ASHRAE Journal - October 2024 - 28

TECHNICAL FEATURE
of WAGs released in the
PACU. High values of (SI)TC
can indicate the spread and
build-up of contaminants
above the target levels of
concentration. The design of
a ventilation system and the
resulting airflow patterns
play an important role in
determining the (SI)TC levels
in the space.9 For this study,
the target concentration of
sevoflurane is assumed to
be 2 ppmv. Additionally,
the Spread Index values for
various target concentrations
are also presented.
FIGURE 3 Distribution of sevoflurane concentration in the breathing plane at a 4 ft (1.2 m) elevation.
Case 1: Overhead Mixing
Case 2: Low Wall Returns
Concentration (ppmv)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Case 3: Ceiling Returns
Case 4: Headwall Returns
Results and Discussion
Airflow Patterns
Figure 2 (page 26) shows airflow patterns in the PACU
for various HVAC layouts. The flow path lines are
colored by the sevoflurane concentration. This CFD
analysis confirms that the traditional overhead mixing
layout (Case 1) with four-way supply diffusers and
ceiling returns forms mixing airflow patterns. Once
the supply air is released from the four-way diffusers it
moves along the ceiling and then flows downward after
encountering another supply airstream or a wall. Such
flow patterns in turn create several air recirculation
zones, which promote the entrainment of WAG plumes
into the supply airstreams. The mixing airflow patterns
spread and accumulate the contaminated air in the
PACU and form zones of high concentration before
exiting through the returns in the ceiling.
In other cases (Case 2 to Case 5), downward flow
patterns are formed once the supply air is released from
the laminar diffusers. Such high-velocity downward
flow also entrains the surrounding air into the supply
stream. However, the locations of returns play a crucial
role in determining the flow path of the contaminated
plumes. In the case of low wall returns (Case 2), the
exhaled plume from the patient's face gets entrained
into the adjacent recirculation zone formed by the
downward airstreams. The contaminated plume
moves toward the back wall, gets lifted upward, and
then mixes with the downward flow. Due to the high
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ASHRAE JOURNAL ashrae.org O CTO B ER 2024
downward momentum of the supply air, only part of this
contaminated air can exit through the low wall returns,
while the rest spreads into the PACU that eventually
exits through other returns.
In Case 3 a part of the clean air supplied through the
laminar diffusers escapes through the adjacent ceiling
return without reaching the PACU space. Such a shortcircuiting
is not desirable. In this case, the airflow
exhibits similar flow patterns as in the previous case.
However, unlike the previous case, the exhaled plume
from the patient's face moves sideward along the wall,
gets lifted upward toward the ceiling returns where it
exits the space. Note that in this case also the supply air
streams get contaminated due to the entrainment of the
exhaled plumes.
The airflow patterns for the headwall return layout
(Case 4) are similar to the low wall return layout
(Case 2). However, the exhaled plume containing
sevoflurane moves toward the back wall and gets readily
captured by the headwall return, which is located
right behind the patient's bed. Such capture of the
exhaled plume close to the source (patient's face) avoids
contamination of the supply air and the eventual spread
into the PACU space. In contrast to the other layouts, the
lower concentration of sevoflurane in Case 4 as depicted
by the blue color of flow path lines indicates that the
supply air streams are not contaminated by the exhaled
plumes from the patients.

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ASHRAE Journal - October 2024

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