure and boiling point of the solution of the latter are lower than those of the original evaporator, the introduced secondary steam can still play a heating role, which is called multi effect evaporation.
Each evaporator in the multi effect evaporation is called one effect. The evaporator that is fed with heating steam is called the first effect, and the evaporator that uses the secondary steam of the first effect as the heating agent is called the second effect, and so on. The purpose of using multi effect evaporator is to save the consumption of heating steam. Theoretically, 1 kg of heated steam can evaporate about 1 kg of water.
However, due to heat loss and the evaporation latent heat of water in the separation chamber is greater than the condensation latent heat in the heating chamber, in fact, more than 1kg of heating steam is needed to evaporate 1kg of water.
According to experience, the economy of steam (U=W/D), single effect is 0.91; Double effect is 1.76; 2.5 for three effects and 3.33 for four effects; Five effects are 3.71, etc. It can be seen that with the increase of the number of effects, the growth rate of W/D decreases gradually. For example, when changing from single effect to double effect, the heating steam can be saved by about 50%; When the four effect is changed to five effect, only 10% of the heating steam is saved. However, with the increase of efficiency, the temperature difference loss of heat transfer increases, which greatly reduces the production intensity of the evaporator and doubles the equipment cost.
When the efficiency is increased to a certain extent, the steam cost saved due to the increase of efficiency may outweigh the equipment cost added.
The industry must weigh the operation cost and equipment cost to determine the most reasonable efficiency. The most commonly used is 2-3 effects, and the maximum is 6 effects.
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