[curiouscat]

Is it necessary for successive effects of a parallely fed Multi Effect Evaporator to be placed under a vacuum? In the absence of a vacuum the vapor generated in stage 1 will still be much hotter than the liquid fed to stage 2, correct?

So why would vacuum be needed?

[Enthalpy]

Under usual operation, the vapour is at the temperature determined by the pressure and the equilibrium with its liquid.

If you want to reuse the first condensation heat as the second evaporation heat, the first condensation must be warmer than the second evaporation, hence at a higher pressure (and so on for the following stages).

This needs evaporation and condensation at a pressure that decreases from the first to the second stage and so on. Whether you start at atmospheric pressure and go on with vacuum, or start at a higher pressure and decrease towards 1atm, depends on your design.

In case evaporation is made without boiling, the vapour partial pressure decreases over the stages.

Pre-heating the feed towards the boiling temperature can help, say by recycling heat through an exchanger, since using the hotter vapour for that would be a waste.

The multi-feed evaporator is a bit tricky because the main heat absorptions and releases happen at fixed temperatures, so it needs conditions less easy than heat exchangers between liquids.

[curiouscat]

If you want to reuse the first condensation heat as the second evaporation heat, the first condensation must be warmer than the second evaporation, hence at a higher pressure (and so on for the following stages).

Yes, I think you are right. I was making a conceptual mistake.