With today’s focus shifted heavily toward sustainability and lower energy costs, choosing the right fume hood becomes even more important. Most common setups employ traditional ducted fume hoods. However, advances in technology have elevated the filtered or “ductless” fume hood, leveling the playing field. So, we thought a discussion on basic design principles and the pros and cons of ductless versus ducted chemical fume hoods was in order.
Chemical hood design—differences between ducted and ductless
Chemical fume hoods are designed to capture and remove the contaminants that result from working with chemicals. The purpose of the hoods is to protect the worker by containing and removing harmful or toxic fumes, gases, vapors, or particulates. The design of the hood attempts to create a capture zone in front of it to keep contaminants within the hood, where they are exhausted or filtered.
The traditional ducted chemical fume hood has an exhaust blower or fan mounted remotely so that laboratory room air is pulled into and through the hood, creating directional airflow into the hood. Any contaminants generated in the hood are exhausted through ducts to the outside. Filtered or ductless hoods use the same principles, but instead of a duct with a remotely mounted blower and stack, they utilize an integrated filter filled with appropriate absorbing media to capture and remove contaminants before exhausting the filtered air back into the lab.
As you can see, there are major differences between ducted and ductless designs. So, how do you know which is best for your application? You have to do some homework. Fear not, we distill the major decisions down for you below.
Determine your specific needs—immediate and long term
As lab managers, we know that the OSHA standard for Occupational Exposure to Hazardous Chemicals in Laboratories, 29CFR1910.1450, commonly referred to as the OSHA Lab Standard, contains statements in the nonmandatory Appendix A that are very general in nature.1 For instance, it states, “Airflow into and within the hood should not be excessively turbulent; hood face velocity should be adequate (typically 60-100 lfm).” Most hoods will provide adequate flows, so we need to be more specific.
First and foremost, we need to know the chemicals anticipated for use, the potential contaminants generated, and the frequency or estimated quantity produced. Is appropriate filtering media available and, if so, how long will the filter pack last before replacement is necessary? (More on the filter replacement to come.)
In general, ducted fume hoods can handle almost everything with proper duct material and installation. However, if radioisotopes, nanoparticulates, or perchloric acid are used, ducted hoods may be your only choice.
The next question is about facility flexibility. Will the hood location ever need to change? If not, either type would work. If a location change is possible, then the ductless hoods offer easier relocation, with no ducts to remove and reinstall.
Cost is always a huge part of the equation, and there are many variables to consider here. Are you installing the hood in an existing building or planning a new facility? Up-front costs differ significantly. A typical five-foot-wide ductless hood from a major vendor might run upward of $25,000 (with the first set of filter media), compared with about $6,000 for a similar-size ducted hood. However, you need to add the costs of needed infrastructure (ducts, blower, exhaust stack, mechanical systems, etc.) as well as engineering and labor, which bring the costs closer together.
In addition to up-front costs, operating costs need to be factored in. For ductless hoods, these include the cost of replacing filtering media as well as disposal of spent media. Ductless fume hood manufacturers are glad to offer a cost analysis based on your specific needs. On the other side, ducted hoods add significantly to the costs of building operation due to the high energy costs of exhausting large quantities of conditioned air.2
Final areas of consideration are user training and typical operation and maintenance. The traditional ducted hood usually requires minor training in proper baffle and sash settings, combined with occasional operation and maintenance of replacing a blower belt or adjustments to the fan. Ductless fume hoods require additional understanding of the filtering media limits, e.g., what can be filtered and what cannot. Knowing how to tell when media needs replacing and how to do it safely, in addition to proper handling and disposal of spent media, are also paramount to safe and proper operation.
Choosing between traditional ducted fume hoods and the newer technology of filtered ductless hoods requires us to do some serious homework. Going with the ductless hoods has the potential for long-term cost savings in terms of significantly lower building energy costs. The ductless hoods also offer more flexibility and are more environmentally sustainable in that they capture and inactivate contaminants instead of diluting and exhausting them into the ambient air.
Your first course of action is to determine whether appropriate media are available for the expected contaminants that are produced. The second is to perform a detailed cost analysis considering all the factors discussed.
1. Occupational exposure to hazardous chemicals in laboratories, Occupational Safety and Health Administration, Washington, D.C. 2006. http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10106
2. Fume hood selection for energy-efficient labs, Blanchard, Richard. Lab Design News, Rockaway, N.J. October 2013. https://www.labdesignnews.com/article/2013/10/fume-hood-selectionenergy-efficient-labs