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Balancing Exhaust and Air Intake Stack Design and Placement Supports Contaminant Dispersion
January, 2007
By Martin Stangl and Glenn Schuyler
The optimal design and placement of exhaust stacks to limit contamination of air intakes and other sensitive locations can be a considerable challenge. Buildings may have several different exhaust types that discharge a variety of contaminants, presenting both health and odour concerns. Often, misconceptions exist that attribute health effects to odours or, conversely, cause the assumption that a health concern does not exist simply because there is no odour.
Exhaust dilution can be used as a tool to assess re-entrainment of exhaust at building intakes or sensitive areas. One of the most prominent issues affecting exhaust dispersion is the effect of recirculation zones on buildings. Winds striking the face of a building will separate over the top, reattaching at the building rooftop or some farther point downwind. This separated zone is called the recirculation zone.
This recirculation zone on the roof will trap exhaust that is discharged within it. In this scenario, contaminants will accumulate within the zone resulting in high concentrations and low levels of dilution. It is desirable to design stacks that discharge above these recirculation zones.
Sometimes a stack discharging within a recirculation zone may be deemed acceptable if the separation distance to air intakes or other locations of concern is far enough or the contaminant source concentration is low enough. Commonly, stacks discharging at least as high as the tallest local recirculation zone are required. If short stacks located within a recirculation zone cannot be avoided, then outside air intakes should be placed out of the zone as far away as is practical.
In most cases, a consultation on the exhaust and intake design from the perspective of exhaust re-entrainment is valuable. Such a consultation can provide insight to design strategies aimed at reducing the level of exhaust re-entrainment.
DILUTION
When exhaust is discharged from a stack, it has certain initial source concentration. As the exhaust travels, it undergoes atmospheric mixing and dispersion, mixing with clean air, which reduces the concentration.
If the concentration was measured at some point downwind, it would be lower than at the source. Dilution is the ratio by which the exhaust concentration is reduced from the stack to the point of measurement.
The goal in exhaust stack design is to ensure that contaminant concentrations are reduced sufficiently to ensure that no applicable health limits are exceeded and odours are reduced to applicable levels. Dilution is a very simple measure that can be used to establish design criteria for various source types to assess the performance of an exhaust stack.
Dilution is a particularly useful measure when numerous contaminants are or can be emitted from the same stack. Since the source concentrations of the various contaminants are typically known, or easily found, they can be compared to applicable air quality standards or limits.
Once all the specific standards or limits are known, there will be a level of dilution for which the applicable standards and limits are met for each contaminant. The highest of these dilutions is typically used as the criterion to assess the adequacy of an exhaust stack. This is call a dilution criterion.
Many difference exhaust sources that exist on buildings can be of concern - i.e. emergency generators, boilers, idling diesel vehicles, biosafety cabinets, specialty chemical and process exhausts etc. Many of these sources require a dilution criterion to be determined on a case-by-case basis.
Generally, the following source-specific ratios apply, based on typical operating conditions and extensive project experience:
Laboratory fume hood: >3,000:1
Cooling tower >20:1
Animal holding rooms and vivaria (odours) >100:1
Commercial kitchens (odours) >600:1
Diesel engine (odours) >4,000:1
LOCATION & DESIGN
Early in the design process it is beneficial to determine some approximate exhaust and intake locations. To achieve a high level of atmospheric dispersion and, therefore, high dilution, the following design features should be considered:
1) Maximize stack height
( This is best accomplished by placing stacks at the highest point on the building
( Ensure stack discharge is above rooftop recirculation zones
2) Maximize separation distance to intakes or sensitive locations
3) Maximize exit velocity and vertical momentum
( Reduce exit diameter, or increase exhaust flow rate by adding clean air to the exhaust stream
( Avoid top-mounted rain caps that eliminate vertical momentum
( Avoid horizontal discharge
4) When appropriate dilution cannot be achieved, other options exist to reduce contaminant concentrations in exhaust streams using either controls or manifolding.
( Controls can include exhaust scrubbers, electrostatic precipitators, filters etc.
( Manifolding with clean, uncontaminated air provides the benefit of pre-dilution (or internal dilution) prior to discharge. This can also be accomplished by using bypass air.
The location and design of exhaust stacks should be assessed prior to the completion of design drawings to ensure that applicable dilution criteria are met. An initial design consultation is useful to provide early feedback and provide recommendations of the type of assessment most beneficial for the design. The assessments include numerical dispersion for single building geometry or a more detailed and accurate wind tunnel study.
The assessment of the proposed stack designs and locations is best performed during the initial stages of the building design when there is flexibility in the mechanical and architectural designs. At this point, solutions can be discussed with the design team to ensure they are practical.
Stack design is often a compromise between maximizing exhaust dispersion and the practical confines of building design. Capital budgets, operating expenses and aesthetics must be balanced with good stack design.
As a minimum, a stack design must provide sufficient dilution to satisfy applicable health standards and limits. Odours are far more subjective. They can often pose a significant nuisance issue.
Martin Stangl, B.A.Sc (Eng), and Glenn Schuyler, M.A.Sc., P.Eng. are with RWDI Consulting Engineers and Scientists, the developers of the RWDI Desktop Dilution Calculator, a modeling tool to provide dilution and recirculation zone estimates . For more information, see the web site at www.rwdi.com/ddc.
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