ncy

ong K

Decit owFault diagnosis

exiystw df a sdempeci

exchangers, 87.67 kW (72.37%) of the total power of pumps on primary and secondary sides of heat

arysed to ofs, espethan tn HVA

ists in almost all large distributed chilled water systems.A series of operational problems might be caused by the decit

ow problem and low delta-T syndrome in practical applications,such as the high supply water temperature, the over-suppliedchilled water, and the increased energy consumption of thesecondary pumps. When the decit ow occurs that meansthe secondary water ow rates exceeds that of the primary loop,

proposed to enhance the energy performance of chilled water sys-tems [1317]. Among the studies, Fiorino [14] indicated that ahigher delta-T can be achieved by proper application of coolingcoils, controls systems, distribution pumps, and piping systems.Up to 25 practical methods were recommended to achieve highchilled water delta-T ranging from component selection criteriato congurations of distribution systems. Wang et al. [16] pre-sented an approach that experimentally validates the feasibilityof using a check valve in the chilled water bypass line to solvethe low delta-T syndrome. Results showed that about 9.2% of totalenergy consumption of the chillers and secondary water pumps is

Corresponding author. Tel.: +852 27665858; fax: +852 27746146.

Applied Energy 98 (2012) 597606

Contents lists available at

lseE-mail address: beswwang@polyu.edu.hk (S. Wang).considerable efforts on increasing the operation and energy perfor-mances of chilled water systems [25]. While in real applications,most of the primarysecondary systems, from time to time, cannotwork as efcient as expected because of the excess secondary owdemand, which causes decit ow problem (i.e., the required owrate of secondary loop exceeds that of the primary loop). When thedecit ow problem exists, the temperature difference producedby the terminal units will be much lower than its design values,which is known as the low delta-T syndrome [69]. Kirsner [6]pointed out that the low delta-T chilled water plant syndrome ex-

increasing to an extremely high value, the indoor temperature can-not be maintained at a comfortable level.

Existing studies [1012] demonstrated a lot of potential causesfor the decit ow problem and the low delta-T syndrome. Thecauses mainly include improper set-points or poor control calibra-tion, the use of three-way valves, improper coil and control valveselection, no control valve interlock, and uncontrolled process load,reduced coil effectiveness, outdoor air economizers and 100% out-door air systems, and so on.

Measures to handle the low delta-T syndrome also have been1. Introduction

Over the last two decades, primsystems have been widely employeenvironment in commercial buildingdue to its higher energy efciencyow system [1]. Many researchers i0306-2619/$ - see front matter 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.03.057exchangers could be saved in the test cases when higher set-points were used. 2012 Elsevier Ltd. All rights reserved.

condary chilled waterfer comfortable indoorcially in large buildings,he traditional constantC&R eld have devoted

the additional return water from terminal units ows back to thesupply chilled water through the bypass line, leading to the highersupply water temperature supplied to the terminal air-handlingunits (AHUs). The supply chilled water with increased temperatureconsequently leads to an increased chilled water ow rate that fur-ther worsens the decit ow, which consumes more energy of sec-ondary pumps. If the supply chilled water temperature continuesChilled water systemLow delta-T syndrome in the in situ experimental tests. The decit ow could be eliminated if temperature set-point was reset

higher. Compared with the original set-point of outlet water temperature on the secondary side of heatDiagnosis of the low temperature differeof a super high-rise building: A case stud

Dian-ce Gao, Shengwei Wang , Yongjun Sun, Fu XiaDepartment of Building Services Engineering, The Hong Kong Polytechnic University, Ho

a r t i c l e i n f o

Article history:Received 3 November 2011Received in revised form 7 March 2012Accepted 29 March 2012Available online 22 May 2012

Keywords:

a b s t r a c t

The low delta-T syndromein the degradation of the sstudy on diagnosing the lothe chilled water system odata during the days whenset-point of outlet water tefault that resulted in the d

Applied

journal homepage: www.ell rights reserved.e syndrome in the chilled water system

ong

sts in many large primarysecondary chilled water systems, which resultsem overall energy performance. This paper presents a method and a caseelta-T problem resulted from the decit ow that frequently occurred inuper high-rise building at its early operation stage. The history operationcit ow and low delta-T syndrome occurred are analyzed. The impropererature on the secondary side of heat exchangers is nally diagnosed as thet ow and low delta-T syndrome. Diagnosis of this fault was also validated

SciVerse ScienceDirect

Energy

vier .com/locate /apenergy

saved in the test case when compared with the case when no checkvalve used.

The above studies demonstrate that low delta-T syndrome anddecit ow problem widely exist in the primarysecondary chilledwater system and the elimination of these problems can improvethe energy performance of the chilled water system. For practicalapplications, it is essential to nd the causes before fully correct-ing them. However, the detailed study for detection and diagnosisof the low delta-T syndrome and decit ow problem, particularlyin real applications, is still insufcient. This paper presents a casestudy that diagnoses the low deltas-T syndrome based on opera-tion data. The associated experiment validation is provided aswell. This paper offers some useful experiences for engineersand operators to nd causes of the low delta T syndrome effec-tively and enhance the energy performance of chilled watersystems. In this study, the history operation data were analyzedand experiments were designed and conducted to diagnose anddetermine the causes for low delta-T syndrome in a complexcentral chilled water system of a super high-rise building in HongKong.

2. System descriptions and operation problems

The central chilled water plant concerned in this study is a com-plex primarysecondary system in a super high-rise building inHong Kong [18]. The building is about 490 m high with approxi-mately 321,000 m2 oor areas, consisting of a basement of fouroors, a block building of six oors and a tower building of 98

for chillers are 5.5 C and 10.5 C respectively. Each chiller isassociated with a constant speed primary chilled water pump.The primary loop is decoupled with the secondary loop throughthe bypass line.

The secondary chilled water is divided into four zones, in whichZone 2 is supplied with the chilled water from chillers directly. Theheat exchangers are employed to transfer cooling energy fromchillers to occupied zones in Zones 1, 3, and 4 to avoid chilledwater pipelines and terminal units from suffering extremely highstatic pressure. Zone 1 is supplied with chilled water through heatexchangers (HX-06) on the sixth oor and Zones 3 and 4 are sup-plied with chilled water through the rst stage heat exchangers(HX-42) on the 42nd oor. Some of the chilled water after HX-42is delivered to terminal units in Zone 3 directly and the other isdelivered to the second stage heat exchangers (HX-78) on the78th oor, which serves as the cooling source for Zone 4. Thepumping congurations after HX-42 and HX-78 are sub primarysecondary chilled water system, which employs primary constantspeeds pumps (PCHWP-42-01 to 07 and PCHWP-78-01 to 03) todeliver chilled water from the heat exchangers (HX-42) to the sec-ondary variable speed pumps that deliver the chilled water to theterminal units. All the secondary pumps are equipped with vari-able speed drivers and all the primary pumps are constant speedpumps. The major specications of all chilled water pumps at de-sign condition are summarized in Table 1.

This central chilled water plant frequently suffered from thedecit ow problem and low chilled water temperature differencedisease after its rst use since the middle of 2008. A ow meter isinstalled in the bypass line, which can measure the ow rate and

C

DE

Primary water circuit

Chiller circuitTO PODIUM & BASEMENT

)

PC

598 D.-c. Gao et al. / Applied Energy 98 (2012) 597606EVAPORATOREVAPORATOREVAPORAROR

WCC-06a-01(2040 Ton)

PCHWP-06-01

FROM OFFICCE FLOORS(7-41)

TO OFFICE FLOORS(7-41)

PCHWP-06-02 PCHWP-06-03

WCC-06a-02(2040 Ton)

WCC-06a-03(2040 Ton)

D

B

E

(S-B

)

HX-06 HX-06

(S-B

)(S

-B

SCHWP-06-03 to 05

SCHWP-06-01 to 02

SCHWP-06-09 to 11oors. As shown in Fig. 1, this central chilled water plant imple-ments six identical constant speed centrifugal chillers to providecooling energy for building. The rated cooling capacity and com-pressor power of each chiller are 7230 kW and 1270 kW respec-tively. The design chilled water supply and return temperatures

AA

BC

Secondary water circuit for Zone 1

Secondary water circuit for Zone 2

Secondary water circuit for Zone 3 and Zone 4FROM PODIUM & BASEMENTCONDENSER CONDENSER CONDENSER

Fig. 1. Schematic of the cdirection of water ow in the bypass line. The reading of the owmeter is negative when the water ows from the return side to thesupply side in the bypass line (i.e., decit ow in the bypass line).The reading of the ow meter is positive when water ows fromthe supply side to the return side in the bypass line. Fig. 2 presents

EVAPORATOR EVAPORATOR EVAPORATOR

HX-42 HX-42 HX-42 HX-42 HX-42 HX-42 HX-42

PCHWP-06-04 PCHWP-06-05 PCHWP-06-06

WCC-06a-04(2040 Ton)

WCC-06a-05(2040 Ton) WCC-06a-06(2040 Ton)

(S-B

)

FROM ZONE 3 FLOORS (43-77)

TO ZONE 3 FLOORS (43-77)(S

-B

)HX-78HX-78HX-78

TO ZONE 4 FLOORSS (79-98)

FROM ZONE 4 FLOORS (79-98)

(S-B

)(S

-B

)

SCHWP-42-01 to 03SCHWP-42-04 to 06

SCHWP-78-01 to 03

PCHWP-78-03PCHWP-78-01 PCHWP-78-02

HWP-42-01 PCHWP-42-02 PCHWP-42-03 PCHWP-42-04 PCHWP-42-05 PCHWP-42-06 PCHWP-42-07

SCHWP-06-06 to 08

BYPASS LINECONDENSER CONDENSER CONDENSER

hilled water system.

EneTable 1Specications of chilled pumps of HVAC system.

Pumps Numbera Flow (l/s)

Primary water pumpPCHWP-06-01 to 06 6 345Secondary pumps for Zone 1SCHWP-06-01 to 02 1(1) 345SCHWP-06-09 to 11 2(1) 155Secondary pump for Zone 2SCHWP-06-03 to 05 2(1) 345Pumps for Zones 3 and 4SCHWP-06-06 to 08 2(1) 345PCHWP-42-01 to 07 7 149SCHWP-42-01 to 03 2(1) 294SCHWP-42-04 to 06 2(1) 227PCHWP-78-01 to 03 3 151SCHWP-78-01 to 03 2(1) 227

a Value in parentheses indicates number of stand by pumps.

200

300

D.-c. Gao et al. / Appliedthe measured water ow rate in the bypass line and the measuredchilled water temperature difference of the main supply and returnpipes (i.e., namely the system temperature difference) in ve con-secutive summer days respectively. It is obvious that the averagesystem temperature difference during the 5 days was very low,only about 3.5 K. It is also noted the decit ow existed in nearlyhalf of the period and the average duration was about 12 h a day.When the decit ow occurred, the system delta-T became muchlower, which indicates that the decit ow and the low delta-Tsyndrome in this system seemed to be highly correlated. Sincethe cooling coils are selected to produce a temperature rise at fullload that is equal to the temperature difference selected for thechillers (i.e., 5 K in this case), the ow rate of secondary loopshould be therefore equal to that of the primary loop under fullload condition and should be less than that of primary loop underpart load condition. However, when the decit ow problem exists,the excessive water ow rate of secondary loop will greatly reducethe temperature difference produced by the terminal units, whichis known as low delta-T syndrome. Therefore, the low delta-T syn-drome existing in this chilled water system was mainly due to theoccurrence of the decit ow problem. In order to raise the systemchilled water temperature difference, it is necessary to nd the ex-act faults that caused the decit ow problem, solve the low delta-T syndrome, and accordingly enhance the energy performance ofthe overall chilled water system.

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0 8 16 24 32 40 48 56Sampling t

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ow r

ate(L

/S)

Fig. 2. Measured water ow rate in the by-pass line and tempeHead (kPa) Power (kW) Remarks

310 126 Constant speed

241 101 Variable speed391 76.9 Variable speed

406 163 Variable speed

297 122 Variable speed255 44.7 Constant speed358 120 Variable speed257 69.1 Variable speed202 36.1 Constant speed384 102 Variable speed

8

9

)

rgy 98 (2012) 597606 5993. Outline of the diagnosis methodology

As mentioned in Section 1, there are a lot of faults that can re-sult in the decit ow and the low delta-T syndrome. The chilledwater system under study is new-built and was properly designed,installed and well commissioned. The following faults were ex-cluded according to the analysis and site investigations, such ascoil fouling, improper sensor calibration, the use of three-wayvalves and the improper selection of components. Therefore, possi-ble faults considered nally are within the category of controlfaults.

A practical diagnosis process is developed in this study to diag-nose the decit ow problem and low delta-T syndrome resultedfrom improper controls in this complex chilled water system,which mainly includes faults detection and diagnosis, and valida-tion of the FDD results as well as evaluation of the energy impacts,as shown in Fig. 3. For detecting faults, the water ow rate in thebypass line and the temperature difference of the entire systemare selected as the indicators to detect whether the low delta-Tsyndrome existed in the chilled water system. If the water owrate in the bypass line is negative and the measured temperaturedifference of the main supply and return pipes are much lowerthan the predened threshold (i.e., 3 K in this study), the chilledwater system can be determined to suffer from decit ow prob-lem and low delta-T syndrome.

64 72 80 88 96 104 112 120ime(hour)

0

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em te

mpe

ratu

re d

iffer

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Water flow rate in the bypass lineTemperature difference

rature difference in secondary system in ve summer days.

phenomenon is triggered as soon as the faults are introduced andis eliminated when faults are released, the previous faults detec-tion/identication results will be nally validated and conrmed.Meanwhile, the energy impacts caused by the faults are also eval-uated by comparing the energy consumption of the system withdecit ow to that without decit ow in the experiments.

4. Results and discussion

An advanced building management system (BMS) is installed inthis building and the operation data of the chilled water system arecollected and stored in the database. The database consists of themain operation data of the HVAC system, such as temperatures,ow rates, air and water pressures at some key points as well asenergy consumptions of chillers, pumps, cooling towers, and AHUs.The history operation data are essential to analyze the operation

Start

Water flow rate in the bypass line 0?

AndDifferential temperature

of the whole systemthreshold?

Yes

Determine faults location

History operation data analysis

Validate the detection/identification results

by experiments

Evaluation of energy impacts by

Faults detection

600 D.-c. Gao et al. / Applied Energy 98 (2012) 597606Faults identication scheme is implemented to identify the ex-act faults that cause the decit ow problem. As there are severalsub-systems in a complex chilled water system, the fault locationwill be rstly determined by comparing the temperature differenceof different sub-systems. Then, the history operation data will beanalyzed to observe whether the set-point used for controlling sec-ondary pumps speed can be achieved when the decit ow oc-

The relationship between the deficit flow and the set-point for controlling

the secondary pumps

Identify the specific faults

faults

Solutions and suggestions

Faults identification

validation of detection/identification results & evaluation of the energy impacts

Fig. 3. Schematic of the diagnosis process.curred. If the occurrence of the decit ow is closely correlatedto the set-point for controlling secondary pumps speed, it can bepreliminarily considered that the decit ow problem is mainly re-sulted from the improper set-point. At last, selected relative oper-ation parameters will be analyzed to identify the faults accordingto the control strategy used in the system, such as valve openings,pumps frequency, pumps sequence, and heat exchangers sequence.

An experimental validation scheme is employed to furthervalidate and conrm the fault detection/identication results.Using this scheme, the specic faults identied preliminarily willbe introduced in the chilled water system. If the decit ow

systems. The temperature differences of the risers serving Zones 1,3, and 4 were much lower. Particularly, during the occurrence of

0

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3

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0 8 16 24 32 40 48 56Sampling t

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pera

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diff

eren

ce (K

)

Fig. 4. Temperature difference of indiv64 72 80 88 96 104 112 120the decit ow, the temperature differences of these two riserswere only about 1 K. It indicates that the low delta-T syndromewas very serious in these two zones. Comparing the temperaturedifferences of the three risers, it was found that the low delta-Tsyndrome was mainly contributed by the riser serving Zone 1and the riser serving Zones 3 and 4. Because the pump congura-tions and the pump control strategies in the two risers are very

Temperature difference of Zone 1Temperature difference of Zone 2Temperature difference of Zone 3&4performance of the chilled water system.The decit ow problem and the low delta-T syndrome have

been detected in this system under study mentioned earlier, asshown in Fig. 2. In the following sections, the fault location andidentication by using the proposed FDD method are presented.Validation of the FDDmethod and evaluation of the energy impactsare also addressed.

4.1. Faults identication by analyzing operation data

Since the temperature difference is a key indicator to evaluatethe operation performance of a chilled water system, temperaturedifference of each sub-system was analyzed rstly through historyoperation data to nd at which sub-system the faults located. Fig. 4shows the temperature difference of each sub-system in ve sum-mer days (i.e., the same period as mentioned in Fig. 2) when thesystem suffered from low delta-T syndrome. It can be observedthat only temperature difference of the riser serving Zone 2 main-tained over 4 K most of time, which is normally accepted in most ofime(hour)idual risers in ve summer days.

similar, Zones 3 and 4 is selected as the example to be analyzedand diagnosed in the followings.

In order to further determine the exact faults of the controlsthat cause the decit ow problem in Zones 3 and 4, the historyoperation data of two typical summer days were selected for com-parison and analysis, in which 1 day experienced signicant decitow problem during daytime and the other day experienced signif-icant decit ow during night. Fig. 5 shows the operation data ofthe day when signicant decit ow occurred during daytime,including water ow rate in the bypass line, the operating numberof heat exchangers and pumps before heat exchangers (i.e., on theprimary side of heat exchangers), the outlet water temperature(Tout,ahx) after heat exchangers (i.e., on the secondary side of heatexchangers), and the set-point of the outlet water temperatureafter heat exchangers. It is obvious that the decit ow began tooccur at about 4:00am and lasted nearly the entire daytime. Whilein another summer day as shown in Fig. 6, the decit ow only oc-curred during the night and disappeared during the daytime. Aninteresting fact is that the set-point of Tout,ahx was closelycorrelated to the occurrence of decit ow. When the decit owexisted, the measured Tout,ahx was signicantly larger than theset-point of Tout,ahx. In contrast, when there was no decit ow,the measured Tout,ahx basically was not higher than the set-pointof Tout,ahx. It is worthy noticing that when the water ow rate of

the bypass line changed from the positive ow to decit ow,the measured Tout,ahx accordingly varied from below its set-pointto above its set-point. Therefore, a preliminary conclusion can bedrawn that the decit ow might be easily triggered when Tout,ahxcannot be maintained at its set-point, and the decit ow will beeliminated when Tout,ahx can be maintained at its set-point. Anotherfact shown in Figs. 5 and 6 is that the operating numbers of heatexchangers and pumps before heat exchangers were signicantlyincreased when the decit ow occurred. The more decit ow oc-curred, the more heat exchangers and pumps before heat exchang-ers were activated.

The above phenomenon observed can be interpreted accordingto the control strategies for pumps and heat exchangers currentlyused in this system, as shown in Fig. 7. The variable speed pumpsbefore heat exchangers distribute the chilled water from the chill-ers to the heat exchangers. The pump speed is controlled to main-tain the measured differential pressure between the main supplypipe and return water pipe before the heat exchanger group atits set-point. A temperature controller is used to keep the outlettemperature (Tout,ahx) after heat exchangers at its set-point by mod-ulating the openings of the valves before heat exchangers. In thecontrol strategy, the differential pressure set-point is constant,while the temperature set-point after heat exchanger has a xedtemperature difference (e.g., 0.8 K) above the chiller supply water

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D.-c. Gao et al. / Applied Energy 98 (2012) 597606 6010102030405060

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Active number of secondary pumps before HX-42e(hour)hen decit ow existed during daytime.

Ene200/S)

602 D.-c. Gao et al. / Appliedtemperature that varies based on the outdoor dry-bulb tempera-ture, as shown in Fig. 8. When Tout,ahx is larger than its set-point,the modulating valves before heat exchangers will widely opento demand more chilled water before heat exchangers. Moreover,

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Fig. 6. Operation data in the typical day w

Fig. 7. Speed control for pumps befrgy 98 (2012) 597606according to the sequence control strategy used in this case, ifone of the valves reach its maximum position and Tout,ahx still can-not reach its set-point, the additional heat exchanger will beswitched on to enhance the heat transfer effect. As the heat

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Active number of HX-42Active number of secondary pumps before HX-42

hen decit ow occurred during night.

ore HX serving Zones 3 and 4.

exchangers are connected in parallel, more operating heatexchangers and more widely opened valves will signicantly de-crease the overall water resistance at the primary side of heatexchangers. When the overall water ow resistance was reduced,the pumps speed before heat exchangers would be continually in-creased to maintain the measured differential pressure across ofthe heat exchangers at its set-point until the set-point was reachedor the pumps reach their full speeds. The over-speeded pumps dis-tributed more chilled water than necessary, which caused the def-icit ow in the bypass line and in turn led to the low delta-T

Ambient dry-bulb temperature ( C)

8.5

5.5

15 34

Temperature set-point ( C)

9.3

6.3

Chiller supply temperature

Outlet temperature after heat exchangers

Fig. 8. The original scheme for determining the set-point of outlet water after heatexchangers.

D.-c. Gao et al. / Applied Enesyndrome. These analyses are based on Figs. 5 and 6, which canbe summarized in Fig. 9.

Based on the above analysis, the preliminary conclusion can bedrawn that the too low outlet water temperature set-point after

Outlet water temperature set-point after heat exchangers (too low to be reached)

Valve opening before heat exchangers=100%

Increase activeheat exchanger numberReduce water resistance of primary side of heat exchangers

Pump speed before heat exchangers is highly increased

Deficit flow

Low delta-T syndrome

More chilled water is requiredto maintain the differential

pressure set-point

Fig. 9. Flow chart for low delta-T syndrome diagnosis.heat exchangers that cannot be reached led to the decit owproblem and the low delta-T syndrome in the system investigated.

4.2. Validation of FDD method and evaluation of energy impacts

In order to validate the FDD method in above analysis, two re-peated eld tests were conducted to verify the cause of the decitow in the bypass line by varying the set-point of the outlet watertemperature (Tout,ahx) after heat exchangers in two separate days.The results of the two tests show the consistent conclusions andthe results presented as follows came from the latest test in the au-tumn of 2010. In this test, the set-point of Tout,ahx was rstly chan-ged from 8.2 C (while no decit ow occurred) to 6.8 C, and 6 Crespectively. Then, the set-point was increased back to 7.4 C and8.2 C respectively. During the whole test period, the chiller supplywater temperature was xed at 5.5 C, and the operating numberof chillers as well as the primary water ow rate remainedunchanged.

Fig. 10a and b shows the water ow rates in the bypass line,Tout,ahx and its set-point. It can be observed that the decit owdid not occur and Tout,ahx basically can be maintained at its set-point before 11:10am. When the set-point of Tout,ahx was reducedto 6.8 C, and 6 C respectively, the water ow rate of the bypassline dropped rapidly from about 170 l/s to negative (25 l/s),which means the decit ow occurred. Meanwhile, Tout,ahx experi-enced a gradual dropping process but could not be low enough toreach its set-point. On the other hand, when the set-point of Tout,ahxwas increased again to 7.4 C and 8.2 C respectively, the decitow was eliminated and Tout,ahx also returned to be controlledapproximately at its set-point. The test results indicate that thedecit ow would most likely occur when the set-point of Tout,ahxis too low to be reached. The decit ow can be eliminated whenthe correct control of Tout,ahx is resumed, which conrms the preli-minary conclusion in Section 4.1.

It is also worthy pointing out in Fig. 10a and b that the actualmeasured outlet water temperature after heat exchangers wasnot signicantly decreased and basically maintained stable atabout 7.2 C although the set-point of Tout,ahx changed from 6.8 Cto 6 C. The reason is that the inlet water temperature on the pri-mary side of heat exchangers was signicantly increased becauseof the decit ow. The more supplied chilled water with highertemperature failed to improve the overall cooling effect of the heatexchangers when the decit ow occurred.

Fig. 10c and d presents the openings of modulating valves be-fore heat exchangers and the operating number of heat exchangersduring the test period. It can be observed that the openings of mod-ulating valves were closely related to Tout,ahx and its set-point. Oncethe set-point of Tout,ahx was lower than Tout,ahx, the modulatingvalves opened rapidly. When one of the modulating valves fullyopened, additional heat exchangers were switched on by the con-trol logic used in this case. When the set-point increased from 6 Cto 7.4 C and further to 8.2 C respectively, the modulating valvesclosed down until reaching the minimum position and the operat-ing number of heat exchangers also was reduced from four to twoeventually. The valve openings and the operating number of heatexchangers greatly affected the controlled speed of pumps at theprimary side of heat exchangers, as shown in Fig. 10e. It can befound that the speed (frequency) of the pumps before heatexchangers signicantly increased when either the valves werewidely opened or extra heat exchangers were switched on. Onthe other hand, when both the valve openings and operating num-ber of heat exchangers were reduced, the speed of pumps beforeheat exchangers dropped again. Both wider valve openings and

rgy 98 (2012) 597606 603more operating heat exchangers mean a lower overall water resis-tance of the heat exchanger group at primary side. Therefore, thepump speed had to be increased to transfer more water to meet

the

M2

t watlet w

Ene-100-50

050

100150200

10:50 11:10 11:30 11:50Wat

er fl

ow r

ate(L

/S)

Water flow rate in

M1

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ture

(C) The measured outle

The set-point of ou

120ing(%

)

Valve opening before HX-42-01Valve opening before HX-42-03

604 D.-c. Gao et al. / Appliedthe predetermined pressure differential set-point. It is worth notic-ing that the variation of overall water resistance of heat exchangergroup was more sensitive to the operating number of heatexchangers compared with the valve openings. As indicated inFig. 10a, water ow variation (i.e., DM2) in the bypass line resultedfrom valve openings was far less than that (i.e.,DM1) resulted fromchanging heat exchanger operating number.

Fig. 11 depicts the dynamic power of pumps on both sides ofheat exchangers during the test period. Clearly, the total energyof the pumps signicantly increased when the set-point of Tout,ahxwas greatly reduced and decit ow occurred. Compared to thatwithout test, 87.67 kW (72.37%) of average power of all pumpson both sides of heat exchangers was wasted during the test. It isalso found that the power of the three pump groups associatedwith heat exchangers varied in different ways when the set-pointof Tout,ahx decreased. The secondary pumps before heat exchangersand the primary pumps after heat exchangers consumed more en-ergy while the secondary pumps after heat exchangers consumedless energy. But the energy wasted by the rst two was far largerthan that saved by the third. The results demonstrated that a toolow set-point of Tout,ahx badly degraded the energy performance

20253035404550

Sampling ti

Freq

uenc

y (H

z)

020406080

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ope

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ratin

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Fig. 10. The operation data12:10 12:30 12:50 13:10

bypass line

(a)

(b)

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ter temperature after HX-42ater temperature after HX-42

Valve opening before HX-42-02Valve opening before HX-42-04

rgy 98 (2012) 597606of the chilled water system although the building cooling load stillcan be satised.

The above test results conrmed the conclusion of the analysisin Section 4.1. It demonstrated that the decit ow and the lowdelta-T syndrome in the chilled water system under study weremainly resulted from the improperly low set-point of the outletwater temperature after heat exchangers.

4.3. Discussion and suggestions

The in situ operations and experiments have demonstrated thatit is not robust and is unreliable to reset the set-point of outletwater temperature after heat exchangers (Tout,ahx) using a xedtemperature difference above the chiller supplywater temperature,particularly when the temperature difference is relatively small. Itis because that the inlet water temperature before heat exchangersis not always equal to the chiller supply water temperature due tothe decit ow problem in practical applications. Once the inletwater temperature before heat exchangers is higher than the chillersupply water temperature, the actual temperature difference be-tween the inlet water temperature before heat exchangers and

me(hour)

Frequency of secondary pumps before HX-42

(c)

(d)

(e)

12:10 12:30 12:50 13:10

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ps before HX

during the test period.

050

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Pow

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f pum

ps(k

W)

bot

D.-c. Gao et al. / Applied Enethe set-point of Tout,ahx will be reduced. When such phenomenonoccurs, more chilled water before heat exchangers will be providedby the control, which makes the decit ow worse. Particularly inthe summer days when the cooling is relatively high, Tout,ahx is moreeasily affected by some uncertainties and disturbances, such as sud-den cooling load increasing or temporarily high inlet water temper-ature before heat exchangers. This also interprets the reason whythe decit ow more easily occurred in summer season than inthe spring season, as observed through eld investigations and his-tory operation data.

Simulation tests on the system under study were conducted tostudy the impact of the temperature set-point of chilled water afterheat exchangers on the operation and energy performances ofpumps. Fig. 12 shows the test results under a xed cooling load(i.e., 60% of design cooling load of Zone 3) when the chiller supplywater temperature was xed at 5.5 C. It can be observed that thetotal power of pumps associated with heat exchangers graduallyincreased while the set-point of Tout,ahx increased from 6.3 C to8 C. When the set-point of Tout,ahx reduced below 6.2 C, the powerof pumps increased dramatically and all the pumps at primary sidewere at maximum speed. The reason is that the decit ow oc-curred when the set-point of Tout,ahx was below 6.2 C, which couldnot be reached under this working condition. The results indicatesthat the set-point of Tout,ahx (around 6.3 C in this study) currently

Fig. 11. Power of pumps onused sometimes is at risk of causing decit ow although it is thedesign value under the design working condition and is thought to

0

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Flow

rat

e of

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ass l

ine

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Power of secondary pumps before HXPower of secondary pumps after HXPower of primary pumps after HXFlow rate of bypass line

Fig. 12. Power of pumps and ow rate of bypass line under different set-point ofTout,ahx under a xed cooling load.be safe in principle. Therefore, a higher temperature differenceabove chiller supply water temperature (Tch,sup) is recommendedfor resetting the set-point of Tout,ahx in this system to ensure thesystem to be maintained at healthy operation condition. Italso can be found in Fig. 12 that proper increase of the set-pointof Tout,ahx had minor effect on the total pump power. Comparedwith the total pump power (207.8 kW) using the current set-pointat 6.3 C (i.e., 0.8 K of temperature difference above Tch,sup), the in-crease was about 0.86 kW only (0.5% of the total pumps energy)when the system worked under the set-point of Tout,ahx at 6.7 C(i.e., 1.2 K of temperature difference above Tch,sup). Obviously, ahigher set-point of Tout,ahx can allow the operation and controlmore robust and reliable while the total power of pumps is almostunchanged. The reset scheme for set-point of outlet water temper-ature after heat exchangers (Tset,out,ahx) is therefore proposed for thestudied chilled water system as expressed by the followingequation.

Tmax P Tset;out;ahx maxTch;sup 1:2; Tmeas;in;bhx 0:8 1where Tch,sup is the chiller supply water temperature, Tmeas,in,bhx isthe measured inlet water temperature before heat exchangers.The proposed scheme for resetting Tset,out,ahx adopts double security.One is to ensure that Tset,out,ahx is 0.8 K higher than the actual inletwater temperature before heat exchangers at least. This ensures

12:10 12:29 12:50 13:10 time(hour)

Power of secondary pumps after HX-42Power of secondary pumps before HX-42Power of primary pumps after HX-42Total power of pumps on both sides of HX-42

h sides of heat exchangers.

rgy 98 (2012) 597606 605the outlet water temperature after heat exchangers be reached eas-ily because 0.8 K is temperature difference used for selecting heatexchangers at design stage. The other insurance is that the temper-ature difference between Tch,sup and Tset,out,ahx is increased from 0.8 Kto 1.2 K. This is because that the use of a higher Tset,out,ahx results inless probability of decit ow while the overall pump energy con-sumption is not increased obviously. The revised set-point resettingscheme provides better chance for the system to resume itself tohealthy mode (surplus ow in bypass line) when possible. A highlimit (Tmax) is set for the set-point of Tout,ahx to guarantee the tem-perature of chilled water supplied to clients within the guaranteedrange and proper for humidity control in occupied zones. It is notedthat the actual coefcients in Eq. (1) are only suitable for the stud-ied chilled water plant and different values may be needed for otherplants.

5. Conclusion

This paper presented a method and a case study to diagnose thelow delta-T syndrome and decit ow problem in a real chilledwater system of a super high-rise building. The improper set-pointreset of the outlet water temperature at the secondary sides of heat

exchangers was found to be the actual fault that caused the decitow problem in the system. The analysis results show that a toolow set-point of outlet water temperature at the secondary sideof heat exchangers would lead to more pumps to be activated withhigher speed on the primary side of heat exchangers, which easilycaused decit ow. Decit ow could be eliminated when this set-point was reset reasonably higher.

Results of an in situ test conrmed the fault detection/identi-cation results. In the meanwhile, a proper set-point of outlet watertemperature on the secondary side of heat exchangers achieved anaverage power saving of 87.67 kW (72.37%) of pumps on primaryand secondary sides of heat exchangers.

It is also suggested to set the temperature difference betweenthe set-point of outlet water temperature and the supply chilledwater temperature at sufciently high level in real applications.

Acknowledgements

The research presented in this paper is nancially supported bya Grant (PolyU5308/08E) of the Research Grant Council (RGC) of theHong Kong SAR, a Grant of The Hong Kong Polytechnic University.The work is supported by Sun Hung Kai Real Properties Limited.

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606 D.-c. Gao et al. / Applied Energy 98 (2012) 597606

Diagnosis of the low temperature difference syndrome in the chilled water system of a super high-rise building: A case study1 Introduction2 System descriptions and operation problems3 Outline of the diagnosis methodology4 Results and discussion4.1 Faults identification by analyzing operation data4.2 Validation of FDD method and evaluation of energy impacts4.3 Discussion and suggestions

5 ConclusionAcknowledgementsReferences