cooling tower fundamentals

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C o o l i n g T o w e r F u n d a m e n t a l s


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Cooling Tower

FundamentalsCompiled from the knowledge and experienceof the entire SPX Cooling Technologies staff.

Edited byJohn C. Hensley

SECOND EDITION

Published bySPX Cooling Technologies, Inc.

Overland Park, Kansas USA


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Copyright 2009by

SPX Cooling Technologies, Inc.

All Rights Reserved

This book or any part thereof must not be reproducedin any form without written permission of the publisher.

Printed in the United States of America


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Foreword

Although the worlds total fresh water supply is abundant, someareas have water usage demands that are heavily out of balancewith natural replenishment. Conservation and efficient reuse ofthis precious and versatile resource are mandatory if such areasare to achieve proper development. And, the need for waterconservation does not limit itself only to arid regions. Recognition

of the detrimental environmental impact of high temperature waterdischarge into an estuary, whose inhabitants are accustomed tomore moderate temperature levels, makes one realize that there-cooling and reuse of water, however abundant, conserves notjust that important natural resourceit conserves nature as well.One helpful means to that end is the water cooling tower.

Those responsible for the specifications, purchasing and operationof plant, station, or building cooling systems must consider manyaspects beyond the primary requirement of dissipating unwantedheat. The following text is devoted to identifying the primary andperipheral considerations and offering approaches refined bysome eighty years of experience in the cooling tower industry.The goal is to assure the implementation of water cooling systems

which will satisfy all design and environmental requirements withsound engineering and responsible cost.

This manual is not intended to be all-encompassing andthoroughly definitive. The entire scope of cooling towers is toobroad, and the technology far too advanced, to permit completecoverage in a single publication. Separate brochures by SPXCooling Technologies, either existing or planned, cover individualtopics in depth. The intent herein is to provide a level of basicknowledge which will facilitate dialogue, and understanding,between user and manufacturer.


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FOREWORD .................................................................................................................................................................................3

SECTION I COOLING TOWER BASICS

A. Background ..........................................................................................................................................................7 B. Types of Towers ...................................................................................................................................................8 C. Nomenclature .................................................................................................................................................... 14 D. The Psychrometrics of Evaporation ............................................................................................................. 17 E. Factors Affecting Cooling Tower Performance .........................................................................................19 F. Materials of Construction ............................................................................................................................... 29 G. Maintaining Water Quality ..............................................................................................................................30 H. Operation in Freezing Weather ..................................................................................................................... 33

SECTION II STRUCTURAL COMPONENTS A. General ............................................................................................................................................................... 37 B. Cold Water Basin ............................................................................................................................................ 37 C. Tower Framework ............................................................................................................................................. 41 D. Water Distribution System ............................................................................................................................. 43 E. Fan Deck ............................................................................................................................................................ 47 F. Fan Cylinders .................................................................................................................................................... 47 G. Mechanical Equipment Supports ................................................................................................................. 48 H. Fill (heat transfer surface)...............................................................................................................................48 I. Drift Eliminators ................................................................................................................................................ 50 J. Casing ................................................................................................................................................................ 51 K. Louvers ............................................................................................................................................................... 51 L. Access and Safety Considerations ............................................................................................................. 52

SECTION III MECHANICAL COMPONENTS A. General ............................................................................................................................................................... 53

B. Fans ..................................................................................................................................................................... 53 C. Speed Reducers .............................................................................................................................................. 56 D. Drive Shafts ....................................................................................................................................................... 58 E. Valves .................................................................................................................................................................. 59 F. Safety Considerations ....................................................................................................................................60

SECTION IV ELECTRICAL COMPONENTS A. Motors ................................................................................................................................................................. 61 B. Motor Controls.................................................................................................................................................. 62 C. Wiring System Design ....................................................................................................................................63 D. Cycling of Motors ............................................................................................................................................. 64

SECTION V SPECIALIZED TOWER USAGE AND MODIFICATIONS

A. General ............................................................................................................................................................... 65 B. Water Conservation.........................................................................................................................................65 C. Visual Impact and Plume Control ................................................................................................................. 68 D. Adiabatic Air Precooling ................................................................................................................................. 71 E. Energy Reduction ............................................................................................................................................ 72 F. Energy Management and Temperature Control ........................................................................................ 72 G. Noise Control .....................................................................................................................................................74 H. Drift Reduction.................................................................................................................................................. 76 I. Abnormal Operating Conditions ................................................................................................................... 76 J. Vibration Isolation ............................................................................................................................................. 81 K. Free Cooling ...................................................................................................................................................... 83 L. Helper Towers ................................................................................................................................................... 86

Contents


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SECTION VI AUXILIARY COMPONENTS A. General ............................ ................................ ................................ ................................ ................................ ....................... 8

B. Extended Oil Fill and Gauge Lines............................................. ............................... ................................ ....................... 8

C. Mechanical Equipment Removal Devices ................................ ............................... ................................ ....................... 8 D. Prevention of Basin Freezing ............................ ................................ ................................ ................................ ................. 8 E. Filtering Systems ............................ ................................ ................................ ............................... ................................. ...... 9 F. Fan Brakes and Backstops ............................... ................................ ................................ ................................ ................. 9 G. Air Inlet Screens ............................. ................................ ................................ ................................ ................................ ...... 9 H. Distribution Basin Covers ............................ ................................ ................................ ................................ ...................... 9 I. Vibration Limit Switch ............................... ................................ ................................ ................................ ........................... 9 J. Fire Protection, Prevention and Control..........................................................................................................................95

SECTION VII THERMAL PERFORMANCE TESTING A. General ............................. ................................ ................................ ................................. ............................... ...................... 9 B. Tower Preparation for Test ............................ ................................ ................................ ............................... ...................... 9 C. Instrumentation for Test ........................... ................................ ................................ ................................ ........................... 9 D. Operating Conditions During Test .............................. ................................ ............................... ................................. ..... 9

E. Conducting the Test ............................ ................................ ................................ ................................ ................................ 9 F. Evaluation of Test Data ............................ ................................ ................................ ................................ ........................... 9

SECTION VIII OWNER RESPONSIBILITIES A. General .............................. ................................ ................................ ................................ ................................ ...................103 B. Covering Specification.............................. ................................ ................................ ................................ ........................10 C. Tower Orientation and Site Services ............................... ................................ ................................ ..............................10 D. Economic Evaluation Parameters ................................ ................................ ................................ ................................ ...10 E. Contractual Information ............................ ................................ ................................ ............................... .........................10 F. Comparing Capability of Proposed Towers .............................. ................................ ................................ ...................10 G. Cleaning and Biological Control.....................................................................................................................................106

SECTION IX TABLES ............................... ................................ ................................ ................................ ................................ ........................10

SECTION X INDEX ............................ ................................ ................................ ................................ ................................ .............................11


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A. BACKGROUNDThe machines and processes of industry, as well

as those devoted to human comfort and well-being,generate tremendous amounts of heat which mustbe continuously dissipated if those machines andprocesses are to continue to operate efficiently.Although this heat is usually transferred to a cool,flowing volume of water, final rejection is always tothe atmosphere and, invariably, is accomplished bysome form of heat exchanger. Many of those termi-nal heat exchangers are not easily recognized assuch because they are better known as creeks,rivers, lakes, etc.

The natural process of evaporation makes themvery effective heat transfer mediums, althoughsomewhat inefficient due to their limited surface

area and their total dependence upon randomwinds.

Although the happy man depicted in Figure 1 manot completely understand the principle of evaporation, he is intuitively making use of this most ancienform of natural cooling. Primeval, perspiring mankind depended upon natural breezes to acceleratthis evaporation process, and was grateful whethey came. At some point in that distant past, however, hands began to manipulate broad leaves tocreate an artificial breeze and the basic concepof a cooling tower was unknowingly founded. Eonlater, the advanced technology which allows MFigure 1 to revel in a mechanically-produced flowof air made finite development of the cooling towepracticable.

Cooling Tower Basics

Figure 1 The principle of cooling by evaporation.


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Figure 2 Atmospheric spray tower.

B. TYPES OF TOWERSCooling towers are designed and manufactured

in several types, with numerous sizes (models)available in each type. Not all types are suitablefor application to every heat load configuration.Understanding the various types, along with theiradvantages and limitations, can be of vital impor-

tance to the prospective user, and is essential tothe full understanding of this text.1. Atmospheric towers utilize no mechanical de-

vice (fan) to create air flow through the tower.The small atmospheric tower depicted in Figure2 derives its airflow from the natural induction(aspiration) provided by a pressure-spray typewater distribution system. Although relatively in-expensive, they are usually applied only in verysmall sizes, and are far more affected by adversewind conditions than are other types. Their useon processes requiring accurate, dependablecold water temperatures is not recommendedand as such has become rarely used.

loads warranting consideration of the use of hy-perbolic towers. However, because natural drafttowers operate most effectively in areas of high-er relative humidity, many such plants located inarid and/or higher altitude regions find mechani-cal draft towers more applicable

2. Mechanical draft towers use either single ormultiple fans to provide flow of a known volumeof air through the tower. Thus their thermal per-formance tends toward greater stability, and isaffected by fewer psychrometric variables, thanthat of the atmospheric towers. The presenceof fans also provides a means of regulating airflow, to compensate for changing atmosphericand load conditions, by fan capacity manipula-tion and/or cycling. (Section V-F)

Mechanical draft towers are categorized aseither forced draft(Fig. 4), on which the fan islocated in the ambient air stream entering the

Conversely, the atmospheric type known asthe hyperbolic natural drafttower (Figs. 3a &3b) is extremely dependable and predictable inits thermal performance. Air flow through thistower is produced by the density differential thatexists between the heated (less dense) air insidethe stack and the relatively cool (more dense)ambient air outside the tower. Typically, these

towers tend to be quite large (250,000 gpm andgreater), and occasionally in excess of 500 feetin height. Their name, of course, derives from thegeometric shape of the shell.

Although hyperbolic towers are more expen-sive than other normal tower types, they areused extensively in the field of electric powergeneration, where large unified heat loads exist,and where long amortization periods allow suf-ficient time for the absence of fan power (andmechanical equipment maintenance costs) torecoup the differential cost of the tower. Thesynfuels industry also potentially generates heat

Figure 3a Counterflow natural draft tower.

Figure 3b Crossflow natural draft tower.


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Figure 4 Forced draft, counterflow, blower fan tower.

Figure 5 Induced draft, crossflow, propeller fan tower.

tower, and the air is blown through; or induceddraft(Fig. 5) wherein a fan located in the exitingair stream draws air through the tower.

Forced draft towers are characterized byhigh air entrance velocities and low exit veloci-ties. Accordingly, they are extremely susceptibleto recirculation (Sect. I-E-6-(c)) and are therefore

considered to have less performance stabilitthan the induced draft. Furthermore, located ithe cold entering ambient air stream, forced draffans can become subject to severe icing (witresultant imbalance) when moving air laden witeither natural or recirculated moisture.

Usually, forced draft towers are equipped

with centrifugal blower type fans which, althougrequiring considerably more horsepower thapropeller type fans, have the advantage of beingable to operate against the high static pressureassociated with ductwork. Therefore, they caeither be installed indoors (space permitting), owithin a specially designed enclosure that provides significant separation between intake anddischarge locations to minimize recirculation.

Induced draft towers have an air dischargevelocity of from 3 to 4 times higher than their aentrance velocity, with the entrance velocity approximating that of a 5 mph wind. Therefore, thereis little or no tendency for a reduced pressur

zone to be created at the air inlets by the actioof the fan alone. The potential for recirculation oan induced draft tower is not self-initiating andtherefore, can be more easily quantified purely othe basis of ambient wind conditions. Locatioof the fan within the warm air stream provideexcellent protection against the formation of iceon the mechanical components. Widespread acceptance of induced draft towers is evidencedby their existence on installations as small as 15gpm and as large as 700,000 gpm.


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3. Hybrid drafttowers (Fig. 6) can give the outwardappearance of being natural draft towers withrelatively short stacks. Internal inspection, how-ever (Fig. 7), reveals that they are also equippedwith mechanical draft fans to augment air flow.Consequently, they are also referred to as fan-assisted natural drafttowers. The intent of theirdesign is to minimize the horsepower requiredfor air movement, but to do so with the least pos-sible stack cost impact. Properly designed, thefans may need to be operated only during peri-ods of high ambient and peak loads. In localitieswhere a low level discharge of the tower plumemay prove to be unacceptable, the elevated dis-

charge of a fan-assisted natural draft tower canbecome sufficient justification for its use.4. Characterization by Air Flow:

Cooling towers are also typed by the rela-tive flow relationship of air and water within thetower, as follows:

In counterflow towers (Fig. 8), air movesvertically upward through the fill, counter to thedownward fall of water. Because of the need forextended intake and discharge plenums; the useof high-pressure spray systems; and the typicallyhigher air pressure losses, some of the smallercounterflow towers are physically higher; requiremore pump head; and utilize more fan power

than their crossflow counterparts. In largercounterflow towers, however, the use of low-pressure, gravity-related distribution systems,plus the availability of generous intake areas andplenum spaces for air management, is tending toequalize, or even reverse, this situation. The en-closed nature of a counterflow tower also restrictsexposure of the water to direct sunlight, therebyretarding the growth of algae. (Sect. I-G-4)

Crossflowtowers (Fig. 9) have a fill configu-ration through which the air flows horizontally,across the downward fall of water. Water to becooled is delivered to hot water inlet basins lo-

Figure 6 Fan-assisted natural draft tower.

Figure 7 Inside of a fan-assisted natural draft tower.

Figure 8 Induced draft counterflow tower.

Figure 9 Induced draft, double-flow, crossflow tower.


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cated atop the fill areas, and is distributed tothe fill by gravity through metering orifices in thefloor of those basins. This obviates the need fora pressure-spray distribution system, and placesthe resultant gravity system in full view for easeof maintenance. By the proper utilization of flow-control valves (Sect. III-E-2), routine cleaning

and maintenance of a crossflow towers distri-bution system can be accomplished sectionally,while the tower continues to operate.

6. Characterization by ConstructionField-erected towers are those on whic

the primary construction activity takes place athe site of ultimate use. All large towers, andmany of the smaller towers, are prefabricatedpiece-marked, and shipped to the site for finaassembly. Labor and/or supervision for final as

sembly is usually provided by the cooling towemanufacturer.Factory-assembledtowers undergo virtuall

complete assembly at their point of manufacturewhereupon they are shipped to the site in as fewsections as the mode of transportation will permit. The relatively small tower indicated in Figur11 would ship essentially intact. Larger, multcell towers (Fig 12) are assembled as cellsor modules (see Nomenclature) at the factoryand are shipped with appropriate hardware foultimate joining by the user. Factory-assembledtowers are also known as packaged or untary towers.

Figure 10 Induced draft, single-flow, crossflow tower.

Figure 11 Small factory-assembled tower.

Figure 12 Multi-cell factory-assembled tower.

Crossflow towers are also sub-classified bythe number of fill banks and air inlets that areserved by each fan. The tower indicated in Fig-ure 9 is a double-flowtower because the fan isinducing air through two inlets and across two

banks of fill. Figure 10 depicts a single-flowtower having only one air inlet and one fill bank,the remaining three sides of the tower beingcased. Single-flow towers are customarily usedin locations where an unrestricted air path to thetower is available from only one direction. Theyare also useful in areas having a dependable pre-vailing wind direction, where consistent processtemperatures are critical. The tower can be sitedwith the air inlet facing the prevailing wind, andany potential for recirculation (Sect. I-E-7-(b)) isnegated by the downwind side of the tower be-ing a cased face.

5. Spray-filltowers have no heat transfer (fill) sur-

face, depending only upon the water break-upafforded by the distribution system to promotemaximum water-to-air contact. The atmospherictower seen in Figure 2 is a spray-fill tower, asis the tower shown in Figure 16. Removing thefill from the tower in Figure 8 would also makeit spray-fill. The use of such towers is normallylimited to those processes where higher watertemperatures are permissible. They are also utilizedin those situations where excessive contaminantsor solids in the circulating water would jeopardizea normal heat transfer surface. (Sect. V-I-2)


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7. Characterization by ShapeRectilinear towers (Fig. 13) are constructed

in cellular fashion, increasing linearly to the lengthand number of cells necessary to accomplish aspecified thermal performance.

Round Mechanical Draft (RMD) towers(Figs. 14 & 32), as the name implies are essential-ly round in plan configuration, with fans clusteredas close as practicable around the centerpointof the tower. Multi-faceted towers, such as the

Octagonal Mechanical Draft (OMD) depictedin Figure 15, also fall into the general classifica-tion of round towers. Such towers can handleenormous heat loads with considerably less sitearea impact than that required by multiple recti-linear towers. (Sect. I-E-7-(d), Fig. 39) In additionto which, they are significantly less affected byrecirculation. (Sect. I-E-6-(a), Fig. 32)

Figure 13 Multi-cell, field-erected, crossflow cooling tower with enclosed stairway andextended fan deck to enclose piping and hot water basins.

Figure 14 Round Mechanical Draft (RMD), crossflow cooling tower.


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8. Characterization by Method of Heat TransferAll of the cooling towers heretofore described

are evaporativetype towers, in that they derivetheir primary cooling effect from the evaporationthat takes place when air and water are broughtinto direct contact. At the other end of the spec-trum is the Dry tower(Sect. V-B, Figs. 98 & 99)where, by full utilization of dry surface coil sec-tions, no direct contact (and no evaporation)occurs between air and water. Hence the wateris cooled totally by sensible heat transfer.

In between these extremes are the PlumAbatement (Sect. V-C, Fig. 103) and WateConservation(Sect. V-B, Figs 96 & 97) towerswherein progressively greater portions of dry suface coil sections are introduced into the overaheat transfer system to alleviate specific problems, or to accomplish specific requirementsDry towers, Plume Abatement, and Water Conservation towers will be discussed in greatedepth in Section V of this manual.

Figure 15 Octagonal Mechanical Draft, counterflow cooling tower.


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C. NOMENCLATUREThe following terms are commonly used in

cooling tower science, many of which are uniqueto the cooling tower industry:

ACFM The actual volumetric flow rate of air-vapormixture. Unit: cu ft per min.

Air Horsepower The power output developed by a

fan in moving a given air rate against a given re-sistance. Unit: hp. Symbol: ahp.Air Inlet Opening in a cooling tower through which

air enters. Sometimes referred to as the louveredface on induced draft towers.

Air Rate Mass flow of dry air per square foot ofcross-sectional area in the towers heat transferregion per hour. Unit: lb per sq ft per hr. Symbol:G. (See Total Air Rate).

Air Travel Distance which air travels in its pas-sage through the fill. Measured vertically oncounterflow towers and horizontally on crossflowtowers. Unit: ft.

Air Velocity Velocity of air-vapor mixture through a

specific region of the tower (i.e. the fan). Unit: ftper min. Symbol: V.Ambient Wet-Bulb Temperature The wet-bulb tem-

perature of the air encompassing a cooling tower,not including any temperature contribution bythe tower itself. Generally measured upwind ofa tower, in a number of locations sufficient to ac-count for all extraneous sources of heat. Unit: F.Symbol: AWB.

Approach Difference between the cold water tem-perature and either the ambient or enteringwet-bulb temperature. Unit: F.

Atmospheric Refers to the movement of air througha cooling tower purely by natural means, or by

the aspirating effect of water flow.Automatic Variable-Pitch Fan A propeller type fanwhose hub incorporates a mechanism whichenables the fan blades to be re-pitched simul-taneously and automatically. They are used oncooling towers and air-cooled heat exchangersto trim capacity and/or conserve energy.

Basin See Collection Basin and Distribution BasinBasin Curb Top level of the cold water basin retain-

ing wall; usually the datum from which pumpinghead and various elevations of the tower aremeasured.

Bay The area between adjacent transverse and lon-gitudinal framing bents.

Bent A transverse or longitudinal line of structuralframework composed of columns, girts, ties, anddiagonal bracing members.

Bleed-Off See Blowdown.Blowdown Water discharged from the system to con-

trol concentrations of salts and other impuritiesin the circulating water. Units: % of circulatingwater rate or gpm.

Blower A squirrel-cage (centrifugal) type fan; usu-ally applied for operation at higher-than-normalstatic pressures.

Blowout See Windage.

Brake Horsepower The actual power output of a mo-tor, turbine, or engine. Unit: hp. Symbol: bhp.

Btu (British Thermal Unit) The amount of heat gain(or loss) required to raise (or lower) the tempera-ture of one pound of water 1F.

Capacity The amount of water (gpm) that a coolingtower will cool through a specified range, at a

specified approach and wet-bulb temperature.Unit: gpm.Casing Exterior enclosing wall of a tower, exclusive

of the louvers.Cell Smallest tower subdivision which can function

as an independent unit with regard to air and wa-ter flow; it is bounded by either exterior walls orpartition walls. Each cell may have one or morefans and one or more distribution systems.

Chimney See Shell.Circulating Water Rate Quantity of hot water enter-

ing the cooling tower. Unit: gpm.Cold Water Temperature Temperature of the wa-

ter leaving the collection basin, exclusive of any

temperature effects incurred by the addition ofmake-up and/or the removal of blowdown. Unit:F. Symbol: CW.

Collection Basin Vessel below and integral with thetower where water is transiently collected anddirected to the sump or pump suction line.

Counterflow Air flow direction through the fill iscounter-current to that of the falling water.

Crossflow Air flow direction through the fill is essen-tially perpendicular to that of the falling water.

Distribution Basin Shallow pan-type elevated basinused to distribute hot water over the tower fill bymeans of orifices in the basin floor. Application isnormally limited to crossflow towers.

Distribution System Those parts of a tower, begin-ning with the inlet connection, which distribute thehot circulating water within the tower to the pointswhere it contacts the air for effective cooling. Mayinclude headers, laterals, branch arms, nozzles,distribution basins and flow-regulating devices.

Double-Flow A crossflow cooling tower where twoopposed fill banks are served by a common airplenum.

Drift Circulating water lost from the tower as liquiddroplets entrained in the exhaust air stream.Units: % of circulating water rate or gpm. (Formore precise work, an L/G parameter is used,and drift becomes pounds of water per million

pounds of exhaust air. Unit: ppm.)Drift Eliminators An assembly of baffles or labyrinth

passages through which the air passes prior to itsexit from the tower, for the purpose of removingentrained water droplets from the exhaust air.

Driver Primary drive for the fan drive assembly. Al-though electric motors predominate, it may alsobe a gas engine, steam turbine, hydraulic motoror other power source.

Dry-Bulb Temperature The temperature of the enter-ing or ambient air adjacent to the cooling toweras measured with a dry-bulb thermometer. Unit:F. Symbol: DB.


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Entering Wet-Bulb Temperature The wet-bulb tem-perature of the air actually entering the tower,including any effects of recirculation . In testing,the average of multiple readings taken at the airinlets to establish a true entering wet-bulb tem-perature. Unit: F. Symbol: EWB.

Evaluation A determination of the total cost of own-

ing a cooling tower for a specific period of time.Includes first cost of tower and attendant devic-es, cost of operation, cost of maintenance and/or repair, cost of land use, cost of financing, etc.,all normalized to a specific point in time.

Evaporation Loss Water evaporated from the circulat-ing water into the air stream in the cooling process.Units: % of circulating water rate or gpm.

Exhaust (Exit) Wet-Bulb Temperature See LeavingWet-Bulb Temperature.

Fan Cylinder Cylindrical or venturi-shaped structurein which a propeller fan operates. Sometimes re-ferred to as a fan stack on larger towers.

Fan Deck Surface enclosing the top structure of an

induced draft cooling tower, exclusive of the dis-tribution basins on a crossflow tower.Fan Pitch The angle which the blades of a propeller

fan make with the plane of rotation, measured ata prescribed point on each blade. Unit: degrees.

Fan Scroll Convolute housing in which a centrifugal(blower) fan operates.

Fill That portion of a cooling tower which constitutesits primary heat transfer surface. Sometimes re-ferred to as packing.

Fill Cube (1) Counterflow: The amount of fill requiredin a volume one bay long by one bay wide by anair travel high. Unit: cu ft.

(2) Crossflow: The amount of fill required in a

volume one bay long by an air travel wide by onestory high. Unit: cu ft.Fill Deck One of a succession of horizontal layers of

splash bars utilized in a splash-fill cooling tower.The number of fill decks constituting overall fillheight, as well as the number of splash bars in-corporated within each fill deck, establishes theeffective primary heat transfer surface.

Fill Sheet One of a succession of vertically-arranged,closely-spaced panels over which flowing waterspreads to offer maximum surface exposure tothe air in a film-fill cooling tower. Sheets may beflat, requiring spacers for consistent separation;or they may be formed into corrugated, chevron,

and other patterns whose protrusions provideproper spacing, and whose convolutions pro-vide increased heat transfer capability.

Film-Fill Descriptive of a cooling tower in which film-type fill is utilized for the primary heat-transfersurface.

Float Valve A valve which is mechanically actuatedby a float. Utilized on many cooling towers tocontrol make-up water supply.

Flow-Control Valves Manually controlled valveswhich are used to balance flow of incoming wa-ter to all sections of the tower.

Flume A trough which may be either totally enclosedor open at the top. Flumes are sometimes usedin cooling towers for primary supply of wateto various sections of the distribution systemFlumes are also used to conduct water from thcold water basins of multiple towers to a common pumping area or pump pit.

Fogging A reference to the visibility and path of theffluent air stream after having exited the coolingtower. If visible and close to the ground, it is referred to as fog. If elevated, it is normally callethe plume.

Forced Draft Refers to the movement of air undepressure through a cooling tower. Fans of forceddraft towers are located at the air inlets to forceair through the tower.

Geareducer See Speed Reducer.Heat Load Total heat to be removed from the circu

lating water by the cooling tower per unit timeUnits: Btu per min. or Btu per hr.

Height On cooling towers erected over a concrete

basin, height is measured from the elevation othe basin curb. Nominal heights are usuallmeasured to the fan deck elevation, not including the height of the fan cylinder. Heights fotowers on which a wood, steel, or plastic basiis included within the manufacturers scope osupply are generally measured from the lowermost point of the basin, and are usually overall othe tower. Unit: ft.

Hot Water Temperature Temperature of circulatingwater entering the cooling towers distributiosystem. Unit: F. Symbol: HW.

Hydrogen Ion Concentration See pH.Induced Draft Refers to the movement of air throug

a cooling tower by means of an induced partiavacuum. Fans of induced draft towers are located at the air discharges to draw air through thetower.

Inlet Wet-Bulb Temperature See Entering Wet-BulTemperature.

Interference The thermal contamination of a towerinlet air by an external heat source. (i.e. the discharge plume of another cooling tower.)

Leaving Wet-Bulb Temperature Wet-bulb temperature of the air discharged from a cooling toweUnit: F. Symbol: LWB.

Length For crossflow towers, length is always perpendicular to the direction of air flow throug

the fill (air travel), or from casing to casing. Focounterflow towers, length is always parallel tthe long dimension of a multi-cell tower, andparallel to the intended direction of cellular extension on single-cell towers. Unit: ft.

Liquid-to-Gas Ratio A ratio of the total mass flowof water and dry air in a cooling tower. (See Total Air Rate & Total Water Rate) Unit: lb per lbSymbol: L/G.

Longitudinal Pertaining to occurrences in the direction of tower length.


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Louvers Blade or passage type assemblies installedat the air inlet face of a cooling tower to con-trol water splashout and/or promote uniformair flow through the fill. In the case of film-typecrossflow fill, they may be integrally molded tothe fill sheets.

Make-Up Water added to the circulating water sys-

tem to replace water lost by evaporation, drift,windage, blowdown, and leakage. Units: % ofcirculating water rate or gpm.

Mechanical Draft Refers to the movement of airthrough a cooling tower by means of a fan orother mechanical device.

Module A preassembled portion or section of acooling tower cell. On larger factory-assembledtowers, two or more shipping modules may re-quire joining to make a cell.

Natural Draft Refers to the movement of air througha cooling tower purely by natural means. Typical-ly, by the driving force of a density differential.

Net Effective Volume That portion of the total struc-

tural volume within which the circulating wateris in intimate contact with the flowing air. Unit:cu ft.

Nozzle A device used for controlled distribution ofwater in a cooling tower. Nozzles are designedto deliver water in a spray pattern either by pres-sure or by gravity flow.

Packing See Fill.Partition An interior wall subdividing the tower into

cells or into separate fan plenum chambers. Par-titions may also be selectively installed to reducewindage water loss.

Performance See Capacity.pH A scale for expressing acidity or alkalinity of the

circulating or make-up water. A pH below 7.0 in-dicates acidity and above 7.0 indicates alkalinity.A pH of 7.0 indicates neutral water.

Pitot Tube An instrument that operates on the prin-ciple of differential pressures. Its primary use ona cooling tower is in the measurement of circu-lating water flow

Plenum Chamber The enclosed space between thedrift eliminators and the fan in induced draft tow-ers, or the enclosed space between the fan andthe fill in forced draft towers.

Plume The effluent mixture of heated air and watervapor (usually visible) discharged from a coolingtower.

Psychrometer An instrument incorporating both adry-bulb and a wet-bulb thermometer, by whichsimultaneous dry-bulb and wet-bulb temperaturereadings can be taken.

Pump Head See Tower Pumping Head.Range Difference between the hot water tempera-

ture and the cold water temperature (HW-CW).Unit: F.

Recirculation Describes a condition in which a por-tion of the towers discharge air re-enters theair inlets along with the fresh air. Its effect is anelevation of the average entering wet-bulb tem-perature compared to the ambient.

Riser Piping which connects the circulating watersupply line, from the level of the base of thetower or the supply header, to the towers distri-bution system.

Shell The chimney-like structure, usually hyperbolic incross-section, utilized to induce air flow througha natural draft tower. Sometimes referred to as a

stack or veil.Speed Reducer A mechanical device, incorporatedbetween the driver and the fan of a mechanicaldraft tower, designed to reduce the speed of thedriver to an optimum speed for the fan. The useof geared reduction units predominates in thecooling tower industry, although smaller towerswill utilize differential pulleys and V-belts for thetransmission of relatively low power.

Splash Bar One of a succession of equally-spacedhorizontal bars comprising the splash surface ofa fill deck in a splash-filled cooling tower. Splashbars may be flat, or may be formed into a shapedcross-section for improved structural rigidity

and/or improved heat transfer capability. Whenflat, the are sometimes referred to as slats orlath.

Splash-Fill Descriptive of a cooling tower in whichsplash type fill is used for the primary heat trans-fer surface.

Spray-Fill Descriptive of a cooling tower whichhas no fill, with water-to-air contact dependingentirely upon the water break-up and pattern af-forded by pressure spray nozzles.

Stack An extended fan cylinder whose primary pur-pose is to achieve elevation of the dischargeplume. Also see Fan Cylinder and Shell.

Stack Effect Descriptive of the capability of a tower

shell or extended fan cylinder to induce air (oraid in its induction) through a cooling tower.Standard Air Air having a density of 0.075 lb per cu

ft. Essentially equivalent to 70F dry air at 29.92in Hg barometric pressure.

Story The vertical dimension between successivelevels of horizontal framework ties, girts, joists,or beams. Story dimensions vary dependingupon the size and strength characteristics of theframework material used. Unit: ft.

Sump A depressed chamber either below or along-side (but contiguous to) the collection basin, intowhich the water flows to facilitate pump suc-tion. Sumps may also be designed as collection

points for silt and sludge to aid in cleaningTotal Air Rate Total mass flow of dry air per hour

through the tower. Unit: lb per hr. Symbol: G.Total Water Rate Total mass flow of water per hour

through the tower. Unit: lb per hr. Symbol: L.Tower Pumping Head The static lift from the eleva-

tion of the basin curb to the centerline elevationof the distribution system inlet; plus the totalpressure (converted to ft of water) necessaryat that point to effect proper distribution of thewater to its point of contact with the air. Unit: ftof water.


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Transverse Pertaining to occurrences in the direc-tion of tower width.

Velocity Recovery Fan Cylinder A fan cylinder onwhich the discharge portion is extended in heightand outwardly flared. Its effect is to decrease thetotal head differential across the fan, resulting ineither an increase in air rate at constant horse-

power, or a decrease in horsepower at constantair rate.Water Loading Circulating water rate per horizontal

square foot of fill plan area of the cooling tower.Unit: gpm per sq ft.

Water Rate Mass flow of water per square foot of fillplan area of the cooling tower per hour. Unit: lbper sq ft per hr. Symbol: L.

Wet-Bulb Temperature The temperature of the entering or ambient air adjacent to the cooling toweas measured with a wet-bulb thermometer. UniF. Symbol: WB.

Wet-Bulb Thermometer A thermometer whose bulbis encased within a wetted wick.

Windage Water lost from the tower because of the

effects of wind. Sometimes called blowout.Wind Load The load imposed upon a structure ba wind blowing against its surface. Unit: lb pesq ft.

Figure 16 Spray-fill, counterflow cooling tower. Figure 17 Typical splash-type fill.

D. THE PSYCHROMETRICS OF EVAPORATIONEvaporation as a means of cooling water is uti-

lized to its fullest extent in cooling towers, which aredesigned to expose the maximum transient watersurface to the maximum flow of air for the longestpossible period of time.

The spray-fill, counterflow tower shown in Fig-ure 16 attempts to accomplish this basic functionby spraying the water into fine droplets, and incontaining those droplets to fall through a mechani-cally-induced, upward-moving stream of air.

It is rather obvious that the overall cooling effectwould be improved by increasing the height of thetower, thereby increasing the distance of water falland maximizing the total time of contact betweenair and water. In utilizing that method, however,structural and economic limitations would soon bereached.

A significantly better way to increase contactime is by the installation of fill within the tower timpede the progress of the falling water. Althoughthe various types of fills, and their configurationswill be discussed in Section II, the basic purposand action of a splash-type fill is depicted in Fig

ure 17. Placed in the horizontal area of the towebelow the sprays and above the air inlet level, istaggered rows, these splash bars retard the fallinwater and increase the surface area exposed to theair, thereby promoting the process of evaporation.

Primary knowledge of how to achieve effectivair and water contact notwithstanding, given th

problem of cooling water from 85F to 70F, howcan one hope to do so when the sensible air tem

perature is 78F at a 50 percent relative humidityUtilizing only sensible heat transfer (as in a

air-cooled heat exchanger) the problem would be


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Figure 18 Psychrometric chart with air-water temperature pursuit curve superimposed.


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impossible because the entering air dry-bulb tem-perature (78F) is higher than the desired coldwater temperature (70F). However, the process ofevaporation that occurs in a cooling tower makesthe solution an easy one.

Understanding the evaporative cooling processcan be enhanced by tracing on a psychrometric

chart (Fig. 18) the change in condition of a poundof air (dry wt.) as it moves through the tower andcontacts a pound of water (L/G = 1), as denotedby the solid line. Air enters the tower at condition 1(78F dry-bulb & 50% R.H.), whereupon it beginsto gain moisture content and enthalpy (total heat)in an effort to reach equilibrium with the water, andcontinues this pursuit of equilibrium until it exits thetower at condition 2.

During the transit of this pound of air through thetower, several notable changes occurred which arepertinent to the study of cooling towers:1. Total heat content increased from 30.1 Btu to

45.1 Btu. This enthalpy increase of 15 Btu was

gained from the water. Since, by definition, aBtu is equal to the heat gain or loss required tochange the temperature of one pound of waterby 1F, this means that the temperature of onepound of water was reduced by the specifiedamount of 15F (85-70).

2. The moisture content of the pound of air in-creased from 72 grains to 163 grains. (7000grains = 1 lb.) This increase of 91 grains (0.013lbs.) represents total evaporation from the water.Therefore, since the latent heat of vaporizationof water (at 85F) is approximately 1045 Btu/lb,this means that 13.6 (0.013 x 1045) of the 15Btu removed from the water (91% of the total)

happened by virtue of evaporation.3. Although the temperature of the water wasreduced 15F, the net sensible (dry-bulb) air tem-perature increase was only 3.3F, from 78F to81.3F. (Indeed, at a somewhat lower L/G ratio,the dry-bulb temperature of the leaving air wouldactually have been lessthan its entering value.)

E. FACTORS AFFECTING COOLING TOWERPERFORMANCE

The atmosphere from which a cooling towerdraws its supply of air incorporates infinitely vari-able psychrometric properties, and the tower reactsthermally or physically to each of those properties.

The tower accelerates that air; passes it through amaze of structure and fill; heats it; expands it; satu-rates it with moisture; scrubs it; compresses it; andresponds to all of the thermal and aerodynamic ef-fects that such treatment can produce. Finally, thecooling tower returns that used up stream of airto the nearby atmosphere, with the fervent intentionthat atmospheric winds will not find a way to rein-troduce it back into the tower.

Meanwhile, the water droplets produced by thetowers distribution system are competing with the Figure 19 Mechanically aspirated psychrometer.

air for the same space and, through natural affinityare attempting to coalesce into a common flowingstream having minimum surface area to expose tthe air.

Obviously, the factors which affect cooling toweperformance are myriad. Those factors whose effects predominate are identified and discussed i

this section. Additional performance-influencinfactors will be discussed in succeeding sections.1. Wet-Bulb Temperature

Important to note in the foregoing exampl(Art. D: Fig. 18) is the fact that precisely thsame amount of enthalpy exchange (cooling effect) would have taken place had the air enteredthe tower at a temperature of 65F and 100%relative humidity (condition 1) which, by defintion, is a 65F wet-bulb temperature. For thireason, the primary basis for thermal design oany evaporative type cooling tower is the wetbulb temperatureof the air entering the tower.


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In point of fact, design air conditions for theexample problem would not have been given toa cooling tower designer as 78F at a 50% rel-ative humidity. Rather, instructions would havebeen to design for a 65F wet-bulb tempera-ture. Only if there was a requirement to know theexact amount of evaporation, or if the selection

were for something other than a normal mechan-ical draft cooling tower, would there have beena need to know the design dry-bulb temperatureof the air, or its relative humidity.

Wet-bulb temperatures are measured bycausing air to move across a thermometerwhose bulb (properly shielded) is encased in awetted muslin sock. As the air moves acrossthe wetted bulb, moisture is evaporated andsensible heat is transferred to the wick, cool-ing the mercury and causing equilibrium to bereached at the wet-bulb temperature. For mostacceptable and consistent results, the velocityof the air across the wick must be approximately

1000 fpm, and the water used to wet the wickshould be as close as possible to the wet-bulbtemperature. Distilled water is normally recom-mended for wetting of the wick.

When a wet-bulb thermometer and a dry-bulbthermometer are combined in a common device,simultaneous coincident readings can be taken,and the device is called a psychrometer.

There are some applications, however, wherea comprehensive study should be made ofthe daily (Fig. 22) wet-bulb temperature cyclethrough critical months and, in some instances,the entire year. (Fig. 23) High-load power gen-

erating stations, applications of the coolingtower to off-season free cooling (Sect. V-K), andcertain critical processes fall into this category.Wet-bulb duration curves (Fig. 24) may be es-tablished from which it is possible to evaluateand compare equipment installed costs, plantoperating costs, efficiencies and capabilitiesat various operating wet-bulb conditions. Fromsuch a study, which would consider both sea-sonal loads and the annual wet-bulb pattern, it ispossible to select the optimum cooling tower forthe installation. In many cases, the study wouldresult in reduced capital expenditure, while stillproviding the desired ultimate operating charac-

teristics.Air temperatures, wet-bulb as well as co-

incident dry-bulb, are routinely measured andrecorded by the United States Weather Bureau,worldwide U.S. military installations, airports, andvarious other organizations to whom anticipatedweather patterns, and specific air conditions, areof vital concern. Compilations of this data existwhich are invaluable to both users and design-ers of cooling towers. One such publication isentitled Engineering Weather Data, compiledby the Departments of the Army, Navy and AirForce, and available at www.wbdg.org. Excerpts

Figure 20 Sling psychrometer.

Figure 21 Sling psychrometer as used to determine wetbulb temperature.

Although the mechanically aspiratedpsychrometer (Fig. 19) is generally used for pur-poses of testing and scientific study, the slingpsychrometer (Fig. 20), used as indicated in Fig-ure 21, can give satisfactory results.

Selection of the design wet-bulb temperaturemust be made on the basis of conditions exist-ing at the site proposed for a cooling tower, andshould be that which will result in the optimum

cold water temperature at, or near, the time ofpeak load demand. Performance analyses haveshown that most industrial installations basedupon wet-bulb temperatures which are exceed-ed in no more than 5% of the total hours duringa normal summer have given satisfactory results.The hours in which peak wet-bulb temperaturesexceed the upper 5% level are seldom con-secutive hours, and usually occur in periods ofrelatively short duration. The flywheel effect ofthe total water system inventory is usually suffi-cient to carry through the above-average periodswithout detrimental results.


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from that and other similar sources, pertinent to

the utilization of cooling towers, are also avail-able.

The wet-bulb temperature determined fromthe aforementioned publications represents theambient for a geographic area, and does not takeinto account localized heat sources which mayartificially elevate that temperature at a specificsite. The codes which govern the sizing and test-ing of cooling towers define ambient wet-bulbtemperature as that which is measured at a dis-tance of 50 to 100 feet upwind of the tower, atan elevation approximately 5 feet above its base,without intervening heat sources. Accordingly,

one can see that an upwind heat source beyondthose limitations could cause a cooling tower toexperience wet-bulb temperatures somewhahigher than would be anticipated from publishedata.

Before making a final decision concerning thproper design wet-bulb temperature, it is good

practice to take simultaneous wet-bulb readingat the proposed tower site, as well as at otheopen, unaffected locations at the same plantThese readings should be compared to one recorded at the same time at the nearest sourcof weather data (airport, weather bureau, etc.)and the apparent design wet-bulb temperaturadjusted accordingly.

Finally, and most importantly, once having decided the correct design wet-bulb temperaturethe specifier must be clear as to whether thcooling tower manufacturer is to treat it as aambientwet-bulb or an enteringwet-bulb in thactual design of the tower. As indicated earlie

the basis for thermal design of any evaporativtype cooling tower is the wet-bulb temperaturof the air actually enteringthe tower. If the design wet-bulb is specified to be ambient, thereputable cooling tower manufacturers will adjust that temperature upward, in varying degreesto compensate for any potential recirculation(Sect. I-E-6)

Conversely, if the design wet-bulb temperature is specified to be entering, then the coolingtower manufacturer will make no adjustment othat temperature in his design, and the wet-bultemperature at the time of test will be the average of multiple readings taken at the tower a

inlets.Currently, cooling tower test codes providprocedures for measuring performance in thcase of either entering or ambient wet-bulb temperature specifications. The entering wet-bulbis nearly always used as the specification whicproduces not only equal competition at the timof bidding, but also provides the least room fodoubt at the time that the tower is tested.

Figure 22 Daily variation of wet bulb temperature.

Figure 23 Annual variation of wet bulb temperature.

Figure 24 Typical wet bulb temperature durationcurve.

Figure 25 Typical performance curve.


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From the length of this dissertation, one cangather that accurate determination of the de-sign entering wet-bulb temperature is vital, if thecooling tower is to perform as planned. This issupported by Figure 25, which shows the directrelationship between wet-bulb and cold watertemperatures. If the actual wet-bulb is higher

than anticipated by design, then warmer-than-desired average water temperatures will result.Conversely, if the actual wet-bulb is lower thanexpected, then the Owner will probably have pur-chased a cooling tower larger than he needs.

2. Dry-Bulb and/or Relative HumidityAlthough it is always good practice to estab-

lish an accurate design dry-bulb temperature(coincident with the design wet-bulb tempera-ture), it is absolutely required only when types oftowers are being considered whose thermal per-formance is affected by that parameter. Thesewould include the hyperbolic natural draft (Fig.3), the fan assisted natural draft (Figs 6 & 7), the

dry tower (Figs. 98 & 99), the plume abatementtower (Fig. 103), and the water conservationtower (Fig. 96). It is also required where thereis a need to know the absolute rate of evapora-tion at design conditions for any type tower. (Fig.18)

Where required, the same thought processand concern should prevail in the establishmentof a design dry-bulb temperature as occurred indetermining the design wet-bulb temperature.

3. Heat LoadAlthough appropriate selection of the cooling

tower size establishes the equilibrium tempera-tures at which the tower will reject a given heat

load, the actual heat loaditself is determinedby the process being served. All else beingequal, the size and cost of a cooling tower isproportional to the heat load.Therefore, it is ofprimary importance that a reasonably accurateheat load determination be made in all cases.If heat load calculations are low, the coolingtower purchased will probably be too small. Ifthe calculations are high, oversized, more costlyequipment will result.

Since volumes of reliable data are readilyavailable, air conditioning and refrigeration heatloads can be determined with considerable ac-curacy. However, significant variations exist in

the realm of industrial process heat loads, eachvery specific to the process involved. (See Table1, Section IX) In every case, it is advisable todetermine from the manufacturer of each itemof equipment involved with, or affected by, thecooling water system the amount of heat thattheir equipment will contribute to the total.

4. GPM, Range and ApproachThe heat load imposed on a cooling tower

(Btu/min.) is determined by the pounds of waterper minute being circulated through the process,multiplied by the number of degrees Fahrenheitthat the process elevates the circulating water

temperature. In cooling tower parlance, this be-comes:

Heat Load = gpm x 8.33 x 60 x R = Btu/hr (1)Where: gpm = Circulating water rate in

gallons per minute.8.33 = Pounds per gallon of water at a typical

temperature.

R = Range = Difference between hot watertemperature entering tower and cold watertemperature leaving tower, in degrees F.

Figure 26 graphically shows the relationshipof range and approach as the heat load is appliedto the tower. Although the combination of rangeand gpm is fixed by the heat load in accordancewith Formula (1), approach (difference betweencold water temperature and entering air wet-bulbtemperature) is fixed by the size and efficiency ofthe cooling tower. A large tower of average ef-ficiency will deliver cold water at a temperaturewhich approaches a given wet-bulb tempera-ture no closer than a somewhat smaller towerhaving significantly better efficiency.

Improving efficiency is, of course, the primaryreason for extensive and continuing research and

development by cooling tower manufacturers, andsubsequent sections of this manual will discussthose factors of design which affect efficiency.Suffice it here to say that increased efficiency willmeasurably improve (decrease) approach.

Given two towers of reasonably equal ef-ficiencies, operating with proportionate fillconfigurations and air rates, the larger tower willproduce colder water, as evidenced by Figure 27.Important to note, from a tower cost standpoint,is the fact that the base tower (15F approach)would have had to be twice as large to producea 7F approach (8F colder water), whereas it

Figure 26 Diagram showing definition of Coolingrange and Approach.


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could have produced a 25F approach (10Fwarmer water) at only 60% of its size.

Note also that the decreasing approach curveis beginning its asymptotic movement toward zeroapproach. For this reason, it is not customary inthe cooling tower industry to guarantee any ap-proach of less than 5F not because towers areunable to produce them but because any errors inmeasurement become very significant when per-formance is calculated at the design point.

As can be seen from an analysis of Formula(1), heat load dissipation can be accomplishedwith almost infinite combinations of flow ratesand ranges. Usually, however, a relatively nar-row band of possible combinations is dictatedby hydraulic limitations and/or temperature-effi-

cient levels of the process being served. Wheresome latitude of choice is given by the process,a smaller, less costly tower will be required whenthe range is increased and the GPM decreased,as shown in Figure 28. Although prudent designresponsibility places flow and temperature re-strictions on cooling towers as well, their latitudeusually exceeds that of the typical processesthey are designed to serve.

Figure 27 Effect of chosen approach on tower size at fixedheat load, gpm and wet-bulb temperature.

Figure 29 Interference.

Figure 28 Effect of varying range on tower size when heatload, wet-bulb temperature and cold watertemperature are constant.

Figure 30 Downwind wet-bulb contour of largexisting cooling tower.

5. InterferenceAs previously indicated, local heat source

upwind of the cooling tower can elevate the wetbulb temperature of the air entering the towethereby affecting its performance. One such heasource might be a previously installed cooling

tower on site, or in the immediate vicinity. Figure29 depicts a phenomenon called interferencewherein a portion of the saturated effluent of anupwind tower contaminates the ambient of downwind tower. Although proper cooling toweplacement and orientation (Sect. I-E-7-(c)) caminimize the effect of interference, many existinginstallations reflect some lack of long range planning, requiring that design adjustments be madin preparation for the installation of a new towe


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Figure 30 indicates the increase in wet-bulbtemperature profile downwind of a large, poorlyoriented cooling tower operating broadside to a10 mph wind, based upon actual readings takenat grade level. If the only available location fora new tower were 300 ft. NNW of this tower,the specifier would be wise to select a design

wet-bulb temperature at least 3 degrees higherthan local conditions would otherwise indicate.However, at a given cold water temperature re-quirement, this would be equivalent to reducingthe approach by about 3 degrees, which Figure27 reveals would have a significant impact uponthe cost of the new tower. Obviously, if any otherlocation for the new tower is available, it shouldbe placed out of the lee of the existing tower.

6. RecirculationArticle E-1 of this Section described the

important differences between ambient and en-tering wet-bulb temperatures. The latter can be,and usually is, affected by some portion of the

saturated air leaving the tower being inducedback into the tower air inlets.

Figure 31 Recirculation.

Figure 32 Round mechanical draft tower operating in asignificant wind. Compare plume rise to flattrajectory of smoke leaving stack.

This undesirable situation is called recircula-tion, and reputable cooling tower manufacturersdevote much research and development timeboth to determining the potential for recirculationunder various wind conditions, and to designingtheir towers in such a way as to minimize its ef-fect.

The potential for recirculation is primarily relat-ed to wind force and direction, with recirculationtending to increase as wind velocity increases.For that reason, accepted codes under whichcooling towers are tested for thermal perfor-mance limit wind velocity during the test to 10mph. Without this, and similar limitations, coolingtower testing and design would become infinite-ly more uncertain and difficult.

Although wind is the primary cause of recir-culation, several other aspects of cooling towerdesign and orientation play important parts in itsreduction and control:

a. Tower Shape:When flowing wind encountersan obstruction of any sort, the normal path ofthe wind is disrupted and a reduced-pressurezone or wake forms on the lee side (down-wind) of that obstruction. Quite naturally, thewind will try to fill this void by means of theshortest possible route. If the obstruction is

tall and narrow, the wind easily compensatesby flowing around the vertical sides. However,if the structure opposing the wind is long andrelatively low, the quickest path for pressureequalization is over the top and downward.

Figure 30 is an exaggerated (althoughactual) example of what can happen in thissituation. Note that the air inlet on the northface of the tower is experiencing wet-bulbtemperatures some 6 to 7 degrees higherthan that seen by the south face. The resultantincrease in enthalpy of the entering air has, ofcourse, degraded thermal performance tre-mendously.

Wind flows in a much more civilizedfashion around a round cylindrical shape(Fig. 32), creating an almost negligible zoneof reduced pressure on the downwind side;the air requirements of which are easily sat-isfied by streamlined flow around the shape.Application of this principle to the design oflarge cooling towers has resulted in extremelystable performance levels for critical projects.(See Fig. 35 also)


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Figure 33 Longitudinal wind direction concentratesseparate stack plumes into one of highbuoyancy.

Figure 34 Effect of wind velocity and dischargevelocity on plume behavior.

Figure 35 Comparative recirculation potential oround and rectangular towers.

b. Orientation with Prevailing Wind:If the wind

indicated in Figure 30 is blowing in its prevail-ing direction, the Owner would have been welladvised to turn the tower 90 degrees from itsindicated orientation, as has been done inFigure 33. With this orientation, the wind firstencounters the relatively high, narrow end ofthe tower, and the small negative pressurezone at the far end is easily filled by wind flow-ing around the vertical sides. Furthermore,wind moving parallel to the line of fans causesthe separate effluents from each fan cylinderto stack up one on another, forming a con-centrated plume of greater buoyancy.

The Round Mechanical Draft tower (Fig.

32) is, of course, unaffected by wind direc-tion, and the centralized clustering of the fansproduces a concentrated buoyant plume.

c.Air Discharge Velocity: At any given atmospheric condition, the velocity at which thedischarge plume from a tower will rise depends upon the kinetic energy imparted bthe fan, and the effluent energy (decrease idensity) imparted to the effluent plume by thetower heat load, both of which are changed to

potential energy by virtue of ultimate elevatioof the plume.The direction that a plume will trave

depends upon the speed, direction, anpsychrometric characteristics of the wind encounters upon leaving the fan cylinder. Lowwind velocities (Va, Fig. 34) will permit an amost vertical plume rise, barring retardation othat rise by unusual atmospheric conditions(For an induced draft tower operating under calm conditions, with a vertically risingplume, entering and ambient wet-bultemperatures can be considered to bequal.) Higher wind velocities will bend the

plume toward the horizontal, where a portioof it can become entrapped in the aforementioned lee-side low pressure zone for re-entrinto the tower. (Figs. 31 & 36)

The velocity ratio indicated in Figure 35is the result of dividing the plume dischargvelocity (Vj) by the velocity of the ambient wind(Va). For all intents and purposes, the recirculation ratio is the percent of total effluent athat is reintroduced into the tower air inlets bvirtue of recirculation. As can be seen, lowevelocity ratios (higher wind velocities) result igreater recirculation. The values for the rectangular tower represent those anticipated fo

an industrial tower of moderate size operatingbroadside to the prevailing wind. The recirculation ratio for that tower would reach minimumvalue with a 90 degree directional change.

Since the velocity ratio is also a functioof plume discharge velocity, ambient wind


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Figure 36 Recirculation potential in a forced draftcooling tower.

force cannot accept all of the blame for re-circulation. At any given wind condition, thevelocity ratio will decrease if the plume ve-locity is decreased, resulting in an increasein the recirculation ratio. This is what makesforced draft towers (Fig. 36) so susceptibleto recirculation. The normal discharge velocity

from an induced draft tower is about 20 mph,whereas the plume velocity leaving a forceddraft tower is approximately 5-6 mph. Figure35 reveals that this 4:1 difference in velocityratios results in considerably greater recircu-lation in a forced draft tower.

d. Fan Cylinder Height and Spacing: Withinstructural limitations, discharge heights of fancylinders can be increased. (Fig 100) Also, thefan cylinders can be spaced somewhat farther

apart to allow for a less restricted flow of windbetween them. Both of these stratagems, usu-ally done in concert, can measurably diminishthe potential for recirculation in most operatingsituations, although not without some impactupon tower cost.

Such specialized modifications will becovered in Section V of this manual.

7. Tower Siting and OrientationA significant portion of this text is devoted to

the effects of recirculation and interference and,with expert guidance from the cooling tower man-ufacturer, it is the responsibility of the Owner/specifier to situate the tower such that these

and other thermal performance influencingeffects will be minimized.Since the long termcapability of a cooling tower is determined byits proper placement on site, the importance ofsuch placement cannot be overemphasized.

Every effort should be made to provide theleast possible restriction to the free flow of airto the tower. In addition to this primary consid-eration, the Owner must give attention to thedistance of the tower from the heat load, andthe effect of that distance on piping and wiringcosts; noise or vibration may create a problem,which can be expensive to correct after the fact;

drift or fogging may be objectionable if the toweris located too close to an area that is sensitiveto dampness or spotting; also easy access andadequate working space should be provided onall sides of the tower to facilitate repair and main-tenance work.

The performance of every cooling tower, large

or small, is dependent upon the quantity and ther-mal quality of the entering air. External influenceswhich raise the entering wet-bulb temperature,or restrict air flow to the tower, reduce its effec-tive capacity. Air restrictions, recirculation andinterferences can be minimized, possibly elimi-nated, by careful planning of tower placementusing the following guidelines:a.Air Restrictions: In residential, commercial,

and small industrial installations, towers arefrequently shielded from view with barriers orenclosures. Usually, this is done for aestheticreasons. Quite often, these barriers restrict airflow, resulting in low pressure areas and poor

air distribution to the air inlets. Sensible con-struction and placement of screening barrierswill help to minimize any negative effect uponthermal performance.

Screening in the form of shrubbery, fenc-es, or louvered walls should be placed severalfeet from the air inlet to allow normal air entryinto the tower. When an induced draft toweris enclosed, it is desirable for the enclosure tohave a net free area opposite each louveredface which is at least equal to the gross louverarea of that tower face.

Screening barriers or enclosures shouldnot be installed without obtaining some input

concerning their design and placement fromthe cooling tower manufacturer.b. Recirculation: Except in the case of single-flow

towers (Sect. I-B-4), the proper placement tominimize recirculation is to orient the towersuch that the primary louvered faces are situ-ated parallel (not broadside) to the prevailingwind coincident with the highest ambientwet-bulb temperature. On towers of relativelyshorter length, this allows the saturated efflu-ent to be carried beyond the air inlets. Longermultiple-fan towers in this orientation benefitfrom the wind having concentrated the sepa-rate cell plumes into one of greater buoyancy.

Because of the restricted siting areasavailable in some plants, the Owner may haveno choice but to orient towers broadside to aprevailing wind, and to adjust his design wet-bulb temperature accordingly. The amount ofadjustment necessary can be reduced by rec-ognizing that recirculation potential increaseswith the length of the tower (Fig. 30) and bysplitting the tower into multiple units of lesserindividual length with a significant air space inbetween. If, for example, the tower in Figure30 had been installed as two 150 foot longtowers in line, with a 50 foot space between


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Figure 37 Proper orientation of towers in a prevailing longitudinal wind. (Requires relatively minimal tower size adjustment tocompensate for recirculation and interference effects.)

Figure 38 Proper orientation of towers in a prevailing broadside wind. (Requires significantly greater tower size adjustment tcompensate for recirculation and interference effects.)


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the ends of the towers, the net amount of re-circulatory effect may well have been halved.

c. Interference: Similarly, multiple towers shouldnot be situated such that any tower is withinthe downwind interference zone (lee) of anoth-er tower or extraneous heat source. If a toweris so located, then its design wet-bulb tem-

perature should be adjusted appropriately.Although the round tower indicated inFigure 32 suffers relatively little from recircula-tion, it is certainly not immune to interferencefrom an upwind tower, nor will it hesitate toimpact a downwind tower under certain at-mospheric conditions.

d. Effect on Site Piping: The need for propersiting and orientation is fundamental to a tow-ers ability to cool water dependably, and musttake precedence over any concern as to thequantity or complexity of site piping requiredto accommodate the appropriate cooling tow-er layout. On relatively small installations, the

extent of cooling tower relocation that may berequired usually has an insignificant impact ontotal piping cost. Large multi-tower projects,however, typically require several hundred feetof pipe of appreciable diameter, representing

a portion of the overall project cost that is any-thing but insignificant.

As will be seen in Section II-D, themultiplicity of water distribution system ar-rangements available on crossflow coolingtower designs coordinate to reduce the re-quired site piping to a minimum for rectilinear

tower layouts. As can be seen in Figure 39,however, most effective reductions in sitepiping requirements occur when either hy-perbolic or round mechanical draft towers arechosen. This is because of their inherent toler-ance to much closer spacing.Obviously, there are no rules of thumb which

will cover every conceivable situation. Nor arethe indicated guidelines intended to take theplace of direct contact and discussion with areputable cooling tower manufacturer. Consider-ing that the location and orientation of the towercan impact the entering wet-bulb temperaturefrom as little as 0.5F, to as much as 3F to 5F,

the user would be wise to invite as much expertassistance as possible. On certain critical proj-ects involving appreciable heat loads, it may wellbe advisable to consider site-modeling for windtunnel study.

Figure 39 Comparison of piping and ground use for both rectilinear towers and round towers. (Both types selected forequal performance.)


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SECTION

F. MATERIALS OF CONSTRUCTIONMention of typical materials utilized for specific

components is made throughout this text, as ap-propriate. This portion, therefore, will be somewhatreiterative in its identification of standard con-struction materials; but will cover in greater depthwhy those materials were chosen, and will indicate

the primary alternatives utilized to satisfy unique re-quirements. The impact of water quality on materialselection will be discussed in Article G following.Other effects will be covered in Section V, Special-ized Tower Usage and Modifications.1. Wood: Because of its availability, workability,

relatively low cost, and its durability under thevery severe operating conditions encountered incooling towers, wood remains a commonly uti-lized structural material.

Having met these requirements admirably,Douglas Fir is used extensively in the coolingtower industry. It began to compete with Cali-fornia Redwood as the preferred material in the

early 1960s, and has since established a recordof success rivaling that of redwood. Douglas Firplywood, in exterior grades, is also widely used asdecking, partitions, and basin flooring materials.

Various other wood species, both domesticand foreign, may be used in cooling towers, pro-vided that they exhibit proven durability in severeexposures and meet the physical and structuralrequirements of the installation.

Regardless of specie, however, any wood usedin cooling tower construction must be treated witha reliable preservative to prevent decay. CCA is awaterborne preservative consisting of inorganicchromium, copper, and arsenic which has been

used to treat cooling tower wood in the U.S. forover 50 years. The copper and arsenic are fungi-cides, while chromium acts primarily as a chemicalfixing agent by reacting with natural chemicals inwood and forming insoluble, leach-resistant com-pounds with the copper and arsenic. Preservativesare diffused into the wood by total immersion in apressure vessel, with pressure being maintainedeither until a prescribed amount of preservative isretained by the wood, or until the wood refuses toaccept further treatment. In either case, laboratorytests, as well as historical records, have proventhe treatment to be adequate.

Creosote treatment is used occasionally, al-

though it adds considerably to the tower cost dueto difficulties encountered in handling the treatedwood. Also, its greater tendency to leach out ofthe wood can lead to heat transfer problems, andmay complicate maintenance of water quality.

Although wood is relatively insensitive to chlo-rides, sulfates, and hydrogen sulfide, it can bedamaged by excessive levels of free chlorine,and is sensitive to prolonged exposure to exces-sively hot water. Design hot water temperaturesshould be limited to 140F, or should be con-trolled to that level by the use of a cold waterby-pass, as described in Section V-I-1.

2. Metals (Hardware): Steel is utilized for mancomponents of the cooling tower where higstrength is required. This would include fan hubfor larger diameter fans; unitized supports fostabilization of the mechanical equipment; mandriveshafts (although stainless steels are normally used for the larger shafts); fan guards and

driveshaft guards; as well as the tie rods, boltsnuts and washers.In the overwhelming majority of cases, cir

culating water conditions will be considerednormal, and the coating of choice for steel itemwill be galvanization. For severe water servicethose components whose requirements are noeconomically met by alternative materials arusually coated with epoxy-coal-tar.

Cast iron and ductile iron are used for geacases, anchor castings, flow control valve bodies, and fan hubs for intermediate sized fansAlthough both cast iron and ductile iron enjogood corrosion resistance, the are usually eithe

galvanized or coated with a high grade enamefor cooling tower use. For severe water servicehowever, they are usually sand-blasted and coated with epoxy-coal-tar. Valve bodies may even bporcelainized to guard against erosion.

Bolts, nuts and washers, of course, do nolend themselves to a pre-coat, other than gavanization or cadmium plating. Severe wateconditions normally dictate a change in materals, and an appropriate grade of stainless steeis the popular choice because of excellent corrosion resistance in the aerated conditions existingin cooling towers.

Copper alloys are sometimes used to resis

special conditions, such as salt or brackish water service. Silicon bronze fasteners are suitablfor service in salt water, but must be protecteagainst erosion. The use of Naval brass is normaly discouraged because of its tendency towardstress corrosion cracking. Utilization of more sophisticated metals, such as monel and titaniumis usually precluded by cost considerations.

Selection of aluminum alloys for use in cooing towers is done with care. Only the morcorrosion resistant alloys are used, and they arutilized only for specific components of significant cross section. Among these componentare fan blades, fan hubs for smaller size fans

some ladder assemblies, and handrail fittings fosteel framed cooling towers.

Most of the smaller towers designed for factory assembly are primarily of steel constructionSome local building and/or fire codes also dictate that larger towers be of steel constructioas well. In these cases, galvanized steel is usedfor structures, basins, partitions, decking, facylinders, and many other major components. Iselected applications of extreme severity, suctowers have been successfully manufacturedof stainless steel, although the cost impact wasignificant as might be expected.


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SECTION I

3. Plastics: The use of selected plastics began tobe investigated in the early 1950s and, since thattime, has accelerated tremendously with pultrud-ed fiberglass becoming the most commonly usedstructural material in recent years. PVC has beenthe most commonly used fill material for sometime. Contributing to this increased usage are their

inherent resistance to microbiological attack, cor-rosion and erosion; their compatibility with othermaterials; their formability; their great strength-to-weight ratio; and their acceptable cost level.

The capability of plastics to be molded intosingle parts of complex shape and dimensions isa distinct advantage, particularly for such close-tolerance components as fan blades and fancylinders. Their many desirable characteristics,combined with the advancements being madein both plastic materials and their production,assure that they will continue to be utilized forcooling tower components, and the rate of us-age is anticipated to increase.

Plastics are currently used in such compo-nents as pultruded structural members, structuralconnectors, fan blades, fan cylinders, fill, fill sup-ports, drift eliminators, piping, nozzles, casing,louvers, and louver supports. Most commonlyused are fiber reinforced polyester (FRP), fiberreinforced epoxy, polyvinyl chloride (PVC), poly-propylene, and fiber reinforced nylon.

Generally speaking, plastic components are ofa dimension, location, or formulation that makesthem least susceptible to abnormal water condi-tions. However, the relatively thin cross sectionutilized for PVC film fill sheets makes them some-what sensitive to temperature, requiring something

more than routine thought in fill support design.Some plastics, such as PVC, are inherently fireresistant. Where required, others may be formu-lated for fire retardancy. The most common suchformulation being fire retardant fiber reinforcedpolyester, utilized for casing and louvers.

4. Concrete: Concrete has been used for manyyears in Europe and other regions of the world,and its use is increasing in the United States. Fol-lowing the first concrete hyperbolic cooling towerinstalled in the USA in 1961, numerous othershave been installed, and the technology has beenexpanded to large mechanical draft towers. Inmany cases, the higher first cost of concrete con-

struction is justified by decreased fire risk and, forlarger structures, higher load carrying capacity.

Basically, design philosophies for concre

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