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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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