With the quantity and sizes of required fans known, the focus can shift to fan placement. While considerations can be made for tall obstructions restricting circulation patterns and any equipment at the roof level that would limit placement (HVAC, exhaust, light fixtures, etc.), HVLS fans should be spaced uniformly in the targeted area and equidistant from adjacent UVGI fixtures (Figure 1), similar to how one would position fans between supply diffusers of standard ductwork. In the vertical plane, the roof deck to airfoil clearance of the HVLS fan takes priority over UVGI fixture height for most applications. Positioning the UVGI fixtures approximately 0.3 m (1 ft) above the fan airfoils and as close to exterior/interior walls as practical is best practice.
The rationale with respect to fixture placement is to target the slowest moving air in the facility with the largest dose of radiation. Air speeds are, at a minimum, at higher interior elevations, just as the recirculated airflow transitions from the boundary wall and begins to turn back to the top side of the fan before being reaccelerated. In Figure 1, average air speeds in the disinfection zone are approximately 0.3 m/s (60 ft/min), with the shortest path from the wall to the swept area of the fan being 4 m (13 ft). Given this, the estimated minimum exposure time for an air particle in the disinfection zone is approximately 13 seconds. For context, if airborne Influenza-A, an RNA virus, is exposed to a range of 30 to 50 μW/cm2 of radiation for 13 seconds, it has an estimated surviving fraction of 63 to 46 per cent after a single pass through the disinfection zone. If the particle makes a second identical minimum exposure pass, the estimated surviving fraction drops to 40 to 21 per cent.
With respect to fan operation, it is recommended to operate the fan in the forward (‘normal’) mode at the same low to moderate speeds typically associated with destratification and air-mixing applications. The reason for this is multifaceted. First, forward operation keeps air particles in the optimal disinfection zones for longer, delivering higher doses of radiation, and inactivating larger percentages of contagions. Second, the reverse operation can ‘short circuit’ recirculation patterns, effectively preventing any of the air cleaned by the UVGI system or delivered by the HVAC system to reach the occupant breathing zone. Third, forward operation of the fan ensures freshly disinfected air reaches the occupant breathing zone as quickly as possible following its exit from the disinfection zone. This mitigates the additional risk of contamination from other sources. Lastly, it is advisable to look for products that have optimized airfoils specifically designed to be operated in the forward orientation. Achieving any level of performance while running a fan in reverse is much more energy-intensive than in the forward operating modes.
Peak UV germicidal wavelength resides between 265 to 270 nm. By damaging nucleic acids (both DNA- and RNA-based) and causing mutations that prevent replication, UV technology can render bacteria and viruses ineffective. Upper-room UV is considered the most effective application for room air disinfection by the Illuminating Engineering Society (IES) and is referenced as the safest, most effective application of UV-C wavelength use, where feasible.2
In summary, upper-room UV applications are the most effective when large volumes of air are constantly and actively mixed by fans. This results in higher equivalent ACH in terms of air disinfection. Thus, even when confined to an upper room application for safe occupation of the space, good air mixing (ideally with low-velocity ceiling fans) results in a high equivalent ACH in the lower, occupied space—estimated to be an additional 24 ACH. When compared to in-duct applications of UV technology, an upper-room system ensures recirculated air in the space is cleaned, effectively reducing the risk of person-to-person transmission in a room where both an infectious source (sick person) and other susceptible persons share the same air. As IES notes, for effective interruption of transmission, air disinfection has to occur in the same room where transmission is occurring and is the safest, most effective manner in doing so in many applications.
Alex Risen heads the public relations department at Big Ass Fans and works closely with the engineering and applications teams to communicate the company’s mission of creating a better, more resilient built environment. He can be reached at firstname.lastname@example.org.