Choosing the Right Fume Hood

Once you have your system in place, it is critical to test and monitor it.

IN any work environment, employers need to consider environmental controls before ever putting an employee in personal protective equipment, such as a respirator, to protect them from dangerous contaminants. A fume extraction system should be one of the first steps to minimize employees' exposure of chemicals and particulates. However, choosing a fume hood is more complicated than just selecting a product out of a catalog.

To start, a thorough hazard analysis needs to be done to determine exactly which chemicals or contaminants need to be removed. This involves consulting Material Safety Data Sheets (MSDSs) to determine exactly which chemicals are present in the workplace. Then, you should do air monitoring to determine the concentrations of the contaminants to which workers are exposed. There are detector tubes, monitoring badges, and digital equipment that can help determine your concentration levels, or you can contract a local industrial hygienist to perform the monitoring. Once you have all of the pertinent data, you can begin to look at the different types of fume extraction systems that may be appropriate.

Ducted Hoods
Ducted hoods are ventilated enclosures that have ducting to connect to the outside so contaminants are pulled out of the building. This type of system is considered the safest for the worker and the easiest to maintain for the employer. Ducted hoods usually consist of a base, a work surface, the hood itself, a blower, and the ducting necessary to get the air outside.

It is important for an engineer or HVAC (heating, ventilation, and air-conditioning) specialist to determine whether your room or area has enough air to "feed" the hood's needs. If the room's air supply is insufficient to provide the necessary volume of air to the hood, the room might create a suction that can pull shut doors that open out of the room or slam open doors that open into the room. Lack of air also puts a drain on heating or air-conditioning systems. In those instances, HVAC specialists might need to work with you to provide a source of "make-up" air to keep the room's air supply balanced. This can require an additional blower system to pump air into the room before it can be pulled into the fume hood.

One category of ducted fume hoods is canopy hoods. This hood does not have an enclosure for the worker. It is either hung from the ceiling or mounted to a wall. These are designed to remove steam, heat, or odors from large pieces of equipment such as tanks or ovens or over processes that might be a nuisance to the worker but are not hazardous. Because these hoods are not enclosed, they pull in massive amounts of air and are very inefficient. The contaminants are not contained, so employees walking by, or shifts in room currents, could expose workers to hazards. Because of these concerns, canopy hoods should be used in very limited applications and never for hazardous or dangerous substances.

Two other major categories of ducted fume hoods are conventional and by-pass hoods. Again, ducted hoods must pull a constant volume of air into the system. In a conventional hood, the air enters the hoods strictly through the sash opening. The disadvantage to this system is that as the sash is lowered, the hood must maintain its constant volume of air supply--and the only way to maintain volume while decreasing the entry area available is to increase air velocity. This can create currents or high-speed winds inside the hood, which can damage equipment, create turbulence, or affect processes running inside the hood (e.g., contaminating samples or cooling down heating instruments). In by-pass hoods, however, the lowered sash opens other air-flow options to the hood, so the velocity is fairly steady at the hood opening. This creates a safer, more consistent working environment.

Hood Materials
Another consideration with fume hoods is the material of construction. Epoxy coated steel hoods have high heat resistance and offer fair compatibility, but the steel can corrode quickly if the epoxy surface is damaged. Fiberglass hoods have good heat resistance, offer high chemical resistance, and are a better choice when working with strong acids, solvents, or bases. Poly resins offer excellent chemical compatibility but poor heat resistance.

Most hoods are sold without blowers, allowing your specialist to match your engineering needs with the correct size and material of construction your blower requires. However, there are hoods packaged with blowers. These hoods usually list the "equivalent duct run" that the blower can handle and still provide enough pull to allow the hood to work adequately. Equivalent duct run takes into consideration the additional resistance generated by elbows and reducers and other in-line items that can reduce air flow and gives those variables a numerical value equal to length of ducting in feet. For instance, a 90-degree elbow in a 6-inch diameter equals about 12 equivalent duct feet. So when choosing a ducted fume hood that already has the blower attached, you need to make sure the blower is going to be appropriate for your engineering and facility needs.

Ducted Extraction Arms
Ducted arms offer the same advantages of ducted hoods in regard to air removal, but they are most appropriate when dealing with specific areas where fumes need to be exhausted, such as over a hot plate or when working with adhesives or soldering applications. Ducted arms can require as little as 75 to 100 CFM (cubic feet per minute) of air compared to more than 700 CFM for a standard 4-foot ducted hood.

They can be table-, wall-, or ceiling-mounted and provide a working radius so the arm can be repositioned over the work area. Generally, ducted arms are sold by component. The arm, mounting hardware, hoods, and blowers are sold separately to give you the versatility to meet your application.

Sizing Blowers
Some hoods are sold with the blowers attached, while some are sold without blowers. It is important to consult with an HVAC specialist or an industrial hygienist to determine what size blower is appropriate based upon the size of hood opening, length of ducting, twists and turns the ducting must make to get to the outside, type of ducting material, diameter of ducting, and in-line filters that may be present. Because of these variables, facilities generally require an on-site consultation with a specialist to properly size a blower.

Ductless Hoods
A ductless fume hood uses filters to remove particulates, such as toxic metals or biological toxins (including spores or bacteria), or vapors, such as acids or organics, from the work area. The filtered air is then exhausted back into the work environment. The advantages of ductless systems are that they do not require expensive installation or duct work and may be placed at an existing work area or on a rolling cart, which can allow sharing between laboratories and brings down the overall cost of the unit. They also save money on energy consumption because of air recirculation and the elimination of make-up air that is required for traditional ducted hoods.

* Particulate filtration. A ductless fume hood can be an excellent choice for removing particulates from the workplace. When you are working with toxins such as heavy metals and biologicals, releasing them to the external environment can pose dangers to the public and violate EPA regulations regarding emissions. Determining when to change the filter simply requires the use of air flow monitors at the face of the hood. Once air flow drops below your acceptable levels, you would want to change the filter to remain protected.

* Vapor removal. A vapor adsorbing ductless fume hood is a self-contained enclosure that uses carbon molecular filters to trap toxic gases and odors from acids, organics, aldehydes, and amines. However, there are many limitations to a vapor adsorbing ductless fume hood. The carbon-based filters used to remove the harmful gases and odors are processed or treated to adsorb specific types of chemicals at low evaporation rates. Because their affinity for various chemicals and adsorption rates varies, ductless fume hoods are fairly chemically specific and are not recommended for applications involving a broad range of chemicals or forced evaporation. Also, knowing when to change the filters in a vapor-removal ductless system can be difficult. Air monitoring is necessary to check saturation levels and ensure safe air quality. Because of the limitations, vapor-adsorbing ductless fume hoods should be considered only if a ducted system isn't viable.

Cabinets, Stands, and Work Surfaces
When choosing a ducted or ductless system, it is important to make sure you get all of the necessary components. Cabinets are sold separately, as are the work surfaces on which the hoods sit. Cabinets are available for regular storage or for meeting special requirements, such as storing flammables, acids, or bases.

If storage isn't necessary, simple base stands are available to elevate and support the hood and work surface. Work surfaces can be purchased with or without spill containment for extra protection.

Testing Equipment
Once you have your hood in place, it is critical to test and monitor your system. Common test agents for fume hoods provide a visible smoke that allows you to test the hood seams and air flow. The smoke is applied from smoke matches, cartridges, generators, or smoke bottles; if it enters the hood in any spot other than the sash, the fume hood would need repair or sealant. Monitors are placed at the front of the hood and monitor the face velocity, or the speed of air at the hood opening. Digital monitors can be set to alarm and provide a visual warning at user-programmed face velocities that are set to meet their requirements. Analog monitors can give users a visual indication of a drop in face velocity.

Where to Go for Help
Starting to create a fume reduction system can seem overwhelming. For most people, calling in a specialist is the best way to make sure the money you are spending on fume removal products is invested wisely.

HVAC specialists can be found in the phone book. Find out whether they have fume removal expertise before bringing them on site. One of the advantages of consulting with an HVAC specialist is if there are additional HVAC concerns your system might create, such as heating and air-conditioning costs skyrocketing because of make-up air or having to reroute existing ducting to make room for the fume ducting, they can give you that information up front and suggest solutions to the potential drawbacks.

Industrial hygienists are another source. Some focus on ventilation issues. The American Industrial Hygiene Association's Web site includes a consultant search page (www.aiha.org/Content/AccessInfo/consult/consultlisting.htm). Simply select your region and the specialty of "ventilation/engineering," and a list of consultants and their contacts are available to you. Advantages to working with an industrial hygienist include having them assist you from the start of your air monitoring program all the way through the implementation and monitoring of your fume extraction system. They also generally are very aware of all the pertinent regulations that may apply in your situation, which may include those of OSHA, NIOSH, or the American National Standards Institute.

This article appeared in the May 2006 issue of Occupational Health & Safety.

This article originally appeared in the May 2006 issue of Occupational Health & Safety.

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