Evaporation occurs inside vertical tubes through a film of liquid established using one of several liquid distribution devices, the selection of which depends on the wetting rate on the tubes.
This is one of the most commonly used evaporator types due to simplicity of operation, dependability of performance, low temperature difference required, and high heat transfer coefficients developed in the film.
- Short product contact time minimizes thermal degradation effects
- Low tube wall temperature and heat flux minimizes fouling
- Performance allows high energy efficient multiple effect designs
- Suitable for heat sensitive products
- High vapor shear reduces foaming products
- Turndown on recirculation designs is close to infinite
In falling film evaporators, the liquid product usually enters the evaporator at the head of the evaporator. In the head the product is evenly distributed into the heating tubes. The liquid enters the heating tube and forms a thin film on the tube wall where it flows downwards at boiling temperature and is partially evaporated. In most cases steam is used for heating the evaporator. The product and the vapor both flow downwards in a parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product from its vapor takes place in the lower part of the heat exchanger and the vapor/liquid separator.