1、2008 ASHRAE 75ABSTRACTThis paper is the first of three papers on the developmentof experimental performance models of variable air volumefan powered terminal units. Tests were conducted on bothparallel and series fan powered terminal units. Data fromthese tests were used to develop empirical models
2、of airflow,power, and leakage of both parallel and series fan powerterminal units. These models are suitable for use in annualenergy use models of variable air volume systems in commer-cial buildings. This paper provides a description of the exper-imental apparatus, the terminal units, and measureme
3、nts forairflow and power. Both 8 in. (203 mm) and 12 in. (304 mm)primary air inlet terminal units from three manufacturerswere evaluated. INTRODUCTIONVariable Air Volume (VAV) systems maintain comfortconditions by varying the volume of primary air delivered toa space. A VAV system (Figure 1) often c
4、onsists of a centralair handling 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
5、an open area. VAVterminal units that include a fan to improve circulation withina zone are called fan powered terminal units. These terminalunits can draw in air from the plenum area and mix it withprimary air from the central Air Handling Unit (AHU) tomaintain comfort conditions in the occupied spa
6、ce. There are two configurations for fan powered terminalunits: series and parallel. The fan can be in the path of theprimary airflow (Figure 2). This configuration is a called a fanpowered series terminal unit. The controller will modulate theterminal unit damper in response to the control signals
7、fromthe thermostat and air velocity sensor. The fans on these termi-nal units output a constant amount of air that does not varywith load because the downstream pressure is constant (Alex-ander and Int-Hout 1998). As a result, when the primary airdamper closes, more plenum air is induced and recircu
8、latedinto the space. When the signal from the inlet air velocitysensor indicates that the primary airflow has reached a prede-termined minimum (because of ventilation requirements), thedamper will not close any more. If the space is still too cold,electric or hot water supplemental heat can be used
9、to meet thethermostat setpoint.When the fan is outside the primary airflow, the configu-ration is called a fan powered parallel terminal unit (Figure 3).During operation, the fan for a parallel terminal unit cycles onand off. During periods of maximum cooling, the fan is off. Abackdraft damper preve
10、nts cold air from blowing backwardsthrough the fan. The terminal unit primary air damper modu-lates the airflow to maintain the space temperature setpoint.An inlet air velocity sensor within the primary air streamallows the unit controller to maintain a consistent volume ofairflow to the zone depend
11、ing on the temperature setpoint.When the primary airflow drops below a specified amount, thecontroller activates the fan. At this point, the terminal unitmixes primary air with air being drawn in from the plenum.Electric or hot water supplementary heat can be used for addi-tional heating. Depending
12、on the control scheme, the control-ler can continue to reduce primary air to the conditioned spaceby adjusting the damper. In the field, the fan on a VAV terminal unit often must befine tuned (test and balancing) to provide the airflow output forPerformance of VAV Fan-Powered Terminal Units: Experim
13、ental Setup and MethodologyJames C. Furr Dennis L. ONeal, PhD, PE Michael A. DavisFellow ASHRAEJohn A. Bryant, PhD, PE Andrew CramletMember ASHRAE Student Member ASHRAEJames C. Furr is a Thermal Management Engineer at Lockheed Martin, Fort Worth, TX. Dennis L. ONeal is Holdredge/Paul Professor andHe
14、ad and Andrew Cramlet is a student at the Department of Mechanical Engineering, Texas A Qfan, the airflow through thefan, and Powerfan, the power consumption of the terminal unitfan. The independent variables were:1. The static pressure upstream of the terminal unit, Pup,2. The static pressure downs
15、tream of the terminal unit, Pdwn,3. The speed of the terminal unit fan controlled by the SCR,as represented by the RMS average voltage to the unit,4. The position of the terminal units damper, and 5. The control pressure from the flow sensor, Piav,. Thisvariable was directly affected by the position
16、 of thedamper and the upstream static pressure. Before testing a unit, each of the independent variableswas assigned a set of specific values. The number of levels foreach of the variables and their values are shown in Table 5. Thevalues for the levels differed across VAV terminal unitsbecause the m
17、aximum and minimum values for certain vari-ables differed across units. The maximum and minimumvalues for the SCR voltage were determined by adjusting theSCR setscrew completely in both directions. The maximumvalue for the damper setting was defined as when the damperwas horizontal, or fully open an
18、d minimum was defined aswhen the damper was closed. The levels for downstream staticpressure varied from 0.1 to 0.5 in w.g. (25 to 125 Pa). Thelevels for upstream static pressure varied depending on the testbeing run. Figure 11 Schematic of experimental test setup.Figure 12 Volumetric airflow balanc
19、e of a terminal unit.ASHRAE Transactions 81The characterization of a terminal unit consisted ofseveral tests. These tests were conducted for each combinationof damper and SCR settings. In every test, data for each combi-nation of upstream and downstream static pressure levels wereobtained. This proc
20、ess was a full-factorial design because datapoints for all combinations of independent variables wereobtained. The sequence of these tests usually consisted ofrunning the tests for all of the SCR speeds at a single damperposition, adjusting the damper to the next position, andcontinuing the sequence
21、.Before starting a test, the damper and SCR were manuallyadjusted to the desired positions according to the test beingrun. Throughout a test, the damper and SCR would remain inthe same position. During a test, the data acquisition systemallowed the user to adjust the VSDs on the upstream anddownstre
22、am blowers to meet desired conditions for a testpoint. The upstream static pressure was first adjusted to thesmaller of the following: the point where the primary airflowwas approximately 5% greater than the terminal units speci-fied maximum or 2 in. w.g. (498 Pa). This pressure was desig-nated as t
23、he maximum level for the upstream static pressurevariable. The minimum upstream static pressure setting wasdetermined by the downstream pressure. It could not be lowerthan the downstream static pressure because primary airwould flow backwards into the terminal unit. Each test hadthree minimum level
24、upstream static pressures. These mini-mums were selected to be approximately 0.25 in. w.g (60 Pa)greater than the corresponding downstream static pressure,except in cases where damper position caused insufficientprimary airflow. For each downstream static pressure, a thirdpoint was obtained for the
25、upstream static pressure approxi-mately halfway between the corresponding minimum andmaximum. This procedure resulted in three data points foreach downstream static pressure level, and nine points per test.The upstream and downstream blowers were manuallyadjusted to the desired conditions for a spec
26、ific data point.After static pressures reached steady state, data were acquired DATA ACQUISITION SYSTEMA computer data acquisition system was used to obtain,process, and store data. This system consisted of a personalcomputer, two separate data acquisition cards, and the termi-nation blocks for all
27、signal wires.An eight channel, sixteen-bit sample-and-hold data cardwas used to measure instantaneous current and voltage. Thesimultaneous sample and hold prevented any introduction oferror due to phase shift between the voltage and currentsignals. The elimination of phase shift allowed for accurate
28、determination of the power factor for the VAV unit fans. Theanalog inputs had a resolution of 16 bits. The other data acquisition card was an eight channel card,with two analog outputs to control the variable speed drives onthe test setup assist blowers. The resolution of the analoginputs on this ca
29、rd was 12 bits.SUMMARYThis paper is the first of three papers. Tests wereconducted on six parallel and six series variable air volume fanpowered terminal units. Both 8 in. (203 mm) and 12 in. (304mm) primary air inlet terminal units from three manufacturerswere evaluated. This paper provides a descr
30、iption of theTable 4. Pressure Transducer SizingPoint Name Transducer SizeDifferential Pressure Across Nozzles, Fig 12 0-6 in. w.g.(0 1.5 kPa)Differential Pressure Across Nozzles, Fig 15 0-6 in. w.g. (0-1.5 kPa)Chamber Static Pressure, Fig 12 0-10 in. w.g. (0-2.5 kPa)Chamber Static Pressure, Fig 15
31、0-10 in. w.g. (0-2.5 kPa)Upstream Static Pressure 0-2 in. w.g. (0-0.5 kPa)Downstream Static Pressure 0-2 in. w.g. (0-0.5 kPa)Inlet air velocity Sensor Pressure 0-2 in. w.g. (0-0.5 kPa)Table 5. Test Variable LevelsIndependent Variable Number of Levels ValuesUpstream Static Pressure 3varied from 0.3 t
32、o 2 in. w.g.(75 to 498 Pa)Downstream Static Pressure 30.1, 0.25, 0.5 in. w.g(25, 62, 125 Pa)SCR Voltage (Fan Speed) 4 Equally spacedDamper Position 4 Equally spaced82 ASHRAE Transactionstwelve fan powered terminal units, the experimental appara-tus, the test procedure, and the data acquisition syste
33、m. ACKNOWLEDGMENTS This work was a part of a project funded by ASHRAEunder RP-1292 and we would like to thank the project moni-toring subcommittee of TC 5.3 and the manufacturers theyrepresent for their support during the project. Several manu-facturers donated terminal units for use in this study.
34、Throughcooperative ventures such as these, ASHRAE research fund-ing can be utilized to the fullest. We appreciate the contribu-tions from these industry leaders.NOMENCLATUREPdwn = downstream static pressure, in. w.gPiav= pressure across inlet air velocity flow sensor, in. w.g.Punit= static pressure
35、inside terminal unit, in. w.g.Pup= upstream static pressure, in. w.g.Powerfan= power consumption of terminal unit fan, WQfan= amount of airflow through terminal unit fan, cfmQinduced= amount of airflow induced from plenum, cfmQleakage= amount of airflow leaking from a terminal unit, cfmQout= amount
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