ASHRAE ST-16-026-2016 Using a Mass and Energy Balance Approach to Model the Performance of Parallel Fan-Powered Terminal Units with Fixed-Airflow Fans.pdf

1、 2016 ASHRAE 253ABSTRACTA mass and energy balance approach was used to charac-terize the performance of parallel fan-powered terminal units(FPTUs) with fixed airflow for applications in building simu-lation programs. The approach included developing relevantmassandenergybalanceequationsforeachcompon

2、entheat-ing coil, fan/motor combination, and mixerin a parallel fan-powered terminal unit. Two locations of the heating coil wereconsidered. One location, designated as the traditional config-uration, was at the discharge of the unit. The second location,designated as the alternative configuration,

3、was at the second-aryairinlet.Fixed-airflowparallelfan-poweredterminalunitsuse fan motors that include either permanent split capacitormotors controlled by silicon controlled rectifiers or electroni-cally commutated motors. This paper demonstrates how toincorporate fan/motor combination performance

4、models forboth permanent split capacitor and electronically commutatedmotors into the mass and energy balance approach. These fanmodels were developed from performance data provided bymultiple FPTU manufacturers. The fan/motor performancedata included an FPTU, a fan airflow range from 250 to3500 ft3

5、/min (0.118 to 1.65 m3/s), and a motor size range from0.333 to 1 hp (249 to 746 W). Leakage was included in themodels. The system was implemented in Engineering EquationSolver(EES)andresultsprovidedtoillustratetheeffectofleak-age in both cooling and heating operations.INTRODUCTIONThe single-duct var

6、iable-air-volume (VAV) system is acommon heating, ventilating, and air-conditioning (HVAC)system used in commercial buildings (ASHRAE 2012). VAVsystemsvarytheamountofairdeliveredtoaconditionedzoneto ensure a desired comfort level. The conditioned air issupplied to each zone by a single terminal unit

7、 based on thesensible load as sensed by a thermostat in the zone.If a terminal unit includes a fan, it is called a fan-poweredterminal unit (FPTU) or powered induction unit (PIU). FPTUsprovide conditioned air to a zone that may include a mixture ofthe primary air with induced recirculated (secondary

8、) air. TheFPTU may provide supplemental heating to the air dependingonwhethertheFPTUisinheatingorcoolingmode.Thesupple-mental heating is typically provided with either a hot-water coil(heat exchanger) or electric resistance (ASHRAE 2012).If the FPTU fan and the primary air fan are in parallel, assho

9、wn in Figure 1, then the FPTU is called a parallel FPTU. ThefanisusedtoinducesecondaryairfromtheplenumspaceintotheFPTU. For a parallel system, the fan is off during cooling opera-tions. Primary air can be supplied to the zone without the fanbeing used. When the fan is off, the back draft damper clos

10、es toprevent air from escaping through the secondary air inlet. Theprimary air damper is located at the FPTU inlet and is used toFigure 1 Parallel configuration for FPTU.Using a Mass and Energy Balance Approachto Model the Performance ofParallel Fan-Powered Terminal Units withFixed-Airflow FansPeng

11、Yin, PhD Carl L. Reid DennisL.ONeal,PhD,PEStudent Member, ASHRAE Student Member, ASHRAE Fellow ASHRAEPeng Yin is an assistant professor in the Department of Mechanical Engineering, University of Louisiana at Lafayette, Lafayette, LA. CarlL. Reid isastaffengineeratBEE,Austin,TX. Dennis L. ONeal isdea

12、nofEngineeringandComputerScience,BaylorUniversity,Waco,TX.ST-16-026Published in ASHRAE Transactions, Volume 122, Part 2 254 ASHRAE Transactionsadjust the supply air entering the FPTU. When the fan is on, itadds recirculated (secondary) air to help control the temperatureof the air supplied to the zo

13、ne. An electric or hot-water heatingcoil can be used to provide supplemental heat to the airstream.The heating coil can be located at the discharge of the FPTU asshowninFigure1foratraditionalconfiguration.TheheatingcoilcanalsobelocatedbeforetheFPTUfanatthesecondaryairinlet.Both permanent-split capac

14、itor (PSC) motors controlledby silicon controlled rectifiers (SCRs) and electronicallycommutated motors (ECMs) have been applied in parallelFPTUs. PSC motors are only used in those applications wherethe airflow from the FPTU fan is expected to be constant. TheSCR chops the voltage supplied to the PS

15、C motor to lower thespeed of the motor and allows an installer to match the airflowfrom the FPTU to the required airflow requirements of thezone. ECM motors can be used for either fixed or variableairflow. An ECM converts alternating current (AC) to directcurrent (DC) and allows for control of the m

16、otor speed.Modeling work by Davis et al. (2012) shows that parallelFPTUs with fixed-airflow ECM fan motors performed simi-larly to parallel FPTUs with SCR-controlled PSC fan motorsinfivedifferentcities:Houston,Phoenix,Chicago,NewYork,and San Francisco. Davis et al. were also able to show that airlea

17、kingfromtheFPTUcabinetcouldhaveasignificantimpacton the annual energy use of the HVAC system. Davis et al.(2012) based their 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 usefu

18、l in estimating savings of ECM FPTUs, itrequired detailed knowledge of the pressures upstream anddownstream of the FPTU. This approach was not directlycompatible with the mass and energy balance (MEB) model-ing approach often used in building energy simulationprograms such as EnergyPlus (2013). Give

19、n the difficulties ofusing the modeling work of Davis et al. (2012) and the datafrom Furr et al. (2008) and Edmondson et al. (2012), ONealet al. (2015a, 2015b) and ONeal (2015) analyzed perfor-mance data on both PSC/SCR and ECM fan/motor data fromfourFPTUmanufacturersanddevelopedperformancemodelstha

20、t could be used with an MEB modeling approach.The purpose of this paper was to combine the PSC/SCRand ECM models developed by ONeal et al. (2015a, 2015b)withamassandenergybalancemodelingapproachtodevelopparallel FPTU models that can be directly implemented inbuildingsimulationprogramssuchasEnergyPlu

21、s(2013).TheFPTU models included leakage and configurations for theheating coil at either the discharge of the FPTU or at the inletto the secondary air. The authors are also working on a modelon variable-airflow parallel FPTUs to complement this paper.MASS AND ENERGY BALANCE APPROACHMassandenergybala

22、nceshavebeenacommonapproachusedtomodelHVACsystemsinbuildingsimulationprograms(Knebel 1983). The MEB approach treats each subsystem ina HVAC system, such as an FPTU, as a set of equations thatdescribe the mass and energy flows into and out of eachsubsystem. A parallel FPTU can then be decomposed into

23、 itsmajor components: mixer, fan/motor, and heating coil.Figure 2 shows a control volume around a traditional parallelFPTU. An analysis can be performed on each component intheFPTUtoestimateoverallairflows(ormassflows)intoandout of the FPTU as well as the energy used by the FPTU fan/motorforeachtime

24、stepofasimulation.ThelargedashedboxintherighttwothirdsofthefigureisthecontrolvolumefortheFPTU. Each FPTU component can be treated as a smallercontrol volume with mass and energy inputs and outputs.An alternate configuration of the traditional parallelFPTU moves the heating coil to the secondary air

25、inlet asshown in Figure 3. One advantage this configuration providesoverthetraditionalconfigurationofFigure2isthattheheatingcoil is outside the primary airstream where it would add to thepressure drop of the primary airstream whenever the primaryFigure 2 Traditional parallel FPTU with heating coil l

26、ocated after the mixing of the primary and secondary airflows.Published in ASHRAE Transactions, Volume 122, Part 2 ASHRAE Transactions 255air handler is operating. This pressure drop would have to beovercome by an increase in static pressure of the central air-handling unit. One potential disadvanta

27、ge of the alternativeconfiguration is that the fan motor is located downstream ofthe heating coil and would be subjected to higher air tempera-tures than in the traditional configuration in the heating mode.Parallel FPTU ModelFor the control volume around the entire FPTU inFigures 2 or 3, energy is

28、input to the FPTU via electricalenergy to the fan, heat energy to the heating coil, and energyassociatedwiththesupplyandsecondaryairstreams.Theonlymass and energy leaving the FPTU is with the airstream at thedischarge of the FPTU and leakage from the housing. Unlikeseries FPTUs that operate at a neg

29、ative pressure with respectto the plenum, parallel FPTUs operate at a positive pressure.The primary air provided to the parallel FPTU has alreadybeen conditioned by the central cooling coil. Thus, air leakingfromtheparallelFPTUoperatinginthecoolingmodeiscolderthan the return air in the plenum and wi

30、ll reduce the tempera-ture of the plenum air. Edmondson et al. (2011) found thatparallel units can leak conditioned air from the housingthrough seams, penetrations, and the back draft damper.The FPTU consists of three major components: mixer,fan, and heating coil. A mass and energy balance must bepe

31、rformed on each of the components to estimate tempera-tures and airflows to determine the performance of the FPTU.There are some basic assumptions we used in developingthe MEB models of parallel FPTUs. First, the system in eitherFigure2or3isassumedtooperateatquasi-steadystateduringeach time step. Du

32、ring a particular time step, the temperatureand airflows remain constant and are averaged over the timestep. Given that the typical time step is an hour, this type ofanalysis cannot capture rapid transients occurring at smallertime steps. Another assumption in the analysis below is thatthe thermophy

33、sical properties are constant. This allows thespecific heat and density of the air to be treated as constants.With the small temperature differences in the airstreams of anFPTU, constant specific heats and densities should introduceless than a 1% error in the analysis. Third, the energy input tothe

34、fan motor is assumed to be completely converted into theheat energy in the airstream. For a parallel FPTU, the fan/motor combination is located in the secondary airstream andis only on during heating operation. SCR fan motors typicallyhave the controller outside of the airstream where the fan andmot

35、or are located. However, based on calculations of powerconsumption of SCR controllers, the controller shouldconsumelessthan1%ofthetotalenergyofthefanmotorifthecontrollersusethetypical1.5W/ampofsuppliedcurrenttothefan motor (Roman and Heiligenstein 2002). ECM controllersare typically located in the s

36、ame airstream as the fan/motor toensure cooling of the electronic components in the controller.Thus, assuming all of the energy of the fan and motor beingdissipated in the secondary airstream should introduce mini-mal error into the analysis. Fourth, the FPTU operates with aminimum amount of primary

37、 air to ensure enough fresh airinto the zone. When the zone calls for heating or a very lowamountof cooling,there willalwaysbe aminimum amountofprimary air provided by the FPTU to the zone.Themassandenergybalancesforageneralcontrolvolumeat steady state are given by Equations 1 and 2, respectively.Th

38、e mass balance is as follows:(1)The energy balance is as follows:(2)For the control volumes shown in Figures 2 or 3, the massentering the system include the primary (supply) air mprifromthe central air handler and the secondary airstream msec. Themass exiting the control volume is total airflow mtot

39、and leak-age mleak. Applying Equation 1 to the control volume yieldsthe mass balance shown below.Figure 3 Parallel FPTU in alternate configuration where the heating coil is located in the secondary airstream.Airflow out of FPTUAirflow into FPTU=Energy out of FPTUEnergy into FPTU=Published in ASHRAE

40、Transactions, Volume 122, Part 2 256 ASHRAE Transactions(3)Leakage is included in the modeling of parallel unitsbecausethehousingisatapositivepressurewithrespecttotheplenum air pressure. Characterizing leakage and it impact onparallel FPTUs is discussed later in this paper. The energytransfer into t

41、he control volume includes the energy providedby the supply and secondary airstreams, energy input into theheating coil, the energy leaving by leakage, and the powerinput into the fan. When these are substituted into Equation 2,it yields the following:(4)Equations 3 and 4 provide the basic equations

42、 thatdescribe the overall mass and energy balance for the parallelFPTU. The unknowns in these equations vary depending onthe mode of operation (heating, cooling, or deadband) of theFPTU. Solving for the unknowns in these equations requiresapplying mass and energy balances to each of the componentsin

43、 the FPTU. The process typically starts from the left at theFPTU discharge to the zone and moves to the right to theprimary and secondary air inlets.With a parallel FPTU, the fan is on when the FPTU is inthe heating mode. During the heating mode, the fan operatesatafixedspeedifthefanmotorisaPSC,orca

44、noperateinfixedor variable speed if the fan motor is an ECM. The analysisprovided in this paper is focused on fixed-airflow fans in thesecondary airstream. The authors are developing a separatemodel for variable-airflow fans in parallel FPTUs.Forcoolingoperation,theFPTUfanisoffandtheprimaryair dampe

45、r is used to vary the amount of supply air that flowsthrough the FPTU. The term mtot, shown in Equations 3 and 4,is the total airflow delivered to the space. In heating opera-tions, the total airflow is the sum of the supply and secondaryairflowsminusleakage.Incoolingoperations,thetotalairflowisjust

46、theprimaryairflowminustheleakage.Thetotalairflowis a fixed value in heating mode if the secondary airflow isfixed.Incoolingmode,thetotalairflowisdeterminedfromtheloadinthezoneandequalstheprimaryairflowminustheleak-ageairflowineitherFigure2or3.Additionalprimaryairmustbeprovidedthat isequaltotheamount

47、ofairleaking outoftheFPTUcabinettoensuretheproperamountofdischargeairmtotisdeliveredtothezonetosatisfythecoolingload.Theprimaryairflow in the FPTU should not drop below a certain percent-age (typically 20% to 30%) of the airflow needed at the designcooling load (ASHRAE 2013). This minimum amount ofp

48、rimary air is used to maintain fresh air requirements in thezone. A recent study found that minimum primary airflows aslow as 10% might still provide acceptable indoor air quality insome applications in California (Zhang et al. 2014).The terms in the left hand side of Equation 4 represent theenergy

49、leaving the FPTU in the airstream either carried by thedischarge airflow or by the leakage airflow. The energy inputinto the FPTU includes heat energy input in the heating coil,power input to the fan (which is assumed to be converted intoheatenergyintheairstream),andenergybeingcarriedintotheFPTU by both the supply and secondary airstreams. If th