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There's a Lot More to It Than You Realized - "Shut the Sash"
Laboratory Fume Hoods and Biosafety Cabinets are essential pieces of equipment when working with chemicals and materials that emit noxious fumes in the lab. The height you set the sash of your fume hood at has an impact on many different things. First and foremost, and the purpose most people associate with the sash height, is that it must be set low enough for proper fume extraction to maintain safe, breathable air throughout the lab. Although the sash height is the determining factor for this, there are a lot of other things that need to be taken into consideration when determining the proper sash height.
Besides setting the sash low enough to make sure that the fumes are extracted effectively, when you close the sash you also protect yourself and others against any chemical accidents that can be contained within the biosafety cabinet or fume hood. Most fume hoods are designed to work more efficiently the lower you go. We’re not talking about typical energy savings, like you get when you switch from a 60 watt lamp to a 40 watt, fume hoods can use as much energy as 4 average homes per year. An open fume hood uses ten times more electricity than a hood with the sash closed. There’s more to it than the additional energy used by the fume hood or the added carbon emissions it produces. The further the sash is open, the more air you vent from your laboratory. Unlike the air loss you’d get with a naturally aspirated chimney, this air is being drawn out by the fume hood motor and, depending on the weather, either the building’s heating system or air-conditioning system may need to work harder as well to keep the laboratory at a comfortable temperature for you to work in.
As you raise the sash the fan speed increases but it does not mean that the higher you raise the sash the more effective the fume hood becomes. It’s the opposite and besides reducing efficiency, you lose effectiveness as more and more air has to be moved to draw out harmful fumes and continue to keep you safe. On top of that, as the sash goes up and the motor continues to work harder and harder, the increased airflow can cause turbulence within the fume hood which increases the likelihood that fumes will escape.
As you can see, there are many reasons to lower, or even close, the sash on your fume hood and we didn’t even consider the additional wear and tear caused when equipment has to work more. Closing the sash is the safest and the most energy-efficient way to work and if you're not working in the fume hood, or before you leave to go home, always remember to lower the sash as far as it can go. Lowering the sash provides the best conditions for venting fumes.