ASHRAE Journal - July 2021 - 18

TECHNICAL FEATURE
supply diffuser. In Case 1, the entire
volume of the supply air (135 cfm
[63.7 L/s]) enters through a single
four-way ceiling diffuser. The air exiting
from the diffuser travels along
the ceiling, descends along the walls
and moves inward along the floor.
Such airflow patterns create large
recirculation loops along and across
the room. These large recirculation
zones are formed inside the cubicles
between the room walls and the
dividing partitions in the occupied
zone. Three-dimensional airflow
patterns (not shown here) are quite
complex. They promote mixing in the
entire space, as is expected from such
diffusers.
Figures 3 shows the resulting risk of
infection computed per Equation 1 at
a breathing level. Figure 4 shows the
extent of infection spread in a space
above 10% probability-a spread index
of SI10. For Case 1, the airflow patterns
described earlier create a nonuniform
distribution of contaminants with
a zone of high concentration in the
vicinity of the infected person. The
stagnation of air in the recirculation
zones form pockets of high concentration.
The lowest concentration occurs
outside the cubicle away from the
return grille. In spite of mixing airflow
patterns, the contaminant distribution
is not uniform and does not
create well-mixed conditions.
For Case 1, as shown in Figure 3, the
risk of infection is above 15% in the
vicinity of the infected individual,
whereas the average and the minimum
probability of infection at the
breathing plane is 11.3% and 7.1%, respectively. The
spread index SI10 of 49.4% indicates that about half
of the space is at or above 10% probability of infection
(Figure 4). It should be noted that all occupants in this
space are covered under the cloud of the high risk of
infection. The zones of high and low risk of infection
A
B
18
ASHRAE JOURNAL ashrae.org J U LY 2021
FIGURE 2 Airflow patterns in an office space. Case 1 shows large air recirculation zones. Case 2 shows that an
HVAC configuration with a symmetric layout of distributed supply and distributed return can form an aerodynamic
containment with identical airflow patterns in both zones.
Supply
Return
Supply
Supply
Return
Return
Case 1: Single Supply and Single Return
Supply
Case 2: Distributed Supply and Distributed Return
Return
Supply
Supply
Return
Return
Case 1: Single Supply and Single Return
Case 2: Distributed Supply and Distributed Return
FIGURE 3 Distribution of infection probability at the breathing plane at 4.25 ft (1.30 m) from the floor. Case 1
shows large zones of high risk of infection. Case 2 shows that aerodynamic containment can reduce the risk of
infection by enhancing the dilution and limiting the spread of airborne contaminants.
Infection Probability (%)
>15.0
13.5
12.0
10.5
9.0
7.5
6.0
4.5
3.0
1.5
0.0
Supply
Infected Person
Return
Supply Infected Person
Supply
Return
Return
Case 1: Single Supply and Single Return
Case 2: Distributed Supply and
Distributed Return
divide the space along the length of the room. The probability
of infection as predicted in this case indicates
that it is not a " single number " for the entire space as
assumed in Equation 1.
With a single point of supply and a single point of
extract, the contaminated air travels farther from the
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ASHRAE Journal - July 2021

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

Contents
ASHRAE Journal - July 2021 - Intro
ASHRAE Journal - July 2021 - Cover1
ASHRAE Journal - July 2021 - Cover2
ASHRAE Journal - July 2021 - 1
ASHRAE Journal - July 2021 - Contents
ASHRAE Journal - July 2021 - 3
ASHRAE Journal - July 2021 - 4
ASHRAE Journal - July 2021 - 5
ASHRAE Journal - July 2021 - 6
ASHRAE Journal - July 2021 - 7
ASHRAE Journal - July 2021 - 8
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ASHRAE Journal - July 2021 - 11
ASHRAE Journal - July 2021 - 12
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ASHRAE Journal - July 2021 - 14
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ASHRAE Journal - July 2021 - 18
ASHRAE Journal - July 2021 - 19
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ASHRAE Journal - July 2021 - 21
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