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In August 2020, Air Movement and Control Association, International (AMCA) was made aware of COVID guidance on operation of circulating fans that was published by U.S. Centers for Disease Control of Atlanta (CDC), National Institute for Health (NIH), and other organizations. The guidance, which did not point to foundations stemming from research or prior practice, advised that circulating fans should be turned off.
AMCA met with several member companies that manufacture large diameter ceiling fans and the consensus position was that the guidance for turning fans off seemed wrong. Wouldn’t mixing / destratification dilute concentrations in breathing zones? Staff contacted the NIH to discuss the guidance to learn about the provenance of the guidance and learned that the agency’s subject matter experts would be consider changing the guidance if AMCA had better information. However, with no specific data in hand with which to respond, AMCA initiated a modeling-based research project for large-diameter ceiling fans operating in mid-size warehouses, which, especially in the context of the pandemic, was strategic given the need for more warehouses in the supply chains of online commerce.
Project Initiation: given the reasonable concern of conflict-of-interest or commercial bias from industry-funded research, AMCA assembled a bi-cameral project team consisting of a science team and an industry team. The industry team consisted of engineers from AMCA and member companies and the science team consisted of experts in computational fluid dynamics modeling, finite element analysis, infectious diseases, indoor air quality, and building science. The science team designed the study, reviewed the initial results and final report, and is participating in post-project peer-review journal publishing.
Modeling: the modeling approach, including assumptions, limitations, validation of the approach, and supplemental discussions are in the final report. The project used two 6-m (20-ft) AMCA-certified fans spaced 120-ft apart at three speeds: 0 m/s, 0.6 m/s (2 ft/s), and 6 m/s (10 ft/s). These speeds are average air speed exiting the fan. At 100% speed provided an airflow of 90 m3/s (192,000 cfm) at 78 rpm. Over 220 parametric simulations were run. The warehouse was modeled with and without racking. Zones were established relative to the workers under the fan so spaces in their vicinity (working zone) and outside their vicinity (breathing zone) could be investigated. Worker and their orientation relative to the fans were varied.
Results: the options for operating the fan (different speed or direction), operating the fans at the highest feasible speed consistently outperforms the other options. However, operating fans at the maximum speed generates high airspeeds in the occupied zone, which will not be practical in some conditions, for example, as it would cause thermal discomfort in cold indoor conditions. Where lower airspeeds are preferred: Fan speed at 3 m/s (10 ft/s) in upward-flow direction is a good option, although it is not as good as 3 m/s (10 ff/s) with downward flow. Reversing fans at higher fan speed (3 m/s) reduces the performance compared to high speed with downward flow. Reversing fans at lower fan speed (e.g., 0.6 m/s (~ 2.0 ft/s)) may reduce the whole warehouse airflow speed and thus lower diluting effect, so the whole warehouse concentration could become higher. Thermal plumes from workers are essential, especially for still air cases; local air mixing and movement could help to dilute and reduce the chance of creating local hotspots.
AMCA has developed COVID guidance for large diameter ceiling fans for warehouses based on these results. The final report and input files for modeling are available at https://bit.ly/COVID_LDCF.