ASHRAE Journal - August 2021 - 24

TECHNICAL FEATURE
often lose efficiency over time (up to 50% of the original
value for small particles sizes).24 ASHRAE Standard
52.2-2017, Appendix J provides optional calculations that
can predict the decrease in efficiency for charged filter
media. Note that filter loading can decrease system airflow
for units with a constant speed fan.
Filter bypass can also greatly affect filter efficiency. A
study by Ward, et al,. modeled the impact of filter bypass
on filter efficiency.25 The study compared two different
size gaps (0.04 in. [1 mm] and 0.4 in. [10 mm]) in two
different configurations and found that the effective
efficiency for the 0.04 in. (1 mm) gap was close to the
filter efficiency, and for the larger 0.4 in. (10 mm) gap
the effective efficiency is close to zero for all submicron
particles.25 The study also calculated effective MERV ratings
with bypass included. The MERV 6 filter remained
a MERV 6 with a 0.04 in. (1 mm) gap, but dropped to
MERV 5 with a 0.4 in. (10 mm) gap. The MERV 11 filter
also remained MERV 11 with a 0.04 in. (1 mm) gap, but
dropped to MERV 8 with a 0.4 in. (10 mm) gap. The
MERV 15 filter dropped to a MERV 14 with a 0.04 in.
(1 mm) gap and also dropped to a MERV 8 with a 0.4 in.
(10 mm) gap.25 The more efficient the filter, the greater
the effect the bypass air had on the filter efficiency.
Ultraviolet Germicidal Irradiation (UVGI)
Ultraviolet germicidal irradiation (UVGI) is a method
of treating air or surfaces to inactivate microorganisms.
The response of microorganisms to light has been
known as early as 1845, but a breakthrough came in
1877 when Downes and Blunt discovered the ability of
sunlight to prevent microbial growth.26 In 1935 Wells
and Fair demonstrated the ability of UVGI to efficiently
inactivate airborne microorganisms.26 The ultraviolet
(UV) spectrum is divided into UV-A, UV-B, UV-C, and
vacuum UV. UV-C energy (280 nm to 200 nm) is used
in UVGI with the optimal wavelength for inactivation of
microorganisms occurring at 265 nm.16 UV-C in commercial
systems is typically supplied by mercury vapor
lamps, which emit a near-optimal 253.7 nm.16 UVGI
effectiveness on a particular microorganism is primarily
dependent on the UV dose.27
When treating an airstream, these UV-C lamps are
placed in the air handler or ductwork with enough
intensity and frequency of placement to provide the
required exposure time for inactivation. The 2019
ASHRAE Handbook-HVAC Applications recommends
24
ASHRAE JOURNAL ashrae.org A U G UST 2021
FIGURE 4 UVGI formulas.27,28
Survival Fraction (S):
S = e-kDUV
Single-pass inactivation rate (h)
h = 1 - S
DUV = UV Dose (μJ/cm2) = I × t
k = Pathogen-dependent inactivation rate constant (cm2/μJ)
I = Average irradiance (μW/cm2)
t = Exposure time (s)
in-duct systems be designed to meet the desired single-pass
inactivation rate (Figure 4) under worst-case
conditions of air temperature and velocity in the irradiated
zone.27 When installed in an air-handling unit
at 500 fpm (2.5 m/s), an irradiance zone 8 ft (2.4 m) in
length results in a 1 s exposure.27
The 2019 Handbook also recommends that these systems
be installed in a location that can provide a minimum
of 0.25 s of UV exposure to minimize system cost
and power consumption (~2 ft [~610 mm] length in the
example above).27 Reflective materials, like aluminum,
can boost the intensity field from direct reflections (as
well as interreflections) and increase the effective UV
dose.27,28 When the UV-C lamps have to be located in the
ductwork (due to space constrictions, etc.), the effects of
interreflections are often used. When the surface reflectivity
is high and the volume is enclosed, the reflections
can bounce between the surfaces and significantly contribute
to the total field.28
Conclusions
Part 3 has looked at several common mitigation strategies
and their application. Lack of proper ventilation
has been shown to increase the chance of infections.
Increasing ventilation above the minimum ventilation
required by code has the ability to decrease particle
concentrations in the space; however, a limit may exist
to how much increased ventilation will decrease the
infection risk. Distance from the contaminant source,
direction of cough and quanta generation rate can
decrease the effect that ventilation has on the infection
rate. Filtration and UVGI are two common ways
to increase a system's ventilation rate without bringing
in additional outdoor air. Part 4 will briefly look at
emerging technology and additional methods for virus
mitigation.
http://www.ashrae.org

ASHRAE Journal - August 2021

Table of Contents for the Digital Edition of ASHRAE Journal - August 2021

Contents
ASHRAE Journal - August 2021 - Intro
ASHRAE Journal - August 2021 - Cover1
ASHRAE Journal - August 2021 - Cover2
ASHRAE Journal - August 2021 - 1
ASHRAE Journal - August 2021 - Contents
ASHRAE Journal - August 2021 - 3
ASHRAE Journal - August 2021 - 4
ASHRAE Journal - August 2021 - 5
ASHRAE Journal - August 2021 - 6
ASHRAE Journal - August 2021 - 7
ASHRAE Journal - August 2021 - 8
ASHRAE Journal - August 2021 - 9
ASHRAE Journal - August 2021 - 10
ASHRAE Journal - August 2021 - 11
ASHRAE Journal - August 2021 - 12
ASHRAE Journal - August 2021 - 13
ASHRAE Journal - August 2021 - 14
ASHRAE Journal - August 2021 - 15
ASHRAE Journal - August 2021 - 16
ASHRAE Journal - August 2021 - 17
ASHRAE Journal - August 2021 - 18
ASHRAE Journal - August 2021 - 19
ASHRAE Journal - August 2021 - 20
ASHRAE Journal - August 2021 - 21
ASHRAE Journal - August 2021 - 22
ASHRAE Journal - August 2021 - 23
ASHRAE Journal - August 2021 - 24
ASHRAE Journal - August 2021 - 25
ASHRAE Journal - August 2021 - 26
ASHRAE Journal - August 2021 - 27
ASHRAE Journal - August 2021 - 28
ASHRAE Journal - August 2021 - 29
ASHRAE Journal - August 2021 - 30
ASHRAE Journal - August 2021 - 31
ASHRAE Journal - August 2021 - 32
ASHRAE Journal - August 2021 - 33
ASHRAE Journal - August 2021 - 34
ASHRAE Journal - August 2021 - 35
ASHRAE Journal - August 2021 - 36
ASHRAE Journal - August 2021 - 37
ASHRAE Journal - August 2021 - 38
ASHRAE Journal - August 2021 - 39
ASHRAE Journal - August 2021 - 40
ASHRAE Journal - August 2021 - 41
ASHRAE Journal - August 2021 - 42
ASHRAE Journal - August 2021 - 43
ASHRAE Journal - August 2021 - 44
ASHRAE Journal - August 2021 - 45
ASHRAE Journal - August 2021 - 46
ASHRAE Journal - August 2021 - 47
ASHRAE Journal - August 2021 - 48
ASHRAE Journal - August 2021 - 49
ASHRAE Journal - August 2021 - 50
ASHRAE Journal - August 2021 - 51
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ASHRAE Journal - August 2021 - 53
ASHRAE Journal - August 2021 - 54
ASHRAE Journal - August 2021 - 55
ASHRAE Journal - August 2021 - 56
ASHRAE Journal - August 2021 - 57
ASHRAE Journal - August 2021 - 58
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ASHRAE Journal - August 2021 - 60
ASHRAE Journal - August 2021 - 61
ASHRAE Journal - August 2021 - 62
ASHRAE Journal - August 2021 - 63
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ASHRAE Journal - August 2021 - Cover3
ASHRAE Journal - August 2021 - Cover4
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