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Commissioning Laboratory Exhaust System Dispersion

Chet Wisner, Ambient Air Technologies, LLC

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 practice.

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.

Findings:

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 projects.

Labs21 Connection:

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.

Biography:

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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