Analysis of Spread of Airborne Contaminants and Risk of Infection
ASHRAE Journal July 2021
The primary goal of HVAC systems for indoor spaces is maintaining a healthy, comfortable environment for occupants. This is achieved by diluting concentration levels of hazardous contaminants and reducing the spread of airborne contaminants. The effectiveness of a ventilation system, however, depends on several factors related to the layout of the air distribution in the space.
With the help of CFD analyses, this study systematically evaluates the impact of the air distribution layout on the airflow patterns and the resulting risk of infection. This study demonstrates that even for a simple layout of a small office, the locations of supply and return air can affect the airflow patterns and the resulting risk of infection of the occupants.
The HVAC configuration with a single four-way supply diffuser and a single return grille can promote the formation of stagnant air recirculation zones, which can form pockets of high concentration of contaminants. With such a layout, the contaminated air can travel farther from the source, spreading the zone of high infection in a space. It indicates that poor airflow distribution can make the measure of social distancing less effective.
A symmetric layout of distributed supply and distributed return can form an aerodynamic containment (airflow envelope), which shows significant promise in improving the ventilation performance by reducing the risk of infection. Since the location of an infected individual is not known a priori, the aerodynamic containment with distributed supply and distributed return can help reduce the probability of infection in indoor spaces.
Based on these analyses the following guiding principles can help improve the ventilation effectiveness and reduce the risk of infection in indoor facilities. They are stated in the order of simplicity of implementation:
- Create a distributed supply layout by increasing the number of supply diffusers and strategically placing them over the occupied zone.
- Create a distributed return layout by increasing the number of exhaust outlets to create a path of least resistance for the contaminated air to exit the space.
- Create an aerodynamic containment by symmetric placement of supply diffusers and return grilles to minimize cross contamination between the symmetric zones of supply and return.
These studies demonstrate that CFD analyses can help identify the potential risk of infection in indoor spaces due to poor airflow distribution. Each space is unique; therefore, the impact of supply and return air configurations should be evaluated by performing such CFD analyses to improve the ventilation effectiveness before increasing the ventilation airflow rates.