hvac chiller_01

7/27/2019 HVAC Chiller_01

1/5

S Y N O P S I SWith the current and future phase-

outs of ozone-depleting refrigerants,chiller replacement is on the minds ofmany building owners and managers.When considering any change in yourchilled water system, you have a goldenopportunity to examine the system as awhole. Many chiller plants are oversizedor not configured

to operate effi-ciently at partload conditions.New technologiesand equipmentcan make significant improvements.When you look at total cost of owner-ship, these improvements can save youconsiderable money, as well as increasethe reliability of your system. But tomake the best choices, you must under-stand how a variety of factors influencechiller plant efficiency.

Managing a chilled water plant is acomplex task. There are many factorsbesides the chiller itself to consider whenlooking at efficiency. Auxiliary equip-ment, such as pumps and cooling towers,affect and are affected by chiller opera-tion. Together these componentscomprise your buildings cooling system.When evaluating or replacing any part ofthe system, it is critical to consider itsinteraction with the other components.Like instruments in an orchestra, individ-ual capability is only a starting point

the key factor is how the instrumentcombines with all the others to producea symphony.

In addition to combined equipmentcapacity, many other factors influence thesystems ability to meet your buildingscooling load efficiently. Such things asweather patterns, number and type of

operating hours,

electric rates andbuilding useshould all beconsidered whenselecting

or retrofitting a chilled water system.And since more than 80% of all

chilled water installations are multiplechiller systems, it is particularly impor-tant to understand the interactionbetween the chillers themselves. Basingchiller selection on single-machine per-formance is a mistake that often leads to

misapplication, which can be costly inboth the short and the long term.

Since the components of a chilledwater system do not operate by them-selves, it does not make sense to evaluatethem this way. Yet, that is exactly whatis typically done. Each component isspecified as if it were operating in avacuum. Not only that, but when speci-fying chillers, designers usually use aformula built on standardized conditionsthat do not reflect the actual operationof the building.

Optimizing Chiller Plant

Efficiency: Factors to Consider

VOLUME THREE NUMBER ONE

A N H V A C N E W S L E T T E R F O R B U I L D I N G O W N E R S A N D M A N A G E R S

Basing chiller selectiononsingle-machineperformance is a mistake that oftenleads

to misapplication, whichcanbe costly in

boththe short and the long term.

Want to know more about opt imizingchiller plant efficiency?Call 1 .8 00 .CARRIER and request thepublication HVAC Analysis, Vol. 3, No. 1,

or visit our website atwww.carrier-commercial.com

In this issue...The Importance of Whole-System

EvaluationConsidering how components worktogether will give the most accuratepicture of your buildings chilledwater system . . . . . . . . . . . . . .1

IPLV and Its ShortcomingsWhy the IPLV formula will notaccurately represent yo urinstallation . . . . . . . . . . . . . . .2

HowWeather Affects

Efficiency . . . . . . . . . . . . . . 2 HowBuilding Loads Vary . . . 3

Multiple Chiller SystemsSince most installations use morethan one chiller, it s important tounderstand the operation of multiplechiller systems . . . . . . . . . . . . .3

The Importance of CondenserWater Temperature . . . . . . . 4

Auxiliary EquipmentThe role of cooling towers andpumps in part load efficiency . . . . .4

More Efficiency GainsCheck out these suggestions forincreasing your chilled watersystems efficiency . . . . . . . . . . .5


2/5

The IPLV FormulaIn December of 1998, the American

Refrigeration Institute (ARI) released a

revised standard for water cooled chillers ARI 550/590-1998. One of the majorchanges in the standard was made to theIntegrated Part Load Value formula, orIPLV. The IPLV is a calculation of pre-dicted chiller efficiency at the ARIStandard Rating Point an estimate ofhow efficiently a chiller will operate atpart load conditions, based on averagecriteria dictated by the standard.

The revisions to the IPLV equationwere designed to make it a more accuraterepresentation of actual operating condi-

tions, such as geographic locations andbuilding types. However, because themany assumptions in the formula cannotexactly match any one particular chillerinstallation, using it will not create a real-istic picture of your building. Using thesame formula for chiller systems in Alaskaand Miami and every place in between cannot produce a very specific result.

The IPLV rating was meant to be arelative guide to single component effi-ciency, not systemefficiency. ARI itselfrecognizes this and recommends per-

forming a comprehensive analysis thatreflects:

actual weather data; building load characteristics; number of chillers; operational hours; economizer capabilities; and energy drawn fromauxiliaries

such as pumps and coolingtowers

when calculating the overall chillerplant efficiency. This becomes increas-ingly important with multiple chillersystems because individual chillers oper-ating within multiple chiller systems aremore heavily loaded than single chillerswithin single chiller systems.

Given that many factors affect theefficiency of a chilled water system, letstake a look at the part each of thesefactors plays.

WeatherThe IPLV formula assumes weather

data for an average U.S. city. Sinceclimate varies considerably in the U.S.and even more around the globe, any for-mula attempting to gather these into anaverage is bound to be inaccurate. Yetaccuracy is important, because weatheraffects several aspects of a chilled watersystem. It is important to remember,however, that there is not a direct corre-lation between outside dry bulb tempera-

ture and building load. Specifying equip-ment based solely on temperature bindata is common, but will not provide acomplete picture of your buildings oper-ating environment.

Run Hours

In colder climates, such as New York,Chicago or Beijing, there is less need forcooling, and many hours are spent withthe building running at part load opera-tion. The chillers may even be shut offfor four or five months out of a year due

to the lack of load, or the system may useeconomizers. In multiple chiller systems,if there is a load, the operator will proba-bly turn all but one of the machines off,and run that one fully loaded.

In contrast, weather in warmer cities,such as Miami, Houston or Singapore,may create year-round system load.Cooling is always required, and the useof economizers may be impractical. Sothe number of run hours would bemuch higher.

Number of Machines RunningIn any multiple chiller system, chillers

spend most of the time running in theloaded position (70% to 100%).Weather affects how many machinesneed to be running to meet set point.

Cooling Tower Temperature

Weather will also greatly affect thetemperature of the water coming fromthe cooling tower. Any Florida servicetechnician will tell you that 65F con-denser water temperature does nothappen very often in Miami. Yet, usingthe IPLV formula, this temperature isassumed to occur 57% of the time.As the map in Figure 1 shows, the vary-ing climate across the U.S. causes awide variance in condenser water temp-eratures. This results in significantdifferences in the percentage of timecondenser water temperatures are below70F a mere 5% in Miami, but 97%in San Francisco.

Building LoadCharacteristicsThe IPLV formula uses a building

load profile that assumes the maximumbuilding load (100%) occurs at the maxi-mum outside temperature. This in itselfis not a bad assumption. But from there,it assumes that building load will beat 20% at the minimum outside tempera-ture and at 38% at 57.5F. Few, if any

S Y N O P S I S

2

Condenser Water Hours Below 70F (24-hr. Operation)

Seattle: 95%

SanFrancisco: 97%

Los Angeles: 78%

Kansas City: 63%

Detroit: 78%

Chicago: 78%New York: 65%

DC: 65%

Charlotte: 78%

Atlanta: 55%

Miami: 5%

FIGURE 1


3/5

buildings will have this exact typeof load profile.

Building load profiles vary accordingto how the building is used. For exam-ple, consider the following typical usagepatterns:

Office buildings: Operate 5.5days per week, from 7:00 a.m.to 6:00 p.m.

Retail: Operate 7 days per week,from 9:00 a.m. to 10:00 p.m.

Hospitals: Operate 7 days perweek, 24 hours per day

Industrial Process facilities:

Operate 7 days per week,24 hours per day

Each building type has its own typi-cal profile. But each also varies accord-ing to weather patterns and individualbuilding usage. Chances are your build-ings load profile will not look anythinglike the ARI load profile. Figure 2 showsthe ARI profile compared to a 24-hourprocess operation. Obviously these twoare not very similiar.

Operational HoursSystem performance is affected by

the number of hours the chillers operate.Consider a school in the northern U.S.It may only use its chilling equipment afew hours a day for a few weeks out ofthe year. Total annual usage may be aslow as 800 hours. In contrast, a 24-hourprocess application right next doorcould be running more than 8000hours annually.

The difference in hours will greatlyinfluence what type of system should beselected for a particular building. Justchanging this one parameter operatinghours will affect a host of others, suchas entering condenser water temperature,electric rate structure, number of chillersrunning, chiller loading and, of course,equipment payback.

Number of Chillers andEconomizer Capabilities

The IPLV calculation assumes singlechiller operation, which is not normally

the case. In fact, approximately 85% ofthe chillers sold are in some type of mul-tiple machine system. For example, if a

1000-ton system is needed, two 500-tonmachines will generally be purchased, asopposed to one 1000-ton machine. Thenumber of chillers in the system andwhether or not an economizer is usedwill affect at what percent of their capac-ity the chillers run in other words, howefficient they are.

Figure 3 shows how a typical applica-tion using three equally-sized chillersmight run without an economizer. Thefirst machine turns on when cooling isfirst needed. It ramps up until it reaches

its full capacity. If the buildings coolingset point cannot be maintained, the sec-ond chiller is turned on. The first andsecond chillers then load up until theyreach full capacity. If set point cannot bemaintained, the third chiller is turned on.All three machines now ramp up until fullbuilding load is met.

If a building operates most of thetime between 30% and 90% of totalbuilding load, the chillers will runbetween 70% and 95% capacity mostof the time. An interesting thing

FIGURE 2

80706050403020100

Min 57.5 Max

Temp. Bin

BuildingL

oad%

Building Load Profiles

90100

Process

ARI

90

100

70

80

60

50

30

40

20

10

0

Building Load %

ChillerLoad

%

Three Chi llers: 33/33/33% O peration

4 12 20 28 36 44 52 60 68 76 84 92100

FIGURE 3


4/5

happens if an economizer is added tothis same system.

Since the economizer takes overat operating hours below 55F, forexample, the chillers will operatewith even less time at low loads.

This scenario is depicted in Figure 4.

Condenser Water

TemperatureDepending on the time of day a

chiller is needed, the condenserwater temperature can varygreatly. A system that runsaround the clock will seemuch more time with cold-er condenser water thanthe same system runningduring typical office hours.

This is due to the largenumber of night timehours, when the outsidetemperature tends to drop.But these machines alsoneed to run at the designdaytime condition, which may have85F condenser water temperature.In a case like this, the designer mustselect chillers that can do both duties(85F and 60F condenser water),but should optimize the selectionswhere the most operational hourswill be spent.

Referring back to Figure 1, wesee that during 24-hour operation, achiller in Chicago will see condenserwater temperatures below 70Fapproximately 78% of the time.It is important to understand thiswhen selecting the proper chillerfor the job.

Some chillers for example,

certain screw and negative pressurecentrifugal chillers cannot runbelow a certain condenser water tem-perature (70F is typical). In areas

like Chicago, these chillers may notbe the best choice.

Other chillers for example,certain positive pressure centrifugalchillers are designed to operateeffectively with colder condenser

water. In Chicago, selecting a chillerthat can operate effectively between55F (below which a typical econo-

mizer would turn on) and 70F wouldbe a critical decision.

Auxiliary EquipmentCooling Towers

Any time a chiller can receivecolder condenser water temperaturefrom a cooling tower, it will operatemore efficiently it has less work toperform and therefore will use lessenergy. A general rule is that forevery one degree drop in condenser

water temperature, chiller efficiencywill increase 2%.It makes sense, then, to consider

sizing your tower as large as possibleto get the coldest condenser watertemperature for the chiller. Towerscan be rated in terms of approachtemperature: the temperature differ-ence between the outdoor wet bulbtemperature and the water tempera-ture leaving the tower. The smallerthe approach, the more efficientthe tower.

Figure 5 shows the exact samesystem with three different coolingtowers. As expected, the chiller

operates most efficientlywith the largest tower.While this is not reflectedin the chillers IPLV rating, itdoes show up when evaluat-ing the operating cost of thesystem as a whole.Although the largest cool-ing tower has the mostexpensive first cost, the sav-

ings in overall costs makefor a quick payback.

4

S Y N O P S I Scontinued on next page

FIGURE 4

90

100

70

80

60

50

30

40

20

10

0

0 10 20 30 40 50 60 70 80 90 100

Building Load %

With Economizer

ChillerLoad%

Large Tower or Small Tower?

Approach

9 deg

7 deg

5 deg

IPLV

.562

.562

.562

Op Cost

$114k

$111k

$108k

Small Tower

Medium Tower

Large Tower

FIGURE 5


5/5

Pumping Requirements

When chillers are selected, theirefficiency rating is expressed inkW/ton. There is also a pressuredrop associated with its heatexchangers. The pressure drop indi-cates how much energy is lost in thewater system as water flows throughthe cooler and condenser. This is theamount of pressure the pumps needto overcome to get water throughthe heat exchanger. The greater thepressure drop, the more pumpingpower needed.

If the flow is lowered on a system for example with a variable speed

drive on the pump motor pressuredrop will come down, and the pump-ing power requirement will bereduced. At the same time, thechiller efficiency will go down. It isimportant to understand the dynam-ics between pumping capacity andchiller capacity in order to strike themost cost-effective and efficient bal-ance between them.

The Importance ofControls

Proper controls can be a keyfactor in meeting constantly chang-ing building requirements in the mosteffective manner. Once equipmentis selected for maximum efficiency,a chilled water control system willboth supervise and optimize theoperation of your chilled waterplant. A good control system willconsider all elements of the coolingoperation, including cooling towers,pumps, variable frequency drives

(VFDs), heat exchangers and thecontrol valves used on the buildingsair handlers.

ConclusionTo thoroughly understand your

application, a comprehensive reviewof the many factors contributing tochiller system efficiency should be

conducted. As we have discussedthese include geographic and climateconditions, building load characteris-tics, anticipated operational hours,economizer capabilities and predict-ed energy drawn from auxiliariessuch as cooling towers and pumps.

This kind of evaluation can help youassess your system-wide efficiencybefore you make purchasing deci-sions. It will show you which

components, if replaced or retrofit-ted, will provide the most improve-ment overall.

The Integrated Part Load Value(IPLV) formula should not be relied

on to accurately represent yourchiller installation. Instead, takeadvantage of the new computer toolsthat can perform system evaluationsnot previously possible. They arecomprehensive, time-efficient, andwill allow your design engineer totruly optimize the chilled watersystem for your specific building.

5

continued from page 4

C O N S I D E R T H E S E

I M P R O V E M E N T S T O G A I N

E F F I C I E N C Y

In addition to correct sizing of chillers, here are some other areas where efficien-

cies can be realized.

Multiple Chiller Configuration. When selecting mu ltiple chillers, be sure toanalyze the difference in system performance between chillers that are equal and

unequal in size. Splitting the load unevenly can result in more efficient operation

at lower building loads.

Cooling Towers. A larger cooling tower can boost chil ler efficiency by lowering

entering cond enser water temperature. Factors such as size, height and fan configur a-

tions wil l affect the efficiency of the tower, and shoul d be considered when making a

replacement selection.

Controls. A good energy management system wi ll o ptimize the efficiency of ind ividual

pieces of equipm ent, as well as how they operate together. It will adju st the delivery of

conditioned air to the specific and varying requirements of your building, maintaining

comfort, whil e conserving energy.

Variable Frequency Drives (VFDs). Adjust the speed of fan and/or pump m otors

to more precisely meet cooling load requirements and reduce power consumption.

They are particularly helpful at part load conditions. If many chiller hours are spent in

deep part load, these may be helpful.

Energy-Efficient Motors. Todays improved motor designs can save energy and

contribute to an overall system downsi zing strategy. When properly matched with their

anticipated load, m otors can run at peak efficiency more of the time, im proving their

performance and saving money.

Variable Air Volume. Replacing a constant volume air distribution system with avariable air volu me system can increase energy efficiency. Variable systems use a

combination of pressure controls and dampers to adjust air flow, rather than varying

the airs temperature.

hvac chiller_01

Documents