November 13, 2020
By Alex Risen
Using an overhead high-volume, low-speed (HVLS) fan’s bi-directional airflow and elevated air speed is a powerful way to keep occupants comfortable in applications ranging from non-conditioned industrial spaces to commercial buildings. Immediately underneath an HVLS fan, airflow is pushed downward. Outside its diameter, the airflow transitions to horizontal air movement, providing comfortable airflow over a large area, according to thermal comfort calculations as per the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) 55, Thermal Environmental Conditions for Human Occupancy.
Directional fan and airflow
Directional fans create a one-way airflow pattern that moves air to the area in front of the fan. Directional fans are most often used to cool occupants in congested areas where overhead fans cannot be safely mounted. Additionally, premium directional fans feature variable speed control, allowing users to adjust airflow to provide optimal comfort.
Improving ventilation with fans
Fresh air is not typically distributed uniformly throughout a space. For example, one of the most common air distribution configurations for HVAC systems is an overhead supply with overhead return. In common heating applications, this ceiling configuration can cause significant stratification of heated supply air and room air layers, preventing fresh air from circulating to occupant level. To compensate, ASHRAE 62.1, Ventilation for Acceptable Indoor Air Quality, requires the increase of ventilation rates by 20 per cent to deliver the necessary amount of fresh air to the room’s occupants. However, in a room destratified by an HVLS overhead fan, the air layers are well mixed so the supplied fresh air can reach the occupant level.
In naturally ventilated spaces, fresh air is passively distributed throughout the space, so localized areas may have stagnant pockets, resulting in poor air quality and buildup of pollutants. Fans disperse these pockets and increase the circulation of fresh air, evenly distributing it throughout the space and improving indoor air quality (IAQ).
Impact of increased air movement on ventilation
Using a computational fluid dynamics (CFD) analysis of an industrial space shows the measurable benefits offered by a large diameter, HVLS overhead fan. In one instance, a 7-m (24-ft) model pushes airflow directly downward in an area of roughly 46 m2 (500 sf). Horizontally, the fan is able to push airflow over a larger area (depending on mounting height, floor-level obstructions, fan model, etc.).
Operating fans during flu seasons
Many variables are at play within each facility and application. Without data and intensive study for each case, it is inconclusive whether elevated air speed has a significant health impact on a facility. Following best practices, as outlined by local public health units and the World Health Organization (WHO), in addition to adhering to building design standards and practices set forth by ASHRAE, ensures proper design and implementation of systems.
Ventilation can reduce the concentration of airborne pathogens through dilution. Increased ventilation rates (outdoor air intake) can provide a higher dilution capability and, potentially, reduce the risk of infection. As such, the following are accepted recommendations to consider:
In addition to using fans on their own, applying modified design guidelines, similar to those used for common destratification applications to increase the rate of air circulation between the upper and lower air zones, can improve the performance of upper-room ultraviolet germicidal irradiation (UVGI) systems by up to 77 per cent.
Improving upper-room UVGI system performance with fans
The use of UVGI technologies in the HVAC industry is well established and understood. The technology has been paired with HVAC equipment use since the early 1940s and the benefits they provide reduce occupant exposure to contagions by disinfecting indoor air near specially designed UVGI devices. These devices resemble standard fluorescent tubes or lighting luminaires and can be ceiling or wall-mounted, portable, installed in ducts, or packaged in the equipment itself.
One of the most common applications is upper-room UVGI. Here, the fixtures are installed above the occupied zone and oriented horizontally such that their radiation output (in the form of UV-C waves) is concentrated to create a disinfection zone above the occupant breathing zone (below), limiting the occupants’ direct exposure to elevated radiation levels. The elevated radiation levels in the disinfection zone damage the DNA/RNA of micro-organisms including that of bacteria, fungi, and viruses. With damaged genetic material, the often contagious micro-organisms are inactivated and unable to replicate in the indoor environment or inside of any host. Viruses are the most susceptible micro-organisms to UV-C irradiation, followed by bacteria and then fungi. Upper-room UVGI is the oldest application, the most easily retrofittable one, and is approved for improved control of highly contagious airborne diseases, such as tuberculosis by the U.S. Centers for Disease Control and Prevention (CDC) and the National Institute for Occupational Safety and Health (NIOSH).
Upper-room UVGI system performance benchmarks
To ensure effective inactivation of airborne contagions, upper-room UVGI design guidelines are provided by NIOSH in partnership with CDC. The most important of these guidelines concerns the selection and positioning of the UV sources. The lamps should be arranged to provide irradiance in a uniform manner. Total UV fluence should be between 30 and 50 μW/cm2. This measure of radiation intensity can be multiplied by the duration of exposure (the time of an air particle in the disinfection zone) in seconds to determine the dose, measured in μJ/cm2. A suggested simplification of this principle is to use 0.18 watts of lamp power per cubic foot (0.18 W/cf) of upper zone air volume or 0.17 watts of lamp power per square foot (0.17 W/sf) of floor area.
Additionally, fixtures with louvers should be a minimum of 2 m (7 ft) above the finished floor, and fixtures without louvers should be at least 3 m (10 ft) above the floor. The minimum height requirements do not guarantee acceptable exposure at occupant levels. While exposure can be estimated in a simulated environment with specialty software, many models fail to capture the complex geometry and the true reflectiveness of every object in a facility. As such, it is recommended radiation levels be measured with calibrated meters to verify intensities are within NIOSH limits for an assumed occupancy duration.
Ambient operating conditions play an important role in the effectiveness of UV-C lamps. For unconditioned spaces in climates where relative humidity (RH) regularly exceeds 60 per cent, higher fluence densities may be required to achieve the desired performance. Likewise, low-temperature applications (below 20 C [68 F]) and scenarios where airspeeds near UV-C lamps exceed 1 m/s (200 ft/min) may cause a decrease in lamp output and require more fixtures than standard applications.
All of these factors combine to drive the system efficacy for upper-room UVGI installations. However, even systems with properly specified and installed fixtures will not perform optimally without being able to effectively circulate air between the disinfection and occupant breathing zones. A significant way to improve system efficiency is to increase the effective rate of air changes per hour (eACH), a measure of how much indoor air is being cleaned by the upper-room UVGI system and subsequently redelivered to the occupant breathing zone by utilizing low-speed ceiling fans to improve the rate at which indoor air is circulated between zones.
Using HVLS fans to improve upper-room UVGI performance
Studies show that in spaces without mechanical ventilation air mixing, the effectiveness of upper-room UVGI systems is modest (12 per cent) and relies only on convection patterns generated by heat gains induced by occupant and equipment sources at floor level. Spaces with mechanical ventilation can also suffer. This is especially true for facilities with high deck heights where systems with high airflow rates can struggle with air mixing and distribution. This issue is problematic during the heating season when the rate of illness among occupant populations tends to be the highest.
Research conducted by the University of Colorado and sponsored by CDC and NIOSH has shown the disinfection of room air requires 6 to 12 ACH, much more than the 0.5 to 2 ACH that is typically required for odour and carbon dioxide (CO2) control in ventilated spaces per ASHRAE 62.1 (See “Relevant Studies,” page 22).1 Bringing in this amount of outdoor air to a facility can have significant energy and equipment implications. By introducing HVLS fans, the volume of air that is actively cleaned in the disinfection zone is more frequently circulated back to the occupant level and replaced in the disinfection zone by air with a higher concentration of contaminants. By continually mixing the disinfection zone and occupant breathing zone air volumes, the effectiveness of the UVGI system is improved, thus increasing eACH. This reduces the concentration of contaminants in the space without the need for a three to six-time increase in outdoor air rates.
In the second stage of the aforementioned study with 12 per cent system effectiveness without air mixing, a ceiling fan was added to ensure air mixing in the space. While operating the fan at a moderate speed, the system’s effectiveness rose to 87 per cent. It is important to note circulating fans, whether operating in forward or reverse, can achieve effective kill rates.
According to a study published in Environmental Health Perspectives, increasing ACH and using a mixing fan enhances UV’s effectiveness in inactivating aerosols of the pathogen S. marcescens. This increase in ventilation rates and mixing of upper and lower air volumes led to a reduction in pathogens, resulting in a safer, healthier indoor environment.
HVLS application design guidance
Overall, the application process for integrating HVLS fans in upper-room UVGI systems closely resembles that of common destratification applications. The key exception is a large change in the desired number of air turnovers per hour (ATH) and how the turnover metrics for each scenario are defined. While a standard destratification application may target two total building air turnovers per hour (ATH) (i.e. total fan airflow versus total space air volume), UVGI applications should target a minimum of 25 zonal air turnovers per hour (zATH) between the upper air volume (disinfection zone) and the lower air volumes (everything below the disinfection zone). This calculation method better represents the efficiency improvement mechanism HVLS fans provide to UVGI systems and more accurately predict required fan quantities. Fans from industrial or commercial/residential product lines can be used, with the largest fan diameter meeting clearance requirements and integrated into the existing building systems being preferred. A UVGI design tool can be used to generate a minimum quantity of fans to meet the desired zATH levels given basic dimensions of the targeted area.
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.
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