Advantages of Mansard Roofs for Exhaust and Intake
Systems Design with Examples from Wind Tunnel Testing
Chet Wisner, Ambient Air
Technologies, LLC
Mansard-style roof façades have become a popular
choice for laboratories. Perhaps serendipitously, this roof style
appears to offer significant advantages for the design of rooftop
exhaust and intake systems. These advantages will be explored in
this presentation for selected variations of the basic Mansard style.
Wind tunnel test results will be used to demonstrate important characteristics
of the air flow and exhaust plume dispersion around these roofs.
The Mansard-style roof façade utilizes sloped roof elements
around the edges of the building leaving the flat roof in the center
of the building open. In true Mansard style, the center roof is
at the elevation of the peak of the sloped elements. However, as
applied to most modern laboratories, the flat center section of
the roof is at or below the elevation of the base of the sloped
elements. This leaves space open on the roof for mechanical equipment
and exhaust stacks. The slope of the elements can vary between 22.5
degrees and 60 degrees, reflecting design motifs ranging from Spanish
Colonial to classic European Mansard. The base of the sloped elements
can be closed or open to allow air flow under the sloped elements
to ventilate the roof.
The sloped elements of the Mansard-style provide a degree of streamlining
for the roof, mitigating the negative impact of rooftop airflows
(roof eddies and corner vortices) found on rectangular roofs. This
tends to allow lower, more aesthetically pleasing exhaust stacks.
Exit velocities can also be lower, reducing first cost, energy usage
and noise. For some applications filtering of intake or exhaust
air may be reduced or eliminated, also resulting in reduced first
cost and energy usage.
Examples from wind tunnel tests of actual lab designs will be presented
as well as idealized examples designed to demonstrate the principals
and significant features of exhaust dispersion around the Mansard-style
roof.
Labs21 Connection:
Use of the Mansard-style roof façade can allow exhaust and
intake system designs which are more aesthetically pleasing and
energy efficient. This presentation introduces the audience to the
advantages of this roof style for exhaust/intake design and to the
use of wind tunnel testing to ensure an acceptable design is achieved.
This presentation is consistent with the Labs 21 Approach of:
- Minimizing overall environmental impacts - The use of inherently-advantageous
roof configurations such as the Mansard style and wind tunnel
testing allow designs which minimize energy usage.
- Protect occupant safety - Exhaust behavior around a laboratory
building can be a serious concern for the safety of occupants
of the lab itself and neighboring buildings. The safety of workers
in the facility, in adjacent facilities, and in surrounding outdoor
areas can be ensured by the use of a well-conceived design and
wind tunnel testing to fine tune and verify the design.
- Optimize whole building efficiency - The use of a wind tunnel
study for laboratories allows the design team to optimize the
size of exhaust system that is required for the building. This
not only reduces the first-costs associated with the construction,
but allows for significant energy savings over the life of the
building.
Biography:
Chet Wisner is the President
of Ambient Air Technologies, LLC, a Colorado based firm specializing
in wind tunnel modeling of laboratory and healthcare facilities.
He has played an active role in the air quality portion of the environmental
industry for over 30 years. Applying his experience and expertise
in meteorology, engineering, and physics to physical modeling using
scale models in a boundary-layer wind tunnel, he has personally
managed or directed many wind tunnel studies. Chet coauthored an
EPA-recommended protocol for the use of environmental wind tunnel
studies to determine plume downwash characteristics for input to
EPA's own regulatory dispersion models. He was responsible for some
of the largest air quality monitoring networks in the U.S., and
has conducted numerous field studies of atmospheric dispersion.
His educational background includes a B.S. in Engineering Physics
from the University of California, Berkeley, an M.S. in Meteorology
from the South Dakota School of Mines and Technology, and an MBA
from the University of California, Los Angeles. He is an active
member of several professional organizations including ASHRAE, the
Air & Waste Management Association, and the American Meteorological
Society.
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