2012_outside_air_measurements.pdf

7/29/2019 2012_Outside_Air_Measurements.pdf

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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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DYNAMICS

Minimum Outside Air - Fixed Minimum Damper set @ 900 CFM (90 FPM)

position set from open position with wind


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DYNAMICS

Mixed Air Plenum Pressure Variations


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DYNAMICS

Supply Fan Speed Changes

(VAV and multi-speed)


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DYNAMICS


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DYNAMICS

Wind Pressure Variations


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DYNAMICS

DirectWind

CrossWind

Wind Effect

NoWind


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DYNAMICS

Minimum Outside Air - Fixed Minimum Damper set @ 900 CFM (90 FPM)

position set from open position with wind


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DYNAMICS

Stack Pressure Variations


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DYNAMICS

No Stack Effect


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DYNAMICS

Winter Stack Effect


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DYNAMICS

Summer Stack Effect


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DYNAMICS

CO2 DCV


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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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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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DYNAMICS


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


OutsideAir(CFM)


ASHRAE 62.1-2010


If metabolic activity > assumed


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DYNAMICS



setpoint

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35


OutsideAir(CFM)


ASHRAE 62.1-2010


If metabolic activity < assumed


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DYNAMICS



setpoint

0

100

200

300

400

500

600

700

0 5 10 15 20 25 30 35


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)


ASHRAE 62.1-2010



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DYNAMICS


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


OutsideAir(CFM)

ASHRAE 62.1-2010


(left axis)

OA Response Using CO2 and OA

Airflow Rate to Calculate Vbz




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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


OutsideAir(CFM) Single Setpoint CO2 DCV Response

Title 24




15 CFM x max population

0.15 CFM/sq.ft.


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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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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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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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SOLUTIONS

l= damper blade width

12 in.304.8 mm

l


6 in.157.4 mm


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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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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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%

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