The Role of Air Conditioning in the Diffusion of Sars-CoV-2 in Indoor Environments: a First Computational Fluid Dynamic Model, based on Investigations performed at the Vatican State Children’s Hospital © 2020 Elsevier Inc. All rights reserved.
More than 60 million people worldwide are affected by the Sars-Cov-2 infection, which already caused 1.500.000 deaths. This virus is mainly transmitted through exhalations from the airways of infected persons, so that Heating, Ventilation and Air Conditioning (HVAC) systems might play a role in increasing or reducing the spreading of the infection in indoor environments.
It has been modelled the role of HVAC systems in the diffusion of the contagion through Computational Fluid Dynamics (CFD) simulations of cough at the “Bambino Gesù” Vatican State Children’s Hospital.
Both waiting and hospital rooms were modeled as indoor scenarios. A specific Infection-Index (η) parameter was used to estimate the amount of contaminated air inhaled by each person present in the simulated indoor scenarios.
Study Conclusions
The study shows that HVAC system in a closed waiting room generates a marked space diffusion of droplets from a cough event. Although the doubling of HVAC’s air flow allows for a marked reduction in the airborn contaminant concentration this also leads to a substantial increase of turbulent air motions resulting in an increased and faster long-range spreading of droplets and air contaminants in the room. In indoor environment, the doubling of the air conditioned flow allows a relevant (up to 77%) reduction in droplets mass concentration within the room compared to nominal air flow. In the first 5 seconds from the cough event, no particular differences in droplets concentration are detectable in the room, while the relevant differences occur within 25 seconds due to the air conditioned systems that induce droplets air dispersion. In absence of air conditioning, larger particles more easily settle on the ground, while for nominal or doulbe-flow HVAC air-flow even the largest particles are subjected to turbulent phenomena and follow the air conditioned flow remaining suspended for longer time (see Scenarios A, B and C here below). Despite that, the calculation of Infection Index for all the subjects in the room proved that a higher air exchange allows to reduce in the long term the overall amount of inhaled contaminated air.
The right fan to allow large airflows?
As best practice denotes, fans will still need to be selected for optimum efficiency at normal working conditions, but also have the capability for a duty increase to supply additional air during high risk periods, or when high occupancy of an area is expected.
The performance charts below highlights the difference in performance envelope between a Large airflow plug fan (in this case our NPA range) vs the normal maximum you could expect to see on an EC equivalent plug fan.
