Objectives
Finish with tracer gas measurement
Blower door and duct bluster measurements
Tracer gas
and IAQ Applications
• Quantification of outside air
• Air distribution system efficiency
– Air change Efficiency
– Contaminant removal effectiveness
• Leak detection House/chamber/duct/…
• Duct flow
• Re-entrainment of exhaust air into ventilation
system
• Simulate toxic pollutant distribution
• Many other applications
A Good Tracer Gas?
•
•
•
•
•
•
Non-toxic
Environmental friendly
Colorless and odorless
Easily detectable
Inert
No other sources
Common Tracer Gases Used
•
•
•
•
•
Carbon Dioxide
Nitrous Oxide
Freon
Helium
Sulfur Hexafluoride
Application 1:Quantification of outside air
Volumetric Air Measurements
• Standard Test
– ASTM E741 - 00(2006)
• Available by the UT library website
• Test Method for Determining Air Change
in a Single Zone by Means of a
Tracer Gas Dilution
ASTM E741 Test Method
• Different methods:
– Concentration Decay (or concentration
increase)
– Constant Injection
Concentration Decay Method
• Inject predetermined volume of gas into room
• Mix room air to get uniform concentration
• Monitor gas concentration decay
• Aim for 10 samples over measured time
• Use reactor model to predict concentrations
Theoretical Basis
Space balance
Vol·dC/d = V·Cin-V·Cout+N
If Cin = constant & Vol/V = ACH
0
dC/(Cin-Cout) = ACH· d
Integrate:
…….
V
V
Cin
N
Cout
Source
ACH =1/Δ·{ln[Cin(=Δ)- Cout)]- ln[Cin(=0)- Cout)]}
Concentration Decay Method
Air Change Rate:
In the case of zero inlet concentrate and
perfect mixing in the space
ACH =- (ln C2 - ln C1)/Δ (in hours)
C1 = Tracer Gas Concentration at start of test
C2 = Tracer Gas Concentration at end of test
Tracer gas result
[minutes]
Decay Test
• Advantages
– Don’t need to release precise amount
– Don’t need to measure volume (if Cout = 0)
• Disadvantages
– Need to keep building well-mixed
– Recontamination from buffer spaces
– House needs to stay in one condition for
entire test
Single zone Example:
Coffee Houses
-1
Air Exchange Rate (h )
2.5
SF6
CO2
2
1.5
1
0.5
0
1
2
3
4
Test Number
5
6
Lohaus and Waring (2006) ArE 381E Course Project
How do you estimate
uncertainty?
1. Use standard error of slope
2. Follow ASTM E741
– ΔACH < 10%
Advanced Tracer Gas Testing
• Multi-zone flows
– Easiest – Use several unique tracer gases
– Harder – Use flow and mass balances
Consider Two-Zone Building
(non-perfect mixing)
º
V4
º
º
º
V1
V3
V5
V6
V2
E
º
º
V1
V2
• Tools
• Mass balance on tracer gas
• Mass (flow) balance on air
• Measured concentrations in each space
vdA
Equations
dC1
Vol
 V2Cout  V6C2  E  V3  V1 C1
1 dt
dC2
Vol
 V4Cout  V3C1  vd AC2  V6  V5 C2
2 dt
V1  V5  V2  V4
and V4  V5  V3  V6
mesure: C1 ( ), C2 ( )
• How many unknowns? Equations?
• Flow direction for interzonal flow
• Air exchange rate for spaces
•Sums of flows
Solution procedure
• Reduce mass balance to one equation by
solving C2 equation for C1 and substituting into
C1 equation
– 2nd order ODE
– Same thing for C1 equation
• 6 unknown flows
– Overall flow balance can be used to get two unknown
flows
– Measured tracer gas concentrations can be used to
eliminate two more flows
– Additional data needed for solvable system
• We need to use multiple tracer gasses!
• Or we need to measure flows with flow meters!
Example 2 Air distribution system efficiency
• How well is outside supply air distributed to
breathing zones in occupied areas?
• Air exchange efficiency
• ASHRAE Standard 129 – Measuring Air Change
Effectiveness
• Uses Tracer Gas Techniques
• Age-of-Air Measurements
Why Worry About Good Mixing?
Poor Mixing
• Occupant complaints
• ASHRAE Standard, Ventilation for Acceptable Indoor Air
Quality
• ASHRAE Standard is based on amounts of outside air
getting to breathing zone not to supply air louvers
Short – circuiting airflow patterns
• Where a significant portion of supply air flows directly to
the exhaust, bypassing the occupied portion (breathing
zone) of the ventilated space.
Air Exchange Effectiveness
• The definition is based on a comparison of
the age of air in the occupied portions of
the building to the age of air that would
exist under conditions of perfect mixing of
the ventilation air.
Age of Air
• The age of the air at a give location is the
average amount of time that has elapsed since
the air molecules at that location entered the
• building.
• Amount of time outside air has been in an area
• Two Methods of determination
– Step-up constant tracer gas injection
– Tracer gas concentration decay
How to measure Age of Air?
Step down method:
Injection and mixing
– Air in the room is marked with tracer gas (injection
and mixing)
– Ventilation turned on
Age of Air Measurements
– Locations of interest
– In the exhaust (C)
Constant Injection
º
V = N / (Cout - Cin)
º
º
V
V
Cin
You need to get to steady state injection
N
Source
Cout
Constant Injection
• Advantages
– Can determine time-dependence of air
exchange rates
• Disadvantages
– Need to keep building well-mixed
– Recontamination from buffer spaces
– Need to have mass flow controller
– Need to measure volume (for ACH())
How to measure Age of Air
and Air Exchange Effectiveness
Age of air at a location =
Average tracer gas level during test
Tracer gas level at beginning of test
Air change effectiveness (E)
E=
avg age of air – Exhaust
avg age of air – age of air in breathing zone
avg age of air =
E = < 1.0 (less than perfect mixing)
E = 1 (perfect mixing)
Significance of
Air Exchange Effectiveness
• ASHRAE Standard 62.1-2004 Ventilation
for Acceptable Indoor Air Quality
- Outside air requirements = QA/E as E
decreases, OA should increase
• US Green Building Council LEED Rating
requires an E > 0.9 in all ventilated zones
Tracer Gas Instrumentation
Tracers which we use
SF6 Gas analyzer
– ppm with IR absorption or photo-acoustic IR
– ppb with GC/ECD
CO2 Tracers gas analyzer ( CO2 sensor )
Infiltration/Leakage Measurements
(Calibrated Fans)
• Flow going through fan is calibrated to
pressure rise across fan
– If you measure pressure, than you know flow
• Requires very smooth inlet and outlet
conditions
• Often have to restrict flow to get
measurable pressure signal for low flows
• Examples:
31
– Blower door and Duct Blaster
31
Fan Pressurization
•
1.
2.
3.
4.
In 1970s, smoke evacuation fans used
to find air leakage
Install blower door
Use fan to create artificial pressure
difference between inside and outside
Use smoke stick (or cigarette, etc.) to
visualize flow patterns.
Seal leaks
Quantitative Blower Door
•
•
How many points should you measure?
How can you estimate uncertainty?
http://www.energyconservatory.com/products/products1.htm#pd16
Test Data
• 1000 ft2 house, 10 ft high ceilings
• Two-point test
– 1688 CFM at 51 Pa
– 1048 CFM at 24.5 Pa
• What is ACH50 (λ50), C?
Answers
• λ50= 1 ACH
• C = 131.07 CFM/Pan
• n = 0.65
Reporting blower door data
• Flow (or air exchange rate) at a given
pressure
• Equivalent Leakage Area
– What are dimensions of c?
Comment
• Blower doors do not measure air
exchange rate
– Why not?
• What do they measure?
Blower Door Misc.
• All windows and doors to outside closed
– Why? What about buffer spaces?
• HVAC off
• Hard to do on windy days
• Watch out for buckling linoleum floors and
fireplaces
• Flow direction change/ring change can be
discontinuous
• Remember to record before and after
40
reference pressures with fan covered
40
More Blower Door Tests
•
Measure component leakage
– What fraction of leakage is due to a certain
component?
– Tape off component and repeat test
•
•
What are issues with this approach?
Series leakage paths (AKA pressure
diagnostics)
– Air leaks to attic and then to crawlspace and
then to outside
Residential Component
Leakage
Component
Range
Average
Walls
18-50%
35%
Ceilings
3-30
13
Windows/Doors
6-22
15
Fireplaces
0-30
12
HVAC
3-28
18
Other Vents
2-12
5