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Reduction of Water Demand in Cooling Towers

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Presentation on theme: "Reduction of Water Demand in Cooling Towers"— Presentation transcript:

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?

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