ASHRAE Journal - October 2021 - 30

TECHNICAL FEATURE
every central HVAC system. The only difference is that
the regular low MERV-rated media filter has lower filtration
efficiency, compared to a high MERV-rated or
HEPA-grade media filter, for particles and microbes riding
on particles, aerosols, etc. One of the simpler options
is to change the HVAC media filter to a higher MERVrated
filter like MERV 13 or above that can filter out these
contaminants and microbes with better effectiveness,
depending on particle size, for example about 50% at
0.1 micron.
A limit exists to how much we can increase the efficiency
of these filters, as the higher rated filters, if not
selected properly, may restrict airflow and cause a
reduction in air pressure. This in turn could compromise
the comfort of occupants. Any such change to the
media filter could call for more frequent filter change,
causing increased recurring operational cost.
Pros
* Single point of intervention, no extra space requirement.
*
Out of sight of occupants, not causing any aesthetic
or acoustic nuisance in the space.
* No complex study needed to design or install.
* Minimal change to maintenance requirements.
Cons
* In-space effectiveness could be limited as the particles
and aerosols still travel within the space before it is
pulled down by gravity, collides into furniture or other
surfaces (such as people), or are pulled into the HVAC
return air vent.
* If a MERV filter is upgraded to a filter with a higher
MERV rating, more frequent filter changes may be
required, and there may be increased recurring operational
cost.
* Since it is out of sight, there is no way for occupants
to know what air quality improvement measures have
been taken.
* For proper effectiveness, it requires well-installed
filters without gaps to achieve the expected removal efficiency.
Portable
Air Purifier
Like any air filtration technology, even a HEPA-based
portable air purifier has a fan and a filter medium.
The reason for the selection of a HEPA-based portable
air purifier is its wide availability and reasonable
price. A HEPA filter-based portable air purifier could
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ASHRAE JOURNAL ashrae.o rg O CTO B E R 2021
comparatively do a better job overall for two reasons:
1) HEPA filters eliminate 99.97% of particles up to 0.3
micron, and NASA research suggests they are very effective
at removing even submicron and nanoparticulate
sizes as well.11 2) Portable air purifiers placed near
people and positioned so their suction inlet faces toward
people can effectively and quickly do the filtration
within the space before the particles and aerosols spread
around. This could be a better alternative, but, as with
everything it has its pros and cons.
Pros
* Portable, plug-and-play and do not need any infrastructure
alteration or special installation skill.
* HEPA filter is more effective than a MERV 13 filter at
filtering even at submicron and nanoparticle size.
* To achieve the same level of particle removal, the
HEPA filter takes less time compared to a MERV 13 filter.
* In-space placement could allow the purifier to be
relatively more effective in reducing the spread further.
* Could add to a visual assurance of healthier space
for occupants.
Cons
* Would take up usable space and wall outlets.
* Could be a nuisance for occupants due to noise
when operating at high speeds.
* May require performing complex CFD study of
airflow in the space prior to their installation to achieve
optimum effectiveness.
* Could be switched off or moved by the users/occupants,
making them ineffective in that area/space.
* Added maintenance to care for the air purifier units
and change filters when needed.
Effective Placement of Air Purifiers
Detailed fluid dynamic models point to three key criteria
in determining the best strategy for air purifier
deployment.
1. The closer the infected person is to the suction inlet
of the air purifier the higher the effectiveness. While
intuitive, the speed with which small airborne fluid
particles can diffuse in a room is a concern. Typical
HVAC systems are designed to efficiently distribute air in
spaces; therefore, the more time that elapses from when
the fluid particles are emitted, the larger the spread
radius of the particles. The droplets will generally move
around the room until they are pulled down by gravity,
collide into furniture or other surfaces (such as people),
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ASHRAE Journal - October 2021

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