Applications for Falling Film Evaporators

Applications for falling film evaporators (Overview)

Falling film evaporators are used extensively in chemical process industry, food and paper industry. Due to the absence of static static head effect caused by liquid column as in other types of evaporators, evaporation can take place at very small effective mean temperature differences. The temperature diffference are tipically between 3 - 8C. This is significantly less than in other devices used for evaporation, e.g. forced reboilers or kettle evaporators, here the effective mean temperature difference is between 15 an 30C. The film heat transfer coefficients are in general high, and characterised by surface boiling.
The absence of hydrostatic head allows this type of evaporator to operate at very low absolute pressures.
Product residence time can be very short, especially in one through operation. These characteristic of short retention time low operation pressure and small required effective mean temperature differences makes this type of evaporator particularly suitable for concentration of heat sensitive liquids. The absence of nucleate boiling under normal operation conditions, and low temperature differences reduces also possible fouling tendencies

The flow pattern can be characterised as a liquid annulus with vapour core.

Conclusion:

In general very high heat transfer coefficients, which can be considerably lower in systems with viscous mixtures.
Very short liquid residence time in tubes, therefore low liquid hold-up and inventory
Small effective mean temperature differences, where evaporation takes place at liquid surface. In general it is recommended to avoid nucleate boiling (fouling)
Pressure drop in tubes is often negligible, therefore only in vacuum applications the calculated pressure drop has to be taken into account
Falling Film Evaporators can be operated at very low absolute pressures, they can be used at absolute pressures approaching the hydrostatic head of film thickness