The working principle of a double-effect evaporator is that the secondary steam generated by the first evaporator is again used as a heating source and introduced into another evaporator. As long as the pressure and solution boiling point in the evaporator are properly controlled and reduced, the secondary steam generated by the first evaporator can be used for heating. The condensation point of the first evaporator is the heating point of the second evaporator. This is the principle of double-effect evaporation. Generally speaking, single-effect evaporation consumes 1.1 to 1.2 tons of steam per ton of water, while double-effect evaporation consumes 0.6 to 0.7 tons of steam per ton of water.

The process of a double-effect evaporator can be divided into three forms: cocurrent, countercurrent, and parallel flow according to the different flow directions of heating steam and materials. The main basis for selecting the double-effect evaporation process is the characteristics of the material.
(1) Cocurrent method: The flow direction of the evaporated material is the same as that of the steam. This method is mainly applicable to the situation where the feed temperature is relatively high and the discharged material after evaporation and concentration is convenient for transportation.
(2) Countercurrent method: The flow direction of the evaporated material is opposite to that of the steam. It is mainly used in process situations where the feed temperature is relatively low but the outlet temperature is relatively high. The feed does not need preheating or only requires a little preheating, saving steam consumption. However, the entire process requires the use of pumps to transport materials, thereby increasing power consumption.
(3) Parallel flow method: The parallel flow method refers to the operation mode in which the feed is added from each effect and the concentrated liquid is discharged from each effect. The concentration of the material in each effect is the same, and the flow direction of the heating steam still follows the order from the first effect to the last effect.