CHAPTER 8
EVAPORATION (cont'd)

VAPOUR RECOMPRESSION

In addition to the possibility of taking the steam from one effect and using it in the steam chest of another, a further possibility for economy of steam is to take the vapour and, after compressing it, return it to the steam chest of the evaporator from which it was evaporated.

The compression can be effected either by using some fresh steam, at a suitably high pressure, in a jet ejector pump, or by mechanical compressors. The use of jet ejectors is the more common. By this means a proportion of the vapours are re-used, together with fresh steam, and so considerable overall steam economy is achieved by reusing the latent heat of the vapours over again.
The price to be paid for the recompression can be either in the pressure drop of the fresh steam (which may be wasted through a reducing valve in any case) or in the mechanical energy expended in mechanical compressors.
Vapour recompression is similar in many ways to the use of multiple effects. The available temperature in the fresh steam is reduced before use for evaporation and so additional heat exchange surface has to be provided: roughly one stage equals one extra effect.