2. A cooling tower is a heat rejection device
which extracts waste heat to the atmosphere
through the cooling of a water stream to a
lower temperature. The type of heat rejection
in a cooling tower is termed “evaporative”.
3. TYPES OF COOLING TOWERS
Cooling Towers
Natural Draft Mechanical Draft
Design of Cooling tower
is such 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 from the side.
Induced Draft
Air is pulled from
cooling tower by
a fan at the top
Counter Flow Cross Flow
4. 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 increased.
•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.
5. 4. Evaporation Losses : (E)
• 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.
5. System Losses : (S)
Circulating water is lost in the plant through
pumps, valves or leakage’s in plant etc.
6. Concentration Cycle : (C)
COC represents the accumulation of dissolved minerals in the
recirculating water.
C.O.C. = T- Hardness in cooling water / T- Hardness in make
up water
.
7. The Corrosion Process
Corrosion is an electrochemical process where metals are converted to their most stable form
(oxides). The process requires an anode, a cathode, and an ionic conduction path through
an electrolyte such as water
In an aerated, neutral solution, the overall reactions are :
Anodic Reaction
Fe Fe++ + 2e- (1)
4OH- (2)
Fe++ + 2OH- Fe(OH)2
Cathodic Reaction
O2 + 2H2O +4e-
Overall Reaction
Fe(OH)3
Fe2O3 (Rust)
Fe (OH)2
ANODE
Fe (OH)3
ELECTRON FLOW CATHODE
Fe ++
2H O
OH-
Water / Electrolyte
O2
O2
8. 1.FORMATION OF PROTECTION LAYER:-
•Ortho phosphate anodic inhibition via promotion of a Gamma-Iron
Oxide due to formation of a Ca3(PO4) film.
•This protective layer is hematite based.
•The simple basis is controlling calcium phosphate precipitation to
allow an inhibitor film to be formed without forming calcium
phosphate sludge. The program depends on using the proper
phosphate stabilizing polymer. Without calcium at levels > 100 ppm (as
CaCO3), the film formation is weak.
PHOSPHATE CORROSION TECHNOLOGY
AND ITS MECHANISM
9. Cathode
Zn (OH)2
Cathodic inhibition
Anode Fe2O3
Fe2(PO4)22H2O
Anodic inhibition
PHOSPHATE CORROSION TECHNOLOGY
AND ITS MECHANISM
2.REDUCING CORROSION CAUSES ELEMENT:-
•Mechanism-These programs also provide corrosion protection because
phosphate will react with ferrous ions (Fe+2) produced at anodic sites to form a
protective barrier, while [Ca3(PO4)2] precipitates in the local alkaline environment
at cathodic sites. Zinc is a common corrosion protection supplement, as zinc ions
will also precipitate (as zinc hydroxide [Zn(OH)2] at cathodic sites) thus
enhancing the barrier film.
10. SCALE FORMATION
• Scaling is the precipitation of hard and adherent salts of water soluble
constituents, like calcium and magnesium, on the metal surface.
• Precipitate on the heat transfer surface and becomes hard.
11. Exchanger tube Scale (Insulation)
Scale formation is accelerated by :
• High temperatures.(But not always true)
• High hardness of water.
• High pH of cooling water.
• High M-Alkalinity
SCALE FORMATION LEADS TO LOW HEAT TRANSFER
Ca(HCO3)2 CaCO3 + CO2 + H2O
Water
12. Scale Formation Leads To -
• Reduction in Water Flow
• Reduces Heat Transfer
Efficiency
• Reduction in Plant Load
• Chemical Cleaning
• Unschedule Shut Down
• Shorten the Life of
Equipments .
WHY SCALE CONTROL ?
13. SCALE 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
• Addition of Scale Inhibitor / Dispersant
14. •Crystal modification:
Prevents regular crystal growth and make them small, softer and irregular
.Dispersion:
Change charge of particulates so they repel each other and do not have an
opportunity to agglomerate.
Charge Reinforcement
Of Polymers
SCALE CONTROL BY ADDITION CHEMICALS
Scale Inhibition Mechanism:
+ +Fe(OH)2
+
Fe(OH)2
+
Fe(OH)3
o
+
+
+ Fe(OH)2
+
Fe(OH)3
o
Fe(OH)2
+
15. ORGANOPHOSPHONATES: THRESHOLD INHIBITION.
Chemical Name Content Action
Scale Inhibitor 1.PBTC(Phosphono
butane-tricarboxylic)
2.HEDP(Hydroxy
Ethylidene
Diphosphonate),
3.PSO(phosphino
succinc
oligomer), etc
Prevents scale formation by
keeping Calcium and Magnesium
in solution.Also PO4 has high
stabilty in high temperature and
Ph,so its effect sustain so long.
Dispersant Polyacrylates,
Polyamide, Maleic,
Sulphonic acid based
copolymers/Ter-
polymer/Tetra-
polymer
1.Crystal Modification.
2. Keeps precipitated Calcium salts in
water.
3.Zinc, Iron, Phosphate stabilizers.
4.Keep suspended solids in suspension
16. MICROBIOLOGICAL FOULING
refers to fouling that occurs when algae, bacteria and fungus grows out of
control in cooling tower systems.
Micro biological fouling refers to fouling that occurs when algae,
bacteria and fungus grows out of control in cooling tower systems.
Factors Influencing Microbiological Growth:-
Temperature of cooling water is ideal for bacterial growth.
- Sun light is accelerate growth of Algae
- Abundant nutrients are available.
17. Microbiological Growth Leads To -
• Fouling of Exchangers .
• Less Heat Transfer.
• Under Deposit Corrosion.
• Reduction in plant load.
• Microbiological Induced Corrosion
• (MIC) -Pitting.(Due to SRB,IRB)
• Unschedule Shutdown.
• Decrease in Cooling Tower Efficiency.
• Damage of Wooden Parts of Cooling
• Tower.
WHY MICROBIOLOGICAL CONTROL ?
18. Sulphate Reducing Bacteria (Anarobic)- pitting type corrosion.
•Brown deposits on top
• Under brown deposit black deposit.
• Under black deposit, silvery shining surface.
Nitrifying Bacteria – Grows in presence of Ammonia / NO2 , leads to pH
drop.
NH3 HNO3, NO2 NO3
Iron Bacteria - Iron fouling.
Oxidize ferrous to ferric iron which precipitate as voluminous brown
slime.
Slime Bacteria
SRB attack
DIFFERENT TYPES OF BACTERIA
SO4 H2S + Fe FeS
SRB
19. MICROBIOLOGICAL CONTROL
Biocides are the chemicals that kill and control the growth of algae,bacteria
And fungus in open cooling tower systems.
BIOCIDE
1.Oxidizing Biocide… 2.Non Oxidising Biocide…
Strip or Burn through the cell wall, Preventing the bacteria from absorbing
Thus exploding the bacteria . food or reproducing
Mechanishm- Mechanishm-
Live Microbe + Untreated
cl2Dead microbe + electron(s) Surface
Biocide loaded
coating that repels
or kills
20. MICROBIOLOGICAL CONTROL
TYPE OF BIOCIDE CHEMICAL
NAME
CHARECTERISTICS
OXIDISING
BIOCIDE
Chlorine 1.Cl2 + H2O - HOCL + HCL
(Also increases pH as HOCL
dissociates)
2. HOCL is mainly responsible for
killing of bacteria.
3. As pH increases above 8, OCl-
formation takes place, effectiveness
decreases.
4. Heavy chlorination bring down the
cooling water pH
NON OXIDISING
BIOCIDE
Methylene Bis
Thiocynate ( MBT
)
Very effective against SRB and
Nitrifying bacteria. It Hydrolizes
above7.5 pH
21. MICROBIOLOGICAL CONTROL
TYPE OF BIOCIDE CHEMICAL
NAME
CHARECTERISTICS
NON OXIDISING
BIOCIDE
•Quaternary
Ammonium
Compounds(QAC)
•Tendency to foam. Ineffective in highly
oil or organic fouled systems. Effective
at high pH.
NON OXIDISING
BIOCIDE
Methylene Bis
Thiocynate ( MBT
)
Very effective against SRB and
Nitrifying bacteria. It Hydrolizes
above7.5 pH
NON OXIDISING
BIOCIDE
Glutaraldehyde (
ALD )
Effective over wide pH range. De-
activated by ammonia / amines in
water.
NON OXIDISING
BIOCIDE
Carbamate
(CARB)
Effective upto pH 8.
Effective against SRB.
22. MICROBIOLOGICAL CONTROL
BIODISPERSANTS
Bio-Dispersants are non-ionic type surface active agents along with slime
solubilizing solvents. It helps to keep cooling systems clean or helps to clean up
fouled systems by dispersing the extracellular material created by bacteria , algae
and fungus.
Role of Biodispersants:-
When Bio-Dispersant is added along with oxidizing or non-oxidizing biocide,
it :
•Increase the effectiveness of biocide (Modify the surface tension of water and
reduce adherence of foulant materials on the surface).
•Removes slime.
•Releases bacteria arrested under slime deposits so that biocides can kill free
bacteria
23. CHLORINE DEMAND
Amount of chlorine consumed by following impurities before free chlorine
appears in Cooling Water.
•Organic Matters
• Ammonia - forms Chloroamines
• Dead Algae, Slime
• Other Oxidizable Substances
•FRC-
24. ORP
ORP stands for Oxidation –Reduction Potential.It measures the
ability of a substance to get electrons or lose electrons from
another substance.That substance can be an oxidizing biocide and
it can be a reducing micro-organism.
In case of disinfection by Chlorine:-
Oxidation-Live microbe + Cl2 Dead microbe + electron(s)
Reduction-Cl2 + 2e 2Cl-
Information Regarding ORP:-
1.Unit is mv and range is -2000 to 2000.
2.The higher the mv reading ,the more powerful is water to oxidize
and disinfect microbes.
3.A drop in ORP indicates an increase in chlorine demand caused
by reducing agents or contaminants(organic matters,microbes)
entering the water
26. COOLING WATER PARAMETERS TO BE
ANALYSED, MONITORED & ITS EFFECTS:
Parameter Low High
PH
(7-8)
Increases scaling, Increases
Microbiological growth, Decreases
effectiveness of Chlorine
a.Increase Acid dosing
b.Give Blowdown
Hardness
(500-700ppm)
increases scaling
a.Give blowdown.
b. Increase antiscalant / dispersant
dose.
Increases corrosion rate
a.Decrease Acid dosing
b.Give Blowdown and Take
Make up water
27. COOLING WATER PARAMETERS TO BE
ANALYSED, MONITORED & ITS EFFECTS:
Parameter Low High
Turbidity
(<15NTU)
Increases deposition / fouling
a.Check for any leak like oil,
Organics. Plug the leaky tubes etc.
b. Operate side sand stream filter
efficiently.
c. Give blowdown.
Zn(1-1.5ppm)
/Meta Phosphate
(2-3ppm)
Deposition,Economical loss
a.Give blowdown.
b. Decrease dosing rate
Damages cathodic layer,
increases corossion
Rate.
a.Increase
dose.
28. Parameter Low High
Inorganic/Ortho phosphate
(7-10 ppm)
Increases corrosion
a. Increase corrosion
Inhibitor dose. a. Check & control corrosion
inhibitor dose.
Delta phosphate
(Filter – Unfiltered)
Phosphate deposition
• Increase dispersant dosage
• > 1.5 ppm
No Film formation /
high pitting corrosion
a. Decrease dispersant dosage
< 0.5 ppm
Organic phosphate
(1.5-3 ppm)
Very high Org. phosphate
increases Corrosion rate.
a. Reduce antiscalant dosage.
Increases deposition /
scaling tendency
a. Increase antiscalant dose
Increases scaling / deposition
COOLING WATER PARAMETERS TO BE
ANALYSED, MONITORED & ITS EFFECTS:
29. Parameter Low High
Chloride
(<250ppm)
Accelerate corrosion rate.
Leads to SSC in SS metallurgy.
a. Increase blowdown.
b. Check make up water chloride.
c. Check for excessive chlorination.
Iron as Fe+++
(0.1-0.3 ppm)
If Iron pickup in cooling water is high
(>0.5ppm) indicates more corrosion in
the system.
a. Increase blowdown.
b. increase corrosion inhibitor dose.
c. Check make up Iron.
d. Dose specific Iron dispersant
e. Check iron bacteria.
Conductivity & TDS Deposition / Scaling tendency
increase.
a. Give blowdown to control in
desired range
COOLING WATER PARAMETERS TO BE
ANALYSED, MONITORED & ITS EFFECTS:
30. Parameter Low High
COD
Increases biological load / biofouling.
Increase Chlorine demand.
a. Check for any source of Organic leak &
attend
a. Increase Biodispersant, chlorination
b. Add chlorine activator( Chlorine Dioxide
c. Give blowdown.
Oil & Grease • Increases biological load / biofouling.
a. Check for any leak.Isolate leaky
exchanger
b. Plug the leaky tube.
c. Add Oil Dispersant
d. Give heavy blowdown.
e. Give extra dose of biocide
f. Increase chlorination.
COOLING WATER PARAMETERS TO BE
ANALYSED, MONITORED & ITS EFFECTS:
31. PARAMETER UNIT
ACT UCT
MIN. MAX. MIN. MAX.
pH - 7 8 7 8
ALKALINITY PPM 70 120 70 120
FRC PPM 0.2 <0.5 0.2 <0.5
CONDUCTIVITY PPM NA 3200 NA 3200
TURBIDITY PPM <5 <15 <5 <15
SILICA PPM NA 125 NA 125
TOTAL HARDNESS PPM 500 700 500 700
CALCIUM HARDNESS PPM 35 450 35 450
MAGNESIUM HARDNESS PPM 150 250 150 250
CHLORIDE PPM NA 250 NA 250
AMMONIA PPM NA <50 NA <20
TOTAL PHOSPHATE PPM 10 14 10 14
DELTA PHOSPHATE PPM 0.5 1.5 0.5 1.5
O- PHOSPHATE PPM 0.7 10 0.7 10
ZINC PPM 1 1.5 1 1.5
ALUMINIUM PPM NILL NILL NILL NILL
IRON PPM 0.6 <1.0 0.6 <1.0
DELTA IRON PPM 0.1 0.3 0.1 0.3
TVC
<100000
organism/ml
SRB Count/100 ml 20 20
COOLING WATER PARAMETERS TO BE MAINTAINED
