Presentation is loading. Please wait.

Presentation is loading. Please wait.

Reduction of Water Demand in Cooling Towers Dr Paul Hirst.

Similar presentations

Presentation on theme: "Reduction of Water Demand in Cooling Towers Dr Paul Hirst."— 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 Evaporation Heat Load Blowdown Makeup Recirculating Pump Cooling Tower Cooling Tower System

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 (m 3 /hr)=RR x C p x T x E f / 556 Where:RR is the Recirculation Rate in m 3 /hr C p is the Specific Heat Capacity (1 kcal/kg/ o C) T is the Temperature Change in o C E f 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 o C

12 Limitations on Cycles Chemical Hydraulic Dynamic

13 Under Deposit Corrosion Corrosion Products Growth Sites Metabolic Products Particle Entrapment Growth Sites 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 ANODIC REACTIONS Chemical Oxidation CATHODIC REACTIONS Chemical Reduction Fe 0 Fe e - 2Fe(OH) 2 + 1/2O 2 + H 2 O 2Fe(OH) 3 In Neutral or Alkaline Water, This is the Cooling Water Reaction : 1/2O 2 + H 2 O + 2e - 2OH - In Acid Media: 2H + + 2e - H 2 (HYDROGEN EVOLUTION) 1/2O 2 + 2H + + 2e - H 2 O WATER (ELECTROLYTE) O2O2 Fe(OH) 2 ANODE Fe 2+ ELECTRON FLOW CATHODE OH - O - O2O2 H2OH2O Fe(OH) 3 Classic Corrosion Cell

18 Factors Affecting Corrosion- Conductivity

19 Effect of Acidic Anions

20 Crevice Corrosion - Initial Stage Na + OH - O2O2 M+M+ Cl - M+M+ M+M+ M+M+ M+M+ O2O2 O2O2 O2O2 O2O2 OH - Cl - Na + O2O2 e-e- e-e- e-e- e-e- e-e-

21 Crevice Corrosion - Later Stage Na + M+M+ OH - Cl - O2O2 e-e- OH - O2O2 O2O2 O2O2 O2O2 O2O2 O2O2 e-e- e-e- M+M+ M+M+ M+M+ M+M+ M+M+ M+M+ M+M+ H+H+ Na + M+M+ M+M+ M+M+ M+M+ Cl - H+H+ H+H+ H+H+ H+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 o C – 400 ppm 50 o C – 200 ppm 60 o C – 150 ppm 316 CPT Chloride Limits 40 o C – 4000 ppm 50 o C – 1500 ppm 60 o C – 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 SO 4 ) in Soil Samples ppm Sulphate (as SO 4 ) in Water Samples Negligible0.00 to to 150 Positive to to 1000 Considerable to to 2000 Severe 2 2 Over 0.50Over 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 CaCO 3 Calcium Sulfate CaSO 4 Calcium Phosphate Ca 3 (PO 4 ) 2 Magnesium Silicate MgSiO 3 Aluminium Silicate Al 2 O 3.SiO 2 Zinc Phosphate Zn 3 (PO 4 ) 2 Iron Phosphate FePO 4 Calcium Magnesium Silicate CaO.MgO.2(SiO 2 ) Silica SiO 2

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 CaCO C 50 C 45 C Scaling (Supersaturated) Non Scaling (Unsaturated)

34 Calcium Phosphate Solubility Orthophosphate ppm PO 4 Calcium ppm CaCO pH = 8.2 pH = 7.0 Non Scaling (Unsaturated) Scaling (Supersaturated)

35 Indices & Guidelines

36 Commonly Used Indices CaCO 3 : –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 = pH a - pH s Where: –pH a = Actual pH –pH s = Saturation pH –pH s 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. pH and Cycles of 140F Untreated

42 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 MoO 4, 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) Time (Days) Concentration (ppm) t 1/2 RT 75 RT 50

48 Calculating System Half Life Half Life t1/2 = Ln 2 x System Volume / System Losses Note Ln 2 = 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 Az Copper or Copper Alloy Metal Surface Cu ++ Cu Az Adsorbed Azole Layer Az Copper Ions Azole Molecules Azole Thin Film

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 CaCO 3 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 CH 2 CH C OO-O- x Polyacrylic Acid (PAA) Effective dispersants for silt and clays NOT Effective for High levels (>1 ppm) of iron or manganese calcium phosphate

62 Polyacrylamide Long retention times and/or high temperatures Break down (hydrolyzes) to Acrylic Acid NH3 liberated When it hydrolyzes it is just like PAA CH 2 CH C ONH 2 x

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

64 HPS-I Polymer Robust ether linkage does not hydrolyse allowing longer retention times (higher cycles) OH O CH SO 3 Na ONa CH 2 (C – C) x C O (C – C) y ETHER LINKAGE

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?

Download ppt "Reduction of Water Demand in Cooling Towers Dr Paul Hirst."

Similar presentations

Ads by Google