Clearing The Air About The Latest Fume Hoods - Part 2
A case in point is Rich Stakutis, strategic marketing manager for Phoenix Controls Corp. (Acton, MA), who believes the term "low-volume" fume hood should cover both the so-called low-flow and high-performance hoods.
"My interpretation of the term 'low-flow hood' is one with a restricted sash opening (e.g., 10 in.). The exhaust system is sized for widely used face velocities (80 to 120 fpm). Then, the exhaust volumetric flow rate, determined by open area and face velocity will be lower than 'conventional' hoods," Stakutis said.
His definition of a high-performance fume hood is one that contains effluents, however, it utilizes lower face velocities (less than 80 fpm) to do so with no restrictions on sash opening. "The result is, again, lower exhaust volumetric flow rates than 'conventional' hoods," he noted.
This month we'll look at the different kinds of "low-flow" and "high-performance" technologies offered by various manufacturers.
The Low-flow StrategyGerhard W. Knutson, president of Knutson Ventilation, Inc. (Edina, MN), said that each laboratory hood manufacturer provides a low-flow hood and that the design differences are as numerous as the manufacturers themselves. "Two general approaches are used. First, the opening is reduced. At the same face velocity, a smaller opening results in a lower volumetric flow. The second approach is to reduce the face velocity. With the same opening, a lower volumetric flow will result."
An example of a low-flow hood is the "Dynamic Barrier Hood," which was introduced in 2000 by Kewaunee Scientific Corporation (Statesville, NC). The company states its low constant volume hood can save 70% of the exhaust volume typically required to operate a conventional bypass fume hood.
"What we do is reduce volumetrics - the amount of air that the fume hood exhausts - by using a smaller flexible opening to work through. If I have a smaller hole, I need to exhaust fewer cubic feet per minute to maintain a robust face velocity," said Bob Haugen, technological director of fume hood systems, Kewaunee Scientific Corporation.
The Dynamic Barrier Hood has a face velocity between 80 and 120 fpm. Haugen doesn't believe anything under that number is safe. He said that Kewaunee basically rejects the notion that it is possible to go to a 40 or 60 fpm face velocity and have a safe fume hood, and that the reason why is vector math.
"Remember those little arrows that you did in high school geometry? Most people walk at a velocity of somewhere between 180 and 250 fpm past the opening of the fume hood. In vector math, this means a javelin-sized arrow of 180 to 250 fpm will beat a wimpy 40 fpm arrow. I don't think with current technology we can go much lower than a face velocity of 80 to 120 fpm," stated Haugen.
The Dynamic Barrier's sash allows access to the entire fume hood interior through a smaller energy-saving opening. However, the sash can open to 37.5 in. tall, in order for laboratory technicians to put equipment into the hood or set up an experiment. Once an experiment is being conducted, the sash is lowered. The company estimates that the energy savings can approach $4,000/yr for each 8-ft bench hood, depending on the local climate and other laboratory air requirements.
Knutson noted that a significant concern with low-flow hoods is the lack of understanding of hood use by the engineer, designer, and even the owner. "Some reduced opening hoods will be a major problem to the user, or will be used at an inappropriate sash height."
The High-performance HoodSome fume hood manufacturers are also introducing what they call high-performance hoods. Smith said that he likes what he sees in some high-performance hoods, but he would never advocate operating a hood at less than 60 fpm. "We just have found from our data there are enough anomalies in the data that say we don't have a high level of confidence and we don't understand the conditions that affect containment well at less than 60. That's my cutoff."
An example of a high-performance hood is the "Air Sentry" hood, which Lab Crafters (Ronkonkoma, NY) introduced in 1997. The company says this hood provides maximum containment while cutting laboratory energy consumption up to 60%. The Air Sentry operates on the bistable vortex theory, said Bob DeLuca, Jr., vice president of technical products for Lab Crafters.
"Through mathematical modeling and ASHRAE 110 testing, we studied the airflow patterns in the fume hood chamber. We determined that if we, by design, created a specific airflow pattern inside the fume hood chamber, the fume hood performance was greatly enhanced. Our design causes the air entering through the sash opening to form a 'roll' in the upper chamber. This roll of air is called a vortex," noted DeLuca.
Lab Crafters determined that the reason why fume hoods lose containment is the vortex becomes turbulent and collapses. The company devised a way to make sure that vortex does not break down and collapse. DeLuca said the two main features of this patented hood are the mathematical formula used to determine the dimensions of the fume hood and the automatically adjusting baffle system.
To enhance containment, Lab Crafters' Air Sentry fume hood uses a control system that measures the pressure differential between the lab space and the fume hood chamber. A fluctuation in the pressure differential indicates whether there will be a collapse of the vortex. If a potential collapse is detected, the controls automatically adjust the rear baffle slots to maintain the airflow pattern and make the vortex more stable.
"People have had it drilled into their heads that 100 fpm face velocity equals safety, and it's not really the case," said DeLuca. "There have been numerous studies done by some of the top fume hood testers, and they've come to the conclusion that there is no statistical correlation between average face velocity and how the hood performs. If you have a superior design you can achieve better containment while operating at lower face velocities, which is the definition of a 'high-performance fume hood'."
Dale Hitchings, president of SAFELAB Corp. (Indianapolis), is familiar with the Air Sentry and said that he has tested it down to a face velocity of 60 fpm. "This hood works well because of the geometry of the enclosure (it is deeper than 'standard' hoods) and because it is relatively well-built and has better than average inlet aerodynamics (airfoils around the opening). The bistable vortex theory, however, does not appear to withstand technical criticism by independent industry experts, nor have the specifics of this 'phenomenon' been published in a reputable peer-reviewed journal."
Lab Crafters' reply to this assertion is that the proof of their theory lies in the performance of their fume hood design. "Industry-wide consensus on the theory of fume hood operation will not likely be achieved," noted DeLuca. "Experts in the aeronautical industry, to this day, do not agree on how airplane wings achieve lift. But they can all agree that the wings perform the function they are designed to perform. We have a similar environment in the fume hood industry, in that each manufacturer may have a different theory on what makes the fume hood perform, but at the end of the day, the proof is in the results of the performance testing. Our vortex theory can be debated, but the performance record of our Air Sentry fume hood cannot be questioned."
Too Different for the Old TestsA brand new fume hood that is not yet on the market is more difficult to define. Engineers at Lawrence Berkeley National Laboratory (Berkeley, CA) designed a fume hood that they say falls into both the low-flow and high-performance categories.
"This fume hood doesn't have an official name yet," said Hitchings. "Dr. Knutson and I independently tested a prototype of this hood at Montana State University under an NIST [National Institute of Standards and Technology] contract. Although it is a truly new concept and has some interesting features, it is also a rather complicated piece of equipment compared to its cousins."
Geoffrey Bell, P.E., a senior energy engineer at Lawrence Berkeley National Laboratory, noted that the Berkeley hood is definitely in a category of its own. "We've spent quite a few hundred thousand dollars testing this hood and verifying its operation and performance, which is basically unprecedented for a new science. This is a heavily tested hood because of its innovative technique and character."
Berkeley's hood design uses a "push-pull" approach to contain fumes and exhaust them from the hood. Small supply fans located at the top and bottom of the hood's sash, or face, gently push air into the hood. These low velocity airflows create an air divider that separates the fume hood's interior from the exterior (unlike an air curtain approach that uses high-velocity airflow). LBNL said its air divider approach of separating and distributing air leads to greater containment and exhaust efficiency. The result, they say, is an extremely effective and energy-efficient unit.
Indeed, engineers at LBNL note that the Berkeley hood reduces airflow requirements by 50% to 70% while maintaining, or enhancing, worker safety. Airflow reduction cuts energy costs about $2,100/yr per hood installation, on average.
Unfortunately, Bell said, this new science is difficult to quantify, because the Berkeley hood doesn't have a standard face velocity. "You can't simply put a face velocity measurement system in our hood, because the air doesn't go into the hood in a perpendicular mode. It actually turns and goes through an arc into the hood."
For this reason, some municipalities have been reluctant to embrace the technology. California OSHA (CAL/OSHA), for example, is one of the only entities in the United States to require a minimum face velocity of 100 fpm. Since the Berkeley hood doesn't have a standard face velocity, it does not comply with CAL/OSHA's face velocity requirement in Standard 5154.1, Ventilation Requirements for Laboratory-Type Hood Operators, with the hood's sash at full open position.
"We're trying to get CAL/OSHA to understand that the face velocity test does not directly correlate to the question of containment," said Bell. "We've had expert witnesses testify that after testing 4,000 fume hoods, they've seen no correlation between face velocity and containment. We're now going to ANSI for an interpretation of an equivalent safety test. Specifically, we're asking what equivalent performance indicator can be used instead of face velocity to determine equivalent performance."
Further, Bell noted, although a hood's energy consumption is an issue, worker safety is LBNL's primary concern. "We believe that verifying containment is the best indicator of a hood working correctly. In many instances, a hood's containment performance is not seen in the context of the whole laboratory's design. Therefore, the best lab designers will consider the challenges that the lab, itself, and its users have on hood containment."
The Berkeley hood may be available as soon as October 2003, through Tek-Air (Danbury, CT), which has the option to license the product.
New Products for a New MarketAnother fume hood manufacturer that has introduced hoods to this "low-flow/high-performance" market is Fisher Hamilton (Two Rivers, WI). Jon Zboralski, director of airflow products, said that its "Concept" hood, introduced in 2001, and the "Pioneer" hood, introduced in 2002, approach both segments of this new market.
"The Concept hood achieves a reduction in volume via application of a combination vertical/horizontal sash or an 18-in. operating opening on the vertical sash design. Special attention to the baffle and sill design enable this hood to achieve proper containment even when the sash is raised to the set-up mode with face velocities as low as 60 fpm," said Zboralski.
The Pioneer hood also achieves volume reduction via an 18-in. vertical operating opening and combination sash options. The company said its unique laminar airflow control module is positioned directly above the operator's breathing zone.
"When the Pioneer sash is opened above the 18-in. operating position, the control module automatically activates, directing a low velocity flow of cleanroom air between the operator and the fume hood interior. This directed airflow helps purge the operator's breathing zone. This hood has been designed to perform and subjected to expanded testing at face velocities as low as 50 fpm," said Zboralski.
Proper usage of a hood sash is to utilize it for physical protection for the user's upper body and face when working at the hood. Both the Concept and Pioneer incorporate a passive sash design that lowers the sash to the 18-in. operating position. The Pioneer also monitors the sash position and provides an audible and visual reminder when the sash is in the set-up positions.
Getting Around the TechnologySo with all this new technology, are endusers rushing to use it in their labs? The manufacturers say that they're seeing increased interest (and sales) in their low-flow and high-performance hoods but that sales are still strong for their "conventional" hoods.
Tom Smith, president of Exposure Control Technologies, Inc. (Cary, NC), said that he hasn't seen a big rush for laboratories to obtain the newer fume hoods. "The only specifications we're seeing with high-performance hoods being specified are the ones where a manufacturer has worked with the enduser to put that in their specifications. We're not seeing it across the board."
Indeed, he noted, numerous pharmaceutical companies have simply written protocols that mandate a maximum sash opening height for its workers in order to save energy. For example, the typical opening of a fume hood is 10 sq ft for a maximum sash opening of 28 in. If the hood has a face velocity of 100 fpm, there will be 1,000 cfm of exhaust. If the enduser lowers the sash to 14 in., that lowers the flow by 50%, and the exhaust will drop to 500 cfm.
The sash height may not be very practical for the user, but many companies have mandated sash openings between 14 and 21 in. "You figure with a high-performance hood that has a face velocity of 60 fpm, you'd only drop the flow by 400 cfm. By lowering the sash, they're saving an additional 100 cfm," said Smith. Endusers who can't work with a reduced fume hood opening, however, might be candidates for other types of hoods.
Meanwhile, some are concerned about the ergonomics of working with either restricted sash openings or deeper hoods (high-performance hoods are typically deeper to enhance containment).
"The fear is that sash-restricting devices will be defeated if they are not workable, and that users will be subject to very low face velocities when sashes are opened beyond the design point. With high-performance hoods, the fear is that users will need to lean into the hood to access the equipment near the rear of the work surface," said Stakutis.
Knutson may have summed up the discussion best when he said, "Assuming all the safety issues are addressed and assuming all the design concerns are addressed, low-flow hoods should be considered as another tool the designer has, and not as an answer to the problem of energy use in a laboratory." ES