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?