Choosing the right dispersion model for your application

Brad Cochran, CPP, Inc.

Over the past 10 years or so a great number of presentations at the Labs21 conference have been made that discuss how dispersion modeling can and has been used to improve air quality and to decrease fan energy costs. However, there has been very little or no discussion on how to select the right type of dispersion model to use for your application. There are basically four types of dispersion models; graphical, analytical, computational, and physical. Graphical models use basic rules of thumb to define acceptable stack heights based on building dimensions and the presence of local recirculation regions. Analytical models use series of equations that are derived from either Gaussian dispersion theory or curve fitting methods applied to field and wind tunnel data bases. Computational models use solutions to the Navier-Stokes fluid motion equations to describe the plume trajectories and downwind concentration profiles. And, physical modeling uses either in-situ monitoring of field releases or reduced scale modeling in an atmospheric boundary layer wind tunnel to simulate plume behavior. Each one of these models has its appropriate applications, but misapplying a model can result in either an inadequate exhaust design or in an excessive effort (i.e., fees). Therefore, the objective of this presentation is to provide the audience with guidelines on how and when each of these models should be utilized, what the limitations are of each model, and some insight in how to determine if models are being properly utilized.

Biography:

Brad Cochran is a registered Professional Engineer in the State of Colorado and has nearly 20 years of experience conducting wind-tunnel and mathematical modeling studies related to laboratory exhaust design. Brad is active in ASHRAE and is currently serving as the lead author of Chapter 9, Exhaust Stack Design, in the upcoming latest edition of the ASHRAE Laboratory Design Guide. He is a voting member of TC9.10 Laboratory Systems. During the past decade, Brad has focused on defining new design techniques to minimize the energy requirements for laboratory exhaust stacks. In 2005 Brad developed the first laboratory exhaust system that utilized local wind data to minimize exhaust fan horsepower requirements. He has authored and presented several papers on the subject of energy efficient laboratory exhaust design for ASHRAE, Labs 21, R&D Magazine, Lab Manager, LabWize, IFMA, AMCA, and AWMA.

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