ASHRAE Journal - August 2021 - 20

TECHNICAL FEATURE
FIGURE 3 High school classroom.
Room area = 800 ft2
Ceiling height =10 ft
Number of students = 27
Number of teachers = 1
Room is served by a 3 ton unit
at 1,200 cfm
pz az
VVoz
ach =
Room uses overhead mixing
ventilation with Ez = 0.8
Table 6-1 in ASHRAE Standard
62.1-2019, people outdoor
air rate = 10, and the area
outdoor air rate = 0.12
V R P RA 10 28 0 12 800 376
bz =+ =⋅ + ⋅ =
ot == = = 470 cfmOA
.
V
E
470
800
bz
z
2
376
08
cfm 60 min
ft 10 ft
⋅
= 35 ach
.
⋅
Vbz = Outdoor airflow required in the breathing zone (cfm)
Rp
Pz = Zone population
Ra = Outdoor air rate required per unit area (cfm/ft2)
Az = Zone floor area (ft2)
Vot = Outdoor air intake flow (cfm)
Voz = Zone outdoor airflow (cfm)
Ez
= Zone air distribution effectiveness
ventilation rates to 12 ach does not necessarily reduce
the infection risk (from coughing) for mixing ventilation.11,12
These studies show the importance of distance
and direction of cough in spaces.
A study by Nardell, et al., predicted the number of
infected occupants in an office based on 67 susceptible
subjects exposed for 160 hours to one source case generating
from 1.25 to 250 infectious quanta per hour
(qph).13 At 1.2 qph, providing more than 15 cfm (7 L/s)
of outdoor air ventilation per occupant produced
very little decrease in infection. At 13 qph, increasing
from 10 cfm (7 L/s) to 25 cfm (12 L/s) per person would
reduce the infection rate by one-third, while decreasing
from 10 cfm (7 L/s) to 5 cfm (2.4 L/s) per person would
increase the infection rate by 78%. Increasing the rate
from 25 cfm (12 L/s) to 35 cfm (17 cfm) reduced the infection
rate by another 19%, and further increases in the
outdoor air ventilation produced progressively smaller
reductions in infection.13 As the quanta generation
rate increased, ventilation offered progressively less
protection.
Table 2 shows quanta generation rates for various
organisms.14 A study by Liao, et al., delves deeper into
the subject and provides a model for evaluating various
20
ASHRAE JOURNAL ashrae.org A U G UST 2021
= Outdoor airflow rate required per person (cfm/person)
.cfm
TABLE 2 Quanta generation rates.14
INFECTIOUS DISEASE
Rhinovirus (Common Cold)
Tuberculosis
SARS
Influenza
Measles
REPORTED QUANTA GENERATION RATES
~1 to 10 Per Hour
~1 to 50 Per Hour
~10 to 300 Per Hour
~15 to 500 Per Hour
~570 to 5,600 Per Hour
control methods and the resulting number of infections
that could occur.15
Filtration (Cleaning)
It may not always be practical to increase the outdoor
air ventilation rate. Filtration can allow you to provide
increased ventilation rates without increasing the outdoor
air. Filters use five main mechanisms for particle
collection:16,17
1. Straining: Particles larger than the filter's openings are
removed from the airstream and deposited on the filter
surface.
2. Interception: Particles that come within one radius of
the fiber's surface and have sufficient contact time with
the fiber can adhere to it.
3. Inertial impaction: Large or dense particles that have
enough inertia to deviate from the airstream path when
it encounters a flow curvature can directly impact onto a
fiber's surface and adhere to it.
4. Diffusion: small-diameter particles are susceptible
to Brownian motion and are knocked off the airstream
path by molecular collisions, bringing them close enough
to the media fibers to be captured by interception.
5. Electrostatic effects: particle or media electrostatic
charges can produce a strong attracting force when they
have opposite charges.
High efficiency particle air (HEPA) filters originated
during World War II. The U.S. military needed a filter
that would protect against chemical, biological and
radiological warfare agents and contain emissions from
nuclear weapons production facilities.18,19 The earliest
filters were made from the same filter paper used in gas
masks of that time.18,19
HEPA filters use different mechanisms for capture for
different particle sizes. For particles less than 0.1 μm,
diffusion is the primary method.17 For particles greater
than 1 μm, inertial impaction and interception are the
primary methods.17 In both of these cases, the HEPA
filter efficiency is close to 100%.17 Between 0.1 μm and
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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
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ASHRAE Journal - August 2021 - 80
ASHRAE Journal - August 2021 - Cover3
ASHRAE Journal - August 2021 - Cover4
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