The document discusses different types of evaporators used to boil liquid solutions. It describes evaporators as equipment that boils liquids through various heating methods like direct fire, steam jackets, or vapor-heated tubes. It provides details on horizontal tube, vertical short tube, basket, long tube, and forced circulation evaporators - outlining their key components, operating principles, advantages, disadvantages and applications.

1. Evaporation and Evaporators

2. Evaporation and evaporators
• The term evaporation is used for boiling of aqueous
or sometimes aqueous-alcoholic solutions either at
atmospheric or at reduced pressure
• An evaporator is a piece of equipment in which a
liquid is boiled
• The conditions under which evaporation is carried
out may widely vary, like:
• the liquid to be evaporated may be less viscous than
water or
• it may be so viscous that it will hardly flow

3. • It may deposit scale on the heating surface
• It may precipitate crystals
• It may tend to foam
• It may have very high boiling point elevation
• Or it may be damaged at high temperatures
• Thus, the construction and design of evaporators
varies according to the requirements

4. Classification of evaporators
• A. Apparatus heated by direct fire
• B. Apparatus with heating medium in jackets,
double walls,etc
• C. Vapor-heated evaporators with tubular
heating surfaces
▫ i) Tubes horizontal, vapor inside tubes
▫ ii) Tubes vertical (Standard type, Basket type,
Long-tube type, Forced-circulation type)
▫ iii) Tubes bent into spherical shapes such as coils,
hairpin tubes, etc

5. • Fire-heated apparatus: The main device in
this category is the steam boiler
• Jacketed apparatus: when liquids are to be
evaporated on a small scale, the operation is
conducted in a jacketed kettle
• They are available in a variety of materials and
constructions like cast iron
• The jacket can be cast in one piece with the
kettle or can be made of sheet metal attached to
the outside of the kettle
• Kettles of stainless steel, aluminium, copper,etc
are widely used in food-process industry with
sheet metal jacket welded/riveted to kettle body

6. Horizontal-tube evaporators
• In 1879, the Wellner-Jelinek type evaporator
was revived
• The usual type of horizontal-tube evaporator is
shown in the following diagram
• The body consists of the liquor compartment in
the form of a vertical cylinder
• It is closed at the top and bottom with dished
heads and the bottom is conical
• The lower part of the cylinder consists of steam
compartments on opposite sides

8. • Steam compartments have cover plates on the
outside and tube sheets (with a number of
horizontal tubes) inside
• Steam comes into the cylinder through the
steam compartment A and flows through the
horizontal tubes, washes off the non-condensed
gases and condensates which leave through the
opposite steam compartment at B and C,
respectively
• The entry point of feed liquid is at D through
which the liquid to be evaporated enters the
evaporator, the liquor level maintained slightly
higher above the tubes

9. • The liquid after being boiled in the evaporator, leaves as
a thicker concentrated liquid at point E at the center of
the bottom, while the vapours from the boiling liquid
escapes at the top through connection G
• Evaporators are also provided with sight glasses F
• The evaporator may be from 36 in. to 12 ft in diameter
and tubes’ diameter ranges from 7/8 to 5/4 in. outside
diameter
• Average evaporator size would be 6-8 ft in diameter and
from 8-12 ft high
• Made of cast iron to resist corrosion
• Use: suitable for non-viscous solutions that don’t deposit
scales or crystals on evaporation and it is economic
compared to other evaporators

10. Vertical short tube evaporators-standard type
• The first vertical tube evaporator was built by
Robert in 1850, and so often known as the
Robert type evaporator
• Consists of a vertical cylinder with tube sheets A
and central downtake B
• Tubes are rolled between these two tube sheets
so that the liquid to be evaporated is within the
tubes and the steam is outside the tubes
• As the liquid boils, it spouts up through the
tubes and returns through the central downtake
• So the liquid gets circulated

13. • Condensate is removed from a place on the bottom
tube sheet, C and non-condensed gases go out near
the upper tube sheet D
• The feed enters through E and the thick liqour exits
through F
• The tubes may range from 1 to 4 in. in diameter and
from 30 in. to 6 ft long; average diameter being 2 in
and length of about 5 ft.
• The circulation depends upon boiling and when
boiling stops the solids in suspensions settle down
• This problem may be overcome by installing a
propeller in the downtake, which causes rapid
circulation of liquid and the capacity of the
evaporator can be doubled

15. • Advantages: high heat transfer coefficients at high
∆t values, easy descaling and less expensive
• Disadvantages: low heat transfer coefficients at
low ∆t values, high floor space and weight, poor heat
transfer with viscous liquids
• Use: mainly for clear liquids, crystallizing solutions
(with propeller), relatively non-corrosive liquids and
mild scaling solutions which require mechanical
cleaning since tubes are short with large diameter
• Basket type evaporator: the downtake is annular
instead of central and the entire heating element is a
single unit which can be removed easily for repairs
and a deflector can be added easily to reduce
entrainment from spouting liquid as the boiling
liquid is quite violent

16. Long tube vertical evaporator
• It consists of a simple one-pass vertical shell and tube
heat exchanger with a tube diameter of 5/4 to 5/2 in. and
the length of the tube varies from 10-20ft
• Feed enters at the bottom of the tube, starts to boil after
sometime and the mixture of liquid and vapour leaving
through the top of the tube impinges upon a deflector at
high velocity
• The deflector breaks the foam and prevents entrainment
and throws down the liquid into lower part from which it
is withdrawn
• the machine has no downtake and the liquid passes
through the machine only once

18. • High liquid velocities in the tube carrying boiling
liquids tend to decrease the thickness of the
viscous film and helps to wipe off steam bubbles
• When very long tubes (L/D > 150) are used in
vertical tube evaporators and low liquor levels
are maintained, the pumping action is violent
and the liquid velocity high
• The liquid level is maintained low not > 2-3 ft

19. Forced circulation type
• The tubes are longer and narrower than the
standard vertical, ¾ in inside diameter and 8 ft
long and are contained in a heating element
between two tube sheets
• The tube bundle projects out partly into the
vapour head B
• A return pipe C for the liquid starts from the
vapour head and connects to the inlet of the
pump D which pumps the liquid into the tubes
with a positive velocity

21. • As the liquid rises through the tubes, it gets heated by the steam
(steam connection near the bottom of tube bundle) and begins to
boil and the vapour and the boiling liquid comes out from the top of
the tubes at high velocity and strikes the deflector E
• the deflector throws the liquid downward and separates it from the
vapour; the liquid comes out through return pipe C to the pump and
is recirculated
• The non condensed gases are removed at G and the condensate
through H
• The evaporator is mainly suited for foamy liquids, viscous liquids
and those that deposit scale or crystals on heating surfaces
• Ref: Introduction to Chemical Engineering, Badger and Banchero,
pg-170
• Pharmaceutical Engineering by K. Sambamurthy, pg-93