ASHRAE Journal - June 2021 - 23

COLUMN IEQ APPLICATIONS
class to make sure we understood the
data and calculations when doing psychrometric
analysis, " Goes into equals
goes outta! " This applies to this Google
sheet since it is built for the system
shown in Figure 1, but the sheet can be
applied, with the appropriate modifi -
cations, to any system type.
The simplest component of equivalent
outdoor air to determine is actual
true outdoor air that the system is
delivering to the space. It is only necessary
to calculate how much of the
system outdoor air is delivered to each
space in terms of air changes per hour. Now, we need
to calculate the remaining equivalent outdoor air for a
specifi c aerosol.
Dampers
Outdoor Air
FIGURE 1 UVGI air-handler schematic.
Airfl ow
Measuring
Station
Heating Coil
UVGI
Filter
Cooling Coil
Supply Fan
Array
Return Air
The equivalent outdoor air produced by cleaning and
inactivation of the recirculated air is dependent on the
HVAC system being evaluated, its components and the
order of those components. It also depends on if any
in-room air cleaners, and what type, are used in spaces
served by the system. Based on Figure 1, we will analyze
the impact of the fi lter and then the UVGI . Finally, we
will consider the ability to use in-room portable HEPA
fi lter units to improve the aerosol removal from the
spaces.
The impact of the fi lter on the recirculated air
depends on the performance of the fi lter at the different
particle sizes that may be carrying the virus.
Ideally, the performance of a fi lter should be based on
its actual test results per ASHRAE Standard 52.2 -2017,
Method of Testing General Ventilation Air-Cleaning Devices
for Removal Effi ciency by Particle Size. Each manufacturer
should be able to provide their fi lters' performance
in the three particle ranges: E1 (0.3 µm to 1.0 µm), E2
(1.0 µm to 3.0 µm) and E3 (3.0 µm to 10.0 µm). Note
that 1 µm = 100 nm.
The next step is to determine the anticipated distribution
of virus-containing particles in the recirculated
airstream in the E1, E2 and E3 sizes ranges of Standard
52.2 -2017, so that a droplet nuclei-weighted effi ciency
of the fi lter can be identifi ed. An article4 that discusses
the myths of the virus and the size it is distributed says
it is best summarized by a June 2020 Tweet5 from Linsey
Marr, Ph.D., a professor at Virginia Tech, that says, in
part, " viruses are not naked. " The virus travels in the
TABLE 1 Anticipated distribution of virus in airflow by particle size.
FILTER RANGES (PARTICLE SIZE)
E1 (0.3 µm to 1 µm)
E2 (1 µm to 3 µm)
E3 (3 µm to 10 µm)
20%
29%
51%
air connected to other materials and " gunk " that makes
them larger.
The Building Readiness guidance used studies6 - 10 of
Infl uenza A to predict the distribution of particles for the
calculations. As of this writing in April 2021 , the research
is incomplete on the distribution of particle sizes that
contain the 0.06 µm to 0.14 µm-sized SARS-CoV-2 virus
exhaled by the infected person .
Several research studies6 - 10 have used quantitative
polymerase chain reaction (q-PCR) to identify the presence
of viruses in expelled droplets and offered insights
not only into what size aerosols exist after expulsion
from the human body, but in what size fractions viruses
are present and are thus of most concern for infectious
disease transmission. One study11 provided a summary
table of recent studies that used particulate matter samplers
to detect infl uenza virus size that shows the mean
viral distribution (Table 1).
This anticipated distribution of virus particles can
then be multiplied by the fi lter's Standard 52.2-2017
effi ciency in each of the ranges and the results combined
to determine the single-pass removal rate. That
percent would be multiplied by the recirculated airfl
ow to determine the effective outdoor air quantity
created by the fi lter. A conservative approach to determine
how much aerosol the fi lter removes would be to
J U N E 2 0 2 1 ashrae .o rg ASHRAE JOURNAL
23
ANTICIPATED DISTRIBUTION OF VIRUS
https://www.ashrae.org/
ASHRAE Journal - June 2021

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