4 Advice to Choose a Laboratory Fume Hood

The fume hood is the best-known local exhaust device used in laboratories. Fume hoods remove hazardous dusts, fumes, gases, and vapors at their point of generation. They are one of the most reliable engineering controls in a laboratory. When properly installed and maintained, a well-designed hood can offer a substantial degree of protection to the user, provided that it is used appropriately and its limitations are understood.

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Fume hoods are often regarded strictly as local exhaust ventilation devices to prevent toxic, hazardous, or offensive chemicals from entering the general laboratory atmosphere. However, hoods offer another significant type of protection by providing an effective containment device. When a chemical manipulation or reaction is performed within a hood (with the sash nearly or fully closed), a physical barrier is created between the workers in the laboratory and the operations inside the hood. Workers, therefore, are not only protected from exposure to harmful air contaminants, but injury will be minimized from splashes, spills, fires, and minor explosions that may occur inside the hood. 

The interior fume hood surface should be constructed of durable, corrosion-resistant, nonporous, noncombustible, and fire-resistant materials such as stainless steel, a unique composite, or polymer material. Corrosive materials can damage many types of materials, shortening fume hood life. In addition, some materials, when exposed to direct flame, emit noxious and toxic fumes. 

Fume hood performance

Fume hoods should be evaluated for performance when installed and before use to ensure adequate face velocities and the absence of excessive turbulence. Performance should be assessed against the design specifications for uniform airflow across the hood face and the total exhaust air volume. Equally important is the evaluation of operator exposure.

Successful hood performance depends on the speed (face velocity) of the air entering the fume hood’s front (sash opening). The hood’s face velocity can be significantly affected by cross-drafts created by the movement of people walking by or even the user’s presence in front of the hood and air currents from open windows and doors. Other factors affecting hood performance include hood design, thermal loading, and the amount and location of equipment in the hood.  

Fume hood face velocity

In most fume hood installations, the exhaust flow rate or quantity of air pulled through the hood is constant. Therefore, the sash can be adjusted to obtain an optimal flow rate for a particular operation. For example, when the sash is lowered and the cross-sectional area of the hood opening decreases, the hood face velocity increases proportionally.

Successful hood performance depends on the speed (face velocity) of the air entering the fume hood's front (sash opening).

Achieving a hood face velocity in the range of 60-120 ft/min is the basis for the successful design of a laboratory fume hood. The face velocity, also known as “control velocity,” is the speed of air through the hood face, which is necessary to contain the contaminants captured by the fume hood, thereby preventing their dispersion into the workplace. 

Too low a face velocity and the hood will not provide adequate exposure control. Too high a face velocity will likely increase the turbulence within the hood and cause contaminants to escape into the laboratory. Therefore, the most meaningful method of evaluating hood performance and obtaining the optimum airflow rate is to measure worker exposure while the hood is being used for its intended purpose.

Make-up air

All local exhaust ventilation systems must have air to exhaust. Since fume hoods exhaust air from the rooms in which they are installed, an adequate supply of air must replenish the exhaust air. Otherwise, the hoods will not be able to exhaust a sufficient volume of air to function efficiently as intended, causing contaminants to escape into the laboratory. To ensure that fume hoods operate correctly, an additional supply of air, known as “make-up air,” is required.  

Work practices

Adequate information and training must be provided at the time of a laboratory worker’s initial assignment so they can safely use fume hoods and ventilation equipment to minimize emissions and employee exposures.

The following is a partial list of guidelines for safe fume hood use and should be followed when using one:

• Do not store chemicals or equipment (which are not being used) or waste in the hood.
• Chemical waste should not be disposed of by evaporation in a hood.
• Keep your head outside the fume hood. Do not walk into a “walk-in” hood when it is operating.
• Use the fume hood with the sash as low as possible, at or below the indicated operating height.
• Periodically check the airflow through the hood face.
• Do not block the rear hood exhaust slots with equipment or materials. 
• Keep combustibles, such as paper towels, out of the hood. Paper items may also become drawn into the hood exhaust system, blocking or restricting airflow.
• Never use perchloric acid in a fume hood not explicitly designed for this purpose.

The OSHA Laboratory Standard, 29 CFR ., Appendix A, Section A. 4., expands on these guidelines. It lists general precautions and engineering controls for handling all laboratory chemicals in a fume hood, thus minimizing the risks from known and unknown hazardous substances.

1) Combine with Corrosive Fume Vent Duct

Lab fume hoods are designed to contain and minimize exposure to hazardous airborne substances. They draw air away from lab work stations via built-in or remote blowers depending on the size and needs of the system.

For optimal safety against hazardous and highly corrosive vapors, ducted lab fume hood systems should be used to expel contaminated air completely from the facility, or to special filtration units like wet scrubbers via corrosive fume exhaust duct.

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2) Position Fume Hoods Wisely

In order for a lab fume hood to perform to its fullest potential, it's imperative that lab engineers, lab designs, and lab managers account for surrounding airflow variables that may negatively impact the efficiency of the system.

Tracer gas testing has shown that the presence of rogue drafts can interfere with the laminar flow of air entering a fume hood system. To minimize the potential for turbulence, it helps to be aware of common sources of competing air currents, and to plan accordingly when positioning or locating fume hoods in a lab design. Such considerations may include:

• Proximity to Traffic:  The movement of a single person through a room can create columns of counter-rotating air at velocities up to 250 fpm behind the person. These air columns can be strong enough to overcome the face velocity of a fume hood system and may draw contaminated air into the surrounding environment. As such, hoods should be placed in low or no-traffic areas. Otherwise, passing by a hood should be avoided when the sash is open and when experiments are underway.

• Proximity to Supply Air Diffusers:  Supply air diffusers are typically located on the ceiling to assist with broader airflow patterns within the lab space. Face velocities of these diffusers can reach 800 fpm. If the diffuser currents are unable to fade to 30-50% of the face velocity of nearby fume hoods, then the competing flow of air will cause turbulence around the hood opening. Moving the diffuser, using a different type, or rebalancing the air volumes between diffusers can mitigate this problem.

• Proximity to Windows and Doors:  Open or drafty windows in a lab space can also be an unwanted source of interference with fume hood flows and containment. Similarly, as with high traffic areas, the presence and location of doorways should also be considered, since these will also contribute to turbulence.

• Orientation of the Face:  If the face of the hood is oriented toward workstations, benches, viewing stations, computer monitors, or any place that personnel are frequently stationed, turbulence could be created and directed at the hood. Conversely, if something goes wrong in the fume hood, it is possible that debris or hazardous material could exceed the draw of the exhaust system and damage whatever is opposite the fume hood opening.

3) Record and Monitor Average Face Velocities

The rate at which air is drawn into the hood through the ‘face’ of the unit is known as the average face velocity. It is helpful to measure and monitor this rate in order to ensure that air is drawn at a careful, yet sufficient rate that will not be impaired by disruptive sources like those mentioned above.

To measure face velocity, air speed is recorded at multiple locations across the plane of the fume hood entrance. These individual point velocities are then averaged.

Once this value is known, exhaust blowers can then be adjusted accordingly until the measured average face velocity is within the desired range. Average face velocity should be measured and recorded for every fume hood unit, and used as a baseline for future fine tuning and maintenance. 

Every lab space should define an acceptable average face velocity, minimum acceptable point velocity, and maximum standard deviation of velocities based on the processes occurring within that space.

4) Map Airflow and Test for Containment

After the average face velocity is established, it is important to test the containment ability of the fume hood for commissioning and/or compliance with ASHRAE/ANSI 110 standards. Testing containment can be done visually or quantitatively.

• To visually test for containment - harmless colored gas or smoke is released within the hood space and the hood's ability to contain and exhaust this gas can be observed once the hood is turned on.

• For a quantitative measurement - tracer gases are placed within the operating fume hood. After a set duration of time, the level and concentration of these tracer gases is measured both within the fume hood and the surrounding environment, producing a quantitative value of the fume hood's containment ability.

Based on the results of either test, modifications to the fume hood face velocity can be made if necessary.

In Summary

It should go without saying that lab fume hood safety and performance can be a key indicator of the safety and performance of your entire lab facility. 

With so many variables to consider on the front end of the fume hood, like making sure average face velocities are properly balanced, containment is in check, and equipment is positioned in harmony with the layout of the lab, wouldn't it be nice to rest easy knowing that the fume hood exhaust is being safely and reliably carried away by the most capable fume exhaust duct system available?

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