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Reducing Stack Height—Understanding the Formation of Roof-Top Re-circulation Regions

Kimberly Thompson, P.Eng, Rowan Williams Davies and Irwin Inc.

Edwina Wong, M.Sc, and Jason Slusarczyk, Rowan Williams Davies and Irwin Inc.


Exhaust from laboratory fume hoods can result in air quality concerns if exhausts are re-entrained into building air supply systems. One of the most important factors influencing the potential for exhaust re-entrainment is the re-circulation region created by wind flowing over a building rooftop. Discharging exhaust within the re-circulation region can result in poor dispersion of the exhaust and can lead to high contaminant concentrations at air intakes. The use of VAV exhausts within re-circulation regions can result in further complications. VAV exhausts can save on energy costs; however, low exit velocities during low fume hood usage can exaggerate the effect of re-circulation regions.

To reduce the potential of exhaust re-entrainment, efforts have been focused on tall stack design and/or increasing flow rate and exit velocity to help the discharged exhaust escape the rooftop re-circulation zone. While these options can effectively accomplish this, they also require increased capital and operational costs over the life of the system and can alter the aesthetics of the building. In contrast, by minimizing the formation of re-circulation regions, exhausts can be effectively dispersed without using tall stacks and can provide more operational flexibility with VAV systems.

This presentation will review factors affecting the formation of re-circulation regions, including building geometry, wind direction and roof top structures such as mechanical penthouses. In addition, building designs that limit the formation of re-circulations regions will be discussed and measurement and modeling will be used to demonstrate that exhausts can be effectively dispersed using reduced stack heights.


Not available at this time.

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