— By Ramez Naguib, P.E. —
Mitigating COVID-19 spread in restaurants using CFD analysis.
The world is having an unprecedented time due to the pandemic. Currently, more than 93 million people have been infected, and over 2 million people have passed away since 2020. SARS-CoV-2 has forced people to change their lifestyles and patterns. Under the pandemic, buildings are no longer safe shelters. The infected transmit infectious viruses to other occupants by direct contact or indirect contact (i.e., indoor airflow).i
Ramez Naguib, Clima-Tech LLC
COVID-19 outbreaks were associated with droplet transmission that was prompted by air-conditioned ventilation. The key factor for infection was the direction of the airflow. To prevent spread of COVID-19 in restaurants, researchers recommended strengthening temperature-monitoring surveillance, increasing the distance between tables and improving ventilation.ii
Some of the research addressing COVID-19 spread in restaurants, along with a framework to mitigate its spread, is outlined in this article.
Computational Fluid Dynamics (CFD)
Unlike other particles, as pathogens are very small with diameters not more than 100 nm, they move under the influence of air particles and, hence, their movement paths are hard to analyze. Computer science advances solved this problem using a computational fluid dynamics (CFD)-based method and developed it well over the years. The CFD solution of nonlinear Navier–Stokes equations enables very complex fluid flows to be simulated in different conditions and regions. CFD is also applied in biological and environmental fields; in early 1994, a new FORTRAN code was developed to simulate the airborne particles and in later decades, many research studies were done, based on this method, for air/environment pollution and emission/transmission of pathogens. In the last 2 years, more than 50 published papers have used CFD simulations to study the SARS-CoV-2 transfer in various conditions/spaces enabling researchers to theoretically model various pathways of sneezing-/coughing-/talking-caused respiratory particles and how they dispersed in open/closed spaces to study the virus behavior more effectively.iii
COVID-19 has shown a high potential of transmission via virus-carrying aerosols as supported by growing evidence. Systematic computational fluid dynamics (CFD)-based investigation of indoor airflow was conducted. The CFD simulation associated aerosol transport in a restaurant setting, where likely cases of airflow-induced infection of COVID-19 caused by asymptomatic individuals were widely reported by the media. The simulation employed an advanced large eddy simulation solver numerical methods to resolve complex indoor processes simultaneously, including turbulence, flow-aerosol interplay, thermal effect and the filtration effect by air conditioners. As shown in Figure 1, using flow structure analysis and reverse time tracing of aerosol trajectories, the simulation was able to further pinpoint the influence of environmental parameters on the infection risks and highlight the need for more effective preventive measures, e.g., placement of shielding according to the local flow patterns.iv
Experimental Verification
As shown in Figure 2, physical modeling studies also demonstrated the most effectual approach is to place the filtration as close to people as possible. The intake and discharge of the units should also be considered. Units should not discharge directly toward a person. An additional benefit is an overall improvement of air quality by removing dust, dander, pollen, smoke and other particles that are problematic for those with allergies or asthma. However, each space is unique and needs to be assessed and addressed individually.v
Hierarchy of Controls
To mitigate the spread of COVID-19 in restaurants based on some of the research outlined above, the field of occupational health and hygiene has long relied on the Hierarchy of Controls as shown in Figure 3 to guide the selection of control measures. The principle is that measures higher up on the hierarchy are more effective in reducing the risk for a greater number of people, and that measures further down the hierarchy (some at the individual level) should be implemented as needed to mitigate any remaining risk.vi
Inspection & Maintenance
Another outcome of the research is the importance of routine inspections and maintenance as appropriate for the system are necessary for adequate mitigation.
Recommended inspection and maintenance measures for air handling systems (including inspection and replacement of filters, if applicable) are essential to follow. Adjustments to ventilation (e.g., increased outdoor air) may require more frequent inspections and filter changes. System humidifiers should be inspected to ensure they are cleaned, maintained, and operating properly. As ventilation systems are complex and changes made have unintended effects, adjustments should be made in consultation with a HVAC specialist to verify intended parameters and appropriate maintenance procedures.
There is a joint standard from the American National Standards Institute (ANSI), ASHRAE and Air Conditioning Contractors of America (ACCA) for HVAC inspection and maintenance. ANSI/ASHRAE/ACCA standard 180-2018 (Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems) details procedures and establishes minimum HVAC inspection and maintenance requirements that preserve a system’s ability to achieve acceptable thermal comfort, energy efficiency and indoor air quality in commercial buildings. This guidance can be considered by facility operators in order to optimize HVAC system operation on a routine basis, regardless of an infectious disease pandemic.
Some buildings may use Energy Recovery Ventilators (ERVs) which are HVAC systems that transfer energy (heat in winter, cold in summer) from indoor exhaust air to outdoor supply air within a building to lower energy costs associated with heating or cooling outdoor air used for ventilation. ERVs should be inspected and properly maintained. Pressure differences and design limitations in some types of ERVs can allow a portion of indoor exhaust air to be transferred into outdoor supply air streams. There are maintenance techniques and modifications that can reduce exhaust air transfer, and ASHRAE has provided guidance on operation of ERVs during epidemics.vii
Path Ahead
As of this writing, the U.S. House of Representatives had voted yes on a bill that would provide $42 billion in relief to foodservice businesses that originally applied for the Restaurant Revitalization Fund, and an additional $13 billion to other businesses that were hit hard during the pandemic, including live event venues, gyms and sports teams. The Senate will soon vote on a similar bill before the Restaurant Revitalization Fund replenishment becomes reality. Passing this will surely help mitigate some of the costs needed to implement the recommendations in this article.viii
COVID-19 has been a humanitarian tragedy and has upended business plans across the globe. Unfortunately, the pandemic is not going to end imminently. Consequently, employers and their workers must continue to remain nimble in how they cope with it. As the local situation dictates, employers must remain vigilant and implement existing and new processes that are proven to keep employees, customers and communities safe while meeting their organizations’ needs.ix
References:
i Yu, J. et. al. (2021). Impact on airborne virus behavior by an electric heat pump (EHP) operation in a restaurant during winter season. Building and Environment Journal 200 (2021) 107951.
ii Lu, J. et al. (2020). COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020. Emerging Infectious Diseases. 2020;26(7):1628-1631.
iii Mohamadi, F., Fazeli, A. (2022). A Review on Applications of CFD Modeling in COVID‑19 Pandemic. Archives of Computational Methods in Engineering.
iv Liu, H. et. al. (2021). Simulation-based study of COVID-19 outbreak associated with air-conditioning in a restaurant. Physics of Fluids 33, 023301 (2021).
v Zhai, Z. et. al. (2021). Central HVAC Filtration vs. Portable Air Purifier Filtration: Mitigating COVID-19In Public Spaces. ASHRAE Journal. October 2021.
vi O’Keeffe, 2021, COVID-19 and indoor air: Risk mitigating measures and future-proofing, Environmental Health Services, BC Centre for Disease Control, Vancouver, BC.
vii Ontario Agency for Health Protection and Promotion (Public Health Ontario). Heating, ventilation and air conditioning (HVAC) systems in buildings and COVID-19. Toronto, ON: Queen’s Printer for Ontario; 2021.
viii Fantozzi, J. (2022). The House passes the $42 billion Restaurant Revitalization Fund replenishment. Nation’s Restaurant News, April 6, 2022.
ix Levin-Scherz, J., Toro, P., 2021, The Delta Variant: How Companies Should Respond. Harvard Business Review, July 27, 2021.
— Ramez Naguib, P.E., LEED AP, CEM, PMP is the president and CEO of Clima-Tech LLC in Los Angeles. He has worked on some of the largest refrigeration, central cooling and heating projects domestically and internationally. He has several published papers on various engineering, economic and management topics. He can be reached at [email protected].