ASHRAE Journal - June 2021 - 24

COLUMN IEQ APPLICATIONS
assume that all viable virus particles are only found in
the E1 range in lieu of using the anticipated virus distribution
because it has the lowest efficiency for a typical
mechanical filter.
The next component in the airstream in Figure 1 is an
ultraviolet germicidal irradiation (UVGI) airstream
disinfection system. The UVGI and particle filter must
be treated like filters in series. This means the UVGI
system only acts on the virus-containing particles in
the recirculated airstream not removed by the filter,
which reduces its impact on overall removal efficiency.
The clean airflow resulting from the combination of a
mechanical filter and UVGI can be captured in the following
equation:
QQ ()1 ηη
ec Rf C
, =−
where
Qe,c = Equivalent outdoor airflow from the air
cleaner (cfm)
QR = Recirculated airflow (cfm)
ηf
ηc
= Single-pass removal of the filter (previously
calculated)
= Single pass inactivation percentage of the air
cleaner.
From Qe,c airflow, the air changes per hour of equivalent
outdoor air from the air cleaner combination can be
calculated using the following formula:
ACHe,c = (Qe,c × 60)/V
where
ACHe,c
V
= Equivalent outdoor air changes from the air
cleaner, after the filter on the recirculated air
=Volume (ft3).
The effect of in-room fan-filter units or other inroom
air cleaners used to increase the equivalent outdoor
air in that space can then be added. If known,
the equivalent clean airflow of fan-filter units should
be assumed to be the clean air delivery rate (CADR)
as determined by AHAM Standard AC-1.12 Placement
of these units and space configuration can impact the
effectiveness of its distribution within the occupied
space.
An adjustment to the equivalent clean air delivery is
required to account for the fact that there is not perfect
mixing in a space. This can be described by the air
distribution effectiveness metric. An estimate of this
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ASHRAE JOURNAL ashrae.o rg
J U N E 2021
correction is the zone air distribution effectiveness (Ez)
given in ASHRAE Standard 62.1-2019 Table 6-4 that
identifies the constant for different air distribution systems.
An Ez value of 1.0 indicates perfect mixing. This
value is typically only multiplied by the air delivered
from the house HVAC system.
Once the equivalent outdoor air changes per
hour (ACHe) for a system or space are determined,
the time it will take to flush it can be calculated.
Equivalent air changes, ACHe, are determined by
finding the sum of the component air change calculated
as described:
ACHe = Ez(ACHoa + ACHf + ACHe,c) + ACHir
where
ACHoa = Air changes from outdoor air
ACHf
= Equivalent outdoor air changes from the filter
on the recirculated air
ACHe,c = Equivalent outdoor air changes from the air
cleaner, after the filter on the recirculated air
= Equivalent outdoor air changes from the inroom
portable HEPA unit.
Flushing time is a function of ACHe and the desired
level of clearance, or contaminant removal. ASHRAE
ETF guidance is based on 95% clearance as minimally
acceptable, but some types of facilities use more stringent
criteria, such as 99% or 99.9%. Clearance of 95%
requires three air changes of equivalent outdoor air that
is adjusted for ventilation effectiveness.
The following shows the equations used to calculate
the contaminant removal:
ACHir
C/C0 = exp(-Qe/V) = exp(-ACH × t)
t = -ln(C/C0)/(ACH)
where
C = Space concentration of contaminants
C0
Qe
ACH = Air changes per hour
V = Volume of the space (ft3)
t
= Space contraction when time equals 0
= Equivalent outdoor airflow in the space (cfm)
= Time to flush the space with Qe.
Using this strategy for aerosol management in a space
between occupancies can help reduce the potential for
virus transmission when the space is reused. Using the
equivalent outdoor air in flushing calculations will help
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ASHRAE Journal - June 2021

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