I'm building a fume hood and wondering how many air exchanges per minute is ideal. Let's assume the fume hood is 25 cubic feet. A 25 CFM fan would
change the air about once per min (actually a lot less considering ducts, filters etc).
Should I be aiming at 1 exchange per min or more?bbartlog - 17-11-2009 at 13:27
Seems like it would depend on what you were doing. In any case, the relevant metric for fume hoods is generally considered to be the face velocity
(not air exchanges per minute), which is the speed of the air moving into the fume hood. See here for example. This is going to depend on the surface area of the opening into the work area.
For example, if you have a 1' x 3' opening (with glass/plastic down), and want to achieve 80fpm airflow (consistent with a Class C fume hood), then
you'll need a fan or fans that can do (3 sq ft x 80fpm) = 240cfm. That's actually not very much; there are computer cooling fans with a 120mm square
form factor that can move over 100cfm, and cost less than $20. Whether they are appropriate for a fume hood in other respects (corrosion resistance,
fire hazard, etc.) I couldn't say, though.
In any case, the relevant metric for fume hoods is generally considered to be the face velocity (not air exchanges per minute), which is the speed of
the air moving into the fume hood.
If your design goal is personal safety, then your design target is
generally face velocity (a good proxy for fume containment). If your design is quality of interior atmosphere, then your design target is air exchange
rate. As a rule, if you are targeting adequate face velocity in a fume hood whose sash is, say, 1/4 open, then you'll get a very high rate of air
exchange.
The only time I can imagine when you'd need to worry about the air exchange rate in the interior is when you're generating a whole lot of something
that is itself hazardous. Boiling off solvent fumes without other containment, for example, might bring you above the autoignition point. If you have
a variable air velocity hood and the sash closed, the air flow could be adequate for fume containment but too low for safety. Another interior product
in this category is smoke, if it obscures visibility enough to hide apparatus faults.User - 18-11-2009 at 04:38
