How a falling film is formed
Feed is distributed at the top of vertical heating surfaces and flows downward as a thin film. Heating medium condenses or cools on the opposite side, while part of the volatile component evaporates from the product film. Vapour and concentrated liquid are then separated in a downstream separator.
Distribution is critical
Uniform wetting of every tube or plate is essential. Maldistribution can create dry areas, fouling and product damage even when the total flow rate is adequate. Distributor design, minimum recirculation, level control and cleanliness therefore have a strong effect on performance.
Why vacuum is used
Operating under vacuum lowers the boiling temperature, which can protect heat-sensitive products and make lower-temperature utilities usable. Vacuum also changes vapour density, pressure drop and condenser duty. The complete system must include vapour handling, condensation, non-condensable removal and leak-tight operation.
Sizing inputs
The heat duty follows the mass and energy balance, including feed heating, evaporation and concentration effects. Required area then depends on the effective temperature difference, overall heat-transfer coefficient and fouling margin. Boiling-point elevation and pressure losses must be considered when defining the real temperature driving force.
Product limits and operation
Viscosity often rises as concentration increases, reducing film quality and heat transfer. Foaming, crystallisation, solids, polymerisation and very low wetting rates may require recirculation, another evaporator type or pilot testing. Start-up, shutdown and cleaning sequences should be included in the design.
Data needed for a proposal
Share feed composition, flow and temperature; target concentration; volatile components; physical properties versus concentration; product sensitivity; available steam or thermal fluid; cooling-water conditions; allowable residence time; cleaning method and required recovery or emissions limits.