ASHRAE Journal - June 2024 - 50

TECHNICAL FEATURE
the spread of contaminants in a lab space. It is the
ratio of the volume of a breathing zone occupied by
the contaminants above a certain target concentration
(TC) to the total volume of the breathing zone. In this
analysis, the breathing zone was defined as a threedimensional
space between 4 ft and 6 ft (1.2 m and
1.8 m) height from the floor (Figure 1). Ideally, the lab
ventilation systems should reduce the concentration
levels below the threshold values everywhere in the lab
space and should limit the spread of contaminants. The
other similar metric, Spread Index (SI)TC, is a ventilation
effectiveness metric that analyzes and quantifies the
extent of the spread of contaminants above a certain
threshold value in the entire three-dimensional space
of a laboratory and not just
in the breathing zone. This
metric is not presented in
this article.
A
Assuming the target
concentration is a safe
exposure limit, then the
concentration levels should
remain below the target
value in the breathing zone
of occupants. It means the
(SI)BZ should be always
close to zero during the
lab operation. The safe level of concentrations can
depend on several risk factors including the type and
quantities of chemicals used in the labs and their safe
exposure limits. Therefore, for each laboratory (SI)BZ
and (SI)TC can be evaluated for various levels of target
concentrations based on the chemical exposure risk
analysis. High values of these indices indicate the
spread and buildup of contaminants above the safe
levels of concentration. The design of an HVAC system
and the resulting flow path of contaminants would
play an important role in determining the levels of
these indices in the space.6 For this study, the target
concentration was assumed to be 25 ppm-an arbitrary
value. The time-varying (SI)BZ25 values were analyzed
with and without demand-control ventilation.
Purge Time (PT)TC. This is the time a ventilation
system requires to reduce the Spread Index (SI)BZ
from its maximum value to zero. In other words
(PT)TC is a measure of the effectiveness of the
ventilation system in clearing the breathing zone
Air Velocity (fpm)
120.0
108.0
96.0
84.0
72.0
60.0
48.0
36.0
24.0
12.0
the ceiling, descends along the walls and moves inward
away from the walls. Such airflow patterns create large
recirculation loops in the vicinity of the walls. Part of this
recirculating air eventually escapes through the exhaust
grilles. Such airflow patterns are typical for four-way
ceiling diffusers. Three-dimensional airflow patterns
are quite complex and have a large impact on the spread
and distribution of contaminants in the space.
Figure 4 shows the concentration of contaminants at
a height of 5 ft (1.5 m) after 720 seconds of the release
of contaminants. Figure 5 shows an associated cloud of
25 ppm (and higher) concentration in the breathing
zone of the occupants. The cloud depicts the threedimensional
spread of the contaminants above the
acceptable level of 25 ppm. It shows that demandcontrol
ventilation can reduce the average concentration
as well as the spread of contaminants in the breathing
zone. The average concentration level due to DCV
reduced from 23.5 ppm to 16.4 ppm (by about 30%) at
a breathing plane (Figure 4). Whereas the Spread Index
50
ASHRAE JOURNAL ashrae.org J U N E 2024
and bringing the concentration to an acceptable
level (target concentration). (PT)TC can provide
valuable guidance in determining the safe period
during which occupants can be evacuated after an
accidental spill of chemicals. In this study, the impact
of demand-control ventilation was analyzed by
evaluating the difference in the (PT)TC values between
with and without demand-control ventilation.
Results and Discussion
Figure 3 shows the airflow patterns in the lab and at
three vertical planes. The airflow patterns' path lines
are colored by the air velocity. As shown in these figures,
the air exiting from the four-way diffusers travels along
FIGURE 3 Airflow patterns colored by air velocity and at three different vertical planes at 5.3 ft (1.6 m), 15 ft (4.5 m), 24.75 ft
(7.5 m) in a lab. It shows air recirculation patterns in the entire space for 6 ach.
B

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ASHRAE Journal - June 2024

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