Fume hood design and operating fundamentals

Common problems we see with hoods include improper installation and setup and its all-too-frequent use for storage, among others. So we thought a discussion on basic design principles, proper operation, and proper installation of the chemical fume hood was in order. To begin, let’s take a look at the design features and proper operational setup.

Chemical hood design—Flow is key

Chemical fume hoods are designed to capture and exhaust contaminants resulting from working with chemicals. They are meant to protect the worker by containing and removing harmful or toxic fumes, gases, vapors, or particulates. Their design attempts to create a capture zone in front of the hood and keep generated contaminants within the hood, where they are exhausted.

The typical chemical fume hood has an exhaust blower or fan mounted remotely so that laboratory room air is pulled into and through the hood, creating directional airflow into the hood. The “pull” at the hood opening is termed the “face velocity” and is usually measured in feet per minute. This is what hood flow refers to—the right face velocity. Proper face velocity of the hood is critical in maintaining protection for the worker. Too little velocity allows room air currents or disturbances to overpower the hood and draw contaminants into the room. Too much velocity can result in turbulence and eddies that also lead to contaminants escaping the hood.

Face velocity is a function of the total exhausted volume and the area of the opening. The basic relationship is: velocity is equal to the flow volume divided by the area. So as the area increases the velocity drops, and vice versa. Also, if the flow volume is reduced, the velocity goes down. The hood sash—the sliding door or window on the front of the hood—controls the area open for flow and thus controls the face velocity.

We frequently get asked to articulate the proper flow or correct face velocity. The Occupational Safety and Health Administration (OSHA) standard for Occupational Exposure to Hazardous Chemicals in Laboratories¹, 29CFR1910.1450, commonly referred to as the OSHA Lab Standard, contains this statement in the non-mandatory Appendix A: “Airflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate (typically 60-100 fpm).”² At the University of Florida, we recommend a face velocity of 100 fpm for general chemical work and 150 fpm for radioactive substances and highly hazardous or toxic chemicals. And the face velocity should never exceed 200 fpm to prevent the chance of eddying and turbulence.

Maintaining proper flow or face velocity requires keeping the working area unobstructed and setting the internal baffles to properly capture the contaminants produced. Let’s examine each of these a little closer.

Working area

This includes not only the internal area where chemicals and equipment are set up, but also the external area immediately in front of the hood. The external area will be discussed below, in the “installation tips” section. One key design feature of a chemical fume hood is the entry. Basic principles of aerodynamics are used to promote a smooth flow of air into the hood. The sides and the sill (the lower lip across the front) are shaped similar to the leading edge of an airplane wing, serving as a foil to guide the airstream into the hood with a minimum of turbulence. The idea is to maintain a laminar, nonturbulent flow. This design helps direct the airflow into the fume hood. The sill is also raised slightly off the bottom, or floor, of the hood to create an airstream across the hood’s floor. So, now we know why not to use the hood workspace for storage. And if our equipment rests on the hood floor, try to elevate any needed equipment slightly, especially if the contaminants produced are heavier than air.

Chemical fume hoods are designed to handle a wide variety of operations and contaminants. Baffles and other aerodynamically designed components guide the airflow into the hood. This is done with a series of baffles or slots on the back wall and/or top of the hood. These slots have adjustable sliding covers that are opened and/or closed to guide the flow across the bottom, into the center slot or toward the top of the hood. The slots are adjusted to direct more flow or scavenging, depending on the expected contaminants.

Installation tips for optimal performance

Since a smooth entry is vital, placement of the hood in the laboratory merits careful consideration. Layout of the laboratory and location of the chemical fume hood are very important for optimal performance and minimal interference. Locate fume hoods away from doorways and exits. The National Fire Protection Association recommends that they be ten feet from any door or exit. The reasons are obvious. The exit would be blocked should a fire or chemical release occur. And the constant traffic can potentially disturb the needed smooth flow into the hood. Therefore, to the extent possible, locate fume hoods away from high-traffic areas and any areas that produce air currents or potential turbulence, such as air supply diffusers, doors and windows, as these could affect the ability of the hood to capture and exhaust contaminants in the way they’re designed to.

Also, do not locate chemical fume hoods opposite workstations, desks, microscope benches or other areas where personnel spend significant time. As above, the reason should be obvious—any incident in the hood could involve or injure anyone seated in front of the hood.

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

In summary

It is up to the operator to know how to adjust flows for a particular need. Pay attention to proper flow and remember to adjust the baffles according to the work being done. Keep the following in mind when routinely checking the hood for adequate flow and velocity:

• Are there dead spots in the face velocity or inside the hood and are they located where capture is needed? We recommend face velocity be checked using a grid pattern with a minimum of six readings, and that readings not differ by more than ten percent. Alternately, air current or smoke tubes are useful in detecting dead or low-flow zones.
• Where is capture needed? Are you working with vapors that are lighter than air? Heavier? If they are heavier than air, then the dampers should be adjusted to capture at the bottom of the hood (i.e., open the bottom slot and close down the upper one). If storage or equipment is blocking the lower slot, this may hinder flow and thus proper capture. One quick fix is to install a shelf above the lower baffle so reagents and chemicals stored on the shelf do not block the lower slot. If the vapors are lighter than air, you may be okay with some storage in the hood. Use smoke tests to confirm this.

References

1. Occupational Exposure to Hazardous Chemicals in Laboratories, OSHA, Washington, D.C. 2006 - http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10106 
2. National Research Council recommendations concerning chemical hygiene in laboratories, OSHA Lab Standard, Appendix A (non-mandatory), OSHA, Washington, D.C. 2006 - http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10107