ASHRAE Journal - September 2020 - 61

COLUMN ENGINEER'S NOTEBOOK

FIGURE 1 Results vs. effort qualitative comparison of analysis methods.

Accurate
Wind Tunnel Validation
Results

3. Fan System Power Consumption. All newly installed
fan exhaust systems serving a laboratory or factory
greater than 10,000 cfm (4719 L/s) shall meet subsection
A and either B, C or D:
A. System shall meet all discharge requirements in ANSI
Z9.5-2012." (Emphasis is the author's.)

ANSI/AIHA Z9.5-2012, Laboratory Ventilation3
"5.4.6 Exhaust Stack Discharge. In any event the discharge shall be a minimum of 10 ft (3 m) above adjacent
roof lines and air intakes and in a vertical up direction.
Exhaust stack discharge velocity shall be at least
3,000 ft per minute (fpm) (15.2 m/s) [emphasis is the
author's] unless it can be demonstrated that a speciﬁc
design meets the dilution criteria necessary to reduce
the concentration of hazardous materials in the exhaust
to safe levels at all potential receptors.
The air intake or exhaust grilles shall not be located
within the architectural screen or mask unless it is demonstrated to be acceptable."
A relationship exists between codes and standards.
Often, codes are mostly sections of existing industry
standards with modiﬁcations made by the authority
having jurisdiction. Therefore, to identify the design
parameters to address with a performance approach to
laboratory exhaust stack design, it is necessary to understand the context and limitations of the referenced standard. It is also essential to keep in mind that the prescriptive criteria for stack height and exit velocity have
substantial disclaimers, such as the one in Appendix 3 of
ANSI/AIHA Z9.5-2012.

ANSI/AIHA Z9.5-2012 Appendix 3, Selecting Laboratory Stack Designs3
"The 10 ft (3.05 m) minimum stack height called for in
the body of this standard is primarily intended to protect maintenance workers from direct contamination
from the top of the stack. However, the minimum height
of 10 ft (3.05 m) is not enough by itself to guarantee that
harmful contaminants would not be re-ingested. (Emphasis
is the author's.)
Similarly, a minimum 3,000 fpm (15.3 m/s) exit velocity is speciﬁed in the body of this standard, but this exit
velocity does not guarantee that re-ingestion will not occur."
(Emphasis is the author's.)
It has been the author's experience that stack height,
exhaust plume exit momentum and physical location
with respect to building massing, site topography and

Computational
Fluid Dynamics

2019 ASHRAE
Handbook-HVAC
Applications
Calculation Method
Conservative

Effort and Resources Required

areas sensitive to air quality, such as outdoor air intakes,
operable windows and populated areas, are critical variables that must be evaluated holistically to achieve optimum design outcomes.
Many laboratory projects are developed as renovations; as such, the site topography and areas sensitive to
air quality are typically ﬁxed variables, and the options
of locations to install the laboratory exhaust systems are
often limited. Therefore, stack height and exit momentum are usually the remaining variables a design team
can explore and evaluate. This column presents a basic
framework for a design process and a case study highlighting the results of this process when applied to a
laboratory renovation project.
Figure 1 presents three conventional methods to evaluate laboratory exhaust plume dispersion performance
and qualitatively ranks what a designer can anticipate
concerning the accuracy vs. effort trade-off between
each method. The designer should exercise extreme
caution when using computational ﬂuid dynamics
(CFD) to model exhaust plumes for laboratory pollutant
sources, as CFD models can both over- and underpredict
concentration levels by orders of magnitude, leading to
potentially unsafe designs.4 The author agrees with the
2019 ASHRAE Handbook-HVAC Applications, Chapter 46
recommendation to use a wind-tunnel analysis to validate the results from a CFD analysis.
Figure 2 describes a process this author has used to
collaborate with design team stakeholders to achieve
optimum laboratory exhaust system design based on
the project priorities and values. Step 5 is optional,
but recommended by the author to achieve optimum
design results; wind tunnel validation can yield substantially more accurate performance results with higher
conﬁdence than computational modeling or manual
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