From the Mechanical Point of View: Fume Hoods

I was thinking about FUME HOODS walking back from class today because my friend Jina is interviewing for some fume hood inspection position over at UCSF. (Fume hoods: the thing you put chemicals in so you don't die from the fumes) I think it's pretty interesting! Fume hoods deal with the fumes of some pretty dangerous stuff. You totally do NOT want to be smelling the stuff that chem/bio majors deal with, right? right?! (Conlo,danny?) So you gotta get that shit out of there otherwise you'll pass out/lab will explode.

So what I thought up! (Figured I'd put all these ideas down in case I ever find myself needing to think about fume hoods while in industry) My thought process:

Air has density (depending on pressure, temperature) and viscosity. Moving air requires moving a mass over a distance = work. Work gives power requirement of fan.

Fume hood is basically the beginning of a large duct. Going to have no-slip conditions (air doesn't move RIGHT at the interface of air/wall... the air right on the surface of the fume hood will not be moving. try washing your car with just water, no scrubbing). No-slip condition creates a boundary layer (A small film of air along the sides that isn't moving quite as fast as all the air in the center of the duct). Should the chemicals get trapped inside this boundary layer, they would stay in the fume hood!

Need laminar flow, that way we know where the fumes are heading (in turbulent flow, we have no idea where the fumes are going, and chemicals could be deposited on everything! dangerous) Need a certain rate of mass transfer to maintain safe operating levels (Don't want to suck out the fumes too slowly... choke!)

If the fume hood door is fully shut, the fume hood can't operate unless it has some other inlet for air. If the fume hood door is somewhere between open and closed, or it is being changed during operation, this causes changes in streamlines inside the fume hood. Another boundary layer on the edge of the fume hood door, possibly causing turbulence/vortices inside the hood, creating chemical deposition?

Need a control system that takes into account minimum operating levels for safety? And then from how this control system works on, i have no idea. Gosh. That's a toughy. Fume hoods are pretty complex.