1. Reduction of Water Demand in Cooling Towers
Dr Paul Hirst

2. Agenda Cooling Tower Basics Water Losses Cycles of Concentration
Limitations on Cycles
Chemical
Hydraulic
Dynamic

3. Cooling Tower System Evaporation Heat Load Cooling Tower Makeup
Blowdown
Recirculating
Pump

4. Cooling Tower Water Losses
Evaporative Water Losses
Non Evaporative Water losses

5. 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/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

6. 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

7. 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

8. Makeup Water
The water added to replace water lost from the cooling system:
Evaporation
Total Non Evaporative Water Losses
MU = E + BD

9. Cooling Tower Cycles of Concentration

10. 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

11. Effect of Tower Cycles on Makeup and Blowdown
RR = 20,000 m3/hr
T = 9 oC

12. Limitations on Cycles
Chemical
Hydraulic
Dynamic

13. Corrosion Deposition Biofouling
Under Deposit Corrosion
Corrosion Products
Growth Sites
Metabolic Products
Particle Entrapment
Corrosion
Deposition
Biofouling

14. Effect of Cycles on Corrosion

15. Factors Affecting Corrosion
Conductivity
Acidic Anions e.g. Sulphate & Chlorides
Materials of Construction
Chlorides with Stainless Steel
Sulphates with Concrete

16. Effect of Conductivity

17. Classic Corrosion Cell
WATER (ELECTROLYTE)
H2O
OH - O2
Fe 2+ O2
Fe(OH)3
O -
Fe(OH)2
CATHODE
ANODE
ELECTRON
FLOW
ANODIC REACTIONS
Chemical Oxidation
CATHODIC REACTIONS
Chemical Reduction
In Neutral or Alkaline Water,
This is the Cooling Water Reaction:
Fe Fe e-
1/2O H2O + 2e OH-
2Fe(OH) /2O2 + H2O Fe(OH)3
In Acid Media:
2H e H2 (HYDROGEN EVOLUTION)
1/2O H e H2O

18. Factors Affecting Corrosion-Conductivity

19. Effect of Acidic Anions

20. Crevice Corrosion - Initial Stage
Na+
OH- O2 M+
Cl- e-

21. Crevice Corrosion - Later Stage
Na+ M+
OH-
Cl- O2 e- H+

22. Crevice Corrosion - Tube Plate Attack at Gasket

23. Crevice Corrosion - Coupons

24. Effect of Materials of Construction

25. Critical Pitting Temperature
304 CPT Chloride Limits
40 oC – 400 ppm
50 oC – 200 ppm
60 oC – 150 ppm
316 CPT Chloride Limits
40 oC – 4000 ppm
50 oC – 1500 ppm
60 oC – 800 ppm
Note: These are guidelines for a clean system

26. Stress Corrosion Cracking - SS + Cl

27. 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

28. Effect of Cycles on Deposition

29. Types of Deposition Scaling Mineral Scale
Increased risk with increased cycles
Fouling (see later)
Suspended Matter
Corrosion Products
Biological

30. Scaling

31. 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

32. 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

33. Calcium Carbonate Solubility
Alkalinity, ppm 4000 mmhos
Calcium, ppm CaCO3
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)

34. Calcium Phosphate Solubility
Orthophosphate ppm PO4
Calcium ppm CaCO3
100
200
300
400
500
600 2 4 6 8 10
pH = 8.2
pH = 7.0
Scaling (Supersaturated)
Non Scaling (Unsaturated)

35. Indices & Guidelines

36. 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!

37. 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

38. Simple Modelling

40. Advanced Modelling

41. Cooling Tower Simulation Calcite supersaturation Vs
Cooling Tower Simulation Calcite supersaturation Vs. pH and Cycles of 140F Untreated

42. Cooling Tower Simulation Calcite supersaturation Vs
Cooling Tower Simulation Calcite supersaturation Vs. pH and Cycles of 140F Treated

43. Hydraulic Limitations

44. 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

45. 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

46. System Dynamics

47. Retention Time (a.k.a. Half life or HTI)80 70
RT50 60 50
RT75
Concentration (ppm) 40 30 20
t1/2
t1/2 10 1 2 3 4 5 6 7
Time (Days)

48. Calculating System 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

49. 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

50. Copper Corrosion Inhibitors

51. Adsorbed Azole Layer Azole Molecules Copper Ions Azole Thin Film++ Cu
Copper
Ions Az Az ++ Cu Az Az Az
Azole Thin Film Az Az
Copper or Copper Alloy Metal Surface

52. Galvanic Plating of Copper onto Steel

53. MS and Copper Corrosion - TTA

54. Copper Corrosion Test - HRA

55. MS and Copper Corrosion - HRA

56. CaCO3 Scale Inhibitors

57. AEC – Chlorine Resistance

58. Dispersants

59. 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

60. Dispersants
Dispersants control particle size by interfering with agglomeration
Adsorb on particle surfaces imparting excess -ve charge
Repulsion

61. Polyacrylic Acid (PAA)
Effective dispersants for silt and clays
NOT Effective for
High levels (>1 ppm) of iron or manganese
calcium phosphate
CH2 CH x C O- O

62. Polyacrylamide x CH2 CH C NH2 O
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 x C
NH2 O

63. AA/AMPS
Copolymer of Acrylic Acid and 2-Acrylamido-2-Methylpropyl Sulfonic Acid; AA/AMPS

64. HPS-I Polymer ETHER LINKAGE OH O CH SO3Na ONa CH2 (C – C)x C (C – C)y
Robust ether linkage does not hydrolyse allowing longer retention times (higher cycles)

65. Treating Long Retention Time Systems

66. Treating Long Retention Time Systems (a.k.a. Running at High Cycles)
The GE Approach
Use continuous chlorination at 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!

67. Questions?