high performance low flow fume hood design

8/3/2019 High Performance Low Flow Fume Hood Design

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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


60 0.000

360 0.000


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Robustness Test (RO)

Frontier Acela 5 Ft LeakCharacteristic

Robustness Test


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)

high performance low flow fume hood design

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