ASHRAE ST-16-025-2016 Characterizing the Performance of Fixed-Airflow Series Fan-Powered Terminal Units Using a Mass and Energy Balance Approach.pdf

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1、240 2016 ASHRAEABSTRACTAtraditionalmassandenergybalancecomponentapproachwas used to characterize the performance of fixed-airflow seriesfan-powered terminal units for applications in building simula-tionprograms.Theapproachincludeddevelopingrelevantenergyandmassbalanceequationsforthecomponentsinafan

2、-poweredterminal unitheating coil, fan/motor combination, and mixer.Fan motors that included permanent-split-capacitor motorscontrolled by silicon-controlled rectifiers or electronicallycommutatedmotorswereincludedinthemodeldevelopment.Thepaper demonstrates how to incorporate the fan/motor combina-t

3、ionperformancemodelsforbothpermanent-split-capacitorandelectronically commutated motors into the mass and energybalance approach. The fan models were developed from perfor-mancedatathatwereprovidedbymultiplefan-poweredterminalunit manufacturers. The fan/motor performance data included afanairflowran

4、gefrom250to3500ft3/min(0.118to1.65m3/s)anda motor size range from 0.333 to 1 hp (248.6 to 745.7 W).INTRODUCTIONA common heating, ventilating, and air-conditioning(HVAC) system used in commercial buildings is the single-duct variable-air-volume (VAV) system (ASHRAE 2012). AVAVsystemvariestheamountofa

5、irdeliveredtoaconditionedzone to ensure the desired thermal comfort level. The supplyair to each zone is modulated by using a single terminal unitbased on the sensible load sensed by a thermostat in the zone.If a terminal unit includes a fan, it is called a fan-poweredterminal unit (FPTU) or powered

6、 induction unit (PIU). Themore general term FPTU is used in this paper. FPTUs mix theprimary air with induced recirculated (secondary) air. TheFPTU may provide supplemental heating to the air dependingon whether the FPTU is in heating or cooling mode. Supple-mental heating can be provided with eithe

7、r a hot-water coil(heat exchanger) or electric resistance (ASHRAE 2012).One configuration of an FPTU is shown in Figure 1. In thiscase, the FTPU fan and the primary air fan are in series. Bothprimary and secondary air pass through the FPTU fan, whichoperates continuously while the system is on. Seco

8、ndary air isinduced into the FPTU from the return air plenum by the FPTUfan.Two different motors are commonly used in FPTUs: apermanent-split-capacitor (PSC) motor controlled by a silicon-controlled rectifier (SCR) and an electronically commutatedmotor (ECM). PSC motors are commonly used in those ap

9、pli-cations where the airflow from the FPTU fan is expected to beconstant.TheSCRchopsthevoltagesuppliedtothePSCmotorto lower the speed of the motor and allows an installer to matchthe airflow from the FPTU to the airflow requirements of thezone. Once the voltage is set, the FPTU fan should operate a

10、t anearlyconstantairflowrate.APSCmotoroperatesatmaximumefficiency when it is at full load.Figure 1 Series configuration for FPTU.Characterizing the Performanceof Fixed-Airflow Series Fan-PoweredTerminal Units Using a Mass and EnergyBalance ApproachCarl L. Reid Dennis L. ONeal, PhD, PE Peng YinStuden

11、t Member ASHRAE Fellow ASHRAE Student Member ASHRAECarl L. Reid is staff engineer at Bee in Austin, TX. Dennis L. ONeal is dean of Engineering and Computer Science, and Peng Yin is a post-doctoral research associate in the Department of Mechanical Engineering at Baylor University, Waco, TX.ST-16-025

12、Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 241ECMs provide an advantage over PSC motors in FPTUapplications because ECMs can be used where the airflow fromthe FPTU is either fixed or allowed to vary to match the airflowrequirements in the zone. An ECM converts alternati

13、ng current(AC) to direct current (DC) to operate the motor. An ECMprovides direct control over the voltage, which allows for moreprecise speed control. An ECM should operate at a higher effi-ciency than the SCR-controlled PSC motors (Int-Hout 2015).Modeling work by Davis et al. (2012) showed that se

14、riesFPTUs with fixed-airflow ECM fan motors outperformed seriesFPTUs with SCR-controlled PSC fan motors in five differentcities:Houston,Phoenix,Chicago,NewYork,andSanFrancisco.The annual total plant energy savings ranged from 5.9% to 8.4%in Chicago for a small, five-zone office building. Davis et al

15、.(2012) based his FPTU models on the power, pressure, andairflow data collected and analyzed by Furr et al. (2008) andEdmondson et al. (2011). While the analysis by Davis et al.(2012) was useful in estimating savings of ECM FPTUs, itrequireddetailedknowledgeofthepressuresupstreamanddown-stream of th

16、e FPTU. This approach was not directly compatiblewith the mass and energy balance (MEB) modeling approachoften used in building energy simulation programs such as Ener-gyPlus (2013). As a consequence, it is difficult to use the model-ingworkofDavisetal.(2012)andthedatafromFurretal.(2008)and Edmondso

17、n et al. (2011) directly in building simulationprograms. ONeal et al. (2015a, 2015b) and ONeal (2015)analyzed performance data on both PSC/SCR and ECM fan/motordatafromfourFPTUmanufacturersanddevelopedperfor-mance models that were compatible with a mass and energybalancesystemmodelingapproachfoundin

18、somebuildingsimu-lation programs.The purpose of this paper was to demonstrate how tocombinethePSC/SCRand ECMperformancedatadevelopedby ONeal et al. (2015a and 2015b) with the mass and energybalance modeling approach used in EnergyPlus (2013). Themodels can be integrated into building energy simulati

19、ons topredicttheannualenergyconsumptioninaVAVsystemusingfixed-airflow series FPTUs. Ongoing work is being done thatextends the mass and energy balance modeling approach tovariable-airflow applications with ECM fan motors applied inseries FPTUs to match the thermal load in the zone.MASS AND ENERGY BA

20、LANCE APPROACHThemassandenergybalanceapproachiscommonlyusedtomodel components in the HVAC system of a building (Knebel1983).ThisapproachiscurrentlyusedinEnergyPlus(2013).TheMEB approach treats each subsystem in an HVAC system, suchasanFPTU, asasetof equationstodescribethe massandenergyflows into and

21、 out of each subsystem. A series FPTU can then bedecomposed into its major components: mixer, fan/motor, andheating coil. Figure 2 shows a control volume around the wholeFPTU.WiththeMEBapproach,ananalysiscanbeperformedoneach component to estimate overall airflows into and out of theFPTUaswellastheen

22、ergyusedbytheFPTUfan/motorforeachtime step of a simulation. The large dashed box is the overallcontrol volume for the FPTU. Each FPTU component can betreated with a smaller control volume with mass and energyinputs and outputs. Figure 2 represents one FPTU. However, abuilding could include dozens of

23、 these in a VAV system.Series FPTU ModelLooking at the overall control volume around the seriesFPTU, energy is input to the FPTU via electrical energy to thefan,heatenergytotheheatingcoil,andenergyassociatedwiththe primary and secondary airstreams. The only mass andenergy leaving the series FPTU is

24、with the airstream at thedischarge of the FPTU.The series FPTU consists of three major components:mixer, fan, and heating coil. A mass and energy balance needsto be performed on each of the components and the condi-tioned zone to estimate temperatures and airflows to deter-mine the performance of th

25、e FPTU. The approach outlinedbelow incorporates empirical models for the FPTU fan/motorto calculate the fan power.Figure 2 Mass and energy flows into and out of a series FPTU.Published in ASHRAE Transactions, Volume 122, Part 2 242 ASHRAE TransactionsBeforestartingananalysis,itisimportanttooutlineso

26、mebasic assumptions we used in developing the MEB model.First, the system in Figure 2 is assumed to operate at quasi-steady state during each time step. During a particular time-step, the temperature and airflows remain constant and areaveraged over the time step. Given that the typical time step is

27、an hour, this type of analysis cannot capture rapid transientsoccurring in a control system at smaller time steps. We alsoassume that the thermophysical properties are constant. Thisallows the specific heat and density of the air to be treated asconstants. Given the small temperature differences in

28、theairstreams, constant specific heats and density should intro-duce small errors (less than a 1%) in the analysis. Third, theenergy input to the fan motor is assumed to be completelyconverted into the heat energy in the airstream. For a seriesFPTU, the fan/motor combination is located completely in

29、 theairstream. Thus, all of the heat energy for the fan should bedissipated in the airstream. This assumption is discussed inmore detail later in the paper. Fourth, the FPTU must alwaysoperate with a minimum amount of primary air to ensureenough fresh air is introduced into the zone. Thus, even when

30、the zone calls for heating or a very low amount of cooling,there will always be a minimum amount of primary airprovided to the FPTU.Themassandenergybalancesforageneralcontrolvolumeat steady state are given by Equations 1 and 2, respectively.Mass balance is as follows:(1)Energy balance is as follows:

31、(2)FortheFPTUcontrolvolumeshowninFigure2,themassflow into the control volume includes the primary, mpri, andsecondary, msec, airstreams. The only mass out of the controlvolume is the total airflow, mtot. Application of the massbalance in Equation 1 to the control volume yields the massbalance shown

32、in Equation 3:(3)The value of mpriis often an unknown that must be esti-matedusingtheanalysisdiscussedbelow.Theonlyexceptionsare when the series FPTU is operating under minimum ormaximum primary flow conditions.The energy transfer into the control volume includes theenergy carried by the two airstre

33、ams, the rate of energy inputin the heating coil (qcoil), and the power input to the fan(POWfan). The energy into and out of the control volume canbe substituted into Equation 2 to obtain the general energybalance for the control volume given in Equation 4.(4)Equations 3 and 4 provide the foundation

34、 for the analysisof the FPTU. The unknowns in these equations vary depend-ing on the mode of operation (heating, cooling, or dead band)of the FPTU. Solving for the unknowns requires applyingmass and energy balances to each of the components in theFPTU. The process typically starts from the left at t

35、he FPTUdischargetothezoneandmovestotherighttotheprimaryandsecondary air inlets.For the case of a fixed airflow with either a PSC/SCR orECM fan/motor, the total airflow, mtot, in the above equationsis a fixed value. With constant air properties, Equation 4 canbe rewritten in terms of temperatures and

36、 specific heats.(5)Proceeding with the analysis requires decomposing theFPTU into its components and performing mass and energybalances on each component as described below.Zone AnalysisFigure 3 shows the mass and energy flows into and outof the conditioned zone. Energy is carried into and out of th

37、ezone via the total airflow, mtot. The load in the zone due topeople, equipment, solar gain, infiltration, etc., is repre-sented by qzs.The value of the FPTU discharge temperature, tout, mustbe calculated. This is done with an energy and mass balanceon the zone in Figure 3. During heating operations

38、, toutis notallowed to drift above 90F (32.2C). The rationale for thisassumption is provided in the “Heating Coil Analysis”section. During cooling operations, this value cannot dropbelow the sum of the primary air temperature plus tempera-ture increase due to the FPTU fan. An energy balance isperfor

39、med on the zone to yield Equation 6, which is used toAirflow out of FPTUAirflow into FPTU=Energy out of FPTUEnergy into FPTU=mtotmprimsec+=mtothout=qcoilPOWfanmprihprimsechsec+mtotcptout=qcoilPOWfanmpricpTprimseccptsec+ +Figure 3 Energy and mass flows into and out of the zonecontrol volume.Published

40、 in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 243determinethedischargetemperatureoftheairattheoutletofthe FPTUunless theheating coilis engaged.The assumptionfor constant properties is used throughout the system.(6)ThewaythatEquation6isusedinananalysisdependsonwhether the system is

41、in heating or cooling mode. For exam-ple, for a fixed-airflow application, mtotis known. In addition,the zone load qzs, specific heat cp, and zone setpoint tempera-ture tz, are also known. For cooling applications, Equation 6can be rewritten and solved for the discharge outlet tempera-ture toutfor t

42、he FPTU:(7)For heating calculations, toutis calculated fromEquation 7 but typically has an upper limit of 15F (8.3C)above the zone setpoint temperature or a fixed value of 90F(32.2C) to help reduce temperature stratification in the zone(Hydeman and Eubanks 2014). Procedures are provided laterin the

43、paper for the calculation of toutfor heating and coolingapplications.Heating Coil AnalysisFigure 4 shows the mass and energy flows into and out ofthe heating coil. It is assumed the system is operating at aquasi-steady state and the mass flow of air entering the coil isequal to the mass flow of air

44、exiting the coil in a given timestep. The energy entering the coil is the heating energy input,qcoil, and is equal to the energy absorbed by the air flowingthrough the coil. Heating energy is often provided by electricresistance or hot water. The energy exiting the coil is carriedby air leaving the

45、coil and supplied to the zone. Applying anenergy balance to the heating coil yields the following:(8)Many times, the variable of interest is qcoil, the heatingenergy input, so the above equation can be rearranged to solvefor qcoil:(9)FPTU Fan AnalysisThe mass and energy flows into and out of the FPT

46、U fanare shown in Figure 5. It was assumed that all of the fan motorand controller power is converted to heat energy in theairstream. This fraction of electric power dissipating intoairstreams as heat energy can be varied by the user in Energy-Plus (2013). With a series FPTU, the electric motor is l

47、ocatedin the primary airstream. With an ECM, the controller is alsoin the airstream, so for an ECM it should be expected that alltheenergyconvertedforthefanmotorandcontrollergoesintothe airstream. With an SCR-controlled fan motor, the SCRcontroller is typically not located in the primary airstream b

48、utin the plenum airstream. Estimates of the power dissipated byan SCR controller are 1.5 W/amp of current flowing throughthecontroller(RomanandHeiligenstein2002).Table1showstheestimatedpercentageoftotalpowerthatanSCRcontrollerconsumesfora0.5hp(373 W)PSCfanmotorfromonemanu-facturer. The power used by

49、 the SCR controller rangedbetween 0.57% to 0.83% of the total power of the motor.Calculations were run for fan motors ranging from 0.167 to1.0 hp (124 to 746 W) with a similar range in percentages ofthetotalpower.Thus,withSCR-controlledfanmotors,assum-ing that all the fan power is dissipated into the primaryairstream introduces less tha