Chilled-Water VAV Systems - HVAC.Amickracing

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Primary System ComponentsTo allow a housed fan to achieve similar discharge sound levels, adischarge plenum can be added to the air-handling unit. While thisplenum helps reduce discharge sound levels, it increases fan input powerand increases the overall length of the unit (Figure 25 and Figure 26). Inthis example, when a discharge plenum is added to the housed fan, theinput power for the housed AF fan is higher than for the direct-driveplenum fan (Figure 26).Figure 25. Plenum fan can reduce overall AHU lengthSAhousedfanRAOASAdischargeplenum18.3 ft (5.6 m)15.0 ft (4.6 m)SARASAplenumfanOASource: Images from Trane TOPSS programFigure 26. Discharge sound levels with multiple discharge connections 1,2housed AF 25 in. (635 mm) 2unit length = 15.0 ft. (4.6 m)input power = 14.7 bhp (10.9 kW)housed AF 25 in. (635 mm) + dischargeplenumunit length = 18.3 ft. (5.6 m)input power = 16.1 bhp (12.0 kW)belt-driven plenum AF 35.56 in. (903 mm)unit length = 15.0 ft. (4.6 m)input power = 17.4 bhp (13.0 kW)direct-drive plenum AF 30 in. (762 mm)unit length = 15.9 ft. (4.8 m)input power = 14.6 bhp (10.9 kW)discharge sound power (Lw), dB ref 10 -12 W110100908070605063 125 250 500 1000 2000 4000 8000octave band frequency, Hz1 Based on a typical VAV air-handling unit configuration with multiple discharge openings (Figure 25),operating at 13,000 cfm (6.1 m 3 /s) and 3 in. H2O (750 Pa) of external static pressure drop.2 Discharge sound power for a housed AF fan with a single (front-top) discharge opening (Figure 24),included here to demonstrate the sound reduction provided by the discharge plenum.34 Chilled-Water VAV Systems SYS-APM008-EN
Primary System ComponentsWhen space is a prime consideration, and multiple supply ductconnections are desired, a housed fan requires a discharge plenum toallow for the multiple connections. If a plenum fan is used, however,multiple duct connections can be made to the fan module itself,eliminating the need for a discharge plenum, and resulting in a shorterair-handling unit (Figure 25).• Belt-driven versus direct-driveHistorically, most large fans used in VAV systems were belt-driven.However, with the increased use of VFDs, direct-drive fans have becomepopular, primarily with plenum fans. With a direct-drive plenum fan, thefan wheel is mounted directly on the motor shaft, rather than using a beltand sheaves (Figure 23). Because there are no belts or sheaves, and fewerbearings, direct-drive fans are more reliable and require lessmaintenance. In addition, there are no belt-related drive losses, so directdrivefans are typically more efficient, quieter, and experience lessvibration (Table 6, Figure 24, and Figure 26).For more information on direct-drivefans and using multiple versus singlefans, refer to the Trane engineeringbulletin titled “Direct-Drive Plenum Fansfor Trane Climate Changer AirHandlers” (CLCH-PRB021-EN).Figure 27. Fan array with direct-driveplenum fansupstream (inlet) sideHowever, the air-handling unit may be slightly longer since the fan motoris mounted at end of the shaft.• Single versus multiple fansMost fans in VAV systems use a single fan wheel. However, usingmultiple fans can shorten the length of the AHU. This is often referred toas a fan array (Figure 27).For a given airflow, a unit with multiple fans uses several, smallerdiameterfan wheels, rather than a single, larger-diameter fan wheel. Thedistance (length) required both upstream and downstream of the fan istypically a function of the fan wheel diameter. Therefore, using multiple,smaller-diameter fan wheels can shorten the required upstream anddownstream spacing required, and can shorten the overall length of theair-handling unit.While the overall length can be reduced significantly by changing fromone to two fans, the potential length reduction diminishes as the numberof fans increases. When more than four to six fan wheels are used, theupstream and downstream spacing requirements begin to be dictated bythe need for access rather than by fan wheel diameter, so there isgenerally little further length reduction benefit.Another benefit of using multiple fans is redundancy. If one fan fails,another fan is available to compensate. In most cases, three or four fansare able to provide the required level of redundancy.However, using multiple fans is typically less efficient and increases thecost of the air-handling unit.Blow-thru versus draw-thru?In a blow-thru configuration, the fan blows air through a cooling coil locateddownstream of the fan (Table 7, p. 37). The heat generated by the fan andmotor is added to the air upstream of the cooling coil.downstream (outlet) sideSYS-APM008-EN Chilled-Water VAV Systems 35
Page 3 and 4: Chilled-WaterVAV SystemsJohn Murphy
Page 5 and 6: ContentsPreface ...................
Page 8 and 9: VAV terminal units ................
Page 10 and 11: Overview of a Chilled-Water VAV Sys
Page 20 and 21: Primary System Componentsunit are t
Page 22 and 23: Primary System ComponentsAvoiding f
Page 24 and 25: Primary System ComponentsFigure 10.
Page 28 and 29: Primary System Componentsweather, a
Page 32 and 33: Primary System Componentsair to a t
Page 34 and 35: Primary System Componentsbarometric
Page 36 and 37: Primary System ComponentsFigure 20
Page 38 and 39: Primary System ComponentsSupply fan
Page 40 and 41: Primary System ComponentsFan typesV
Page 44 and 45: Primary System ComponentsIn a draw-
Page 46 and 47: Primary System ComponentsSupply fan
Page 48 and 49: Primary System ComponentsThe modula
Page 50 and 51: Primary System ComponentsTable 8. A
Page 54 and 55: Primary System Componentssystem. So
Page 56 and 57: Primary System ComponentsPhotocatal
Page 58 and 59: Primary System ComponentsPCO techno
Page 60 and 61: Primary System Componentsand upstre
Page 62 and 63: Primary System ComponentsThermal pe
Page 66 and 67: Primary System ComponentsFan-powere
Page 68 and 69: Primary System Componentspowered VA
Page 70 and 71: Primary System Componentsinterfere
Page 72 and 73: Primary System ComponentsIf the des
Page 74 and 75: Primary System ComponentsTypically,
Page 76 and 77: Primary System ComponentsInterior z
Page 78 and 79: Primary System ComponentsInterior z
Page 80 and 81: Primary System ComponentsEqual fric
Page 82 and 83: Primary System Components• If nee
Page 84 and 85: Primary System ComponentsIn additio
Page 88 and 89: Primary System ComponentsAir-cooled
Page 90 and 91: Primary System Componentstemperatur
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Primary System ComponentsFigure 74.
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Primary System ComponentsVariable-
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Primary System ComponentsFreeze pre
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Primary System Componentsrow coil i
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Primary System ComponentsThermal st
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Primary System Components(although
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Primary System ComponentsFigure 80.
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Primary System ComponentsA variable
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System Design Issues and Challenges
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System Design VariationsThis chapte
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System Design Variationspressure dr
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System Design VariationsImpact on o
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System Design Variations• Run 7 v
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System Design Variationsmechanical
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System Design Variations• Impleme
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System Design Variationsconventiona
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System Design VariationsFigure 113.
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System Design Variationsbuildings (
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System Design VariationsFor most ap
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System Design VariationsWhile the s
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System Design VariationsFigure 120.
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System ControlsThis chapter discuss
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System Controlssensor from being in
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System ControlsFigure 126. Modulate
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System Controlsfixed enthalpy contr
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System ControlsIf the air-handling
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System ControlsFigure 133. Floor-by
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System ControlsVAV terminal unitsSi
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System Controlspeople in a zone, an
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System ControlsAlternatively, if th
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System Controlsvariable-speed drive
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System ControlsFigure 137. System-l
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System ControlsTable 31. Example of
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System ControlsFigure 139. Morning
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System Controlszone to the unoccupi
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System ControlsThe optimal starting
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System ControlsFigure 143. Fan-pres
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System ControlsTable 36 lists condi
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System Controlsreset to be used dur
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System ControlsFigure 147. Ventilat
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System Controlsdecrease overall sys
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System ControlsFigure 150. Optimize
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GlossaryACH. Air changes per hour.A
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Glossarycondensing boiler. A type o
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Glossaryelectronic air cleaner. Par
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GlossaryMERV. Minimum Efficiency Re
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Glossarypump-pressure optimization.
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GlossaryTR. Cold-spot thermal resis
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ReferencesAir-Conditioning and Refr
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References______. 2009. Direct-Driv
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References______. 2008. “ASHRAE S
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Indexchilled-water system 79air-coo
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Indexhumidifiers 126humidity contro
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IndexVAV controlpressure-dependent
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Tranewww.trane.comFor more informat
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