Fume re-entry is an important concern for many types of facilities such as hospitals and laboratories that emit pathogens and toxic chemicals that may impact public health by being re-entrained into the building though nearby air intakes. Numerical methods can be used to evaluate dispersion of pollutants from stacks at sensitive receptors. However, numerical methods have limitations and simplifications that can significantly affect its predictions. An alternate way of analyzing stack re-entrainment is with physical modeling in a wind tunnel. In such a study, a scale model that accounts for buildings, topography, and vegetation is used with planned and alternate stack designs to determine the toxic emission impacts on air intakes and other sensitive locations. In a wind tunnel study different stack designs and possible mitigation options can be evaluated. This method is superior to numerical methods (e.g., dispersion models) because it accounts for the immediate structures, topography, and vegetation that is often ignored or oversimplified in numerical methods. This presentation will show a hypothetical case study evaluating a site with toxic air emissions using AERMOD and physical modeling.

1. Using Physical Modeling to
Evaluate Re-entrainment of
Stack Emissions
Pacific Northwest International
Section of the A&WMA
2017 Annual Conference
Boise, ID
Sergio A. Guerra, PhD
Ron Petersen, PhD, CCM
November 2, 2017

2. Outline
1. Background on re-entrainment studies
2. Two case studies comparing predicted impacts from
using:
– AERMOD
– Wind tunnel testing
3. Benefits of using WT testing instead of AERMOD
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions2

3. Public Concern
Business Weekly - May 2, 1988
3

4. Health & Liability
Chicago Daily Herald - April 17, 1998
• Study finds that rare cancer in
Amoco employees is probably
work related
• Incidence in 503 wing was
four times that of general
population
• Incidence of the second and
third floors was seven times
that of general population
• The incidence of such tumors
on the rest of Amoco’s
campus was actually lower
than that of the general
population
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions4

5. • Chloroform fumes from
capped stacks
• Fumes reenter building
roof top units
• High incidence of
miscarriages
• Litigation
Petrochem lab
Capped
Stack
Air Intake
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions5

6. Air quality - why the concern in labs?
Accidental Spill
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions6

7. Why the concern?
Fume Re-entry
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions7

8. Design Guidelines
http://www.i2sl.org/documents/toolkit/bp_modeling_508.pdf
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions8

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Wind Tunnel Modeling Starts With the Basic
Equations of Motion
(Navier Stokes Equations)
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions9

10. • Used to validate CFD and analytical methods
• Compares well with the atmosphere
• Analogous to a field study
• Controlled meteorological conditions
• Results sensitive to site specific features
• Accurate for near-field applications
Wind Tunnel Modeling
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions10

11. Accuracy
From EPA Fluid Modeling Guideline, 1981
• Basic equations are solved by simulating the flow at a
reduced scale, then measuring the desired quantity
• An analog computer with near infinitesimal resolution
and near infinite memory
• If a mathematical model cannot simulate the results of
an idealized laboratory experiment, how can it possibly
be applicable to the atmosphere
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions11

12. 12
Compliance?
BPIPBuilding Geometry
Meteorological Data
Terrain Data
AERMET
AERMAP
Operating Parameters AERMOD
OtherInputs
Building
Inputs
AERMOD Modeling Approach
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions

13. Building Downwash
13 Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions
Image from Lakes Environmental Software

14. Building Profile Input Program (BPIP)
Figure created in BREEZE ® Downwash Analyst
BREEZE is a registered Trademark of Trinity Consultants, Inc.
14 Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions

15. 15
PRIME
AERMOD’s Building Downwash Algorithm
• Used EPA wind tunnel data base
and past literature
• Developed analytical equations
for cavity height, reattachment,
streamline angle, wind speed and
turbulence
• Developed for specific building
dimensions
• When buildings outside of these
dimensions, theory falls apart
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions

16. Evaluation
Investigate the effect on nearby air intakes from:
1. Combustion sources at a hospital and
2. A laboratory stack at a university
Evaluated with:
– AERMOD v16216r (screening mode)
– Wind tunnel testing
Compared to normalized concentration thresholds for:
– National Ambient Air Quality Standards
– The National Institute of Occupational Safety and Health recommended
exposure limit for NO2.
– Occupational Safety and Health Administration permissible exposure limit for
NO2.
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions16

17. Case Study 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions17
Boiler Stack
Diesel Engine
Stack
Co-gen Stack Rec1
Rec4
Rec18
Rec7

18. Model of Site
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions18

19. Stack Parameters
Source Q (g/s) Hs (m) T (K) Vs (m/s) D (m)
Boiler 1 28.45 414.8 12.29 0.60
Diesel Generator 1 28.45 414.8 31.67 0.60
Co-Gen 1 28.35 599.8 22.15 0.76
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions19

20. Boiler Stack Comparison
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions20
Boiler Stack
Rec1
Rec4
Receptor
AERMODv16216r
(µg/m3 / g/s)
Wind Tunnel
Testing
(µg/m3 / g/s)
AERMOD/
WT Factor
Receptor 1 2066 560 3.7
Receptor 4 1608 508 3.2
Health Thresholds µg/m3 / g/s
NO2 NAAQS 540
NIOSH 5169
OSHA 25,843

21. Boiler Stack: Receptor 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions21

22. Boiler Stack: Receptor 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions22

23. Boiler Stack: Receptor 4
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions23

24. Boiler Stack: Receptor 4
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions24

25. Diesel Generator Comparison
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions25
Diesel Engine
Stack
Rec4
Rec7
Receptor
AERMODv16216r
(µg/m3 / g/s)
Wind Tunnel Testing
(µg/m3 / g/s)
AERMOD/WT
Factor
Receptor 4 1159 204 5.7
Receptor 7 1186 242 4.9
Health Thresholds µg/m3 / g/s
NO2 NAAQS 38
NIOSH 363
OSHA 1,817

26. Diesel Generator: Receptor 4
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions26

27. Diesel Generator: Receptor 4
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions27

28. Diesel Generator: Receptor 7
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions28

29. Diesel Generator: Receptor 7
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions29

30. Co-Gen Stack Comparison
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions30
Co-gen Stack
Rec1
Rec18
Receptor
AERMODv16216r
(µg/m3 / g/s)
Wind Tunnel Testing
(µg/m3 / g/s)
AERMOD/WT
Factor
Receptor 1 1328 483 2.7
Receptor 18 665 244 2.8
Health Thresholds µg/m3 / g/s
NO2 NAAQS 34
NIOSH 326
OSHA 1,632
Odor
(1:4000 dilution)
25

31. Co-Gen Stack: Receptor 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions31

32. Co-Gen Stack: Receptor 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions32

33. Co-Gen Stack: Receptor 18
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions33

34. Co-Gen Stack: Receptor 18
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions34

35. Case Study 2
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions35
Laboratory
Fume Hood
Stack
Rec1

36. Model of Site
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions36
Source Q (g/s) Hs (m) T (K) Vs (m/s) D (m)
Fume Hood Exhaust 1 20.5 294.3 20.33 0.77

37. Lab Stack Comparison
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions37
Laboratory
Fume Hood
Stack
Rec1
Receptor
AERMODv16216r
(µg/m3 / g/s)
Wind Tunnel Testing
(µg/m3 / g/s)
AERMOD/WT
Factor
Receptor 1 267 321 0.8
Health Thresholds µg/m3 / g/s
Phosphine
7803-51-2
264
Ethylamine
75-04-7
298
Carbon disulfide
75-15-0
306
Methyl Hydrazine
60-34-4
320

38. Lab Stack: Receptor 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions38

39. Lab Stack: Receptor 1
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions39

40. Conclusions
• Numerical methods like AERMOD have limitations when used
in re-entrainment studies
• Building created by BPIP and used by AERMOD is not actual
building
• BPIP’s artificial building may over or under estimate
concentrations when used by AERMOD
• Effects from building structures and nearby terrain are best
characterized with a wind tunnel study
Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions40

41. Sergio A. Guerra, PhD Ron Petersen, PhD, CCM
sguerra@cppwind.com rpetersen@cppwind.com
Mobile: + 612 584 9595 Mobile:+1 970 690 1344
wwww.SergioAGuerra.com
CPP, Inc.
2400 Midpoint Drive, Suite 190
Fort Collins, CO 80525
+ 970 221 3371
www.cppwind.com @CPPWindExperts
Questions?
41 Using Physical Modeling to Evaluate Re-entrainment of Stack Emissions