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Fume Hood Exhaust Stacks - Developing Design Criteria for Specific Conditions to Save Energy

Simona Besnea, P.E., and Michael Ratcliff, Ph.D., P.E., Rowan Williams Davies & Irwin, Inc.

Fume hood exhaust containing toxic and odorous chemicals is typically discharged to the atmosphere to protect the safety of laboratory workers. Reentrainment of insufficiently diluted exhaust back into the emitting building or into surrounding buildings may result in odors or adverse health impacts. Atmospheric modeling is often performed to evaluate fume hood exhaust dispersion. The modeling results must be compared to design criteria based on the chemical usage in the laboratory. In particular, if the chemicals used can be grouped by health risk levels using control banding, then the design criteria can be similarly adjusted to satisfy the requirements associated with the control bands. Where fume hood exhaust can be designed to reflect the control banding approach, savings in energy costs and stack height requirements can be possibly achieved.

This presentation discusses recommended design criteria for fume hood exhaust, considerations involved in adjusting the criteria and potential energy savings as they relate to safe implementation of VAV systems and manifolded exhausts. The presentation will show that based on the specific needs of the project, design criteria can be developed in a number of ways:

  • By conducting a detailed analysis of the chemicals to determine limiting exposure limits
  • By recommending, where practical (i.e., only a few toxic chemicals require high dilutions) usage controls to reduce emissions
  • If requested by the design team, considering only specific potential effects (e.g., only health and not odors) in the assessment

The presentation will also discuss example design criteria developed for a specific set of conditions and will quantify implications in terms of stack height reduction, fan power required and energy savings. To address potential changes in chemical usage, we will present a tool that can be made available to lab owners/users for reevaluation of the criteria.

Labs21 Connection:

Our approach of the fume hood exhaust stack design focuses on two important aspects: safety and energy efficiency. In providing a balanced approach, we present methods for evaluating and refining the design criteria to answer the specific needs of the project. Stack height optimization, flexibility for future use, and energy savings are derived benefits.

This presentation directly reflects the following aspects of the Labs21 Approach:

  • Energy-efficient design/reduced operating costs - Detailed assessment of the fume hood exhaust and development of design criteria for specific set of conditions will help limit over-designing, and will result in lower energy and equipment costs.
  • Minimize environmental impacts - Implementation of an exhaust system that provides appropriate levels of dispersion will prevent degradation of local air quality and environmental impacts.
  • Adopt voluntary goals - Optimization of the exhaust design relies on due diligence and voluntary compliance of building designers and owners.
  • Community support and relations - A high performance stack design will facilitate understanding of the project by the community and enhance public support.


Simona Besnea has a B.Sc. in Engineering Physics from the University of Bucharest, Romania, and is currently completing her M.Eng. in Environmental Engineering at the University of Guelph, in Ontario, Canada. The focus of Ms. Besnea's research is air quality and dispersion modeling. Ms. Besnea is a Professional Engineer within the province of Ontario and is currently a Senior Engineer in the Wind Air and Microclimate Division at Rowan Williams Davies & Irwin Inc. in Guelph, Ontario, Canada. She focuses primarily on numerical and physical air quality modeling, specializing in exhaust reentrainment studies for the design of building exhaust and air intake systems for laboratories, hospitals, and other related facilities.

Michael Ratcliff has a Ph.D. in Civil Engineering from Colorado State University and has been a practicing consultant in air quality and building design for 22 years. Dr. Ratcliff is a registered Professional Engineer in four states and has been a Senior Specialist with Rowan Williams Davies & Irwin, Inc. since 1998. He has been an active member of ASHRAE since 1991. The focus of his consulting work is numerical and physical air quality modeling for exhaust systems at laboratories, hospitals, and other institutional facilities.

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