Cooling Training

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COOLING OPERATOR TRAINING

MAR206 9901 Page 5-1 1997, BetzDearborn A Division of Hercules Incorporated. All rights reserved.CHAPTER 5

CHAPTER 5DEPOSITION

INTRODUCTION

The gradual accumulation of deposits in cooling watersystems directly affects production. Deposition prob-lems can lead to:

reduced tower efficiency

decreased heat transfer

reduced carrying capacity of pipelines

Ultimately, if unchecked, deposition can result in pro-duction losses, shortened equipment life, and increasedcosts, due to frequent cleaning or added pumpingrequirements.

FACTORS AFFECTING DEPOSITION

Deposit formation is strongly influenced by a number offactors. The key factors are:

water composition

pH

water and exchanger skin temperatures

water velocity

residence time

system metallurgy

These factors are interrelated. The most severe deposi-tion is normally encountered in process equipment,operating at high skin temperature and/or low watervelocity. For cooling towers with high efficiency film fill,deposit accumulations are another area of concern.

SCALE AND FOULANTS

In general, deposits are broadly classified as scale orfoulants.

Scale is a coating of predominantly inorganic (salt-like,or mineral-like) materials, formed by precipitation andsubsequent crystal growth at a surface in contact withwater. Precipitation occurs when the solubilities ofdeposit-forming minerals are exceeded, either in thebulk water or at the surface. The most commonscale-forming salts that deposit on heat transfer sur-

faces show retrograde, or inverse solubility with temper-

ature. Lets look at this more closely.Most salts become more soluble as temperature isincreased. Common table salt (sodium chloride) showsthis kind of behavior: the hotter the solution, the higherthe concentration of salt that dissolves in it. Some saltsshow inverse solubility: as the temperature increases,their solubility decreases (See Figure 5-1). With saltshaving inverse solubility, the potential for scalingproblems is greatest in the hottest part of the coolingsystem the heat exchanger surfaces which alsohappen to be the most critical for efficient heat transfer tooccur.

Calcium carbonate, calcium sulfate, calcium phos-phate, and magnesium silicate are examples of saltsthat have inverse solubility. They may be completelysoluble in the lower temperature bulk water of thecooling system, but they are not soluble in the highertemperature water at the heat transfer surface of theexchangers and they precipitate on the surface. Notethat calcium and magnesium salts are particular prob-lems in this respect. Most s odium salts show normalsolubility. Most hardness- based salts show inversesolubility.

Theres more at work here than just temperature effects.Scales form by a multistep process which begins withthe production of tiny crystals of a hardness salt asshown in Figure 5-2. This initial step is termed nucle-ation. The initial crystals grow and can actually providemore nucleation sites, accelerating the scale-forming

Figure 5-1: Mineral solubility

Temperature

S o l u b i l i t y

CommonSalts

HardnessSalts

InverseSolubility

NormalSolubility


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DEPOSITION

Page 5-2 MAR206 9901 1997, BetzDearborn A Division of Hercules Incorporated. All rights reserved. CHAPTER 5

process. If you can interfere with the nucleation step,you can interfere with scale production. Conditions atmetallic surfaces, however, are ideal for crystal nucle-ation. The surface of the metal is rough from an atomicviewpoint, providing sites for nucleation. In addition,water velocities are lower next to a surface, which

prevents turbulence from breaking up nucleation sites.Further compounding the problem, corrosion cells on ametal surface produce local areas of relatively high pH,promoting the precipitation of many cooling waterhardness salts. Once formed, scale deposits provideadditional nucleation sites, and crystal growth acceler-ates.

Calcium carbonate is the most common scale formed incooling water systems, because it is formed from twomaterials present in virtually all makeup water: calciumhardness and bicarbonate alkalinity. The chemistry ofcalcium carbonate formation, however, depends onseveral factors:

calcium hardness

bicarbonate alkalinity

total dissolved solids (TDS)

pH

temperature of the water

Figure 5-2: Crystal growth leading to scale

CO 32

Ca +2Ca +2

Ca +2

CO 32

CO 32

DissolvedAtoms

Nucleation(Suspended

Small Crystals)

Crystal Growth(Suspended

Large Crystals)

Scale(Deposition and

Continued Growthof Large Crystals)

Ca +2

Ca +2

Ca +2

CO 32

CO 32

Ca +2

Ca +2

Ca +2Ca +2

CO 32

CO 32CO 32

CO 32Ca +2

Ca +2

Ca +2

Ca +2

Ca +2CO 32

CO 32

A university professor, W. F. Langelier, explored howthese factors are all interrelated. He developed anindex, or mathematical short-cut that we use to predictthe tendency of calcium carbonate to either precipitateor remain soluble in cooling water systems. This index isthe Langelier Saturation Index, LSI. If its positive,

calcium carbonate tends to deposit. If the LSI isnegative, calcium carbonate tends to dissolve or remainsoluble.

Basically, the LSI measures the gap or difference in pHbetween the water s actual pH and the pH at whichcalcium carbonate would start to precipitate. The pH atwhich calcium carbonate would start to precipitate iscalled the saturation point, and that s where thesaturation part of the name LSI comes from. And, all thefactors affecting scaling listed above can act to changethat saturation point. The use of LSI tables and a samplecalculation are provided as enrichment reading at theend of this chapter.

One way to prevent calcium carbonate scaling is tomaintain a negative LSI. That sounds easy, but it is quiteoften not feasible. Incoming makeup water may containenough calcium, alkalinity, and TDS to push the indexwell into the positive range. In addition, the morenegative the index, the higher the potential for corrosion.It does not make much sense to prevent a depositionproblem by promoting corrosion! As we will see later inthis chapter, there are special deposit control agentswhich we can add to interfere with the normal scalingtendency of positive LSI waters.

Another common deposit is calcium phosphate. Likecalcium carbonate, it becomes less soluble with in-creasing pH and temperature. Typical phosphatesources include river or city water, partially-treatedsewage waters or phosphate-based water treatmentprograms. As we saw in Chapter 4, phosphate is oftenadded to prevent corrosion. High levels of phosphateprovide protection to anodic areas at the metal surfaceand controlled precipitation of calcium phosphate pro-tects cathodic areas. If phosphate precipitates as anuncontrolled scale on a heat exchanger surface, notonly do we get reduced heat transfer, but the loss ofphosphate in the bulk water may promote further

corrosion.Dissolved silica is found naturally in water. As water isconcentrated in a cooling tower system, high levels ofsilica can build up. Levels of silica greater than 200 ppm[mg/L] often lead to silica scales. Unlike most otherscalants, silica shows normal solubility its solubilityincreases with increasing temperature. Therefore, if weare going to have silica scale, it usually deposits in thecoldest parts of the cooling system. We sometimes see


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DEPOSITION


silica scale on the tower fill and transfer lines. Silica canalso react with itself, forming large aggregates.

Calcium and magnesium can react with silica to formvery tough magnesium silicate or calcium/magnesiumsilicate scale. These compounds show inverse solubility

and deposit on exchanger surfaces. Deposits formed bythese scalants are highly tenacious slight amountscan drastically decrease heat transfer.

Iron in well water often shows low solubility in coolingsystems, leading to the formation of iron depositsthroughout the cooling system. They can be hard,dense, tightly adhering scales, or loose, highly porousdeposits. Both forms can drastically reduce heattransfer efficiency.

Manganese is often seen in conjunction with iron.Manganese deposition depends on system metallurgy.

Manganese deposits form preferentially on admiraltyand stainless steel surfaces. Once formed, thesedeposits are very difficult to remove and they can lead tosevere under-deposit corrosion.

Aside from the ions occurring naturally in water,treatment chemicals, added to control corrosion, cancause deposit problems if not fed properly. It really ispossible to have too much of a good thing in thesetreatment programs. That s why testing and monitoringthe cooling system are critical to maintaining reliableoperations with minimal deposition.

FOULING

Fouling is the accumulation of suspended materials inwater as opposed to dissolved materials in waterwhich usually form scales. Suspended materials includemud, silt, organic compounds, oils, dust and dirt,corrosion products, biological slimes, and generaldebris.

Suspended solids enter a cooling system in a number of

ways. If the makeup water contains suspended solids,these become concentrated during tower operation asthe water is cycled. As air passes through the tower, it isscrubbed by the tower water. Any dust or particlescarried in the air are scrubbed into the water. In addition,microorganisms carried by the air have a primebreeding ground in the cooling water because of itswarmth and nutrient loading. Biological-based foulingcan severely reduce heat transfer efficiency if notcontrolled.

DEPOSIT CONTROLJust as there were mechanical and chemical methods tocontrol corrosion, there are also mechanical andchemical methods to control deposition.

In terms of mechanical control methods, improvingmakeup water quality to the tower is of prime impor-tance. Lime/soda softening, zeolite softening, ion ex-change and reverse osmosis are options; however, agood cost/benefit analysis is needed to make the mostappropriate treatment choice.

Another alternative is to allow the impurity to precipitateas a removable sludge rather than as a hard deposit.Clarifiers or solids separators are designed to removethese sludges. Many systems use sidestream filters onthe tower water to remove, or at least reduce suspendedsolids or corrosion products, substantially decreasingthe potential for deposition.

Process adjustments provide another means of control-ling deposits. These adjustments include increasingblowdown, decreasing pH, increasing water velocity orreducing temperatures. There is a cost limitation toincreasing blowdown: operation of a tower at too few cycles can be uneconomical. Acid is often added tocooling systems to reduce pH, but many customers donot want to handle acid in their plants. Velocities are veryimportant in shell-side exchangers, where suspendedsolids in the cooling water can settle in low flow areas.

Even with good process adjustments, however, achemical deposit control program is required for optimalsystem operation. There are 3 types of chemical control:

inhibitors dispersants surfactants

Inhibitors delay or retard crystal growth of scale-formingsalts by adhering to the surfaces of crystals. They distortcrystal structure as it is forming, during the nucleationstep. This makes the initial crystal fragile, breaking intosmaller units or even redissolving. Use of an inhibitorallows higher salt concentrations in the system withoutdanger of deposition on hot exchanger surfaces. Themicrophotographs in Figure 5-3 show the powerfuleffect of crystal inhibitors.

A tremendous research effort has been expended bythe water treatment industry to discover new, morepowerful crystal growth inhibitors. The objective is tofind more cost-effective molecules that can be applied atlower dosages, or to extend the range of chemistry inwhich a deposit control program works. A brief history ofthese deposit control agents is given in Chapter 7.


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DEPOSITION


Figure 5-3:Example of crystal distortion using

phosphonate

a. Untreated

b. Treated

Dispersants control particle size by interfering withparticle-to-particle attraction. Normally, particles areattracted to each other and combine to produce largerparticles, eventually leading to deposition. The use ofdispersants interferes with this process. Dispersantsattach to particles, while they are still small, and givethem a greater negative charge. As you may recall fromusing magnets: like charges repel. The same effectapplies: negative charges on the particles repel eachother and particles are prevented from growing to a

dangerous size. Figure 5-4 shows this effect. If we cankeep small particles dispersed, they can be removedfrom the cooling system by blowdown. Again, consider-able research work continues to find the right moleculeto disperse specific particles or foulants.

Surfactants are also useful in a deposit control program.These chemicals act like soap, reacting with greasesand oils, and dispersing them. Surfactants have aninteresting structure: part of the molecule has a

Figure 5-4: Deposit control agents

CO 32

Ca +2Ca +2

Ca +2

CO 32

CO 32Inhibitors

ReduceNucleation

Force SmallCrystals toRedissolve

Ca +2Ca

+2

Ca +2Ca +2

CO 32

CO 32

Ca +2

Ca +2CO 32

CO 32

Dispersants

Prevent LargeCrystals fromGrowing

Ca +2

Dispersants also

Prevent Crystalsfrom Attaching toMetal Surfaces

moderately charged end and prefers to dissolve inwater, like other charged species. The other part of themolecule has a non-charged end that does not likewater very much. This end of the molecule prefers oilsand greases, which are also not very highly charged

chemicals. As shown in Figure 5-5, when surfactantmolecules react with grease or oil, the non-chargedends line up and surround the oil globule, leaving thecharged ends free to disperse themselves and the oil in the water. This is the same mechanism whichmakes soap effective at removing grease and dirt fromyour hands. The word surfactant is an abbreviation ofsurface active agent. Surfactants are effective be-cause they act on the surfaces of a mixture of oil andwater.


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DEPOSITION


Figure 5-5: Surfactants

SurfactantMolecule

Charged End(Prefers Water)

Uncharged End(Hates Water)

SurfactantSurroundingand DispersingOil Droplet

Water

Oil

Biodispersants are a special class of surfactants used inmicrobiological control programs. As you will see in thenext chapter, part of the problem with microbiological

growth in cooling systems is the production of slimelayers on cooling system surfaces. Biocides are used tocontrol microbiological populations, but they are oftenused with biodispersants. Biodispersants break up theslime layer by a soap mechanism, as described above.Once the slime layer is broken, biocides are used to kill

the microbiological populations. Use of biodispersantspermits lower overall dosages of biocides, saving theplant money, and posing less of a problem for ultimatedisposal.

In this chapter of the workbook we examined thedifferences between mineral-based scale and otherfoulants. We introduced you to the LSI measurement, animportant consideration in deposit control. We alsoreviewed the three main chemical approaches we use toreduce deposition and fouling: inhibitors, dispersants,and surfactants. Your attention to chemical feed levelsand monitoring tests is a vital factor in the success of anydeposit control program.


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DEPOSITION


KEY WORDS

Dispersants

Fouling

Inhibitors

Inverse Solubility

Langelier Saturation Index (LSI)

Nucleation

Scaling

Surfactants


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DEPOSITION


CHAPTER QUIZ

1. When crystals start to form, the first step is termed nucleation. True False

2. An important factor in the LSI calculation is the water velocity in a heat exchanger. True False

3. An important factor in the LSI calculation is the total dissolved solids of the water. True False

4. In the following list of compounds, identify which show normal solubility (solubility increaseswith temperature) and which show inverse solubility (solubility decreases with temperature)

Compound Normal Solubility Inverse Solubility

Sodium Chloride

Calcium Carbonate

Calcium Sulfate

Sodium Bromide

Magnesium Silicate

5. Sodium chloride deposition is very difficult to treat in a cooling system. True False

6. All cooling towers should be operated with a negative LSI to avoid deposition of

calcium carbonate. True False

7. Dispersant chemistry is based on the principle that like charges repel. True False

8. In the surfactant molecule, both ends are highly charged and prefer to dissolve in water. True False

9. It is impossible to interfere with the way crystals grow. True False

10. If scaling is the accumulation of mineral salts, what is fouling?


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DEPOSITION


11. What are some of the steps you can take to reduce the level of fouling in a cooling system?

Figure 5-6: LSI Calculator

P ar t s

p er Mi l l i on

( m g / L )

C Scale

pAlkpAlk and pCa Scale

P a r t s p e r

M i l l i o n

( m g

/ L )


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DEPOSITION


ENRICHMENT READING

Langelier Saturation Index

Many makeup waters for cooling systems containsignificant calcium hardness and natural alkalinity. Thisis a dangerous combination. When cycled up in arecirculating cooling system, calcium carbonate candeposit on heat exchanger surfaces. As scale accumu-lates, heat exchange efficiency decreases, eventuallyresulting in production losses.

The mechanism for the precipitation of calcium carbon-ate scale involves several factors: calcium concentra-tion, alkalinity levels, temperature, pH, and totaldissolved solids. The Langelier Saturation Index incor-porates these factors into a very helpful mathematicalformula which measures the tendency of calciumcarbonate to precipitate or remain dissolved with anygiven combination of factors.

The first step in the use of LSI is the calculation of thetheoretical pH at which calcium carbonate begins toprecipitate as a scale. This is the pH of saturation,abbreviated pH s . Here is an example, taken from the 9thedition of the Betz Handbook.

Suppose the recirculating water has the followingcomposition:

Calcium Hardness 200 ppm [mg/L]as calcium carbonate

M-alkalinity 160 ppm [mg/L]

as calcium carbonateTotal solids 400 ppm [mg/L]

pH 7.80

and the skin temperature of a critical exchanger is140 F [60 C].

The value for pCa is read from the ppm [mg/L] scale onFigure 5-6, proceeding horizontally to the left diagonalline and reading the pCa from the scale below the line.The value for pAlk is read from the same lower scale, butusing the right-hand diagonal line. The contribution from

total solids is also read from the same ppm [mg/L] scale,but read the value on the top C scale corresponding tothe appropriate temperature of the exchanger. Someinterpolation is needed for temperatures between thoselisted on the chart.

In the example, the pCa corresponding to a calciumconcentration of 200 ppm [mg/L] is 2.70. The pAlkcorresponding to an alkalinity of 160 ppm [mg/L] is 2.50.The C scale value for 400 ppm [mg/L] total solids at140 F [60 C] is 1.56

The pH of saturation, pH s , is the sum of these threevalues:

pHs = pCa + pAlk + C = 2.70 + 2.50 + 1.56 = 6.76

The LSI is the difference between the pH s and the actualwater pH:

LSI = pH actual pH s = 7.80 6.76 = + 1.04

In this case, the positive LSI indicates a tendency of thewater to form calcium carbonate precipitate. Note that itis an indication , not an absolute prediction that the waterwill form scale on the exchangers. If the LSI werenegative, calcium carbonate would probably not precipi-tate; however, the water could be corrosive. If the LSIwere close to zero, the water would be in equilibrium .

Due to the mathematical calculations above, somecorrosive waters may actually have positive LSI values.To remove this ambiguity, Ryznar modified the LSI,proposing a Ryznar Stability Index, calculated asfollows:

Ryznar Stability Index = 2 (pH s ) pH actual

Waters with a Stability Index of 6.0 or less tend to formscale and the potential for corrosion decreases. Waterswith a Stability Index exceeding 7.0 usually do not formscale. When the Stability Index exceeds 7.5, theprobability of corrosion increases.

In some cooling treatment systems, it may be necessaryfor you to calculate the LSI as part of your monitoringprogram. Your BetzDearborn representative will reviewthe calculations with you.


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CHAPTER 6MICROBIOLOGICAL FOULING

INTRODUCTIONOur focus in this chapter of the workbook is very differentfrom all the other chapters. In the other chapters, we talkabout non -living processes: corrosion, deposition,pumps, monitoring tools, and treatment programs. All ofthese, although complex, do not involve life. When lifeis introduced into the equation, things get very compli-cated.

Cooling systems provide conditions which support thegrowth of microscopic life predominantly algae, fungi,and bacteria. You are already familiar with these lifeforms. Algae grow in swimming pool water or standingponds, turning the water green or yellow. Fungi show upin the form of bread molds. Mushrooms are largerspecies. As yeasts they make bread dough rise. Andbacteria, well, they are everywhere. They are responsi-ble for many serious, human infections, such aspneumonia and tuberculosis. They also help to makecheese and are responsible for the natural fertilization ofsoil, in the form of nitrogen-fixing bacteria. Although theyhave good and bad implications for human life, incooling systems they are only bad news.

We distinguish between two major classes of microor-ganisms:

planktonic free-swimming organisms, live in the water

sessile prefer to live attached to a surface

Microorganisms that attach to wetted surfaces growthere and, in time, form larger communities. Thesecommunities, called biofilms, consist of microbial cellsand material secreted by the cells as a protective layer.This layer consists of complex biological polymers.These materials are gelatinous and stickyslimy.Another word for biofilm is slime. This process is

illustrated in Figure 6-1.If their growth is not controlled, biofilms interfere withequipment performance: biofouling can reduce or evenblock water flow, reduce heat transfer and increasecorrosion rates. Some biofilm organisms attack wood,which weakens structural members of wooden coolingtowers. Plus, dirty cooling systems increase the risk ofairborne disease from inhalation of cooling tower driftladen with microorganisms.

Figure 6-1: Development of biofilm

Water

Cooling Tower Surface

STAGE 1 Individual microorganisms attach to a surface

Water


STAGE 2 Original organisms multiply into small colonies

Water


STAGE 3 Colonies secrete protective layer

Water


STAGE 4 Colonies merge to form biofilm

Water


STAGE 5 Slime layer traps debris

Without adequate microbiological control, effectivenessof corrosion inhibition and deposition control programsis seriously compromised. For example, a thick protec-tive slime layer can prevent corrosion inhibitors fromreaching the surface of the metal beneath the organ-isms. Worse yet, the organisms often secrete acidicwaste products which actually accelerate corrosion ofthe metal. Protective slime layers are sticky and trapsuspended solids from the bulk water, increasing thelevel of fouling in the cooling system.


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MICROBIOLOGICAL FOULING


Where do microorganisms come from? In open recircu-lating systems, they are primarily introduced into thetower by being scrubbed out of the tremendous volumesof air drawn through the towers. They also enter bothopen and closed systems by way of contaminatedmakeup water or by inleakage from process streams.

Why do they thrive in cooling systems? Cooling systemsprovide optimal conditions for growth of microorgan-isms:

water

nutrients for growth

optimal temperature

preferred pH range

The nutritional needs of microorganisms are simple.They primarily need sources of carbon, nitrogen, and alittle phosphorous. These elements can be in themakeup, leaked into the cooling system from processstreams, scrubbed out of the air, leached from the towerwood, or even added as corrosion or depositiontreatment chemicals. Many species need oxygen forgrowth, but oxygen is abundant in an open recirculatingsystem. Some organisms actually thrive in the absenceof oxygen. As we will see below, closed systems are afavorite breeding ground for such species.

Before we discuss each particular type of microorgan-

ism, let s look at their general characteristics. First, asthe name implies, they are quite small. Typically, amicroscope is needed to see the individual organisms.Unfortunately, a second characteristic is a very rapid growth potential. Microorganisms reproduce at a phe-nomenal rate because they typically grow by celldivision: a given cell grows and divides into two offspringcells. These two cells then grow and divide into four cells, then eight, etc. This leads to explosive growth andlarge populations, the third characteristic. It s whenpopulations grow very large that we see resultingcolonies with the unaided eye.

Microorganisms are also environmentally flexible. Notonly are there very many species, but they mutaterapidly, changing their basic biochemistry as the envi-ronment changes. These creatures are also nutritionallyversatile in that they adapt to changes in available foodand can make use of thousands of different compoundsto survive.

Now let s look at the three major classes of microorgan-isms which invade cooling systems.

ALGAEThe first class is algae. This class of organisms is a formof simple plant life and is characterized by photosyn-thesis algae use sunlight to provide energy and tosynthesize much of what they need to grow. Using light,they take CO 2 from the air and react with water to makesugars and other compounds. They use chlorophyll, ayellow-green compound, to assist with this chemistry.Because they need sunlight to grow, they are generallyfound on the tower deck and support members of thetower. Chlorophyll gives the colonies their characteristiccolor. Algae can plug nozzles, upset the tower waterbalance, plug screens, and reduce tower efficiency.Because they convert CO 2 to organic compounds whichthey and other organisms use, algae are often referredto as pioneer colonizers of a cooling system. They setthe stage for the growth of other organisms which follow.A microscopic view of a small colony of algae is shown inFigure 6-2.

FUNGIFungi can best be described as simple plants, likealgae, but they lack chlorophyll. Their inability tosynthesize all their food requirements compels them tolive off the byproducts of other creatures, or to get theirnutrition from nonliving materials. Fungi include moldsand yeasts. They require less moisture and survive atlower pH than algae or bacteria.

The major class of fungi we are concerned with incooling towers is the wood destroyers. These includespecies which produce soft rot, white rot and brown rot.The white rot mold and yeast species attack and eatcellulose which makes up wood fibers. Brown rotattacks lignin , the binder holding wood fibers together.Often the decay is internal, weakening timbers with littleor no outward sign of the condition. Other species, while

Figure 6-2: Algae


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not directly attacking wood, add to the slime mass ontower surfaces. A good illustration of cooling tower woodattacked by fungi is shown in Figure 6-3.

Figure 6-3: Cooling Tower Wood Attacked By Fungi

BACTERIA

One of the largest classes of living microorganisms onthe Earth is bacteria. Unlike algae and fungi, bacteriatend to grow throughout the entire cooling system. The

enormous variety of bacterial types allows them tosurvive in a wide range of environmental conditions andnutrient sources. Most bacteria require oxygen; theseare called aerobic. Some live in the absence of oxygen;these are called anaerobic. This group includes sulfate-reducing bacteria. These organisms, because they donot need oxygen for growth, are often found in sludgesor underneath deposits. They give off hydrogen sulfide,H2S, which has the smell of rotten eggs. H 2S is verycorrosive; consequently, these bacteria can cause a

tremendous amount of under-deposit corrosion dam-age. Special testing is needed to measure the presenceof these organisms. An example of metal damaged bycorrosion under a deposit of sulfate-reducing bacteria isshown in Figure 6-4.

Figure 6-4: Metal corroded by sulfate-reducing bacteria

The main characteristics of the three classes ofmicroorganisms are summarized in Table 6-1.

Table 6-1: Microbiological Groups

Examples

Type CharacteristicsEveryday

LifeCoolingTowers

Algae Microscopic plants Photosynthesis

(only need sunlightand CO

2)

Yellow, yellow-greenin color (chlorophyll)

Greenswimmingpool

Slime ondistributiondeck andsupportmembers(sessile)

Fungi Simple plants, butlack chlorophyll

Bread mold,mushrooms

Wood rot(sessile)

Bacteria Largest class ofliving organisms

Enormous variety

Infections,Cheese

Turbid water(planktonic)

Under-de-posit corro-sion (sessile)


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POPULATION DYNAMICSOne of the reasons microorganisms are a problem incooling systems is because of their incredible versatility.A change in system chemistry can have disastrousresults. For example, suppose there is a stable popula-tion of organisms in a tower. A shift in pH may upset thebalance and accelerate their growth into an unbalanced,troublesome phase. There are many operational factorswhich can affect pH. A process leak may drop the pH.Too low a pH may encourage growth of molds and yeast.Raising the pH to reduce corrosion may encourage analgae bloom in the cooling system. There are strongseasonal effects in open systems. In the fall, contamina-tion of the cooling system with falling leaves can depresspH, encouraging bacteria to grow at the expense ofalgae. Whenever changes occur in cooling systemoperations, it s always a good idea to be on guard forincreases or changes in the nature of microbial activity.

MONITORINGA detailed discussion about the measurement of thenumber of microorganisms in cooling systems is given inChapter 9. Historically, the water treatment industryfocused on methods for measuring planktonic or free-swimming organisms. It was direct and easy to do platecounting. Modern improvements on the earlier tech-niques include Petrifilm, serial dilution vials, shown inFigure 6-5, and DipSlides. Unfortunately, most of theproblems in cooling systems stem from sessile organ-isms biofilms, discussed above. There really is not a

good correlation between bulk water populations andbiofilm presence. Therefore, the water treatment industryhas changed the focus of its monitoring efforts to includespecialized techniques for sessile populations Betz-Dearborn Biobox, MonitAll , Delta P monitor, and afouling monitor (these are all discussed in Chapter 9).

Figure 6-5: Serial dilution vials

The most commonly used measure of microbiologicalcontrol is counting the number of microorganisms permilliliter of sample. This number is just like pH or calciumor alkalinity: it is a numerical test value and must becompared with a designated control range. If a value isbelow the range unlike pH or calcium or alkalinity

that s OK. If a value is above the range, we must killsome of the organisms.

CONTROLThere are three general types of products used tocontrol microbiological fouling:

biocides: (1) oxidizing biocides(2) nonoxidizing biocides

surfactants: (3) biodispersants

Biocides kill microorganisms. They do so in an

oxidizing manner, such as with chlorine and bromine, orthey do so by a nonoxidizing mechanism. Biodisper-sants are special surfactants which break the protectiveslime layer of biofilms and help the biocide get to theorganisms to kill them. Biodispersants, in themselves,are not toxic to microorganisms; however, they are aclever way of enhancing biocide performance at rela-tively low cost, with minimal environmental impact.

Oxidizing biocides are very reactive chemicals which,in effect, burn whatever compound the oxidizer attacks.Oxidizers typically go after complex carbon-containingstructures. They break open cell walls. They destroylife-supporting materials, such as proteins and enzymesand DNA. Because they are nonselective, oxidizers areeffective against the widest range of organisms.

Oxidizing biocides include chlorine gas, sodium hy-pochlorite or liquid bleach, bromine-chlorine com-pounds, and ozone. In the United States, chlorine gashas been the main disinfecting compound for manyyears for water-based applications. It has been used totreat incoming raw water, cooling systems, and evensome wastewater streams. Recent concerns over theliability of storing chlorine gas cylinders on site have ledmany users to consider alternative biocides.

Sodium hypochlorite, liquid bleach, is chlorine in liquidform. It provides more or less the same disinfectingchemistry as chlorine gas, but it eliminates the need forpressurized gas cylinders. It is still a very dangerouschemical in its own right and extremely corrosive. It isavailable commercially as a dilute solution. A consider-ably larger amount of bleach is needed than chlorinegas for the same level of disinfection. This means thatbleach requires large, expensive storage and feedfacilities.


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There is a new chemistry which has many of the benefitsof oxidizing-type kill but is much safer to store and feed.It is based on a bromine-chlorine organic compound.The material is furnished as a solid which slowlydissolves in a feeder, releasing both bromine andchlorine into the water. Although the product itself is

more expensive than bleach, feed systems are consid-erably less expensive and the material is far safer than bleach or chlorine gas. A typical feed system isshown in Figure 6-6.

The new bromine-chlorine chemistry works well in highammonia waters and at high pH. And it works well wherecontact time is limited. It is also proving to be lesscorrosive than chlorine to system metallurgies, espe-cially yellow metals.

Ozone is another oxidizing biocide used in coolingsystems. It is so unstable that it has to be generatedonsite, next to the cooling system. Ozone generatorsare capital-, maintenance-, and energy-intensive; how-ever, ozone is an effective biocide against mostmicrobiological populations. Unfortunately, it is incom-patible with many cooling treatment chemicals and, likeany oxidizer, overfeed can cause corrosion.

Nonoxidizing biocides are much more specific in theway they attack microorganisms. Instead of the burn chemistry of chlorine, nonoxidizers either interact withthe membrane surrounding an organism or they inter-fere with its metabolic activity.

Figure 6-6: BetzDearborn brominator

The cell membrane is critical to survival of the organism.The membrane regulates materials entering and leav-ing the cell. Some biocides selectively damage the cellmembrane. With a defective membrane, the cell losescontrol over its internal environment and dies.

Other biocides enter the cell and poison a specific

biochemical activity within the cell. There are manybiochemical reactions going on in every cell: producingenergy, moving nutrients into the cell, using nutrients tomake new cell material, and cell division. Interfere withenough of these reactions, or damage a critical reaction,and the cell dies.

Because nonoxidizing biocides are more specific thanoxidizers, a given nonoxidizing biocide may not beeffective against all organisms in a cooling system. Wehave to test for the right choice of biocide and the mostcost-effective dosage. We often have to switch biocidesas microbial populations change. Special combinations

of biocides are available. Multiple actives expand the killrange and are sometimes more effective than theirindividual components used alone. Multiple actives mayalso provide better microbial control at lower netdosages.

CONTACT TIME AND CONCENTRATIONRegardless of what biocide is used, there is an importantprinciple at work. We need the correct concentration ofbiocide sustained over a certain minimum period of time(the contact time). Whether oxidizing or nonoxidizing,biocides need time to do their work. If we achieve the

correct concentration but cut the contact time short, thebiocide does not give us maximal kill.

Oxidizing biocides are typically fed continuously orsemi-continuously at low levels, although a higher dosemay be needed to bring a population back under control.Nonoxidizing biocides are typically slug-fed at relativelyhigh levels. But these are generalizations. Each biocidehas a preferred feed profile and a lot depends on systemdynamics, system chemistry, and biocide chemistry.Your BetzDearborn representative is an excellentsource for this kind of information.

CHEMICAL SAFETYBiocides are, by definition, toxic. Anyone handlingbiocides must observe proper safety procedures. Theseinclude the right choice of gloves (both style andcomposition), splash-proof goggles and faceshield. Thelabel on a biocide container and the MSDS (MaterialSafety Data Sheet) provide specific safety precautionsfor each product and should be reviewed prior to biocideuse.


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KEY WORDSHere is an alphabetic listing of the key words for this chapter of the workbook. Write your own definitions in the spaceprovided and then check your answers with the text. The words were defined when first introduced in the chapter, andthis was indicated by bold text .

Algae

Bacteria

Biocides

Biodispersants

Biofilm

Colonies

Fungi

Nonoxidizing Biocides

Oxidizing Biocides

Photosynthesis

Planktonic

Sessile

Slime (Layer)


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CHAPTER QUIZ

Here is a test to determine your understanding of the material in this chapter. Check your answers with the text or checkwith your BetzDearborn representative.

1. There are three general classes of microorganisms which cause trouble in cooling systems. They are

_________________, ______________________, and __________________.

2. Cooling tower support members are often made of wood because it is resistant tomicrobiological attack. True False

3. All organisms on the Earth need oxygen from the air to survive. True False

4. Some microorganisms prefer to live in the recirculating water or bulk water of a cooling system.

These organisms are termed ________________________.

5. In contrast to the organisms in question 4, others prefer to live attached to or growing on a surface.

These organisms are termed ___________________________.

6. Wood rot is caused by a fungus attack on wood. True False

7. There is always a direct relationship between the number of microorganismsgrowing in the bulk water and the number of microorganisms growing oncooling tower system surfaces. True False

8. Chlorine is one of the most powerful biocides available. True False

9. Nonoxidizing biocides are very selective in the types of microorganisms they kill . True False

10. Because it is very stable, ozone is often purchased in bulk containers. True False


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11. The best source for information regarding chemical characteristics,

handling precautions, and special safety requirements for biocides is the

M_____________________ S___________________D______________ S_________________

available at your plant.

12. Biodispersants are often used in conjunction with biocides for effective control ofmicrobiological fouling. True False

13. All biocides work by the same chemical mechanism. True False

14. In your own words, describe the stages in the development of a biofilm or slime layer.


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CHAPTER 7TREATMENT PROGRAMS

INTRODUCTION

All water treatment, whether related to cooling or boilersystems, is directed toward important plant goals: to extend equipment life to minimize downtime and associated production

losses to minimize maintenance costs to avoid system upsets to maximize heat transfer/energy efficiency

We have to achieve these goals using every opportunityfor recycling, minimizing wastewater, maintaining envi-ronmental compliance, and providing a safe workingenvironment for you, other plant personnel, and our ownrepresentatives who visit your plant. And, the cost of thetreatment program has to be economical.

Before we chemically treat a cooling system, there areseveral non chemical considerations to review. As dis-cussed in Chapter 4, Corrosion, its important to selectsystem metallurgies which are exactly suited for theintended operation. Mechanical engineers at equipmentdesign firms are usually responsible for this step. Metalstrength, exchanger temperature, heat transfer rates,flow velocitiesall these factors have to be considered.

Even the best designed equipment and systems can be

subjected to contamination that might have beenunforeseen in the design stages. For example, airleaking into a system that is supposed to be closeddramatically changes the corrosion activity. Processcontamination can lead to deposition and microbialgrowth. Even the wrong kind of makeup water, fed to acooling tower, can drastically upset things. A suddenintroduction of high hardness water in a system that issupposed to see low hardness makeup can lead tosignificant hardness-scale problems.

Speaking of makeup water, there are a lot of specialconsiderations that affect the choice of a cooling water

treatment program: Is there readily available soft waterin the plant? Can the system be economically made-upwith reverse osmosis water, low in conductivity anddissolved minerals? Was the tower designed with asidestream filter to handle a heavy silt load? Similarly, atthe other end of the system, can the waste treatmentplant handle blowdown from the cooling towers?

We are all seeking to recycle as much water as possible.Sometimes small recycled streams have a big impact on

system chemistry, requiring a total treatment programchange.

HVAC VS. OPEN RECIRCULATINGSYSTEMSWe have to make a distinction early in this discussionbetween HVAC systems and larger, open recirculatingsystems. HVAC systems typically operate at lowertemperatures and contain relatively high amounts ofcopper metallurgy. In addition, varying loads requirethrottled flow. Open recirculating systems often operate ata constant flow rate. Seasonal operation of HVAC systemsrequires equipment to sit idle for extended periods of time.These conditions create a severe challenge for a water

treatment program, because of increased hazards fromcorrosion and microbiological activity.

Closed systems, air washers, and potable/once-through systems also have their own unique mechanicaland chemical requirements.

PRETREATMENTThe best treatment program can fail if new piping andequipment are not properly cleaned of mill scale andcutting oils before being put into service. There are avariety of procedures for precleaning, pretreating, andpassivating cooling systems as part of startup. Thatdiscussion is beyond the scope of this chapter. Anyquestions about pretreatment should be directed to yourBetzDearborn representative.

Small comfort cooling towers typically use more galva-nized metal than larger systems. These galvanizedtowers need to be seasoned as part of the startupprocedures.

You play a critical role in the startup of any plant. Yourattention to detail and concern for pretreatment proce-dures ensures that the cooling system is getting a goodstart in its life and lays the foundation for the success of

the chemical treatment program.

COOLING WATER TREATMENTThe selection, construction, and implementation of acooling water treatment program bring together manyconcepts developed in preceding chapters. An impor-tant principle to understand is the interrelationship between corrosion, deposition, and microbiologicalfouling. This is illustrated by the triangle in Figure 7-1.


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Figure 7-1: Interrelationships between corrosion,deposition and biofouling

Deposition

CorrosionBiofouling

This diagram shows that corrosion can cause microbio-logical problems by providing growth sites and corrosioncan be caused by microbiological problems as a result ofwaste products given off by organisms. Similarly,deposition can cause under-deposit corrosion anddeposition can be caused by the accumulation ofcorrosion byproducts. Deposition and biofouling aresimilarly interrelated.

It also makes sense from a treatment standpoint toensure that a chemical added to the cooling tower tofight corrosion does not cause additional depositionsomewhere else in the system. That s BetzDearborn s

job. Your representative knows how to put all thecomponents together to ensure an effective blend thataddresses all concerns for the cooling system.

OPEN RECIRCULATING COOLINGSYSTEMS

In terms of the evolution of chemical treatment, it has beenmentioned that chromate is an excellent corrosion inhibi-tor. Chromates were used primarily in the 1950s and1960s, until environmental considerations forced thechemical treatment industry to develop better alternatives.

The use of zinc phosphate was one of the firstalternatives to chromate treatment. It was supplement-ed by Polynodic treatment programs in the early 70s. Abig breakthrough in cooling water treatment was theintroduction of Dianodic II by BetzDearborn in the late1970s. It is still used today in many cooling systems.Zinc/alkaline treatments were developed in the early1980s to handle higher pH ranges. Many plants now useContinuum products, introduced in the 1990s.

Polynodic programs combat corrosion by using addi-tives to shut down both the anode and cathode of acorrosion cell. These programs contain phosphate toreduce corrosion at anodic surfaces of the metal and topromote formation of a small amount of calciumphosphate precipitate to turn off the cathodic surfaces of

the metal. The strategy in any program using calciumphosphate is to make sure you use enough to do the job,but not too much. That s where polymeric dispersantscome into play.

POLYMERIC DISPERSANTS

Polymers are complex chemicals composed of manyrepeating simple units, strung together to form a chain.There is a tremendous number of polymers available forindustrial use. You may be familiar with polymers used inthe influent clarifier in your plant or in the wastetreatment plant. Polymers used in these applications

promote the formation of larger particles in water so theysettle and are removed from the flowing stream.

Polymers can also be used for the exact oppositepurpose. By choosing the correct composition andlength of chain, we can design polymers that actuallyinhibit the formation of precipitates and that s a keyusage in cooling water treatment: precipitation inhibi-tion . In the case of calcium phosphate, the polymercontrols the amount of calcium phosphate which coats ametal surface. Again, we want just enough for corrosionprotection, but not so much that we form a deposit whichinterferes with heat transfer. This is not easy to do.

Actually, the evolution of cooling water treatment isbased on the discovery of various new polymers, eachone opening the door for new treatment options.

Polynodic programs were effective but were limited tocertain ranges of pH and alkalinity. If the pH were too low,corrosion could not be controlled. That was not a commonproblem because most natural makeup waters (as we sawin Chapter 1) contain alkalinity that, when cycled in acooling tower, tends to raise the pH. So the bigger problemis controlling calcium phosphate deposition at elevatedpH. That s where BetzDearborn Dianodic II comes in.

DIANODIC II

This program uses high levels of phosphate, which actas an excellent anodic inhibitor. On steel, it is actually aseffective as chromate! The aim of this program is tomaintain solubility of calcium phosphate in the presenceof high levels of phosphate and calcium hardness. Thesuccess of the program was entirely dependent onfinding the right polymeric dispersant, better than thosepreviously available. In the case of Dianodic II programs,


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the polymer was a genuine breakthrough; it introduced awhole new chemistry to cooling water treatment.

The components of the treatment program includephosphate, polyphosphate, organic phosphonates anda special polymeric dispersant. These components acttogether to control iron-based corrosion. An azole(organic nitrogen compound) is often added to protectcopper-bearing components in the system. And, on topof that, biocide treatment is also needed but, we do notwant to use a biocide which interferes with corrosion/de-position control.

The Dianodic II program handles tower calcium levelsbetween 50 and 1200 ppm [mg/L] and carries phos-phate levels between 10 and 20 ppm [mg/L]. Thissubstantially exceeds the normal solubility limits forcalcium phosphate and allows us to operate coolingsystems at the highest number of cycles for optimalefficiency. The special polymeric dispersant works wellup to a pH of 7.8.

As good as Dianodic II treatment is, there may be a needto operate at pH higher than 7.8. There are manyreasons for wanting to do this in a cooling system.Alkaline waters, in general, are less aggressive towardsteel. A higher operating pH range tends to buffersystem pH upsets, without requiring the feed ofexcessive amounts of acid, a constant concern withother programs.

ALKALINE/ZINC TREATMENTPROGRAMS

Programs for higher pH were introduced by BetzDear-born as BAT/zinc. BAT stands for balanced alkalinetechnology , indicating that we could operate in a higher,more alkaline pH range, while preserving a balancebetween corrosion and deposition.

These programs employ zinc (which forms a precipitateat cathodic areas on a metal surface), phosphonates(which control scale), molybdate (also for anodiccorrosion protection), azole (to inhibit copper corrosion),and an all-important polymeric dispersant. The hallmarkof this program is that it eliminates or significantly

reduces the need for acid feed in the recirculating towerwater. However, it s not magic and it does have somelimitations. For example, we cannot increase LSI muchover +2.5 (see Chapter 5, Deposition), and we have towatch the overall level of silica. The alkaline technolo-gies let us operate with pH values up to 9.

You can see the need for chemical testing. We willreview this in detail in a later chapter, but we wanted tomention it now. It s critically important that you do the

required chemical testing to make sure the toweroperating conditions are within control ranges of theprogram being used. This is one of your main responsi-bilities. If you have any questions about what to do whentest results are outside control ranges, you shouldreview them with your BetzDearborn representative.

CONTINUUM TREATMENT

The latest development in cooling water treatmenteliminates zinc from the program ( any metal is aconcern in some areas of the world), and allows thealkaline program to operate under a wider variety ofconditions. New Continuum programs contain phos-phate, phosphonate, and azole as corrosion inhibitors,with molybdate as needed, and a polymeric dispersantfor precise scaling and fouling control. Again, a biocideprogram is also required.

SURFACTANTSSometimes we need to add surfactants as a supplementto the main treatment program. For example, in asystem which becomes contaminated with oils andgrease, a surfactant is the most cost-effective way todisperse grease. Surfactants perform like soap in acleaning operation by dissolving grease.

CHEMICAL TREATMENT FORMULATIONS

There is an important subtopic of chemical treatmentwhich is useful for you to understand at this point. When

BetzDearborn formulates treatment components to-gether, sometimes concentrated actives cannot bemixed with each other because of chemical incompati-bilities. The components are designed to work together,but under diluted conditions in recirculating water (or in aclosed loop or other systems). Therefore, some pro-grams require the feed of separate products. Thatmakes your job more complicated we understand that.But sometimes, it just cannot be helped.

Many programs are available in what is called aone-drum formulation, where everything needed forthe program is furnished in one, highly concentrated

product. This is convenient, but there is a disadvantage.Suppose your chemical testing indicates that morephosphate is needed. With a one-drum approach, youhave no alternative but to add more of everything:inexpensive phosphate, and relatively expensive dis-persant. So convenience may come at a higher price.With major separation of components into two or threeproducts, we tailor the amount of additive needed for agiven change in system chemistry. This helps keepcosts in line and makes your plant more profitable.


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A summary of treatment options is provided in Figure7-2.

Figure 7-2: Cooling water treatment

Deposition

CorrosionBiofouling

Biocides Surfactants

Anodic/ cathodicinhibitors

Adsorbedfilms

Crystal growth inhibitors Polymer dispersants Surfactants

Water treatment programs address three interrelatedproblems with a variety of options.

Cycles, LSI, pH, TDS Makeup water hardness, silica Cost Feed and disposal of blowdown Degree of monitoring

Plus special considerations of:

CLOSED COOLING SYSTEMSThe treatment of closed cooling systems is not asdemanding as open systems. Deposition and scaleformation are usually not a concern, because ofwidespread use of softened water or condensate asmakeup; however, if you are using hard water makeup,then the potential for scale formation must be ad-dressed. Also, oxygen concentration in closed systemsis lower than in aerated, open systems, which reducesthe corrosion potential.

The corrosion inhibitor program of choice for many

closed systems is based on molybdate. In closedsystems, we operate with molybdate at the high levelsneeded for good control. However, molybdate is expen-sive. In a closed system, it s cost is justified anothergood reason for you to keep alert to leaks in thesesystems.

Molybdate is often combined with an organic azole ornitrite. Nitrite gives rapid passivation of steel surfaces;unfortunately, it is prone to degradation (mostly biologi-

cal). Molybdate then takes over as a long-term corrosioninhibitor. The azole is a film-former and protects coppermetallurgies.

There are also some sulfite-based programs whichborrow from boiler water treatment technology. Sulfitereacts very quickly with oxygen and actually removes itfrom a system. No oxygen, no oxygen corrosion. Theseprograms are usually run at an alkaline pH to preventacid attack of the metal. Of course, air in-leakage has tobe minimized.

One of the biggest problems with closed systems iscontrolling microbiological growth. There are manyspecies of bacteria which thrive in reduced oxygenenvironments (See Chapter 6, Microbiological Fouling).A biocide may be recommended as part of a closedsystem program.

THE TREATMENT OF HVAC SYSTEMSThe unique operating conditions of HVAC systems posespecial problems for treatment. As indicated in Chapter3, they are characterized by variable flow. HVACsystems have to respond to varying atmosphericconditions, varying heat loads, and major seasonalchanges. For control purposes, flows are highly throttledand often reduced. Sometimes they are reduced tozero.

Stagnant branches of a chilled water system are abreeding ground for anaerobic bacteria. In addition,periodic contamination with glycol and hydrocarbons

furnishes food for these microorganisms, compoundingthe problem.

Treatment options include Dianodic II and BAT pro-grams (as discussed above), as well as molybdate andazoles as needed. Because exchanger skin tempera-tures and heat transfer rates tend to be lower in HVACsystems, the potential for scale formation is lesssevere but it still exists. Consequently, the thrust oftreatment programs for HVAC units focuses on corro-sion and microbiological control.

Special products have been developed for HVACsystems, providing effective treatment at affordable

prices. One of the main problems in treating thesesystems is that they are often not monitored as closelyas open recirculating systems. Therefore, treatmentprograms have to be able to work under wide operatingconditions and still be effective when system upsets andchanges occur.

There are two additional topics we need to review toround out our understanding of cooling water treatmentprograms: air washers and potable applications.


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AIR WASHERSAir washers are common in the textile industry. Theyprovide filtration (removal of textile fibers and lint) andhelp control humidity, temperature, and static charges inthe air. The main problem in these systems is acombination of microorganisms and particulate fibers,wrapped up in a slime deposit. Sometimes this slimecreates objectionable odors in the air.

The ultimate objective for treatment is to increase theservice time between air washer cleanings. Treatmentof these systems combines an effective corrosioninhibitor and a good microbiocide, in conjunction with asurfactant. You might recall from Chapter 6, that weoften use surfactants to break up microbiologicaldeposits, allowing the biocide to be more effective. Achemical antistat is added to adjust the static charge inthe air.

There is a strong seasonal effect. When an air washer isin the humidification phase, evaporation is occurring,allowing solids to build up. Blowdown of the system isrequired in this phase to control scale. In the dehumidifi-cation phase, using refrigeration cooling to promotecondensation, the condensation creates an overflowcondition. The mineral content of the condensate isquite low, so scale is not a problem, but low conductivitywater can be quite corrosive. This provides a goodopportunity for recycle, once treated. Consequently,operators responsible for these systems must be awareof the phase in which the air washer is operating.

POTABLE WATER TREATMENT

The requirements for potable water treatment are drivenby federal guidelines for drinking water. Many of theregulations focus on maximum levels of heavy metals,such as lead and copper.

Corrosion inhibitors have been reviewed by the NationalSanitation Foundation (NSF). Approved componentsinclude phosphates, polyphosphates, and low levels ofzinc. Products are applied at what is termed threshold levels, levels low enough to meet guidelines, yet just high enough to control corrosion and deposition. As youmay guess, operating at threshold levels requiresprecise control of system chemistry and an attention togood testing practices.

REGULATORY AND ENVIRONMENTALCONSIDERATIONSThe discussion of potable water treatment leads us to abroader discussion of regulatory and environmental

issues. Selecting an effective treatment program goesbeyond selecting chemicals to feed. Every plant has aNPDES permit, regulating discharge from the facility.This permit focuses on heavy metals, phosphates, andsometimes chlorine and chlorides.

We control corrosion for all the system integrity andoperating efficiency reasons mentioned at the begin-ning of this chapter. But we also have to factor in how theplant is going to remove blowdown or accidentaldischarge from treated systems. Uncontrolled iron andcopper corrosion can actually put a plant out ofcompliance when corrosion byproducts are discharged.But to control corrosion, we need phosphate-basedchemicals, sometimes in large amounts, and dischargeof these phosphates is also regulated. You see howcomplicated the balancing act can be.

The success of many of our treatment programs isbased on high-performance polymeric dispersants.What happens when these complex products arereleased into the environment? BetzDearborn hasembarked on a massive research effort dedicated toidentify the ultimate fate of these products and theirpotential effect on the environment. This puts us in amore responsible position to recommend treatmentprograms for industry.

CONCLUSION

We do not expect you to become a chemical expert.

That s our job. You should now have an appreciation ofhow treatment programs work, and why we incorporatecertain chemicals in them to perform specific tasks. Youshould also have a better understanding of the relation-ships between corrosion, deposition, and microbiologi-cal fouling.

Two important things we would like you to take from thischapter:

support system maintenance. The better job youdo maintaining mechanical integrity and cleanli-ness of cooling systems, the easier and less

expensive the chemical control program will be.This directly carries to the bottom line reliable,profitable operation of your plant.

some chemical control programs are compli-cated. We need YOUR help in making theseprograms work: consistent chemical feed, accu-rate testing, timely reaction to test results,working closely with your BetzDearborn repre-sentative. That s a winning combination.


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KEY WORDS

Polymeric Dispersant

Precipitation Inhibitor


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CHAPTER QUIZ

1 HVAC systems require special attention to prevent hardness deposition. True False

2 A one-drum treatment formulation, while convenient, often costs more than thefeed of separate products. True False

3 The best corrosion protection can be achieved by using additives that shut downboth the anode and cathode of a corrosion cell. True False

4 Chemical treatment can always overcome design deficiencies in a cooling system. True False

5 Chemical treatment can always overcome operating deficiencies in a cooling system. True False

6 Polymers can be used to promote the formation of particles or to inhibit theformation of particles. True False

7 Although calcium phosphate is an excellent corrosion inhibitor, care must be takento prevent excess accumulation of the material as scale on exchanger surfaces. True False

8 Because closed cooling systems are closed to the atmosphere, it is impossible for

microbiological organisms to grow in these systems. True False

9 Azoles are organic nitrogen compounds added to a cooling treatment program toprovide corrosion protection for which particular metal? Answer

10 Before a new cooling system or a new part in a cooling system is put into service, a special chemicaltreatment step is performed. From the following list, identify this important step:

A Deionization

B Disbursement

C PretreatmentD CoagulationE Bioaugmentation

Answer


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11. Explain, in your own words, the interrelationship between deposition, biological fouling, and corrosion.


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CHAPTER 8FEED SYSTEMS

INTRODUCTIONEvery cooling system is different. Water chemistry andcomponent metallurgies can be significantly different,even within the same plant. Therefore, as watertreaters, we must tailor treatment programs specificallyfor each system. There is usually a need for corrosionprotection, deposition control, and microbiological con-trol.

Corrosion inhibitors need to be fed at a treatment level toprotect all the various metallurgies within a system.Dispersants need to be added at a high enough level todisperse suspended solids and inhibit scale formation,and to act as an adjunct to corrosion inhibitors to preventchemical inhibitors from depositing on heat transfer

surfaces. Biocides are maintained at dosage levels tocontrol biological growth, but not at feedrates thatinterfere with corrosion or deposition program compo-nents.

To meet these difficult treatment requirements, we needto maintain precise levels of treatment chemicals inrecirculating systems. This is not easy to do with thenormal system changes and chemistry swings that arepart of everyday plant operations.

In previous sections, we detailed the results of anunderfeed of corrosion inhibitor: the ensuing corrosioncan cause damage and losses, ranging from a small

loss in efficiency of a minor exchanger, to a major,unscheduled plant shutdown. The economic losses canbe staggering.

Decreased feed of a deposit control agent shows asimilar pattern: deposit buildup in an exchanger maycause an undetectable loss of efficiency or an economiccatastrophe, with lost production and replacement orcostly cleaning of capital equipment.

Control of microbiological growth is no exception to theneed for accurate feed control. Uncontrolled growth ofmicroorganisms generates the same list of economiclosses to a plant, with a few additional problems unique

to biofouling.No one doubts the potential danger of underfeeding achemical treatment program. Many customers, howev-er, are unaware of the danger and economic lossesresulting from overfeed of treatment. The first penalty isthat you are using more chemicals than requiredthatsa waste of money.

But there can be unexpected consequences to overfeedof treatment chemicals. Dispersant fed alone is usually

not a chemical problem. Some corrosion inhibitors,however, can form deposits if they are overfed, unless

balanced by an overfeed of dispersant. Recall fromChapter 4 that many of the newer corrosion inhibitorswork by deliberately forming a controlled precipitate atthe cathode. Overfeed of these inhibitors causes a toomuch of a good thing problem, and undesirable scale iscreated.

Overfeed of biocide can limit the effectiveness ofcorrosion and deposition control programs. Somebiocides, such as chlorine and bromine, indiscriminatelydestroy all carbon molecules. This is normally goodbecause microorganisms are composed of complexcarbon molecules. Chemical components of the treat-ment program, some of which are also complicatedcarbon molecules, are usually tough enough to with-stand normal use levels of oxidizing biocides. But if theoverfeed surpasses the ability of the treatment chemicalto survive, it is also destroyed. Once a deposit controlagent is destroyed, deposition can proceed unchecked.In addition, byproducts from the reaction with chlorineare corrosive.

Underfeed is a problem; overfeed is a problem. Erraticfeed can produce highly variable results and make itimpossible to determine the effectiveness of a treatmentprogram. Inconsistent feed can also make it difficult toidentify system changes which affect performance.

Many plants subscribe to the idea of continuousimprovement, seeking to constantly improve productionpractices. With ineffective control of chemical feed, suchquality improvement programs are undermined from thestart.

FEED POINTS

In addition to maintaining required levels of treatmentchemicals, it is also important to know where in thesystem a treatment is applied. There are a number ofconcerns you need to consider.

Its important to feed chemical treatment programs to ahighly agitated area; this promotes good mixing. It isfoolish to feed the right amount of chemical and have itremain in a pool, unmixed in a quiet area of the system.Although its the right dosage, unless it gets dispersedthroughout the circulating system, its of no use. Evenworse, because undispersed treatment is usually alocal overfeed situation, it may even cause damagealthough it was the right amount for the system.


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In cooling systems, the preferred area to feed treatmentchemicals is to the suction side of the recirculatingpump. The area is highly mixed. Passage through thelarge recirculating pumps assures proper dilution of theconcentrated actives. In fact, many chemical treatmentadditives are added to the pump sump as concentrates,

avoiding the need for dilution or a day tank arrange-ment. The pump sump area is also mechanicallyaccessible, and usually has room to house bulk productstorage tanks. In addition, it s easy to check the feedsystem while performing other duties in the area. Ifgravity feed of treatment chemicals is required, abelow-grade cold well is an excellent choice for a feedpoint.

A major consideration for chemical treatment of boilersis the need for high pressure pumps. The treatmentprogram is applied to high pressure boiler feed lines orpressurized steam lines. However, this is not the case

with cooling systems. Atmospheric pumps are sufficientto feed chemical treatment programs. These pumps aremuch less expensive than high pressure pumps, easierto maintain, and generally safer to use.

TYPES OF FEED AND CONTROLSYSTEMS

The simplest chemical feed option is called bucketfeed . You fill a bucket with treatment chemical and dumpit into a tower. The only positive comment about bucketfeed is that it is easy and cheap. Actually, in the long runits not really cheap. Consider the results with bucket

feed: underfeeding and overfeeding are inevitable witha large percentage of inaccurate feeding. Underfeedingmay promote corrosion and deposition or encourageuncontrolled microbiological growth. Overfeedingwastes chemical treatment and can cause deposition.Ultimately, all these problems cost money.

A more acceptable method for feeding chemicaltreatment is with a pump. Small chemical feed pumpsare found in virtually every industrial plant in the world. Atypical example is shown in Figure 8-1. Few stop to thinkabout the drawbacks of such systems. There are three:(1) The pump setting is based on some assumed

constant system conditions that is rarely the realsituation. (2) Everyone assumes the output from a pumpis constant again, that s not the case. Not only theoutput, but also the discharge pressure varies. Did wemention that pumps leak? That s number (3).

A variation on bucket feed and pump feed is the shotfeeder . This is an intermittent method, used to rapidlydevelop some maximum treatment concentration. Dis-persants and biocides are sometimes fed to open

Figure 8-1: Chemical Feed Pump

recirculating systems in this manner. Automated shotfeeding uses a chemical feed pump and a controllerwhich actuates the pump to achieve a desired treatmentlevel. There is no feedback from the system beingtreated, and the chemical profile depends entirely on thequality of the actuating timer.

Another intermittent variation is specific for once-through cooling water systems and is called semi-con-tinuous. Sometimes it s difficult to maintain effectivetreatment levels with reasonable economics in once-through systems. As effective as the treatment chemi-cals are, the nature of once-through systems (with norecycle of treatment) represents a treatment challenge.Chemical addition to these systems is often controlledby a pump and timer that feed chemical for several hoursand then stop the pump for a given period of time.

There have been many attempts over the years tosomehow target the performance of a feed system to animportant, changing parameter in a system. One suchattempt is called the meter/counter/timer (MCT) feedsystem. Another name for this system is meter/accu-mulator/timer (MAT) . It is superior to an unmodulatedpump because it follows specific system changes, suchas makeup water flow. An MCT system is illustrated inFigure 8-2.

MCT is normally based on a targeted treatment level. Itattempts to maintain this level by tracking lost chemical,that is, blowdown. Most of the treatment in an openrecirculating system is lost by blowdown and this feed


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Main Flow

FlowMeter

Counter Timer

ChemicalFeed Pump

Figure 8-2: Meter/counter/timer feed system

system is designed to account for such losses. Makeupflow is metered and sends a pulsed signal to a counter.The counter accumulates a set number of pulses andthen actuates chemical feed for a preset time. Unfortu-nately, even with higher installation costs, feed is stillbased on a theoretical model of the system and thefeed system does not verify that the pump is feedingwhat it is supposed to feed.

One of the most cost-effective ways to achieve targetedcontrol levels (in the face of changing system condi-tions), as well as meeting the need for verification ofpump delivery, is with a BetzDearborn PaceSetterPlus . This control system is based on a targetedtreatment level; however, its on-board logic changes thetarget in a feed-forward action to meet a variety ofchanging system conditions. In addition to adjusting thetreatment pump (in response to a changing target), italso continuously calibrates the pump with a verifieddraw down assembly and corrects for inevitable pump

output inaccuracies.The PaceSetter Plus system reacts instantly to a pumpor feed failure and other alarm conditions. Somesystems have been set up to call the local BetzDearbornrepresentative on a beeper system, should the needarise. The device has easy-to-use graphics, logssystem and feed data, and interfaces with statisticalprograms to analyze data. It can also transfer data byway of a modem hookup. Most installations have ademonstrably short payback time in terms of operatingequipment protection and conservation of treatmentchemicals.

Chemical feed and control is a major concern toindustry. We at BetzDearborn are proud of the PaceSet-ter Plus controller and the benefits it provides.

OPERATOR RESPONSIBILITYYou have a big part to play in the success of a chemicaltreatment program. You are probably involved inday-to-day adjustment of pumps in the chemical feed

system. And you make changes based on results ofchemical testing (reviewed in Chapter 9). It s not enoughto just perform the tests and write the numbers in alogyou have to react to the test results. Resultsout-of-range are telling you something: a low result for acorrosion inhibitor is putting your cooling system at risk.

Follow the directions set up for your particular coolingtreatment program. They indicate what steps youshould take with low test results. A high test result couldmean an overfeed and your plant is wasting money. Oursuccess as a team is absolutely tied in with the successof your plant and that includes staying within budget.Follow the procedures set up by your plant and yourBetzDearborn representative. The proper feed of treat-ment chemicals is everybody s business.

SAFETY

No discussion of chemical feed is complete withoutmention of safety concerns. Some chemicals used incooling water treatment contain hazardous materialsthat may be dangerous to handle. You should not comeinto direct contact with these materials.

We are eager to fulfil our ethical and legal obligation toyou to fully inform you of the hazards associated with ourproducts. You have access to Material Safety DataSheets (MSDS) which identify safety concerns and listexplicit safety precautions you need to take.

In addition to BetzDearborn products, some coolingsystems require the feed of acid or caustic to maintain adesired pH. Some systems may require chlorine orliquid bleach for microbiological control. These chemi-cals are extremely dangerous. Extreme caution needsto be exercised when working around them. Your owncompany has safety procedures which you need toknow and follow.

There is also a built-in safety hazard in our business:cooling water and electrical pumps or electronic feedsystems. Water and electricity, when mixed, are not agood combination. The electrical and electronic aspectsof a cooling system and chemical feed delivery systemsshould be serviced only by a qualified technician. Majorelectrical work must be done under the auspices of themaintenance department. Check with your supervisorabout the proper procedures for your plant.

Safety is everybody s business. If you have anyquestions about the safe handling of BetzDearbornchemicals in your plant, contact your BetzDearbornrepresentative. An emergency number is available tohandle serious accidents with BetzDearborn chemicals.Be sure it is posted where you can call quickly, if theneed arises.


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KEY WORDS

Bucket Feed

Chemical Feed Pump

Material Safety Data Sheets (MSDS)

Meter/Counter/Timer (MCT)

Meter/Accumulator/Timer (MAT)

Shot Feeder


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CHAPTER QUIZ

1. As an operator, you should only be concerned when a pump is underfeedinga chemical program. True False

2. The preferred area to feed cooling water chemicals is to the discharge sideof the recirculating pump. True False

3. Low pressure pumps are safe to use to feed chemicals to cooling water systems. True False

4. The only drawback to the PaceSetter Plus feed and control system is itsinability to verify that the pump is feeding what it is supposed to feed. True False

5. MCT or MAT feed systems are superior to shot feeders because

.

6. Chemical treatment to an open recirculating cooling system is lost by

.

7. Before working on a chemical pump that is out-of-service or not functioning,

it is a good practice to consult the

because

.

8. Underfeeding a deposit control agent can cause deposit buildup, resulting in

.


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CHAPTER 9MONITORING

INTRODUCTION

The previous chapters in this workbook have reviewedbasic cooling system problemscorrosion, deposition,and microbiological foulingand mechanical andchemical approaches to solve those problems. In thediscussion of mechanical issues, we identified theimportance of monitoring temperature, pressure, veloc-ity, fan horsepower, etc., and stressed the importance ofyour job in ensuring that the system performs as well asit was designed to perform. Proper chemical treatmentfollows efficient mechanical operation.

In applying effective chemical treatment, the sameconcerns are raised about monitoring. We have to watch

feedrates, incoming and recirculating water chemistry,and perform special tests which certain treatmentprograms require. Some tests in large systems need tobe performed several times a shift. Large or criticalsystems require close attention to keep them at optimalperformance. Smaller systems are more lenient. Youstill have to watch them, but mechanical and chemicalparameters are set more broadly and the treatmentprograms we use for these systems already have a lot ofbuilt-in safeguards. Comfort cooling systems, althoughrelatively small, are no less important for the efficientoperation of a business than the cooling loops on criticalprocess heat exchangers.

You may find that some of the monitoring toolsmentioned here are not used in your plant. In that case,its certainly OK to skip some of the subsections.BetzDearborn representatives have been trained in theuse of all the tools. From a complete monitoring menu,your representative selects the best tools for yourcooling systems and treatment programs. If you have aquestion regarding a monitoring program, please reviewit with your BetzDearborn representative.

Lets start with some routine control tests. Then wellprogress to more complicated monitoring devices.

CHEMICAL TESTING

It seems that everybody has t

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