Fume hoods are so integral to day-to-day laboratory operations that operators tend to think of the devices as “part of the furniture.” We hardly notice fume hoods until something happens inside them—an explosion, the release of toxic or radioactive materials, or a reaction running out of control that could have caused injury, property loss, or worse had it occurred on a benchtop.
Ironically, what makes fume hoods invaluable to worker health comes at significant environmental cost. Unlike other laboratory safety enclosures, which recirculate air after scrubbing out contaminants, a typical fume hood vents 250 cubic feet per minute of conditioned (heated or cooled, dehumidified) air to the great outdoors. That comes to roughly the amount of air in a large home every hour. Additional energy is required to keep exhaust fans and various control functions operating.
Minimizing the environmental impact of fume hood operation while maintaining or improving the devices’ safety features has been a top priority for fume hood manufacturers for at least 20 years. Luckily, “greening” of fume hoods has been a rather easy sell since owners see an immediate return on investment in the form of lower energy consumption.
“The largest contributor to fume hood energy consumption is the quantity of air it exhausts,” says Kasey Fulmer, product specialist at LABCONCO (Kansas City, MO). One measure of airflow is face velocity, or the speed of air entering the hood. “The challenge is how low this value can be without compromising safety. The design of the airfoil, corner posts, baffle, and bypass are critical to sustaining constant containment levels in a wide range of laboratory environments.”
Standards-setting organizations (e.g. OSHA, California OSHA, NIOSH) recommend that fume hoods have a face velocity of about 100 linear feet per minute (FPM). But even within that recommended airflow range, designers have come up with innovations that improve energy performance. For example, Sentry Air Systems’ 40” ductless fume hood provides an eight-inch-tall access area, 100 FPM face velocity, ductless technology, and numerous other customizable features.
Fume hoods must also pass ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) 110 testing for certification, which includes inflow velocity tests and smoke testing.
Omar Ilsever, PhD, vice president of marketing at Sentry Air Systems (Houston, TX), identifies fan efficiency, access area opening dimensions, and ductless technology as significant contributors to energy efficient fume hood operation. “Utilizing an energy efficient fan that requires just two amps peak reduces electrical consumption.
Also, limiting the size of the access area to eight inches reduces the outside airflow while maintaining the required air velocity.” Ductless technology, he adds, reduces energy costs by eliminating both the venting of conditioned air, and the need to produce make-up air to replace it.
Ductless fume hoods recirculate laboratory air after filtering out particulates and fumes, instead of venting contaminants and the large volumes of air containing them. Ductless designs use airflow and air velocity meters to ensure the hood provides air velocity suitable to the job, while maintaining a safe working environment.
“Ductless systems allow air to move through a fume hood by a self-contained blower motor and filter system located at the top of the unit,” says iQ Labs’ (Muskegon, MI) co-owner and president, Nathan Thornton. “Instead of exhausting air out of the building via duct work, clean air recirculates within the room itself. These units also provide more flexibility in lab planning, as it eliminates roof-mounted blower motors and ducting.”
Ductless fume hoods are typically offered in fixed-location or caster-fitted configurations that allow moving the units around the room or facility, as needed. “They also maintain the normal productivity for lab technicians or students without changing the way they work,” Thornton adds.
Variable air velocity systems
According to Thornton, significant improvements in fume hood efficiency is achievable through variable air velocity (VAV) systems. VAV uses a butterfly or venturi-style valve in the duct system, located just above the fume hood itself. This valve connects to the fume hood controller that monitors air flow at the hood face and the position of the sash. “As the sash rises, the valve system opens, allowing more air to be drawn through the booth and exhausted. As the sash is lowered, the valve system begins to close in relation to the sash position and airflow at the face, thus moving and exhausting less air through the hood from the lab to the environment.”
VAV thus reduces the volume of makeup air and conditioned air needed by the building’s HVAC system to maintain comfortable working conditions. Some VAV installations deploy a proximity camera to determine the presence of workers, and to adjust the sash opening accordingly to maximize safety while minimizing airflow.
With VAV systems, the tradeoff is unit complexity and upfront acquisition costs. “Understanding the true ROI on these systems based on facility size, number of hoods, and layout is critical for making the correct purchase decision,” Thornton says. “VAV hoods also require additional preventive maintenance (e.g. cleaning, greasing) of the duct valve system. Ductless systems are also slightly more expensive to operate than constant air velocity fume hoods, since the filters must be regularly changed to allow the blower motor to draw adequate volumes of air through the hood, and filter out the necessary contaminants.”
One advantage of VAV is the potential to lower airflow requirements while maintaining safety. LABCONCO’s Protector XStream VAV fume hood operates at an air velocity of just 60 feet per minute and exceeds most safety and user exposure standards. Energy savings vary with the installation, but according to the company, they can be quite substantial. LABCONCO’s Protector Echo, a ductless, filtered chemical fume hood, eliminates exhaust air entirely and, says Fulmer, “preserves 100 percent of conditioned air within the laboratory.”
The main drawback of VAV technology is its implementation costs. “It’s important to evaluate ROI before considering a VAV installation,” says Fulmer. “Buildings with existing constant air volume exhaust systems may be prohibitively expensive to convert to VAV.”
Every improvement in laboratory efficiency comes at financial cost, at least initially, and may involve operational or safety tradeoffs as well. Fume hoods are no exception. A smaller hood access area, for example, reduces the flow of air out of the building but reduces workspace accessibility. “Fume hoods may be customized with larger openings,” Ilsever notes, “but that requires a fan that is less energy efficient. Also, ductless units require the operator to monitor filter saturation and change the filter when static pressures reach a certain level.”