Updating the Labs21 Best Practice Design Guide: Modeling Exhaust Dispersion for Specifying Acceptable Exhaust/Intake Design
Brad Cochran, CPP, Inc.
The Laboratories for the 21st Century: Best Practices Guide "Modeling Exhaust Dispersion for Specifying Acceptable Exhaust/Intake Design" (772 KB, 12 pp) was first written in 2005. Changes in the industry have necessitated an update of this document to better reflect more current, state-of-the-art best practices.
The updates incorporated into the new best practices fall under three categories. The first is primarily a bookkeeping exercise: updating the references and influence that they have on laboratory exhaust system design. The second reflects the recent update in the analytical dispersion equations presented in the ASHRAE Applications Handbook used to predict downwind concentrations of a laboratory exhaust plume at nearby air intakes. And third, the best practices have been expanded to include descriptions of how dispersion modeling can be used to define methods where VAV technology can be safely applied to laboratory exhaust stacks to save significant energy.
The primary update in the references involved the much-anticipated latest version of the ANSI/AIHA, American National Standard for Laboratory Ventilation Standard Z9.5, which will hopefully be published by the time of this presentation. The updated standard includes changes that remove some of the energy inefficient "rules of thumb" that have often be applied to laboratory exhaust systems and replaces them with design tools that can be applied to employ safe and energy-efficient designs. The plume dispersion modeling techniques described in the 2007 ASHRAE Handbook—HVAC Applications have been significantly upgraded in the 2011 version. While the new equations are slightly more complex, they increase the accuracy of the downwind concentration predictions. This will enhance a design engineer's ability to define a safe and energy-efficient laboratory exhaust system. The simple plume rise equation that has been often misinterpreted by both design engineers and manufacturers has been eliminated and replaced with a more accurate transitional plume rise equation that correlated the height of the plume with the distance traveled downwind.
Finally, the new best practices include discussions of techniques that can be safely applied to reduce the energy consumption associated with the laboratory exhaust systems by utilizing VAV technologies. The techniques described include simple turndown, where the systems are designed to operate under the minimum potential HVAC loads; wind responsive systems, which set the minimum flow rate based on the local wind speed and wind direction; and a demand/control system that sets the minimum flow rates based on the presence of chemicals within the exhaust stream.
Biography:
Brad Cochran has 20 years of experience conducting wind-tunnel and mathematical modeling studies related to laboratory exhaust design. Mr. Cochran has managed projects for such clients as Harvard University, the National Institutes of Health, University of Texas Medical Center, Loyola University, Genentech, and several University of California campuses, to name a few. During the past decade, Mr. Cochran has focused on defining new design and analysis techniques to safely minimize the energy requirements for laboratory exhaust stacks. Mr. Cochran has authored and presented several papers on the subject for ASHRAE, Labs 21, Labwize, R&D Magazine, Air Movement and Control Association, Campus Safety Health and Environmental Management Association, International Facility Management Association, Real Property Institute of Canada, and Air and Waste Management Association.