Endless factors go into facility design. From an air quality standpoint, it is essential to consider how wind directions, facility size, location and surrounding buildings impact air quality dispersion. These factors directly inform the appropriate height of an exhaust stack.

To optimize stack height, it must be understood that surrounding buildings block and change the wind’s direction and speed, fundamentally affecting how a plume mixes with the atmosphere. This concept is called downwash. Building downwash occurs as the wind flows over and around buildings and impacts the dispersion of pollution and emissions. It can be difficult to predict which buildings will cause dominant downwash impacts.

While air dispersion modeling can often help projects determine the right stack height, sometimes projects are well-suited for simply utilizing the information gathered in a downwash analysis. A downwash analysis is an equation that looks at 36 different wind directions for potential stacks, as well as the height and presenting face of the buildings in each direction. Building height and width are major factors, along with the proximity of surrounding buildings. This analysis also considers the terrain, because a hilly site could change the height of the building relative to sea level.

While a downwash analysis is often a significant first step for many facilities, some flexible and optimal projects may be able to stop there. Smaller-scale projects — such as peak shaving facilities, or campuswide institutions such as universities or medical facilities — often have design flexibility on stack location or size. Through a downwash analysis, the necessary information can be gathered to provide high-level guidance on ways to minimize stacks without having to spend the time and money on a full air dispersion model.

A modeling tool such as BPIP-Prime — the air quality dispersion modeling program recommended by the U.S. Environmental Protection Agency — can manipulate plans to see the 3D effects of design layouts and refine the estimate of impacts. This program includes an algorithm for calculating downwash values for input into the steady-state plume models, or AERMOD models.

It’s essential to have the appropriate tools and perspective to understand what level of air quality modeling is needed for a new project. For the right scenarios, downwash analyses alone could provide owners and operators enough information to determine where to site a stack so it won’t be adversely impacted by other buildings. These analyses also help identify where to locate new auxiliary boilers. This downwash analysis information also plays a large role in a facility’s decision matrix when deciding between two site layouts. This unique approach offers a flexible, inexpensive option to owners and operators of smaller-scale facilities.


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Robynn Andracsek, PE, is an associate environmental engineer at Burns & McDonnell. She specializes in air quality permitting for industrial and utility clients.