ASHRAE Journal - June 2022 - 36

TECHNICAL FEATURE
FIGURE 5 Face velocity measurement results with the small clutter and different thermal load/tracer gas emission character.
Face Velocity Values
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
0.44
1 2
3
4
5
6
7
8
Sampling Point
(a) Without Thermal Load, 4 L/m Emission Rate
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
0.44
1 2
3
Face Velocity Values
Average Face Velocity Values
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
0.44
4
5
6
7
8
9 10 11 12 13 14 15
Sampling Point
(c) 200°C Thermal Load, 4 L/m Emission Rate
clutter. A similar distribution trend of face velocity
values in each row is observable for large clutter in
Figure 6. The face velocity values in the left middle row
(points 6 and 7) are always the highest, while points 8
and 13 are the lowest values regardless of thermal load
and tracer gas emission rate.
The blockage effect from the large clutter tends to
be more prominent than that from the small clutter,
and as a result, the face velocity drops dramatically to
0.13 m/s (26 fpm) at its lowest point, with a variation of
69%, which deviates significantly from the background
variation without any clutter (Figure 1) and the suggested
maximum variation from Standard 110 (7.41% and 15%,
respectively).11 As expected, Table 3 shows that with large
clutter, the corresponding tracer gas concentrations at
the bottom center of the sash (from 0.2 to 19.32 ppm
under different conditions) are far beyond the control
level, even with the lower tracer gas emission rate and
no thermal load (0.2 ppm).
This finding is consistent with the finding of Cheng,
et al.,23 that large inside clutter can contribute to the
spread of contaminant, and also alerts laboratory technicians
that inadequate setback of any large object inside
36
ASHRAE JOURNAL ashrae.o rg
J U N E 2022
1 2
3
Average Face Velocity Values
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
0.44
9 10 11 12 13 14 15
1 2
Face Velocity Values
Average Face Velocity Values
3
4
5 6
7
8
9 10 11 12 13 14 15
Sampling Point
(b) 100°C Thermal Load, 4 L/m Emission Rate
Face Velocity Values
Average Face Velocity Values
4
5 6
7
8
9 10 11 12 13 14 15
Sampling Point
(d) 200°C Thermal Load, 8 L/m Emission Rate
the hood is detrimental to airflow stability at the hood
face. Personal exposure sampling and assessment might
be a valid next step so that laboratory operators would
know whether proper respiratory protective equipment
(RPE) should be used to reduce their occupational
exposure.
The average face velocity by row (upper, middle
and bottom) of the sash opening area under different
scenarios is illustrated further in Figure 7. As shown in
this figure, average face velocity values in the upper
and middle rows are always higher than the values in
the bottom row, demonstrating the same trend as face
velocity values in the corresponding rows. The potential
issue of contaminant leakage associated with the low
face velocity at the bottom row area could be alleviated
through optimized aerodynamic design of the airfoil
and baffle structures as well as by a bracket (for large
objects to stand on) to facilitate a uniform and inward
airflow pattern at the hood entrance.22 The maximum
difference in average face velocity between the three
rows under the influence of the small clutter is relatively
slight (0.05 m/s [10 fpm]), whereas the impact of the
large clutter doubles the maximum difference to reach
Face Velocity Values (m/s)
Face Velocity Values (m/s)
Face Velocity Values (m/s)
Face Velocity Values (m/s)
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ASHRAE Journal - June 2022

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