Cooling Water TreatmentFundamentals

Cooling Water Systems

Once Throughe.g large utility plantslarge quantity of water required.Water taken from plant supply, passed throughcooling system and returned to receiving bodyof water.

Closed Recirculatinge.g hot and chilled water loops used for heating,

WATERWATEROUTOUT

PROCESSPROCESSOUTOUT

WATERWATERININ

PROCESSPROCESSININ

e.g hot and chilled water loops used for heating,refrigerating and air conditioning; also critical cooling applications.Negligible evaporation or exposureto atmosphereHigh chemical treatment levels but economicalHeat removed from closed loop by second cooling water cycle e.g. evaporative

Open (Evaporative) RecirculatingHot process water cooled by evaporation ofwater in contact with air. Cooling tower designedto maximise water / air contact. Evaporation of water leads to an increase in the concentration of solids.

PROCESSIN

PROCESSOUTOUT IN

Make up

M

E

B

RR

Water Balance In An Open Evaporative Cooling Water System

Recirculating Cooling Water (R)

Evaporative Loss (E)Windage Loss (W)

Environment:Health & Safety

BLOWDOWN(B)

MAKEUP(M)

Holding WaterVolume (H)

Heat Exchanger

Quality Availability &Cost

Environment:Chemical Discharge

Heat Exchanger Efficiency

Terms & Symbols Related To Water Balance In Open Evaporative Cooling Water Systems

Recirculating WaterQuantity

R, m3/hr total cooling water circulated by pumps per hour

Holding WaterVolume

V, m3 total water volume held in cooling water system including pipingand cooling water tower basin

Evaporative Lossess E, m3/hr water lost by evaporation per hour

Windage Loss W, m3/hr water lost by windage + drift

Blowdown Water B, m3/hr water quantity discharged per hour in order to controlconcentration

Make Up Water M, m3/hr water supplied to the cooling system to maintain system waterMake Up Water M, m3/hr water supplied to the cooling system to maintain system watervolume

Temperature DropThrough CoolingTower

T, C difference of cooling water temperature between the coolingtower inlet and outlet

Concentration Factor(Cycles ofConcentration)

CF concentration of dissolved solids in circulating water comparedto makeup water

Retention Time TR time required for water to make one trip around the circulatingloop

Holding Time Index,or Half Life

HTI represents the time required to dilute an added chemical to50% of its concentration

time required to concentrate makeup solids by a factor of 2 an important factor for establishing effective biofouling, scale

& corrosion control programme

CalculationsMakeup M = E + B + W

Concentration Factor CF = [X] Circulating Water

(or Concentration Factor) [X] Makeup Water

Evaporative Losses E = R x T H

Blowdown B = E ,

(CF -1)

Holding Time Index = 0.693 x V ,

Or Half Life B+W

H

H = Latent heat of water evaporation,

Approx 578 kcal/kg at 40C

As a rule of thumbE = 1.4% x R if T = 10CE = 0.8% x R if T = 10F

Calculation continued/...e.g. an open recirculating system has following operating and chemical parameters. Calculate evaporative losses, concentration factor, blowdown and makeup.

R = 3,500 m3/hr [Cl] make up water = 250 ppm

T = 9C [Cl] recirculating water = 750 ppm

ANSWERS

1) Evaporative Losses (E) = R x T = 55 m3/hr578

2) Concentration Factor (CF) = 3 (from chloride analysis)

3) Blowdown (B) = E = 55 = 27.5 m3/hr(CF - 1) 2

4) Makeup (M) = E + B = (55 +27.5) = 82.5 m3/hr

Problems in Cooling Water Systems

Corrosion(mild steel,

yellow metal)

Efficiency drop in heat exchangers

Leakage from heat exchangers

Reduction ofmaterials strength

Microbiological(algae, fungi& bacteria)

Plugging of heat exchangers

Scale(Calcium carbonatecalcium phosphate

Silicates and sulphates)

Increased pumppressure and reduction

of flow rate

Acceleration of corrosion

Dirty Appearance

Sludge(general deposits)

These problems occur most frequently in open evaporative cooling water systems since the dissolved solids are concentrated in the cooling water by evaporation.

Adsorption and wasteof chemicals

Estimation of Calcium Carbonate Tendency and Corrosivity of Cooling Water

Tendency for calcium carbonate formation increases as following water characteristics increase:

skin temperature (water temperature at heat exchanger surface) calcium hardness (calcium concentration in the water normally expressed as calcium carbonate)

alkalinity (bicarbonate + carbonate + hydroxide normally expressed as alkalinity (bicarbonate + carbonate + hydroxide normally expressed as calcium carbonate

total dissolved solids (tds) pH

Tabular methods exist to calculate pHs = pH at which calcium carbonate is at saturation

pH - pHs = Langelier Saturation Index (LSI):

2 pHs - pH = Ryznar Stability Index (RSI) :

Index Tendency of Water

LSI RSI

2.0

0.50-0.5

-2.0

Example ..

Calculate LSI and RSI of a cooling water at 60C, pH 8.5,calcium hardness = 100 mg/l as calcium carbonatetotal alkalinity = 80 mg/l as calcium carbonatetds = 200 mg/l

pHs = (9.3 + A + B) - (C + D)

Answer:

pHs = (9.3 + A + B) - (C + D)

From tablespHs = (9.3 + 0.1 + 1.35) - (1.65 + 1.95) = 7.15

LSI = 8.5 - 7.15 = +1.35

RSI = 14.3 - 8.5 = 5.8

Therefore this water is slightly scale forming and a programme is required to primarily control calcium carbonate formation

Approaches to Efficient Operation of Open Recirculating Cooling Systems

Operation at Alkaline pH range (8.0 - 9.2)

Higher pH substantially reduces natural corrosivity,buffer capacity provided by water reduces impact of system upsets

BUT deposit control becomes more difficult stabilisation of zinc and phosphate becomes more difficult

Properties required from additives used for effective treatment and maintenance of open

cooling water systems

Calcium carbonate inhibition and control LSI multifunctional inorganic scale inhibition effective dispersancy of silt/sludge/corrosion debris stabilisation of zinc and phosphate in alkaline programmes robust mild steel and yellow metal corrosion inhibition

under broad operating conditions broad spectrum biocidal properties particularly at pH 8-9 economical non-toxic to environment

BWA Water Additives: Product for Industrial Cooling Water Treatment

Belclene 200 Maleic homopolymer providing outstanding calcium carbonate control under severe service conditions (high LSI)

Belclene 283 Multifunctional maleic terpolymer providing stabilisation of zinc and phosphonates: provides effective dispension and calcium carbonate control over broad operating conditions

Belclene 400 Sulphonated copolymer providing stabilisation of extended phosphate programmes, zinc, phosphonates: good dispersant for ironoxide & silt/sludge.

Polymeric Scale

Inhibitor

& Dispersants

Belclene 499 Provides both phosphonate and sulphonated copolymer functionality in one molecule; building block for new formulations

Belcor 575 Cathodic corrosion inhibitor used as basis of all-organic, zinc and phosphate band Belcor 575 Cathodic corrosion inhibitor used as basis of all-organic, zinc and phosphate band

corrosion inhibitor programmes.

Belclene 500 Building block for zinc based corrosion programmes for soft water applications

Corrosion Inhibitors

Belcor 593 Tricarboxylic acid used in combination with Belcor 575 in closed cooling systems

Bellacide 325 Terbuthylazine for control of algae in open recirculating cooling water systems,

ornamental pools and fountains. Exhibits synergistic effect with halogen.

Bellacide 350 Quaternary phosphonium chloride with broad spectrum fast kill performance; hard surface cleaning properties

BromiCide Hydantoin bromine release product for controlled release of biocidal hypobromous

acid.

Oxidising

& Non-Oxidising BioCides

LiquiBrom Sodium Bromide providing safe efficient release of biocidal hypobromous acid

when activated by a chlorine source.

Belclene Phosphonates

Belclene 640: nitrilotris (methylene phosphonic) acid (ATMP)

Belclene 650: phosphonobutane tricarboxylic acid (PBTC)(PBTC)

Belclene 660: 1-hydroxyethylidene diphosphonic acid (HEDP)