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High Performance Low Flow Fume Hood Design
Chandra Manik Senior Mechanical Engineer of ESCO Micro Pte Ltd
Peh Jingpeng Product Specialist of ESCO Micro Pte Ltd
Abstract
Fume hoods have always been an integral part of a chemical laboratory. For many years,
face velocity for fume hoods has been set at 0.5m/s (100FPM ). With escalating energy
cost, it has become prudent to explore lower face velocity to ensure fume hood
containment. Esco Micro Pte Ltd performs research focusing on fluid flow characteristicssuch as: reverse flow, turbulence intensity and boundary layers. It was concluded that
these significantly influenced the containment performance of the fume hood.
Therefore changes were made to the design of our fume hood to increase the overall
aerodynamics. Reverse flow in the fume hood were reduced and turbulence was minimized
with the newly engineered designs.
The new design established was a new high performance low velocity fume hood. Final
prototype designed was named as Frontier Acela (for 5 foot size) and tested according to
fume hood performance test protocol, ASHRAE 110:1995 and EN14175: 2003. Frontier
Acela passes the tests when operating at a face velocity of 0.3 m/s with the same resultsfor other sizes of Frontier Acela: 4,6,8 foot.
1.Introduction
History of fume hoods
Fume hoods are one of the important equipment in the laboratory. It must guarantee the
operator safety from being exposed to hazardous gas when performing their experiments.
Its working principles are divided into three major concepts, extract, contain and finally
exhaust into the environment. According to history, the first fume hood was a fireplace used
by alchemist [1]. It was connected to very tall chimneys where the hazardous gas orparticles were exhausted to the environment by thermo-lift effect. In mid 1800s ventilation
engineers added gas burning rings in the stack to achieve greater thermo-lift. During the
industrial age, the gas ring gave way to mechanical fans.
The first major improvement to the fume hood was the addition of the baffle system. With
this addition, fume hoods started to work like a safety device. Thereafter other types of
fume hood were introduced, like auxiliary fume hoods and variable air volume fume hood.
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Measuring fume hood performance
As fume hoods have an important role in protecting the operators, its performance chart iscompulsory and should be presented both qualitatively and quantitatively. The main
protocols to measure the performance of fume hoods are ASHRAE 110:1995 and
EN14175: 2003. These protocols measure the performance at specified operational face
velocity. Performing smoke tests on the fume hoods provides qualitative assessment, while
tracer gas containment provides quantitative data. Despite the difference in methods of
these protocols to each other, they principally represent the same concept.
Factors influencing fume hood performance
Fume hood performance is influenced by two factors; first the fume hood designs itself and
secondly the environment or the system where it is installed. Understanding well thecorrelations of how fluid flow behaves when passing such a shape and its effects on fume
hood performance is the key for advance development.
2.Existing problems
Presence of vortex
Present designs of fume hoods are unable to fully reduce the vortex in the fume hood
chamber, especially at the sash opening. To reduce the vortex, the sash handle, airfoil and
even the edges of the sidewall should be aerodynamically designed to improve uniform
flow into the internal chamber.
VAV installation and operating cost
Variable air volume (VAV) fume hoods were introduced to lower running cost of a fume
hood by reducing outflow air. Initial installation cost for VAV fume hoods were high, as cost
for the control system is high. Furthermore, annual calibration of the controller incurs further
running costs.
Motorized baffles with sensor
Motorized baffles that change the angle of incline according to flow of air was installed in
some current designs of fume hoods. These fume hoods are acting on the principles of
changing the flow pattern based on the airflow to improve the uniformity of flow and reduce
turbulence in the fume hood. The main drawback of this system is that it is a feedback
control system, therefore response time is relatively slow and will not be able to react to
changes fast enough to reduce vortex formation in the fume hood. Since containment of
chemical fumes is the main objective of a fume hood, this is not a viable solution.
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Laminar flow fume hoods
Recently, a new type of fume hood was introduced. It is the laminar flow fume hood, whereair flow horizontally across the work surface and into the back of the fume hood. This is a
highly idealized design and will not necessarily work, particularly in cases where hot plates
are used and will interfere with the horizontal flow. In addition, there is very slow evacuation
of air at the top of the internal fume hood chamber.
Face velocity and energy saving
Although when a fume hood is operated under the specified face velocity is used properly,
this does not mean the fume hood is capable to contain the hazardous gases. When air
mixes with chemical substances inside fume hood chamber produces aerosol or other
mixing types, it can be seen that the velocity will not be the only factor to determine thesuccessfulness in transporting the fluid into exhaust system. Other factors like turbulence
intensity and flow behavior should be taken into consideration. Some approach should be
engaged to seek the correlation to each other: changes on fume hood feature to be more
aerodynamically, baffle orientation and additional auxiliary features.
3.Objective
The main purpose of this research are divided into three major objectives as follows:
1. Recognize the factors influencing fume hood containment performance instead of
dogmatic operational face velocity at 0.5 m/s
2. Building features to increase fume hood containment performance
3. Test fume hood performance according to ASHRAE 110:1995 and EN 14175:2003
4. Current Fume Hood Concepts
Push Pull Concept
Push-pull concept uses auxiliary fan tubular type. This concept consumes less energy
compared to raising the face velocity of fume hood to meet traditional dogmatic operationalface velocity at 0.5 m/s (100 FPM). Energy saving was achieved by changing the area of
breathing zone and push the vortex to the back of the fume hood and finally exhausted into
environment. It is well recognized that vortex is the source for leakage.
This concept faces challenges when dealing with bigger sizes of fume hood where tubular
fan at more than 4 feet length is unavailable in market. So in reality, though theoretically
sound, it is hard to apply.
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Air dilution and laminar flow concept
Air dilution concept is purposely used to align the airflow direction from bypass and aimedto counter the area of vortex above the fume hood chamber by becoming an air curtain to
avoid excessive vortex flow in this zone. The air curtain layers need to be more than one
and to achieve this condition high density perforated holes on baffle should be applied. In
fact this method decrease the negative pressure behind the baffle, further creates low
speed flow as if it is a laminar flow regime.
When dealing with high-density gases compare to air, this concept faces a difficult
challenge. The heavy gases tend to occupy the top portion of fume hood and have little
chance to be exhausted to the environment immediately since there is no slot opening in
that zone.
Moving baffle concept
This concept was generated by deliberating on the possibilities to create laminar flow inside
fume hood at 0.3 m/s (60 FPM) face velocity. The vortex can be reduced and stabilize
using Bi-vortex Stable Concept. The main action to make stable vortex is by re-orientation
of the baffle mechanically using servomotor and with Laplace transformation formula, the
orientation of baffle adjusted according to the reading of the vortex sensor and counter it by
such appropriate orientation of baffle.
Although this concept is sound, the response of baffle is not fast enough to the changes of
the fume hood performance. Besides, difficulties in maintenance of motor make it less
appealing.
Bypass concept
The concept is simple; introduce air from bypass to sweep the breathing zone of operator.
Flow was very active even though the airflow from bypass was not as purposed. After
studying the design concept, it was found that manufacturing a reduce depth of fume hood
by about 20% compare to other fume hoods help reduce strong negative pressure and
reduce the flow losses.
This concept works properly but geometric design will have significant effect in market
since reduction in depth also reduce the comfort and design space in daily use of the fume
hood.
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Supportive flow concept
The supportive flow concept is similar to push pull concept. An approach to replace thetubular fan using centrifugal fan works properly. Discharge port of the centrifugal pump was
connected to the armrest airfoil using flexible PVC tube.
There is a small hole nozzle placed on the side wall of the hood to discharge air from
blower to sweep the wall area, thus clearing possible contaminant by strong momentum of
air to the back of the baffle. This concept tried to counter the reverse flow occurring near
sidewall.
The concept is a success in countering reverse flow near sidewall and the working table but
the baffle orientation does not guarantee the user safety when dealing with high-density
gases. Also high noise from top wall perforated reduces the user comfort.
5.Computational Fluid Diagram
Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical
techniques and algorithms to analyze problems that involve fluid flows. Computers do these
analyses, as it requires many complex equations. Navier Stokes equations govern the
CFD problems, and these equations will define any single phase fluid flow. Making use of
these equations, a stimulation program can do and used to produce a CFD diagram as
shown below.
CFD Diagram of Frontier Acela
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6.Designing the ESCO Frontier Acela
Frontier Acela was designed to increase energy savings. To achieve this, operational face
velocity was reduce to 0.3 m/s (60 FPM).
Concepts and Ideas
After performing careful research by performing numerical calculation and literature study, it
was found that changes in design of baffle orientation, sash airfoil, armrest airfoil, exhaust
collar and bypass can increase the performance of Frontier Acela.
Compromise Solution(s)
The various features to support the concepts and ideas generated in previous stage are
selected to meet the technical merit with regards to mechanical and manufacturability,
electrical and electronic issues where the high reliability, humble and simple design are the
focus.
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Aerodynamic designs of Frontier Acela
Baffle Orientation
The mid-baffle was inclined to meet the
requirement when users deal with high-density
gases compared to air. The inclined baffle
leads the flow into exhaust system smoothly
where its inclined orientation avoid the
possibility of rolling inside the fume hood
chamber. A perforated down baffle is designed
by the consideration to maintain sufficient
negative pressure for easy transportation of
hazardous gases and particles directly to the
back of the baffle.
Sash Airfoil
Aerodynamic sash airfoil helps to reduce the
turbulence intensity at the breathing zone. The
shape of airfoil was designed after much
testing and numerical calculation with the
objective of avoiding vortex flow behind the
sash and reducing the turbulence intensity.
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Armrest Airfoil
One of the most important features in a
fume hood is armrest airfoil. A correctly
designed armrest airfoil enhances the
flow characteristic entering the fume
hood face by eliminating the vortex
above working table.
Bypass
Bypass maintains the location behind sash, which is close to breathing zone, free from
contamination by introducing fresh air from the upper baffle. The bypass is also used as an
air curtain to minimize the possibilities of sudden backflow into operator zone due to the
environmental changes in laboratory.
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Exhaust Collar
Exhaust collar shall be designed to reduce pressure drop and create uniform flow above
working surface. Saving the energy by lowering face velocity will be useless if high
pressure drop at exhaust collar still exists. The Frontier Acela (EFA) exhaust collar was
designed by considering on how to reduce flow resistance, pressure drop and the
excessive noise that may occur.
Frontier Acela 5 Ft Pressure Drop
Diameter exhaust: 305 mm
Exhaust flow rate: 664 m3/h
Face Velocity*: 0.3 m/s
DP sash 500 mm open: 7.64 pa
DP sash closed: 9.74 pa
*) About 5-10% additional flow from
Bypass.
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Model and Prototype
The model was built to prove that the design meets the technical challenges. Variousfeatures were analyzed and summarized in protocol based on factory standard in quality,
manufacturability, electrical and electronic.
Testing According to ASHRAE 110
Table 1 show the summary where Frontier Acela was tested according to ASHRAE
110:1995
Table.1. Frontier Acela performance test according to ASHRAE 110:1995
The test performed under condition As Manufactured in ESCO Fume Hood laboratory at
various face velocity from 0.5 m/s (100FPM) to 0.3 m/s (60 FPM).
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Testing According to EN14175
The test set-up of EN 14175 divided into outer grid test (OG), robustness test (RO) and theInner grid test (IG) as shown in Figure
Outer Grid Test (OG)
Frontier Acela 5 Ft leak characteristic
Outer Grid Test
t(s) Average Leak (ppm)
60 0.000
360 0.000
420 0.003
600 0.004
Inner Grid Test (IG)
Frontier Acela 5 Ft Leak
Characteristic
Inner Grid Test
t(s) Average Leak (ppm)
60 0.000
360 0.000
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Robustness Test (RO)
Frontier Acela 5 Ft LeakCharacteristic
Robustness Test
t(s) Average Leak (ppm)
60 0.237
240 0.000
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7.Conclusion
To increase Frontier Acela containment performance when operated at 0.3 m/s face
velocity, several ideas and concepts relating to fluid mechanics were developed and
applied as follows:
- Design aerodynamic Side Wall to avoid reverse flow where less aerodynamic design
will lead to failure.
- Proper design on sash handle airfoil eliminates separation at boundary edge, which
enhances protection to the operator breathing zone.
- Designing carefully and numerically on armrest airfoil was proven to increase the
protection to operator by countering flow separation above working table.
- Inclined middle baffle to enhance the reduction of vortex flow inside fume hood
chamber.
- Bypass from top baffle helps to reduce the risk of reverse blow into operator breathing
zone and also used as air curtain to protect sudden back flow due to the laboratory
environment change.
- Unique exhaust collar shape, statistically proven to reduction in pressure drop and
noise levels thus increase energy savings and uniformity of air stream above working
table.
Final prototype Frontier Acela 4Ft, 5Ft, 6Ft and 8Ft pass the performance test according to
ASHRAE 110:1995 The average leak was below the acceptance level of 0.05 ppm for
condition As Manufactured (AM) according to the protocol ASHRAE 110:1995. Frontier
Acela also pass the performance test according to EN 14175 (except the EFA 8Ft whichwas not typically familiar in Europe Market, in this case EN14175 was not executed). The
average leak (see table below) pass the acceptance criterion where the value in Europe
(Especially in Germany) for Outer grid and robustness test shall be below 0.65 ppm, for the
inner grid test the acceptance value was referred to French standard that demands the
maximum leak is below 0.1 ppm.
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Type of Test EFA-4 EFA-5 EFA-6 AcceptanceValue
Inner Grid Test 0.000 0.000 0.000 0.100(French)
Outer Grid Test 0.006 0.004 0.109 0.650(Germany)
Robustness Test 0.111 0.237 0.242 0.650(Germany)
8.References
[1] Saunders GT. Laboratory fume hood-a user manual. New York;Wiley;1993
[2] Fletcher,B and Jhonson, A.E (1992). Containment testing of fume cupboards-II. Test
room measurement. British Occupational Hygiene Society.UK.
[3] Munsen 1989
[4] Hitching 1996, Hitching and Maupins 1997, Caplan and Knutson 1997; Saunders 1993
[5] William Peters. Performance Review- the journal of the controlled environment testing
association; San Antonio; CETA 2008.
[6] Bell et al 1996
[7] Lars E. Ekberg, Jan Melin. Required response time for variable air volume fume hood
controllers. Department of Building Service Engineering, Chalmers University of
Technology; Sweden;1999
[8] Bell G, Sartor D, Mills E. The Berkeley hood: development and commercialization of an
innovative high-performance laboratory fume hood. Berkeley, CA : Lawrence Berkeley
National Laboratory; 2002 (Report No. 48983)
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