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Selecting VAV Terminals for
Acceptable IAQ
Selecting VAV Terminals forSelecting VAV Terminals for
Acceptable IAQAcceptable IAQ
Dan Int-HoutChief Engineer, Krueger
Richardson, Texas
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Where We Are Today:Where We Are Today:
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The Rules
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Air Distribution DeviceAir Distribution Device
Selection GuidelinesSelection Guidelines
The ASHRAE fundamentals handbook, chapter 20 (AirDistribution), provides guidance on several methods of airdistribution.
Methods include overhead fully mixed, as well as fullystratified and partially mixed systems from below, and even
task /ambient personal air delivery systems. Described delivery systems include constant and variable
volume, UFAD, displacement ventilation and chilled beamsystems.
All have advantages and disadvantages, which must beunderstood by the design engineer and architect.
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Well Mixed system design
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Air Distribution DeviceAir Distribution Device
Selection GuidelinesSelection Guidelines
For overhead well mixed systems, one shouldselect a unit with throw at max, and minimum, flowthat meets ADPI guidelines based on diffuserspacing and T50 (throw to 50 fpm).
Additionally, select for maximum mixing:
Noise can be good.
Dirt on the ceiling is not bad.
Air Distribution Effectiveness (ADE) is a new term
describing room air mixing, and is a parameterwith all delivery methods.
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ADE (Ez)ADE (Ez)
Air Distribution Effectiveness replaces ventilationefficiency and other terms.
Older ASHRAE 62-99 minimum outside air quantitieswere based on an Ez = 1.
Tests using ASHRAE 129 show that with ceilingcooling systems, Ez always = 1.
Heating system performance, however, can reduce Ezto less than 70% (as low as 20%).
Current Std 62 (from addendum N) wording sets Ez =0.8 when heating when rules not followed.
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POORLY ADJUSTED / SELECTED DIFFUSER
THERMOSTAT
Air Distribution, Poor PatternAir Distribution, Poor Pattern
ExampleExample
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Air Distribution, Poor PatternAir Distribution, Poor Pattern
ExampleExample
THERMOSTAT
POORLY ADJUSTED / SELECTED DIFFUSER
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PROPERLY ADJUSTED DIFFUSER
THERMOSTAT
Good Pattern ExampleGood Pattern Example
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ADPI Air DiffusionADPI Air Diffusion
Performance IndexPerformance Index Single Number Rating: % of points in a room that meet a
comfort criteria based on temperature differences andairspeeds.
In practice, a measure of well mixed.
In ASHRAE fundamentals since 1967.
Agrees with ASHRAE & ISO comfort requirements.
Can be predicted from a table in ASHRAE fundamentals,Chapter 20.
Can be measured using ASHRAE 113.
Selection to achieve acceptable ADPI was first recommended inreview draft of std 62-89, and is in the current ASHRAE
Fundamentals handbook, Chapter 20. ADPI is the ONLY means of documenting compliance to
Standard 55 in the design phase.
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ASHRAE Journal, 2004ASHRAE Journal, 2004
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2 x L2 x L
(Separation)(Separation)
Area Conditioned by DiffuserArea Conditioned by Diffuser
44--way Diffuserway Diffuser
SelectionSelection
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Perforated 24X24, 10 inlet, 4 way, 20 Delta-T
2
4
6
8
10
12
14
16
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
CFM/Sq.Ft.
1/2UnitSeparationDis
tance
420350
300
250
160
Range
Spacing for 80% ADPI
NC=35
CFM
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Prism 24x24
0
4
8
12
16
20
24
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
cfm/SqFt
1
/2
U
nitS
e p a ra tio n
Dis
ta nc e ,L
600
545
436
273
109
10in. Neck
NC=37
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ADPI and LEED
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THERMOSTATCold
Outside
Window
Common Overhead Heating Design
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Perimeter Considerations:Perimeter Considerations:
Maximum delta-t for effective mixing when heating,per ASHRAE handbook = ?.
= 15F (90F discharge), continuous operation.
Throw toward and away from glass.
150 FPM should reach 4-5 feet from the floor.
ASHRAE 62.1 requires that ventilation be increasedby 25% when heating, if the above rules are notfollowed.
Typical perimeters require only 8Fdelta-t @ 1 cfm/sq.Ft.
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Perimeter Considerations:Perimeter Considerations:
From 2009 ASHRAE Fundamentals, Chapter 20:
Diffusers should be located such that the published 150 fpm
isothermal throw (which is typically unaffected by t) extends
to within 4.5 ft of the floor.
According to ASHRAE Standard62.1, if throw does not meet
this requirement, and the discharge-to-room temperature
differential exceeds 15F, the ventilation rate must be increased
by 25%. Furthermore, when the room-to-discharge differential
exceeds 15F, it is unlikely that the vertical temperature
limitation of ASHRAE Standard55 will be met.
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ASHRAE Standard 62.1ASHRAE Standard 62.1
Table 6.2Table 6.2