Field Measurements of Plume Rise From Various Laboratory Exhaust Systems

Chet Wisner, Ambient Air Technologies, LLC
Jim Meats, Loren Cook Company

When designing an exhaust stack installation, it is important to understand the degree to which the plume from the stack will penetrate vertically into the ambient airflow. For many years, the analysis tool relied on for this purpose by many engineers has been the Briggs' plume rise formula. However, there are questions as to what degree the "Briggs' Equation" is truly applicable to many of the current laboratory exhaust fan installations. It is based on observations of exhaust stacks which (1) present a fully-developed pipe flow velocity profile at their exit plane, and (2) are immersed in wind regimes of generally horizontal straight-streamline flow. Both of these key conditions are violated in the typical modern laboratory applications where the exhaust system is often immersed in the complex flows caused by the building and presents velocity profiles at the exit plane which clearly cannot be characterized as fully-developed pipe flow. The work to be presented addresses primarily the first point, using field data to investigate the effect of the exit velocity profile on the degree of penetration of the plume into the ambient flow--the plume rise. This knowledge will allow the HVAC engineer to select the exhaust fan configuration which maximizes plume dispersion while minimizing the fan energy required.

This presentation will show the results of recent field testing of laboratory exhaust fans/stacks of a variety of designs--non-induced air (traditional vertical stack), induced-air lab fans, and powered-induction (fan-at-the-top) exhaust systems--providing a wide variety of exit velocity profiles. The exit velocity profile of each exhaust system tested will be presented. Visible smoke from the exhaust systems is photogrammetrically documented to determine the trajectories of the plumes in field experiments in undisturbed boundary-layer wind regimes. The applicability of the Brigg's equation will be evaluated with respect to each of these exhaust systems.

This study is intended to allow evaluation of the plume rise provided by each exhaust system given similar expenditure of fan energy, thereby allowing the design engineer to better ensure the predicted plume rise will match system performance, and to evaluate that performance on an equivalent energy basis for the application being considered.

Learning Objectives

  • Optimize the selection of laboratory exhaust fans and stacks to maximize plume dispersion with minimum energy expenditure.
  • Recognize potential shortcomings of traditional rule-of-thumb exhaust stack design approaches.
  • Understand the relationship between exit-plane velocity profile and plume rise.
  • Understand the performance differences among various exhaust fan and stack designs currently available.


Chet is the President of Ambient Air Technologies, a Colorado firm specializing in wind tunnel modeling of laboratory and healthcare facilities. In several 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 Engineering Physics, an MS in Meteorology from South Dakota School of Mines, and an MBA from UCLA. He is a frequent presenter and member of ASHRAE, AMS, AWMA.

Jim holds an engineering degree from Kansas State Univ and is a licensed P.E. Jim started his career as a consulting engineer, designing mechanical and electrical systems for commercial, industrial and institutional projects. He has been in the HVAC manufacturing arena since 1987 with responsibility for managing engineering, manufacturing, as well as sales and marketing. Jim has been active in AMCA and ARI/AHRI since 1988 and has been an ASHRAE guest speaker around the country.


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