practicalities in applying the astm tracer gas … exterior door n 17 feet common ... boiler room -...
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Practicalities in Applying the ASTM Tracer Gas Method to Measure Air Change Rates
M. T. Weeks, CIHF.W. Boelter, CIH, PEC. E. Simmons, CIH
American Industrial Hygiene Conference and ExpositionPhiladelphia, Pennsylvania – June 5, 2006
Introduction
ASTM Method E741 is a standardized method used to measure the air change rate of a single-zone space using a tracer gasHigh air change rates (e.g., >20 air changes/hour) and other conditions may make achieving the required equilibrium concentration of tracer gas difficultModifications and liberties in applying the method may be necessary in order to obtain valid results
ProblemIndustrial hygiene monitoring was to be performed in a small boiler roomBecause of the monitoring, no mechanical mixing of the air (e.g., fans) could be doneThe room had multiple points of air entry and exit, along with the boiler’s use of room airAir change rates were probably consistently >20 air changes per hour (ACH) and were highly variableBecause of equipment in room, directional airflow, and variable conditions, room air was not well-mixed
18 Ft
Boiler#1
Boiler#2 Interior
Door
Exterior Door
N
17 Feet
CommonExhaust
Flue
Open Louvered Vents
Work Area
Fans
Fans
Air Change Rate – ASTM E741
Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution (2000)
Allows direct measurement of air change rateConstant Injection Method used
Background measurements takenTracer gas introduced continuously at a known flow rateConcentration measured over time
Method requires equilibrium concentration of tracer gas
ASTM Method E741-00, Standard Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution. American Society of Testing and Materials, 2000.
Our Solution
Introduce tracer gas at multiple points throughout space to overcome mixing problemSample tracer gas at multiple points throughout space and create a composite sampleWhen air sampling was not being conducted or at times when room air could be mixed using fans, test to see if results were consistent
Equipment
Tracer gas – sulfur hexafluoride (SF6)Extremely low background levels (10-20 ppb)Easily detectable with proper equipmentNon-toxic (PEL = 1000 ppm) and inert
Innova 1412 Multi-Gas MonitorMeasures SF6 at ppb level
Manifold system with plastic tubing to distribute and sample gas
Needle valves used at injection end to ensure even distribution of the gas
Glass mixing jar and high volume pump to create composite air sample
Field Testing
Previous testing with no mechanical mixing showed air change rates to be approximately 20-35 air changes/hour (ACH)
Dependent on weather and work conditionsSeven tests were performed over two consecutive days
Three tests with fans offFour tests with fans on
Boiler Room - Air Change Rate AnalysisSulfur Hexaflouride Concentration vs. Time
Entire DayOctober 3, 2005
0
50
100
150
200
250
300
350
400
10:00 10:30 11:00 11:30 12:01 12:31 13:01 13:32 14:02 14:32 15:02
Time
Con
cent
ratio
n Su
lfur
Hex
aflo
urid
e (p
pm)
Boiler Room - Air Change Rate AnalysisSulfur Hexaflouride Concentration vs. Time
Entire DayOctober 3, 2005
0
50
100
150
200
250
300
350
400
10:00 10:30 11:00 11:30 12:01 12:31 13:01 13:32 14:02 14:32 15:02
Time
Con
cent
ratio
n Su
lfur
Hex
aflo
urid
e (p
pm)
24.4
19.2
27.3 21.5 26.7
Testing – Day 1
26.7On
21.5Off
27.3On
19.2Off
24.4On
Air Changes per HourFan Condition
Results were 22% lower without mechanical mixing
Boiler Room - Air Change Rate AnalysisSulfur Hexaflouride Concentration vs. Time
Entire DayOctober 4, 2005
0
50
100
150
200
250
300
350
400
8:51 9:13 9:35 9:57 10:18 10:40 11:02 11:24Time
Con
cent
ratio
n Su
lfur H
exaf
lour
ide
(ppm
)
Boiler Room - Air Change Rate AnalysisSulfur Hexaflouride Concentration vs. Time
Entire DayOctober 4, 2005
0
50
100
150
200
250
300
350
400
8:51 9:13 9:35 9:57 10:18 10:40 11:02 11:24Time
Con
cent
ratio
n Su
lfur H
exaf
lour
ide
(ppm
)
21.3 19.3
Testing – Day 2
19.3Off
21.3On
Air Changes per HourFan Condition
Results were 9% lower without mechanical mixing
Questions
Did introducing mechanical mixing alter the air change rate?How much of a factor were the weather conditions?Would more injection points have brought the results closer together?Were multiple sampling points necessary?
Conclusions
When conducting tracer gas testing, an equilibrium concentration should be the goalWhen good mixing is not achievable, equilibrium can be approached by using multiple injection pointsStandard methods provide excellent guidance, but can be applied to situations outside their scope if necessary
Practicalities in Applying the ASTM Tracer Gas Method to Measure Air Change Rates
Thank you!
Michael Weeks, CIHBoelter Associates, Inc.
Park Ridge, Illinois(847) 692-4700