These operate on a "thermo-siphon" principle. Feed product (A) enters the bottom of the heating tubes and as it heats, steam begins to form. The ascending force of this steam produced during the boiling causes liquid and vapors to flow upwards in parallel flow. At the same time the production of vapor increases and the product is pressed as a thin film on the walls of the tubes, and the liquid rises upwards. This co-current upward movement against gravity has the beneficial effect of creating a high degree of turbulence in the liquid. This is advantageous during evaporation of highly viscous products and products that have a tendency to foul the heating surfaces.

Figure 1: Rising Film Evaporator

A: Product
B: Vapor
C: Concentrate
D: Heating Steam
E: Condensate

Figure 1: Rising Film Evaporator

Usually there must be a rather high temperature difference between the heating and boiling sides of this type of evaporator. Otherwise the energy of the vapor flow is not sufficient to convey the liquid and to produce the rising film. The length of the boiling tubes will typically not exceed 23 ft (7m).

Figure 2: 5-effect falling film evaporation plant for monosodium glutamate, heated by thermal vapor recompression. Evaporation rate is 50,000 lbs/hr (~22,690 kg/h).

Figure 2: 5-effect falling film evaporation plant for monosodium glutamate, heated by thermal vapor recompression. Evaporation rate is 50,000 lbs/hr (~22,690 kg/h).

This type of evaporator is often used with product recirculation, where some of the formed concentrate is reintroduced back to the feed inlet in order to produce sufficient liquid loading inside the heating tubes. A number of different designs have been developed using this basic principle. A good example is the Roberts evaporator, which is the oldest type of circulation evaporator. This type of evaporator has a wide circulation tube in the center of the heating tube bundle through which concentrate flows back to the bottom of the tube bundle. The Roberts evaporator is still widely used in the sugar industry.

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