1、2008 ASHRAE 83ABSTRACTEmpirical models of airflow output, power consumption,and primary airflow were developed for parallel fan poweredvariable air volume terminal units at typical operating pres-sures. Both 8 in. (203 mm) and 12 in. (304 mm) primary air inletterminal units from three manufacturers
2、were evaluated.Generalized models were developed from the experimentaldata with coefficients varying by size and manufacturer.Fan power and airflow data were collected at down-stream static pressures over a range from 0.1 to 0.5 in. w.g.(25 to 125 Pa). Upstream static pressures ranged from 0.1 to2.0
3、 in. w.g. (25 to 498 Pa). Data were collected at fourprimary air damper positions and at four terminal unit fanspeeds. Model variables included the RMS voltage enteringthe terminal unit fan, the inlet air differential sensor pressure,and the downstream static pressure. A model was also devel-oped to
4、 quantify air leakage when the unit fan was off.In all but one of the VAV terminal units, the resultingmodels of airflow and power had R2values greater than 0.90.For the exception, excessive air leakage from the unit appearedto limit the ability of the airflow and power models to capturethe variatio
5、n in the experimental data. These performancemodels can be used in HVAC simulation programs to modelparallel fan powered VAV systems.INTRODUCTIONVariable Air Volume (VAV) systems maintain comfortconditions by varying the volume of primary air that is deliv-ered to a space. A VAV system often consist
6、s of a central airhandling unit (AHU), where air is cooled by cooling coils(Wendes 1994). This air, referred to as primary air, is sentthrough a single-duct supply system to VAV terminal units bythe supply fan. Each terminal unit is ducted to air outlets,usually serving two or more offices or an ope
7、n area. VAVterminal units that include a fan to improve circulation withina zone are called fan powered terminal units. These terminalunits can draw in warm air from the plenum area and mix itwith primary air from the central Air Handling Unit (AHU) tomaintain comfort conditions in the occupied spac
8、e.When the fan in a VAV fan powered terminal unit isoutside the primary airflow, the configuration is called a paral-lel terminal unit. During operation, the fan for a parallel termi-nal unit cycles on and off. During periods of maximumcooling, the fan is off. A backdraft damper prevents cold airfro
9、m blowing backwards through the fan. The terminal unitprimary air damper modulates the airflow to maintain thespace temperature setpoint. An inlet air differential sensorwithin the primary air stream allows the unit controller tomaintain a consistent volume of airflow to the zone dependingon the tem
10、perature setpoint. When the primary airflow dropsbelow a specified amount, the controller activates the fan. Atthis point, the terminal unit mixes primary air with air beingdrawn in from the plenum. Electric or hot water supplementalheat can be used for additional heating. Depending on thecontrol sc
11、heme, the controller can continue to reduce primaryair to the conditioned space by adjusting the damper.There is a need to develop a better understanding ofsystems using parallel and series fan powered terminal units.To model a VAV system properly in a commercial buildingenergy use model, it is impo
12、rtant to be able to characterize theindividual terminal units.This paper is the second of three papers that describe thedevelopment of experimental models of VAV fan poweredterminal units. The first paper (Furr et al. 2008a) described thePerformance of VAV Parallel Fan-Powered Terminal Units: Experi
13、mental Results and ModelsJames C. Furr Dennis L. ONeal, PhD, PE Michael A. DavisFellow ASHRAE John A. Bryant, PhD, PE Andrew CramletMember ASHRAE Student Member ASHRAEJames C. Furr is a thermal management engineer with Lockheed Martin, Fort Worth, TX. Dennis L. ONeal is Holdredge/Paul Professor andH
14、ead, Department of Mechanical Engineering, Texas A&M University, College Station, TX. Michael A. Davis is a research engineer with andJohn A. Bryant is a visiting associate professor in the Department of Mechanical Engineering, Texas A&M University Qatar, Doha, Qatar.Andrew Cramlet is a research ass
15、istant, Department of Mechanical Engineering, Texas A&M University, College Station, Texas.NY-08-013 (RP-1292)2008, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 114, Part 1. For personal use only. Additiona
16、l reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.84 ASHRAE Transactionsexperimental setup and methodology used to measure theperformance of parallel and series fan powered units. The thirdpaper (Furr et al. 2008b)
17、 describes the measured results andmodels developed for series fan powered terminal units. In thispaper, the performance of six parallel fan powered terminalunits from three manufacturers (labeled A, B, and C) ismeasured and models developed from the data. These unitsincluded three 8 in. (203 mm) an
18、d three 12 in. (304 mm) units.An 8 in. (203 mm) unit from manufacturer A has the designa-tion P8A. One from manufacturer B that is 12 in. (304 mm) isP12B, etc. As described in the first paper (Furr et al. 2008a),there were small differences between the terminal units thatincluded the rated power of
19、the terminal unit fan, the style ofthe primary airflow damper, and the style of the backdraftdamper. Statistical analyses of experimental data wereperformed and used to develop generalized models that can beapplied to the different manufacturers terminal units. Theempirical models were developed for
20、 units from three manu-facturers and two sizes to obtain representative samples of fanpower terminal units installed into the field. In addition to themodels of airflow output and energy consumption, a modelwas developed to characterize the air leakage that occurred inthe parallel terminal units whe
21、n unit fan was off.RESULTS AND MODELSOne goal of this research was to determine if a singlegeneralized model could be used for all terminal units testedfor a given size. Because of design differences in the units,performance varied dramatically. Thus, no single model couldbe used to describe a given
22、 size unit. However, the models thatwere developed had the same form, but used different coeffi-cients for the different sizes and manufacturers.Variables were first identified that were expected to besignificant in explaining fan airflow and power. Models werethen developed by determining the most
23、statistically influen-tial independent variables using multiple linear and non-linearregression techniques. For the multiple linear regression, thevariable with the largest F statistic was added first. Statisti-cally significant variables were continually added to the modelprovided their respective
24、F statistic was above 4.0. Betweeneach step, models were compared against each other accord-ing to their adjusted coefficient of determination, R2adj(Neteret al. 1996). In developing the models for the parallel units, severalvariables were considered: the SCR voltage, Piad, Pdwn, Pup,and Qprimary. T
25、he models for all of the parallel terminal unitswere compared against each other. Any differences in termsincluded in the airflow or power models were investigated inan effort to create a single form model that would be applicableto all of the terminal units.Leakage ModelDuring the cooling mode, the
26、 terminal unit fan is off andthe backdraft damper was supposed to prevent any air fromcircuiting backwards through the fan. For this case, the airflowoutput downstream of the terminal unit should have been equalto the inlet primary airflow. However, it was discovered that airleakage occurred at the
27、backdraft damper and through thesheet metal seams of the terminal unit. A leakage model wasdeveloped to quantify the amount of air leakage from theterminal units.The primary factor that was expected to influence airleakage was the pressure inside the terminal unit. Becausethere was no physical obstr
28、uction at the outlet of the terminalunits, the static pressure inside the terminal units was assumedto be very close in value to the downstream static pressure.Therefore, the downstream static pressure was used as a proxyfor the pressure inside the box and was expected to be the mostsignificant vari
29、able in the leakage model.Initial analysis of the data confirmed that the downstreamstatic pressure played a significant role in air leakage. Air leak-age increased with an increase in downstream static pressurefor the 8 in. (203 mm) and 12 in. (304 mm) units (Figures 1and 2). The response between a
30、ir leakage and downstreamFigure 1 Air leakage for 8 in. (203 mm) inlet parallelterminal units.Figure 2 Air leakage for 12 in. (304 mm) inlet parallelterminal units.ASHRAE Transactions 85static pressure was very similar among the six terminal units.However, terminal unit P8A showed more scatter than
31、theother units.Air leakage occurred either through the sheet metal seamsof the terminal units or at the backdraft damper. The leakageat the seams was affected mainly by the static pressure insidethe terminal unit. The primary air velocity across the damperwas expected to influence the leakage around
32、 the backdraftdamper. Terminal units from group A utilized the primary air-operated backdraft damper. A change in primary air wouldhave an effect on the operation of this damper. In the terminalunits from groups B and C, the backdraft dampers were gravityoperated, and primary air velocity was expect
33、ed to have alesser effect, or possibly no effect on leakage. The pressure atthe inlet air differential sensor, Piad, was approximately linear(Appendix) with respect to the primary airflow entering theterminal over the ranges studied in this paper. Piadwas used toapproximate the influence of primary
34、air velocity.A leakage model using only Pdwnwas developed forparallel terminal unit P8C with a resulting R2adjof 0.917.Upon further analysis of the F-statistics, another model usingPdwnand Piadas explanatory variables was developed and theR2adjimproved to 0.970. Similar results were found for all of
35、the group A and C units. The results indicate that primaryairflow, as represented by Piad, played a statistically signifi-cant role in the air leakage from the terminal units. Similar analysis was conducted for unit P8B. The Piadterm failed the F-statistic test (F = 1.3 4.0), did not improvethe R2ad
36、jstatistic from 0.767, and was not included in themodel. The backdraft damper was not located in the primaryairstream for this unit as it was for group A and C units.The P12B terminal unit, with the backdraft damper out ofthe primary airstream, did not respond in the same way. Theaddition of the Pia
37、dvariable (with an F statistic of 87.6)increased the R2adjstatistic from 0.7398 to 0.9454 which indi-cated that Piadshould be included in the model. While the twogroup B units had the same backdraft damper configuration,the larger terminal unit had air dynamics acting on the back-draft damper that d
38、id not occur in the smaller terminal unit.More investigation would need to be conducted regarding theair dynamics within the terminal units.Air leakage was found to be dependent on Pdwn, and Piad(Equation 1). Table 1 provides the coefficients for each of theterminal units. In this model, the Pdwnter
39、m accounts for theeffect of the internal terminal unit pressure on leakage, whilePiadaccounts for the effects of primary air on the backdraftdamper.Qleakage= C1+ C2Pdwn+ C3Piad(1)Airflow ModelThis model quantified the amount of airflow goingthrough a terminal unit fan during the heating mode when th
40、efan was on. The fans on each of the terminal units were centrif-ugal, forward-curved style fans. The model for these fans wereexpected to follow typical fan curves and the fan laws(ASHRAE 2001).The SCR settings of the fans were a variable in the modelthat had to be quantified first. Each SCR settin
41、g correspondedto a different fan speed. A simple experiment was conductedto determine the relationship between the SCR setting and thespeed of the fan. A tachometer was instrumented to terminalunit P8A and at several different voltage settings, the RPM ofthe fan was measured. During this testing, th
42、e upstream anddownstream static pressures were held constant to eliminatethe effects of pressure on the fan speed. A quadratic equationwas fitted to the data for unit P8A (Figure 3) and had an R2of0.999.This test was conducted on two other terminal units, P12Band P8C, which resulted in R2values of 0
43、.994 and 0.997,respectively. Because of the high R2values for the variety ofgroups and sizes, it was assumed that a general quadratic rela-tionship would remain true for all of the terminal units even iftheir coefficients differed. A linear relationship between airflow and fan speed wasexpected (ASH
44、RAE, 2001). Because a quadratic equation hadbeen used to show the relationship between SCR voltage andfan speed, it was assumed that a different equation of the sameform could be used for the relationship between SCR voltageand fan airflow.Table 1. Coefficients for the Leakage ModelNameC1, cfmC2, cf
45、m/V2C3, cfm/in. w.g.R2adjP8A 23.15 101.70 12.31 0.937P8B 13.8 37.41 0 0.767P8C 16.86 77.55 10.76 0.970P12A 14.4 97.94 37.9 0.858P12B 17.83 58.26 27.16 0.945P12C 22.30 100.83 15.02 0.989Figure 3 Effect of SCR voltage on fan speed for parallelterminal unit P8A.86 ASHRAE TransactionsFrom an understandi
46、ng of fan curves and the fan laws, theonly other factor that should influence the fan output would bethe pressure across the fan. For all parallel terminal unitstested, the pressure on the front side of the fan was atmo-spheric. The pressure at the fan output was assumed to beapproximately equal to
47、the downstream static pressure. There-fore, this pressure would have the other significant influenceon the terminal fan capacity. The results typical for the termi-nal units of groups B and C confirm the effect on fan airflowdue to the downstream static pressure and the SCR voltage(Figure 4).The res
48、ults from group A (Figures 5 and 6) differed withthat of groups B and C. Two reasons possibly explain thisdifference in results. First, parallel terminal unit P8A appearedto have significant air leakage. Second, both terminal units hada different style backdraft damper that could have affected thefa
49、n performance.Parallel unit P8A leaked more air than any of the otherunits (Figure 1). Additionally, the coefficient, C2, of the leak-age model for P8A was the highest of the 8 in. (203 mm) units(Table 1). This part of the model estimates the leakage relatedto the internal static pressure of the terminal unit. It would beexpected that a terminal unit with greater leakage would havea model that gave more weight towards Pdwn. The leakagemodel for P8A displayed this characteristic, confirming theleakage that occurred when the unit fan was off.At higher downstream static pressures, the
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