Efficiency, materials of construction, operating costs prime considerations
“Fume hoods are tremendously expensive to run,” notes Dana Dahlgren, VP of sales and marketing at Kewaunee Scientific (Statesville, NC). “The last decade has been all about fume hoods that are more efficient, that do not require the same air flow as older models.”
Facility managers increasingly specify variable air volume (VAV) systems for new buildings. VAV uses a combination of sensors and controls that ramp down exhaust in individual hoods according to demand.
VAV may be combined with proximity sensors coupled to automatic sash closing mechanisms, which engage after the user has been away from the hood face for a specified time. Automated sash closers only make sense, from the perspectives of safety and energy economy, when used with VAV. Contrary to popular belief, closing a sash in a conventional hood provides a safety barrier but does not save energy, because air flow remains constant. This is known as a “bypass hood” or constant volume hood. Otherwise, small sash openings would create a wind vortex within the work space that could disrupt or ruin processes.
VAV and automatic sashes are expensive, but the return on investment is remarkably short, Dahlgren says. “Depending on the climate, the payback is a couple of years at worst. If you’re building a 30- or 40-year building, adding those features is a bargain.”
The emergence of the SEFA 9 standard from the Scientific Equipment and Furniture Association (SEFA), an industry group, is expected to improve acceptance of ductless hoods. The standard puts ductless hoods and filtered fume hoods into one of three categories: DHI, DHII, or DHIII. The DHIII category calls for advanced sensors and monitoring and for catch-all filters. “Many organizations have adopted ductless hoods for lab renovations, and that trend will continue as the user base becomes better educated about the benefits,” says Kevin McGough, president of AirClean® Systems (Raleigh, NC).
SEFA members, including AirClean Systems, Labconco, Erlab, Kewaunee Scientific, and others, are hoping the international standards organization ANSI will accept the SEFA 9 standard, which should boost even greater acceptance of ductless hoods.
Instead of exhausting conditioned air out of buildings, ductless hoods recirculate air that has been cleansed via HEPA and/or carbon filters. Thus, ductless hoods save energy to an even greater degree than do VAV-based systems with automated sashes.
Ductless hoods are appropriate in two situations. The first involves very low-risk dilute aqueous chemistry, as one would encounter in a freshman chemistry course. The second category includes higher-risk processes where the hazards are quantifiable and well-understood, and where hazardous vapor concentrations fall within the system’s capability for removal.
McGough is adamant about understanding risks. “If you’re getting into real R&D,” he says, “and you don’t know what your products or byproducts will be, or their concentrations, we will not sell you a ductless hood.” Even in those situations ductless designs will probably work, but users will need to change filters more frequently and sensors may not detect contaminants at their threshold limit values. For these reasons, AirClean Systems requires purchasers to submit an application worksheet as part of the sale.
Traditionally, hoods were fabricated from wood, steel, and stone, and were dedicated to a specific task. New materials greatly broaden a hood’s utility. NuAire (Plymouth, MN) holds the distinction of constructing its fume hoods entirely from polypropylene rather than metal, stone, or wood. “Metal eventually rusts,” says Polypro sales manager Terry Thompson, “sometimes in as little as two years.”
Aside from degradation, metalfree hoods are preferred by labs that conduct trace metal analysis and are concerned about steel and rust particles, agitated by air flows, contaminating their assays.
Metal-free design involves some alterations in how manufacturers build hoods. Instead of chain and sprocket sashes, polymer hoods employ a polypropylene pulleyand- rope assembly. Handles, latches, even screws are fabricated from the durable plastic. Advice: Ask for extra screws if polypropylene hoods are in your future.
“Plastic-lined hoods are 20 to 30 percent more expensive than metal designs, which is something to keep in mind,” Thompson advises. “But they’re virtually a onetime installation.”
Usage & operation
Aside from cleaning and sash operation, users do not need to worry much about fume hood usage and operation. Following best practices will ensure that usage remains trouble-free.
In her visits to customer labs, Beth Mettlach, sales engineer at Labconco (Kansas City, MO), often notices inappropriate fume hood usage. “They have a lot of things that shouldn’t be in there,” she says.
Kevin McGough has seen similar abuses. “Labs put rotary evaporators inside hoods,” he explains. “What terrible utilization of a critical asset.”
To ensure optimal operation, Mettlach advises that lab workers remove from hoods everything that does not belong in a protected area. “A fume hood is foremost about safety,” she says. “You have to let air through, and that will in turn provide better containment.”
Part of safe operation includes working within the exhaust space, beginning about six inches beyond the sash. Hoods will generally protect workers outside those areas, but will not prevent spills from dropping onto users’ laps or the floor.
Mettlach is very big on airflow monitors as an essential feature of hoods. Monitors ensure that the device operates within exhaust specifications, and they can diagnose numerous problems related to the blower, the sash, and the exhaust. Some hoods come equipped with monitors; others rely on VAV systems that incorporate facility-wide monitoring. If your lab’s hoods lack airflow monitors, consider installing them as an add-on.
Other usage tips include the following:
- Fume hoods are taken for granted, but users still require training in operation and safety. YouTube (www. youtube.com) hosts numerous videos illustrating safe hood usage.
- Organizations that maintain many hoods will probably need a state or local certification that air flow is sufficient for user safety. Certification involves checking exhaust and air flow systems.
- It is important to keep sashes closed when lab personnel are not working directly in the hood. This will not save money (unless your lab has a VAV installation), but it will protect users from explosions and sudden gas releases.
- For labs with automated sashes, Dana Dahlgren advises against automated opening for obvious safety reasons. “The sash should never open unless the user specifically wants it to,” she says.
Safety is always the first factor to consider. What will your lab be using the hood for? High school chemistrytype experiments require a minimum of user protection, if any, whereas high concentrations of heated acid demand the utmost in protection.
Kevin McGough advises purchasers to consider their overall goals. Particularly during renovations, they should consider a ductless design when they cannot afford to stop working for extended periods. Ductless hoods can be located in unoccupied space throughout a facility, allowing end users to keep processes going at full capacity while the renovation is taking place.
Ductless systems cost more to purchase than conventional hoods, but they often pay for themselves within two years through energy savings. “Ductwork removal and installation, HVAC and damping systems, VAV controls, add-on features, and the cost of heating and cooling the air you’re blowing outside represent serious costs,” McGough explains.
Mettlach concurs. “Steer away from considering upfront costs,” she says. “Operating costs are far more significant and persist for as long as you own the hood.”
New labs increasingly specify a mix of conventional and ductless hoods, a strategy that maximizes facility utilization. The high-risk processes with unknown byproducts still occur in conventional hoods, while less risky and better defined operations go ductless. “It’s a better way to allocate resources,” McGough says.
Finally, consider epoxy resin hood liners and floors. Polymers resist most caustic chemicals and are easy to clean, while stainless steel will corrode when exposed to strong acids. Stainless steel lining is suitable for dilute acids or isotope work, where corrosion risk is low, because stainless decontaminates easily.
Users who fully understand their processes, who shop around, and who exploit vendors’ wealth of knowledge will be better positioned to acquire the right hood for everyday operation, as well as for exceptional workflow events.