2012_outside_air_measurements.pdf
TRANSCRIPT
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Outside Air Intakes
Airflow Measurement Challenges
David S. Dougan
President, EBTRON Inc.
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Challenges
Intake systems are dynamic, not static.
Airflow rates are typically very low.
There is little distance between the
louver and damper to measure.
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Outside Air Intakes:
DYNAMICS
Damper Issues
Linkage and Actuator Hysteresis
will result in airflow variations
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Outside Air Intakes:
DYNAMICS
Minimum Outside Air - Fixed Minimum Damper set @ 900 CFM (90 FPM)
position set from open position with wind
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Outside Air Intakes:
DYNAMICS
Mixed Air Plenum Pressure Variations
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Outside Air Intakes:
DYNAMICS
Supply Fan Speed Changes
(VAV and multi-speed)
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Outside Air Intakes:
DYNAMICS
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Outside Air Intakes:
DYNAMICS
Wind Pressure Variations
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Outside Air Intakes:
DYNAMICS
DirectWind
CrossWind
Wind Effect
NoWind
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Outside Air Intakes:
DYNAMICS
Minimum Outside Air - Fixed Minimum Damper set @ 900 CFM (90 FPM)
position set from open position with wind
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Outside Air Intakes:
DYNAMICS
Stack Pressure Variations
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Outside Air Intakes:
DYNAMICS
No Stack Effect
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Outside Air Intakes:
DYNAMICS
Winter Stack Effect
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Outside Air Intakes:
DYNAMICS
Summer Stack Effect
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Outside Air Intakes:
DYNAMICS
CO2 DCV
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Outside Air Intakes:
DYNAMICS
N / (Ci Co) = Vo / P = scfm/person
CO2 Contaminant or Surrogate for Ventilation ?
CoVo
CiVo
NP
Co = Outside air CO2 level (scfm CO2/scfm Air)Ci = Inside CO2 level (scfm CO2/scfm Air)Vo = Outside air flow rate (scfm Air)N = CO2 production rate (scfm CO2/person)P = Number of peopleR = Rate (scfm OA/person)
Mass Balance Equation
In = OutCoVo + NP = CiVo
Simplifies to:N / (Ci-Co) = Vo / P = R
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Outside Air Intakes:
DYNAMICS
ASHRAE 62.1-2010 CO2 DCV
OA CFM provided at using fixed 1,000 ppm CO 2 setpoint
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35
Number of Adult Students
OutsideAir(CFM)
Single Setpoint CO2 DCV Response
ASHRAE 62.1-2010
Required Ventilation
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Outside Air Intakes:
DYNAMICS
ASHRAE 62.1-2010 CO2 DCV
OA CFM provided at using fixed 1,000 ppm CO2
setpoint
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35
Number of Adult Students
OutsideAir(CFM)
Single Setpoint CO2 DCV Response
ASHRAE 62.1-2010
Required Ventilation
If metabolic activity > assumed
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Outside Air Intakes:
DYNAMICS
ASHRAE 62.1-2010 CO2 DCV
OA CFM provided at using fixed 1,000 ppm CO2
setpoint
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35
Number of Adult Students
OutsideAir(CFM)
Single Setpoint CO2 DCV Response
ASHRAE 62.1-2010
Required Ventilation
If metabolic activity < assumed
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Outside Air Intakes:
DYNAMICS
ASHRAE 62.1-2010 CO2 DCV
OA CFM provided at using fixed 1,000 ppm CO2
setpoint
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35
Number of Adult Students
OutsideAir(CFM)
Vbz=Rp Pz + Ra Az
Vbz=Ra Az
(greater if local exhaust exceeds Ra Az)
Minimum OA airflow rate cutoff
Maximum OA airflow rate cutoff (when economizer is not active)
Single Setpoint CO2 DCV Response
ASHRAE 62.1-2010
Required Ventilation
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Outside Air Intakes:
DYNAMICS
ASHRAE 62.1-2010 CO2 DCV
OA CFM using CO2
and OA airflow to estimate the population
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35
Number of Adult Students
OutsideAir(CFM)
ASHRAE 62.1-2010
Required Ventilation
(left axis)
OA Response Using CO2 and OA
Airflow Rate to Calculate Vbz
Minimum OA airflow rate cutoff
Maximum OA airflow rate cutoff (when economizer is not active)
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Outside Air Intakes:
DYNAMICS
CA Title 24 CO2DCV
OA CFM provided at using fixed 1,000 ppm CO2setpoint
and 0.15 CFM/sq.ft. minimum
0
100
200
300
400
500
600
700
0 5 10 15 20 25 30 35
Number of Adult Students
OutsideAir(CFM) Single Setpoint CO2 DCV Response
Title 24
Required Ventilation
Maximum OA airflow rate cutoff (when economizer is not active)
Minimum OA airflow rate cutoff
15 CFM x max population
0.15 CFM/sq.ft.
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Outside Air Intakes:
LOW AIRFLOW RATES
0 250 500 1,000 2,000 5,000 10,000 15,000
Airflow Rate (FPM)
A
pplication
Technology
Thermal Dispersion Probes
Vortex Shedding
Pitot Tubes and Arrays
Piezo Rings
Outside Air Intakes
Duct Tracking
Fan Inlets
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Outside Air Intakes:
SOLUTIONS
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Solutions
Install permanently mounted airflow
measuring devices for control or, at a
minimum, alarm.
Flow rates?
Placement?
Other considerations?
Verification of performance? Field adjustment?
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Outside Air Intakes:
SOLUTIONS
OA Measurement & ControlGuidelines
Follow these basic rules and you will be successful: Select and apply airflow measuring devices suited for the
measurement of intake flow rates. Make certain the flow meter can measure the flow rates you
are trying to control.
Make certain that the flow rates are high enough to control andare not affected by transient wind gusts (> 150 FPM atminimum [ 200 FPM preferred] )
Choose the best measurement location.
Select and size quality control dampers. Use high quality, extruded aluminum blades, with long-lasting
and non-binding linkage.
Implement a control strategy that optimizes theperformance of your system.
Use the right sequences and slow it down!
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Outside Air Intakes:
SOLUTIONS
l= damper blade width
12 in.304.8 mm
l
l= damper blade width
6 in.157.4 mm
l= damper blade width
l
6 in.157.4 mm
6 in.157.4 mm
Probe(s)
Probe(s)
Probe(s)
Probe(s)
l
6 in.157.4 mm
Important: Actual plenum depth should bedetermined based on louver data and maximumairflow rates to minimize water carry-over into theintake system.
AIRFLOW
AIRFLOW
AIRFLOW
AIRFLOW
12 in.304.8 mm
OA Intake Placement
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Outside Air Intakes:
SOLUTIONS
Damper Interaction
Downstream Louver with Parallel Blade Damper (5% error)
0.0
100.0
200.0
300.0
400.0
500.0
600.0
0.0 100.0 200.0 300.0 400.0 500.0 600.0
REFERENCE FPM
UUTFP
MDamper at 100%
Damper at 80%Damper at 60%
Damper at 40%
Damper at 20%
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Outside Air Intakes:
SOLUTIONS
Damper Interaction
Downstream Louver with Opposed Blade Damper (5% error)
0.0
100.0
200.0
300.0
400.0
500.0
600.0
0.0 100.0 200.0 300.0 400.0 500.0 600.0REFERENCE FPM
UUTFPM
Damper at 100%
Damper at 80%Damper at 60%
Damper at 40%
Damper at 20%