Saving Energy by Reducing Exit Velocity With a Suitable Stack Design

Simona Besnea, Rowan Williams Davies & Irwin, Inc.

The primary purpose of a laboratory fume hood exhaust is the safe release of toxic contaminants to the atmosphere. Safety is first. Large volume flow rates and high stack exit velocities are typically required to achieve safe levels of dilution. Energy savings can be achieved by reducing the volume of conditioned air exhausted through use of variable air volume fume hoods. In many cases, safe release of toxic contaminants requires that a high stack exit velocity be maintained. This is commonly achieved through the addition of bypass air. Designing exhaust stacks to minimize the amount of bypass air needed will result in energy savings. Design options such as stack/intake placement, stack height and exit velocity have a direct effect on the fan energy required to discharge the fume hood exhaust safely.

Others have shown that, for a given stack height and placement, fan energy can be saved by controlling the amount of bypass air based on wind conditions. However, greater savings can be achieved through improved stack placement and use of stack height to supplement plume rise.

This presentation will show that with some investment in stack height, the required amount of bypass air, and thus energy requirements, can be reduced. The decrease in exit velocity and the potential for stack tip downwash will be addressed. This presentation will demonstrate, based on wind tunnel testing of actual buildings, that a safe and energy-efficient stack design can be achieved even under typical worst-case wind conditions. Practical design guidance for various stack heights and locations, flow rates, and building configurations will be provided.

This approach focuses on two important aspects: safety and energy efficiency. This presentation shows that energy savings can be derived by operating laboratory exhaust stacks at lower exit velocities without compromising the health and comfort of building occupants. This presentation will explore findings based on wind tunnel data for stack height optimization, reduced exit velocity, flexibility for future use, and energy savings.

This presentation reflects the following aspects of the Labs21 Approach:

• Energy-efficient design/reduced operating costs—By using design guidance that allows safe operation at reduced exhaust stack velocity, energy savings can be derived.
• 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 facilitates project understanding by the community and enhances public support.

Biography:

Simona Besnea has a Bachelor of Applied Science in engineering physics from the University of Bucharest, Romania, and holds a Master of Engineering in environmental engineering from 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. (RWDI), in Guelph, Ontario, Canada. She focuses primarily on numerical and physical air quality modeling, specializing in exhaust re-entrainment studies for the design of building exhaust and air intake systems for laboratory, hospital, and other related facilities.

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