The falling-film evaporator allows a greater number of effects than the
rising-film evaporator. For example, if steam is available at 110 °C and the
boiling temperature in the last effect is 50°C, then the total available temperature
differential is 60°C. Since rising-film evaporators require 14°C temperature
differential across the heating surface, only four effects are feasible. However, as
many as 10 or more effects may be possible using a falling-film evaporator. The
falling-film evaporator can handle more viscous liquids than the rising-film type.
This type of evaporator is best suited for highly heat-sensitive products such as
orange juice. Typical residence time in a falling-film evaporator is 20-30 seconds,
compared with a residence time of 3-4 minutes in a rising-film evaporator.