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Foreword

A properly designed cooling tower promotes the maximum possible contact between air and waterand does so for the

maximum possible time period. This effort on the part of the designer results in an efciency which, although greatly appreciated

in the summertime, has the capability to produce performance-degrading ice formations during winter operation. Therefore,

means by which the cooling tower's efciency can either be controlled, or can be made to work toward the management of

ice formations, must be incorporated into its design, and must be properly utilized by the operator.

In addition to describing the basic concept of ice control, this paper will describe the potential for ice formations in various

types of towersand will make specic recommendations for the reader's consideration.

Types Of Ice

As on any outside structure, ice can form on a coolingtower in the wintertime purely by natural effect. In addition,being both a water cooling and an air moving device, acooling tower can promote the formation of ice by its veryoperation.

In either case, whether caused by nature or by thetower itself, the owners concern for an ice formation ona cooling tower should be a reection of both its locationand its amount. Ice on exposed working platforms can bea personnel hazard, and should be corrected manually.Light random ice on the louvers, structure, and the leadingedges of ll, is usually of minor concern. Ice allowed to formon fans and other mechanical equipmentnot to mentionthe shrouds and control devices associated with thatmechanical equipmentcan lead to catastrophe.

Generally speaking, acceptable ice is of relatively thincross-section which may have formed on the louvers or airintake structure of an induced draft tower. Figure 1 shows

what might be considered an acceptable amount of icehaving formed inside the air intake structure of a counterowcooling tower, and Figure 2 indicates a relatively lightcurtain of ice on the louvers of a crossow tower. Sincethis amount of ice would normally have been anticipated ina cooling towers design loading, it is customarily of littlestructural concern and, in some cases, its retardation ofair ow through the tower achieves a result similar to theairside control procedures about to be discussed. However,although this ice may still be considered acceptable, it hasproceeded to a point where measures for its limitation orremoval should be undertaken.

If allowed to grow unchecked, ice can achieve massivecross-section, encroaching upon the ll (Fig. 3) or totallyblocking air ow (Fig. 4). Its weight alone can overloadaffected members and, when ice of such mass dislodges,it is obviously capable of doing signicant damage.

Operating Cooling Towers

In Freezing Weather

SUBJECT:

Number: H-003C Date: April 2012

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Figure 1Acceptable Counterow Ice

Potential For Ice

Although the methods of ice control vary somewhat

with type of tower, and the type of ll with which it isequippedas well as the water distribution system andmechanical equipment arrangementsthey are all basedupon the following points of logic:

1. The potential for ice varies inversely with outsideair temperatures. The lower temperatures become belowfreezing (32F wet bulb, which is often below dry bulb) thegreater the probability of ice.

2. Within design limits, the potential for ice variesinversel y with the amount of water owing over the ll. Areduced water ow rate increases the probability of ice.

3. The potential for ice varies directly with the quantityof air owing through the tower. Reducing the air owretards the formation of ice.

4. Where air ow is uncontrolled (as in the case ofnatural draft towers), the potential for ice formation variesinversely with the heat load imposed on the tower. In thatcase, a reduced heat load will increase the probability ofice.

5. Where air ow is controlled to maintain a speciccold water temperature, the potential for ice varies directl ywith head load, and inversel y with the selected cold watertemperature. Increasing the heat load, or lowering therequired cold water temperature, will increase the probabilityof ice. (See airside control following).

Figure 2Acceptable Crossow Ice

Figure 3Unacceptable Counterow Ice Figure 4Unacceptable Crossow Ice

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All mechanical draft towers permit some degree of airow manipulation for controlling ice, the extent of whichdepends primarily upon the number of cooling tower cells,and the speed-change characteristics of the motors. Largertowers designed to be operated in cold climates usually alsoinclude means by which to control placement of water overthe ll. In mechanical draft towers, airside and watersidecontrol arrangements can be mutually supportive. However,natural draft towers offer no reasonable opportunity forairside control and, for that reason, the methods will bediscussed separately.

Airside Control

Manipulation of the air ow (see Technical ReportH-001-A ) is an invaluable tool, not only in the retardationof ice formation, but in the reduction or elimination of icealready formed. In addition to bringing less cold air intocontact with the circulating water, reducing the enteringair ow velocity alters the path of the falling water, allowingit to impinge uponand meltice previously formed byrandom droplets which wind gusts or normal splashingmay have caused to escape the protection of the relativelywarm mainstream of water. This aspect will be claried inthe ll behavior section of this paper.

There are three choices available to the designer forcontrolling airow thru the tower. Single-speed fans affordthe least opportunity for air ow variation, and towersso equipped require maximum vigilance on the part ofthe user to determine the proper cyclic operation of thefans which will result in best ice control. Two-speed fan

motors offer greater operating exibility and should beconsidered the minimum level of control for coolingtowers used in cold climates . Fans may be individuallycycled back and forth between full speed and half speedas required to achieve balance between cooling effectand ice control, limited only by the maximum allowablemotor insulation temperature which an abnormal numberof speed changes per hour may cause to be exceeded.Since cooling tower capacity varies directly with the fanspeed, while fan hp varies with the cube of the speed,operating all fans at half speed provides the same levelof cooling as operation of half the fans at full speed, butuses only an eighth of the energy. On towers having twoor more fans evacuating a common plenum (such as theround towers depicted in Figure 5), those fans should bebrought to the off position in unison to prevent a down draftof cold moisture-laden air from icing up the mechanicalequipment of an inoperative fan. On multicell towers (in-lineconguration) equipped with a separate plenum for eachfan, individual fans may be cycled as necessary to controlice. However, it must be understood that cycling the fanon a particular cell accomplishes nothing with respect todeicing of adjacent cells. Individual cell ice control mustbe accomplished independently . This is because of thetemperature gradients about to be discussed.

Variable Frequency Drives (VFDs) offer the greatest levelof exibility, since they permit all fans to operate at the samespeed for minimum energy consumption. In addition, allcells will then produce the same cold water temperature,eliminating the temperature gradient between cells.

Figure 5Multifan common plenum cooling towers

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Temperature Gradients

Understanding how to anticipate and control ice requiressome knowledge of the water temperature gradientsthat occur in an operating cooling tower. Without suchknowledge, operators often assume that controls whichwill automatically cycle fans to maintain a leaving cold

water temperature well above freezing are sufcientinsurance against the formation of ice. Occasionally, theyare bewildered to nd ice beginning to form even beforethe cold water basin temperature has depressed to thatpresumably safe level.

The reason, of course, is the aforementioned temperaturegradients that occur transversely in all towers, andlongitudinally in multicell towers where fans are cycledprogressively. Figure 6 indicates the typical transversetemperature gradients in a bank of crossow ll. In thisparticular case, water is entering the tower at 64.5F andleaving at 44.5F; temperatures which would seem toindicate to an operator that a 12.5F safe zone (44.5-32) exists between his operating point and freezing.

Obviously, such is not the case. As can be seen, the netoutlet temperature of 44.5 results from a mixture of watertemperatures varying from about 53F at the inboard edgeof the ll to about 33F at the outboard edge. Consequently,the real margin of safety is only about 1F in this case.

Readers must not assume from this that 44.5F coldwater temperature is the magic point of control for alloperating situations. Water temperatures at the coldestpoint of the ll are very sensitive to therange (difference

between entering hot and leaving cold water temperaturesthrough which the tower is cooling).At a given cold watertemperature control point , reduced ranges (i.e. reductionsin heat load at a constant water ow rate) will cause thewater temperature at the coldest point of the ll to rise.Conversely, increased ranges (i.e. reductions in water owat a xed heat load) will cause water temperature at thecoldest point of the ll to depress.

For example, if the tower in which the Figure 6 ll isinstalled were operating at a 10F range (cooling the waterfrom 54.5F to 44.5F), the entering wet-bulb temperaturewould be 29F and the water temperature at the coldest pointof the ll would be about 38.5F. As wet-bulb temperaturefurther reduces, measures would be taken to diminish airow through the ll (by fan manipulation) and the coldestwater in the ll would reduce only negligibly below that level.

There is also a longitudinal temperature gradient(actually, stepsrather than a gradientas individual fans aremanipulated) in a multicell tower. This is because cells withfans operating at full speed contribute much more to the

towers overall cooling effect than do cells with fans eitheroperating at reduced speedor off. For example, if waterwere entering the tower in Figure 7 at 80F and leavingat a net 60F (one fan running-one off), the actual watertemperature produced by cell #1 would be 50F and waterat the coldest point of its ll would be at or near freezing.

Figures 8 and 9 indicate net performanceand thermalgradientsof a two cell tower cooling through a 20Frange, equipped with single-speed and two-speed fansrespectively. These curves are drawn on the premise thatthe operator will manipulate fans to prevent the net cold

water temperature from going below 60F, and the winterwet bulb temperature can routinely depress to 0F. Thesolid line indicates the net water temperature sensed by theoperators thermometers or control devices; the dashed lineindicates the net water temperature from the cell operatingat the greatest fan speed; and the dotted line indicates thecoldest water temperature in the ll.

Looking at Figure 8, one can see that the situationdepicted in Figure 7 would occur at about a -2F wet-bulbtemperature for that particular tower. Comparing Figure 8against Figure 9, one can also see the tremendous advantageafforded by two-speed motors; both in operating exibility,and in the reduction of ll temperature gradients.

64.5F Entering Water Temperature

Water Temp.6 4 F

6 0 F

5 5 F

5 0 F

4 5 F

4 0 F

3 4 F

E n t e r

i n g

A i r a t

1 0 F W e t - B u l

b T e m p e r a

t u r e

44.5F NetOutlet WaterTemperature

Figure 6Fill Transverse Temperature Gradients

Water Entering at 80F

Cell #1Fan at Full Speed

Net 50FWater Temp.

Net 70FWater Temp.

60F Water

Cell #2Fan Off

Figure 7Longitudinal Temperature Gradient

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Top Curve = Net Cold Water Temperature From Tower

Wet-Bulb Temperature F

O n e F a n

R u n n i n g

a n d O n e

O f f

B o t h F

a n s R u

n n i n g

Net Water Temperature FromCell with Fan Operating

Water Temperature atColdest Point in Fill

C o l

d W a t e r

T e m p e r a t u r e

F

Top Curve = Net Cold Water Temperature From Tower

Wet-Bulb Temperature F

B o t h F a n s

a t H a l f S p e

e d

B o t h F

a n s R u

n n i n g F u l l

S p e e d

O n e F a n

a t F u l l S

p e e d

a n d O n e

a t H a l f S

p e e d

Water Temperature atColdest Point in Fill

Net Water TemperatureFrom Full Speed Cell

C o l

d W a t e r

T e m p e r a t u r e

F

Figure 8Operating Characteristics of a Two-Cell Tower with Single-Speed Motors (20F range)

Figure 9Operating Characteristics of a Two-Cell Tower with Two-Speed Motors (20F range)

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Icing Characteristics vs. Types of Fills and Towers

The falling water pattern associated with various typesof towers has much to do with both the type of ice formedand its location. Crossow towers equipped with splashll tend to form the louver ice depicted in Figures 2 and 4,wherein random water droplets generated by the splashing

action may impinge upon the louvers and be frozen almostinstantaneously. Ice typical of Figure 2 can usually becontrolled merely by reducing the fans speedor turning itoff for a short period. The reduction in air velocity enteringthe louvers causes the waters pattern of fall to becomevertical, and the louvers are subjected to a cascade ofrelatively warm water for deicing.

To the degree that such ice blocks air ow, it canoftentimes be self-limiting. Water behind a series of blockedlouvers will fall vertically, and will effect a certain amountof deicing. Many wintertime operators of crossow towerswill have observed the kaleidoscopic effect of this alternatefreezing and thawing.

Left to progress to the magnitude indicated in Figure4, such ice would require that the fans be reversed (on amechanical draft tower) for an interim of time, the extent ofwhich must depend upon the tendency for ice to form onthe fan cylinder and mechanical equipment. This reversalof air ow not only tends to inundate the louvers with warmwater, but also bathes the louvers with warm exit air. In anatural draft tower, where air ow reversal is impossible,removal of such ice would require a special distributionsystem whereby water could be periodically diverted todeluge the louvers.

Because of a typically contained ow pattern, randomwater droplets rarely escape lm-type ll, and crossowtowers so equipped tend toward little or no self-producedlouver ice. In those cases, louver ice is usually the productof high winds, snow, sleet, and other natural forces.

Used in crossow conguration, PVC lm ll also tendsto limit the amount of ice that it will permit to form on thell itself. In extended tests, conducted at zero heat loadin freezing weather, attempts at forming signicant ll icehave met with almost no success. In these tests, thin icewould form at the leading edge of the ll, turn to slushas air ow became blocked; and shortly disappear. Thiswas particularly true of the congurations having louversmolded integrally with the ll. Therefore, given normal controlmeasures, these towers have proved to be quite civilizedin their wintertime operation. This is especially true in thelow-load, low-temperature situations encountered in free

cooling. Such towers routinely deliver 40F cold water atimposed ranges varying from 10F to less than 2F.Since the ll of a counterow tower is elevated

appreciably above the cold water basin level, the generationof random water droplets produced by this free fall tendsto be irrespective of the type of ll utilized. Droplets whichsplash in an outboard direction will freeze on the basincurb and lower structure. Wind gusts cause falling water tomomentarily encounter, and freeze upon, the intermediatestructure (or louvers, if so equipped). Also, water whichnormally encounters the inside of the casing can continuedown the inboard side of the exposed structure, where itbecomes subject to freezing. The combination of these

effects initially results in the formations typied by Figure1. Given no concern (and a sufciently low ambient), theformations will tend to grow toward that depicted in Figure3, particularly where the normal ll temperature gradientresults in water near freezing at the lls coldest point.

Deicing measures for counterow towers are similar tothose utilized for crossow towers, but tend to be somewhatless effective. The normally vertical sides of a counterowtower place air inlet areas beyond the reach of the fallingwater pattern with fans off. That operating mode, therefore,usually removes only that ice which has begun to encroachinward from the air inlets. Because of a counterow towersstructural nature, however, this limited deicing capability mayprove to be enough in most cases. Peripheral ice of concerncan be removed by fan reversal, but acceptable results mayrequire several attempts. This is because air ow reversalcan bring only a relatively minimal amount of warm water tobear on the ice. Therefore, warm air must accomplish mostof the work, which slows down the process considerably.The number of attempts necessary, of course, depends onthe tendency for icing of the mechanical equipment duringbackward air ow.

Many operators are reluctant to reverse fans on acounterow tower because of the small amount of watercaused to escape the air inlets by the outward ow of air.This may produce sufcient ice in the immediate region ofthe tower to be considered hazardous, requiring separatemeasures for its control.

Although fewer icing tests have been run on lm ll forcounterow towers, there is every reason to believe itsresponse will be similar to that encountered in crossowtowers. Fill icing is expected to be relatively little, with atendency toward self-limitation.

Waterside Control

Larger towers designed for operation in freezing weathershould be equipped with a water distribution system whichcan be manipulated to place the greatest concentrationof owing water nearest the air intakes of the tower. Thisapplies particularly to natural draft towers, where no meansof airside control is typically available, and the airow rateactually increases with lower temperature due to the drafteffect. Not only does this give the most difcult cooling

job to the coldest air, but it also assures a rapid rise in airtemperature to preclude freezing on the ll. Most importantly,it places the maximum amount of relatively warm owingwater in close proximity to the areas of greatest ice concern.

To provide for start-up and operating exibility, provisionfor total water bypass directly into the cold water basin (Fig.10) is advisable on mechanical draft towers, and shouldbe considered mandatory on natural draft towers. Duringcold weather start-up, the basin water inventory may be ata temperature very near freezing, at which time the total water ow should be directed back into the cold waterbasin upon return from the process load, without goingover the ll. This bypass mode should be continued until thetotal water inventory reaches an acceptable temperaturelevel (usually about 80F), at which time the bypass maybe closed to cause total ow over the ll.

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Even during operation, combinations of low load, lowambient, and high winds can promote ice formations despitenormal airside and waterside control procedures. In thosecases, it may intermittently become necessary to divert totota l bypass ow to build a heat content in the circulatingwater. Modulation of the bypass whereby a portion of thewater ow is allowed to continue over the ll,must not bepermitted on a natural draft tower, and its utilization onmechanical draft towers should be discouraged .

Where reduced ow rates cannot be avoided, either bypump manipulation or through bypass modulation, crossowtowers can be provided with a longitudinal dam in the hotwater basins to concentrate water outboard on the ll.The height and location of this dam would be based upona predetermined minimum allowable water ow rate. Atincreased ow rates, the dam would become submerged,allowing water access to the inboard areas of the ll.

Comparison of Induced Draft and Forced Draft

As indicated previously, of the two basic types oftowers utilized (counterow and crossow), neither can beconsidered to have an overriding advantage with respect tocold weather operation. Of far greater importance is the typeof fan used, and its location with respect to air ow throughthe tower. Consequently, the remainder of this paper willcompare the icing characteristics of induced draft, propellerfan towers and forced draft, blower fan towers.

In the induced draft, propeller fan tower depicted inFigure 11, cold air enters through the louvers at relatively

low velocityis heated by the water owing over the llandexits through the fan at a relatively high velocity. Thisincreased exit velocity assures that the amount of saturatedair recirculating into the intake louvers will be minimal.

Therefore, only the passive air-intake area is subjectedto any potential for ice formation related to cooling toweroperation. The fan and mechanical equipment are exposedonly to warmed air. This is true even when the fan is turnedoff. Convection currents induced by the warmth containedin the owing water bathe the mechanical equipment withheated air.

Figure 11 also illustrates the ll-louver relationship in acrossow tower. Note that the space occupied by ll anglesinward on the tower from top to bottom. During full fan

operation, water passing over the ll will tend to fall at thatangle due to the velocity of the horizontal ow of air withwhich it is in contact. When the fan is slowed, or broughtto a stop, the incoming air velocity reduces signicantlyand the water attempts to fall vertically. In doing so, thewater impinges upon the louvers, cascading successivelydownward to the cooling tower basin. Except in the mostsevere situations, this cascade of warm water is usuallysufcient to accomplish deicing.

In situations of extreme cold weather and/or a very lightheat load, it may become necessary to reverse the directionof fan rotation. (This is easily accomplished on a three-phasemotor by switching two of the three leads.) In this mode ofoperation, air ows downward through the fan picks up

heat passing through the lland exits through the louvers.This reverse passage of air shifts the falling water patternoutward causing an increased cascade down the louvers.The combination of warm water and warm air, of course,effects complete deicing.

Fan reversal should be limited to a time period of nomore than 1 or 2 minutes to preclude excessive icing of thefan. Unless an inordinate buildup of ice has been allowedto accumulate, this is usually more than sufcient timeto accomplish louver deicing. Monitoring is required todetermine the time required to melt accumulated ice. Also, toprevent possible damage to the fan, drive train, and electricalsystem, an interval of no less than two minutes should existbetween the instant that the motor is de-energized and thetime it is restarted in the opposite direction. This allowsthe fan time to lose rotational momentum.

The least desirable cooling tower for operation in freezingweather is the forced draft, blower fan type, as depicted inFigure 12. Its lack of adaptability to cold weather operationstems primarily from the fact that the fans are located in theentering cold air stream, rather than in the warm leavingair stream. Because of the location of the fans care mustbe taken to ensure proper operation and orientation toprevent recirculation .

Figure 10Typical Water Bypass Arrangement Figure 11Induced Draft, Propeller Fan Tower

AIR OUT

AIRIN

AIRIN

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To those of you who may be unfamiliar with recirculation,it is the reintroduction of a portion of the saturated leavingair stream back into the relatively dry entering air stream.In summertime, such a situation causes an undesirableelevation in cold water temperature by increasing the wet-bulb temperature of the air entering the tower. In wintertime,the articially-increased moisture content of the enteringair quickly condenses and freezes on the towers coldestpointthe air intake (fan) area.

Obviously, in any given wind condition, recirculation isa function of the relative velocities of the air entering andleaving the tower. Where the exiting air velocity exceedsentering velocity, the tower's tendency to recirculate isreduced. Conversely, higher entering velocities producea localized zone of low pressure into which recirculationis induced. (See Technical Report H-004 ).

In the typical induced draft, propeller fan tower (Fig.11), the average exit velocity is approximately 1900 ft/min, compared to an entering velocity of 685 ft/min. Thispositive ratio exceeding 2.5/1 minimizes the potential for

recirculation. In contrast, the air velocity at the fan region ofa forced draft, blower fan tower (Fig. 12) is approximately2000 ft/min, compared to a tower exit velocity of about660 ft/min. This negative ratio exceeding 3/1 virtuallyassures some recirculation of saturated air back into thefan intakes.

An adverse wind condition compounds the problem,as seen in Figure 13. A wind velocity as little as 7.5 mph(660 ft/min) will deect the exit plume an amount sufcientto insure entrapment within the negative pressure zonecreated by the fans high entering velocity.

Under no circumstances should the direction of airow be reversed in a forced draft tower. (In the case of acentrifugal blower fan, the point is academic because itis impossible to do so.) Reversing the direction of air owmerely concentrates moisture on the mechanical equipment,which refreezes instantaneously upon return to normal airow. Unfortunately, merely bringing the fan to a stop canalso produce this unwanted situation. The spray-type waterdistribution system normally utilized in forced draft towerscauses a downward aspiration of saturated air, which exitsthrough the inoperative fan. Although this movement of airis insufcient to effect deicing, it does contribute to furthericing upon fan restart.

Finally, forced draft towers can be operated in freezingweather, but extreme care must be taken to ensure proper

operation and orientation to prevent recirculation

.

Figure 12Forced Draft, Blower Fan Tower Figure 13Recirculation Potential in a Forced Draft Tower

WIND

AIR IN

AIR OUT AIR OUT