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Fundamentals of Fundamentals of
Cooling WaterCooling Water TreatmentTreatment
chembond
Cooling WaterCooling Water TreatmentTreatment
Chembond Ashland Water Technologies Limited
Ashland
Cooling
Producing Steam DM water
Use of Water In The Industries:
Producing Steam DM water
Process
Drinking
Process fluids are required to be cooled at
various stages
Water is the Cheapest Coolant Available
Process Fluids are cooled in heat exchanger,
cooler, condenser
Water Used for Cooling Process Fluids:
cooler, condenser
Reactor / DG set Jacket Cooling
Heat is transferred from Hot Process Fluid to
Cold water and it becomes hot
Hot water is cooled in Cooling Tower and cold
water re-used for cooling
Types of Cooling Systems
Once through Cooling System
In this system:
cooling water passes through heat exchangers only once before
it is discharged.
T is very negligible.
it is discharged.
Large volumes of water are required in this type of systems.
Closed Recirculating Cooling SystemIn this system:
Water is reused continuously
water loss is very low.
There is no evaporation loss.
Example :
Chilled water system, Engine Jacket Cooling System.Chilled water system, Engine Jacket Cooling System.
Cooled water
Closed circuit water tank
Process
Warmed water
Air/ C.W. Cooled
water
Secondary cooler
chiller
Open Recirculating Cooling System
Water is re-circulated again and again
Heat from hot water removed with the help of cooling tower.
Permits extensive reuse of water and reduces the volume of make up
water required.
Due to evaporation, concentration of dissolved salts takes place.
H2O
MAKE UPWATER HEAT EXCHANGER
Air, Dust, Bacteria
HOT WATERProcess fluid out
Blowdown
COLD WATER
OPEN RECIRCULATINGCOOLING SYSTEM
TYPES OF COOLING TOWERSTYPES OF COOLING TOWERS
Cooling Towers
Natural Draft Mechanical Draft
Design of Cooling tower is such
that cold air of the bottom of towerForced Draft Induced Draft
that cold air of the bottom of tower
push the warmer air out from top.Forced Draft
Air is pushed in
the tower with a
fan at the side.
Induced Draft
Air is pulled from
cooling tower by
a fan at the top
Counter Flow Cross Flow
CROSS FLOW COOLING TOWER
COUNETR FLOW COOLING TOWER
MAKE UP WATER
HEAT EXCHANGER
Air, Dust, Bacteria
H2O
HOT WATER
Process fluid out
Process fluid in
OPEN RECIRCULATING COOLING WATER SYSTEM
BlowdownProblems in
HEAT TRANSFER Equipments
COLD WATER
CORROSION SCALE/ DEPOSITS MICROBIOLOGICAL FOULING
Normal Terminology used in Open Recirculating Cooling Water System
Normal Terminology used in Open Recirculating Cooling Water System
1. Hold up Capacity of the system : (V)
Hold up capacity of the system = water contained in basin
+
sump of cooling tower
+
water contained in piping and equipments.
2. Blowdown : (B)
Due to evaporation, concentration of Impurities / dissolved solids takes place.
Part of water is removed from system as a blowdown to control concentration of
impurities / dissolved solids in water.
3. Drift / Windage loss : (D)
Some water droplets escape alongwith air and water vapours. A usual drift loss in
conventional cooling towers is in the range of about 0.05 -0.2% of the recirculation
rate.
Contd.4. Evaporation Losses : (E) Water lost to the atmosphere in the cooling process is evaporation.
The rate of evaporation depends upon the temperature difference across tower.
For each 5.6 C Delta T across tower, evaporation rate is 1% of Circulation Rate.
5. System Losses : (S)Circulating water is lost in the plant through
pumps, valves or leakages in plant etc.
6. Concentration Cycle : (C) 10.012.0
%
M
A
K
E
U
P
W
A
T
E
R
(
b
a
s
e
d
o
n
c
i
r
c
u
l
a
t
i
o
n
)
Makeup Water requirements vs. cycles of
concentration
6. Concentration Cycle : (C)
Mg or Silica in cooling water
C =
Mg or Silica in make up water
Blowdown E
& Windage losses = -----
C-1
where E = Evaporation rate.
C = Cycle of Concentration.
00
1 2 3 4 65 7 8 9 10
2.0
4.0
6.0
8.0
10.0
50F DROP
25F DROP
10F DROP
%
M
A
K
E
U
P
W
A
T
E
R
(
b
a
s
e
d
o
n
c
i
r
c
u
l
a
t
i
o
n
)
CYCLE OF CONCENTRATION
Contd.7. Make-up Water : (M)This is the water which is to be added to replace the water lost by evaporation, blow down, drift and
leakage.
M = E + B + D + S
8. Holding Time Index :Time required to reduce the concentration
of any constituent in cooling water to half.
0.693 x (Hold up capacity )
HTI =
Blowdown
Each programme has maximum allowable HTI beyond which chemical lose its effective.
9. Approach : Indicate efficiency of cooling tower.
Lesser is approach better is cooling tower efficiency.
Approach = Supply of C. W. temperature -
Wet bulb temperature
Water impurities and its effectWater impurities and its effect
Impurity Effect
Total Hardness Scale formation
(Calcium + Magnesium)
M-Alkalinity Corrosion - Low Alk.
Scale/Deposition. -
High Alk.
Chlorides Corrosion / SCC of SSChlorides Corrosion / SCC of SS
Suspended Solids Deposition
Sulphate Corrosion / Scale
formation
SiO2 Scale
Organic Matters Fouling
Iron and Manganese Deposition
Micro-Organisms Fouling / Corrosion
Ammonia Nitrifying bacteria / fouling
The Corrosion ProcessThe Corrosion ProcessOccurs due to the presence of local cells with anodic and cathodic sites on the metalmediated by electron transfer through an electrolyte.In an aerated, neutral solution, the overall reactions are :Anodic Reaction
Fe Fe++ + 2e- (1)
Cathodic ReactionO2 + 2H2O + 4e- 4OH- (2)
Fe++ + 2OH- Fe(OH)2
Overall Reaction
Fe(OH)3
Fe2O3 (Rust)
Fe (OH)2 ANODE
Fe (OH)3
ELECTRON FLOW CATHOD
Fe ++ H2OOH-
Water / Electrolyte
O2
O2
Pitting:Most destructive
Caused by Localized Deposition & Differential Oxygen cells.
Most Common types of Corrosion inMost Common types of Corrosion inCooling Water SystemsCooling Water Systems
Most Common types of Corrosion inMost Common types of Corrosion inCooling Water SystemsCooling Water Systems
General Type:Uniform in nature.
Less dangerous
Galvanised Corrosion :
It is caused due to the presence of Dissimilar metals.
Stress Corrosion Cracking :
It occurs in S.S. or Copper Alloys and is caused by high chloride and high
Contd.Contd.Contd.Contd.
It occurs in S.S. or Copper Alloys and is caused by high chloride and high
temperature & pressure.
Contd.Contd.Contd.Contd.Crevice Corrosion:
Tube - Tube sheet joint
Under deposit or tubercles
Threaded joints
Erosion Corrosion:Erosion Corrosion:
Normally restricted to copper based
alloys.
High water velocity, High suspended solids, Turbulence accelerates.
Chloride in water.
Dissolved oxygen in water.
High temperature
Corrosion is Accelerated by:Corrosion is Accelerated by:Corrosion is Accelerated by:Corrosion is Accelerated by:
Suspended solids (under deposit corrosion)
Bacteria (Sulphate reducing & nitrifying )
Contd.Contd.Contd.Contd.
Low pH of water.
Corrosion Leads To -
Leakage of Exchangers
Un-schedule Shut Down
Why Corrosion Control ?Why Corrosion Control ?Why Corrosion Control ?Why Corrosion Control ?
Loss of Production
High cost of Equipment Replacement
Selection of proper metal of construction.
Applying protective coating.
Galvanic corrosion control
Corrosion Control :Corrosion Control :Corrosion Control :Corrosion Control :
Using sacrificial anodes (cathodic protection)
Galvanic corrosion control
Corrosion Control of water box in power
plant.
Chemical Treatment
Most economical and widely used.
The commonly used corrosion inhibitors in an Open Recirculating System are :
i. Chromates : Anodic type.
ii. Orthophosphate : Anodic type
iii. Molybdate : Anodic type
iv. Silicate : Anodic type
v. Zinc : Cathodic Type
Corrosion lnhibitor:Corrosion lnhibitor:
v. Zinc : Cathodic Type
vi. Polyphosphate and
glassypolyphosphates : Cathodic Type
Selection criteria
Water analysis
Metallurgy of equipments
process parameters
Environmental restrictions
Scale : Scale : Scale : Scale :
Dense, adherent and hard material composed most commonly of
Calcium & Magnesium salts.
Precipitate at high temperature and get deposited on the heat transfer surfaces.
FollowingFollowing areare somesome typestypes ofof scalescale::
CaCO3, Ca3 (PO4)2
MgCO3
Silicates - Not very common
280
240
200
160
120
pH = 7.5
ppm TDS = 500
ppm Alk. = 100
p
p
m
S
o
l
u
b
l
e
C
a
C
O
3
Chembond
Silicates - Not very common
Magnesium Silicate - very hard
60 80 100 120 140 160 180
120
80
40
0p
p
m
S
o
l
u
b
l
e
C
a
C
O
TEMPERATURE OF
Water
Exchanger tube Scale (Insulation)
Scale formation is accelerated by :
Scale Formation leads to Low Heat Transfer.Scale Formation leads to Low Heat Transfer.
Scale formation is accelerated by :
High temperatures.
High hardness of water.
High pH of cooling water.
High M-Alkalinity
Ca(HCO3)2 CaCO3 + CO2 + H2O
Water Indices Water Indices Water Indices Water Indices
Water indices make it possible to predict the tendency of
water either to precipitate or to dissolve Calcium Carbonate.
Types of IndicesTypes of IndicesLangalier Index :
Chembond
Langalier Index :
L. I. = pHa - pHs
Where, pHa = Actual pH of cooling water and pHs = Saturation pH is the function
of the Total solids, Temperature, Calcium & Total alkalinity
When L.I. is positive, it denotes scale forming; and When L.I. is negative, it
denotes corrosion tendency of water.
5
Stability Index [Stability Index [Ryznar Stability IndexRyznar Stability Index] ] Stability Index [Stability Index [Ryznar Stability IndexRyznar Stability Index] ] R. S. I. = 2pHs - pHa
Water Tendency :Index Tendency of
waterLSI RSI
2.0 8 Undersaturated, very aggressive
WHEN WATER IS TREATED WITH CHEMICALS, THESE
INDICES DO NOT HAVE INDICES DO NOT HAVE ANY IMPORTANCE.
Scale Formation Leads To -
Reduction in Water Flow
Poor Heat Transfer
Why Scale Control ?Why Scale Control ?Why Scale Control ?Why Scale Control ?
Poor Heat Transfer
Reduction in Plant Load
Chemical Cleaning
Unschedule Shut Down
Shorten the Life of Equipments.
Scale ControlScale ControlScale ControlScale Control
Removal of Ca, Mg by Ion Exchange - Cost is High.
Addition of Sulfuric Acid
Ca(HCO3)2 + H2SO4 CaSO4 +2CO2 + 2H2O
CaSO4 Higher solubility than CaCO3
Contd..Contd..Contd..Contd..Formation of Scale can be controlled by addition of :
Polyphosphate (SHMP):
Less stable at high temperature
Revert to Orthophosphate as P-O-P bond is weak.
Reverted Orthophosphate causes deposition.
O O
Zinc glassy polyphsophate:
Zinc glassy polyphosphate has high X value and no definite structure.
More stable than SHMP.
Negligible orthophosphate formation at high temperature.
NaO P O P ONa
ONa ONa x
Organophosphonates :
HEDP: ( Hydroxy Ethylidene Diphosphonate)
P-C-P bond is strong.
High stability with respect to pH and
temperature.
Keeps Calcium & Magnesium in solution.
Scale ControlScale ControlScale ControlScale Control
H
O HCH O
OH P C P OH
OH OH OH
Synthetic Polymers:
Low molecular weight polyacrylates copolymers/Ter-polymer.
- Crystal Modification.
- Keeps precipitated Calcium salts in water.
PBTC :
Phosphono butane-tricarboxylic acid more effective than HEDP
OH OH OH
Deposition / Fouling Deposition / Fouling Deposition / Fouling Deposition / Fouling
Causes :1. Corrosion Products.
2. Suspended solids in cooling water.
3. Dead algae in cooling tower.
4. Slime produced by microorganisms in water.
5. Low velocity.
6. Process Contaminants - Ammonia, Oil, Hydrocarbons.
Control Agents/ Methods:1. Dispersants based on low molecular weight polymer.
2. Bio-Dispersant.
3. Providing proper velocity to water (3-5ft/sec)
4. Providing Side Sand Filter.
Capacity 3-5% of Circulation rate.
5. Backflushing / Air rumbling arrangement.
Deposition causes under deposit corrosion.Deposition causes under deposit corrosion.
Microbiological FoulingMicrobiological FoulingMicrobiological FoulingMicrobiological Fouling
The major classes of microorganisms which are associated with recirculating Cooling
System are :
Bacteria - In Cooling Water.
Algae - On cooling towers structure / distributing deck.
Fungus - On wooden structures of cooling towers.
Delignification of wood. (phto27.8)Delignification of wood. (phto27.8)
-Temperature of cooling water is ideal for bacterial growth.
- Sun light is accelerate growth of Algae
-Abundant nutrients are available.
Different Types of Bacteria :
Sulphate Reducing Bacteria(Anarobic)- pitting type corrosion.
SRB attack
Brown deposits on top
Under brown deposit black deposit.
Under black deposit, silvery shining surface.
Nitrifying Bacteria - pH drop.
Iron Bacteria - Iron fouling.
Slime Bacteria
Why Microbiological Control ?
Microbiological Growth Leads To -
Fouling of Exchangers .
Less Heat Transfer.
Under Deposit Corrosion. Under Deposit Corrosion.
Reduction in plant load.
Microbiological Induced Corrosion (MIC)
-Pitting.
Un-schedule Shutdown.
Decrease in Cooling Tower Efficiency.
Damage of Wooden Parts of Cooling Tower.
Microbiological controlMicrobiological controlMicrobiological controlMicrobiological control OXIDISING BIOCIDE
Chlorination.
Microbiological action of Chlorine :
Cl2 + H2O - HOCL + HCL
HOCL is the active species and dissociates as pH
Increases HOBr and OBr- are more toxic to bacteria compared to HOCl, OCl-
Reaction Of Chlorine ActivatorReaction Of Chlorine Activator
5 6 7 8 9 104
HOCl HOBr
0
20
40
60
80
100MBr + HOCl HOBr + MCl
pHIncreases
HOCl + OH- = OCl- + H2OFormation of HOCl, OCl- depends on pH of water
HOCL is mainly responsible for killing of bacteria OCl- has very low action on bacteria As pH increases above 8, OCl- formation takes place, chlorination
effectiveness decreasesHeavy chlorination bring down the cooling water pHDosage : 0.2 - 0.5ppm for 3 - 4hrs. Per day
Chlorine does not reach all the parts of the Cooling System.
At pH = 8.5HOCl / OCl- = 10 / 90HOBr / OBr- = 60 / 40
Bromine
Chlorine Demand:
Amount of chlorine consumed by following impurities before free
chlorine appears in Cooling Water.
Organic Matters
Ammonia
Dead Algae, Slime
Other Oxidizable Substances
Bromine-Effective at high pH
-Effective in ammonia contaminates system compare to chlorine
Ozone
-pH is sensitive-Produced at site by electrolysis
Chlorine Dioxide - Very effective in presence ofAmmonia
Organic
In-Situ Generation of Chlorine Dioxide
NaClO2 + 1/2 Cl2 - ClO2 + NaCl
(Chlorine Dioxide )
Chlorine Dioxide:
Chlorine Activator Chlorine Activator ( Chlorite based)( Chlorite based)Chlorine Activator Chlorine Activator ( Chlorite based)( Chlorite based)
Chlorinated
water inlet
Electric Control Box
2Auto Switch
Flow meter
Dosing Pump
13
Emergency Shutdown Switch
ClO2
Loss of Cl2 Switch.
GENEROX
Flow Indicator
C
l
O
2
G
e
n
e
r
a
t
o
r
Pressure gauge
Filter
BSP
connection
BSP
connection
Advantages :
Do not react with Ammonia, Organics. Hence
more effective than chlorine in contaminated
system.
Reduces chlorine consumption.
Reduces the chloride builtup due to chlorination.
Effective in high pH range also.
Negligible delignification of wood.
Precursor Source 48"H x 42"W x 17"D
Reaction Of Chlorine ActivatorReaction Of Chlorine ActivatorReaction Of Chlorine ActivatorReaction Of Chlorine Activator
HOCl HOBr
40
60
80
100MBr + HOCl HOBr + MCl
HOBr and OBr- are more toxic to bacteria compared to HOCl, OCl-At pH = 8.5HOCl / OCl- = 10 / 90HOBr / OBr- = 60 / 40
5 6 7 8 9 1040
20
40
pH
Non-Oxidising BiocidesNon-Oxidising Biocides Methylene Bis Thiocynate ( MBT )
Very effective against SRB. It Hydrolizes above7.5 pH
Quaternary Ammonium Compounds(QAC)
Tendency to foam. Ineffective in highly oil or organic fouled systems. Effective at high pH.
Glutaraldehyde ( ALD )
Effective over wide pH range.
Isothiozoline ( THIO )
pH InsensitivepH Insensitive
Dichlorophene
effective over wide pH range.
DBNP EFFECTIVENESS
Bacteria Fungi Algae E = Excellent G = Good
MBT E S S S = SlightQAC E G EALD E E ETHIO E G EDBNP E S S
NonNon--Oxidising BiocidesOxidising BiocidesNonNon--Oxidising BiocidesOxidising Biocides
Dosage : Once in 8 - 10 days
Bacteria develop resistance to non-oxidizing biocide.
Use of more than 1 non-oxidizing biocide is preferred to avoid immunity. Use of more than 1 non-oxidizing biocide is preferred to avoid immunity.
Covering the top of the distributing deck of the cooling tower will
eliminate the sunlight resulting in a reduction in the formation of algae.
Bacterial Growth & its Control with Biocides Bacterial Growth & its Control with Biocides Bacterial Growth & its Control with Biocides Bacterial Growth & its Control with Biocides T
o
t
a
l
B
a
c
t
e
r
i
a
l
c
o
u
n
t
/
m
l
.
Acceptable
Range
Biocide
Dosing
T
o
t
a
l
B
a
c
t
e
r
i
a
l
c
o
u
n
t
/
m
l
.
Time ( in days )
7 14 21
Bio Bio Dispersant Dispersant Bio Bio Dispersant Dispersant What is Bio-Dispersant ?
Bio-Dispersants are non-anionic type surface active agents alongwith slime
solublizing solvents.
Functions :
When Bio-Dispersant is added alongwith oxidizing or non-oxidising biocide, it :
Increase the effectiveness of biocide.
Removes slime.
Releases bacteria arrested under slime deposits so that biocides can kill free
bacteria.
Addition of Bio - Dispersant increases turbidity, increases total bacterial
count & generates foam.
Monitoring methodsMonitoring methodsMonitoring methodsMonitoring methodsCorrosion : Corrosion Meter & Coupon Holding Rack.
COUPON
EBONITE
FLOW
OUTLET
W ATER IN LET
TEE
18
1 1/2
22.3 x WCORRO SIO N R ATE ( mpy) = -------------
d x a x tW - Loss in we ight in m g.d - Density of m etal in gm/cm 3a - Area of coupon in in2t - T im e in days
d for CS = 7.85Copper = 8.9Brass = 8.17
Coupon holding rack22.3 x w
Corrosion rate = ------------
d x a x t
Mpy = mills per year
w = Loss in weight in mgs
d = Density of test coupon in gms/cm3
a = area in Inch2
t = time in days
Scale and deposition : Test Heat Exchanger / Deposit Monitor
t - T im e in days
FLOW METER
BALL VALVE
FLOW CONTROL VALVE
SENSOR ( t 2 )
ACRYLIC TUBE
JUNCTION BOX
TEMP. IND ICATOR
SENSOR ( t 1)
Mains Restart Heater
Actual
Set
Temp. Set
AmmeterAmmeter
Thermostat
ELECTRIC ITY SUPPLY 230V, 50HZ, 1Ph
HEATER
OUTLET
INLET
1/2 TUBE
RUBBER TUBE
HOT CONDENSATE IN
Length about 2 - 3 feet
COLD CONDENSATE OUT
COOLING WATER IN
COOLING WATER OUT
Test Heat ExchangerDeposit Monitor
t = time in days
Contd..Contd..Deposition : Temperature & Pressure monitors of exchangers.
Bacteria analysis : T.B.C. S.R.B. N.R.B.
Flow Meter
Outlet
DPG
Inlet
PG
Pressure drop tube
S Y M B O L S
= Ball Valve= Flow Control Valve
D.P.G. = Differential Pressure Gauge
= Strainer ValveBiofouling Monitor
Online Measures for Overall improvementOnline Measures for Overall improvementin performance of Treatment Programme.in performance of Treatment Programme.Online Measures for Overall improvementOnline Measures for Overall improvementin performance of Treatment Programme.in performance of Treatment Programme.
Side Sand Filter
About 5% of circulating water is to be passed through a side sand filter so as to
reduce suspended solids.
Air/Nitrogen Bumping and Back Washing :
Air bumping and back washing is very necessary for shell side water heat exchangers
and compressor jackets.
Annual Water Flushing of Heat Exchangers and cleaning sump, basin.
C ooling W a ter In To D ra in
P rocess O utP rocess In
C oo ling W a ter O u t
COOLING WATER ON SHELL S IDE
Cooling W ater In
Air / N 2 In jectorpoint
Process O ut
Process In
Coo ling W ater O ut
COOLING WATER ON SHELL S IDE
Alternate AirIn jector P oint
Biocleaning ( On-Line )Biocleaning ( On-Line ) Addition of Biodispersant ( 5-10ppm)
Maximum Circulation
Addition of NonOxidizing Biocide
Blowdown after 8-12hrs. Circulations.
Turbidity of water increases. Turbidity of water increases.
Target Dosing:Target Dosing: Dosing of Biodispersant / Dispersant at inlet cooling
water of fouled Exchanger.
PRECLEANING: - Removal rust etc and makingsurface active for passivation.
Addition of oil dispersant ( For New System )
Reduction of pH ot 5.5 - 6.0
Addition of Dispersant
Monitoring of Iron Content
Blowdown after Fe remains constant.
Precleaning / Passivation Precleaning / Passivation Precleaning / Passivation Precleaning / Passivation
Bare metal
With proper passivationBlowdown after Fe remains constant.
Maximum circulation.
PASSIVATION : - Film formation on cleaned surface.
Control of pH
Addition of Corrosion Inhibitor (PO4= 60-70ppm )
Circulation without blowdown for 24-48hrs.
New Exchanger (CS) - Individual precleaning/ passivation essential.
Without proper passivation
Performance LimitsPerformance LimitsPerformance LimitsPerformance LimitsGood treatment programs should ensure the following results for the parameters :
1.Corrosion rate of