Chilled-Water VAV Systems - HVAC.Amickracing

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Primary System ComponentsSupply fan and return fanThe final configuration uses a fan in the return-air path, rather than in theexhaust air path. When the system includes a return fan, the supply fan muststill create the same pressure at its outlet (A) to overcome the pressure lossesassociated with the supply-air path (Figure 22).Figure 22. VAV system with supply and return fans (25% outdoor air)zonereturn-air grille,ceiling plenum,return ductworkAHUmain supplyductworkVAVterminalunitsupply ductrunouts,diffuserszonestatic pressure relative to outdoors+0-returnfanFCAsupplyfanBreturn-airdamperHowever, the pressure at the inlet of the supply fan (B) only needs to be lowenough to overcome the pressure losses associated with drawing the returnair through the return-air damper, filter, and coils (or the pressure associatedwith drawing the outdoor air through the outdoor-air damper, whichever ishigher). The return fan is used to overcome the pressure losses associatedwith drawing the return air out of the zones and through the return-air grilles,open ceiling plenum and/or return ductwork. It must also generate enoughpressure (F) to push any relief air out through the relief damper.Should the system use a relief fan or a return fan?The “supply fan and relief fan” configuration usually works best in VAVsystems that use an open ceiling plenum for part of the return-air path(Table 5). Systems with relief fans are easier to control, typically lower thecost of the air-handling unit, and are often less costly to operate than systemswith return fans. (See “Building pressure control,” p. 178.)When the pressure drop through the return-air path is very high (which maybe the case in a larger system with a fully ducted return-air path), evaluateboth the relief- and the return-fan configurations. If the supply fan is capableof handling the pressure drop of both the supply- and return-air paths, thenthe relief-fan configuration is preferred for the reasons mentioned above. Usea return fan only if the return-air path adds more pressure drop than thesupply fan can handle.30 Chilled-Water VAV Systems SYS-APM008-EN
Primary System ComponentsTable 5. Return fan versus relief fan in a VAV systemReturn FanAdvantages:Disadvantages:• Lower differential pressure across the supply fan, if thepressure drop of the return air path is greater than thepressure drop of the outdoor air path (which is likely in mostVAV systems).• Potentially lower installed cost for a system with a fullyducted, return-air path. A smaller differential pressure acrossthe supply fan can result in a smaller fan motor and variablespeeddrive.Relief fanAdvantages• Lower operating cost. In some applications, the relief fan canremain off during “non-economizer” hours 1 and operate at lowairflow during many “economizer” hours. Also, the return-airdamper can be sized for a lower pressure drop.• Simpler control scheme. One less sensor and one lessactuator simplifies installation and air balancing. Applicationswith a relatively low pressure drop through the return-air path(which is common in a system that uses an open ceilingplenum for part of the return-air path) can use lower-cost fansas well as fewer (less costly) controls.• Greater layout flexibility. The relief fan can be positionedanywhere in the return-air path because the supply fan drawsthe return path negative (relative to the occupied spaces)during modulated economizer operation. An air-handling unitinstalled in the basement with a central relief fan installed onthe roof can take advantage of winter stack effect to loweroperating cost.• Higher operating costs, especially in applications with extended hours ofeconomizer cooling 1 (the return fan must run whenever the supply fanoperates).• Potential for air to leak out through the relief damper, because the returnairplenum inside the air-handling unit operates at a positive pressure (Fin Figure 22). Note: Using low-leak relief dampers can minimize airleakage to the outdoors.• More complex (expensive) fan-speed control. Controlling the pressure inthe return-air plenum inside the air-handling unit requires an additionalpressure sensor and modulating device (either a damper actuator orvariable-speed drive), and the pressure sensor is difficult to situatebecause this plenum is usually small and turbulent. (See “Return-fancapacity control,” p. 181.)• Requires more fan power at part load. The pressure in the return-airplenum inside the air-handling unit must always be high (positive) enoughto force air out through the relief damper. The return-air damper musttherefore create a significant pressure drop between the positive returnairplenum and the negative mixed-air plenum. This added pressure droprequires more combined fan power than a system with a relief fan.• Limited layout flexibility. The return fan must be situated between the airhandlingunit and the closest leg of the return-air path (usually near theair-handling unit) because it must draw the entire return path negativerelative to the occupied spaces. It must also discharge into the return-airplenum during modulated economizer operation.Disadvantages• Potential for negative building pressure at low loads. This condition canoccur when a variable-speed drive controls the relief fan, supply airflow isvery low, and required relief airflow is less than is delivered with the relieffan operating at lowest speed. Note: Using a constant-speed relief fanwith a modulating relief damper avoids this problem.• Potential for air to leak in through the relief damper because the return-airplenum inside the air-handling unit operates at negative pressurewhenever the relief fan is turned off (C in Figure 21). Note: Using low-leakrelief dampers can minimize air leakage from outdoors.• Higher differential pressure across the supply fan than in a system with areturn fan. The supply fan must overcome the pressure drop of the returnpath as well as the supply path. For this reason, an air-handling unit witha relief fan may not be capable of delivering the pressure differentialrequired in a system with a fully ducted return-air path.1 In many applications, when the system is bringing in minimum ventilation airflow (not in airside economizer mode), local exhaust fans (in restrooms and copycenters, for example) and exfiltration (due to positive building pressurization) are often sufficient to relieve all the air brought into the building for ventilation. Inthis case, no central relief is needed and the central relief fan can be turned off, only needing to operate during economizer mode.SYS-APM008-EN Chilled-Water VAV Systems 31
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 40 and 41: Primary System ComponentsFan typesV
Page 42 and 43: Primary System ComponentsTo allow a
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
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Primary System ComponentsAir-cooled
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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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