P • A • R • T • 4

SPECIALIZED WATERTREATMENT

TECHNOLOGIES

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CHAPTER 38COOLING WATER TREATMENT

Most of the water employed for industrial purposes is used for cooling a productor process. The availability of water in most industrialized areas and its high heatcapacity have made water the favored heat transfer medium in industrial andutility type applications. Direct air cooling is finding increasing use, particularlyin water-short areas but is still far behind water in total numbers of applicationsand total heat transfer loading.

During recent years, the use of water for cooling has come under increasingscrutiny from both environmental and conservational points of view and as aresult, cooling water use patterns are changing and will continue to do so. Forexample, many systems pass cooling water through the plant system only onceand return it to the watershed. This creates a high water withdrawal rate and addsheat to the receiving stream. On the other hand, cooling towers permit reusingwater to such a large extent that most modern evaporative cooling systems reducestream withdrawal rates by over 90% compared to once-through cooling. Thissubstantially reduces the heat input to the stream but not to the environment,since the heat is transferred to the air.

These changes in cooling water system design and operation have a profoundimpact on the chemistry of water as it influences corrosion, deposition, and foul-ing potential in the system. This chapter reviews the industrial operations whichuse water for cooling purposes, the problems of corrosion, scale, and fouling inthese systems and how these problems affect plant production through loss of heattransfer, equipment failures, or both. In addition, various cooling water treatmentconcepts are examined and the control procedures required for their success arediscussed.

HEATTRANSFER

Heat transfer is simply the movement of heat from one body to another, the hot-ter being the source and the cooler the receiver. In cooling water systems, theproduct or process being cooled is the source and cooling water the receiver.

Cooling water usually does not contact the source directly; the materials areusually both fluids, separated by a barrier that is a good conductor of heat, usuallya metal. The barrier that allows heat to pass from the source to the receiver iscalled the heat transfer surface, and the assembly of barriers in a containmentvessel is a heat exchanger.

In many industrial heat exchangers both the source and receiver are liquids. Ifthe source is steam or other vapor that is liquefied, the heat exchanger is called a

condenser; if the receiver is a liquid that is vaporized, the exchanger is called anevaporator.

The simplest type of heat exchanger consists of a tube or pipe located con-centrically inside another—the shell. This is called a double pipe exchanger (Fig-ure 38.1). In this simple exchanger, process liquid flows through the inner tubeand cooling water through the annulus between the tubes. Heat flows across themetal wall separating the fluids. Since both fluids pass through the exchanger onlyonce, the arrangement is called a single-pass heat exchanger. If both liquids flow

FIG. 38.2 Simple detail of shell-and-tube heat exchanger. The water box may bedesigned for as many as eight passes, and a variety of configurations of shell-sidebaffles may be used to improve heat transfer, (a) Several water box arrangements fortube-side cooling, (b) Assembly of simple two-pass exchanger with U-tubes.

FIG. 38.1 Double-pipe heat-exchanger units like this may be assembled from a num-ber of common modules to provide the necessary heat transfer rate.

Cooling water out

Hot fluid in

Cooled f lu id out

Cooling water inInner tubeOuter tube

End caps

Cool ing waterf l o w

Two-pass head

Legend: •- F lowing away Flowing toward

Four -pass head

Cooling water in

Tube sheet ShellProcess fluid out

Shellside baff lesProcess f luid in

Cooling water out

Channel(wa te r box)

Flat head

in the same direction, the exchanger is parallel or cocurrent flow; if they move inopposite directions, the exchanger is a countercurrent type.

Progressing from this exchanger, more sophisticated units are designed toimprove the efficiency of the heat exchange process. Figure 38.2 shows a shell-and-tube exchanger. Process fluid and cooling water could be located on eitherside of the barrier.

Another simple heat exchange device is the jacketed vessel, with cooling waterpassing through the space between the double walls of a chemical reaction vessel,removing heat from the process. This design is like a thermos bottle, but in thiscase, the double wall is used for heat removal instead of insulation. Plate-typeheat exchangers, somewhat resembling plate-and-frame niters, are used in manychemical process industries because of their compact design and availability in awide range of materials of construction.

Removing Undesirable Heat

Once the water completes its job and cools the source, it contains heat that mustbe dissipated. This is accomplished by transferring heat to the environment. Inonce-through systems cool water is withdrawn, heated, and returned to a receiv-ing stream, which subsequently becomes warmer. In this type of system eachpound (0.454 kg) of cooling water is heated I0F (0.560C) for each Btu (0.252 cal)removed from the source.

In open recirculating systems, water is evaporated; this phase change from liq-uid to gas discharges heat to the atmosphere instead of to a stream. Evaporatingwater dissipates about 1000 Btu per pound (555 cal/kg) of water converted tovapor. When evaporation is used in the cooling process, it can dissipate 50 to 100times more heat to the environment per unit of water than a nonevaporative sys-tem. (This is explained in more detail in a later section of this chapter.)

Sensible Heat Transfer

The two most common ways heat is transferred from process fluid to coolingwater in the heat exchange process are conduction and convection. Heat flowsfrom a hot fluid through a heat exchange surface to the other side by conduction.Heat is then removed from this hot surface by direct contact with cooling wateri.e., by conduction. Subsequently this heated water then mixes with other coolerwater in a heat transfer process called convection.

The five factors controlling conductive heat transfer are:

1. The heat transfer characteristics (thermal conductivity) of the barrier.2. The thickness of the heat transfer barrier.3. The surface area of the barrier.4. The temperature difference between the source and the cooling water (the driv-

ing force).5. Insulating deposits on either side of the barrier.

Of these five factors, the first three are inherent in the design of the exchanger.Items 4 and 5 are operational characteristics that change depending on the con-ditions of service. Deposits on either side of a metal barrier have a lower thermal

conductivity than the metal itself, so the rate of heat conduction is reduced byany deposit. For example, a buildup of 0.1 in. (0.25 cm) of calcium carbonatescale on a heat exchanger tube wall may reduce the rate of heat transfer by asmuch as 40%.

This reduction means that the cooling water may not remove sufficient heatfrom the process. Therefore, production must be slowed or the flow of coolingwater must be increased to maintain the same cooling rate that prevailed beforefouling developed. Frequently the latter is not possible, and the productivity ofthe process unit or the entire plant is reduced.

Heat Exchanger Design

In the simple heat exchanger noted earlier (Figure 38.1), both fluids pass throughthe exchanger only once, so this is known as a single-pass exchanger. The coun-tercurrent arrangement in which source and receiver liquids flow in oppositedirections is superior to the cocurrent design because it provides a greater drivingforce (measured as a mean temperature difference) for the same terminal temper-atures; less surface area can transfer the same amount of heat. Counterflow shell-and-tube exchangers are thus more commonly found because more heat can betransferred for a given set of conditions.

The principal disadvantage of the double-pipe exchanger is the small heattransfer surface provided by a single tube relative to the total space required forinstallation. To offset this limitation, modern exchangers increase the effectiveheat transfer surface area by using multiple tubes in the shell. This allows moreintimate contact between the source and receiver. Also, most industrial heatexchangers employ multiple pass designs. Both of these techniques combine effi-cient heat transfer with practical equipment sizing. Figure 38.2 shows a double-tube pass, single-shell pass exchanger. In almost all units of this kind, water flowsinside the tubes, with process fluids on the shell side, outside the tubes. However,there are notable exceptions, and any investigation of heat exchanger perfor-mance must begin with identification of the shell side and tube side fluids. Forconvenience, this text concentrates on conventional flow, with cooling water inthe tubes and process fluid in the shell.

In the design of heat exchangers, the engineer obtains from handbooks heattransfer rates in Btu per hour per square foot of surface, per inch of barrier thick-ness, per degree of temperature difference, or U1 = Btu/h/ft2/in/°F (cal/h/m2/cm/0C). For a given heat exchange tube size, which may be standardized in specificdesigns such as condensers, since the wall thickness of the tube is known, thetransfer rate may be shortened to

U2 = Btu/h/ft2/°F

= (cal/h/m2/°C)

The expression may be further reduced for fixed process temperature conditionsto

U3 = Btu/h/ft2

= (cal/h/m2)

And finally, a given exchanger in a fixed application which shows little change intemperature conditions may simply be rated in total heat transfer in Btu/h (cal/

Tubeside velocity ( f t /s )with constant shellside velocity

FIG. 38.3 Heat transfer coefficients change withvelocity, both on shell side and tube side. Uniformshell-side velocity cannot be achieved because of lim-itations in geometry.

Cooling water temperature varies across the cross section of a tube, the hottestwater being that contacting the tube wall. The temperature of the tube wall, calledthe skin temperature, is important in designing chemical treatment programs. Infact, skin temperature is the most important variable controlling corrosion anddeposition. The individual contributing factors of water velocity, heat flux, andwater and process temperatures all combine to define skin temperature. Forexample, where high skin temperatures occur [above 20O0F (930C)] the plant canfind the most probable locations for scale formation and corrosion. Many com-pounds found in water-formed deposits are less soluble at increased temperatures,and corrosion reactions proceed faster at elevated temperatures.

Cooling water can be on either the tube side (inside the tubes) or shell side(surrounding the tubes) of an exchanger. From a water treatment perspective,there are significant advantages to having tube-side water. With this type ofexchanger, water velocity is usually maintained above 2 to 3 ft/s (0.6 to 0.9 m/s)to as high as 7 to 8 ft/s (2.1 to 2.4 m/s) to help keep the tube walls free of sus-pended solids deposition. Lower velocities encourage tube deposits to form by thesettling out of suspended solids. In a bundle of tubes, perhaps only one tube mayhave a low velocity due to plugging or poor distribution.

Occasionally, high pressures on the process side make it more economical todesign the exchanger with cooling water on the shell side. One major problem insuch exchangers is the low flow velocities frequently encountered around baffles,tube supports, and tube sheets even when the average flow velocity through theshell appears acceptable. These low velocity areas influence skin temperature andgreatly increase the potential for deposits and rapid metal deterioration. For

h). If the heat flux in Btu/h falls off, then an investigation is needed to determineif this is because of changed temperature conditions, insulating deposits, pluggedtubes, or some other factor. These problems can occur on either the water or pro-cess side.

In most process heat exchangers, heat flux averages 5000 to 6000 Btu/h/ft2 (120to 150 cal/h/m2). However, heat flux in some exchangers exceeds 30,000 Btu/h/ft2 (750 cal/h/m2) when vapor is condensed on the process side. Transfer ratesalso vary considerably with water flow velocities (Figure 38.3).

Ove

rall

coef

ficie

nt,

Btu

/hr/

sqft

/°F

example, mild steel exchangers with water on the shell side have been known tofail from perforations in as little as 3 months, even in the presence of a strongcorrosion inhibitor like chromate.

COOLING WATER SYSTEM: PROBLEMS &TREATMENT

The principal water side problems encountered in cooling systems are:Corrosion is a function of water characteristics and the metals in the system.

Corrosion causes premature metal failures; deposits of corrosion products reduceboth heat transfer and flow rates.

Scale is caused by precipitation of compounds that become insoluble at highertemperatures, such as calcium carbonate. Scale interferes with heat transfer andreduces flow.

Fouling results from the settling out of suspended solids, build up of corrosionproducts, and growth of microbial masses. Fouling has the same effect on the sys-tem as scaling, but fouling also promotes severe corrosion under deposits.

The treatment of cooling water follows the same basic principles for all typesof cooling systems. The first step is to properly identify the problem as scale, cor-rosion, fouling, or combinations of these factors. The next step is a thorough sur-vey to understand both the process and water side of the system. This establishesthe system design, operating characteristics, and water chemistry, important con-siderations for selecting and applying a reliable, economical treatment program.Special considerations are given to systems restricted to specific treatments; thepotential for cross-contamination of water with process or product may not per-mit employing the most effective treatment. There are three basic types of coolingwater systems: once-through, closed recirculating (nonevaporative), and openrecirculating (evaporative).

Once-through Cooling

Once-through water is taken from the plant supply, passed through the coolingsystem, and returned to the receiving body of water. Heat has been picked upfrom the source. The chief characteristic of once-through water systems is the rela-tively large quantity of water that is usually used for cooling. A simple flow dia-gram for a once-through cooling water system is shown in Figure 38.4. Some once-through systems use plant water for drinking as well as cooling, thereby requiringchemicals that are safe for potable use.

A typical chemical treatment program for corrosion control may use varioustypes of inorganic phosphates alone or synergized with zinc. When applied at thelow levels required for economical treatment of once-through systems, thesematerials form no visible film on the metal surface; nevertheless, they can reducethe corrosion rate by as much as 90% over nontreated systems. Corrosion protec-tion is provided because the chemicals act at the point of potential metal loss,hindering the corrosion reaction and thereby reducing the amount of metalremoved from the surface.

Where scale is~a problem, it is most often calcium carbonate resulting from achange in the stability index of the water.

Polyphosphates are typically used for scale control in potable water systems.

In nonpotable applications, phosphonates, specific acrylate polymers or phos-phonate/acrylate combinations are more effective scale inhibitors. These inhibi-tors function in two ways to prevent calcium carbonate scale on heat transfer sur-faces and in distribution lines:

1. They interfere with the potentially scaling ions and prevent crystal growth.Inorganic polyphosphates and organophosphorus compounds are normallyused alone or together for this purpose (threshold treatment). Occasionallyacid is used to adjust the stability index of the water thereby preventing CaCO3scale. Acid will not control iron and manganese scales. Usually it is not themost economical method for treating high-volume once-through systems forprevention OfCaCO3 scale.

2. They condition crystal nuclei to prevent their growth on heat transfer surfacesand transmission lines. This process of crystal modification uses various poly-mers and phosphate compounds—both organic and inorganic—and some-times natural organics.

Fouling, the deposition of particulate matter, iron, manganese, or microbialmasses, is a complex mechanism governed by variables such as particle size andcharge; water velocity, composition, and temperature; and bacterial populations.

FIG. 38.4 Typical once-through cooling in a smallindustrial plant.

One approach to handling this problem is to condition foulants such as iron andmanganese as they develop by continuously applying specific polymers so that theconditioned material will be carried out of the system. The success of thisapproach depends on adequate water velocities throughout the system. Lowvelocity areas, such as in shell side exchangers, reactor jackets, and compressorjackets, are likely to accumulate some sludge and may not be amenable toprotection.

A second approach involves dispersing the suspended solids into very tiny par-ticles thereby preventing their agglomeration into sufficiently large particles thatwould readily settle out of the water. These small particles can be more easilycarried through the system. Chemicals frequently used include surfactants andlow molecular weight polymers. The choice of the best dispersant depends on the

To sewerCompressorjacket

Bearing

Mill watersupply

Pump seal

Heatexchanger

problem to be solved. Polymers can be tailor-made to optimize dispersant per-formance for specific foulants. This is especially true for foulants such as iron andmanganese.

Most fouling problems in all types of cooling systems are complicated bymicrobial activity. Slime deposits on tubes not only interfere with efficient heattransfer, but act as a trap to enmesh suspended solids, further impeding heattransfer. In addition, by-products of bacterial metabolism influence water chem-istry, including the tendency for scale to form or metal to corrode. Proper use ofbiocides and biodispersants can be a major step toward solving a once-throughfouling problem.

Rarely do corrosion, scale, and fouling occur independently of one another.Usually two or all three develop together to cause loss of heat transfer and pre-mature metal loss. For example, microbial fouling can cause scaling and corrosionto occur; corrosion can contribute to iron fouling and encourage more corrosionto occur. To break this cycle, proper problem identification is important forselecting and applying a practical, economical solution to any deposit problem.

Closed Recirculating Systems

A closed recirculating system is one in which the water is circulated in a closedloop with negligible evaporation or exposure to the atmosphere or other influ-ences that would affect the chemistry of the water in the system. These systemsusually require high chemical treatment levels, and, since water losses are negli-gible, these levels are economical. High-quality makeup water is generally usedfor best system operation. These systems are frequently employed for critical cool-ing applications, such as continuous casters in the steel industry where the slight-est deposit from any source could cause equipment failure.

FIG. 38.5 For many critical applications of heattransfer, water in a closed loop is used for fail-safechemical control, and this water is cooled by anopen system.

Figure 38.5 shows a simplified closed recirculating system. Heat is transferredto the closed cooling water loop by typical heat exchange equipment and isremoved from the closed system loop by a second exchange of heat from theclosed loop to a secondary cooling water cycle. The secondary loop could useeither evaporative or once-through water cooling, or air cooling.

Velocity of water in closed systems is generally in the 3 to 5 ft/s (0.9 to 1.5 m/s) range. Temperature rise usually averages 10 to 150F (6 to 90C), although some

Open loop Closed loop

Critical heatload

systems can exceed this substantially. Generally closed systems require little orno makeup water except for pump seal leaks, expansion tank overflows, and sur-face evaporation from system vents. This periodic makeup requires regular anal-ysis for control of correct treatment chemical residuals.

Closed systems usually contain a combination of different metals which pro-vide a high potential for galvanic corrosion. The potential for dissolved oxygenattack is generally quite low in closed systems because of the small amount ofmakeup water—the main oxygen source. However, in systems that require sub-stantial makeup because of loss of water from leaks, oxygen is continually sup-plied and oxygen corrosion presents a serious problem. Oxygen can, at elevatedtemperatures or at points of high heat transfer, cause severe pitting corrosion.

Since relatively little makeup is added to most closed recirculating systems, itis practical and desirable to maintain the system in a corrosion-free condition.This is normally achieved by applying chromates, nitrite/nitrate-based inhibitors,or soluble oil-type treatments at rather high concentrations.

Theoretically, scale should be a minor problem in a closed system since thewater is not concentrated by evaporation. In a tightly closed system, none of thecommon scale-forming constituents deposit on metal surfaces to interfere withheat transfer or encourage corrosion.

With high makeup rates, however, additional scale forms with each new incre-ment of water added so that in time, scale becomes significant. In addition, thereis opportunity for sludge, rust, and suspended solids to drop out at low flow pointsand bake on heat transfer surfaces to form a hard deposit. Therefore, scale retar-dants and dispersants are usually included as part of a closed system treatmentprogram where makeup rates are high. Often soft water or condensate is used formakeup to closed systems depending on the characteristics of the system beingprotected.

Because water circulating through a closed system is not exposed to the atmo-sphere, fouling by airborne silt and sand is rare. However, fouling by microbialmasses may occur in closed systems where makeup rate is significant or processleaks encourage bacterial growths. These are controlled with biological controlagents formulated to be compatible with the chemical treatments and operatingconditions found in closed systems.

It is desirable as a part of routine maintenance to flush closed water systemswith high-pressure high-velocity water to remove accumulated debris if makeuprates are high.

Open Recirculating Systems

An open recirculating system incorporates a cooling tower or evaporation pondto dissipate the heat it removes from the process or product. An open recirculatingsystem (Figure 38.6) takes water from a cooling tower basin or pond, passes itthrough process equipment requiring cooling, and then returns the water throughthe evaporation unit, where the water evaporated cools the water that remains.The open recirculating system repeats this process of reuse, taking in sufficientfreshwater makeup to balance the water evaporated and that blown down fromthe system to control the chemical character of the recirculating water. Thisgreatly reduces water demand (e.g., withdrawal from a river) and discharge, asshown by Figure 38.7.

The following definitions are used to explain the operation of an evaporative

Concent ra t ion r a t i o ( C R )

FIG. 38.7 Reduction of makeup flow by concentration in an evaporative coolingsystem.

system and to permit the plant operator to calculate performance. Figure 38.6helps to clarify these definitions.

1. Recirculation rate (Qc): This is the flow of cooling water being pumpedthrough the entire plant cooling loop, usually cooling a number of exchangers.Qc can usually be estimated from the recirculating pump nameplate data;however, actual measurements are more accurate. The actual recirculation isseldom more than the nameplate data and frequently may be 10 to 20% less.A pump curve, usually available from the manufacturer, plots recirculatingflow against head; a pressure gauge on the pump discharge should provide areasonably accurate estimate of flow.

2. Temperature differential or range (&T): This term refers to the differencebetween the average water temperature returning to the tower from the plant

Evaporat ion -E

FIG. 38.6 Evaporative cooling system using a cooling tower. Waterand solids balances are shown.

A T

Cooling towerHeatloads

Makeup (M)

S lowdown - M-E

Evapora t ion (E)

Upper l imit - once- th rough f l o w

E x a m p l e - 10,000 gpm at 2O0F temperature r ise

Ma

keu

p,g

pm

Lower l im i t -evapora t i on rate

Basin

M at CMQ c - g p m at T1,0F

exchangers (T2) and the average water temperature following evaporation (T1)(tower basin).

3. Evaporation (E): E is the water lost to the atmosphere in the cooling process[gallons per minute (m3/min)]. The evaporation rate is dependent on theamount of water being cooled (Q0) and the temperature differential, AT. As arule of thumb, for each 1O0F (5.60C) temperature drop across the evaporationprocess, 1% of the recirculation rate (Qc) is evaporated. Therefore, a 2O0F(11.20C) AT across a cooling tower produces an evaporation loss of 2% of therecirculation rate (0.02 Qc = E).

E-O x (T* ~ T1)E~&x iooo

E= QCX (Tl~Tl) (metric)JOU

The amount of evaporation that can take place over a given tower is limitedprimarily by the relative humidity of the air. Relative humidity is determinedby measuring the wet and dry bulb temperatures of the air. E can be as lowas 0.75% per 1O0F of AT in high humidity areas like the Gulf Coast in theUnited States. Conversely, E may be as high as 1.2% per 1O0F AT in regionsof very low humidity, such as the arid regions of the southwestern UnitedStates. E depends to a lesser degree on the liquid-to-gas flow ratio, and con-ductive heat losses in other areas of cooling systems.

4. Makeup (M): The input of water required to replace the water lost by evap-oration plus that being lost through blowdown, tower drift, and other mis-cellaneous losses. It is usually measured by a flow meter; if not it may becalculated as shown below:

-HoN5. Concentration ratio (CR): Makeup to a recirculating cooling water system

contains dissolved impurities. The evaporating water produces pure H2Ovapor, leaving behind these impurities. The ratio of the concentrations ofsalts in the circulating water (C8) to those of the makeup (CM) is the concen-tration ratio.

CR = CB/CM

Since the input solids must equal the output solids,

M X CM = B X C8

where M is the makeup flow and B represents loss of concentrated water.Therefore, the concentration ratio is also

CR = M/B

The CR should be calculated for several individual components of thewater to determine if the system is "in balance." In the ideal case, the systemis in balance when the CRs of all ions in the water (Ca, Mg, alkalinity, etc.)are equal. If the concentration ratios are not equal, it can indicate that some

mineral (CaCO3, SiO2, etc.) is precipitating from the recirculating water. Forexample, if the CR for calcium or alkalinity is more than 0.5 below the CRfor magnesium, then CaCO3 is probably precipitating in the cooling system.By knowing what may precipitate, the CR can be a valuable indicator that aproblem is occurring.

The concentration ratios of some ions will be affected by chemicals addedto the cooling system. The CR for SO4

2" would be increased when sulfuricacid is added or where the plant atmosphere contains SO2. In these cases, theCR for alkalinity would be decreased because alkalinity is destroyed by acidadded to the tower. Chlorination of the cooling water will increase the CR forCl". A CR based on conductivity will also be increased by some of the treat-ment chemicals added.

As shown in Figure 38.7, even a small degree of concentration enormouslyreduces water demand, and the greater the CR, the lower the demand as theevaporation rate is approached as a limit.

6. Slowdown (B): Since pure water vapor is discharged by evaporation, the dis-solved and suspended solids left behind concentrate. If there were no waterloss other than evaporation, these solids would concentrate to brine, causingmassive scale and corrosion. To balance this, regulated flow is bled from thecirculating system. This blowdown (BR) is calculated and controlled toremove solids at the same rate at which they are introduced by the makeup.

There are other uncontrolled losses from the system. One is drift (BD)i theother is leakage (BL), sometimes deliberate, but usually accidental. These areincluded in the total blowdown calculation,

B — BR H- BD H- BL

Blowdown is related to other factors thus:B = M-E

and B = M/CR

7. Drift (BD): Even though evaporating water is pure, some water dropletsescape as mist through the evaporation equipment. In modern cooling tow-ers, very intricate mist and drift eliminators may be added to reduce thisdroplet loss to about 0.0005% of the recirculation rate. A more usual drift lossin conventional cooling towers is in the range of 0.05 to 0.2% loss based onthe recirculation rate. Since drift contains dissolved solids it is really a por-tion of the blowdown. In the absence of a controlled blowdown, as when theblowdown valve is deliberately closed, drift establishes the maximum con-centration ratio in the absence of other system losses.

8. System losses (B1): Circulating water may be lost in the plant through pumpor valve leaks; by tap-off for once-through cooling of pump glands, compres-sor jackets, or bearings; or draw-off for such uses as equipment or floor areawashup when the cooling water line happens to be close to where water isneeded. In many plants, miscellaneous draw-off of recirculating cooling wateris so great that it is impossible to build up the concentration ratio over 1.2 to1.5. This severely limits the selection of an economical chemical treatmentprogram and prevents effective conservation of water.

9. Holding capacity of system (V): Usually most of the water in a system iscontained in the cooling tower basin or spray pond. An approximation of theholding capacity can be obtained by calculating the volume of water in the

basin and adding an extra 20 to 30% for the water contained in the lines andequipment. Additional increases may be required if the system has anunusually large number of open box condensers, jacketed vessels, or furnaceswith substantial water holding capacity.

10. Time/cycle (t): One cycle is denned as the time required for water to makeone trip around the circulating loop. This time is a function of the holdingcapacity and the recirculation rate.

t = VIQC

11. Holding time index (HTI): The holding time index is an expression of thehalf-life of a treatment chemical added to an evaporative cooling system.Mathematically, this index represents the time required to dilute an addedchemical to 50% of its original concentration after the chemical addition isdiscontinued. It is also the time required to concentrate the makeup solids bya factor of 2. This is an important factor in setting control limits where chem-ical feed may be interrupted. It is also important for establishing an effectivedosage for biological control agents, which are slug fed into the system. Adilution curve for a chemical slug fed into a cooling tower system is illustratedby Figure 38.8.

Calculation of Holding Time Index

The half-life of a system depends on the capacity and the rate at which the com-ponents are leaving the system. For a cooling tower, the half-life depends primar-ily on the system capacity and the blowdown rate. In its simplest form, the equa-tion to calculate holding time index is

HTI = 0.693 X £f l̂iJE!>B (gal/mm)

where 0.693 = In 2.0, a number derived from standard half-life equations.To illustrate how the holding time index depends on other elements of a cool-

ing system, the factors used to calculate blowdown (B) can be substituted:

HTI = 0.693 X capacity X (CR - 1)A!I

or

HTf - 06Q3 v capacity X (CR - 1)HTI - 0.693 X a x A r x o.ooi

This illustrates that a change in several factors (Qc, AT, CR, or capacity) will allaffect the HTI.

Another method of calculating holding time index is illustrated in the follow-ing example.

1. Calculate the time per cycle, t.2. From Table 38.1 determine the number of cycles required to reach the pre-

vailing concentration ratio, based on the temperature drop through the toweror spray pond.

3. Multiply this by the time per cycle to get the holding time index expressed inminutes, then divide by 60 or 1440 to convert to hours or days, respectively.

The following example illustrates this calculation:

1. Recirculation rate: Pump data show a recirculation rate of 5500 gal/min (21m3/min). Use 5000 gal/min (19 m3/min) as a good estimate of actualrecirculation.

Example; A slug of biocide at 50 mg/1would be diluted to 25 mg/l in 34 passesat 2% evaporation (2O0F)

Percent of original concentration remaining

FIG. 38.8 The effect of time and makeup dilution on slug appli-cation of a chemical to an evaporative cooling water system.

Pas

ses

acro

ss t

ower

at

2O

0F

2. Temperature drop:

1050F - 850F = 2O0F

(410C- 3O0C = U0C)

3. Evaporation loss: 2O0F (U0C) is equivalent to 2% evaporation loss:

0.02 X 5000 = 1OO gal/min evaporation loss

4. Concentration ratio (see analyses on Figure 38.9): Tests on the makeup andrecirculating water show concentration ratios of 2.8 to 8.3.

The approximate concentration ratio is 3.0 based on magnesium and silica,since magnesium and silica remain soluble at the prevailing pH and concen-tration conditions. Chlorine and sulfuric acid are both being added to the sys-tem and thereby eliminate the use of Cl" and SO4

2"" as valid CR indicators.5. Makeup:

-HoNM= 100 X - = 150 gal/min

(M = 0.38 X I = 0.57 mVmin)

6. Holding capacity of the system: Basin contains 72,000 gal (284 m3). The totalholding capacity of the system is estimated to be 100,000 gal (379 m3).

TABLE 38.1 Cycles Required to Concentrate in theAbsence of Blowdown

Concentrationratio

1.11.31.51.82.02.53.03.54.04.55.0

Temperature drop through tower

10°

6.5 cycles20.234.555

69105138174

208241324

20°

3.2 cycles10.217.22834527087

104120162

30°

2.1 cycles7.0

11.719

23354758708193

* Not valid, since Cl^ end H^SO^ are being fed.

I I I I l I I l

FIG. 38.9 Concentration ratios in an evaporative cooling system.

7. Time/cycle:

100,000 ^ .' = ^ooo~ = 20mm

( 379m3 ™ • \t = TT;—T,—~ = 20 mm

V 19m3/mm J

8. Holding time index: IfCR = 3, and Ar = 2O0F (U0C), Table 38.1 showsaverage cycles at 70; at 70 cycles,

HTI = 70 X 20 = 1400 min, or 23 h

Identification of Analyses Tabulated Below:

A. Raw water D

B Recirculated cooling water E

C. Concentration ratios F

ConstituentCalciumMagnesium

Sodium (by difference)

Total Electrolyte

Bicarbonate

CarbonateHydroxylSul fateChloride

NitrateChromate

M AIk.

P AIk.

Carbon Dioxide

PH

SilicaIron

TurbidityTDSColor

As

CaCO3

CaCO3

CaCO3

CaCO3

Fe

A182

7840

305

190OO

8429

2O

190TRTR

8. 3

220 .3

10360Nil

B526240120

886

80OO

69694

610

80O

7 .5

68

ND

C D E F2 . 83.1

2 . 9

8 . 3 *3 .2*

1

3.1

COOLING TOWERS

Cooling towers are designed to evaporate water by intimate contact of water withair. Cooling towers are classified by the method used to induce air flow (naturalor mechanical draft) and by the direction of air flow (either counterflow or cross-flow relative to the downward flow of water).

In natural draft towers, air flow depends on the surrounding atmosphere,which establishes the difference in densities between the warmer air inside thetower and the external atmosphere; wind velocity also affects performance. Mostnatural draft towers in modern utility service are of hyperbolic design (Figure38.10), which has been used for many years in European installations. These talltowers provide cooling without fan power, and they also minimize plume prob-lems and drift.

FIG. 38.10 Hyperbolic towers cooling condenser water in a utility station. (Courtesyof The Marley Company.)

Mechanical draft cooling towers use fans to move air instead of depending onnatural draft or wind. This speeds the cooling process and increases the efficiencyof the tower by increasing the air velocity over droplets of water falling throughthe tower. Mechanical towers can, therefore, evaporate much more water thannatural draft towers of the same size.

There are two designs of mechanical draft towers, forced and induced draft. Inforced draft towers (Figure 38.11) fans mounted on the side of the tower force airthrough the tower packing, producing intimate mixing of air with the fallingwater.

Induced draft cooling towers (Figure 38.12) are either counterflow or crossflowwith fans on top pulling cooling air up through or horizontally across the fallingwater. The choice between forced draft and induced draft is based on engineeringconsiderations that take prevailing weather patterns into account. A major con-sideration is to avoid recirculation of the warm air discharge, which would greatly

FIG. 38.12 Induced-draft tower. Air enters the tower louvers and is well distributed beforeexiting at top.

FIG. 38.11 Forced-draft tower design. This design was widely used prior todevelopment of the induced-draft design. Fans and motors are convenientlylocated for maintenance.

To cooling waterc i rcu lat ing pump

Basin with level controlsfor makeup regulation

Makeup

Enclosedsides

Towerpack ing-

Spray piped is t r ibu to rs

D r i f t e l iminatorsect ion

Warm moistair discharge

Hot waterreturn

Fans

Airintake

reduce tower performance. The main advantage of a counterflow tower is that thecoldest water contacts the driest air, providing the most efficient evaporationsequence. A complete survey is shown in Figure 38.14.

COOLING WATER TREATMENT AND CONTROL

Every cooling water system presents a unique combination of equipment, waterchemistry, contaminants, blowdown, and control considerations. Proper selectionof a sound cooling water treatment program requires collecting a considerableamount of information. This is often a painstaking task because of the complexityof the mechanical equipment involved and the variations encountered in oper-ating conditions. Figure 38.13 and Table 38.2 show an example of a system sur-

FIG. 38.13 A condensed summary of data required in making a cooling system survey. A com-plete survey is much more extensive (Figure 38.14) and includes flow diagrams and tabulation ofindividual heat exchanger data (Table 38.2).

TABLE 38.2 Performance Data for Individual Heat Exchangers Tabulated on General Survey(Figure 38. 13)

Process sideCooling water

Materials of construction

SheetHeadChannelTubesPressure,

lb/in2

Designoutlet

temp, 0F

Max.inlet

temp, 0FProduct

nomenclaturePressure,

lb/in2

ATacrossexc.

Maxexit

temp, 0FVelocity

Numberof

passes,ft/s

General

Heatflux,Btu,ft2-h

Shellside ortubeside

coolingwater

Nomenclature

FCCExc.

number

Adm

Adm

SS

SS

CS

CS

CS

CS

CS

CS

SS

SS

CS

CS

CS

CS

CS

CS

SS

SS

CS

CS

CS

CS

Adm

Adm

SS

SS

CS

CS

Adm

CS

70

25

25

30

120

40

65

40

154

159

159

159

154

154

115

110

262

245

245

230

510

225

320

200

Hydrocarbonvapor

Oil

Oil

Oil

Hydrocarbonvapor

Oil

Air

Air

35

35

40

35

30

40

40

40

5

10

8

10

25

12

10

5

100

120

120

120

135

118

125

115

3

5.5

6.5

4.5

3.0

6.5

2.5

5.0

1

2

2

2

4

2

1

9,475

6,000

3,780

4,600

12,000

8,750

10,250

8,000

Tube

Tube

Tube

Shell

Tube

Tube

Shell

Shell

Compressor inletstage cooler

Compressor sealoil cooler

Compressor sealoil cooler

Bearing lube oilcooler

Main overheadtrim cooler

Main air blowerlube system

Main air blowerturbineexhaust

Main air blowerturbine glandcooler

307

309

314

316*

322

330

333

337

* Information was taken from the most critical exchanger in a series of similar exchangers.

vey, divided into five major categories: (1) cooling system data, (2) cooling waterchemistry, (3) heat transfer data, (4) effluent considerations, and (5) present treat-ment and control methods.

Cooling System Data

This section identifies physical aspects of the system such as number and type ofheat exchangers, materials of construction of exchangers and piping, type oftower, maximum temperatures of the water and process, tower operating temper-atures, and system characteristics such as velocities, makeup, bleed-off, and hold-ing time index. This section should be supplemented with the process flow andwater flow diagram as well as complete specifications on critical heat exchangers.

Cooling Water Chemistry

This section identifies the chemical environment of the system. The informationis divided into the chemistry of the makeup and recirculating water, including a

FIG. 38.14 Complete survey of a cooling water system.

description of pretreatment processes and sources and types of contamination ofthe recirculating water. Causes of poor makeup water quality and persistentsources of contamination should be examined; these are critical to the treatmentprogram, so the possibility of correction is a decisive factor in program selection.

Heat Transfer Data

This survey section is organized into four parts.

1. Results monitoring: Defines how heat transfer is evaluated, including the useof corrosion coupons and test heat exchangers; data on plant heat exchangers,permitting calculation and monitoring of heat transfer rates.

2. Control methods: Indicates how heat transfer is controlled. For example, acommon method in many plants is to throttle cooling water entering certainheat exchangers in the winter to prevent overcooling of the process. However,throttling reduces velocities and promotes fouling which leads to a loss of heattransfer that cannot always be recovered by reopening the throttled valve.Alternate control methods, such as water recycle or process stream bypass,should be considered in preference to throttling water flow.

3. Present conditions: Defines the physical conditions of the heat exchangeequipment inspected during the survey, supplemented by the analysis of sig-nificant deposits. This information provides a basis for recommendations forcleaning, preconditioning of metal surfaces, and application of chemicals forproper system maintenance.

4. Cleaning procedure: This includes mechanical and chemical cleaning proce-dures currently employed.

Plant Effluent Considerations

Some cooling systems are bled off directly into a receiving stream; others are dis-charged to various kinds of waste treatment processes; and some discharge tomunicipal sewage systems. Each imposes considerations on the choice and appli-cation of a chemical treatment program.

Control Monitoring and Followup

Chemical control, monitoring of results, and corrective action are required for aneffective cooling water treatment program. A wide variety of analytical tools andmonitoring devices are available to aid in developing and maintaining a chemicalprogram that will provide an efficient operation.

The goal of analysis and monitoring is to identify potential problems beforethey occur. The major diagnostic tools include:

1. Water analyses (on-site and laboratory)2. Deposit analyses (organic, inorganic, and microbial)3. Corrosion and deposition monitoring devices4. Metallographic analyses5. Microbial analyses

Table 38.3 lists some of the more important variables that must be controlledin cooling systems. Calcium and magnesium hardness define the scaling tendencyof the water. Total alkalinity, pH, and temperature define the concentrations ofcarbonate and bicarbonate ions in the water, and also the solubility of calciumcarbonate. All of these must be controlled within acceptable ranges for each sys-tem to ensure scale-free operation. Concentrations of sulfate and silica must alsobe controlled at reasonable levels to prevent formation of gypsum and silica scale.

Many systems contain suspended solids which concentrate in the tower andcause fouling. Dispersants may be used to control this once the problem has beenproperly defined. Some of the solids may come from the makeup, some may bewashed from the air, and some may be precipitation products or microbialmasses. If the suspended solids are excessive, as evidenced by plugged tubes, afilter system must be added to the circuit to filter a portion of the circulatingwater, approximately equal to the evaporation rate, Figure 38.15. This side-stream filter must be designed to avoid excessive backwash, as this represents

FIG. 38.15 This 15-ft-diameter automatic valveless gravity unit filters a cooling towersidestream at a Texas natural gas plant.

Variable

Ca, MgM, pH, T

SO4, SiO2

Suspended solidsContaminants:

Hydrocarbons,glycols, NH3,SO2, H2S

Effects

Define scaling tendency of waterDefine concentrations of carbonate and bicarbonate, and

solubility of calcium carbonateMust be controlled to prevent sulfate and silicate scales

Cause fouling, require dispersantsCause fouling and microbial growth, high chlorine demand,

precipitate chemical treatments

TABLE 38.3 Important Cooling Water Variables

uncontrolled blowdown loss. These filters can also become incubators for micro-bial growths if the biocidal treatments are not assiduously applied.

Table 38.4 lists some of the major chemical components available for coolingwater treatment. In using this chart note that no one of these components is usefulby itself. For example, good corrosion control is difficult or impossible in a dirty,scaled system. Good scale and fouling control to maintain clean surfaces mini-mizes the dosage of corrosion inhibitors. Many chemicals help to solve more thanone problem, as shown in Table 38.4. Whether a particular chemical is the bestchoice for a given system depends on the specific conditions in that system.

AMBIENT AIR EFFECTS

Cooling towers scrub the air passing through them to provide the evaporativeconditions, handling about 200 ft3 of air per gallon of water (about 1500 m3 airper cubic meter of water). It is not surprising, then, that the atmospheric environ-ment has profound effects on the performance of the cooling system.

In some areas, the air contains a large mass of dust, as in arid sections of thecountry, especially where dust storms are common. The western and southwest-ern areas of the United States are prone to this problem, and cooling systems therecannot work effectively without side-stream filtration. In a complex industrialplant, solids may become airborne from dirt on roads, open areas between plantbuildings, or from open storage of solids (e.g., ore or coal), and this self-generatedsource of particulates is as damaging as silt, also requiring side-stream filtration.

A more subtle, difficult problem is the presence of acidic or alkaline gases inthe atmosphere. These gases affect the pH of the system, a critical control factorin the chemical treatment program that has a direct bearing on the scale-formingor corrosive tendencies of the water. An unusual, but pertinent, example is anammonia plant cooling tower located between the ammonia process and the nitricacid process. When the wind came from one direction, ammonia in the atmo-sphere raised the pH of the system water; when the wind came from the opposite

TABLE 38.4 Chemical Components of Cooling Water Treatments

Problems

Chemical treatments

ChromatesZincMolybatesSilicatesPolyphosphatesPolyol estersPhosphonatesAll-organicsNatural organicsSynthetic polymersNonoxidizing biocidesChlorine/bromineOzone

Corrosion

X

X

X

X

X

X

X

Scaling

X

X

X

x

X

Fouling

X

X

X

X

Microbes

X

X

X

direction, acidic nitrogen-oxide gases reduced the pH. Although this is an exag-gerated case, it makes the point that in most plants, the pH of the system may beaffected by wind direction.

The most prominent of the atmospheric gases are acidic, and chief amongthem is carbon dioxide, which occurs at an average concentration of about 0.03%by volume in the atmosphere. The amount of CO2 supported in water at about680F (2O0C) by this partial pressure is less than 1 mg/L. However, the actual CO2level in a cooling tower system varies considerably from one plant to anotherbecause of local atmospheric conditions, such as the presence of industrial stackgas discharges.

There is a definite relationship between CO2, alkalinity, and pH. (See Chapter4.) If CO2 is variable, alkalinity must be varied to maintain a pH control point.This makes it difficult to predict the alkalinity concentration that will be requiredto achieve a specific control pH value. It must be done empirically.

If experience shows that the atmospheric CO2 concentration is constant, thenthe correct alkalinity for a selected pH can be chosen. The pH will then vary as alogarithmic function: i.e., if the alkalinity doubles, the pH will increase by the logof 2, or a value of 0.3; similarly, if the atmospheric CO2 doubles with a fixedalkalinity, the pH will be reduced by the log of 2, or 0.3 pH unit. The expectedequilibrium pH of any new system can only be established empirically, unlessthere is a record of previous experience in the vicinity of the new cooling toweror strong evidence that the average atmospheric CO2 concentration of 0.03%applies.

CORROSION AND SCALE CONTROL

Corrosion in recirculating cooling water systems is controlled by employing eitherinorganic or organic inhibitors. The four major inorganic inhibitors are chromate,zinc, orthophosphate, and polyphosphate. Minor supplements include molyb-date, nitrite, nitrate, various organic nitrogen compounds, silicate, and naturalorganics.

The earliest chemicals for treating recirculating cooling waters were inorganicpolyphosphates and natural organic materials. The concept was to add a smallamount of acid to control the stability index to a slightly scale-forming value.Organic corrosion inhibitors include organic phosphorus compounds, specificsynthetic polymers, organic nitrogen compounds, and long-chain carboxylicacids.

Polyphosphate and natural organic materials were added to the program toprovide both corrosion protection and scale inhibition. The scale inhibitionstemmed from the use of the polyphosphate as a threshold treatment. In addition,the polyphosphate combined with calcium to form a cathodic inhibitor thatreduced the corrosion rate. The natural organic material tended to keep the metalsurface relatively clean and aid the inhibitor in establishing a protective film. Italso dispersed suspended solids, and modified calcium carbonate and tricalciumphosphate precipitates if they tended to develop on hot surfaces.

The greatest disadvantage of this treatment approach is the reversion of poly-phosphate to orthosphosphate, which can combine with calcium to form calciumphosphate scale. For this reason, this type of program has evolved into the sta-bilized phosphate program. In this treatment, both ortho- and polyphosphate areused as corrosion inhibitors. To prevent calcium phosphate deposition, the pH is

generally controlled at 7.0 and specific synthetic polymers are added to disperseand stabilize calcium phosphate.

The next cooling water treatment was chromate, an exceptionally reliable cor-rosion inhibitor. Initially, chromate was applied at very high dosages, frequentlyin the range of 200 to 300 mg/L as CrO4. Acid was added to the system to lowerthe pH to between 6 and 7, preventing calcium carbonate from precipitating. Thistreatment was quite effective in both scale inhibition and corrosion protection,but one shortcoming was that pitting attack tended to occur if the chromate resid-ual became low. It was found that if chromate were combined with other inhibi-tors, particularly cathodic types (e.g., zinc and polyphosphate), the chromate levelcould be reduced to 20 to 30 mg/L CrO4 with better results than obtained at 200to 300 mg/L CrO4 used alone. The synergized chromate approach also employedacid, frequently controlling the pH to 6 to 7. An additional advantage of syner-gized chromate was the margin of safety provided against pitting attack shouldthe chromate be momentarily underfed.

These synergized chromate formulations are still considered among the bestcorrosion inhibitors in use today. However, increasing environmental pressuresare forcing the development of innovative synergized chromate formulations thatpermit carrying chromate levels in a recirculating system substantially below 10mg/L CrO4 while continuing to provide acceptable corrosion protection. Toachieve results with this approach the system pH must be controlled precisely,and dispersants and biocides used to keep the system clean. An obvious limitationto this approach is that the reservoir of protection available with the higher CrO4levels does not exist. Therefore, process contamination, uncontrolled microbialactivity, fouling, and deposition will disrupt the system much more quickly thanat the more traditional 20 to 30 mg/L CrO4 levels.

Although chromate has done an outstanding job for years, increasing environ-mental concerns have brought pressure on research into new corrosion inhibitorswith potentially less environmental impact. An early result of such research wasthe development of organozinc combinations. Since zinc, a cathodic inhibitor,has a lower film strength than chromate, the pH of the system for an organozincprogram was increased to between 7 and 8 to make the water less corrosive, allow-ing the zinc to form a satisfactory inhibitor barrier. The organic portion of thetreatment was a dispersant to keep the system free of deposits, thereby encour-aging formation of an adequate zinc film. In addition to dispersancy, certain typesof organics increased zinc solubility at the higher pH required for this method oftreatment. These programs were adequate in many industrial plants, but becausethe inhibitor film at the operating pH was not as effective as a chromate film, theseprograms did not substantially replace traditional chromate-type treatments.

Subsequently, an innovative concept in cooling water chemistry arrived withthe introduction of organophosphorus compounds. Like inorganic polyphos-phates, these prevent scale formation by the threshold effect. However, there thesimilarity ends; inorganic polyphosphates easily revert to orthophosphates, withincreasing holding time, temperature, and microbiological attack. Organophos-phorus compounds do not revert under normal cooling tower conditions exceptunder severe microbiological attack. Further, unlike the inorganic polyphos-phates, the organophosphorus compounds are generally able to inhibit precipita-tion of calcium carbonate and other scale-forming species at a higher pH and alka-linity than tolerated by the inorganic polyphosphates. This development openedthe door to what is now known as the alkaline approach to treating cooling watersystems.

The basic treatment concept is to raise the pH of the operating system to 7.5to 9.0, thereby substantially reducing the natural corrosivity of the recirculatingwater. Experience has shown that although the higher pH provides a less corrosivewater, frequently this reduction is not of sufficient magnitude to protect all mildsteel systems, especially mild steel heat exchangers with high heat flux or low flowvelocities. Thus a specific all-organic inhibitor package is required to control cor-rosion and scale. In general, all-organic inhibitors combine organic phosphoruscompounds, synthetic polymers, and aromatic azoles. These combinations pro-vide corrosion control for steel and copper alloys, scale control, and depositcontrol.

Another approach to alkaline treatment involves the use of modern scale anddeposit control agents along with more traditional corrosion inhibitors. Organicphosphorus compounds and polymers can be supplemented with inorganics likechromate or zinc. These programs can provide the performance of an all-organicprogram at a lower cost, where chromate or zinc can be used.

The significant advantage provided by alkaline operation over earlier treat-ments is the buffer capacity provided by the water that reduces the impact of sys-tem upsets on performance. Another particular advantage of the alkaline conceptof treatment is the substantial reduction or occasional elimination of acid feed.This, of course, depends on the chemistry of the system.

FOULING CONTROL

Deposit control in cooling water systems is absolutely essential for maintenanceof heat transfer rates. However, control of deposits is often more difficult in alka-line systems than in lower pH systems. The makeup water may contain dissolvedsolids, organic matter, and suspended solids, any of which can contribute to foul-ing. A system may become grossly contaminated with microbes; for example,makeup water with a high BOD, such as a recycled municipal or industrialeffluent, is particularly susceptible to fouling from slime-forming bacteria.

Table 38.5 shows some sources of foulants in a typical recirculating system.The raw water and air inoculate a system with colloidal organic matter, silt, sol-uble iron, and microbes. Hydrogen sulfide, sulfur dioxide, and ammonia mayenter from the plant atmosphere.

The selection of the proper dispersant for any operating system is based onactual analysis of a deposit. Synthetic organics, including polymers and surface-active agents, are generally applied for dispersing microbial and organic deposits.

TABLE 38.5 Sources of Fouling Deposits

Raw water

Colloidal organics

Silt, dirt

Soluble iron

Microbial contamination

Airborne

Dirt

Reactive gases — H2S,SO2, NH3

Microbial contamination

Recirculating water

Scale: CaCO3, CaSO4,MgSiO3

Corrosion products: Fe2O3

Process leaks —hydrocarbons, sulfides

Microbial deposits

Synthetic polymers such as polyacrylates or polyacrylamides are dispersants forsilt, sand, iron, and other inorganic deposits. These polymers can be tailor-madeby varying the components and molecular weights to maximize dispersant per-formance on specific foulants. Organophosphorus compounds, including polyolesters and phosphonates, are inhibitors for calcium carbonate and calcium sulfateprecipitates. However, once deposits form, any scale removing action by thesedispersants takes place slowly, so the best approach is to prevent the scale fromforming in the first place.

MICROBIAL CONTROL

Microbial deposits present a special case of fouling. Treatment often requires bio-cides to kill microbe colonies and dispersants to loosen and wash them away. Themost common biocide employed in all systems is chlorine. In general, chlorine isthe only biocide required in most systems. If applied continuously at a residualof 0.2 to 0.4 ppm it will provide effective control at all cooling water pH values.At alkaline pH, the continuous presence of chlorine species in the water will pro-vide the required microbial killing power because of the infinite contact timeavailable. In intermittent chlorination, such as utility cooling systems, the chlo-rine contacts the microbial organisms for short periods of time. In this case pHcan be more important. Sterilization studies have shown that chlorine kills fasterat pH 7 than above pH 8. This may be due to the greater amount of HOCl presentin the hypochlorite equilibrium at pH 7. Thus slug chlorination may be moreeffective at neutral pH because HOCl has a faster killing power than OC1~.

There are problems associated with the use of chlorine. It can react with someorganic materials, particularly phenolic compounds, to form reaction productsthat are nonbiodegradable or refractory, presenting potential effluent problems.Generally speaking, chlorine can be applied to most recirculating systems withoutdanger of tower lumber delignification if free chlorine residuals do not exceed 1mg/L. It is seldom necessary to continually carry a free chlorine residual over 0.2to 0.3 mg/L to control microbial growths in most systems. Bromine is often amore practical treatment than chlorine because it remains effective at higher pHvalues and avoids formation of the kinds of halogenated by-products resultingfrom chlorination.

Although chlorine and bromine are excellent killing agents, their performancecan be significantly improved by the use of biodispersants. Biodispersants aid thetoxicant by breaking loose the biofilms and enabling them to contact more micro-bial organisms. In cases of gross contamination or loss of toxicant feed, a contin-gency nonoxidizing biocide may be required (See Chapter 22).