ASHRAE Journal - May 2020 - 14

TECHNICAL FEATURE

These values were calculated using the correlations
for the gap method presented in the Handbook of Smoke
Control Engineering6 for different door gaps, varying
from 1 mm to 4 mm (1/16 in. to 1/8 in.) for top and side
gaps, and from 5 mm to 12.7 mm (0.2 in. to 0.5 in.) for
bottom gaps, and a reference pressure difference of
5 Pa (0.02 in. w.g.).
These values of ELA were then used as flow area in
the orifice equation with C = 0.65. Three scenarios
were modeled with the multizone software, with the
same envelope cracks and different door leakage area,
representing the different door airtightness scenarios.
The minimum outdoor airflow rate was then fitted to
match the ventilation requirements for each room and
the relative pressurization criteria. The minimum pressure difference across a closed door was set to 2.5 Pa
(0.01 in. w.g.), as this is the minimum value proposed
for protective environments by the infection control
guideline for health-care facilities.7 The maximum
pressure difference across a closed door was set to 50
Pa (0.2 in. w.g.) to limit the door-opening force.6 For
this analysis, the coupled CFD solution is not necessary,
and the multizone software was used (CONTAM ONLY
option).

Contaminant Purge Analysis for Different Ventilation Strategies

This test was set to compare the performance of different ventilation strategies on contaminant purge, after a
concentration buildup has been set inside an OR, due to
the door opening and personnel movement transport.8
Two air distribution arrangements were compared:
with the use of unidirectional flow diffusers (UD) and
mixed ventilation (MV). The UD scenario was selected
since it is recommended for ORs with specialized surgeries by North American2 standards. MV was selected
since it is widely used in health-care applications,9 and
because the use of unidirectional diffusers is not a global
standard. Both scenarios were analyzed with the same
airflow of 556 L/s (1,178 cfm), which provides 27 ach, and
the same room thermal load of 1.9 kW (6,483 Btu/h). The
resultant flow rate was determined by the minimum
discharge velocity at the unidirectional diffuser.
The basic difference between the two scenarios was
related to the air distribution method. In the UD scenario, supply air delivery is through a single unidirectional flow diffuser located over the operation table,
since this strategy provided the best results of reducing
14

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TABLE 1 Adopted values of effective leakage area (ELA) for door gaps, m² (ft²).
DOOR AIRTIGHTNESS
SCENARIO

SINGLE DOOR:
0.9 m × 2.1 m (3 ft × 7 ft)

DOUBLE DOOR:
1.6 m × 2.1 m (5 ft × 7 ft)

Tight

0.008 (0.09)

0.013 (0.14)

Base

0.019 (0.21)

0.032 (0.35)

Loose

0.039 (0.42)

0.064 (0.69)

the microbial contamination on the surgical site in a
reference experimental study.10 The supply air diffuser
was sized to produce a discharge velocity of 0.18 m/s
(35 fpm), which is in the range used in similar studies.11,12 Return air is provided by two low wall-mounted
grilles on opposite walls.
In the MV scenario, supply air distribution is through
a high sidewall-mounted linear slot, sized to produce
a discharge velocity of 1.50 m/s (295 fpm). Return air is
provided by one low wall-mounted grille. In theory, a
mixed ventilation system produces little or no thermal
stratification of air within the space, and the return air
placement would not be a critical factor for a fully mixed
room.
This study assumed that particles of interest are less
than 5 µm, which follows the flow path. Particle settling
and deposition were not considered, to focus on airborne distribution. The tracer gas technique describes
well the behavior of suspended particles with an aerodynamic diameter of less than 5 µm.13 Therefore, SF6 was
selected for contaminant modeling, given that the tracer
gas technique has been used in ventilation effectiveness
reference studies.14
Little information exists about the contaminant rate
through the opening of OR doors, and this contamination depends on several parameters outside the scope
of this study. Therefore, for each scenario, contaminant
burst was modeled as a constant source for 10 seconds,
located close to the door, to represent the contaminant
migration from the corridor through the opening-closing door procedure. There are some reports from applications where the reported door-opening time is higher
than this selected value. But from an infection control
perspective, careful space planning along with an effective administrative control could be used to minimize
the door-opening period.
Therefore, to focus on ventilation performance,
the door-opening time was selected to represent the
minimum time that would be necessary for a person
to pass through a closed door. The value of 10 seconds

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ASHRAE Journal - May 2020

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