Ductless (recirculating) Fume Hoods
Mainly for educational or testing use, these units generally have a fan mounted on the top (soffit) of the hood, or beneath the worktop. Air is sucked through the front opening of the hood and through a filter, before passing through the fan and being fed back into the workplace. With a ductless fume hood it is essential that the filter medium be able to remove the particular hazardous or noxious material being used. As different filters are required for different materials, recirculating fume hoods should only be used when the hazard is well known and does not change.
Air filtration of ductless fume hoods is typically broken into two segments:
- Pre-filtration: This is the first stage of filtration, and consists of a physical barrier, typically open cell foam, which prevents large particles from passing through. Filters of this type are generally inexpensive, and last for approximately six months depending on usage.
- Main filtration: After pre-filtration, the fumes are sucked through a layer of activated charcoal which absorbs the majority of chemicals that pass through it. Ammonia and carbon monoxide will, however, pass through most carbon filters. Additional specific filtration techniques can be added to combat chemicals that would otherwise be pumped back into the room. A main filter will generally last for approximately two years, dependent on usage.
Recirculating fume hoods are often not appropriate for research applications where the activity, and the materials used or generated, may change or be unknown. As a result of this and other drawbacks, some research organizations, including the University of Wisconsin, Milwaukee, Columbia University, Princeton University, the University of New Hampshire ,and the University of Colorado, Boulder either discourage or prohibit the use of ductless fume hoods.
A benefit of ductless fume hoods is that they are easy to install since they require no ductwork and can be plugged into a 110 volt or 220 volt outlet.
In a survey of 247 lab professionals conducted in 2010, Lab Manager Magazine found that approximately 22% of fume hoods are ductless fume hoods.
| Pros | Cons |
|---|---|
| Ductwork not required. | Filters must be regularly maintained and replaced. |
| Temperature controlled air is not removed from the workplace. | Greater risk of chemical exposure than with ducted equivalents. |
| Contaminated air is not pumped into the atmosphere. | The extract fan is near the operator, so noise may be an issue. |
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Famous quotes containing the words ductless and/or fume:
“What we feel and think and are is to a great extent determined by the state of our ductless glands and viscera.”
—Aldous Huxley (1894–1963)
“Love is a smoke made with the fume of sighs,
Being purged, a fire sparkling in lovers’ eyes,
Being vexed, a sea nourished with lovers’ tears.
What is it else? A madness most discreet,
A choking gall and a preserving sweet.”
—William Shakespeare (1564–1616)