Numerical Modeling for Exhaust Stack Design
Michael Ratcliff, Rowan Williams Davies and Irwin, Inc. (RWDI, Inc.)
The Labs21 Environmental Performance Criteria (EPC) recognizes that good design of laboratory exhaust stacks and air intakes is important for avoiding air quality problems from contaminants emitted into the atmosphere. In evaluating stack and intake design, several methods are available to predict the dispersion of contaminants. One class of methods lies in the simple algebraic model based on previous wind tunnel experiments on simple buildings. Several variants of these methods have been published in ASHRAE handbooks and in an appendix of ASHRAE Standard 62.1 (Ventilation for Acceptable Indoor Air Quality). These simpler methods have the appeal of being very efficient to use, both from a time and a cost perspective. Another dispersion method is that of physical wind tunnel modeling, where the dispersion of exhausts on a physical scale model of the building and surroundings is evaluated in a boundary layer wind tunnel. This method is less expensive and time consuming to use.
During this presentation, the speaker will compare dispersion results from simplified numerical methods to results from wind tunnel modeling for a variety of test cases, including laboratories, healthcare central utility plants, and commercial buildings. In general, the results show that the numerical models are stricter than wind tunnel modeling, as has been expected. However, there are several situations where the numerical models are too strict, which hurts their usefulness for providing cost-effective stack and intake designs. There is also uncertainty in how to account for rooftop obstacles and adverse flow patterns that affect dispersion. These results point to areas for future development of these numerical methods.
Biography:
Mike Ratcliff has 27 years of experience in the field of air quality, exhaust stack design, and airflow around buildings. Dr. Ratcliff has a Ph.D. in civil engineering from Colorado State University and is a registered professional engineer in the state of Florida. Dr. Ratcliff has been an active member of ASHRAE since 1991; employed with RWDI Inc. since 1998, Dr. Ratcliff is currently a technical director. Dr. Ratcliff has performed numerous studies on stack design and air quality for laboratories and healthcare facilities, as well as for the oil and gas industry.