There is nothing magical about 18 inches, or 15 or 20 or 10.  The number is basically an arbitrary selection from back in the days before ASHRAE 110 was around.  Whatever sash height was found to produce 100 FPM face velocity was acceptable.  When I worked in the OHS (now EHS) group at Princeton University, back in the dark ages, we settled on a standard of 20 inches and set sash motion stops and warning signals for that height for new hoods as an energy saving measure.  Later, 18 inches became a widely used standard, but there is nothing sacred about either.

In addition to the issue of clearance of the work volume mentioned in an earlier response, you do want to be aware of the bio-mechanics of standing, or sitting, in front of a hood with the sash at whatever position you set as maximum.  Depending on how high the equipment is in the hood, what manipulations are required, and how tall the user is, the bottom of the sash ought to be high enough so as not to interfere with reasonably comfortable access to the work volume.

Also note that the smaller the perimeter of the open face the less susceptible the face will be to interference from cross-drafts in the lab. 

Peter Zavon, CIH

Penfield, NY

(on location in Cincinnati, OH.)

From: ACS Division of Chemical Health and Safety [mailto:DCHAS-L**At_Symbol_Here**PRINCETON.EDU] On Behalf Of Margaret Rakas
Sent: Friday, November 16, 2018 12:12 PM
To: DCHAS-L**At_Symbol_Here**PRINCETON.EDU
Subject: [DCHAS-L] Operating a fume hood w/Sash Height 15 inches max...

 

hi,

 

I wanted to check whether there are any health and safety concerns or any 'lessons learned' that would preclude using a continuous volume fume hood with a maximum sash height of 15 inches (rather than 18) if it passed ASHRAE 110 testing and had an average face velocity between 80-120 fpm.  We would train users regarding why the sash has to be at the 15 inches and there would be a sticker indicating maximum sash height.  It  is the sole fume hood in a lab, and would be used for mixing aqueous acidic solutions and dissolving rocks rather than complicated organic chemistry setups...I realize manufacturer's specifications are likely 18 inches (this is an old fume hood, probably from the 1980's) so looking up documentation on this is probably not going to be possible and might not be helpful, anyway.

 

A renovation several years ago turns out to have futzed the ventilation in this room; we're planning another renovation in the next year or so, but in the meantime it would be great to be able to use this fume hood if there are no regulations/guidance which would indicate it should not be used.

 

Many thanks,

Margaret

--

Margaret A. Rakas, Ph.D.
Manager, Inventory & Regulatory Affairs
Clark Science Center
413-585-3877 (p)