Reduction of Water Demand in Cooling Towers Dr Paul Hirst
Mar 26, 2015
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
Evaporation
HeatLoad
Blowdown
Makeup
RecirculatingPump
CoolingTower
Cooling Tower System
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) = RR x Cp x T x Ef / 556
Where: RR is the Recirculation Rate in m3/hrCp is the Specific Heat Capacity (1 kcal/kg/oC)T is the Temperature Change in oCEf is evaporation factor and depends on Wet bulb temperature, Relative Humidity556 = 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
Effect of Tower Cycles on Makeup and Blowdown
0100200300400500600700800
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Cycles
MU
(m
3/h
r) MU
BD
RR = 20,000 m3/hr
T = 9 oC
Limitations on Cycles
•Chemical•Hydraulic•Dynamic
Unde
r Dep
osit
Corr
osio
n
Corr
osio
n Pr
oduc
ts Grow
th Sites
Metabolic Products
Particle EntrapmentGrowth Sites
Corrosion
Deposition 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
ANODIC REACTIONSChemical Oxidation
CATHODIC REACTIONSChemical Reduction
Fe0 Fe2+ + 2e-
2Fe(OH)2 + 1/2O2 + H2O 2Fe(OH)3
In Neutral or Alkaline Water, This is the Cooling Water Reaction:
1/2O2 + H2O + 2e- 2OH-
In Acid Media:
2H+ + 2e- H2 (HYDROGEN EVOLUTION)
1/2O2 + 2H+ + 2e- H2O
WATER (ELECTROLYTE)
O2
Fe(OH)2
ANODE
Fe 2+
ELECTRONFLOW
CATHODE
OH -
O -
O2
H2O
Fe(OH)3
Classic Corrosion Cell
Factors Affecting Corrosion-Conductivity
Effect of Acidic Anions
Crevice Corrosion - Initial Stage
Na+
OH-
O2
M+
Cl-
M+ M+ M+
M+
O2O2O2
O2
OH-OH-OH-
Cl-
Cl-
Na+
Na+ Na+
O2
e-
e-
e-e-
e-
Crevice Corrosion - Later Stage
Na+
M+
OH-
Cl-
O2
e-
OH-OH-OH-
OH-
O2O2
O2
O2
O2
O2
e-
e-
M+
M+
M+
M+
M+
M+
M+ H+
Na+
M+
M+
M+
M+
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-Cl-
H+
H+
H+
H+
Crevice Corrosion -Tube Plate Attack at Gasket
Crevice Corrosion - Coupons
Effect of Materials of Construction
Critical Pitting Temperature
304 CPT Chloride Limits•40 oC – 400 ppm•50 oC – 200 ppm•60 oC – 150 ppm316 CPT Chloride Limits•40 oC – 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 Use Type II cement 2 Use Type V cement
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
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
Alkalinity, ppm CaCO3 @ 4000 mmhos
Cal
ciu
m,
pp
m C
aCO
3
0
100
200
300
400
500
600
700
800
900
1000
150 250 350 450 550 650
60 C 50 C
45 C
Scaling (Supersaturated)
Non Scaling (Unsaturated)
Calcium Phosphate Solubility
Orthophosphate ppm PO4
Cal
ciu
m p
pm
CaC
O3
0
100
200
300
400
500
600
2 4 6 8 10
pH = 8.2
pH = 7.0
Non Scaling (Unsaturated)
Scaling (Supersaturated)
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
TemperatureGuidelines:
– Positive (+) = scale is likely to form– Negative (-) = scale is not likely to form
Simple Modelling
GE Water & Process Technologies Acme VOL 300 (m3)
TOWER WATER CYCLING ANALYSIS ANZ RR 1,500 (m3/hr)
( VERSION 8.0 Metric ) D T 10 (deg C)
60 oC (Hotest Skin) 50 oC (Bulk Water) Cooling Description EVAP 27 (m3/hr)
0.9 M-ALK FACTOR 30/04/2007 F 1.00
CYCLES pH M-ALK Ca Mg SiO2 COND Cl SO4 LSI MgSi CMSi RT75 B.D. M.U.
(day) (m3/hr) (m3/hr)
MAKEUP 7.80 100 60 40 25 500 50 20 -0.56
2.0 8.23 180 120 80 50 1000 100 40 0.38 ok ok 0.01 27 54
3.0 8.55 270 180 120 75 1500 150 60 1.03 ok ok 0.02 14 41
4.0 8.78 360 240 160 100 2000 200 80 1.49 ok ok 0.03 9 36
5.0 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 9.34 720 480 320 200 4000 400 160 ***** ***** ***** 0.07 4 319.0 9.43 810 540 360 ***** 4500 450 180 ***** ***** ***** 0.09 3 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 1440 960 640 ***** 8000 800 320 ***** ***** ***** 0.16 2 2917.0 9.94 1530 1020 680 ***** 8500 850 340 ***** ***** ***** 0.17 2 2923.0 10.18 2070 1380 920 ***** 11500 1150 460 ***** ***** ***** 0.24 1 28
29.0 10.37 2610 1740 1160 ***** 14500 1450 580 ***** ***** ***** 0.30 1 28
Advanced Modelling
Cooling Tower SimulationCalcite supersaturation Vs. pH and Cycles of Concentration @ 140FUntreated
22.2
2.42.6
2.83
3.23.4
3.63.8
4
7
7.2
7.4
7.6
7.8
80
5
10
15
20
25
30
S(CaCO3)
CyclespH
22.2
2.42.6
2.83
3.23.4
3.63.8
4
7
7.2
7.4
7.6
7.8
80
5
10
15
20
25
30
S(CaCO3)
CyclespH
Cooling Tower SimulationCalcite supersaturation Vs. pH and Cycles of Concentration @ 140FTreated
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
Retention Time (a.k.a. Half life or HTI)
010203040
50607080
1 2 3 4 5 6 7
Time (Days)
Co
nce
ntr
atio
n (
pp
m)
t1/2 t1/2
RT75
RT50
Calculating System Half Life
Half Lifet1/2 = 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
AzAz
Copper or Copper Alloy Metal Surface
Cu++
++Cu
AzAz
Az
Az
Az
Adsorbed Azole Layer
Az Az Az Az Az Az Az AzAz
Az
Az Az Az Az Az Az Az AzAz
CopperIons
AzoleMolecules
Azole Thin Film
Galvanic Plating of Copper onto Steel
MS and Copper Corrosion - TTA
Time (hr)
Co
rro
sio
n R
ate
(mp
y)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 20 40 60 80 100 120 140
TTA/MSTTA/Cu
Chlorination Starts
Time (hr)
Co
rro
sio
n R
ate
(mp
y)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 20 40 60 80 100 120 1400 20 40 60 80 100 120 140
TTA/MSTTA/CuTTA/MSTTA/Cu
Chlorination StartsChlorination StartsChlorination Starts
Copper Corrosion Test - HRA
Add 5 ppm NaOCl
0 5 10 15 20 25 30 35 40Time (hrs)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Co
rro
sio
n R
ate
(mp
y)
TTAHRABZT
Add 5 ppm NaOClAdd 5 ppm NaOCl
0 5 10 15 20 25 30 35 40Time (hrs)
0 5 10 15 20 25 30 35 40Time (hrs)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Co
rro
sio
n R
ate
(mp
y)
TTAHRABZT
MS and Copper Corrosion - HRA
Time (hr)
Co
rro
sio
n R
ate
(mp
y)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 20 40 60 80 100 120 140
HRA/MSHRA/Cu
Chlorination Starts
Time (hr)
Co
rro
sio
n R
ate
(mp
y)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 20 40 60 80 100 120 1400 20 40 60 80 100 120 140
HRA/MSHRA/CuHRA/MSHRA/Cu
Chlorination StartsChlorination StartsChlorination Starts
CaCO3 Scale Inhibitors
AEC – Chlorine Resistance
Free Free ChlorineChlorine, ppm, ppm
%
% In
hib
itio
nIn
hib
itio
n
1107 1107 ppmppm Ca; pH 8.6; Temp 70CCa; pH 8.6; Temp 70C
5050
6060
7070
8080
9090
100100
00 22 55 1010
AECAEC
HEDPHEDP
Free Free ChlorineChlorine, ppm, ppm
%
% In
hib
itio
nIn
hib
itio
n
1107 1107 ppmppm Ca; pH 8.6; Temp 70CCa; pH 8.6; Temp 70C
5050
6060
7070
8080
9090
100100
00 22 55 1010
AECAEC
HEDPHEDP
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 shell-side
– Plate & Frame exchangers
Dispersants
•Dispersants control particle size by interfering with agglomeration•Adsorb on particle surfaces imparting excess -ve charge
Repulsion
CH2 CH
C
OO-
x
Polyacrylic Acid (PAA)
•Effective dispersants for silt and clays•NOT Effective for>High levels (>1
ppm) of iron or manganese
>calcium phosphate
Polyacrylamide
•Long retention times and/or high temperatures>Break down
(hydrolyzes) to Acrylic Acid
>NH3 liberated•When it hydrolyzes it is just like PAA
CH2 CH
C
ONH2
x
AA/AMPS
Copolymer of Acrylic Acid and 2-Acrylamido-2-Methylpropyl Sulfonic Acid; AA/AMPS
HPS-I Polymer
Robust ether linkage does not hydrolyse allowing longer retention times (higher cycles)
OH
O
CH
SO3Na
ONa
CH2
CH2
CH2
(C – C)x
C
O
(C – C)y
ETHERLINKAGE
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?