ASHRAE Journal - May 2020 - 19

TECHNICAL FEATURE

The results show that relative pressurization is very
sensitive to door airtightness. The application of poorly
ﬁtted doors would require an additional amount of outdoor airﬂow to meet the relative pressurization criteria
listed in "Room Pressurization Analysis" (Page 13). For
the studied scenarios, this addition would be 25%, when
comparing the doors from the "loose" to the "base" scenario. Investment in a tighter door may reduce these
HVAC operational costs, due to the need for lower outdoor airﬂow rates. Savings possibilities are up to 16% for
the studied scenarios when comparing the doors from
the "tight" to the "base" scenario. This approach is generally attractive, given the higher initial and operational
costs of the HVAC system, when compared to the doors'
cost.
Therefore, this study shows that the multizone software may be used in a similar analysis to produce rational solutions, ﬁtted to a speciﬁc case. It may also be used
to assess door performance for a given airﬂow condition.
For instance, if the surgery center HVAC was designed to
meet the "base" scenario speciﬁcations, project changes
that lead to the application of doors with lower airtightness (e.g., like the "loose" scenario) would result in failing to meet the relative pressurization criteria if the outdoor airﬂow rate is not revised. It is also relevant to note
that manual calculations would be impractical for this
analysis, given a large number of interconnected zones.
The CFD study shows that the use of the unidirectional ﬂow diffusers reduces the cross-contamination
risk on the surgical site due to the door-opening and

FIGURE 9 Results of relative contaminant concentration on a position located at
h = 0.1 m (4 in.) over the operation table, during contaminant burst (max = 1).

MV
Relative Concentration (C/Cmax)

Discussion

1.00
0.80
0.60
0.40
0.20
0.00

120 180 240 300 360 420 480 540
Elapsed Time (s)

600

personnel movement transport when compared to the
use of mixed ventilation for the studied case. It may be
seen from Figure 10 that the contaminant source spreads
throughout the entire room with the mixed ventilation, while the use of the unidirectional ﬂow diffusers
restricts the contaminant concentration on the surgical
site. These characteristics stem from the speciﬁc features of the indoor airﬂow pattern provided by each air
distribution solution, depicted in Figure 6.
The UD solution provides a relatively unidirectional,
clean airﬂow at the surgical site, despite some entrainment ﬂow from the surrounding air. The contraction of
the central jet in the UD scenario was minimized by the
high airﬂow rate and the relatively low temperature difference to surrounding air. The timescale of room contaminant purging is sufﬁciently higher than the transport by entrainment ﬂow to the surgical site, leading to

FIGURE 10 Results of iso-surface of contaminant concentration for the two ventilation strategies; MV: (a) t = 30s, (b) t = 60 s, (c) t = 180 s, (d) t = 240 s; UD: (e) t = 30 s,

(f) t = 60 s, (g) t = 180 s, (h) t = 240 s.

M AY 2020

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ASHRAE Journal - May 2020

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