Even more than the colocation of lab and cafeteria, I'm concerned about the way the discussion morphed from "how can we be safe" to "how can we comply"-as you certainly know, they're not the same. Always an alarm bell for me.
You might try refocusing the conversation a bit. The ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Handbook has material on stack heights, inlets, and reentrainment that you could probably use for initial support. If you have to fall back on regs, the ASHRAE material is certainly something OSHA would look at in determining that it was a "recognized hazard" and thus a General Duty Clause issue. The relevant material is probably in the volume on "HVAC Applications."
The location of the kitchen exhaust is immaterial as long as the fans stay on and it doesn't become an inlet. A low-down inlet and high stack MIGHT work if there is no factor in the area that could disturb the airflow and cause downwashing. Which could be an issue if the buildings on your site are close together (relative to the length and height of the buildings). If the downwash is strong enough, you can short-circuit the hood outlet to the main building air inlets.
If the lab is next to the cafeteria, you're going to need as few penetrations in the wall as possible and the lab kept under negative pressure; wall construction costs might become an issue as well. (Typical cafeteria walls aren't built to laboratory tightness and so might need to be rebuilt.) If you end up having to put the lab there, might want to restrict what can be used or done in there-one wouldn't want to have the A/C fail, have the lab get positively pressurized, and not be able to shut down whatever reaction they're running. Alternatively, if you can rely on the researchers to follow SOPs, including an air failure alarm with instructions to shut all chemical bottles and quench any vapor-emitting reactions would work.
I personally wouldn't do SF6 tracer work unless you'd already built the stacks, in which case you're already out X tens of thousands for construction in sunk cost and you're trying to see if it can be salvaged. If Michelin has access to fluid-dynamics software like Comsol, somebody with expertise in that could probably run some simulations of various conditions for you much more cheaply.
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I would appreciate your help again with a lab design issue. My fabulous engineering colleagues want to locate a new lab next to a cafeteria, sharing a wall. I do not like this idea. The cafeteria is a full kitchen cafeteria where it will have its own exhaust system right next to the chemical laboratory. Their solution is to have the kitchen exhaust stacks "short" and the chemical lab exhaust stacks "high" to try & eliminate the occurrence of any chemical exhaust being drawn into the kitchen intake. I have objected to this but they want a "regulation" that says they can't do this. My professional judgment isn't enough.
Melissa Ballard, MSPH, CIH, CCHO
Industrial Hygienist / Industrial Hygiene Chemist
IH Lab - Bldg 150 - 1st