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Laboratory Chemical Hood User's Guide

by schmidy last modified Oct 04, 2013 02:11 PM

 

 

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INTRODUCTION

The laboratory chemical hood is often the primary control device when using flammable and toxic chemicals in the laboratory. It is vital that lab personnel understand how chemical hoods work so they can use them properly and avoid exposure to hazardous chemicals.

Throughout this document we have strived to use only the modern terminology “chemical hood” as opposed to the outdated though more familiar “fume hood.” However, some of the illustrations may contain the older terminology.

New laboratory workers and even experienced personnel will benefit from a thorough understanding of these important control devices. Any questions about chemical hoods can be directed to the Lab Safety Coordinator.

 

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SAFE OPERATION OF CHEMICAL HOODS

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Confirm that the hood is operational. If fitted with a local on/off switch, make sure the switch is in the "on" position; check the airflow gauge if so equipped. In the absence of a gauge, observe the plastic "flow check ribbon" taped to the lower corner of the sash. Airflow can be visually assessed by noting that the ribbon is pulled gently into the hood. The most recent hood test data and optimum sash height are indicated on the yellow label affixed to the hood face. Never work with a malfunctioning hood; report problem hoods to Physical Plant Work Control. Advise DEHS of chemical hoods that malfunction repeatedly.

Maintain operations at least 6" inside the hood face. Vinyl tape can be attached to the work surface to serve as a visual reminder.

Lower sash to optimum height. Optimum height is the sash height at which airflow is maximized without creating turbulence, generally 100 feet per minute. A yellow label placed on the hood face indicates the most recently recommended sash height. Exception -- variable volume exhaust hoods maintain 100 fpm at any position at or below the sash stop. With unattended or potentially explosive processes, conduct the operation behind a lowered sash or safety shield.

Keep head out of hood except when installing and dismantling equipment.

Keep hood storage to an absolute minimum. Keep only items needed for the ongoing operation inside the hood. Keep the back bottom slot clear at all times as it serves as an exhaust port for chemicals generated near the work surface. Raise large objects at least two inches off the hood surface to minimize air flow disruption.

Minimize foot traffic around the chemical hood. A person walking past a chemical hood can create competing currents at the hood face, causing vapors to flow out. Other sources of competing air currents such as open windows and fans must also be avoided while using a chemical hood.

Use extreme caution with ignition sources inside a chemical hood. Ignition sources such as electrical connections, Variac controllers and open flame can be used inside a chemical hood as long as there are no operations involving flammable or explosive vapors. If possible, ignition sources should remain outside the hood at all times.

Replace hood components prior to use. Every component of a chemical hood, whether airfoil, baffle, utility panel or sash, plays a vital role in preventing the escape of hazardous materials from the hood. Any hood components removed to conduct maintenance or repair activities, or to set up experimental apparatus must be replaced prior to using the hood for contaminant control.

 

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THE BASICS OF LABORATORY CHEMICAL HOODS

A laboratory chemical hood is a partially enclosed workspace that is exhausted to the outside of the building. When used properly, hazardous gases and vapors generated inside the hood are captured before they enter the breathing zone. This serves to minimize your exposure to airborne contaminants. The typical components of a chemical hood and their major functions are defined below.

 

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Hood Body -- The visible part of the chemical hood that serves to contain hazardous gases and vapors

Baffles -- Moveable partitions used to create slotted openings along the back of the hood body. Baffles keep the airflow uniform across the hood opening, thus eliminating dead spots and optimizing capture efficiency.

Sash - The sliding "door" to the hood. By using the sash to adjust the front opening, airflow across the hood can be adjusted to the point where capture of contaminants is maximized. Each hood is marked with the optimum sash configuration. The sash should be held in this position when working in the hood and closed completely when the hood is not in use. The sash may be temporarily raised above this position to set up equipment, but must be returned to the optimum sash height setting prior to generating contaminants inside the hood.

Airfoil - Located along the bottom and side edges the airfoil streamlines airflow into the hood, preventing the creation of turbulent eddies that can carry vapors out of the hood. The space below the bottom airfoil provides source of room air for the hood to exhaust when the sash is fully closed. Removing the airfoil can cause turbulence and loss of containment.

Work surface -- Generally a laboratory bench top, but also the floor of a floor-mounted hood, this is the area under the hood where apparatus is placed for use.

Exhaust plenum -- An important engineering feature, the exhaust plenum helps to distribute airflow evenly across the hood face. Materials such as paper towels drawn into the plenum can create turbulence in this part of the hood, resulting in areas of poor airflow and uneven performance.

Face -- The imaginary plane running between the bottom of the sash to the work surface. Hood face velocity is measured across this plane.

 

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USES OF CHEMICAL HOODS

Chemical hoods are approved for three general types of uses: General Purpose, Radioisotope, and Perchloric Acid. Hoods approved for each of these uses will appear alike but require different functional and operating parameters.

  • General-purpose hoods, the most common use type, are used to prevent exposure to toxic, irritating, or noxious chemical vapors and gases. A face velocity of 100 feet per minute (fpm) provides efficient vapor capture while reducing hood turbulence.

  • Radioisotope hood systems are ideally made from welded stainless steel to ensure against absorption of radioactive materials. In order to comply with licensing requirements, iodinations using radioactive sodium iodide (I-125) must be performed in a hood with a minimum face velocity of 125 fpm. The yellow label on the hood will indicate the hood is "Approved for Iodine-125" with a minimum face velocity of 125 fpm. This higher face velocity is not required for use of RIA kits with low activity.

  • Perchloric acid hoods have wash-down capabilities to prevent the buildup of explosive perchlorate salts within the exhaust system. Researchers heating perchloric acid must use a perchloric acid hood. At this time there is only one perchloric acid hood at U of L, located in the laboratory of Dr. Teresa Fan, room 336 of the Belknap Research Building. Contact the Lab Safety Coordinator, 852-2830 if you have an experiment or procedure that uses heated perchloric acid.

  • Wet benches with Laminar Flow for Semiconductor Applications - (reserved)

 

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DESIGN STYLES OF CHEMICAL HOODS

Variable Air Volume Hoods - Variable air volume (VAV) hoods are the most sophisticated of the hood types, requiring technically proficient design, installation and maintenance. The primary characteristic of VAV hoods is their ability to maintain a constant face velocity as sash height changes. As the sash is moved the exhaust volume is adjusted so that the average face velocity is maintained within acceptable parameters. It is best to use this type of hood with the sash positioned at the sash stop (half-open position), as this provides more even airflow, and a degree of face protection in case of an unexpected spill, fire or explosion in the hood. When not in use the sash should be closed to save energy. Currently the chemical hoods in the Baxter Biomedical Research Buildings, the Cardiovascular Innovation Institiute and most of the chemical hoods in the Belknap Research Building are VAV hoods.

Constant Volume Hoods - With constant volume (CV) hoods the volume of airflow into the hood remains more or less constant. Unlike VAV hoods, the velocity of the airflow entering a CV hood increases as the sash is closed. Proper positioning of the sash is vital to maintaining the optimum face velocity (100 or 125 feet per minute). Raising the sash too high lowers face velocity, allowing contaminants to escape from the hood. Setting the sash too low will result in high face velocities. Face velocities in excess of 200 feet per minute cause excessive turbulence and loss of containment. The black arrow on the yellow certification label indicates the proper sash position for constant volume hoods.

  • Conventional Hoods - Conventional hoods represent the original and most simple of the hood design styles. With a conventional hood the volume of air exhausted is constant, regardless of sash height. As the sash is lowered the opening area decreases, resulting in an increase in face velocity. Since face the sash at its velocity changes linearly with sash position it is particularly important when working with conventional hoods to maintain optimal height as indicated by the yellow label attached to the hood frame. (Figure 3 below) 

  • Bypass Hoods -- Bypass hoods have an added engineering feature and are considered a step up from conventional hoods. An air bypass incorporated above the sash provides an additional source of room air when the sash is closed. As the sash is lowered the bypass area becomes exposed, effectively increasing the face opening and reducing the rate of increase of the face velocity. This reduces the chance for turbulence and loss of containment as the sash is lowered. However, it remains important to utilize the optimum sash height as indicated on the yellow label attached to the hood frame.

  • Auxiliary Air Hoods -

    With this type of hood a dedicated duct supplies outside air to the face of a bypass hood. The main advantage of an auxiliary air hood is the energy savings realized by reducing the amount of heated or air-conditioned room air exhausted by the hood. While energy savings can be substantial, the unconditioned airflow can cause discomfort for those working near the hood. It is important to keep in mind, however, that the auxiliary air supply is necessary for proper functioning of the hood. Any alteration of the air supply system, such as sealing off the auxiliary air duct, will adversely affect hood operation and may result in hazardous chemical exposures. If the sash of an auxiliary air hood is kept closed most of the unconditioned air will bypass through the hood, reducing its effect on room temperature and humidity. Remember to check the optimum sash height since it will affect face velocity in a manner similar to that for bypass hoods. (Figure 4 below).

    While energy savings can be substantial, the unconditioned air flow can cause discomfort for those working near the hood. It is important to keep in mind, however, that the auxiliary air supply is necessary for proper functioning of the hood. Any alteration of the air supply system such as sealing off the auxiliary air duct will adversely affect hood operation and may result in hazardous chemical exposures. If the sash of an auxiliary air hood is kept closed most of the unconditioned air will bypass through the hood, reducing its effect on room temperature and humidity. Remember to check the optimum sash height since it will affect face velocity in a manner similar to that for bypass hoods.

  • Perchloric Acid Hoods - When heated above ambient temperature, perchloric acid will vaporize and may condense on hood, duct and fan components. In addition to being highly corrosive, condensed vapors can react with hood gaskets, greases and other collected materials to form explosive perchloric salts and esters. A perchloric acid hood is built with welded stainless steel hood surfaces, ductwork, and fan to minimize the corrosive and reactive effects. More importantly, perchloric acid hoods have a wash-down system of water fog nozzles dispersed throughout the hood and exhaust system. By washing down the hood following each use of heated perchloric acid, any materials deposited within the system are removed, preventing the buildup of hazardous perchlorates. At this time there is only one perchloric acid hood at U of L, located in the laboratory of Dr. Teresa Fan, room 336 of the Belknap Research Building.

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MONITORING HOOD FUNCTION

The ultimate goal of chemical hood use is to contain the contaminants generated within the hood. While direct measurement of containment is difficult and costly, there exists a surrogate measurement for hood performance that is simple and straightforward -- hood face velocity. A face velocity of 100 fpm with a minimum of turbulence is generally considered a good compromise between competing air forces and the creation of turbulent eddies. With the exception of VAV hoods, face velocity is strongly dependent on sash height. It is therefore important when using the hood to position the sash at the height indicated by the arrow on the yellow hood certification label. When not in use the sash should be closed completely. This will reduce noise levels and ease the load on the heating or air conditioning system.

The face velocity of each chemical hood is tested annually by DEHS's Industrial Hygiene staff. Hoods approved for Iodine-125 are tested every six months. The yellow certification label affixed on the left side of the hood indicates the most recently measured hood face velocity and the date by which the hood should be retested. Contact the Lab Safety Coordinator or 852-2830 if your hood has not been retested by the expiration date.

This face velocity information represents airflow results at the time of the test. Hood function can change from one moment to the next due to system irregularities such as hood storage and use, broken belts, electrical malfunctions, or maintenance activities. For this reason it is important that you always verify air flow prior to conducting procedures inside the hood. This can be done by checking the airflow gauge on the hood, if so equipped. In the absence of a gauge, DEHS has taped a lightweight plastic ribbon to the lower corner of the sash. Airflow can be visually assessed by noting that the ribbon is pulled gently into the hood. If you suspect a chemical hood is not working contact the Physical Plant Work Control.

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TO REPORT A MALFUNCTIONING HOOD

  • Belknap Campus- Call Physical Plant Work Control, 852-6241
  • Health Science Campus- Call HSC Physical Plant, 852-5695
  • If a fume hood repeatedly malfunctions or cannot be repaired, contact the Lab Safety Coordinator at 852-2830.

 

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COMMON CHEMICAL HOOD MISCONCEPTIONS

Myth - With an auxiliary air hood lab temperature problems can be remedied by covering the supply air duct.

While this might provide marginal temperature control, it will cause a stream of air to be forced down the face of the hood that will actually draw contaminants out of the hood. For hoods with unconditioned make up air the best solution is to keep the sash closed whenever the hood is not being used. With the sash closed all of the unconditioned air is exhausted by the chemical hood. With the sash open some unconditioned air will escape into the room.

 

Myth - When working with highly hazardous materials, the higher the face velocity the better

While it is important to have a face velocity between 80 and 120 feet per minute (fpm), velocities higher than this are actually harmful. Excessive face velocity causes turbulence and eddy currents are which allows contaminants to be drawn out of the hood, increasing worker exposures.

 

Myth - A chemical hood can be used for storage of volatile, flammable, or odiferous materials when an appropriate storage cabinet is not available.

While it is appropriate to keep chemicals that are being used during a particular experiment inside the chemical hood, hoods are not designed for permanent chemical storage. Each item placed on the work surface interferes with the directional airflow, causing turbulence and eddy currents that allow contaminants to be drawn out of the hood. Even with highly volatile materials, as long as a container is properly capped evaporation will not add significantly to worker exposures. Unlike a chemical hood, flammable materials storage cabinets provide additional protection in the event of a fire.

 

Myth - The airfoil on the front of a hood is of minor importance. It can safely be removed if it interferes with my experimental apparatus.

Airfoils are important to efficient operation of a chemical hood. With the sash open an airfoil smoothes flow over the hood edges. Without an airfoil eddy currents form, causing contaminates to be drawn out of the hood. With the sash closed, the opening beneath the bottom airfoil provides for a source of exhaust air.

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BIOSAFETY CABINETS, TISSUE CULTURE HOODS AND CLEAN BENCHES

Although chemical hoods, biosafety cabinets, tissue culture hoods and clean benches can look similar, they have very different uses.

  • A chemical hood is designed to contain hazardous vapors and gases and exhaust them outside the building.

  • A clean bench or tissue culture hood is designed to protect biological specimens by bathing the work area with a laminar flow of air free of particulate contamination. Because these devices force air out across the work surface and toward the worker they protect only the specimen, not the user.

  • A biosafety cabinet provides biological protection for both specimen and user. A laminar flow of HEPA-filtered air is passed down from the top of the hood and across the work surface, and is exhausted or recirculated without entering a worker's breathing zone. The air is then re-filtered before being exhausted, usually back into the laboratory. This filtration only removes particulates and aerosols, not gases and vapors. Because all clean benches and most biological safety cabinets exhaust air back into the work area, they cannot safely be used with hazardous gases and vapors. Only Class II Type B2 (total exhaust) biosafety cabinets can be used with significant quantities of volatile hazardous chemicals.

  • For more information about biosafety cabinets, see Biological Safety.


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