ASHRAE Journal - July 2021 - 19

TECHNICAL FEATURE
source, and the physical barrier
formed by the cubicle walls promote
formation of air recirculation zones,
which in turn promote the accumulation
of contaminants. Cubicle partitions
offer little barrier to the transport
of airborne contaminants.
Case 2: Distributed Supply and Distributed
Return-A Concept of Aerodynamic Containment
As mentioned before, in the Case
2 confi guration each pair of supply
diffusers and return grilles forms
independent zones of airfl ow. The
airfl ow distribution shown in Figure 2
indicates two identical airfl ow patterns
within each zone of supply
and return. Unlike in Case 1, in Case
2 the air recirculation zones are
formed locally within each zone.
Similarly, the travel of contaminated
air is mostly limited within each
zone.
As shown in Figure 3 for Case 2, the
risk of infection is reduced in the
second cubicle, and the zone of high
infection above 15% remains in the
vicinity of the infected individual.
The average and minimum values of
FIGURE 4 Spread Index SI10 indicating the extent of the space volume at or above the 10% infection probability.
Case 1 shows that almost half of the space is at high risk of infection with all the occupants under the cloud of
high infection. Case 2 shows that aerodynamic containment can reduce the zone of high risk infection by limiting
the spread of airborne contaminants.
Supply
SI: 49.4%
Return
Supply
SI: 39.3%
Supply
Return
Return
Case 1: Single Supply and Single Return
Case 2: Distributed Supply and Distributed Return
FIGURE 5 Impact of HVAC configuration on the probability of infection (%) for various locations of the individuals.
It indicates that poor airflow distribution can make the measure of social distancing less effective.
Aerodynamic containment with distributed supply and return locations can significantly reduce this risk.
Infected Person
16
12
8
4
1
2
3
Person Location
Person Locations
probability of infection are reduced from 11.3% to 9.1%
and from 7.1% to 2.7%, respectively. The spread index
SI10 as shown in Figure 4 for Case 2 is reduced from 49.4%
to 39.3%. Unlike the previous cases, only about onethird
of the room space is at or above 10% probability of
infection.
In this case, dividing the total supply of air through two
diffusers created two distinct aerodynamic containment
zones. The airfl ow patterns from each diffuser create
their own zone of containment, which minimizes bidirectional
air movement between the zones. Providing
returns for each zone reduced the long travel of contaminated
air through the occupants. Additionally, moving
the supply diffusers away from the returns helps sweep
the clean air through occupied zones. Such aerodynamic
containment with a symmetric layout of distributed supply
and distributed return alters the fl ow path of contaminated
air and moves it away from the occupants.
This analysis indicates that creating sweeping airfl ow
patterns, increasing the number of returns and placing
returns away from the occupied zone can reduce the
spread of contaminants and minimize the risk of infection.
However, infection risk cannot be entirely eliminated
without removing the source.
Location of Occupants
The impact of the HVAC confi guration on the probability
of infection varies with the location of a person in the
space. Figure 5 shows the probability of infection for various
individuals for two cases. The location of each individual
is numbered from 1 to 5, and the infected person's
location is noted.
The infected person, Person 2 and Person 3 are located
in the same cubicle. Persons 3, 4 and 5 are located in the
other cubicle. The distance from the infected person
to Persons 1, 2 and 3 is about 4.6 ft (1.4 m); the distance
J U LY 2 0 2 1 ashrae.org ASHRAE JOURNAL
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Case 1
Case 2
2
1
3
4 5
4
5
Probability of Infection (%)

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ASHRAE Journal - July 2021

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