PHOTO COURTESY OF K C A (NATIONAL CENTER FOR SUPERCOMPUTING APPLICATIONS,LOCATED ON THE CAMPUS OF THE UmïERSITV OF ILLINOIS AT URBANA-CHAMPAIGN)TECUNICAL FEATUR

EnergyCooling TowersBY FRANK MORRISON, MEMBER ASHRAE

Water-cooled systems offer lower energy use than air-cooled alternatives. Many yearsago, the first water-cooled systems used potable water directly in the condenser toprovide heat rejection with the cooling water wasted to a drain. Cooling towers weredeveloped to recycle more than 98% of this water, resulting in tremendous reductionsin water and energy use as these systems grew in both size and popularity.

Since then, water-cooled systems have steadilyimproved their performance. For instance, the effi-ciency of a 500 ton (1757 kW) water-cooled centrifugalchiller has improved by over 50% since 1975' as indi-cated by the requirements of ASHRAE/IES Standard 90.1(hereafter referred to as Standard 90.1). Cooling towershave also evolved from centrifugal fan units to muchmore energy-efficient axial fan designs with improvedheat transfer surfaces, known as fill. In addition, inde-pendent certification of thermal performance for opencircuit cooling towers per the Cooling TechnologyInstitute's Standard 201* has become widely accepted inthe marketplace and became required by Standard 90.1in the 2007 edition.

While the efficiency improvements of individual sys-tem components have certainly lowered overall energyuse, even greater improvements are possible by optimiz-ing the way cooling systems are designed and operated.For instance, the full load energy use in a 500 ton (1757

kW) water-cooled chiller system, based on Standard90.1-2013 minimum efficiencies, is roughly brokendown as follows: chiller - 77%, cooling tower - 8%, con-denser pump - 7%, and chilled water pump - 8%. Withthe chiller accounting for the majority of the energy use,many contend that it makes sense to operate the coolingtower fan and condenser pump such that compressorenergy use is reduced—since it is by far the largest motorin the system.

For instance, to lower chiller energy, the cooling toweris often operated at full fan speed and flow until ambi-ent conditions allow the minimum condenser watertemperature limit to be reached. Below this level, thefan speed of the cooling tower is modulated, typicallyby a variable speed drive (VSD), to maintain the set-point. This is essentially the operating sequence for thewater-cooled baseline buildings found in AppendixG of Standard 90.1-2013, which uses 70°F (21.1°C) asthe lower condenser water setpoint (though this value

*CTI Standard 201 has been replaced by two 2013 standards: STD 201-OM, Operations Manual for Thermal Performance Certification of Evsporative Heat Rejection Ei]uißinent ^nú STD 201RS, PerformanceRating of Evaporative Heat Rejection Equipment

34 A S H R A E J O U R N A L a s h r a e . o r g FEBRUARY 2014

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is above the low limit for almost all chillers). In prac-tice, this lower limit varies and is dependent on thetype of chiller. The closer to full load the system runs,the greater the energy savings from such strategies.However, most chiller systems operate at less than fullload for the majority of time.

While it may seem counterintuitive, many designersand operators have found that using less cooling towerenergy reduces overall system energy at many off-designconditions. At such conditions, ancillary equipment(condenser pumps and cooling tower fans) operating atfull design speed becomes a larger portion of the systemenergy use, especially when variable speed chillers areused. Reducing cooling tower fan speed can reduce asso-ciated fan power significantly while increasing chillerpower only marginally. For example, slowing the towerfan speed to 80% of design reduces tower fan power byabout half, while only raising the cooling tower leavingwater temperature about 3°F (I.7°C). Depending on thespecific load point, the increase in chiller energy con-sumption from the higher condenser water temperaturemay or may not be less than the reduction in coolingtower energy. The key is to balance the performanceof the system components so overall performance isoptimized. Articles such as Taylor's '̂̂ '''' excellent serieson chilled water system design provide more details onsuch strategies.

Closer Approach SelectionsAnother method to reduce system energy is to select a

cooling tower using a closer approach than might be typ-ical for a particular area. The tower approach is definedas the difference between the water temperature leav-ing the cooling tower minus the entering wet-bulbtemperature. When a closer design approach is chosen,the resulting cooling tower provides colder water to thechiller condenser, even on a design day, which in turnreduces compressor energy. This, of course, assumes thesystem designer has not taken advantage of the colderdesign water temperature to reduce the condenser sur-face area of the chiller.

The added cooling tower cost and potentially greatertower fan horsepower and pumping head must be evalu-ated versus the expected chiller energy savings. Facilitieswith constant year-round loads, such as those experi-enced in data centers or certain manufacturing facilities,typically derive the greatest benefit from this technique.

PHOT01 Four cell crossflow open circuit cooling tower.

Fan Speed ControlThere are several specific methods to optimize cool-

ing tower energy use. First, as required by Standard90.1-2013, cooling tower fan speed must have the capa-bility to be controlled proportional to the leaving fluidtemperature or condensing temperature/pressure.^This can be accomplished in several ways, includingthe use of two-speed motors or variable speed drivetechnology. For multi-cell cooling towers, all of thefans should be operated simultaneously at the samefan speed, maximizing the heat transfer surface areaused in the evaporative cooling process, for the lowestenergy use. This is opposed to the traditional mannerof fan cycling (on/off) that provides step control (forexample. Tower 1 fan on, then Tower 2 fan on, etc., asthe load increases). This operating sequence becomeseven easier to apply today with the widespread use ofcost-effective variable speed drives (VSD), though thesequence can also be used with either multi-speed orpony motors.

To illustrate the potential energy savings, let'slook at the case of a four-cell cooling tower withand without variable speed fan drives as shown inPhoto 1. With full water flow over all cells, operat-ing the fans in two cells at full speed with the fansin the other two cells idle produces essentially thesame leaving water temperature off the tower aswhen running the fans in all four cells at approxi-mately 56% fan speed. However, by running all fanssimultaneously at the lower speed, the fan energy isreduced by more than 60% compared to step control

ABOUT THE AUTHOR Frank Morrison is manager, global strategy at Baltimore Aircoil Companyin Jessup, Md. He is chair of ASHRAE TC 3.6, Water Treatment, and a voting member ofASHRAE SSPC 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings.

FEBRUARY 2014 ashrae.org A S H R A E J O U R N A L 35

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thanks to the fan laws.^ In addition to the significantenergy savings, this control sequence has other ben-efits, including:

• Significantly improved condenser water tempera-ture control;

• Fan system maintenance is minimized thanks to thelower average fan speed and reduced starts and stops;

• The cooling tower has a lower sound profile due tothe lower average fan speed coupled with the soft fanstarts and stops; and

• All fan motors are regularly exercised without theneed for a lead/lag arrangement.

This simple energy-saving technique can be effec-tively applied on both new and existing installations. Ofcourse, both the minimum fan speed and flow require-ments for the specific cooling tower design must be fol-lowed per the manufacturer's recommendations.

Note that this fan control method for multi-cell cool-ing towers and other heat rejection equipment hasbeen added as a requirement in Standard 90.1-2013

f ü u í í i . Multi-cell counterflow open circuit cooling tower installation.

along with a 5% increase in the minimum efficiencyfor axial fan, open circuit cooling towers. Additionally,a limitation on the use of centrifugal fan, open circuitcooling towers over approximately 300 nominal tons(1318 kW) was incorporated in the 2010 edition of theStandard helping to accelerate an important market

tAssumes fuli design water flow over each ceil in both cases: exact percentage savings wiil vary depending on the specific wet buib. load conditions, and tbe estimation of tbe naturai draft cooiing capacityof tbe two ceils with tbe fans off.

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36 A S H R A E J O U R N A L asbrae.org FEBRUARY 2014

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trend towards the use of cooling towers with lowerenergy axial fans. It is important to note, however,that centrifugal fan units can still be used on projectsunder 300 nominal tons (1318 kW) and, without a sizerestriction, on installations where unit sound attenu-ation is required, when like-for-like replacements arenecessary, or for indoor ducted applications. The latteris often used in colder climates or where building secu-rity is an issue.

Lower Energy Cooling TowersAnother effective technique to reduce cooling tower

fan energy is to increase the amount of heat trans-fer surface in the cooling tower thereby reducing therequired airflow and associated air pressure dropthrough the tower. This, in turn, reduces the fan motorsize required for the same thermal duty. Motor sizereductions of 25% to 50% are often economically prac-tical on many projects. The additional first cost of thecooling tower, along with the larger support grillage, isoffset in part by lower fan motor wiring and VSD costs.Depending on the specific model chosen, the towerheight may increase resulting in higher pumping costs,but this is usually a relatively small factor and can beminimized by the judicious selection of the coolingtower. Combining all of these factors, the higher netfirst cost is paid for by the large fan energy savings,often producing a simple payback of two years or less.These energy savings can also contribute toward earn-ing LEED points for the building.

Tower Flow TurndownAfter optimizing chiller and cooling tower fan

energy, designers and operators should then evalu-ate condenser water flow turndown on open circuitcooling towers as a further means for reducing sys-tem energy use. Several important factors must beconsidered. First, the fill in the cooling tower mustbe properly wetted at all times to avoid wet/dry areasthat can lead to scaling in the fill pack. Scaling is theaccumulation of solids from the water at the wet/dryinterface. The amount of scaling depends on how wellthe entire fill pack is wetted as well as the recirculat-ing water quality in the cooling tower, which is mea-sured by such indicators as the level of total dissolvedsolids.

Uncontrolled scale can block both air and waterflow through the heat transfer media, which forces

the cooling tower fan to use more energy to meetthe design condenser water temperature setpoint.Under high load conditions, this can also result in anincrease in chiller energy consumption or, if severe,chiller surge. In addition, too low a flow over thetower can result in winter icing issues, which will becovered in more detail in the March issue in an articleby Paul Lindahl.

To avoid such issues, the minimum flow rateover the cooling tower as specified by the manu-facturer must be maintained, which may involveshutting down some cells as chillers are cycled off.Additionally, the cooling tower must be designed tohandle the expected flow turndown, which usuallyinvolves features such as weir dams and/or a com-bination of spray nozzles that can properly span theexpected range of water flows while providing ade-quate wetting of the fill pack at all times. This appliesto whether a crossflow {Photo 1) or counterflow {Photo2) design is specified. Crossflow towers typically usegravity flow basins to distribute the cooling water overthe fill pack as shown in Figure 1, while counterflowtowers have pressurized water distribution systemsas shown in Figure 2. Similarly, the flow limits throughthe condenser must also be checked as low velocitiesbelow the manufacturer's recommended minimumcan result in fouling of the condenser tubes. Thisin turn increases system energy consumption byincreasing chiller energy use while forcing the cool-ing tower to work harder to meet the necessary con-denser water temperature that is being called for bythe system.

Second, a portion of the pumping head on an opencircuit cooling tower is fixed, which lowers the poten-tial pump energy savings compared to those that areachievable with closed loop systems such as the chilledwater piping or a closed condenser loop that uses aclosed circuit cooling tower. Though important, thesefactors typically place condenser pump savings lastin line after the chiller, cooling tower fan, and chilledwater pump. Thus the designer and operator mustevaluate the potential energy savings of reducing thewater flow over the cooling tower with the operatingrisk to the system. These cautions apply whether thesystem uses 2.0 gpm/ton or 3.0 gpm/ton (0.036 L/s-kWto 0.054 E/s-kW) on the condenser loop. While in manycases the system energy is reduced with the lowercondenser design flow,^ note that there is even less

FFBRUARV2014 ashrae.org A S H R A E J O U R N A L 37

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Warn Air Out

Air In I

Hot Water In

I

Air InletLouvers

Cooler! Water Out

FIGURE 1 Induced draft, axial fan, crossflow open circuit cooling tower.

Drift Eliminators

Warm Air Out

Hot Water In

Air in

Cooied Water Out •

Fiii Surface

Typicai on Ail Four Sides

FISUIIE 2 Induced draft, axial fan, counterflow open circuit cooling tower.

potential condenser pump energy savings to capture,which further alters the risk/reward ratio.

Standard 90.1-2013 now requires that all open cir-cuit cooling towers on water-cooled chiller systemswith either multiple or variable speed condenserwater pumps have the capability of a minimum 50%flow turndown. Note, however, that there are norequirements or guidelines for implementing con-denser flow turndown in the standard at this timeas each system is unique. As such, system design-ers and operators should consult with their chillerand tower manufacturer for specific flow turndownrecommendations for their system. As mentionedpreviously, the local makeup water quality, theexpected water treatment program, and the sophis-tication of the control system required should alsobe evaluated.

Water Economizers for Computer RoomsJanuary's article by Mick Schwedler'' covered the

proper application of water economizers, cautioningthe reader to properly account for the changes in cool-ing tower approach temperature at lower wet-bulbtemperatures. Reflecting this same phenomenon, achange to the requirements for water economizers thatare primarily used for computer room applications wasimplemented in Standard 90.1-2013. Unlike a comfortcooling application where the load decreases in winter,the load in a typical computer room remains relativelyconstant throughout the year, independent of climatevariations. The high year-round load coupled with the

increasing approach temperature at lower wet bulbsresults in a cooling tower selection that is oversized forthe summer duty.

Since many computer facilities are lightly loadedduring their first few years of operation, systemcontrol can suffer, which is a serious concern forcomputer system operators due to their need forhigh reliability and system uptime in such facili-ties. To reduce the oversizing, the switchover designtemperature requirement for full economizationwas revised from a single fixed design temperaturefor all computer rooms (wet bulb for cooling towersand dry bulb for dry coolers) to one that varies byclimate zone. The lower design temperature reducesthe size of the heat rejection device required for thewater economizer which in turn reduces first costas well as improves system control under low loadconditions.

Closed Circuit Cooiing Towers and Evaporative CondensersWhile this article primarily discussed open circuit

cooling towers, the fan speed control requirementsmentioned earlier also apply to closed circuit coolingtowers, evaporative condensers, dry coolers, and aircooled condensers. Minimum efficiencies and thermalperformance certification requirements for closed cir-cuit cooling towers were added in the 2010 edition ofStandard 90.1. Closed circuit cooling towers combinethe function of a cooling tower and heat exchanger inone compact device, keeping the process fluid clean ina closed loop.

38 A S H R A E J O U R N A L ashrae .o rg FEBRUARY 2014

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Furthermore, minimum efficien-cies for evaporative condensers usedin both ammonia and halocarbonapplications have been added withthe 2013 edition. Evaporative con-densers are similar to closed circuitcooling towers except that a refrig-erant is condensed within the coil.Evaporative condensers are oftenused in cold storage warehouses,food processing facilities, supermar-kets, industrial processes, and, to alimited extent, HVAC systems.

PHOTO 3 Regular cooling tower maintenance pays cff.

Maintenance BenefitsFinally, no matter what the specific system design,

paying regular attention to the cooling tower (along withother system components) through a comprehensivemaintenance and water treatment program can savetime, money, and energy while increasing the cooling

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tower's life expectancy. Owners andoperators with a working knowl-edge of cooling tower preventivemaintenance and upgrade tech-nology can also take advantage ofcost-saving ideas and procedures,^such as replacement fill kits or themore energy efficient operatingsequences described earlier. Besure to refer to the cooling tower'soperating and maintenance manualfor the appropriate maintenancerequirements and service intervals.In addition, the article on cooling

tower maintenance in the Bibliography is a good sourceof best practices that can be used to keep your coolingtowers operating at peak efficiency.

ConclusionWater-cooled systems save energy compared to air

cooled alternatives for cooling duties. Proper selec-tion, design, operation, and maintenance of evapora-tive heat rejection equipment used in such systemsoffer the opportunity to further optimize systemenergy usage. Evaluating sub-system and systemperformance, as opposed to the performance of indi-vidual system components, is a path to significantlyimprove overall building performance. This energysaving opportunity will be explored by SSPC 90.1 asthe Committee begins development of the 2016 edi-tion of Standard 90.1.

References1. Path B water cooler chiller per Richard Lord, Carrier Corporation

and Vice Chair. SSPC 90.1.

2. Taylor. S. 2011. "Optimizing Design and Control of Chilled Water

Systems—Part 2: Condenser Water System Design" ASHRAFfournal

53(9):26.

3. Taylor. S. 2012. "Optimizing Design and Control of Chilled WaterSystems-Part 4: Chiller & Cooling Tower Selection." ASHRAE

Journal 54 (4):60.4. Taylor, S. 2012. "Optimizing Design and Control of Chilled Water

Systems—Part 5: Optimized Control Sequences." ASHRAF Journal

54(6):56.

5. ANSI/ASHRAE/IES Standard 90.1-2013, Energy Standard for Buildings

Except low-Rise Residential Buildings. Paragraph 6.5.5.2.

6. Copeland. C.C. 2012. "Improving Energy Performance of NYC'sExisting Office Buildings." ASHßA£/ouma/54(8):38.

7. Schwedler, M. 2014. "Effect of Heat Rejection Load and Wet Bulb on

Cooling Tower Performance." AS/iiMEJourna/56(1):16.

8. Babcock. C. 2005. "Because temperature matters: maintaining

cooling towers." ASHRAFJownal 47(3):51. •

40 A S H R A E J O U R N A L a s h r a e . o r g F E B R U A R Y 2 0 1 4

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