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In a falling-film evaporator, the heating chamber consists of one or several vertical long tubes; typically, the diameter of these heating tubes is 25–50 mm, with a tube length-to-diameter ratio of 100–150. After preheating, the feed liquid enters from the bottom of the evaporator, while the heating steam condenses outside the tubes. When the solution is heated to boiling point and rapidly vaporizes, the resulting secondary steam rises at high speed inside the tube, driving the liquid to flow upward in a film along the inner wall of the tube; this rising liquid film continues to evaporate due to the heat. Thus, as the solution rises from the bottom to the top of the evaporator, it becomes progressively more concentrated; the concentrated solution enters the separation chamber where it is separated from the secondary steam, and then is discharged from the bottom of the separator. At atmospheric pressure, the velocity of the secondary steam at the outlet of the heating tube should not be less than 10 m/s; it is generally between 20 and 50 m/s. During reduced-pressure operations, this velocity can sometimes reach 100 to 160 m/s or even higher. Film-evaporator type evaporators are suitable for applications involving large evaporation rates (i.e., dilute solutions), heat-sensitive solutions, and solutions that tend to foam, but they are not suitable for solutions with high viscosity, those in which crystals precipitate, or solutions prone to scaling.