> They are usually used with VAV systems, where we would otherwise be at less than full system capable exhaust most of the time,

 

That's an important point that hadn't occurred to me before. Purge buttons make sense to me in VAV hood settings, but not CAV settings, assuming the fans are appropriately sized to provide additional exhaust.  Providing purge capability for general lab ventilation would require significantly more redesign of the overall ventilation system.

 

The original question, as I recall, was whether we require purge settings in laboratories. At Cornell, we don't because about half of our lab hoods are VAV and the other half CAV. As several people noted, our VAV fume hoods often (but now always) have purge settings within their control systems, and we are still (unfortunately) occasionally installing CAV hoods due to hood configuration or project budget limitations.  

 

The next challenge, of course, is helping lab workers understand what the purge setting of a hood does and the disadvantages of using it on a regular basis…

 

Thanks to all for their responses on this question.

 

- Ralph

 

 

Ralph Stuart CIH

Chemical Hygiene Officer

Department of Environmental Health and Safety

Cornell University

 

rstuart**At_Symbol_Here**cornell.edu