As already explained it is very important that the product is distributed evenly into all the tubes in the calandria to get a good coverage. The distribution system is therefore given special attention when designing an evaporator.

In principle there are two different systems:

• Dynamic distribution system
• Static distribution system

Dynamic Distribution System

In the dynamic distribution system, see Fig. 18, the necessary kinetic energy for distribution is obtained by a pressure drop of the product over a full-cone nozzle. As the product is superheated in relation to the pressure inside the tubes, flash vapour will instantaneously be formed. The mixture of product and vapour is sprayed into the inlet of the tubes thus being covered by product.

Fig. 18 Dynamic product distribution system

The ability of a nozzle to spray optimally is depending upon the pressure drop determined by the quantity of the liquid to be handled and the degree of flashing when en-tering the room above the tubes. This distribution system is therefore not considered adequate in modern evaporators where flexibility, and thereby differences in the feed quantity are required. Further, in multi-effect evaporators the flashing, and therefore the distribution, is minimal due to low ∆t.

Static Distribution System

In the static distribution system the incoming superheated product is first separated in flash vapour and product. The product enters a distributor plate placed inside an open cone, as the product enters the calandria. The cone is placed above a distributor bowl with a number of holes. Here a certain level of product is maintained. The product flows through the holes in the plate by gravity. Each hole is placed just above the area between the tubes. Thus the product flows onto the tube plate and then over the edge down along the surface of each tube. The flash vapour also enters the tubes and pushes the product against the inner surface of the tubes giving it its initial velocity. See Fig. 19.

Fig. 19 Static product distribution system

This distribution system is much more flexible in respect of capacity, as an increase in the level in the distributor bowl - as a result of increased capacity - will make the product flow through the holes at a higher velocity, thus maintaining the level.