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A Planner’s Guide to Filtered Fume Hoods

Thu, 02/14/2019 - 11:00am
by Michele S. Pollio, AIA, HERA Laboratory Planners
 

As lab planners, HERA spends a lot of time discussing fume hoods, considering type, cost, use, sustainability and installation, to name just a few aspects. The introduction of filtered hoods has made the discussion much more complex. Filtered fume hoods are a relatively new technology, so some users are reluctant to consider using them; however, in certain situations, they do offer some compelling advantages. We can help users determine if using filtered fume hoods is right for them by evaluating if the hoods will filter the chemicals that they use, if the maintenance is practical for their situation and if it makes sense economically.

Fume Hood Basics

Fume hoods provide a protected environment for working with hazardous materials by moving a lot of air through the device away from the user. Typically, fume hoods are specified with face velocities of 60 to 100 fpm, which is achieved by pulling 500 to 800 CFM through the hood. Building systems must supply enough air into the lab to achieve that volume of air through the hood.

Filtered fume hoods, or ductless hoods, are containment devices that filter fumes, odors and particles and re-circulate air back into the lab. This is done by moving air through carbon filters that capture hazardous molecules. When the filters become saturated, they are removed and discarded. Typical installations include a primary and a secondary filter with monitoring capabilities. When the primary filter is saturated, it is discarded and the secondary filter moves to the primary position and a new secondary filter is installed. Because filtered hoods do not contribute to the components of the HVAC system, they do not affect the size requirements for air handlers and exhausts systems.

Factors to Consider

 

Chemical Usage. In evaluating if filtered hoods are the right device, one must consider the hood’s use and setting. We start by considering what chemicals are being used in the hood. Users complete a chemical evaluation form such as SEFA 9 for evaluation by the fume hood manufacturer’s chemist. The hoods work well with most chemicals; however, those with low molecular weight or low boiling points may not be captured or are easily liberated from the surface of the filter, rendering the filter ineffective.  The filters have capacity for a certain amount of adsorption based on weight and vapor pressure of the various chemicals, so the longevity of the filters will be determined when a single chemical or chemical family saturates the filter, even if the filter still has capacity for capturing other chemicals. Finally, one must consider interactions between chemicals that are being used in the filtered hoods. Unlike ducted fume hoods, where chemicals are exhausted and removed from the hood, in a filtered hood, chemicals remain in the filter until the filter is removed. Chemical pairings that become toxic or explosive when they interact should not be used in the same hood. 

Applications. Ideal applications for filtered hoods are ones where the chemicals to be used are known, such as those in teaching labs and processing labs like histology and forensic labs. Research labs dealing with life science processes or using organic solvents would also be appropriate. Applications that are inappropriate would be experimental labs/discovery labs where scientists do not know what chemicals will be required in future experiments or in labs in which scientists are working with chemicals with unknown properties. Some of these labs could be served by a combination of filtered and standard hoods, however these should only be used in labs with experienced users who understand the requirements and limitations of the different kinds of hood.

Types of Filtered Hood. Filtered hoods come in several different types—those that provide a filtered enclosure and those that behave like traditional fume hoods. Filtered enclosures, such as the Erlab Captair hood, provide a significantly lower price-point option and simpler installation, but have limitations. They have glass at the front and sides, so they contribute to an open-feeling lab, but they do not have the structure to accommodate pre-piped utilities. In contrast, filtered fume hoods such as the Labconco Echo or Air Master System Green Solution Filtration Hood, have a similar look and feel to ducted fume hoods. Like ducted fume hoods, they can have pre-piped utilities, but they are more expensive than the Captair hoods and require installation by a contractor.

Maintenance. Filters typically need to be changed every one to three years, so although this is not a frequent activity, the evaluation of the appropriateness of filtered hoods should include an evaluation of how the filters will be changed. Because filtered hoods require filters to be changed to maintain a safe work environment, facility directors need to consider how maintenance will be accomplished. Large companies usually have facility departments with personnel who have specialized experience, whereas a small company, start-up, college or high school may have maintenance done by staff tasked with all maintenance facility-wide. Because changing a ductless hood’s filter must be done within the lab, it is important to consider the implications of sending maintenance personnel into a lab and how scheduling maintenance affects the work to be done in a lab. This may be inappropriate in restricted-access labs or those used intensively.

 

Cost

The decision to use a filtered hood should include an evaluation of the cost implications that includes consideration of operating cost and first cost. The cost of standard fume hoods is significantly lower than the cost of filtered hoods; however, we must also consider the cost of ducts, valves, fans and additional air-handling capacity requirements to support the ducted hoods. The additional cost will vary widely based on facility size, number of hoods and type of facility, but analysis of the installation cost for the entire system generally shows filtered hoods to be less expensive.  An analysis of operating expenses should compare the cost of infrequent replacement of filters, to the cost of additional energy required for supply and exhaust support systems, which can be many times higher.  

One situation in which standard hoods would not require significant additional ductwork or HVAC capacity would be in exhaust driven spaces. In labs with few fume hoods, the exhaust needs for the lab may be supplied through the hoods. If the hood’s exhaust need is equal to or less than the exhaust requirements from the room, than the ducting that would be used to serve the lab would be used instead as the ducting for the hood. This means that no additional ducting would be required for the hoods, even if standard hoods are in use—negating potential savings for a filtered hood installation.

Installation

Installing standard fume hoods into new construction requires coordinating the location of the duct runs, but is done relatively easily. It is much more difficult in existing construction, requiring consideration of feasibility, demolition and schedule. Not so for filtered hoods. Adding a filtered hood to an existing operating lab could be as simple as finding the right location, clearances and benchtop space. Adding piped utilities adds some complexity; however, routing piping to and from a hood is relatively simple compared to adding ductwork and potentially exhausting from the building. Even in more complex renovations impacting large sections of building, adding new ductwork requires careful consideration of routing both in and outside of the construction zone, as well as potential added load to an existing system. Installation of a fume hood without ducts would be quicker and would minimize construction disturbance.

Filtered fume hoods are a technology that should be considered as an alternative to traditional, ducted fume hoods. Users, the design team and the manufacturer will evaluate safety, maintenance, cost and installation. Although evaluation of the appropriateness of the technology for a particular installation takes time and effort, often the most difficult hurdle to overcome is fear of the unknown.

Author’s note: Cost analysis evaluation was done with estimating help from John Frondorf of Becker Frondorf and HVAC system strategy was done with help from John Bovencamp of Stanley Consultants. Jesse Coiro of Erlab and Andrew Paez of Labconco provided information about hoods and filters.

References

  1. Erlab Hood Selection Guide.
  2. How to determine if Green Fume Hoods are right for your laboratory, Ken Crooks, January 2015.
  3. Esco Guide to Ductless Fume Hoods.

About the Author

Michele S. Pollio, AIA, is Director of Philadelphia Office, Associate, with HERA Lab Planners. Michele’s diverse laboratory experience ranges from higher education academic environments to corporate research facilities to clinical laboratory spaces. With each project she manages, Michele leads the team from programming through construction administration, helping clients achieve all their goals. As a science enthusiast, Michele loves working with end users and learning about their work.

 
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