ASHRAE Journal - July 2021 - 20

TECHNICAL FEATURE
from the infected person to Persons 4 and 5 is 9.5 ft
(2.9 m). Virtual sensors were placed in front of the occupants'
faces to record the contaminant concentration to
determine the level of infection risk.
These analyses indicate that the HVAC configuration
(layout of the supply and return locations) has a significant
impact on the risk of infection even for individuals
who are located away from the source. The aerodynamic
containment with distributed supply and distributed
return can reduce the infection risk for all individuals,
even for those who are in the vicinity of the source.
These analyses further indicate that the poor airflow
distribution caused by discrete locations of supply and
return can make the social distancing measure less effective
even with a physical barrier. Such a layout promotes
the spread of contaminants, making occupants vulnerable
to a high risk of infection; the proper distribution of
the supply air and creating a path of least resistance for
the contaminated air can reduce the risk of infection.
Summary and Conclusions
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
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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.
Acknowledgments
The author is thankful to AnSight LLC for providing
funding to perform these studies and to Mr. Thierry
Marchal, Ansys, for providing software support.
References
1. CDC. 2020. " Science Brief: SARS-CoV-2 and Potential Airborne
Transmission. " Centers for Disease Control and Prevention. https://
tinyurl.com/3w3bk5ma
2. Khankari, K. 2021. " CFD Analysis of Enclosed Parking Garage
Ventilation. " ASHRAE Conference Paper. 2021 ASHRAE Virtual
Winter Conference.
3. Khankari, K. 2018. " Analysis of Spread Index: A Measure of
Laboratory Ventilation Effectiveness. " ASHRAE Conference Paper.
2018 ASHRAE Annual Conference.
4. Yuguo, L. P. Nielsen, M. Sandberg. 2011. " Displacement
ventilation in hospital environments. " ASHRAE Journal 53(6):86 -88.
5. Riley, E., G. Murphy, R. Riley. 1978. " Airborne spread of
measles in a suburban elementary school. " Am. J. Epidemiol.
107(5):421 -432.
6. Buonanno1, G., L. Stabile, L. Morawska. 2020. " Estimation of
airborne viral emission: Quanta emission rate of SARS-CoV-2 for
infection risk assessment. " Environment International 141:105794.
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ASHRAE Journal - July 2021

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