Exhaust Dispersion: Myths and Misconceptions Illustrated

Chet Wisner, Ambient Air Technologies, LLC
Delores Wisner, Ambient Air Technologies, LLC

Many well-intentioned architects and engineers base initial design decisions for their laboratory exhaust systems on myths and misconceptions that have taken root over years of use. Some of these have even been included in codes and guidelines. Since wind tunnel testing of laboratory exhaust systems has become the general norm, most of the resulting safety issues are corrected before design finalization and construction. However, the design changes are larger than would otherwise be needed, and fan energy consumption is often increased to compensate for an initially-flawed design.

Does a 10-foot stack with 3,000 foot-per-minute exit velocities provide any assurance of acceptable dispersion? Are minimum separation distances between exhausts and intakes any guarantee of acceptable concentration levels? Can induced-air laboratory exhaust fans effectively replace tall stacks? What is 'effective stack height' and can it be accurately calculated for stacks on a typical laboratory roof? Are small, specialty exhaust stacks less challenging to design than the primary laboratory exhaust? How effectively can exit velocity trade off against taller stacks? Can moving the outside air intake location a few feet significantly improve its air quality?

This presentation explores these and similar questions, illustrating fact versus fiction using the wind tunnel to show where real exhaust plumes go and how they disperse. The shortcomings of ingrained rules-of-thumb, guidelines and common practice will be examined and demonstrated, providing a better basis for making initial design decisions.

Learning Objectives

  • Recognize potential shortcomings of traditional rule-of-thumb exhaust stack design approaches.
  • Understand the general features of typical airflow patterns affecting laboratory exhaust plume dispersion.
  • Avoid energy penalties involved in resolving exhaust dispersion issues late in the design process.

Biography:

Chet is the President of Ambient Air Technologies, a Colorado firm specializing in wind tunnel modeling of laboratory and healthcare facilities. In several recent cutting-edge studies, AAT has produced data supporting energy reduction initiatives which are reaping significant energy savings for numerous clients. He has a BS from UC Berkeley in Eng Physics, an MS in Meteorology from the SD School of Mines, and an MBA from UCLA. He is a frequent presenter and member of ASHRAE, A&WMA and AMS.

Delores is a vice president with Ambient Air Technologies in Colorado and has over 25 years' experience working in the area of wind tunnel modeling and consulting for environmental applications. With both a practical and pragmatic approach to solutions in projects involving laboratory and healthcare facilities, her working relationships with architects, engineers, lab designers, facilities planners, and end-user clients is well known and respected in the industry.

 

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