ASHRAE Journal - October 2021 - 29

TECHNICAL FEATURE
through fi ltration and circulation. The EPA has stated,
" Consider using portable air cleaners to supplement
increased HVAC system ventilation and fi ltration, especially
in areas where adequate ventilation is diffi cult to
achieve. " 8 One CDC example concluded that " adding the
portable HEPA unit increased the effective ventilation
rate and improved room air mixing, resulting in an 80%
reduction in time for the room to be cleared of potentially
infectious airborne particles. " 9 Another study for
a reception setting showed " high-volume HEPA fi ltering
decreases [the individual risk of infection] by a factor of
ten. " 10
For our comprehensive study, computational fl uid
dynamics (CFD) models such as the one in Figure 1 were
developed to simulate airfl ow patterns while people
were breathing and talking in public spaces under normal
conditions. Portable air purifi ers were introduced
into the models to analyze the benefi ts and determine
their optimal number and locations . Field experiments
using thermal mannequins were also carried out to validate
and visualize the cleaning performance of portable
air cleaners.
The overall goal was to improve air quality and mitigate
airborne transmission risks in restaurants and public
spaces with these three objectives:
* Instill confi dence in guests that they are safer in
public spaces such as hotels so they will want to travel,
and have associates feel safer about returning to work.
* Improve indoor air circulation and dilute concentrations
of airborne contaminants.
* Remove harmful particles from the air where it is
most important-where the people are.
Public Spaces in Hospitality
Hotel public spaces are designed primarily for aesthetics
and function, especially in lobbies, restaurants
and ballrooms. Architects and designers create these
beautiful spaces with a lot of open space and high ceilings.
Then they hand the drawings over to a mechanical
engineer and ask them to make the spaces comfortable
for the occupants; they do this by controlling the temperature
and humidity. Hotel HVAC systems are not
typically designed to give a perfect mixing of air, nor are
they designed for zoning, isolation and pathogen control
like hospitals.
Although hotel HVAC systems provide more air dilution
for given contaminant sources due to both larger
FIGURE 1 Layout of the lobby where the simulation is performed. It includes the
furniture in the lobby with bar, lounge furniture, meeting table, reception desk
and HVAC ducting. Supply air vents are represented with the airflow coming out
from it. One at the top edge and another at left bottom corner vents with red
color dots around it represents return air vents.
space volume and potential vertical air stratifi cation,
higher ceilings introduce larger volumes of air to condition
and supply, and returns are further away from the
occupant's breathing zone. Filtration and purifi cation
provided by the central system will have to overcome
these challenges.
Without social distancing, ballrooms can have high
occupant density, especially when the setup is for large
meetings and conferences. And, the confi guration of
the space can change throughout the day. In the morning,
the space can be set " in rounds " for breakfast, then
changed to a classroom setup for the meeting that lasts
throughout the day. Then it fl ips to a stand-up reception
for the evening.
Restaurants have an added challenge, as the occupants
will take their masks off to eat and will likely be talking
to the people they are dining with. If an infected person
is sitting among them, they can potentially introduce
large numbers of infectious particles into the air.
Pros and Cons of Central HVAC Filtration
Vs. Portable Air Purifi er Filtration
Improving fi ltration can either be done at the central
HVAC level or by using portable air purifi ers in the
space. Both options have their pros and cons.
Central HVAC Filtration
Any air fi ltration technology generally has at least the
following two components: a fan to move the air and a
fi lter medium to perform the fi ltration. Both exist in
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ASHRAE Journal - October 2021

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