Page 174 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

172 Chapter 6 Evaporators

shorter residence time as compared to rising film evaporators for any given set of conditions. The
temperature is also more uniform in falling film types as there is no problem of hydrostatic head, and the
only critical concern is the feed distribution system.
Due to the combined feature of functioning at the low-temperature difference and short residence
time, the falling film evaporator is highly suitable for temperature-sensitive products. Typical appli-
cations include the concentration of dairy products, sugar solutions, urea, phosphoric acid, and black
liquor.
The Climbing-and-Falling film Evaporator (Fig. 6.11) combines the configuration of the rising-film
and the falling film evaporators and offers the advantages
of both. When a high ratio of evaporation to feed is
required resulting in a viscous product, a tube bundle can
Climbing and Falling Film Evaporator
be divided into two sections, in which, the first, functions
as a climbing-film evaporator and the second as a falling
film evaporator. Feed enters at the bottom of the tube-
sheet of the climbing-film portion. Boiling starts as the liquid rises through the tubes. A mixture of
liquid and vapor is discharged and redistributed over the top of the tubes for the falling film pass. The
vapors from the climbing-film aid in the distribution of liquid in the tubes and increase the velocity of
the liquid, which increases heat transfer. The discharge from the falling film tubes go to a vapor-liquid
separator located at the bottom of the calandria, and the vapor outlet is connected to a vacuum system
or a condenser.

Heating
steam
Upflow Downflow
tubes tubes

Vapour to vacuum
system/condenser
Demister
pad
Condensate

Vapour-Liquid
separator

Feed

Product

FIGURE 6.11
Climbing & Falling film evaporator.