COOLING WATER TREATMENT

COOLING SYSTEMS

Once through Cooling System

• cooling water passes through heat exchangers
only once before it discharged.
• Large volumes of water are required in this type
of systems.
 COOLING SYSTEMS

Closed cooling System

• Water is reused continuously
• water loss is very low.
• There is also no evaporation loss.
• Example : Chilled water system, Engine Jacket
Cooling System.
 COOLING SYSTEMS

Open cooling System

• Water is recirculated again and again
• Heat from hot water removed with the help of
cooling tower.
• Due to evaporation, concentration of
dissolved salts takes place.
 Types of Cooling Towers

Cooling Towers

Natural Draft Mechanical Draft

Design of Cooling tower is
such that cold air of the Forced Draft Induced Draft
bottom of tower push the
Air is pulled in
warmer air out from top. Air is pushed in
cooling tower by
the tower with a
a fan at the top
fan at the side.

Counter Flow Cross Flow

Cooling Water
 Normal Terminology used in cooling water system

• Hold up Capacity of the system

Hold up capacity of the system = water contained in basin
+
sump of cooling tower
+
water contained in piping and equipments.
• Blowdown
Due to evaporation, concentration of Impurities / dissolved solids
takes place. Part of water is removed from system as a blow down
to control concentration of impurities / dissolved solids in water.
 Normal Terminology used in cooling water system

• Drift / Windage loss

Some water droplets escape along with air and water vapors. A
usual drift loss in conventional cooling towers is in the range
of about 0.05 -0.2% of the recirculation rate.
• Evaporation Losses
Water lost to the atmosphere in the cooling process is
evaporation. The rate of evaporation depends upon the
temperature differential. 1% of the circulation rate evaporates
for each 5.6 Delta T.
 Normal Terminology used in cooling water
system
• System Losses
Circulating water is lost in the plant through pumps, valves or
leakage’s in plant etc.
• Concentration Cycle
How many times cooling water is concentrated as compared
to make up water.
• Make-up Water
This water which is to be added to replace the water lost by
evaporation, blow down, drift and leakage.
M=E+B+D
 Normal Terminology used in cooling water
system
• Approach
Indicate efficiency of cooling tower. Lesser is approach better is
cooling tower efficiency.
Approach = Supply C.W. temperature - Wet bulb temperature

• 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 + Windage loss
 Problems in cooling water treatment

Main Problems in cooling water treatment are

• Corrosion
• Scaling
• Microbiological Fouling
 Corrosion Process

Loss of metal from metal surface by chemical or

electrochemical reaction with its surrounding.
Anodic Reaction
Fe Fe++ + 2e-
Cathodic Reaction
O2 + 2H2O + 4e- 4OH-
Overall Reaction

Fe++ + 2OH- Fe(OH)2 Fe(OH)3 Fe2O3(Rust)

 Factors Influencing Corrosion
• Soft water is more corrosive (LSI– negative)
• Hard water occurs deposition which results under
deposit corrosion
• Chloride accelerates corrosion and stress
corrosion cracking
• High pH increase corrosion
• Suspended solids promotes under deposit
corrosion
• Bacterial Growth promotes MIC
 Why Corrosion Control ?

Corrosion leads to
• Leakage of Exchangers
• Unscheduled Shut Down
• Loss of Production
• High Equipment Replacement Cost.
 Types of Corrosion

General Corrosion
• Less dangerous & uniform in nature .
Pitting Corrosion
• Most destructive
• Caused by Localized Deposition & Differential Oxygen cells.
• Tuberculation leads to pitting
Microbiological Induced Corrosion
• Leads to pitting
• Cause by mainly sulphate reducing bacteria
 Types of Corrosion
Galvanized Corrosion
• It is caused due to the presence of dissimilar metals.
• When two dissimilar metals are in contact, more active metal
corrode first.
Stress Corrosion Cracking
• It occurs mostly in S.S. and some time in Copper Alloys and is
caused by high chloride, high temperature & pressure.
Erosion Corrosion
• High water velocity, High suspended solids, Turbulence
accelerates
 Classification of Corrosion Inhibitors

Anodic Inhibitor Cathodic inhibitor

• Chromate • Poly, Meta/Pyro Phosphate
• Nitrite • Zinc
• Orthophosphate • Controlled deposition of
• Molybdate CaCO3
 Corrosion Inhibitor

Selection criteria
• Make up water parameter
• Cycle of concentration
• MOC of exchangers
• Water velocity in exchangers
• process parameters – Temperatures
• Environmental restrictions
 Scaling

Dense, adherent and hard material composed

most commonly of Calcium/ phosphate salts
precipitate on the heat transfer surface and
becomes hard.
 Why Scale Control ?

Scale formation leads to

• Reduction in Water Flow
• Poor Heat Transfer
• Reduction in Plant Load
• Unscheduled Shut Down
• Shorten the Life of Equipments.
 Factors Influencing Scaling

Scale formation is accelerated by :

• High temperatures
• High hardness of water
• High pH of cooling water
• High M-Alkalinity
 Types of scale

• CaCO3, Ca3 (PO4)2, Iron Phosphate – Most Common

• MgCO3 – Less Common

• Silicates – Less Common

• Magnesium Silicate - Very hard

 Scale Control

• Removal of Ca, Mg by Ion Exchange - Cost is

High.

• Addition of Sulfuric Acid

Ca(HCO3)2 + H2SO4 CaSO4 +2CO2 + 2H2O
CaSO4 has Higher solubility than CaCO3

• Addition of Scale Inhibitor / Dispersant

 Microbiological Fouling
• Temperature of cooling water is ideal for bacterial
growth. Sun light is accelerate growth of Algae.
• 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.
 Why Microbiological Control ?

Microbiological Growth Leads To

• Fouling of Exchangers .
• Less Heat Transfer.
• Under Deposit Corrosion.
• Reduction in plant load.
• Microbiological Induced Corrosion (MIC)
• Unscheduled Shutdown.
• Decrease in Cooling Tower Efficiency.
• Damage of Wooden Parts of Cooling Tower.
 Different Types of Bacteria

Anaerobic Bacteria
Takes place in absence of oxygen
Sulphate Reducing Bacteria
• Pitting type corrosion.
• Brown deposits on top. Under brown deposit black
deposit. Under black deposit, silvery shining surface.
SO4 H2S + Fe FeS
 Different Types of Bacteria
Aerobic Bacteria
Takes place in presence of Oxygen
• Nitrifying Bacteria
Grows in presence of Ammonia / NO2 , leads to pH drop.
NH3 HNO3 , NO2 NO3
• Iron Bacteria
Oxidize ferrous to ferric iron which precipitate as brown slime.
• Slime Bacteria
Produces sticky gelatinous mass which traps the suspended
solids.
 Microbiological Control

• Oxidizing biocide
Chlorine
Microbiological action of Chlorine :
Cl2 + H2O HOCl + HCl
HOCL is the active species and dissociates as pH increases
Bromine
Effective at high pH and in ammonia contaminated system
compare to chlorine.
HOCl + NaBr HOBr + NaCl
 Microbiological Control
Ozone
• pH in sensitive
• Produced at site by electrolysis
Chlorine Dioxide
NaClO2 + 1/2 Cl2 ClO2 + NaCl

• It can be generated without Chlorine gas also.

• It is a true gas hence does not dissociates like Cl2 and it is 2.5 times
stronger than Cl2
• Do not react with Ammonia, Organics. Hence more effective than
chlorine in contaminated system.
 Microbiological Control
• Non oxidizing biocides

Methylene Bis Thiocynate ( MBT )

• Very effective against SRB and Nitrifying bacteria.
• It Hydrolyzes above7.5 pH
Quaternary Ammonium Compounds(QAC)
• Tendency to foam. Ineffective in highly oil or organic fouled
systems. Effective at high pH.
DBNPA
• Works quickly and De-activated by strong reducing agent.
 Microbiological Control
• Bio-Dispersant
Bio-Dispersants are non-ionic type surface active agents along
with slime solublizing compound.
Functions :
• When Bio-Dispersant is added along with 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.
THANK
YOU