in hot water: a cooling tower case study

Journal of STEM Education Vol. 6 • Issue 3 & 4 July-December 2005 41

In Hot Water: A Cooling Tower Case StudyJustin Cochran, P.K. Raju, and Chetan SankarAuburn University

Problem Statement

Vogtle Electric Generating Plant operated bySouthern Nuclear Operating Company, a subsid-iary of Southern Company, has found itself at adecision point. Vogtle depends on their natural draftcooling towers to remove heat from the power cycle.Depending on the efficiency of the towers, the cyclecan realize more or less power output. The effi-ciency of the cooling tower is loosely defined byVogtle personnel as how well the tower’s actualperformance compares to its original design per-formance. The primary goal here is to have thecooling tower produce the coolest water possibleto re-enter the condenser. The performance, there-fore, can be described as how close the tempera-ture of this water gets to the expected (predicted)temperature of the original tower design. A one ortwo degree decrease in the cold water tempera-ture can have a very significant impact on thecompany’s ability to compete in the deregulatedmarket.

When Plant Vogtle began producing power in1989, the cooling tower only performed at 76% ofexpected efficiency. The original design was modi-fied by the manufacturer in 1990 and resulted in anincreased efficiency of 91%. In an effort to improvethe efficiency of the towers from 91% to 100%, twomodifications to the nozzle sizes and distributionpattern had been suggested by a cooling towerconsultant, John Cooper, but had resulted in a de-creased efficiency of 86% after the first modifica-tion and 79% following the second modification.To control the distribution of the water in the tower,over 10,000 nozzles were used and nozzle sizesvary in diameter, allowing more or less water to fallin certain areas of the tower. The figures at rightshow the placement of nozzles of different diam-eter during 1989-1998.

The outlet water temperature from the coolingtower can be affected by the spray nozzle configu-ration inside the tower. The diameter of the nozzlescan be changed to affect where water is releasedin the tower and plays a vital part in the performanceof the tower. Selection of the appropriate nozzlesizes and their placement in the tower, or distribu-tion pattern, for maximum performance is at thecenter of Southern Company’s dilemma. Calcula-

tions indicate restoring the tower to 100% capabil-ity has a present worth value of $10.2 to $11.9 mil-lion (in 1989 dollars) from 80% of design capability.

During Sept. 1999, John Cooper made a newrecommendation for a further modification of thenozzle sizes in this tower, but it was unclear howeffective the change would be. Therefore, theSouthern Company had to make a decision: eitherchoose to revert to the previous nozzle configura-tion of the tower with guaranteed 91% efficiency,or try the new modification in an attempt to reachthe tower’s expected capability of 100%.

Interact: Summarize and restate the problem state-ment.

July 23, 1999. 7:27 A.M.

Patrick Conley was standing by the windowlooking out toward the cooling towers in the eastwhen Kerry Walton, Plant Vogtle’s performance en-gineer, rushed into the conference room. The sunwas peeking around the twin hyperbolic structuresand after two years as a project engineer at Plant


Vogtle, Patrick was still enchanted by this view.“I have some bad news, Patrick. We were able

to get the efficiency measurements for the 1998redesign only recently and it shows that the effi-ciency decreased from 86% in 1995 to 79% for the1998 modification. The best efficiency we had ob-tained on this unit was during 1990 when it reached91%. It is going to cost us a bundle until we comeup with a better design.”

Patrick’s day had suddenly taken a turn for theworse. A strict, detailed analysis had been con-ducted by John Cooper, a cooling tower consult-ant. The Southern Company had reviewed this in-formation and the modification was approved.

“Kerry, this isn’t the ‘good morning’ I was ex-pecting. Do we have any indications so far aboutthe problem’s source?”

Kerry now regretted his lack of cordiality, butthere was a major problem and cordiality had towait.

“Good morning,” Kerry retorted with a smirk. “Iapologize for my anxiety this morning. I know thatwe can’t rush out to the tower and correct this prob-lem today. After all, we have a planned shut downof each reactor only once every eighteen months.It may be more than seventeen months before wecan fix this.”

Robert Moye, the engineering supervisor,strolled into the conference room. He’s a tall man,though he enters standard doorways without duck-ing. Robert always seems quite relaxed. This of-ten perplexes his engineering team because of thepressure that supervisors can come under. ‘Givethe engineers some leeway to work,’ Robert be-lieves, ‘and the engineers will surprise you with theiroutput.’

“I have heard the news guys. What’s the re-port?” Robert calmly asked.

Kerry stepped forward. “The cooling tower forunit two is not performing efficiently. At this pointthe only things we are sure of are that efficiency isdown and we are losing money.”

Robert turned toward the window and gazedintently at the cooling towers. As relaxed and asbusiness-like as usual, he said, “We know the con-ditions. Let’s go locate the problem and find a so-lution.”

Interact: How could the modifications hurt thenuclear plant’s efficiency?

Role of Cooling Towersin a Power Plant

Conceptually, there are two ways to get morepower from an existing power plant: either producemore power by consuming fuel at a higher rate orreduce the amount of energy consumed by powerproduction. Many power plants operate at their

maximum capacity already, so the second alterna-tive becomes the only alternative. Furthermore, ata nuclear facility, the maximum energy producedfrom the nuclear reactions is capped both by reac-tor capacity and regulations set by the NuclearRegulatory Commission. Therefore, to improve thenet output of a nuclear plant, the efficiency of theplant must increase.

There are many systems in a power plant wherepower is consumed, as it requires a certain amountof power to operate the facility. In some instances,power is simply lost due to inefficiencies. Somesources of inefficiency include the pumps that areused to circulate water, the friction that exists invarious types of moving equipment and, heat en-ergy that is not completely converted to electricity.Some of the inefficiencies are inherent in the equip-ment and processes, but others can be improved.One example of an area that can be improved iswithin the circulating water cycle, specifically thecooling tower.

This study focuses on reducing the tempera-ture of the circulating water cycle. If this tempera-ture can be reduced through improved cooling towerperformance, it will result in improved condenserperformance by reducing the exhaust pressure onthe turbine. A reduction in turbine exhaust pres-sure, also called backpressure, increases theamount of work performed by each pound of steamand therefore increases the overall plant efficiency.By reducing the circulating water return tempera-ture by 2 degrees, it can be shown that plant willgain approximately 5 MW. (Figure 1)

Figure 1.


According to John Cooper, a cooling tower con-sultant, cooling tower performance is an importantaspect in plant efficiency for several reasons. Theprime reason relates to the negative pressure thatis desired for the exhaust side of the turbine. Tomake electricity, the thermal energy of the reactoris converted to mechanical energy, which turns theturbine. The turbine is directly coupled to the gen-erator, which produces electricity. By reducing theturbine exhaust pressure more energy can be ex-tracted from the steam cycle, which increases theplant efficiency. To improve the steam cycle effi-ciency the cooling tower efficiency must be opti-mized. A cooling tower is a heat exchanger, whichtransfers heat out of the condenser into the envi-ronment. A cooling tower can be thought of as awaste heat disposal device.

John Cooper continues, “At a time when en-ergy is at a premium some argue that somethinguseful should be done with this heat. With deregu-lation and the heightened competition between utili-ties, all power plants in the United States are pres-ently looking for ways to make power in a morecost effective way and the cooling tower is one ofthe first places that a lot of utilities are looking at toimprove plant efficiencies. A one or two degreedecrease in the cold water temperature can have avery significant impact on the electric utility’s abil-ity to compete in today’s market.”

Interact: Describe the basic necessity of a cool-ing tower at a power plant.

Describe how the cooling tower can im-prove plant performance.

Define enthalpy, temperature, pressure,power, BTU, and MW.

Cooling Tower Operation

In a pressurized water reactor (PWR) nuclearpower plant there are three distinct water cycles(Figure 2). These include: reactor water cycle (yel-low), turbine water cycle (white), and circulating(circ) water cycle (blue). The water, utilized for itshigh heat capacity and non-corrosiveness, is usedas a medium for transferring heat to different areasof the plant. The circulating water cycle is describedbelow since it is important in understanding thisstudy. The reactor water cycle and the turbine watercycle are described in Appendix A.

The main component of the Circulating WaterSystem (Figure 3) is the cooling tower, which is usedas a heat removal device, and can be categorizedas either mechanical draft or natural draft andcrossflow or counterflow. Mechanical draft coolingtowers use huge fans to force air across the hotwater to cool it. Natural draft cooling towers gen-erally are much larger than mechanical draft tow-

Figure 2.

ers and use the tendency of heated air to rise formoving air across the hot water. Counterflow tow-ers are designed so that the air path is generallyparallel with the path of the hot water, while crossflow towers are designed to pass air perpendicularto the flow of the hot water. The cycle requires alarge water source (Figure 2), but is contained pri-marily in a cyclical process. Some water, however,is lost due to evaporation and must be replenished.

At Plant Vogtle, a natural draft, counterflow cool-ing tower (Figures 3 and 4) is used to remove theheat from the circulating water system that was ab-sorbed from the turbine water cycle. The concretetower stands 541 feet tall and has a base diameterof 444 feet. The tower has its characteristic hyper-

Figure 3.


Figure 4. Figure 5.

bolic shape because of the shape’s unique abilityto support high wind loads (this is a crucial issuefor large structures) with less material than a cylin-drical tower. This shape also helps ensure the natu-ral draft effect is maximized. The water passes fromthe condenser to the cooling tower through a pipethat is about 14 feet in diameter (Figure 5, behind Patrick).

This pipe carries approximately 535,000 gallonsof water per minute. The hot water flows throughthis pipe underneath the cooling tower and splitsinto four risers, one of which is shown in Figure 6.

These risers are approximately 50 feet tall andeach riser feeds two horizontal concrete flumes thatspan the tower. Smaller pipes split out the sides ofthe flumes (Figure 7) to distribute the water to theareas in between the flumes.

Figure 6.

Figure 7.


The spray nozzles are connected to these pipes(Figure 8) and consist of a nozzle ring, nozzle, anda splashplate (Figure 9). The nozzle is insertedinto the nozzle ring from the top (Figure 10). Theconical shape of the nozzle combined with gravityand water pressure ensures that the nozzle staysin place within the nozzle ring.

For a tower of this size, the distribution of wa-ter is important. There can be areas where waterdoes not fall from the nozzles allowing air to flowby without cooling any water. There can also beareas where too little or too much water falls fromthe nozzles so that the heat transfer between thewater and air is not maximized. To control the dis-tribution of the water in the tower, over 10,000nozzles are used and nozzle sizes are varied indiameter, allowing more or less water to fall in cer-tain areas of the tower.

Figure 9.

Figure 8.

Figure 10.


The splashplate aids in the effective distribu-tion of water. As water pours out of the nozzle andimpacts the splashplate, the water splatters in theform of a hollow cone leaving an area without wa-ter directly beneath the splashplate. Thesplashplate breaks up the water flow, creating aspray of water particles, thereby increasing the to-tal surface area available for heat transfer. In aneffort to utilize the full area of the tower, the nozzlesare placed (Figure 11) so that the cones of wateroverlap each other to reach the areas beneath theneighboring nozzles. To prevent the nozzle ringsfrom detaching due to water pressure (once a com-mon occurrence), the piping was retrofitted withlarge hose clamps (Figure 11).

After the water splatters against the splashplate,the water particles fall onto the fill matrices visiblebeneath the nozzles in Figure 11. The fill is madeof PVC (Figure 12) and provides many surfaces forthe water to flow along and many cavities for air topass through. Since heat transfer is heavily de-pendent on surface exposure, or surface area, themore surfaces for water to travel on, the better theheat transfer. This is why there are several layersof fill (as much as 9 feet thick in areas) in most ofthe tower. The fill also slows the water’s descent,allowing more time for the water and air to exchangeheat, affecting heat transfer also. Much of the heatis removed through partial evaporation of this wa-ter. Although this evaporative process removes80% percent of the heat, only about 3% of the wa-ter actually evaporates.

As water flows over the fill at a rate of 535,000gallons per minute, the structure becomes veryheavy. To support this weight, a massive supportstructure (Figures 13, 14) for the fill must be pro-vided underneath the tower. Large concrete col-umns are necessary throughout the tower for struc-tural stability. Occasionally, deterioration developsin the support structure and part of the fill falls andmust be replaced (Figure 15).

Figure 13.

Figure 12.

Figure 11.

Figure 14.


John Cooper states, “The purpose of fill is firstto increase the hang time of the water inside thetower. Water droplets that are sprayed down hitthe surface of the fill media and it takes a finiteamount of time for the water to flow down throughthe matrix of surfaces. So it increases the timethat the water has in contact with the cooling airstream. The second purpose of the fill material isto spread the water out over as large of a surfacearea as possible. In the Plant Vogtle tower, thereare arrays of asbestos cement sheets, each 16inches tall, which are stacked one on top of theother. At the deepest point, there are 7 tiers of 16inch high sheets (thickness evident in Figure 16)that the water is sprayed on to give you as muchsurface area for contact between the air and thewater.”

Realizing that increased surface area meansgreater heat transfer, one might think that addingmany layers of fill would help this process. Thedrawback of too much fill is the increased air resis-tance and a tendency to stifle the airflow throughthe tower. This resistance to flow would reducethe cooling tower’s efficiency by eliminating thenatural draft effect. After water trickles over the fill,it falls into the basin of the tower. The region be-tween the fill and the basin is called the rainzone(Figure 17). The rainzone is the final opportunityfor the water to be significantly cooled by air in the

Figure 15.

Figure 16.

Figure 17.


cooling tower. Cooled water flows from the basininto the channel (Figure 18) that leads to the circu-lating water pumps. The water is filtered (Figure19) for large debris before it enters the two largepumps. These two large pumps (Figure 20) areresponsible for pumping the circulating waterthroughout its journey. Each 50 percent capacitypump will force half of the water into a manifoldunderground that unites the two streams. The uni-fied pipeline is about 14’ in diameter.

Figure 18.

Figure 19.

Figure 20.


Apparent in the channel picture (Figure 18), thewater used in the circulating water cycle appearsmurky like river water. This is because the water isdrawn as needed from the Savannah River to makeup water lost during the cycle (Figure 21). Thisleads to some accumulation of silt in the tower ba-sin that must be cleaned out (Figure 22) when theentire unit is shutdown for a refueling outage. Thenearby river provides a good source to replace thewater that is lost due to evaporation during the cool-ing process. This make-up water is pumped intothe circulating water cycle as needed using threelines (Figure 23) that simply dump the fresh waterinto the basin. The water level in the circulatingwater basin is maintained at a level between 32and 33 feet, or approximately 6.1 million gallons ofwater. Once in operation, approximately 22,000gallons per minute is used to maintain the appro-priate water level in the circulating water canal.

Tower performance is a figure that relates howwell the tower does its task of cooling water com-pared to what it is expected to do. In other words,the tower might be expected to cool the hot water20 degrees Fahrenheit, and the tower performanceis a reflection of how well this is achieved. To de-termine the expected performance of the coolingtower, many variables are used. The temperatureof the incoming hot water from the condenser, thedesired temperature of the cooled water leaving thetower, the mass flow rate of water, the size of thetower, and the typical climate for the area are themajor factors. In the design phase of the tower,the size and water distribution pattern is chosen tomaximize the cooling effect of the tower. Once thetower is constructed, the tower’s performance, orefficiency, is compared to the design expectations.In normal operation, the mass flow rate of the wa-ter and its incoming temperature varies little. How-ever, the temperature of the air and the humidity,or the amount of water vapor in the air, varies fromday to day and seasonally, which varies the perfor-mance of the tower day to day and seasonally.Therefore, since the weather plays such an impor-tant role in the tower’s efficiency, the design has tobe based on typical climate conditions for the area.Consequently, the tower may have days that it can-not cool the circulating water to the expected, anddesired, temperature, or there may be days thatthe tower, if configured properly, can exceed theexpected capability, yielding an efficiency of greaterthan 100 percent. (This is why cooling towers aredesignated with the term design efficiency.) Theclimate figures used in the design are typically fig-ures for the summer, when the load on the plant ishighest and conditions are hot and humid.

If the existing tower is not performing as de-signed, or expected, and assuming that the toweris properly sized to begin with, the primary way to

Figure 21.

Figure 22.

Figure 23.


try to improve performance of the tower is to modifythe water distribution pattern.

Interact: What are the benefits of natural draft-cooling towers over mechanical draft-cooling towers?

What is the purpose of fill matrices?

How can a cooling tower achieve over100% design performance?

What are the variables that affect towerperformance?

Intuitively, how should the water distribu-tion pattern in the tower be configured?

Why is the fill needed? How much fill isused? What is the purpose of the col-umns in Figure 16?

Why not pour hot water into the Savan-nah River?

Why use cooling towers and recirculatewater? Why not use direct feed from theriver?

Are there other additional cooling meansthat are used in other power plants todecrease the circulating water tempera-ture?

July 24, 1999. 12:12 p.m.

Kerry and Patrick often sit together at lunch andconversations range anywhere from football to chil-dren. Today, however, the tower’s decreased per-formance was still on their minds.

Picking up a french fry, Kerry mused, “I thoughtof the tower all night. I really don’t like to carrywork home with me, but when something puzzlesme, I can’t get it out of my head.”

“I know what you mean.” The cafeteria hadsomething different to choose from every day, butPatrick sat down with the routine, but surprisinglytasty, chicken fingers and fries. Patrick continued,“I thought about it myself for quite a while last night.I think what I would like to do is sit down and take alook at the cooling tower’s history up until this point.I hope that by writing down exactly what changeshave taken place, we can get a better feel of wherewe’ve been and where we are going.”

“That will be an interesting report. I have someof the old information at my desk I can share withyou. I’ve always heard that you learn more frommistakes than success, but we need to make surethat this is the last time this lesson is taught.”

Patrick’s History Report

The cooling tower for unit 2, manufactured byResearch Cottrell Inc., began operating in 1989when the second nuclear reactor went on-line forthe first time. At this point, tests were run to deter-mine how the towers were performing in relation tothe design specifications. The tower was perform-ing at only 76% of the expected performance. Thiswas not satisfactory to the Southern Company andin the words of John Cooper, “Georgia Power heldtheir tower designer’s feet to the fire after the tow-ers failed at the performance test.”

A document (Figures 24 and 25) was generatedby Russell Noble of Southern Company Services andpresented to R. J. Bush, a project engineer at PlantVogtle, on October 30, 1989. It outlined the expectedgains in useful power output by reclaiming losses dueto “tower malperformance.” A decision was made tomodify the tower water distribution pattern in an effortto reach the design performance.

Figure 24.


Figure 25.

Modification during 1990:In 1990, the tower manufacturer (now Ecodyne,

a subsidiary of Research Cottrell) came in with amodification idea that created more nozzle zonesin an effort to more precisely distribute volumes ofwater according to their distance from the center.Five nozzle sizes were used in this tower arrange-ment. The fine-tuning of the water distribution wasexpected to lead to a more even air resistance anda more uniform cooling pattern across the towerarea. This would result in a lower temperature forthe circulating water prior to its return to the plant.This modification resulted in an improvement intower efficiency. An efficiency of 91% wasachieved.

Modifications during 1995:Further improvements could be made. To ob-

tain increased performance, John Cooper was con-tracted as a consultant in 1995. He measured thetemperatures and air velocities at about 1000 pointsinside the operating tower. He used a computer tocreate color mappings (Figures 26, 27) to try anddetermine why the tower was underperforming thedesign. From John’s temperature and velocitymappings, he determined that the air was too coldin the center of the tower, thereby indicating un-even cooling in the perimeter region. In an inter-view, John Cooper stated, “my upgrade design wasan extrapolation of their [Research Cottrell] design.”This design also used five nozzle sizes in a similardistribution pattern. However, all of the nozzles ineach zone were larger in an attempt to shift morewater toward the center. This modification resultedin a tower efficiency of 86%. This value was lessthan the previous scenario.

Modifications during 1998:Obviously this modification was still better than

the original design of the tower, but was a reduc-tion from the previous layout and led to a degrada-tion in the plant output. In 1998, the tower wasagain modified and smaller nozzles were placed inthe center of the tower to force the water flow biasback toward the perimeter. The tests which fol-lowed this modification indicated that the tower haddecreased in performance yet again to 79%.

Thus we arrive at the present situation. Obvi-ously there are some unknowns (John Cooper evenrefers to it as partially an ‘art’) in dealing with cool-ing tower setup, and in this case the expected re-sults have failed to materialize. To graphically de-pict the modifications as they have occurred, thefollowing diagram has been included (Figure 28).The numbers in Figure 28 represent nozzle sizesin the different regions of the tower. The differentregions that are created by various nozzle sizes,denoted with diameter measurements, were an at-tempt to configure the flow pattern to improve effi-

Figure 26.

Figure 27.


ciency. John Cooper’s new recommendation is in-cluded at the bottom of Figure 28 and will be dis-cussed later. Within the parentheses, the efficiencythat was achieved with each modification is in-cluded.

Interact: How has performance responded tothe gradual zones of nozzle sizes?What do the patterns in the temperatureand air velocity mappings tell you aboutthe tower?

July 30, 1999. 3:26 p.m.

“I talked to John Cooper and he has come backto us with a new modification for the tower. Heregrets that he was mistaken in his last modifica-tion, but wants to make it right, if we will allow him,”Patrick began. “He says he knows where his analy-sis went wrong and is willing to provide the infor-mation for the new modification free of charge.”

Robert leaned back in his chair a little. “Well Ishould hope so. It has cost us lots of money sincethe modification. It will cost us lots more if we don’tcorrect it. Has anyone done an analysis on theamount of money we are losing due to the lastmodification?”

Kerry flipped through some papers. “With thetower performing at 79% efficiency, we are about 5MW deficient based on our tower design expecta-tions.”

Patrick interjected, “I have been able to arrangea meeting with John Cooper to get more detailsabout his opinion on what the error was and hisnew design. He will meet with us in two days.”

Robert stood up. “In the meantime, to refreshour minds, let’s look at how much a modificationlike this costs.”

Kerry’s Report on TowerModification Costs

Beginning with the tower modification in 1995,the Minor Design Change (MDC) No. 97-V2M047lists that the material costs associated with thechange was $116,000. The installation cost wasabout $50,000. This modification included adding828 new nozzle locations in the tower for better dis-tribution of water. It also included adding more fillin some regions and 8 stainless steel flow diverters.These items will not be repeated in subsequentmodifications.

It can be assumed that since the labor costsare in-house, that those costs remained about thesame as in 1998. Several aspects of the modifica-tion were not carried out in 1998, therefore reduc-ing the material costs somewhat. The total cost ofthe 1998 modification was $90,000.

Figure 28.

August 1, 1999. 1:08 p.m. : TheMeeting with John Cooper

Patrick, Kerry, and Robert entered the smallconference room to meet with John Cooper. Johnseemed quite relaxed considering the potential forharsh criticism for previous miscues.

Patrick began the meeting. “John, thank youfor coming in to meet with us about this. We arelooking forward to hearing your opinion about ourproblem, both what created it and how we can fix it.”

John smiled. He had known this audience forsome time and expected calm behavior, but wasunsure if that was what he would receive this time.After all, it was his recommendation before that hurtthe tower’s, and therefore the plant’s, efficiency. “Ishould be thanking you for allowing me this oppor-tunity to come back here in light of the present situ-


ation. I will begin by talking some about my previ-ous approach. I will also talk about what I think theproblem is and how I plan to fix it.

“The problem with the Vogtle tower and all ofthese Belgian-designed towers that I know of, andI’m pretty familiar with most of them, is that all ofthem fail to meet their original performance-guar-anteed levels because of water distribution prob-lems. It’s not that the towers are too small or nottall enough, it’s just that there is not sufficient de-tail, not enough attention was paid to detail for thedesign of the distribution systems.

“Consequently, Georgia Power held their towerdesigner’s feet to the fire after the towers failed atthe performance test. The tower manufacturerspent over two million dollars out of their own pock-ets to try to upgrade their towers. They success-fully raised their design performance from about76% (1989) to about 91% (1990), which left themabout a degree and a half deficient even after theirbest efforts.

“After the Plant Vogtle tower had been modi-fied by the original designer of the tower, the hy-draulics were pretty fouled up. My upgrade designis an extrapolation of their design. Although I didall the things that I’ve done in all my other upgradepackages, I didn’t undo what the manufacturer haddone to the tower. So my design didn’t work. Ittook some thought and additional analysis for meto understand why my upgrade did not work for thiscooling tower. Once I realized what I had done, itwas a simple matter of correcting it.

“One of the things that the tower manufactur-ers did in the Plant Vogtle towers was to bias thewater loading towards the perimeter of the tower.I’ve seen this done before. It is not an unusual thingto do. The coolest air in the cooling tower is in theperimeter. Air that goes through the center of thetower is warm because it’s preheated by flowingthrough all the rain zone of the fill. The tower de-signer biased the water loading to the extreme how-ever. In the process, they raised the static headover the water distribution system by about 8inches, which caused some problems in the riserpipes.

“Excessively high riser water levels didn’t existuntil after the upgrade mods were put in the towerby the tower designer in 1989. In fact, that highwater level was created when they reduced thenozzle size of the interior of the tower. Originally,all of the nozzles in the interior of the tower were1.33 inches in diameter. The tower manufacturerreduced them all to 1.09 inches in their modifica-tion. My exit air mappings back in 1995 showedthat the entire interior of the tower was cold. I hadassumed that was by virtue of the fact that all ofthe interior nozzles were reduced in size. But, Iknow now that’s only partially true. The riser waterlevel is high because of the back pressure that’s

put on the riser pipes at the flume-riser interface.“The cold air that I saw in the mappings (Fig-

ures 29, 30) in the interior of the tower was largelycaused by the presence of large eddies interferingwith the normal flow of water between the risersand the flume. The flow vectors, or the direction of

Figure 29.

Figure30.


flow, were no longer such that water flowed fromthe riser, making a gradual bend of 90 degrees inthe flumes (Figure 31), but instead water in the riserwas flowing upward, bypassing the entrance to theflume, then bending down in a circular motion (Fig-ure 32). It is a vertical eddy in front of the flumeface. There is a significant head loss associatedwith that flume and riser flow pattern. Once I real-ized that was the root cause of the hydraulics prob-lem, it’s a simple matter for me to formulate a cor-rective action to eliminate the turbulent flow at theriser-flume interface.

“When I first approached the Vogtle coolingtower performance problem, I noted that the riserwater levels were excessively high. Generally thesewater levels should be only two inches, but withrespect to walkway elevations, the water levels ofthe Vogtle towers were almost five feet above that.Unlike all the other upgrade designs I’ve been in-volved in, rather than trying to maintain the exist-ing water levels, I tried, along with redistributing thewater, to reduce the water level in the riser. Thatwas a critical mistake. By attempting to reduce thewater level by a foot, I, in effect, reduced the statichead of all the nozzles by a foot. That put the statichead in the water distribution system at a level thatwas lower than it was in the original design. Afteradding over a thousand nozzles to the original de-sign, it created water distribution problems at theperimeter of the tower and at the center of tower.Some nozzles in the longer pipes had no waterwhatsoever coming out of them. So there was notenough static head to reach all of the lateral distri-bution pipes.

“As I say, it took me a while to understand whereI went wrong. After modeling the original designand discovering that the tower manufacturer hadraised the static head by 8 inches, it was clear tome that what needed to be done was the originalwater level needed to be reestablished. The ex-treme bias of the water from the center of the towerto the perimeter of the tower had to be dissolved.In other words, the nozzle sizes had to be varied torelease more water in the center of the tower andless at the perimeter.

“I am currently working on a computer model ofthe Vogtle tower to consider both the water distri-bution and the need to lower the riser water level. Ifeel quite certain that the modified computer pro-gram will produce results better than this facility hasseen. Once the model has been verified, I will gen-erate a report citing the history we have discussedhere today and the problems I believe to exist inthe tower. It will also include my suggestion formodification of the cooling tower.”

Figure 31.

Figure 32.

Interact: What does John Cooper consider theproblem with the Vogtle cooling

towers?

Intuitively, how might high riser waterlevels affect the tower’s performance?

How might the riser-flume interfacebe configured differently to reduceturbulence?


Kerry’s Memo on the Cost Impact ofCooling Tower Malperformance

For a tower operating at 80%, the cold watertemperature exiting the tower is about 2.5 to 3 de-grees too hot. This translates to 5-7 MW of powerin the summertime. With the loads that the plantnormally has in the summer, every Megawattcounts. We have had to start up our peaking plantsto supply additional power during the summers andthis power costs the company more to produce thanit would at Plant Vogtle. For the sake of compari-son, according to the Georgia Power website, thecost of power at a nuclear plant per Kilowatt-hour(KWh) is $0.005. The cost of power per KWh at anatural gas or diesel plant, like our peaking plants,is between $0.0417 and $0.0466. This shows theimportance of producing as much of the neededpower as possible at a base load plant like Vogtle.Furthermore, if our base plants in conjunction withour peaking plants cannot supply the needs of thepower grid, we have to buy power from our com-petitors. This power may cost as much as $1.00per KWh.

Therefore, in extreme demand periods we arefaced with two costs: the unrealized profits for inef-ficiency and the cost of buying from our neighbors,either internal or external to the Southern Company.

For a plant producing 1200 MW, a few extraMegawatts of power seems insignificant. However,this incremental saving extended over the life ofthe facility, which is expected to be another 30 yearsor so, is quite substantial.

Interact: Compute potential cost saving per yearfor 1% improvement in efficiency.

September 21, 1999. 7:45 a.m.John Cooper’s 1999 Recommendation

“Thermal performance testing of Plant Vogtle Unit 2cooling tower carried out in July 1999 proved conclu-sively that upgrade design modifications developed byJohn Cooper and Associates, P.A. for Georgia PowerCompany in 1995 and in 1998 have resulted in a reduc-tion in cooling tower thermal performance efficiency.” —John Cooper, Technical Report No. JCA-GPC-1099082,August 23, 1999

Patrick had been eagerly awaiting the reportfrom John Cooper for several days and was ex-cited to hear that it had been delivered late yester-day afternoon. Everyone had been wondering ex-actly what John’s recommendation would be thistime for the tower. Patrick took the time to siftthrough the report and found that much of the pre-liminary material was similar to the August 1stmeeting’s discussion, only with more numbers. Ashe further perused the document, Patrick came tothe new recommendation and some new tower

mappings. Since the team was meeting at 10:00this morning anyway, Patrick decided to make him-self familiar with the report.

1999 Cooling Tower MappingsPatrick looked carefully at the color mappings

(Figures 29, 30) of the cooling tower. At first glance,there was an obvious difference between the tem-peratures (about 15-20 degrees) at the center ofthe tower and at the perimeter. This told Patrickthat the air reaching the center of the tower had aneasy path out of the tower, never fulfilling its pur-pose to cool the hot water. Increasing the flow ofwarm water in the tower’s center should block thispath of least resistance that the air was taking.

The velocity mapping further confirmed whatwas occurring in the tower. The blue regions dem-onstrated that cold air was rushing through at a highvelocity, making the center part of the tower lessefficient. From the data shown graphically in Fig-ures 29 and 30, Patrick realized that the towerneeded more water flow in the center, but not somuch as to make the perimeter ineffective. Thisadditional water would provide more resistance tothe cold air rushing through the center, increasingcontact time between the water and the air. Theideal temperature mapping would show a uniformtemperature, i.e. a single color.

10:00 a.m.“I have had an opportunity to look over the re-

port,” Patrick began. “I think we should all look at itin detail later. For now, I will just highlight someexcerpts about the actual tower modification he sug-gests.”

“In order for the Vogtle Unit 2 cooling tower tobe upgraded to 100%, or better, thermal perfor-mance capability, it is imperative that the head ver-sus flow characteristics of the cooling tower waterdistribution system be restored to the original set-ting. This will eliminate the excessively high riserwater levels in the cooling tower and the associ-ated abnormal water flow patterns at the flume-riserinterface points. A computer hydraulic model of theoriginal Vogtle cooling tower design shows that theaverage effective flume water elevation required toproduce the original measured circulating water flowrate value of 535,200 gallons per minute (gpm) is51.69 feet.” (Cooper Report, August 23, 1999, p.11-12)

“If we completely eliminate the outward biasingof water flow to the perimeter of the cooling towerby using a single nozzle orifice size (Figure 28)throughout the cooling tower and select a nozzleorifice size that maintains our anchor point hydrau-lic conditions, i.e., 535,000 gpm at effective flumewater elevation of 51.69 feet, then our thermal per-formance enhancement goals should be achieved.”(Cooper Report, August 23, 1999, p.12)


Robert smiled. He wondered if this was reallythe solution to the cooling tower problem. “I wouldlike for everyone here, including myself, to becomefamiliar with as much of the cooling tower story aspossible including this new report. Kerry, could youplease look at the potential risks involved with tak-ing on this project. Patrick, could you make somephone calls and find out how much money we’retalking about to do this modification, parts and la-bor. Feed these numbers to Kerry for his financialassessment of this change.”

Interact: How does the suggested water distribu-tion configuration compare to previousconfigurations?

Financial Assessmentfor the Modification

Patrick walked into Kerry’s office and placed afax down on his desk. “Kerry, I’ve gathered thenumbers that you need for the financial analysis ofthe modification. I’ve talked to maintenance andthey’ve estimated the labor costs to be about$40,000 for this change. I also talked to Mid-SouthNuclear and this is the quote for the nozzles neededaccording to the report.”

Kerry thumbed through some files to find theminor design change (Appendix B) for the coolingtower modification back in 1995. He hoped to findthe complete analysis, but was unsure if it was in-cluded in this document. The front page of the MDClisted the savings for the plant as about $3 millionover the plant’s lifetime of at least 32 more years.

“The benefit gained from modifying the towerin 1995 was expected to be $3 million over the life-time compared to no benefit if the tower was notmodified,” Kerry began. “Similar analyses can bedone for the new proposed modification. Note thatthe benefit is based on expected gains in perfor-mance, and since 1995, we haven’t realized those.Future analyses for modifications of the coolingtower should possibly consider the benefits of par-tial success.”

September 27, 1999. 9:45 a.m. : RiskDiscussion and Decision Point

Robert had been thinking for some time aboutthis project. There were two options to choose.Robert had asked Patrick and Kerry to present thetwo alternatives.

Patrick presented Option 1. “The first option isthe safe option. This is simply to revert to the modi-fication that produced a tower efficiency of 91%during 1990. This modification is 100% reliable.The configuration is known to us and will requirethe labor to implement, but little material cost sincethe old nozzles were stored.”

Kerry presented Option 2. “The second optionis to modify according to John Cooper’s new rec-ommendation to implement a single nozzle orificesize. This option may result in one of three out-comes: an increase in performance, a decrease inperformance, or no change. The likelihood of thesethree outcomes is unknown.”

Kerry continued, “To simply accept the tower’soperation below design performance for option 1seems passive. Higher demand on the power in-dustry from both competition and consumptionmakes this option impractical from a financial per-spective. We know the payoff if the modification issuccessful.”

Patrick interjected, “How many times can wespend the money trying this? 91% is a solid per-formance number. Don’t get me wrong, I like JohnCooper, but whether or not he knows what he’s talk-ing about is a different story altogether. We haveheard of his success, but we have also seen hismodifications not only fail to improve our tower, butactually hurt its performance. Twice.”

Robert entered the discussion. “We know therisk is minimal if we choose Option 1, but we alsoknow its limitations. We also know there is morerisk in choosing Option 2 and based on the trackrecord we have here, that’s scary. However, thepotential payoff is greater than Option 1. A goodquestion is how to assess the risk involved here,the reliability of John Cooper’s ideas, and the like-lihood he’s wrong for a third time. This is critical tothe decision and I have no idea how to get a handleon it.”


Glossary

backpressure – see exhaust pressure.

basin – the reservoir beneath the cooling towerwhere cooled water collects and flows to the pumpsfor recirculation through the condenser.

circulating (circ) water cycle – the water cycle thatis responsible for removing the heat from the tur-bine water cycle. This cycle flows through the cool-ing tower and condenser.

control stage – initial set of turbine blades that con-trol the amount of steam admitted to the turbine.

cooling tower – a large heat exchange device uti-lizing air and evaporative properties of water to re-move heat from the water and place it in the atmo-sphere.

counterflow cooling tower – a cooling tower in whichthe air path is the opposite of the water path duringheat exchange.

crossflow cooling tower – a cooling tower in whichthe air path flows perpendicular to the water pathduring heat exchange.

deregulation – the recent governmental change thatopened the power industry to competition from otherpower producers.

electric heated pressurizer - a tank or vessel thatacts as a head tank (or surge volume) to controlthe pressure in a pressurized water reactor.

exhaust pressure – (also backpressure) the pres-sure on the condenser side of the turbine that re-sults in less work output per pound of steam fedinto the turbine. Reducing this pressure has a posi-tive effect on power output.

feedwater - water supplied to the reactor pressurevessel (in a Boiling Water Reactor) or the steamgenerator (in a Pressurized Water Reactor) thatremoves heat from the reactor fuel rods by boilingand becoming steam. The steam becomes the driv-ing force for the plant turbine generator.

fill matrix – a volume of many different surfaces usedto increase the contact surface area between theair and water for maximum heat transfer. This as-bestos or plastic matrix also increases the durationof the water’s descent for greater heat transfer.

flumes – in the Plant Vogtle towers, horizontal, rect-angular concrete pipes that carry water from therisers to the lateral distribution pipes. These con-crete pipes also serve as walkways inside the cool-ing tower.

generator – a device utilizing the principles of in-duced current (rotating a magnet inside a coil ofwire to produce an electric current) to producepower for distribution to customers. The generatorrotor is rotated by its connection to the turbine ro-

tor, which is driven by steam pressure.

governor valves - hydraulically controlled valves thatcontrol the admission of steam to the turbine.

heat exchanger – a device that passes two fluidsby each other to cool one stream and heat the other.The streams may or may not mix. A heat exchangerthat mixes streams is found in the Plant Vogtle cool-ing towers. A heat exchanger that does not mixstreams acts as the condenser at Plant Vogtle.

hotwell - attached to the bottom of the condenser,collects water as it is condensed from the turbineexhaust.

hyperbolic – refers to a shape that follows the gen-eral function, x=y2. This shape has some uniquestrength properties and it is used commonly in largenatural draft cooling towers to gain more strengthusing less materials.

main and auxiliary feedpumps - pumps that areused to raise the feedwater pressure before it en-ters the steam generator.

make-up water – the stream of water added to thecirculating water cycle to replace the amount thatis lost due to evaporation during cooling tower op-eration.

manifold – a union of multiple pipes typically re-sulting in a larger single pipe.

mechanical draft cooling tower – a cooling towerthat uses fans to force air through the tower to coolwater.

natural draft cooling tower – a cooling tower thatutilizes the principles of gas laws to cause air flowthrough the tower. More specifically, hot air risesdue to lower density, so as air enters the coolingtower and is warmed, it has a tendency to rise, simi-lar to a chimney effect, thereby requiring no exter-nal input to force air flow.

nozzle – in Plant Vogtle towers, the plastic orificethat sprays water onto the splashplate. The orificesize can be varied to distribute water in certain pat-terns across the tower.

nozzle ring – the circular plastic component thatholds the nozzle and splashplate in place. It is con-nected to the lateral distribution pipes.

nuclear reactions – in a nuclear power plant, thisinvolves bombarding uranium atoms with neutronsin order to split the atoms, a process called fission,which releases large amounts of heat. This heat isused to drive the power cycle at a nuclear plant.

Nuclear Regulatory Commission — Federal Agencythat oversees all nuclear related activities through-out the United States. See http://www.nrc.gov TheNuclear Regulatory Commission (NRC), part of ourgovernment, makes sure nuclear power plants in


the United States protect public health and safetyand the environment. The NRC licenses the use ofnuclear material and inspects users to make surethey follow the rules for safety.

pressurized water reactor (PWR) - a reactor inwhich heat is transferred from the core to an ex-changer by high temperature water kept under highpressure in the primary system. Steam is gener-ated in a secondary circuit. Many reactors produc-ing electric power are pressurized water reactors.

rainzone – the region between the fill matrices andthe basin where the water is in freefall. The regionhas an appearance similar to rain. This is the finalsignificant opportunity for heat transfer in the cir-culating water cycle.

reaction blade stages – the turbine stages wherethe design of the turbine blades equally divides thetotal pressure drop between the stationary and ro-tating blades.

reactor vessel - cylindrical vessel at the heart ofthe nuclear reactor with a hemispherical bottom anda flanged and gasketed, removable upper head.The vessel contains the core (fuel cells), core sup-port structures, control rod clusters, thermal shield,and other parts directly associated with the core.

reactor water cycle – the highly purified water cyclethat is used to transfer the immense amount of heatfrom the nuclear reactions to the turbine water cyclefor power production.

regenerative feedwater heating cycle - series ofheat exchangers that preheats the feedwater be-fore returning it back to the steam generators. Usedto improve efficiency.

reheat steam - steam that passes from the highpressure turbine through the moisture separatorreheater before entering the low pressure turbine.

riser – the large vertical pipe that carries water fromthe base of the cooling tower to the distribution level.

riser water level – the level that water is pumped toin the vertical riser pipe. According to John Coo-per, the riser water level should be about 1 footabove the flumes. At Plant Vogtle, the riser levelshave been about 5 feet above the flumes.

splashplate – a surface for the nozzle to splatterwater on to produce more water particles, therebyincreasing the surface area of the water stream andincreasing the resulting heat transfer. Thesplashplate creates a hollow cone pattern beneathitself, requiring overlap of cones from neighboringspray nozzles.

spray nozzle – consists of the nozzle ring, nozzle, andsplashplate and is connected to the lateral distributionpipes in the cooling tower. Constructed of plastic.static head – the vertical distance in feet between

the free level of the source of supply and the pointof free discharge or to the level of the free surfaceof discharge. At Vogtle, this refers approximatelyto the distance vertically from the pump level to theheight in the risers that water reaches. Some erroris involved, as it is unknown here how far beneathpump level the water travels.

steam generator - the heat exchanger used in somereactor designs to transfer heat from the primary(reactor coolant) system to the secondary (steam)system. This design permits heat exchange withlittle or no contamination of the secondary systemequipment.

steam jet air ejector - used to remove air and non-condensable gases from the condenser. It consistsof a suction chamber, diffuser, and steam nozzle.

turbine water cycle – the water cycle that is turnedto steam in the steam generator. This steam is usedto spin the turbine rotor and, in turn, spin the gen-erator to produce power. The cycle continuesthrough the condenser for cooling by the circulat-ing water cycle.

turbine shaft steam seals - uses low pressure steam(4 psi) to seal the turbine openings where the rotorpenetrates the housing. Necessary to keep air leak-age to a minimum.

Bibliography

Baker, Donald. Cooling Tower Performance. NewYork: Chemical, 1984.

Beiser, Arthur. Physics. 5th Ed. Reading, MA:Addison-Wesley, 1991.

Cengel, Yunus A., and Michael A. Boles. Thermo-dynamics: An Engineering Approach. 2nd Ed. NewYork: McGraw-Hill, 1994.

Li, Kam W., and A. Paul Priddy. Power Plant Sys-tem Design. New York: John Wiley & Sons, 1985.

Willenbrock, Jack H., and H. Randolph Thomas.Planning, Engineering, and Construction of Elec-tric Power Generation Facilities. New York: JohnWiley & Sons, 1980.


Reactor Water Cycle:The reactor water cycle consists of a reactor

and four closed reactor coolant loops connected inparallel to the reactor vessel, each loop containinga reactor coolant pump and a steam generator. Thereactor water cycle also contains an electric heatedpressurizer and other auxiliary equipment. Thereactor core at Plant Vogtle is capable of produc-ing 3565 MWt (Megawatts thermal).

High pressure reactor coolant (highly purifiedwater) circulates through the reactor core to removethe heat generated by the nuclear chain reaction.The heated water exits from the reactor vessel andpasses via the coolant loop piping to the steamgenerators. Here it gives up its heat to thefeedwater to generate steam for the turbine gen-erator. The cycle is completed when the water ispumped back to the reactor vessel. The entireReactor Coolant System is composed of leaktightcomponents to ensure that all radioactivity is con-fined to the system.

Turbine Water Cycle:The Westinghouse PWR system (used at Plant

Vogtle) utilizes saturated steam for transportingthermal energy from the steam generators to theturbine, where it is converted to mechanical andthen electrical energy. Energy is conserved as thesteam expands through the nozzles and blades ofthe turbine on its way to the condenser.

The main steam system transports the steamfrom the outlet of the steam generators to the vari-ous system components throughout the turbinebuilding. The steam is used for operational auxil-iaries such as turbine shaft steam seals, drivingmain and auxiliary feedpumps and steam jet airejectors. The principal purpose of the steam is todrive the main turbine and provide reheat steam.

Steam admission to the double flow, high pres-sure turbine is controlled by governor valves andcan be quickly isolated by quick acting stop valvesin an emergency. These valves are not an integralpart of the turbine. Four separate pipes direct thesteam from the governor valves to the nozzle cham-bers. Thermal energy is converted to mechanicalenergy by expansion through a control stage and anumber of reaction blade stages within the turbine.On leaving the last row of high pressure blades,the steam has a moisture content of approximately10 percent. The steam is passed through a mois-ture separator and reheater to improve the turbineefficiency, reduce the low pressure turbine exhaustmoisture, and reduce maintenance on the low pres-sure blades of the turbine rotor.

The moisture separator and reheater arehoused in one pressure vessel. Wet steam entersthe vessel and passes through the moisture sepa-

rator where the moisture is removed and drainedto a heater drain tank. The steam rises above themoisture separator, passes over the tube bundleand is reheated by high pressure and high tem-perature steam. The hot reheat steam is conveyedto the double flow, low pressure turbines. Thesteam expands across the turbine blades and isexhausted into the condenser.

The condenser is a large heat exchanger thathas steam exhaust on the shell side and circulat-ing water on the tube side. As the steam hits thecool tubes it is condensed into water. The con-densed water (condensate) is collected in the bot-tom of the condenser hotwell. The condensate andfeedwater system returns the condensed steamfrom the turbine condenser and drains from the re-generative feedwater heating cycle to the steamgenerators, while maintaining the water inventoriesthroughout the cycle.

Appendix A: Description of the Reactor and Turbine Water Cycles


Appendix B: Minor Design Change

in hot water: a cooling tower case study

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