Objectives
• Finish air cleaner problem
• Measure ventilation rates• Focus on smaller buildings
Measuring air exchange rates
• Two general strategies to get λ• Direct measurement
• Tracer gas (constant injection or decay)
• Apply well-mixed reactor model
• Indirect measurement • Blower door
• Apply infiltration model
Tracer Gas Testing
• Release gas and measure concentration
• Use model to estimate ventilation rate
• Properties of a tracer• Nontoxic• Non-reactive (chemically inert)• Not present at significant concentrations in typical
environments• Easy to measure over wide concentration range
Common tracers
• SF6
• ppm with IR absorption• ppb with GC/ECD
• Tracers of chance (eg. CO2)
Tracer Gas Decay Test
• Seed tracer gas
• Mix well
• Aim for 10 samples over ~1 time constant
• Use reactor model to predict concentrations
Δt
CC
eCC
C
iΔti
t
dtdC
lnln0
Decay Test
• Advantages• Don’t need to release precise amount• Don’t need to measure volume (if you just want air
exchange rate)
• Disadvantages• Need to keep building well-mixed• Recontamination from buffer spaces• House needs to stay in one condition for entire test
Building Air Exchange Rates
0
0.5
1
1.5
2
2.5
3
0 20 40 60 80 100 120
% Less Then
Air
Exc
han
ge
Rat
e (1
/hr)
Residential Commercial
Murray and Burmaster (1995) Risk Analysis
Constant Injection
• Continuously release known amount of tracer gas into space
• Continuously mix air
• Use model to determine air exchange rate
Q
EC
eQ
EtC
ss
tVQ
1)(
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• Typically uses more tracer gas• Need to measure volume
Common problems
• Contamination from buffer spaces
• HW #2 Problem 3 (last year)
Summary
• Tracer gas can be used to measure ventilation rate• Assumes reactor model• Constant injection or decay
• Measures ventilation rate at time of test• Varies with time, ΔT ,wind
Fan Pressurization – The Early Years
• In 1970s, smoke evacuation fans used to find air leakage
1. Install blower door
2. Use fan to create artificial pressure difference between inside and outside
3. Use smoke stick (or cigarette, etc.) to visualize flow patterns.
4. Seal leaks
Ref: Home Energy Magazine, 1995
Limitations
• Flow rate (can use multiple fans…)
• Not good for complex leakage paths• Multizone buildings
• Does not give you air exchange rate• Independent of wind, ΔT, time• Useful for comparison of different buildings
Blower Door (quantitative)
house averagean for 65.0,5.01
leaks of Size
1 Refor constant, 1 Refor
2
5.0
nn
c
PcQ
Pcc
PAcQ
ndd
d
Procedure for Blower door test
1. Install blower door
2. Use fan to create artificial pressure difference between inside and outside
3. Measure flow at several inside-outside pressure differences
4. Find n and C
Ref: Sherman and Dickerhoff (1994, 1998)
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
Reporting blower door data
• Flow (or air exchange rate) at a given pressure• Why is air exchange rate preferable?
• Leakage Area
• What units are c in?
)4(2
)4(
Pa
PacELA
n
Where does that leave us?
• ELA is at specific reference pressure.
• So, use LBL model and improvements to get infiltration and ventilation rates• ASHRAE 2001 Handbook of Fundamentals 26.21-26.24
• Requires L, ∆T, wind and shelter information• More advanced forms require distribution of leakage
LBL Model
22swws QQQ Ref: Sherman (1992) Indoor Air
2VCtCAQ wsL
Q = Ventilation air flow (CFM)
AL = ELA from blower door test (in2)
Cs = stack coefficient f(height) (CFM2/(in4 °F))
Δt = temperature difference (°F)
Cw = wind coefficient f(height, shielding) (CFM2/(in4 mph))
Ref: ASHRAE Fundamentals (2001) ch. 26
Summary
• In order to measure air exchange rates• Need to use direct tracer gas measurements to get a
snapshot of λ• Use reactor model
• Need to use a blower door to compare to other buildings
• Use LBL (or other) model to get λ
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