Wind Tunnel Testing and Other Strategies to Increase Laboratory Exhaust Fan Energy Savings
Aimée Smith, M.Eng., P.E., Rowan Williams Davies & Irwin Inc.
Along with high ventilation rate requirements, the energy consumed by the exhaust fans on the roof is an important piece of the laboratory energy use puzzle. A reduction in the exhaust flow rates discharged from these rooftop fans is crucial to reduce energy and operational costs, but must be balanced against the potential for undesirable re-entrainment of hazardous exhausts into the outside air supply system. The risk of undesirable re-entrainment can be evaluated through detailed dispersion modeling using mathematical or physical (wind tunnel) modeling. Of the two modeling approaches, the use of wind tunnel testing is less conservative, and in most cases will demonstrate a higher potential for exhaust turn-down.
Modern laboratory designs will often include the use of variable flow exhaust fans in order to reduce the fan horsepower requirements during unoccupied periods. This strategy is positive; however, much more can be done to fully optimize the energy used by the laboratory exhaust system.
Important factors contributing to the success of safe laboratory exhaust turn-down strategies include the following:
- Concentration of chemicals in the exhaust stream: One needs to define a credible emission profile for the chemicals present in the exhaust stream to evaluate the risk of chemical exposure and nuisance odors with exhaust turn-down.
- Placement of the exhaust stacks and outside air intakes: It is difficult to adopt a general approach for exhaust stack and intake placement because every case is different and the site specifics, including the local wind climate, building geometries (including those of surrounding buildings), and other potential contaminant sources and nearby existing air intakes will have a significant effect on the optimal locations.
- Number of exhaust fans, fan selection, and staging/sequencing: For laboratory fume hood exhausts, industry practice generally recommends a minimum outlet velocity of 3,000 feet per minute at full load. Fans cannot typically be run at less than 50 percent of their full flow capacity without undesirable re-entrainment issues, due to poor exhaust dispersion resulting from low exhaust momentum. There is a need to consider how to achieve an overall reduction below 50 percent of the exhaust system capacity through fan selection and sequencing.
Through the use of selected case studies and specific examples, this presentation will focus on how an in-depth consideration of these factors, combined with detailed physical (wind tunnel) exhaust dispersion modeling, can significantly increase the opportunity for laboratory exhaust fan turn-down.
Aimée Smith has a Bachelor of Science in environmental engineering from the University of Guelph and a master's degree in civil engineering from Carleton University, and is a registered professional engineer in the province of Ontario. Ms. Smith is a principal of Rowan Williams Davies & Irwin Inc. (RWDI) in Guelph, Ontario, Canada, and a project director in RWDI's Building Science division. Ms. Smith provides design ideas to clients and focuses on different aspects of building performance as it is influenced by the local microclimate. In particular, Ms. Smith specializes in exhaust re-entrainment studies for the design of building exhaust and air intake systems for laboratory, healthcare, and other related facilities.