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Product out

Ij-Feed

Figure 2. Natural circulation.

Figure 2. Natural circulation.

The external heater has the advantage that its size is not dependent on the size or shape of the vessel itself. As a result, larger evaporation capacities may be obtained. The most common application for this type of unit is as a reboiler at the base of a distillation column.

Rising Film Tubular. The first of the "modern" types of evaporators is the rising film unit, although its use by industry goes back to the early 1900s. The rising film principle was developed commercially by using a vertical tube with steam condensing on its outside surface (Fig. 3). Liquid inside the tube is brought to a boil, the vapor generated forming a core in the center of the tube. As the fluid moves up the tube, more vapor is formed, resulting in a higher central core velocity that forces the remaining liquid to the tube wall. Higher vapor velocities, in turn, result in thinner and more rapidly moving liquid film. This provides higher HTCs and shorter product residence time.

The development of this principle was a giant step forward in the evaporation field, particularly from the viewpoint of product quality. Its further advantage of high HTCs resulted in reduced heat-transfer area requirements and consequently, in a lower initial capital investment.

Falling Film Tubular. Following development of the rising film principle, it took almost a further half century for a falling film evaporation technique to be perfected (Fig. 4). The main problem was to design an adequate system for the even distribution of liquid to each of the tubes. Distribution in its forerunner, the rising film evaporator, was easy as the bottom bonnet of the calandria always was pumped full of liquid, thus allowing equal flow to each tube.

While all manufacturers have their own techniques, falling film distribution generally is based on the use of a perforated plate positioned above the top tube plate of the calandria. Spreading of liquid to each tube sometimes is

To condenser or vacuum

To condenser or vacuum

Product out

I— Condensate

Figure 3. Rising film tubular.

Product out

I— Condensate

Figure 3. Rising film tubular.

Steam -M

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