Dedicated Outdoor Air Systems (DOAS) Stanley A Mumma Ph D PE Fresno ASHRAE Chapter Sept. 14, 2011 1 Stanley A. Mumma, Ph.D., P .E. Prof. Emeritus, Architectural Engineering Penn State University, Univ. Park, PA [email protected]Web: http://doas-radiant.psu.edu Key Presentation Points Problems with common VAV systems. DOAS defined. Parallel sensible terminal equipment choices. DOE Report: DOAS ranks first. System Selection Matrix. Issues 2 Conclusions.
Transcript
Dedicated Outdoor Air Systems (DOAS)
Stanley A Mumma Ph D P E
Fresno ASHRAE Chapter Sept. 14, 2011
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Stanley A. Mumma, Ph.D., P.E.Prof. Emeritus, Architectural Engineering
Key Presentation PointsProblems with common VAV systems.DOAS defined.Parallel sensible terminal equipment choices.DOE Report: DOAS ranks first.System Selection Matrix.Issues
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Conclusions.
Current HVAC system of choice: VAV
OA
Std. VAV AHU
VAV
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Space 1,VAV w/ single air
delivery path
Inherent Problemswith VAV Systems
Poor air distributionPoor humidity controlyPoor acoustical propertiesPoor use of plenum and mechanical shaft spaceSerious control problems, particularly with tracking return fan systemsPoor energy transport medium: air
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Poor energy transport medium: airPoor resistance to the threat of biological and chemical terrorismPoor and unpredictable ventilation performance
.
AHU% OAB=?
OAB=3,600 cfmOA=? 60
Poor & unpredictable vent’n performance.
OAreq’d=900 cfm
based on table 6-1
Z1=900/1,500
OAreq’d=1,350 cfm
6,000 cfm 1,500 cfm 4,500 cfm
Over vent=?1,350 cfm, Unvit
OA+(6,000-OA)*0.225=3,600OA=2,903, ~30% more, but no
LEED i t
OA=2,250? (900+1,350) No! OA=3,600? No! Why not?Eq. for OA?
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Z1=0.6 Z2=0.3, ,
Unvit ratio = 0.2251,350/6,000
LEED point
2,903-(900+1,350)=653more than table 6-1 value
Where does the 653 cfm go?
Can VAV limitations be overcome?
AHU% OAB =100OA=2,250 Condition of supply air, DBT & DPT?
OAreq’d=900 cfm OAreq’d=1,350 cfm
2,250 cfm 900 cfm 1,350 cfm
How is the space load handled,
when 6,000
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Z1=1 Z2=1cfm required for a VAV?
DOAS Defined for This Presentation
20%-70% Highless OA,than VAV
DOAS Unit w/ Energy Recovery
Cool/Dry Supply
P ll l
High Induction Diffuser
Building with Sensible
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Parallel Sensible
Cooling System
Sensible and Latent
Cooling Decoupled
Pressurization
Key DOAS Points1. 100% OA delivered to each zone via its
own ductworkl ll b d2. Flow rate generally as spec. by Std. 62.1
or greater (LEED, Latent. Ctl)3. Employ TER, per Std. 90.14. Generally CV5 U d l S/L l d D
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5. Use to decouple space S/L loads—Dry6. Rarely supply at a neutral temperature7. Use HID, particularly where parallel
system does not use air
TotalEnergy Energy
Recovery (TER)Wheel
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High Induction Diffuser
Provides complete air mixing
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p gEvens temperature gradients in the spaceEliminates short-circuiting between supply & returnIncreases ventilation effectiveness
Parallel Terminal SystemsDOAS air
Induction Nozzle
S C li C il
Radiant Cooling PanelsRadiant Cooling Panels
Chilled Beams
Sen Cooling Coil
Room air
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Fan Coil UnitsFan Coil Units
Air Handling UnitsCV or VAV
Air Handling UnitsCV or VAV
Unitary ACsi.e., WSHPsUnitary ACsi.e., WSHPsVRV
Multi-SplitsVRV
Multi-Splits
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Std. VAV AHU
OA Economizer
DOAS with Parallel VAV
OA
Outdoor air unit with TERVAV
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Space 2, DOAS in
parallel w/ VAV
Poor air distributionPoor humidity control
VAV Problems Solved with DOAS/Parallel VAV
Poor humidity controlPoor acoustical propertiesPoor use of plenum and mechanical shaft spaceSerious control problems, particularly with tracking return fan systemsP t t di i
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Poor energy transport medium: airPoor resistance to the threat of biological and chemical terrorismPoor and unpredictable ventilation performance
DOAS with Parallel FCUOther ways to
introduce OA at FCU? Implications?
Outdoor air unit with TER
OA
FCU
Implications?
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Space 3, DOAS in
parallel w/ FCU
Parallel vs. Series OA introduced for DOAS-FCU applications?
16Parallel, Good Series, Bad
Advantages of the correct paradigm parallel FCU-DOAS arrangement
At low sensible cooling load conditions, the terminal equipment may be shut off—saving fan energyThe terminal device fans may be down sized since they are not handling any of the ventilation air, reducing first costThe smaller terminal fans result in fan energy savingsThe cooling coils in the terminal FCU’s are not derated since they are handling only warm return air, resulting
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y g y , gin smaller coils and further reducing first cost.Opportunity for plenum condensation is reduced since the ventilation air is not introduced into the plenum near the terminal equipment, for better IAQ
Poor air distributionPoor humidity control
VAV Problems Solved with DOAS/Parallel FCU
Poor humidity controlPoor acoustical propertiesPoor use of plenum and mechanical shaft spaceSerious control problems, particularly with tracking return fan systemsP d
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Poor energy transport medium: airPoor resistance to the threat of biological and chemical terrorismPoor and unpredictable ventilation performance
DOAS with Parallel Radiant, or Chilled Beam
Outdoor air unit with TER
OA
Radiant Panel
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Space 3, DOAS in
parallel w/ CRCP
Poor air distributionPoor humidity control
VAV Problems Solved with DOAS/Radiant-Chilled Beam
Poor humidity controlPoor acoustical propertiesPoor use of plenum and mechanical shaft spaceSerious control problems, particularly with tracking return fan systems
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Poor energy transport medium: airPoor resistance to the threat of biological and chemical terrorismPoor and unpredictable ventilation performance
Additional Benefits of DOAS/Radiant-Chilled Beam
Beside solving problems that have gone unsolved for nearly 35 years with unsolved for nearly 35 years with conventional VAV systems, note the following benefits:Greater than 50% reduction in mechanical system operating cost compared to VAVEqual or lower first cost
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Equal or lower first costSimpler controlsGenerates up to 80% of points needed for basic LEED certification
DOAS Equipment on the Market TodayI: Equipment that adds sensible energy
recovery or hot gas for central reheatII E i t th t t t l II: Equipment that uses total energy
recoveryIII: Equipment that uses total energy
recovery and passive dehumidification wheels
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w ee sIV: Equipment that uses active
dehumidification wheels, generally without energy recovery
DOAS Equipment on the Market Today
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4850 gr
80 80% 60%
90.028
.024
020140
168
196
OA
EW
RA1 2 3 4
5
PH CC
Space
2H & h id
Wet Bulb
(F)
50
60
70
40%
20%
.016
.012
008
HUMIDITY RATIO
(Lbv/Lba)
.020140
112
84
56 (gra
ins/
lb)3
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Hot & humid OA condition
24DRY BULB TEMPERATURE (F)
40
40
6050
50
70 80 90 100 120
.004
.008
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56
Hum
idity
ratio
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DOAS & Energy Recovery
ASHRAE Standard 90.1 and ASHRAE’s new Standard for the Design Of High Performance Green Buildings (189.1) both require DOAS systems to utilize TER for almost the entire USA regardless of system size as illustrated in the next slide. They also require that the total effectiveness be at least
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require that the total effectiveness be at least 50 and 60% respectively.
The Stds permit the use of class 1-3 air w/ TER.
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Climate Zone 60% TER Req’d Std. 189.1-2009 Design Air flow when >80% OA1A, 2A, 3A, 4A, 5A, 6A, 7, 8 (Moist E. US + Alaska) > 0 cfm (all sizes require TER) 6B > 1,500 cfm1B, 2B, 5C > 4,000 cfm3B, 3C, 4B, 4C, 5B > 5,000 cfm
48.7 M 18.7 M 85.4 M 141.6 M
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1,603 hours in hot/humid region. 70% ε EW reduces peak load from 69T to 44T, (25T reduction) and saves
11,680 TH or $1,168/yr assuming 10,000 cfm OA, g , ,
• System performance in a category (i.e. 1st cost) rating 1-8 (8 Best): i.e. FCUw/ DOAS meeting 1st cost earns a 7
• Importance weighting of a category 1-5 (5 most important)
• Score: in a cell: product of importance weighting and system performance. i.e. for CRCP-DOAS in the category of Op $, the score is 4*8=32
Conventional VAV 145 pts: DOAS-Rad 254 pts
DOAS IssuesReserve capacityEW issues, including controlSA C ditiSA Conditions30% surplus OA for a LEED pointLost air side economizerFiltration/Terror resistanceP i i /fl
