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Short-tube vertical evaporators consist of a short tube bundle (about 4 to 10 ft in length) enclosed in a cylindrical shell. This is called calandria. A evaporator of this type is shown in Figure 3.1 . The feed is introduced above the upper tube sheet and steam is introduced to the shell or steam chest of the calandria. The solution is heated and partly vaporized in the tubes.

The central tube in a calandria is of longer diameter. Typically it's downcomer area is taken as 40 to 70% of the total cross sectional area of tubes. The circulation rate through the downcomer/downtake is many times the feed rate. The flow area of the downtake is normally approximately equal to the total tubular flow area.

Figure 3.1 . Calandria type evaporator.