Par t One

Maintain

Don’t let ‘out of site, out of mind’ prevent you from keeping cooling towersrunning efficiently

BY GLENN A. BABCOCK

How to

a TOWERof POWER

Û The cooling tower needs routine inspection and mainte-nance to continue performingas designed.

TThe cooling tower is often the forgot-ten component of the water-coolingsystem when it comes to maintenance.It’s a good example of the phrase “outof sight, out of mind.” A newlyinstalled cooling tower reliably deliv-ers the design fluid temperature andflow rate.

But because its heat transfer opera-tion creates a “hurricane-like” environ-ment and is a natural “air-washer,” thecooling tower needs routine inspectionand maintenance to continue perform-ing as designed. This two-part reportwill cover routine maintenance tipsand suggest ways to improve coolingtower performance.

Owners and operators who have aworking knowledge of cooling towerpreventive maintenance and knowhow to upgrade technology will getthe most out of their cooling towers.Their efforts can yield beneficialresults, including:

➤ Keeping the cooling tower run-ning smoothly and reliably.

➤ Increasing cooling tower lifeexpectancy.

➤ Maintaining and potentially im-proving performance.

Operation fundamentals In an open-circuit cooling tower, warmwater from the heat source is evenlydistributed via a gravity or pressurizednozzle system directly over a heattransfer surface called “fill” or “wetdeck.” Air is simultaneously forced ordrawn through the tower. This causesa small percentage of the water toevaporate.

The evaporation process removesheat and cools the remaining water,which is collected in the tower’s coldwater basin and returned to the heatsource (typically a water-cooled con-denser or other heat exchanger).

April 2006 ~ RSES Journal 17

Similarly, in a closed-circuit cool-ing tower or evaporative condenser,the heat is rejected indirectly from afluid or vapor flowing through thecoil section by spraying re-circulatedwater over the coil section, againevaporating a small percentage of thewater in the process.

The temperature at which thecooled fluid is returned to the systemmeasures tower performance. Thetemperature can vary depending onthe cooling load, water flow, airflowand the entering air conditions.

Because temperature matters somuch, owner confidence in coolingtower performance will be enhancedby selecting a tower model whose per-formance is certified by the CoolingTechnology Institute (CTI).

Also, performing routine preventivemaintenance is paramount for consis-tently achieving the desired tempera-ture and flow rate, and plays an impor-tant role in maximizing cooling toweroperating life.

To perform properly, all tower com-ponents must be kept clean and free ofobstructions. Neglecting the coolingtower will lead to higher-than-desiredreturn water temperatures to the sys-tem, which will result in higher energyusage from two perspectives.

First, the system (chiller) will con-sume more energy because it mustoperate at a higher-than-necessarycondensing pressure to satisfy the loaddue to the higher fluid temperaturesprovided by the cooling tower. As littleas 2° F higher temperature can resultin 6 percent more energy being con-sumed by the chiller. Second, thetower must operate longer at higherfan horsepower, while trying to attainthe design cold water temperature.

Maintenance proceduresThe following sections describe stan-dard maintenance procedures for opti-mized circulating water system opera-tion. These procedures can preventloss of efficiency in the heat transfersection by maintaining proper waterand airflow, as well as preventing cor-

Ý In an open-circuit cooling tower the evaporation processremoves heat and cools the remaining water, which is collected inthe cooling tower.

Ý In a closed-circuit cooling tower or evaporative condenser, theheat is rejected indirectly from a fluid or vapor.

Open-circuit cooling tower

Closed-circuit cooling tower

Ý The tech checks the strain-ers in the cold-water basin.

Û The technician inspects theprestrainer, which preventsclogging of nozzles.

This simple cleaning procedureeliminates the need to access the dis-tribution nozzles. Routinely cleanand inspect both strainers as neces-sary. Some tower designs allow exter-nal access to the strainers, whichallows maintenance without turningoff the unit.

The water distribution systemshould evenly distribute water over thefill section or coil section via either agravity distribution system or a pres-

18 RSES Journal ~ April 2006

rosion in the cooling tower. Maintenance frequency will depend

largely upon the condition of the circu-lating water, the cleanliness of theambient air utilized by the tower andthe environment in which the tower isoperating. More detailed informationis provided by the specific manufac-turer.

Fundamentally important to theperformance of a cooling tower is amethod to minimize contact between

air- and water-borne debris and thesystem components. This is accom-plished with strainers.

Strainers in the tower provide ameans of keeping debris out of the condenser system water loop.Strainers in the cold-water basinoutlet prevent debris from reachingthe pump. Some towers feature low-pressure drop prestrainers upstreamof the hot water basin to preventclogging of distribution nozzles.

Ý The water distribution systemshould evenly distribute waterover the fill section or coil sec-tion via either a gravity distribu-tion system or a pressurizedspray system.

Ý The tech removes the nozzle prior to inspec-tion to make sure it’s clean.

Þ The sump sweeper piping keeps the cold-waterbasin clean.

April 2006 ~ RSES Journal 19

surized spray system, utilizing anarray of nozzles. If you determine thatwater distribution is uneven, check thenozzles. Clean clogged nozzles accord-ing to the manufacturer’s recommen-dations.

In a gravity-distribution system,you can externally access, visuallyinspect and clean by removing thehot-water basin covers on the fandeck. Most pressurized spray distri-bution systems utilize nozzles andbranches held in place by snap-inrubber grommets, which allow easyremoval to clean and flush debris ifnecessary.

Keeping it cleanBecause some debris will make itsway into the cooling tower, the unitdesign should facilitate debrisremoval. A well-designed cold-waterbasin is sloped toward the strainer tokeep dirt (which can accelerate cor-rosion) from accumulating through-out the basin.

Keep the basin clean by occasional-ly flushing the dirt out of the systemthrough the tower drain. Another wayto accomplish this is to install sumpsweeper piping in conjunction withwater filtration or separator devices.Water filtration saves maintenancecosts by reducing the dirt in the cool-ing water system, which in turnreduces the time required to clean thecold-water basins.

It also reduces water treatmentcost, as water treatment chemicalstend to work more effectively in cleanwater. Foreign particles in dirty watercan absorb treatment chemicals, thusrequiring the distribution of evenmore chemicals to properly treat thetower water.

Though most of the water in thesystem is recirculated, some watermust be added to replace what is lostthrough evaporation and “bleed,”which is the water that is discharged toprevent the accumulation of solids inthe cooling water. The makeup watersystem, replaces the water via amechanical float ball and valve assem-bly or an electronic water level probeassembly (with solenoid valve), which

measures water depth in the cold-water basin.

Maintain the makeup water supplypressure between 15 psig and 50 psigto ensure proper valve shut-off and toavoid “chatter.” If the supply pressureis higher than 50 psig, install a pres-sure reducing valve.

The operating water level of thecooling tower will vary with systemthermal load (evaporation rate), the

bleed rate employed and the makeupwater supply pressure. Some towerdesigns offer access to the makeupassembly external to the coolingtower, which allows easy basin waterdepth inspection and adjustmentwithout the need to turn off the unit.

Set the tower water level in accor-dance with the manufacturer’s recom-mendations to ensure no air enters thepump suction. Do not set it so high

20 RSES Journal ~ April 2006

that water is wasted through the over-flow when the tower is shut down.

To prevent the accumulation ofsolids in the recirculating water, makesure the tower is equipped with a bleedline (including a metering connectionand globe valve) connected to a nearbydrain. In a closed-circuit cooling toweror evaporative condenser with a circu-lating pump, a metering valve to con-trol the bleed rate should be providedat the pump discharge.

While a manually adjusted bleedvalve is the simplest system, gettingthe proper bleed rate can be a problembecause cooling tower loads varythroughout the day. A conductivitymeter connected to a solenoid valvesolves this problem by maintaining theproper cycles of concentration at alltimes.

Also, it is recommended that youinstall a separate meter to measurebleed volume because less water isdischarged to drain than supplied tothe cooling tower. This can reducesewer water charges.

Adjust the bleed rate to prevent anexcessive buildup of impurities inthe recirculating water. This largelydepends on the local water qualityand the evaporation rate. Constantbleed and replacement with fresh

water will prevent the accumulationof impurities. To obtain specific rec-ommendations, contact a competentwater treatment professional.

The mechanical drive systemThe mechanical fan-drive system hasseveral components that you need toregularly check. Many of these compo-nents operate at high speed. To maxi-mize safety, follow proper lock-out/tag-out procedures, including locking out all motor disconnect switches beforeworking on the mechanical system.

Cooling tower fans are typically driv-en by belt- or gear-drive systems. Bothrequire routine maintenance to ensurereliable, trouble-free performance.

Belt drive systems are popularbecause they are inexpensive, reliable,offer single-point adjustment and haveno limit on turndown capabilities forvariable-speed applications. If a prob-lem occurs, usually changing a belt isall that is required. Replacement com-ponents are readily available.

Gear drives provide reliable opera-tion as well and perform consistentlywhen properly maintained. If a prob-lem occurs, resolution may be moreinvolved if a gear box rebuild orreplacement is required. Some manu-

facturers offer both systems to meetuser needs or preferences.

To ensure proper operation of abelt-drive system, tighten the drivebelts to manufacturer’s specifications.In gear-drive systems, regularly checkthe oil level and quality, and shaftalignment in accordance with themanufacturer’s recommendations.

When starting up a new unit, lubri-cation for the fan-shaft bearings is typically not necessary becausemost units leave the factory alreadygreased. For a seasonal startup, purgethe fan-shaft bearings with newgrease per the manufacturer’s recom-mendations. Lubricate fan-shaft bear-ings every 2,000 hours of operation orevery three months, whichever occurssooner.

Lubricate motor bearings as recom-mended by the manufacturer. For max-imum life, it is best to install motorswith a cooling tower duty rating.

Part two in next month’s issue willcover how to handle common cool-ing tower problems such as elevatedleaving water temperatures, drift andcorrosion.◆

Glenn A. Babcock is manager of mar-keting communications and trainingdevelopment for Baltimore Aircoil Co.

Ý The makeup water system replaces the watervia a mechanical float ball-and-valve assembly oran electronic water level probe assembly, whichmeasures water depth in the cold-water basin.

Ý Some tower designs offer access to themakeup assembly external to the cooling tower.