2.4. Falling Film Evaporators

In a falling film evaporator, the liquid is fed at the top of the tubes in a vertical tube bundle. The liquid is allowed to flow down through the inner wall of the tubes as a film. As the liquid travels down the tubes the solvent vaporizes and the concentration gradually increases. Vapor and liquid are usually separated at the bottom of the tubes and the thick liquor is taken out. Evaporator liquid is recirculated through the tubes by a pump below the vapor-liquid separator. This type of evaporator is illustrated in Figure 3.3 . The distribution of liquid in the inner wall of the tubes greatly affects the performance of this type of evaporator.

The falling film evaporator is largely used for concentration of fruit juices and heat sensitive materials because of the low holdup time. The device is suitable for scale-forming solutions as boiling occur on the surface of the film.

Figure 3.3 . Falling-film evaporator.