Regarding the secondary steam in multi-effect evaporation
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In a multi-effect evaporation system, the secondary steam from the top of the previous effect is used as a heat source for the material in the bottom of that effect, and it enters the next effect. However, the temperature of the secondary steam is not much different from that of the material in the bottom of the tower; in some cases, the temperature of the material in the bottom is even higher than that of the steam at the top. How can heat transfer from the secondary steam to the material in the bottom of the tower take place? Is it because the pressure of the material entering the next stage is lower than the pressure of the secondary steam?
Yes. In a multi-effect evaporation system, as the number of effects increases, the operating pressure gradient of the evaporators decreases progressively. The last effect is usually vacuum evaporation.
This has nothing to do with the material pressure. Taking three-effect evaporation as an example, the first effect is heated by primary steam, the second effect is heated by the secondary steam generated in the first effect, and the third effect is heated by the secondary steam generated in the second effect. As mentioned above, the operating pressure of the evaporator decreases stage by stage; in fact, the pressure of the secondary steam also drops gradually. A prerequisite for multi-effect evaporation is that as the number of effects increases, the boiling point of the evaporation medium decreases progressively (the last effect usually employs a vacuum-based negative pressure environment)
I think the original poster must have misidentified the craftsmanship
Under normal circumstances, evaporation occurs under negative pressure after the second effect.