nbec 2014 - flow exponent values and implications for air leakage testing
TRANSCRIPT
Flow Exponent Values and Implications for Air Leakage Testing
NBEC 2014 ROBIN URQUHART, MBSC, MA NRES, RDH BUILDING ENGINEERING
DR. RUSSELL RICHMAN, P.ENG, RYERSON UNIVERSITY
Outline
à Introduction to air leakage testing
à Relationship between flow and pressure
à Case study building
à Abnormal flow exponents
à Data extrapolation to operating pressures
à Conclusions/Implications
à Further study
Air leakage testing
à Air leakage = Uncontrolled air movement through the
building enclosure à Can account for up to 50% of a buildings heat loss/gain
à Affects assembly durability
Stack Wind
Fan induced pressure
à Induced pressure differentials using fans à Mechanical
à Blower door
à ASTM E779-10 (10-60 Pa) à multi-point
à ASTM 1827-12 (50 Pa) à single point
à USACE (75 Pa) à single point
Why test?
à Evaluate performance and determine retrofit options à Single building
à Building stock
à Determine leakage rates for energy models à 4 Pa – 10 Pa
à Code or other
specification compliance
à LEED, Energuide, etc.
The Relationship between Flow and Pressure
Pressure (P) and Flow (Q)
10 Pa 50 Pa
Power Law Equation (Bernoulli):
𝑄=𝐶(∆𝑃)↑𝑛
Where, Q = flow C = flow coefficient P = pressure n = flow exponent (dimensionless).
n = flow exponent
Flow exponent (n)
Power Law Equation:
2
2.05
2.1
2.15
2.2
2.25
2.3
2.35
2.4
2.45
2.5
0.8 1 1.2 1.4 1.6 1.8 2
log Flow
log Pressure
log Flow vs log Pressure
Series1 Linear (Series1)
> 0.5 and < 1.0 Slope = n
0.5 = Turbulent • flow resistance varies with the square of velocity
1.0 = Laminar flow
• flow resistance proportional to velocity
𝑦 = 𝑚𝑥 + 𝑏
à 13-story MURB
à Enclosure retrofit 2012
à focus on air tightness
à Air leakage tested
à Pre-retrofit
à Post-retrofit
Case study building
Guarded-zone method
Flow exponent values – Case study building Suite 101 Suite 102 Suite 301 Suite 302 Suite 303 Pre-press 0.59 0.56 0.60 0.68 0.54 Post-press 0.56 0.67 0.75 0.85 0.85 Change2 -0.03 +0.11 +0.15 +0.17 +0.31 Pre-depress 0.92 0.80 0.50 0.62 0.61 Post-depress 0.79 0.69 0.60 0.57 0.37 Change -0.13 -0.11 +0.10 -0.05 -0.24 Suite 1101 Suite 1102 Suite 1103 Suite 1301 Suite 1302 Pre-press 0.52 0.49 0.53 0.74 0.52 Post-press 0.71 0.56 0.64 0.77 0.72 Change +0.19 +0.07 +0.11 +0.03 +0.20 Pre-depress 0.47 0.46 0.78 0.57 0.63 Post-depress 0.61 0.43 0.59 0.57 0.47 Change +0.14 -0.03 -0.19 0.00 -0.16 1 pre-press = pre-retrofit pressurization, post-press = post-retrofit depressurization, pre-depress = pre-retrofit depressurization, post-depress = post-retrofit depressurization 2 positive change indicates a move towards laminar flow (smaller cracks), negative change indicates a move towards turbulent flow (larger cracks)
“…blower door tests occasionally yield exponents less than the
Bernoulli limit of 0.5. In fact, it is physically impossible for such
low exponents to occur.”
Sherman, Wilson and Kiel. 1986
Flow exponents outside of theoretical range
So why do we get them?
Flow Exponents <0.5
Series 1: n=0.5-1.0 Series 2: n<0.5
Q P logQ logP n r-sq Q P logQ logP n r-sq
800 10 2.9 1 0.63 0.999 800 10 2.9 1 0.42 0.999
1600 30 3.2 1.48
1300 30 3.11 1.48
2200 50 3.34 1.7 1590 50 3.2 1.7
2500 60 3.4 1.78 1710 60 3.23 1.78
Flow exponents <0.5
R² = 0.9999
R² = 0.9993
2.8
2.9
3
3.1
3.2
3.3
3.4
3.5
0.9 1.1 1.3 1.5 1.7 1.9
Log Flow
Log Pressure
n = 0.63 n = 0.42
Geometry of the Enclosure
à Higher pressure differentials change leakage path geometry
à Resulting in abnormal flow exponent (n) values.
50 Pa 10 Pa à What effect on the data does a changing n
value cause?
Extrapolating to lower pressure differentials
40 L/s
Difference of ~35%
n = 0.40
n = 0.65
n = 0.50
n = 0.80
Q P
650 30 865 50
1100 70 1180 75 1250 80 1375 90
Enclosure change at high pressure
Q P
650 30 865 50
1100 70 1150 75 1200 80 1250 90
N = 0.68, Rsq = 0.99
N = 0.62, Rsq = 0.99
600700800900100011001200130014001500
20 40 60 80 100
Flow
Pressure
static enclosure enclosure changes
Enclosure change at higher pressures
1000
1050
1100
1150
1200
1250
1300
1350
1400
65 70 75 80 85 90 95
Flow
Pressure
static enclosure enclosure changes
600700800900100011001200130014001500
20 40 60 80 100
Flow
Pressure
static enclosure enclosure changes
Adjusted values using n
0
200
400
600
800
1000
1200
1400
0 20 40 60 80
Flow
Pressure
static enclosure changing enclosure
0
50
100
150
200
250
2 3 4 5 6
Flow
Pressure
static enclosure changing enclosure
25 l/s
14% difference
à The enclosure may not remain static throughout the
test
à The linear relationship may not be accurate at operating
pressure
à Substituting n values can have magnified impacts at
lower/higher pressure differentials
à R-squared values not perfect indicators
Conclusions
à Sizing mechanical systems
for predicted in operation
flow rates
à Retrofit options
Implications for the industry
à Energy models using extrapolated flow rates
may be inaccurate
à Multi-point testing still important
à Order of magnitude for comparison purposes
à Flow exponents tell us something, we might not know
exactly what it is
à Standards at higher pressures can be used for
compliance purposes
The Good
In-test corrections
à Check R-squared values during testing
à Check n values at the termination of each test
Post-test corrections
à Review data points and remove lowest pressure
differential point(s)
à If not reconciled, single point test at 50 Pa
Flow exponent corrections
à Sensitivity analysis: R-value threshold
à The use of flow exponent values in forensics
à Standardized leakage metric and pressure differential
for comparison purposes
à Effect on flow exponent of enclosure geometry
changes
Further study
