Cooling Tower Tech Report H-004

7/28/2019 Cooling Tower Tech Report H-004

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External Infuences on

Cooling Tower Perormance

SUBJECT:

Number: H-004 Date: January 1983

Foreword

Owners and their system designers, quite naturally, want a specied cooling tower to

perorm as expected. Otherwise, the system served by the tower will suer. Thereore, they

are meticulous in determining the imposed heat load; the preerred water fow rate; the

required cold water temperature; and a design wet-bulb temperature which they anticipate

will seldom be exceeded.

The manuacturer, equally concerned, wants ull-rated perormance rom his cooling

tower in order to maintain his reputationand that o the industry. Given the established

design parameters with which to work, he too is meticulous in the selection o a propercooling tower, type and size; the calculation o a required air fow rate; and the application

o sucient an horsepower to assure delivery o that quantity o air through the tower.

Why, then, do a signicant number o cooling towers appear to be perorming inadequately

in spite o this dedication by both the user and the manuacturer? In many cases, the problem

can be traced to environmental actors which aect the quantity and/or thermal quality

o the air entering the tower. This report will describe situations contributing to a poor air

supply, and will suggest the preplanning necessary to ameliorate those situations.

A cooling tower reacts much like a human being to its air supply. I it is caused to reinhale

its own exhaust, it begins to smother. I that air supply is restricted, it begins to choke.

However, a tower is unlike a human being in two very important ways: it cannot walk away

rom a jeopardizing situation; and it has a propensity to contribute to its own demise.


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Recirculation

Mother Nature, in her innite wisdom, made man'sbreathing pattern such that exhaust air is given a momentto disperse beore a resh supply is inhaled. Conversely,a cooling tower's respiration is continuous. It dischargeswarm, saturated air into the very atmosphere rom which itattempts to draw cool, relatively dry air. Occasionally, part othe discharge air will recirculate into the entering air stream,and the system begins to experience problems associatedwith elevated water temperatures.

The net result o recirculation is an unexpected rise inwet-bulb temperature o the air entering the cooling tower,and a corresponding increase in the temperature o thewater leaving the tower. Depending upon the severity othe recirculation, cold water temperatures can be caused toincrease one to ve degrees, or more. At best, this causesthe system to operate above design parameters. At worst,system tolerance may be exceeded, causing shutdown tobecome necessary. Accordingly, it behooves cooling towerusers to be aware o this phenomenon, and to know themeasures to take to minimize its impact.

All cooling towers oer the potential or recirculation, the

extent o which depends primarily upon the entering and exitingair velocitiesand their relationship to each other. Higherentering velocities increase the potential or recirculation,while higher exit velocities decrease its opportunity. Thisis because higher entering velocities result in an increasedlow-pressure zone at the air intake, into which exhaust vaporcan be induced; whereas higher exiting velocities tend topropel the discharge vapors beyond the infuence o theintake areas low-pressure zone.

Cooling tower types representing opposite ends o thescale or recirculation potential are depicted in Figures 1 and2. In the induced drat tower (Fig. 1), exhaust air is propelledupward by the an at an imparted velocity o approximately2000 t/min, while resh air is drawn into the air intakes at avelocity less than 700 t/min. This velocity relationship givesreasonable assurance that the tower will be subjected to verylittle sel-imposed recirculation. (As will be seen, however,external infuences can adversely aect even the best odesign intentions.)

The velocity relationships in the orced drat tower (Fig2) are essentially the reverse o those encountered in theinduced drat tower. Air enters the an region at velocitiesusually exceeding 2000 t/min, and exits the top o thetower at velocities normally less than 700 t/min. This highentrance velocity creates a signicant low-pressure zone atthe intake, in which a portion o the exhaust air is likely tobe captured.

AIR OUT

AIR IN AIR IN

WATER IN WATER IN

WATER OUT

Figure 1Cut-away of an Induced Draft Tower

Air Restriction

At a given heat load, a xed water fow rate, and a particularwet-bulb temperature, the cold water temperature producedby a cooling tower is totally dependent upon the quantity oentering air. Decrease the amount o air fow and the coldwater temperature will rise. Because o the importance oair fow, manuacturers are painstaking in their calculation othe amount required to meet specied perormance, and inthe application o a an/motor combination which will movethat quantity o air against the static pressure encounteredwithin the tower.

Static pressure is the measure o a systems resistanceto a given fow o air. It results rom restrictions in the system(which increase the airs velocity), and rom changes in thedirection o air fow. Within the cooling tower are manyrestrictions to air fow (ll, drit eliminators, piping, structure,etc.), and a multitude o directional changes. These combineto produce a total net static pressure which determines thean horsepower applied by the manuacturer. I somethingshould happen to cause an increase in the static pressure,air fow through the tower will decreaseand a higher coldwater temperature will result.

AIR OUT

WATER IN

WATER

OUT

AIR

IN

Figure 2Cut-away of a Forced Draft Tower

(extended fan-section style)


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External Infuences

Having dened the recirculation-which smothers a coolingtower, and the imposition o static pressure- which chokesit, we can now look at two primary external infuences whichaect these conditions. Namely, wind and air obstructions.

Wind

Wind, depending upon its speed and direction, tends tomagniy a cooling tower's potential to recirculate. Not onlydoes it bend the exhaust air stream in the direction o windfow, it also creates a low-pressure zone at the lee (downwind)side into which a portion o the saturated plume may bedrawn. I the downwind side o the tower happens to be anair inlet ace, then that portion o the plume can be expectedto thermally contaminate the entering air.

The degree to which wind can aect plume rise dependsupon the ratio o a cooling towers discharge (jet) velocity(Vj) compared to the velocity o the ambient wind (Va). Figure3, or example, depicts the behavior o a plume as it leavesthe an cylinder o an induced drat tower and encounters awind o equal velocityapproximately 20 mph. Not shown (or

reasons o scale) is the depression o some aected portiono this plume into the tower's lee side low-pressure zone.

Fortunately, since most induced drat towers (Fig. 1) haveseparate air inlets on opposite aces, recirculation will normallyaect but one inlet at time, and its net eect tends to be

minimized. Nevertheless, it is sucient to have warrantedstudies, resulting in the recirculation-potential curve shownin Figure 4. This curve plots percent o net recirculation asa unction o the ratio o discharge velocity (Vj) to ambientwind velocity (Va). It was developed or towers o rectangularconguration (Figs. 1 and 2), but having air inlet aces ontwo opposite sides o the tower. Thereore, the indicated

recirculation ratio doubles its impact on towers with butone inlet ace (Fig. 2). Bear in mind that the curve does notpredict a constant amount o recirculation, but identies thenominal amount that would be anticipated under conditionso adverse wind direction.

Recirculation ratios are applied in the ollowing ormulato determine the anticipated entering wet-bulb or any givenoperating condition:

Where ha = Enthalpy o ambient air. (Btu/lb)

he = Enthalpy o air entering the tower (net).

(Btu/lb)

L = Mass water rate. (Lb/min) (=gpm x 8.33)

G = Mass air rate. (lb/min)(= cm/specic

volume)

R = Range= Dierence between hot & cold

water temperatures. (F)n = Number o principal tower aces

equipped with air inlets. (= 1 or single-fow towers, and 2 or double-fowtowers)

(Entering wet-bulb temperatures are worked out in theappendix or both induced drat and orced drat towersoperating at given conditions.)

2 x % Recirc. x L x R

(100 % Recirc.) x G x nhe = + ha

WIND

VA

VA

VJ

Figure 3Effect of Wind on Plume Behavior

VELOCITY RATIO (VJ/VA)

RECIRCULATIONRATIO

(%OFTOTALCFM)

RECTANGULARCONFIGURATION

Figure 4Potential for Recirculation in Rectangular Towers


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The process o recirculation is depicted in Figure 5 or alow velocity discharge, single air entry tower operating in awind velocity similar to that utilized or Figure 3. At a velocityratio less than 0.5, its recirculation ratio (double that shownon the Figure 4 curve) would be approximately 10 percent.On the other hand, the induced drat tower would have avelocity ratio exceeding 1.0, resulting in a recirculation ratioo approximately 4 percent. These values would be typical oa tower situated in an open environment, but poorly oriented

with respect to the prevailing wind.

Although air restrictions can take the orm o walls,structures, even shrubbery, located in close proximity to theair intake o a cooling tower, enclosures utilized to hide thetower rom public view are o primary concern and will bethe subject o the remainder o this paper. The reader shouldunderstand, however, that any "rules o thumb" developedor clearances within an enclosure apply equally to randomadjacent structures or installation pockets.

Enclosures oering the greatest potential impact on a

cooling tower's thermal perormance are those in which thetower is surrounded on all our sides by solid walls, suchthat the total air supply to the tower enters the enclosure in adownward direction. Figures 6 and 7 depict such enclosuresas they might be applied to induced drat and orced drattowers respectively. Recommended clearance dimensionsshown between walls and the tower's air inlet(s) are thoserequired or towers o one or two cells only. (For purposes othis paper, the number o cells is considered to be equal tothe number o an motors with which the tower is equipped.)For towers o more than two cells, the dimension should beincreased by 15 percent or each additional cell. Alternativeenclosures will be discussed in relationship to these solidenclosures.

Interference

Localized heat sources situated upwind o a cooling towercan interere with its expected thermal perormance. Theseintererences may be exhausts rom various building systems

or processes. Many times, they consist o thermal contributionrom the efuent o other cooling towers in the vicinity.

Intererence, like recirculation, tends to elevate a coolingtowers entering wet-bulb temperature, but its eect tends tobe somewhat more pervasive. This is because intererencecan exist without recirculation, and it aects all o the air inletso a cooling tower equally. Despite the most sophisticateddesign eorts to reduce recirculation, many cooling towerswill experience higher-than-expected wetbulb temperaturespurely due to intererence. Fortunately, the eect ointererence on cold water temperature can oten be nulliedby preplanning, as will be seen in the Recommendationssection o this paper.

Walls or Enclosures

In the broad sense, air restrictions are a orm o intererencesince they interere with the ree fow o air into the coolingtower. To the degree that they redene the source o incomingairor increase its velocityair restrictions also tend tointensiy recirculation. Which o the two eects is o greatestconcern depends upon the type o tower, as will be seen.

WIND

VAVA

VJ

Figure 5Recirculation

A A

A

Figure 6Induced Draft Tower Enclosed by Four Solid Walls


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In the induced drat enclosure (Fig 6), the minimum distancerequired rom an air-opposing wall is indicated to be equal tothe height o the tower's air inlet (louvered) ace. In the orceddrat enclosure, however, the required distance is indicatedto be equal to twice the tower's width. The reasons or theserecommended distances are as ollows:

Although induced drat towers tend to be less susceptibleto recirculation, they also develop less total static pressurethan do orced drat towers and are, thereore, more sensitive

to the imposition o external air losses. This means thatenclosures for induced draft towers must be governedby their potential to add system air losses. In the Figure 6enclosure, with clearances as shown, the maximum possibledownward air velocity will not exceed the lowest velocitycomponent o the tower, the air inlet ace. Thereore, sincepressure is a direct unction o velocity, the added air losscan be considered negligible. Furthermore, although thedownward movement o air imposed by the enclosure haseectively relocated the tower's intakes in closer proximityto the exhaust, the 3/1 ratio between discharge and inletvelocities has been maintained, and only slightly greaterrecirculation potential results.

Conversely, orced drat towers are characterized byrelatively high internal static pressures, so the imposition o asmall amount o external air loss is o little importance. Werethis the primary concern, the clearance distance indicated inFigure 7 would need to be only one tower width. However,because o a orced drat tower's propensity to recirculate,enclosures for forced draft towers must be governed bytheir potential to contribute to recirculation. The clearanceindicated in Figure 7 assures an upward discharge velocity at

least twice that o the downward velocity into the enclosure.Although a greater out/in velocity dierential is desirable inorder to diminish the possibility o sel-imposed recirculation,urther increasing the overall enclosure area usually becomesimpracticaland does nothing to improve the dischargeplume's velocity relationship to that o the ambient wind. Thenal step, thereore (not shown in Figure 7), should be to equipthe orced drat tower's discharge area with a constrictingcowl designed to increase the velocity o the air leaving thetower. Such cowls are available rom the manuacturer, andusually require an increase in an horsepower to overcomethe increased static loss. They also necessitate an increasein enclosure height to preclude line-o-site exposure.

Although the obvious purpose o an enclosure is to hide

the tower, the height o the enclosure must never exceedthe installed height o the tower. That is to say, the elevationo the enclosure walls must be equal toor less thantheelevation o the tower's point o air discharge. Depressinga tower within an enclosure will increase recirculationtremendously.

Enclosures can also be provided with openings in the walls(either louvered or totally open) to permit some horizontalmovement o air into the tower area. In many cases, theseopenings are designed to compensate or an otherwiseinadequate wall distance rom the tower's air intakes.However, the trade-off should never be considered adirect one. In any opening through which air fows, wall

eects and eddy currents always prevent some portion othe apparent net ree open area rom being an eectivefow area. This eective fow area, o course, varies with thedesign and conguration o the opening. Openings ree oobstructions may lose only 5 to 10 percent o the apparentnet ree area to these eects, whereas walls o peroratedbuilding blocksor busy with louversmay lose as much as50 percent. These losses must be compensated or eitherby limiting wall distance encroachment on the tower, or byappropriately increasing the wall opening's net ree area.

Figures 8 and 9 show enclosures in which openings havebeen provided where possible to ventilate the enclosure.In Figure 8, the ar end o the enclosure provides ampleclearance or that air inlet ace o the tower. Although the

towers near air inlet ace would appear to be obstructed bya set-back penthouse wall, the elevation o the tower withinthe enclosure aords generous clearance or most o theair inlet acewith the slotted enclosure wall compensatingor the remainder.

2 x W

W

Figure 7Forced Draft Tower (indented-fan style)

Enclosed by Four Solid Walls


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Figure 8

Figure 9


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The Figure 9 enclosure is somewhat more restrictive, withcompensating air slots having been provided in a side wall.Adequately sized, ventilating slots so located may add but anegligible amount o system static pressure. However, in anadverse wind, they can become an entryway or recirculation.I the prevailing summer wind were rom the upper rightblowing toward the lower let o the photo, or example,some o the tower's efuent air might have a tendency tobe induced into the enclosures lee-side low pressure zone

beyond the enclosure, rom which its escape route wouldbe through the slots and into the tower inlets.

Recommendations

1. Recommended design wet-bulb temperatures orselected geographical areas are tabulated in variouspublications, principal among which is Engineering WeatherData published by the U.S. Government Printing Oce.In many cases, however, these tabulations refect readingstaken at military installations and Class A airports somewhatremoved rom the heat island eect o an urban center.Neither, o course, do they include heat sources within theimmediate vicinity o a planned cooling tower site.

Where possible, it is good practice to take wet-bulbtemperature readings at the proposed tower site and comparethem to simultaneous readings taken at the nearest sourceo weather data. The dierence between these readings willprovide justication or adjustment o an apparent designwet-bulb temperature obtained rom the most trustworthypublication. Where such measurements would prove awkward

or impossible, it would be recommended that the apparentdesign wet-bulb temperature be increased by at least 1Fto adjust or an urban area.

2. Unless limited tower siting is available, towers should besituated such that the lee side o the tower (when operatingin the prevailing summer wind o highest coincident wet-bulb) is not an air inlet ace. This assures that whateverrecirculation occurs with seasonal wind shits will probablyoccur at some reduced wet-bulb temperature, at which time

an elevation in expected cold water temperature might notbe considered critical.

3. Where critical recirculation is likely to be unavoidable (asin an enclosure) the design wet-bulb temperature determinedin Recommendation No. I should be urther increased by 1For an induced drat tower, or 2F in the case o a orceddrat tower. (See appendix)

4. At the prospect o an unusually tight enclosure oradjacent restrictions, conducive to excessive recirculationor static losses, the cooling tower manuacturer should beconsulted with regard to a possible change o tower size-oran increase in its operating horsepower.

Acknowledgment

With minor changes, this paper was published in theJanuary 1983 issue o ASHRAE Journal.

Appendix

Conditions: Towers selected to cool 2100 gpm rom 95F to 85F (700 tons) at an ambient air wet-bulb temperatureo 78F. Select Marley Model 8818 NC and BAC Model VXT-700. Towers to operate in a 10 mph wind (Va=880 t/min)situated such that an air inlet ace is downwind, or towers are located in an enclosure.

Problem: Determine anticipated wet-bulb temperature o air entering each tower.

Solution: Induced Forced

Item (and source) Draft Draft

cm at Fan (rom catalog data) ..........................................................................................................................150,889 145,400

cm at Exit (sp. vol. gains about 2% thru tower) ............................................................................................150,889 148,308

G (assume exit sp. vol at 14.5 cu. t./lb) .......................................................................................................... 10,406 10,228

Discharge area (sq. t.) (rom catalog data)

= an area o I.D. tower = x 52 ..................................................................................................................78.5

= top plan area o F.D. tower = 24 x 9.5 ......................................................................................................................228

Vj (= exit cm/discharge area) ................................................................................................................................1,922 650

Vj/Va ............................................................................................................................................................................2.18 0.74

Percent recirculation (rom Fig. 4) ........................................................................................................................3.15 4.6ha (rom psychrometric chart or table) ................................................................................................................41.58 41.58

L (=2100 gpm x 8.33) ...........................................................................................................................................17,500 17,500

R(= 95 - 85) .................................................................................................................................................................10 10

n (= number o opposing air inlets) ...........................................................................................................................2 1

he (rom ormula on Page 3) ...................................................................................................................................42.13 43.23

Entering wet-bulb temperature (F) (rom psych. table) ..................................................................................78.52 79.51

Recommendation:

Increase design wet-bulb temperature 1F or induced drat towers, and 2F or orced drat towers.


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Cooling Tower Tech Report H-004

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