Considering the Effect of Rooftop Structures in Modeling
Exhaust Dispersion
Amit Gupta, M.Eng, Concordia
University
The presentation deals with the effect of rooftop
structures (RTS) on exhausts released from rooftop stacks. Most
buildings have one or more RTS, which can be in the form of HVAC
equipment, a penthouse, or mechanical room. Such structures can
significantly alter the airflow on the building roof and may lead
to plume downwash for certain geometries. Concentrations produced
by exhaust sources located in the wake of an RTS may exceed the
recommended standards and lead to health and odor problems depending
on chemicals released.
Wind tunnel modeling was carried out for a range of RTS sizes,
with aspect ratio, w/h, varying from 2.5 to 12.5, where w is the
crosswind length and h is the height of RTS. The exhaust source
was located at a distance of 0.5h downstream of RTS. Concentration
measurements were gathered for various combinations of exhaust momentum,
stack height and wind directions.
The 'amplification factorm,' a, defined as the ratio of concentration
obtained on the building roof with and without RTS was obtained
for all configurations. A comparison or present findings with the
current design standards is also presented. A field study showing
the effect of downwash caused by a roof skylight on a downwind stack
is also discussed.
The study reveals that the current minimum recommended stack height
of 3m suggested by standards like ASHRAE and ANSI/AIHA may not be
sufficient to avoid reingestion problems when an RTS is present.
It was found that in the presence of a RTS the concentrations on
a high-rise building roof can increase by a factor of 100 at locations
closer to the stack and a factor of 20 for locations near the leeward
edge of building. For a low-rise building the amplification factor
can vary from 10 to 30. However, these factors are quite sensitive
to the stack location, wind direction, and exhaust parameters. It
is also demonstrated how the current ASHRAE dispersion models are
incapable of taking into consideration the effect of an RTS. Simple
guidelines are suggested for designing an efficient exhaust and
intake system for a building with rooftop structures.
Labs21 Connection:
A wind tunnel can be used as a powerful tool to model building
dispersion applications. Full-scale tests are expensive and time
consuming but they are still needed to validate the results predicted
by empirical or physical modeling.
This presentation is consistent with the Labs21 Approach of:
- Improved environmental quality - Designing a proper exhaust
system will eliminate the problem of re-ingestion and improve
air quality for the emitting building and the surrounding structures.
- Increased health, safety, and productivity - Proper exhaust
design will be beneficial to the health and safety of workers
working inside and also outside the building.
- Better understanding of air flow around buildings - The first
approach for a good exhaust and intake design is to have a proper
understanding of air flow around buildings.
Biography:
Amit Gupta holds a
B.Eng. in civil engineering and a M.Eng in structural dynamics from
Indian Institute of Technology Roorkee and currently working on
his Ph.D. at Concordia University, in Montreal, Canada, in the field
of environmental aerodynamics. Amit is working on issues related
to exhaust and dispersion modeling. He worked previously with Risk
Management Solutions India a subsidiary of RMS, California, USA
in developing loss estimation models for hurricanes and earthquakes.
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