AP Government GATE Online
Classes
Heat Transfer
Day-10 (04.06.2020)
Dr. R. Srikanth
Professor
ANITS-Visakhapatnam
Evaporation
Evaporation
• Evaporation is the
conversion of a liquid to
a vapor.
• Evaporation is the
removal of volatile
solvent from a solution or
relatively dilutes slurry by
vaporizing the solvent.
Evaporators are used for
this process.
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Evaporators
• 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.
• Evaporation concentration means removal of water by
boiling. Evaporation finds application in a variety of food
processing operations. A primary application is
concentration of fruit juices (orange juice concentrate),
vegetable juices (tomato pastes and purees), and dairy
products (condensed milk). Evaporation is also used to
concentrate salt and sugars prior to refining.
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Requirements for optimal
evaporation
(l) rapid rate of heat transfer.
(2) low-temperature operation through
application of a vacuum.
(3) efficient vapor-liquid separation.
(4) efficient energy use and recovery.
Performance of Evaporator
• There are three main measures of
evaporator performance:
– Capacity (kg vaporized / time)
– Economy (kg vaporized / kg steam input)
– Steam Consumption (kg / hr)
• Note that the measures are related, since
Consumption = Capacity/Economy
Boiling Point Elevation
• The vapor pressure of an aqueous solution is less than
that of pure water at the same temperature; so the
boiling point of the solution will be higher than that of the
water. This is called Boiling Point Elevation (BPE) or
vapor pressure lowering.
• The boiling point of a solution is a colligative property - it depends on the concentration of solute in the
solution, but not on what the solute and solvent are.
• When working problems involving heat transfer to or
from boiling solutions, it is necessary to adjust the
temperature difference driving force for the boiling point
elevation
Boiling Point Elevation
• The extent of the boiling-point elevation
depends upon the nature of the material
being evaporated and upon the
concentration changes that are produced.
• The extent of the rise can be predicted by
Raoult's Law, which leads to:
ΔT = kx
where ΔT is the boiling point elevation, x is the
mole fraction of the solute and k is a constant
of proportionality.
Duhring's rule
• There is no straightforward method of predicting the
extent of the boiling-point elevation in the concentrated
solutions that are met in some evaporators in practical
situations.
• Many solutions have their boiling points at some
concentrations tabulated in the literature, and these can
be extended by the use of a relationship known as
Duhring's rule.
• Duhring's rule states that the ratio of the temperatures at
which two solutions (one of which can be pure water)
exert the same vapour pressure is constant.
Duhring plot
Duhring plot for boiling point of sodium chloride solutions
Types of Evaporators
1) Long-Tube Vertical Evaporators
2) Short-Tube (Calandria) Vertical
Evaporators
3) Horizontal-Tube Evaporators
4) Forced Circulation Evaporators
5) Agitated, Thin Film Evaporators
6) Plate Evaporator.
7) Centrifugal/Conical Evaporator.
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LONG-TUBE VERTICAL (LTV)
EVAPORATORS
• These types of evaporators are used more than
all other types combined for evaporation
because of
 low cost per unit capacity
 simplicity of construction
 heat transfer performance
• The LTV-type evaporator cannot handle
crystallizing solutions but excellent for foaming
solution.
• There are two main types of LTV evaporators:
a)Climbing (Rising) film evaporators
b)Falling film evaporators
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Long Tube Vertical Rising
Film Evaporator
• A thin film of liquid food is formed on the
inside of the long tubes, with steam
providing heat transfer from the outside.
The vaporizing bubbles of steam cause
film of concentrate to rise upwards inside
the tubes. Vapor and concentrate are
separated, as they exit the top, in a
separate chamber.
Climbing (Rising) Film Evaporators
• LTV rising-film evaporator is
used primarily to concentrate
non-salting liquors.
• Climbing film evaporators
are used mainly in the paper
industry, food industry to
condense milk.
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Long Tube Vertical Falling
Film Evaporator
• Using gravity to make liquid flow
downwards. Steam condensing on the
outside of the tubes causes evaporation of
a thin film of product flowing down the
inside of the tubes. Product and steam exit
the bottom of the tubes together, then are
separated.
Falling Film Evaporators
• LTV falling film evaporator
can be used to concentrate
the non-salting liquids and
more viscous liquors.
• Principal applications have
been for citrus juices. They
are also used to
concentrate heat-sensitive
materials and corrosive
solutions.
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Short tube Evaporator
• A short but wide steam chest in the form
of a shell and tube heat exchanger
characterize this type of evaporator.
Steam is fed to the inside of the internal
tubes. Circulation is generated naturally.
Density differences due to heating
around the steam pipes cause the
warmer fluid to rise and the colder fluid
to sink. A vacuum source maintains to
reduce boiling temperature.
SHORT-TUBE (CALANDRIA)
VERTICAL EVAPORATORS (STV)
• The short tube evaporators
were the first developed
commercially and still represent
probably the largest number of
unit in operation.
• Used in sugar factory.
• Used for noncorrosive
materials.
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Advantages
Inexpensive
Efficient heat transfer at high temperature
Easy to de-scale
Disadvantages
× Poor heat transfer at low temperature
differences or with viscous liquids
× Require a great deal of floor space
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HORIZONTAL TUBE EVAPORATORS
• Horizontal tube evaporators use a tube and
spray method of heat transfer.
• The tubes are arranged so as to maximize
the heat transfer area between the steam and
the liquor.
Symbols:
 C = condensate
 F = feed
 N = noncondensibles’ vent
 P = product or concentrate
 S = steam
 V = vapor
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• 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.
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Horizontal Tube Evaporator
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Forced Circulation
Evaporator
• Fluid is pumped from the main evaporator
chamber through an external steam chest.
Vapor-liquid separation occurs in the main
chamber, Dilute feed is added to the
recirculation loop, and sent through the steam
chest
• Since external pumping is used to maintain
fluid flow, excellent heat transfer can be
obtained, But, recirculation of the fluid
through the steam chest causes long
residence times
FORCED CIRCILATION
EVAPORATORS
• The liquor in a forced-circulation evaporator is
pumped through the tubes to minimize tube
scaling or salting when precipitates are
formed during evaporation.
• Submerged-tube type is the most common
type forced circulation evaporator.
• Forced circulation evaporators are used in
the mining industry, and also they are used to
evaporate corrosive or highly viscous
solutions.
• Forced circulation evaporators are efficient
the transfers heat from steam to liquid,
continuous liquid flow, low amount of salting,
scaling and fouling. They are expensive and
need power for circulating pump.
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Forced Circulation Evaporator
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Wipe Film or Agitated Thin
Film Evaporator
• Very viscous foods are difficult to
evaporate efficiently using any of the
previously discussed evaporators.
Products such as thick fruit or vegetable
purees, or even highly concentrated sugar
syrups, can be efficiently evaporated when
a thin film at the heat transfer surface is
continuously agitated or wiped to prevent
buildup.
AGITATED THIN (WIPED-FILM) FILM
EVAPORATORS
Agitated film evaporators use mechanical to promote heat transfer.
They employ a single large diameter straight or tapered tube as the
heating surface, in which a set of the blades is rotated. The cost of
these evaporators is very high and the capacity relatively low.
 These evaporators are used highly viscous liquids or liquids requiring
very low residence times. These evaporators exhibit poor heat transfer
performance on low viscosity fluids.
 Their maintenance are difficult because of internal moving parts.

Symbols:
 C = condensate
 F = feed
 N = noncondensibles’ vent
 P = product or concentrate
 S = steam
 V = vapor
Agitated Thin Film Evaporator
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Plate Evaporator
• A series of metal plates and frames forms the
heat exchange surface both product and steam
are directed in alternate gaps. Evaporation can
take place within the plate and frame system, or
evaporation can be suppressed by maintaining
sufficient pressure and allowing evaporation to
occur as the heated product flashes into a lower
pressure chamber.
Evaporator Modeling
• In an evaporator, heat is transferred from a
heating medium (usually steam) to a solution by
conduction through a solid surface (the tube
walls). As the solution boils, mass and heat are
simultaneously transferred into the vapor
phase. Consequently, an evaporator model
consists of
– an overall material balance
– component material balances
– energy balances (usually one each on the process
side and the heating side)
– heat transfer equations
Energy and material balances on evaporation
units
Material balance:
F=E+P
Solid component balance : F(xf) = P(xp)
Heat balance:
Mss = Fcpf (Tb-Tf) + E v
Influence of feed properties on
Evaporation
1.
2.
3.
4.
5.
6.
Viscosity,
Fouling-scaling tendency,
Foaming,
Temperature sensitivity,
vapor loss,
Corrosion characteristics.
Heat conservation in evaporator
systems
Mainly three methods:
1.
2.
3.
Feed preheating,
Multiple effect operation,
Vapor recompression.
The key factor in determining the economy of an evaporator is
the number of effects. The economy of a single effect
evaporator is always less than 1.0. Multiple effect
evaporators have higher economy but lower capacity than
single effect
Single Effect Evaporation
• The simplest mode of evaporation is to use a
single stage, where steam is fed into the steam
chest, concentrate and vapor are removed, and
the vapor is condensed into hot water.
• However, the vapors produced are still steam,
and thus can be used to provide the heat for
evaporation in a subsequent stage. Therefore,
steam can be used many times to provide
evaporation in a series of operations.
multiple-effect evaporation
• In a two-stage evaporator, the vapors
produced by evaporation of water in the
first stage are fed into the steam chest of
the second stage to provide further
evaporation. Since there is no driving force.
Thus, operating pressure in the second
stage must be reduced to lower the boiling
temperature
Triple effect evaporation system
Forward-feed multiple - effect evaporator
Backward-feed multiple - effect evaporator
Parallel-feed multiple-effect evaporators
Mechanical Vapor
Recompression
• Mechanical compression can be used to
improve the quality of vapors. The vapors
from a single stage are compressed to higher
pressure in a mechanical compressor and
then reused as steam in the steam chest .
Reuse of compressed vapors makes up most
of the steam addition. Only a small portion of
fresh steam is needed to account for
inevitable energy losses. Steam economies
can be obtained.
Thermal Vapor
Recompression
• The quality of the vapors produced during
evaporation can be recompressed. One
alternative is to use fresh steam to enhance the
value of a portion of the vapors. This combined
steam is then fed into the steam chest. High
pressure steam is passed through a nozzle (or
ejector) before entering the evaporator chamber.
As the fresh steam passes through the nozzle.
Thermal recompression
in evaporation
kg steam consumption per kg of
water evaporated
No. of Without vapor
Effects recompression
With vapor
recompression
1
1.1
0.6
2
0.6
0.4
3
0.4
0.3
Previous GATE Questions
SOLVED PROBLEMS
Q1:
Economy of evaporators used for concentrating
sugarcane juice is
A. kg of concentrated juice produced/ kg of
steam supplied
B. kg of steam supplied/kg of sugarcane juice
fed
C. kg of water vaporized/kg of steam supplied
D. kg of sugarcane juice fed/kg of water
vaporized
Answer: C
Q2:
Multiple effect evaporator has capacity to
process 4000 kg of solid caustic soda per
day when it is concentrating from 10% to
25% solids. The water evaporated in kg per
day is,
A. 800
B. 24,000
C. 60,000
D. 48,000
Q3:
It is desired to concentrate a 20% salt
solution (20 kg of salt in 100 kg of solution)
to a 30% salt solution in an evaporator.
Consider a feed of 300 kg/min at 30°C. The
boiling point of the solution is 110°C, the
latent heat of vaporization is 2100 kJ/kg, and
the specific heat of the solution is 4 kJ/kg-K.
The rate at which heat has to be supplied (in
kJ/min) to the evaporator is
(A) 3.06×105
(B) 6.12×105
(C) 7.24×105
(D) 9.08×105
Q4:
A dilute aqueous solution is to be concentrated in an evaporator
system. High pressure steam is available. Multiple effect
evaporator system is employed because
A. total heat transfer area of all the effects is less than that
in a single effect evaporator system
B. total amount of vapour produced per kg of feed steam in
a multi-effect system is much higher than in single effect
C. boiling point elevation in a single effect system is much
higher than that in any effect in a multi-effect system
D. heat transfer coefficient in a single effect is much lower
than that in any effect in a multi-effect system
Answer: B
Q5:
Steam economy of a multiple effect evaporator system is
defined as
A. kilogram of steam used per hour
B. kilogram of steam consumed in all the effects for
each kilogram of steam fed
C. kilogram of steam used in all the effects for each
kilogram of water vaporized per hour
D. kilogram of water vaporized from all the effects for
each kilogram of steam fed to the first effect
Answer: D
Q6:
An aqueous sodium chloride solution (10 wt %) is
fed into a single effect evaporator at a rate of
10000 kg/hr. It is concentrated to a 20 wt% sodium
chloride solution. The rate of consumption of
steam in the evaporator is 8000 kg/hr. The
evaporator capacity (kg/hr) and economy are
(A) 5000, 0.625 (B) 10000, 0.625
(C) 5000, 1.6
(D) 10000, 1.6
Q7:
An aqueous solution of a solute is concentrated from 5% to
20% (mass basis) in a single effect short-tube evaporator.
The feed enters the evaporator at a rate of 10kg/s and at a
temperature of 300 K. steam is available at a saturation
pressure of 1.3 bar. The pressure in the vapor space of the
evaporator is 0.13 bar and the corresponding saturation
temperature is 320K. if the overall heat transfer coefficient
is 5000 W/m2K, calculate steam economy and heat transfer
surface area required.
Enthalpy
(kJ/kg)
Saturated steam (1.3 bar, 380K)
Saturated steam (0.13 bar, 320K)
Feed (5%, 300K)
Concentrated Liquid (20%, 325K) B.P.E is
5K
2200
80
400
Heat of vaporization
(kJ/kg)
2000
Q8:
The capacity of multiple effect evaporator when
compared with a single effect evaporator both
operating with significant boiling point rise at the
same terminal temperatures and surface area in
each effect equal to surface area of single effect
evaporator is,
A. Decreases
B. Increases
C. Remains the same
Answer: B
Q9:
A multiple effect evaporator as compared to a
single effect evaporator of the same capacity has
A. Lower heat transfer area
B. Lower steam economy
C. Higher economy
D. Higher solute concentration in the product
Answer: C
Q10:
An aqueous suspension at 60C is fed to the first
effect of double effect evaporator with a mass flow
rate of 1.25 kg/s. The sum of the rates of water
evaporated from the first and second effects is 1.0
kg/s. Temperature of exit streams from the first
and second effects are 100 C and 60C
respectively. Consider the specific heat of aqueous
suspension, and the latent heat of phase change
for water to 4 kJ/kg K and 2200 kJ/kg, respectively,
over this temperature range. The steam economy
(in kg per kg) is _______