Evaporadores

Heat Transfer

Selecting Evaporators
for Process Applications
Evaluate product characteristics
and process requirements to select
among the various evaporator designs.

William B. Glover
LCI Corp.

E

VAPORATORS ARE USED IN A WIDE RANGE
of processes, including pharmaceuticals, foods and
beverages, pulp and paper, chemicals, polymers and
resins, inorganic salts, acids, bases, and avariety of other
materials. There are many types and variations of evaporators,
and the best for a particular application depends on the product characteristics and desired results.
This article first explains the process requirements that an
evaporator must meet, and discusses the critical operational
and product characteristics and their effects on evaporator
selection. It then reviewsthe various types of evaporator
designs, their modes of operation, features, advantages and
limitations, and it provides guidance on how to select an evaporator for a particular application.

The basics
Evaporation is an operation used to concentrate a solution
of a nonvolatile solute and a volatile solvent, which in many
cases is water. A portion of the solvent is vaporized to produce
aconcentrated solution, slurry or thick, viscous liquid.
Evaporation differs from drying in that the residue is a
flowable liquid instead of a solid. Evaporation is different
from distillation in that there is no attempt to separate the
vapors into individual components.
Either the vapor or the concentrate stream, or both, may be
the desired product (1). Therefore, the evaporator should bedesigned to provide a clean separation of the vapors from the
condensate and the feed.
An evaporator consists of a heat exchanger or heated
bath, valves, manifolds, controls, pumps and condenser. The
most common designs are jacketed tanks, tubular heat
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exchangers, plate-and-frame heat exchangers, and agitated
thin-film evaporators.
A properlydesigned evaporator must, at a minimum:
• be designed to effectively transfer heat at a high rate with
minimum surface area to be cost-effective for installation,
operations and maintenance
• effectively separate the vapor from the liquid concentrate
• meet the conditions required by the product being
processed
• produce a product that meets the required quality
• be energy efficient,where possible making effective use
of steam with multiple-effect evaporation or vapor recompression
• minimize fouling of heat transfer surfaces
• be constructed of materials that are adequate to minimize
corrosion.

Critical operational and product characteristics
Critical operational and product characteristics of the solution to be evaporated have a major effect on the selection of
theevaporator type most suited for the application.
Heat sensitivity. Many foods, pharmaceuticals, chemicals
and resins are heat- or temperature-sensitive and require either
low heating temperatures or a short residence time exposed to
the heat, or both. This can be accomplished by a combination
of minimizing the volume of product in the evaporator at any
one time, minimizing the time in theevaporator, and reducing
the product’s bulk boiling temperature by operating the evaporator at reduced pressures. Reducing the internal operating
pressure may also allow operation at lower heating temperatures while still maintaining a reasonable heat-transfer driving

force (the temperature difference between the boiling point of
the bulk product and the temperature of the heating medium).Fouling. Fouling of the heat transfer surfaces is usually
caused by solids in the feed, precipitating solids in the concentrate, or degradation of the product. A slow buildup of a film
on the heat transfer surfaces will cause a gradual reduction in
the overall heat-transfer coefficient. Eventually this will
require shutdown of the process and cleaning of the heat transfer surfaces, which...