INTRODUCTION

The cooling tower is often the forgotten component ofa water cooling system when it comes to maintenance.It’s a good example of the phrase “out of sight, out of mind.” A newly installed cooling tower reliablydelivers the design fluid temperature and flow rate.However, since its heat transfer operation creates a hurricane-like environment and is a natural air-washer, the cooling tower needs routine inspectionand maintenance to continue performing as designed.This article discusses routine maintenance measuresand suggests some ways to improve cooling towerperformance.

Let’s take a moment to recall how a cooling towerworks. In an open-circuit cooling tower (see Figure 1below), warm water from the heat source is evenlydistributed via a gravity or pressurized nozzle systemdirectly over a heat transfer surface called the fill orwet deck.

Air is simultaneously forced or drawn through thetower, causing a small percentage of the water toevaporate. The evaporation process removes heat andcools the remaining water, which is collected in thetower’s cold water basin and returned to the heatsource (typically a water-cooled condenser or otherheat exchanger).

1

BECAUSE TEMPERATURE MATTERS: MAINTAINING COOLING TOWERS

by Glenn A. BabcockBaltimore Aircoil Company

Reprinted with permission from the Refrigerating Engineers & Technicians Association (RETA).

Service Application ManualSAM Chapter 650-013

Section 8D

© 2012 Refrigerating Engineers & Technicians Association. No reprint is permissible without the express written consent of RETA.Supplement to the Refrigeration Service Engineers Society.

Warm air out

Fill surface

Air in

Cooled water out

Warmair

Warmair

Cold water basin

Water Water

Air inletlouvers

Sump

Hot water inHot water in

Air in

Figure 1. Open-circuit cooling tower

The process is similar in a closed-circuit coolingtower (see Figure 2) or evaporative condenser. Heatis rejected indirectly from a fluid or vapor flowingthrough the coil section by spraying recirculatedwater over the coil section—again evaporating asmall percentage of the water in the process.

The temperature at which the cooled fluid is returnedto the system measures tower performance. Thistemperature can vary, depending on the actualcooling load, water flow, air flow, and the entering air conditions. Because temperature matters so much,owner confidence in cooling tower performance willbe enhanced by selecting a tower model whoseperformance is certified by the Cooling TechnologyInstitute (CTI).

PREVENTIVE MAINTENANCE

Performing routine preventive maintenance isparamount for consistently achieving the desired

temperature and flow rate, and also plays animportant role in maximizing cooling tower operatinglife. Cooling tower manufacturers conscious of theimportance of maintenance offer many features thatsimplify these procedures, saving both time andmoney.

To perform properly, all tower components must bekept clean and free of obstructions. The followingparagraphs describe a number of standardmaintenance procedures for optimized circulatingwater system operation. These routine maintenanceprocedures can prevent loss of efficiency in the heattransfer section by maintaining proper water flow and air flow, as well as preventing corrosion in thecooling tower.

The frequency with which preventive maintenance is performed depends largely on the condition of thecirculating water, the cleanliness of the ambient airused by the tower, and the environment in which thetower is operating. More detailed information isprovided by the specific manufacturer’s operating and maintenance instructions.

2

Warmair

Warmair

Fill surface Spray pump

Water distribution

system

Combinedinlet

shields

Fluidout

Fluidin

Coil

Air in

Air in

Warm air out

Water

Water

Figure 2. Closed-circuit cooling tower

Figure 3. Inspecting a cold water basin strainer

STRAINER SYSTEMS

Fundamentally important to the performance of a cooling tower is a method to minimize contactbetween air- and water-borne debris and the systemcomponents. Strainers in the tower provide a meansof keeping debris out of the condenser system waterloop. Strainers in the cold water basin outlet preventdebris from reaching the pump (see Figure 3).

Some towers feature low pressure drop pre-strainersupstream from the hot water basin to preventclogging of distribution nozzles. This simple cleaning procedure eliminates the need to access the distribution nozzles. Both strainers should beroutinely inspected and cleaned as necessary. Sometower designs allow external access to the strainer,which enables maintenance to take place without theneed to turn off the unit.

WATER DISTRIBUTION

The water distribution system should distribute waterevenly over the fill package or coil section via either

a gravity distribution system or a pressurized spraysystem, using an array of nozzles. If water distributionis found to be uneven, the nozzles need to bechecked. Clogged nozzles should be cleaned inaccordance with the manufacturer’s recommendations.Since even water distribution is important to theperformance of the tower, consideration should begiven to the ease with which the distribution nozzlescan be accessed, inspected, cleaned, and replacedwhen selecting a cooling tower model.

In a gravity distribution system, the nozzles can be externally accessed and visually inspected andcleaned by removing the hot water basin covers onthe fan deck (see Figure 4). Most pressurized spraydistribution systems use nozzles and branches heldin place by snap-in rubber grommets, which alloweasy removal to clean and flush debris if necessary(see Figure 5).

COLD WATER BASIN

Since some debris eventually will enter into thecooling tower, the unit design should facilitate debris

3

Figure 4. Gravity water distribution Figure 5. Pressurized spray water distribution

removal. A well-designed cold water basin is slopedtoward the strainer to keep dirt (which can acceleratecorrosion) from accumulating throughout the coldwater basin. The basin should be kept clean byoccasionally flushing the dirt out of the systemthrough the tower drain.

Another way to accomplish this is to install sumpsweeper piping in conjunction with water filtration ofseparator devices. Water filtration saves maintenancecosts by reducing dirt in the cooling water system,which reduces the time required to clean the coldwater basins. It also reduces water treatment cost,since water treatment chemicals tend to work moreeffectively in clean water. Foreign particles in dirtywater can absorb treatment chemicals, thus requiringthe distribution of even more chemicals to treat thetower water properly.

MAKEUP WATER SUPPLY

Although most of the water in the system isrecirculated, some water must be added to replacewhat is lost by evaporation and by bleed, which is thewater that is discharged to prevent the accumulationof solids in the cooling water. The makeup watersystem provides the means to replace the water via a mechanical float ball and valve assembly, whichmeasures water depth in the cold water basin. Anelectronic water level probe assembly with solenoidvalve also can be used (see Figure 6). The makeupwater supply pressure typically should be maintainedbetween 15 and 50 psig (103 and 345 kPa) to ensureproper valve shutoff and avoid chatter. If the supplypressure is higher than 50 psig (345 kPa), install apressure-reducing valve.

The operating water level of the cooling tower willvary with system thermal load (evaporation rate), the bleed rate used, and the makeup water supplypressure. Some tower designs offer access to themakeup assembly external to the cooling tower,which allows easy basin water depth inspection and adjustment without the need to turn off the unit.The tower water level should be set in accordancewith the manufacturer’s recommendations, but not so high that water is wasted through the overflowwhen the tower is shut down.

BLEED

To prevent the accumulation of solids in therecirculating water, the tower should be equippedwith a bleed line (including a metering connectionand globe valve) connected to a nearby drain.

In a closed-circuit cooling tower or evaporativecondenser with a circulating pump, a metering valveto control the bleed rate should be provided at thepump discharge. While a manually adjusted bleedvalve is the simplest system, getting the proper bleedrate can be a problem, since cooling tower loads varythroughout the day. A conductivity meter connectedto a solenoid valve solves this problem by maintainingthe proper cycles of concentration at all times. Also,it is recommended that a separate meter be installedto measure bleed volume, since less water is

4

Figure 6. Water level control

discharged to drain than supplied to the coolingtower. This can reduce sewer water charges.

The bleed rate should be adjusted to prevent anexcessive buildup of impurities in the recirculatingwater. This largely depends on the local water qualityand the evaporation rate. Constant bleed andreplacement with fresh water will prevent theaccumulation of impurities. To obtain specificrecommendations, contact a competent watertreatment professional.

MECHANICAL DRIVE SYSTEM

The mechanical fan drive system has severalcomponents that should be checked regularly. Many of these components operate at high speed. So, to maximize safety, follow proper lockout/tagoutprocedures, including locking out all motordisconnect switches before working on themechanical system.

Cooling tower fans typically are driven by belt-drive or gear-drive systems. Both require routinemaintenance to ensure reliable, trouble-freeperformance. Belt-drive systems are popular, sincethey are inexpensive yet reliable, offer single-pointadjustment, and have no limit on turndowncapabilities for variable-speed applications (seeFigure 7). If a problem occurs, a simple change of the belt is usually all that is required, andreplacement components are readily available.

Gear drives provide reliable operation as well, andperform consistently when properly maintained. If aproblem occurs, resolution may be more involved if agear box must be rebuilt or replacement is required.Some manufacturers offer both systems to meet userneeds or preferences. To ensure proper operation of a belt-drive system, tighten drive belts to themanufacturer’s specifications. In gear-drive systems,the oil level and quality, as well as shaft alignment,should be checked regularly in accordance with themanufacturer’s recommendations.

When starting up a new unit, lubrication of the fanshaft bearings is typically not necessary, since mostunits leave the factory already greased. However, for

seasonal start-up, purge the fan shaft bearings withnew grease (per the manufacturer’s recommendations).Fan shaft bearings should be lubricated after every2,000 hours of operation or every three months(whichever occurs sooner). Motor bearings should belubricated as recommended by the manufacturer’sinstructions. For maximum life, it is best to installmotors with a cooling tower duty rating.

IMPORTANCE OF CLEAN OPERATION

Cooling tower components must be kept clean andfree of obstructions, because temperature matters.Neglecting the cooling tower will lead to higher thandesired return water temperatures to the system,which will result in higher energy use from twoperspectives. First, the system (chiller) will consumemore energy because it must operate at a higher thannecessary condensing pressure (head) to satisfy theload, due to the higher fluid temperatures providedby the cooling tower. A temperature higher by aslittle as 2°F (1°C) can result in 6% more energybeing consumed by the chiller. Second, the towermust operate longer at higher fan horsepower whiletrying to attain the design cold water temperature.

COMMON PROBLEMS: CAUSES, EFFECTS, ANDSOLUTIONS

Regardless of how often routine maintenance isperformed, like any other mechanical component,problems with cooling towers may materialize

5

Figure 7. Mechanical belt-drive system

unexpectedly. These include elevated leaving watertemperatures, drift, and corrosion. Should any ofthese problems occur, contact the cooling towermanufacturer’s representative or water treatmentsupplier for assistance.

High leaving water temperature

High leaving water temperature will cause a drop inprocess cooling efficiency, increasing system powerconsumption and creating physical discomfort forpeople within the air conditioned area. It will have a detrimental effect on other system components aswell. Major factors affecting cooling tower leavingwater temperatures are cooling load, water flow, airflow, entering air conditions, and the condition of thetower itself.

Cooling load

If the actual cooling load exceeds the design load for which the tower was selected or surpasses thecertified thermal performance of the tower, theleaving water temperature will exceed the designspecifications for the towers.

Water flow and distribution

Visually inspect the water distribution system inorder to ensure that the spray distribution nozzles are clean and correctly installed (see Figure 8). The nozzles should be distributing a uniform spraypattern over the wet deck surface. The cooling towercirculating water flow rate can be measured either by measuring the pressure differential between thecirculating pump discharge and suction, or bymeasuring the pressure drop across the condenser(which should be relatively free of scale and fouling),and comparing it to the manufacturer’s specifications.

In counterflow towers, measure the pressure at thecooling tower inlet connection and compare it to the design pressure provided by the cooling towermanufacturer. For towers with a gravity distributionsystem, the operating level in the hot water basin—typically between 2 and 5 in. (51 and 127 mm)—can be correlated to a specific flow rate based on the manufacturer’s information.

Air flow

Cooling towers should be located where anunimpeded supply of fresh air is available to the air inlets. The cooling tower air discharge also shouldbe a least as high as any surrounding walls to reducethe possibility of hot, moist discharge air beingrecirculated into the air inlets, creating artificiallyelevated entering wet-bulb and leaving watertemperatures. To ensure full design air flow, thecooling tower drive system must be adjustedaccording to the manufacturer’s operating andmaintenance instructions.

The cooling tower and surrounding area should beexamined for air flow restrictions that may causeblockage of the air inlets. Check for clogging orimproper distribution of water across the tower fillsurface and check for proper operation of capacitycontrol dampers in centrifugal fan towers to ensureproper air flow. The dampers (airfoil blades locatedin the discharge of the fan housing) help achievetight temperature control and energy savings bymatching cooling tower air flow to actual loadrequirements.

Although you may encounter dampers in olderexisting units, today’s towers tend to take advantage

6

Figure 8. Inspecting spray nozzles

of variable-frequency drive (VFD) technology tocontrol capacity. VFDs help save energy, do a betterjob of following the load, and help reduce wear andtear on the drive system.

Ambient conditions

Cooling towers are selected to produce the requiredleaving water temperature at the design cooling loadand entering wet-bulb temperature. Whenever theactual entering wet-bulb temperature is higher thandesign conditions, the leaving water temperature also will be higher. The result is decreased energyefficiency and possible downtime for the system.

Drift

Drift occurs as air flows through the cooling towerand carries water droplets out of the tower. Drifteliminators are installed in the discharge stream to strip entrained water droplets from the air. In aproperly maintained system, efficient eliminators will reduce drift loss to a negligible percentage of the design flow rate.

If excess drift occurs, check the drift eliminators forproper installation, spacing, and overall condition(see Figure 9). Examine the wet deck surface

for even spacing, inspect the wet deck surface or water distribution system to ensure no clogging orblockage, and check water and air flow as describedpreviously. Repair or replace eliminators asnecessary.

Corrosion

Because of their ability to wash the air of impurities,corrosion is always a concern with cooling towers.Long-term exposure to impurities left on the wet decksurface or in the cold water basin can cause scale,corrosion, and eventually damage to systemcomponents.

Historically, galvanized steel construction has been commonly used for factory-assembled coolingtowers. Its structural rigidity and corrosion-resistantproperties are needed in the corrosive environment in which recirculating water becomes oxygenated.Current industry standards use G-235 galvanizedsteel, which has 2.35 ounces of zinc galvanizedcoating per 2 ft (717 g per 2 m) of steel.

The formation of white rust is a type of corrosion thatmay be promoted because of recent changes in waterquality and water treatment programs. White rustappears as a white and waxy adherent, non-protectivedeposit on steel surfaces. White rust forms on a newtower if it is operated at high pH levels (above 8.0)for an extended period of time before the zincgalvanizing can form a protective passivation layer. If white rust remains unchecked, the galvanized steelcoating will continue to corrode, eventually leading toan early failure.

Many additional cost-effective construction optionsare available to provide increased cooling towerprotection and equipment longevity. For example,fuse-bonded thermosetting polymers that areelectrostatically applied on galvanized steel canprovide enhanced protection from corrosion. Theyalso serve as a deterrent to white rust.

Stainless steel construction offers the ultimate incorrosion protection. Options of entire stainless steelunits or units with only stainless steel cold waterbasins are effective in negating corrosion damage.

7

Figure 9. Inspecting a drift eliminator

However, stainless steel is not maintenance-free.Submerged surfaces must be kept reasonably clean toprevent under-deposit oxygen deprivation and rapidpitting. For areas of the cooling tower which do notprovide structural support, such as the casing walls,fiberglass reinforced polyester (FRP) panels providea non-corroding, cost-effective construction material.

If a constant bleed of the system is ineffective incombating scale or corrosion, chemical treatmentmay be necessary. A successful chemical or watertreatment program should satisfy the specificguidelines set by the manufacturer, provide effectivemicrobiological control, and be compatible with thesystem’s materials of construction as an integral partof the total water treatment program.

Potential airborne impurities and biologicalcontamination (such as Legionella) should becontrolled through the use of biocides, and suchtreatment should be initiated at system start-up andcontinued regularly. ASHRAE has taken proactivesteps to understand and deal with Legionella throughits publication, ASHRAE Guideline 12-2000,Minimizing the Risk of Legionellosis Associated

with Building Water Systems. To obtain specificrecommendations of water treatment programs,contact a competent water treatment supplier.

Although basic cooling tower problems can either be alleviated or avoided altogether by using therecommended maintenance procedures, the processof servicing and maintaining the factory-assembledcooling tower is by no means a simple, one-stepoperation. Many interrelated factors must beconsidered, including periodic inspections and use of a competent specialist such as a water treatmentservice engineer. These factors serve as a valuableresource in the total service program and mayrepresent the margin of success throughout thelifespan of the cooling tower.

PERFORMANCE IMPROVEMENTS

Product improvements (retrofit kits) can improve theperformance of yesterday’s cooling towers with today’stechnology. Older, structurally sound cooling towerscan be retrofitted with upgrade kits to conserveenergy, restore or improve performance, and facilitatemaintenance (see Figure 10).

8

Figure 11. Access platforms and ladderFigure 10. Installing a retrofit fill kit

To conserve energy, two-speed motors, VFDs, and/orpony motors can be added to the mechanical drivesystem to take advantage of moderate operatingconditions with lower horsepower. VFDs offer a range of speeds to more closely parallel operatingrequirements, and pony motors provide the addedbenefit of redundancy in the event of a motor failure.A popular energy conservation approach uses a ponymotor system with a VFD controlling the lowerhorsepower motor.

To improve performance on water distributionsystems, kits are available to replace older, smallerdistribution nozzles or troughs with large-orifice,clog-free design replacements. Retrofit fill kits easily replace the original fill that may be cloggedwith scale or airborne debris. Access platforms also can be added to existing cooling towers tofacilitate maintenance and improve worker safety(see Figure 11).

CONCLUSION

Paying regular attention to the cooling tower througha regular, comprehensive maintenance program cansave time, money, and energy, while increasing thetower’s life expectancy. A well-maintained tower is aviable candidate for retrofit kits designed to enhanceperformance and lengthen its life.

Owners and operators with a working knowledge ofcooling tower preventive maintenance and upgradetechnology can take advantage of cost-saving ideasand procedures. If you are not regularly performingroutine maintenance on your cooling tower, implementa comprehensive maintenance program today—because temperature matters.

All illustrations courtesy Baltimore Aircoil Company.

9

1911 Rohlwing Road, Suite A Rolling Meadows, IL 60008 800-297-5660 www.rses.org