An Evaluation of the TruView Two-Sided Glass-Walled
Fume Hood
Dr. Robert K. Haugen, Technological Director, Fume Hood Systems
Rudolph Poblete, Fume Hood Design Engineer
Karole Clanton, Sales System Administrator
Kewaunee Scientific Corporation
Statesville, North Carolina
Introduction and Background:
In the past decade, there has been increased interest in new fume hood designs
for use in teaching labs and other applications where additional hood interior
visibility is required. This interest is created by several things a standard fume
hood does poorly in such a setting:
1)
Students using standard hoods are difficult to observe and
monitor. A standard fume hood (fig. 1) has solid walls and a back
exhaust baffle system that is made out of solid non-transparent
chemically resistant panels. The only way an instructor can monitor
students doing experiments is to observe them through the front
transparent sash opening. If one or more students are occupying this
sash area, it is almost impossible for the instructor to see around the
student’s body.
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2)
Mounted in a normal fashion, standard fume hoods require a very
large classroom. (A minimum of 47 square feet per student) A
standard hood is usually mounted against a wall. A teaching lab with 30
students would require 138 lineal feet of fume hood to simultaneously
accommodate the 30 students. This would, at minimum, require a
classroom 38 feet on a side, or 1428 square feet. (fig. 2)
In a typical glass-backed or glass-walled fume hood, space can be saved by mounting
hoods back to back on penninsular benching.
3)
Solid-walled and solid-baffled fume hoods are tough to maintain. It
is next to impossible to look at a standard fume hood interior and
determine if tissue, papers, or other foreign materials have become
lodged between the baffles and the back or roof of the fume hood. If
such a problem is discovered, it is also difficult to dismantle the hood for
maintenance. These types of “blowing scrap” problems are bad enough
in laboratory fume hood applications. From my experience, they are
even worse in a teaching lab where inexperienced students are all taking
independent notes on paper.
The most common contemporary solutions to teaching hood problems have been
less than satisfactory.
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Initially, “box” structures with sashes on both sides were employed on island or
peninsula benching. (fig. 3) Such units had glass walls for greater supervisory
scrutiny, plus they could be mounted on island or peninsular benches to take
greater advantage of floor space. These units, however, did not contain fumes
well, mostly due to the lack of a baffle system. Most failures in containment
happened when both sashes were opened simultaneously. Also such units
exhausted large amounts of air and were therefore not economical to operate.
Most importantly, while such hoods were frequently used as two independent
hoods, the two sides were very dependent upon each other. Opening the right
side would diminish the left side face velocity.
4
Such problems really became apparent when economy-minded schools installed
two-sided “box” hoods inside a wall between classrooms (fig. 4). In such an
arrangement, exhaust load on each room became a complex function of hood
exhaust, sash position, room static pressure, and room make-up air. More often
than not, such systems were next to impossible to get working properly.
More modern versions of this original design have been devised with a single
glass wall separating the two halves of the hood. (fig. 5) While both halves of
such a hood now function with reasonable flow independence depending on their
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design, the exhaust ducting is complicated as the lower exhaust air follows a
different general route than the upper air.
While these modified two-sash “boxes” were considerably better than the
original, their general performance was still less than what was expected from a
standard fume hood. Many times, manufacturers would call these devices
“ventilated work stations” to emphasize this diminished performance expectation.
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Modifications Undertaken by Kewaunee in the Single
Baffle Student Hood Design:
The researcher used criticisms and shortcomings of earlier student fume hood
designs to develop the following updated single baffle student fume hood (fig.6):
1)
Instead of no central dividing wall or a single transparent dividing wall, a
double tempered glass wall with slots is used (fig. 6). Such a wall
effectively isolates aerodynamic performance of the right and left sides of
the hood while allowing a simplified single exhaust connection.
2)
A front-to-back bypass is used to vector bypass air behind the sash
plane on both fume hood sides. By placing less contaminated air between
the laboratory worker and the contaminated air in the hood interior, this
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modified bypass improves fume hood containment, particularly during
walk-bys and other dynamic challenges.
This down wash of uncontaminated laboratory air behind the sash plane
has previously been used by Kewaunee in its "dynamic barrier" low
constant volume fume hood. 1
3)
The flush sill airfoils produce an effective worktop-clearing air current
without causing wasted airflow under the airfoil when all sashes are
closed. Effective clearing action is provided even when the sashes are
both opened to their operating limits.
4)
A special velocity alarm is provided with a perforated pressure
equalization tube that mechanically averages static pressures in the fume
hood interior. This feature allows the utilization of any sidewall-sensor
type velocity alarm with good sensitivity and response time to reliably
report hood face velocity whether the sash has a pure vertical opening or
is equipped with optional combination vertical/horizontal sashes or an
optional safety panel. In addition, the Kewaunee alarm has a scrolling
one-hour timeline indicating fume hood face velocity over that period of
time.
5)
A slanted sash is added to assist internal air change rate, diminish
turbulence, and move the student closer to operations inside the hood.
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Energy Savings: Another Expectation of the Single
Baffle Design
Because the two halves of the unibaffle hood are aerodynamically isolated, both
sides can be equipped with an 18” sash stop without worry that a sash “violation”
on one side will lower the face velocity on the other. What follows is a graphical
comparison between the unibaffle design at 18” versus a 30” opening on a “box”
design.
TABLE 1: COMPARISON OF “BOX” AND UNIBAFFLE EXHAUST REQUIREMENTS
2-Side
Hood
Size
Unibaffle
opening
height on
each side
Unibaffle
CFM
@80 FPM
“Box”
hood
opening
height on
each side *
“Box”
CFM
@100
FPM
CFM Savings
with
unibaffle
Yearly Savings
assuming $3/CFM
1) 4'
2) 5'
3) 6'
4) 8'
18”
18”
18”
18”
880
1120
1360
1840
31”
31"
31"
31"
1720
2240
2760
3790
840
1120
1400
1950
$ 2520
$ 3360
$ 4200
$ 5850
•
31” required for 30” clearance due to 1” airfoil
Can a Single Baffle Hood Effectively Contain Fumes
in This Exhaust Range?
While the above table shows impressive potential annual savings, the following
questions need to be affirmatively answered before this specific technology can
be recommended for classroom use:
1)
2)
3)
4)
Does the single baffle really isolate the two halves of this hood so that the
face velocity on one side is not changed by sash movement on the
opposite side?
Are the flow dynamics and face velocity distributions stable and
consistent?
Does this fume hood contain under the conditions outlined in Table 1?
Will this fume hood contain under dynamic challenge?
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Test Methodology:
A. INDEPENDENCE OF FACE VELOCITY AS OPPOSING SASH IS RAISED
AND LOWERED:
The researchers took an average face velocity on one side of the fume hood
with the opposing sash lowered. A second average face velocity was then
taken with the opposing sash completely raised. Results are shown below:
Hood Sash Set-up
1) A= 18” open; B= Closed
2) A= 18” open; B= 30” open
Average FV Side A
101.3 FPM
99.7 FPM
Average FV Side B
NA
59.5 FPM
As a result of these data, the independent aerodynamic behavior of both
sides of the unibaffle hood is established.
B. CONSISTENT FLOW DYNAMICS AND UNIFORM FLOW
The researchers tested face velocity profiles for this unit both at 100 FPM full
open (31.5”; 2975 CFM) and at 80 FPM at 18” open (1360 CFM).
In all cases, velocity profiles were even and laminar as shown below:
Face Velocity at 31.5” Open, East Side:
99 FPM
102 FPM
91 FPM
104 FPM
106 FPM
117 FPM
Vav = 102.5 FPM
106 FPM
91 FPM
104 FPM
101 FPM
102 FPM
107 FPM
Face Velocity at 31.5” Open, West Side:
99 FPM
100 FPM
100 FPM
104 FPM
116 FPM
113 FPM
Vav = 99.9 FPM
98 FPM
91 FPM
91 FPM
10
102 FPM
85 FPM
100 FPM
Face Velocity at 18” Open, West Side:
77 FPM
80 FPM
80 FPM
72 FPM
Vav = 80.0 FPM
88 FPM
83 FPM
75 FPM
85 FPM
Face Velocity at 18” Open, East Side:
80 FPM
89 FPM
71 FPM
72 FPM
Vav = 79.0 FPM
84 FPM
76 FPM
C. Does this hood contain fumes?
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75 FPM
85 FPM
The researchers decided to use ANSI /ASHRAE 110-1995 as the test
methodology for evaluating the TruView Fume Hood.
The tracer gas used is 100% SF6. The detection instrument used is a Miran
103 by Foxboro with 13.5 M path length and a 10.7-micron filter. Meter
response was set at 1 second and 10x was expansion control setting.
The test was divided into 2 parts:
1) A five minute "static" test where the ASHRAE Manikin is not moved
2) A two-minute "full-up" period followed by one rapid sash opening followed by
a one-minute observation period. (SME test)
The above tests were done on the 6' two-sided single baffle TruView fume hood
in the following orientations:
1) East & West Vertical open 18" (left, center, & right manikin positions)
2) East & West Vertical open 31.5" (left, center, & right manikin positions)
Test results for the 6 data runs are shown in Table 2:
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OBSERVATIONS:
TABLE 2: ASHRAE TEST RESULTS
#
Hood
Type
6'Unibaffle
6'Unibaffle
6'Unibaffle
6'Unibaffle
6'Unibaffle
6'Unibaffle
Vert
Sash
&
Manikin Position
31.5", l
31.5", c
31.5", r
18", l
18", c
18", r
Face
Velocity
100 FPM
100 FPM
100 FPM
80 FPM
80 FPM
80 FPM
1
2
3
4
5
6
Key:
1) l, c, r = left, center, right
2) All tests done on west face of fume hood
CFM
5 min. ASHRAE PPM
SME Max. PPM
2975
2975
2975
1360
1360
1360
0.002 PPM
0.001 PPM
0.002 PPM
0.002 PPM
0.000 PPM
0.004 PPM
0.005 PPM
0.007 PPM
0.005 PPM
0.003 PPM
0.004 PPM
0.011 PPM
ASHRAE CHARTS:
1) 31.5”; 100 fpm; LEFT POSITION
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2) 31.5”; 100 FPM; CENTER POSITION
3) 31.5”; 100 FPM; RIGHT POSITION
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4) 18”; 80 FPM; LEFT POSITION
5) 18” 80 FPM; CERTER POSITION
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6) 18”; 80 FPM; RIGHT POSITION
CONCLUSIONS:
The 6' Single Baffle TruView bench fume hood showed excellent containment
and stability to dynamic sash challenge in all positions detailed in Table 2. Test
results obtained were comparable to 100 FPM full-open standard fume hood
performance.
In addition, opening and closing one sash did not affect the average face velocity
on the opposite side of the fume hood.
While these containment results are both exciting and positive, the application of
this technology to any laboratory must be thoughtfully undertaken.
The following issues are of particular relevance:
1.
As with any fume hood product, users should be trained in the safe
operation of this device.
2.
Emergency gas and electrical cutoff switches should be installed in any
teaching lab to guard against accidents and runaway reactions.
3.
Existing face velocity guidelines for fume hoods are not to be ignored!
Table 3 shows published recommended face velocity minima.
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TABLE 3: PUBLISHED FACE VELOCITY
RECOMMENDATIONS
Organization
1) ACGIH
2) ASHRAE
Citation
Industrial Ventilation 19th edition p.5.24
1999 ASHRAE Handbook, 13.5
Face Velocity
60-100 FPM
3)
4)
5)
6)
ANSI/AIHA Z9.5, Sect 5.7
CCR Title VIII, Subchapter 7.5454.1
Prudent Practices, p.187
NFPA 45: 6-4.5 & A6-4.5
80-120 FPM
Min 100 FPM
80-100 FPM
7) NIOSH
8) NRC
Recommended Indust. Ventil. Guidelines p166
100-150 FPM
NRC Guide, 6.3
100 FPM for hospital
radioactives
9) OSHA
10) SEFA
29 CFR 1910 Appendix A Sec. A.C.4.g
SEFA 1.2: 5.2
60-100 FPM
75-100 FPM
ANSI/AIHA
CALOSHA
Nat. Rsrch.Cnc.
NFPA
20%-50% of exterior
disturbance velocities.
(60-175 FPM) if 300 FPM
walkby used to calculate)
"Sufficient to prevent
escape from hood; 80120 FPM;
40 CFM/lin foot min
While the research here demonstrates the TruView Fume Hood can work
effectively, reduced face velocity fume hoods of this design are a different story.
A slow walk producing a turbulence wake of 200 FPM behind a student can
overpower a low input vector at a fume hood face of, say, 40 FPM. It is this
researcher's opinion that HVAC savings and safety can be achieved by using
smaller fume hood openings (say 18”) at face velocities at the level shown in
Table 3. These smaller openings can be made flexible, offer more protection
from spatters and small debris, and can be opened very wide for equipment setups when no fume-evolving experiment is taking place. In addition, the single
baffle design allows for set-up mode on one side, while an experiment is being
run in containment mode on the other.
Footnote:
1)
Laboratory Design, April,2000, “Containment Study Shows Performance of Dynamic Barrier Low Flow
Hoods”
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