Initial Dilution and Effective Stack Height for Induced Air Fans

John Carter, CPP, Inc.

Induced air fan manufacturers provide in their specifications both an "effective stack height" and information regarding the "initial dilution" their fans provide. Both of these specifications can be misleading and may provide designers with a false sense of safety from a fume reentry perspective. This talk will document how the quoted extra dilution does not decrease concentration levels at sensitive downwind locations and will demonstrate that the same concentration levels are achieved using the fan flow or the fan flow plus induced flow. It will also demonstrate that the "effective stack height" value quoted by the manufacturers is based on a simplified equation that is incorrect for rooftop building environments. The equation used by the manufacturers is only appropriate for a low ambient turbulence environment, like a grass field or airport, and only provides a final plume height value that occurs 100 to 200 feet downwind of the stack.

This information is important to designers because they may currently be making fan selections based on incorrect information. The best way to select a fan is to determine the optimum combination of physical stack height, volume flow rate, and exit velocity needed to achieve acceptable air quality at building air intakes and other sensitive locations. Often, a lower volume flow and/or exit velocity exhaust system can be specified by simply raising the stack height a few feet, thus potentially saving significant energy.

This presentation will present two examples where designers initially selected an induced air fan system and later found that an alternate system would provide either a lower cost solution, both in first cost and energy consumption, or a solution that would prevent odors. For the first example, a wind tunnel modeling evaluation demonstrated that acceptable air quality could be achieved using a conventional manifold exhaust system with reasonable stack heights and lower exit velocities in lieu of manifold-induced air fans. This resulted in significant first cost and energy savings. The second example discusses a project where an induced air fan was specified because the designer thought it would solve an existing odor problem. After the fan was installed, it did not perform as the designer thought it would, and the odors were still present. The high turbulence environment at the site brought the plume down to ground level sooner than predicted by the manufacturer's "effective stack height" specification. The only practical solution for this problem was a taller stack combined with bypass air.

For some applications, an induced air fan is the best solution; however, the best solution can only be determined using the appropriate analysis.

Biographies:

John Carter, an associate at CPP, Inc., has over 10 years of experience conducting wind tunnel and numerical modeling studies related to laboratory exhaust design for such clients as General Electric Global, Johnson and Johnson, Allergan, Applied Bio, University of Missouri, Duke University, the University of California (UC) at Irvine, UC Davis, and UCSM to name a few. While employed at CPP, he has been instrumental in developing tools to aid in modeling of internal building airflow patterns using pressure coefficient data measured in CPP's wind tunnels. This technique, which uses the pressure data in conjunction with existing numerical models, has been used to evaluate natural ventilation in high-rise buildings as well as to troubleshoot and provide solutions for ventilation problems in existing facilities. In addition, he has conducted numerous wind tunnel dispersion studies to determine "Equivalent Building Dimensions" to provide clients with greater accuracy in estimating concentrations due to downwash using EPA's Industrial Source Complex model, as well as studies of "Good Engineering Practice" stack height and building ventilation.

Prior to his arrival as a full time engineer at CPP, John participated in ASHRAE's Research Project, investigating the impact of architectural screens on rooftop concentration levels (ASHRAE Research Project 805-TRP). This work was the basis of his Master's thesis. John has written several technical reports and is an active member of ASHRAE, ASWE, and the American Society of Mechanical Engineers. He holds a Bachelor of Science in Mechanical Engineering from South Dakota School of Mines and Technology and a Master of Science in Wind Engineering from Colorado State University.

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