Chemical Fume Hood Design Pros and Cons

A while back, in response to a reader’s question regarding storage inside the exhaust cabinet, we wrote about the fundamentals of chemical fume hoods. In that article, we discussed the basic design principles and operation of chemical fume hoods. (If your memory is like ours and needs refreshing or you require another copy, just let us know.)

Since exhaust hoods are among the major expense items for research laboratories and have a huge impact on continuing operational costs, we’ve decided to provide you with information on some of the newer hood designs that offer good performance and energy conservation.

Laboratory exhaust systems fall into three main classes: chemical fume hoods, for working with corrosive acids and bases, volatile solvents, and other hazardous chemicals; biological safety hoods, which can be designed to protect the work (clean-air bench) or the worker (true biosafety cabinet); and standard exhaust hoods, typically used in mechanical or machine shops and their production areas. We are going to limit this discussion to the first category, the chemical fume hood, since this is the cornerstone of most research laboratories.

Laboratory fume hoods are designed to protect the worker by containing and exhausting harmful or toxic fumes, gases, or vapors emitted by chemicals used in the hood. A typical fume hood has an exhaust blower mounted so that air from the room is pulled into and through the hood, creating directional airflow. The “pull” at the hood opening is termed “face velocity” and usually is measured in feet per minute (fpm).

Proper face velocity of the hood is critical to the protection of the worker. Too little flow allows currents or disturbances in the laboratory air to overpower the hood and draw contaminants into the room. Too much flow can result in turbulence and eddies that also can lead to contaminants escaping the hood. Baffles and other aerodynamically designed components determine how air moves into and through the hood. Contaminants inside the hood are diluted with room air and exhausted to the outside via the hood’s duct system, where they are dispersed.

The volume of air exhausted by the hood depends on a number of factors, the most important of which are hood size and design. With the average chemical fume hood exhausting around 750 to 1,000 cubic feet per minute of conditioned air, you can see how hoods put a large load on the laboratory’s heating, ventilating and air-conditioning (HVAC) system, thus impacting operational costs. Let’s look at some of the different chemical fume hood designs available, along with their pros and cons.

Constant air volume (CAV)

There are two basic types of laboratory fume hoods: conventional and bypass. Conventional hoods consist of a basic enclosure with a movable sash (or window). Since the face velocity, or “pull,” is a function of the total volume divided by the area of the sash opening, closing the sash on a conventional CAV hood will increase the face velocity. The conventional hood’s performance depends primarily on sash position.

However, as the sash is closed, velocities can increase to the point where they disturb instrumentation and delicate apparatuses, cool hot plates and slow reactions, or create turbulence that can force contaminants into the room.

Bypass hoods contain openings above the sash, in addition to an airfoil sill that will redirect the airflow as the sash is closed. The bypass openings reduce changes in face velocity to a narrow range by keeping the area for airflow equal (within the limits of the bypass) as the sash is moved up or down. Therefore, face velocities do not reach the detrimental levels often seen with conventional hoods. For this reason, bypass hoods hold a major share of the market today.

Recent models of bypass hoods, called high-performance or “low-flow” hoods, display improved containment and safety features as well as energysaving designs. These design features vary by manufacturer but generally have one or more of the following: sash stops or horizontal-sliding sashes to limit the openings; sash position and airflow sensors that can control mechanical baffles; small fans to create an air-curtain barrier in the operator’s breathing zone; and refined aerodynamic designs and variable dual-baffle systems to maintain laminar (undisturbed, nonturbulent) flow through the hood. Although the initial cost of a high-performance hood is slightly more than that of a conventional bypass hood, the improved containment and flow characteristics allow these hoods to operate at a face velocity as low as 60 fpm, which can translate into $2,000 per year or more in energy savings, depending on hood size and sash settings.1

Reduced air volume (RAV)

In laboratory settings where the tasks may be very specific and unchanging, the reduced air volume hood (a variation of the low-flow hood) is an option to consider. This design incorporates a bypass block to partially close off the bypass, reducing the air volume and thus conserving energy. Usually, the block is combined with a sash stop to limit the height of the sash opening, ensuring a safe face velocity during normal operation while lowering the hood’s air volume. By reducing the air volume, the RAV hood can operate with a smaller blower, which is another costsaving advantage.

One downside to the RAV hood is that its restricted sash movement and reduced air volume also constrain its flexibility and narrow the realm of tasks that can be performed. Another major caution to note is the potential to override or disengage the sash stop. If this occurs, the face velocity could drop to an unsafe level. To counter this condition, operators must be trained never to override the sash stop while in use, and only to do so when loading or cleaning the hood. In addition, an airflow monitor is always recommended.

Variable air volume (VAV)

The newest generations of laboratory fume hoods vary the volume of room air exhausted while maintaining the face velocity at a predetermined level. Variable air volume hoods change the exhaust volume using different methods, such as a damper or valve in the exhaust duct that opens and closes based on sash position, or a blower that changes speed to meet air-volume demands. Most VAV hoods integrate a modified bypass-block system that ensures adequate airflow at all sash positions. They are connected electronically to the laboratory building’s HVAC, so hood exhaust and room supply are balanced. In addition, VAV hoods feature monitors and/or alarms that warn the operator of unsafe hood-airflow conditions.

Although VAV hoods are much more complex than traditional constant-volume hoods, and correspondingly have higher initial costs, they can provide considerable energy savings by reducing the total volume of conditioned air exhausted from the laboratory. Since most hoods are operated the entire time a laboratory is open, this can quickly add up to significant cost savings.

1. How to Select the Right Laboratory Hood System, Labconco Corp., Kansas City, Mo., 2003.
Chemical Fume Hood Handbook, Northwestern University, Chicago, Ill. Last revision, May 2007. http://www.research. northwestern.edu/research/ors/labsafe/hoods/index.htm
National Research Council Recommendations Concerning Chemical Hygiene in Laboratories, U.S. Department of Labor, Occupational Safety and Health Administration, Washington, DC. http://www.osha.gov/pls/oshaweb/owadisp.show_ document?p_table=STANDARDS&p_id=10107