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Lab bench extract arm

Lab bench extract arm

Lab bench extract arm

Hi, I am trying to design the local extract ventilation system in a lab. Over the lab benches the user want ventilaiton arms with a hood to stretch down over the bench to where chemical etc are been mixed & tested.
What velociaty should I be looking for at the extract point? is there any code or standard out there to guide me on this? Also if I get a reccomended velocity, is this the face velocity at the widest point of the hood (200mm dia) or at the arm (100mmdia)?

RE: Lab bench extract arm

Don't forget the OSHA Z tables.

The face velocity is the mean velocity at face of local exhuast ventilation. You will probably need a capture velocity and capture distance. The ACGIH explains this well, though I don't think it will cite either.

I've had a lot of bad luck with flexible arm exhausts-they tend to have very high statics (and noise) and if using manual shutoff dampers (if allowed by NFPA 45) end up being continuous exhausts or abandoned. I've had a lot better luck going through lab safety to follow GLP and use the lab hood for mixing, or using downdraft tables, or using a configuration closer to the evaporation bench/hood to intercept chemicals before the breathing zone (helps to have directed supply behind the user as well, getting a push-pull effect to keep things out of the breathing zone).

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