Environmental Safety

Environmental Safety

Chemical Fume Hoods

A chemical fume hood is a local exhaust unit with rear and side walls designed to effectively control atmospheric contamination (hazardous gases, vapors and/or fumes) at its source. The chemical fume hood depends upon the creation of air flow past the source of contaminant sufficient to remove the highly contaminated air around the source or issuing from the source and to draw the air into an exhaust system vented directly to the outdoors. It should have an average linear face velocity of 80 to 120 linear feet per minute (lfpm); hoods used for radioactive materials activities must have a minimum of 125 lfpm.

A chemical fume hood is a partially enclosed work space that is exhausted to the outside. 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 common parts of a fume hood and their major functions are:

standard fune hood image

Hood Body -- The visible part of the fume 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 -- By using the sash to adjust the front opening, air flow 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 work involving the fume hood is being performed and closed completely when the hood is not in use.

Airfoil -- Found along the bottom and side edges airfoils streamline air flow 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.

Work surface -- Generally a laboratory bench top, but also the floor of a walk-in 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 air flow 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 air flow 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.

Chemical fume 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 certification label on the hood will indicate the hood is "Approved for Iodine-125" with a minimum face velocity of 125 fpm.

fume hood test label image

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 are no perchloric acid hoods at Clemson University. Please contact Robin Newberry at Environmental_Safety@clemson.edu if you have an experiment protocol that uses heated perchloric acid.

Hoods are checked annually by EHS and labeled for approved use. The arrow on the certification label indicates the proper sash position for constant volume 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 velocity changes dramatically with sash position it is particularly important when working with conventional hoods to maintain the sash at its optimal height as indicated by the yellow label attached to the hood frame. Optimal sash height represents the point where face velocity equals 100 fpm

standard hood image

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 dampening face velocity fluctuations. Because variations in face velocity still occur, it remains important to utilize the optimum sash height as indicated on the yellow label attached to the hood frame.

Auxiliary Air Hoods have a dedicated duct to supply 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.

auxiliary air system image

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 are specifically designed for work with perchloric acid. When heated above ambient temperature, perchloric acid will vaporize and 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, duct work, 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. There are currently no perchloric acid hoods with wash-down capabilities at the University.

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. Sash height is continuously monitored and exhaust volume adjusted so that the average face velocity is maintained within acceptable parameters. The kindest thing to be said about VAV systems is that they are a technology which is at its early stages of development. Currently they don't work very well, requiring lots of maintenance and careful design.

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

A Chemical Fume Hood is designed to contain hazardous vapors and gases and exhaust them outside the building.

A Clean Bench is designed to protect biological specimens by bathing the work area with air free of particulate contamination. Because a clean bench forces air out from the back of the hood, across the work surface and toward the worker it protects only the specimen, not the user.

A Biological Safety Cabinet (BSC) provides biological protection for both specimen and user. Particulate-free air is passed down from the top of the hood and across the work surface, and is captured before entering a worker’s breathing zone. The air is then re-filtered before being exhausted, usually back into the laboratory.

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.

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 air flow gauge if so equipped. In the absence of a gauge, observe the plastic "flow check ribbon" taped to the lower corner of the sash. Air flow 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 certification label affixed to the hood face. Never work with a malfunctioning hood; report problem hoods to University Facilities (656-2186).

Maintain operations at least 6" inside the hood face. Barricade tape should 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 air flow is maximized without creating turbulence, generally 100 feet per minute. A certification label placed on the hood face indicates the most recently recommended sash height. With unattended or potentially explosive processes, conduct the operation behind a lowered sash or safety shield.

Keep your 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 fumes 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 fume hood. A person walking past a fume 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 should also be avoided while using a fume hood.

Use extreme caution with ignition sources inside a fume hood. Ignition sources such as electrical connections, Variac controllers and open flame can be used inside a fume 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 fume hood, whether airfoil, baffle, 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.

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 fume hood. With the sash open some unconditioned air will escape into the room.

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

While it is important to have a face velocity between 80 and 120 feet per minute, velocities higher than this are actually harmful. When face velocity exceeds 125 feet per minute eddy currents are created which allow contaminants to be drawn out of the hood, increasing worker exposures.

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

While it is appropriate to keep chemicals that are being used during a particular experiment inside the fume hood, hoods are not designed for permanent chemical storage. Each item placed on the work surface interferes with the directional air flow, 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 fume 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 critical to efficient operation of a fume hood. With the sash open an airfoil smooths 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.

End Notes

This concludes the orientation to Chemical Fume Hoods. For additional information or training on Chemical Fume Hoods and their use, contact Robin Newberry at Environmental_Safety@clemson.edu .