ASHRAE ST-16-024-2016 Low Evaporator Airflow Detection Using Fan Power for Rooftop Units.pdf

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1、 2016 ASHRAE 233ABSTRACTLow evaporator airflow is one of the common faults forrooftop units. It can be caused by dirty filters, evaporatorfouling, or loose belts. Low airflow could result in frozenevaporator coils, reduced cooling capacity, and indoorcomfort issues. Accordingly, more fan power is co

2、nsumed aslongeroperatingtimeisrequired.Withthewidespreaduseofvariable-frequency drives (VFDs) on rooftop units (RTUs),low evaporator airflow can be potentially detected by moni-toring the fan-power variation. In this paper the principle offan-power-based detection is introduced first. Then, thedetec

3、tion algorithm is proposed, including development ofthe baseline and comparison of operational data with thebaseline. Finally, the conducted field test is discussed, veri-fying the proposed methods. The test results indicate that thefan-power-based method can effectively detect low evapora-tor airfl

4、ow for rooftop units.INTRODUCTIONProper airflow is an important factor to ensure the contin-uous and healthy operation of a rooftop unit (RTU). However,low airflow is a common fault in most units (Breuker andBraun 1998). Low airflow can be caused by many reasons,such as a dirty air filter, evaporato

5、r coil fouling, ductworkblockage, or loose fan belt. For a rooftop unit equipped witha constant-speed fan, a rule of thumb is that measured airflowlower than 300 cfm/ton (0.04 m3/s/kW) is considered to be alow airflow fault (Cowan 2004). Reduced fan airflow couldresult in evaporator coil freezing an

6、d system trip-off at highburner temperatures. More importantly, it will cause thermaldiscomfort and increase energy consumption.Low airflow is a very common issue but is usually not paidattention to until comfort issues occur. Plenty of studies showthat the effect of air-side fouling of a heat excha

7、nger on pressuredropismorepronouncedthanonheattransfer.Detectionofdirtyfilters and evaporator coil fouling can be converted to help withdetection of reduced airflow (Lankinen et al. 2003; Bell et al.2009).Thereareseveralwaystodetectlowairflow,suchaspres-sure or temperature detection. With pressure-b

8、ased detectionmethods, an air pressure switch is used to measure the differen-tial pressure across the filter or cooling coil. When the filter orcoil is dirty, the blocked airflow results in a higher pressure dropthan the normal value. An alarm can be generated to indicate ablocked filter or coil. T

9、his method works better under constant-fan-speedoperations,especiallyforlarge-sizeair-handlingunitsor rooftop units. However, for small-sized RTUs or unitsequipped with a variable-speed fan, the pressure switch may notsensethepressurechangeunderlow-speedconditions.Forroof-top units without a filter

10、pressure switch, the common way tomaintain their functions is routine inspection and replacing witha new filter regularly. Besides the pressure-based method, thetemperature-based method (Li and Braun 2007; Wichman andBraun 2009) is the most-used method to detect the evaporatorairflow restriction. Se

11、veral temperatures are measured, includ-ing evaporator inlet air temperature and condenser inlet airtemperature. The evaporator airflow is derived based on thesetemperature measurements and system models.Researchers from Massachusetts Institute of Technology(Armstrong et al. 2006) used electrical me

12、asurements such asreal power and reactive power to detect the reduced airflowfault caused by blockage. The transient power value iscollected by a nonintrusive load monitoring (NILM) device.The idea of electrical measurement is pretty valuable, as theLow Evaporator Airflow Detection UsingFan Power fo

13、r Rooftop UnitsYunhua Li, PhD Bei Zhang, PhDAssociate Member ASHRAE Associate Member ASHRAEJosephine Lau, PhD MingshengLiu,PhD,PEMember ASHRAE Member ASHRAEYunhua Li is a mechanical product engineer, Bei Zhang is a project engineer, and Mingsheng Liu is the president and CTO at Bes-Tech Inc.,Omaha,

14、NE. Josephine Lau is an associate professor in the Department of Architectural Engineering at the University of Nebraska-Lincoln,Omaha, NE.ST-16-024Published in ASHRAE Transactions, Volume 122, Part 2 234 ASHRAE Transactionselectrical signal measurements are more reliable comparedwith temperature or

15、 pressure measurements. However, theNILMdeviceisrequired,whichmaynotrealisticforallRTUs.As variable-frequency drives (VFDs) have been installedon supply fans more and more widely, they can be treated asan electrical meter to measure the instantaneous fan parame-ters, such as frequency or power. Thes

16、e readings can be trans-mitted to a stand-alone controller or building automationsystem (BAS), which is helpful to further analyze the perfor-mance of units and detect the reduction of airflow.PRINCIPLEAt a fixed speed, the fan power is determined by airflowandfanhead.WhenaVFDisinstalledforasupplyfa

17、n,thefanspeed should be always considered. Centrifugal fans arecommonly used in rooftop units. There are several bladeshapes: airfoil, backward inclined or backward curved, radial,and forward curved. For radial and forward-curved fans, thefanpowerincreaseswhentheairflowrateincreases.However,for airf

18、oil and backward-inclined or backward-curved fans,there is a point where the fan power is at the maximum value.If the airflow rate is higher than this point, the fan powerdecreases. Figure 1 shows the fan performance curve for ageneric fan. This turning point is on the right of the maximumfan-power

19、point. This means that as long as the airflow variesin the selection range, the fan power still increases as theairflow increases.Figure 1 shows the fan performance curve from one manu-facturer.Assumingthedesignairflowis8000cfm(3.8 m3/s),thefanpowerwillbearound3.2hp(2.4kW).Ifthefanspeedisfixedandthe

20、airflowisreducedto6000cfm(2.8 m3/s)(whichisequiv-alent to a 25% reduction), the fan power will be 2.8 hp (2.1 kW),whichisequivalenttoa12.5%reduction.Iftheairflowisreducedto4000cfm(1.9m3/s),whichisequivalentto50%reduction,thefan power will be around 2.4 hp (1.8 kW), which is equivalent to25%reduction

21、.Thisexampleclearlyillustratesthatthefanpowercan reflect the change of airflow.In a single-zone variable-air-volume (VAV) system, thesupply fan is usually controlled by the VFD to maintain thespace air temperature. The airflow reduction caused by a dirtyfilter or a dirty evaporator can be identified

22、 by comparing theactual fan power and fault-free fan power.Figure 2 shows the change of working point and powercomparison, where c1 and c2 are system resistance curvesunder normal and fault conditions (dirty filter or evaporator);and arethefanheadcurvesofaforward-curvedfan,under speed and ; and are

23、the fan-powercurves under speed and . At normal conditionsthatis, when both the filter and evaporator are cleanthe fanworking point is 1 with speed . If the filter or evaporator isdirty, then the pressure drop across the filter and evaporatorwill be higher, and also the system resistance will increa

24、se.The system curve will be changed from c1toc2. The workingpoint becomes 2. It can be seen clearly that the airflow dropsfrom Q1to Q2, and the fan power is reduced from W1to W2.Figure 1 Fan performance curve (ASHRAE 2008).P1P21 2 W1W21 21Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE T

25、ransactions 235The reduced airflow causes the reduction of coolingcapacity, which results in the increase of space air tempera-ture. As the fan speed and compressor speed are adjusted tocontrol the space temperature, the thermostat calls for morecoolingand,consequently,theVFDspeedgoesup.Assumingthat

26、atspeed theairflowincreasestoQ1andthesystemcanprovide the required cooling capacity, the new working pointwill be 3 and the power curve is changed to . It can beseen that the fan power under the new working condition willbe higher than the initial fan power. The increment is asfollows:(1)However, th

27、e fan power at Point 3 is still lower than atPoint 4, which is at the normal system curve. When the fanspeed increases to compensate for the lost cooling capacity,the fan power at the new working point is always lower thanthat at the original curve c1. Therefore, this relationship couldbe used to de

28、tect the low-airflow fault.FAN-POWER MEASUREMENTIt has already been demonstrated that the change of fanairflow is reflected by the variation of fan power. Therefore,low airflow can be indicated by the reduction of fan power.Usually,thefanpowerisnotdirectlymeasured.Thefanmotorpower measurement is an

29、approximation of fan power with adifferenceofbeltpowerlossandmotorpowerloss.Themotorinput power can be measured by a power meter installed forthefanmotor.However,themotorinputpoweroftencannotbetrended automatically by the third-party controller or BAS.With the application of VFD, it is feasible to u

30、se a VFDto measure the fan power and speed. In recent years, VFDshave been more and more widely applied in the HVAC indus-try because of the needs of building energy efficiency and thefast development of VFD technology. Most VFDs have theability tomeasure either the currentor power input ofa motor.T

31、he measured motor power can be used to estimate the fanpower, while the error is the power loss of a motor. The motoroperating power could be sent out from VFDs through eitheranalog signal or digital communication (e.g., Modbus,BACnet). In this study, Modbus communication is directlyused to receive

32、the fan motor power data.ALGORITHMFirst,thebaselinemustbedeveloped.Then,theactualfanpower can be measured by VFD to compare with the baselinevalue. The difference of actual and base values will indicatethe magnitude of airflow reduction.Baseline DevelopmentThe rooftop unit can run at cooling mode, h

33、eating mode,or fan mode (ventilation mode). The VFD can be installed forfanonlyorbothfanandcompressor.Inthispaper,wefocusonthe second scenario, that is, a VFD is installed on both fan andcompressor. The baseline model is described as follows.Heating Mode and Free-Cooling Mode (Outdoor AirTemperature

34、 OAT 55F 12.8C). In both modes, onlythe fan is running and the compressor is off. The fan-power-speed curve can be built using the measured data (Li 2012):(2)The experimental results reveal that the difference in thefan power between the normal and faulty conditions will belarger at higher speeds th

35、an at lower speeds. Therefore, thefull-speed fan power is used as the baseline:(3)Fan Mode. In fan mode, both compressors and heatersare off and only the fan is running at the minimum speed. Thefan power at minimum speed is used as the baseline.Actual Fan-Power PerformanceIn heating and free-cooling

36、 mode, the VFD reading willbe the fan-only power. The full-speed fan power can be deter-mined by the following:(4)where and are motor efficiency under speedand60Hz.Thesevaluescanbedeterminedbasedonthemotorspeed ratio and load ratio (Li et al. 2015).In fan mode, the VFD power reading is the fan power

37、 atminimum speed:(5)Figure 2 Fan-power analysis.2W2WW3W1=Wf , na02a1 a2+=Wf , n,60Wf , n,60a02a1 a2+ 1=a0a1a2+=Wf , a,60Wf , a, m, 3m,60-=m, m,60Wf , a, minWVFD=Published in ASHRAE Transactions, Volume 122, Part 2 236 ASHRAE TransactionsLow Evaporator Airflow DetectionThe low-airflow fault will be d

38、etected by comparing theactual fan power and the baseline. The control limit method isused to evaluate the fan power.The ratio of actual power to its normal value is used:(6)The actual value is xa, and its normal value is xn; x is notlimited to fan powerit could be speed, total power, or anyother pa

39、rameter. Total power is the sum of fan power andcompressor power.The upper and lower control limits are established usingthe baseline data:(7)(8)At normal conditions, the upper and lower control limitscan be obtained from the baseline data:(9)(10)During actual operations, the fan-power ratio is asfo

40、llows:(11)where Wf, meas, iand Wf, pred, iare the measured and predictedfull-speed fan power, respectively. Wf, a, iand Wf, n, i, are theactual and normal full-speed fan-power ratios. Similarly, theupper and lower limits of total power ratio could be calculatedas well:(12)(13)(14)If the ratio is var

41、ied between the upper and lower limits,no“change”isdetected.Otherwise,thefan-powerchangewillbe detected.FIELD DEMONSTRATIONA field test was conducted to demonstrate the proposedmethod. The rooftop unit was designed as a constant-volumesystem originally and retrofitted to a VAV system before thefield

42、 testing. A VFD was installed on this unit with commu-nication with a third-party controller to control both fan andcompressor speeds. The system performance data wereobtained from an energy management and control system(EMCS),whichgetsthepowerreadingfromtheVFDthroughModbuscommunication.Whenthecompr

43、essorisrunning,theVFDpoweristhesumoffanandcompressorpower.Whenthecompressor is off, the VFD power reading is the fan power.One VFD was used during this testing. The accuracy offrequency and power measurements are 0.1 Hz and 0.1 kW,respectively.The low-airflow fault is simulated by blocking the evap-

44、orator coil with papers.Baseline Development. When the room temperaturereacheditssetpointandthethermostatwassetatthe“fanon”mode, the compressor was off and only the fan was running,at minimum speed. The minimum speed of the tested unitwas 45 Hz. The average fan power at minimum speed was0.62 kW, as

45、shown in Figure 3. Figure 4 shows the distribu-tion of total power ratio during the baseline period. Of totaldata, 98.7% are located within 6% of the predicted value.The minimum value of total power ratio in this case is 0.94.Therefore, rwt, n, ll= 0.94.During this test, the dirty evaporator coil wa

46、s simulated byblockingthecoilwithpaperswhichwereaddedevery10minutes.The minimum fan speed was 45 Hz. Because the test was carriedout in late July and early August, the supply fan ran with thecompressor together in the cooling mode or ran itself in ventila-tion mode. There is no free-cooling mode ava

47、ilable. Therefore,theminimum-speedfanpowerwasusedtodevelopthefan-powerbaseline.Figure5showsthefan-powervariationduringthetest.Thex-axis represents the time, and the y-axis represents the actualfan power. The test started with one paper blocking the coil,which was approximately equivalent to 6% coil

48、blockage, at8:05 a.m. More papers were used to block the coil as the timeincreased. The blockage percentages are marked in Figure 5.When the evaporator is clean or no coil was blocked, the fanpowerwasabout0.62 kW.Itisclearlyshownthatthefanpowerdecreased with more coil surface blocked. At around 9:50 p.m.,around 84% of the evaporator coil was blocked. The fan powerwas 0.56 kW at this time. The corresponding airflow was633 cfm (0.3 m3/s), which was 41% of the normal value.rxaxn-=rulmaxxa, ixn, i-=rllminxa, ixn, i-=rwf , n, ulmaxWf , meas, iWf