If the temperature difference between the heating medium and the solution to be evaporated shall be low in order to avoid an overheating of the solution, the plant should be operated with vapour compression. Depending on the cost ratio between steam and electric power, the following methods may be applied:
Thermal vapour compression:
A steam ejector sucks in a part of the vapour which develops during evaporation, compresses it by the aid of live steam to a higher pressure and temperature and leads it back into the heater of the evaporator to serve as heating steam. Thus the heating steam consumption can be reduced under the steam consumption of a two-stage circulation evaporation plant.
Mechanical vapour compression:
If the cost of electric power are lower than the cost for the production of steam it is advisable to operate the vapour compression by means of electric power. A mechanical turbo compressor does the compression work. In this case live steam is only needed for starting of the plant. Besides single-stage radial compressors also multi-stage machines and axial compressors may find application.
Vapour compression may also be applied in multi-stage evaporation plants and evaporation crystallization plants.