Cooling Water

Watertreatment and chemical conditioning of

open and closed cooling systems

Dr. K. Nachstedt, Weidner Wassertechnik, Herten

Recirculation of process watervolume of utilization is increasingvolume of delivery is nearly constant

Rate of cooling water at direct wastewater disposal

Cooling Systems 1

Flow-through system

must be close to watersno recirculation of watervery large volume of water consumptionmarginal expenses for water treatmentecological problem: temperature rise of watersrare used at the present state of technology

Cooling Systems 2

closed/semi-closed circulation systemtypical useage: hydraulics, engine, injection mouldsneglectibal loss of water very strong corrosion inhibition possibleconnected to heat exchangers of secondary cooling systems (refrigerating machine / cooling tower

Cooling Systems 3

Open evaporating cooling systems mainly used system: cooling towertarget: high rate of recirculation (cycles of conc.)consumption of water: evaporaton + blowdownwatertreatment is always necessary

Cooling Systems 4Cooling towers

more than 80% of cooling capacity: evaporation

very high cooling capacity: use of latent heat of evaporation

consumption of water depends directly on water treatment

Problems to solve

Closed circuitshigh temperatures may cause scaling and corrosionlarge copper surfaces in heat exchangersdifferences in electrical potentials (e.g. target cooling)

Open circuits (cooling towers)high concentration of minerals in circulation water

deposits of calcium carbonate (scale formation)

biological growth (organic contamination, favorableenviroment for microorganism)

Priority tasks in cooling water treatment

avoidance of inorganic deposits (scale inhibition)

prevention / inhibition of corrosion

control of biological growth (fouling control)

Steps to take Cooling equipment:

Careful selection of materials and -combinationsFlow velocity within the range 1,5 – 2 m/slow temperature of water and materials < 45 °C (large surfaces of heat exchangers)

Water: Outer water treatment (Units and equipment)

filtration, softening, desalination

Internal water treatment (chemical products)Inhibitors for corrosion and scaling, antimicrobials

Closed / semi-closed systems (1) Usually easy to treat:

small systemvolume: typically < 200 m³no sunlight: suppression of biol. growth only small entry of pollutionneed only small amounts of makeup water(small costs of treatment products)

Makeup water:Town water / water from wells at T< 45 °Csoftening / desalination at T> 45 °C

Closed / semi-closed systems (2)Chemical treatment

in the past: chromates, nitrites, zinc, polyphosphatetoday: combinationproducts

dispergators (polycarbonic acids)corrosion inhibitors (molybdates, phosphonates, organic N-heterocyclic compounds)scaling inhibitors (phosphonates, polycarbonic acids)

Typical products & equipment necessary:dosing system: metering pump + water meter with impulse outputtreatment product: ca. 3-4 kg/m³ watercontroling: testkits / online-analysis / corrosion testsbiological contamination (well-water) : Biocides/ UV-burner

Closed / semi-closed systems (3)

Most common mistake of closed circuit treatmentLow concentration of corrosion inhibitor (Mo > 40 ppm)too large time – intervals between the control measurements

Closed / semi-closed systems (4)

Destructed surface of cast iron: 3 weeks after setting in operation with dangerous inhibitor concentration

Open evaporation cooling systems (1)

Water treatment can be very complicatedSystemvolume range 500 – 2000 m³

Makeup-water often above 100 m³/h

frequently large pre-destructed pipesystems. “historical material mix” (iron, steel, zinc-coatings, brass, copper, aluminium)

large amounts of dirt may enter the system

water is sometimes in direct contact with the product

seasonal large fluctuation of biological exposure.

Open evaporation cooling systems (2)

First priority: Prevention of depositsEconomical target: maximum of concentration cycles as possible ! (saving of makeup water):

Exceeding the limit of solubility of inorganic compounds:calciumcarbonate deposits (lime scale)

concentration of chloride content in the circulation water:promoting of corrosion (pitting at passive metals)

concentration of dirt in the circulation water:sludge-deposits can cause biological problems

Open evaporation cooling systems (3)

Target: cycle no. CN > 2,5Volume makeup (VM) = evaporation (VE) + blow down

VM = VE x (CN/(VN-1)) (Hyperbel)

CN = 2 = 100% VMCN = 3 = 75% -25%CN = 4 = 64% -11%

Even at small cooling towers:

enormous potential to save water and money !

Open evaporation cooling systems (4)

Practical Limits of concentration cycels:“carbonate-hardness” < 20° dH (tot. alkal. < 7 mmol/l) (better, if available: SI < 2,8)Chlorid concentration < 200 mg/l (danger of pitting)

Is it possible to operate an open cooling system without hardness-stabilizer?

15 mg/l = ca. 0,7 °dH

Deposits (1)Effects of deposits

Corrosion and microbiol. growth at and below depositsLoss of energy (flow resistance in pipes)Faulty flow (cavitation, erosion, promotion of corrosion)Poor heat transfer in heat exchangers (increas. tranf. coeff.)

Ablagerungen (2)

Plate of heat exchanger Pipe of coolingsystem

Products and methods of treatment (1)Basics of treatment products (state of the art)

phosphonic acids and phosphonatesscale inhibitioncorrosion protection

Polycarbonic acids / polycarbonatesdispersing agent / mobilisation of sludge scale inhibition

organic N-heterocyclic compounds (Triazoles)corrosion inhibition of yellow metals

additives for stabilisation of the formulation

Products and methods of treatment (2)Phosphonic acids: most effektive scale inhibitors

Threshold-inbitors (nonstoichiometric application)

Example: stoichiometric usage of known chelates

Products and methods of treatment (3)Phosphonic acids / Phosphonates

Threshold-inhibition (far below stoichiometric ratios)principle of operation: blocking of crystal growthunder-stoichoimetric ratio 1:<1000 underdosing: no hard deposits (easy to purge away)synergistic effect in combination with polycarbonic acidsusual concentration 5 - 10 g/m³ makeup water

Inhibition / prevention of corrosionamplification effect for other inhibitorsforming of protective membranes: anodic and cathodic corrosion inhibition

Products and methods of treatment (4)Phosphonic acids / phosphonates

Important difference: no polyphosphates ! (From the past!)

Easy hydrolysis !Degradation to ortho-phosphate !

Phosphonic acid are stable to hydrolysis in cooling water

Products and methods of treatment (5)Most effective phosphonic acid: PBTC

Products and methods of treatment (6)

Dipersing of sludges: anionic polymeres geringe Molmassen (1000 -4000)water soluble (highly negative charged)electrostatic effects mobilise sludge and slurry

R & D of formulationsTube-Blocking-Test / Scale inhibition test (NACE)

Practical application

Scale inhibitors / corrosion inhibitorsDosing proportional to addition of makeup watertypical dosing: 6-20 g/m³ makeup watercontrol of product concentration: total phosphate or special polymertest (if polymers are detectable)limit of scale stabilisation: 20 °dKH / 7mmol/l TAabove limit of stabilisation: dosing of mineral-acid (internal decarbonisation)Ion-exchange / reverse osmosis:decarbonisation / softening / desalination

Biological Control

Antimicrobials / biocidesNot oxidizing biocides

Isothiazolinones (CIT/MIT, deliv. 1,5%)DiBromNitriloPropionamid (DBNPA, deliv. 20%)

Oxidizing biozides Chlorine (sodium hypochlorite)Chlor/Brom-Hydantoin (halogen donor; tablettes)Bromine (Flow through reactor with chlorine)Chlordioxid (“one component”, production system)Ozon (AOX / CSB-reduction possible; expensive)

Applikation of antimicrobials

Nonoxidizing antimicrobials

always shockdosing (avoiding of resistence!)interval time after dosing: 7 - 14 daysnecessary concentration 0,1 kg / m³ circuit volumeuse with surfactant to combat biofilmsexpensive for large circuit volumes

Oxidizing antimicobials

continuous dosing possiblemay promote corrosionchlordioxide excellent for biofilms

Control of treatment success:

microbiological examination in regular intervals

Treatment of open circuits

Automatic system for open cooling circuits

Dosing of scale inhibitors Control of concentration cycles via conductivityPH-Control and pH-regulationTreatment with microbialsnonoxidizing: dosing at selectable timesoxidizing: dosing depends on continuously measurement (sensor, on-line analysis) Meeting WHG Anhang 31 (luminicence test)

Blocking of blow down / pre-blow-down

Automatic management system for cooling water

sensor for halogen

T

fluorescence

carboxylate

ortho - PO4

total- PO4

pH

total alkalinity

conductivity

Cooling tower

water circuit

Analytical Parameter Information

concentration cycles

concentration cycles

acid dosing / decarb.

Oxid. biozide content

cooling capicity

conc. treatment prod.

phosphonate x F =

conc. treatment prod.

KWMS 1: Coolingwater-Managementsystem

KWMS 1:

maximum of information formaximum of reliability

Keywords: External Water TreatmentIf no economical internal treatment is possible:

Softening of waterworks continuouslylarge consumption of regeneration salt(often more than 50.000 Kg/year in medium circuits)

Decarbonisation (partial desalination)internal decarbonisation (decreasing pH, acid dosing)external decarbonisation (weak acid ion exchanger)

DesalinationFull desalination (Ion-exchanger, anionic + cathionic)Reverse Osmosis (Costs of energy! 75% yield )

SummaryBasic schematics of open and closed circuits have been introduced.Targets of cooling water treatment in open and closed circuits were pointed out.Internal (chemical) and external (equipment based) treatment possibilities and limitations have been discussed. Problems of cooling systems and possible solutions of suitable water treatment have been shown.Modern treatment substances and their properties in formulated products have been listed and discussed.Automatic management systems or cooling water and projects of R & D have been introduced.

Further questions + informations

Dr. K. NachstedtTel. 0178/6011062k.nachstedt@wasserchemie.de

Weidner Wassertechnik GmbHZechenstrasse 2545699 Herten