ASHRAE Journal - June 2022 - 32

TECHNICAL FEATURE
Testing Environment and Experimental Equipment
The indoor pressure was maintained as slightly
negative (-5 to -10 Pa [-0.02 to -0.04 in. w.c.]) via the
interlock between air supply and general exhaust
ventilation in the laboratory, which met the expectation
of a standard testing environment from ASHRAE
Standard 110-2016. The operational frequency of the
blower was set at 35 Hz, and its operation failure alarm
was set up. The impacts of air supply diffusers, exhaust
outlets, traffic and doors were carefully considered, and
on-site cross-draft measurement was deemed necessary
to prevent negative effects on fume hood containment
performance.
To that end, the on-site cross-draft velocities were
measured following ASHRAE Standard 110-2016,11 using
a calibrated hot wire anemometer. The anemometer's
transducer was factory-calibrated and is traceable to
National Institute of Standards and Technology (NIST)
standards, with velocity corrections to the room temperature
and standard atmospheric pressure allowed in
the calibration. The instrument is accurate to ±3% of the
reading or ±0.015 m/s (±3.0 fpm), whichever is greater.20
Measured cross-draft velocities ranged from 0.06 to 0.12
m/s (12 to 24 fpm), well below the limit of 0.15 m/s (30
fpm) recommended by ASHRAE Standard 110-2016 and
other technical references for an ideal environment for
fume hood face velocity testing.
A portable electron capture detector was used to check
the initial level of indoor air contaminants. It was field
calibrated before the test using a 0.9 ppm standard
tracer gas, sulfur hexafluoride (SF6). The dual-reservoir
SF6 leak kit is also NIST traceable and American
Association for Laboratory Accreditation (A2LA) accredited.
The gas was assayed using NIST standards, resulting
in an expected calibration accuracy of ±0.01 ppm.
Aggregate experimental errors were expected to be less
than 0.02 ppm at the industry standard control level of
0.1 ppm.10 During a 5 min period of continuous monitoring,
SF6 concentrations were kept at " not detected "
(below the limit of detection [LOD] of 0.01 ppm) constantly.
This confirmed that total indoor air contaminants
sensed by the detector as the tracer gas remained
at less than 10% of the recognized control level (0.1 ppm),
which is the testing environment required by ASHRAE
Standard 110-2016.
An aerodynamically designed, commercially available
laboratory chemical fume hood (1590 mm × 550 mm ×
32
ASHRAE JOURNAL ashrae.o rg
J U N E 2022
FIGURE 1 Background fume hood face velocity profile without inside clutters and
thermal loads.
0.60
0.58
0.56
0.54
0.52
0.50
0.48
0.46
1 2
3
Face Velocity Values
Average Face Velocity Values
4
5
6 7
8
9 10 11 12 13 14 15
Sampling Point
870 mm [62.6 in. × 21.6 in. × 34.3 in.]) with an exhaust
duct diameter of 300 mm (12 in.) was properly commissioned
before the face velocity investigation in this
study. The sash of the hood was raised 435 mm (17.1 in.)
to keep it 50% open, as it is normally operated in the
laboratory. Figure 1 shows the background face velocity
profile with no testing variables established; the
variation was 7.41%, well below the maximum variation
(15%) suggested by Standard 110.
Meanwhile, to allow analysis of the impact of various
influential factors in real applications, two identical
thermostatic heaters (100 mm × 100 mm × 145 mm
[3.9 in. × 3.9 in. × 5.7 in.]) were placed 50 mm (2 in.)
away from the tracer gas ejector bilaterally in symmetry
to simulate thermal load; and one small corrugated box
(220 mm × 160 mm × 160 mm [8.7 in. × 6.3 in. × 6.3 in.])
and one large corrugated box (300 mm × 70 mm ×
380 mm [11.8 in. × 2.8 in. × 15.0 in.]) were placed 50 mm
(2 in.) behind the sash plane to simulate small and large
inside clutter, respectively. The purpose of the placement
and dimensions of the clutter was to illustrate the
severe impact of clutter on fume hood performance and
to emphasize the need for adequate setback in laboratory
hood use.
While responsible laboratory hood users follow operating
procedures that require a proper setback (i.e., 15 cm
[5.9 in.]) for all objects inside the hood to avoid disrupting
airflow at the hood face, it is a not-uncommon poor
practice in the laboratory for some instruments to be
placed fairly close to the sash due to either operational
difficulty or space limitations. The size of the mocked-up
clutter well represents the dimensions of typical
instruments used inside the fume hood in chemical
laboratories, such as a digital rotator, an analytical
balance, a vacuum pump and a spectrometer.10
Face Velocity Values (m/s)
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