Commissioning Laboratory Exhaust System Dispersion
Chet Wisner, Ambient Air Technologies,
Rooftop stacks are generally assumed to perform as designed, safely
carrying polluted laboratory exhaust streams clear of people and
research animals. In a growing population of laboratory design efforts,
wind tunnel testing or numerical techniques are used to evaluate
the dispersion of plumes from the stacks prior to construction.
If the dispersion evaluation is performed correctly and the exhaust
stacks are built and operated as designed, plumes from the stacks
can reasonably be expected to safely disperse without exposing people
or research animals to potentially harmful concentrations of the
pollutants they contain. However, there are a number of ways in
which things can go wrong, allowing potentially harmful exposures
to occur, so it would be wise to verify that the plumes actually
behave as designed.
Commissioning of laboratory buildings does not typically include
verification of plume dispersion from exhaust stacks. However, it
has been reported that about five percent of fume hood tests fail
as a result of recirculation of air from rooftop exhaust stacks
into air intakes. This suggests that recirculation, and possibly
other modes of exposure, are not adequately dealt with by current
This presentation makes a case for including verification tests
of exhaust plume dispersion in the commissioning of laboratory exhaust
systems. It explores the challenges of obtaining meaningful measurements
in the real world, and suggests practical approaches to conducting
plume verification studies to support laboratory commissioning.
Conducting a meaningful measurement effort to verify plume dispersion
requires overcoming several challenges including the variability
of wind conditions, specifying practical acceptance criteria, and
the high expense of field measurement efforts.
Since the measurements are constrained to use the wind nature provides,
the test protocol cannot require that the worst case of plume dispersion
be tested. That case may not occur during the field measurement
effort. So a test protocol must be devised which provides reasonable
assurance of acceptable performance using the wind conditions presented
during the test period. This requires evaluating the site climatology
to ensure a reasonable expectation of useable wind conditions during
the field measurement effort. It also requires specification of
acceptance criteria based on less than worst case wind conditions
and specification of what combinations of wind conditions are deemed
to provide a sufficient basis for declaring a successful test.
Meteorological field measurement efforts tend to be very expensive
because they require crews of trained technicians to travel to the
measurement site and remain until the desired weather conditions
appear. This produces a highly variable and large cost for labor.
This untenable cost element can, however, be mitigated through the
use of automated field measurement equipment to a level which should
make these tests a viable component of laboratory commissioning
Laboratories for the 21st Century encourages minimization of energy
utilization while ensuring the safety of the laboratory workplace.
This tends to drive the design of rooftop exhaust systems away from
the historically conservative and energy-consumptive approaches
and toward more sophisticated designs which will, by their nature,
include smaller margins of safety. Smaller errors in the system
design and implementation are then required to produce failure of
the exhaust dispersion system in actual operation. So as the principles
of Labs21 receive broader acceptance, the need for including plume
dispersion verification testing as part of laboratory commissioning
becomes more pressing.
Chet Wisner is the President of Ambient Air Technologies,
LLC, a Colorado based firm specializing in wind-tunnel modeling
of laboratory and healthcare facilities. He has played an active
role in the air quality portion of the environmental industry for
over 30 years. Applying his experience and expertise in meteorology,
engineering, and physics to physical modeling using scale models
in a boundary-layer wind tunnel, he has personally managed or directed
many wind tunnel studies. In addition, he has personally managed
a number of large scientific projects including a $1.2 million field
project for the Baton Rouge Ozone Task Force to study the formation
of ozone in the Baton Rouge vicinity. This project was touted by
users of its data as "producing the most useful data per dollar
of any project of its kind."
Chet coauthored an EPA-recommended protocol for the use of environmental
wind tunnel studies to determine plume downwash characteristics
for input to EPA's own regulatory dispersion models. He was responsible
for some of the largest air quality monitoring networks in the US,
and has conducted numerous field studies of atmospheric dispersion.
His educational background includes an MBA in Management Strategy
and Policy/Marketing from UCLA, an MS in Meteorology from South
Dakota School of Mines and Technology, and a BS in Engineering Physics
from UC Berkeley. He is an active member of several professional
organizations including the Air & Waste Management Association
(A&WMA) and the American Meteorological Society (AMS).
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