Multi-effect evaporation involves串联 operating several evaporators, allowing the heat energy of steam to be utilized multiple times, thereby improving the thermal energy utilization rate, commonly used in the treatment of aqueous solutions. In the process of three-effect evaporation (see diagram), the initial evaporator (referred to as the first effect) uses raw steam as the heating steam, while the other two (referred to as the second effect and third effect) use the secondary steam from the previous effect as the heating steam, significantly reducing the amount of raw steam required. The temperature of the secondary steam in each effect is always lower than that of the heating steam, so during multi-effect evaporation, the operating pressure and boiling temperature of the solution decrease sequentially in the direction of steam flow. Depending on the direction of flow of the secondary steam and the solution, the multi-effect evaporation process can be divided into: ① Concurrent flow. The solution and secondary steam pass through each effect in the same direction. Due to the higher pressure of the previous effect, the feedstock can flow by pressure difference. However, the final effect has a high concentration of solution and a low temperature, resulting in a high viscosity and a low heat transfer coefficient. ② Counter-current flow. The solution and secondary steam flow in opposite directions. A pump is required to send the solution to the previous effect with higher pressure, and the effects' concentrations and temperatures roughly offset the impact on viscosity, resulting in similar heat transfer conditions in each effect. ③ Cross-current flow. The secondary steam passes through each effect sequentially, but the feedstock enters and exits each effect separately. This process is suitable for feedstock that precipitates crystals.
Under conditions where the feed steam temperature is the same as the final effect condenser temperature (i.e., the total temperature difference is the same), switching from single-effect evaporation to multi-effect evaporation increases the number of effects in the evaporator, reduces the amount of feed steam, but the total evaporation rate does not increase; instead, it decreases due to increased temperature difference losses. Multi-effect evaporation saves energy consumption but reduces the production intensity of the equipment, thereby increasing equipment investment. In actual production, a comprehensive consideration of energy consumption and equipment investment should be made to select an optimal number of effects. The evaporation of alkali solutions such as caustic soda, due to significant temperature difference losses, is typically carried out in 2 to 3 effects; non-electrolyte solutions like sugar can be evaporated in 4 to 6 effects; for large volumes of water to be evaporated in seawater desalination, up to 20 to 30 effects can be used after implementing various measures to reduce temperature difference losses.