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

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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.

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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' 18” 880 31” 1720 840 $ 2520 2) 5' 18” 1120 31" 2240 1120 $ 3360 3) 6' 18” 1360 31" 2760 1400 $ 4200 4) 8' 18” 1840 31" 3790 1950 $ 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) 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?

2) Are the flow dynamics and face velocity distributions stable and consistent?

3) Does this fume hood contain under the conditions outlined in Table 1? 4) 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 Average FV Side A Average FV Side B 1) A= 18” open; B= Closed 101.3 FPM N A 2) A= 18” open; B= 30” open 99.7 FPM 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 106 FPM 101 FPM 91 FPM 104 FPM 91 FPM 102 FPM 106 FPM 117 FPM 104 FPM 107 FPM

Vav = 102.5 FPM

Face Velocity at 31.5” Open, West Side:

99 FPM 100 FPM 98 FPM 102 FPM 100 FPM 104 FPM 91 FPM 85 FPM 116 FPM 113 FPM 91 FPM 100 FPM

Vav = 99.9 FPM

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Face Velocity at 18” Open, West Side:

77 FPM 80 FPM 88 FPM 75 FPM 80 FPM 72 FPM 83 FPM 85 FPM

Vav = 80.0 FPM

Face Velocity at 18” Open, East Side:

80 FPM 89 FPM 84 FPM 75 FPM 71 FPM 72 FPM 76 FPM 85 FPM

Vav = 79.0 FPM C. Does this hood contain fumes?

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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 Vert Sash & Manikin Position

Face Velocity

CFM 5 min. ASHRAE PPM SME Max. PPM

1 6'Unibaffle 31.5", l 100 FPM 2975 0.002 PPM 0.005 PPM 2 6'Unibaffle 31.5", c 100 FPM 2975 0.001 PPM 0.007 PPM 3 6'Unibaffle 31.5", r 100 FPM 2975 0.002 PPM 0.005 PPM 4 6'Unibaffle 18", l 80 FPM 1360 0.002 PPM 0.003 PPM 5 6'Unibaffle 18", c 80 FPM 1360 0.000 PPM 0.004 PPM 6 6'Unibaffle 18", r 80 FPM 1360 0.004 PPM 0.011 PPM Key: 1) l, c, r = left, center, right 2) All tests done on west face of fume hood

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 Citation Face Velocity 1) ACGIH Industrial Ventilation 19th edition p.5.24 60-100 FPM 2) ASHRAE 1999 ASHRAE Handbook, 13.5 20%-50% of exterior

disturbance velocities. (60-175 FPM) if 300 FPM walkby used to calculate)

3) ANSI/AIHA ANSI/AIHA Z9.5, Sect 5.7 80-120 FPM 4) CALOSHA CCR Title VIII, Subchapter 7.5454.1 Min 100 FPM 5) Nat. Rsrch.Cnc. Prudent Practices, p.187 80-100 FPM 6) NFPA NFPA 45: 6-4.5 & A6-4.5 "Sufficient to prevent

escape from hood; 80-120 FPM; 40 CFM/lin foot min

7) NIOSH Recommended Indust. Ventil. Guidelines p166 100-150 FPM 8) NRC NRC Guide, 6.3 100 FPM for hospital

radioactives

9) OSHA 29 CFR 1910 Appendix A Sec. A.C.4.g 60-100 FPM 10) SEFA SEFA 1.2: 5.2 75-100 FPM 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 set-ups 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”