In falling film evaporators the liquid product (A) usually enters the evaporator at the head (1) of the evaporator. In the head the product is evenly distributed into the heating tubes. A thin film enters the heating tube are it flows downwards at boiling temperature and is partially evaporated. In most cases steam (D) is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product (C) form its vapor (B) is undergoing in the lower part of the heat exchanger (3) and the separator (5).

A: Product B: Vapor C: Concentrate D: Heating Steam E: Condensate 1: Head 2: Calandria 3: Calandria, Lower part 4: Mixing Channel 5: Vapor Separator

Figure 1: Falling Film Evaporator

Falling film evaporators can be operated with very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These characteristics make the falling film evaporator particularly suitable for heat-sensitive products, and it is today the most frequently used type of evaporator.

Figure 2: 5-Effect Evaporator for Grape Juice with Thermal Vapor Recompression

However, falling film evaporators must be designed very carefully for each operating condition; sufficient wetting (product film thickness) of the heating surface by liquid is extremely important for trouble-free operation of the plant. If the heating surfaces are not wetted sufficiently, dry patches and incrustations will occur; at worst, the heating tubes will be completely clogged. In critical cases the wetting rate can be increased by extending or dividing the evaporator effects, keeping the advantages of single pass (no recirculation of product) operation.

The proper design of the product distribution system in the head of the evaporator is critical to achieve full and even product wetting of the tubes.

Because of the low liquid holding volume in this type of unit, the falling film evaporator can be started up quickly and changed to cleaning mode or another product easily.

Falling film evaporators are highly responsive to alterations of parameters such as energy supply, vacuum, feed rate, concentrations, etc. When equipped with a well designed automatic control system they can produce a very consistent concentrated product.

The fact that falling film evaporators can be operated with small temperature differences makes it possible to use them in multiple effect configurations or with mechanical vapor compression systems in modern plants with very low energy consumption.