A SHRAE JOURNAL

M a y 1 9 9 8 A S H R A E J o u r n a l 3 9

By Scott E. MayesMember ASHRAEandTom Ruisinger

hether produced by na-ture or humans, ozone isone of the most powerfuldisinfectants available.

Properly applied, it destroys most, if notall, bacteria, viruses and other pathogenicorganisms.

Ozone is an unstable formof oxygen. In the upper atmo-sphere, it is contained in themuch-publicized ozonelayer. In this layer, the sunsultraviolet light forms ozoneby splitting oxygen moleculesinto atoms. Some of these at-oms combine with separateoxygen molecules to formozone.

Natural and artificial pro-cesses manufacture ozone inthe lower atmosphere. Light-ning is the most familiar wayozone is naturally manufac-tured. Some artificial sourcesof ozone include electric motors, copiers,laser printers and fluorescent lights.

Ozone in Water TreatmentIn 1857, Werner Siemens developed a

method of producing ozone by silent elec-trical discharge. His invention paved theway for ozones use in water treatment,including the first full-scale applicationof ozone in drinking water treatment in1906.

Ozone was first used in cooling towerwater treatment in the late 1970s whengovernment and private concerns con-ducted research on its effective use.

Today, ozone is gaining rapid accep-tance in hospitals, commercial office build-ings and industrial plants for a wide rangeof applications including cooling tower

water treatment, food and food process-ing, waste water cleanup, smoke removal,swimming pools and spas, drinking andbottled water, and pulp and paper bleach-ing. Ozone is used where reliable biologi-cal control is needed and residual chemi-cal compounds are undesirable.

When ozone reacts with an organic,the byproduct is oxygen and an oxidizedform of the organic. In addition, unusedozone naturally and quickly decays tooxygen. These factors make using ozonefor biological control an attractive choice.

Ozone in a Cooling SystemOzone is an excellent biocide. It oxi-

dizes not only bacteria, but also organicfood sources for those bacteria. For ex-ample, in one study, ozone killed existingalgae in the cooling tower basins andsidewalls. A mat of algae over 0.25 in. (6mm) thick was allowed to grow in a cool-ing tower basin. When ozone was appliedand the oxidation reduction potential(ORP) level maintained at 750 millivolts,the algae turned from bright green to darkgreen within one day, dark green to brownwithin four days and sloughed off withina week.

Maintaining an ORP above 500 milli-volts generally is necessary to maintainan acceptable bacteria count. In thestudy, the ORP was lowered gradually

Using OzoneFor Water Treatment

from the standard 750 millivolts. Below500 millivolts, bacteria and algae growthwas essentially uncontrolled. In a cool-ing tower properly treated with ozone,total bacteria counts of 103 are achievedroutinely. The clarity of the water rivalsthat of the best-maintained swimmingpools.

Scale control in a cooling system isimportant for efficient operation. Ozonesprimary contribution to scale control isthe fact that it is an excellent biocide.Scale adheres to a biological layer, often

called slime. Cooling systemsproperly treated with ozone willhave no slime layer for scaleparticles to adhere.

Maintaining water quality isalso essential to maintaining ascale-free system. At highercycles of concentration, cal-cium and carbonate will precipi-tate from solution to suspen-sion as the most common formof scale, calcium carbonate. Ina clean system, these scale par-ticles will settle in the low ve-locity area of the cooling sys-temoften the cold water ba-

sin of the cooling tower. Re-moval of suspended minerals byblowdown, filtration or physical cleaningmay be necessary.

In general, maintaining a positiveLangelier Saturation Index (LSI) of 1.5 to

W

About the AuthorsScott Mayes is a product manager at theMarley Cooling Tower Company and isthe chairman of ASHRAE TechnicalCommittee 3.6, Corrosion and WaterTreatment.

Tom Ruisinger is a consultant on watertreatment and cooling tower performanceissues. He has been involved in the de-sign, research and application of ozonefor cooling tower water treatment for 10years.

Figure 1: Ozone generation by corona discharge.

The following article was published in ASHRAE Journal, May 1998. Copyright 1998 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE.

4 0 A S H R A E J o u r n a l M a y 1 9 9 8

Figure 2: Air or oxygen is passed throughthe gap between two conductors, which re-sults in some of the oxygen converting toozone.

2.5 in the circulating water allows reason-able water savings while keeping scaledeposition to a minimum. Water veloci-ties, temperatures of the heat transfersurfaces and system configuration alsoaffect the tendency of a system to formand accumulate scale.

Generally, ozone is not considered acorrosion inhibitor. However, experiencebased on corrosion studies performedon over 75 sites across the countrysome with over eight years of couponanalysisindicate corrosion rates in acooling water system can remain wellwithin industry acceptable limits.Most of these sites operate with mildsteel corrosion rates, measured bycoupon analysis, of less than 2.5 milsper year (mpy). Copper corrosionrates average less than 0.3 mpy, withsome so low they cannot be measuredon a 90-day coupon. Extremely softwaters that cannot be cycled up, orwaters with high levels of chlorideswill require the use of corrosion inhibi-tors to maintain acceptable corrosionrates by industry standards.

For example, on one tower installa-tion the copper corrosion rates ap-proached 1 mpy. This system had softwater, a low heat load, a large watervolume and major leaks in the system.This combination kept the water fromcycling to normal hardness levels.

A commercially-available coppercorrosion inhibitor was introducedinto the system in a single dose andthe corrosion rates remained accept-able throughout the year. After the leakswere corrected, cycles of concentrationelevated and corrosion rates decreasedto acceptable levels.

A clean system will have lower corro-sion rates due to less micro-biologically-induced corrosion. Maintaining a hard oralkaline water quality where calcium car-bonate has a tendency to precipitate re-sults in a less corrosive water than a softor acidic water. Removing the precipitatedsolids from the water reduces the occur-rence of under-deposit corrosion.

Ozone, although an excellent treatmentoption, does not work as a stand-alonetreatment on all systems. High heat ex-changer skin temperatures can causescale to adhere in a manner that ozonecannot prevent.

Well water may be so hard that it can-not run through the system even one timewithout scaling. In the following example,a cooling tower cools electrical leads fora foundry. Although the makeup watersource is city water, it is drawn from wells.The LSI of the makeup water is 2.75. Thishigh LSI coupled with the high skin tem-perature of the electrical leads, caused anunacceptable scaling on these leads.

The amount of ozone required to treata cooling water system is dependent on anumber of factors: water flow rate to thetower, water volume, water temperature(higher temperatures shorten the life of

ozone), bacteria count and organics in theincoming water, and organics in the air(cooling towers clean the air well).

Waters that are high in organic con-tent, such as river or gray water (prima-rily treated sewage), are sometimes usedas makeup water. These waters requiresignificantly larger amounts of ozone thanthe normal city water used in most HVACand light industrial systems. This is be-cause the city water treatment processhas already removed the bacteria presentin river or gray water, which reduces theozone demand.

In this case, the scaling reduced thewater flow, increasing temperature andscale deposition until the water passagewas totally blocked. Partially softeningthe makeup water reduced the scaling ten-

dency adequately on this system andactually resulted in overall cost savings.

Manufacturing OzoneOzone for use in cooling water treat-

ment is made by corona discharge or ul-traviolet light. Corona discharge involvesgenerating an electric field between twoconductors. The ultraviolet method usesultraviolet light to break the bond betweenoxygen molecules. Some of these oxygenradicals then combine with other oxygenmolecules to form ozone. Figure 1 showsa simplified reaction. For most coolingwater applications, corona discharge is

the method of choice because ultra-violet light produces less than 0.5%ozone versus about 1.5% for coronadischarge for the same energy input.

Production of ozone using coronadischarge occurs when a high-energyelectric field exists between two con-ductors (with gas containing oxygenbetween the conductors). Typically a4,000 to 10,000 volt electrical potentialis placed on two conductors spacedabout 0.3 to 0.6 in. (10 to 15 mm) apart.Air or oxygen is then passed throughthis gap with the result that some ofthe oxygen is converted to ozone(Figure 2).

A typical generator produces about1.5% ozone by weight for air. Using90% or higher oxygen concentrationsinstead of air will generate up to 13%ozone by weight depending on gasflows and cooling efficiency. The en-ergy required to produce a pound of

ozone is about 11.5 kW using air and isabout 4.5 kW using oxygen. Most of theenergy input (80% to 90%) is convertedto heat and must be removed by someform of cooling.

How an Ozone Generator WorksThe main components of an ozone

system are the gas feed preparation sys-tem, the generator and the contacting anddelivery system (see Figure 3).

Clean, dry feed gas (air or oxygen) isimportant to ozone generator operation.Water vapor in the air combines with ni-trogen and forms nitric acid. In air with adew point above 40F (40C), the nitricacid formed will not be enough to depressthe pH of the circulating water, but it usu-ally will corrode the electrodes.

M a y 1 9 9 8 A S H R A E J o u r n a l 4 1

Using an air compressor with a pro-duction capacity of at least twice that ofthe air consumption of the generator pro-longs the life of the air compressor. A highquality air compressor minimizes mainte-nance and downtime. Using oil-free com-pressors where practical helps to mini-mize the chance of getting oil in the ozonegenerator. An auto-drain on the compres-sor tank automatically removes watercondensed in compression.

Because more than 80% of the electric-ity used in the production of ozone is con-verted to heat, adequate cooling is impor-tant. Common methods of cooling theelectrodes include air cooling, refrigerat-ing and water cooling. With refrigeration,a small refrigeration coil removes heat fromthe electrodes. Cooling water generallycomes from one of three sources: the cool-ing tower cold water basin, the makeupwater source or the chilled water loop. All

of these methods are effective at remov-ing heat from the electrode. The genera-tor manufacturer incorporates coolinginto the generator and can recommendalternative methods if necessary.

Each manufacturer controls ozone pro-duction differently. Three common waysof control are using a potentiometer, ORPcontroller or no control. With a potenti-ometer the user must adjust the outputbased on some method of determiningozone requirements. With an ORP (oxida-tion-reduction potential) controller, aprobe (similar to a pH probe) measuresthe level of oxidants in the water. Al-though this is not a direct measure ofozone, ozone is the only oxidant normallyadded to the system. The ORP controlleruses high and low set points to controlthe production of ozone. The downsideof ORP is that the probe requires mainte-nance to be accurate.

Figure 3: Major components of ozone water treatment system.

Generally, it is not recommended to op-erate an ozone generator without moni-toring or some form of automatic andmanual control. Usually, the ozone de-mand varies significantly throughout thecooling season. The demand may swingfrom 10% of the generator capacity to100% with the changing conditions ofoutside temperature, heat load and organiccontent of the air and water. Excess ozoneproduction wastes energy, can acceleratecorrosion and result in excessive off gas-sing in the area of the distribution piping.

Methods to determine the ozone de-mand include visually inspecting the towerfor algae and/or making actual bio-counts.If there is algae growing in the water, thereis probably slime throughout the system,which can be removed by increasing theozone output. Making bio-counts, usu-ally by dip slides, is a 48-hour process,which delays monitoring by two days.

Advantages of Ozone Water Treatment

Reduced risk of Legionnaires Disease: ozone is anextremely efficient biocide.

Reduced operating costs: chemical and watersavings coupled with improved operatingefficiencies reduce system operating costs.

No chemical handling, storage or dischargeproblems. Ozone is generated on-site as needed.

Automatic control insures adequate treatment.

Disadvantages of Ozone Water Treatment

Initial capital expenseHowever, as the technologyof ozone generation and mass transfer improve,systems that five years ago had paybacks of five tosix years now have paybacks of two to three years.

Limitations of temperature and water quality:1. Hot water temperatures above 120F (48C) and

cold water temperatures above 100F (38C)should be avoided.

2. Plan to partially soften water using pH control orother means to reduce scaling tendencies ofmakeup waters with a LSI of 2.0 or higher.

C O O L I N G T O W E R S

4 2 A S H R A E J o u r n a l M a y 1 9 9 8

The use of ORP, while an indirectmethod of measuring ozone, is the mostcommon method of determining theamount of ozone needed and produced.This method minimizes the time requiredfrom plant personnel, provides a constantlevel of oxidant with changing conditionsand uses the least amount of energy tomaintain the proper ORP level. ORP sen-sors are commonly placed in the sidestream. It is important that the watersample passing over the probe is repre-sentative of the amount of ozone goingto the heat exchanger equipment.

A typical contacting system consistsof a side-stream pump and a venturi educ-tor (Figures 4 and 5). The water in whichthe ozone is dissolved is usually a 2% to7% side-stream of the circulating rate ofthe tower. The side-stream pump suctionusually is placed somewhere close to thesupply to the heat load. The water ispumped through the eductor where ozoneand water are mixed. Over 90% of theozone can be transferred to the water withthis type of mixer. From the eductor thewater containing ozone and the ozone-depleted air are pumped to the coolingtower cold water basin. Distribution nor-mally is done by using a PVC pipe withdistribution holes at the louver face ofthe cooling tower.

Telemetry is common and can be usedto remotely monitor the system status.Several types are available and multipleinputs allow monitoring of conductivityand pH as well as ORP and fault lights.

The telemetry system is a data logger thatrecords data over several days. This datacan be examined to establish the nor-mal rhythm of generator operation. Manypotential service problems such as fouledprobes or faulty blowdown valves canbe diagnosed by evaluating the data. Thisminimizes downtime and emergency ser-vice requirements.

Conductivity and pH controllers areother peripheral devices installed on sys-tems. These devices maintain water qual-ity automatically. Maintaining water qual-ity is important to prevent scale buildupon heat exchange surfaces. The conduc-tivity controller eliminates the need tomanually measure the conductivity andblowdown to acceptable limits. Conduc-tivity controllers are highly recommendedfor any cooling tower system.

In some installations the ozone needsto be injected into an indoor storage tank.Because not all of the ozone dissolves inthe water, off-gas control is important toprevent ozone concentrations from ex-ceeding the OSHA limits of 0.1 for an eight-hour time weighted average, or a 0.3 short-term exposure limit.

The process of preventing ozone frombeing released into closed spaces isstraightforward. Undissolved ozone is re-moved from indoor tanks by venting cov-ered tanks, adding a cover and vent to othertanks, or adding a de-gas tank and vent inother situations. If desired or needed, avent fan and ozone destruct device canalso be added.

Please circle the appropriate number onthe Reader Service Card at the back ofthe publication.

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Figure 5: Ozone distribution/piping layout for a typical cross-flow cooling tower cold water basin.

ConclusionUsing ozone in cooling tower systems

is an excellent method of water treatment.In a large number of applications, ozonecan effectively replace chemicals and helpto maintain the high quality water neededin cooling towers.

BibliographyG. Coppenger, B. Crocker, D. Wheeler. 1989.Ozone Treatment of Cooling Water: Resultsof a Full-Scale Performance Evaluation, Cool-ing Tower Institute Technical Paper No.TP89-07.

K. Mortensen. 1996. Comparison of Labo-ratory and Field Observations: Ozone WaterTreatment for Cooling Tower Systems.ASHRAE Transactions. 102, Part 1: pp. 591-599.

Ozone Treatment for Cooling TowersNewEnergy and Water Saving Technology to Re-duce Cooling Tower Operating Costs. A Fed-eral Technology Alert produced for the U.S.Department of Energy by the Pacific North-west Laboratory.

Figure 4: Typical ozone/water mixing usinga venturi eductor.