ASHRAE Journal - September 2020 - 65

COLUMN ENGINEER'S NOTEBOOK

are essential because they establish
upper and lower boundaries on the
laboratory exhaust system operation
for analysis. Designers can eliminate
bypass air if the volumetric airﬂow
rate for acceptable plume dilution
is less than the absolute minimum
volumetric airﬂow rate required
by the laboratory exhaust system
as determined by the wind-tunnel
analysis (Table 3).

Wind Tunnel Analysis Results

FIGURE 8 Figure 6, "Design Procedure for Required Stack Height to Avoid Contamination," from Chapter 46 of
the 2019 ASHRAE Handbook-HVAC Applications.4
Z1 Roof Recirculation Region
Undisturbed
Flow
Z2 High-Turbulence Region
Z3 Roof Wake Boundary

Building Wake
Recirculation
Region

1.5R
UH
H

The owner's design requirement stipulated that the laboratory
exhaust system shall use a minimum
of three fans with an operational
redundancy of N + 1; N is the number of fans required
to meet the design load. Our design team determined
that a capacity of 26,000 cfm (12 271 L/s) was needed to
meet the demand of the project. Therefore, a laboratory
exhaust system composed of three 13,000 cfm (6136 L/s)
capacity fans with individual exhaust stacks terminating
at 10 ft (3 m) above the ﬁnished roof were studied under
this option to represent an exhaust stack design that
prescriptively complied with local code requirements.

Prescriptive Design Performance Observations
The 2019 ASHRAE Handbook-HVAC Applications4 publishes a general use laboratory exhaust dilution value of
400 µg/m3 per g/s as an evaluation benchmark for locations sensitive to air quality. Our wind tunnel validation
testing determined that a minimum exhaust stack exit
velocity of 16,552 fpm (84.1 m/s) would be required to
achieve this criterion at the outdoor air intake of the
new air-handling unit. When compared to the commonly referenced prescriptive approach of 3,000 fpm
(15.2 m/s) for exhaust stack exit velocity, these results
highlight ANSI/AIHA Z9.5's disclaimer that the use of
their prescriptive criteria does not guarantee that reingestion will not occur. Designers should carefully consider these disclaimers when applying broad, generalized rules for their applications.
For this project, a prescriptively designed exhaust
stack would have operated with an exit velocity that is
essentially an order of magnitude slower than the project conditions require to achieve acceptable dilution

HC
XC

10:1

5:1

hs

LC

A

H

Lr

L

TABLE 3 Facility exhaust airflow demands for various operational scenarios.
OPERATIONAL SCENARIO

MIN. EA FLOW

MAX. EA FLOW

Day 1

10,060 cfm

13,125 cfm

Day 1 + Shell Spaces

11,455 cfm

17,300 cfm

Day 1 + Shell Spaces + Spare Capacitya

11,455 cfm

26,000 cfm

aTotal overall system capacity provided is 39,000 cfm including redundant fan.

performance. The resultant air quality would have
been signiﬁcantly compromised using a prescriptive
approach.
Our team then sought to determine at what height
would an exit velocity of 3,000 fpm (15.2 m/s) yield our
target dilution criteria. We discovered through additional wind tunnel testing that a 20 ft (6 m) stack would
be needed, which is twice as tall as the minimum prescriptive criteria required by local code (Figures 9 and 10).

Evaluating the Impact of Stack Height and Volumetric Flow on Performance
The results from the single stack, single fan scenario
show there is a strong correlation between stack height
and associated minimum exit velocity to achieve acceptable exhaust plume dispersion performance (Table 4).
This relationship has diminishing returns with the most
signiﬁcant potential reduction in minimum exit velocity achieved up to a height of 22 ft (6.7 m) and rapidly
diminishing reductions beyond ~22 ft (6.7 m) for this
speciﬁc project. While it has been the author's experience that this trend of diminishing returns is generally
applicable, the height and rate of performance gains are
particular to each project and should be evaluated for
each project.
SEPTEM BER 2020

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