Falling film
product distribution generally is based around use of a perforated plate
positioned above the top tube bundle of the calandria. The main problem in this design
is figuring how to design for the even distribution of liquid to each of the tubes.
Once designed, spreading of liquid to each tube is oftentimes further enhanced by
generating flash vapor at this point. The falling film evaporator does have the
advantage that the film is 'going with gravity' instead of against it. This results in
a thinner, faster moving film and gives rise to an even shorter product contact time
and a further improvement in the value of heat transfer coeficient.
One advantage that falling film evaporators have over rising film evaporators is in
efficiency. The rising film evaprator requires a driving film force, and requires a
temperature difference of at least 25 degree F across the heating surface. In contrast,
the falling film evaporator does not have a driving force limitation-permitting a
greater number of evaporator effects to be used within the same overall operating
limits. However, using the falling film technique, it is feasible to have 6 or more
effects.
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