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21 Cooling towers

Original by John NellerFilm Cooling Towers Ltd

Revised by Eur Ing Dennis A. Snow CEng,MIMechE, HonFSOE, HonFIPIantE, HonFIIPE

21/1

Contents

21.1 Background 21/3

21.2 Theory 21/3

21.3 Design techniques (see Appendix 21.1) 21/3

21.4 Design requirements 21/4

21.5 Materials and structure design 21/421.5.1 Counterflow 21/421.5.2 Crossflow 21/4

21.6 Specification 21/5

21.7 Water quality and treatment 21/6

21.8 Operation 21/7

21.9 Modifications – retro-fits 21/8

21.10 Consultation 21/8

21.11 Environmental considerations 21/821.11.1 Aesthetic 21/821.11.2 Noise 21/10

21.12 Problem areas 21/1221.12.1 Installation 21/12

21.13 Summary 21/14

Appendix 21.1 Theoretical calculations 21/14

Appendix 21.2 Evaluation of the MDF 21/15

Appendix 21.3 Technical requirements 21/15

Acknowledgements 21/16

21.1 BackgroundThe use of water as a cooling medium has been longestablished, but its importance, in an industrial sense,was emphasized with the introduction of steam power.Cooling ponds are still widely used and spray ponds,which incorporate a degree or so of evaporative cooling,can be found, but the increasing requirement for controlon water-cooling temperatures heralded the developmentof the modern cooling tower. The cost of land and theincreasing expense of abstracting and of returning water,as well as its availability, ensured that the engineering anddesign techniques employed were sufficient to satisfy theeconomic factors imposed by these constraints.

The state of the art in cooling tower design is beingconstantly improved. Correctly designed, installed andmaintained, today's cooling tower still remains the opti-mum selection in the great majority of cases where heatdissipation is required.

Historically, the first pack designs were random timber,to be followed rapidly by ordered timber splash bars. Theconcept of filming water as opposed to splash or concreteoriginated in England in the 1930s. The introductionof plastic packings dates from the 1950s, but this wasconfined to mechanical-draught towers until the 1970s,when experiments started with plastic packs in natural-draught cooling towers. Asbestos ceVnent in flat sheets,corrugated sheets and flat bars, although widely used inthe past, are now out of favour on health grounds in mostdeveloped countries. Plastic-impregnated paper is used incertain Eastern countries in air-conditioning towers, buthas been unsuccessful in the West. In most cases thechanges have been due to economics, but water qualityand type of process can significantly affect the selectionin individual cases.

Developments over the years have brought about thenecessity for very close control over the water quality.This important subject is referred to later, but at this stage,it is important to draw attention to the latest publicationfrom the Health & Safety Executive - the new issue ofthe L8 - APPROVED CODE OF PRACTICE - (ACOP)which incorporates in Part 2 'Guidance on the control ofLegionella in water systems' which replaced the separateHS(G)VO document.

21.2 TheoryWater cooling in towers operates on the evaporative prin-ciples, which are a combination of several heat/masstransfer processes. The most important of these is thetransfer of liquid into a vapour/air mixture, as, forexample, the surface area of a droplet of water. Con-vective transfer occurs as a result of the difference intemperature between the water and the surrounding air.Both these processes take place at the interface of thewater surface and the air. Thus it is considered to behaveas a film of saturated air at the same temperature as thebulk of the water droplet.

Finally, there is the transfer of sensible heat fromthe bulk of the water to the surface area. This is soslight in terms of resistance that it is normally neglected.

Radiant heat transfer is also ignored for all practicaldesign purposes.

Thus the two main processes are evaporation of waterand convective cooling. The first is based on the differ-ence in partial vapour pressure and the second upon thetemperature difference.

Merkel's analysis in 1924-1925 demonstrated that forpure counterflow it is possible to combine these processesinto a single term by using the enthalpy difference as adriving force. Experience over many years supports this,and cooling tower design in counterflow is universallyrepresented by temperature-enthalpy diagrams.

For crossflow designs the additional factor of the hor-izontal depth of packing has to be included in the basiccalculations. The accuracy of the design is directly relatedto the number of calculations in the selection programme.Whereas counterflow can be dealt with as a single entity,crossflow has to cope with the changes that occur at everylevel of pack, both vertically and horizontally.

21.3 Design techniques (seeAppendix 21.1)

The performance of any cooling tower can be assessedagainst the following:

KaV _ CL ~ (LIGY

where KaV/L is normally given as the performance indexor is quoted as KaV/L demand. LIG = water/dry air massratio and, taking n as average value 0.6, the constantC is proportional to the height of packing. This wouldcover most design requirements. Excessive temperaturesand extremely high air rates will require further factorswhich are not necessary in the great majority of cases.

The Cooling Tower Institute (CTI) publishes sets ofgraphs which give demand in terms of three design tem-peratures and LIG. The CTI graph first published in1967 gives KaVIL demand plotted against LIG withthe approach temperature as a parameter. Each curveapplies to a specified combination of wet bulb temper-ature and cooling range. Each cooling tower pack has itsown KaVIL and responsible suppliers will supply per-formance graphs similar to those of the CTI.

Having selected a type and height of pack, the aboveequation can be plotted to intersect with the requireddemand curve to obtain the LIG. With the LIG and thegiven amount of water to be cooled, the air requirementcan be calculated for:

Design water flowAir= UG

The following steps should then be:

1. Correct calculated air volume to conditions at the fan.2. Select the air rate and calculate the plan area of the

tower.3. Select a fan and calculate the power requirements

from the known air volume and the pressure dropcharacteristics of the selected pack.

4. Assess the cost factors applicable, in terms of fan,pump cost, price of land, maintenance and treatmentcosts.

5. Optimize all known factors in terms of efficiency andthen economy.

21.4 Design requirementsThe factors which should be applied at the design stagecover the water flow rate, the design wet bulb figure,the required return temperature at the design point, thecost of power and land, and the water analysis. Waterflow is normally determined by the equipment that thecooling tower is serving (for example, heat exchangers).Process designers historically leave the cooling tower untillast (it is, after all, the final heat sink). When watercosts were negligible, this was acceptable, but with theincrease in costs and, in certain cases, the restrictions onavailability of water, this approach has had to be modified.Greater consideration must be given to the overall system.Experience in the last ten years has shown that economicoptimization can lead to a more efficient cooling tower,with a corresponding drop in the cost of heat exchanger.This is particularly true in power generation and industrialprocesses.

Design wet bulbs can be determined from publishedmeteorological data for the area concerned. The difficultyis deciding how to relate the annual coverage to the towerperformance at any given time.

For some years it was common practice to quote threedifferent figures, based on the tower's performance as apercentage of the year. For example, in air conditioning itcould be shown that the tower would achieve its design for95% of the year. Alternatively, a tower costing 15% lesscould obtain its design parameter for 85-90% of the year.Only the operator would know whether the 85-90% orless was acceptable, while the economists would welcomethe saving of financial capital.

The tendency nowadays is to design direct for thethree warmest months of the year or as specified bythe requesting purchaser, or to meet the specified bythe requirements of the plant supplier's necessity. Theselection of a tower no longer gives the 3 wet bulbalternatives, as the selection of the final specification mayinfluence the obtaining of an incorrectly suited tower. Theeconomic argument therefore no longer enters into theselection process.

The frequent failures to achieve even the quotedreduced percentage figures led to a reappraisal, and cur-rent design is more accurate. In some respects this is alsodue to the improvement in pack designs, particularly inthe European and American markets. However, it must besaid again that in optimizing cooling tower selection thedesigner must be advised of all appropriate factors. Dis-cussions with cooling tower designers at the outset cansave time and money in the future.

Water quality is important, not only from an environ-mental point of view but also in relation to the typeof packing to be specified. Analysis of the circulatingwater is simple to obtain, but it is very seldom offeredto the cooling tower designer. The quality, or lack of it,

will determine the type of pack to be used, the selectionof structural materials and whether the tower shouldbe induced or forced draught, counterflow or crossflow.Water treatment, in the shape of chemicals to control pHand to act as counter-corrosion agents or as biocides, allhave a bearing on tower selection.

Modern film packs can be offered for a range of TotalSuspended Solids' (TSS) levels in recirculating wateri.e. typically using the most efficient pack designs. TSSconcentrations should not exceed 50mg/l.

Alternative film flow designs can be supplied for con-centrations levels up to lOOmg/1 and 180mg/l. Obviouslyother factors may modify these parameters, but are goodenough for a general rule. Over 180mg/l splash packdesigns would be required And such designs would berequired and such designs are now based on plastic splashgrids, rather than timber splash bars/laths.

The 'Legionella syndrome' has resulted in healthauthorities in the UK applying statutory regulations, whichare directly reflected in terms of capital cost and towermaterial selection. To safeguard against this, responsibledesigners have already produced cooling tower designswhich not only meet the regulations but anticipate future,more stringent, legislation.

The following list of information factors should bemade available to any supplier so that discussions onthe technical requirements can be carried out prior tooptimization (see Appendices 21.1 and 21.2).

21.5 Materials and structure designThe great majority of towers available fall into one oftwo categories: counterflow or crossflow (co-current flowis available but is seldom used) (see Figure 21.1).

Natural Draught Towers, once associated with the gen-eration of electricity, this type has not been installed inthe UK for some years, although they have been installedabroad. This is primarily because they are considered as'Visual Pollution', and do tend to give the uninitiated theimpression that they are also environmentally pollutingthe atmosphere, although they are not.

21.5.1 Counterflow

Counterflow designs are used throughout the entire designfield, i.e.:

Natural draught - hyperbolic concrete shellsMechanical draught - induced draughtForced draught

21.5.2 Crossflow

Crossflow designs are also used throughout the entirefield, i.e.:

Natural draught - hyperbolic concrete shellsMechanical draught - induced, single and double sidedand circularForced draughtAdvanced fan-assisted natural draught.

Figure 21.1 Counter- and crossflow modes

Axial or centrifugal fans can be applied in most cases andare significant factors in the final selection and optimiza-tion (Figures 21.2 and 21.3).

Hybrid towers, combining wet and dry cooling, aredesigns to meet specific problems and require exper-tise from specialist suppliers. Structural materials includeconcrete, timber, various forms of metal (including gal-vanized and alloys), GRP, PVC and, again, variationsand combinations of other materials. Packing materialshave an almost similar pattern but must include com-pressed paper and compressed asbestos cement paper,but the great majority of towers currently employ plas-tic, in some form or another, unless the water conditionsare such that timber (or even concrete) must be used asalternatives.

Figure 21.3 Improved design of crossflow unit (note easy-removalpanel)

21.6 Specification

The purchase of the cooling tower is, in most cases, aonce in a decade operation. Where towers are bought ona regular basis, specifications are determined either by theuser or by the consultant, incorporating their experienceof operation and any changes required as a result ofproduction/process alterations.

Figure 21.2 Basic crossflow units

Counter-flow

Natural draught

Induced draught (rectangular)

Induced draught (circular)

Forced draught

Crossflow

Natural draught

Double-sided crossflow

Single-sided crossflow

Circular

Forced-draught crossflow

Fan-assisted natural draught

In air conditioning circles, the tower normally repre-sents the final heat sink in a turnkey package which wouldinclude compressors/condensers, pipework, ducting, fans,pumps, control gear, etc. Where consultants and experi-enced contractors are concerned, the tower specificationis well defined and the purchases based upon economicsrelated to efficiency.

Where a tower is to be purchased for a one-off situation,it is worth considering the various factors which can affectthe final choice:

Location (both geographically and in elevation)Restrictions (i.e. planning, structural, physical and envi-ronmental)Design wet bulbDesign dry bulbWater flow rateWater qualityWater treatment to be used/cycles of concentration?ProcessConstant or cyclic heat loadCriticality of return temperatureNoise restrictionsAdditional local environmental factorsWater discharge regulations (quantity and quality)

For the periodic or once-off buyer it is essential to obtainadvice from a reputable supplier or a consultant withexperience of cooling tower usage (Figure 21.4).

As an example, the reputable cooling tower designerwould establish most of the above parameters from hisown experience. In addition, he could determine withhis client the economic factors which could influencehis selection, i.e. low capital cost with high runningcosts, or a higher capital cost with more acceptablepower costs (a 12-month or a 5-year payback period).This particular factor is often understandably ignored byturnkey contractors, and end users should always obtainalternative designs to make their own selection.

The costs of water, energy and land are all contributingfactors in economic assessment. The emphasis placed oneach differs from the varying viewpoints, but it is the end

user who has to 'foot the bill'. It is therefore in his owninterests to acquire some knowledge of the cooling towersbeing offered.

21.7 Water quality and treatment

As, in most cases, the circulating water in any systemincorporating a cooling tower is recycled (the loss beingmade up as a percentage of total flow), the subsequentconcentration will affect the water condition. This, in turn,determines the type of treatment required to maintain pHand to control any potential biogrowth. Acidic or alkalinewaters pose their own problems in terms of corrosion andmaterial attack. The larger users probably employ theirown chemical specialists, while others rely on consultantsto determine the type of treatment the system requires.However, the majority of tower users have very littleknowledge of the chemistry involved and depend onwater-treatment organizations.

The following definitions are useful for reference tofamiliarize the end user with the terminology currentlyemployed:

pH The acidity or alkalinity of the waterexpressed as a scale of O (acid) to 14(alkaline). pH 7 is regarded as neutral.

TDS Total dissolved solids, expressed as ppm(parts per million) or as mg/1 (milligramsper litre). Evaporate the water from asample and the residue can be weighed.

TSS Total suspended solids: expressed in sim-ilar terms to TDS but representing a con-centration of insoluble particles.

Conductivity Used as a measure of mineral impurities.LSI Langelier Saturation Index: indicates the

corrosive (negative) or scale-forming(positive) characteristics.

Hardness Expressed as CaCO3, this is the totalcalcium and magnesium salts in the water.Hardness figures given as ppm or mg/1 areimportant, as the compounds of these twoelements are responsible for most scaledeposition.

Alkalinity Expressed as CaCO3, this is the total con-centration of alkaline salts (i.e. bicarbon-ate, carbonate and hydroxide).

BOD Biological Oxygen Demand: expressed asppm or mg/1, it is used as a measure ofpollution.

COD Chemical Oxygen Demand: as above, butrelated to chemical impurities.

Oxygen Sag The level of oxygen in a polluted watersystem. Normally shown in a graph form.

Fouling Factor This is generally applied to plastic packsin natural-draught towers but can relateto larger mechanical draught and to thebiological fouling that can occur. It alsoreflects on the thermal performance ofthe packing (In the days when the nat-ural draught towers associated with thegenerating industry the CEGB published

Figure 21.4 Architect-designed tower (note water pattern inbasin). The office windows are affected by chemicals in carry-over

fouling factors for certain high-densitypacks where the supply of water wasprone to seasonal biological growth andsilt deposition, along with calcium hard-ness deposition).

While other terms are, of course, employed, the above canbe useful for most end users.

21.8 Operation

Having selected and purchased a cooling tower, it needsregular maintenance, as does any other part of the plant.This is true of every cooling tower, from the largestnatural-draught tower to the smallest packaged unit.

All cooling tower operators should keep a full main-tenance and working log to enable proper control to bemaintained on the water quality and treatment and theoperating efficiency, enabling planning of maintenanceand operating procedures to be continually be monitoredand updated as required. In air-conditioning installations,with the experience of Legionella, it is now mandatory tokeep such a log, as well as a record of hygiene testing todetermine the non-existence of bacteria. Chlorine dosing,as recommended by certain local authorities (includingLondon), is essential. Tower materials have to be assessed

in regard to chlorine residuals which can be damagingto galvanized metals and timber. Mechanical equipmentrequires regular checks, apart from the commonsenseordinary maintenance (it is surprising how often this isignored).

The fault-finding chart (Figure 21.5) was originallyproduced by the Cooling Water Association (which sub-sequently became the Industrial Water Society and thenThe Water Management Society), and it is practical andsimple to follow. Water treatment checks (apart from theLegionella requirements referred to earlier) must be car-ried out and water samples should be analysed on a regularbasis. How frequently this takes place depends on the crit-icality of the tower for the end user. Cooling towers arewater-conservation tools as well as heat dissipators: withwater costs increasing, continuous tower performance isessential. In any case, a down-time caused by lack ofmaintenance is costly and careless.

Remember that outside influences (for example, newbuilding work in the vicinity of the installation) canincrease air-based pollution, such as cement dust enteringthe tower at air inlet levels or via the forced-draught fan.Extra cleaning of the tower pack and distribution systemshould be undertaken under these circumstances and closechecks kept on the efficiency of the overall system.

Figure 21.5 Fault-finding chart

Blockageof packing

Iced-upair inlets

Restrictionsto air inlets

Restrictionsto air outlets

Towercladding

Blockage ofeliminators

Non-returndampers betweenfans stuck

Incorrectrotation

Incorrectblade angle

Impellerdeterioration

Driveslip

Insufficientfans run

Insufficientfan rev/min

Structure defects Fan defects

Insufficient air flow

Valvesshut

Brokendrive shaft

Brokenimpeller

Startercontrol fault

Drive motorfault

Failure of high-temp, cut-out

Wrongvoltage

Seizedimpeller

Seizedbearing

Drive motorfault

Frozenwater inlet

High-temperature cut-outFuses blownNoYes

Water head pressurePump motor running Pump motor stopped

No water flowAll OK

Damagedpump impeller

Strainerrestricted

Airentrainment

Strainerrestricted

Slowdownvalve open

Non-returnvalves stuck open

Leakingbasin

Make upsupply failure

Drain cockleft open

Packingdamage

Packingblocked up

Distribution ,system restricted^

Insufficient pump pressure

Good

Non-flooded suction and/or vortexing

Water flow patternYes

Poor

Is the water flowing?No

Leakingbasin

Drain valvesleft open

Basinfrozen over

Brokendrive

Non-returndampers stuck

Impellerloose

iDrive motorfault

iStarter motorfault

Wrongvoltage

Seizedbearing

iImpellerrotation blocked

Yes No Fan motor stoppedfuses blown

Fan motor running

No air flowNo

Are the fans running?Yes

Is there water in basin?

OK

Changes of process or modification to the productcan introduce new design parameters, which can affectthe tower. Overloading the tower in both a thermal andhydraulic sense may be acceptable as a temporary mea-sure, but 'temporary' is the critical word. Too long expo-sure to an increased temperature can affect plastic pack-ings, unless they have been designed to withstand such anincrease. (Remember to discuss future predicted problemswith the cooling tower designer before installation.)

Excessive water loads can lead to malfunction of thedistribution system. Higher loadings in one area can leadto pack collapse and rapid fall-off in performance.

In the majority of cases common sense, combined withbasic engineering principles, should be sufficient to ensuregood service from the tower on a continuous basis. Thereputable supplier will always be ready to help and advise.If the advice is sought in time, many of the problemsassociated with the changes mentioned need never arise.

21.9 Modifications - retro-fitsChanges in circumstances will frequently require mod-ifications to an existing cooling tower. Additional heatloads may be needed, and changes in the end processmay cause the return temperature to increase, necessitat-ing a new thermal load on the tower. Most frequently itis a requirement for an increase in hydraulics.

21.10 ConsultationAll too often, the supplier is not consulted at the planningstage, with the inevitable result of a tower failure, not onlyin performance but also in pack collapse, structural fail-ures and total shutdown. The original tower was suppliedagainst a design water and thermal load, and changes tothose parameters will affect the performance. It is possibleto change existing towers, and not only is it possible, itis well-established practice. The successful amendmentsare those where the tower designer has been advised (inadvance) of the proposed changes. He knows the limita-tions of his product and can advise on what can or cannotbe done.

There is one area where improvements can be achieved,namely the increase in thermal performance by changingto a more advanced design of packing. Care has to betaken to ensure that the new pack configuration is com-patible with the quality of water as well as its quantity.The water treatment conditions could change, and almostcertainly the distribution system will need amendments.The benefits to be obtained can be listed as follows:

1. Improvement in thermal load.2. Possible reduction in pumping head (e.g. the change

from a splash pack to a high-density film-type pack cansave power by installing the new pack at the bottomof the former splash area and lowering the pumpinlet). If the correct design is used it may be possibleto leave existing fans, thus incurring no additionalpower penalty. Even if fan changes are required, theeconomics have to be studied, but in most cases suchchanges will be beneficial.

Changes to a distribution system can be of assistance inminor improvements. Nozzle designs are under constantreview but it is in the context of pack changes thatamendments are important.

21.11 Environmental considerationsThe principal areas where cooling design is affected byenvironmental requirements are visual and audible (i.e.aesthetic (plume) and noise).

21.11.1 AestheticPlanning regulations can be rigid in their attitude tothe visual impact that a cooling tower may have onthe surrounding area. This is not, however, confinedto air-conditioning installations. Certain industrial areas,within the UK and elsewhere, limit the number of towers,their height and configuration in relation to the existingbackground situation. While this is understandable inenvironmental amenity terms, it leads to major designdifficulties on the part of the supplier.

Consultations with all interested parties is essential atthe planning stage. Solutions can normally be found,but the cooling tower designer must be included inthese discussions. Towers can always be designed tomeet the planning regulators and the demands of archi-tects. Low towers, tall narrow towers, circular towers,multi-faceted towers, towers built into the buildings, eventowers installed under ground, are all practical examplesof modern installations (Figures 21.6-21.8). Obviously,

Figure 21.6 Cooling plant, including towers, installed as a sepa-rate unit on a supermarket satisfies the architect, town plannersand the end-user

Figure 21.8 Cranes can be eliminated!

the design characteristics have to be reassessed. Hence theemphasis on cooling tower designers' involvement fromthe outset.

As cooling towers operate on the evaporative principle,at certain temperature conditions the discharged heatvapour will appear as a plume. The amount of the plumingcan be accurately assessed against the temperature condi-tions (both inside and outside the tower), the volume of

air and the velocity of the discharge. The extent to whichthis can be classified as a 'nuisance' depends entirely onthe location of the tower and its proximity to sensitiveareas (i.e. housing, office blocks, roads etc.). There havebeen cases where medium to large conventional wet tow-ers have been installed, and under certain atmosphericconditions, the plume can fall down to ground level, caus-ing poor visibility or even icing on neighbouring roads orthoroughfares. In such cases, Hybrid towers should beconsidered.

However, as with commercial considerations madewhen selecting the wet bulb temperature for the towerdesign, such a commercial evaluation is required whendeciding on the plume point for Hybrid towers. To supplya Hybrid tower that would not plume under all climaticconditions would be expensive, if not uneconomical.

Looking towards the smaller towers, there have beenquite a few smaller towers used in dry cleaning instal-lations, within sensitive areas, where the location of atower has been very restricted, not only because of theavailability of space, where inlet has been ducted intothe tower, and the outlet, has been combined with warmventilation extract fan discharge air, thus avoiding allpossibilities of moisture deposition and the associatedproblems. Necessity being the driving force of the deci-sion, sometime brought about by aesthetic considerations,especially where the location has been in the newer shop-ping precinct location.

One variation is the natural-draught tower, where geo-graphical location may cause the plume to affect ambi-ent conditions downwind, such as moisture depositionon roads (Figure 21.9) or, in one instance in Switzer-land, where the plume from a large natural-draught towerlocated in a narrow valley restricted sunlight on the farm-ing area downwind. (The predicted 7% restriction wasaccurate.) The drift can normally be confined withinmore than acceptable limits by the use of efficient drifteliminators.

The height of the discharge is important, and inair-conditioning projects it can be critical in relation tosurrounding buildings. Consideration has to be given to

Figure 21.7 Design requested by architect - low plume, no splash(note catchment tray), clear of office block

Figure 21.9 Congested site. Restrictions on air inlet, increasedpressure and air velocity results in drift deposition on road

the volume of discharge, the possibility of entrainmentof water-treatment chemicals in the drift (these can cause'etching' on glass windows), the siting of the tower inrelation to the fresh air inlets to the building and the visualimpact, in architectural and aesthetic terms.

Eliminator design can vary from the non-existent toan efficiency which can limit drift to 0.00005% of flow.Whilst such a level can be achieved, in certain cases,it would be rare that any manufacturer would offer aguarantee of anything better than 0.001% whilst achievinga more common level of around 0.0005%. Such drifteliminator levels would normally be offered for towersto be installed in a sensitive area or for sea water coolingtowers. A more typical (as specified by clients) guaranteelevel would be 0.005%. In other words, drift can bealmost undetectable, but the difference is obviously inthe economics involved.

Pressure groups in the environmentally aware politicalparties can make excessive demands. Often these canbe met by good public relations meetings, but the morereputable cooling tower suppliers can be invaluable indealing with such matters.

Plume abatement is possible, even to the point of'invisible plumes'. While this is not a problem in hotcountries, the temperate zones (for example, Europe andthe USA), will always have seasons when the plume isnormally visible. To change this situation to a non-plumeeffect is again possible but it is expensive. As an example,one installation in the centre of a North American city,where non-pluming throughout the year was a mandatoryclause in the planning permission, resulted in the towercost being increased by a factor of 3! (A not-insignificantamount in the overall cost of the project.) Plume controlcan be achieved at the expense of larger installations andpossible changes in the temperature levels, all of whichrequire prior consultation with the designer.

The other source of possible drift or precipitation fromcooling towers is caused by windage or blow-out fromthe air inlets, noticeably under strong or gusty conditions.This can occur in both mechanical- and natural-draughtcooling towers, but the effect is normally localized tothe immediate tower area. In the natural-draught coolingtower the extent of the problem varies according to thedesign of the packing.

In the case of pure counterflow packing, where theentire packing is positioned above the air inflet, the largeair inlet opening can give rize to discharge of waterby air entering the air inlet on the windward side, thenbeing sucked out in the vortex depression, which generallyoccurs at approximately 90° to the wind direction. Theresulting spray is carried as a fairly narrow band for adistance of perhaps 20 to 30 m downwind of the tower. Ina mixed-flow packing, where the cooling tower packingextends down into the air inlet, there is considerableresistance to the free passage of air through the air inlet.Therefore the depressed area of the vortex is reduced andthe resulting spray tendency is likely to be restricted to afine spray, literally being blown off the peripheral packlaths. Various techniques have been adopted to minimizethis effect, such as external radial baffles, internal bafflesand louvres. All of these will, of course, incur additional

cost and some of them may also increase the pressuredrop through the air inlet and thus affect the thermalperformance. Additionally, they can increase the overalldimensions of the installation.

Mechanical-draught cooling towers are normally sup-plied with either central baffles or inlet louvres. Thisdepends on the tower dimensions. On these towers thewind or spray blow-out is generally confined to relativelysmall single-cell units where an inlet may be provided onall four faces. In this case the major remedy is to pro-vide internal diagonal baffles to prevent crossflow of airthrough the air inlets.

On larger multi-celled mechanical-draught towers ofboth counterflow and crossflow variety, the air inlets areconfined to the two opposing faces and windage or driftloss is unlikely to occur, except under exceptionally highwind conditions. Here again, remedial work, dependingupon the location, can be applied but at additional cost(see Figure 21.10).

21.11.2 Noise

Perhaps the most common environmental requirementin modern cooling tower installations is that of noise.Cooling tower noise is generated by the fan equipmentand the falling water. In large mechanical- or natural-draught cooling towers the water noise is at a level whereit could exceed the noise generated by the fan equipment(particularly if steps have been taken to reduce fan noiseto a minimum). In the case of smaller cooling towers theprediction of noise intensity at a distance in excess ofbetween 30 and 50m can be taken as a hemisphericalradiation from a point source, i.e. a reduction in the levelof 6dB for every doubling of the distance. However,with the large multi-cell mechanical- and natural-draughtcooling towers the noise is radiating from a considerablylarger area, and therefore the sound pressure level fallsby only 3 dB for every doubling of the distance up to \d(where \d = pond diameter of the hyperbolic shell in anatural-draught tower or the tower length for the multi-cell mechanical-draught type). For distances greater than\d the sound pressure will then fall by 6dB for everyfurther doubling of the distance.

Fan noise is likely to be more obtrusive than the so-called 'white sound' emitted by falling water due tothe presence of discrete frequencies arising from blade-passing frequency, tooth frequency on the gearboxes bear-ing and rumble from the gearboxes and motors and otherelectrical noises. It can, of course, be minimized by cor-rect choice of fan. In general, the use of broad-cordmulti-bladed fans enables the fan to be operated at aminimum possible speed compatible with the duty per-formance. Reduction of bearing noise in gearboxes canbe eliminated, as far as possible, by careful design in themounting system, and motor noise can always be shieldedby acoustic enclosures. For extremely quiet operation onmechanical-draught towers recourse may have to be madeto the use of silencers or attenuators on the air inlets to thecooling tower to minimize the water noise, radiation andmechanical noise break-out, with further acoustic attenu-ators on the fan discharge.

Figure 21.10 Configurations of cooling tower air flows

Any acoustic treatment on the fan discharge is requiredto operate under potentially corrosive conditions, withwarm moist air passing through the attenuators. Precau-tions are needed to ensure that these are adequately treatedto prevent risk of condensation and that the structuralmedia are also protected against water pick-up and poten-tial damage.

In the case of natural-draught towers the total noisesource is very considerable and has to be assessed asa large-area source relating from the whole diameter,through the entire height of the air inlet. Falling waternoise in that situation can be as high as 85 dB. Thenoise level at 70m reduces to 66 dB and at 48Om it isapproximately 46-47dBA. On the largest natural-draughttowers the figures increase. For example, at the pond leveland at the side of the tower they can be in excess of91 dBA and can actually reach 55 dB at 50Om.

Sound is defined as any pressure variation that thehuman ear can detect. This variation can occur in air,water and other media. To determine noise, it is necessaryto assess the frequency of the variation which, in turn, canbe related to the speed. For most applications the speed ofsound is expressed at 340 m/s. Speed and frequency givethe wavelength, i.e. the physical distance in air from onewave to the next. For example, at 20Hz this gives 17mwhile at 20 KHz one wavelength is 1.7 cm.

For convenience, the usual measurement of sound isexpressed in decibels (dB), and ratings go from 'thresholdof hearing' to 'threshold of pain' (135 dB). Figure 21.11illustrates the common noise criteria, which can beexpressed in sound-pressure levels (SPL). The human earcan detect 1 dB but 6 dB represents a doubling of the SPL,although it would need a 10 dB increase to make it 'sound'twice as loud.

Forced-draughtcounterflow

Half an induced-draught crossflow

Standard induced-draught counterflow

Induced-draught counterflowincreased air entry

Single entry crossflow

Air entry on one side onlyAir entrythree sides

Air entryfour sides

Forced-draughtcounterflowback to back

Induced-draughtcounterflowincreased air entry

Forced-draughtcounterflow pair

Special induced-draught counterflow

Air entry two adjacent sidesAir entry two opposite sides

Double-entry induced-draught crossflow

Figure 21.11 Threshold ratings for sound

In assessing the noise emanating from a cooling tower itis necessary to measure the main points of emission - thefan, the motor, the gearbox and the falling water. As noisebounces and can be absorbed by certain materials, it isusual, where noise restrictions apply, to map the area andcalculate the SPL at a large number of points, taking intoconsideration interferences, bounce and absorption. Thenumber of points measured is reflected in the accuracy ofthe resultant topograph.

Remember also to take background noise into yourcalculation. Too frequently, specifications are made whichignore this, with the result that equipment is applied to amore rigid design than is absolutely necessary.

The siting, as well as the selection of type of tower, canbe critical. Rotating the tower, shielding the motor, useof baffles can all help in meeting environmental noiserequirements. If in doubt, consult your cooling towerdesigner.

21.12 Problem areas

21.12.1 Installation

Cooling towers have been called the Cinderella of theplant scene - usually unnoticed (if not even unseen),

forgotten and sadly ignored. While cooling tower design-ers may have other ideas, they generally recognize thisas being true in many cases. Designers therefore try toachieve the impossible, i.e. to build a piece of mechanicalequipment that can be left alone and perform its functionwithout fuss and attention.

While designers can claim some degree of success,there are many occasions when their products aremisinstalled.

21.12.1.1 Don'ts

Mix products - placing a forced-draught tower beside aninduced-draught one causes problems for both designs(Figure 21.12).Place access panels incorrectly - the access panel is for theuser's benefit. Ensure that it is accessible (Figure 21.13).Starve the air inlets - insufficient air results in poorperformance (Figure 21.14).Ignore the bleed - inadequate bleed means concentrationof salts, change of pH and pack fouling.Forget about make-up - water starvation means poorperformance, vortexing, pump and motor failure.Fail to check on water treatment - inadequate treatmentand haphazard slug dosing can lead to poor performance,damage to associated equipment and failure to meetdischarge requirements.Forget to install safety cut-outs, for overload and ambientchanges - ice damage can be disastrous (Figure 21.15).Allow corrosion to develop - metal failure can be costlyand, at times, dangerous (Figure 21.16).

Figure 21.12 Mixing forced draught with induced. The overloadedforced-draught tower with excess plume results in elevated wetbulb at air inlets on new tower. The problem is resolved byremoving the forced draught and adding one more cell to theinduced draught

Threshold of hearing

Birdsong

Home

Office

Vehicular traffic

Pneumatic drill

Aircraft

Threshold of pain

Figure 21.14 An attempt to conceal tower and reduce noise,resulting in starvation of air and failure in performance

Do not locate the tower, where access can be made bymembers of the public as many a tower has had a bottleof detergent emptied into the water with disastrous effect.

27.72.7.2 Do's

Ensure maintenance checks are carried out.Check on power consumption, water usage, water costs.

Figure 21.16 The effects of corrosion. Structural failure is visible(holes in fan casing)

Carry out monthly inspection - inside and out, wherepossible.Check water analysis; the frequency depends on individ-ual cases but it should be no less than quarterly.

Figure 21.15 Ice on natural-draught tower (no de-icing ring fitted)

Figure 21.13 Cyclic heat load using small tower and large waterstorage. This is a good idea and sound engineering, but the accesspanel is on the wrong side (30ft drop to ground level)

Check mechanical equipment (i.e. fans, motors, drives).(Remember to check belt tensions where applicable.)Check for vibration, both mechanically and structurally.Ensure that access panels are used and replaced correctly.Make certain that repairs, when necessary, are carriedout efficiently and quickly.If it in doubt, call the cooling tower designer.

21.13 Summary

While the majority of cooling tower installations workefficiently, the normal requirements of maintenance andgood efficiency practice have still to be applied. Thismay not always be the case. Time for maintenance islimited, and plant engineers have other pressing problemsor, as is well known, forget about the towers! With theincreasing economic and environmental pressures on theuse of water, this situation must change.

It may be appropriate to quote the old engineering termof 'KISS' (Keep It Simple, Stupid!) and, recognizing thatthe cooling tower designer does his utmost to comply, itis the responsibility of the operator to 'co-operate'!

Regular checks and an efficient logging system willensure that the cooling tower, correctly planned, effi-ciently installed and adequately maintained, will givevaluable service for many years. The economic returnsjustify a little more thought and attention than has beengiven to the subject in the past.

Appendix 21.1 Theoretical calculations

LdT = KadV(hL-hG)

where K = the coefficient of heat transfer for thepacking in question,

a = the effective transfer surface area perunit pack volume,

V = depth of packing,/ZL = enthalpy of boundary air layer in contact

with and at the same temperature as thewater, and

he = enthalpy of bulk air passing through thepacking.

Integrating this for the full depth of packing, the expres-sion becomes:

L - AT = KaV -Ahm

where Ahm is the mean enthalpy difference, otherwiseknown as the Mean Driving Force (MDF) (see Appendix21.2).

This can be rearranged as:

KaV _ ATL ~ MDF

which is the form in which it usually appears. KaVIL iscommonly called the 'tower characteristic'.

Now let us refer to the right-hand side of the aboveexpression. The mean driving force varies with the spec-ified design temperatures and also the ratio of water/air

loading (LIG). If we take a low air flow, the air soonrises in temperature and tends to reach equilibriumconditions with the boundary layer. Thus the driving forceis reduced. On the other hand, excess air is unneces-sary. Therefore we must adjust the air flow that supplyjust meets demand. A plot of LIG versus ATIMDFis shown in Figure 21.17. This is known as a demandcurve.

The left-hand side of the above expression is a measureof the quality and quantity of the packing being used, andhas been shown empirically to obey the law (KaVIL) =c(LIG)~n for counterflow applications only.

Cooling in the crossflow mode requires an incremen-tal 'trial and error' technique, best suited to computeranalysis. The tower characteristic KaVIL can then beplotted against varying LIG ratios, and this gives a mea-sure of the ability of the packing to effect the transfer(Figure 21.18).

We have already equated KaVIL with ATIMDF,therefore we can superimpose the 'supply' curve over the'demand', the intersect being the optimum LIG ratio forthe packing being considered for the duty (Figure 21.19).

It is interesting now to examine the effect of usinggreater or lesser depths of packing, and to consider theirsuitability for duties of different degrees of difficulty. InFigure 21.20 we can see the effect of using three differentpack depths on a moderately easy duty. By changing from

Hard

Easy

Figure 21.17

Figure 21.18

AT/

MD

FKa

V/L

UG

UG

Figure 21.20

pack depth (1) to pack depth (2) we are able to use amuch higher LIG ratio (which, in turn, means less airand/or a smaller tower). The increment from depth (2) todepth (3) gives a less significant increase in L/G, andtherefore suggests that the optimum depth has perhapsbeen exceeded.

Now let us examine the same pack depths but appliedto a more difficult duty (Figure 21.21). The increase inLIG is almost constant from (1) to (2) and from (2) to

(3), showing that the optimum depth has not been passed,and may not yet have been reached.

Thus there is an optimum depth of packing for eachindividual duty and, in practice, it is usually found thatan intersection near the knuckle on the demand curveproduces the most economic selection.

Appendix 21.2 Evaluation of the MDF

Several methods for the evaluation of the MDF havebeen put forward, notably that processed by Tchebycheff,which gives a high degree of accuracy in the case of largecooling ranges. In the form in which it is most commonlyused, it reads:

MDF = 4/(1/AhI + l/Ah2 + l/Ah3 + I/AM)

where hi = value of hL - hG at T2 + 0.1 AT,h2 = value of hL + hG at T2 + 0.4A Thi = value of hL + hG at T1 + 0.4AJh4 = value of hL + /IG at TI + 0.1 AT

Graphically, this can be represented as in Figure 21.22.The expression for determining KaV/L is

(AI/4)(l/Ahl + l/Ah2 + l/Ah3 + l/Ah4).

Appendix 21.3 Technical requirements

LocationMeteorological dataWind roseWater flow rateTemperature to tower (T\)Temperature from tower (T2)Cooling rangeApproachDesign wet bulbDesign dry bulbWater analysis - circulating

- make-upCycles of concentrationBlowdown/purge rate

Figure 21.21 Figure 21.22

Temperature

RangeApproach

Figure 21.19

KaV

/L

Ent

halp

y

KaV

ILK

aV/L

UG

UG

L/G

Power cost analysisDrift loss requirementLocal authority requirementsDischarge qualitiesStructural specificationsPack specificationsMechanical specifications (if applicable)Noise specificationImpedance by adjoining structures (if applicable)

AcknowledgementsThe authors wishes to thank the following for theirconsiderable input: Mr Richard Clark and Mr AnthonyKunesch (FCT Ltd), The Cooling Water Association (nowThe Industrial Water Society), Mr John Hill (Director ofBEWA) and the many understanding people who gavepermission for publication of the photographs.