PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila

College of Engineering and Technology
EVAPORATOR TYPES AND APPLICATIONS
Evaporators may be classified as follows:

1. Heating medium separated from evaporating liquid by tubular heating surfaces.

2. Heating medium confined by coils, jackets, double walls, flat plates, etc.
3. Heating medium brought into direct contact with evaporating liquid.
4. Heating by solar radiation.

By far the largest number of industrial evaporators employs tubular heating surfaces.
Circulation of liquid past the heating surface may be induced by boiling or by mechanical
means. In the latter case, boiling may or may not occur at the heating surface.

FORCED-CIRCULATION (FC) EVAPORATORS

FC evaporators are used if boiling of the product on the heating surfaces is to be
avoided due to the fouling characteristics of the product, or to avoid crystallization. The flow
velocity in the tubes must be high, and high-capacity pumps are required. Although it may not
be the most economical for many uses, the forced-circulation evaporator is suitable for the
widest variety of evaporator applications. The use of a pump to ensure circulation above the
heating surface makes possible separating the functions of heat transfer, vapor-liquid
separation, and crystallization. The pump withdraws liquor from the flash chamber and forces it
through the heating element back to the flash chamber. Circulation is maintained regardless of
the evaporation rate; so this type of evaporator is well suited to crystallizing operation, in which
solids must be maintained in suspension at all times. The circulating liquid is heated when it
flows through the heat exchanger and then partially evaporated when the pressure is reduced in
the separator, cooling the liquid to the boiling temperature corresponding to this pressure. The
liquid is typically heated only a few degrees for each pass through the heat exchanger, which
means the recirculation flow rate has to be high. This type of evaporator is also used in
crystallizing applications because no evaporation, and therefore no concentration increase,
takes place on the heat transfer surface. Evaporation occurs as the liquid is flash evaporated in
the separator or flash vessel.

A: Product
B: Vapor out
C: Concentrate out
D: Heating System
E: Condensate

1: Calandria
2: Separator (Flash Cooler)
3: Circulation Pump
4: Concentrate Pump

Figure: Forced Circulation Evaporator

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

The main applications for a forced circulation evaporator are in the concentration of
inversely soluble materials, crystallizing duties, and in the concentration of thermally degradable
materials which result in the deposition of solids. In all cases, the temperature rise across the
tube bundle is kept as low as possible, often as low as 3F to 5F (2C to 3C). This results in a
recirculation ratio as high as 220 pounds to 330 pounds (100 kg to 150 kg) of liquor per pound
(kilogram) of water evaporated. These high recirculation rates result in high liquor velocities
through the tube which help to minimize the buildup of deposits or crystals along the heating
surface. Forced circulation evaporators normally are more expensive than film evaporators
because of the need for large bore circulating pipework and large recirculating pumps.
Operating costs of such a unit also are considerably higher.
The liquid velocity above the heating surface is limited only by the pumping power
needed or available and by accelerated corrosion and erosion at the higher velocities. Tube
velocities normally range from a minimum of about 1.2 m/s in salt evaporators with copper or
brass tubes and liquid containing 5% or more solids up to about 3 m/s in caustic evaporators
having nickel tubes and liquid containing only a small amount of solids. Even higher velocities
can be used when corrosion is not accelerated by erosion.

Advantages of forced-circulation evaporators:

1. High heat-transfer coefficients
2. Positive circulation
3. Relative freedom from salting, scaling, and fouling

Disadvantages of forced-circulation evaporators:

1. High cost
2. Power required for circulating pump
3. Relatively high holdup or residence time

Best applications of forced-circulation evaporators:

1. Crystalline product
2. Corrosive solutions
3. Viscous solutions

Frequent difficulties with forced-circulation evaporators:

1. Plugging of tube inlets by salt deposits detached from walls of equipment
2. Poor circulation due to higher than expected head losses
3. Salting due to boiling in tubes
4. Corrosion-erosion

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

SHORT-TUBE VERTICAL EVAPORATORS

This is one of the earliest types of
evaporators that are still widely used.This
evaporator consists of a vessel (or shell)
which contains vertical tubes through
which the product passes by natural
circulation. Liquid in the tube is heated by
steam outside the tube.
The tubes are usually 50.8 to
76.2mm in diameter and 1.2 to 1.8m long.
The body is a vertical cylinder, usually of
cast iron. Around 1000 tubes are fitted in a
vessel, the level of the liquid is maintained
slightly above the top of the tubes so that
the space which is left in the evaporator
can be used for the purpose of separation
of vapour from the boiling liquids.
Circulation and heat transfer in this type of evaporator are strongly affected by the liquid level.
Highest heat-transfer coefficients are achieved when the level as indicated by an external
gauge glass is only about halfway up the tubes. Slight reductions in level below the optimum
result in incomplete wetting of the tube walls with a consequent increased tendency to foul and
a rapid reduction in capacity. Circulation in the standard short-tube vertical evaporator is
dependent entirely on boiling and when boiling stops, any solids present settle out of
suspension. Product is removed from the evaporator when the desired concentration is has
been attained. The vapour produced escapes to the top of the dome. The short tube vertical
evaporator is mainly used for concentrating cane sugar solutions, syrups, salt and fruit juices.

Advantages of Short Vertical tube Evaporator

1.
2.
3.
4.

High Heat transfer coefficients at high temperature differences

Low headroom
Easy mechanical descaling
Relatively inexpensive

Disadvantages of Short Vertical tube Evaporator

1. Poor heat transfer at low temperature differences and low temperature.
2. High floor space and weight

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
3. Relatively High holdup
4. Poor heat transfer with viscous liquids

Best Applications of Short-tube vertical evaporators

1. Clear liquids
2. Crystalline product if propeller is used
3. Relatively noncorrosive liquids, since body is large and expensive if built of materials
other than mild steel or cast iron.

4. Mild scaling solutions requiring mechanical cleaining, since tubes are short and large in
diameter.

LONG-TUBE VERTICAL EVAPORATORS

The Long-tube Vertical (LTV) evaporator consists
of a simple one-pass vertical shell-and-tube heat
exchanger discharging into a relatively small vapor head.
Normally, no liquid level is maintained in the vapor
head, and the residence time of liquor is only a few
seconds.

Long-tube Vertical Evaporator

The tubes are usually about 50.8 mm (2 in) in

diameter but may be smaller than 25.4 mm (1 in). Tube
length may vary from less than 6 to 10.7 m (20 to 35 ft)
in the rising film version and to as great as 20 m (65 ft)
in the falling film version. The evaporator is usually
operated single-pass, concentrating from the feed to
discharge density in just the time that it takes the liquid
and evolved vapor to pass through a tube. An extreme
case is the caustic high concentrator, producing a
substantially anhydrous product at 370C (700F) from
an inlet feed of 50 percent NaOH at 149C (300F) in one
pass up 22-mm- (8/8-in-) outside-diameter nickel tubes
6 m (20 ft) long.

The largest use of LTV evaporators is for concentrating black liquor in the pulp and
paper industry. Because of the long tubes and relatively high heat-transfer coefficients, it is
possible to achieve higher single-unit capacities in this type of evaporator than in any other.
The LTV evaporator shown is typical of those commonly used, especially for black liquor.
Feed enters at the bottom of the tube and starts to boil partway up the tube, and the mixture of
liquid and vapor leaving at the top at high velocity impinges against a deflector placed above
the tube sheet. This deflector is effective both as a primary separator and as a foam breaker.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
In many cases, as when the ratio of feed to
evaporation or the ratio of feed to heating surface is low, it is
desirable to provide for recirculation of product through the
evaporator. This can be done in a Long Tube Evaporator by
adding a pipe connection between the product line and the
feed line. Higher recirculation rates can be achieved in the
Recirculating Long Tube Vertical Evaporator which is used
widely for condensed milk. By extending the enlarged
portion of the vapor head still lower to provide storage space
for liquor, this type can be used as a batch evaporator.
Liquid temperatures in the tubes of an LTV
evaporator are far from uniform and are difficult to predict.
At the lower end, the liquid is usually not boiling, and the
Recirculating Long Tube
liquor picks up heat as sensible heat. Since entering liquid
Vertical Evaporator
velocities are usually very low, true heat-transfer coefficients
are low in this non boiling zone. At some point up the tube, the liquid starts to boil, and from
that point on the liquid temperature decreases because of the reduction in static, friction, and
acceleration heads until the vapor-liquid mixture reaches the top of the tubes at substantially
vapor-head temperature. Thus the true temperature difference in the boiling zone is always less
than the total temperature difference as measured from steam and vapor-head temperatures.
Although the true heat-transfer coefficients in the boiling zone are quite high, they are
partially offset by the reduced temperature difference. The point in the tubes at which boiling
starts and at which the maximum temperature is reached is
sensitive to operating conditions, such as feed properties,
feed temperature, feed rate, and heat flux.

Falling Film Evaporator

The falling-film version of the LTV evaporator

eliminates these problems of hydrostatic head. Liquid is fed
to the tops of the tubes and flows down the walls as a film.
Vapor-liquid separation usually takes place at the bottom,
although some evaporators of this type are arranged for
vapor to rise through the tube counter currently to the
liquid. The pressure drop through the tubes is usually very
small, and the boiling-liquid temperature is substantially the
same as the vapor-head temperature. The falling-film
evaporator is widely used for concentrating heat-sensitive
materials, such as fruit juices, because the holdup time is
very small, the liquid is not overheated during passage

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
through the evaporator, and heat-transfer coefficients are high even at low boiling
temperatures.
The principal problem with the falling-film LTV evaporator is that of feed distribution
to the tubes. It is essential that all tube surfaces be wetted continually. This usually requires
recirculation of the liquid unless the ratio of feed to evaporation is quite high. An alternative to
the simple recirculation system of the falling Film Evaporator is sometimes used when the feed
undergoes an appreciable concentration change and the product is viscous and/or has a high
boiling point rise. The feed chamber and vapor head are divided into a number of liquor
compartments and separate pumps are used to pass the liquor through the various banks of
tubes in series, all in parallel as to steam and vapor pressures. The actual distribution of feed to
the individual tubes of a falling-film evaporator may be accomplished by orifices at the inlet to
each tube, by a perforated plate above the tube sheet, or by one or more spray nozzles.
Both rising- and falling-film LTV evaporators are generally unsuited to salting or
severely scaling liquids. However, both are widely used for black liquor, which presents a mild
scaling problem, and also are used to carry solutions beyond saturation with respect to a
crystallizing salt. In the latter case, deposits can usually be removed quickly by increasing the
feed rate or reducing the steam rate in order to make the product unsaturated for a short time.
The falling-film evaporator is not generally suited to liquids containing solids because of
difficulty in plugging the feed distributors.
Because of their simplicity of construction, compactness, and generally high heattransfer coefficients, LTV evaporators are well suited to service with corrosive liquids.

Advantages of Long-tube vertical evaporators:

1.
2.
3.
4.
5.
6.

Low Cost
Large heating surface in one body
Low holdup
Small floor space
Good heat-transfer coefficients at reasonable temperature differences(rising film)
Good heat transfer coefficient at all temperature differences (falling film)

Disadvantages of long-tube vertical evaporators:

1.
2.
3.
4.

High headroom
Generally unsuitable for salting and severely scaling liquids
Poor heat-transfer coefficients of rising-film version at low temperature differences
Recirculation usually required for falling-film version

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

Best applications of long-tube vertical evaporators:

1.
2.
3.
4.
5.
6.
7.

Clear liquids
Foaming liquids
Corrosive solutions
Large evaporation loads
High temperature differencesrising film, low temperature differencesfalling film
Low-temperature operationfalling film
Vapor compression operationfalling film

Frequent difficulties with long-tube vertical evaporators:

1. Sensitivity of rising-film units to changes in operating conditions

2. Poor feed distribution to falling-film units

HORIZONTAL TUBE EVAPORATOR

This is the oldest type of chemical
evaporator, although other improved types are now
more commonly used. It is the only type of chemical
evaporator in which the heating medium is inside
the tubes. The principal advantage of horizontal
tube evaporators lies in the relatively small
headroom they require. The horizontal evaporator is
least satisfactory for fluids that form scale or deposit
salt, which would build up on the outside of the
tube. It is well-suited for processes where the final
product is a liquid instead of a solid, such as sugar
syrups where the large volume of liquid stored in the
evaporator allows a close adjustment of the final
density by changing the hold-up in the evaporator.

Figure: Horizontal Tube Evaporator

In this type, the steam is inside and the liquor outside the tubes. Low entrainment loss is
the primary aim: the horizontal cylindrical shell yields a large disengagement area per unit of
vessel volume. Special versions use deformed tubes between restrained tube sheets that crack
off much of a scale deposit when sprayed with cold water. Originally called the Lillie, this
evaporator is now also called the spray-film or horizontal tube evaporator. Liquid distribution
over the tubes is accomplished by sprays or perforated plates above the topmost tubes.
Maintaining this distribution through the bundle to avoid over-concentrating the liquor is a
problem unique to this type of evaporator. It is now used primarily for seawater evaporation.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

VAPOR-LIQUID SEPARATION

FOAMING
LOSSES

SPLASHING
LOSSES

ENTRAINMEN
T LOSSES BY
FLASHING

ENTRAINMEN
T
SEPARATORS

KNITTED
WIRE
MESH

Product losses in evaporator may result from foaming, splashing losses, and
entrainment losses by flashing. Primary separation of liquid from vapour is
accomplished in the vapour head by making the horizontal plan area large enough so
that most of the entrained droplets can settle out against the rising flow of vapour.
FOAMING LOSSES
Usually result from the presence in the evaporating liquid of colloids or surface-tension
depressants and finely divided solids. Solutions for this may include antifoam agents, other
means of combating foam include the removal of product at the surface layer, where the
foaming agents seem to concentrate, and operation at a very low liquid level so that the hot
surfaces can break the foam. Operating at lower temperatures and/or higher-dissolve solids
concentrations may also reduce foaming tendencies.

SPLASHING LOSSES
Splashing Losses are usually insignificant if a reasonable height has been provided
between the liquid level and the top of the vapor head. The height required depends on the
violence of boiling. Heights of 2.4 to 3.6 m (8 to 12 ft) or more are provided in short-tube
vertical evaporators, in which the liquid and vapor leaving the tubes are projected upward. Less
height is required in forced-circulation evaporators, in which the liquid is given a centrifugal
motion or is projected downward as by a baffle. The same is true of long-tube vertical
evaporators, in which the rising vapor-liquid mixture is projected against a baffle.

ENTRAINMENT LOSSES BY FLASHING

Entrainment losses by flashing are frequently encountered in an evaporator. If the feed is
above the boiling point and is introduced above or only a short distance below the liquid level,
entrainment losses may be excessive. This can occur in a short-tube-type evaporator if the feed
is introduced at only one point below the lower tube sheet. The same difficulty may be
encountered in forced-circulation evaporators having too high a temperature rise through the

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
heating element and thus too wide a flashing range as the circulating liquid enters the body.
Poor vacuum control, especially during startup, can cause the generation of far more vapor than
the evaporator was designed to handle, with a consequent increase in entrainment.

ENTRAINMENT SEPARATORS
When a bubble of vapor rises to the surface of liquid and
bursts, the liquid film that forms the top of the bubble is usually
sprayed as very fine droplets along with the stream of vapor.
These droplets greatly vary in size. Some of them drop back
quickly into the liquid from which they came; some settle more
slowly; and some will not settle at all, at any vapor velocity (that is
practicable to maintain). Such finely divided liquid carried along with
the stream of vapor is called entrainment, and may cause serious
losses from the liquid being evaporated and contamination of the
condensate.
Entrainment Separators are frequently used to reduce
product losses. The most common separator is the cyclone, which
may have either a top or a bottom outlet or may even be wrapped
around the heating element of the next effect. The separation
efficiency of a cyclone increases with an increase in inlet velocity,
although at the cost of some pressure drop, which means a loss in
available temperature difference. Pressure drop in a cyclone is from
10 to 16 velocity heads, based on the velocity in the inlet pipe. Such
Cyclone Separator
cyclones can be sized in the same manner as a cyclone dust collector
(using velocities of about 30 m/s (100 ft/s) at atmospheric pressure)
although sizes may be increased somewhat in order to reduce losses in available temperature
difference.

KNITTED WIRE MESH

Knitted wire mesh is a metal wire or plastic
strand knitted into a mesh structure in very much
the same way as stockings or sweaters. Mesh is
normally knit from wires ranging in diameter from
.0035 inch to .0200 inch. In special applications,
Metal Textiles has knit wires ranging in diameter
from .0005 to .0350.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
This serves as an effective entrainment separator when it cannot easily be fouled
by solids in the liquor. This is installed as a blanket across the top of the evaporator or
in a monitor of reduced diameter atop the vapour head. These separators have low
pressure drops usually on the order of 13mm(1/2 in) of water, and collection efficiency
is above 99.8 %in the range of vapour velocities from 2.5 to 6 m/s. Chevron (hook and
vane) type separators are also used because of their higher-allowable velocities or
because of their reduced tendency to foul with solids suspended in the entrained liquid.
EVAPORATOR ARRANGEMENT
SINGLE EFFECT EVAPORATOR
Single effect evaporators are used when the
required capacity is small. The typical evaporator is
made up of three functional sections: the heat
exchanger, the evaporating section, where the
liquid boils and evaporates, and the separator in
which the vapour leaves the liquid and passes off
to the condenser or to other equipment.
In many evaporators, all three sections are
contained in a single vertical cylinder. In the centre
of the cylinder there is a steam heating section,
with pipes passing through it in which the
evaporating liquors rise. At the top of the cylinder,
there are baffles, which allow the vapours to
escape but check liquid droplets that may
accompany the vapours from the liquid surface. In the heat exchanger section, called a calandria
in this type of evaporator, steam condenses in the outer jacket and the liquid being evaporated
boils on the inside of the tubes and in the space above the upper tube plate.
The resistance to heat flow is imposed by the steam and liquid film coefficients and by
the material of the tube walls. The circulation of the liquid greatly affects evaporation rates, but
circulation rates and patterns are very difficult to predict in any detail. Values of overall heat
transfer coefficients that have been reported for evaporators are of the order of 1800-5000 J m2 -1
s C-1 for the evaporation of distilled water in a vertical-tube evaporator with heat supplied by
condensing steam. However, with dissolved solids in increasing quantities as evaporation
proceeds leading to increased viscosity and poorer circulation, heat transfer coefficients in
practice may be much lower than this.
As evaporation proceeds, the remaining liquors become more concentrated and because
of this the boiling temperatures rise. The rise in the temperature of boiling reduces the available

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
temperature drop, assuming no change in the heat source. And so the total rate of heat transfer
will drop accordingly. Also, with increasing solute concentration, the viscosity of the liquid will
increase, often quite substantially, and this affects circulation and the heat transfer coefficients
leading again to lower rates of boiling. Yet another complication is that measured, overall, heat
transfer coefficients have been found to vary with the actual temperature drop, so that the
design of an evaporator on theoretical grounds is inevitably subject to wide margins of
uncertainty.
Single effect evaporators may be operated in batch, semibatch, or continuous batch
modes or continuously.

Batch evaporators- are the ones in which the filling, evaporating and emptying are consecutive
steps. This method of of operation is rarely used since it requires that the body be large enough
to hold the entire charge of feed and the heating element be placed low enough so as not to be
uncovered when the volume is reduced so that of the product.

Semi-batch evaporators- more usual method in of operation in which feed is continually

added to maintain a constant level until the entire charge raches final density.

Continuously batch evaporators- usually have a continuous feed and over at least part of the
cycle, continuous discharge. One method of operation is to circulate from a storage tank to the
evaporator and back until the entire tank is up to desired concentration and then finish in
batches.

Continuous Evaporators- have essentially continuous feed and discharge, and concentrations
of both feed and product remain substantially constant.

THERMOCOMPRESSION
The simplest means of reducing the
energy requirements of evaporation is to
compress the vapour from a single effect
evaporator. In order to keep the
compressor cost and power requirements
within reason, the evaporator must work
with a fairly narrow temperature difference
usually from about 5.5 to 11 . o reduce
energy consumption, water vapor from an
evaporator is entrained and compressed
with
high
pressure
steam
in
a
thermocompressor so it can be condensed
in the evaporator heat exchanger. The

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
resultant pressure is intermediate to that of the motive steam and the water vapor. A
thermocompressor is similar to a steam-jet air ejector used to maintain vacuum in an
evaporator. Only a portion of the vapor from an evaporator can be compressed in a
thermocompressor with the remainder condensed in the next-effect heat exchanger or a
condenser. A thermocompressor is normally used on a single-effect evaporator or on the first
effect of a double- or triple-effect evaporator to reduce energy consumption.

MECHANICAL THERMOCOMPRESSION
In Mechanical recompression, it
employs a reciprocating rotary
positive displacement, centrifugal or
axial flow compressors. Positive
displacement
compressors
are
impractical for all but the smallest
capacities, such as portable seawater
evaporators, axial-flow compressors
can be built for capacities of more
than 472
Centrifugal compressors
are usually the cheapest for the
intermediate capacity ranges that are
normally encountered. The principle
of this is that, cold feed is preheated
almost to its boiling point by
exchange with hot liquor and is
pumped through a heater as in a
conventional forced0circulation evaporator. The vapour evolve however is not
condensed; instead it is compressed to somewhat higher pressure by a positivedisplacement or centrifugal compressor and becomes the steam is fed to the heater.
Since the saturation temperature of the compressed vapour is higher than the boiling
point of the feed, heat flows from the vapour to the solution, generating more vapour.
The optimum temperature drop for a typical system is about 5 .

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

MULTIPLE-EFFECT EVAPORATION

Multiple-effect evaporation is the principal means in use for economizing on energy

consumption. Most such evaporators operate on a continuous basis, although for a few difficult
materials a continuous-batch cycle may be employed. In a multiple-effect evaporator, steam
from an outside source is condensed in the heating element of the first effect. If the feed to the
effect is at a temperature near the boiling point in the first effect, 1 kg of steam will evaporate
almost 1 kg of water. The first effect operates at (but is not controlled at) a boiling temperature
high enough so that the evaporated water can serve as the heating medium of the second
effect. Here almost another kilogram of water is evaporated, and this may go to a condenser if
the evaporator is a double-effect or may be used as the heating medium of the third effect. This
method may be repeated for any number of effects. Large evaporators having six and seven
effects are common in the pulp and paper industry, and evaporators having as many as 17
effects have been built. As a first approximation, the steam economy of a multiple-effect
evaporator will increase in proportion to the number of effects and usually will be somewhat
less numerically than the number of effects.
The increased steam economy of a multiple-effect evaporator is gained at the expense
of evaporator first cost. The total heat-transfer surface will increase substantially in proportion
to the number of effects in the evaporator. This is only an approximation since going from one
to two effects means that about half of the heat transfer is at a higher temperature level, where
heat-transfer coefficients are generally higher. On the other hand, operating at lower
temperature differences reduces the heat-transfer coefficient for many types of evaporator. If
the material has an appreciable boiling-point elevation, this will also lower the available
temperature difference. The only accurate means of predicting the changes in steam economy
and surface requirements with changes in the number of effects is by detailed heat and material
balances together with an analysis of the effect of changes in operating conditions on heattransfer performance.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

The approximate temperature distribution in a multiple-effect evaporator is under the

control of the designer, but once built, the evaporator establishes its own equilibrium. The total
available temperature drop is divided between the effects in proportion to their resistances. If
one effect starts to scale, its temperature drop will increase at the expense of the temperature
drops across the other effects. This provides a convenient means of detecting a drop in heattransfer coefficient in an effect of an operating evaporator. If the steam pressure and final
vacuum do not change, the temperature in the effect that is scaling will decrease and the
temperature in the preceding effect will increase.
The feed to a multiple-effect evaporator is usually transferred from one effect to another
in series so that the ultimate product concentration is reached only in one effect of the
evaporator.
Backward Feed
In backward-feed operation, the
raw feed enters the last (coldest) effect;
the discharge from this effect becomes
the feed to the next-to-the-last effect, and
so on until product is discharged from the
first effect. This method of operation is
advantageous when the feed is cold, since
much less liquid must be heated to the
higher temperature existing in the early
effects. It is also used when the product is
so viscous that high temperatures are
Backward Feed Operation
needed to keep the viscosity low enough to
give reasonable heat transfer coefficients. When product viscosity is high but a hot product is
not needed, the liquid from the first effect is sometimes flashed to a lower temperature in one
or more stages and the flash vapor added to the vapor from one or more later effects of the
evaporator.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

Forward Feed
In forward-feed operation, raw
feed is introduced in the first effect and
passed from effect to effect parallel to the
steam flow. Product is withdrawn from the
last effect. This method of operation is
advantageous when the feed is hot or when
the concentrated product would be
damaged or would deposit scale at high
temperature. Forward feed simplifies
operation when liquor can be transferred
by
pressure
difference
alone,
thus
eliminating all intermediate liquor pumps.
When the feed is cold, forward feed gives a low steam economy since an appreciable part of the
prime steam is needed to heat the feed to the boiling point and thus accomplishes no
evaporation. If forward feed is necessary and feed is cold, steam economy can be improved
markedly by preheating the feed in stages with vapor bled from intermediate effects of the
evaporator. This usually represents little increase in total heating surface or cost since the feed
must be heated in any event and shell-and-tube heat exchangers are generally less expensive
per unit of surface area than evaporator heating surface.
Forward Feed Operation

Mixed-Feed
Mixed-feed operation is used only
for special applications, as when liquor at
an intermediate concentration and a
certain temperature is desired for
additional processing. In the mixed feed
operation, the dilute feed liquid enters at
an intermediate effect and flows in the
next higher effect till it reaches the last
effect of the series. In this section, liquid
flows in the forward feed mode. Partly
concentrated liquor is then pumped back
Mixed-Feed Operation
to the effect before the one to which the
fresh feed was introduced for further concentration. Mixed feed arrangement eliminates some of

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
the pumps needed in backward configuration as flow occurs due to pressure difference
whenever applicable.
Parallel Feed

Parallel Feed Operation

Parallel feed operation involves

the introduction of raw feed and the
withdrawal of product at each effect of the
evaporator. It is used primarily when the
feed is substantially saturated and the
product is solid containing slurry. In parallel
feeding, there is no transfer of liquid from
one effect to another effect. An example is
the evaporation of brine to make common
salt. In this case, parallel feed is desirable
because the feed washes impurities from
the salt leaving the body.

Heat-Recovery Systems
Heat-recovery systems are frequently incorporated in an evaporator to increase the
steam economy. Ideally, product and evaporator condensate should leave the system at a
temperature as low as possible. Also, heat should be recovered from these streams by exchange
with feed or evaporating liquid at the highest possible temperature. This would normally require
separate liquid-liquid heat exchangers, which add greatly to the complexity of the evaporator
and are justifiable only in large plants. Normally, the loss in thermodynamic availability due to
flashing is tolerated since the flash vapor can then be used directly in the evaporator effects.
The most commonly used is a condensate flash system in which the condensate from each
effect but the first (which normally must be returned to the boiler) is flashed in successive stages
to the pressure in the heating element of each succeeding effect of the evaporator. Product
flash tanks may also be used in a backward- or mixed-feed evaporator. In a forward-feed
evaporator, the principal means of heat recovery may be by use of feed pre heaters heated by
vapor bled from each effect of the evaporator. In this case, condensate may be either flashed as
before or used in a separate set of exchangers to accomplish some of the feed preheating. A
feed preheated by last-effect vapor may also materially reduce condenser water requirements.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

SEAWATER EVAPORATORS
The production of potable water from saline waters represents a large and growing field
of application for evaporators. Steam economies on the order of 10 kg evaporation/kg steam
are usually justified because (1) unit production capacities are high, (2) fixed charges are low on
capital used for public works, (3) heat-transfer performance is comparable with that of pure
water, and (4) properly treated seawater causes little deterioration due to scaling or fouling.
Figure (a) shows a multiple-effect (falling-film) flow sheet as used for seawater. Seawater
is used to condense last-effect vapor, and a portion is then treated to prevent scaling and
corrosion. Treatment usually consists of acidification to break down bicarbonates, followed by
deaeration, which also removes the carbon dioxide generated. The treated seawater is then
heated to successively higher temperatures by a portion of the vapor from each effect and
finally is fed to the evaporating surface of the first effect. The vapor generated therein and the
partially concentrated liquid are passed to the second effect, and so on until the last effect. The
feed rate is adjusted relative to the steam rate so that the residual liquid from the last effect can
carry away all the salts in solution, in a volume about one-third of that of the feed. Condensate
formed in each effect but the first is flashed down to the following effects in sequence and
constitutes the product of the evaporator.
As the feed-to-steam ratio is increased in the flow sheet of Figure (a), a point is reached
where all the vapor is needed to preheat the feed and none is available for the evaporator
tubes. This limiting case is the multistage flash evaporator, shown in its simplest for in Figure
(b). Seawater is treated as before and then pumped through a number of feed heaters in series.
It is given a final boost in temperature with prime steam in a brine heater before it is flashed
down in series to provide the vapor needed by the feed heaters. The amount of steam required
depends on the approach-temperature difference in the feed heaters and the flash range per
stage. Condensate from the feed heaters is flashed down in the same manner as the brine.
Since the flow being heated is identical to the total flow being flashed, the temperature
rise in each heater is equal to the flash range in each flasher. This temperature difference
represents a loss from the temperature difference available for heat transfer. There are thus two
ways of increasing the steam economy of such plants: increasing the heating surface and
increasing the number of stages. Whereas the number of effects in a multiple-effect plant will
be about 20 percent greater than the steam economy, the number of stages in a flash plant will
be 3 to 4 times the steam economy. However, a large number of stages can be provided in a
single vessel by means of internal bulkheads.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

In the flow sheet of Figure (b) all this seawater must be deaerated and treated for scale
prevention. In addition, the last stage vacuum varies with the ambient seawater temperature,
and ejector equipment must be sized for the worst condition. These difficulties can be
eliminated by using the recirculating multistage flash flow sheet of Figure (c). The last few
stages, called the reject stages, are cooled by a flow of seawater that can be varied to maintain a
reasonable last-stage vacuum. A small portion of the last-stage brine is blown down to carry
away the dissolved salts, and the balance is recirculated to the heat-recovery stages. This
arrangement requires a much smaller makeup of fresh seawater and hence a lower treatment
cost.
The multistage flash evaporator is similar to a multiple-effect forced-circulation
evaporator, but with all the forced-circulation heaters in series. This has the advantage of

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
requiring only one large volume forced-circulation pump, but the sensible heating and short
circuiting losses in available temperature differences remain. A disadvantage of the flash
evaporator is that the liquid throughout the system is at almost the discharge concentration.
This has limited its industrial use to solutions in which no great concentration differences are
required between feed and product and to where the liquid can be heated through wide
temperature ranges without scaling. A partial remedy is to arrange several multistage flash
evaporators in series, the heat-rejection section of one being the brine heater of the next. This
permits independent control of concentration but eliminates the principal advantage of the
flash evaporator, which is the small number of pumps and vessels required. An unusual feature
of the flash evaporator is that fouling of the heating surfaces reduces primarily the steam
economy rather than the capacity of the evaporator. Capacity is not affected until the heatrejection stages can no longer handle the increased flashing resulting from the increased heat
input.

EVAPORATOR ACCESSORIES
CONDENSER
The vapor from the last effect of an evaporator is usually removed by a condenser.
Surface condensers are employed when mixing of condensate with condenser cooling water is
not desired. Tubular condensers used in power plant to condense exhaust steam are known
as surface condensers. They are for the most part shell-and-tube condensers with vapor on the
shell side and a multi-pass flow of cooling water on the tube side. Heat loads, temperature
differences, sizes, and costs are usually of the same order of magnitude as for another effect of
the evaporator. Surface condensers use more cooling water and are so much more expensive
that they are never used when a direct-contact condenser is suitable.
In direct contact condensers, the exhaust steam and cooling water come in direct
contact and mix together. The steam condenses suddenly, achieving the temperature of the
cooling water. Then, the condensate and cooling water are continuously taken out. Direct
contact condenser is used when the dry cooling towers are used, in geothermal power plants
and in plants that use temperature differences as in ocean waters.

Advantages of Direct contact condenser:

1. Heat exchange through direct contact, thus less water quantity required
2. Construction is simpler thus less costly.
3. Maintenance is simple and cheap.
4. Requires small floor space.
Disadvantages of Direct contact condenser:
1. If condensate contains impurities, it cannot be reused.
2. Low vacuum efficiency, hence, it is not suitable for larger plants.
3. More power required for air pump

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

There are mainly three types. These are

spray condenser, barometric condenser, and jet
condenser. Spray Condenser is the modern direct
contact condenser. In this type, water is sprayed
into the steam. Part of the condensate, which is
equal to the turbine exhaust flow, is sent back as
feed water. The rest is cooled in a cooling tower.
Then, the cooled water is sprayed into the turbine
exhaust, process is repeated. The continuously
circulated cooling water must be pure. In simpler
terms, the coolant is sprayed, using nozzles, into a
vessel to which the vapor is supplied. It is important
that the spray nozzles and vessel are designed to
produce a fine spray of liquid (to give a large
interfacial area for heat transfer), and a long enough
residence time of liquid droplets in the vessel.
The most common type of direct contact condenser is the countercurrent barometric
condenser, in which vapor is condensed by rising against a rain of cooling water. It is also an
early type of direct contact condenser. The
condenser is set high enough so that water
can discharge by gravity from the vacuum in
the condenser. Such condensers are
inexpensive and are economical on water
consumption. They can usually be relied on
to maintain a vacuum corresponding to a
saturated-vapor temperature within 2.8C
(5F) of the water temperature leaving the
condenser. The vacuum of this condenser is
obtained by an air pump that pulls air out
from the top. Baffles are used which
increases the surface to volume ratio. It has
a long vertical tail pipe (at least 10.23 m)
making it difficult to access. The outlet of
the tail pipe must be at least 6 below the
minimum level of the water in the hot valve.
The condensate cannot be reused in boiler
since, it is contaminated by cooling water.
The cooling water is made to pour down a
series of baffles in the form of water
curtains or sheets. The static head
compresses the mixture to atmospheric pressure and thus, replaces the pump. The steam
condenses and the mixture goes down a tail pipe to the hot well. The larger the enthalpy, the
easier it is for the condensate to flow down to the hot well.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

Jet condenser makes use of high-velocity jets of water both to condense the vapor and
to force non-condensable gases out the tailpipe. This type of condenser is frequently placed
below barometric height and requires a pump to remove the mixture of water and gases. Jet
condensers usually require more water than the more common barometric-type condensers and
cannot be throttled easily to conserve water when operating at low evaporation rates. It is a
kind of direct contact condenser in which cooling water and steam mix in a series of combining
cones. The kinetic energy of steam is expended to drain off the condensate and cooling water
from the condenser. It reduces the tail pipe height in barometric condenser. Jet condenser also
removes the need of air pump. The condensate cannot be reused in boiler since it is
contaminated by cooling water. Water enters from the top through nozzles. The steam entering
from the side is guided on to the surface of water through guiding cones. Due to decreasing
area of guide cones, the potential energy is converted into kinetic energy. Decrease in potential
energy causes pressure drop resulting in suction. Suction pulls more and more steam for
condensation. Water and condensate then, enter diverging section where the velocity decreases
and pressure increases. The built up pressure enable the mixture of condensate and water to be
discharged automatically in the hot well.

Figure: Jet Condenser

VENT SYSTEMS
Noncondensable gases may be present in the
evaporator vapor as a result of leakage, air dissolved in the
feed, or decomposition reactions in the feed. When the
vapor is condensed in the succeeding effect, the
noncondensables increase in concentration and impede
heat transfer. This occurs partially because of the reduced
partial pressure of vapor in the mixture but mainly because
the vapor flow toward the heating surface creates a film of

Vent Valve

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology
poorly conducting gas at the interface. The most important means of reducing the influence of
noncondensables on heat transfer is by properly channeling them past the heating surface. A
positive vapor-flow path from inlet to vent outlet should be provided, and the path should
preferably be tapered to avoid pockets of low velocity where noncondensables can be trapped.
Excessive clearances and low-resistance channels that could bypass vapor directly from the inlet
to the vent should be avoided.

SALT REMOVAL
When an evaporator is used to make a crystalline product, a number of means
are available for concentrating and removing salt from the system. The simplest is to
provide a settling space in the evaporator itself. Sufficiently high slurry densities can
usually achieved in this manner to reach the limit of pumpability. The evaporators are
usually placed above barometric height so that the slurry can be discharged
intermittently on a short time cycle. If the amounts of salt
crystallized is on the order of a ton an hour or less, a salt
trap may be used. This is simply a receiver that is
connected at the bottom of the evaporator and is closed of
the evaporator and is closed off from the evaporator
periodically for emptying. Such traps are useful when
insufficient headroom is available for gravity removal of the
solids.

PAMANTASAN NG LUNGSOD NG MAYNILA

University of the City of Manila
College of Engineering and Technology

REFERENCES
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Freeze, H. L. and Glover, W. B. Mechanically Agitated Thin-Film Evaporators. Chem. Eng.
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Hyde, W. L. and Glover, W. B. Evaporation of Difficult Product. Chemical Processing, 60 (2),
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Kern, D. Q. Process Heat Transfer. McGraw-Hill, New York, NY, (1950).
McCabe, W. L., and Smith, J. C. Unit Operations of Chemical Engineering. McGraw-Hill, New
York, NY, (1958).
McNaught, J. M. CONDENSERS. Thermopedia. 9 February 2011. Retrieved 17 July 2015, from
<http://www.thermopedia.com/content/654/>.
Minton, P. E. Handbook of Evaporator Technology. Noyes Publications, New York, NY, (1986).
Perry, R. H., Chilton, C. H. and Kirkpatrick, S. D. Chemical Engineers Handbook. 4th Ed.,
McGraw-Hill, New York, NY, (1963).
Perry, R. H., Chilton, C. H. and Kirkpatrick, S. D. Chemical Engineers Handbook. 7th Ed.,
McGraw-Hill, New York, NY, (1963).
Shah, F. A. "Direct Contact or Open Condensers". Slideshare. LinkedIn Corporation, 21 May 2013.
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