ASHRAE Journal - June 2022 - 35

TECHNICAL FEATURE
FIGURE 4 Illustration of the virtual sampling grid and 15 sampling points at the
fume hood opening (unit: mm).10
318
1
145
6
11
7
12
8
13
9
14
10
15
Large
chromatograph-thermal conductivity detector in the
laboratory. Due to the limitation of extremely high cost
and large amount of greenhouse gas emission from the
tracer gas test, it was not appropriate to conduct the test
at each sampling point (15 total) and under all scenarios
(eight total).
Although laboratory analysis did not show the exact
variation in the concentration of the tracer gas, this
method was supplementary to the on-site monitoring,
enhancing its reliability and facilitating better understanding
of the time-weighted average (TWA) exposure
to the contaminants.10
The details of the testing variables and conditions
are specifi ed in Table 2, and the testing setup
is demonstrated in Figure 4. Testing operators had to
remain vigilant against the potential risk of equipment
fi re and personnel scald because the thermostatic heaters
kept relatively high temperature (>60°C [>140°F]) for
a fairly long time (>20 min). The fume hood remained
operational after the testing was completed, until the
surfaces of the thermostatic heaters returned to room
temperature.
Results and Discussion
To fac ilitate a better understanding of the comparative
analysis in this section, pertinent parameters are
defi ned fi rst:
VV ni
i
= ∑


=1
n


(1)
where Vis the average face velocity, Vi is the face velocity
measured at the center of an individual grid rectangle
and n is the number of grid rectangles.
The test results are presented here with discussion of
the impact of inside clutter, thermal load and tracer gas
No thermal challenge
100°C at the surface of
the thermostatic heaters
200°C at the surface of
the thermostatic heaters
200°C at the surface of
the thermostatic heaters
2
3
4
5
TABLE 2 Demonstration of the testing variables and conditions.
SASH OPENING INSIDE CLUTTER
THERMAL LOADS
Half Open
Small
No thermal challenge
100°C at the surface of
the thermostatic heaters
200°C at the surface of
the thermostatic heaters
200°C at the surface of
the thermostatic heaters
TRACER GAS
EMISSION CHARACTER
4 L/m
4 L/m
4 L/m
8 L/m
4 L/m
4 L/m
4 L/m
8 L/m
emission rate on the robustness of face velocity for fume
hood containment performance evaluation.
The limited impact of thermal l oad on the face velocity
can be seen in both Figure 5 and Figure 6. There is a minor
and slow downward trend in the average face velocity
with the increase in thermal load, regardless of the
size of the inside clutter, leading to a maximum difference
of 0.07 m/s (14 fpm) and a minimum difference of
0.01 m/s (2 fpm) only. Please refer to Figure 5a and 5d. The
maximum difference is derived from 0.48 m/s - 0.41 m/s
(95 fpm - 81 fpm). Likewise, the minimum is 0.42 m/s -
0.41 m/s (83 fpm - 81 fpm). Similarly, Table 3 shows
that the tracer gas leakage concentrations under the
infl uence of the small clutter, 4 L/m (1 gpm) tracer gas
emission and different thermal loads were constantly
below the LOD of SF6 (10 ppb) or the control level
(10 ppb); this also demonstrates the minor impact of the
thermal load. Even when the large clutter was placed
inside of the fume hood, the maximum increment of
tracer gas leakage due to increased thermal load is still
negligible (0.16 ppm).
Figure 5 shows that with small clutter, face velocity
values in the upper (points 1 to 5) and middle rows
(points 6 to 10) of the sampling grid (Figure 4) are
relatively close to each other, and that all of them are
higher than the values in the bottom row. Further, the
face velocities at point 10 and points 12/13/14 become the
dominant highest and lowest value(s), respectively, in all
scenarios with a thermal challenge. This is mainly due
to the formation of the center of the doorsill boundary
layer separation and the blockage effect from the inside
J U N E 2 0 2 2 ashrae .o rg ASHRAE JOURNAL
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ASHRAE Journal - June 2022

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