Steam condenses on the outside of the tubes supplying the required energy to the inside of the tubes. Calculating pressure drops in falling film evaporators has been investigated since the late 1940's. A universal equation is really not agreed upon. Typically, a constant dependent on the percentage of vapor exiting the evaporator is used in a pressure drop relationship. Falling Film Evaporators in the Food Industry Evaporating fruit and vegetable juices presents a special challenge for chemical engineers.
Juices are heat sensitive and their viscosities increase significantly as they are concentrated.
Small solids in the juices tend to cling to the heat transfer surface thus causing spoilage and burning. High flow circulation rates help avoid build-ups on the tube walls. Some of the volatile, flavor-containing components are lost during evaporation. In this case, some of the raw juice is mixed with the concentrate to replace the lost flavors. Considering that the components of juices have close boiling points, a standard, single evaporator is seldom sufficient.