1. Falling Film Evaporators
In falling film evaporators, liquid and vapors flow downward in parallel flow. The preheated concentrated liquid reaches boiling temperature. It enters the heating tubes through a distribution device in the evaporator's head, flows downward at boiling temperature, and undergoes partial evaporation. The co-current vapor flow augments the gravitationally-induced downward movement. Falling film evaporators can operate with very small temperature differences between the heating media and the boiling liquid. They have very short product contact times, typically only a few seconds per pass. The falling film evaporator's properties render it ideal for heat-sensitive products, making it the most commonly employed evaporator type.
However, falling film evaporators must be carefully designed for each operating condition. Adequate wetting of the heating surface by liquid is critical for the plant's trouble-free operation. Dry patches and incrustations will form if the heating surfaces are not sufficiently wetted; at worst, the heating tubes will become completely clogged. In critical cases, extending or dividing evaporator effects while maintaining the benefits of single-pass operation can increase the wetting rate. To achieve full and even product wetting of the tubes, the liquid distribution system must be properly designed.
2. Rising Film Evaporators
These work on the basis of the "thermo-siphon" principle. As the feed enters the bottom of the heating tubes, steam begins to form. Because of the ascending force of the steam produced during boiling, liquid and vapors flow upwards in parallel flow. At the same time, the product presses as a thin film on the walls of the tubes while increasing vapor production. As a result, the liquid rises upwards. This co-current upward movement has the advantageous effect of increasing turbulence in the liquid. This is beneficial when evaporating highly viscous products or products that tend to foul the heating surfaces.
Typically, the temperature difference between the heating and boiling sides of this type of evaporator must be quite large. Otherwise, the vapor flow's energy is insufficient to convey the liquid and produce the rising film. Typically, the length of the boiling tubes will not exceed 23 feet. Product recirculation often accompanies the use of this evaporator type, wherein the feed inlet reintroduces some of the formed concentrates. This is to produce sufficient liquid loading inside the boiling tubes.
3. Forced Circulation Evaporator
To prevent the product from boiling on the heating surfaces due to its fouling characteristics, we employ forced circulation evaporators. The heat exchanger heats the circulating liquid, which is then partially evaporated by reducing the pressure in the separator. This reduction in pressure allows the liquid to cool to the boiling temperature corresponding to the current pressure. Since the liquid undergoes only a slight temperature increase with each pass through the heat exchanger, maintaining a high recirculation flow rate is necessary.
This evaporator is suitable for crystallizing applications because it does not experience evaporation and, consequently, does not see a concentration increase on the heat transfer surface. In separator vessels, the liquid undergoes flash evaporation, resulting in the formation of crystals in crystallizer applications. Special separator designs are employed to separate crystals from the recirculated crystal slurry. The arrangement of the heat exchanger, whether horizontal or vertical, depends on specific requirements.
4. Plate Evaporators
Plate evaporators are small in size. Short interconnecting pipework connects the separators to the plate packages. Minimal space requirements result in typically limiting heights to 20 to 25 feet (6-8 meters). As a result, plate evaporators can be installed in the majority of buildings. You can use framed plates as a heating surface instead of relying on tube and shell heat exchangers. These plate assemblies are similar to plate heat exchangers but have larger vapor flow passages. These units alternate the connection between a product plate and a steam plate.
The design of the product passage ensures even distribution of liquid on the plate surfaces and minimizes pressure drop in the vapor phase. The easy-to-open plate package allows for surface inspection, changing of individual plates as needed, and adjustment of the evaporation rate by adding or removing plates. The units can be customized to meet USDA Dairy sanitation standards.