1. Ventilation Measurement Tracer Gas Decay Testing
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Ventilation Measurement
Tracer Gas Decay Testing
Sonya Milonova
Research Fellow
Harvard School of Public Health
July 9, 2015

2. What we’ve learned so far…
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What we’ve learned so far…
The main objective of ventilation: supply clean air and remove contaminants as efficiently as possible
How good is our ventilation system? How effective is the ventilation system at removing contaminants from the room? How do we measure this?

3. Assessing Ventilation Performance
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Assessing Ventilation Performance
Does the system provide sufficient ventilation rate?
Is the overall airflow direction in the building from clean to dirty zones?
How efficient is the system in delivering outdoor air to each location in the room?
How efficient is the system in removing the airborne contaminants from each location in the room?
Flow rate or ventilation rate: amount of outdoor air and quality of outdoor air
Can be thought of as air distribution in a space
air distribution or airflow pattern — the external air should be delivered to each part of the space in an efficient manner and the airborne pollutants generated in each part of the space should also be removed in an efficient manner.

4. Ventilation Effectiveness
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Ventilation Effectiveness
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The ability of the system to exchange air in the room
Air change efficiency
Local air change index
The ability of the system to remove airborne contaminants from the room
Contaminant removal effectiveness
Local air quality index

5. 1/2/2019
Air Change Efficiency
“A measure of how quickly air in the room is replaced”
ε 𝑎 = 𝑠ℎ𝑜𝑟𝑡𝑒𝑠𝑡 𝑝𝑜𝑠𝑠𝑖𝑏𝑙𝑒 𝑎𝑖𝑟 𝑐ℎ𝑎𝑛𝑔𝑒 𝑡𝑖𝑚𝑒 𝑎𝑐𝑡𝑢𝑎𝑙 𝑎𝑖𝑟 𝑐ℎ𝑎𝑛𝑔𝑒 𝑡𝑖𝑚𝑒
Shortest possible air change time: (almost) plug flow (on left) has shortest possible air change time; tau n, depends only on room volume and the ventilation flow rate
Normally expressed as a percentage… Ideal piston flow = 100%... Fully mixed flow = 50%; if below 50, not perfectly mixed
The shortest possible air change time can also be compared with the local air change indices – a measure of how quickly the air in a point is replaced
The local air change index, ε 𝑎 𝑝 , characterises the conditions at a particular point.
Drawn by Tobias van Reenen, adapted from [1]

6. Drawn by Tobias van Reenen, adapted from [1]
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Contaminant Removal Effectiveness
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“A measure of how quickly an airborne contaminant is removed from the room”
ε 𝑐 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑒𝑥ℎ𝑎𝑢𝑠𝑡 𝑚𝑒𝑎𝑛 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 𝑟𝑜𝑜𝑚
𝑸 𝟐
𝑸 𝟏
Both figures show short circuiting; contamination release is the same in both cases, as is concentration in exhaust. But the mean concentration in the room is different --
On your left: contamination is in a stagnant zone; mean concentration is high
On right: contamination source is near exhaust; mean concentration is low
In a fully mixed room, conc in exhaust is same as conc in room, therefore CRE = 1
If you move contaminant closer to exhaust, local air quality index improves
CRE: you want concentration of contaminants in the space to be low compared with the concentration in the exhaust air.
The exhaust concentration can also be compared to local values of the concentration of contaminants, these indices are called local air quality indices – a measure of the local concentration of a contaminant in a point
Drawn by Tobias van Reenen, adapted from [1]

7. 1/2/2019
Consider a point, P…
Local mean age of air: average time required for the supply air to reach P.
A measure of the air quality at a given point.
Room mean age of air: the average of the age of air at all points in the room.
Consider a point P in a room. The age of air is the length of time required for the supply air to reach the point. As air can reach the point through various paths, the mean value of the ages at the point is called the local mean age (LMA) of the air at P. Likewise, the length of time required for the contaminant located at P to reach an exhaust is called the residual lifetime of the contaminant at P. The mean value through various paths is the local mean residual lifetime (LMR).
Air change efficiency—how to measure… by measuring mean age of air
With CRE… age of air is low in left picture
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8. Used to “trace” flow of air through a space.
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Tracer Gas Techniques
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Used to “trace” flow of air through a space.
Used to measure ventilation rates in buildings from all sources (mechanical ventilation, natural ventilation, infiltration, etc.)
Calculate air changes per hour, age of air, and air exchange efficiency.

9. CO2 Tracer Gas Techniques Selection of appropriate tracer gas:
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Tracer Gas Techniques
Selection of appropriate tracer gas:
Should not be naturally present in room in high concentrations
Should be similar in density to air
No reactions with room materials
Cost effective gas
Cost effective measuring equipment
Relatively Safe
(The ideal tracer gas does not exist, but CO2 is a reasonable substitute if background concentration is consistent.)
CO2
Other gases: Sulfur Hexafluoride, Nitrous Oxide

10. Tracer Gas Decay Testing
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Tracer Gas Decay Testing
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Infrared gas analyser
Cylinder of gas
Mixing fan
Tubing
Anemometer

11. Tracer Gas Techniques
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12. Tracer Gas Techniques
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13. Tracer Gas Step-Down (Decay) Method
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Tracer Gas Step-Down (Decay) Method
Ensure all inlets and exhausts have been closed.
Measure the background concentration of tracer gas in the room.
Start measuring tracer gas at a point in the room or the exhaust at predetermined intervals.
Switch ON the fan.
Release the required amount of tracer gas into the room.
Allow the fan to run for an additional minute or two after the tracer has been released to ensure adequate mixing.
Switch OFF the fan.
Open the inlets and exhausts of the ventilation system.
Allow the tracer gas to decay to the background concentration.
Most widely used tracer gas method
Room is SEALED
A tracer gas is introduced into the room, mixed with the room air to a uniform concentration, and allowed to decay.
Fan ensures uniform distribution
The decay allows for the calculation of several indices, including ventilation effectiveness, contaminant removal effectiveness, nominal time constant, and the mean age of air.
Fan used to ensure uniform initial concentration – can check conc. in several places before starting to check
Measure in one or multiple locations

14. Tracer Gas Step-Down (Decay) Method: Calculations
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Tracer Gas Step-Down (Decay) Method: Calculations
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Plot measured concentration as a function of time in a logarithmic chart in Excel.
Use only straight line after an initial elapse
Ignore initial and “tail” values
Denote last usable concentration 𝑐 𝑛
Air changes per hour is calculated from the weighted area under the curve and the slope of the curve

15. REHVA Guidebook Number 2 : Ventilation Effectiveness
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REHVA Guidebook Number 2 : Ventilation Effectiveness

16. Tracer Gas Techniques
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17. Tracer Gas Step-Up Method
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Tracer Gas Step-Up Method
Tracer gas is released into the room in a continuous and constant flow.
Monitoring of the tracer gas at a point in the room or at the exhaust is continued until steady state is reached.
Doesn’t require full sealing of room.
Requires better equipment (accurate rotameter calibrated for CO2, more accurate sensor); more expensive.
Passive step up: just use the people in the room to approximate how much CO2 is being released
The work that Dirk has done in the prisons… pretty much what they did there

18. Tracer Gas Techniques
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19. Tracer Gas Pulse Method
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Tracer Gas Pulse Method
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Measurement period begins before the tracer is injected.
A certain amount of tracer gas is introduced in a short period of time.
Total volume of tracer gas injected must be known.
A peak concentration is detected at a monitoring point (with a time delay).
Monitoring continues until concentration decays to initial concentration.
Doesn’t require full sealing of room.
Doesn’t require as much tracer gas volume as step-down method.

20. Conclusions Ventilation effectiveness:
Air change efficiency: a measure of how quickly air in the room is replaced
Contaminant removal effectiveness: a measure of how quickly an airborne contaminant is removed from the room.
Use tracer gas techniques to calculate ventilation effectiveness.
Step-down (decay) method is simplest and most commonly used, but has limitations.
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21. Questions? References:
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Questions?
References:
[1] REHVA REHVA Guidebook Number 2 : Ventilation Effectiveness.
[2] WHO Natural Ventilation for Infection Control in Health-Care Settings.