As you note, Ralph, most issues are not related to hood containment if face velocities are properly set per manufacturers' and end users' recommendations.

 

Some more relevant issues are:

 

1)  Air changes in lab.  Unless special detection and flushing technology is present, a small number of HE hoods in a lab may not produce enough air changes to promptly clear a lab proper from accidental spill vapors.  Also, lab thermostatic and humidity control may become issues.

 

2)  Low lab air changes may also produce an excessive thermal gradient from ceiling to floor.

 

3)  Errors in setting hood flows become more significant when nominal flow is a lower number. 

+ 10 FPM is fairly inoccuous at 100 FPM (+10%), but downright serious at 60 FPM (+18%).

 

4)  Solvents tend not to be problems as long as duct concentrations can never exceed 10% of the LEL.

 

I have had other experiences with HE hoods and would be glad to elaborate for any members wishing more detail.

 

Dr. Bob Haugen

labdr1**At_Symbol_Here**yahoo.com