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cell no: 0300-70244
Institute Of Chemical Engineering And Technology Lahore Pakistan
Evaporation is a unit operation that consists of
elimination of water of a fluid food by means of vaporization
Evaporation is the removal of solvent as a vapor from a
solution or slurry
Evaporation is used for concentration of aqueous solutions,
it involves removal of water from solution by boiling the
liquor in suitable vessel called evaporator and withdrawing
The equipment used to remove water from the food
product is called evaporator
Evaporators are used to separate materials based
on differences in their boiling temperatures.
Its purpose is to concentrate nonvolatile solutes
such as organic compounds, inorganic salts, acids
or bases. Typical solutes include phosphoric acid,
caustic soda, sodium chloride, sodium sulfate,
gelatin, syrups and urea.
Why we need evaporation
Reduces transportation cost
Prepare for the next Unit operation – drying,
Reduces deteriorative chemical reactions
Better microbiological stability
Recovery of solvent
Driving force: temperature difference in between
steam chest temperature and product temperature
Result: volatile solvent is removed from the feed
Solution(high volatile solvent+non volatile solute)
Concentrate(non volatile solute)
Concentration of milk to produce condensed milk
Concentration of juices
Concentration of NaOH , NaCl from aqueous solutions
to produce salt.
Ether recovery from fat extraction
Steam heat is used for transfer of heat for subsequent
vessels. Steam has a very high heat content Heat is
given up at constant temperature.
It can be used at high pressure to generate electric
power and low-pressure exhaust steam is used for
Evaporation is a process of vaporizing large quantities
of volatile liquid to get a concentrated product.
Evaporation is a surface phenomenon, i.e., mass
transfer takes place from the surface.
Basic Parts of an Evaporator
Vaccum for non
An evaporator is used to evaporate a volatile
solvent, usually water, from a solution. Its
purpose is to concentrate non-volatile solutes
such as organic compounds, inorganic salts, acids
or bases. Typical solutes include phosphoric acid,
caustic soda, sodium chloride, sodium sulphate,
gelatine, syrups and urea.
In many applications, evaporation results in the
precipitation of solutes in the form of crystals,
which are usually separated from the solution
with cyclones, settlers, wash columns, elutriating
legs, filters or centrifuges. Examples of
precipitates are sodium chloride, sodium sulfate,
sodium carbonate and calcium sulphate. The
desired product can be the concentrated
solution, the precipitated solids, or both.
Multiple Effect Evaporators
Heat Transfer Coefficient, h
A coefficient which indicates the amount of heat flow
that is exchanged across a unit area of a medium or
system in a unit amount of time with a unit of
temperature difference between the boundary of the
S.I. unit W m-2 K-1.
Overall Heat Transfer Coefficient, U
In cases of combined heat transfer for a heat
exchanger, there are two values for h.
Convective heat transfer coefficient for the fluid
film inside the tubes
Convective heat transfer coefficient for
the fluid film outside the tubes.
The thermal conductivity (k) and thickness
(Dx) of the tube wall must also be accounted for.
So an additional term (Uo), called the overall heat
transfer coefficient, must be used instead.
Oldest type and used for limited application
Either jacketed/internal coils/heaters
Product residence time is many hours
Boil at low temp and high pressure for heat
heat transfer area and coefficients are low under
Evaporation capacities are low
Large temp differences not achieved b/c of rapid
fouling of heating surface
Used for concentration of Jams and Jellies, also
for some pharmaceutical products
3) Calandria type Evaporator:
Commonly it is known as short tube or single effect evaporator.
Vertical type of evaporator.
Tubes sheets extending across the body and central down take.
Material which has to be evaporated is introduced in tubes.
The tube may be about 1.2m long and 5 cm in diameter, but the size
varies with the nature of the substances.
There is space below containing a steam coil to give extra heating
capacity and large enough to afford circulation of liquid.
*Large floor space and weight
*Poor heat transfer at low temperature
*Not use for thermo-labile products
Natural Circulation Units
Circulation is obtained by convection currents
arising from heating surface
A) Tubes are horizontal with steam inside
B) Tubes are vertical with steam outside
a. solution to be evaporated boils outside the tubes and
steam condenses inside of tubes. These tubes interfere
with natural circulation and minimize liquid agitation
Overall heat transfer coefficients are lower
Cylindrical section above heating portion is used
to separate vapor from liquid
vapor leaves through de-entraining device to
prevent carry over of liquid droplets with vapor
Steam enters through one side chest and leaves
through opposite chest
Steam condensate out via steam trap
The major use is for making distilled water for boiler
feed. Horizontal tube evaporators are used in the
pharmaceutical industry, pulp and paper industry.
They are relatively low cost. They have very low
headroom. Horizontal tube evaporators are not
suitable for salting or scaling liquids, and they have
smaller capacity than other evaporators.
2. Easy to install
3. Require less space for installation
4. Suitable for liquids that not crystallize
5. Can be used for batch/continuous operation
1. Not suitable for viscous liquids b/c of poor
Solution boils inside vertical tubes with heating
media, usually steam, held in steam chest, through
which tubes pass.
Boiling of liquid in tubes, causes liquid flow upward
through tubes and un-evaporated liquid flows
downward through central hole
These overcome disadvantages of horizontal tube
1. natural circulation is promoted(1-3 ft/s)
2. heat transfer coefficients are higher
3. solid built inside tubes is removed by mechanical cleaning
4. Viscous liquids can be used, but circulation is slow
5. Used in sugar and salt industries
These are impractical when solution is very viscous or form
foams or is heat sensitive
Climbing film Evaporators:
It is also known as long tube or rising film evaporator.
The theory of climbing film evaporator’s working is that the ascending force of this steam
produced during the boiling causes liquid and vapors to flow upwards in parallel flow. At
the same time the production of vapor increases and the product is pressed as a thin film
on the walls of the tubes, and the liquid rises upwards. This co-current upward
movement against gravity has the beneficial effect of creating a high degree of turbulence
in the liquid. This is advantageous during evaporation of highly viscous products and
products that have a tendency to foul the heating surface.
Usually there must be a rather high temperature difference between the heating
and boiling sides of this type of evaporator. Otherwise the energy of the vapor flow is not
sufficient to convey the liquid and to produce the rising film. The length of the boiling
tubes will typically not exceed 23 ft (7m).
High heat transfer rates at high temperature differences
Ease of cleaning
Large floor space and weight
Poor heat transfer at low temperature differences
Not use for thermolabile products
Horizontal Film Evaporator:
Also known as falling film evaporator.
This type of evaporator is generally made of long tubes (4-8 meters in length)
which are surrounded by steam jackets. The uniform distribution of the
solution is important when using this type of evaporator. The solution enters
and gains velocity as it flows downward. This gain in velocity is attributed to
the vapor being evolved against the heating medium, which flows downward as
well. This evaporator is applicable to highly viscous solutions so it is frequently
used in the chemical, food, and fermentation Falling Film Evaporator with
boiling inside the tubes. The feed to the evaporator is fed to the top of the
Calandria through efficient liquid distributors. Steam is given on the shell side.
The concentrate is collected at the bottom. The same operation is repeated in
multiple effects to achieve steam economy.
) Rising-falling film Evaporator:
Rising and falling film evaporators
are some time combined in to a same unit. When a
high ratio of evaporation to feed is required and
concentrated liquid is viscous, a tube bundle can be
divided in to two sections with the first section
functions as a rising film evaporator and the second
section serves as falling film evaporator.
Low cost and small floor space
Suitable for viscous material
Large units for handling high evaporation load
High headroom required
Recirculation is frequently required
Water is boiled in a sequence of vessels, each held at a
lower pressure than the last.
Because the boiling point of water decreases as
pressure decreases, the vapor boiled off in one vessel
can be used to heat the next
Generally the first vessel (at the highest pressure)
requires an external source of heat
A multiple effect evaporator system for concentrating a process liquid comprises:
(a) a plurality of evaporator effects arranged in series, each effect including a process liquid
inlet and a process liquid outlet; a heating fluid inlet and heating fluid outlet;
(b) heat exchange means in each effect for passing said process liquid in heat exchange
relationship with heating fluid for evaporating water out of said process liquid; and
wherein evaporated water from one effect serves as heating fluid for an adjacent effect; and
(c) an evaporative condenser provided with liquid inlet means for receiving process liquid
from one of said evaporator effects, and liquid outlet means for transmitting said process
liquid to another of said evaporator effects; and means for receiving heating fluid vapor
and for passing said heating fluid vapor in heat exchange relationship with cooled process
liquid in a cooling circuit, for condensing said heating fluid vapor.
(2) Steam for heating
(3) Feed in
(5) Feed out
(6) Vapour Separator
(9)Cooling water in,
(10) Cooling water
Multiple effect evaporator Due to heat transfer, the liquid
temperature increases & reaches the B.P. during this process,
vapour well be generated from the liquid feed.
So, formed vapour displaces air in the upper part of 1st evaporator.
Moreover, the vapour also displaces the air in the steam space of
the 2nd evaporator.
After complete displacement of air by vapour in the steam
compartment of 2nd evaporator, the second
valve is closed.
The vapour of 1st evaporator transmits its heat to the liquid of 2nd
evaporator & gets condensed.
Condensate is removed through the second condensate valve.
These steps continue in the 3rd evaporator also.
As the liquid in 1st evaporator gains temperature the difference in
temperature between the liquid & steam decreases, hence, the
rate of condensation decreases.
As a result, the pressure in the vapour space of 1st evaporator
gradually increases to P1 by increasing temperature to T1 , which is
the B.P. of the liquid in first evaporator & decreasing the
temperature difference (t0-t1).
A similar change takes place in the 2nd evaporator & the liquid
reaches the B.P.
similarly, the process will be repeated in 3rd evaporator. Finally 3
evaporators come to a steady state with the liquid boiling in all the
As boiling proceed, liquid level in 1st evaporator comes down. Feed is
introduced through the feed valve to maintain the liquid level constant.
Similarly evaporation of liquid takes place in 2nd & 3rd evaporators.
To maintain the liquid levels constant, feed valves F2 & F3 are used for
2nd & 3rd evaporator respectively.
This process is continued until the liquid in all the evaporators reaches
the desired viscosity.
Now the product valves are opened to collect the thick liquid.
Thus in this evaporators, there is continuous supply of feed, continuous
supply of steam & continuous withdrawal of liquid from all 3
evaporators. Hence, evaporators work continuously.
Efficiency of multiple effect
It is the quantity of vapour produced per unit steam
Feed is admitted at its B.P. so it does not require any more
heat to raise its temp.
Hence, the supplied steam is condensed to give heat of
condensation. This heat will then transferred to the liquid.
The heat transferred now serves as latent heat of
vaporization, i.e. liquid undergoes vaporization by
receiving heat. Loss of heat by means is negligible.
Feeding of Multiple Effect Evaporators
There are three types of feeding methods:
The pressure in the second effect must be reduced below that in the
first. In some cases, the first effect may be at a pressure above
atmospheric; or the first effect may be at atmospheric pressure and
the second and subsequent effects have therefore to be under
increasingly lower pressures. Often many of the later effects are
under vacuum. Under these conditions, the liquid feed progress is
simplest if it passes from effect one to effect two, to effect three, and
so on, as in these circumstances the feed will flow without pumping.
This is called forward feed
In the case of a forward feet operation, the raw feed is
introduced in the first effect and is passed from effect to
effect parallel to steam flow. The product is withdrawn
from the last effect. This procedure is highly advantageous
if the feed is hot. The method is also used if the
concentrated product may be damaged or may deposit
scale at high temperature
Alternatively, feed may pass in the reverse
direction, starting in the last effect and proceeding
to the first, but in this case the liquid has to be
pumped from one effect to the next against the
pressure drops. This is called backward feed and
because the concentrated viscous liquids can be
handled at the highest temperatures in the first
effects it usually offers larger evaporation capacity
than forward feed systems, but it may be
disadvantageous from the viewpoint of product
In the backward operation, the raw feed enters the last (coldest) effect and
the discharge from this effect becomes a feed for the next to last effect. This
technique of evaporations is advantageous, in case the feed is cold, as much
less liquid must be heated to the higher temperature existing in the early
effects. The procedure is also used if the product is viscous and high
temperatures are required to keep the viscosity low enough to produce good
heat transfer coefficients.
A hot saturated solution of the feed is directly fed
into each of the three effects in parallel without
transferring the material from one to another. This
is commonly used in the concentration of the salt
solution, where the solute crystallizes on
concentration without increasing the viscosity.
Operations :-The equipment is at room temp. & at
atm. Pressure at the beginning. The liquid feed is
introduced into all the 3 evaporators up to the
level of upper tube sheets.
Multiple effect evaporator
Suitable for large scale & for continuous operation.
Highly economical when compared to single effect.
Multiple effects, or stages, are now used to minimize the
energy input required to evaporate or boil off undesirable
The total evaporation achieved in these systems is
approximately the number of effects times the energy input
to the first effect.
Typical materials of construction for a number of
evaporator applications are shown below:
MATERIAL OF CONSTRUCTION
Most dairy and food products
304/316 stainless steel
Most fruit juices
316 stainless steel
Carbon steel /304/316
Foods containing high salt (NaCl)
High alloy stainless steels
Duplex stainless steels
Caustic soda < 40%
Stress relieved carbon steel
Caustic soda high concentration
Graphite/Rubber lined carbon steel
Design criteria and processing
Must prevent entrainment due to product loss
Contamination of the vapour phase (polution)
Condensation of vapour onto surfaces (corrosion and fouling)
Overhead mist or spray may cause troublesome deposits
Vortices increase pump head requirements and therefore equipment
Short circuiting a big problem as it presents the problem of cavitation
(there must be a net positive suction head)
Liquid concentration -> relates to viscosity and heat transfer
Temperature and Pressure
Boiling temperature is inversely proportional to pressure.
Boiling points may increase as solution get concentrated (boiling
Foaming -> will determine the height of your freeboard in the
Solubility of materials -> May be the limit to the concentration that
you can achieve.
Scale deposits -> decrease your heat transfer coefficient.
Criteria for selection of
Multiple Effect Evaporator plant:
During the design of evaporation plants, numerous, sometimes
contradictory, requirements have to be considered. They determine
which type of construction and arrangement is chosen, and the
resulting process and economic data. The most important
requirements are as follows:
Capacity and operational data, including quantities, concentrations,
temperatures, annual operating hours, change of product, controls
Product characteristics, including heat sensitivity, viscosity and flow
properties, foaming tendency, fouling and precipitation, boiling
Required operating media, such as steam, cooling water, electric power,
cleaning agents, spare parts, etc.
Capital and other financial costs
Personnel costs for operation and maintenance
Standards and conditions for manufacture, delivery, acceptance,
Choice of materials of construction and surface finishes
Site conditions, such as available space, climate (for outdoor
sites), connections for energy and product, service platforms,
Legal regulations covering safety, accident prevention, sound
emissions, environmental requirements, and others, depending
upon the specific project.
Drive motor trips
§ The product may be backing up in the
evaporator body due to choking.
§ Open discharge lines to check and
eliminate cause of choking.
§ The evaporator body may be heavily
§ Dismantle the unit and check for
§ The feed rate is excessive.
§ Control the feed rate.
§ There might be a problem with the
§ Check for malfunctioning of the drive
System starts vibrating
§ There may be heavy scaling.
§ Go through the cleaning procedure.
§ The product is backing up in the body
due to choking.
§ Check and eliminate the cause of
§ Uneven heating of evaporator shell
causing thermal stresses on its body.
§ Check the steam pressure of