1. Vertical Falling Film Evaporators
In general Falling Film Evaporators are build vertically.
Typically, liquor is fed to the tops of
tubes arranged as a bundle and flows down the walls as a film under gravity.
The liquid to be vaporized flows in the majority of designs inside the
tube. To balance the in general high heat transfer of the evaporating film,
condensing steam is often used as heating medium. In the majority of designs the
vapour generated is flowing concurrent with the liquid film. The pressure and
the vapour flow direction is dictated by the location and the temperature level
of the applied condenser. In general the liquid and vapour is separated at the
bottom of the unit.
In vertical falling film evaporators the tube length is
The tube length is typically 6 m to 11 m (20 ft to 36 ft) but
can be as short as 1.5 m to 3 m (5 ft to 10 ft) for example in
deep vacuum applications. Diameters are typically 20 mm to 64 mm (1 in to 2½
2. Horizontal Film Evaporators
This type of evaporator is used mainly in applications for
seawater desalination and refrigeration technology. The liquid is evaporated at
the outside of the tubes. It flows from one tube to the other in form of
droplets, jets or as a continuous sheet. Due to the impinging effect when
flowing from one tube to the other the heat transfer is higher compared to
vertical falling film evaporators. In addition this unit type can be operated
with even lower pressure drops compared to the vertical design.
Therefore this type is often used in seawater desalination and refrigeration
technology with heat recovery. Thermal or mechanical vapour compression can be
used in order to increase the effectiveness of such units.
It is also possible to design a higher heat transfer area for a
given shell compared to the vertical units. Perforated plates or specially designed
spray nozzles can be used in order to guarantee a even liquid distribution for
Cleaning of the out side tubes can be difficult, therefore this
type of evaporators is not used for processes with dendency to foul.
Tube dimensions are typically 0.75 to 1''.