Evaporation of Cane Juice

Module 5
Evaporation
Module 5: Evaporation
Table of Content
Unit One: General ......................................................................................................................... 6
1.1 Introduction ........................................................................................................................ 6
1.2 Amount of Water Evaporator.............................................................................................. 7
1.3 Arrangement of Evaporators ............................................................................................. 8
1.3.1 Single Effect Evaporators ........................................................................................... 8
1.3.2 Single Effect Evaporators with Vapor Recompression .............................................. 9
1.3.3 Multiple Effect Evaporation ..................................................................................... 10
1.3.3.1 Rillieux Principles ............................................................................................. 10
1.3.3.2 Arrangement of a Multiple Effect Evaporator .................................................. 11
1.3.3.3 Juice Evaporation under Vacuum in the Multiple Effect Evaporators ............. 12
1.4 Accessories of Evaporators ................................................................................................ 16
1.4.1 The Heating Calandria .......................................................................................... 17
1.4.1.1 The Calandria Tubes ....................................................................................... 17
1.4.1.2 The Upper and Bottom Tube Plates (Sheets) .................................................. 17
1.4.1.3 The Down Take (Center Wall) ....................................................................... 17
1.4.2 The Vapor Body........................................................................................................ 18
1.4.3 Calandria Part Provisions of an Evaporator .............................................................. 18
1.4.4 The Vapor Body Part Provisions of an Evaporator .................................................. 21
1.5 Types and Construction of Evaporator ............................................................................. 23
1.5.1 The Robert Vessels .................................................................................................... 24
1.5.2 The Kestner Evaporator Vessel (L.T.R.F) ................................................................. 26
1.5.3 The Long Tube Falling Film Evaporator (L.T.F.F.) .................................................. 29
1.5.4 Plate Evaporator ......................................................................................................... 30
1.5.5 Recent Trends ............................................................................................................ 33
Unit Two: Factors Covering Evaporation Operation............................................................ 38

2.1 Introduction ...................................................................................................................... 38
2.2 Juice Temperature ............................................................................................................ 38
2.3 Juice Level and Juice Level Control System ................................................................... 38
2.4 Temperature and Pressure Distribution ........................................................................... 41
Sugar Engineering and Manufacturing Training Team 2

2.4.1 Boiling Point Elevation (B.P.E) ................................................................................ 41

2.4.2 Temperature Distribution .......................................................................................... 42
2.4.3 Influence of Retention Time ..................................................................................... 42
2.4.5 Other Requirements .................................................................................................. 42
2.4.6 Instrumentation ......................................................................................................... 43
2.4.7 Influence of Operator ................................................................................................ 43
2.5 Starting and Shut Down Procedures ............................................................................. 43
Unit Three: Checking Performance and Steam Requirements ............................................ 47

3.1 Introduction ...................................................................................................................... 47
3.2 Evaporation ...................................................................................................................... 48
3.3 Heat Transfer .................................................................................................................. 51
3.4 Simple Calculations ........................................................................................................ 59
3.4.1 Heat Transfer ............................................................................................................. 59
3.5 Cleaning of Evaporator Tubes ......................................................................................... 65
3.5.1 Scaling (Fouling) of Evaporators’ Calandria Tubes ................................................ 65
3.5.3 Reducing Scaling ..................................................................................................... 67
3.5.4 Tube Cleaning .......................................................................................................... 68
3.5.4.2 Chemical Cleaning .................................................................................................. 69
3.6 Soda Boiling and Mechanical Scrapping Procedures ................................................... 71
3.7 Procedures for Testing Evaporators .............................................................................. 73
Unit Four: Cooling Ponds and Cooling Towers ....................................................................... 81

4.1 Introduction ..................................................................................................................... 81
4.2 Spray Ponds: Practical Considerations ......................................................................... 83
4.3 Cooling Towers ............................................................................................................... 84
Unit Five: Condensers ................................................................................................................ 87

5.1 Introduction ...................................................................................................................... 87
5.2 Types of Condenser ......................................................................................................... 88
5.2.1 Co-Current Condensers .............................................................................................. 91
5.2.2 Counter-Current Condensers .................................................................................... 91
5.2.2.1 Dimension of Counter-Current Condensers...................................................... 92

5.2.2.2 Height of Condenser ...................................................................................... 92

5.2.2.3 Cross-Section Area ...................................................................................... 92
5.2.2.4 Cooling Water Ratio .................................................................................... 92
5.2.2.5 By Heat Balance .......................................................................................... 93
5.2.2.5 Approach Temperature ................................................................................ 93
5.2.2.6 Incondensable Gases and Air ....................................................................... 93
5.2.2.8 Volume of Air to Be Extracted .................................................................... 94
5.2.3 Rain Type Counter–Current Condenser..................................................................... 95

Module Introduction
Dear trainees! I welcome you to module three of the course “Cane Sugar Manufacturing” In this
fourth module we will discuss evaporation.
The module is divided into five units. The first unit deals with objectives and stages of
evaporation of clarified juice. It explains how to calculate the percentage of water evaporated
and the different arrangement of evaporators. In addition to these the unit tries to describe the
accessories of evaporator, different types and constructions of evaporators and their advantages
and disadvantages.
The second unit of the module focuses on the issues of juice level and juice level controlling
systems and temperature and pressure distribution. It also deals about capacity equation for the
performance of evaporators. Finally it describes the work procedure of the starting and shut
down of evaporators.
The third unit is mainly devoted on the calculations associated with evaporation station. It
explains factors that govern the performances of evaporators and the main sources of losses on
the evaporator station. It also describes about scale formation and the different methods of
removing scales from evaporator tubes. Finally it discuss about the work procedures of soda
boiling and mechanical scrapping and tests for the performance of evaporators.
The fourth unit of the module deals about cooling circuits of the evaporators. It tries to elaborate
the two types of cooling, the spray ponds and cooling tower systems and why cooling system is
used in the sugar industry
The fifth unit, the last unit of the module focuses on condensers that are used in the evaporator
station. It describes the advantages and disadvantages of each types of condenser. In addition to
these, it explains the working principles and approach temperature of condensers.

Unit One
General
Unit Objectives
At the end of this unit training, the trainees will be able to:
Explain the objectives and stages of evaporation of clarified juice.

Knows the composition of clear juice.
Calculate the percentage of water evaporated during a Brix change from clear juice to
syrup.
Differentiate and briefly discuss the different arrangement of evaporators.
State all the three principles of Rillieux and associate with the practical aspect.
Describe the accessories of evaporators and their connection.
Discuss the main types and constructions of evaporators
Explain the advantages and disadvantages of each type of evaporators.
1.1 Introduction
Evaporation generally refers to the removal of water, by vaporization, from aqueous solution of
non-volatile substances.
The aim of evaporation is to concentrate clarified juice to a Brix, which will allow
crystallization. Evaporation is done in two stages:
i) In the evaporator to a Brix of 70 0B, to give syrup.
ii) In the vacuum pans where the syrup is further concentrated, to allow crystallization to
proceed.
In the evaporator, juice is concentrated to syrup of Brix between 650 and 700, the upper limit of
evaporator being determined by the fact that sucrose will start to crystallize at between 780 and
800 Brix.

In practice, it is preferable not to exceed 70o Brix at the syrup stage. At this concentration, the
syrup is capable of melting crystals which have remained in the pan from a previous strike.
Composition of Clarified Juice
Clear juice consists of:
Water (85 %) – Source - From cane and imbibition water
Solids (15 %)
Sugar
Non –Sugar
Water is the major constituent (85 %) of clear juice while the total dissolved solids
in the clarified juice are only about 15 %.
1.2 Amount of Water Evaporator
The amount of water evaporator from clear juices is calculated from a Brix balance. It is
assumed that there is no loss of solids between clear juice and syrup, so that the amount of solids
in clear juice should be equal to the amount of solids in syrup.
For instance, let:
E = Mass of Water to be Evaporated – % Clear Juice

J = Mass of Clear Juice ( = 100)
S = Mass of Syrup
Bj = Brix of Clear Juice
Bs = Brix of Syrup
Then, by Brix balance:
100 Bj = S . Bs
100 Bj = (100 – E) Bs
100 Bj = (100 Bs – E . Bs)

100 ( BS − BJ )
Then, E =
BS
(BS − BJ )
So, Evaporation % Juice = × 100
BS
If we have a clear juice with a Brix of 13.5 and we want syrup of 680 Brix, the % evaporation
required will be:
100 (68 − 13.5)

E =
68
E = 80.1 %
This is the largest consumption of thermal energy in process. It is important that the evaporation
station be as efficient as possible.
1.3 Arrangement of Evaporators
Based on the concept of steam economy three evaporator arrangements are realized:
I. Single effect evaporators
II. Single effect evaporators with vapor recompression
III. Multiple effect evaporators
1.3.1 Single Effect Evaporators
The required capacity is so small.
Steam is cheap.
The vapor is so contaminated that it cannot be reused.

1.3.2 Single Effect Evaporators with Vapor Recompression

They are the simplest means of reducing the energy requirements of evaporators in compressing
the vapor from the single evaporator so that the vapor can be used as heating medium in the same
evaporator.
The pressure and consequently the temperature of vapor from the evaporator is increased to a
certain extent (i.e., to the pressure and temperature that prevail in the heating calandria of the
evaporator) by the aid of a compressor.
Evaporators with vapor recompression are of two types based on the vapor compressing element:
a) The Turbo-Compressor / Motor-Compressor
b) The Thermo-Compressor
Figure 1.1: Turbo-/Motor-Compressor

Figure 1.2: Thermo-Compressor
1.3.3 Multiple Effect Evaporation

The greatest and most striking advance in the history of sugar manufacture was no doubt the
development of multiple effect evaporation. The principle of the multiple effect evaporators were
conceived by Norbert RILLIEUX (USA 1830) and put into particle in Louisiana in 1844.
Rillieux Principles
1. In a multiple effect evaporator, for each kilo of steam used, as many kilos of
evaporation will result as there are units in the set.
2. If vapours are withdrawn from any unit of a multiple effect evaporator to replace
steam in a concurrent process, the saving of steam will be equal to the amount of
vapour so used, divided by the number of units in the set, and multiplied by the
sequence position of the unit from which vapour has been withdrawn.
3. In any apparatus in which steam or vapour is condensed, it is necessary to

continuously withdrawn the accumulation of non-condensable gasses.

The first statement is a simple but rough approximation. Rilleux did not fully consider the fact
that the incoming juice is usually at a temperature below boiling point in the first effect. The
second statement is found to be almost correct when checked against heat balance. The third
statement is quite obvious; otherwise we cannot introduce any vapour.
1.3.3.2 Arrangement of a Multiple Effect Evaporator

An evaporator consists of several vessels in series as shown in Fig.1.3 and 1.4. These vessels are
connected so that the vapour from one goes to the calandria of the following vessels and
ultimately to the last vessel which is connected to a condenser. Steam is admitted to the first
vessel. Juice enters below the calandria of the first vessel where it boils and goes out by means
of a down take to the next vessels an ultimately out of the last vessels as syrup.
Multiple-effect evaporators are the principal means (arrangement) in use for economizing on
energy consumption. They can be double, triple, quadruple, quintuple, sextuple, etc. As the
number of effects increases, energy (steam) saving increases, but the rate of evaporation (which
is minimum when the number of effects is greater than five and maximum when the number of
effects is three) decreases. The rate of evaporation falls with the number of effects because the
driving force, log-mean temperature difference, for heat transfer decreases.
An arrangement of four vessels in series is known as a Quadruple Effect Evaporator, whereas a

Quintuple Effect Evaporator consists of five vessels in series.
An effect means a step or a stage of evaporation. There are four stages of evaporation in a
quadruple effect evaporator. The effects are known as 1st, 2nd, 3rd and 4th in a quad, with a 5th
stage in a quint.
There may be more than single vessels in an effect, depending on certain factory set ups.

1.3.3.3 Juice Evaporation under Vacuum in the Multiple Effect Evaporators

Across a multiple-effect evaporators, the operating pressure and correspondingly the boiling
point of juice decreases from the first to the last effect. Evaporation of juice under such condition
presents the following main advantages:
It increases the driving force for heat transfer, the total temperature difference between
the steam and the juice, which in turn increases the rate of heat transfer thereby
increasing the evaporation rate.
Low temperature operations generally require less energy (heat) than high temperature
operations, i.e., it saves energy.
Low temperature operations are less dangerous as the juice becomes more and more
concentrated and viscous from the point of view of:
Inversion - is favored at high temperatures.
Caramalization (Coloration) – is the charring of sugar manifested at

high temperature operations.
Such low temperature operation increases the evaporation capacity of an evaporator.

Evaporation in the multiple - effect evaporators is not only due to the heat supplied to the
heating surface of the calandria but also occurs as a result of the flash that takes place
when the juice passes from one body into another where lower pressure exists, as a
consequence of progressively increasing vacuum and decreasing temperature in the
evaporator set.
The pressure differences that exist between the vessels of the multiple - effect
evaporators are the driving forcers for the juice to flow from one vessel to the other
across the multiple- effect evaporator.
Creating the necessary vacuum in the multiple- effect evaporators requires installation of
vacuum creating equipments called CONDENSERS. Condensers are dealt with in unit 5 of
this module

Figure 1.3: Vapor Cell Followed by Quadruple Effect Evaporators with Vapor Bleeding
The Vapor Cell
Receives clarified juice at a brix of about 15 0C and a temperature range of 115 - 120 0C.
The heating medium is exhaust steam at 125 0C.
Evaporates as maximum water as possible from the juice. The vapors so furnished are at
112 - 115 0C and are entirely used for vacuum pans as heating medium.
First Effect Evaporator
Also heated by exhaust steam.
Furnishes vapors at 112 – 115 0C which are used:

Partly for vacuum pans as make up.
Partly for secondary juice heating.
And the balance for second – effect evaporator as a heat medium.

Second Effect Evaporator
Generates vapors at 100 – 101 0C.
Is heated by vapors from the 1st – effect.
The vapors so furnished are divided between the 3rd –effect evaporator and primary
juice heater.
Third Effect Evaporator
Is heated by vapors from 2nd –effect.
The vapors generated in this vessel are at about 90 0C and are totally supplied to the
calandria of 4th – effect.
Fourth Effect Evaporator
Is the last effect, which delivers syrup at a brix range of 60 – 650.
It works at a temperature range of 65 – 70 0C and pressure of 400 to - 500 mm Hg.
The vapors from this effect are condensed by the multijet condenser and lost to the ditch.

Figure 1.4: Multiple Effect Evaporator Station

1.4 Accessories of Evaporators

An evaporator has two main body parts as indicated in the figure above.
• The heating calandria, and

• The vapor body
The evaporator is not complete without the following accessories.
i) Clear Juice Feed Pump
ii) Syrup Extraction Pump
iii) Condensate Extraction Pump
iv) Barometric Condenser
v) Incondensable Gas Extraction Pump (air Pump)
vi) Water Circulation Pump for Condenser
vii) Cooling Pond/Tower
viii) Level Control (For Efficient Performance)
ix) Brix Control ( Recommended for Automated Vacuum Pans)
Figure1.5: Body Parts of Evaporators

1.4.1 The Heating Calandria
It is the heating element in an evaporator. It consists of:
I. A large number of vertical tubes with open ends.
II. Upper and bottom tube plates into which tubes are expanded on both ends.
III. A large down take pipe situated at the center.
1.4.1.1 The Calandria Tubes
The calandria tubes of each of the five evaporators are identified with the following
specifications.
Tube Outer Tube Length Total Heating

Evaporator Diameter (mm) (mm) Surface (m2)
Vapor Cell 38 2.13 1400

1st -Effect 38 2.13 1400
nd
2 –Effect 38 2.13 1100
3rd – Effect 38 2.13 850
4th - Effect 38 2.13 850
1.4.1.2 The Upper and Bottom Tube Plates (Sheets)
The calandria tubes are fixed on both ends into upper and bottom tube sheets as indicated is
Fig.1.5.
1.4.1.3 The Down Take (Center Wall)
The role of the down take is to return to the bottom the juice which has been projected over the
top tube plate in order to remove the concentrated juice from one vessel to the following vessel
as shown is Fig.1.4.

The lower opening of the down take is fitted with a CONICAL FUNNEL which forms the outlet
point for the juice passing to the following vessel without mixing with the incoming juice as
indicated is Fig.1.5. At the lower end of the funnel is placed a RING PLATE with slots so as to
direct the entering juice towards the bottom of the tubes near the periphery of the calandria,
Fig.1.5.
1.4.2 The Vapor Body

The vapor space/body represents the greater part of the volume taken up by the vessel to
diminish the risk of entrainment of droplets of juice projected by evaporation. “High vapor space
is the best entrainment separator.” Height of the vapor space is 1.5 - 2.5 times the height of the
calandria tubes.
1.4.3 Calandria Part Provisions of an Evaporator

The calandria part of an evaporator has the following provisions:
Cold water connections
Exhaust steam (vapors) admission connection pipe
Incondensable gases removal connection pipes
Equalizer connection
Condensate removal connection
Insulation
Safety valve
Cold Water Connection Pipe

Just as juice heaters, each of the multiple-effect evaporator calandrias is provided with cold-
water (pond water) connection line for pressing the calandria tubes for leakage.

Exhaust Steam (Vapors) Admission Connection Pipe
Exhaust steam (vapors) admission pipes are provided for each of the evaporators for evaporation
purpose. Moreover, 2nd -, 3rd _
and 4th – effect evaporators are provided with exhaust steam
supply lines in addition to the vapors admission lines for the purpose of caustic soda (NaOH)
solution boiling. For the vapor cell and 1st – effect evaporators, the same exhaust steam
connection pipes used for juice evaporation are also utilized for caustic soda solution boiling.
Incondensable Gases Removal Connection Pipe Lines

The vapor/steam which arrives in each calandria introduces with it air and foreign gases. These
gases and air are of three origins:
A. Gases dissolved in juice, which are released on boiling. It is air that is mainly released
from cane juice.
B. Air brought in by exhaust steam, which is introduced into exhaust steam by the make-
up water, which carries air in solution.
C. Air, which enters by leaks at the joints of the calandrias and of the vessels at valves,
sight glasses, etc.
In the calandrias of the vapor cell and 1st – effect evaporators incondensable gases of (B) origin
are common. Gases (air) of (A) and (C) origin are not expected in these vessels’ calanderias
because:
They are using exhaust steam and not vapor bleed, i.e., no (A) origin gases/air.
They operate at relatively higher pressure than atmospheric pressure and thus entrance of
air (gases) to them through joints at valves, sight glasses, etc is not possible, i.e., there is
no air (gas) of (C) origin.
In the calandrias of the 2nd – to the 4th – effect evaporators, (A) and (C) origin gases are common
for they are using vapors (A) and work under vacuum (C) (there is significant admission of air
from atmosphere into the calandrias through the joints).

Every of the multiple – effect evaporators calandrias is provided with incondensable gases
withdrawal pipes at three different levels in the calandria:
At the bottom of the calandria for removal of heavy incondensable
At the middle for medium weight gases
At the top just below the upper tube plate for withdrawal of light incondensable
The incondensable gases removal pipes of the vapor cell and 1st – effect evaporators discharge
the gases into the atmosphere because the pressure in these calandrias is greater than the
atmospheric pressure as a result of which the gases can easily be released into atmosphere. In the
case of 2nd -, 3rd – and 4th – effect evaporators, this is impossible because they operate under
vacuum, i.e., at pressures lower than atmospheric pressure implying that the gases/air won’t be
simply released into the atmosphere. Therefore, the gas/air withdrawal pipes of these effects are
expanded into the vapor pipe connecting the 4th – effect evaporator and the multijet condenser,
which is at a lower pressure than the calandrias of these effects.
Equalizer Connection Pipe
The pressure equalizer connection lines are also provided for multiple – effect evaporators for
the same reason as those of juice heaters. But unlike that of the juice heaters, the equalizer of the
vapor cell and 1st – effect evaporators is connected to the calandria of the 2nd – effect, that of the
2nd – effect to the 3rd -effect and the equalizers of the 3rd – and 4th – effect evaporators are
expanded into the calandria of the 4th – effect. Such connection of equalizer of one evaporator to
the calandria of the following evaporator has the following advantage: Since the flash tank is
connected through the equalizer line to a lower pressure calandria than from which it receives
condensate, part of the condensed vapor and the uncondensed vapor flash out into this lower
pressure calandria which in turn boosts the evaporation capacity of the evaporator.

Condensate Removal Connection Pipe Lines
Condensates are removed from the bottom of the evaporator calandrias into condensate flash
tanks by means of syphons.
A syphon (U – tube) is inserted between a calandria and its condensate flash tank so as to
balance the pressure difference existing between them. If they were absent, the condensates
would cause the flash tanks to vibrate because of the turbulence in the flash tanks, which is in
turn accounted to the pressure difference between the calandria and the flash tank.
1.4.4 The Vapor Body Part Provisions of an Evaporator
The vapor body provisions of an evaporator are:
Sight glasses
Caustic soda solution, hot and cold water supply lines
Entrainment separator
Vacuum breaker
Soda vent pipe
Manholes
Temperature and pressure gauges
Slight Glasses
Every evaporator in the multiple effect is furnished with three (except the 4th - effect which has
only two) circular windows or sight glasses (one located above the other) formed of thick glass
enclosed between the wall of the evaporator and brass frame, and bolted on with a soft joint
between in order to observe the working of the evaporator, and to see what is happening, inside
it. In order to see what is happening in the vessel against a window not against a dark
background, two lateral windows, one on the front and the other on the backside each at 450 from

the vertical are installed at a certain height from the sight glasses. Outside these lateral windows,
powerful lights with reflectors that light up the interior of the vessel are placed.
Caustic Soda Solution, Hot and Cold Water Supply

Caustic soda solution of 20 – 30 % concentration is prepared in the caustic soda solution tank
and pumped to a common header for evaporators. From the header a pipeline is connected to the
upper part of the vapor body of each evaporator for admission of the caustic soda solution into
each vessel during soda boiling.
There are two hot/cold water supply lines to the vapor body of each evaporator vessel; one is
connected to the caustic soda solution line and the other is located just above the upper tube
sheet. The hot/cold water provisions to an evaporator have the following major purposes:
To maintain the level of juice in the vessels whenever there is shortage of clear juice.
To rinse the calandria tubes before and after soda boiling as well as after scrapping.
To maintain the level of caustic soda solution in the evaporators during soda boiling.
Soda Vent Pipe

Each evaporator vessel is provided with soda vent pipe near the top part of its vapor body so as
to collect vapors during soda boiling. The soda vent pipe from each vessel expands into a
common header, which then damps the vapors into ditch.
Manholes
An evaporator is provided with three manholes on its vapor body for the purpose of inspection,
maintenance and scrapping activities.
A side manhole located just above the upper tube sheet.
A rear side manhole located at the top part of the vapor body, and
A manhole to the primary catchall.

Entrainment Separator Vapor

Entrainment is the carry over of fine (very light) droplets of juice, some of which are minute
bubbles of vapor enclosed in a film of juice like soap bubbles, towards the following vessel or
towards the multijet condenser by current of vapor in the vapor space of an evaporator.
In general,
As the vacuum in the vessels increases, the danger of entrainment increases.
The smaller the size of the droplets the higher the entrainment and vice versa.
In order to avoid (minimize) losses of sugar by entrainment all evaporators are provided with a
device placed in the vapor bodies at the topside. Such a device is termed Entrainment
Separator or Save all or Catchall. There are two types of catchalls installed at FSF: the primary
catchall and secondary catchall. All the primary catchalls on the five evaporators are umbrella
type. Secondary catchall is provided only for the 4th - effect evaporator in addition to the primary
catchall since it is the 4th - effect that produces the most serious entrainments because of the
higher vacuum existing in it. The secondary catchall is installed in the vapor pipe expanding
between the primary catchall and the multijet condenser. It is a rectangular box in which
vertically arranged metallic grilles are placed side by side with their hollow sides facing the
vapor stream.
1.5 Types and Construction of Evaporator
Nowadays, we can classify evaporators according to:
a) Their Heating Surfaces:
1. Short Tubes (Robert ) 2 – 3 meters
2. Long Tubes (Kestner) 6 – 8 meters
3. Plate Heaters

b) Their Juice Circulation
1. Rising Film( Robert, Kestner)
2. Falling Film (Tubular and Plates)
1.5.1 The Robert Vessels
The Roberts vessel is the normal and most common evaporator vessels used universally (See Fig.
1.6). It consists of four distinct parts, namely:
i) The Calandria
ii) The Vapour Space
iii) The Top Space
iv) The Bottom Part
i) The calandria consists of tubes inserted between the upper and lower tube–plates with a
down take positioned centrally or on the side of the vessel. The tubes make up the heating
surface, which allows the transfer of heat from the steam or vapour on the outside to the juice
inside the tubes. The clandaria is provided with:
a) A steam or vapour inlet
b) A condensate outlet, to remove the condensate as it is formed.
c) Incondensate gas outlets, to remove the gasses continuously.
Calculate tubes are arranged in a rhombic pattern and very in length between 1.6 to 2.3 meters
with outside diameters from 38 to 51 mm. Brass tubes made from copper and zinc are now too
expensive and have mostly disappeared. They have been replaced by stainless steel tubes.
ii) The Vapour space is that section of the vessel extending from the upper tube-plate to the top
dome of the vessel. It is provided on the working front with sight glasses and a manhole. The
height of the vapour space is at least twice the length of the tubes, to reduce the risk of
entrainment.

iii) The top part of the vessels is fitted with an entrainment arrestor and is provided with
distributor and outlet.
An evaporator vessel is also known as an evaporator “body”. Most modern’s vessels are made of
mild steel and are entirely welded.
Figure 1.6: Roberts Evaporators Vessel

1.5.2 The Kestner Evaporator Vessel (L.T.R.F)

This type of vessels was invented in the beginning of the 20th century by the French engineer,
Paul KESTNER. It found wide application in the beet sugar industry, but took a long time to be
installed in the cane Sugar industry (See Fig.1.7).
It consists of a tall vessel with tubes 7 m long and is based on the “Climbing Film Principle”.
Juice, boiling at the base of a tube, produce vapour bubbles, which rise and expand to almost the
full tube diameter, thus forming a thin film juice against the tube, rising in it is entrained by the
vapours. The Kestrner vessel is defined as a long tube rising film evaporator or L.T.R.F. it is
mostly used as first vessels.
The Advantages of a Kestner are:
a) It gives a short juice retention time, which prevents color formation and reduces inversion
at high temperature.
b) It provides a high concentration of heating surface due to the tubes.
c) It does not require a heavy steel structure for support and is erected on a thick concrete
foundation at ground level.
The Disadvantages are:
a) It used to require an external separator to separate juice from vapour. Nowadays, this
can be avoided.
b) It must be preceded by a juice heater to supply it with juice at a temperature that will
ensure flashing the tube-plate.
c) Depending on its construction, it may require chemical cleaning.

Figure 1.7 (a): Kestner Evaporator

Figure 1.7 (b): Kestner Evaporator with Independent Tangential Separator

1.5.3 The Long Tube Falling Film Evaporator (L.T.F.F.)

This type of evaporator was introduced in the 1970s. This vessel is shown in Figs. 1.8(a) and
1.8(b). It uses tubes of 10 m length and 51 mm o.d. and requires a circulation pump to ensure
proper juice distribution and adequate wetting of the sides of the tubes at all times. It takes
advantages of the absence of Boiling Point Elevation (B.P.E) in the juice due to hydrostatic
pressure.
Automatic control and chemical cleaning are required.
Figure 1.8(a): Falling Film Evaporator

Figure 1.8(b): Falling Film Vessel with Mattress Separator and Turbo Compressor
1.5.4 Plate Evaporator
This type of evaporator was introduced in the 1990s and is based on the design of the well-
known plate heat exchanger (See Fig.1.9). It is also made by Alfa-Laval, Sweden, and uses the
rising film principle. Juice travels upwards between steel plates with stream on each side of the
juice plates. Improved clip-on gaskets are used on the juice side. The evaporator gives higher
heat transfer coefficients than Roberts vessels, but cleaning has to be done chemically. Plate
evaporators can be used to upgrade existing units or can be used independently in their open

outlet design for mounting inside a vessel. A manufacture (Balcke – Durr) also proposes a
“falling film” version, making possible a 6 - effect evaporator (See Fig.1.10)
(A) Principle
Figure 1.9: Plate Evaporator

(B) Flow Pattern
Figure 1.9: Plate Evaporator
Figure 1.10: Balcke - Durr Evaporator (Principle of Operation)

1.5.5 Recent Trends
Despite their advantages, plate evaporators are not very popular in cane sugar factories owing to
scaling problems. Roberts and Kestner remain the more common types of vessels.
a) Kestner: With the use of poly-baffle separators, they are now constructed without
independent juice separators, like the ones at Omnicane Milling at La Baraque.
b) Roberts: Cheap and easy to manufacture, they have been much improved by the
Australians (performance increase of ± 30 % has been achieved). Interesting
modifications made are illustrated in Fig. 1.10.
1. Peripheral juice inlet.
2. Use of several 150 mm down pipes to favor recirculation and improve the brix
profile in the apparatus.
3. Elimination, in some cases, of the central down take and use of level control to
improve performance.
4. Reduction of pressure drop at the steam inlet by the use of an annular box and
multiple inlets to the calandria.
5. Removal of condensates and in condensable gasses at the centre of the calendria ,

as per the co-current principles
The Australians also use magnetic flow meters on the individual condensate discharge to monitor
continuously the heat transfer coefficient.

(a) Cyclone Type Arrester (b) Louvre or Channel Type Arrester
Figure 1.11: Roberts (Australian vs. Indian)

Figure 1.12: Entrainment Separator

Figure 1.13: Heat Transfer Coefficients in Sugar Solutions in Robert and Falling Film
Evaporators
Compression
1 Bar = 100 KPa = 1.02 Kgf/Cm2
= 14.15 lbf/Cm2
1 Kilo Calories = 4.18 Kilo Joule
Kilo Cal/hr/m2/OC = 1.16 W/m2. OK

Self Check Exercises
1. What is the main problem of plate heating surfaces in evaporators?
2. State the “Principles of Rillieux” governing the operation of a multiple effect evaporators?
3. What is the purpose of “Flash Pots”? Sketch one indicating its position on the evaporator
station. Explain the purpose and advantages of “Flash Pots”.
4. Briefly describe the various types of heating surface found in evaporators.
5. Why do we perform bleeding on an evaporator?
6. Make a sketch of a long tube falling film “LTFF” evaporator. What is its main advantage?
7. Explain the “Film Effect” in the evaporator? How does it favor good performance?
8. Sketch the diagram of a Kestner (LTRF) evaporator body. What are the advantages of
Kestner when compared to Roberts?
9. Why should we not exceed a certain maximum Brix for the syrup? What is approximately
that limit?
10. Why do we perform bleeding of vapour from evaporator station? What are the limiting
factors?
11. Describe briefly the evolution of juice evaporation process from the beginning to the
present time. (From open fire to now days).
12. Why is it advisable to supply the evaporator with juice at or as close to the boiling point?
In what case is it of prime importance?
13. Make simple sketch of a Robert and a Kestner evaporation cells to show the difference
between the two.
14. Describe briefly the plate type evaporator. Give its advantages and disadvantages.
15. What are the problems caused by incondensable gases in evaporators? How do we remove
these gases?

Unit Two
Factors Covering Evaporation Operation
Unit Objectives
Explain juice kevel and juice level control system.

Discuss about temperature and pressure distribution in multiple effect evaporators.
Describes the capacity equation for the performance of evaporator.
Discuss the boiling point elevation problems in evaporators.
Apply practical technique and describes properly the starting and shut down
procedures of evaporators.
Discuss the boiling point elevation problems and factors associated with it.
2.1 Introduction
The evaporator bodies being interconnected are interdependent. It is the quantity of steam
supplied to the first body that drives the whole apparatus and governs its performance. The
evaporator is also subjected to the influence of various operating conditions given below:
2.2 Juice Temperature

To have the best performance, it is desirable to have the juice temperature at the evaporator inlet
at least 3 – 5 0C above boiling point. A juice heater on the evaporator feed in necessary, although
Roberts’ type bodies may do without it.
2.3 Juice Level and Juice Level Control System
In the case of rising film evaporators (Roberts and Kestners), there is optimal level for their
operation (See Fig 2.1). For Roberts, it is between 30 – 40 % and for Kestners 20 – 25 % of the
tube lengths. This is to provide the “film effect” which favors heat transfer. A level control or
indication systems helps a lot. It is also recommended to have a control on the juice flow to

achieve stable conditions. In case of juice shortage, it I essential to compensate with hot
condensates to avoid drying up of the evaporator
Conditions of Steam to the Set
a) Steam from the turbo-alternator and other steam turbines will be superheated and has a
poor heat transfer coefficient. Consequently, steam has to be de-superheated before
entering the 1st effect. A de-superheated is installed in the exhaust main to reduce the
degree of superheat by spraying hot water in a very fine mist inside the main. A level of
superheat of not more than 30 0C is considered acceptable.
If the level of juice in the calandria tubes is too low, the boiling juice cannot reach the top
of the tubes and as a result there will not be circulation of juice in the calandria. On the
other hand, if the juice level is too high, the tubes will be fully submerged, i.e. increasing
the hydrostatic level of the juice in the calandria will result in the boiling point elevation
of the juice and thus prevents evaporation. Moreover, too low or too high juice level
causes entrainment.
Therefore, the optimum juice level that must be maintained in the evaporator vessels is
between 30 – 40 % of the height of calandria tubes.
All evaporators are provided with automatic level control, which consists of:
The controller
The level transmitter (Sensor)
The pneumatically actuated valve

Figure 2.1: Variation of Heat Transfer as a Function of Juice Level in the Calandria
b) We may need a “let down” station using high pressure steam to stabilize the exhaust
steam pressure. To optimize the thermal balance of the factory, it is better to get that
steam from the first stages of a condensing turbo alternator.
c) Co-generating sugar factories get their exhaust steam at a fixed pressure from a separate
bagasse–coal power station.

2.4 Temperature and Pressure Distribution
The performance of the evaporator is governed by the “Capacity Equation”.
Q = c . A . T
Where/ Q = Amount of Evaporation

c = Coefficient of Evaporation
A = Heating Surface Area
T = Temperature Difference
Consider that the 1st effect is fed with saturated exhaust steam at 121 0C, and 205 KPa, and that
the condenser is at 54 0C and 15 Kpa. The overall temperature difference across the evaporator
( T 0C) is then:
T 0C = 121 0C – 54 0C = 67 0C
This overall temperature difference will be reduced by the total amount of the boiling point
elevation in the set so that we effectively have T = (67 – Sum of B.P.E). That “ T ”
will be distributed in the various effects according to their evaporation rate.
2.4.1 Boiling Point Elevation (B.P.E)

As a sugar solution becomes more concentrated, its boiling point increases relative to water at
the corresponding pressure. This increase is known as the Boiling Point Elevation or B.R.E. and
is directly proportional to the Brix or solids content of the solution. Extra amounts of heat will be
needed to raise the boiling temperatures of the liquor at various concentrations in an evaporator,
the B.P.E. being small in the first effect and increasing to a maximum in the last. B.P.E is given
by:
(2.4 × Brix )
=
(100 − Brix )
Brix is the mean of inlet and outlet Brix in each effect) or can be obtained more
accurately from tables:

2.4.2 Temperature Distribution

The available net temperature difference will distribute itself across the different effects in
relation to the amount of evaporation in each one. The T between the last effects is greater
than the T in the 1st effects, due o the increasing viscosity as concentration of the liquor
increases: this will reduce the heat transmission across the heating surface. The vapour bleeding
duties in a evaporator results in unequal heating surfaces in effects. Pressure and corresponding
vapor temperatures will be higher in the 1st and 2nd effects as vapour bled from them will be used
for juice heating and pan boiling. The amount of vapour bled is sometimes important and may
require larger heating surface in these effects.
2.4.3 Influence of Retention Time

The vapour temperature in a 1st effect ranges normally from 108 0C to 112 0C, if final juice
heating is done on first vapour. Second vapour temperatures would be from 104 0C to 105 0C in
cane diffusion factories. Juice retention ties at these temperatures could be too high with
consequent destruction of reducing sugars and inversion of sucrose, especially at low juice pHs.
In order to reduce these influence, it is essential to keep the products of Temperature × pH ×
Time as small as possible.
In respect of time, the use of Kestner or eventually falling film for 1st and 2nd vessels will provide
much shorter residence times than with Roberts’s vessels. As regards the pH of juice entering the
evaporator, a value of 7.2 to 7.3 is satisfactory.
There is not much that the can be done regarding juice temperatures as these depend on vapour
bleeding requirements of the factory.
2.4.5 Other Requirements

a) It is understand that the effect removal of non-condensable gasses is done continuously
and carefully regulated as not to waste steam. A thin steam exhaust to atmosphere from
pressure vessels and the use of a manometer up stream of the control valve in vessels
under vacuum.
b) Condensates should be withdrawn automatically and flash recovery carried out where
possible.

2.4.6 Instrumentation
It is also accepted that instrumentation and automatic control of working pressures, juice levels
and syrup Brix is provided and overviewed by a competent operations supervisor. Juice levels
should be controlled at 20 % of the height of tubes in a Kestner evaporator and between 30 – 40
% in Roberts’s vessels (See Fig.2.2 (a) and Fig .2.2 (b)].Final syrup Brix of 680 to 700 should be
aimed at.
2.4.7 Influence of Operator

In countries where sugar factories are relatively small and evaporators have not been provided
with sufficient instrumentation, it is essential that experienced and competent personnel operate
the evaporator. Above all, one of the greatest mistakes made by operators is to open and close
feed valves much too often. An evaporator stabilizes itself and will run better if not distributed
after valves have been properly set.
2.5 Starting and Shut Down Procedures
When starting, we first put the condenser and the air pump in operation. Gradually through the
incondensable gasses line vacuum is produced in each effect. Juice and then steam is admitted in
the first body. The juice, on boiling, overflows in the down take and goes to the next body. At
the same time, vapour produced in the first body goes to the second body. Gradually, all the
bodies warm up and start evaporating and a pressure gradient is established. The condensates and
syrup extraction pumps are put in operation.
When stopping the evaporator, we just do the reverse. As the juice flow diminishes, we also
reduce the steam admission, until syrup stops overflowing from the last body. For short stops,
just discharge syrup from the last body and replace with water to prevent crystallization. For
longer stops, we empty all the bodies. The thin juices from the first bodies are sent to the pan
station for concentration. The apparatus is then filled with water and flushed before proceeding
with mechanical or chemical claiming.

Figure 2.1(a): Principle of Juice Level Control in Evaporators

Sugar Engineering and Manufacturing Training Team
Figure 2.2 (b): Juice Level in a Multi-Effects Evaporator. (Five Cail – Babcock)
45
1. Explain how the performance of evaporators is affected by juice level.
2. What is “Boiling Point Elevation (B.P.E)” What are the factors that influence it in
multiple effect evaporators?
3. Is it a good practice to apply superheated steam to the evaporator?
4. Why is “Retention Time” in an evaporator a point to consider when evaluating the merits
of an evaporator station?
5. Give the “Capacity Equation”? Explain the influence of each component?
6. Some times the clear juice has to be heated to the boiling point in the first body of an
evaporators. How this does affects its performance?
7. What do you understand by “Retention Time” in an evaporator? How does it affects

sugar losses?
8. What are the factors that affect the boiling point elevation (BPE) of the juice in
evaporator? How BPE does affect the capacity of evaporators?
9. Describe the stepwise work procedure to start up and shut down the evaporator.
10. Explain why superheated exhaust steam on the evaporator station has a poor
performance. (Low Efficiency).
11. Why must we remove incondensable gases from the calanderia of evaporators and how do
we do it?
12. How does a poor vacuum (Low Vacuum) affect the performance of the evaporator
Station?

Unit Three
Checking Performance and Steam Requirements
Unit Objectives
Explain factors that should be considered while discussing the performance of

evaporators.
Discuss about heat balance in evaporation.
Describe briefly the “DESSIN” Formula.
Describe the two main sources of sugar losses.
Calculate heat transfer rate, steam requirements, mass of water evaporated and other
quantities related to evaporation.
Describe the effect of scale formation in the heat transmission of heating medium..
Discuss the composition of scale formed inside the calandria tube.
Explain different methods of reducing scales in evaporator tubes.
List out the major steps for soda boiling and mechanical scrapping procedure.
Describe all the important points or procedures to follow to test the performance of
evaporators
3.1 Introduction
Just as for any equipment, we must have means of calculating the performance of our evaporator.
We may consider performance under three aspects:
a) Evaporation
b) Heat transfer and Steam Consumption
c) Sugar Losses

3.2 Evaporation
The simplest measure of performance is the percentage of evaporation obtained. This can be
calculated from the Brix of the juice and that of the syrup.
( BS − BJ )
E% = × 100
BS
Good performance rates at about 80 %
If we sample the juice leaving each body of the evaporator, we may calculate the percentage of
evaporation obtained in each one.
Knowing the juice flow, we can make a Material Balance and calculate the amount of vapour
from each effect. A pictorial representation of what is taking place in the evaporator is given in
Figs. 3.1 and 3.2.
We may also calculate:
a) The Evaporation Coefficient (Kg Evaptn/h/m2)
b) The Specific Evaporation Coefficient (Kg /h/m2/0C)
We must distinguish between the apparent and real S.E coefficient depending on the temperature
drop between vapour and juice that we are using.

W = Quantity of Liquor which Enters D4 = d4 + s4 + δ4 + λ4 = Total Steam from Vessel IV
Do = Total Hot Steam for Vessel I s4 = Produced by Self Evaporation in Vessel IV
dh = Steam for Heating b1, b2, b3, b4 = Condensed Water from the Four Vessel
do = Steam for Evaporating (Produces d1 = D1) t1, t2, t3, t4 = Temperature of the Steam in the Four Vessel
D1 = Steam from Vessel I. (Produces d2) tm1, tm2, tm3,, tm4 = Temperature of the of the Liquor in
D2 = d2 + s2 = Total Steam from Vessel II the Middle of Each Vessel
d2 = Steam Produced from d1 (Produces d3) tu1, tu2, tu3, tu4 = Temperature at the Bottom of Each Vessel
s2 = Produced by self evaporation in Vessel II c1, c2, c3,, c4 = Total Heat in One Kilo of Steam
(Produces δ3).
D3 = d3 + s3 + δ 3 = Total Steam from Vessel III
d3 = Produced by d2 (Produces d4)
δ3 = Produced by s2 (Produces λ4)
s3 = Produced by self evaporation in Vessel III (Produces δ4)
Figure 3.1: Straight Quadruple Effect

de = Heating Steam for the Production of Extra Steam e3 = Extra Steam Taken from Vessel III
(Produce e, E1, n1) (Produced from E1 which is from E2)
e1 = Extra Steam Taken from Vessel I D3 = d3 + s3 + δ3 = Total Steam from Vessel
n1 = Produced from de (Produces e2) III to Vessel VI
n1 = Produced from “de” (Produces e2) δ4 = Produced from s3
D1 = d1 + E1 + n = Total Steam from Vessel II s4 = Produced from δ3
e2 = Extra Steam Taken from Vessel II D4 = d4 + s4 + δ4 + λ4
c2 = Produced from c1 (Produces e3) = Total Steam from Vessel IV
D2 = d2 + s2 + δ3 d4 = Produced from d3
= Total Steam from Vessel II to Vessel III
Figure 3.2: Quadruple Effect with Bleeding

3.3 Heat Transfer
For heat transfer, we must perform at heat balance (See Fig.3.3) assuming no heat losses (or else
take the losses as being 2.5 % of heat supplied to the first effect for a fully lagged evaporator and
5 % for a partially lagged one).
Figure 3.3: Heat Balance
Heating Entering = Heat Leaving
(Total Heat of Steam) (Total Heat of Vapour)

(Total Heat of Juice) (Total Heat of Juice)
(Total Heat of Condensate)
(Total Losses If Considered)

To make a complete analysis of the performance, we must record the working conditions of the
evaporator, namely:
Brix of the juice at each stage
Temperature conditions in each effect
Temperature and pressure conditions of the steam supply
Temperature rise on each juice heater using “bled–vapour”
Juice flow rate
Having all these data and working out material and heat balances, we can calculate the gross heat
transfer coefficient (W/m2/0C), either the apparent or the real value.
Some conservative apparent G.H.T.C. values given by certain authors are:
a) Pre-evaporator working on steam at 250 - 300 KPa = 3,490 W/m2/0C
b) Quadruple Effect: I = 2,325 III = 1,050 – 1,160

II = 1,628 IV = 465 – 580
However, the transfer of heat is favored if we work at higher temperature and when we deal with
lighter liquors. Consequently, we ought to apply a correction for these conditions, and DESSIN, a
French engineer, proposed his equation.
DESSIN formula reads thus:
c = 0.001 (100 – B) (T – 54)
Where/ c = True Specific Evaporation Rate (Kg Evaptn/h/m2)

B = Brix of Juice Leaving the Effect (See note below)
T = Temperature of the Heating Steam
54 = Temperature equivalent to the vacuum assumed in the last effect. We
can use the actual temperature which is observed
0.001 = Assumed for good normal conditions. More conservative value are used
for design purpose.

Note: Coutanceau (Mauritius) consider that the mean Brix of the juice in the effect should be
used, and is found to give results closer to reality.
To check the performance of our apparatus, we then use the Capacity Equation:
Q = c × A × dt
Where/ Q = Evaporation Obtained

A = Area of Heating Surface
c = f (100 – B) (T – 54)
c = True Specific Evaporation Rate
dt = Temperature Drop between Heating Steam and Boiling Liquid
f = Actual DESSIN Factor Achieved
f
Efficiency = × 100
0.001
Example; c = × 100 ; f =
A × dt A × dt (100 − B) (T − 54)
15,000 Kg / hr
f =
(100 − 16.5 Brix) (115 O − 54 O C )
O
× 444 m 2 × 9.2 O C
= 0.00072
0.00072
Efficiency = × 100
0.001
= 72 % Theorotical Value
Note: Nowadays, we frequently have values of more than 100 % meaning that we are achieving
conditions better than those proposed by DESSIN).
Figs. 3.1 and 3.2 shows the various flows of steam occurring during the process of evaporation
When working on evaporator calculations, we must take certain points into consideration. The
main ones are dealt with below;

Juice Heating
The amount of steam or vapour used is calculated thus:
Heat Given by Steam = Heat Taken by Juice.
Msteam × hfg = Mjuice × Cj × dt
Where/ hfg = Enthalpy of Evaporation (Latent Heat)

cj = Specific Heat of Juice = 4.187 (1 – 0.006 Brix ) KJ/Kg/0C
dt = Temperature Rise Obtained
MJ × CJ × dt
M Steam Used = Assu min g 5 % Heat Losses
h fg × 0.95
Juice Heating in First Effect
The temperature of the juice has first to be raised before evaporation. Consequently, part of the
heating surface can be assumed to be used for juice heating only and has to be deducted when
calculating coefficients. This area is determined thus:
S = 0.1 × Mj × (t1 – t0)
Where/ S = Heating Surface Area in m2

Mj = Mass of Juice in Tones /Hour
t1 = Boiling Point of Juice
t0 = Temperature of Juice Entering the First Effect
Boiling Point Elevation
As a result of both the Brix and the hydrostatic pressure, the juice in the tube boils at a higher
temperature than that of the vapour at the surface. When calculating steam requirement, it need
not be considered, but when calculating performance, it has to be taken into consideration to
obtain the true values (compared to the apparent values). The mean Brix and the mean depth of
the liquid column are taken and BPE obtained from Table A.
Note: We assume optimum level, i.e. 35 %. So depth to be taken 17.5 % of tube length.

Condensate Flash
The flashing of condensates under reduced pressure as it moves from the conditions of one effect
to that of following is used to recover part of the energy of the condensate. The vapour recovered
is equivalent to the change in liquid enthalpy (or sensible heat).
Superheated Steam
This has a very bad heat transfer coefficient (Only 0.8 % of that of saturated steam). A superheat
of 30 – 50 OC is said to be acceptable. However, above 160 OC, it is recommended to cool (or
de-superheat) the steam. In the case of superheat, we must also record the pressure in the main
steam line some distance away from the calandria so as to calculate its enthalpy.
Steam Consumption
The method used to calculate steam demand is sketched in Fig. 3.4. The total evaporation “E” is
found by a Brix balance.
We must calculate “j” the steam required to raise the juice to boiling point.
Figure 3.4: Methods Used to Calculate Steam Demand

j = Juice Heating in First Body Evaporation I = e + h1 + h2
II = e + h1
III = e
IV = e__________________
Total (E) = e + 2h1 + h2
E = % Evaporation Mass of Juice
Sugar Losses
Two main sources of sugar losses are:
a) Inversion Losses
b) Entrainment Losses
a) Inversion Losses
For inversion losses, little can be done apart from keeping watch on the pH of the juice and the
temperature reached. We can keep watch on these losses by calculating the reducing sugar to
sucrose ratio. It should fall slightly from clarified juice to syrup (About 4 % according to some
authors)
It is the transformation of sucrose (non- reducing sugar) into reducing sugars, glucose and
fructose. These inverted sugars are “lost” in the final molasses, as they never crystallize under
any circumstances in the vacuum pans and crystallizers.
Inversion is mainly a function of two factors: pH and Temperature.
In general,
As temperature increases, the rate of inversion increases and vice versa. It is
pronounced at temperatures above 100 0C.
The lower the pH (or the higher the acidity) of juice, the higher the rate of inversion is
and vice versa. At pH lower than 6.5, inversion becomes more and more significant.

b) Entrainment Losses
Entrainment losses are monitored by frequently analyzing the condensates from each effect. A
recommended method of analysis is the RESORCINOL method described in Appendix 2. High
sucrose content of condensate indicates inefficient save-alls or else a too high evaporation rate.
Appendix 1.
A. Losses by Inversion at Normal pH (6.5 to 7.2)
i) After HONIG
Losses at 100 OC = Index of 100

0
120 C = 520 980 = 85 800 = 15
0
110 C = 250 940 = 60 700 = 5
0
105 C = 170 920 = 50 650 = 3
0
100 C = 100 850 = 25 550 = 1
ii) After LASSEN
Vapour Admitted to 1st Effect at 112 0C
Losses at 1st Effect = 0.0020 % Sugar in Juice
Losses at 2nd Effect = 0.015 % Sugar in Juice
Losses at 3rd Effect = 0.010 % Sugar in Juice
Losses at 4th Effect = 0.005 % Sugar in Juice
Total 0.050 % i.e. about 0.006 % Cane
B. Losses by inversion at 100 OC at different pHs (% Inverted Sugar per Hour
pH = 6.0 6.2 6.4 6.6 6.8 7.0

0.21 0.13 0.084 0.053 0.034 0.021

Appendix 2.
CONGDON, J.V. et ZAIATZ, S.S. (1961)
Development of a new auto-analyzer method for continuous measurement of sugar in refinery

waste water in the presence of interfering agents. Sug.Ind. Tech. Proc.20:30 - 37
Reagents
i) Resorcinol, 1 % w/v solution. This must be freshly prepared. It may be kept for a few
days in complete darkness.
ii) Hydrochloric acid, special grade 1.15.
iii) Sucrose, Analar.
Preparation of Standard Curve
Transfer 1.000 g of dry Analar sucrose to a 1-litre volumetric flask. Dissolve with distilled water;
add 0.5 mL of juice preservative and make up to the mark.
Pipette 0, 10, 20, 30, 40 and 50 mL of the above solution into six 100 mL flasks. Make up to the
mark with distilled water. These solutions represent a range of 0 to 500 ppm.
Pipette 4.0 mL of each of standard sucrose solutions into a Quick fit stopper glass tube; add 2.5
mL of hydrochloric acid and 1.5 mL of 1 % w/v resorcinol solution. Stopper and mix
Immerse the stopper tubes in boiling water for three minutes, stirring from time to time.
Allow to cool, mix and read absorbance at 480 mm against blank of water, hydrochloric acid and
resorcinol.
Plot a curve of sucrose concentration (ppm) against absorbance.

Analysis of Factory Waste Water
Waste water is sampled and preserved with mercuric iodide juice preservative at the rate 1.0 mL
per liter. If composted, it should be kept in a refrigerator.
Filter the sample. Pipette 4.0 mL into a Quickfit stoppered tube. Add 2.5 mL of hydrochloric acid
and 1.5 mL of 1 % w/v resorcinol solution. Stopper and mix gently. Immerse the stopper tube in
boiling water for three minutes, stirring from time to time.
Allow to cool, mix and read absorbance at 480 mm against a blank of hydrochloric acid,
resorcinol and water. Calculate sucrose content of the sample from the standard curve. If the
level of sucrose is too high, dilute the filtrate and repeat the procedure above.
3.4 Simple Calculations
To enable factory staff to follow the performance of the evaporator, a simplified method of
approach is discussed in this section.
3.4.1 Heat Transfer

During evaporation, the amount of heat flowing through the heating surface in order to boil the
liquor and cause evaporation is governed by the equation of heat transfer.
Q = U × A × t
Where/ Q = Quantity of Heat Transmitted
U = Heat Transfer Coefficient (kW/m2/0C)
A = Heating Surface (m2)
t = Temperature of Steam Condensing on the Outside of a Tube (ts),

minus Temperature of Juice Boiling in the Tube (tj), Expressed in 0C.
From the heat transfer equation, we have:

 Q 
U =  
 A ∆t 
But Q = (Kg Evaporation/Hour) × (Latent Heat of Vapour)
Then,
 Kg Evaporation / Hour KJ / Kg 1
U =   × × K W / m2 / OC
 A  ∆t 3600
Kg Evaporatio
n / Hour
  Is the evaporation coefficient and is represented by symbol E.
 A 
 E × Latent Heat 
Then, U = 
2 O
 KW / m / C
 ∆t × 3600 
 3600 
From which, E =   × U ∆t Kg / m 2 / h
 Latent Heat 
 3600 
The expression,   almost equal to 1.56 across the evaporator
 Latent Heat 
E = 1.56 U ∆ t Kg / m 2 / h

Sample Calculation
The following figures are from a factory operating a quintuple effect:
Tonnes Cane/Hour = 200
Brix Clear Juice = 13.52
Required Syrup Brix = 66.0
Tonnes Mixed Juice and Filtrate = 253 t/h (126.5 %)
Tonnes Clear Juice = 226 t/h (113 %)
% Evaporation = 100 (66.0 - 13.52) / 66.0
Therefore, % Evaporation = 79.5 %
Tonnes Evaporation per Hour = (79.5 × 226) /100 = 179.7
The Heating Surfaces Area;
1st Effect = 23000 m2

2nd Effect = 2000 m2
3rd Effect = 1000 m2
4th Effect = 740 m2
5th Effect = 740 m2
Vapour Bleeding
i) Primary Juice Heating on V3 = 17,500 Kg/h
ii) Limed Juice Heating on V2 = 10,600 Kg/h
iii) Final Juice Heating on V1 = 3,600 Kg/h
iv) 1st Stage Clear Juice Heating on V1 = 3,800 Kg/h
v) Vapour 1 to Pans Estimated 14 % on Cane = 28,000 Kg/h
Tones Vapour to Juice Heaters and Pans = 63,500 Kg/h

Estimated Steam /Vapour Temperatures
Exhaust (or Process) Steam = 121 0C
Vapour1 = 110 – 112 0C
Vapour2 = 104 – 105 0C
Vapour3 = 90 – 92 0C
Vapour 4 = 75 – 80 0C
Vapour 5 = 54 0C
These estimations are necessary for vapour bleeding calculations
Evaporation /Effect
Effect Kg Evaporation/h Per Effect

5 x 14,120
4 x 14,120
3 x + 17,500 31,620
2 x + 17,500 + 10,600 42,220
1 x + 17,500 + 10,600 + 3,600 + 3,800 + 2,800 77,620
5x + 52,500 + 21,200 + 3,600 + 3,800 + 28,000 179,700
5x + 109,100 179,700
Then, x 14,120
Calculations of “E” and “ t” per Effect
i) E = Mass Evaporated (above)/Heating Surface
ii) U = Generally Accepted Value
iii) t = Derived from Equation, E = 1.56 U . t

Effect H.S.m2 E, (Kg/m2/h) U (KW/m2/0C) t 0C

5 740 19.08 0.70 17.4
4 740 19.08 1.14 10.7
3 1000 31.62 1.56 12.9
2 2000 21.10 1.99 6.7
1 2200 35.28 2.55 8.8
Total 6680 26.90 56.5 0C
Add Vapour 5 54.0 0C

110.5 0C
Add B.P.E 9.8 0C
Exhaust Steam Temperature 120.3 0C
It can be seen that the temperature difference required for working the evaporator is fully
utilized. In other words, the evaporator has no capacity for expansion.
Let us work out the individual temperatures in effects:
Vapour Temperature, 120.3 0C 1st Effect 2nd Effect 3rd Effect 4th Effect 5th Effect
t 0C 8.8 6.7 12.9 10.7 17.4
Juice 111.5 104.3 90.4 77.9 58.0
B.P.E 0.5 1.0 1.8 2.5 4.0
Vapour Temperature 111.0 103.3 88.6 75.4 54.0
Pressure (KPa) 148 113 65 39 15
But assume now that the two last vessels were each 1,000 m2, and then the dt would be as
follows:
t in 5 = (14120/1000) / (1.56/0.70) = 12.9 0C
And, t in 4 = (14120/1000) / (1.56/1.14) = 7.9 0C

The sum of t’s across the evaporator now is:
(12.9 + 7.9 + 12.9 + 6.7 + 8.8) = 49.2 0C
Add vapour 5 = 54.0 0C
103.2 0C
Add BPE = 9.8 0C
Required Exhaust Temperature = 113.0 0C
Actual Exhaust Temperature = 121.0 0C
Difference = 8.0 0C
So, now with enlarged last vessels, there is a “freeboard” of 8.0 0C. This means our evaporator is
capable of larger throughput to absorb this freeboard. Its capacity could then be good for a
crushing rate of approximately:
(121 − 54)  67 
=   × 227 TCH
(113 − 54)  59 
The importance of “ t” determination can be appreciated from the example given above. In
factory practice, it is possible to install thermometers on the vapour pipes between effects and
measures the differences regularly to have a scale of temperatures under operating conditions.
When this has been established, any significant increases in a “ t” will indicate a problem in an
effect. With the use of E = 1.56 t, a solution to the problem may be found. We have:
E
∆t = O
C
1.56 U
The above calculations are meant to be used with an ordinary hand calculator. They can be
useful to check the evaporator in operation quickly. There are now computer programmes which
are used for plant calculating heat balances, etc. In is advised to consult some well known
authors on those subjects.

3.5 Cleaning of Evaporator Tubes
3.5.1 Scaling (Fouling) of Evaporators’ Calandria Tubes

Scales (incrustations) reduce heat transmission from the heating medium (steam/vapors) to the
juice thereby decreasing the evaporation rate and consequently the crushing rate of the factory.
In operation the tubes of a multiple effect become fouled in two ways:
• On the outside, a deposit of oil carried by the steam is formed.
• Inside, scales derived from the juice are deposited.
The exterior deposit of oil on Calandria tubes hardly exists except in the case of factories, which
still use reciprocating steam engines, particularly if these are lubricated by atomization of oil at
the steam entry. The most troublesome deposit is formed by scales inside the tubes. The scales
formed inside the Calandria tubes originate from:
Materials in suspension in juice, poorly separated by clarification. These materials

deposit mainly in the first vessel (vapor cell).
Non- sugars in solution, which become insoluble as the juice becomes

concentrated. These scales are formed mostly in the latter vessels.
The composition of scales formed inside the calandria tubes are:
Calcium salts: phosphate, sulphate, oxalate, and carbonate
Metallic oxides: oxides of magnesium, aluminum and iron
Silica: this forms the greater part of the deposit in the last vessel.
Sulphites: are common in factories using sulphitation process.
The main reason for stopping sugar cane factories every week or fortnight is the scaling of
evaporator tubes. During the concentration of the juice, the various mineral salts present in juice

are brought to their point of crystallization and are deposited in the tubes as scales. This scaling
of the tubes reduces their heat transfer coefficient and lowers the capacity of the evaporator, and
may lead to a reduction in the Brix of the syrup.
The following heat transfer coefficients (in W/m2/0C) are quoted by certain authors:
Steel = 45 – 55 Silicates = 0.08 – 0.175

Brass = 80 – 115 Carbonates = 2.20
Copper = 295 – 395 Scales in General = 2.55
A 50 % reduction in heat transfer has bee noted with a 0.57 mm deposit of calcium carbonate in
the first body and a 0.17 mm deposit of silicate in the last body. The Fig.3.5 illustrates the effect
of scales on heat transfer.
3.5.2 Composition of Scales
Scales are made up of amorphous organic substance, and calcium salts of different composition
for the different bodies.
a) In the first body, we find much organic substance and a high proportion of calcium
phosphate, giving a scale that is fairly easy to remove.
b) Calcium carbonate and sulphate are found in increasing amount in the later bodies.
c) Silica, as calcium silicate, is found mainly in the last two bodies.
d) Oxalates are deposited in the last body. The hardest scales are those of calcium sulphate
and silicate, as well as carbonate and oxalate.

Figure 3.5: Effect of Scale on Heat Transfer
3.5.3 Reducing Scaling

In the past, various means have been used to reduce scaling or else to produce softer scales.
1. Magnesia has been used to partially replace lime, giving managerial slats scales that are
softer than those of calcium.

2. Using anti-scalant, that is organic polymers that interfere in the crystalline structure of scales
to produce “muddy” deposits instead.
3. Using “softeners” on the “hard “juice so as to eliminate scale deposits (all sodium salts are
soluble), and using the “soft “syrup to regenerate the softeners.
All theses methods represents a certain cost and been displaced by chemical cleaning.
3.5.4 Tube Cleaning
The cleaning of tubes is usually done mechanically. More and more, the cost of labour and its
availability is leading sugar factories towards chemical cleaning. New equipment fitted with
plate heaters has to be cleaned chemically.
3.5.4.1 Mechanical Cleaning
Mechanical cleaning of tubes is done by means of brushers or tool heads equipped with cutters
that are rotated (1500 or 3000 rpm) by means of a flexible drive coupled to an electric motor
placed outside the evaporator (See Fig.3.6). Safety regulations recommended the use of low
voltage (24 V) motor. Differential relays (earth leakage relays) may also be used to trip the
motors.
The evaporator is filled with water up to the upper tubular plate and the rotating tool is inserted
in each tube, one after the other. A stop on the flexible cable helps to make sure that the tube has
been brushed through its whole length and prevent the tool from leaving the tube.
The success of the operation depends on the sharpness of the cutters, the stamina of the cleaners
and their experiences in that fairly hard job.

Figure 3.6: Equipments for Cleaning Evaporator Tubes
3.6 Chemical Cleaning
This technique has been well developed in Australia and is general use. In South Africa, it is
much used and in Mauritius, its use is spreading despite the cost. The method used differs from
one factory to another, but the principle remains the same. Treat the scales with a hot solution of
caustic soda, wash with water, and follow with an acid wash. Some factories still perform a
mechanical claiming every two or three weeks.
Methods Used
a) Spraying the tubes, by means of a special nozzle placed at the top of the evaporator cell
(Robert), with a concentrated caustic soda solution (20 – 30 %), heated by circulating
through a juice heater.
b) Circulating a hot dilute solution of caustic soda (5 – 10 %), to which a wetting agent has
been added (0.2 %) through the tubes. The solution is passed through a juice heater to

maintain its temperature (between 70 – 90 0C). This method is specially suited for falling
film and plate evaporators.
c) Boiling a caustic solution (as above) in the calandria of the evaporator by adding steam in
the calandria itself. Suitable for Roberts.
Observations:
The following have been observed during studies on chemical cleaning in South Africa;
a) The length of treatment varies from 3 – 6 hours depending on the particular case.
b) Boiling gives better results than spraying
c) The addition of a wetting agent (ex. Busperse 47) allows the use of dilute solutions (a 6
% solutions being as effective as a 24 % solutions without wetting agent).
d) Plate heating surfaces require an acid cleaning with phosphoric acid.
Note that periodically sludge must be drained from the caustic soda tank and the concentration
readjusted.
Precautions:
It is essential to follow safety measures prescribed by regulations. The following arte general
rules:
1. For mechanical cleaning, avoid any loose clothing. A bathing trunk (slip) is the most
suitable one.
2. To avoid electric shocks, use 24 V lighting as well as 24 V motors. Otherwise, use

differential relays (earth leakage relays) to protect the motors.
3. Never use open flames.
4. For chemical claiming, use appropriate gloves, goggles and overalls for caustic solutions.
Provide for eye–wash and showers.

3.7 Soda Boiling and Mechanical Scrapping Procedures
Soda boiling is carried out in evaporators so as to soften the scales formed in the calandria tubes.
Then the softened scales are scrapped using scrapping machines, which are driven by electric
motors. Procedures for soda boiling and scrapping activities are outlined hereunder.
1. Prepare 20 - 30 % concentration caustic soda solution and circulate the soda solution in
the caustic soda solution storage tank for at least four hours before soda boiling starts.
2. All evaporator vessels should be shut down properly and made ready for soda boiling.
3. Make sure that the caustic soda drain valves of all vessels are closed and leakage free.
4. Close the caustic soda solution drain 900 mm diameter hand valve to ditch.
5. Make sure that the injection water to evaporator's station main isolation hand valve is
also closed. Check also the hot water (body and spray) and pond water provision valves
to all vessels are closed.
6. Make all valve arrangements from the caustic soda solution storage tank to the
evaporators and the soda solution return line from the evaporators to the storage tank
for caustic soda solution pumping: all the caustic soda solution return line valves should
be closed.
7. Start the caustic soda solution pumps following centrifugal pump start up procedures.
8. Fill each evaporator vessels with caustic soda solution turn wise using the caustic soda
solution inlet valves up to the vessels’ 1st sight glasses.
9. After all vessels are properly filled with caustic soda solution, stop the caustic soda
solution pumps following centrifugal pump stopping procedures.
10. Check that the non-condensable gases withdrawal pipe valves of all evaporators are
open.
11. Open soda vent valves of all evaporators.
12. Open the exhaust steam admission valves to vapor cell and 1st – effect evaporators.
Also open the 150 mm exhaust steam admission valves to 2nd -, 3rd – and 4th –effect
evaporators.
13. Check for proper soda boiling in all vessels after some time.
14. Soda boiling should be carried out for about 8 to 9 hours by maintaining the following
parameters:

• Keep the soda solution levels in all vessels to the 1st sight glass using pond
water as make up.
• The soda boiling temperatures in all vessels should be maintained to 110 0C
or greater by keeping the exhaust steam pressure to 100 - 125 KPa. .
15. After effective soda boiling is carried out for the first 7 to 8 hours, concentrate the soda
solution in all vessels for the last one hour by not using pond water as make up.
16. Then close the exhaust steam admission valves to all vessels. Open all the caustic soda
solution return line valves to the caustic soda solution storage tank and close all valves
from the storage tank to evaporators including the caustic soda discharge valve from the
storage tank. The soda drain 900 mm diameter hand valve to ditch should be closed
before returning the soda solution.
17. Return the concentrated caustic soda solution to the storage tank after
opening the soda drain valves of all evaporators. After the solution is totally recovered
to the storage tank, close the soda solution returning valves.
18. Then cool down all vessels to a temperature lower than 30 0C and rinse them with cold
water.
19. Keep showering (rinsing) all vessels to ditch by cold water until risings are clear.
20. Close the cold-water supply valves for rinsing and open the side manholes of all
evaporators.
21. Close the 900 mm diameter soda drain hand valve to ditch and refill all evaporators
with cold-water turn wise up to the rims of their calandria tubes for scrapping.
22. Now, start scrapping of all vessels with the already made ready scrapping machines.
23. After scrapping activity is over, open the 900mm diameter soda drain hand valve to
ditch and drain the contents of all vessels to ditch by opening their soda drain valves.
24. Then open the bottom cone manholes of the evaporators and shower the calandria
tubes by cold water until rinsing are clear.
25. Use extension light at the bottom calandria tube plate to inspect the calandria tubes
from above, i.e., being on the top calandria tube plate.
26. If there are insufficiently scrapped or unscrapped calandria tubes, repeat instructions
22, 24 and 25 until all calandria tubes become shiny.
Now, the evaporators are ready for testing.

3.8 Procedures for Testing Evaporators
1. Close the condensate withdrawal valves of all vessels to ditch. Check also that condensate
withdrawal valves to the flash tanks are closed. Confirm that exhaust steam/vapors
admission valves to all vessels are closed. Close the non-condensable gases withdrawal
valves and equalizer connection valves of all evaporators.
2. Check that no spray/ body water leaks to the calandria tubes.
3. Open the steam chest pond water provision valves of the vessels to press the calandria tubes
for leakage(s).
4. While pressing check the calandria tubes from the bottom as well as from the top tube plates
for leakage(s) using extension lights. Check also the non-condensable gases withdrawal lines
(both inside and outside the vessels) and equalizer connection pipes of all evaporators for
leakage(s).
5. If there are leaky calandria tubes, mark them and drain the pond water contained in the steam
chests after opening the condensate withdrawal valves to ditch and closing the pond water
provision valves to the steam chests. Mark also if there are leakages on the non-condensable
gases withdrawal lines and equalizer pipes.
6. Replace, expand or in rare cases plug the leaky calandria tubes(s). Take also corrective
measure if there are leaking non-condensable gases withdrawal lines and equalizer pipes.
7. Repeat procedures 3 - 6 until all the calandria tubes, non-condensable gases withdrawal
lines and equalizer pipes are leakage free.
8. Now, close the side as well as the bottom manholes of all vessels. Close the 900 mm
diameter soda drain valve to ditch.
9. Fill all evaporators with cold-water turn wise up to their top manholes using the soda drain
and inlet valves.
10. Check all the sight glasses and manholes (bottom, side and top manholes) for leakages and
take corrective measures if there are any leakages.
11. Drain the cold-water (injection water) in the evaporator vessels up to their second sight
glasses by opening their soda drain valves turn wise after opening the 900 mm diameter
soda drain hand valve to ditch.
12. Check the soda drain valves of all vessels are closed and close the 900 mm diameter soda
drain valve to ditch. Now, the evaporator vessels are ready for start up.

1. What are the main sugar losses occurring in evaporators?
2. What are the means we can used to reduce scaling on an evaporator station?
3. Describe briefly chemical cleaning of evaporator tubes.
4. How can we reduce scaling on the evaporator station?
5. Give the DESSIN formula for specific evaporation coefficient? What was its purpose?
How does it illustrate the variation of that coefficient from the first to the last
evaporator body?
6. What is “Specific Evaporation Coefficient”? What is the difference between its

apparent and real value?
7. Describe briefly how we remove scales in evaporators?
8. What are the possible sugar losses on an evaporator station? What are the factors that
are these losses?
9. What are the problems caused by the scaling of the evaporator heating elements? How
do we remedy these scaling problems?
10. Explain all the important steps to test the performance of evaporators.
11. Discuss the procedure of soda boiling and mechanical scraping procedure.

12.
The diagram above illustrates an evaporator station performing bleeding for the heaters and pan
station. The Brix of clear juice, syrup and the juice leaving the first body are indicated.
Please calculate the following values:
(a) The overall percentage evaporation of the station.
(b) The amount of steam (S) required by the first body if the juice arrives at boiling point.
(c) The amount of vapour (P) bled to the pan station.
(d) The mass of the water (W) used to operate the rain type condenser if it cooling water
ratio is 24:1.
(e) The capacity of the syrup pump (In liters/minute) if the bulk density of syrup is 1.35
Kg/L.

13.
The above is the uintuple of a 350 TCH factory. Bleeding is performed to supply the vacuum
pans and three heaters as indicated. The Brix of the clear juice and that of syrup are given.
Assume the vapour for the pans to be 13 % cane.
Calculate:
(a) The overall evaporation % juice obtained.
(b) The amount of steam (S) required to drive the station if clear juice is already at the
boiling point.
(c) The steam economy we would achieve if we feed the pan station with VP2 instead of
VP1.
(d) The mass of the water (W) used y the old condenser if the cooling water ratio is 35:1.
(e) The capacity of the single condensate pumps removing the condensate of the last three
cells, assuming the mean density of the condensate to be 0.95 Kg/L.

14.
In the above quadruple evaporator we bleed a vapour for the pans and two heaters. The Brix of
the clear juice; that of the juice leaving the first body and that of the syrup are given.
Calculate:
(a) The overall evaporation percent juice achieved on the station.
(b) The mass of steam (S) required by the first body if the clear juice arrives at boiling point.
(c) The mass of VP1 (P) used by the vacuum pan.
(d) The mass of the water (W) used by the rain type condenser if the cooling water ratio is
25:1.
(f) The capacity in (m3/h) of the syrup pumps if the density of the syrup is 1.30.

15.
The evaporator station above receives clear juice at the boiling point at the rate of 150 tonnes/hr.
Syrup is produced at a brix of 72o.
Calculate:
(a) The percentage evaporation achieved and the total mass of the water evaporated.
(b) The mass of steam required per hour (S) to operate the evaporators station.
(c) Calculate the saving in the steam consumption if we modify the set up to bleed steam
from bodies I, II, III for the three heaters.
(d) Calculate the capacity of the syrup pump if we assume the bulk density of syrup to be
1.35 Kg/L.
(e) Calculate the mass of water (W) required by the rain type condenser, if the cooling water
ratio is 25:1 .

16.
A factory of 150 tonnes juice/hour capacity has the above evaporation station: a Kestner driving
a conventional quadruple. Juice arrives at a boiling point at a Brix of 15o, and leaves the Kestner
at 20o Brix. The syrup leaves at 72o Brix.
Calculate:
(a) The percentage evaporation obtained in the Kestner and in quadruple.
(b) The amount of steam required (S) by the station.
(c) The mean steam consumption (P) of the pan station.
(d) The amount of cold water (W) required by the rain condenser which has a cooling water
ratio of 22:1.
(e) The capacity of the syrup pump in (m3/h) if the syrup has a density of 1.30.

17.
Assume in the above that the juice arrives in the first body at boiling point and bleeding is done
for the heaters only.
Calculate:
(a) The amount of steam (S) required by the evaporator?
(b) The condenser is of the rain type and has a cooling water ratio of 25:1. Calculate “W” the
mass of the water required by the condenser in tones/hours.
(c) Estimate the capacity of the syrup pump (in liters per minute) if the density of the syrup
is 1.20 Kg/L.
(d) Calculate the evaporation coefficient of the last evaporator body (Kg/m2/hr) if its surface
area is 500 m2.

Unit Four
Cooling Ponds and Cooling Towers
Unit Objectives
Describe cooling circuit of evaporation station, cooling system and why need for
cooling system required for evaporation station?
Differentiate the different types of condensers and discuss their advantages and
disadvantages in cooling system.
Recognize the spray ponds practical considerations.
Describe the cooling tower working principle.
Compare and contrast the two types of cooling systems.
4.1 Introduction
In most sugar factories, warm water coming out of condensers must be cooled and used again,
due to the fact that there is not enough water available.
Cooling is performed in most cases in larger spray ponds (See Fig.4.1 and 4.2) or in cooling
towers (See Fig. 4.3). Both systems depend on the relative humidity of the surrounding air as
well as air movement to cool down the warm water.
Evaporation of the warm water is the most important factor in the cooling process and depends
on the ability of the air to absorb moisture, when it I not yet saturated.
The lowest temperature to which, in theory, the water can be cooled is that of the wet bulb
temperature. This temperature is never reached in practice, but with reference to it, the efficiency
of the cooling can be calculated.
Let t2 = Temperature of Warm Water Entering the Cooling System

t1 = Temperature of Cooled Water Leaving the System
t0 = Wet Bulb Temperature
E = Efficiency
100 % ( t 2 − t1 )
Then : E % =
( t2 − t0 )
Efficiency obtained varies from 50 – 70 % poor efficiency would be below 60 %.
When a wet–bulb thermometer is not available, a temperature difference of 2 0C between the

cooled water and the ambient air is considered satisfactory.
Figure 4.1: Cooling Circuit

4.2 Spray Ponds: Practical Considerations
i) A spray pond should be long and narrow in order to improve efficiency at the center.
ii) All lateral branches (supporting the nozzle risers) should be provided with easily
accessible quick acting flushing valves at their ends.
iii) Nozzles should be preferably arranged in star patterns.
iv) Fresh water make-up controlled by a ball cock.
v) A floating platform should be available for repairs. A small outboard motor is a

necessity.
vi) Pump suction must be protected from the accumulated and blockage by trash bagasse
and other flotsam. Easy access for regular cleaning of screens must be provided.
Spraying surface of the cooling pond is about 1.33 m2/tonne water/hour. It must be situated in a
well exposed location to benefit from prevailing winds.
Figure 4.2: Spray Pond

4.3 Cooling Towers
Cooling towers operate on the same principles of evaporative cooling. Water is dispersed in a
chamber by various means (trays, honeycombs, and mesh) and air is blown into the chamber.
Cooling towers are very compact but often less efficient than cooling ponds. A serious problem
can be fouling by proliferation of large.
Cooling towers are very compact but often less efficient than cooling ponds. A serious problem
can be fouling by proliferation of algae. Algicide treatment may be necessary.
Figure 4.3: Cooling Tower

Figure 4.4: Condenser Water Cycle

1. Why do we need a cooling system for the evaporator station?
2. What are the two main types of cooling systems?
3. Sketch a cooling pond or a cooling tower and explain how we achieve the cooling of
condenser water.
4. What factors affects the efficiency of a water cooling system (Cooling pond and
cooling tower)?
5. Make a simple drawing of a cooling tower to explain how it works.

Unit Five
Condensers
Unit Objectives
Differentiate the different types of condensers of evaporators.

Describe briefly the working principle and construction of different types of
condensers.
Explain the advantages and disadvantages of the different types of condensor.
Discuss about approach temperature and describe its result on the basis of
performance of condenser.
5.1 Introduction
A condenser is equipment, which creates vacuum. 1 Kg of water at 100 0C occupies 1 liter.

When the water is converted to steam (vapor), its volume increases to 1673 liters. Therefore,
condensation of 1 Kg of vapors causes a volume contraction of 1673:1. It is this volume
contraction when carried out in a sealed space that gives rise to vacuum. Air, which got through
leaks and non- condensable gases in the vapor don’t condense and would accumulate in the
condenser. Such incondensable gases are removed by an extraction pump (vacuum pump) or
water stream jet ejectors.
About 40 - 50 tons of water is required to condense 1 ton of vapor. The barometric

leg of a condenser deeps into a sealing well that is always filled with water.
Previously the barometric counter current condenser type condensers had been
installed and in operation at FSF. But recently all the barometric condensers (except
that of filtration plant) have been replaced by multi-jet condensers.
Unlike all the rest types of condensers, multi-jet condensers do not require vacuum
pumps to extract the air and incondensable gases.

The multi-jet condenser for the multiple effect evaporators at FSF is installed on the
4th – effect vapor pipe after the secondary catchall.
The vacuum in the evaporator is created by condensing the vapour coming from the last effect in
a condenser.
A condenser is a vertical steel vessel of welded construction in which vapours are condensate by
close contact with cold water [See Figs. 5.1].
Fig.5.1: (a) Dry Air Co-Current Condenser Fig.5.1: (b) Dry Air Counter-Current Condenser
5.2 Types of Condenser
In modern sugar mill the most common type of condenser in use is the barometric counter
current condenser. Other types of condensers are;
1. Barometric counter current condenser

2. The Dry-Air Co-Current Condenser
3. The Wet-Air Co-Current Multi-Jet Condenser
4. The Rain–Type Counter Current–Condenser

Fig.5.1(c): Short Counter-Current Condenser Fig.5.1(d): Barometric Counter Current Condenser
Fig.5.1(e): Wet Air Parallel-Current Condenser Fig.22(f): Weber-Type Condenser

(Jet Condenser)
Figure 5.1: Different Types of Condensers

Figure 5.2(a): Temperatures in Condenser
Figure 5.2(b): Variation of Water Consumption with (t2 – t1)

5.2.1 Co-Current Condensers

In co-current condenser, vapour and air travel in the same direction. In the dry-air co-current
condensers, vapour and water enter at the top of the condenser, while air is removed towards the
base by means of an air pump. In the wet-air co-current condenser, water is admitted in a system
of nozzles at the top, below the vapour inlet. The water jets spray large amounts of water at the
inlet to the tail pipe to maintain adequate extraction of air with the water, there is no necessary
for an air extraction pump.
Disadvantage of Co-Current Condensers
i) The amount of cold water used is much greater than in counter-current condensers due
mainly to the absence of baffles and poor contact between vapour and water and the
necessity of large volumes of water in multi-jet condensers to ensure the removal of air.
Larger water than for counter-current condensers are required.
ii) In a dry –air co-current condenser the air has not been cooled sufficiently and its specific
volume is greater than for air removed from a counter –current condenser. This requires
of a large volumetric pumps.
iii) Their approach temperature is not less than 6 0C or more.
iv) They are much less controllable than counter–current condensers
5.2.2 Counter-Current Condensers

In the counter-current condensers, water is admitted at the uppermost part of the condenser and
cascades down baffles. Vapour enters at the base of the condensers and air travels in the opposite
direction to the water. Air is extracted at the highest point of the condensers. Condensed vapour
and water go down the barometric leg to a seal well and a cooling systems.
Advantages:
i) Air is removed at the coldest part of the condensers and the size of air pumps is
reduced.

ii) It is possible to control the vacuum (pressure/temperature) in the condenser and pan
or evaporator by increasing or reducing the amount of water in to the condenser. This
allows automatic control and a minimum use of cold water.
5.2.2.1 Dimension of Counter-Current Condensers
The following figures are guidelines that could be useful in evaluating condensers with baffles.
5.2.2.2 Height of Condenser
The height determines the time of contact between water particles and vapour. It is normally
between 3.0 to 4.5 m.
5.2.2.3 Cross-Section Area
The more important dimension is the cross-section of the chamber. It can be found by calculation
to be 0.28 m2/ton vapour/h but values of between 0.15 – 0.20 m2/ton vapour/h have been
reported.
5.2.2.4 Cooling Water Ratio
This is the mass of water required to condense one kilo of vapour. The amount of water required
can be calculated by equating the heat lost by the condensing vapour with the heat gained by the
water. If we take:
M = Mass of Water/Kg Vapour to be Condensed
t1 = Cold Water Temperature - 0C
t2 = Tail Water Temperature - 0C
tv = Temperature of Vapour - 0C
LHV = Latent Heat Condensing Vapour Taken at 13.5 KPa (26” Vacuum)
( = 2,380 KJ/Kg)

5.2.2.5 By Heat Balance
Heat Gained by Cooling Water = Heat Lost by Vapour
4.19 M (t2 – t 1) = LHv + (tv – t2) 4.19
 2380 
=  + (tV − t 2 ) 4.19
 4.19 
568 + (tV − t2 )
M =
(t 2 − t1 )
573
=
(t 2 − t1 )
When, (tv – t2) = 5 0C
5.2.2.5 Approach Temperature
The function (tv – t2) is called the “Approach Temperature“. The closer “t2”, tail water
temperature approaches “tv“ (temperature of vapour), the more efficient the condenser is for
condensers with baffles (tv – t2) = 5 0C is considered satisfactory. [See Fig 5.2(b)].
5.2.2.6 Incondensable Gases and Air

In a counter-current condenser, the air upwards through the curtains of cold water and gets
cooled all the way to the air extraction pipe. Apart from gases, air also enters condensers and
must be removed at the same time. The air comes from the injection water in which air is
dissolved and from leakages into the pans, evaporator bodies or condensers themselves, air
pumps used for extraction were formerly of the reciprocating type. Nowadays, liquid ring
centrifugal pumps are used (See Fig.5.3)

Principles of Operation
Figure 5.3: Liquid Ring Vacuum Pump
5.2.2.8 Volume of Air to Be Extracted

The volume of air to be extracted is given on average to be 0.15 m3/Kg of vapour.
Height of Barometric Leg
This height taken between the bases of the cone to the level of water in the seal well should be 11
to 11.3 m.

Seale Well
The volume of water in the seal well should e such as to fill the tail pipe bottom of the condenser
cone, while the lower end of the pipe should be about half a meter below the water under this
condition. To allow for the free exit of the waters, at least a distance of one diameter of the pipe
should be allowed below the end of the tail pipe.
5.2.3 Rain Type Counter–Current Condenser

This condenser was conceived and developed in Australia to wards 1968 and has replaced all
former designs in that country. Much work has been done on development of the rain type
condenser both in Australia and South Africa. (Fig .5.4)
Brief Description
i) There is a perforated tray at the tope of the condenser to produce the rain effect.
ii) The tray is surrounded by a weir with holes around it to irrigation the wall.
iii) A gap his provided around the weir and between the tray and condenser body. This
gap is designed to accept a large water overflow.
iv) The perforated tray has a circular sold plate at its centre, on which the cold water
from the water inlet pipe impinges.
v) Gassing pipes (or chimneys) are provided through the tray to allow incondensable
gases to travel freely to the air exit pipe.
vi) Vapour to be condensed enters at the base of the condenser either as a single entry or
by a circumferential entry system
vii) the angle of the bottom cone is steeper than for conventional condensers, from 600 to
700 with the horizontal

Figure 5.4: Rain Type Condenser

Performance and Advantages
i) In well- designed single tray condensers an approach temperature of 3 0C is common.
ii) They are easier to build and can made in a conventional factory workshop.
iii) Construction is simpler as only one perforated tray is required.
iv) Costs are reduced due to less material being used than for ordinary condensers.
v) The design of Rain–type condenser are now computerized programs and are available
from many sugar Research institute in Australia, South Africa and Mauritius.

1. Sketch the various types of condensers found on the evaporator station?
2. What is Approach Temperature of a condenser? How does it affect its efficiency?
3. Sketch the various types of condensers that we can find in the sugar industry?
4. Sketch a water jet condenser? What are its main advantage and its disadvantages?
5. What is the “Cooling Water Ratio” of a condenser? How do we calculate it?
6. What do you understand by “Approach Temperature” in a condenser? How does it

influence the efficiency of the condenser?
7. Sketch a simple diagram of a “Rain Type Condenser” and explain the reason for its
good performance.
8. How do we evaluate the efficiency of a cooling system of condenser water? What are the
factors that can influence that efficiency?
9. Explain briefly the purpose of cooling system for condenser water and the way of
functions. Sketch simply one cooling system.
10.

Evaporation of Cane Juice

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Module 5

Unit Two: Factors Covering Evaporation Operation............................................................ 38

Sugar Engineering and Manufacturing Training Team 2

2.4.1 Boiling Point Elevation (B.P.E) ................................................................................ 41

Unit Three: Checking Performance and Steam Requirements ............................................ 47

Unit Four: Cooling Ponds and Cooling Towers ....................................................................... 81

Unit Five: Condensers ................................................................................................................ 87

Sugar Engineering and Manufacturing Training Team 3

5.2.2.2 Height of Condenser ...................................................................................... 92

Sugar Engineering and Manufacturing Training Team 4

Sugar Engineering and Manufacturing Training Team 5

Explain the objectives and stages of evaporation of clarified juice.

i) In the evaporator to a Brix of 70 0B, to give syrup.

Sugar Engineering and Manufacturing Training Team 6

Composition of Clarified Juice

Clear juice consists of:

Water (85 %) – Source - From cane and imbibition water

1.2 Amount of Water Evaporator

For instance, let:

E = Mass of Water to be Evaporated – % Clear Juice

Then, by Brix balance:

Sugar Engineering and Manufacturing Training Team 7

100 (68 − 13.5)

1.3 Arrangement of Evaporators

I. Single effect evaporators

II. Single effect evaporators with vapor recompression

III. Multiple effect evaporators

1.3.1 Single Effect Evaporators

The required capacity is so small.

The vapor is so contaminated that it cannot be reused.

Sugar Engineering and Manufacturing Training Team 8

1.3.2 Single Effect Evaporators with Vapor Recompression

a) The Turbo-Compressor / Motor-Compressor

Figure 1.1: Turbo-/Motor-Compressor

Sugar Engineering and Manufacturing Training Team 9

Figure 1.2: Thermo-Compressor

1.3.3 Multiple Effect Evaporation

3. In any apparatus in which steam or vapour is condensed, it is necessary to

Sugar Engineering and Manufacturing Training Team 10

1.3.3.2 Arrangement of a Multiple Effect Evaporator

An arrangement of four vessels in series is known as a Quadruple Effect Evaporator, whereas a

Sugar Engineering and Manufacturing Training Team 11

1.3.3.3 Juice Evaporation under Vacuum in the Multiple Effect Evaporators

Inversion - is favored at high temperatures.

Caramalization (Coloration) – is the charring of sugar manifested at

Such low temperature operation increases the evaporation capacity of an evaporator.

Sugar Engineering and Manufacturing Training Team 12

The Vapor Cell

The heating medium is exhaust steam at 125 0C.

First Effect Evaporator

Also heated by exhaust steam.

Furnishes vapors at 112 – 115 0C which are used:

Partly for secondary juice heating.

And the balance for second – effect evaporator as a heat medium.

Sugar Engineering and Manufacturing Training Team 13

Second Effect Evaporator

Generates vapors at 100 – 101 0C.

Is heated by vapors from the 1st – effect.