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Figure 1: Wind interacting with building massing and rooftop exhaust stacks.
 
  

It’s no secret lab facilities carry the burden of a large energy demand. Reasons for this high demand include the significant plug loads of specialized lab equipment, the high ventilation air change rates often implemented in lab spaces and the large volumes of hazardous exhaust air that must be moved out of the building.

Of course, proper equipment and adequate ventilation in labs is essential for the success, health and safety of the building and its users. However, no less important is the safe release of hazardous exhaust air from the building so it doesn’t adversely impinge on nearby air-sensitive locations or the lab building itself via re-ingestion into air intakes.

Many labs have addressed this concern by situating manifolded exhaust fans on the building roof, which is an excellent design step. However, the optimum fan design that harmonizes safety with fan energy and cost is less straightforward, given the many factors that contribute to the best possible dispersion of hazardous exhaust.

Exhaust fan
The exhaust fan design is of great importance as it dictates how well hazardous contaminants will be dispersed away from the building. It will also determine how much energy will be consumed.

Exhaust fans can be operated in two primary ways: constant air volume (CAV) and variable air volume (VAV). In CAV mode, the fan always exhausts the same air volume to maintain a constant momentum. If fume hoods that feed into the exhaust fan are turned down/off, fresh bypass air is provided to the fan to maintain the same air volume. Conversely, VAV mode reduces the air volume released from the fan during low fume hood usage, saving fan energy.

Manifolding fume hoods together so they discharge from a common exhaust fan is an excellent design step for a few important reasons. First, the resultant exhaust plume released from the fan is much larger in volume, which means the plume will have much improved dispersion momentum. Second, less ductwork and fewer fans are required for installation and maintenance, thus saving on first and ongoing costs. Third, additional internal dilution of the contaminants is afforded to the exhaust plume, given that hazardous constituents won’t be released simultaneously from all hoods.

With respect to the type of fan, centrifugal fans are often used as they can supply high exhaust airflows and high pressures. Axial fans can be pursued, although they often consume more energy because the fan spins faster for the same airflow.

Exhaust stack
No less important from an exhaust dispersion perspective is the proper design of the stack tied to the exhaust fan. Figure 1 illustrates this importance, showing the complicated rooftop turbulence that can impact proper plume dispersion. This turbulence is created by the approaching wind interacting with the massing of the building itself and any surrounding massing nearby.

Several options can be considered to limit the influence of this turbulence. First, exhaust stacks should be vertical and uncapped and discharged at a reasonably high exit velocity, ideally greater than 3,000 fpm. This will afford the plume good discharge momentum to overcome the turbulence. Second, stacks should be situated away from nearby obstructions such as stair towers, mechanical rooms or adjacent higher roof levels. Ideally, the stack should be situated on the highest roof possible to minimize impacts on dispersion. Third, exhaust stacks should discharge at a minimum height of 10 ft above the roof for good dispersion and the protection of the breathing space of any personnel working in the immediate vicinity.

Dispersion modeling
Wind tunnel dispersion modeling is one means of navigating these many factors to determine the optimal exhaust fan and stack design for any lab building. This type of modeling is well suited to account for the significant impacts surrounding buildings and local terrain can have on plume dispersion. Wind tunnel modeling can also be used to quantify fan energy savings by demonstrating what level of fan turndown can be pursued, while still meeting applicable health and odor thresholds.

Mark Hallman, LEED AP, is a Senior Project Engineer with Rowan Williams Davies & Irwin Inc. (RWDI). Hallman has provided consulting services on exhaust and intake designs for a variety of new and existing lab facilities in the educational, medical and research sectors.

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