time inside the tubes is measured in seconds, making it ideal for heat-sensitive products such as milk, fruit juice, pharmaceuticals, and many others. Falling film evaporators are also characterised by very low pressure drops; therefore, they are often used in deep vacuum applications. Fouling Due to the intimate contact of the liquid with the heating surface, these evaporators are sensitive to fouling from precipitating solids. Low liquid velocity at the inlet is usually not sufficient to perform an effective self-cleaning of the tubes. Falling film evaporators are therefore used in clean, non-precipitating liquids. A typical application, in chemical industry, is for concentration of caustic soda. Falling film evaporators versus flooded evaporators Falling film evaporators have a number of advantages over their flooded evaporator counterparts. They require a lower charge, as the entire shell (in the case of horizontal evaporators) or all the tubes (in the case of a vertical evaporator) need not be filled with liquid as a thin film is now used to cover the surfaces. In industries such as heating and air-conditioning this can save significant money due to the high costs of a refrigerant charge. Falling film evaporators also show improved heat transfer characteristics over their flooded counterparts, particularly in cases with low heat flux. A number of disadvantages exist, primarily being the comparable lack of understanding of falling film evaporators compared to flooded evaporators, particularly for horizontal falling film evaporators. Furthermore the fluid distribution for horizontal falling film evaporators is a challenge, as the performance is severely limited if an uneven distribution of film over the tubes is created. Horizontal versus vertical falling film evaporators Horizontal falling film evaporators have a number of potential advantages over their vertical counterparts in the petrochemical industry, such as the ability to use tubes with external enhancements; while internally-enhanced tubes are available for vertical falling film evaporators, external enhancements are typically superior for boiling applications. The chief disadvantage of horizontal falling film evaporators is that if a corrosive or fouling liquid is to be evaporated, it will have to placed on the shell side. This is against best practice, as it is easier to clean fouling found on the inside of tubes rather than the outside. See also Climbing and falling film plate evaporator References External links Falling Film Evaporators Wolverine Tube Heat Transfer Databooks Evaporators