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Keeping The Flow: Getting The Most From Your Chemical Fume Hood

Thu, 02/17/2011 - 8:43pm
Vince McLeod, CIH

Chemical fume hoods are one of the most important fixtures in a forensic laboratory.One focus of this issue of Forensic Magazine is laboratory equipment. And one of the most important fixtures in any laboratory, including a well-outfitted forensic lab, is the chemical fume hood. Fume hoods are available in many different designs, such as constant air volume, variable air volume, ductless bench top hoods, and auxiliary or supply air hoods. Regardless of the specific type selected, there are a few design parameters and basic operational principles that should be used across the board. Chemical fume hoods are for working with gases, acid fumes, and chemical vapors.

Chemical fume hoods are designed to capture and exhaust contaminates resulting from working with chemicals. They are sometimes referred to as wet benches as the chemicals used (solvents, corrosives, etc.) are usually liquid. Fume hood design endeavors to create a capture zone in front of the hood to draw contaminates away from the worker and into the hood where they are exhausted. This column provides guidance on design features and proper operation.

Laboratory Ventilation Design Considerations
Before we get to the specifics of fume hoods there are a few basic ventilation rules that should be incorporated into all labs where potentially toxic or hazardous chemicals are used. First is to provide adequate general ventilation. This means a non-recirculating supply with exhaust to the outside.1,3 This general supply is not to be relied on for protection from exposures but ensures a continual supply of fresh outside air to prevent the buildup of contaminate concentrations. One recommendation for general ventilation is to provide at least six air changes per hour for your chemical laboratory.1 This means that six times the total volume of air in the lab is supplied every hour.

Next, laboratories where chemicals are used must be under negative pressure in relation to the adjacent non-laboratory rooms, offices, and corridors.2 In the event of a spill or release the contaminates will be kept in the lab and exhausted rather than spread into surrounding areas. You generally want the air to flow from low hazard areas to high hazard areas. Proper pressurization is maintained by balancing the ventilation system. A good rule of thumb is to provide make up air equal to 90% of exhaust volume.1

One final recommendation is to make sure cabinets and equipment do not block or interfere with supply or exhaust vents. Too often we inspect labs to find things stored on top of cabinets or in front of vents, completely disrupting the air flow.

Fume Hood Design Principles
Basic principles of aerodynamics are used to promote a smooth flow of air into the hood. This is termed laminar flow, and the sides and the sill (bottom edge) of the hood are shaped like a foil to allow the airstream into the hood with a minimum of turbulence. In addition, the sill is also raised slightly off the bottom or floor of the hood to create flow across this surface.

Since undisturbed entry is key for hood performance, placement of the hood in the laboratory should receive attention. Do not locate fume hoods near doors, busy walkways, or room air supply and exhaust vents. Try to keep room air currents to less than 20% of the hood face velocity by separating the hood from supply and exhaust locations to the extent practical.1 Air turbulence, especially near the hood entry can affect its ability to contain and exhaust contaminates.

The next most important design parameter for good hood performance is face velocity, or the speed that air flow enters the hood. Air velocity needs to be just right. If it is too slow contaminates will not be contained or exhausted out. Too fast and turbulence and eddies can lead to slipstreaming and dumping contaminates into the laboratory. Face velocity is a function of the total exhausted volume and the area of the opening. The basic equation from physics is that velocity is equal to the volume divided by the area. So if the area increases, for example by opening the hood sash, the velocity drops. Conversely, if the area is reduced by closing the sash the face velocity increases for constant volume hoods. In addition, since the velocity is directly related to the exhaust volume, if the volume is reduced the velocity does likewise. Keep in mind however that different hood designs use this principle differently so bypass hoods, auxiliary air hoods, and VAV (variable air volume) hoods operate slightly differently. Full descriptions of these new hood types are beyond the scope of this column but may appear in future articles.

The third major design feature is the baffling or guiding of the hood flow. Chemical fume hoods are designed to handle a wide variety of operations and contaminates. Typically this is done with a series of baffles on the back wall and or top of the hood. These are slots with adjustable sliding covers, usually located near the bottom, center, and top of the hood’s back panel. By opening and closing the appropriate baffles more flow can be guided across the bottom, thru the center, or toward the top of the hood.

Maintain Performance with Proper Operation
Commonly referred to as the OSHA Lab Standard, the OSHA standard for Occupational Exposure to Hazardous Chemicals in Laboratories, 29CFR1910.1450, does not specify procedures for safe hood operation, exhaust volumes, or face velocities.4 Basically, it requires that a chemical hygiene plan be prepared for every covered laboratory and provides the items that a complete CHP must contain. Regarding fume hoods it states, “fume hoods and other protective equipment are functioning properly and specific measures shall be taken to ensure proper and adequate performance of such equipment.” In addition, the non-mandatory Appendix A contains this statement: “airflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate (typically 60-100 lfm).”3

As the operator, the lab worker must know how to adjust flows for his or her particular need. Where is capture needed for the particular experiment or task being conducted? Are you working with vapors that are lighter than air or heavier? If they are heavier than air then the dampers should be adjusted to capture at the bottom of the hood (e.g. open the bottom slot and close down the upper one). Second, check to see if storage is blocking the lower slot that may hinder flow and thus prevent proper capture. Although we do not recommend storing chemicals in the hood, one quick fix is to install a shelf above the lower baffle so that reagents and chemicals stored on the shelf do not block the lower slot. A final check for dead spots in the face velocity and hood flow is highly recommended.We recommend face velocity be checked using a grid pattern and that readings not differ by more than 10% between points. Alternately, air current or smoke tubes could be used to detect dead or low flow zones.

These chemical fume hood basics should get you started. Pay attention to proper flow and remember to adjust the baffles according to the work being done. Finally, routinely check the hood for adequate flow and velocity and recheck if you suspect a problem.

References

  1. Stanford Laboratory Standard and Design Guide, Stanford University, Environmental Health and Safety. 2006 http://www.stanford.edu/dept/EHS/prod/mainrencon/Labdesign.html
  2. Laboratory Ventilation Z9.5-2003, American National Standards Institute. New York. 2003 http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI/AIHA%20Z9.5-2003
  3. National Research Council Recommendations Concerning Chemical Hygiene in Laboratories, CFR 1910.1450 Appendix A, Occupational Safety and Health Administration. Washington D.C. 2003 sha.gov/pls/oshaweb/ owadisp.show_document?p_table=STANDARDS&p_id=10107
  4. Occupational Exposure to Hazardous Chemicals in Laboratories, Occupational Safety and Health Administration, US Department of Labor. Washington, D.C. 2006 http://www.osha.gov/pls/oshaweb/owadisp.show_document? p_table=STANDARDS&p_id=10106


Vince McLeod is an American Board of Industrial Hygiene Certified Industrial Hygienist and the senior IH with the University of Florida’s Environmental Health and Safety Division. He has 22 years of experience in all facets of occupational health and safety and specializes in conducting exposure assessments and health hazard evaluations.

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