Laboratory fume hoods are designed to protect the worker by containing and exhausting harmful or toxic fumes, gases, or vapors from chemicals used in the hood. With average chemical fume hoods exhausting around 750 to 1000 cubic feet per minute of conditioned air, you can see how hoods put a large load on a laboratory’s HVAC system and thus impact the operational costs. Let’s look at some of the different chemical fume hood designs available and their pros and cons.

Constant Air Volume
There are two basic types of laboratory fume hoods: conventional and by-pass. Conventional hoods consist of a basic enclosure with a movable sash (or window). The conventional hood’s performance depends primarily on sash position. However, as the sash is closed down velocities can increase to the point where they disturb instrumentation and delicate apparatus, cool hot plates, and slow reactions or create turbulence that can force contaminates into the room.

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

Recent models of by-pass hoods, called high-performance or “low flow” hoods, have improved containment and safety features as well as energy saving designs. These design features vary by manufacturer, obviously, 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; refined aerodynamic designs and variable dual-baffle systems to maintain laminar (undisturbed, non-turbulent) flow through the hood. Although the initial cost of a high performance hood is slightly more than a conventional by-pass hood, the improved containment and flow characteristics allow these hoods to operate at face velocities 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.

In laboratory settings where the tasks might be very specific and unchanging for the most part, a variation of the “low flow” hood referred to as the reduced air volume (RAV) hood is an option to consider. This design incorporates a by-pass block to partially close off the by-pass 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 cost saving advantage.

One down side to the RAV hood is that with its restricted sash movement and reduced air volume, the realm of tasks and flexibility of use is also constrained. 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, train operators to never override the sash stop while in use, only for loading or cleaning the hood. In addition, an air flow monitor is always recommended.

Variable Air Volume
The newest generations of laboratory fume hoods vary the volume of room air exhausted while maintaining the face velocity at a pre-determined level. Variable air volume (VAV) 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 by-pass block system that will ensure adequate air flow 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 with corresponding 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.

From Flow Down! by the Safety Guys