Stack HeightUnderstanding the Formation of Roof-Top Re-circulation
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
Not available at this time.