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Optimum quality is obtained when processing times and temperatures are kept

as low as possible during concentration of the products. The most critical portion

in the process occurs during the brief time that the product is in contact with a

heat transfer surface which is hotter than the product itself.

In a rising lm design, liquid feed enters the bottom of the heat exchanger and

when evaporation begins, vapor bubbles are formed. s the product continues up

the tubular passage and the evaporation process continues, vapour occupies an

increasing amount of the passage. !ventually, the entire center is lled with

vapor while the liquid forms a lm on the heat transfer surface.

The e"ect of gravity on a rising lm evaporator is twofold. It acts to keep the

liquid from rising in the passage. #urther, the weight of the liquid and vapor in

the channel pressuri$es the %uid at the bottom and with the increased pressure

comes an increase in the boiling point.

The ma&ority of the liquid residence time occurs in the lower portion of the

passage before there is su'cient vapor to form a lm. If the liquid is not

preheated above the boiling point, there will be no vapor. nd since a liquid pool

will ll the entire area, the residence time will increase.

(here y)*ocal weight fraction of vapour

) vapour density

) liquid density

lso

(here t ) time

) cross sectional area

* ) tube length

) liquid rate

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