Scraper film evaporator

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This type of evaporator uses the scraping action of rotating scrapers to distribute the liquid over the walls of the heating tubes. Its main advantage is its strong adaptability to various materials; it can be used with materials that are highly viscous, thermosensitive, or prone to crystallization and scaling. The structure of the scraper thin-film evaporator is shown in Figure (5-11). Its shell is equipped with a heating steam jacket on the outside, and it contains rotatable stirring blades inside; these rotating blades come in both fixed and movable types. The gap between the former and the inner wall of the shell is 0.75~1.5 mm, while the gap between the latter and the wall of the vessel changes with the rotation speed of the stirring shaft. After the feed liquid is introduced tangentially from the upper part of the evaporator, driven by gravity and rotating scrapers, it forms a downward-moving film on the inner wall of the shell; as it descends, it is continuously evaporated and concentrated, resulting in the finished product at the bottom. In some cases, the solution can be evaporated to dryness to directly obtain the solid product from the bottom. The disadvantages of this type of evaporator are its complex structure, high power consumption, and small heat transfer area, which is generally 3–4 m2, with a maximum of no more than 20 m2; as a result, its processing capacity is low.