Fume hood: Encyclopedia II - Fume hood - Recirculating Fume Hoods

Fume hood - Recirculating Fume Hoods

Mainly for educational use, or where the design of a building does not permit the fitting of external ductwork, 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.

Fume hood - Pre-Filtration

The first stage of filtration consists of a physical barrier, typically of open cell foam, which prevents large particles from passing through. A filter of this type is generally inexpensive, and would last for approximately six months, dependent on usage.

Fume hood - 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.

• Ductwork not required.
• Heated air is not removed from the workplace.
• Contaminated air is not pumped into the atmosphere.

• Filters must be regularly maintainted and replaced.
• Greater risk of chemical exposure than with ducted equivalents.
• The extract fan is near the operator, so noise may be an issue.

Adapted from the Wikipedia article "Recirculating Fume Hoods", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki