ASHRAE Journal - September 2020 - 69

COLUMN ENGINEER'S NOTEBOOK

TABLE 6 Stack height vs. reduction in exit velocity required for two clustered

FIGURE 13 Performance exhaust stack design installed.

stacks in operation.
STACK HEIGHT

AVERAGE VELOCITY
REDUCTION PER FOOT INCREASE

EXIT AIRFLOW RATE PER STACK

20 ft to 21 ft

497 fpm/ft

6,250 cfm

21 ft to 22 ft

379 fpm/ft

6,250 cfm

22 ft to 23 ft

294 fpm/ft

6,250 cfm

23 ft to 24 ft

233 fpm/ft

6,250 cfm

24 ft to 25 ft

189 fpm/ft

6,250 cfm

25 ft to 26 ft

154 fpm/ft

6,250 cfm

TABLE 7 Airflow rate increase vs. exit velocity reduction for two clustered stacks

in operation.
STACK HEIGHT

AVERAGE VELOCITY REDUCTION
PER 1,000 CFM INCREASE

STACK EXIT AIRFLOW
RATE RANGE TESTED

20 ft

80 fpm per 1,000 cfm

6,250 to 13,000 cfm

TABLE 8 Prescriptive vs. performance design results summary.
PRESCRIPTIVE EXHAUST SYSTEM RESULTS

PERFORMANCE EXHAUST SYSTEM RESULTS

10 ft stack height.

24 ft stack height.

16,552 fpm minimum exit velocity
required per stack when in use.

2,049 fpm minimum exit velocity
required for single-stack operation.

13,000 cfm minimum volumetric airﬂow
rate required per fan when in use.

1,462 fpm minimum exit velocity
required per stack for two-stack operation; less if all three stacks are running
in parallel.
10,060 cfm minimum volumetric airﬂow
rate required for single stack operation.
6,250 cfm minimum volumetric airﬂow
rate required for two-stack operation; less
if all three stacks are running in parallel.

Prescriptive vs. Performance Design Results Summary
Table 8 summarizes the ﬁnal results between the
prescriptive vs. performance-based approach design
options for the project evaluated within this case study.
Figures 13 and 14 show images from the performancebased approach design option installed.

Conclusions
A laboratory exhaust system prescriptive-based
design approach does not yield consistent results

across different appliFIGURE 14 Close-up of cluster stack
from performance design option
cations and could result
installed
in higher contaminant
concentrations than
are acceptable. Using
a performance-based
design approach will
give greater conﬁdence
in achieving higher
performance and
acceptable air quality.
Laboratory exhaust system performance targets can be quantiﬁed
and achieved through
design team collaboration and modeling of stack dispersion performance
with the performance-based approach. Design strategies using taller stack heights and clustering exhaust
stacks can result in substantial improvement of laboratory exhaust dilution performance.

References

1. IAPMO. 2019. "2019 California Mechanical Code."
2. ICC. 2019. "2019 California Energy Code."
3. ANSI/AIHA/ASSP Z9.5-2012, Laboratory Ventilation.
4. 2019 ASHRAE Handbook-HVAC Applications.
5. Briggs, G.A. 1984. "Plume rise and buoyancy effects."
"Atmospheric Science and Power Production," D.
Richardson, Editor. U.S. Department of Energy,
DOE/TIC-27601.
6. Petersen, R.L., J.D. Reifschneider, 2008.
"The effect of ganging on pollutant dispersion
https://bit.ly/3k9ecVE
from building exhaust stacks." ASHRAE
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