ASHRAE Journal - June 2022 - 38

TECHNICAL FEATURE
FIGURE 6 Face velocity measurement results with the large clutter and different thermal load/tracer gas emission character.
Face Velocity Values
0.60
0.50
0.40
0.30
0.20
0.10
0.00
1
2
3
4
5
6
7
8
Average Face Velocity Values
0.60
0.50
0.40
0.30
0.20
0.10
0.00
9 10 11 12 13 14 15
Sampling Point
(a) Without Thermal Load, 4 L/m Emission Rate
0.60
0.50
0.40
0.30
0.20
0.10
0.00
1
2
Face Velocity Values
Average Face Velocity Values
0.60
0.50
0.40
0.30
0.20
0.10
0.00
3
4
5 6
7
8
9 10 11 12 13 14 15
Sampling Point
(c) 200°C Thermal Load, 4 L/m Emission Rate
0.1 m/s (20 fpm). This means that the stability of the face
velocity is significantly reduced under the influence
of large inside clutter; this instability consequently
increases potential personal exposure to hazardous
vapors (19.32 ppm under 200°C [392°F] thermal load
and 8 L/m [0.28 cfm] tracer gas emission rate vs.
0.1 ppm of SF6 control level).
Conclusions
This study investigated the impact of various
influential factors (thermal load, small and large
inside clutter, tracer gas emission rate) on the
robustness of face velocity for fume hood containment
performance evaluation through comprehensive face
velocity measurements and tracer gas tests. Pertinent
parameters and equations of face velocity measurement
were defined and then comparative analysis of test
results was conducted to analyze the impact. The
important findings of this study are summarized below.
There was only a minor and slow declining trend in
the average face velocity (0.01 to 0.07 m/s [2 to 14 fpm])
and a subtle increase of tracer gas leakage (0.16 ppm)
with the increase of the thermal load regardless of the
size of the inside clutter, demonstrating that the thermal
38
ASHRAE JOURNAL ashrae.o rg
J U N E 2022
1
2
Face Velocity Values
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
Average Face Velocity Values
3
4
5 6
7
8
9 10 11 12 13 14 15
Sampling Point
(d) 200°C Thermal Load, 8 L/m Emission Rate
load has very limited impact on the robustness of fume
hood face velocity. While there was no major difference
between average face velocities in the upper and middle
rows, the values in these two rows were much higher
than the values in the bottom row. The minimum face
velocity (0.13 m/s [26 fpm]) and maximum tracer leakage
(19.32 ppm) were found close to the doorsill with the
large clutter placed inside of the fume hood.
This may be explained by the combined effect of
boundary layer separation at the fume hood inlet and
the blockage effect from the large inside clutter. The
blockage effect from the large inside clutter placed with
little setback from the sash led to significant variation
of the face velocity (69%) compared to the background
and suggested maximum values (7.41% and 15%, respectively).
Tracer gas leakage was also exacerbated by the
blockage effect, with up to a 190-fold exceedance of the
standard control level. This is an important reminder
for laboratory technicians that a proper setback (15 cm
[5.9 in.]) should be maintained for instruments inside
the fume hood and that large instruments should be put
on a shelf or an equivalent device that can smooth the
airflow at the fume hood inlet and mitigate turbulence
intensity.
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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