Dairy Ingredients for Food Processing

Concentrated Fluid Milk Ingredients 127

Falling fi lm evaporators are often used

with product recirculation, in which some of

the formed concentrate is reintroduced back

to the feed inlet to produce suffi cient liquid

inside the boiling tubes.

Forced Circulation Evaporators

Forced circulation evaporators are used if

boiling of the product on the heating surfaces

is to be avoided due to the fouling character-

istics of the product, or to avoid crystalliza-

tion. Forced circulation evaporators are

usually for viscous liquids, corrosive liquids,

and concentrated liquids that cause fouling

and scaling problems.

Plate Evaporators

Framed plates can be used instead of tube

bundles as a heating surface. These plate

assemblies are similar to plate heat exchang-

ers, but they are equipped with large passages

for the vapor fl ow. In these units a product

fl ow plate and a steam fl ow plate are con-

nected alternately. The product passage is

designed for even distribution of liquid on

the plate surfaces and low pressure drop in

the vapor phase.

Vapor Recompression Evaporators

To minimize energy consumption, two sys-

tems for vapor recompression are used:

Thermal vapor recompression (TVR) and

mechanical vapor recompression (MVR)

evaporators. MVR evaporators are single -

effect evaporators divided into several stages

(approximately fi ve to eight), using a high -

pressure fan for recompression of vapor.

Thus, the heating medium in the fi rst effect of

MVR evaporators is vapor developed in the

same effect, compressed to a higher tempera-

ture using turbocompressor or high - pressure

fans. TVR evaporators are multiple - effect

evaporators with two to seven effects using a

steam jet compressor for re - compression of

• Falling fi lm evaporators


• Rising fi lm evaporators


• Forced circulation evaporators


• Plate evaporators


• Vapor recompression evaporators

Falling Film Evaporators

The falling fi lm tubular evaporator is primar-
ily used in the dairy industry. In this type
of evaporator, liquid and vapors fl ow down-
ward in parallel fl ow. The liquid to be con-
centrated is preheated to boiling temperature.
An even and thin fi lm of the liquid enters
the heating tubes via a distribution device
in the head of the evaporator, fl ows down-
ward at boiling temperature, and is partially
evaporated. This gravity - induced downward
movement is increasingly augmented by the
co - current vapor fl ow.
Falling fi lm evaporators can be operated
with very low temperature differences
between the heating media and the boiling
liquid, and they also have very short product
contact times, typically just a few seconds/
pass. These characteristics make the falling
fi lm evaporator particularly suitable for heat -
sensitive products such as milk.

Rising Film Evaporators

In rising fi lm evaporators, the feed enters
the bottom of the heating tubes and as it gets
heated, steam begins to form. The ascending
force of the steam produced during the
boiling causes liquid and vapors to fl ow
upward in parallel fl ow. At the same time
the production of vapor increases and the
product is pressed as a thin fi lm on the walls
of the tubes, and the liquid rises. The co -
current upward movement helps to create a
high degree of turbulence in the liquid,
making the process advantageous for evapo-
ration of highly viscous products and those
that have a tendency to foul the heating
surfaces.