Thin-film evaporation is applied in heat exchangers called thin-film or thin-layer
evaporators. Actually thin-layer evaporators may be considered as heat and mass
exchangers, since molecules from liquid phase are transferred to gas phase during process
of vaporization and the movement of molecules from gas phase to liquid phase is probable
during process of condensation. This kind of devices are commonly applied in chemical,
pharmaceutical and food industries since 1950. Vertical thin-layer evaporator, example of
which is presented in Fig.1, is characterized by small pressure drop and short residence time
of the phases in the apparatus, which means also that there is short contact time of the liquid
with hot surface of the evaporator wall. These features of the evaporator cause that it is
applied for the concentration of heat sensitive liquid solutions especially with high viscosity.
Because of low value of pressure drop during gas flow inside the evaporator the boiling
temperature of liquid, which is evaporated, depends only on its composition and does not
depend on liquid position in the evaporator, which is of great importance in case of
evaporation at low pressure