Reduction of Water Demand in
Cooling Towers
Dr Paul Hirst
Agenda
•Cooling Tower Basics
– Water Losses
– Cycles of Concentration
•Limitations on Cycles
– Chemical
– Hydraulic
– Dynamic
Cooling Tower System
Evaporation
Heat
Load
Makeup
Blowdown
Cooling
Tower
Recirculating
Pump
Cooling Tower Water Losses
•Evaporative Water Losses
•Non Evaporative Water losses
Evaporative Water Losses
•Water that is evaporated from the cooling tower
(does not carry solids)
E (m3/hr)
Where:
=
RR x Cp x T x Ef / 556
RR is the Recirculation Rate in m3/hr
Cp is the Specific Heat Capacity (1 kcal/kg/oC)
T is the Temperature Change in oC
Ef is evaporation factor and depends on
Wet bulb temperature, Relative Humidity
556 = kcal/kg to evaporate water
Non Evaporative Water Losses
•Drift - The water lost from the tower as
entrained droplets in the exhaust air
•Windage - The water lost from the tower as a
result of wind action
•Blowdown - The water deliberately purged
from the system to control water chemistry
Total Non Evaporative Water Losses
•All water that is lost from the cooling system (carries
solids)
– Controlled Losses
• Blowdown
– Uncontrolled losses
• drift & windage
• leaks
• side stream filter backwashes
• sample coolers
Makeup Water
•The water added to replace water lost from
the cooling system:
> Evaporation
> Total Non Evaporative Water Losses
MU = E + BD
Cooling Tower Cycles of
Concentration
Cycles of Concentration (Cycles)
•The dissolved solids concentration in the blowdown
relative to the makeup
– 5 cycles – BD has 5x concentration of MU
– 12 cycles – BD has 12x concentration of MU
•A measure of how efficiently the water is used
– Evaporation fixed by heat load & climate
– Non evaporative losses controlled by cycles
– Increased cycles = reduced losses
MU (m3/hr)
Effect of Tower Cycles on Makeup
and Blowdown
800
700
600
500
400
300
200
100
0
RR = 20,000 m3/hr
T = 9 oC
MU
BD
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Cycles
Limitations on Cycles
•Chemical
•Hydraulic
•Dynamic
Corrosion
Deposition
Particle Entrapment
Growth Sites
Biofouling
Effect of Cycles on Corrosion
Factors Affecting Corrosion
•Conductivity
•Acidic Anions e.g. Sulphate & Chlorides
•Materials of Construction
– Chlorides with Stainless Steel
– Sulphates with Concrete
Effect of Conductivity
Classic Corrosion Cell
WATER (ELECTROLYTE)
Fe 2+
O
2
ANODIC REACTIONS
Chemical Oxidation
Fe2+
2
CATHODE
ELECTRON
FLOW
CATHODIC REACTIONS
Chemical Reduction
In Neutral or Alkaline Water,
This is the Cooling Water Reaction:
+ 2e-
2Fe(OH)2 + 1/2O2 + H2O
O
O-
Fe(OH)3
Fe(OH)2
ANODE
Fe0
OH
H2
O
-
2Fe(OH)3
1/2O2 + H2O + 2e-
-
2OH
In Acid Media:
2H+ + 2e1/2O2
+ 2H+
H2 (HYDROGEN EVOLUTION)
+ 2e-
H2O
Factors Affecting CorrosionConductivity
Effect of Acidic Anions
Crevice Corrosion - Initial Stage
O2
OH-
Cl-
e-
O2
O2
Na+
e-
M+
Na+
Cl-
O2
Na+
O2
M+
OH-
Cl-
e-
Na+
O2
M+
OH-
e-
M+
M+
OH-
e-
Crevice Corrosion - Later Stage
Na+
O2
OH-
O2
OH-
ee-
Cl-
O2
O2
Na+
M+
Cl-
M+
Cl-
Cl-
ClCl-
O2
O2
M+
OH-
OH-
H+
OH-
e-
Cl-
M+
H+
ClH+
Cl-
M+
H+
Cl-
O2
M+
M+
M+
ClM+
M+
M+
H+
ClM+
Cl-
Crevice Corrosion Tube Plate Attack at Gasket
Crevice Corrosion - Coupons
Effect of Materials of Construction
Critical Pitting Temperature
304 CPT Chloride Limits
o
•40 C – 400 ppm
•50 oC – 200 ppm
•60 oC – 150 ppm
316 CPT Chloride Limits
o
•40 C – 4000 ppm
•50 oC – 1500 ppm
•60 oC – 800 ppm
Note: These are guidelines for
a clean system
Stress Corrosion Cracking - SS + Cl
Sulphate Attack on Concrete
Attack on Concrete by Soils & Waters Containing Sulphate
1
Relative Degree
of Sulphate
Attack
Percent Water-Soluble
Sulphate (as SO4) in
Soil Samples
ppm Sulphate
(as SO4) in
Water Samples
Negligible
0.00 to 0.10
0 to 150
Positive1
0.10 to 0.20
150 to 1000
Considerable2
0.20 to 0.50
1000 to 2000
Severe2
Over 0.50
Over 2000
Use Type II cement 2 Use Type V cement
Effect of Cycles on Deposition
Types of Deposition
•Scaling
– Mineral Scale
– Increased risk with increased cycles
•Fouling (see later)
– Suspended Matter
– Corrosion Products
– Biological
Scaling
Common Scales
•Calcium Carbonate
CaCO3
•Calcium Sulfate CaSO4
•Calcium Phosphate
Ca3(PO4)2
•Magnesium Silicate
MgSiO3
•Aluminium Silicate
Al2O3.SiO2
•Zinc Phosphate
Zn3(PO4)2
•Iron Phosphate FePO4
•Calcium Magnesium
Silicate CaO.MgO.2(SiO2)
•Silica SiO2
Factors Affecting Scale Formation
•Scale forms when solubility is exceeded
•Rate depends on degree of super saturation
– Concentration of Ions
– Temperature, most salts increase in solubility
with increasing temperature except for Ca and
Mg Salts
– pH/Alkalinity, most salts decrease in solubility
with increasing alkalinity/pH except for Silica
– Oxidation State, Fe and Mn salts increase in
solubility with decreasing oxidation state
Calcium Carbonate Solubility
1000
45 C
Calcium, ppm CaCO3
900
800
700
60 C
50 C
Scaling (Supersaturated)
600
500
400
300
200
Non Scaling (Unsaturated)
100
0
150
250
350
450
Alkalinity, ppm CaCO3 @ 4000 mmhos
550
650
Calcium Phosphate Solubility
600
Scaling (Supersaturated)
500
Calcium ppm CaCO3
400
pH = 7.0
300
200
pH = 8.2
100
Non Scaling (Unsaturated)
0
2
4
6
Orthophosphate ppm PO4
8
10
Indices & Guidelines
Commonly Used Indices
CaCO3:
– Langelier Saturation Index (LSI)
– Ryznar Stability Index (RSI)
– Stiff-Davis Stability Index (S&DI)
• High Conductivity Waters > 10,000 ppm TDS
•Calculated Using:
– Several charts, nomograms & formulae
– Some give quite varied answers!
Langelier Saturation Index
LSI = pHa - pHs
Where:
– pHa = Actual pH
– pHs = Saturation pH
– pHs is a function of Ca, M-Alk, TDS, and
Temperature
Guidelines:
– Positive (+) = scale is likely to form
– Negative (-) = scale is not likely to form
Simple Modelling
GEWater &ProcessTechnologies
Acme
VOL
TOWER WATER CYCLING ANALYSIS
ANZ
RR
1,500 (m3/hr)
DT
10 (deg C)
EVAP
27 (m3/hr)
( VERSION 8.0 Metric )
o
60 C (Hotest Skin)
o
50 C (Bulk Water)
Cooling Description
0.9 M-ALK FACTOR
CYCLES
pH
M-ALK
30/04/2007
Ca
Mg
SiO2
COND
Cl
SO4
MgSi
CMSi
RT75
(day)
7.80
100
60
2.0
8.23
180
3.0
8.55
270
4.0
8.78
5.0
(m3)
1.00
F
LSI
300
B.D.
M.U.
(m3/hr) (m3/hr)
40
25
500
50
120
80
50
1000
100
40
0.38
ok
ok
0.01
27
54
180
120
75
1500
150
60
1.03
ok
ok
0.02
14
41
360
240
160
100
2000
200
80
1.49
ok
ok
0.03
9
36
8.96
450
300
200
125
2500
250
100
1.85
ok
*****
0.04
7
34
6.0
9.11
540
360
240
150
3000
300
120
2.14
ok
*****
0.05
5
32
7.0
9.23
630
420
280
175
3500
350
140
2.39
ok
*****
0.06
5
32
8.0
9.0
9.34
9.43
720
810
480
540
320
200
360 *****
4000
4500
400
450
160
180
*****
*****
*****
*****
*****
*****
0.07
0.09
4
3
31
30
10.0
9.52
900
600
400 *****
5000
500
200
*****
*****
*****
0.10
3
30
11.0
9.59
990
660
440 *****
5500
550
220
*****
*****
*****
0.11
3
30
12.0
9.66
1080
720
480 *****
6000
600
240
*****
*****
*****
0.12
2
29
13.0
9.73
1170
780
520 *****
6500
650
260
*****
*****
*****
0.13
2
29
14.0
9.78
1260
840
560 *****
7000
700
280
*****
*****
*****
0.14
2
29
15.0
9.84
1350
900
600 *****
7500
750
300
*****
*****
*****
0.15
2
29
16.0
9.89
17.0
9.94
23.0 10.18
1440
1530
2070
960
1020
1380
640 *****
680 *****
920 *****
8000
8500
11500
800
850
1150
320
340
460
*****
*****
*****
*****
*****
*****
*****
*****
*****
0.16
0.17
0.24
2
2
1
29
29
28
29.0 10.37
2610
1740
1160 *****
14500
1450
580
*****
*****
*****
0.30
1
28
MAKEUP
20 -0.56
Advanced Modelling
Cooling Tower Simulation
Calcite supersaturation Vs. pH and Cycles of
Concentration @ 140F
S( C aC O3) w it h T e m p =140. an d 0. p p m o f UNT REA T ED
Untreated
30
25
20
S( CaCO3)
15
10
5
0
8
4
3 .8
7.8
3 .6
3 .4
Cy c les
7.6
3 .2
3
2 .8
7.4
2 .6
7.2
2 .4
2 .2
2
7
pH
Cooling Tower Simulation
Calcite supersaturation Vs. pH and Cycles of
Concentration @ 140F
S( C aC O3) w it h T e m p =140. an d 2. p p m o f A EC
Treated
30
25
20
S( CaCO3)
15
10
5
0
8
4
3 .8
7.8
3 .6
3 .4
Cy c les
7.6
3 .2
3
2 .8
7.4
2 .6
7.2
2 .4
2 .2
2
7
pH
Hydraulic Limitations
Hydraulic Limit on Cycles
•All systems have uncontrolled losses (e.g. leaks,
drift, windage)
•When: Uncontrolled Losses > Blowdown Required to
Control Water Chemistry
•Then: Actual Cycles < Target Cycles
•Cycles no longer limited by chemical constraints,
said to be hydraulically limited
Hydraulic Limit on Cycles
•Hydraulic limit on cycles can be determined:
– Measure “Total Non Evaporative Water Losses”
• Decay study using MoO4, LiCl
– Measure “Controlled Losses” (blowdown)
• Flow meter, Rotameter
– Calculate “Uncontrolled Losses” (difference)
– Calculate Hydraulic Limit from Uncontrolled
Losses
•Need to reduce Uncontrolled Losses to further
increase cycles beyond Hydraulic Limit
System Dynamics
Concentration (ppm)
Retention Time (a.k.a. Half life or
HTI)
80
70
60
50
40
30
20
10
0
RT50
RT75
t1/2
1
t1/2
2
3
4
5
Time (Days)
6
7
Calculating System Half Life
Half Life
t1/2 = Ln 2 x System Volume / System Losses
Note Ln 2 = 0.693
Retention Time
– RT50 = t1/2
– RT75 = 2 x t1/2
Impact of Half Life
•Typical half life for industrial cooling tower
– 2-4 days design
– 5-7 days actual
•Long half life can cause:
– Degradation of treatment chemicals
– Persistence of upset conditions
• Wind Blown Solids
• Process Contamination
Copper Corrosion Inhibitors
Adsorbed Azole Layer
Az
Az
Az
++
Cu
Az
Azole
Molecules
Copper
Ions
++Az
Cu
Az
Az
Az
Azole Thin Film
Az
Az Az Az Az Az Az Az Az Az
Az Az Az Az Az Az Az Az
Copper or Copper Alloy Metal Surface
Galvanic Plating of Copper onto
Steel
MS and Copper Corrosion - TTA
4 .0
T T A /M S
3 .5
C o rro s io n R a te ( m p y)
T T A /C u
3 .0
2 .5
C h lo rin a tio n S ta rts
2 .0
1 .5
1 .0
0 .5
0 .0
0
20
40
60
80
T im e (h r)
100
120
140
Copper Corrosion Test - HRA
4 .0
TTA
C o rro s io n R a te ( m p y)
3 .5
HRA
BZT
3 .0
2 .5
2 .0
Add 5 ppm N aO C l
1 .5
1 .0
0 .5
0 .0
0
5
10
15
20
T im e (h rs )
25
30
35
40
MS and Copper Corrosion - HRA
4 .0
3 .5
H R A /M S
C o rro s io n R a te (m p y)
H R A /C u
3 .0
2 .5
2 .0
C h lo rin a tio n S ta rts
1 .5
1 .0
0 .5
0 .0
0
20
40
60
80
T im e (h r)
100
120
140
CaCO3 Scale Inhibitors
AEC – Chlorine Resistance
1 1 0 7 p p m C a ; p H 8 .6 ; T e m p 7 0 C
100
AEC
% In h ib itio n
90
HEDP
80
70
60
50
0
2
5
F re e C h lo rin e , p p m
10
Dispersants
Fouling
•Sources
> Suspended Solids in makeup or wind blown
> Corrosion Products generated in the system
> Organic materials and microorganisms can act as
binding agents
•Settle in low flow areas (obey Stokes Law)
– Shell & Tube exchangers with water on shellside
– Plate & Frame exchangers
Dispersants
•Dispersants control particle size by interfering with
agglomeration
•Adsorb on particle surfaces imparting excess -ve
charge
Repulsion
Polyacrylic Acid (PAA)
CH2 CH
C
-
O
O
x
•Effective dispersants for
silt and clays
•NOT Effective for
> High levels (>1 ppm) of
iron or manganese
> calcium phosphate
Polyacrylamide
CH2 CH
C
NH2
O
x
•Long retention times
and/or high temperatures
> Break down
(hydrolyzes) to Acrylic
Acid
> NH3 liberated
•When it hydrolyzes it is
just like PAA
AA/AMPS
Copolymer of Acrylic Acid and 2-Acrylamido-2-Methylpropyl Sulfonic Acid;
AA/AMPS
HPS-I Polymer
(C – C)x (C – C)y
C
ONa O
CH2
ETHER
O
LINKAGE
CH2
CH OH
CH2
SO3Na
Robust ether linkage does not hydrolyse allowing
longer retention times (higher cycles)
Treating Long Retention Time
Systems
Treating Long Retention Time
Systems (a.k.a. Running at High
Cycles)
•The
GE Approach
> Use continuous chlorination at 0.5-1.0 ppm for Legionella
control (CTI best practice)
> Use halogen stable treatment chemistries
•The Alternative Approaches
> Overdose products to compensate
– Costly
– Beware misleading monitoring and control data!
> Use non oxidising biocides or weak (stabilised) oxidising
biocides
– Beware Legionella!
Questions?