Heat Transfer in Falling Film Evaporators





Types of evaporators

Working Principles


Residence time

Heat Transfer

Liquid distribution


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In general Falling Film evaporators are designed to operated under conditions were no nucleated boiling takes place. Heat from the tube wall is transfered via convection or conduction through a liquid film to the vapour-liquid interface were evaporation takes place. In the majority of applications evaporation takes place on the tube side, with vapour liquid separation at the bottom of the tube. The film heat transfer coefficient depend on the hydrodynamic of the annular liquid film. The film can be divided in pure laminar, laminar-wavy and full turbulent flow. (see also flow characteristics) The transition is characterised by the Film Reynolds number and the Kapitza number.
In laminar film flow, the heat transfer mechanism is one of conduction through the film. Indicting that with thinner films at lower circulations the heat transfer increased.

In this scenario the Nusselt solution for Film flows can be used:
 Nu (film, laminar) = 1.1 Re(film) (-1/3)

Whereby the film Reynolds number is defined as follows:

Re(film) = 4 G / h

G = mass flow per circumference 

h = fluid viscosity 

With increasing Reynolds number due to lower viscosity or higher mass flow the liquid film becomes wavy laminar, here according to Chun Seban the heat transfer can be calculated as follows:

Nu (film, wavy laminar) = 0.82 Re(film) (-0.22)

The waviness of the film adds turbulents to the flow and therefore compared to the pure laminar the heat transfer increases.

In turbulent films the mechanism which can be found outside the laminar sub-layer, is one of convection and with increasing Reynolds number an increase in heat transfer can be observed. In turbulent flow the effect of the liquid film properties has to be taken into account and the heat transfer according to Chun Seban(1) can be expressed as follows:

Nu (film, wavy laminar) = 0.0038 Re(film) (0.4)  .   0.0038 Pr(film) (0.65)

This correlation works satisfactorly for single component fluids up to Pr numbers of 7.

Other authors suggest:

Nu (film, wavy laminar) = 0.0097 Re(film) (0.29)  .   0.0038 Pr(film) (0.63)

Side notes:
- More recently Alhusseini (2) who used water and propylene glycol was able to demonstrate that the Chun Sean correlation overpredicts the heat transfer at high Prandtl numbers. He proposed take into account also the Kapitza number when calculating the heat transfer in liquid films. 

- Heat transfer can be enhance in case vapour shear at the liquid vapour interface leads to more agitated thiner liquid films.



(1) Chun, K. R. and Seban, R. A., Heat transfer to evapaporating liquid films.  Journal of Heat Transfer, 197 I, 93, 391-396

(2) Falling film evaporation of single component liquids

Inr. J. Hear Mass Transfer. Vol. 41, No. 12, pp. 1623-1632, 1998